Rubber filler types

Antioxidants of the polyhydroxy aromatic type can be adsorbed on fine silica of the rubber-filler type and dispersed in polyethylene. It proved to be as good an antioxidant as carbon black yet the film was clear (649). 

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. 

Analysis of inorganic fillers in plastics and rubber materials is normally accomplished by ashing material in a muffle furnace at a temperature of 550°C. An IR spectrum of the resulting ash, sometimes as a paraffin oil mull is then obtained to identify the filler type. Examination of the ash by XRF and/or X-ray diffraction can also provide useful information to help identify complex systems. 

The carbon black generated by a fire from a rubber source increases the smoke density other products are highly toxic and often corrosive. The halogens, phosphates, borates, and their acids evolved during a fire corrode metals and electrical and electronic equipment. Hence many of the fire retardants described below cannot be used in situations where the toxic gases evolved will create their own hazards. In these cases inorganic hydroxides are used, at filler-type addition levels. Aluminium hydroxide and magnesium hydroxide are used as non-toxic fire retardant systems. 

In multiphase filled polymer compositions, which may contain mixed filler types, combinations of fillers and fibres, or proportions of filler and a secondary modifying polymer, such as an elastomer, the spacial distribution of the phases has a direct bearing on the properties of the composite. In the case of the last mentioned system, the rubber may encapsulate the filler, be present as discrete droplets within the thermoplastic matrix or co-exist in both structural forms [80,81]. 

Tensile strength, modulus at a given elongation and elongation at break depend on the rubber type and the reinforcing filler type and... 

Fig. 3.3 Pipette fillers (a) rubber-bulb type (b) Pi-Pump . ...

Finger joint replacements are divided into three types (1) hinge, (2) polycentric, and (c) space-fiUer. The most widely used are the space-filler type. These are made of high performance silicone rubber (polydimethylsiloxane) and are stabilized with a passive fixation method. This method depends on the development of a thin, fibrous membrane between implant and bone, which allows pistoning of the prosthesis. This fixation can provide only minimal rigidity of the joint [Swanson, 1973). Implant wear and cold flow associated with erosive cystic changes of adjacent bone have been reported with silicone implants [Carteret al., 1986 MaistrelU, 1994]. 

As an example, the graphic representation in Figure 5.13. shows crack growth curves of natural rubber compounds reinforced with layered silicate and silica. It can be seen that the crack propagation behaviour is different in dependence on the filler type and on the content of the layered silicate, too. The slope of the crack propagation curve is decreased by adding 60 phr in comparison to 15 phr layered silicate and to 60 phr silica. This means, the crack growth speed is reduced. 

In the same studies, Moehlenpah et al (1970, 1971) obtained master curves for the stress relaxation of their epoxy systems, at least into the glass-to-rubber transition region (Figure 12.4), and demonstrated similar behavior of both the stress relaxation modulus and the tensile modulus as a function of strain rate. As with the strain rate studies mentioned, no effect of filler type on the WLF shift factor was observed. All solid fillers increased the modulus of the system, the fibers being more effective than the spheres. The bubbles, as expected (Nielsen, 1967a), decreased the modulus. 

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

The tensile strength of filled elastomers over the tensile strength of the gum rubber is much greater for SBR. A greater increase in tensile strength is attainable for non-crystallizable polymers. Filler distribution is also affected by filler type. Elaborating ... 

In recent years, lamellar nanofiUers have been established as the most important filler type for barrier and mechanical reinforcement. Dal Point et al. reported a novel nanocomposite series based on styrene-butadiene rubber (SBR latex) and alpha-zirconium phosphate (a-ZrP) lamellar nanofiUers. The use of surface modified nanofiUers improvement the mechanical properties. However, no modification of the gas barrier properties is observed. The addition of bis(triethoxysilylpropyl) tetrasulfide (TESPT) as coupUng agent in the system is discussed on the nanofiUer dispersion state and on the fiUer-matrix inteifacial bonding. Simultaneous use of modified nanofillers and TESPT coupling agent is found out with extraordinary reinforcing effects on both mechanical and gas barrier properties [123]. 

The strength of rubbers is influenced strongly by the effect of filler type and loading. This is discussed further in Section 7.6.1. The addition of plasticisers is known to reduce... 

Figure 7.4 Reinforcement of rubber by fillers. Filler type versus strength.

The importance of the relevance of specification parameters to applicability should be borne in mind at all times. As we have seen, fillers play a vital role in the formulation of most rubber products. They are produced using a wide variety of processes and may have either natural or synthetic origins. Hence, fillers vary enormously in their chemical characteristics and in particle size, which, in turn, influences the filler s overall behaviour in rubber. Fillers provide the formulator with a range of materials that can modify processing behaviour, and physical and chemical properties of the polymer. Details of the production routes used and the characteristics of the individual filler types are shown in greater detail in Chapter 2. This chapter will concentrate on those aspects of particular importance to elastomers. 

Carbon blacks [44] are a form of carbon produced by controlled pyrolysis of hydrocarbon oil or gas. They are the most important filler type for use in rubber as they are the main agents for providing high-strength compounds. These materials also have a pronoimced effect on the processing behaviour of rubbers. 

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