Reinforcement by filler

This surprising feature of crack growth appears to be a major aspect of reinforcement by fillers... 

Some classification terms which may be encountered in some texts discussing reinforcement by fillers include ... 

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... 

The reinforcement by fillers increases as the filler concentration increases since the reinforcing mechanism is related to the presence of active sites on the filler surface which are available for reaction or interaction with matrix polymer. But this increase is limited by the effect a filler has on the rheological properties of a mixed material. There is a certain filler concentration above which the reinforcing effect of the dispersed filler is lost. Carbon black can serve as a simple example. Acetylene black has many useful properties but it cannot be used effectively for reinforcement because its structure does not permit high loadings whereas some furnace blacks can be loaded to high concentrations. 

In general, when two or more different types of polymer are mixed, they show immiscibility (separation of phases), and exhibit a dispersion of the minor phase (the dispersed phase) in the major phase (the matrix). The cohesion between these different phases is low in most cases, and so the mechanical performances of the blends produced by simple blending are generally low, and need improvement. This is of prime importance in the preparation of mnlti-layered prodncts (packaging films, bottles, and so on) or multi-material parts, as well as where plastics are reinforced by fillers or by fibres. In such cases, interfacial cohesion between different phases can be improved by the use of special additives, called adhesion promoters or compatibilisers, that act at inter-phases. 

Wolff, S., Chemical aspects of rubber reinforcement by fillers, Rubb. Chem. Technol., 69, 325 346, 1996. 

It should be noted that the difficulties associated with brittleness of plastics can become more severe when they are reinforced by fillers. NR is also capable of toughening the plastic composite this was studied by Nematezadeh where toughening of a PA6-montmorillonite (MMT) nanocomposite was performed by blending with ENR. As can be seen in Figures 24.6 and 24.7, the presence of ENR and MMA in PA6 was similarly examined by measuring elongation at break and by Izod impact test. 

Free Moisture. The free moisture of a filler is the water present on the surface of the particles. This weakly bound water can sometimes contribute to iaterparticle bonding (reinforcing) or filler—matrix iateraction, ie, biader adsorption or catalysis. A determination of free moisture is usually made by measuriag the percent loss on drying the sample at either 100 or 110°C. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Polypropylene can be fabricated by almost any process used for plastics (see Plastics processing). The extmsion of pipe and injection mol ding of fittings present no unusual problem. However, there is no way to bond the fittings to the pipe except by remelting the polymer, which is impractical on most constmction sites. The resin can be reinforced by glass fibers, mineral fillers, or other types of fillers and can be pigmented readily. 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

These characteristics can be further enhanced and their applications widened by fillers, additives, and reinforcements. Compounding properly will yield an almost limitless combination of an increased loadcarrying capacity, a reduced coefficient of friction, improved wear resistance, higher mechanical strengths, improved thermal properties, greater fatigue endurance and creep resistance, excellent dimensional stability and reproducibility, and the like. 

The developed micromechanical model of reinforcement by active fillers allows for a better control of material properties and a more fundamental engineering praxis in mbber industry. In particular. 

The use of reinforcing fillers was examined by Seed Wilson (1980). An alumina-fibre cement had a flexural strength of 44 MPa, while one reinforced by carbon fibre had a flexural strength of 53 MPa. Metal reinforcement has also been examined. Seed Wilson (1980) found that a cement reinforced with silver-tin alloy had a flexural strength of 40 MPa. 

Improvement of the mechanical properties of elastomers is usually reached by their reinforcement with fillers. Traditionally, carbon black, silica, metal oxides, some salts and rigid polymers are used. The elastic modulus, tensile strength, and swelling resistence are well increased by such reinforcement. A new approach is based on block copolymerization yielding thermoelastoplastics, i.e. block copolymers with soft (rubbery) and hard (plastic) blocks. The mutual feature of filled rubbers and the thermoelastoplastics is their heterogeneous structure u0). 

The two systems discussed above demonstrate two mechanisms whereby the tensile strength of elastomers can be reinforced by the presence of rigid fillers. For the polymeric fillers dispersed within a vulcani-zate, the filler operates by raising the viscosity of the matrix, analogous to a decrease in temperature, but without affecting the dynamic, high frequency response (there is ample experimental evidence of the independence of Ty on presence of filler). There is also some indication that the rigidity of the filler affects the extent of reinforcement. 

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