Silane coupling agents for enhanced silica performance.
The carbon black surface is primarily carbon. Other hydrocarbon or oxygen-containing groups make up a low percentage of the surface. This relatively non-polar surface is very compatible with the hydrocarbon polymers that are widely used in rubber compounding.
On the other hand, the surface of silica aggregates or particles is virtually covered by silanol groups (-Si-OH) that are more polar and considerably more active chemically. Compared to carbon black, silica is much less compatible with general purpose polymers and gives much lower cohesive bonding forces. The silica particles also tend to link to each other to form a filler-filler network. This strong filler network can give a rigid uncured compound that is difficult to process in extrusion and forming operations.
Moisture also readily attaches to the silica surface through hydrogen bonding and probably contributes in the formation of the filler-filler network. In normal storage conditions, silicas will have 4% to 7% absorbed and adsorbed moisture. Surface moisture reduces the chemical activity of the silica and the associated "side reactions" that may affect the vulcanization chemistry. Some of the moisture is driven off during mixing; altering the chemical effects. Variations in mixing temperature can contribute to variations in rubber modulus and cure rate from batch to batch.
Chemistry of silicas and silicates
The chemically active silanols on the surface of silicas and silicates (including clays and talcs) contain active hydrogen atoms that can react with various chemical groups. In high surface area silicas this can have a dramatic effect on in-compound reactions; especially sulfur vulcanization.
The silanol groups are acidic in nature and reactive. Silanols show similarities to carboxylic acid groups in their reactions with amines, alcohols and metal ions.
The higher surface area silicas have many more reactive silanols and are thus more active chemically.
At elevated temperatures, the silanol groups on the surface of silicas, silicates, clays and talcs will react with a number of chemical groups present in rubber compounds. Silanes are known to form strong chemical bonds, while others, such as water or glycols, form fairly weak adsorption bonds via Van der Waals forces or hydrogen bonding.
As mentioned earlier, water adsorbed on the surface of filler particles reduces the reactivity of the silanols. During hot mixing, some of the adsorbed water is removed, leaving a very reactive filler surface. If diethylene glycol (DEG) or polyethylene glycol (PEG) is present in the recipe, it can replace the volatilized water and reduce the reactivity of the filler surface.
Some of the reactions with silanols can have a profound 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 or reaction 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, because zinc is involved as an activator in the cure system. These negative effects on the cure system can be reduced or completely removed by adding other chemicals that will tie up the silanol groups and reduce their activity. Besides the glycols mentioned earlier, other additives commonly used in non-black compounds include hexamethylene tetramine (hexa), hexamethoxy methyl melamine (HMMM) and triethanol amine (TEA). Magnesium oxide in nitrile (NBR) and polychloroprene compounds also reduce the tendency for the fillers to rob zinc from the cure system. Some of these additives also reduce the polarity of the filler surface and thus improve wetting and dispersion in non-polar polymers. Using polar oils or aromatic resins also generally improves dispersion and the properties of compounds containing silicas.
If the surface silanols are not tied up by one of the chemical groups mentioned above, the silica particles tend to agglomerate to form a filler-filler network. The relatively non-polar hydrocarbon polymers such as NR, BR and SBR are at the bottom of the preference list of chemical groups that interact with silica surfaces.
Non-black fillers can reduce the amount of accelerator available due to attractive forces (or reactions) between silanols and the amine group of the accelerator. The amount of interaction probably varies with the activity of the amine group of the accelerator. The effect on cure is considerably more evident with high surface area silicas than with low surface area fillers.
One way to overcome this effect is simply to increase the amount of accelerator. Other strong amines such as TEA, hexa or HMMM are sometimes utilized. Polyethylene glycol (Carbowax) is often effective in reducing the adsorption of accelerator, and generally is lower in cost.
The guanidine accelerators have such a strong interaction with silicas that they can give reduced viscosity in silica filled compounds, as well as act as an accelerator. Many recipes containing silica also contain DPG or DOTG as part of the cure system.
The reaction of silanol groups with zinc ion is important in sulfur cured compounds filled with silica. Zinc oxide and stearic acid are important ingredients in sulfur vulcanization systems. They react to produce zinc stearate, an intermediate in the vulcanization mechanism. The active silanols on the surface of a silica particle will react with zinc stearate. If the reaction is given enough time to progress, it would eventually rob all the zinc -- leaving none to activate the vulcanization reaction.
Because of the reaction of silica with soluble zinc, it is important to add zinc oxide as late as possible in the mixing sequence (to reduce the available reaction time). The rheometer curves shown in figure 1 illustrate the effect on the cure rate and state of cure between adding zinc oxide early (1st mixer) and adding it in a later mix stage. Early addition of zinc oxide resulted in a slower cure rate and lower maximum torque because active silanols were free to react with zinc and rob it from the compound. Polyethylene glycol (Carbowax) and other ingredients in the compound can tie up silanols and reduce their reactivity with zinc.
[Figure 1 ILLUSTRATION OMITTED]
The data in table 1 illustrate the effect on modulus with zinc oxide added early vs. late. The data also demonstrate that, with early addition of zinc oxide, the reaction of zinc with silanols contributes lower viscosity, presumably by reducing the tendency for silica particles to attach to each other and form a strong silica network. Other metal ions, such as magnesium (from MgO), are sometimes used in compounds -- added early -- to minimize the effects from the zinc reactions. Adding other zinc compounds, such as zinc octoate or zinc stearate, may also give improvements in cure characteristics.
Table 1- nitrile compound-peroxide cure N550 black vs. silica (55 S.A.), silanes Filler (50 phr) N550 Silica Silica Silica Methacryl silane A174 -- -- 1.00 -- Mercapto silane A189 -- -- -- 1.00 t90% cure at 165[degrees]C 11.8 10.5 10.5 11.9 Hardness 70 69 72 68 100% modulus, MPa 5.8 3.4 5.0 4.5 Tensile strength 21.2 14.3 19.5 14.2 % elongation at break 270 385 267 282 Die C tear, N/mm 46.5 37.1 39.9 39.2 Pico abrasion index 145 68 167 79 Compr. set, 72h/100[degrees]C 13.0 14.5 12.5 12.5 Aged 14 days/135[degrees]C Hardness 89 86 85 85 Tensile 16.3 17.7 18.0 13.8 Elongation 42 78 70 59 Volume change #3 oil, 72h/150[degrees]C 6.2 3.9 3.8 3.6 Fuel B, 48h/23[degrees]C 28.6 30.6 29.3 29.8
Carbon black and a silica with similar surface area produce compounds with dramatically different stress-strain responses when compounded at 40 phr in an SBR compound. Although the silica gave similar tensile strength (stress at break), it stretched much farther before it broke and showed much lower modulus at 100% or 200% elongation (strain).
Modifying the silica with 3% silane coupling agent (3% by weight of the silica) shifted the stress-strain curve closer to that obtained with carbon black. With additional silane modification it should be possible to match the carbon black stress-strain curves.
The different stress-strain response with silica is primarily due to the difference in surface chemistry vs. carbon black. Silica's lesser compatibility with SBR and the resulting reduced cohesive forces at the filler-polymer interface contribute. The reactive silica also ties up more of the rubber chemicals that are needed for sulfur crosslinking reactions and thus yields a compound with lower crosslink density.
Silane coupling agents are effective agents for modifying the surface chemistry of silicas and other non-black fillers. These react with the silanols on the filler surface to give a strong bond, and also contain a functional group that will link to the rubber during vulcanization. This gives a chemical "coupling" between the silica on one end and the polymer on the other end of the molecule. The end result is additional filler-polymer bonding that increases modulus and tensile strength, and improves abrasion resistance.
Modification of the filler surface also improves wetting and dispersion, and reduces the tendency to tie up ingredients of the cure system. The modified silica also has reduced filler-filler network attraction and thus gives lower viscosity compounds with improved processing characteristics.
The silane coupling agents are expensive and as a result they are used primarily in premium rubber compounds; usually where abrasion resistance is an important performance requirement.
Silane modification of the silica surface reduces its affinity for neighboring silica particles. With sufficient silane modification of the silica, the tendency to form a silica-silica network can be nearly eliminated.
Filler-filler network effects can be demonstrated using dynamic testing methods to look at the strain amplitude dependence, commonly referred to as the "Payne Effect." The data shown in figure 3 are from studies by Degussa AG comparing N110 carbon black and a silica with a similar surface area (Ultrasil VN2) in a simple natural rubber recipe with a peroxide cure system. The peroxide cure system was used to insure that the compounds had equivalent crosslink density. The substitution of silica for carbon black in a sulfur cured recipe could result in considerably altered crosslink density that would influence the dynamic results.
[Figure 3 ILLUSTRATION OMITTED]
At low strain, the silica filled compound in the figure showed considerably more stiffness or resistance to deformation (E') than an equivalent carbon black filled compound. At higher strain, the silica network was broken up and the dynamic modulus was nearly the same as the carbon black filled compound.
The silane modified silica gave considerably reduced E' in the low strain region, and showed nearly the same dynamic modulus regardless of the strain that was applied (very little strain dependence). For these studies sufficient TESPT silane (Si69) was used to give nearly 100% modification of the silica surface.
Stiff, high viscosity silica filled compounds can lead to processing difficulties. Viscosity can be reduced temporarily with high shear mixing, but the silica-silica network can reform as the compound sits, and its stiffness can return. Modifying the surface chemistry with silane or another chemical that attaches securely to the silica surface will reduce the formation of the silica-silica network and give lower viscosity compounds.
On the down side, the silica network probably contributes significant resistance to tearing and cut growth in the cured compound. This typical silica advantage may be lessened as the tendency to form a silica-silica network is reduced.
The energy consumed in the breaking and re-forming of filler-filler networks is believed to be a significant contributor to hysteresis of the rubber compound. Tangent delta reflects the energy loss per unit of applied force during dynamic testing.
For tire tread compounds, tangent delta measured at tire operating temperatures has been shown to correlate well with the tread's contribution to the rolling resistance of the tire and subsequently to fuel consumption by the engine. The critical strain region associated with tire tread rolling resistance is felt to be in the 1% to 10% range; where the carbon black filled compound shows its highest tangent delta in figure 4. Although N110 carbon black has less tendency to form a filler-filler network than the silica, it does form some; and the carbon black network is not as strong. Energy is consumed as the carbon black network breaks and re-forms during deformation. The stronger silica network resists breakage until higher strains are applied.
[Figure 4 ILLUSTRATION OMITTED]
Silane modified silica forms a lesser amount of filler networks to be broken and re-formed, and thus the compound consumes less energy than the carbon black filled compound. With the use of bifunctional silane coupling agents, the covalent bonds to the polymer chains add strength in the high strain region that is critical for abrasion resistance (tread wear). Compounds used in power transmission belts should show similar performance benefits from the use of silane modified silica.
Silicas can be modified with silane coupling agents to achieve a wide range of dynamic performance properties. Generally, the best cut growth resistance is obtained with relatively low silane modification and the maximum abrasion resistance is achieved with higher coupling agent modification.
Several silane coupling agents are commonly used in sulfur cured compounds filled with non-black fillers. These are A-189 mercaptosilane (from OSI), Si264 thiocyanatosilane and Si69 tetrasulfide silane (from Degussa). The methoxy or ethoxy groups (left end of each structure as shown in figure 5) react during mixing with the silanol groups on the surface of silica, silicate or clay particles to give a strong bond. Alcohol is released as a by-product of the reaction. The sulfur-containing group of each structure reacts during vulcanization to give bonding to the polymer. All three silanes probably yield similar final structures for the coupled linkage between the filler and polymer.
[Figure 5 ILLUSTRATION OMITTED]
Si69 contributes some additional sulfur to the compound that the others do not. The resulting longer, more flexible sulfur linkages can give improved performance in a number of dynamic applications. It is also the largest molecule and thus requires more time and temperature during mixing for adequate reaction with the silica filler. However, mixing temperatures in excess of 170oC can lead to premature breakup of the tetrasulfide group and undesirable sulfur reactions. X50S, with 50% Si69 carded on carbon black, is commonly used in compounds that contain both carbon black and silica.
A-189 has a strong mercapto odor that makes working with the "neat" liquid product unpleasant and often gives short scorch times. It is normally used as a concentrate carried on an inert carrier. The "blocked mercapto" structure of Si264 nearly eliminates the odor and gives longer scorch delay in rubber compounds.
The mixing sequence can affect the efficiency of the silane-silica reaction. The most efficient use of the silanes is obtained if the polymer, fillers and silane coupling agent are allowed to mix for a short period of time before adding the other chemicals that might compete for the silica surface (amine antioxidants, zinc compounds, etc.).
The final silane coupling bond is depicted in figure 6. The chemistry of the bond would be similar whether the filler was a silane treated clay or a high surface area silica that had Al 89, Si264 or Si69 added during mixing. The sulfur functional group of the coupling agent should react with the sulfur and accelerators during vulcanization to form covalent sulfur bonds to the polymer chain (length of sulfur chain is determined by the amount of sulfur available).
[Figure 6 ILLUSTRATION OMITTED]
Silanes with other functional groups such as chloro, amino, vinyl, methacryl or epoxy groups are also used -- primarily with non-sulfur cure systems. The functional group should be tailored to the type of cure system to maximize bonding to the polymer during vulcanization.
Si230, with a chloro functionality, is commonly used with halogenated polymers such as chloroprene, Hypalon or chlorinated polyethylene.
For peroxide cured compounds, the silane of choice from Degussa would probably be Si225, a vinyl silane produced in Europe.
Table 1 shows a silane application in a peroxide-cured nitrile (NBR) automotive hose compound designed for use in high temperature, oil resistant service. N550 carbon black was replaced by a silica with 55 m2/gram surface area, and two silane coupling agents are evaluated. The details of the recipe are shown in table 2 (the peroxide was added on a mill).
Table 2 - recipe Ingredients phr NBR-40 ACN, 65 ML4 100 Filler 50 Magnesium Oxide 5 DOP plasticizer 10 Stearic acid 2 Polyethylene glycol 1 TMQ antioxidant 1.5 ODPA antioxidant 1.5 Zinc oxide 5 Sulfur 0.1 Sartomer SR-350 5 DiCup 40[degrees]C (40%) 4
The silica compound with no silane showed properties that were deficient in several areas. But with 1 phr of A174 methacryl silane added, the properties were better than the N550 control compound for abrasion resistance, heat aging resistance and volume change in ASTM #3 Oil. This would be a good candidate for a colored hose compound with equal or better performance than the original black compound.
The silica compound with 1 phr of A- 189 mercapto silane did not perform quite as well in this peroxide cured compound; illustrating that the silane functional group should be matched to the cure system. In a sulfur cured NBR compound, the mercapto silane would probably give better performance than the methacryl silane.
Development activities are very brisk with silicas and silanes in lower rolling resistance "green tires" that give better fuel consumption efficiency. A typical patented all-silica tread is contrasted in table 3 to a conventional black-filled tread compound. The TESPT silane (Si69) is typically used at about 8% by weight of silica.
Table 3 - conventional (European) vs. "green tire" tread 1 2 3 4 E-SBR (1500) 100 100 -- -- S-SBR, Hi Vinyl -- -- 70 70 BR, 96% 1,4-cis -- -- 30 30 N234 carbon black 80 -- 80 -- Silica MN 3 GR -- 80 -- 80 X 50-S (50% TESPT) -- 12.80 -- 128 Aromatic oil 36 36 36 36 ZnO - St. acid 3-2 3-2 3-2 3-2 Sulfur- CBS 1,5-1,5 1.5-1.5 1.5-1.5 1.5-1.5 DPG -- 2 -- 2 DIN abrasion loss 70 82 100 97 Tangent delta 0[degrees]C 0.39 0.38 0.46 0.43 Tangent delta 60[degrees]C 0.28 0.18 0.27 14
The dynamic test results for tangent delta at 60oC correlate well with the rolling resistance contributed by the tread compound in a tire. The data show a 50% reduction in the tangent delta at 60 [degrees] C; with some trade-off in abrasion losses resulting from the combination of filler and polymer modifications (1 vs. 4). Tire tests have proven that fuel economy can be improved without compromising the wet stopping traction, a previously unobtainable feat.
New silicas that give improved abrasion resistance are being developed to overcome the loss in treadwear life (new "highly dispersible" silicas).
The silica-silane tread compounds demand additional mixing time and temperature to achieve their improved performance. An additional mixing stage is commonly added to accomplish the silica-silane reaction step in a more controlled manner. The data in table 4 illustrates the influence of mixing temperature on abrasion resistance and dynamic properties.
Table 4 - mixing procedure for TESPT-modified silica compounds Stage 1 0'-1' Polymer 1'-2' 1/2 filler, 1/2 silane, ZnO, stearic acid, oil 2'-3' 1/2 filler, 1/2 silane, antioxidant 3'-3.5' Sweep down, dump at about 160[degrees]C Stage 2 Mixer 2.5' dump at about 160[degrees]C remill Stage 3 0'-1.5' Batch from stage 2, accelerator, sulfur 1.5' Dump at about 100[degrees]C Influence of mixing temperature 1 2 3 Rotor speed (stage 1), rpm 50 65 80 Final temperature (stage 1 ), [degrees]C 140 149 160 Final temperature (stage 2), [degrees]C 158 155 152 DIN abrasion loss, mm3 96 81 71 E', MPa, 60[degrees]C 8.80 8.80 9.50 Tan delta, 60[degrees]C 0.13 0.12 0.10 E', MPa, 0[degrees]C 14.10 3.00 13.40 Tan delta, 0[degrees]C 0.30 0.30 0.30
A new di-sulfide silane (TESPD) has recently been introduced that offers better stability at higher mixing temperatures than the TESPT (tetrasulfide). The TESPD should allow hotter, shorter mixes to achieve the silica-silane reaction step. The reduced sulfur donor effect may necessitate a slight increase in elemental sulfur to achieve equivalent dynamic performance.
As discussed earlier, the differences in dynamic force responses are the key to the performance advantages of the new silica-silane tread compounds. The differences between carbon black and silica significantly alter their dynamic performance in the low strain region associated with rolling resistance. The trends that are observed as dynamic strain is varied are felt to be due to differences in the amount, as well as the strength, of filler-filler networks that are formed between filler aggregates.
The performance advantages cannot be realized unless the desired degree of silane modification of the silica is achieved during mixing and the sulfur functional group reacts during vulcanization. Pre-reacted silicas have been introduced as an alternate choice to the in situ silanization reaction during mixing.
Several polymer manufacturers are also busy trying to develop an alternative choice via the silica masterbatch route. In this writer's opinion, the manufacturing process for a silica masterbatch will need to include the silanization reaction.
Several varieties of pre-reacted silicas have been produced in Europe. At this time, their availability is limited and the additional manufacturing steps necessitate premium pricing. Time and experience will determine whether they will achieve commercial success and be the silica fillers of the future. The concept certainly looks attractive.
We have illustrated the differences between carbon black and silica and the resultant negative effects that can result from simply substituting silica in place of carbon black.
However, silicas can offer distinct advantages in some applications that cannot be obtained with carbon black alone. Silica without silane modification, or modification at low levels, can give enhanced tear and cut growth resistance. Adhesive bond strength is usually improved with silica, and may be primarily due to improved tear and cut growth at the adhesion interface. Many of these types of applications use combinations of a carbon black and 10-20 phr of silica in order to achieve the best trade-off in properties.
The "green tire" tread application of silane-modified silica illustrates unique dynamic performance capabilities that probably could be applied to enhance the performance of power transmission belts and possibly other dynamic applications.
Either with or without silane modification, silica generally gives improved aging resistance. Silica can also be used to shift typical hardness/stiffness relationships in useful ways in some cases. Silica reinforcement is required for high quality white or colored rubber compounds.
The off-the-road tread compound for giant mining or construction fires in table 5 illustrates the type of carbon black-silica combination typically used to obtain enhanced tear and cut growth properties. The carbon black is replaced by enough silica to enhance the desired properties, but not so much silica that the sulfur crosslinking reactions are seriously affected.
Table 5 - OTR tread formulation Additive phr Natural rubber 100 N-220 carbon black 45 Silica 15 Zinc oxide 5 Stearic acid 2 Aromatic oil 3 Cumarone indene resin 3 Antiozonant 1.5 Antioxidant 1.5 PE glycol 0.3 Sulfur 2.0 TBBS 1.1
Various silicas covering a range of surface areas were evaluated at 15 phr in the OTR tread recipe and an all carbon black (N220) compound was included for comparison.
The tear strength data in figure 7 show a maximum range for silicas with a surface area of about 140 to 180 [m.sup.2]/g. These gave higher tear strength than the equivalent all carbon black compound. The data show a decreasing trend for the silicas with very high surface area. These are more difficult to disperse and they would also interact more with the cure system ingredients, leading to reduced crosslink density.
[Figure 7 ILLUSTRATION OMITTED]
DeMattia cut growth results indicate improved cut growth resistance (less growth) as surface area of the silica increases. All of the silica-containing compounds showed better cut growth resistance than the all-carbon black compound.
Heat build-up results increased with increasing surface area of the silica, illustrating the typical trade-off in properties (as cut growth resistance is improved, heat build-up is increased). Silicas with 140 to 180 surface gave heat build-up approximately equal to the N220 control compound, but showed improved tear and cut growth resistance.
Silica gives enhanced adhesion performance in a number of applications. The usual level of silica is 10-20 phr, similar to tear and cut growth applications, and much of the increase in adhesion may simply be the enhancement of tear and cut growth at the adhesion interface.
A wire adhesion compound with or without special bonding agents is generally enhanced with some silica replacing part of the carbon black.
A customer wanted a silica filled NR compound with certain hardness, tear and abrasion resistance properties. A recipe that seemed to fit the need was recommended based on some previous lab work.
After a lab evaluation, the customer was pleased with the performance of the compound. However, when he went to a plant trial mix, he found that he could not handle the compound in his cold feed extrusion process. Although the ML4 viscosity looked reasonable, the compound was simply too tough and "boardy" to run.
The rheometer curve in figure 8 illustrates the higher viscosity of the VN3 compound compared to a black-filled version of the same compound. Despite the compound modification with 4 phr Si69 and additional TBBS accelerator, it was still too stiff to process.
[Figure 8 ILLUSTRATION OMITTED]
Additional mixing should solve the processing problem with the VN3 silica filled NR compound. The rheometer curves in figure 9 show the considerable effect of adding a third high temperature mix stage to obtain a more complete reaction between Si69 and the silica.
[Figure 9 ILLUSTRATION OMITTED]
The effect of adding DPG accelerator is also illustrated. DPG is a strong amine and has some effect on viscosity, as well as accelerating the sulfur cure reaction. The VN3 compound with additional DPG may be processable without the extended high temperature mixing. However, the level of DPG that was used increased the crosslink density too much and resulted in poorer DeMattia cut growth performance. Further optimization of TBBS and DPG levels is needed.
Si 264 silane was also evaluated in our lab and showed a faster cure rate than Si 69. Other tests indicated similar performance when using either Si 69 or Si 264 silane. A switch to Si 264 would improve the slow cure rate of the extra-mixing version of the VN3 compound. Some DPG could also be added to speed up the cure rate.
Silica is different from carbon black and requires additional consideration of its chemical interactions with various other compounding ingredients.
Silicas can be modified more readily than carbon blacks to obtain a wide range of performance properties that often cannot be achieved with carbon black alone.
The silane coupling of silica offers alternative choices in rubber compounding that have proven to be advantageous in a number of applications.