World Of Rubber

Elastomeric materials
that meet tough challenges
Dr. Banja Junhasavasdikul

Elastomeric materials
that meet tough challenges
---------------------------------------------------------
W O R L D
OF RUBBER

WORLD OF RUBBER
Author
Co-authors
Ms. Jutarat Phanmai
Mr. Wittayanipon Chittanakee
Dr. Wattana Teppinta
Dr. Phattarawadee Nun-anan
Edited by
Prof. Dr. Suwabun Chirachanchai
Prof. Dr. Vernon Platts
Designed by
Ms. Nuchsarawadee Waed-udom
Published no. 1 : August 2022
© Copyright 2022: Innovation Group (Thailand)
ISBN: 978-616-92530-4-4
Published by Innovation Group (Thailand)
18 Soi Ramkhamhaeng 30 (Ban Rao),
Hua Mak, Bang Kapi, Bangkok
www.elastomer-polymer.com

Contents
Preface
Introduction
About the Author
Chapter 1 6
Natural Rubber
Chapter 2 36
Synthetic Rubber
Chapter 3 80
Vulcanization of Rubber
Chapter 4 103
Rubber Compounding
Chapter 5 120
Rubber Sponge
Wheel and Tire
Chapter 6 128
- Chapter 1
- Chapter 2
- Chapter 3
- Chapter 4
- Chapter 5
- Chapter 6
- Chapter 7
Chapter 7 134
Rubber Shaping and Curing
References
144
148
153
156
157
158
159
161

Rubber is a miracle elastomeric material for which there are hardly any alternatives
because of its elastomeric properties. Natural rubber and synthetic rubbers have been
developed to serve man-kind in sealing, transporting, conveying and containing solid, liquid
and gas that other materials find difficult to do.
How could we live in this world without rubber? How would we drive our cars without
rubber? Rubber products are everywhere and offer practical solutions for a wide variety of
design challenges.
However, before natural rubber and synthetic rubbers become useful, they have to
be converted from thermoplastics to thermosets. They have to pass through a long process
of mixing, shaping and curing. The process involves polymer and engineering knowledge in
product design, formulation design and process design to obtain rubber products with the
physical and mechanical properties that are required.
Innovation Group has been involved in rubber for over 40 years. Nowadays, our
Technical Center provides total rubber and polymer technical solutions to customers and
industries. We intend to offer a practical rubber handbook to the public, especially to industrial
partners, academic researchers and university students. We also hope that this will serve
as an educational tool to bring rubber chemists and technologists to increase knowledge in
rubber technology.
Technology Center
Innovation Group
August, 2022
PREFACE

WORLD OF ELASTOMER
Elastomers are a group of materials with viscoelastic properties which have
extremely weak intermolecular forces and low Young’s modulus. Compared to many other
materials, elastomers have high failure yield strain. They are amorphous when they are above
their glass transition temperature and allow significant motion. They can be stretched 1.5
times their original dimension and return to their original dimension when the force applied
to them is released. Elastomers are relatively soft when they are at ambient temperature.
Elastomers can be classified into 6 types:
1. Thermoset-elastomers. This is a group of elastomers that has unmatched energy
return elastomers. It is the largest group of elastomers that are used in various industries
and applications; over 12.7 million tons of natural rubber and 14.5 million tons of synthetic
rubbers are consumed every year.
2. Block copolymers. They are formed by both rigid and soft blocks in their structures.
Manipulating the types of blocks and relative ratios allows the creation of various types of
block copolymers. Advantages of block copolymers that they are recyclable and are comparable
with plastic processing equipment; for examples: block copolymers on styrene bases (SIS,
SEBS, SBS, and SEPS), block copolymers of polyamide block, and polyether block highly
engineered properties of block copolymers make 3D printing possible.
3. Ionic-linked elastomers. These are products from ionic cross-linking and have
outstanding dynamic and mechanical properties. An example is the ionic-cross-linking of
BR using zinc diacrylate in producing the inner cores of golf balls.
4. Thermoplastic Elastomers (TPE). This is a group of elastomers developed by blending
rigidthermoplasticwithelastomersi.e.,blendsofEPDM/PP,andblendsofnylonwithelastomers.
5. Thermoplastic vulcanizates (TPV). This is the blend of elastomers with thermoplastics
and elastomers that have been dynamically vulcanized during the mixing step. Examples of these
materials: PP/ EPDM, PP/ NR, and polyester/ acrylic rubber. Properties of the materials depend
largelyonthestructure,contentandparticlesizesofthevulcanizedrubbers.Theycanbeprocessed
like thermoplastic and recyclable elastomers.
6. Thermoplastic urethane elastomers (TPU, TPE-U). It is group of materials which
is formed when the isocyanate group reacts with the hydroxyl group of the alcohol. TPU and
TPE-U have long chains of soft segments of linear polyester (TPE-AU) or polyether (TPE-EU)
and short, hard segments of urethane that are formed of di-isocyanate and small alcohol
molecule chain extenders (i.e., butanediol). Thermoplastic urethane elastomers are tough
materials with excellent abrasion, tear and chemical resistances.
These elastomers are found in our daily life; they are consumed because of their
elastomeric property, being soft and can be formed into specific shapes. The low compression
set property makes these elastomers suitable for seals and gaskets to retain gases and liquid
in systems. Thermoplastic elastomers and the alloys of elastomers are recyclable and able
to be processed like thermoplastics becoming green material that is recyclable. Elastomers
are miraculous materials that can hardly be replaced with plastic or metal.
INTRODUCTION

7
Natural Rubber
1.1) Early History of Natural Rubber
The history of natural rubber began on June 11, 1496, when Christopher Columbus
returned from his second voyage to the West Indies, bringing back rubber balls. The people
of South and central America used natural rubber to make balls, containers, and shoes. They
called the rubber tree ‘Caoutchouc’, or the weeping tree. While the Spanish observed that
a rubber ball had a very good bounce property, it was merely a novelty. Natural rubber did
not catch the attention of Europeans until 1765, when a French scientist, Charles Marie de la
Condamine, went to study the rubber tree in the dense forests of Central America. He brought
back a milky liquid that he tapped from the rubber tree and called it “latex”, which in Spanish
means “milk.” He converted latex into rubber sheets that became widely used by scientists to
cover their scientific instruments during sea voyages to the West Indies. Throughout its history,
rubber has been variously known as elastomer, tree gum, Indian rubber, or caoutchouc.
In 1770, the British scientist who discovered oxygen, Joseph Priestley, developed
an eraser from natural rubber. In 1799, Antoine France de Fourcroy discovered to dissolve
natural rubber in turpentine, and later on Samuel Peal applied this technique to coat natural
rubber on leather, cloth, and paper. These coated materials were used for soldiers’ rubber
boots and raincoats. However, the products had a strong rubber smell and became very
sticky in summer.
During the seventeenth and eighteenth centuries, Brazil was the major country
exporting natural rubber, but not in large quantity until Thomas Hancock began to grind the
waste rubber from his rubber process. The waste rubber could be minced up very small
and the heat was generated during the grinding process. He found that he could more easily
fabricate the rubber into finished products after grinding. This method was known as
“mastication” In which the long chain of rubber polymer was ground into a shorter chain. The
ground rubber was easily dissolved in naphtha (hydrocarbon solvent). Thomas Hancock built
a rubber factory to produce rubber belts, rubber boots, stockings and medical equipment as
demand for natural rubber increased rapidly after Hancock’s discovery.

8
1.2) Discovery of Vulcanization led to the Growth of many Industries
in the 20th
Century
The discovery of vulcanization by Charles Goodyear was a landmark in the early history
of rubber. By nature, products made from natural rubber were sticky at high temperatures and
became rigid at low temperatures. Charles Goodyear was an American hardware merchant
who became very interested in natural rubber. In his book, Gum-Elastic and Its Varieties,
he called natural rubber a miracle material with many fantastic properties. He did research
aiming at modifying the properties of rubber to avoid its temperature defects by mixing
natural rubber with magnesia bronze powder or nitric acid, and even boiled it in lime water.
But nothing improved the property of natural rubber until his friend, Nathaniel Hayward, told
him to try sulphur. In 1841, while he was mixing natural rubber with sulphur and lead oxide
powder in his wife’s kitchen, he found that the rubber that had accidentally overheated had
very good elastomeric properties and did not harden in winter nor soften in summer. He called
this process “vulcanization”, from the name of Greek God, Vulcan. He patented this process;
patent no 3633 for “metallic gum elastic composite” in December 6, 1842.
From history, we know that many scientific discoveries were discovered accidentally.
Archimedes discovered the theory of specific gravity when he immersed himself into the
bathtub. He found that the volume of overflow water was equal to the volume of his body. He
calculated the specific gravity of a subject by using the weight of the subject (himself) divided
by the volume of overflow water. Sir Isaac Newton discovered the gravitational force when
an apple dropped on his head as he slept underneath an apple tree. In the case of Charles
Goodyear, he accepted that his discovery was accidental. After that discovery, a series of
developments in natural rubber science and technology in 1840-1860 led to a range of new
materials derived from rubber as well as new uses. Demand for natural rubber grew very
fast in 1880 to 1920 in Europe, a few thousand metric tons of natural rubber was brought from
Brazil in 1886 and increased to 15,000 metric tons in the next 5 years. During that time, the
USA was another country that consumed large quantities of natural rubber because of the
development of the automotive industry by Henry Ford. In 1890, 16,000 metric tons of natural
rubber were imported into the USA.

9
Natural rubber was an important material used in the twentieth century. After the
invention of tires made of canvas bonded with liquid rubber by John Boyd Dunlop in 1888,
natural rubber was used in making automotive tires and aircraft tires were first marketed
in 1910. The first rubberized bitumen was laid in the Rue Ferrier in Geneva in 1947. Rubber
became an important material used in machinery, highway and bridge construction, and
manufacturing in the industrial development period in Europe. Rubber-metal laminated bridge
bearings were used in 1957 on the Pelham Bridge in Lincoln, UK. Because of its nonslip and
its elastomeric properties, it was practical for seals and O-rings in machinery components.
Rubber conveyor belts were widely used to convey coal from the mines.
In the USA, consumption of natural rubber grew rapidly during 1900-1930 because
of the growing automotive industry developed by Henry Ford. To replace the American
conventional transport, the horse and wagon, Ford offered Americans a new transportation
that they could afford, the Ford Model T. Before the First World War, Ford produced 15 million
Model T a year which consumed a large amount of natural rubber from Brazil. As the imported
volume increased very fast. Ford established a rubber plantation and a rubber industrial
complex, called Fordlandia, at Tapajos in Brazil, to secure the supply of natural rubber.
Although he used his successful Detroit management concept to operate the rubber plantation
in Brazil, the project was not successful.
1.3) The Industrial Revolution
The Industrial Revolution marked a major turning point in the history of mankind for
two phases. The first happened during the period of 1700 to 1840 when and was the transition
of the manufacturing process from hand-made productions. The second phase happened
during 1840 to 1940, when more manufacturing processes were upgraded through the use of
processing lines and management systems to increase productivity. The Industrial Revolution
started in England after the invention by James Watt of the steam engine which came to use
in the cotton and metal industries. The change of cotton spinning from manpower to steam
engine consequently created a demand of metal parts in the machinery, as well as machining
tools along with batch process for small production.
Coal became the major source of energy because it produces more heat of
combustion than wood does. The need for increased chemical production was also an
important development during the first phase of the Industrial Revolution as evidenced from,
the large-scale production of sulphur, alkaline, and hydrochloric acid to support the textile,
soap, glass, steel industries, etc. Additionally, the transportation that changed from small boats
along canals and rivers to steamboats enable the travel to faraway lands to be faster and
easier. The invention of the internal combustion engine by Carl Benz, led to the development
of the passenger car in the second phase of the Industrial Revolution, whereas Henry Ford car
production was advanced through the concept of the assembly line for higher productivity and
cheaper costs. Growth in the automotive industry led to the need for supporting industries.
In USA, upstream and downstream petrochemical industries were developed along the Gulf
of Mexico, while steel industries were arise along Lake Michigan and rubber industries were
grown in Ohio.

10
Rubber was one of the important materials needed for the Industrial Revolution.
Without rubber and the discovery of vulcanization by Charles Goodyear, it would have been
impossible for James Watt to build the steam engine. Engines driving the movement of
machines in factories and cars needed rubber seals and belts; rubber seals stopped the
leakage of steam or oil in the engines, and belts were used to transfer the energy generated
by engines to drive machines. The development of the pneumatic tire by John Boyd Dunlop
led to the production of the passenger-car tire, which made riding in cars more comfortable,
thus increasing demand for Henry Ford’s Model T and created a high demand for rubber,
plastic, and steel.
During 1900 to 1910, people rushed to Santarem, South America, to seek their fortune
from rubber. Revenue from exporting rubber from Rio Negro was as high as 14 million pounds
sterling in 1906. People believed that there were millions of rubber trees in the dense forests
of South America, but those forests became a killing zone i.e., the powerful individuals hired
workers, including natives of the Amazon, to tap rubber latex and forced these people to
work for them by using firearms and might kill them when tried to run away.
From 1880 to 1930 was an important period in the economic and social history of
Brazil and the Amazonian regions. The extraction and commercialization of natural rubber
caused a rubber boom resulting from the high demand for natural rubber Industrial Revolution in
Europe and America. A large workforce was needed for the latex tapping and rubber sheets
production on the rubber plantations. Many people moved to Brazil and Peru but they were
struggled with many unforeseen problems. Finally, Ford left Brazil with a vast empty land
behind.
1.4) Natural Rubber Processing
After Charles Goodyear developed the process of vulcanization, natural rubber
became an important material in industrial development during the nineteenth century. Prices
of natural rubber rose quickly, and South America was then the only source of natural rubber.
Trade was well protected and exporting seeds from Brazil became a capital offense. In 1876,
Henry Wickham smuggled 70,000 para rubber seeds from Brazil and delivered them to Kew
Garden, England, then seeds were sent to India, Ceylon, and Singapore which were part of
British Empire. Before the Second World War, para trees were planted in Malaysia, Indonesia,
and the southern part of Thailand. The commercial production of rubber in Malaysia and
Singapore was heavily promoted by Sir Henry Nicholas Ridley who served as Scientific
Director of the Singapore Botanic Garden from 1886 to 1911, and Malaysia became the largest
producer of natural rubber. However, after 1985, the Malaysian government decided that
palm oil had more commercial value than natural rubber. Many natural rubber plantations
in Malaysia were converted to palm oil plantations since then.
In 1900, Phraya Ratsadanupradit Mahison Phakdi, the Governor of Trang (Thailand)
brought 22 rubber trees from Malaysia to plant at Trang. After that those rubber trees were
planted in 14 provinces in southern of Thailand.

11
Thailand is the world’s largest natural
rubber producer, with the production at 4.6
million metric tons per year. Indonesia is the
second largest producer, producing 3 million
metric tons per year, followed by Malaysia,
India, Vietnam, and China [1]. The total worldwide
production of natural rubber is around 13 million
tons with 75% produced in three countries i.e.,
Thailand,Indonesia,andMalaysia[2].Agricultural
researchers in Malaysia and Thailand have
worked hard to develop a rubber tree with
highly resistant to the “Toura” fungus that
spreads easily in rubber trees. They have also
studied how to get rubber trees to produce
high yields of rubber latex. They even went
back to the Amazon to collect seeds from para
rubber trees to bring back to their research
laboratories.
The average lifespan of a rubber tree is about 35 years. It takes 6-7 years before
a tree can be tapped for latex. After about 25 years, the trees produce less and less latex,
and they are then cut down and new trees are planted. The wood from para rubber trees is
useful and can be treated with chemicals and used for furniture. In the early morning when
the internal pressure of the tree is high, the farmers extract latex from rubber trees by using
sharp knives to cut away small pieces of bark. Trees are usually tapped on alternate days, but
during the summer when the leaves fall, farmers do not tap latex for 2 months. Latex, which
contains 25-30% dry rubber, slowly drips from the tapping cut for 3-4 hours and is collected
in a small container placed underneath the cut. The collected latex is transferred to a larger
container and carried back to the farmer’s home where it is cleaned by filtering through a
mesh screen and diluted with mineral-free water to 15% solid content in a coagulation tank.
Formic acid is added to the latex while stirring, until the pH of the solution reaches 4-5. At
this acidity, the latex coagulates and is left in the tank for another 4-5 hours. Then water is
squeezed out from the coagulant by using a two roll mill to form 5 millimeters wet rubber
sheets thick which are then dried in the sun for a few days before being sent to the rubber
smoking house where rubber sheets are dried at 60°C for 3 days. Hot air in the smoke house
comes from burning dry wood, and then hot air is pumped into the smoking room. Dried
smoked rubber sheets are pressed into 102 kilograms/bale. The product from this process
is called “ribbed smoke sheet (RSS)” which is widely used in tire manufacturing [3]. The
complete manufacturing process of RSS is shown in Figure 1.1

12
There are other processes to produce clean dry rubber. Instead of using hot air
from burning wood, hot air from burning liquefied petroleum gas (LPG) gives clean “air dried
sheet.” Block rubber is another type of dry rubber that is used to make colored rubber
products; small pieces of chopped wet rubber from the coagulation tank are dried in an oven
for 24 hours at 70°C as recommended by the Rubber Research Institute of Thailand (RRIT)
[4]. Besides dry rubber, natural rubber latex is used to produce rubber tubes, sheeting, film,
and dipped goods such as rubber gloves and condoms. Products from natural rubber latex
tend to be clean and have the excellent physical properties in terms of elongation, tear
resistance, and recovery. Natural latex is normally supplied to latex product factories as
60% dry rubber content (%DRC). The process for concentrating latex is simple as illustrated
in Figure 1.2. Fresh latex is centrifuged to obtain a 60% concentration which is known as
concentrated latex. Ammonia is added to the concentrated latex to adjust pH to higher than
10, and sometimes chemicals, such as methyl tuods (TMTD) and zinc oxide (ZnO), are also
added as preservatives [5]. Then, the concentrated latex is shipped in liquid form to factories,
where it is used for dipping, coating, and other products. Today, people who are routinely
exposed to rubber products, such as healthcare workers and medical doctors, often develop
an allergic reaction after the routine contacts with the products. Therefore, surgical and
examination gloves are alternatively made from synthetic latex such as chloroprene latex
and nitrile butadiene rubber latex (NBL) and are more acceptable as allergic-free products.
Figure 1.1 Manufacturing process of ribbed smoke sheet (RSS).

13
By nature, natural rubber has a very high molecular weight and contains small
amounts of proteins and enzymes. During storage, the protein and phospholipid contents of
the rubber, which makes the rubber harder [6, 7]. Therefore, to soften the rubber, it is
necessary to shorten the polymer chain at the beginning of the mixing process by “mastication,”
the mechanical shearing of the natural rubber between rollers inside the mixer
[8]. This process results in the reduction of the molecular weight and Mooney viscosity
(a measurement named after American physicist, Melvin Mooney) of the rubber, allowing
compounding ingredients to be easily mixed into the rubber. Sometimes special chemicals
(peptides) such as aromatic mercaptans (i.e., sulphur containing compounds) are added to
natural rubber at the beginning of the mixing process to reduce mastication time and to
achieve a constant Mooney viscosity. In order to manufacture natural rubber with consistent
quality, a small amount of the mercaptan additive is added during the coagulation step to
produce natural rubber with constant Mooney viscosity.
Because of the four electrons in the cis-1,4-isoprene double bonds in natural rubber,
chemical reactions are easily activated. This is the reason why natural rubber can be
vulcanized with sulphur at the positions of the double bonds. However, these double bonds
can also be activated by UV radiation, other chemicals, and heat, thus causing natural rubber
to have poor resistance to deterioration from those sources. Scientists have improved the
quality of natural rubber by creating epoxidized natural rubber. The epoxidized natural rubber,
with different percentages of epoxide group is commercially available with more higher price
than the natural rubber.
1.5) Natural Rubber: Structure and Function
In 1963, Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry for the
development for their eponymous catalyst for the production of stereoregular polymers from
propylene. The catalyst of organoaluminum compounds coupled with a transition metal. This
led to the development of synthetic rubber with a structure to that of natural rubber. But the
structure of natural rubber, which is known to be provenance of nature’s enzymatic control,
could not be duplicated by synthetic pathway because of the unique structure of natural rubber.
Figure 1.2 Concentrated latex supply chain

14
Natural rubber is a long chain polymer that contains repeated units of isoprene as a
result of a series of biochemical reactions starting form isopentenyl pyrophosphate within the
tree [9]. Natural rubber has very high molecular weight which results in less chain ends and
more entanglement than an equal weight of synthetic rubber [10]. The chain ends are weak
points at the molecular scale, because they do not transmit the strength of covalent bonds in
the molecular chain. Therefore, tensile strength is high for high molecular weight polymers
like natural rubber. In general, microstructural characteristic related to the branching lead to
a decrease of glass transition temperature (Tg) of a polymer. Natural rubber has these large
bulky groups of branched chains, thus causing the Tg of natural rubber to be as low as -72ºC.
As the microstructure of natural rubber, consists almost entirely of cis-1,4 polyisoprene
(Figure 1.3) [11], it is very stereoregular and confers many of the mechanical properties.
Crystallinity of natural rubber is a characteristic provided by this microstructure. If the units
of polymer chain are in a regular enough for spatial arrangement, the interactions between
units i.e., hydrogen bond, hydrophilic-hydrophilic or hydrophobic-hydrophobic, including
ionic-interaction among functional groups will lead to the crystalline structures which stiffen
the polymer. Because of stereo regularity, natural rubber form crystals upon storage (rubber
becomes harder after a period of storage) causing some processing difficulties and upon
stretching. The reversible crystallization upon stretching, ‘strain induced crystallization’,
which is caused by intermolecular forces in the polymer, provides many unique properties,
especially in excellent green strength, tensile strength, building tack, cut resistance, tear
resistance, cut and crack growth [12, 13]. These properties of find natural rubber are very
useful in tire applications.
1.6) Strain Induced Crystallization of Natural Rubber
Low-temperature performance of rubber is determined not only by the presence of
Tg, but also the presence of crystallinity. Polymers may show crystallization upon cooling of a
homogeneous melt, if the polymer chain (partially) aligns in an exothermic process. Melting is
an exothermic process occurring at the characteristic melting temperature with the enthalpy
released being a direct measure of the degree of crystallization. For crystallization formation,
the loss in entropy from the increased order has to be overcome by a sufficiently large gain
in the enthalpy. The present of crystallization in polymer results in a much higher modulus
and rigidity and a lower toughness, i.e., in the less rubbery behavior. In NR, isoprene rubber
(IR), butadiene rubber (BR), nitrile rubber (NBR), butyl rubber (IIR) and chloroprene rubber
(CR), the crystallization occurs significantly.
Figure 1.3 Chemical structure of cis-1,4-polyisoprene from natural rubber [11]

15
Strain-induced crystallization (SIC) may occur
for highly stereoregular rubber with melting temperature
below room temperature, such as IR, IIR, CR, and
especially NR [14]. At high strain levels, the polymer chain
aligns, which facilitates crystallization [15]. Stretching the
rubber chain results in a shift of melting temperature, to
temperature above the room temperature. The crystalline
regions formed result in self-reinforcement of the rubber
as evidenced from the higher tensile strength and
elongation at break. Natural rubber is renowned for its
high degree of SIC and its excellent ultimate properties
[15, 16].
Because of the resilience of natural rubber after
cross-linking upon curing, the elasticity and flexibility,
combined with crystallization induced toughness when
stretched. This mean less kinetic energy is lost during
repeated stress deformation. In tires, natural rubber is
used extensively to provide low heat build-up. For instance,
the shoulder temperature of heavy-duty truck tires may
rise up to 100ºC, and the heat of this magnitude increased the risk of a blow-out or other
delamination related to crock growth. Tremendous stresses also occur on the tread and
sidewall of the truck tire if it backs up over the curb, causing strain in the sidewall.
Besides hydrocarbons, natural rubber contains approximately 6% non-rubber
components (i.e., 2.2% protein and 3.4% lipid and other substances). It was found that these
small amounts of non-rubber also exhibit crystallization effects and affect associated
properties [17, 18]. In conclusion, it is difficult to replace natural rubber with synthetic rubbers
in tire applications.
1.7) Composition of Natural Rubber Latex
Although many species of plants are found to exude NR latex (NRL) on tapping, only
‘Hevea Brasiliensis’ plant is of commercial importance and accounts for 99% of World’s NR
production. NRL is mainly consists of rubber molecules as the major fraction and other
fraction called as non-rubber components such as protein, lipid, carbohydrate, etc. as
summarized in Table 1.1 [19, 20]. These rubber and non-rubber components may vary due to
various factors such as season, weather, soil and especially rubber clone [19, 21]. Some of
these components are suspended and dissolved in the aqueous phase of the latex, while
the others are adsorbed on the rubber surface of rubber particles. It is noted that 5% of
non-rubber components can be removed or degraded during dry rubber processing [20].

16
1.8) Rubber and Mastication
The very high molecular weight of natural rubber results in less chain ends and more
entanglement than equal weight of synthetic rubber. Its Mooney viscosity varies from type of
rubber tree, fertilizer used, season of rubber tapping and especially staging time. Its viscosity
affects rubber breakdown during plasticization which in turn affects the mold filling condition
in finished product production. In tire production, natural rubber has to be masticated to obtain
the required Mooney viscosity before the rubber compounding step. Mastication is a polymer
chain breakdown process which is related to mechanical breakdown at low temperature and
a thermo-oxidative effect at high temperature [8, 22, 23] as shown in Figure 1.4.
Latex
%w/v fresh latexa
%w/w dry matter of latexb
Figure 1.4 The effect of mastication in variation of temperature [22]
Table 1.1 Composition of NRL [20]
Composition
Rubber hydrocarbon 35.0 84.0
Lipids 1.3 3.2
Protein 1.5 3.7
Carbohydrate 1.5 3.7
Organic substances
0.5 1.1
Inorganic substances 0.5 1.2
a
Averaged from data published by Wititsuwannakul, D.; Wititsuwannakul, R. In Biopolymers. Vol. 2:Polyisoprenoids; Koyama, T., Steinbüchel,
A., Eds.; Wiley-VCH: Weinheim, 2001; pp 151–201.25
b
Calculated.

17
Figure 1.5 The reactions for the mastication of natural rubber [23]
The first effect occurs during low temperature shearing in the internal mixer (<80ºC).
The long-chain branched networks do not have time to relax and break by the reaction of
stresses, therefore, the shorter chain molecules are formed and the viscosity decreases [23,
24]. The rate of emission of heat depends upon the distribution of both shear and elongation
stresses, as well as the nature of the polymer and the temperature, the natural of polymer,
and the temperature. When the temperature increases, the polymer chains are more mobile
resulting in the decrease in relaxation time. The higher temperature, the lower the effect of
mechanical breakdown. The second effect is thermo-oxidation breakdown. This chemical
oxidative reaction happens when the temperature of mastication increases beyond 80ºC.
As consequence, the radicals species (R*) are form [25]. These free radicals react with
oxygen to form proxy radicals (ROO*) followed by transforming to be a cyclic diperoxide
or hydroperoxide group (ROOH). As a consequence of the hydrogen atom abstraction, the
free radicals are formed along the chains to propagate the chain rupture (Figure 1.5), as
described in the previous work [23].
The use of peptizing agents can accelerate the breakdown of natural rubber polymer
chains. The peptizing agents act by the mechanochemical and thermo-oxidative breakdown
of natural rubber as radical acceptor at low temperatures and as oxidation catalysts at high
temperature [23]. Peptizing agents usually are compounds of thiophenol or aromatic disulfides
combined with metal complexes of Fe, Cu, and Co as the catalysts for the oxidative breakdown.
1.9) Latex Compounding Technology
Latexes (or latex compounds) are complex colloid systems containing polymer
molecules as the major fraction. The polymer may be a homo-polymer or co-polymer
(random/ block/ graft) having stereo regularity with complexity related to the polymer chains
i.e., linear, branched and molecular weight distribution of the latex depend on the grade of
selected. Latex is rubbery or resinous in nature while the rubber molecules in the latex can
be cross-linked, plasticized and oil extended. The type of latex lead to the different mechanical
properties and temperature limits of serviceability [26]. The rubber particles is generally oval
with the particle size less than 5 micron under a typical distribution. The aqueous phase
consists of dissolved and suspended matter and the information about the concentration and
pH are essential for successful latex compounding. Latex products are subject to degradation,
therefore, an adequate antioxidant protection is necessary.

18
1.9.1 Preservation of NR latex
(I) High ammonia NR latex (HANR latex)
NR Latex causes coagulation within a few hours due to acidity through micro-organisms,
and the pH of latex decreases to roughly 5.0. Ammonia at a concentration of 0.7-1.0% is mostly
used for long term preservation [26], but it also serves as a latex bactericide and sequestering
agent for Mg2+
and PO4
3-
ions.
The weak points of using ammonia are from its strong odor, the additional cost,
including the thickening when compounded with ZnO which, interferes the gelation of latex
foam while further compounded with sodium silicofluoride (SSF). Therefore, the excess
ammonia has to be driven off before compounding.
(II) Low ammonia NR latex (LANR latex)
 Centrifugation is used to generate low ammonia NR latex (or LA-TZ) which is then
preserved with low ammonia coupling with other preservatives such as tetramethylthiuram
disulphide (TMTD) and zinc oxide (ZnO). With 0.025% TMTD/ZnO and 0.05% of ammonium
laurate on latex, the ammonia level is less than 0.29% [26]. The LA-TZ latex is commonly used
in all dipped products application.
 Zinc dialkyl dithiocarbamates i.e., zinc dimethyldithiocarbarmate (ZDMC) and
zinc diethyldithiocarbarmate (ZDEC), at 0.1-0.2 % concentration along with 0.2% ammonia and
0.2% lauric acid enhance the stability of NR latex to be in the similar level as 0.7% ammonia
preserved latex without any significant effect on vulcanization [26]. However, the latex may
discolor badly on aging, in some cases, the color develops from the trace amount of copper
contamination, so-called (copper staining).
 For other types of low ammonia latex, the latex is preserved with a low ammonia
content (about 0.2%) and 0.2% boric acid coupling with 0.05% lauric acid, which is a common
LA-preservative system [26]. The benefits of LA-TZ latex are its ease of use, low cost, low
toxicity and free from discoloration. However, the un-vulcanized deposits tend to soften more
quickly.
 Low ammonia latex can also be preserved by coupling 0.2% sodium
pentachlorophenate with 0.2% ammonia as a stabilizer [26] although this method is not
widely used due to toxicity of the stabilizer which is linked to pentachlorothiophenol.

19
1.9.2 Destabilization of latex or gelation
Chemical methods (acidification, addition of salts of polyvalent metals, higher
concentration of salt) and mechanical methods (mechanical agitation and dehydration) can
both destabilize NR latex. Destabilization produces that generate homogenous destabilization,
also known as gelation, of a three-dimensional aggregate of rubber particles. The NR latex
has good gel strength that most suitable for latex applications and foamed products, though
excessive stabilization results in weaker gels and slower gelation rate. Gels do not have a
consistent composition (or structure), and the film shrinks due to aqueous phase exudation,
which is retained in the interstices of the gel.
Three methods of Gelation [26]
(I) Organic acid/acids or acid liberating substances
(II) Use of salts of multivalent cations
(III) Application of heat
(I) Gelation By Acids
Rubber particles are stabilized as a result of the net negative charge (from both
lipids and protein structure on the surface of rubber particles) which creates repulsive
forces between rubber particles. The pH of latex is dropped once acid is added, resulting in
a decrease in the ionization of adsorbed anions and a decrease in repulsive forces between
rubber particles, resulting in gelation.
(II) Gelation by Salts
Normally, calcium nitrate is used in the dipping process to create gels of the
compounded latex on the ‘formers’. Calcium ions destabilize the latex by forming insoluble
salts with all fatty acid soaps and protein solution. Calcium ions also produce a high
concentration of ions, reducing the colloidal stability of NR latex.
(III) Heat sensitized gelation
Immersing a hot former in to a suitably heat sensitized latex compound is knowns
as heat sensitized gelation. Zinc amine ions and hydrophilic polymers can both help to heat
sensitize NR latex [27]. Zinc oxide-ammonium salt process (ZOA) facilities the role of ammonia
solutions to increase the solubility of zinc ions [26]. The addition of an ammonium salt also
increases the ionic strength of the aqueous phase and contribute to destabilization. Because
the gelation occurs at room temperature in a short period, the ZOA process is truly heat
sensitive. However, heat accelerates the process. The gelling time or the thickness of dipped
product can be used to determine the degree of heat sensitization. The thickness of the dry
film is influenced by a number of processing parameters such as temperature of the former,
rate of immersion, dwell time and heat capacity of the former including the compounding
latex characteristics [27].

20
1.9.3 Film formation and structure
Formation of film involves immobilization of free-moving polymer particles when
brought into contact. In most latex-goods manufacturing processes, the contact between
polymer particles is achieved by the ‘gelation’ process as a consequence of pH dropping with
ammonia stabilizer during the initial stages of drying. Note that if the pH is maintained by
using a fixed alkali stabilizer like KOH, this process will not proceed. Gelation does not affect
the rate of drying of the film or any other characteristics.
Beforedrying,thefilmisleachedwithcleanwatertoremovewater-solublechemicalin
compoundingresiduesfromthecompoundingprocess,aswellasresidualcoacervant(i.e.,aqueous
calcium nitrate) and other surface-active substances [26]. This improves the film’s feel,
eliminates porosity defects and makes it resistance to water absorption and aging. The film
thickness of dipped goods varies between 0.1 mm to 0.2 mm depending on the viscosity of
the latex compound, a multi-dip process is used to achieve the necessary film thickness.
1.9.4 Chemical modification of NR latex
(I) Pre-vulcanized natural rubber (PVNR) latex
PVNR latex is a chemically modified NR latex, which on drying gives a vulcanized
film, and can be produced latex stage with concentrated latex (or HANR latex). During the
maturation process, the PVNR latex compounds containing ZnO and ultra-fast accelerators
(i.e., ZDBC or ZDEC) and other ingredients usually achieve some degree of pre-vulcanization.
The formulation of produced PVNR latex is prepared as follows [26]:
In the jacked mixing tank, the HANR latex and ingredients are mixed with stirring.
When the desired degree of cross-linking has been achieved, the latex is heated up to
50°C-60°C by flowing hot water through the jacket for 3-5 hr or when QC the QC tests
confirmed the as-desired of cross-linking. The latex compound is cooled to room temperature,
then filtered or collected.
The degree of cross-linking of latex compounds is determined by swelling method,
combinedsulphuranalysisortensilepropertyevaluation[28].PVNRlatexispopularinthemedium/
Compounding Ingredients
60%HANR latex 100 167
50% ZnO dispersion 1 2
50% ZDEC dispersion 1 2
50% Sulphur dispersion 2 4
10% KOH Solution 0.3 3
10% Sodium Caseinate 0.2 2
Dry
(by weight)
Wet
(by weight)
Table 1.2 Compounding ingredients of PVNR latex

21
Figure 1.6 Poly(methyl methacrylate)-grafted-natural rubber (Heveaplus MG) [30]
smallsectordippedproductbecauseisdoesnotnecessaryorislimitedtoincorporatethepigments
such as balloons, medical product and feeding bottle teats. The cross-linking can be achieved
by reaction with sulphur, sulphur donors (i.e., dithiodimorpholine (DTDM), tetraethylthiuram
disulfate (TETD) and tetramethylthiuram disulfate (TMTD)) or by gamma radiation. The
compounding formulations can be varied to be suitable for the application. ZnO is not
necessaryif ZDEC/ZDBC is present. ZnO reduces the film clarity but it be substituted by ZnCO3
to improve the clarity. For peroxide cross-linking, tert-butyl hydroperoxide and tetraethylene
pentamine are used. Maximum film clarity is obtained by using ZDBC alone. The cross-links
found in the pre-vulcanized latex are predominantly polysulphidic (except for sulphurless /
sulphur donor cures).
(II) Heveaplus MG graft polymers
Poly(methyl methacrylate)-grafted-natural rubber (Heveaplus MG) grafted side
chains of polymethyl-methacrylate on NR molecule [29] as shown in Figure 1.6.
The polymethyl-methacrylate used are 15% (MG15), 30% (MG30) and 49% (MG49) [31].
As the amount of polymethyl methacrylate in MG increases, their capacity to form declines.
This form of latex can be blended in any proportions with unmodified latex. The modulus,
tensile strength and tear strength of such blends are significantly improved. When methyl
acrylate is partially substituted with butyl methacrylate in the grafting reaction, the film
forming properties improve [29, 31].

22
(III) Hydroxylamine modified latex (HRH latex)
Because of the cross-linking process, the viscosity of the concentrated latex increases
during storage. During the first 20-30 days, the rate of storage hardening is faster (and over
100 days the process is completed). When hydroxylamine is added for 0.15 at the latex
production stage; the storage hardening effect is inhibited [32]. The vulcanizates show slightly
low modulus (resulting in less volume shrinkage of latex foam), whereas other properties
are unchanged. This type of latex is used to make latex foam and latex adhesives for use in
footwear [26].
1.9.5 Latex compounding ingredients
The transformation of wet NR latex into a final product is accomplished using various
processes, which are based on the criteria of minimum energy consumption during various
stages (High energy consumption during drying has been a major concern).
In all the processes, a stable colloidal system is maintained for a desired time after
which the system is made unstable to convert the same to a solid product. Maintaining the
balance of stability is the major challenge.
The latex compounds contain four or more distinct dispersed phases and are highly
polydispersed with several different surfactants. The aqueous phase of high ionic strength
gives the NR latex compounds a relatively low colloid stability but facilitate conversion to
solid products. For dipped goods, the latex compounds used must produce continuous films
on the former and maintain film integrity during drying and vulcanizing stages.
No. Function
1. Vulcanizing agents Sulphur, Sulphur donors & others.
2. Accelerators Dithiocarbamates, Thiazoles, Thiurams,
Xanthates
3. Antioxidants Amine derivatives, Phenolic derivatives.
4. Fillers & pigments Inorganic, Organic.
5. Surface-active agents Anionic, Cationic, Amphoteric, Non-ionogenic.
6. Viscosity modifiers Plant hydrocolloids, Proteins, Polyvinyl alcohols,
Cellulose derivatives, Starches, Polyacrylates,
Carboxylate polymers, Colloid clays, etc.)
7. Other ingredients: Mineral oils, Waxes, Resins, Antifoaming agents,
Antiwebbing agents, Corrosion inhibitors, etc.
Ingredients
Table 1.3 Latex compounding ingredients

23
1.10) GAP/GMP in Natural Rubber
Natural rubber is a product produced by photo-synthesis of carbon dioxide and water
inside the rubber tree. Its quality varies from type of rubber tree, fertilizer used, season in
tapping and method of tapping and process in making rubber sheet. Therefore, the quality of
natural rubber varies widely from various factors. The total process of natural rubber is very
labor-intensive. It is different from synthetic rubber that is produced from a continuous
operating process in petrochemical plants that have high processing control. However, natural
rubber has its unique properties that are hardly replaced by other materials. That is why natural
rubberis 42% of total rubber consumed every year. Currently, natural rubber is grown in many
countries and its supply is over demand. Consumers buy natural rubber firstly because of
price and quality. Large tire companies will select natural rubber suppliers based on price,
consistent supply, and quality management in the total supply chain.
Thailand is the largest natural rubber producer. Total area of the rubber plantation
is 20.58 million rai. (8.13 million acres.). Over 1.2 million people are involved in natural rubber
plantations. Yield of production is 234 kilograms/rai/year is lower than what it should be of
300 kilograms/ rai / year. Meanwhile the quality of rubber sheets varies from time to time.
It passes through many processes such as latex tapping, latex collection and rubber sheet
process in the factory. Without good knowledge, farmers produce natural latex using hazardous
chemicals and wrong process in latex tapping, rubber coagulation and drying. Natural rubber
produced in the conventional method normally has lower yield and contamination by foreign
materials and chemicals, so the rubber sheets are classified as substandard grades. This
reflects incomes of rubber farmers which have been at a low level for years. They need
financial support from the government from time to time.
Ms. Preprame Tassanakul, Director of Rubber Testing and Certification Center, Southern
region, Rubber Authority of Thailand, analyzed those problems. She found that method of
tapping latex, contamination in the latex during tapping, and processing method in making
rubber sheets, etc. are not well managed. Main reasons behind this are:

24
1. Thai rubber farmers do not have sufficient knowledge in rubber plantation.
2. They lack knowledge and good practice in tapping latex and how to collect clean
latex to deliver to rubber drying plants.
3. In the rubber processing plant, is operating in fully manual process and there is
no quality control system etc.
Ms. Preprame set up standards and educated these farmers how to produce good
latex to supply to rubber plants so they could produce consistent quality rubber sheets
supply to the market. She wrote a handbook of ‘How to produce rubber smoked sheet’ and
enhance it to the agricultural standard (TAS 5906-2013) [33] and a handbook in producing
crepe rubber in the following five years (TAS 5907-2018) [34]. Those handbooks’ contents are
related to the process of Good Agricultural Practice (GAP) and Good Manufacturing Practice
(GMP) to encourage good practice by rubber farmers in their farms, latex tapping, collecting
and delivery to rubber plants. In the rubber processing plants, GMP guides them how to have
good manufacturing practices to produce consistent rubber sheets. She also provides
standards in the process of producing good quality latex (TAS 5908-2019) [35], cup-lump
rubber to supply to the rubber plants (TAS 5910-2020) [36] and excellent latex collection
centers (TAS 5911-2021) [37].
Innovation Group realizes the value that Ms. Preprame Tassanakul produces by GAP
and GMP programs and gives strong support to GAP and GMP programs.
1.10.1 What is GAP?
Good agricultural practice (or GAP) is good practice in rubber plantation [35].
1. Land to plant rubber trees: Owner must have the legal right on those lands (not
from the deforested area). The suitable planting area is the tropical monsoon area with
average rainfall 1,250 mm per year, pH in soil in the range of 4.5-5.5
2. Hazardous chemicals and herbicides are not allowed to be used.
3. Starting from clones of rubber tree that are recommended by Rubber Authority
of Thailand. Rubber trees to tap latex must be mature, rubber trees must have a minimum
circumference of 50 cm and height not less than 1.5 meter from the ground level.
4. Tapping latex by barking half of the circumference of a rubber tree with an angle
of 30-35 degrees. Tapping should be done 2 days and stop 1 day or in a dry period or the dry
areas, tapping should be done 1 day and stop 1 day (alternate days). This should be done after
the mid-night till 6 o’clock in the morning.
5. Collection of latex should be done and delivered to the process plant, not over 8
hours after tapping.
6. Equipment to use; latex cups and containers for collecting latex have to be kept
clean and avoid any contamination from unwanted material.
7. Filter the latex from latex cups into the stainless steel container with 7 mesh-filters.
8. Record quantity of latex collected every time as a production statistic.
9. Clean rubber cups and place upside down, every time after collecting the latex, to
prevent contamination of foreign material.

25
1.10.2 What is GMP?
Good manufacturing practice (or GMP) is the instructions on good manufacturing
practice in rubber plants [37].
1. Condition of rubber plant: it should have a proper design and construction according
to standards of processing plant with good utilities.
2. Stainless steel is recommended to be used in the latex receiving station and the
coagulation tank.
3. Every arriving latex batch is required to go through standard quality control.
4. Arriving latex must pass through filtration with 10 mesh-filters.
5. Take a sample of each arriving lot and check dry rubber content (DRC) to calculate
weight of formic acid to use. Only formic acid is used in rubber coagulation (adding 4% by
weight of formic acid to 100 parts DRC).
6. Check volatile fatty acid number (VFA no.) of latex accordingly to ISO 506
7. Stainless steel coagulation tank is recommended.
8. Squeeze water out from the sponge sheets by a clean two roll mill. Dry those
sheets under the shade before sending to the smoking room.
9. Smoke rubber sheets at a controlled temperature of 55-60ºC for 3 days.
10.Packthesheetinplasticbagsof25or30kilograms/pack(accordingtotherequirement
of customers)
1.10.3 Benefits of GAP/ GMP
1. Good properties control: dirt, ash content, volatile materials and has a good initial
Mooney viscosity range.
2. Contamination free
3. Traceability of rubber sheet, right from original plantation.
4. Easy to handle (a 30 kilograms/bale instead of a 110 kilograms/bale) and power free.
5. Non-hazardous chemicals.
6.Themostimportantarehigheryield,betterproduct,andfarmerscansellatahigherprice.
GAP and GMP are good standard practices in producing consistent quality of natural
rubber. However, this needs cooperation from rubber farmers in practicing GAP/GMP standards.

26
1.11) Value Chain of Natural Rubber
Natural rubber is one of the strategic agricultural products of the country. Thailand is
the largest rubber producer harvesting 4.2 million tons each year involving 1 million families
in the upstream industry of natural rubber. However, natural rubber has a very long value
chain. In order to develop a sustainable growth of natural rubber, we have to consider how
to create values to this total value chain. The total value chain of rubber can be divided into
three sectors:
1. Upstream industries involve the growers and tappers. This has to add value to primary
production of rubber latex or basic dried rubber sheet, such as cup lump, scrap raw sheet and
crepe rubber. Starting from diagram 1, there are 1 million families or approximately 6 million
of the Thai population living in rubber plantations and working in the upstream industries.
2. Intermediate or midstream industries may be called rubber processors. They
produce rubber on plantations and convert it into semi-finished products, such as ribbed
smoked sheet rubber (RSS), standard Thai rubber (STR) (or block rubber) and concentrated
latex
3. Downstream product industries produce various rubber products and latex dipped
goods
Over 85% of Thai intermediate rubber (dried rubbers and concentrated latex) is
exported. The main importing countries of Thai intermediate rubber are China (58%), Malaysia
(15%) Japan, and Korea. The main use of rubber sheet is for the production of tires (about
49%) followed by latex dipped goods (13%). Natural rubber supply is largely impacted by
market pricing, which is mainly set up by buyers, China, Singapore and Japan. Because the
price setting by buyer markets is done irrespective of production costs, artificially low prices
disincentivize producers from sustainable production and replanting, further compromising
the supply of natural rubber. Meanwhile two international organizations, Forest Stewardship
Council (FSC) and Global Platform for Sustainable Natural Rubber (GPSNR) are becoming
influencers to the manufacturing and marketing of natural rubber. In order to sustain growth
of natural rubber, the Rubber Authority of Thailand should support upstream and midstream
of natural rubber to qualify for FSC and GPSNR certification.
Figure 1.7 Total supply chain of natural rubber (NR) [38]

27
1.11.1 Global platform for sustainable natural rubber (GPSNR)
The GPSNR is an international, multi-stakeholder, voluntary membership driven
platform for improvements in socioeconomic and environmental performance of the natural
rubber value chain by defining and implementing industry wide standards on fairness, equity
and environmental sustainability. The development of the GPSNR was initiated by the CEO’s of
the World Business Council for Sustainable Development (WBCSD) and Tire Industry Project
(TIP) in 2018. The members of the platform include tire manufacturers, rubber suppliers,
rubber processors and vehicle makers. The GPSNR policy frame works are composed of 8
sustainable natural rubber principles, [39]:
Commitment to legal compliance, forest sustainability, respect human rights,
community livelihoods, increase production efficiency, traceability and management, systems
and processes to drive effective implementation of policy, and reporting [40].
1.11.2 Forest stewardship council (FSC)
FSC is an independent, not for profit, non-governmental organization established to
support environmentally appropriate, socially beneficial, and economical management of the
world’s forest. To achieve the mission of FSC, the FSC has developed a set of 10 principles
as follows [41]:
Commitment to legal compliance, respect worker and human rights, enhancing the
social and economic well-being of local communities, indigenous peoples’ rights, benefits
from the forest, environmental values and impacts, management planning, monitoring and
assessment, forest conservation, implementation of management activities [41].
1.11.3 How to apply the FSC trademark in the finished goods
Chain of custody certification (COC) of any company which is the process to ensure
that all the processes or transformations of FSC-certified products have been checked at
every processing stage (Traceability system) [42].
Remark: FSC certification applies to plantation owners. COC certification applies to
manufacturers, processors and traders of FSC-certified forest products.

28
1.12) Rubber Gloves
In 1894, William Stewart Halsted, the first chief of surgery at Johns Hopkins Hospital,
invented surgical gloves for his wife as he noticed her hands were affected by the daily
operations she performed and in order to prevent medical staff from developing dermatitis
from surgical chemicals [43]. In 1965, Ansell Rubber Co. Pty. Ltd. developed the first disposal
medical gloves. Nowadays, over 300 billion pairs of rubber gloves are consumed every year
and demand remains high for medical gloves that protect against virus [44]. USA, Europe and
Japan consume 60% of the rubber gloves produced. Malaysia is the largest glove manufacturer,
accounting for 63% of the total world production. Although, Thailand has the main source of
raw materials, Thailand produces only 18% of the world’s glove market. China is the third
highest producer at 10%, while others such as Indonesia, Belgium and Vietnam share 9% [44].
The major materials in producing rubber gloves are natural rubber (NR) latex,
acrylonitrile latex, neoprene latex, isoprene latex and polyvinyl chorine latex. The main reason
why people use rubber gloves is to protect their hands from contacting fluid when they are
performing an operation; for example: household gloves are used by housewives in their
daily works, washing, cooking and gardening. Household gloves protect their hands from
detergents, contact with food and soil during their operations and NR latex or chloroprene
gloves are widely used. Industrial gloves such as latex gloves protect workers from contact
with chemicals, but NBR (nitrile) or chloroprene gloves, which have oil and chemical
resistances, are the preferred gloves. Surgical gloves need good resistance to fluids and
have good permeability resistance; therefore, chloroprene gloves are preferred. Medical and
examination gloves, which are consumed in very large quantities are typically made the areas
of nitrile and NR latex. However, the highest demand for disposal gloves; they are vital goods
in the healthcare environment. They not only protect healthcare providers and patients from
potentially harmful microorganisms, but they also assist set a standard for hygiene and care
in the business. Different materials and design choices make certain products better suited
for different medical environments. Nitrile, NR and vinyl latex are most common material
used in disposable gloves. For decades, NR latex gloves have been used as the medical
disposable gloves worldwide, because they had been recommenced for protection since 1980
against blood borne pathogens like HIV. Disposable gloves have to be comfort, able have a
high degree of touch sensitivity and be relatively cheap. NR latex gloves were most popular
until it was found that many people were allergic to the NR latex. Vinyl gloves are prepared
by using PVC liquid, as a petroleum-based material. The advantage of vinyl gloves is their
low cost, but they are less durable than NR latex gloves and have limited protection against

29
biomedical exposure. Nitrile gloves came into prominence in the market in the 1990s as the
leading rubber gloves, being the ideal choice for disposable gloves because they have
exceptional puncture-resistance and are not allergenic to human skin.
Figure 1.8 Market demand and trend–overall NR vs NBR gloves [44-46]
1.12.1 Overview of the global glove market
In 2020, Thailand emerged as the world’s second largest rubber gloves manufacturer
and exporter behind Malaysia. Thailand exported over 20.5 billion pairs of rubber gloves, an
increase of approximately 22% from the same period the previous year due to the Covid-19
outbreak [44]. According to EIC, Thailand earned approximately 700 million USD per year from
glove exports, while Malaysia’s earned more over 1 billion USD per year from rubber glove
product exports [39]. Malaysia imports concentrated latex from Thailand in order to manufacture
rubber products (i.e., glove, condom, glue and pharmacy products). However, Malaysia
produces rubber products for domestic consumption as well as export to other countries on
the international market [47]. Malaysia’s gain could be viewed as Thailand’s economic loss,
as Thailand lack the ability to transform concentrated latex into a valued-added product [44].

30
Figure 1.9 Market demand and trend – by Segmentation [48]
Product (1) Latex glove (1) Nitrile glove (I) enhancing the concen trated latex
(2) Nitrile glove (2) Latex glove producers into the rubber glove market
(II) improve the drawback of NR product
(i.e., reducing-proteins)
Manufacturers Not many Various The support from Thai government and
manufacturers (i.e., manufacturers (i.e., the related rubber industry
only 1 manufacturer 4 manufacturers
among 5 largest among 5 largest
manufacturers 2021) manufacturers2021)
Efficiency 6-7 million gloves/ 20 million gloves/ Increased advanced technology for
(glove/month) month (type on month (based on faster and higher production capacity
different number different number
of factories) of factories)
Export market Mostly exported to Mostly exported to Extend distribution to new markets (i.e.,
USA,anapproximate USA,andapproximate Asia Pacific, South-East, Middle East
rate 36% rate 33% and Africa), due to lower price of latex
glove and lower number of proteins-
allergic cases
Rubber glove
industry
Strategic adjustment
of Thailand
Thailand Malaysia
Table 1.4 Thailand’s and Malaysia’s rubber gloves industries comparison [44, 47]

31
Figure 1.10 Comparison of latex glove and synthetic gloves (i.e., nitrile and vinyl gloves)
1.12.2 Types of rubber gloves
Rubber gloves are protective items to cover the hands of the wearer and provide
physical protection for the wearer. There are two main categories of rubber gloves by material;
namely, (I) latex glove and (II) synthetic gloves (i.e., nitrile and vinyl gloves). In addition,
rubber gloves can be classified into two types: (I) medical and (II) non-medical gloves. The
market is divided into end-user industries such as healthcare, food and beverage, automotive,
machinery, and others. The key information for the two major categories of rubber glove by
materials are illustrated in the figure below:
1.12.3 Rubber glove manufacturing processes
Rubber glove manufacturing processes are normally comprised of seven steps: (I)
raw material testing, (II) compounding, (III) dipping, (IV) leaching and vulcanizing, (V) stripping,
(VI) quality control, and (VII) packing [49-51]. These process steps are shown in the figure
below:

32

‹ Raw material testing
Before the latex compounding process, raw materials are tested in the factory’s
laboratory, where materials are subjected to various detailed and stringent quality tests (i.e.,
latex and chemical properties testing).

‹ Latex compounding
Ingredients and their functionalities for rubber glove manufacturing
processes are summarized in Table 1.5 [51]. All ingredients are generally mixed with latex (i.e.,
concentrated latex, HANR latex) such as surfactant or stabilizer, activator, accelerators,
antioxidants, and vulcanizing agent (i.e., sulphur) based on the specified formulation. Before
being fed to the production line, the compounded latex is tested to ensure that it meets the
specification requirements.
Figure 1.11 Rubber glove manufacturing process model [52]

33
60% HANR latex 100.0
10% KOH 0.30
20% K-laurate 0.20
50% ZnO 0.40
50% Wing stay L 0.75
50% ZDC 0.75
40% SDBE 0.50
50% Sulphur 0.50
Total 103.40
Ingredients phr
Table 1.5 Rubber formulation of rubber glove (surgical glove) [50, 51]
‹ Dipping
Before forming the gloves on glove molds (or formers), cleaning and dipping
processes of the rubber glove molds are required. Firstly, the rubber glove molds are cleaned
with acid (i.e., H2SO4 or HCl) or alkali (i.e., NaOH or KOH) to remove contaminant and dust,
followed by leaching with clean water, and then dried at 50ºC. After that, the glove molds are
dipped into the coagulant tank (i.e., the mixed solution of CaNO3 or CaCl2 and CaCO3), which
contains a processed chemical.
Next, the glove molds are then dipped into the latex dipping tank to coat them with
a thin layer of latex. Coagulant and compound latex tanks are both controlled for properties
and conditions such as total solid content (%TSC) and temperature, and so on.
‹ Leaching and vulcanizing
The gel films on rubber glove molds are beaded, further dried, and then leached in
the pre-leached tank before they are further vulcanized to provide good physical properties
and reduce moisture content.
In the leaching step, all gloves are moved through a water bath to leach excess additives
from previous stages (i.e., coagulant and other ingredients) and reduce protein content. The
effectiveness of this process is dependent on three major factors: (I) the temperature of the
water, (II) the duration of the process, and (III) the rate of water exchange.
In the vulcanizing step, the latex film is vulcanized by the combination of sulphur
and accelerator generating cross-linking of the rubber molecules while being heated (i.e., 100-
120ºC); this provides high elasticity and good tensile strength to the rubber film.
‹ Stripping
In this step, the leached gloves are dipped into a closely controlled wet-slurry tank
to remove any protein buildup, including, bacterial and other contaminants which remain on
the rubber glove. Finally, auto-stripping lines are used to remove the gloves from the formers.

34

‹ Quality control
The quality control process is carried out by random sampling after all the batch
of products has been finished. Several methods are used to inspect the products. The first
method is called “inspection”. In this method, air blowers are used in this method to investigate
for pin holes on the glove’s surface; if any are found, these gloves are rejected. For this method,
air is blown into the gloves for about 1 h.
Furthermore, the watertight test is the second quality control method to which the
gloves are subjected. This method is similar to the air blower inspection test, except that
water is poured inside the gloves to investigate any product defects.
The third method of quality control is a visual inspection to check for stain marks on
the gloves and/or misshaped gloves. Gloves with defects are rejected. Finally, size, thickness
and aesthetic appeal are all inspected to ensure that the gloves produced is in accordance
with specifications.

‹ Packaging
The rubber glove packing area is under a tightly controlled dust-free environment
by using a hygienic filtered air system. Before packaging, packing operators must perform
one final visual inspection and remove any defective products before packing the gloves. Lot
continuing a hundred pairs of a specific size are first weighed and, if within specification are
packed into small boxes. Finally, these boxes are loaded into larger cardboard boxes ready
for delivery to customers.

‹ Standard specification of rubber gloves
The specifications for rubber surgical gloves (in terms of physical requirements)
are summarized in Table 1.6.
Table 1.6 Physical requirements of surgical gloves
I 5.5 24 750 18 560
II 7 7 650 12 490
Before aging
500%
modulus (MPa)
Tensile
strength (MPa)
Elongation at
break (%)
Elongation at
break (%)
Tensile
strength (MPa)
After aging
Type
Note: Type 1 is compounded primarily from natural rubber latex and Type 2 is compounded from a rubber cement or from synthetic rubber latex.

35
1.12.4 Future & market trend of rubber glove
Although demand for rubber gloves varies by product, continued growth in demand is
possible against a background of rising hygiene and safety consciousness among consumers.
The following significant shifts in disposable gloves demand have occurred in recent years [53]:
ƒ HighdemandforsyntheticglovesduetotheriskoftypeIhypersensitivityinlatexglove
ƒ More light weight or less thickness of rubber glove (especially in NBR glove)
ƒ Ease of working while wearing gloves
ƒ More sustainable production of degradable gloves
COVID-19 health workers use approximately 80 million gloves per month, but the
synthetic material from which they are primarily made is disposed of in landfill for 100 years.
However, latex gloves can biodegrade 100 times faster than the synthetic glove and produce
30% less waste during production [54]. This has resulted in more sustainable and green ways
of life; slowly but steadily, everything is shifting towards biodegradable products.
Previous works has reported the potential of biodegradable natural rubber latex
gloves for commercialization. It was found if a new green additive was included in the NRL
gloves formulation to accelerate the biodegradation process, the biodegradable gloves could
be decomposed in soil in four weeks [55]. Therefore, a special formulation of latex glove
(more biodegradable material) is an ideal future composition for sustainable manufacturing,
providing a greener future to benefit everyone.

37
Synthetic Rubber
2.1) Synthetic Rubber
Throughout the nineteenth century, many scientists tried to determine the structure
of natural rubber. The English scientist Michal Faraday (1791-1867) found that natural rubber
has the chemical formula C5H8. Charles Williams (1829-1910) was another English scientist
who analyzed hurl rubber by destructive distillation and obtained a large quantity of light oil
that he called isoprene. In 1879, Gustave Bouchardat, a French chemist, heated isoprene with
hydrochloric acid, so was the first person to obtain a synthetic rubber-like product. The first
truly synthetic rubber was made by William Tilden 3 years later, when he heated turpentine
with a catalyst and obtained isoprene. Incidentally, Carl D Harris and Francis Matthews along
with E. Halford Strange discovered that isoprene could be polymerized more rapidly by sodium.
In the early 1900s when prices and demand for natural rubber increased readily, Carl
Duisberg, CEO of Bayer in Germany, realized the opportunity and the benefit of synthetic rubber
as a substitute for natural rubber, he then formed a research group headed by Fritz Hofmann,
the chief chemist in the pharmaceutical division of Farbenfabriken. In 1909, his research
team developed a synthetic rubber from the polymerization process of pure isoprene using
p-cresol, a component from coal tar, as the starting material. However, the cost of isoprene
production was very high. The research team applied dimethyl butadiene (CH3-CH2-CH=CH3)
as the starting material and became successful in 1910 to name it ‘K Rubber’. This synthetic
rubber was relatively hard and easily degraded. In 1918, Germany was cut off from natural
rubber supply, so Bayer produced 2400 metric tons of K Rubber for the German Army.
After the First World War, demand for rubber increased significantly because of the
sharp rise in the automotive industry. BASF, Bayer, Agfa Farwerke, Hoechst and Chemische
Fabrik Kalk decided to resume research on synthetic rubber. Eduard Tschunkur and Walter
Bock were assigned to lead this research, which managed to polymerize styrene and
butadiene in the aqueous emulsion. In June 21, 1929, IG Farben obtained a patent on the
invention of butadiene-styrene copolymers (SBR). They named it Buna S. The name Buna came
from the first two letters ‘butadiene’ and ‘natrium’ which meant sodium. In 1930, the team of
Helmut Kleiner, Erich Konrad and Eduard Tschunkur succeeded in developing acrylonitrile-
butadiene rubber (NBR or Buna N) Buna N has high oil and petroleum resistance and is widely
used in petrochemicals and automotive industries in oil seals and O-rings.

38
On the other side of the world Neoprene was discovered by Dr. Wallace Carothers
of DuPont in 1930. While Carothers started his research on polymerization, Father Arthur
Nieuwland of the University of Notre Dame carried out on a research on acetylene chemistry
and produced divinyl dichloride, an elastomeric compound. DuPont decided to purchase the
patent rights from the university and continued to work on divinyl dichloride. In April 1930,
Collins discovered an elastomeric material which has excellent elastic and chemical
resistance properties; DuPont called it DuPrene but it was changed to Neoprene later on.
Buna S, Buna N and Neoprene were three synthetic rubbers that had been discovered
before the Second World War. After the war, the automotive industry had been well developed
in the USA, consumption of rubber grew very fast and many medium and high performance
rubbers were developed to serve requirements in automotive, petroleum, petrochemical,
construction industries.
2.2) Development of Synthetic Rubber in the United States of America
On the other side of the world, President Franklin D. Roosevelt was well aware of
the vulnerability threatened supply of natural rubber in USA. In June 1940, he declared rubber
as a strategic material for the country and formed the Rubber Reserve Company (RRC) to
stockpile and conserve the use of natural rubber. He encouraged research and development
in synthetic rubber. During the Second World War, the USA was cut off from the supply from
Southeast Asia. The RRC called for the production of synthetic rubber in the country, and on
December 19, 1941, Jersey Standard, Firestone, Goodrich, Goodyear, and United States Rubber
Company signed a patent and information sharing agreement under the auspices of the RRC.
Before that time, the giant oil company of Standard Oil of New Jersey, who had a
working relationship with IG Farben, had go through a transatlantic transfer of synthetic
rubber technology. In the 1930s, chemists at Jersey Standard Oil started the research and
development on the production of butadiene, the basic material for producing synthetic
rubbers, from petroleum. Their research allowed large-scale production.
DuringtheSecondWorldWar,the naturalrubbersupplysituationbecame moreserious.
Roosevelt appointed a Rubber Survey Committee to speed up the production of synthetic rubber.
The technology chosen for synthetic rubber production was based on Buna S research because
Buna S could be mixed with natural rubber and milled on the same machinery as natural
rubber. Research and development to produce this all-purpose substitute for natural
rubber was dominated by the big four rubber companies. Fifty-one plants were constructed
to produce the monomers and polymers needed for the manufacture of synthetic rubber (GR-S,
or government rubber-styrene rubber) using a recipe that consisted of 75% butadiene and
25% styrene with potassium per-sulfate as a catalyst. Right after the Second World War, the
United States was the largest producer of synthetic rubber at 920,000 metric tons per year.

39
2.3) Rubber Structure and Related Properties
Natural rubber and synthetic rubbers before cross-linking show properties of
thermoplastics, but after being cross-linked the chemical structures will change and thus
offer different characteristics and performance. Rubber chemists have to fully understand
the background for the structure/properties relationship of each rubber in order to predict
the final properties of the product to be produced. The chemical structure determines the
chemical reactivity toward the design of cross-linking. It also affects the mixing behaviour,
processability and the properties of products.
Rubber has an organic hydrocarbon structure as the backbone of the polymer which
then forms a long chain structure in the polymerization process. Natural rubber has its single
unit of isoprene, though by photosynthesis, isoprene monomers are polymerized to polyisoprene
structureinthecolloidalformoflatex.Man-maderubberorsyntheticrubbersallstart from small
units of monomer which through the process of polymerization, form long chain polymers
that can have many combinations of monomers. These polymerized monomers can be the
same monomers or different kinds of monomers, either only two kinds of monomer or more
than two kinds of monomers. Some monomers have only hydrocarbon structure while other
monomers may contain heteroatoms of N, Cl, O, F or Si. Each chemical structure of a monomer
will exhibit different physical and chemical properties in the final polymer’s properties and
performance. Therefore, rubber chemists have to know the chemical structures of polymers
to predict the properties of final products.
First, we have to start to study the skeleton of polymer, starting from its backbone
of polymer which is the main C-C chain of polymers. Atoms of carbon have 8 electrons at the
outer layer. In order to form a bond between C-C, two carbon atoms share two outer electrons
to form a covalent bond which is a stable bond. Polymers with the single C-C bond are more
stable than polymers which share four outer electrons to form a double bond, or C=C bond.
Four electrons that are sharing are active, so when polymers are exposed to heat, radiation
or oxidizing agents, one electron will leave the sharing bond to become a negative charge
at the site of the polymer. This negative charge of free radical is very active in promoting
further reactions, which cause the breakdown of the polymer main chain, or easily react
with positive charge chemicals to form a side chain. Therefore, polymers with double bonds
(unsaturated bonds) are prone to be active and not stable when they are exposed to ozone,
oxygen sunlight, heat, UV and oxidizing chemicals. Natural rubber is one of the examples, the
main chain of natural rubber has double bonds which cause natural rubber to be unsuitable
when exposed to sunlight and heat. However, the double bond is essential for sulphur curing.

40
Secondly, rubber chemists have to study the chemical structures of each type of
polymer. The type and content of heteroatoms, such as N, O, F, Si and Cl atoms affect the
heat resistance, polarity and density of rubber. The polarity in turn affects the compatibility
of rubber with other rubbers, polymers and compounding ingredients, and also its resistance
against liquid media, especially oil.
Thirdly, the low temperature performance, flexibility and elasticity of rubber are
governed by the glass transition temperature of the polymers which depends on the
stereoregularity of the structure of polymers. The presence of some crystallinity and
especially the occupancy of strain-induced crystallinity are beneficial to the strength and
abrasion resistance.
Fourthly, other structural features of the rubber polymer, such as molecular weight,
molar mass distribution, number of branching chains and size of branched chains, all affect
the production conditions of rubber processing and the final performance of both rubber
compounds and vulcanized rubber. Molecular weight affects the Mooney viscosity and strength
of vulcanizates. The flexibility of a polymer chain affects the entanglement density, which in
turn determines the rubber compound viscosity and the acceptance of fillers and plasticizers
by the polymer and the modulus and hardness of the vulcanizates.
Fifthly, rubbers are polymers with a glass transition temperature (Tg), below their
application temperatures, having no or hardly any crystallinity. They are soft and flexible with
modulus in the MPa range at their application temperature. Dynamic mechanical properties
of a polymer are characterized by the storage modulus (G’) and the loss modulus (G”).
Sixthly, rubber polymer is different from thermoplastic polymer, which has two
thermal transition temperatures, Tg, and melting temperature (Tm) whereas rubber polymer
has only Tg, does not melt, but at temperatures above Tg, becomes soft and flows. Mooney
viscosity of rubber is the measurement of rubber polymer at a certain temperature.
At temperatures below Tg, molecules inside the thermoplastic polymer form
crystalline structures which become more rigid and dense. If high strength properties are
needed, thermoplastic with high Tg is used at room temperature and at service temperatures
of the plastic product up until reaching its heat distortion temperature. In case the properties
of flexibility and softness of thermoplastic, are required, thermoplastic with very low Tg is the
choice. Rubber polymers are used because of the flexibility as their Tg is below the service
temperature.
Sevenly, molar mass (MM) and molar mass distribution (MMD) and long chain branching
(LCB) have to be put into the selections of polymers. MM, MMD and LCB which are governed
by monomer structures, types of catalysts and polymerization processes, have their effects
on the mixing, processing and vulcanization as well as the final properties of the products.
MM increases with the Mooney viscosity of the polymers. MM here with is the
mass-average molar mass. In rubber industry, the Mooney Viscosity (MV) correlates with
MM but is also affected by MMD and LCB. Selection of MV of a rubber polymer for a particular
rubber application is often a compromise in rubber compounding mixing, processing and
the final properties of the products. Lower rubber MM and MV gives a faster wetting of solid
filler particles, resulting in a shorter mixing cycle and is easier to process and shaping. The
higher rubber MM and MV gives benefit in the higher physical and mechanical properties of

41
the final rubber product, compression set and improves solvent resistance. Higher MV rubber
gives higher shear force during compound mixing. Therefore, the selection of MM and MV
of rubber in an application, one has to identify the right balance between compound mixing,
process, vulcanizate performance and total cost of product.
Eighty, strain-induced crystallization may occur for highly stereo-regular rubber with
melting temperature below room temperature, such as IR, IIR, CR, and especially NR. At high
strain levels, the polymer chain aligns, which facilitates crystallization. Stretching the rubber
chain results in a shift of melting temperature, to temperature above the room temperature
and forms crystalline region. The crystalline regions formed result in self-reinforcement
of the rubber, resulting in a higher tensile strength and elongation at break. Natural rubber
is renowned for its high degree of strain-induced crystallization and its excellent ultimate
properties.

42
2.4) Neoprene or Chloroprene Rubber
Neoprene is a trade name of DuPont for this excellent weather, ozone, chemical
corrosion and oil resistant synthetic rubber. In 1934, DuPont announced its success in
developing chloroprene rubber through the emulsion polymerization process. DuPont called
it neoprene [1]. The construction of a 2000 tons/year plant was completed in 1940, providing
the first synthetic rubber used by the US military during the Second World War.
In 1957, Bayer produced Perbunan C (chloroprene rubber) by a continuous production
process. The first commercial production of Denka chloroprene with a capacity of 200 tons/
month was established in 1962. On November 4, 2005, Denka announced its acquisition of
DuPont chloroprene Rubber, which made Denka the largest chloroprene supplier.
Chloroprene is the common name for 2-chloro-1,3-butadiene with the formula
CH2=CCl-CH=CH2 [2]. Acetylene was used as a feed stock in the older manufacturing process,
while the modern processes use butadiene. By 1980, most of the chloroprene was produced
from butadiene in the USA and Western Europe, while in Japan, Denki-Kagaku still used the
acetylene route. Acetylene came from the production of calcium carbide and its dimerization
to form mono-vinylacetylene in the aqueous hydrochloric acid solution of CuCl2 and NH4Cl at
80ºC in the reactor tower. Then chloroprene was formed by a direct hydrochlorination step,
a process that needed high investment and was very energy intensive. The more recent
chloroprene process is based on butadiene, a less expensive feedstock. The initial step is a
gas-phase free-radical chlorination with chlorine at 250ºC and 1-7 bar to give a mixture
of 3,4-dichlorobutene and 1,4-dichlorobutene. Step 2 is the isomerization and de-hydro
chlorination of 3,4-dichlorobutene-1, followed by treatment with base to induce
dehydrochlorination to 2-chloro-1,3-butadiene in the last step of the process [3]. The
dehydrochlorination entails loss of a hydrogen atom in the carbon 3 position and chlorine
atom in the carbon 4 position, therefore a double bond between carbon 3 and 4 is formed.
Currently, the polymerization of chloroprene is a free-radical polymerization in an emulsion
process which allows good temperature control of the reaction and is a stable process [3].
This leads to better batch-to-batch control of variations in structure and rheological
properties of grades of chloroprene produced.
2.4.1 Structures and properties
Chloroprene has four different chemical structures, 1,4-trans, 1,4-cis 1,2 and 3,4 in
the polymer chain as shown in Figure 2.1 [2, 4]. The composition of these types of polymer
which determines the grade of chloroprene is dependent on the reaction temperature. Lower
polymerization temperature, produces a higher yield of 1,4-trans incorporation as resulting
in higher crystallization rate of the polymer. Grades of chloroprene from low temperature
polymerization are needed for adhesive applications benefiting from strain-induced
crystallization. More grades of 1,4-cis incorporation are produced at higher temperatures for
various grades elastomeric properties.

43
Figure 2.1 Chemical structure of chloroprene rubber [2, 4]
The high chlorine content with its high density of 1.23 g/cc [4] and mid-range polarity
makes chloroprene inherently fire resistant and gives moderate fluid swelling resistance.
Although chloroprene has an unsaturated polymer backbone, the chlorine atom which is
attached to the tertiary carbon atom of the double bond, withdraws electrons from the
unsaturation in the backbone. This chlorine atom reduces the reactivity in oxidation reactions
of chloroprene, therefore chloroprene, in spite of the double bond in its structure, offers
a reasonable resistance to weathering, ozone and heat. The high amorphous grade of
chloroprene has very low glass transition temperature of -40ºC to be an excellent rubber
for low temperature applications [4]. Chloroprene also has high green strengths, high tear
resistance and overall good mechanical and excellent dynamic properties [4].
Generally, the grades of chloroprene offered by DuPont, (acquired by Denka) Arlanxeo,
and Denka, can be separated into three groups [5].
1. Adhesive grades which have high crystalline structure are offered in different
Mooney viscosities. As a consequence of crystallization, the adhesive strength of the adhesive
film is considerably greater than the amorphous grades of chloroprene.
2. Chloroprene grades which have more 1,4-cis give amorphous structures; various
Mooney viscosities are offered. For extrusion applications, a small quantity of pre-crossing
agent is added to the polymer to provide good extrusion processing [5].
3. Chloroprene grades with sulphur from the peptisation process contain polysulphide
bridges, giving unique properties [5]. They have high initial viscosity allowing faster and better
filler dispersion. These compounds are good for the extrusion and calendaring process. They
also promote good bonding between rubber and substances such as fabric.

44
Structures of G type and W type are shown in Figure 2.2 and 2.3, respectively.
Presence of chlorine atoms in the backbone of chloroprene rubber make its
properties as follows [4, 7]:
1. Chloroprene is resistant to non-polar oil such as paraffinic and naphthenic oil, but
swells in aromatic and engine oil.
2. Chlorine atoms in chloroprene can help to provide flame resistance.
3. Chloroprene is resistant to oxidation and ozone.
4. Chloroprene can be self-scorching at high temperature. This is because
chloroprene releases HCl to react with metal oxide in the compound.
5. Chloroprene rubber has poor dielectric resistance.
Adhesive
AD
none Low to high rapid Fast crystallization
AG
Sulphur Medium slow Rheological control
W polymer
W
none 42-51 fast General purpose
WHV 100
none 90-105 fast General grade
medium Mooney
viscosity
WRT
2,3-dichloro- Medium very slow
Low temperature app.
WD 1,3-butadiene High very slow High viscosity WRT
GRT
containing S Medium medium High tear
GW
Co-monomer Mooney Viscosity Crystallization rate
Distinguishing
Features
Type
Table 2.1 Grades of Neoprene offered by DuPont [6]
Figure 2.2 G type: contains sulphur in the structure
Figure 2.3 W type: contains chlorine atom in the structure

45
Figure 2.4 Vulcanization of chloroprene rubber by the action of zinc oxide and magnesium
oxide [10]
2.4.2 Applications
Because of its high crystalline structure in the adhesive grades, chloroprene finds
application in solvent adhesives [7, 8]. In the moulded rubber area, chloroprene is well
recognized for its high gum vulcanized strength due to the strain-induced crystallization.
It has good chemical stability, excellent weathering and ozone resistance [4], and good thermal
resistance properties which it can retain at low temperatures. It has excellent dynamic
propertiesandgoodgaspermeabilityresistance[8].Itisconsideredforageneral-purposerubber
goods produced by molding, extrusion and calendaring. Chloroprene is self-extinguishing
hence it has been used in high power-cable jacketing and mining belt applications. Because
of its excellent dynamic property, Neoprene GRT is also used in V belts. Neoprene WRT and
WD are used in bridge bearing pad, wiper blade and seal joint [8]. Neoprene W finds its
applications in general rubber applications, such as industrial hoses, axle boots, wire and
cable jacketing, engine mountings and bearing pads. Neoprene GRT and GW types have
excellent tear resistance, soft touch feeling and are used in sport wet-suit applications.
Neoprene latex finds its application in industrial and surgical glove applications.
2.4.3 Vulcanization of chloroprene
Chloroprene is generally vulcanized by metal oxides; the common cross-linking
agent is usually zinc oxide, along with magnesium oxide which is necessary to give scorch
resistance [9]. The reaction involves the allylic chlorine atom, which is the result of the small
amount of 1,2 polymerization, as shown in the chemical reactions below (Figure 2.4):
Metal oxide vulcanization with accelerated-sulphur vulcanization is common.
Tetramethylthiuram disulphide (TMTD), N, N’-di-o-tolyguanidine (DOTG) and sulphur are
commonly used for high resilience and good dimensional stability, while metal oxide with
ethylenethiourea (ETU) is a curing system being used to avoid carcinogens.

46
2.5) Styrene-butadiene Rubber (SBR)
Styrene-butadiene rubber (SBR) is a general purpose synthetic rubber, produced
from a copolymerization of styrene and butadiene
Figure 2.5 Chemical structure of SBR [2]
Exceeding all other synthetic rubber in consumption, SBR is used in large quantities
in automotive and truck tires. It is the most consumed type of synthetic rubber, widely used
in place of natural rubber because of its good abrasion resistance. However, because of the
butadiene content, SBR become swollen and weakened by hydrocarbon oil and is degraded
over time by atmospheric oxygen and ozone.
2.5.1 History of styrene butadiene rubber
SBR was developed and commercialized in Germany in 1930 with the name of Buna S.
It was produced by emulsion polymerization which contained 68-70% butadiene and 30-32%
styrene. Around 1942, GR-S (Government rubber styrene) was produced in the United States.
The product was designed to be similar to the German Buna S but lower in molecular weight
for easier processing. It was also polymerized by emulsion polymerization. Immediately
after the Second World War, the United States was the largest producer of synthetic rubber
at 920,000 metric tons per year.
2.5.2 Polymerizations of styrene butadiene rubber
SBR can be produced either by emulsion polymerization (E-SBR) or solution
polymerization (S-SBR) techniques which lead to different properties. Emulsion SBR can be
produced by both hot and cold polymerizations. Hot emulsion SBR has more branch chains
than the cold emulsion SBR, which makes it better for extrusion, be more stable and have less
shrinkage. Cold SBR is more abrasion resistant and has higher tensile strength. The S-SBR
was developed with higher molecular weight and a smaller molecular weight distribution.
S-SBR has better flexibility and tensile strength with lower rolling resistance than E-SBR.
Therefore, S-SBR finds its application in tire manufacturing.
In E-SBR, there are two processes, hot polymerization and cold polymerization. The
polymerization temperature of E-SBR hot polymerization is 50ºC or higher, while the cold
is about 5°C or even lower (-10°C or -18°C). In the cold polymerization process, more active
initiators are used. Cold polymerization gives better controlled structures, such as less
branching which will give better abrasion resistance and higher tensile strength (E-SBR
hot polymerization produces more branching which makes it better for extrusion and less
shrinkable). Cold polymerized E-SBR is more popular in the market than hot polymerized
E-SBR.

47
SBR 1000 “Hot” emulsion grades
SBR 1100 “Hot” black masterbatch +oil (≤14 phr)
SBR 1500 “Cold” emulsion grades
SBR 1600 “Cold” black masterbatch +oil (≤14 phr)
SBR 1700 “Cold” oil masterbatch
SBR 1800 “Cold” oil black masterbatch +oil (≤14 phr)
SBR 1900 Emulsion resin rubber masterbatch
SBR 2000 “Hot” lattices
SBR 2100 “Cold” lattices
Grade Type
Table 2.2 Grades of SBR
Solution styrene butadiene S-SBR is a copolymer of styrene and 1,2-butadiene
produce via anionic polymerizations in a solution process. The main chain of S-SBR is highly
unsaturated which give high sulphur utilization, but poor ozone, oxygen and heat resistances.
But S-SBR has good resistance properties toward polar media, acid and alkaline. The unique
nature of the insertion of butadiene on the growth 1,4- and 1,2-addition of butadiene, as well
as the formation of the two cis-1,4 and tran-1,4 stereo isomers, implies the presence of a
four-monomer units comprising copolymer. The ration of the structural unit content of
styrene and the butadiene inserted in 1,4- and 1,2-addition mole along the chain is the important
parameter influencing the glass transition temperature of S-SBR. The styrene content of
S-SBR typically range of 10 to 40 wt.% with the variation of the vinyl content (1,2-butadiene)
for the butadiene part of the copolymer of butadiene part of the copolymer of between 10
and 70 wt.%. Both the styrene content and the vinyl content influence the glass transition
temperature (Tg) of the polymer. S-SBR produces narrower molecular weight distribution,
higher molecular weight, higher cis-1,4-polybutadiene content and lower non-rubber content.
These give improvement in flexibility, dynamic properties and, superior mechanical properties
like tensile strength, low rolling resistance but S-SBR more difficult to process than E-SBR.
In the early stage of development of S-SBR, it was not very popular. Until 1980s, there were
requirements of green tires for lower fuel consumption, so low rolling resistance and good
traction of tires were required. Good control of microstructure of butadiene especially vinyl
and styrene contents and molecular weight in later developments of S-SBR were able to
achieve the requirements of green tires and also balance processing problems and properties.
The latest generation of S-SBR can raise Tg by adjusting the vinyl and styrene contents which
can help to improve grip and rolling resistance in tire performance.
2.5.3 Grades of SBR
The last two numbers of grades indicate staining and non-staining antioxidants
being used i.e., 1500 which contains staining antioxidant while 1502 contains non staining
antioxidants.

48
Emulsion Grades
Krylene 1500 23.5 16 12 72 -50 0
Krylene 1502 23.5 16 12 72 -50 0
Krylene 1509 23.5 16 12 72 -50 0
Krynol 1712 23.5 16 12 72 -50 27
Krynol 1721 40.0 16 12 72 -35 27
1st
Gen’ Solution Grades
Buna SL 704 Bayer 18.0 10 38 52 -75 0
Buna SL 705 Bayer 24.0 10 38 52 -65 0
Buna SL 750 Bayer 18.0 10 38 52 -75 27
Buna SL 751 Bayer 25.0 10 38 52 -65 27
Buna SL 754 Bayer 18.0 10 38 52 -70 33
2nd
Gen’ Solution Grades
Buna VSL 1924 S25 Bayer 25 33 25 42 -50 0
Buna VSL 1939 S20 Bayer 20 50 19 31 -45 0
Buna VSL 1940 S20 Bayer 20 50 19 31 -40 27
Buna VSL 1944 S15 Bayer 15 53 18 21 -50 0
Buna VSL 1945 S15 Bayer 15 53 18 29 -50 27
Buna VSL 1950 S25 Bayer 25 67 13 21 -25 27
Buna VSL 1954 S25 Bayer 25 73 10 17 -20 0
Buna VSL 1955 S25 Bayer 25 73 10 17 -20 27
Styrene
content (%)
Microstructure of 100% BR
Vinyl content
(%)
Cis-1,4
content (%)
trans-1,4
content (%)
Tg (°C) Oil (%)
Grade
Table 2.3 Microstructure of some SBR grades [11]
High aromatic oil (HA oil) was added to the SBR in the past. HA oils contain high
concentrations of polycyclic aromatic hydrocarbons (PAH) which have been identified as
carcinogenic. In 2010, European Legislation controlled extended oils in E-SBR and processing
oil in tire compounding to contain not more than 1 mg/Kg (0.0001% by weight) of Benzo[a]pyrene
and not more than 10 mg/kg (0.001% by weight) of all listed PAH. Currently, treated distilled
aromatic extract (TDAE), residue aromatic extract (RAE), and treated residue aromatic extract
(TRAE) have been used as substitutes for HA oils [12]. SBR grades do not used HA oils now.

49
Table 2.4 Grades of emulsion styrene-butadiene rubber (E-SBR)
Buna@ SE 1500 Caxias (BR) 23.5 52 none - staining in bales
Buna@ SE 1502 H Triunfo (BR) 23.5 53 none - non-staining in bales
Buna@ SE 1502 L Triunfo (BR) 23.5 49 none - non-staining in bales
Buna@ SE 1723 Caxias (BR) 23.5 50 TDAE 37.5 staining in bales
Buna@ SE 1739 Caxias (BR) 40.0 53 TDAE 37.5 staining in bales
Buna@ SE 1783 Caxias (BR) 23.5 49 RAE 37.5 non-staining in bales
Buna@ SE 1793 Caxias (BR) 23.5 51 TRAE 37.5 staining in bales
Buna@ SE 1799 Caxias (BR) 40.0 55 TRAE 37.5 staining in bales
Production site
Styrene
(%)
ML
(1+4)
Type of
oil
Oil
(phr)
Physical
form
Stabilization
Name
Table 2.5 Grades of solution styrene butadiene polymer (S-SBR)
Buna@ VSL 4526-2 Pt.Je ́ro ̂me (FR) 26.0 44.5 TDAE 37.5 -30 in bales
Buna@ VSL 4526-2 HM Pt.Je ́ro ̂me (FR) 26.0 44.5 TDAE 37.5 -30 in bales
Buna@ VSL 2538-2 Pt.Je ́ro ̂me (FR) 38.0 25 TDAE 37.5 -31 in bales
Buna@ VSL 2438-2 HM Pt.Je ́ro ̂me (FR) 38.0 24 TDAE 37.5 -32 in bales
Buna@ VSL 3038-2 HM Pt.Je ́ro ̂me (FR) 38.0 30 TDAE 37.5 -26 in bales
Production site
Styrene
(%)
ML
(1+4)
Type of
oil
Oil
(phr)
Physical
form
Tg (ºC)
Name
2.5.4 Compounding and vulcanized properties
SBR compounding is similar to NR compounding but does not require mastication
as the molecular weight is designed to be not too high for mixing and processing. The same
curing ingredients can be used as for NR. However, SBR curing is slower than that for NR so
more accelerator or more active accelerator is required [11]. Also, SBR cannot crystallize on
stretching like NR, therefore it needs reinforcing filler.
Even though most properties of SBR are comparable to NR, some properties are
lower like gum tensile strength, elongation at break, tack, hysteresis and resilience.
However, reinforcing fillers and designed compound formulation can improve these
properties. The better properties of SBR over NR are processability, slightly better abrasion
and aging resistance together with less scorch problems.
2.5.5 Applications
The main applications of SBR are tires and the rest are hoses, belts, adhesives,
footwear, rollers and molded rubber goods. SBR can be used in many applications as a
replacement of NR but not in severe dynamic applications and very low heat build-up. This
is one of the reasons why SBR cannot be substitute for NR in tire manufacturing. However,
blending SBR with NR or BR can improve some properties.

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