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Natural rubber-silica combinations for low rolling resistance truck tire treads.

In recent years, the increasing demand for low-energy consuming and low rolling resistance tires has led to growing use of silica in tread compounds. Four essential elements in silica-robber technology include the robber polymer, a special type of silica, an effective coupling agent and the appropriate mixing technology which are interconnected in expanding the magic triangle of tire technology; the compromise between rolling resistance, (wet) traction and wear. Compared to carbon black, mixing silica compounds involves many difficulties due to the large polarity difference between silica and rubber. A bifunctional organosilane such as bis(triethoxysilylpropyl) tetrasulphide (TESPT) or its disulphide equivalent is commonly used as coupling agent in enhancing the compatibility of silica and rubber, by chemically modifying silica surfaces and eventually creating a chemical link between silica aggregates and the rubber chains (ref. 1). Complications arise during mixing of silica compounds as several chemical reactions need to take place, all at their appropriate time slots during rubber processing, namely the silica and silane reaction or silanization, silane-rubber coupling and crosslinking between the rubber chains (ref. 2).

The high-dispersion silica technology, as it is used today, employs mainly solution-polymerized synthetic rubber, and is still not commercially feasible with natural rubber (ref. 3). It was postulated that non-rubber constituents contained in natural rubber such as proteins compete with the coupling agent for reaction with the silica during mixing, so disturbing its reinforcement action (ref. 3).

Commercial natural rubber (NR) comes from the milky sap or latex that exudes from the rubber tree, Hevea Brasiliensis, which coagulates on exposure to air. Hevea latex consists of rubber hydrocarbon for about 30-45 weight %, non-rubber constituents for about 3-5 weight %, and the rest is water. The non-rubber constituents are comprised of proteins, amino acids, carbohydrates, lipids, amines, nucleic acids, as well as other inorganic and mineral components (ref. 4). The work by Tanaka and coworkers has revealed that the fundamental structure of a linear NR chain consists of a long sequence of 1,000-3,000 cis- 1,4 isoprene units, with, at the [alpha]- and [omega]-chain ends, specific other groups (refs. 5-7). The a-terminal is composed of mono-and diphosphate groups linked with phospholipids by hydrogen or ionic bonds (ref. 8). The co-terminal entails two trans-l,4 isoprene units (ref. 9) and a modified dimethylallyl unit linked with functional groups, which is associated with proteins to form crosslinks via hydrogen bonding (ref. 10). Both non-robber constituents, i.e., proteins and phospholipids, are presumed to be the origin of branching and gel formation in NR (ref. 11). These secondary structures play a significant role in the strain-induced crystallization of unvulcanized and vulcanized natural rubber (figure 1) (refs. 12 and 13).

In the present investigation, the influence of non-rubber constituents in NR, particularly proteins on silica reinforced compounds in the presence and absence of coupling agent, is illustrated. NR is compared with deproteinized natural rubber (DPNR) for reduced protein content, as well as skim rubber with high protein content.



The compound was based on a truck tire tread compound recipe consisting of 100 phr of NR (Malaysian Rubber Board), 55 phr of silica Ultrasil 7005 (Evonik) and 5 phr of bis (triethoxysilylpropyl) tetrasulphide or TESPT silane coupling agent (Evonik). NRs with different protein contents were compared, as shown in table 1. For skim rubber, the formulation is adjusted to 112 phr to take into account the high protein content.

The compounds were mixed in two stages: In the first stage mixing, all ingredients except the curatives were mixed in a Brabender Plasticorder 350S internal mixer with 60 rpm rotor speed and 0.7 fill factor. The starting temperature was varied from 70[degrees] to 120[degrees]C to obtain variable temperature histories and dump temperatures. The curatives were added during the second stage of mixing on a two-roll mill.

Testing and characterization

Mooney viscosity was measured at 100[degrees]C with a Mooney viscometer 2000E (Alpha Technologies) using a large rotor for compounds and a small rotor for masterbatches. Vulcanization curves were measured using a rubber process analyzer, RPA 2000 (Alpha Technologies) at 150[degrees]C, under a condition of 0.833 Hz and 2.79% strain. The Payne effect was measured prior and after curing in the same equipment. Before curing, the sample was heated to 100[degrees]C in the RPA and subsequently subjected to a strain sweep at 0.5 Hz. The Payne effect was calculated as the difference between the storage modulus, G', at 0.56% and at 100.04% strain. The Payne effect after cure was measured after vulcanization in the RPA at 150[degrees]C for 10 minutes and subsequent cooling to 100[degrees]C, making use of the same strain sweep conditions.


The bound rubber content (BRC) measurements were performed on unvulcanized samples by extracting the unbound rubber with toluene at room temperature for seven days in both normal and ammonia environments. The ammonia treatment of BRC was done in order to obtain the chemically BRC as ammonia cleaves the physical linkages between rubber and silica (refs. 14-15). The amount of BRC (g/g filler) was calculated by:


Where [w.sub.o] is the initial weight of the sample, [W.sub.dry] is the dry weight of the extracted sample, [W.sub.insolubles] is the weight of insolubles (mainly filler) in the sample, [W.sub.filler, phr] is the total filler weight in phr and [, phr] is the total compound weight in phr. The physically BRC was taken as the difference between untreated BRC and ammonia treated BRC.

Wolff's filler structure parameter, [[alpha].sub.f] was determined from the ratio between the increase in rheometer torque of the filled compounds and that of the unfilled gum (ref. 16):

[D.sub.max] - [D.sub.min]/[D.sup.o.sub.max] - [D.sup.o.sub.min] - 1 = [[alpha].sub.f] [m.sub.f]/[m.sub.p] (2)

Where [D.sub.max] - [D.sub.min ]is the maximum change in torque for the filled rubber, [D.sup.o.sub.max]-[D.sup.o.sub.min] is the maximum change in torque for the unfilled gum rubber, and [m.sub.f]/[m.sub.p] is the weight ratio of filler to polymer, [[alpha].sub.f] is a filler specific constant which is independent of the cure system and closely related to the morphology of the filler.

The vulcanizates were prepared by curing the compounds for their respective [t.sub.95] at 150[degrees]C using a Wickert laboratory press WLP 1600/5*4/3 at 100 bar. Tensile properties of the vulcanizates were measured using a Zwick Z020 tensile tester according to ISO-37. The hardness of the cured samples was determined according to DIN-53505. The tan delta value calculated as the ratio of the loss modulus G" to the storage modulus G' at 60[degrees]C was measured using the RPA by applying a frequency sweep at 3.49% strain after first curing in the RPA at 150[degrees]C.


Results and discussion


In terms of processability of the masterbatches, NR and DPNR are comparable, but skim rubber has a lower viscosity. In figure 2, the increase in the viscosities of the masterbatches with increasing mixer dump temperature up to an optimum of 150[degrees]C is a combination of the hydrodynamic effect and silanization rate of the silica. More silica is hydrophobized by TESPT when the dump temperature is raised, and this results in a higher compatibility between silica and rubber, and consequently an increment in the viscosity. However, the viscosity of the masterbatches of NR and DPNR start to decrease above the optimum dump temperature and, in the case of skim rubber, it levels off. One explanation is the degradation of the NR chains at high temperature.

Once the curatives are added to the compounds, the viscosities drop to acceptable levels, mainly due to the remilling step. In spite of the overall lower Mooney viscosities of the skim rubber masterbatches after the first mixing step, the Mooney viscosities with curative included by mill mixing are almost comparable with those of the NR and DPNR compounds.

Vulcanization properties

The influence of protein in NR on the silica-silica interaction can be clearly observed from the cure curve at 150[degrees]C as depicted in figure 3. The clear two-step curve for NR-silica and DPNR-silica compounds without silane is due to the silica flocculation or re-agglomeration (refs. 17 and 18), and strong silica networking. With high amounts of protein present in the compound, the silica-silica interaction is disrupted and this is shown with no sign of flocculation at the beginning of vulcanization for the skim rubber-silica compound without silane.

The use of silane TESPT in the NR-silica compound results in less pronounced silica flocculation, and this is demonstrated by only a small initial torque rise at the beginning of vulcanization (figure 3). As compared to the silica compounds without silane, the flocculation of silica in the compounds with TESPT is small due to hydrophobation of the silica surface by TESPT. The effect of protein on the cure behavior of the silica compounds fades with the presence of TESPT.

Filler-filler interaction

Filler-filler interaction is commonly measured by the so-called Payne effect: the drop in storage modulus of filled rubber as the strain increases, due to the breakage of physical bonds between filler particles, for example van der Waals, hydrogen bonds and London forces. The Payne effect was measured in the present study by a strain sweep in the RPA2000 (Alpha Technologies) at 100[degrees]C and 0.5 Hz.




The use of silica without silane modification in the rubber results in a high Payne effect due to strong interaggregate interaction of silica. With silane TESPT modification, the Payne effect of the silica-filled compounds is greatly reduced as the silica surface is hydrophobized by TESPT, and the silica-silica network is disrupted, as schematically shown in figure 4. What is interesting in this study is that the same effect can be seen with protein. With a high amount of protein present in the rubber, the Payne effect of the silica compound without silane is lowered. There is a relation between the amount of protein and the decrease of silica-silica interaction, as shown in figure 5. This indicates a strong interaction of proteins and silica, as well as the role of proteins in hydrophobizing the silica surface.

For silica compounds with silane TESPT, the Payne effect decreases sharply with increasing mixing dump temperature, as is also seen in synthetic rubber/silica compounds and taken as a sign of reaction and consequent hydrophobation of the silica by the silane coupling agent (refs. 19 and 20). No effect of mixing temperature is perceived on filler-filler interaction for the skim rubber compound (figure 6). This again indicates a strong interference of the proteins in skim rubber with the silica-silica network. The silica-silica network is not influenced by the dump temperature for skim rubber with silane because silanization is hindered. The proteins in the skim rubber prevent the modification of the silica surface by the silane coupling agent. The logical explanation is that the interaction between silica and protein overrules the coupling agent, and that protein is shielding the silica surface.


Rubber to filler interactions

The difference between the NR compounds with different protein contents is further illustrated by the "filler in rubber structure," as shown in figure 7. As observed earlier with the Payne effect, [[alpha].sub.f] is reduced with increasing dump temperature for the NR and DPNR compounds. This is due to the increased hydrophobation of the silica by the silanization reaction at higher mixing temperature. Better hydrophobation leads to a decrease in silica-silica interaction, and consequently results in reduced [[alpha].sub.f]. The DPNR compound shows a higher [[alpha].sub.f] than the NR compound, indicating a different type of filler and rubber network in the two compounds. For the high protein content skim rubber compound, the [[alpha].sub.f] is much smaller than the NR and DPNR compounds, and is constant regardless of the changes in the dump temperature. This corresponds with the results of its Payne effect earlier. The non-rubber or protein in the skim rubber again plays the main role in the interactions between the filler and the polymer molecules, and mixing temperature has little influence on the compound.


The filler to rubber interaction of silica-filled NR with varying protein content can also be judged on the basis of the chemically and physically bound rubber. Most of the bound rubber formed in a NR-silica-TESPT compound is chemically bound (table 2). This is obviously due to the hydrophobation of the silica surface as a result of silanization with TESPT. The increase in silica-TESPT coupling consequently results in more filler-to-rubber interaction. This corresponds well with the lower Payne effect of the silica compounds with TESPT. Without silane in the compounds, there is still silica-rubber interaction, as indicated by the physically bound rubber. It demonstrates that the proteins contained in NR do interact with the silica to make it less hydrophilic to increase the rubber-silica interaction. However, no chemical bound rubber was obtained for the silica compound without silane after ammonia treatment. This means that without silane in the compound, only loosely or physically bound rubber is formed. This again indicates that the interaction of filler to NR in a silica compound without silane coupling agent is much less than in a compound with silane.

The trends seen in chemically bound rubber correlate surprisingly well with the Mooney viscosities of the masterbatches (figure 2). Apparently, when more chemically bound rubber is formed, the molecular motion of the rubber chains is restricted and this results in higher Mooney viscosity. This emphasizes the importance of chemical coupling of the rubber to the silica surface.

Physical properties

The use of TESPT as a coupling agent improves the vulcanizate properties of silica-filled compounds. Vulcanizates without silane exhibit inferior tensile strength compared to those with silane (figure 8). NR-silica-TESPT vulcanizates mixed at higher dump temperature exhibit somewhat lower tensile strength and modulus. On the other hand, DPNR-silica-TESPT vulcanizates show slightly more constant physical properties. The skim rubber vulcanizate performs overall much worse compared to NR and DPNR in physical properties due to lack of sufficient silica-rubber coupling and lower molecular weight of the polymers to start with.

Commonly, the dynamic mechanical loss angle tan [delta] at 60[degrees]C of cured compounds are employed as indications for the rolling resistance of tires. The lower the tan [delta] at 60[degrees]C, the lower the rolling resistance expected in real tire performance. Natural rubber vulcanizates show a strong decrease in tan [delta] at 60[degrees]C with increasing dump temperature, regardless of the amount of protein in the rubber (figure 9). Improvement in tan [delta] at 60[degrees]C can still be achieved with higher mixing temperature, like with synthetic rubber. This must obviously be the result of more coupling of silica to the rubber with greater silanization efficiency at high mixing temperatures. With low protein content, the DPNR vulcanizates exhibit the lowest tan a at 60[degrees]C at high dump temperature. This actually relates well with the higher chemically bound rubber content of DPNR than of the NR compound. Still, with all the protein contained in skim rubber, the tan [delta] at 60[degrees]C is significantly lowered by mixing temperature history, and only marginally worse than for NR and DPNR.


Both coupling agent and proteins show an antagonistic effect in silica reinforcement of rubber. When high amounts of proteins are present in NR, the interactions between proteins and silica are already able to disrupt the silica-silica networking. The effect of proteins is most pronounced when no silane is used in NR-silica compounds. The temperature development is an important parameter in mixing NR-silica with the aid of TESPT as a coupling agent, as silica-silica interaction is reduced through silanization at sufficiently high mixing temperature. This is clearly the case for NR and low protein content rubber DPNR. However, mixing temperature has little influence on the properties of a high protein-content skim rubber compound. Consequently, the hydrophobation of the silica surface due to silica-protein interactions.




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by S. Salina Sarkawi, University of Twente and Malaysian Rubber Board, and Wilma K. Dierkes and Jacques W.M. Noordermeer, University of Twente
Table 1--protein content of natural rubbers used

Rubber type Nitrogen Protein
 content, wt. % content, wt.

NR (SIVIR 20) 0.21 1.3
DPNR (Pureprena) 0.07 0.4
Skim rubber 2.06 12.9

It is generally accepted that the conversion factor from
nitrogen to protein is 6.25 (refs. 4 and 21).

Table 2--bound rubber content (BRC)

 Without silane With silane

Rubber type Physically Chemically Physically Chemically
 BRC (%) BRC (%) BRC (%) BRC (%)

NR 57 0 11 68
DPNR 45 0 11 76
Skim rubber 51 0 13 80
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Author:Sarkawi, S. Salina; Dierkes, Wilma K.; Noordermeer, Jacques W.M.
Publication:Rubber World
Date:Nov 1, 2012
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