Rubber Compounding - Chemistry and Applications Part 9 pptx

In this chapter we discuss the following compoundingingredients with respect to their influence on processing behavior and theirrelevant compound vulcanizate properties: Physical and chem

Note: Throughout this chapter the authors make reference to suppliers of particular materials and their trade names Mention of any company does not imply that it is the sole supplier of this material.

Table 1 Rubber Processing—Performance Factors

Raw materials Storage, handling, 1 Temperature control

weighing, blending, 2 Humidity control

7 Homogenization

8 Sticking and release

9 Mix time

2 Sticking and releasecalendering, sheet/fabric

3 Shrinkage and stretchingcalendering, profile

4 Die swellcutting/joining fabric,

5 Dimensional stabilitybuilding

11 Fabric cord penetration

Vulcanization Compression molding, 1 Scorch

2 Flowtransfer molding,

3 Component state of cureinjection molding,

4 Curative migration anddispersion

continuous vulcanization

5 Mold release, fouling,cleaning

6 Surface appearanceSource: Schill+Seilacher, Hamburg, Germany.

A Raw Materials Handling

Chemicals are frequently dusty powders that are difficult to handle and todisperse They can become electrostatically charged, and as a result incorpo-ration into a product is made more difficult Also, dusty powders areundesirable for environmental reasons, and this has led to the use of bindersand dispersing agents to improve materials handling and weighing Generallypreparations are coated, nondusting powders, granules, and masterbatches

C Forming

Down-line processing, i.e., shaping of semiproducts, requires compoundswith good flow properties Profile compounds should calender and extrudeeasily, fast, and uniformly The profiles should exhibit dimensional stability,smooth surface appearance, and exact edge definition Temperature and dieswell or shrinkage should be controllable and acceptable For sheet calen-dering, a smooth surface, uniform shrinkage, and freedom from blisters arerequired For metal wire or textile calendering, cutting, and joining, good flowproperties and acceptable tack are required Last but not least, bloom should

be avoided

D Vulcanization

In the vulcanization process good flow properties are needed in order to

1 Obtain adequate compound–compound adhesion

2 Obtain compound–metal and/or compound–textile adhesion

3 Fill the mold quickly, uniformly, and free of blisters or trapped air,particularly with transfer and injection molding equipment

Finally, the vulcanizates should demold easily without tear and not producemold-fouling residues.

Processing additives may be subdivided according to their chemicalstructures (Table 2), or according to their application (Table 3) Severalclasses of substances can have more than one application For example, fattyacid esters act as lubricants and dispersing agents Mineral oils act as physicallubricants in rubber compounds, reducing viscosity, and also help in the fillerdispersion process In this chapter we discuss the following compoundingingredients with respect to their influence on processing behavior and theirrelevant compound vulcanizate properties:

Physical and chemical peptizers

Mastication is the process whereby the average molecular weight of a polymer

Table 2 Processing Additives—Chemical Structure

Paraffin waxesPetroleum resinsFatty acid derivatives Fatty acids

Fatty acid estersFatty alcoholsMetal soapsFatty acid amides

Polybutenes

Source: Schill+Seilacher, Hamburg, Germany.

facilitates the incorporation of fillers and other compounding ingredients andcan improve their dispersion Because it is often difficult to homogeneouslyblend rubbers with very different viscosities, mastication of the higher vis-cosity rubber will enable improved blending with other, lower viscosity elas-tomers Improved compound flow leads to easier down-line processing such

as calendering and extrusion Shorter processing time and lower power

con-Table 3 Processing Additives—Applications

Chemical peptizer Reduces polymer viscosity

by chain scission

2,2V disulfide

-Dibenzamidodiphenyl-PentachlorothiophenolPhysical peptizer Reduces polymer viscosity

by internal lubrication

Zinc soaps

Dispersing agent Improves filler dispersion Mineral oils

Reduces mixing time Fatty acid estersReduces mixing energy Metal soaps

Fatty alcoholsLubrication agent Improves compound flow

and release

Mineral oilsMetal soapsFatty acid estersFatty acid amidesFatty acidsHomogenizing

agent

Improves polymer blendcompatibility

Improves compounduniformity

Resin blends

Phenolic resinsPlasticizer Improves product

performance at lowand high temperatures

Aromatic di- and triestersAliphatic diestersAlkyl and alkylether monoestersStiffening agent Increases hardness High styrene resin rubber

MasterbatchesPhenolic resinsTrans-PolyoctenamerSoftening agent Lowers hardness Mineral oils

Mold release

agent

Eases product releasefrom moldDecreases moldcontamination

OrganosiliconesFatty acid estersMetal soapsFatty acid amidesSource: Schill+Seilacher, Hamburg, Germany.

Because most of today’s synthetic rubbers are supplied with process viscosity levels, the mastication process is mainly restricted to naturalrubber.

easy-to-Although the natural rubber mastication process may be accomplished

on an open mill, it is generally carried out in an internal mixer Duringmechanical breakdown the long-chain rubber molecules are broken under theinfluence of high shear from the mixing equipment Chain fragments withterminal free radicals are formed, which recombine to form long-chainmolecules if they are not stabilized (Fig 1) Through atmospheric oxygenthe radicals are saturated and stabilized The chains are shorter, the molecularweight is reduced, and the viscosity drops The course of the chain breakdown

of natural rubber is shown inFigures 2 and3

Temperature is an important factor in the mastication of naturalrubber When the breakdown of natural rubber is plotted versus temperature(Fig 4), it can be seen that the effect is lowest in the range of 100–130jC.Chain cleavage by the mechanical process is more efficient at low temper-Figure 1 Physical peptization of rubber (Courtesy of Schill+Seilacher.)

Figure 2 Physical peptization of rubber—reaction sequence (Courtesy ofSchill+Seilacher.)

atures (below 90jC) because, owing to the viscoelastic nature of elastomers,the shear is higher the lower the temperature With increasing temperaturethe mobility of the polymer chains increases; they slide over one another,and the energy input and generated shear force drop However, although themechanical breakdown process is minimal around 120jC, above this temper-ature another breakdown process with a different mechanism, thermo-oxidative scission of the polymer chains, takes over and becomes more severe

as temperature increases An envelope curve is formed by the curves of thethermomechanical mastication and thermo-oxidative breakdown at elevatedtemperatures In practice, the two reaction modes superimpose Whereas themechanical breakdown at low temperatures largely depends on the mixingparameters, the thermo-oxidative breakdown is accelerated by temperatureand catalysts, i.e., peptizing agents

Free radicals are generated when the molecular chains of the rubber arebroken by mechanical or thermo-oxidative means These radicals may re-Figure 3 Physical peptization of polyisoprene (Courtesy of Schill+Seilacher.)

Figure 4 Peptization of NR Viscosity reduction vs temperature (Courtesy ofSchill+Seilacher.)

combine, and consequently no reduction in molecular weight and viscositywill be observed Moreover, branching is likely to occur The peptizing agentscan act as radical acceptors, thus preventing recombination of the generatedchain-end free radicals.

All peptizing agents shift the start of thermo-oxidative breakdown tolower temperatures Of the peptizing agents used in former times (Fig 5), onlycombinations of specific activators with thiophenols, aromatic disulfides, andmixtures of the activators with fatty acid salts are now available Note that forenvironmental reasons the chlorine-containing or polychlorinated thiophe-nols have largely been removed from use

The activators used in combination with a peptizing agent permitbreakdown to start at lower temperatures and accelerate the thermo-oxida-tive process They are chelates—complexes of ketoxime, phthalocyanine, oracetylacetone with metals such as iron, cobalt, nickel, or copper, but now-adays almost exclusively iron complexes These chelates facilitate the oxygentransfer by formation of unstable coordination complexes between the metalatom and the oxygen molecule This loosens the OUO bond, and the oxygenbecomes more reactive Because of the high effectiveness of the activators orboosters they are used only in small proportions in the peptizing agents.During recent times physical peptizers have gained major importance.They act as internal lubricants and reduce viscosity without breaking the

polymer chains Generally, zinc soaps have proved to be very effective in thisrole Mechanical and chemical breakdown of the elastomer results in chainscission, lower molecular weight, broader molecular weight distribution, and

an increased number of free chain ends Normally this leads to an increase incompound heat buildup and a decrease in abrasion resistance Lubricants donot change the molecular chains, i.e., the chains are not broken

As mentioned previously, synthetic rubbers are normally supplied witheasy-to-process viscosity levels If viscosity reduction is needed, mechanicalmastication in an internal mixer has virtually no effect In comparison tonatural rubber, viscosity reduction of synthetic rubbers is more difficult owing

to the 1) lower number of double bonds (SBR, NBR); 2) electron-attractinggroups in the chain, which stabilize the double bonds; 3) vinyl side groups,which foster cyclization at high temperatures (NBR, SBR, CR); and 4) lowergreen strength due to the absence of strain-induced crystallization (NBR,SBR)

Synthetic rubbers can be broken down by means of peptizing agents.However, they require higher dosage levels and temperatures than naturalrubber For this reason they are nowadays mostly physically peptized withsalts of unsaturated fatty acids

B Processing with Peptizing Agents

At one time it was common practice to have a separate mastication stagewhereby the peptizer was added to the NR and the mixing cycle was con-trolled to obtain an acceptable viscosity reduction Nowadays normally onlyone stage is used, with the filler addition being delayed in order to allow thepeptizing agent to be incorporated in the rubber The early addition of thefiller, while enhancing shearing and breakdown, also has a positive effect

on dispersion However, as the activators used in combination with the tizing agent may be adsorbed by the filler, it is normal to increase loadingslightly

pep-When natural rubber is blended with synthetic rubber that has a lowerviscosity, it is useful to peptize the natural rubber before the synthetic rubber

is added

Because antioxidants inhibit the oxidative breakdown of rubber, theyshould be added late in the mixing cycle during the processing of naturalrubber With synthetic rubbers an early antioxidant addition can avoidcyclization

Figure 6shows the influence of a number of chemical and physicalpeptizing agents on the breakdown, as measured by Mooney viscosity, ofnatural rubber (RSS1) in a 1 L laboratory internal mixer at 65 and 49 rpm and

a start temperature of 90jC Samples for Mooney viscosity testing were taken

Comparable results are obtained when physical peptizers are used athigher dosage levels than the chemical peptizers The raw RSS1 had aMooney viscosity of 104.

C Influence of Peptizing Agents on Vulcanizate Properties

The effects of a chemical peptizer (STRUKTOLR* A 86, an aromatic sulfide in combination with a metal organic activator), a physical peptizer(STRUKTOLR A 60, based on unsaturated fatty acid salts of zinc), andmechanical mastication on viscosity reduction and the tensile properties of

di-NR (SIR 5 L) have been investigated Apart from the usual evaluation ofviscosity at low shear rates (i.e., Mooney viscosity, ML 1 + 4V, 100jC), vis-cosity at higher shear rates, using a rubber processing analyzer (RPA), wasmeasured The data are shown inTable 4

Under low shear conditions the chemical peptizer is by far the moreeffective method for viscosity reduction However, under higher shear, which

Figure 6 Chemical vs physical peptizers in NR STRUKTOLR A 82 is a chemicalpeptizer containing an organic metal complex booster STRUKTOLR A 86 combines

a chemical peptizer and a booster Its composition is similar to that of STRUKTOLR

A 82 but with a higher concentration of active substance STRUKTOLR A 50 Pcontains zinc soaps of high molecular weight fatty acids STRUKTOLR A 60 is similar

to STRUKTOLR A 50 P but has a lower melting range, allowing open mill mixing.(Courtesy of Schill+Seilacher.)

* STRUKTOL is a registered trademark of Schill+Seilacher ‘‘Struktol’’ AG, Hamburg,

is a better simulation of factory conditions, the physical peptizer performsbetter This is of special importance because there are sometimes concernswith the use of chemical peptizers regarding their effect on long-term physicalproperties of compounds The changes in modulus and tensile strength(Table 5) show a greater degradation of natural rubber with the chemicalpeptizer system, whereas the physical peptizer highlights better retention ofphysical properties.

Chemical peptizers give the greatest fall in rubber viscosity, but theycause an increase in the amount of very low molecular weight polymer Theyalso adversely affect dynamic heat buildup, increasing tan delta in comparisonwith mechanical mastication, especially on overcure and aging Similarrubber viscosities can be achieved by masticating the rubber in the presence

of fatty acid soaps without a significant change in tan delta (1)

In summary, the benefits of peptizing agents are as follows TheyAccelerate viscosity reduction, decreasing mixing time

Reduce power consumption

Promote batch-to-batch uniformity

Facilitate blending of elastomers

Table 4 Influence of Chemical and Physical Peptizers on Viscosity Reduction

Mechanicalmastication

Chemicalpeptizer

Physicalpeptizer

Source: Schill+Seilacher, Hamburg, Germany.

Table 5 Influence of Chemical and Physical Peptizers on Tensile Properties

Mechanicalmastication

Chemicalpeptizer

PhysicalPeptizer

Reduce mixing costs

Modern lubricants on the market are normally composed of the listed basic materials Among the fatty acids, stearic acid still finds widespreadapplication as a material that improves both the processability of compoundsand their curing characteristics Because of their low melting point andwaxlike character, fatty acids enhance both mixing and down-line processing.They reduce the stickiness of compounds The fatty acids produced fromvegetable oils and animal fats are predominantly mixtures of C16–C18fattyacids Even though they have a higher volatility, fatty acids having a shorterchain length, such as lauric acid (C12), are occasionally used

above-The limited compatibility of stearic acid with synthetic rubbers and theneed for specialty products to solve complex processing problems have beenthe driving force for the development of more modern lubricants Rawmaterials for most lubricants are mixtures of glycerides such as vegetable oilsand animal fats Typical examples are listed inTable 6 Through saponifica-tion of the glycerides, mixtures of fatty acids are obtained that vary in carbonchain length distribution and in their degree of unsaturation

The most important fatty acids are listed inTable 7 Separation andpurification processes lead to specified technical grade fatty acids that are thebasis for tailor-made lubricants in rubber processing

The fatty acids tend to be incompatible and therefore insoluble in therubber hydrocarbon, and consequently they can migrate to the surface of theuncured rubber to form a bloom This will be detrimental to the tack-buildingability of the component and may lead to down-line assembly problems Thishas led to the development of fatty acid esters, fatty acid amides, and metalsoaps that are soluble in rubber and minimize bloom formation

Fatty acid esters are produced through reaction of fatty acids withvarious alcohols Apart from good lubricating effects they promote thewetting and dispersion of compounding materials The carbon chain lengths

of the acid and alcohol components vary between C16and C34

Metal soaps are produced through the reaction of water-soluble fattyacid salts with metal salts (e.g., ZnCl2) in an aqueous solution (precipitationprocess) Metal soaps are also obtained via a direct reaction of fatty acid withmetal oxide, hydroxide, or carbonate

The most important metal soaps are zinc and calcium soaps, with thezinc soaps having the largest market share Because calcium soaps have lessinfluence on the cross-linking reaction and scorch time, in most cases they areused in compounds based on halogen-containing elastomers such as CR or

Table 7 Important Fatty Acids

Fatty acid Chain lengtha Double bonds

Source: Schill+Seilacher, Hamburg, Germany.

Table 6 Important Raw Materials for

Fatty Acids

Palm kernel oil Rapeseed oil

Tallow

Source: Schill+Seilacher, Hamburg, Germany.

halobutyl The metal soaps are mostly based on C16–C18fatty acids Because

of better solubility in the rubber and lower melting points, modern lubricantsfrequently contain the salts of unsaturated fatty acids

When 2–5 phr of a metal soap is present in a compound, the stearic acidlevel should be reduced to 1 phr to minimize bloom

The most well known soap, zinc stearate, is also used as a dusting agentfor uncured slabs based on nonpolar rubbers Owing to its high crystallinitythe compatibility of zinc stearate is often limited Bloom can occur, whichmay lead to ply separation in assembled articles

In general metal soaps are also good wetting agents Under the influence

of higher shear rates they promote compound flow, but without shear theviscosity remains high (green strength) As discussed in the previous section,soaps of unsaturated fatty acids are also used as physical peptizers because oftheir lubricating effect; they exhibit high compatibility with rubber

Mixtures of zinc salts based on aliphatic and aromatic carboxylic acidsare cure activators, strongly delaying the reversion of NR compounds Theeffect is most pronounced in semi-EV systems

Fatty alcohols are obtained through reduction of fatty acids Straightfatty alcohols are rarely used as processing additives for rubber compoundsbecause of their very limited solubility They act as internal lubricants andreduce the viscosity

Fatty acid amides are reaction products of fatty acids or their esters withammonia or amines All products of this group reduce scorch safety, whichneeds to be allowed for in compound development

Organosilicones are relatively new in the range of lubricants They areproduced through condensation of fatty acid derivatives with silicones andcombine good compatibility through the organic component with the excel-lent lubricating and release properties of the silicones Depending on theirstructure they can be adapted to standard or specialty elastomers They havehigh thermal stability Because of their high compatibility the organosiliconesare not prone to reduced adhesion, delamination, or general contamination,which are generally associated with the presence of silicones in a rubberfactory They significantly improve calendering and demolding and reducemold fouling in critical polymers such as ethylene oxide epichlorohydrincopolymer (ECO) or fluoropolymers such as FKM

Polyethylene and polypropylene waxes of low molecular weight areeasily dispersed in natural rubber and synthetic rubbers They act as lubri-cants and release agents They improve the extrusion and calendering of drycompounds in particular and reduce the stickiness of low viscosity com-pounds Their compatibility with polar rubbers such as polychloroprene oracrylonitrile butadiene copolymer (NBR) is limited This can lead to adhesionand knitting problems when higher dosage levels are used

B Properties and Mode of Action of Lubricants

The major positive effects that can be achieved in various processing stages byusing lubricants are listed in Table 8

A strict classification of the products into internal and external cants is difficult, because practically all lubricants for rubber compoundscombine internal and external lubricating effects This depends not only ontheir chemical structure but also on the specific polymer in which they areused In general, the solubility in the elastomer is the determining factor

lubri-A processing additive predominantly acting as an internal lubricant willserve mainly as a bulk viscosity modifier and improve filler dispersion Slipperformance is influenced only to a minor extent A lubricant with predom-inantly external action will greatly improve slip and reduce friction betweenthe elastomer and the metal surfaces of the processing equipment Itsinfluence on compound viscosity is marginal Filler dispersion can beimproved through accumulation at the interface between elastomer and filler.Higher dosage levels, however, can lead to ‘‘overlubrication’’ (overconcen-tration) and subsequent blooming

Lubrication is achieved through a reduction of friction In the initialphase of addition the lubricant is coating the elastomer and possibly othercompounding materials, and friction against the metal parts of the processingequipment is reduced With increasing temperature the lubricant begins tomelt and is worked into the matrix by the shearing action of the mixer Therate and extent of incorporation of the lubricant into an elastomer is

Table 8 Lubricants—Processing Benefits

Mixing Faster filler incorporation

Less energy consumption

Molding Faster cavity fill at lower operating pressure

Reduced stress in molded parts through easy cavity fill

Shorter cycle times

Improved release

Source: Schill+Seilacher, Hamburg, Germany.

determined by its melting point, melt viscosity, and solubility These factorsdepend on its chemical structure and polarity.

The chemical criteria for the efficiency of organic lubricants are thelength of the hydrocarbon chain, the degree of branching, the unsaturation,and the structure and polarity of the terminal groups The action of fatty acidbased lubricants has been explained by means of the micelle theory ofsurfactant chemistry (2)

Rubber may be considered to be mostly nonpolar and as such is similar

to a mineral oil but with far higher molecular weight When dispersed in thismedium, metal soaps that have a sufficiently long hydrocarbon chain canform spherical or lamellar micelles The nonpolar hydrocarbon chain of thesoaps is soluble in the rubber whereas the polar terminal group remainsinsoluble Because of their limited solubility the micelles can aggregate instacks (Fig 7) Under the influence of the high shear rates that occur duringrubber processing, these layered aggregates can be shifted against oneanother, and the rubber compound flows more easily (Fig 8)

Relatively strong cohesion of the aggregates formed by zinc stearate can

be noted through a slight increase in the green strength of NR compoundsthat contain this metal soap at higher concentrations

The structure related effect of fatty acid based lubricants is shown inTables 9and10

The polar groups of certain fatty acids and their derivatives exhibit ahigh affinity to metal surfaces and are adsorbed easily A film is formed at themetal surface The film is extremely thin, is quite stable, and withstandsrelatively high shear The formation of a film should in theory facilitatedemolding, and the high thermal stability of the lubricant should reduce mold

Figure 7 Metal soaps as surfactants in a polymer matrix (Courtesy of Schill+Seilacher.)

contamination This is not, however, always the case in practice Becauselimited compatibility is the essential and determining factor for the effective-ness of external lubricants, an overdosage has to be avoided, otherwiseundesirable bloom will occur The lubricant concentration required, underpractical conditions, depends on the processing procedures used and inparticular on other compounding materials included in the formulation andtheir individual dosage levels Therefore it is necessary to check the compat-

Table 9 Structure–Property Relationships of Zinc Soaps

Carbon chain length

Carbon chain length distribution (blend)

Higher M.P

Poor dispersibilityCan bloom easily

Lower M.P

Disperses readilyReduced bloom tendencySolubility is increasedPolarity

High (functional groups, metal salts) Increased affinity to metal surfaces

Figure 8 Metal soaps as rheological additives (Courtesy of Schill+Seilacher.)

ibility of the lubricant chosen for a specific formulation Additives are easilyadsorbed by fillers Therefore higher dosages are required when highly activefillers or high filler loadings are used Certain plasticizers can reduce com-patibility and make the additives bloom.

Many commercial zinc soaps are indeterminate blends resulting fromthe ‘‘cut’’ of natural fatty acids used in the manufacture

Most lubricants are easily incorporated In some cases they are added at thebeginning of the mixing cycle, along with the fillers, to make use of theirdispersing effects Many of them can also be added at the end of the cycle.Because of their relatively low melting points the products will soften earlyand give a good and uniform dispersion

When the lubricating effect is of major importance, the processingadditives should be incorporated in the final stage The effects of selectedlubricants on spiral mold cavity fill when they are added in the first pass orfinal stage are shown inFigure 9

Depending on requirements and compatibility, the dosage varies tween 1 and 5 phr Usually the minimum dosage is 2 phr For an exceptionallyhigh lubricating effect in tacky compounds or where high extrusion rates andeasy demolding are critical, even higher dosages might be useful This appliesalso to compounds with high filler loadings

be-D Influence of Lubricants on Vulcanizate Properties

The effects of the lubricants STRUKTOLR WB 16 and STRUKTOLR A 50

P on the physical properties of a natural rubber compound are shown inTable 11 The lubricants lead to a decrease in 300% modulus in conjunctionwith a small drop in tensile strength and increase in elongation to break Nodifference is observed in Shore hardness, but compression set increasesslightly (3)

Table 10 Structure–Property Considerations of Zinc Soaps

Most zinc soaps are rubber-soluble, therefore act as intermolecular lubricants.Increased hydrocarbon chain length improves surfactant action

Presence of unsaturation improves dispersibility

Source: Schill+Seilacher, Hamburg, Germany.

Figure 9 Spiral mold cavity fill with lubricant-added in first or final stage Struktol

WB 222 is an ester of saturated fatty acids It is a lubricant and release agentpredominantly used for polar elastomers STRUKTOLR WB 16 is a mixture ofcalcium soaps and amides used as a lubricant for nonpolar polymers STRUKTOLR

A 50 P is a zinc soap of unsaturated fatty acids It is used as a physical peptizer in

NR compounds (Courtesy of Schill+Seilacher.)

Table 11 Influence of Lubricants on NR Physical Properties

Source: Schill+Seilacher, Hamburg, Germany.

IV HOMOGENIZING AGENTS

A Examples and Function

Homogenizing agents are used to improve the homogeneity of blend elastomers They assist in the incorporation of other compoundingmaterials, and intrabatch and batch-to-batch viscosity variation are reduced

difficult-to-by their use

They are resin-based mixtures that exhibit good compatibility withvarious elastomers and facilitate blending through early softening andwetting of the polymer interfaces Because the softening resins exhibit acertain tackiness, polymers that tend to crumble and polymer blends willcoalesce more easily, energy input is maintained at a high level, i.e., mixing ismore effective, and mixing times can often be reduced

Fillers are incorporated at a faster rate and are more evenly distributedowing to the wetting properties of the homogenizing agents Filler agglom-eration is minimized

Apart from their compacting effects the homogenizers lead to increasedgreen strength when used as a partial replacement for processing oil, andcompound flow is facilitated through improved homogeneity and a certainsoftening effect They increase the green tack of many compounds and boostthe efficiency of tackifying agents

In summary, homogenizing agents promote 1) the blending of tomers; 2) batch-to-batch uniformity; 3) filler incorporation and dispersion;4) shortening of mixing cycles; 5) energy savings; and 6) the building oftack

elas-The greater the difference in the solubility parameter and/or viscosity ofeach component elastomer in a blend, the more difficult it is to produce ahomogeneous mix (Table 12) Blends of plasticizers that are each compatiblewith different elastomers can in theory be effective at improving blendhomogeneity, provided that they have a viscosity sufficiently high to maintainhigh shear on mixing Plasticizers have the disadvantage of being prone tomigration and bloom Therefore mixtures of high molecular weight productssuch as resins are more often used

The homogenizing resins are themselves complex blends and containparts that are compatible with aliphatic and aromatic structures in a blend.Potential raw materials for use as homogenizing resins can be divided intothree groups:

1 Hydrocarbon resins including coumarone-indene resins, leum resins, terpene resins, bitumens, tar, and copolymers, e.g.,high styrene reinforcement polymers

petro-2 Rosins and their salts, esters, and other derivatives

3 Phenolic resins of various kinds, such as hyde resins, alkylphenol and acetylene condensation products, andlignin and modifications thereof

alkylphenol-formalde-Coumarone resins, produced from coal tar, were the first syntheticresins used as processing additives because of their ability to act as dispersingagents to improve filler incorporation and as tackifiers They are typical aro-matic polymers, consisting mainly of polyindene The structural elements ofthese copolymers are methylindene, coumarone, methylcoumarone, styrene,and methylstyrene (Fig 10) The melting range of these products is between35jC and 170jC

Petroleum resins are relatively inexpensive products that are often used

at fairly high dosages, up to 10 phr or more They are polymers produced fromthe C5cut of highly cracked mineral oils The petroleum resins are relativelysaturated and are also available with a high content of aromatic structures.Grades with a lower content of aromatic compounds have a strongerplasticizing effect The highly saturated grades are used by the paint industry.Apart from cyclopentadiene, dicyclopentadiene and its methyl derivatives,styrene, methylstyrene, indene, methylindene, and higher homologs of iso-prene and piperylene are found in these resins This may explain their highcompatibility with different elastomers

Table 12 Solubility Parameters of Elastomers and Plasticizers

AU, EU11

NBR (high nitrile) Polar ethers

Highly polar estersNBR (medium nitrile)

Copolymers such as high styrene resin masterbatches are used for highhardness compounds Whereas straight polystyrene can hardly be processed

in rubber compounds, copolymers of styrene and butadiene with higherstyrene contents have proven their worth

Terpene resins are very compatible with rubber and give high tackiness.However, they are used mainly for adhesives The polymers are based on a-and h-pinene The cyclobutane ring is opened during polymerization andpolyalkylated compounds are formed (Fig 11) Terpene resins improve agingperformance and resistance against oxidation of rubbers

Asphalt and bitumen are products that have been used since the verybeginning of rubber processing Their tackifying effect is not very distinct

Figure 11 Terpene resins—main constituents (Courtesy of Schill+Seilacher.)Figure 10 Coumarone resins—structural members (Courtesy of Schill+Seilacher.)

They are relatively inexpensive products Whereas asphalt is a naturally curring product, bitumen is produced from the residues of mineral oil pro-duction Blown bitumen, oxidized to achieve higher solidification points, isalso known as mineral rubber and is a good processing additive, for example,

oc-in difficult-to-process compounds that have a high percentage of diene Mineral rubber is also successfully used to improve the collapseresistance of extrusions

polybuta-Rosins are natural products obtained from pine trees They aremixtures of organic substances, for the most part doubly unsaturated acids,such as abietic acid, pimaric acid, and their derivatives (Fig 12) To reducetheir sensitivity to oxidation, resins are partially hydrogenated ordisproportionated Because of their acidity they have a slight retarding effect

on vulcanization Abrasion resistance, in particular that of SBR, is said to be

Figure 12 Rosin acids (Courtesy of Schill+Seilacher.)

improved Rosin acid is widely used (as a salt) in the production of syntheticrubbers (SBR) because of its emulsifying properties.

Phenolic resins (Fig 13) are used mainly as tackifiers, reinforcing resins,curing resins, and in adhesives Their use is determined by the degree of para-substitution and the presence of methylol groups

Lignin has a complex structure and is based on various substitutedphenols that are in part linked via aliphatic hydrocarbon units As a by-product of the cellulose industry and especially the paper industry, it isavailable in large quantities and is quite cheap It is often used in shoe soles,where it improves the incorporation and dispersion of high mineral fillerloadings

Modern homogenizing agents are blends of rubber-compatible hardening synthetic resins of different polarities With their specific compo-sitions they promote the homogenization of elastomers that differ inmolecular weight, viscosity, and polarity They may also be used in homo-polymer compounds where, among other effects, they can improve processinguniformity and filler dispersion

The homogenizing agents are usually added at the beginning of the mixingcycle, particularly when elastomer blends are used They exhibit optimumeffectiveness at around their softening temperature The recommendeddosage is between 4 and 5 phr Difficult-to-blend elastomers will require anaddition of 7–10 phr

As an example, the processing of butyl compounds may be improvedthough the use of a mixture of aromatic hydrocarbon resins, such asSTRUKTOLR 40 MSflakes, as a homogenizing agent Filler dispersion,splice adhesion, physical properties, and impermeability are significantlyimproved through the use of this resin blend

Figure 13 Alkylphenol resins (Courtesy of Schill+Seilacher.)

V DISPERSING AGENTS

A Properties of Dispersing Agents

Because dispersing agents are mostly fatty acid derivatives they can be looked

at as a subgroup of lubricants The central property, however, is dispersbility

In particular, they improve dispersion of solid compounding materials Theyreduce mixing time and have a positive influence on subsequent processingstages

Dispersing agents have distinct wetting properties They are often lesspolar fatty acid esters Because often a combination of dispersing propertiesand good lubrication is desirable, the dispersing agents available on themarket are occasionally mixtures of higher molecular weight fatty acids andmetal soaps Most products on the market are offered as ‘‘dispersing agentsand lubricants’’ and are not listed separately in the product ranges Theirmode of action has already been described in the section on lubricants

B Processing with Dispersing Agents

Dispersing agents are usually added together with the fillers Their productform and low melting point facilitate easy incorporation When fillers areadded in two steps, the dispersing agents should be added at the beginning.The dosage of these parts is between 1 and 5 phr Because of their higheffectiveness, however, low dosages are often sufficient Very high fillerloadings may require higher dosages

A typical product is STRUKTOLR W 33, a mixture of fatty acid estersand metal soaps that allows fillers to be rapidly incorporated and dispersed,particularly when high loadings have to be processed Agglomerations areavoided and batch-to-batch uniformity is significantly improved Their lu-bricant action leads to shorter mixing cycles, less power consumption, andlower mixing temperature Down-line processing is facilitated, and releaseperformance is improved

A Definition and Manufacturing Importance

The use of tackifiers in the tire industry has been reviewed by Lechtenboehmer

et al (4) Tack is considered the ability of two uncured rubber compoundsurfaces to adhere together or resist separation after being in contact undermoderate pressure for a short period of time Two types of tack may bedefined: autohesive tack, in which both materials are of the same chemical