Elastomers stiff fillers

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

However, not all properties are improved by filler. One notable feature of the mechanical behaviour of filled elastomers is the phenomenon of stresssoftening. This manifests itself as a loss of stiffness when the composite material is stretched and then unloaded. In a regime of repeated loading and unloading, it is found that part of the second stress-strain curve falls below the original curve (see Figure 7.13). This is the direct opposite of what happens to metals, and the underlying reasons for it are not yet fully understood. 

Fig. 19. Improved stiffness and impact resistance in PP composites containing a filler and an elastomer (A) usual behavior, (O) improved properties... 

The incorporation of carbon black into elastomeric systems is a process of significant commercial importance. However, the additional stiffness of the sample imparted by the reinforcement effect of fillers is not favourable in terms of the experimental conditions for high-resolution NMR spectroscopy. Electric conductivity of the carbon black may also interfere to some extent. Under these circumstances, filled formulations are not widely used for the study of elastomer vulcanisations where high resolution and signal-to-noise ratios are required to detect small amounts of vulcanisation products. 

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... 

Elastomers are long chained polymers that are bent back upon themselves many times. They have moduli of less than 1 GPa. The segments of the molecules will freely slide back across each other and are strengthened by fillers and the occasional cross-link. Table 2.7 shows bond type versus stiffness and modulus. With increasing temperature, entropy or disorder increases. Thus, stretching these types of parts decreases their entropy because molecules are aligned to become more crystalline. 

Minerals, such as calcium carbonate, talc, silica, are quite common fillers in plastic industry. They, often at abont 6-15 cent/lb, replace a much more expensive plastic, increase stiffness of the filled product, and render the plastic more flame resistant. The world filler market for plastics is dominated by carbon black and calcium carbonate. Of abont 15 billion pounds of filler in America and Europe, about half the filler volume goes into elastomers, a third into thermoplastics, and the reminder into thermosets. About 15% of all manufactured plastics contain fillers. 

The blends could also contain inorganic fillers, other resins, elastomers, and additives. The materials had good impact and notched impact strength, combined with stiffness and toughness. They have been used for the same applications as new materials, instead of having to be downcycled [Timmermann et al., 1994]. 

The very presence of the high-modulus filler imparts a certain stiffness to the system, as given by the lower-bound-type Guth-Smallwood relationship. In normally reinforced systems, however, the thermodynamic reinforcement mode, by way of internal energy increases, is much more important. Thermodynamic reinforcement acts in addition to the Guth-Smallwood mode. While the presence of the filler tends to increase the crosslink density slightly, this is apparently not a major cause of elastomer modulus increases. 

Reinforcements are used to enhance the mechanical properties of a plastic or elastomer. Finely divided silica, carbon black, talc, mica, and calcium carbonate, as well as short fibers of a variety of materials, can be incorporated as particulate fillers. Incorporating large amounts of particulate filler during the making of plastics such as polypropylene and polyethylene can increase their stiffness. The effect is less dramatic when temperature is below the polymer s Tg. 

As described above, NMR can be used to measure chemical crosslink concentrations however, the spectra are unaffected by physical entanglements. Since these entanglements contribute to the elastic response of cured rubber (see Chapter 4), the difference between crosslink determinations from NMR and modulus measurements can be used to quantify the entanglement concentration. Most applications of this method have been directed toward assessing the role of filler-polymer interaction on the stiffness of elastomers [95]. 

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