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KAOLIN CLAYMineral Fillers for Rubber

R.T. Vanderbilt Company, Inc.

Before viewing this presentation, please be sure to read the usage statement.

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Mineral Fillers For Rubber: KAOLIN CLAY

FILLER BASICS

A rubber compound contains, on average, less than 5 lbs. of chemical additives per 100 lbs. of elastomer, while filler loading is typically 10-15 times higher. Of the ingredients used to modify the properties of rubber products, the filler often plays a dominant role. The term “filler” is misleading, implying, a material intended primarily to occupy space and act as a cheap diluent of the more costly elastomer. Most of the rubber fillers used today offer some functional benefit that contributes to theprocessability or utility of the rubber product. Styrene-butadiene rubber, for example, has virtually no commercial use as an unfilled compound.

Kaolin clay is the primary non-black filler in rubber, accounting for more than 50% of all non-black filler used. Of this, about 80% is airfloat hard clay, with calcined clay, delaminated clay and water-washed clay making up the balance.

A review of kaolin clay and its performance in rubber compounds is best undertaken within the context of certain general considerations regarding the ways that fillers and elastomers interact.

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Effects of Mineral Fillers Depend On

Particle Size

Particle Shape

Particle Surface Area

Particle Surface Activity(Compatibility With/Adhesion To Matrix)

The characteristics which determine the properties a filler will impart to a rubber compound are particle size, particle shape, surface area and surface activity. Surface activity relates to the compatability of the filler with a specific elastomer and the ability of the elastomer to adhere to the filler.

Functional fillers transfer applied stress from the elastomer matrix to the strong and stiff mineral. It seems reasonable then that this stress transfer will be better effected if the mineral particles are smaller, because greater surface is thereby exposed for a given mineral concentration. If these particles are needle-like, fibrous or platy in shape, they will better intercept the stress propagation through the matrix. Likewise, intimate adhesion of the matrix onto mineral particles is essential, since air gaps represent points of zero strength. Thus, compound strength can be further enhanced if the matrix is adhered to the mineral surface via chemical bonding.

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Particle SizeSmaller is Better

>10,000 nm (10µµm): Degradants

1000-10,000 nm (1-10µµm): Diluents

10-1000 nm (0.1-1µµm): Semi-reinforcing

10-100 nm (0.01- 0.1µµm): Reinforcing

If the size of the filler particle greatly exceeds the polymer interchaindistance, it introduces an area of localized stress. This can contribute toelastomer chain rupture on flexing or stretching. Fillers with particle size greater than 10,000 nm (10 µm) are therefore generally avoided because they can reduce performance rather than extend or reinforce. Fillers with particle sizes between 1,000 and 10,000 nm (1 to 10 µm) are used primarily as diluents and usually have no significant affect, positive or negative, on rubber properties. Semi-reinforcing fillers range from 100 to 1000 nm (0.1 to 1µm). The truly reinforcing fillers, which range from 10 nm to 100 nm (0.01 to 01 µm) significantly improve rubber properties.

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Median Particle Size = Half the particles are larger, half are smaller

Size (top and bottom) Counts

ESD!!!

Particle Size Distribution

The particle size of a mineral filler is often reported in terms of median or average size and in most cases is actually measured as equivalent spherical diameter rather than actual size or dimensions. For round or block-shaped, such as natural calcium carbonate, there is no significant difference. For platy minerals, such as clay, talc and mica, or needle-like minerals, such as wollastonite, most automated particle sizing instrumentation will match the behavior of a particle to that of an idealized round particle of specific diameter. For example, a clay platethat is reported as 200 nm eqivalent spherical diameter might be 1 nmthick by 600 nm across. Comparing the particle size of two fillers by comparing their respective median esd can therefore be very misleading if differences in particle shape are not accounted for. Further complicating the issue is the fact that median (or average) particle size can poorly represent particle size distribution, as suggested here. Both distributions have the same median size, but the lower one may have particles large enough to compromise compound properties.

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Basic Particle Shapes

5-20+11Aspect Ratio

NeedleCubeSphereShape

20-200

Plate/Flake

20-200+2-4Aspect Ratio

FiberBlockShape

A particle’s shape plays a significant role in its ability to intercept stress applied to the elastomer composite. In general, anisometric particles -those having a significant difference in length vs width - are more effective as reinforcements than isometric particles - those similar in length and width. The basic particle shapes characteristic of mineral fillers are shown here along with their typical aspect ratios, the measure of their anisometry.

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Needle/Fiber Aspect Ratio:Ratio of mean length to mean diameter

Plate Aspect Ratio:Ratio of mean diameter of a circle of the same area as the face of the plate to the mean thickness of the plate

L

D

TD

Aspect Ratio

For needle and fiber shaped fillers, aspect ratio is the ratio of length to diameter. For kaolin clay and other platy minerals, it is the ratio of the diameter of a circle with the same are as the face of the plate to the thickness of the plate.

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Particle ShapeBroader (and Longer) is Better

Isometric: calcium carbonate

Platy: kaolin, talc, mica

Acicular: wollastonite

Fiber: glass fiber

Chain: carbon black, ppt silica

Isometric fillers that are approximately round, cubic or blocky in shape, are considered low aspect ratio. Low, in this context, means less than about 5:1 aspect ratio. Platy, acicular (needle-shaped) and fibrous fillers are considered high aspect ratio.

Aspect ratio is not applied to carbon black and precipitated silica. The primary particles of these fillers are essentially spherical, but these spheres aggregate in such a way that the functional carbon black and precipitated silica filler “particles” are chains or bundles with various degrees of branching. The anisometry of these fillers is described in terms of “structure”, which incorporates aggregate shape, density and size. The higher the structure, the greater the reinforcement potential.

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Particle Surface AreaBigger Is Better

From smaller particle sizeFrom higher aspect ratio

A filler must make intimate contact with the elastomer chains if it is going to contribute to reinforcement of the rubber-filler composite. Fillers that have a high surface area have more contact area available, and therefore have a higher potential to reinforce the rubber chains. The shape of the particle is also important. Particles with a planar shape have more surface available for contacting the rubber thanspherical particles with an equivalent average particle diameter. Clays have planar-shaped particles that align with the rubber chains during mixing and processing, and thus contribute more reinforcement than spherical-shaped calcium carbonate particles of similar average particle size. Carbon black and precipitated silica chains and bundles are considerably smaller than the particles of clay. They thus have more surface area per unit weight available to make contact with the polymer. Rubber grade carbon blacks vary from 6 to 250 m2/g. Most reinforcing precipitated silicas range from 125 to 200 m2/g; a typical hard clay ranges from 20 to 25 m2/g.

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Particle Surface ActivityMore is Better

Active sites on filler surface

Reactive surface treatments

A filler can offer high surface area, high aspect ratio and small pariclesize, but still provide relatively poor reinforcement if it has low specific surface activity. The specific activity of the filler surface per cm2 of filler-elastomer interface is determined by the physical and chemical nature of the filler surface in relation to that of the elastomer. Nonpolar fillers are best suited to nonpolar elastomers; polar fillers work best in polarelastomers. Beyond this general chemical compatibility is the potential for reaction between the elastomer and active sites on the filler surfaces. Carbon black particles, for example, have carboxyl, lactone quinone, and other organic functional groups which promote a high affinity of rubber to filler. This, together with the high surface area of the black, means that there will be intimate elastomer-black contact. The black also has a limited number of chemically active sites (less than 5% of total surface) which arise from broken carbon-carbon bonds as a consequence of the methods used to manufacture the black. The close contact of elastomer and carbon black will allow these active sites to chemically react with elastomer chains. The carbon black particle effectively becomes a crosslink. The non-black fillers generally offer less affinity and less surface activity toward the common elastomers. This can be compensated to a greater or lesser extent by certainsurface treatments.

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Si

OHSi

OH

Si

OH

SiOH

SiOH

Si

OHSiOH

SiOH

SiO-H+

=

Silanol

Clays, silicas, silicates have chemically active surface silanols

Kaolin clay and all of the non-black, non-carbonate reinforcing or semi-reinforcing fillers have one feature in common. The clays, silicas, and silicates all have surface silica (SiO2) groups which have hydrolyzed tosilanols (-SiOH). These silanol groups behave as acids (-SiO-H+) and are chemically active. The higher surface area fillers have more silanolsavailable and are thus more reactive.

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Si

OHSi

OH

Si

O-

Si

O

Si

O

Si

O- Si

O- N+ -activator

ZnTEADEGPEG

Silanols will interfere with sulfur cures,

but their activity can be controlled.

Silanols show similarities to carboxylic acid groups in their reactions with amines, alcohols, and metal ions. While water adsorbed on the surface of filler particles will reduce silanol reactivity, hot compounding volatilizes some of this, leaving a reactive surface. Some of the reactions with silanols can have a significant effect on the properties of the rubber compound, especially where the chemical involved is an important part of the cure system. Most of the accelerators used in sulfur cure systems contain an amine group. Strong adsorption orreaction with filler particles can decrease the amount of accelerator available for vulcanization reactions. This can give slower cure rates and a reduced state of cure. Similar effects can result from the reaction of zinc ions with filler particles.

The adsorption or reaction of accelerators by hard clay usually requires about 15-25% increase in acceleration. These negative effects on the cure system can be reduced or completely avoided, however,by adding other chemicals that will tie up the silanol groups and reduce their activity. Triethanolamine (TEA), diethylene glycol (DEG) and high molecular weight polyethylene glycol (PEG) typically serve this function. These are mixed into the compound prior to the addition of the accelerators.

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Si

OHSi

OH

Si

OH

Si

O

Si

OSi CH2CH2CH2 S

Polymer

Si

OHSi

OH

ModulusTensile Strength

Abrasion Resistance

Silanes = Rubber Reactivity

One effective means of modifying the surface chemistry of non-black fillers is by the addition of silane coupling agents. These react with thesilanols on the filler surface to give a strong bond, and also contain afunctional group that will bond to the rubber during vulcanization. The result is filler-polymer bonding (crosslinking) that increases modulus and tensile strength and improves other compound properties.Modification of the filler surface also improves dispersion.

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Sulfur CureMercapto

TetrasulfideThiocyanate

Non-Sulfur CureAminoEpoxy

SILANES

There are three silane coupling agents commonly used in sulfur-cured compounds filled with kaolin clay and other non-black fillers. These have mercapto, tetrasulfide and thiocyanate functionalities, respectively. The sulfur-containing group of each reacts during vulcanization to bond to the polymer. For non-sulfur cures, amino silane and epoxy silane are typically used.

Clay pretreated with silane is available, but in situ treatment of clay and other non-black fillers is common. In this case, the silane is added during compounding so that it can react with the filler. The addition sequence is very important when adding silane coupling agents, particularly with sulfur cures. The polymer, filler, and silane coupling agent should be mixed for 1 to 2 minutes before adding any otheringredients that will interfere with the reaction between the filler and thesilane.

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Fillers EffectsIncreasing Surface Area (Decreasing Particle Size)

Higher Mooney viscosity, tensile strength, abrasion resistance, tear resistance, and hysteresis .Lower resilience

Increasing Surface ActivityHigher abrasion resistance, chemical adsorption or reaction, modulus (at elongation >300%), and hysteresis (except silated clay).

Increasing Aspect RatioHigher Mooney viscosity, modulus (at elongation <300%) and hysteresis .Lower resilience and extrusion shrinkage; longer incorporation time.

The principal characteristics of rubber fillers – particle size, shape, surface area and surface activity – are interdependent in improving rubber properties. In considering fillers of adequately small particle size to provide some level of reinforcement, the general influence of each of the other three filler characteristics on rubber properties can be generalized as follows:

1) Increasing surface area (decreasing particle size) gives: higher Mooney viscosity, tensile strength, abrasion resistance, tear resistance, and hysteresis; lower resilience.

2) Increasing surface activity (including surface treatment) gives: higher abrasion resistance, chemical adsorption or reaction, modulus (at elongation >300%), and hysteresis (except for silane treated clay, as we’ll see).

3) Increasing aspect ratio gives: higher Mooney viscosity, modulus (at elongation <300%), and hysteresis; lower resilience and extrusionshrinkage; longer incorporation time.

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Tensile Properties

MovableGrip

FixedGrip

Strain GaugeTest

Specimen

Stretching Force

The primary filler effects in rubber can be related to tensile properties: in simplest terms, the ability of the filler-rubber composite to respond to stretching forces.

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300% Elongation

Stretching

ForceResistance to Stretching

Modulus: Resistance to stretching

Uncompouded elastomer: disentangle polymer chains, break weak inter-chain bonds

Vulcanized, unfilled elastomer: break sulfur crosslinks

The force required to stretch a defined specimen of rubber to a given percent elongation is measured as modulus. Most often, modulus is reported at 300% elongation (four times original length). This can be alternatively viewed as the resistance to a given elongating force. For an uncompounded elastomer, elongation is primarily a function of stretching and disentangling the randomly oriented polymer chains and breaking the weak chain-chain attractions. Vulcanized, but unfilled,elastomers, for example, more strongly resist elongation because the sulfur crosslinks must be stretched and broken to allow chain extension and separation.

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Break

Elongation at Break

Tensile Strength: Resistance to stretching rupture

If stretching continues, rupture ultimately results.

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Viscous Drag

Coupling

The introduction of a filler into the vulcanizate provides additionalresistance to elongation. A filler with low surface activity will increase resistance to elongation by the viscous drag its surface provides to the polymer trying to stretch and slide around it. Higher surface area, greater aspect ratio, and higher loading (the latter two effectively increasing the surface area exposed to the elastomer) will all increase the modulus. Fillers with strong chain attachments, via active sites or coupling agents, provide the most resistance to the chain extension and separation required for elongation and ultimate rupture.

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B4 B5

A4

B6

A2

B1B2

A1

B3

A3

A1A2

B1B2

B3

A3

A4

B4

B5

B6

B3

A1

B2

A4

B5

B6

A2

B1

A3

B4

(1)

(2)

(3)

The effect of filler particles on the ability of the compound to stretch can be pictured as shown here. Before stretching, as in step 1, the polymer chains are in random configuration. Chains A and B have multiplepoints of attachment to the filler particles, corresponding to the particle’s active sites. On elongation, resistance is supplied as the energy required to detach the chain segments from these active sites, as in steps 2 and 3.

The amount of energy required to attain maximum elongation, and then required to cleave chain-chain and chain-filler attachments, accounts for the tensile strength of a filled system of this type.

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B4 B5

A4

B6

A2

B1B2

A1

B3

A3

(3)

A1

B3 B6

A4

(4)

After the elongating force has been removed, the elastomer chains return to their preferred random orientation, as in step 4, except that now they have the minimum number of points of attachment to the filler as a consequence of having been extended in Step 3. Less force would now be required to return these chains to ultimate extension, because the intermediate points of attachment that existed in Steps 1 and 2 have been eliminated. This explains the phenomenon known as stress softening. With repeated stress-relaxation cycling, a decrease in modulus from the initial maximum is obtained. Stress softening is a temporary effect. After a period without strain, the rubber will recover to near its original modulus, as the active filler sites again attach to polymer segments. A percentage of the original modulus is permanently lost, however, due to irrecoverable chain and bond cleavage.

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Resilience

HysteresisResilience

Reinforcement

Resilience / Hysteresis

Resilience is essentially a measure of rubber elasticity – the characteristic ability to quickly return to original shape following deformation. Unfilled elastomers are at their peak resilience because there is no obstacle to elastomer chain extension and contraction (return to randomness). The introduction of a filler creates such an obstacle in proportion to the strength of the particle-polymer interaction. Resilience is therefore generally in inverse proportion to filler loading and reinforcement.

If resilience is considered the ratio of energy release on recovery to the energy impressed on deformation, hysteresis can be viewed as a measure of the amount of impressed energy that is absorbed. An unfilled (elastic) elastomer will convert most of the energy of deformation into the mechanical energy used in returning to the original shape. The degree to which input energy is converted to heat, due to friction from polymer chains sliding past each other, is a function of polymer chain morphology. All fillers tend to increase the hysteresis of rubbers, the most reinforcing fillers generally producing the largest increases. This is due to the energy consumed in polymer-filler friction and in dislodging polymer segments from active filler surfaces. This is heat energy which would otherwise have a mechanical manifestation as elastic rebound. In short, hysteresis is inversely related to resilience.

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Related to Tensile Properties

Abrasion ResistanceImpact StrengthTear StrengthFlex Resistance

Soft Edge of Depression

Filler size, shape and matrix adhesion are also reflected in abrasion resistance, impact strength, tear strength and flex resistance.

Loss of large or poorly bound filler particles by abrasion exposes the relatively soft surrounding elastomer matrix to wear. The effect is acute on the edge of the depression left by the dislodged particle. This is the area most susceptable to elongation, crack initiation and ultimate loss.

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Abrasion ResistanceImpact StrengthTear StrengthFlex Resistance

Impact strength is the measure of a hard rubber’s resistance to fracture under sudden impact or force. The impact force is one of compression on one side of the rubber and tension on the other, so that fracture is a result of failure similar to that occuring when a tensile sample is elongated to break.

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Abrasion ResistanceImpact StrengthTear StrengthFlex Resistance

Compression

TensionTear

Tear strength and flex resistance are essentially a measure of resistance to crack or slit propagation. Both typically measure the growth of a crack under tension. Large or poorly bound fillers will act as flaws and initiate or propagate cracks under test conditions. Small particles size, high surface area, high surface activity and high aspect ratio allow the filler particles to increase flexural modulus (i.e., resistance to bending), act as barriers to the propagation ofmicrocracks, and provide the higher tensile strength required to resist failure.

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Mineral Fillers For Rubber: KAOLIN CLAY

CLAY BASICS

9 million tons of US kaolin clay:90% from GA 5% from SC

225k tons of rubber clay:30% from GA 70% from SC

The United States is the leading supplier of kaolin filler clay. At 9 million tons per year, it provides about 40% of world production. Nearly all US filler kaolins are produced in the world’s major kaolin belt, the 250 miles between Aiken, SC and Eufala, AL, which has estimated reserves o f up to 10 billion tons. Nearly 90% of the kaolin produced in this country, comes from Georgia, and about 5% from South Carolina. However, of the approximately 225,000 tons of kaolin clay sold to the US Rubber Industry annually, 70% comes from South Carolina.

An additional 34,000 tons of kaolin clays for rubber are exported.

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= O = Si

OH

OH

OH

OH

OH

OH

= Al or MgSilica tetrahedron Al, Mg Octahedron

Kaolin clay and related platy silicates (talc, mica, pyrophyllite) can be considered inorganic polymers based on two basic “monomer” structures - the silica tetrahedron and the alumina or magnesia octahedron. These minerals contain a continuous octahedral layer with the joined octahedra tilted on a triangular side. This layer is bound on one side (as in kaolin) or both sides (as in talc, mica and pyrophyllite) by a continuous silica layer.

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= O = Si

Linked Silica

Rings

The continuous silica layer is composed of tetrahedra with three sharedoxygens and every fourth (apical) oxygen pointed in the same direction.This forms a layer of linked rings with hexagonal openings, as illustrated here.

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Kaolinite

OH

OH

OH

= O = Shared O = Si = Al

OH OH

OH

~0.72nm

The prototypical clay mineral is the aluminum silicate, kaolinite. Its structure, as illustrated here, is composed of an octahedral aluminalayer joined to a tetrahedral silica layer via shared apical oxygens.

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Kaolin “Books”

The continuous sheet structure produces thin particles which are often found in nature as overlapping flakes. These are informally called “books” because of their resemblance under magnification to stacks of paper. Kaolin books are bound via hydrogen bonding of the octahedral layer hydroxyl face of one flake to the tetrahedral layer oxygen face of the adjacent flake. Separation of books into individual clay flakes is therefore difficult.

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Kaolin Types

Kaolin clays are classified in various ways, with terminology specific to certain industries. We’ll preface the descriptions of the clays commonly used in the Rubber Industry, with a brief review of several clay aliases.

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GeologicallyPrimary kaolin: The clay occurs in the deposit where it formed.

Sedimentary kaolin: The clay has been eroded and transported from its site of formation. The GA/SC clays are all sedimentary.

Primary or residual kaolin clay occurs in the deposit where it formed. The world’s major source of primary kaolin is Cornwall, England.

Sedimentary or secondary kaolin clay has been eroded and transported from its site of formation and deposited at a distant location. The eastern US kaolin belt consists of sedimentary kaolin.

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In the Ceramics IndustryChina clay: Pure white or near white, usually well crystallized kaolin with low plasticity and low green strength. (Soft clay)

Ball clay: Fine-grained sedimentary clay containing natural organic matter that confers high plasticity and high green strength.

Fire clay: A heat resistant kaolin used to raise thevitrification temperature of ceramics.

Flint clay: A highly refractory hard rock composed mainly of well ordered kaolinte.

The Ceramics Industry differentiates four types of kaolin:

China clay is pure white or near white, usually well crystallized kaolin with low plasticity and low green strength. China clay generallycorresponds to what is called soft clay in the Rubber Industry.

Ball clay is fine-grained sedimentary clay containing at least 70% kaolinite and also natural organic matter. The organics give ball clay high plasticity and high green strength, which means that, when mixed with water, it has the properties associated with modeling clay.

Fire clay is a refractory (heat resistant) kaolin used to raise thevitrification temperature of ceramics.

Flint clay is a highly refractory hard rock composed mainly of well ordered kaolinte.

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Multi (Paint, Paper, Rubber) IndustryWater-washed clay: Kaolin slurried in water and centrifuged or hydrocycloned to remove impurities and produce specific particle size fractions. Water-washed clays are often treated to improve brightness.

Water-washed kaolin has been slurried in water and centrifuged orhydrocycloned to remove impurities and produce specific particle size fractions. The refined slurry is either dewatered (to reduce soluble impurities) and dried, or concentrated to 70% solids and sold in slurry form to the Paint and Paper Industries. Water-washed clays are often treated to improve brightness. This includes chemical bleaching and/or high-intensity magnetic separation to remove iron and titanium impurities. Although most of the clay used in rubber is airfloat, some water-washed clay is used because its lower level of impurities provides better color and less die wear with rubber extrusions.

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Rubber ClaysHard clay: Very fine-grained, relatively poorly crystallized. Provides good tensile properties, stiffness and abrasion resistance. Improves properties of GCC compounds. Low cost substitute for portion of carbon black or ppt silica without loss of properties.

Soft clay: Better crystallized, low reinforcing effect. Higher loadings, quicker extrusions.

Rubber filler clays are classified as either “hard” or “soft” in relation to their particle size and stiffening affect in rubber. A hard clay will have a median particle size of approximately 250 to 500 nm, and will impart high modulus, high tensile strength, stiffness, and good abrasion resistance to rubber compounds. Soft clay has a median particle size of approximately 1000 to 2000 nm and is used where high loadings (for economy) and faster extrusion rates are more important than strength.

More hard clay than soft is used in rubber because of its semi-reinforcing effect and its utility as a low cost complement to other fillers. It is used to improve the tensile and modulus of ground calcium carbonate compounds and will substitute for a portion of the more expensive carbon black or precipitated silica in certain compounds without sacrificing physical properties.

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Airfloat clay: Dry-ground kaolin that has been air separated to minimizeimpurities and control particle size distribution.

Airfloat clay is dry-ground kaolin that has been air separated to minimize impurities, such as quartz, mica and bentonite, and control the particle size distribution. About 80% of the kaolin used in the Rubber Industry is airfloat hard clay.

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24/25507.0 (40)594201.4 (200)1.1 (160)15024/23455.3 (30) 534101.6 (2301.2 (170)10024/21406.2 (35)493901.7 (250)1.4 (200)50

Calcium CarbonateC

23/405422.9 (130)677307.6 (1100)3.3 (480)15024/325020.2 (115)627408.8 (1270)2.8 (410)10027/234311.4 (65)505403.3 (480)1.8 (260)50

Soft ClayB

19/475137.0 (210)6770010.1 (1470)4.8 (700)15025/325024.6 (1406181012.8 (1860)3.2 (460)10027254613.2 (75)507709.0 (1310)1.7 (250)50

Hard ClayA

MooneyT5/ML

3Comp.Set2, %

Tear1

kN/m (pli)HardnessShore A

Elong.%

TensileMPa (psi)

300%MPa (psi)

Fillerphr

1Die A 222 hours, 70oC 3132oC AAirfloat, DIXIE CLAY BAirfloat, McNamee CLAY CAtomite

Comparison of Clay and Calcium Carbonate in SBRSBR 1006 100 AGERITE® STALITE® S 1.5VANFRE® AP2 2 Sulfur 2Zinc Oxide 5 METHYL CUMATE® 0.1Stearic Acid 2 ALTAX® 1.5

This table compares calcium carbonate, hard clay and soft clay in an SBR compound. The effects of particle size and shape are evident. The calcium carbonate represents large (approximately 2500 nm average) isometric particles providing little improvement in compound properties compared to the clays. The hard clay provides greater tensile and tear strength than the soft clay.

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605054Rebound, 100oC494348Rebound, 23oC486768Pico Abrasion Index82527Trouser Tear, kN/m

720770735Elongation, %20.420.920.5Tensile Strength, MPa3.13.13.6300% Modulus, MPa656566Hardness101115Cure Time, 150oC3712t90% Cure, 150oC

23----Calcium Carbonate (dry-ground), phr--23--Hard Clay, phr303045HI-SIL® 233 (ppt silica), phr

Silica Plus Lower Cost Fillers in Shoe Sole CompoundSBR 1502 100 ALTAX® 0.7Diethylene Glycol 3 CAPTAX® 0.4Stearic Acid 1 UNADS® 0.3Zinc Carbonate 1 VANAX® DPG 0.1Sulfur 2

This compound compares the ability of a hard clay and a dry-ground calcium carbonate to reduce the cost of a 65 Durometer shoe sole compound reinforced with precipitated silica. All three compounds show equivalent modulus and tensile strength. The compound with calcium carbonate, however, has significantly reduced tear strength and abrasion resistance while the clay maintains these properties.

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Delaminated clay: Coarse clay fraction from water washing is milled to reduce the kaolinite stacks into thin, wide individual plates. This improves brightness and opacity.

To produce delaminated clay, the coarse clay fraction from water washing is attrition milled to break down the kaolinite stacks into thin, wide individual plates. This improves brightness and opacity.

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APolyfil® DL BSuprex®

150100Die Swell(% comparison)

1727Olsen Stiffness(2” span, in-lbs)

2853NBS Abrasion Index, %390360Elongation, %30003080Tensile Strength, psi22002400300% Modulus, psi

7074Hardness, Shore A

Hard ClayBDelaminated ClayA

Comparison of Delaminated Clay to Hard ClayNarural Rubber 100 Stearic Acid 4(Smoked Sheets) Cumar® MH 2½ 7.5 Clay 156 Sulfur 3Zinc Oxide 5 CAPTAX® 1

Delaminated clay is the highest aspect ratio form of clay available, and it is used for high stiffness and low die swell, as demonstrated in this natural rubber compound.

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Calcined clay: The kaolin, usually water-washed soft clay, iscalcined to partiallyremove surface hydroxyls. Used in wire and cable coverings for excellent dielectric and water resistance properties.

To produce calcined clay for filler uses, the kaolin, usually water-washed soft clay, is calcined to partially remove surface hydroxyl groups. Calcining increases brightness and opacity, both useful in the Paint Industry, as well as oil absorption and hardness (i.e., abrasivity or abrasion resistence). The partial or complete removal of surface hydroxyls provides a corresponding decrease in surface activity, and thus reinforcement. Calcined clay is commonly used in wire and cable coverings because it provides excellent dielectric and water resistance properties.

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Flexible Cord InsulationRoyalene® X3751 100 Parafin Wax 5(EPDM) Vinyl Silane A-172 2Whitex® Clay 180 Kadox® 911C 5(Calcined Kaolin) SR-350 2Naugard® Q 1 VAROX® DCP-40KE 7.5Sunpar® 2280 55

0.00.00.01 Day/14 Days/Change, 1-14 DaysStability Factor

-0.31-14 Days, %0.37-14 Days, %

Capacitance Change1.51.53.2214 Days1.61.73.217 Days1.61.63.231 Day

PF @ 40 v/milPF @ 80 v/milSIC @ 80 v/milSIC, PF @ 75oC Water

This EPDM insulation compound illustrates how calcined clay is used to maintain electrical properties.

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Surface-treated clay: Processed kaolin that has been surface modified (e.g., with silanes) to improve compatibility with and performance in elastomer matrices.

High Modulusand

Low Hysteresis

Silane treated hard clays provide better reinforcement than untreated clay, and in some applications can rival furnace blacks. The combination of platy shape and chemical reactivity enable the silane-treated kaolins to impart a unique blend of properties to elastomers. These include high modulus, low hysteresis, good abrasion resistance, low viscosity, low set, and resistance to heat and oxidative aging. The unusual combination of high modulus and low hysteresis, in particular, have earned them a place alone or as a partial carbon black replacement in products requiring good dynamic properties, such as transmission and V belts, and non-tread tire components.

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Comparison of Treated to Untreated Hard ClayNATSYN® 400 100 AGERITE® White 1Hard Clay 75 Sulfur 2.75 Zinc Oxide 5 AMAX® 1.25Stearic Acid 5 METHYL TUADS® 0.2

182319Mooney Scorch, min.(MS-3/121o C)

ANulok® 321 BNucap® 200 CSuprex®

334258Mooney Viscosity(ML-4/100oC)

29.614.012.4Compression Set B(22 hours, 70oC)

155235245Die C Tear, ppi580480480Elongation, %

308032402960Tensile Strength, psi77017701630300% Modulus, psi606562Harness, Shore A

NoneCMercaptosilaneBAminosilaneAClay Treatment

Aminosilane and mercaptosilane treated clay are compared here to an untreated hard clay in a synthetic polyisoprene compound. The improvements in modulus, tear strength, compression set and the reduction in elongation with the treated clays are all indicative of the improved elastomer-clay bonding. The ultimate tensile strength is not significantly affected since the rupture strength of the polymer-polymer linkages remains unchanged.

45

Comparison of Treated Hard Clay to Carbon BlackNarural Rubber 100 Cumar® MH 2½ 7.5(Smoked Sheets) Sulfur 3Zinc Oxide 5 CAPTAX® 1Stearic Acid 4

2047983NBS Abrasion Index, %

N330----Carbon BlackMercaptosilaneBAminosilaneAClay Treatment

10912Mooney Scorch, min.(MS-3/121o C)

ANulok® 321 BNucap® 200

614546Mooney Viscosity(ML-4/100oC)

219142183Heat Buildup, oF(Firestone)

660215235Die C Tear, ppi590560520Elongation, %388033203320Tensile Strength, psi140013601630300% Modulus, psi

575556Harness, Shore A

4575 75Level, phr

The high modulus with low hysteresis of the treated clays vs. carbon black in this natural rubber compound suggests that the silane mediated polymer-clay bond is stronger than the polymer-black bonds at the carbon black active sites. Deformation results in loss of fewer polymer-filler bonds with the treated clays, so that energy loss as heat is less compared to the carbon black. This stronger polymer-clay bond is also consistent with the higher modulus and abrasion resistance.

46

KAOLIN CLAYMineral Fillers for Rubber

R.T. Vanderbilt Company, Inc.

For all the functionality it provides in rubber, kaolin clay, and airfloat hard clay in particular, can convincingly claim to be the biggest bargain in rubber additives.

47

KAOLIN CLAYMineral Fillers for Rubber

R.T. Vanderbilt Company, Inc.

Use of Information

The information presented herein, while not guaranteed, was prepared by technical personnel and, to the best of our knowledge and belief, is true and accurate as of the date hereof. No warranty, representation or guarantee, express or implied, is made regarding accuracy, performance, stability, reliability or use. This information is not intended to be all-inclusive, because the manner and conditions of use, handling, storage and other factors may involve other or additional safety or performance considerations. The user is responsible for determining the suitability of any material for a specific purpose and for adopting such safety precautions as may be required. R. T. Vanderbilt Company does not warrant the results to be obtained in using any material, and disclaims all liability with respect to the use, handling or further processing of any such material. No suggestion for use is intended as, and nothing herein shall be construed as, a recommendation to infringe any existing patent or to violate any federal, state or local law or regulation.[ Click to First Page ]