Rubber melting

Then, one day in 1839, Charles was holding a piece of rubber combined with sulfur while talking with friends near a hot stove.The rubber fell onto the stove, but instead of melting, it became hard Charles was amazed. Maybe a little bit of heat made rubber melt, but a lot of heat made rubber hard, yet still flexible. Here was the key to making rubber useable. 

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. 

Some additives are added to the rubber melt in the extender for avoiding the pellets to stick downstream during pelletisation, or storage (e.g. a wax). 

The viscosities of thermoplastic rubber melts are strongly non-Newtonian, decreasing as... 

Let us consider the expansion of the extrajwlated melt/rubber line from absolute zero. It was postulated that melt/rubber expansion was primarily due to free-volume expansion. The thermal expansion of the rubber/melt thermal expansion multiplied by the temperature also seems to reach a fixed value at the glass transition of about 0.164. 

No.20, 1998, p.4915-21 MATHEMATICAL MODELLING OF THERMOPLASTIC NATURAL RUBBER MELT FLOW IN A DOUBLE FEED EXTRUSION SYSTEM... 

With the increase of silicon dioxide content, the average cell diameter of silicone rubber foam decreases (Table 2). The foam density declines to a minimum value, then increases (Figure 2). This is also because with the increase of silicon dioxide content, the elastic modus of rubber melt increases, the bubble expanding is suppressed, and the expansion ratio is decreased. With... 

B. Rubber pellet feed —> rubber melt — addition of 80%... 

Fig. 32). Using a fine pipette insert about i cm. length of the liquid into the bottom of the tube. Now place in the tube A a fine inverted melting-point tube B of about i mm. diameter, sealed at the upper end. Fasten the capillary tube to the ther- Fio. 32. mometer by means of a rubber band and place in a melting-point apparatus. Heat slowly until a stream of bubbles rises from the bottom... 

J. A. Brydson, Flow Properties of Polymer Melts, 2nd ed., Godwin/Plastics and Rubber Institute, 1981. 

Prepa.ra.tlon, There are several methods described in the Hterature using various cobalt catalysts to prepare syndiotactic polybutadiene (29—41). Many of these methods have been experimentally verified others, for example, soluble organoaluminum compounds with cobalt compounds, are difficult to reproduce (30). A cobalt compound coupled with triphenylphosphine aluminum alkyls water complex was reported byJapan Synthetic Rubber Co., Ltd. (fSR) to give a low melting point (T = 75-90° C), low crystallinity (20—30%) syndiotactic polybutadiene (32). This polymer is commercially available. 

An unusual method for the preparation of syndiotactic polybutadiene was reported by The Goodyear Tire Rubber Co. (43) a preformed cobalt-type catalyst prepared under anhydrous conditions was found to polymerize 1,3-butadiene in an emulsion-type recipe to give syndiotactic polybutadienes of various melting points (120—190°C). These polymers were characterized by infrared spectroscopy and nuclear magnetic resonance (44—46). Both the Ube Industries catalyst mentioned previously and the Goodyear catalyst were further modified to control the molecular weight and melting point of syndio-polybutadiene by the addition of various modifiers such as alcohols, nitriles, aldehydes, ketones, ethers, and cyano compounds. 

Japan Synthetic Rubber Co. noncross-bnked melt flow iadex, 150°C, 2.160 is 3 g/10 min for all grades. 

Well, that is the case at the low temperature, when the rubber has a proper modulus of a few GPa. As the rubber warms up to room temperature, the Van der Waals bonds melt. (In fact, the stiffness of the bond is proportional to its melting point that is why diamond, which has the highest melting point of any material, also has the highest modulus.) The rubber remains solid because of the cross-links which form a sort of skeleton but when you load it, the chains now slide over each other in places where there are no cross-linking bonds. This, of course, gives extra strain, and the modulus goes down (remember, E = [Pg.61]

Fig. 6.2. How Young s modulus increases witl) increasing density of covalent cross-links in polymers, including rubbers above tbe glass temperature. Below To, be modulus of rubbers increases markedly because tbe Van der Waals bonds take hold. Above Tq they melt, and the modulus drops.

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. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

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