Bonding solid rubber

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. 

A tyre which is composed of solid rubber bonded to a suitable wheel and without an internal air space, as in the pneumatic tyre or an internal cavity as in the semi-solid tyre. The main uses of solid tyres are on forklift trucks and armoured fighting vehicles (tanks). 

The mat is a smooth unicellular base bonded to a solid rubber or plastic top covering. 

It is also possible to bond silicone rubbers to various substrates which are not injection moulded. Such materials range from steel and aluminium to ceramics, glass or any other solid. 

The adhesion of RFL-coated tire cords to rubber can be adversely affected if the dipped cords are exposed to ozone, UV light, nitrogen oxides, sulfur dioxide, or air before vulcanization into rubber. lyengar proposed that ozone exposure of RFL reduces adhesion because ozone attacks the double bonds of the butadiene component of the rubber latex and impairs its cocuring with the solid rubber compound. Infrared studies by Solomon reinforced this argument. When typical RFL films were exposed to ozone, the IR spectrum showed an increase in IR absorption at 1720 cm corresponding to an increase in the carbonyl content in the exposed film. An RFL film with no ozone exposure did not show this absorption at 1720 cm The increased carbonyl content is due to the reaction of some double bonds in the rubber with ozone and therefore, would leave fewer unsaturation sites for rubber crosslinking and adhesion. 

The reason why I take up these polymers in this review is that (1) CB-BR is a cure type solid rubber carrying OH groups and Cl atoms as active species besides double bonds, (2) viologen elastomers are thermoplastic elastomers (TPE) derived from liquid rubber, indicating some characteristic behaviors, i.e., high tensile properties and special functions, and (3) ABA type liquid crystalline elastomers are also new TPE at room temperature, but behave as liquid rubber at high temperature. 

Chem. Descrip. Polymerisate of unsat. aromatic C8/C9 hydrocarbons Uses Dispersant, tackifier for solid rubber and bonding solutions Features Improves dispersion char. 

A comparison of these predicted values of E with the measured values plotted in the bar-chart of Fig. 3.5 shows that, for metals and ceramics, the values of E we calculate are about right the bond-stretching idea explains the stiffness of these solids. We can be happy that we can explain the moduli of these classes of solid. But a paradox remains there exists a whole range of polymers and rubbers which have moduli which are lower - by up to a factor of 100- than the lowest we have calculated. Why is this What determines the moduli of these floppy polymers if it is not the springs between the atoms We shall explain this under our next heading. 

All polymers, if really solid, should have moduli above the lowest level we have calculated - about 2 GN m - since they are held together partly by Van der Waals and partly by covalent bonds. If you take ordinary rubber tubing (a polymer) and cool it down in liquid nitrogen, it becomes stiff - its modulus rises rather suddenly from around lO GNm" to a proper value of 4GNm . But if you warm it up again, its modulus drops back to 10 GNm . ... 

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]

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. 

Solvent-borne adhesives. Although the NR polymer is inherently tacky, tack-ifying resins are generally added to improve bonding to polar surfaces. Because the solids content in these adhesives is lower than 35 wt%, they are not suitable for gap filling. The quick-grab (cements) adhesives are particular because they contain about 65 wt% rubber, and set within a few seconds under finger pressure. 

Nitrile rubber/phenolic resin blends. Blends of equal parts by weight of a nitrile rubber and a phenolic resin in methyl ethyl ketone (at a 20-30 wt% total solids content) is suitable for many adhesive purposes. The more phenolic resin in the formulation, the greater the bond strength and brittleness of the NBR adhesive [67]. Table 10 shows the effect of phenolic resin on nitrile rubber properties. On the other hand, the higher the acrylonitrile content in the rubber. 

We conclude that high internal stresses are generated by simple shear of a long incompressible rectangular rubber block, if the end surfaces are stress-free. These internal stresses are due to restraints at the bonded plates. One consequence is that a high hydrostatic tension may be set up in the interior of the sheared block. For example, at an imposed shear strain of 3, the negative pressure in the interior is predicted to be about three times the shear modulus p. This is sufficiently high to cause internal fracture in a soft rubbery solid [5]. 

---