Influence of mixing procedures on the properties of a silica reinforced agricultural tire tread.

Carbon black is the most universal reinforcing filler. In some tire applications it does, however, show a few weak points which can be avoided if it is used in combination with precipitated silica.

It is particularly true for off-the-road tire applications, such as agricultural tire treads, where basic requirements are good stone cutting resistance and good block chipping, chunking and tearing resistances, together with low heat build-up and good wear resistance.

Precipitated silica is usually used to enhance the resistance of such treads to tearing (refs. 1-6). Increased quantity of silica produces improved tread cutting and chipping performances (refs. 1 and 7). Precipitated silica may be used to lower heat build-up (refs. 3, 8 and 9). Nevertheless, a coupling agent and appropriate modifications of the curing system are necessary in order to hold tread properties to a good level (refs. 1-14). Improved dispersion may accompany the use of silane (ref. 1) or stearic acid (ref. 8) to decrease heat build-up (ref. 8). A sufficient quantity of accelerator provides correct crosslinking level (refs. 9-10). Network and coupling bonds cause similar reductions in heat build-up (ref. 1). A coupling agent is necessary in order to obtain low abrasion loss (refs. 1, 3, 9 and 13). The modification of the silica leads to filler-to-rubber bonds, which increase wear resistance and reduce heat build-up (refs. 3, 12 and 13).

Therefore, combinations of carbon black and silica, as long as certain rules are observed, provide an improved compromise with higher performance level than when using carbon black alone (ref. 9). It becomes possible to minimize heat build-up without sacrificing tear (refs. 9 and 11) and wear (refs. 12-13) properties.

Moreover, resistance to cut propagation can be increased by fine-particle filler (refs. 15-16). In general, the smaller the particle size, the higher will be the reinforcement, but also the heat build-up (ref. 16). By reinforcement is meant the enhancement of wear resistance and tear resistance.

Where a risk of physical cutting is involved, the use of high gum strength polymers such as natural rubber is desirable (ref. 16).

Consistent with such guiding principles, partial substitution of the carbon black by a high specific surface area silica (Silica B, table 1) in an adjusted natural rubber formula (table 2), is one way to obtain agricultural tire treads with an improved compromise in specifications.

Nevertheless, all details of the mixing procedure are important in determining the final properties of the compound (refs. 18-19), particularly if the rubber is natural rubber reinforced with precipitated silica (refs. 1, 3, 4, 14, 17 and 20). A first problem is the sequence of adding chemicals (refs. 1, 3, 14 and 17). A second problem is choice of the dump temperature to obtain good agricultural tire tread properties (refs. 3, 4, 17, 20 and 21). A third problem is to achieve a final mix with the necessary processing characteristics (refs. 13 and 19).

Therefore, we made a study in order to achieve good processing characteristics and a full range of rubber performance features through judicious compounding.

Unfortunately, there are currently no reliable laboratory tests available to measure and predict the extent of tearing, cutting, chipping and chunking (ref. 3). Field visual ratings are still necessary (refs. 1 and 5).

The laboratory test which points in the direction of field cutting and chipping tests and seems the least objectionable is trouser tear as a function of temperature (refs. 1-6). Correlation is remarkably good for one elastomer, but a separate curve is needed to accommodate a change in base elastomer from NR to SBR (ref. 5).

Initiation and development of cracks within a tread subjected to cyclic flexion and compression is the cause of chunking and delamination chipping. Therefore, fatigue test methods were developed to simulate operating conditions (refs. 2 and 22). A pin drum fatigue test method appeared to simulate delamination chipping better than fatigue by both compression force and heat (ref. 22).

Furthermore, we have developed, in our Collonges (France) Rubber Laboratory, an original test to evaluate the fatigue block tearing resistance of off-the-road and agricultural tire treads.

Experimental

Formula

A typical agricultural tread NR formula was selected and modified to improve stone cutting and block tearing resistance and to decrease abrasion loss and heat build-up (table 2).

Such compounds are usually highly filled (refs. 1, 2, 4, 5, 6 and 7) and contain aromatic processing oil (refs. 2, 4, 5 and 6). The total loading is selected on the basis of hardness, wear, processing and cost considerations (ref. 5).

As said before, useful modifications involved partial substitution of the carbon black by a high specific surface area amorphous silica in an all natural rubber elastomer system (Silica B, table 1). Designing NR compounds for optimum combinations of tear strength, cutting resistance and heat build-up requires a reinforcement of 20 to 30 phr of fine particle precipitated silica (refs. 3 and 5).

Owing to adsorption of ingredients on the silica and non-coupling between silica and NR, adjustment of the formula is always necessary as soon as precipitated silica is used (refs. 1-14).

Accelerator, stearic acid and silane coupling agent must be added to a basic quantity of ingredient "[K.sub.i]," required to cure the elastomer reinforced with carbon black alone, in proportion to the specific surface area ([S.sub.BET]) and to the quantity (phr) of silica. In order to obtain low heat build-up together with high tear resistance and good wear resistance, some previously experimentally determined relations (refs. 8, 9 and 11) have been adapted ([K.sub.1] = 1.25 phr; [K.sub.2] = 3 phr):

* phr accelerator (CBS) = [K.sub.1] + 0.08 [10.sup.-3] * [S.sub.BET] * phr silica ( 1 )

* phr stearic acid =[K.sub.2] + 0.13 [10.sup.-3] * SBET * phr silica (2)

* phr coupling agent (TESPT) = 0.7 [10.sup.-3] * [S.sub.BET] * phr silica (3)

A monomolecular layer theoretically requires roughly 13 to 14.5 phr of TESPT on 100 phr of silica (175 [m.sup.2]/g) (refs. 3 and 12). Therefore, a 0.7 [10.sup.-3] coefficient corresponds to a modification degree of 89%. Higher than 70% modification seems necessary to compound tread stocks without loss in treadwear when compared to highly reinforcing carbon blacks (ref. 3).

An additional feature of the basic compound is the antidegradant system used. This is comprised of 1.75 phr each of IPPD and TMQ, a combination found to be effective for a natural rubber compound (refs. 3 and 6).

Mixing

Mixers - All experimental mixings, including curative addition, were carried out in internal mixers (table 3). Some preliminary laboratory work was carried out using a Farrel Bridge BR (Mixer A). The main investigation was carried out using a Francis Shaw K2A Intermix of batch volume ca 26 litres, which is believed to give reasonably comparable performance to that in considerably larger-scale production equipment (ref. 24). Some complementary information was obtained by mixing in an industrial (net chamber volume 270 d[m.sup.3], batch weight 180 kg, rotor speed 40 rpm) Farrel Bridge internal mixer (Mixer C).

Basic mixing procedure - sequence of adding ingredients - The sequence of adding ingredients is important for in-rubber modification.

Chemicals that can react with either the alkoxisilyl or the polysulfidic group should not be present during the modification mixing step since the resulting side reactions consume TESPT to produce unwanted effects. Chemicals containing amino or to a lesser degree aromatic or OH-groups are suspected of! causing side reactions (ref. 3). Nevertheless, processing oils, plasticizers, antioxidants and stearic acid were apparently without significant effect on coupling (refs. 1 and 7).

The preferred two-stage mixing procedure is the incorporation of rubber, silica and silane coupling agents in the first mixing cycle (refs. 1, 3, 4 and 7) in order to get the highest modification with silane and to make best use of the reaction time dependency (ref. 4). Comparisons between this method and pretreatment of the silica showed no significant differences (ref. 1 ).

The order of zinc oxide and silica addition to the mix has a profound effect on compound viscosity and modulus (ref. 17). When stearic acid is present and zinc oxide is added early in the mix, the compound viscosity is lower (ref. 14). TESPT is to be added together with the siliceous filler and zinc oxide/stearic acid (ref. 3).

Therefore, we have chosen such a two-stage procedure (table 4) as our basic mixing sequence. The addition order of the ingredients in the first stage was rubber, antioxidant, a mixture of silica, coupling agent, stearic acid and oil, and a mixture of zinc oxide and carbon black. The mixtures were loaded in very rapid succession (ref. 20). Sulfur, accelerators and retarding agent were added during the second step. Throughout this work, the above mixing schedule was considered as a basis.

Each batch was homogenized by banding on a twin roll mill. Between each mixing, the batches were left to rest for 24 hours.

Dump temperature - A second problem is the choice of dump temperature to obtain good agricultural tire tread properties. It depends mainly on the coupling agent reaction with precipitated silica and with rubber. Thermal reaction starts at 170 degrees C and consumes TESPT. Up to 160 degrees C, the incubation time for the thermal crosslinking reaction of NR with TESPT is thought to assure safe processing (ref. 3).

The modification rate within the rubber of the precipitated silica depends on time and temperature (ref. 4). The modification time can be shortened by increasing the temperature, but this rise must be limited to avoid the onset of thermal crosslinking reaction of the tetrasulfidic group (ref. 3).

Consistently, with natural rubber, the optimum temperature giving the best filler-rubber to rubber-rubber bond ratio is about 160 degrees C (refs. 3, 4 and 12).

This temperature gives considerably higher 300% modulus values (ref. 4), which means high wear resistance (ref. 1). Nevertheless, trouser tear decreases with increasing 300% modulus caused by the increasing number of filler/rubber bonds (ref. 4).

R.N. Datta, P.K. Das, S.K. Mandal and D.K. Basu showed with gas chromatographic evidence that the reaction between TESPT and NR occurs even as low as 140 degrees C (ref. 21). This reaction generates gels and long ramifications. With a high amount of silica, the above effect is magnified (ref. 20). A higher temperature encountered during mixing generates more gel and long branching, sources of processing problems.

Moreover, N. Nakajima, W.J. Shied and Z.G. Wang showed with DSC evidence that at 140-160 degrees C the reaction between silica and TESPT is completed within about two minutes (ref. 20). Completion of the modification reaction is not a problem as long as the degree of modification is less than a monomolecular layer of filler (ref. 4). This suggests that the dump temperature need not be 160 degrees C, but that 140-145 degrees C may be sufficient to complete the reaction between silica and TESPT (ref. 20).

Therefore, the dump temperature after the first stage was fixed as low as 145 degrees C/150 degrees C for our basic mixing procedure (table 4).

The dump temperature was measured by inserting a probe into several places of the compound.

Curing

Curing characteristics were determined on the Monsanto Rheometer Model M100S at desired curing temperature according to ASTM D-2084.

As off-the-road tires require extended mold times due to their considerable thickness (ref. 2), they are subjected to longer cure periods to minimize reversion effects on the physical data. In practice, cure time is prolonged rather than cure temperature (ref. 12).

Therefore, physical testing was carried out on samples which had been cured for 40 minutes at 140 degrees C.

Properties

Processing characteristics - Processing characteristics were measured on the Monsanto Mooney viscometer at 100 degrees C (ASTM D-1646). According to G.M. Bristow, for remilled treadstock, a fair correlation is evident between Mooney viscosity and shear stress at 360 [s.sup.-1] (ref. 23).

Some compounds were extruded through an AMF Orbitread Series 200c machine (screw rate 73 rpm) and extrudate swell was measured.

Usual vulcanizate properties - Stress strain properties were measured according to ASTM D-412. Hardness (Shore A) was obtained after 15s using ASTM D-2240. Goodrich heat build-up data were obtained using the Goodrich Flexometer (ASTM D-623). The temperature rise inside the sample is measured with a thermocouple (ref. 8).

Abrasion was measured using the DIN abrasion tester (DIN 53516). It is considered that the DIN technique measures only simple abrasive wear (ref. 3).

Tear resistance was determined according to ASTM D-624 at low (20 degrees C) and high (80 degrees C) temperature.

Fatigue block tearing resistance - This new test intends to simulate the behavior of off-the-road and agricultural tire tread blocks. To get the condition for a reliable fatigue laboratory test to work, a lugged wheel test die (figure 1 ) was chosen.

In turn, each lug meets a heavy stainless steel cylinder (diameter: 38 mm; length: 100 mm) turning on two very strong bearings that deform it on the perimeter of the wheel. The elevation of the cylinder determines the severity of the test.

A slow speed (2 rps) was chosen, corresponding roughly to the back-tire of a 1.5 m diameter farming-tractor at a speed of 30 km per hour.

The propagation of a crack is observed at each lug bottom. Fatigue block tearing resistance is determined as a percentage from the ratio "mean crack length/lug width." Extreme resistances correspond to 0% (no crack at all) and 100% (totally chunked lugs).

Dispersion - Classification of carbon black agglomeration size and dispersion range was determined according to ASTM D-2663.

Results and discussion

All details of the mixing procedure are important in determining the final properties of the compound, particularly if the rubber is natural rubber reinforced with precipitated silica.

Two-stage mixing sequences

Two-stage basic mixing sequence - In order to mix the ingredients of the adjusted natural rubber formula (table 2), we first tried a two-stage mixing sequence (table 4). TESPT is added, together with the siliceous filler and zinc oxide/stearic acid. The dump temperature after the first stage was fixed as low as 145 degrees C/150 degrees C.

Owing to our formulation rules, we logically obtained the good compromise in properties that we expected (table 5) - low heat build-up, acceptable abrasion loss, excellent trouser tear properties and good block tearing resistance.

Like S. Wolff (ref. 3), we obtained a positive temperature gradient for a silica-carbon black filled compound, as opposed to a negative gradient for carbon blacks.

Nevertheless, such a compounding sequence appeared to be industrially impractical. The compound was excessively stiff and.difficult to work with. The sheet stuck badly to the mill, had a very irregular aspect and was not continuous.

Such problems were not observed at a laboratory scale (Mixer A). They appeared with a larger-scale internal mixer (Mixer B). Vulcanizate properties are quite similar (table 5).

A high Mooney viscosity of the compound at each stage of compounding can explain bad processing behavior (table 5).

Moreover, the compound was very difficult to extrude, with large swelling and a bad aspect. It should be noted that the complete compound viscosity does not lie within the easy processing range (Mooney 75 --> 45).

Precipitated silicas generally produce higher Mooney (low shear rate) viscosity in rubber mixes than carbon blacks (refs. 1, 13 and 14). There are at least two separate phenomena which contribute to the extrusion problem of compounds containing silica.

One is due to a poor dispersion or poor wettability of silica. With either carbon black or silica, the poorer dispersion results in a stiffer compound (ref. 20). The silica agglomerates are more difficult to break down into aggregates, as compared to carbon black agglomerates, because the silica aggregates are held together by hydrogen bonding involving the surface silanols (refs. 1 and 8). Together with a large effective volume of unbroken agglomerates, the poorly dispersed filler particles may interfere with each other like a log-jam (ref. 20).

The other phenomenon is a network, i.e. ge1 or long-chain-branch formation, involving the coupling agent (tel. 20). A high dump temperature must be avoided (ref. 20).

For natural rubber, mechanical working during compounding may be necessary to achieve a final mix with the necessary processing characteristics (ref. 19). The conversion of any raw natural rubber into a fully compounded mix inevitably involves mechanical working of the material.

The longest reaction times produce the lowest compound viscosities, and the highest end temperatures produce the largest drop in viscosity with mixing time (ref. 4). There is a breakdown of rubber chains by mechanochemical and oxidative reactions as determined by mechanical energy input and temperature.

Moreover, mechanical working is essential for the dispersion of reinforcing fillers and the distribution of other ingredients (ref. 19).

Nevertheless, mechanical working is both time- and energy-consuming (ref. 24) and there have been comments from the industry that a direct correlation exists between high levels of mechanical working during the preparation of natural rubber tire compounds and decreased tire service life (ref. 19).

Consistently, a decrease of the Mooney viscosity of the compound can be obtained if we decrease the natural rubber viscosity and/or if the precipitated silica is better dispersed.

There are undoubtedly many materials, including accelerators and antioxidants, which, by helping dispersion or decreasing matrix viscosity, will act to reduce compound viscosity (ref. 17). Significant reduction in viscosity occurs when processing oils (ref. 25), silanes (ref. 2), fatty acid soaps (ref. 25), activators (ref. 1) and multifunctional additives (ref. 13) are used. Nevertheless, we decided to avoid a modification of the formula.

We first premasticated and/or peptized the natural rubber to decrease rubber viscosity before filler incorporation. Otherwise, we remilled the compound to obtain better filler dispersion and to masticate the natural rubber at the same time.

Premastication - The simplest method of lowering viscosity is natural rubber premastication as a stage separate to that of mixing (ref. 24). Raw natural rubber (Mooney [ML.sub.1+4] = 95-98) was subjected to mastication in order to obtain a Mooney [ML.sub.1+4] = 48-50. Two mechanisms are responsible for the reduction in viscosity (refs. 16 and 26). At low temperature, there is a mechanical breaking of the macromolecules. At high temperature, there is thermooxidative depolymerization. This may be called the zero stage (ref. 19). Therefore, the mixing process becomes a three-stage mixing sequence.

The first-stage viscosity is low enough to adequately mill a considerably rough sheet (table 7). The second-stage viscosity lies within the easy processing range (Mooney 75 --> 45). Extrudate swell is reasonably high. Extrudate aspect is good.

Silica-filled vulcanizate prepared from raw rubber subjected to mastication shows some effects attributable to the mastication. Rheometer delta torque (ref. 19), and thus hardness, 100% and 300% moduli are subsequently higher. Elongation at break (tel. 19), trouser tear resistances and abrasion loss are lowered. Fatigue block tearing resistance, heat build-up and dispersion are not modified.

Dispersion lies in the same range (table 7). These changes can be attributed to higher crosslinking level and/or better coupling.

Peptization - Premastication is both time- and energy-consuming. Peptizers, which are designed to act as catalysts for the breakdown which occurs during mastication, are industrially used to reduce the time needed to obtain a particular viscosity with a saving in energy consumption under a given set of machine conditions (ref. 24).

An appropriate level of chemical peptizer for NR compounds lies between 0.05 and 0.15 phr (refs. 24, 25 and 27). At such levels, the properties of the carbon black reinforced vulcanizates are very similar to data obtained from masticated rubber of similar viscosity (refs. 24-26).

High rotor speed peptization of natural rubber results in an unhomogeneous product which exhibits extreme surface stickiness (ref. 24). Peptization as part of a normal internal mixer cycle did not give the stickiness associated with peptization as a separate stage (ref. 24).

However, since peptizers tend to be de-activated by some compounding ingredients (ref. 26) and since with only the rubber present the mixer is relatively underloaded for peptization within the batch, relatively larger amounts of chemical peptizer (0.25 --> 0.5 phr) are necessary to give a useful reduction in viscosity (refs. 24 and 26). In addition, some rubber molders are now using levels in excess of 0.15 phr (0.6 phr) to achieve good flow properties, thus eliminating the costs involved in extra mastication (ref. 27).

The continued action of peptizers after mixing and excessive rubber breakdown before black addition leading to poor black dispersion can lead to inferior physical properties in NR vulcanizates (ref. 27).

Blends of fatty acid soaps with chemical peptizers are intended to achieve peptization by lubrication of the molecular chain (also acting as dispersant and processing aids) and by chain scission (ref. 25). They are used in higher concentrations (--> 2 phr [ref. 25]).

We used such a peptizer (2 phr Struktol A82, Rhein Chemie, Mannheim, Germany; detailed composition not disclosed), after natural rubber premastication, in order to decrease the first- and second-stages Mooney viscosities.

Peptization was done as a part of the first-stage internal mixer mixing cycle. A period of three minutes (ref. 27) was allowed for peptization to occur before addition of the other ingredients (table 6, mixing sequences n degrees 3 and 4). It is important to design the mix cycle to give sufficient time for the peptizer to act before other ingredients are added (ref. 24).

The first-stage viscosities are low enough to adequately mill (table 7). The second-stage viscosities lie near the lower border of the easy processing range - extrudate swelling is low; extrudate aspects are very good. However, whe n too low Mooney viscosity (mixing sequence 4) is obtained, the sheet is not easily released from the mill.

Silica-filled vulcanizates prepared from raw rubber subjected to peptization show some effects attributable to peptization (table 7). Rheometer delta torque, hardness, 100% and 300% moduli are subsequently higher. Elongation at break, trouser tear resistances, fatigue block tearing resistance, tensile strength and abrasion loss are lowered.

Noakes similarly observed that trouser tear strengths at 23 degrees C and 100 degrees C and ring fatigue were somewhat lower for highly peptized compounds (ref. 27).

These changes can also be attributed to higher crosslinking level, particularly for mixing sequence n degrees 4 (table 6) - high first-step dump temperature; second-stage coupling agent addition. Coupling action is not helped. According to B. Wijayarathna (ref. 18), effective crosslink density does increase with mixing temperature.

Moreover, carbon black dispersion showed a high proportion of undispersed black in the peptized compound (ref. 27). It is observed with mixing sequence n degrees 4. We think it is a consequence of the second stage silane addition.

In summary, for a silica filled compound, processing and wear are improved while tear properties and fatigue block tearing resistance fall when compound viscosity is decreased by lowering (premastication and/or peptization) the viscosity of raw natural rubber. Such a compromise in properties is hardly satisfactory for an agricultural tire tread.

Three-stage mixing procedures

In order to decrease elastomer viscosity and to improve filler dispersion, three-stage mixing procedures were followed (table 9). This kind of mixing procedure follows general industrial practice (ref. 20).

Remilling - Remilling (mixing sequence n degrees 5, table 8) of an unpeptized compound to a viscosity similar to that of a peptized compound has been shown to have no adverse effect on vulcanizate properties and would therefore be a preferable technique in circumstances where the normally recommended levels of peptizer are insufficient to achieve a low enough viscosity for processing (ref. 23).

The first-stage viscosity is not low enough to adequately mill (table 9). The second stage viscosity (after remilling) is low enough to mill a continuous rough sheet that sticks well to the mill. The third stage lies at the upper limit of the easy processing range. There is a small viscosity decrease. Extrudate swell is reasonably high. Extrudate aspect is good.

Reworking improves carbon black dispersion (table 9, ref. 23). A slight increase in moduli is observed (ref. 20). The reason is unclear. Any increase in black dispersion resulting from the reworking should have given a reduction in modulus (ref. 20). Coupling could be increased by more mixing time (ref. 4). Other usual properties are unaffected. Remilling has no adverse effect on vulcanizate properties. Moreover, it improves fatigue block tearing resistance.

Second-stage filler addition - To decrease first stage viscosity, we tried second stage filler addition mixing sequences.

Second-stage silica addition scorched. Therefore, we studied first-stage silica addition with second-stage carbon black addition mixing sequences (table 8).

Oil addition - Oil must be added during the first stage, with precipitated silica. Indeed, if it is added with carbon black (mixing sequences n degrees 7 or 9, table 8), the first stage compound is too dry; the compound viscosity is high and the sheet is crumbly, resembling cardboard. Such a compound is rather difficult to process industrially.

Nevertheless, filler dispersion and vulcanizate properties are excellent (table 9).

Zinc oxide addition - Some mixing cycles used (mixing sequences n degrees 8 and 9, table 8) are characterized by often recommended early addition of silica and late addition of zinc oxide (refs. 1, 2, 13, 14 and 17).

A specific, or so-called zinc oxide, mixing cycle is used in silica-containing compounds to prevent the formation of a ZnO-silica complex which, if it were formed, would produce inadequately cured rubber vulcanizates (refs. 13-14). Zinc oxide in the presence of organic acids greatly reduces compound viscosity and state of cure (tel. 14). It is reasonable to assume that zinc oxide which is solubilized by the organic acids furnishes zinc ion which is adsorbed on the silica (ref. 14).

For maximum coupling efficiency, zinc oxide must not be present initially (refs. 1 and 7). After the silica and silane had sufficient time to interact, zinc oxide no longer appeared to interfere (ref. 1 ).

As expected, second-stage zinc oxide addition subsequently increases rheometer delta torque (table 9). Therefore, hardness, 100% and 300% moduli are increased; elongation at break, trouser tear resistances and abrasion loss are lowered. It has no effect upon fatigue block tearing resistance and dispersion. Surprisingly, thirdstage viscosity is lowered. Reaction of soluble zinc with silica is influenced not only by the mixing order of addition, but also by the mixing temperature (ref. 17). This could explain discrepancies.

Silane addition - Silane must be added during the first stage together with precipitated silica. Moreover, silane dispersion and coupling actions are not fully used (mixing sequence n degrees 10, table 8). Consistently, dispersion is very bad, vulcanization time is longer and rheometer delta torque is lowered.

The main observed effects are high abrasion loss and low fatigue block tearing resistance. Moreover, processing becomes harder - high compound viscosities and extrudate swelling (table 9).

Optimum mixing sequence - Optimum mixing cycle is sequence n degrees 6 (table 8). Zinc oxide and silane are added during the first stage together with precipitated silica. Therefore, it does not affect usual vulcanizate properties. Oil is added during the first stage. Therefore, the first stage viscosity is low enough to mill a continuous rough sheet that sticks well to the mill and does not crumble. It is a three-stage mixing cycle. Therefore, dispersion and fatigue block tearing resistance are improved. Heat build-up is low.

The first-, second- and third-stage compounds are correctly milled. Nevertheless, the third-stage viscosity lies near the upper limit of the easy processing range. Extrudate swell is reasonably high. Extrudate aspect is good.

An increase in the mixer size does not modify the conclusions (table 10). It is an attractive compromise if very low viscosity is not necessary.

Peptization - In order to decrease the third-stage viscosity, we combined peptization during the first-stage sequence and the optimized three-stage mixing procedure (mixing sequence n degrees 11).

We used 0.25 phr of a chemical peptizer (Renacit VII [Bayer]: 47% wt pentachlorotiophenol; 48% wt kaolin; 5% wt oil; 0.075% wt iron complex [ref. 26]).

Peptization was done as part of the first-stage mixing cycle. A period of 1.5 minutes (tel. 24) was allowed for peptization to occur before addition of the other ingredients.

The first- and second-stage viscosities are low enough to adequately mill a sheet (table 9). The third stage viscosity lies within the easy processing range (Mooney 75 --> 45). Extrudate swell is low. Extrudate aspect is good.

As exptected from previous peptization results, rheometer delta torque is increased and abrasion loss is improved, compared to mixing sequence n degrees 6 results; dispersion and fatigue block tearing resistance are worse, but then of course, similar to mixing sequence n degrees 1 results (table 9).

Trouser tear resistances are not modified. It is another attractive compromise if low viscosity is necessary.

Low specific surface area silica - In order to avoid processing problems, a lower specific surface area silica could be used (Silica A, table 1 ).

From our optimized three-stage mixing sequence (mixing sequence n degrees 6), quite low viscosities are obtained (table 11). Abrasion loss is slightly increased. Heat build-up is slightly decreased.

The main differences are lower trouser tear properties and lower fatigue block tearing resistance. Resistance to cut propagation is in fact increased with high specific surface area filler.

No IPPD - If IPPD is not mixed into the compound (Formula 2, table 2), the vulcanization ([t.sub.s+2], [t.sub.90]; table 11) is slower, rheometer delta torque and tensile moduli are lowered, and tear resistance is increased. Such an antioxidant interferes with the vulcanization and acts as an accelerator. It also acts as a dispersing aid (table 11, ref. 17).

Nevertheless, the main effect is to protect against flexion fatigue. If not incorporated into the compound, the vulcanizate fatigue block tearing resistance is very bad (table 11).

From our results, it appears that trouser tear and fatigue block tearing are two different properties. Trouser tear depends mainly on rheometer delta torque. Fatigue block tearing depends mainly on dispersion.

Conclusion

A way has emerged to make agricultural tire tread compounds with an attractive compromise in properties - excellent tear properties and block tearing resistance, good wear resistance and low heat build-up.

It involves partial substitution of the carbon black by a high specific surface area precipitated silica in an adjusted all-natural rubber formula.

It also involves judicious compounding to obtain easy processing and to maintain good properties. Key features include a three-stage mixing sequence in order to decrease viscosity and help dispersion; silane must be added with silica; zinc oxide, silica and oil are incorporated during the first stage; carbon black is incorporated alone during the second stage; rubber dump temperatures lie around 145/150 degrees C. Our block tearing resistance test appears to be useful to differentiate mixing sequences.

References

1. M.P. Wagner; Rubber Chem Technol. 49, 703 (1976).

2. K.M. Davies, R. Lionnet, Rubbercon '81, Harrogate, England, paper G4, June 8-12 (1981).

3. S. Wolff, Tire Sci. Technol. 15, 276 (1987).

4. S. Wolff, Rubber Chem. Technol. 55, 967 (1982).

5. N.L. Hewitt, Rubber World 193, 24 (1982).

6. L.A. Walker, ACS Rubber Division Meeting, paper 35, Montreal, Canada, May 2-5 (1978).

7. M.P. Wagner, Rubber Chem. Technol. 50, 356 (1977).

8. J. Machurat, IRC '90, Paris, France, June 12-14 (1990).

9. F. Bomo, J. Machurat, Third Chemical Congress of North America, Toronto, Canada, June 5-10 (1988.).

10. F. Bomo, Makromol. Chem. Macromol. Symp. 23, 321 (1989).

11. J.Y. Germain, J. Machurat, Rubber World 51, October (1985).

12. S. Wolff, Kautsch. Gummi Kunstst. 34, 280 (1981).

13. C. Hepburn, M.H. Halim, M.S. Mahdi, Kautsch. Gummi Kunstst. 43, 794 (1990).

14. D.D. Dunnom, Rubber Age 49, May (1968).

15. A.D. Robens, "Natural rubber science and technology," Oxford Universit)., Press (1988).

16. C.M. Blow, "Rubber technology and manufacture," Newnes-Butterworths, London ( 1975 ).

17. N.L. Hewlit, Elastomerics 33, March (1981).

18. B. Wijayarathna, W.V. Chang, R. Salovey, Rubber Chem. Technol. 51, 1008 (1979).

19. A.G. Sears, NR Technol. 19, 68 (1988).

20. N. Nakajima, W.J. Shied, Z.G. Wang, Intern. Polvm. Processing 6, 90 (1991).

21. R.N. Datta, P.K. Das, S.K. Mandal, D.K. Basu, Kautsch. Gummi Kunstst. 41, 157 (1988).

22. Y. Udagawa, 139th ACS Rubber Division Meeting, paper 46, Toronto, Canada, May 21-24 (1991).

23. G.M. Bristow, K.N.G. Fuller, A.G. Thomas, NR Technol. 14, 69 (1983).

24. B.G. Crowther, NR Technol. 12, 27(1981).

25. B.G. Crowther, NR Technol. 14, 1 (1983).

26. H.W. Engels, M. Abele, Rubber World 14, October (1991).

27. Noakes T.C.Q. NR Technol. 19, 10 (1988).

Acknowledgements

"Improved black sidewall compound performance using precipitated silica" is based on a paper given at the May, 1992 Rubber Division meeting. "Influence of carbon black morphology and surface activity on vulcanizate properties" is based on a paper given at the November, 1992 Rubber Division meeting. "Influence of mixing procedures on the properties of a silica reinforced agricultural tire tread" is based on a paper given at the May, 1992 Rubber Division meeting.

Table 1 - characteristics of the precipitated
          silica used
                                  Silica A (*)   Silica B (**)
Loss ignition (900 degrees C)       9.5             10.5
 (%) (ISO 3262/11)
Moisture (loss after 2              5.5                6
hours 105 degrees C) (%) (ISO
 787/2)
 pH (ISO 787/9)                     6.5              6.5
Specific surface area               175              240
 ([m.sup.2]/g) (BET; ISO
 5794/1 single point)
Specific surface area               165              220
 ([m.sup.2]/g) (CTAB)
Sodium sulfate (%)                  1.2              1.4
 (*) Rhone-Poulenc Commercial precipitated silica
 (**) Rhone-Poulenc high specific surface area
precipitated silica (development silica)
Table 2 - agricultural tire tread formulations
               (phr)
                   Formula 1  Formula 2  Formula 3
Natural rubber     100         100        100
 SMR20
Carbon black        35          35         35
 N339
Precipitated         0           0         25
 silica A (175
 [m.sub.2]/g)
Precipitated        25          25          0
 silica B (240
 ([m.sup.2]/g)
Aromatic oil        10          10         10
 729FC
Zinc oxide           3.5         3.5        3.5
Stearic acid         3.5         3.5        3.35
Silane X50S          8.75        8.75       6.1
Antioxidant          1.75        0.0        1.75
 IPPD
Antioxidant          1.75        1.75       1.75
 TMQ
 Sulfur              1.7         1.7        1.7
Accelerator CBS      2.0         2.0        1.8
Retarder PVI         0.2         0.2        0.2
  X50S: Bis-(triethoxysilyl-propyl)-tetrasulfide (TESPT)
50% on carbon black N330
  IPPD: N-isopropyl-N'-phenyl-paraphenylenediamine
  TMQ: Polymerized 2,2,4-trimethyl, 1,2-dihydroquinoline
  CBS: N-cyclohexyl-2-benzothiazyl-sulphenamide
  PVI: N-(cyclohexylthio)-phthalimide
Table 3 - laboratory internal mixer characteristics
                               Mixer A        Mixer B
Mixer type                   Fartel Bridge    Francis Shaw
                                BR             K2A Intermix
Net chamber volume             1.6                46
 (d[m.sup.3])
Ram pressure (MPa)             0.8               0.55
                                            0.35 (last stage)
Rotor speeds (rpm)           80/130              25/50
Starting temperature           85                 60
 ( degrees C)
Cooling water ([m.sup.3]/h)     -                 10
Table 6 - two-step mixing sequences with lower viscosity NR
Mixing    Low viscosity NR           Specificity
sequence
n degrees
2         Premastication -->         As mixing sequence
          Mooney [ML.sub.1+4] = 48   n degrees 1
3         Premastication -->         First stage
          Mooney [ML.sub.1+4]  = 48  peptization followed
          + peptization 3 min.       as mixing sequence
          2 phr Struktol A82         n degrees 1
4         Premastication -->         Stage 1 rubber
          Mooney [ML.sub.1+4] = 48   dump temperature
          + peptization 3 min.       185 degrees C
          2 phr Struktol A82         Silane added step 2
Table 10 - mixing sequence n degrees 6 -influence of
            mixer size (Formula 1)
Mixer                         A              B               C
Processing characteristics (ASTM D-1646)(Mooney
  [ML.sub.1+4] 100 degrees C)
First stage                   -              87             78
Second stage                  -             100            105
Third stage                   76             76             76
Extrudate swell
Swelling (%)                  -              97              -
Curing characteristics (ASTM D-2084) (140 degrees C)
Delta torque                  59             56            60.2
[t.sub.s+2] (scorch)          9.3           10.1           10.2
(min.)
[t.sub.90] (min.)            21.5           22.1           25.3
Stress strain properties (ASTM D-4 12)
M100% (MPa)                  3.5            3.4             3.8
M300% (MPa)                  16.3           15.4           15.8
T.S. (MPa)                   26.9           28.1           26.9
E.B. (%)                     508            510             520
Hardness Shore A (ASTM D-2240)
Sh. A 15 sec.                 70             69             69
Trouser die tear resistance (ASTM D-624) (kN/m)
20 degrees C                  35             42             40
80 degrees C                  60             63             63
Abrasion loss (DIN 53516)
Loss ([mm.sup.3])            112            101            101
Heat build-up (center of the die)
Temperature                   80             79             78
( degrees C)
Fatigue block tearing resistance
% tearing after               23             18             23
200 kcs
Cabot black dispersion
                              -             A1-2             -
Table 11 - three-stage mixing sequence n degrees 6-properties
            (Formulas 2 and 3]
 Formula                            2                 3
 Mixer                              B                 A
Processing characteristics (ASTM D- 1646) (Mooney
[ML.sub.1+4] 100 degrees C)
 First stage                       78                 72
 Second stage                      98                 90
 Third stage                       74                 67
Extrudate swell
 Swelling %                       100                  -
Curing characteristics (ASTM D-2084) (140 degrees C)
 Delta torque                    54.4                61.7
 [t.sub.s+2] (scorch) (min.)     12.5                11.6
 [t.sub.90] (min.)               30.2                24.3
Stress strain properties (ASTM D-4 12)
 M100% (MPa)                      2.9                 4.2
 M300% (MPa)                     13.8                16.6
 T.S. (MPa)                      27.0                26.1
 E.B. (%)                        546                  472
Hardness Shore A (ASTM D-2240)
 SH. A 15 sec.                    69                  70
Trouser die tear resistance (ASTM D-624) (KN/m)
 20 degrees C                     48                  26
 80 degrees C                     85                  46
Abrasion loss (DIN 53516)
 Loss ([mm.sup.3])               104                 114
Heat build-up (center of the die)
 Temperature (degrees C)          79                  75
Fatigue block tearing resistance (RP test)
 % tearing after                  53                  51
 200 kcs
Cabot black dispersion
 Dispersion                      B1-2                 -

[Tabular Data Omitted]