Natural rubber melting temperature

While the melting point of undeformed natural rubber is just over room temperature, the melting point of crystallites in stretched samples is much higher. At the highest elongations possible, natural rubber melts at about 100 C (Greensmith et a/., 1963, esp. p. 264), placing a temperature limitation on this mode of reinforcement. 

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

Elastomer-plastic blends without vulcanization were prepared either in a two roll mill or Banbury mixer. Depending on the nature of plastic and rubber the mixing temperature was changed. Usually the plastic was fed into the two roll mill or an internal mixer after preheating the mixer to a temperature above the melting temperature of the plastic phase. The plastic phase was then added and the required melt viscosity was attained by applying a mechanical shear. The rubber phase was then added and the mixture was then melt mixed for an additional 1 to 3 min when other rubber additives, such as filler, activator, and lubricants or softeners, were added. Mixing was then carried out with controlled shear rate... 

When we compared the viscosities of solutions of natural rubber and of guttapercha and of other elastomers and later of polyethylene vs.(poly)cis-butadiene, with such bulk properties as moduli, densities, X-ray structures, and adhesiveness, we were greatly helped in understanding these behavioral differences by the studies of Wood (6) on the temperature and stress dependent, melting and freezing,hysteresis of natural rubber, and by the work of Treloar (7) and of Flory (8) on the elasticity and crystallinity of elastomers on stretching. Molecular symmetry and stiffness among closely similar chemical structures, as they affect the enthalpy, the entropy, and phase transitions (perhaps best expressed by AHm and by Clapeyron s... 

Attempts have been made with some success to produce other polymers that exhibit this property of natural rubber. Although the melting temperature can be matched by appropriately disrupting the crystallizable structure through controlled introduction of another monomer, an exact match is not possible because the extent of crystallinity and the kinetics of crystallization will differ. 

It is important in this or any other heat stability test to mold the natural resin as a control. Some resins, especially acrylonitrile butadiene styrene (ABS), darken considerably when heated above a particular threshold temperature. In the case of ABS, the butadiene rubber component may begin to go yellow and dark over 450°F. Although the customer may set its machines below 450°F barrel temperatures, shear heating or heating in the hot runner manifold could yield local melt temperatures in excess of 500°F and burn the natural resin. 

This compounded cured elastomer or rubber99 shares with all the other methyl silicone products the common characteristic of exceptional thermal stability. The material does not melt when heated in air at 300° C., which is far above the decomposition temperature of natural rubber or of any of the synthetic organic elastomers. Service over long periods of time at 150° C. does not destroy its elasticity. 

Caffrey and Bilderback have made a similar study for natural rubber. Using a Vidicon camera they concluded that the amorphous halo disappears while the preferentially oriented powder pattern appears at the same time. Holl et all., have studied the reversibility of this process in more detail. Thus in Fig. 51 the variation of the modulus and the draw ration are compared with selected Vidicon patterns. corresponds to the onset of the crystallization, 3- to the maximum in crystallisation and 3-i to the melting of the last crystallites upon relaxation. Note that and Xj, occurr, at different draw ratios. This is obviously due to the nucleation process which demands a certain overdrawing while the melting occurs at the equilibrium melting temperature. 

Mixing of natural rubber with polyolefin is one of the methods used to prepare thermoplastic natural rubber (TPNR). TPNR behaves like vulcanized rubbers at ambient conditions, but at elevated temperatures they melt and flow like a thermoplastic material. Thus TPNR could be processed using conventional thermoplastic processing machinery without requiring vulcanization, and these materials could be reprocessed. Thus there is low level of wastage, as scrap too can be recycled. 

Cold-seal adhesives are members of a general category sometimes termed coadhesive substances that have a great tendency to stick to themselves, but often not to much else. Cold-seal adhesives are typically based on natural rubber, which has been applied in a latex (suspension in water) form. One major application is in plastic packaging for chocolate candy. The seal between the two parts of the wrap can then be activated with pressure, as an alternative to heat-sealing, which is problematic with the low-melting temperature candy. 

Figure 10-30. Thermal conductivity k of natural rubber (NR), poly(oxyethylene) (PEOX), and poly (ethylene) (PE) of various densities as a function of temperature. Tg is the glass transition temperature, Tm is the melting temperature. (From data of various authors in the compilation of W. Knappe.)...

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