Very useful article for rubber latex processors.

1. LATEX INGREDIENTS AND COMPOUNDING WITH
FORMULATIONS OF PRODUCTS WITH BRIEF
MANUFACTURING PROCESS
By
PRIYABRATA GHOSH
B.Sc.(Cal);LNCRT, LPRI, Grad.PRI, ANCRT, APRI,
MPRI(London);FIRI(India)
Consultant Rubber Technologist, priyabrata01@yahoo.co.in

2. LATEX : Latex is a colloidal solution of a polymer particles in water. The colloidal materials have the
properties that they may be readily recognized. Those are the characteristics motion of the particles
(called Brownian movement visible when examined under microscope), the electrical charge which
they carry and light is transmitted through the solution and reflected from colloidal solution (Tyndall
effect). The average particle size is about 600 nm or 6000 A.
COAGULATION : It is a irreversible formation of solids from a colloidal solution in form of lumps
(coagulum) separating out from the liquid (serum), whereas in gelation (gelling) whole mass sets into
a semi-solid state .
NATURAL RUBBER LATEX : NR latex is obtained by tapping the trees called Hevea Brasiliensis.
Composition of NR latex : Being a natural product, the composition of field latex varies between
wide limits. The following is a typical composition.
Percentage
Dry Rubber Content (DRC) -------------------- 33
Proteinous Substance -------------------------- 1 – 1.5
Resinous Substance ----------------------------- 1 – 2.5
Ash --------------------------------------------------- Up to 1
Sugar ------------------------------------------------- 1
Water ------------------------------------------------ Rest

3. Protein and Lipids : The protein and lipid components are present as absorbed layers on the surface
of rubber particles – the lipid forming the first layer and the protein the second in a concentric spherical
shells. The lipids are effective adhesive which bonds the protein to the rubber. The protein layer being
negatively charged contributes to the colloidal properties to the rubber particles.
COLLOIDAL STABILITY OF LATEX :
Three factors are responsible for colloidal stability of latex.
(i) Reduction of interfacial tension between the polymer and aqueous phase.
(ii) Presence of similar electric charges which cause the particles to repel each other.
iii) Presence of hydration layer (a layer of tightly bound water molecules) which acts as a mechanical
barrier.
CONCENTRATION OF LATEX :
NR latex as it comes from tree contains only 30 – 38% DRC (Avg. 33%). It is uneconomical to transport
this over long distances. Moreover, many important latex processes require more concentrated latex
(60% DRC). Concentrated latex tends to be more uniform and purer in quality due to partial removal of
non-rubber constituents during the concentration process.
There are four general methods for concentration of latex.
(i) Evaporation, (ii) Creaming, (iii) Centrifuging and (iv) Electro-decantation . Roughly 90% of latex
concentrate is produced by centrifuging and 5% each by evaporation and creaming processes. Electro-
decantation is now obsolete.

4. PRESERVATION OF LATEX :
Concentrated NR latex is preserved by either ammonia alone (HA latex containing 0.7% ammonia) or by
ammonia in presence of a bactericide (LA latex containing 0.2% ammonia).
Three types of LA latex are commercially available :
i) SPP type : 0.2% NH3 + 0.2% pentachlorophenate
ii) ZDC type : 0.2% NH3 + 0.1 to 0.2% ZDC + 0.2% lauric acid
iii) BA (Boric Acid) type : 0.2% NH3 + 0.2% Boric acid + 0.02% lauric acid
Lauric acid will form laurate ions imparting desired stability of latex.
SYNTHETIC LATICES
These are aqueous dispersions of polymer obtained by the process of emulsion polymerization. Such
lattices are of low total solids content ( of the order of 25%) and small particle size (usually in the range of
400 – 800A0) .
SBR LATEX : SBR latex is not suitable for dipped articles or any product requiring good tensile strength.
This can be used in foam manufacture in mixture with NR latex. Other uses are in treatment of tyre cord
and fabrics and backing of carpets.

5. NEOPRENE LATEX : CR latex is superior to NR in resistance to heat, ozone, oils and solvents and they
burn less easily and in low impermeable to gases. Its applications are industrial gloves (resistance to oils
and solvents), foam (resistance to heat, solvents and flame) and meteorological balloons (resistance to
air impermeability and ozone attack).
NITRILE LATEX : NBR latex is used where high level of oil resistance is required. It has low gum
strength and finds its application in foams, but not in dipped articles where high strength is required.
TERPOLYMER LATEX : Terpolymer NBR lattices containing carboxylic acid as the third monomer, have
high tensile strength. So it can be used in dipped articles. Such lattices may also be used to form oil-
resisting coatings over products with NR latex. But care is necessary to prevent any de-lamination.
Carboxylated SBR : lattices are also available which are widely used in carpet backing. Vinyl pyridine
latex (styrene – butadiene – vinyl pyridine), another terpolymer latex finds wide application for cord
dipping solution for nylon, polyester and rayon.

6. Water insoluble ingredients and water immiscible liquids must be converted into dispersion and
emulsion before addition to latex.
DISPERSION OF SOLIDS : It is aqueous suspension of solid in finely divided form to
produce no or little agglomerating tendency. Those materials whose particles are fine, but have
formed loose agglomerates (e.g. Clays) can be dispersed simply by high speed stirrer to make
slurry containing solid, water and a dispersing agent. The slurry may also be passed through a
colloid mill to produce high quality dispersion. Other materials require actual grinding of the
particles and for this a ball mill is very commonly used.
BALL MILL : It consists of a cylindrical vessel approx. half filled grinding charge of unglazed
porcelain or glass balls of about 1” diameter. The mill is charged with solid, water and a
dispersing agent until the balls are just covered. On discharging the mill after grinding, some
paste will adhere to the balls, but if further batches of the same dispersion are to be made,
negligible loss is incurred. Large mills must rotate more slowly than the small ones. Slower
speeds may be used, but the time of milling will be more since grinding is achieved by a definite
number of rotations of the mill. A mill having 24” internal diameter should have 38 RPM where a
mill of 10” dia. should have 60 RPM. No fixed time for milling can be specified. A few hours
milling is sufficient for clays, while hard materials such as Sulphur require a minimum of 48
hours.
PREPARATION OF DISPERSIONS AND EMULSIONS

7. DISPERSING AGENTS :
Main function of a dispersing agent is to prevent re-agglomeration of the dispersed particles. It lowers
the viscosity of the slurry which enables to prepare a dispersion of high solid content (in order of
50%). A dispersing agent must not produce forthing during milling. Best agents are lignosulphenates
and Na-naphthalene sulphonate-formaldehyde condensate. These will not produce any forthing, but
wet out the solid particles.
COLLOID MILL : It consists of two circular plates, one is stationery and the other rotates at a very
high speed (10,000 – 20,000 RPM). Clearance between the plates is kept within 0.025 to 0.2 mm. It is
provided with water cooling device to prevent overheating of the material being dispersed.
EMULSION OF LIQUIDS : It is a suspension of fine droplets of a liquid in water in which it is
immiscible. A simple equipment consists of a tank and a high speed stirrer. The liquid to be emulsified
is stirred with water and an emulsifying agent. Soaps are very good emulsifying agents. In this
method cationic part of the soap is dissolved in water and the anionic part is dissolved in the liquid to
be emulsified.
Very fine and stable emulsions can be prepared with a colloid mill.
The liquid, water and emulsifying agent are mixed together and passed through the colloid mill.
Recycling for several times is adequate to produce satisfactory emulsion. The stability of an emulsion
can be improved by adding thickening agent like casein. Ammonium polyacrylate or sodium
carboxymethyl cellulose.

8. Latex chemicals can be divided into two groups –
(1) Auxiliary chemicals (2) Compounding ingredients.
Auxiliary chemicals are required to achieve the desired characteristics
of a latex mix during compounding and processing and do not effect on the physical properties of the
vulcanized product. These are dispersing/emulsifying agents, stabilizers, wetting agents and
thickening agents. The compounding ingredients are Sulphur, accelerators, activator, antioxidants,
fillers, softeners and colours.
STABILISERS : Stabilizer is to increase the colloidal stability of the latex during compounding. It
functions by producing negative charge to the latex particles, thus increasing the repulsive force
between particles and/or by forming hydration layer around the particles. The former action increases
the mechanical stability and the latter chemical stability. Mechanical stabilizer are anionic surface-
active agents like fatty alcohol sulphates, Na-olyl sulphate etc. and chemical stabilizers are non-ionic
surface-active agents like ethylene oxide/fatty alcohol condensate.
LATEX CHEMICALS, COMPOUNDING OF LATEX AND PROCESSING

9. WETTING AGENTS : It is to reduce the surface tension of the latex compound. This facilitates
penetration of the latex compound into the textile fabrics during impregnation. Typical examples are
alkyl aryl sulphonates and sulphonated oils.
THICKENING AGENTS : This is to increase the viscosity of the latex compound, thus preventing or
retarding penetration of the compound into textile substrate during any spreading process. These are
cationic surface-active agent, e.g. cetyl trimethyl ammonium bromide, ammonium polyacrylate and Na-
carboxymethyl cellulose.
Casein is a versatile material, used as dispersing agent, stabilizer, wetting and thickening agent, though
the effectiveness in each application is not good as synthetic chemicals. One disadvantage of casein is
that it is attacked by bacteria. So a solution of casein should contain bactericide if required to be stored.

10. SULPHUR : It is the common vulcanizing agent. Finely ground Sulphur grade should be
used. It is a very hard material and takes at least 48 hrs. for grinding in a dispersing mill.
50% sulphur dispersion : (P B W )
Sulphur 100
- Dispersing Agent 4
- Water 96 ;
Ball mill for 48 hours.
ACTIVATOR : Zinc oxide is the common activator. For transparent articles zinc carbonate is
recommended.
50% Zinc oxide dispersion : ( P B W )
Zinc oxide 100
- Dispersing agent 2
- Water 98 ;
Ball mill for 24 hours.
COMPOUNDING INGREDIENTS

11. ACCELERATORS : There is no scorch problem in latex compounding. Moreover, latex products are
cured at low temperature, not exceeding 1000C. So, ultrafast accelerators are used in latex
compounds. Dithiocarbamates are common primary accelerators ( zinc diethyl dithiocarbamate ). 0.5
to 1.0 phr is normally used. Mercaptobenzo thiazole (MBT) is used as secondary accelerator along
with ZDC which increases the modulus and reduces set properties of the vulcanisates. MBT is slightly
acidic, so causes thickening/coagulation of the latex compound. Zn salt of MBT ( ZMBT ) is widely
used. Tetramethyl thiuram disulphide ( TMTD ) is also used for improved heat ageing properties of the
vulcanisate.
50% dispersion of accelerators : ( P B W )
Accelerator 100
D Dispersing agent 2
W Water 98 ;
Ball mill for 24hrs.

12. ANTIOXIDANTS : It is to prolong the life of the products protecting from various deterious agents
including heat. Non-staining types are generally used are phenolics, e.g. styrenated phenol and staining
type are amines, e.g. beta-naphthylamine and acetone diphenylamine condensates. Non-staining types
are generally weaker antioxidants.
50% Emulsion of Styrenated phenol :(P B W)
- Part A Styrenated phenol 50
- Oleic acid 2
- Part B Triethanol amine 1.5
- Water 46.5
Part A is heated to about 600C and is added with high speed stirring to Part B at the same temperature.
FILLERS : Reinforcement of latex products by adding carbon blacks and silica fillers are not possible
as there is no mastication of rubber particles. Hence cheap and soft fillers are used to cheapen and to
stiffen the products. Carbon black is merely used as a pigment at 5 phr.
High filler loadings will reduce the tensile properties and resistance to cyclic deformations of a product.
Low cost mineral fillers, ground whiting and Kaolinite clays are popular in latex compounding. These
can be easily dispersed in water with the help of a high speed stirrer or colloid mill. Max. 30 phr per 100
phr rubber can be loaded, e.g. carpet backing. Wet ground mica powder is very effective in foam
manufacturing as it gives less post- moulding shrinkage and facilitates easy removal of product from
the mould after vulcanization.

13. SOFTENERS : Softeners are required to reduce the stiffness of the product, eg. toy balloons which
require to be soft for easy inflation. Mineral oils are commonly used and are easily emulsified.
50% Emulsion of mineral oil: ( P B W )
Part A Mineral oil 100
Oleic acid 2.5
Part B Conc. NH3 solution 2.5
Water 45
Part A is mixed into Part B and passed through a colloidal mill. A further improvement is achieved by
replacing 1 part of water by 1 part of a thickening agent.
COLOURS : For black and white articles carbon black and titanium dioxide are used. Other colours
can form aqueous dispersions and show no coagulation effect on the latex. Colours should not leach
out in water.

14. COMPOUNDING OF LATEX
The compounding of latex is very simply done in a mixing of mild steel lined with a vitreous material
provided with a slow speed stirrer. Ammonia is to be removed by passing air. A small amount of KOH
may be added to compensate for the loss of ammonia. High ammonia may lead to various problems,
thickening of the compound in presence of ZnO and rapid skin formation. Ammonia content is reduced
to 0.2%. Then a suitable stabilizer, if needed, is added followed by the dispersions and emulsions. The
mix should be continuously stirred during addition and then allowed to stand for several hours for
removal of air bubbles and maturing.
PREVULCANISED LATEX : Latex particles can be vulcanized in the fluid state without affecting its
colloidal stability. The products can be made from this prevulcanised latex, say by dipping and then
dried. No further curing is necessary.
Minimum quantities of curing ingredients dispersions are used.
60% NR latex 167
- 10% KOH solution 3
- 10% solution of Na-naphthalene formaldehyde 5
- 50% Sulphur dispersion 1.2
- 50% ZDC dispersion 1.5
- 50% ZnO dispersion 0.4
The above mix is heated at about 80C with constant stirring
on a water bath for 2 to 2.5 hours. The mix after vulcanization is to be cooled rapidly. To this latex,
dispersions of fillers, softeners, pigments, etc. as required may be added. Such latex is especially
suitable for manufacture of cheap products like toy balloons.

15. After latex compounding different processes can be classified as under :
1) Dipping 5) Extrusion
2) Casting and Moulding 6) Gelling
3) Spreading 7) Electrodeposition
4) Spraying
# In dipping process, a former in the shape of the article is dipped in the latex compound whereas in
the casting and moulding process, the article is built up on the inside walls of a mould.
# Spreading of latex is used on proofed fabrics by applying latex compound on the fabric with the help
of a Doctor’s knife. This process is also used in tufted carpets.
# Spraying process is applied for bonding paper, cloth, leather, fiber, etc. This process is largely used
in the manufacture of rubberized coir products.
# Extrusion process is mainly used in the production of elastic thread.
# In ‘gelling’ process slow setting of a mass of compounded latex into a solid jelly like mass of soft
coagulum of the same shape and size of the mould. Latex foam products are exclusively manufactured
by this process.
Electrodeposition : The negative charge on the rubber particles makes it possible to deposit rubber
as in the electroplating process for metals. This process has very little commercial application.
LATEX PRODUCT PROCESSING

16. A) DIPPED ARTICLES
1)SURGICAL GLOVES P B W
60% NR Latex 167
10% KOH Solution 2
50% Sulphur dispersion 2
50% ZDC dispersion 2
50% ZnO dispersion 2
50% Accinox B dispersion 2
Compound is heated at 450C for 1 hour, cooled and then following added :
50% Antioxidant SP emulsion 2
Distilled water 55
30% Silicone oil emulsion 0.1
a) Dipping : Coagulant dipping using calcium nitrate solution in alcohol/water mixture. Normally two
dips are given.
b) Former : Aluminum or glazed porcelain, sometimes wood coated with corrosion resistance paint is
used.
c) Washing : Washed in warm water ( 60 – 700C ) for 15 – 20 minutes.
d) Drying : Dried at 700C for 15 minutes.
e) Curing : Cured at 100 – 1050C for 25 – 30 minutes.
f) Stripping : Stripped with the aid talc powder.
SOME LATEX PRODUCTS WITH FORMULATIONS AND PROCESSES

17. CONDOMS
P B W
60% NR latex 167
50% ZnO dispersion 1.5
50% ZDEC dispersion 1.0
50% ZDBC dispersion 0.5
20% SP a/o emulsion 5.0
50% Sulphur dispersion 2.2
10% KOH solution 1.0
Distilled water ( to adjust viscosity ) As required
A) Dipping machine consists of endless chain having glass formers take dip and dried in hot chamber
at about 900C and pass through rotating brush for bead formation.
B) After beading it is again heated in drying chamber and passed through a leaching tank containing
ammonia solution and then passed through a anti-sticking solution bath. Then the condoms are
stripped out by nozzle spray from the glass formers.
C) Stripped condoms then pass through a rotating drum where powder slurry is sprayed and collected
in plastic bins. Then it is centrifuged in a machine to remove powdered water from the condoms.
D) Condoms are then taken for vulcanization at 100 – 1200C for 1 hour. After that these are taken to de-
powdering machine for cooling for 10 –15 minutes.
E) Then it is kept in plastic bins and waiting for electronic testing and packing. Condoms are very thin,
thickness being 0.045 – 0.075 mm.

18. B) LATEX THREAD
PBW
60% NR latex ( LA ) 167
10% KOH solution 3.5
20% Potassium oleate soap solution 5
50% Sulphur dispersion 2
50% ZDC dispersion 2
40% ZnO dispersion 2.5
50% SP emulsion 2
Colour As required
Latex thread, for use in elastic garments, is manufactured by extrusion process. Latex compound is
passed through glass nozzles under pressure into a bath of coagulant. The extruded latex gels in the
form of round thread which is drawn out of bath to washing, drying and vulcanizing operations. The
threads are wound in bobbins and kept in air ovens for about 24 hours at 600C.

19. C) RUBBERISED COIR PRODUCT
PBW
60% NR latex ( LA ) 167
20% KOH solution 1
20% Stabilizer solution 6
50% ZDC dispersion 2.5
50% Sulphur dispersion 2.5
50% ZnO dispersion 2.5
50% SP emulsion 2
50% Kaolin clay slurry 20
Distilled water To adjust viscosity
The coconut fibers are made into sheets, not exceeding 2” thickness, sprayed on both sides with the
latex compound and dried. The sheets are then plied up and compressed, filled in the moulds and heated
to get the shape. The products are then vulcanized in a vulcanizer at 100 – 1050C for 30 – 45 minutes.

20. D) CARPET BACKING
P B W
60% NR latex ( LA) 167
20% KOH solution 1.5
20% RD paste solution (anionic stabilizer ) 5
50% RVL solution ( non-ionic stabilizer ) 5
50% Sulphur dispersion 3
40% ZnO dispersion 7.5
50% ZDC dispersion 2
50% SP emulsion 2
60% slurry of clay/talc/ground whiting Up to 200
5% solution of thickening agent As required
( Na- carboxymetyhyl cellulose )
The tufted carpets are given a coating of latex compound on the back by spreading process. The
object is to anchor the plies to the hessian backing.
Spreading is carried out with the help of a Doctor’s knife. The coated carpet passes through a hot air
tunnel at 90 –1200C to dry and vulcanize the latex coating.

21. E) LATEX FOAM
P B W
60% NR latex ( LA ) 167
20% K-oleate soap solution 5
50% Sulphur dispersion 4
50% ZDC dispersion 2
50% ZMBT dispersion 2
50% SP a/o emulsion 2 – 3
50% Filler slurry Up to 40
50% ZnO dispersion 8
50% dispersion of cetyl trimethyl ammonium bromide 1 or as reqd.
(a secondary Gelling agent, prevents foam collapse)
20% Sodium silicofluoride dispersion 5.0 – 7.5

22. a) This process consists of expanding (foaming) a suitable latex compound to a desired volume, setting
(gelling) and then vulcanize the product.
b) There are two processes : (1) Dunlop process and (2) Talalay process which differ in the foaming and
gelling methods.
c) Dunlop process : Sodium silicofluoride is used as gelling agent. Latex compound is mechanically
beaten up to a foam and then dispersion of Na- silicofluoride is added which in presence of ZnO will
set the foam to the size and shape of the mould within a few minutes, and then vulcanized in steam.
This method is mostly used for foam products.
d) Talalay process : This process involves mechanical foaming in a mixer. Partially foamed latex
compound is poured into mould foaming is completed by vacuum. The foamed latex is then chilled and
gelled by passing CO2 gas. Curing is done by passing suitable heating fluids.
This type of plant requires very heavy investment. It is widely used in the USA and UK.

23. HAVE A GOOD DAY
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THANK YOU