Plastics rubber elasticity

Elastomers are often blended with plastics either to improve the impact resistance or to develop new materials having both plastic and elastic behavior. When the elastomer in the blend is dynamically vulcanized, the product is called a thermoplastics vulcanizate (TPV). Blends with unvulcanized mbber phase are usually known as thermoplastic elastomers. TPVs are discussed in another section of this book. This section will deal with recent developments in rubber-plastic blends. 

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

According to the change of strain rate versus stress the response of the material can be categorized as linear, non-linear, or plastic. When linear response take place the material is categorized as a Newtonian. When the material is considered as Newtonian, the stress is linearly proportional to the strain rate. Then the material exhibits a non-linear response to the strain rate, it is categorized as Non Newtonian material. There is also an interesting case where the viscosity decreases as the shear/strain rate remains constant. This kind of materials are known as thixotropic deformation is observed when the stress is independent of the strain rate [2,3], In some cases viscoelastic materials behave as rubbers. In fact, in the case of many polymers specially those with crosslinking, rubber elasticity is observed. In these systems hysteresis, stress relaxation and creep take place. 

Flexible plastics and rubbers can, as a matter of fact, only be treated with rubber-elastic lacquers, mainly on the basis of polyurethane, which, moreover, should be resistant to oxidation, oils, fuel and UV light. Besides, polyurethane lacquers are often used for several other plastics, such as PVC, polyamides, ABS and glass-fibre reinforced resins. 

The polyvinylethers form a further group of thermoplastics which are not used as containers or packaging films. They are atactic polymers forming oils, sticky soft resins or nonsticky rubber elastic materials according to their molecular weight and composition. All polyvinylethers are very resistant to saponification by dilute acids and alkalis. They can subsequently be used as unsaponifiable polymer plasticizers and for the manufacture of glues. 

These propellants can be manufactured by casting or by pressing. The grain fineness of the salt employed affects the combustion properties to a significant extent. The mechanical (preferably rubber-elastic) properties of the plastic binders must satisfy special requirements. 

In manufacturing explosive charges which are required to have a certain mechanical strength or rubber-elastic toughness, Cyclonite is incorporated into curable plastic materials such as polyurethanes, polybutadiene or polysulfide and is poured into molds (-> Plastic Explosives). 

There has been the question why the TPV materials with ductile thermoplastic matrix display rubber elasticity. Several models have been suggested to answer this question (41 7). Inoue group first analyzed the origin of mbber elasticity in TPVs (43). They constructed a two-dimensional model with four EPDM mbber inclusions in ductile PP matrix and carried out the elastic-plastic analysis on the deformation mechanism of the two-phase system by finite-element method (FEM). The FEM analysis revealed that, even at highly deformed states at which almost the whole matrix has been yielded by the stress concentration, the ligament matrix between mbber inclusions in the stretching direction is locally preserved within an elastic limit and it acts as an in-situ formed adhesive for interconnecting mbber particles. 

The ease of deformation called plasticity characterizes the unvulcanized rubber compound, and causes the highly plastic rubber to flow easily. Viscosity is the resistance to plastic deformation or flow and, hence, the inverse of plasticity. Unvulcanized rubbers are not totally viscous but exhibit some elastic behavior. Rubber does not exhibit Newtonian flow as the shear rate is not proportional to the shear stress. Moreover, measurements of flow properties should be made at the shear rate of interest. 

Secondly, polymers are known to possess multilevel structures (molecular, topological, supermolecular, and floccular or block levels), the elements of which are interconnected [43, 44]. In addition, an external action on a polymer can induce the formation of new (secondary) structural elements — cracks, fractured surfaces, plastic flow regions, etc. These primary and secondary structural elements as well as the processes forming them are characterised by miscellaneous parameters therefore, only empirical correlations have been obtained, at best, between these parameters. If each of the above-mentioned elements (processes) is described by a standard parameter, for example, fractal dimension, one can derive analytical equations relating them to one another and containing no adjustable parameters. This is very significant for the computer synthesis of structure and for the prediction of properties and behaviour of polymeric materials during performance. Note that fractal analysis has been used successfully to describe the phenomena of rubber elasticity [16, 45, 46] and fluidity [25, 47-49]. 

According to rheology (the science of flow), viscous flow and energy elasticity are only two extreme forms of the possible types of behavior of matter. It is appropriate to consider the entropy-elastic (or rubber elastic), viscoelastic, and plastic bodies as other special cases. 

Once the glass transition temperature (7 ) has been exceeded, the intermolecular forces have become so weak that the influence of external forces can cause the macromolecules to slip apart from one another. The strength declines steeply, while the elongation leaps upward. In this temperature range, the plastic exists in a rubber-elastic or thermoelastic state. 

Plasticizers are employed as softeners in plastics. In a physical sense, softening means moving the glass transition temperature (Tg) in highly polymeric materials towards lower values, generally to a point below room temperature. Thus a rigid, brittle material becomes extensible, rubber-elastic (at room temperature)... 

In principle, as in metals it is possible to measure indentation magnitude after unloading or under load whereas the latter method is preferred for plastics and in the case of elastomers unavoidable due to the rubber-elastic redeformation. 

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

Improved adhesion is obtained by galvanizing, but this is only suitable for ABS polymers. When the plastic surfaces are pickled, the rubber elastic components are anodized. This produces pores and channels in which, for example, silver can be deposited chemically. The silver then forms the adhesive base for the copper layers subsequently deposited electrochemically, and these layers are then reinforced by the galvanized coating. Here, too, it is difficult to manufacture metal layer thicknesses of more than 10 fJLm because the different thermal coefficients of expansion of plastics and metals can easily lead to stresses, and thence to bubbles or cracks. 

Allen G, Bevington J (eds) (1989) Comprehensive polymer science, vol 6. Pergamon, Oxford Eisenbach CD, Baumgartner M, Guenter C (1986) In Lai J, Mark JE (eds) Advances in elastomers and rubber elasticity (Polyurethanes). Plenum, New Yrak Gachter R, Muller H (eds) (1993) Plastics additives handbook, 4th edn. Hanser, Munich Houben-Weyl (1987) Methoden der organischen Chemie, vol E20, Makromolekulare Stoffe, Thieme, Stuttgart... 

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