Elastomers and Silica

In contrast with carbon black, silica has particular surface properties that bring a number of problems when using such materials as reinforcing fillers, particularly in hydrocarbon elastomers. Indeed the surface of silica, either fumed or precipitated, is strongly polar and hydrophilic, owing to its polysi-loxane structure with numerous silanol groups. This particular surface chemistry of silica has several immediate consequences  

Silica surface can adsorb significant quantities of water, as reflected by the well known usage of the material as drying agent (or moisture absorber) 

Interparticle interactions are very strong because of hydrogen bonding 

Silica is the ideal filler for silicone polymers, for instance polydimethylsiloxane 

When used as filler for diene elastomers, chemical modification of silica surface is required firstly to promote mixing through decreased inter-particles interactions and secondly to establish adequate rubber-filler interactions. 

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The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

H. Ismail, Effect of palm oil fatty acid additive (POEA) on curing characteristics and vulcanizate properties of silica filled natural mbber compounds. Journal of Elastomers and Plastics, 32, 33, 2000. 

Recent work has focused on a variety of thermoplastic elastomers and modified thermoplastic polyimides based on the aminopropyl end functionality present in suitably equilibrated polydimethylsiloxanes. Characteristic of these are the urea linked materials described in references 22-25. The chemistry is summarized in Scheme 7. A characteristic stress-strain curve and dynamic mechanical behavior for the urea linked systems in provided in Figures 3 and 4. It was of interest to note that the ultimate properties of the soluble, processible, urea linked copolymers were equivalent to some of the best silica reinforced, chemically crosslinked, silicone rubber... 

Exfoliating layered particles such as the clays, mica, or graphite is being used to provide very effective reinforcement of elastomers at loading levels much smaller than in the case of solid particles such as carbon black and silica [228-231]. Other properties can also be substantially improved, including increased resistance to solvents, and reduced permeability and flammability. 

Atomic oxygen is a highly reactive species that can pose serious problems for spacecraft in low Earth orbit due to degradation of materials of construction. In the case of siloxane elastomers and coatings88,89, it has been shown that surface oxidation occurs to give a silica layer which then protects the bulk material from further oxidation, with the result that little loss of physical properties occurs. 

TG-MS and TG-FTIR have been used for ageing characterisation of the methylphenyl silicone elastomers GE 566 (containing ferric oxide and silica filler) and GE 567 with the... 

A low-resolution proton NMR method is one of the few techniques that have so far proved to be suitable for studying elastomer-filler interactions in carbon-black-filled conventional rubbers and silica-filled silicon rubbers [20, 62, 79]. It was pointed out by McBrierty and Kenny that Many of the basic characteristics of filled elastomers are revealed by low resolution spectra while the more sophisticated techniques and site specific information refine interpretations and clarify motional dynamics [79]. 

Elastomers require, in most applications, to be reinforced by fillers in order to improve their mechanical properties. Carbon black and silica have been used for a long time in the rubber industry to prepare composites with greatly improved properties such as strength, stiffness and wear resistance. These conventional fillers must be used at high loading levels to impart to the material the desired properties (1). The state of filler dispersion and orientation... 

Halasa [2] reduced hysteresis in tires by anionically copolymerizing functionalized butadiene, (I), and styrene, (II), to enhance elastomer compatibility with carbon black and silica fillers. 

Through the combination of special basic silicas, suitable silicone elastomers, and adequate hydrophobization conditions, it is possible to generate high-performance silicas with new properties at competitive prices. 

Mixtures of rubber latices or elastomer foams were modified with colloidal silica to give improved properties. Typical processes involved drying, gelling, or coagulating the colloidal silica within the elastomer system. Silica sols were used with phenolic, formaldehyde-based, melamine, polyester, acrylic, vinyl or styrene polymer-copolymer, polyamide, and styrene-butadiene rubber systems to provide strength to films and coatings. 

Many of the silicone elastomers that are used in biomedical applications are produced by Dow Chemical Corp., under the trade name SILASTIC . For example, a typical medical-grade silicone (like SILASTIC MDX4-4210 Medical grade elastomer) contains, after curing, cross-linked drmethylsiloxane polymer and silica for reinforcement. Silcones are also reinforced with PET (Dacron) fiber meshes for certain biomedical applications. For implantable medical devices, it is important to realize that the cured polymer contains residual catalysts and silicone cross-linkers, which are necessary for the polymerization. 

This result is shown for polyisoprene in Figure 3.5 (Akcasu et al., 1980). Direct SANS measurements of the size of chains in the melt validated Flory s original hypothesis that polymers behave ideally in the bulk. Other applications of SANS to elastomers include investigations of microscopic aspects of network deformation (Gronski et al., 1990 Boue et al., 1991 Westermann et al., 2001) and the effect of fillers, such as carbon black and silica, on network deformation (Botti et al., 2003 Westermann et al., 1999 Zhang et al., 2001). 

Compared to morphology, filler chemistry has been only slightly studied, partly because of the difficulty of such characterizations and more probably because since the 1970s reinforcement is broadly considered as a physical interaction between elastomer and filler. So carbon black chemical characterizations mainly date from the 1960s, and few new technical methods have been applied to carbon black surface characterization since then. The situation is somewhat different for silicas, because silica reinforcement is the consequence of a chemical reaction of silane with silica surface. Few studies have been published in the elastomer reinforcement area, probably because silica surface was already well characterized for other applications. 

Chem. Descrip. Fumed silica, surf, treated Uses Rheology control agent, reinforcing agent, tlow aid, hydrophobing agent for powd. coatings, elastomers, and toners Properties Wh. fine powd., bulk dens. 6 Ib/ff surf, area 212 028 m /g ... 

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