Useful article for students as well as the latex product manufacturers.

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
LATEX : Latex is a colloidal solutionof a polymer particles inwater. The
colloidal materials have the propertiesthat they may be readily recognized.
Those are the characteristics motionof the particles (calledBrownian
movement visible whenexaminedunder microscope), the electrical charge
which they carry and light is transmittedthroughthe solutionandreflected
from colloidal solution(Tyndall effect). The average particle sizeis about 600
nm. Or 6000 A0
.
COAGULATION : It is a irreversible formationof solids froma colloidal solution
in form of lumps (coagulum) separating out from the liquid(serum), whereas in
gelation(gelling) whole mass sets intoasemi-solidstate.
NATURAL RUBBER LATEX: NR latex is obtainedby tapping the trees called
HeveaBrasiliensis.
Compositionof NR latex : Being a natural product, the compositionof fieldlatex
varies betweenwide limits. The following is a typical composition.
Percentage
Dry Rubber Content (DRC) -------------------- 33
Proteinous substance -------------------------- 1 – 1.5
Resinous substance ----------------------------- 1 – 2.5

2. Ash --------------------------------------------------- Upto 1
Sugar ------------------------------------------------- 1
Water ------------------------------------------------ Rest
Proteinand Lipids : The proteinand lipidcomponents are present as absorbed
layers on the surface of rubber particles –the lipidforming the first layer and
the proteinthe secondina concentric spherical shells. The lipids are effective
adhesive whichbonds the proteintothe rubber. The proteinlayer being
negatively chargedcontributes to the colloidal propertiestothe rubber
particles.
COLLOIDAL STABILITY OF LATEX :
Three factors are responsible for colloidal stability of latex. (i) Reductionof
interfacial tensionbetweenthe polymer andaqueous phase. (ii) Presence of
similar electric charges whichcause the particlestorepel eachother.
iii) Presence of hydrationlayer (a layer of tightly boundwater 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 processesrequiremore concentratedlatex (60%DRC). Concentratedlatex
tends tobe more uniform and purer inquality due to partial removal of non-
rubber constituentsduring the concentrationprocess.
There are four general methods for concentrationof latex.
(i) Evaporation, (ii) Creaming, (iii) Centrifuging and(iv) Electro-decantation.
–Roughly 90% of latex concentrate is producedby centrifuging and5% each by
evaporationand creaming processes. Electro-decantationis nowobsolete.
PRESERVATION OF LATEX :

3. ConcentratedNR latex is preservedby either ammoniaalone (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 desiredstability of latex.
SYNTHETIC LATICES
These are aqueous dispersions of polymer obtainedby the process of emulsion
polymerization. Suchlatticesare of low total solids content ( of the order of
25%) and small particle size (usually inthe range of 400 – 800A0
)
SBR LATEX : SBR latex is not suitable for dippedarticles or any product requiring
good tensile strength. This can be usedin foam manufacture in mixture withNR
latex. Other uses are intreatment of tyre cordand fabrics and backing of
carpets.
NEOPRENELATEX: CR latex is superior toNR inresistance toheat, ozone, oils
and solvents andthey burn less easily andin low impermeable togases. Its
applications are industrial gloves (resistance tooils and solvents), foam
(resistance toheat, solvents andflame) and meteorological balloons (resistance
to air impermeability andozone attack)
NITRILELATEX : NBR latex is usedwhere highlevel of oil resistance is required.
It has lowgum strengthandfinds its applicationinfoams, but not indipped
articles where highstrengthis required.
TERPOLYMER LATEX: Terpolymer NBR lattices containing carboxylic acidas the
thirdmonomer, have high tensile strength. Soit can be usedin dippedarticles.
Such lattices may alsobe usedto form oil-resisting coatings over products with
NR latex. But care is necessary toprevent any de-lamination.
CarboxylatedSBR : lattices are alsoavailable whichare widely usedincarpet
backing. Vinyl pyridine latex (styrene –butadiene – vinyl pyridine), another

4. terploymer latex finds wide applicationfor corddipping solutionfor nylon,
polyester andrayon.
PREPARATION OF DISPERSIONS AND EMULSIONS
Water insoluble ingredients andwater immiscible liquids must be converted
intodispersionandemulsionbefore additiontolatex.
DISPERSION OF SOLIDS : It is aqueous suspensionof solidinfinely dividedform
to produce no or little agglomerating tendency. Those materialswhose particles
are fine, but have formedloose agglomerates (eg. Clays) canbe dispersed
simply by high speedstirrer tomake slurry containing solid, water anda
dispersing agent. The slurry may also be passedthrougha colloidmill to
produce high quality dispersion. Other materials require actual grinding of the
particles andfor this a ball mill is very commonly used.
BALL MILL : It consists of a cylindrical vessel approx. half filledgrinding charge
of unglazedporcelainor glass balls of about 1” diameter. The mill is charged
withsolid, water and a dispersing agent until the balls are just covered. On
discharging the mill after grinding, some paste will adhere tothe balls, but if
further batches of the same dispersionare tobe made, negligible loss is
incurred. Large mills must rotate more slowly thanthe small ones. Slower
speeds may be used, but the time of milling will be more since grinding is
achievedby a definite number of rotations of the mill. A mill having 24” internal
diameter shouldhave 38 RPM where a mill of 10”dia. Should have 60 RPM. No
fixedtime for milling can be specified. A fewhours milling is sufficient for clays,
while hard materials suchas sulphur require a minimum of 48 hours.
DISPERSING AGENTS :
Mainfunctionof a dispersing agent is toprevent re-agglomerationof the
dispersedparticles. It lowers the viscosity of the slurry whichenables toprepare
a dispersionof high solidcontent (inorder of 50%). A dispersing agent must not
produce forthing during milling. Best agents are lignosulphenatesandNa-

5. naphthalene sulphonate-formaldehyde condensate. These will not produce any
forthing, but wet out the solidparticles.
COLLOID MILL : It consists of twocircular plates, one is stationery andthe other
rotates at a very high speed(10,000 –20,000 RPM). Clearance betweenthe
plates is kept within0.025 to0.2 mm. It is providedwithwater cooling device to
prevent overheating of the material being dispersed.
EMULSION OF LIQUIDS : It is a suspensionof fine droplets of a liquidin water in
which it is immiscible. A simple equipment consists of atank and a high speed
stirrer. The liquidtobe emulsifiedis stirredwithwater andan emulsifiedagent.
Soaps are very good emulsifying agents. Inthis methodcationic part of the soap
is dissolvedinwater and the anionic part is dissolvedinthe liquidtobe
emulsified.
Very fine and stable emulsions canbe preparedwitha colloidmill. The liquid,
water and emulsifying agent are mixedtogether andpassedthroughthe colloid
mill. Recycling for several times is adequate toproduce satisfactory emulsion.
The stability of an emulsioncan be improvedby adding thickening agent like
casein. Ammoniumpolyacrylate or sodium carboxymethyl cellulose.
LATEX CHEMICALS, COMPOUNDING OF LATEX AND PROCESSING
Latex chemicals canbe dividedintotwo groups – (1) Auxiliary chemicals and
(2) Compounding ingredients.
Auxiliary chemicals are requiredtoachieve the desiredcharacteristicsof a latex
mix during compounding and processing anddo not effect on the physical
properties of the vulcanizedproduct. These are dispersing/emulsifying agents,
stabilizers, wetting agentsandthickening agents. The compounding ingredients
are sulphur, accelerators, activator, antioxidants, fillers, softenersand colours.
STABILISERS : Stabiliser is toincrease the colloidal stability of the latex during
compounding. It functions by producing negative charge tothe latex particles,
thus increasing the repulsiveforce betweenparticles and/or by forming
hydrationlayer around the particles. The former actionincreases the

6. mechanical stability andthe latter chemical stability. Mechanical stabilizer are
anionic surface-activeagents like fatty alcohol sulphates, Na-olyl sulphate etc.
and chemical stabilizersare non-ionic surface-active agentslike ethylene
oxide/fatty alcohol condensate.
WETTING AGENTS : It is to reduce the surface tensionof the latex compound.
This facilitatespenetrationof the latex compoundinto the textilefabrics during
impregnation. Typical examples are alkyl aryl sulphonates and sulphonatedoils.
THICKENING AGENTS : This is to increase the viscosity of the latex compound,
thus preventing or retarding penetrationof the compound intotextile substrate
during any spreading process. These are cationic surface-active agent, eg. cetyl
trimethyl ammoniumbromide, ammonium polyacrylate andNa-carboxymethyl
cellulose.
Caseinis a versatile material, usedas dispersing agent, stabilizer, wetting and
thickening agent, thoughthe effectiveness ineachapplicationis not good as
synthetic chemicals. One disadvantage of caseinis that it is attackedby
bacteria. So a solutionof caseinshouldcontain bactericide if requiredtobe
stored.
COMPOUNDING INGREDIENTS
SULPHUR : It is the common vulcanizing agent. Finely groundsulphur grade
shouldbe 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 )
u 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 )
Z Zinc oxide 100
- Dispersing agent 2
- Water 98 ; Ball mill for 24 hours.
ACCELERATORS : There is no scorchproblemin latex compounding. Moreover,
latex products are curedat lowtemperature, not exceeding 1000
C. So, ultrafast
accelerators are usedinlatex compounds. Dithiocarbamates are common

7. primary accelerators ( zinc diethyl dithiocarbamate ). 0.5 to1.0 phr is normally
used. Mercaptobenzothiazole(MBT) is usedas secondary acceleratoralongwith
ZDC which increases the modulus and reduces set propertiesof the
vulcanisates. MBTis slightly acidic, socauses thickening/coagulationof the latex
compound. Zn salt of MBT( ZMBT) is widely used. Tetramethyl thiuram
disulphide ( TMTD ) is also usedfor improvedheat ageing properties of the
vulcanisate. 50% dispersionof accelerators : ( P B W )
A Accelerator 100
D Dispersing agent 2
W Water 98 ; Ball mill for 24hrs.
ANTIOXIDANTS : It is to prolong the life of the products protecting fromvarious
deterious agents including heat. Non-staining types are generally usedare
phenolics, eg. styrenatedphenol andstaining type are amines, eg. beta-
naphthylamine and acetone diphenylamine condensates. Non-staining typesare
generally weaker antioxidants. 50% Emulsionof Styrenatedphenol :(P B W)
- Part A Styrenatedphenol 50
- Oleic acid 2
- Part B Triethanol amine 1.5
- Water 46.5
Part A is heatedto about 600
C and is added withhighspeedstirring toPart B at
the same temperature.
FILLERS : Reinforcementof latex products by adding carbon blacks and silica
fillers are not possible as there is nomasticationof rubber particles. Hence
cheap and soft fillers are usedtocheapenand tostiffenthe products. Carbon
black is merely usedas a pigment at 5 phr.
Highfiller loadings will reduce the tensile properties andresistance tocyclic
deformations of a product. Low cost mineral fillers, groundwhiting and
Kaolinite clays are popular in latex compounding. These can be easily dispersed
in water withthe helpof a high speedstirrer or colloidmill. Max. 30 phr per 100
phr rubber can be loaded, eg. carpet backing. Wet ground mica powder is very
effective infoammanufacturing as it gives less post-moulding shrinkage and
facilitates easy removal of product from the mould after vulcanization.
SOFTENERS : Softeners are requiredtoreduce the stiffness of the product, eg.

8. toy balloons whichrequire tobe soft for easy inflation. Mineral oils are
commonly usedand are easily emulsified.
50% Emulsionof mineral oil: ( P B W )
Part A Mineral oil 100
O Oleic acid 2.5
Part B Conc. NH3 solution 2.5
W Water 45
Part A is mixedinto Part B and passedthrougha colloidal mill. A further
improvement is achievedby replacing 1 part of water by 1 part of a thickening
agent.
COLOURS : For black and white articles carbonblack and titaniumdioxide are
used. Other colours can form aqueous dirpersions andshowno coagulation
effect onthe latex. Colours shouldnot leachout in water.
COMPOUNDING OF LATEX
The compounding of latex is very simply done in a mixing of mildsteel lined
witha vitreous material providedwithaslow speedstirrer. Ammoniais tobe
removedby passing air. A small amount of KOH may be added to compensate
for the loss of ammonia. Highammonia may lead tovarious problems,
thickening of the compound inpresence of ZnO and rapid skinformation.
Ammonia content is reducedto 0.2%. Thena suitable stabilizer, if needed, is
added followedby the dispersions andemulsions. The mix shouldbe
continuously stirredduring additionand thenallowedto standfor several hours
for removal of air bubbles and maturing.
PREVULCANISED LATEX:
-Latex particles canbe vulcanized inthe fluidstate without affecting its colloidal
stability. The products canbe made from this prevulcanisedlatex, say by
dipping and thendried. Nofurther curing is necessary.
–Minimumquantities of curing ingredients dispersions are used. Anexample:
60% NR latex 167
- 10% KOH solution 3

9. - 10% solutionof Na-naphthalene formaldehyde 5
- 50% Sulphur dispersion 1.2
- 50% ZDC dispersion 1.5
- 50% ZnO dispersion 0.4
The above mix is heatedat about 800C withconstant stirring ona water bath
for 2 to2.5 hours. The mix after vulcanizationis tobe cooledrapidly. To this
latex, dispersions of fillers, softeners, pigments, etc. as requiredmay be added.
Such latex is especially suitable for manufacture of cheap products like toy
balloons.
LATEX PRODUCT PROCESSING
After latex compounding different processes canbe classifiedas under :
1) Dipping
2) Casting and Moulding
3) Spreading
4) Spraying
5) Extrusion
6) Gelling
7) Electrodeposition
-In dipping process, a former inthe shape of the article is dippedin the latex
compound whereas inthe casting and moulding process, the article is built up
on the inside walls of a mould.
–Spreading of latex is usedon proofed fabrics by applying latex compound on
the fabric withthe helpof a Doctor’s knife. This process is alsousedintufted
carpets.
–Spraying process is applied for bonding paper, cloth, leather, fibre, etc. This
process is largely usedinthe manufacture of rubberizedcoir products.
–Extrusionprocess is mainly usedinthe productionof elastic thread.
–In ‘gelling’ processslowsetting of a mass of compounded latex intoa solidjelly
like mass of soft coagulum of the same shape and size of the mould. Latex foam
products are exclusively manufacturedby this process. --
Electrodeposition:The negative charge on the rubber particles makes it

10. possible todeposit rubber as inthe electroplating process for metals. This
process has very little commercial application.
SOME LATEX PRODUCTS WITH FORMULATIONS AND PROCESSES
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 heatedat 450
C for 1 hour, cooledand then following added:
50% Antioxidant SP emulsion 2
Distilledwater 55
30% Silicone oil emulsion 0.1
a. Dipping : Coagulant dipping using calciumnitrate solutionin
alcohol/water mixture. Normally twodips are given.
b. Former : Aluminumor glazedporcelain, sometimes woodcoatedwith
corrosionresistance paint is used.
c. Washing : Washedin warm water ( 60 – 700
C ) for 15 – 20 minutes.
d. Drying : Driedat 700
C for 15 minutes.
e. Curing : Curedat 100 – 1050
C for 25 – 30 minutes.
f. Stripping : Strippedwiththe aidtalc powder.
2. CONDOMS
P B W

11. 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
Distilledwater ( toadjust viscosity ) As required
a. Dipping machine consists of endless chainhaving glass formers take dip
and driedin hot chamber at about 900
C and pass through rotating brush
for bead formation.
b. After beading it is again heatedin drying chamber and passedthrougha
leaching tank containing ammonia solutionand thenpassedthrougha
anti-sticking solutionbath. Thenthe condoms are strippedout by nozzle
spray from the glass formers.
c. Strippedcondoms thenpass througha rotating drum where powder
slurry is sprayedand collectedinplastic bins. Thenit is centrifugedina
machine to remove powderedwater from the condoms.
d. Condoms are thentakenfor vulcanizationat 100 – 1200
C for 1 hour.
After that these are takentode-powdering machine for cooling for 10 –
15 minutes.
e. Then it is kept inplastic bins and waiting for electronic testing and
packing. Condoms are very thin, thickness being 0.045 –0.075 mm.
B. LATEX THREAD
60% NR latex ( LA ) 167
10% KOH solution 3.5
20% Potassiumoleate soapsolution 5
50% Sulphur dispersion 2
50% ZDC dispersion 2
40% ZnO dispersion 2.5
50% SP emulsion 2
Colour As required

12. Latex thread, for use inelastic garments, is manufacturedby extrusionprocess.
Latex compound is passedthrough glass nozzles under pressureintoabath of
coagulant. The extrudedlatex gels inthe formof round threadwhich 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 600
C.
C. RUBBERISED COIR PRODUCT
60% NR latex ( LA ) 167
20% KOH solution 1
20% Stabiliser 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
Distilledwater To adjust viscosity
The coconut fibres are made into sheets, not exceeding 2”thickness, sprayed
on both sides withthe latex compoundand dried. The sheets are then pliedup
and compressed, filledinthe moulds and heatedto get the shape. The products
are then vulcanizedina vulcaniser at 100 – 1050
C for 30 – 45 minutes.
D. CARPETBACKING
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/groundwhiting Up to 200
5% solutionof thickening agent As required
( Na- carboxymeyhyl cellulose )

13. The tuftedcarpets are givena coating of latex compound on the back by
spreading process. The object is toanchor the plies tothe hessianbacking.
Spreading is carriedout with the helpof a Doctor’s knife. The coatedcarpet
passes througha hot air tunnel at 90 –1200
C to dry and vulcanise the latex
coating.
E. LATEX FOAM
P B W
60% NR latex ( LA ) 167
20% K-oleate soapsolution 5
50% Sulphur dispersion 4
50% ZDC dispersion 2
50% ZMBTdispersion 2
50% SP a/o emulsion 2 – 3
50% Filler slurry Up to 40
50% ZnO dispersion 8
50% dispersionof cetyl trimethyl ammoniumbromide 1 or as
(a secondary Gelling agent, preventsfoamcollapse) reqd.
20% Sodium silicofluoride dispersion 5.0 – 7.5
a. This process consists of expanding (foaming) a suitable latex compoung
to a desiredvolume, setting (gelling) andthenvulcanise the product.
b. There are two processes :(1) Dunlopprocess and (2) Talalay process
which differ inthe foaming and gelling methods.
c. Dunlop process :Sodium silicofluoride is usedas gelling agent. Latex
compound is mechanically beatenupto a foam and then dispersionof
Na-silicofluoride is addedwhichinpresence of ZnO will set the foam to
the size and shape of the mould withina fewminutes, and then
vulcanizedinsteam. This method is mostly usedfor foam products.
d. Talalay process : This process involves mechanical foaming ina mixer.
Partially foamedlatex compound is poured intomould foaming is
completedby vacuum. The foamed latex is thenchilledand gelledby
passing CO2 gas. Curing is done by passing suitable heating fluids.
This type of plant requires very heavy investment. It is widely usedin
the USA and UK.