Rubbers amorphous

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

The Tg will determine the maximum temperature of use of the material as a rigid thermoplastic. For amorphous rubbers the will determine the minimum temperature. 

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

The underlying nonlinearity function m (A). which is independent of the type of deformation, is very similar for different amorphous rubbers. For SBR, it is independent of the cross-link" density over moderate changes in cross-link density (62) and independent of the temperature down to —40°C, a temperature where the modulus has increased by a factor of 2 to 3 over the room-temperature value (61). The function A) is insensitive to the presence of moderate amounts of carbon black filler for strains up to about 100% (63). 

The principles were refined by Meyer in a second paper (70). In it he proposed that the micelles occurred at regular intervals. He also included an explanation of the elasticity of rubber based on the assumption that the molecular chains tended to roll together in knots in unstretched rubber, but line up when stretched. This explanation was especially elucidating since it agreed well with Katz s discovery (53) that amorphous rubber crystallizes when stretched. 

Figure 9 shows SEM photomicrographs of ion-etched surfaces of blends of LDPE/amorphous Epcar 845 and of LDPE/crystalline Epcar 847. Nodule diameters of the LDPE/amorphous rubber blend average... 

One might expect the nodule diameter of pure LDPE to be the same as that in the amorphous rubber/LDPE blend. This could result if the same proportion of LDPE nucleated the crystals and if no amorphous EPDM lay inside the LDPE crystallites. However, the concentration of crystallites would be lower in the blend. It is impossible for us to measure the concentration of crystallites in this blend. The resolution is inadequate and the etching depth is not accurately known. We will have to look at blends containing less LDPE to see if the crystallite concentration decreases. No spherulites are seen in these blends by polarized optical microscopy. However, these nodules are too small for optical resolution, and may indeed be spherulites or aggregates of lamellae. 

TPEs are blends of various amorphous rubbers such as ethylene-propylene and of polyolefin semicrystalline plastics such as PP. They are part of the family of TP olefins (TPOs). TPOs are mechanical blends consisting of a hard plastic and softer rubber. They are considered different from blends that are dynamically thermoplastic vulcanized (TPV) a process in which the elastomer phase is cured during mixing of the polymers.84 94... 

The reinforcement of elastomers with particulate fillers is a process of great practical and technological importance. Most finished rubber articles are made from filled elastomers and, with a few exceptions, all amorphous rubbers which are incapable of crystallizing under strain require fillers to impart to them technologically useful mechanical properties. 

Bueche and Halpin (126, 203, 215-217) have developed a fracture theory for amorphous rubbers. Their model pictures rupture as the result of the propagation of tears or cracks within the material. The growth of a tear is viewed as a process in which molecular chains at the tip of the tear stretch viseoelastieally, under the influence of a high stress concentration, until they rupture. The failure process is a non-equilibrium one, developing with time and involving consecutive rupture of molecular chains. The principal result of the theory is embodied in the equation... 

Fig. 24. Schematic stress-strain curves of an amorphous rubber reinforced with various fillers. See text for identification of curves A to D...

Table 4.1 Amorphous rubber fractions versus loss relaxation areas (DMA analysis of PP/C2C3 rubber blends)...

Loss relaxation area versus amorphous rubber content... 

All these commercially produced BR systems are amorphous rubbers under atmospheric conditions. The Tg-value of these systems, depending on their structure, is described by the Gordon-Taylor relation, see Chapter 1. BR becomes a semicrystalline polymer under atmospheric conditions if the 1,4 trans-BR content is higher than about 70 %wt. or if a syndio-or isotactic 1,2-BR phase is present. This is shown by the results of thermo-analytical measurements on experimental BR systems with a high trans content and with a high syndiotactic 1,2-BR content which are reported in this chapter. Moreover, the Tg-values of two series of IR samples containing 1,2- and 3,4-IR are used to determine the Tg/structure relation for non-polar polymers with side-chains. 

Rather peculiar to the rubber industry is the use of the fine particle size reinfordng fillers, particularly carbon black. Fillers may be used from 50 phr to as high as 100-120 phr or even higher proportions. Their use improves such properties as modulus, tear strength, abrasion resistance, and hardness. They are essential with amorphous rubbers such as SBR and polybutadiene that has Kttle strength without them. They are less essential with strain-crystallizing rubbers such as NR for many applications but are important in the manufacture of tires and related products. 

With mixtures of alkoxy substituents having longer alkyl chains, crystallization can be avoided to produce an amorphous rubber. The product is referred to as phosphonitrilic fluoroelastomer, a semiorganic rubber. The rubber can be cross-linked with free-radical initiators or by radiation. A... 

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