Rubber formulation Hardness

Fillers are added to the elastomer in order to add bulk, lower cost and/or to improve physical properties such as hardness, strength and abrasion resistance. Typical fillers are materials such as carbon black, talc, china clay and whiting. Carbon black has been shown to contain polynuclear aromatics (PNAs) and there is concern regarding their carcinogenicity (Lee and Hites, 1976). However, despite extra controls there has been a move away from the use of carbon black as a filler in applications involving the primary packaging of parenterals. Its use continues as a pigment or colourant in rubber formulations but at substantially lower levels than that as a filler. 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

Table 3. Effect of Cure Time and Filler Type on Development of Hardness in a Rubber Formulation...

Hard products may also be made by vulcanising rubber (natural or synthetic) using only about two parts of sulphur per 100 parts of rubber. In these cases either the so-called high-styrene resins or phenolie rubber compounding resins are ineorporated into the formulation. These compounds are processed using the methods of rubber technology but, like those of ebonite, the produets are more akin to plastics than to rubbers. Examples of the usage of these materials are to be found in battery boxes, shoe heels and ear washer brushes. 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

Okumoto [89] has reported an analytical scheme (Scheme 2.8) for automotive rubber products (ENB-EPDM vulcanisates). For high-resolution PyGC analysis, organic additives are first removed from the rubber/(CB, inorganics) formulation. Carbon-black and inorganic material hardly interfere with pyrolysis. For the analysis of the additives the extracted soluble... 

Castable polyurethanes can be formulated to contain conventional crosslinks, as in rubbers and epoxies. They may be either in the form of the liquid system or in the form that requires conventional rubber-processing equipment. The introduction of conventional cross-links will break the formation of the hydrogen bonds and normally reduce the hardness of the finished product. In very hard grades, they are used to increase the compression resistance. 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

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