Rubber filler particle shape

The matrix is usually polypropylene and it is this which melts during processing to permit shaping of the material. The rubber filler particles then contribute the flexibility and resilience to the material. The other type of TPR is the polyamide and the properties of all five types are summarised in Table 1.4. 

Liquid-solid transitions in suspensions are especially complicated to study since they are accompanied by additional phenomena such as order-disorder transition of particulates [98,106,107], anisotropy [108], particle-particle interactions [109], Brownian motion, and sedimentation-particle convection [109], Furthermore, the size, size distribution, and shape of the filler particles strongly influence the rheological properties [108,110]. More comprehensive reviews on the rheology of suspensions and rubber modified polymer melts were presented by Metzner [111] and Masuda et al. [112], respectively. 

The shape factor ( d/hg) reflects the boundary condition s constraint on rubber flow during deformation, and can be considered as a measure of tightness for a junction. The shape factor, or the ratio d/hg, can be used to calculate the stored energy with a junction rubber between two spherical filler particles [86,87] ... 

The CCA-model considers the filler network as a result of kinetically cluster-cluster-aggregation, where the size of the fractal network heterogeneity is given by a space-filling condition for the filler clusters [60,63,64,92]. We will summarize the basic assumptions of this approach and extend it by adding additional considerations as well as experimental results. Thereby, we will apply the CCA-model to rubber composites filled with carbon black as well as polymeric filler particles (microgels) of spherical shape and almost mono-disperse size distribution that allow for a better understanding of the mechanisms of rubber reinforcement. 

This is the well-known Einstein-Smallwood equation, where c is the volume fraction of the filler and Gn, is the elastic modulus of the rubber matrix. The equation is obtained based on three idealized assumptions, such as (i) freely dispersed particles, i.e. low volume fraction, (ii) a spherical shape (leading to the constant 2.5) and (iii) entirely non-elastic filler particles, i.e. their elastic modulus has to be infinitely large. The reinforcement term contains two factors one is a simple number related only to the geometry of the particles, the other is linear in the volume fraction of the filler particles. 

In the case of filled vulcanizates, the reinforcement efficiency depends on a complex interaction of several filler-related parameters including particle size, particle shape, particle dispersion, surface area, surface activity, structure of the filler, and interactions between the fillers and the rubber matrix. 

For hydrophobic elastomers such as NR and styrene butadiene rubber, carbon black usually has been selected as filler due to the hydrophobic surface characteristics and special particle shapes of carbon black which provide good dispersion. However, the dispersion of polar filler in hydro-phobic rubbers matrix is difficult because of its hydrophilic surface. The hydroxyl groups exist on the surface of polar filler provide strong filler-filler interactions which resulted in poor filler dispersion. The polar surface of filler formed hydrogen bonds with polar materials in a rubber compound. As known, the silica surface is acidic and forms strong hydrogen bonds with basic materials. ... 

Examples of inert or extender fillers include china clay (kaolin), talc, and calcium carbonate. Calcium carbonate is an important filler with a particle size of about 1 iJim. It is a natural product from sedimentary rocks and is separated into chalk, limestone, and marble. In some cases the calcium carbonate may be treated to improve the bonding with the thermoplastic. Glass spheres are also used as thermoplastic fillers. They may be either solid or hollow, depending on the particular application. Talc is an important filler with a lamellar particle shape. It is a natural, hydrated magnesium silicate with good slip properties. Kaolin and mica are also natural materials with lamellar structure. Other fillers include wollastonite, silica, barium sulfate, and metal powders. Carbon black is used as a filler primarily in the rubber industry, but it also finds application in thermoplastics for conductivity, UV protection, and as a pigment. Fillers in fiber form are often used in thermoplastics. Types of fibers include cotton, wood flour, fiberglass, and carbon. Table 1.11 shows the fillers and their forms. 

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. 

The characteristics which determine the properties filler that will impart to a composite are particle shape, particle size, surface area, and particle-matrix compatibility (Fig. 1). Particle-matrix compatibility relates to the ability of the polymer to coat and adhere to the filler. The shape of most mineral filler particles can be a sphere, cube, block, plate, needle, or fiber whereas some filler also contain a mixture of shapes. Mineral particles resembling plates, needles, and fibers are further characterized by their aspect ratio (http //www.rtvanderbilt.com/ fillersintroweb.pdf). In rubber/polymer composites, applied stress is transferred from the rubber/polymer matrix to the strong and stiff mineral. It seems reasonable that this stress transfer will be better affected if the mineral particles are smaller, because greater surface is thereby exposed for a given mineral concentration. Moreover, if these particles have a high aspect ratio (are needle-like, fibrous or platy in shape), they will better intercept the stress propagation through the matrix (Fig. 2) (http //www.rtvanderbilt.com/fillersintroweb.pdf). 

Study the effect of particle shape on the filler reinforcement of rubber vulcanizates, using the model polymeric fillers described above,... 

Fillers have been used in the formulation of rubber compounds since the early days of the rubber industry. Whilst their primary function is to reduce cost, it has been found that fillers have a reinforcing effect in the rubber mechanical properties sueh as tensile strength, modulus, tear resistance and abrasion resistanee and thus very few mbber compounds are prepared without substantial quantities of filler. The performance of filler in the rubber matrix is governed by its characteristics, such as the particle size and concentration, particle shape, surface activity, degree of interactions with rubber matrix and structure of the particle agglomerates. 

Geometrical factors - structure, void volume and porosity will affect the filler s ability to be wetted by the rubber into which it is incorporated. The shape of the particle will depend on the crystal structure of the mineral being used. 

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