Filler/elastomer interface

Surface Activity - A filler can offer high siuface area and high structure, but still provide relatively poor reinforcement if it has low specific surface activity. The specific activity of the filler siuface per cm of filler-elastomer interface is determined by the physical and chemical nature of the filler surface in relation to that of the elastomer. Nonpolar fillers are best suited to nonplar elastomers polar fillers work best in polar elastomers. Beyond this... 

The failure behavior of composite propellants, which are filled elastomers, is complicated by the presence of filler particles. Under loading, phenomena such as cavitation and debonding can arise at or near the filler-matrix interface. (1, 2) Identification of a practical failure criterion for such... 

Tensile properties of composite propellants depend on the tensile properties of the matrix, concentration of the components, particle size, particle-size distribution, particle shape, quality of the interface between fillers and polymeric binder, and, obviously, experimental conditions (strain rate, temperature, and environmental pressure). Many authors (2, 3) have explained the effect of fillers on the mechanical properties of composites, the importance of the filler-matrix interface on physical properties, and the mechanism of reinforcement of the material. Other efforts have examined the effect of experimental conditions on the failure properties of filled elastomers. Landel and... 

Fumed silica is widely used for the reinforcement of polydimethylsiloxane (PDMS) elastomers. The intermolecular interaction of the filler surface with the PDMS matrix controls this process [1, 2] so that understanding which factors influence the interaction at the filler/PDMS interface has become a crucial point for further development of the technology. Among the factors of interest there are the... 

In ternary nanocomposites, compatibilizers have been mostly used to improve the adhesion at the polymer/filler interface rather than to modify the polymer/elastomer interface. Mishra et al. [80] compounded PP/EPDM/organoclay (75/25/5 wt%) and added PP-g-MA (1 wt% MA) as a compatibilizer with a clay/PP- -MA ratio of 1/3. They characterized the interlayer spacing of the clay platelets by XRD and observed that it increased from 3.4 to 4.3 nm for systems without and with compatibilizer. This was attributed to a better diffusion of the PP-g-MA chains inside the interlayer spacing thanks to their functional groups. Numerous other authors prepared and characterized ternary composites with a compatibilizer. Examples include Lim et al. [81] and Lee et al. [5] on PP/PP- -MA/POE/ organoclay systems, Mehta et al. [23] on PP/PP-g-MA/EPR/organoclay systems, and Liu and Kontopoulou [24,45] on PP/PP-g-MA/ethylene-octene copolymer/silica composites. It should be noted that the compatibilizer itself may affect the properties of the matrix... 

Berriot J, Martin F, Montes H, Monnerie L, Sotta P (2003) Reinforcement of model filled elastomers charactetizatirai of the cross-linking density at the filler-elastraner interface by IH NMR measurements. Polymer 44(5) 1437-1447... 

In the rubber industry the distribution of particle size is considered to be important as it affects the mechanical properties and performance. Aggregate size also varies with particle size. Aggregates can have any shape or morphology. The fundamental property of the filler used in a filled elastomer is the particle size. This affects the reinforcement of elastomer most strongly. One of the sources of reinforcement between the carbon black surface and the rubber matrix is the van der Waals force attraction. Also, rubber chains are grafted onto the carbon black surface by covalent bonds. The interaction is caused by a reaction between the functional group at the carbon black particle surface and free radicals on polymer chains. Hence, filler-rubber interface is made up of complex physical-chemical interaction. The adhesion at the rubber-filler interface also affects the reinforcement of rubber. When the polymer composites are filled with spherical filler (aspect ratio of the particle is equal to unity), the modulus of the composite depends on the modulus, density, size, shape, volume ratio, and number of the incorporated particles. 

From the discussion in Chapter 2, it follows that both physical and chemical bonds play an important role in reinforcement, determining the adhesion at the filler-matrix interface. Kraus made a detailed study of various aspects of interaction between elastomers and reinforcing fillers, in particular, on the influence of the chemical properties of carbon black particles on the reinforcement. It was found that the character of interaction of carbon black with pol5aner differs, depending on chemical properties of the surface of the black particles. In particular, there is a possibility of chemical grafting of the polymer molecules onto the surface. 

At present, a large area of interest in silane coupling agents in elastomers is in wire and cable applications. The electrical properties of white filled EPM and EPDM cables deteriorate rapidly under conditions of high humidity, due to adsorption of water at the filler/rubber interface. The use of silanes prevents water ingress and maintains the electrical properties. Table 20 shows the effect of various silanes on the... 

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

Galuska, A.A., Poulter, R.R., and McElrath, K.O., Eorce modulation AEM of elastomer blends Morphology, fillers and cross-hnking. Surf. Interface Anal., 25, 418, 1997. 

The single selection of particle diameter for the characterization of a reinforcing filler is, however, not appropriate, because, on the one hand, only fillers exhibiting a very poor reinforcing effect consist of independent spherical particles, and, on the other hand, gum-filler interactions taking place at the elastomer-filler interface are thus conditioned by the accessibility of the surface. The latter may, indeed, be restricted either by the presence of micropores or by the size of the macromolecule. The knowledge of the specific surface area of the filler is thus a prerequisite. Insofar as the determination of the filler specific surface area, performed by low-temperature gas adsorption or iodine adsorption, takes into account its microporosity, the adsorption of larger tensioactive molecules will often be favored 12,13). 

Molecular mechanisms for stress-softening are also discussed. It is shown that this phenomenon is not related to the chain slippage or to a conversion of a "hard" adsorbed phase to a soft one. The obtained results assume that the stress-softening in silicon rubbers is caused by two possible reasons changes in the positions of filler particles relative to the direction of stretching at the first deformation and by a re-distribution of the topological hindrances. It is shown that the tensile strength at break as a fiinction of temperature is closely related to the chain dynamics at the elastomer-filler interface. 

On one hand, it is generally believed that knowledge of chain dynamics at the elastomer-filler interface is of major importance for the molecular understanding of the reinforcement effect. On the other hand, some other reasons for the reinforcement should also be considered. They are ... 

The characterization of surface activity of fillers is obtained by use of several analytical techniques [1]. Examples of them are inverse gas chromatography [1, 2], the adsorption of a low molecular weight analog of elastomers [3], the adsorption of elastomer chains fi om dilute solutions [4], the wettability, viscosity of PDMS fluids in the boundary layer at the surface of solids [5], the determination of the specific surface area, and the analysis of surface groups [1]. It should, however, be mentioned that the results obtained by these methods do not provide direct information on the elastomer behavior at the interface, due to the use of small probe molecules or the presence of a solvent in the systems studied. 

This paper is devoted to the study of a part of the complex phenomena of reinforcement, namely the behavior of the host elastomer in the presence of filler particles. The results of solid state NMR experiments and some other methods for filled PDMS are reviewed. The short-range dynamic phenomena that occur near the filler surface are discussed for PDMS samples filled with hydrophilic and hydrophobic Aerosils. This information is used for the characterization of adsorption interactions between siloxane chains and the Aerosil surface. Possible relations between mechanical properties of filled silicon rubbers on the one hand and the network structure and molecular motions at flie PDMS-Aerosil interface on the other hand are discussed as well. 

To sum it up it can be said, that solid state NMR is a very sensitive tool for the study of chain dynamics at the elastomer-filler interface as well as the network structure resulting from chemical Junctions, adsorption junctions and topological hindrances from the filler particles. The method is of interest for establishing structure-property relations for filled elastomers. 

Diamine salts of fatty acids are used as multifunctional additives in natural rubber compounds filled with carbon black.They affect the elastomer-carbon black interface. With an increased concentration of multifunctional additive, the concentration of bound rubber decreases but dispersion of carbon black is improved. In silica filled rubber, multifunctional additive also improves the dispersion of silica, but in addition, it decreases the negative influence of silica filler on vulcanization rate. 

Position Dependence. In polymers with heterogeneous structures— for example, semicrystalline polymers and filled elastomers— the transport process is complicated by the generally impermeable dispersed phase. Not only does the crystallite or filler particle create a larger path for the diffusing molecule to traverse, but also the presence of a high area interface within the polymer changes the nature of the continuous phase from that of the pure homogeneous state. These effects are related by the expression (4) ... 

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