The challenge ahead - new polymer/filler systems. (Tech Service).
* The colloidal filler has to be dispersed as well as possible to create the largest possible polymer/filler contact surface; and
* phase adhesion of the polymer on the filler surface has to be of a high level and, to some extent, reversible in nature.
Nowadays it is recognized that filler dispersion strongly depends on the balance of two main but opposite interactions:
* On the one hand, the filler-filler interaction that keeps colloidal filler particles in agglomerates or aggregates;
* on the other hand, the filler-matrix interaction that exploits the energetic interaction potential of both the filler surface and the polymer chain segments.
Mechanical mixing shifts the balance of these two opposite interactions as much as possible in favor of extensive filler-matrix interaction and better dispersion of the filler.
In the ideal case - one in which these demands are completely met - it can be assumed that dynamic properties as well as ultimate ones would achieve a maximum level. But as in many other areas as well, rubber technology was and still is far from dealing with such cases. Being a capital-intensive industry forever operating under price pressure, and belabored by problems in defining quality and performance, development was and is made by reacting to the automakers' call for faster and better products. Frankly, basic research into the physics and chemistry of the polymer/filler interaction and development of new filler systems, dispersion agents or polymer systems was almost exclusively the domain of large suppliers and some university institutes.
Unlike carbon blacks, where the surface energy of the primary particles offers quite good possibilities for anchoring rubber chains, silica, with its very high polarity, is not well dispersed and demonstrates a poor interaction potential vis-a-vis the non-polar general purpose rubber used in the tire industry. In this respect, the introduction of Si69, the well known bifunctional triethoxysilane, triggered a substantial revolution in both tire technology and rubber research.
Irrespective of the quite long period of promoting Si 69 for the tire industry in the seventies and eighties, or the hurdles for tire producers arising mainly from the tricky nature of the reactive process, success was guaranteed by the sequence of two of this chemical's specific capabilities, namely:
* Firstly, reduction of filler-filler interaction through silanization of the silica surface, thereby promoting deagglomeration and resulting in better filler particle dispersion and distribution;
* secondly, the molecular coupling mechanism of the initially silanized filler with the rubber chains of the matrix.
The right sequence of these two processes was a major obstacle for some tire producers. It soon became obvious that while providing for excellent dispersion, silanization alone had the disadvantage of resulting in poor ultimate properties, especially with respect to abrasion resistance but also in regards to rolling resistance and traction.
If phase bonding of silica particles on the matrix takes place at a very early stage of dispersion, the resulting filler network and network topology exhibit large filler lumps. These cause premature damage and reduce the life of products.
With the introduction of Si69 into rubber technology, and especially with the performance jump realized with the "green tire," it became evident that better filler dispersion opened the door to the creation of larger polymer-filler interphases. What's more, the coupling of the polymer - preferentially solution SBR - onto the filler surface reduces chain slippage and therefore hysteric processes. The compound effect of these mechanisms is lower rolling resistance and better wet traction (figure 1).
[FIGURE 1 OMITTED]
The latter effect is attributed mainly to polar silanol groups that also improve the high frequency components of rubber friction.
The success of this coupling agent is topped by its ability to act as an antireversion agent. The overwhelming success of Si69 proved the driving force for challenging new developments in the last decade, all of which can be summarized under the heading of functionalization.
Despite the search for new silanes with improved properties, recent developments have to be mentioned that specifically change the surface energy of the filler and functionalize the polymer with specific interacting groups that can be accepted by original or modified fillers. It has been shown that introducing functional groups into existing polymers that can interact specifically with particulate fillers is a proper technique for reducing rolling resistance and for improving wet skid resistance or wear resistance in tread compounds.
The idea of chain modification has provided positive input for the development of new types of emulsion rubber after more than 50 years in which very little occurred on this front. The first approach was chain-end functionalization. Amine groups placed at the chain end improve the dispersion of silica in E-SB, and thus its dynamic properties as well.
Emulsion-polymerized chains can also be modified by copolymerization with a low mol-% using suitable termonomers. The creation of a new all purpose rubber, styrene-butadiene-nitrile rubber (SBNR), exploits especially the capacity of nitrile groups to interact with carbon black's high energetic sites or with the silanol groups of silica.
The functionalization of S-SBR by hydroxylic groups leads to a significant increase in the dynamic loss factor in the low temperature range and a reduction in this factor in the high temperature range, pointing to decreased rolling resistance (figure 2).
[FIGURE 2 OMITTED]
Both modification of the nitrile groups and of the hydroxyl groups increases polymer/filler phase bonding, resulting in better filler dispersion and higher wear resistance.
Parallel to the innovations in synthetic rubbers, one has come to recognize the heterogeneous distribution of activity on the surface of carbon blacks. This has proved important in exploring the specificity of rubber-carbon black interactions.
Static gas absorption experiments and infrared spectroscopy of adsorbed hydrocarbon molecules on carbon black surfaces led to the detection of four main adsorption sites with discrete energies (figure 3).
[FIGURE 3 OMITTED]
For each furnace black, the lowest energy of 15 kJ/mol corresponds to the most abundant energy sites located on graphitic layers. The next higher energy levels are amorphous carbon structures with 20-21 kJ/mol. High energetic adsorption sites with 25-26 kJ/mol and 31-32 kJ/mol correspond to the topographic characteristics of carbon black's primary particles. In addition to the fact that all four energy sites do not exploit specific interactions, it is obvious that the high energetic sites contribute to the interaggregate interactions. They thus promote agglomeration.
The desired effects - high dispersion and strong phase adhesion to the polymer - can be obtained by modifying the surface activity of carbon black to such an extent that filler-matrix interactions are more favorable than interaggregate interactions.
This can be done by modifying the furnace process sufficiently that less graphitic microcrystallites and more amorphous carbon structures are formed. A successful approach was undertaken by Degussa in developing "inversion" blacks. Qualitatively, a similar result is obtained when CB is subjected to post-treatment with only traces of some chemicals. It can be shown that in this case, the high energetic sites are diminished and the amorphous carbon structures became relatively more abundant. This leads to a better promotion of filler/polymer interactions. If strain amplitude dependence is considered, surface treatment of this kind leads automatically to a substantial increase in dispersion and a further reduction in energy dissipation (figure 4).
[FIGURE 4 OMITTED]
If the temperature sweep of the dynamic moduli and tan [delta] is considered, the surface treatment of CB results in higher tan [delta] values in the low temperature range (indicating better traction) and lower tan [delta] values in the high temperature range (as required for better rolling resistance). What's more, the tan [delta] vs. T curve matches perfectly the curve obtained with a silica-filled green tire tread (figure 5).
[FIGURE 5 OMITTED]
This, of course, opens new perspectives for the use of modified CB grades. It is believed the surface modification of CB will come to enjoy a significance equal to that of morphology performance and economic value.
In addition to surface activity, one has to consider the nanorheology of polymer chains in porous fillers. It is a striking fact that branched E-SBR always leads to higher energy dissipation than linear, unbranched L-SBR with a similar microstructure. The same filler and the very similar microstructure determine the same wet-grip properties of the respective tires. The difference in energy dissipation reflected in rolling resistance could be due to the differences in the macrostructure - the chain architecture, in other words. Modern polymer physics connect the nanostructure of the pores (or the distribution of pore size) with the respective polymer structure and molecule topology (figure 6).
[FIGURE 6 OMITTED]
The spectral dimensions of the linear molecules simply provide a better "match" for the pore distribution of the silica. In the case of branched molecules, on the other hand, pore penetration during mixing is impeded. Alongside the fractal dimensions of the fillers, the spectral dimensions, which are still insufficiently known, play a fundamental role in the characterization of filled elastomers.
So far we have considered improvements on only one part of the elastomer system. Contemporary models show that specific interacting groups can be designed according to the key-lock principle. An interacting system of this type normally exhibits thermoreversible effects. Placing one group type on the filler surface and the other in the polymer chain makes it possible to control phase bonding and energy dissipation in elastomers via temperature and, above all, frequency (figure 7).
[FIGURE 7 OMITTED]
Today, this is achieved by means of functionalized polymeric fillers and special polymers. Work is in progress. The results give us reason to hope that such systems will demonstrate adaptivity to external excitation. This is sometimes referred to as "smart elastomers." As a model system of this kind is self correcting, it should be particularly effective for non-destructive energy dissipation in high-frequency ranges, as is required for traction and skid resistance (figure 8).
[FIGURE 8 OMITTED]
Synergistic effects can be expected when the two main routes presented - namely the modification achieved in rubber by introducing specifically interacting groups and the modification in the specific surface and morphology of colloidal fillers - are fused together in one global concept.
In order to do that, we have to pinpoint the most beneficial energy sites on the filler surface and learn how to modify them economically. We also have to gain a better understanding of the specific interaction between functional groups in polymer chains and surface modifications of particular fillers.
The profusion of changes cannot be solved by any one party alone. We need to learn how to establish a bridge between the concepts of filler producers and those of polymer producers. In addition, we need to establish a bridge between polymer chemistry and polymer physics, solid state physics and advanced processing. In other words, we have to start to develop systems with specific properties.
The above makes abundantly clear the magnitude of the R&D potential in rubber technology unleashed by technical exploitation of the multifarious functionality present in the Si69 molecule. No more than 10% of this potential is being exploited, and this academically.
There will be shifts in emphasis and new areas to concentrate on. This will lead to the creation of adaptive systems using functionalized polymers and fillers for specific purposes. That in no way conflicts with silane technology and its improvements in the near future. Anyone who is convinced of the technical and economic future of high-performance elastomers has no alternative but to accept the challenges dictated by the market and create new systems by interdisciplinary cooperation. Understanding the mechanisms is part of the challenge. The more important part, however, is to create and deliver optimal interacting systems to the customer.
Dr. Robert H. Schuster is the director of Deutsches Institut fur Kautschuktechnologie.
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|Title Annotation:||rubber technology|
|Comment:||The challenge ahead - new polymer/filler systems. (Tech Service).(rubber technology)|
|Author:||Schuster, Robert H.|
|Date:||Sep 1, 2001|
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