Rubbery adhesives temperature

Inspection of Figure 5 shows a very broad glass-to-rubber transition range which extends from below -100°C to above 0°C for the polyurethane adhesive. The relaxation modulus E(t) - 400 Kg/cm which occurs at the rubbery inflection temperature - 40°C - 313 K describes an effective molecular weight M as defined by kinetic theory of rubber elasticity ... 

In the case of a rubbery adhesive the work of Gent and Kinloch [11] has shown that the adhesive fracture energy, Gc, is independent of the type of specimen. They used several of the joint geometries illustrated in Fig. 7.7 and confirmed that the value of Gc, when measured for a given value of temperature and crack velocity, is independent of the details of the test specimen. 

Rubbery adhesives and rubber-modified structural adhesives have lower temperature limits (the glass transition temperature). In many case, the development of thermal mismatch stresses upon cooling will increasingly weaken joints. 

Thermal properties of adhesives affect their behavior in a number of diverse ways. Firstly cure processes, pot life and shelf life depend on temperature. High temperatures can limit the lives of adhesives by causing chemical degradation and in some cases by introducing stresses due to the differences in thermal expansion. Glass transition is the most important temperature for an adhesive, in that it demarcates rubbery adhesives from rigid ones, and is something to be avoided in service. 

It is important to note that structural adhesives as a family of polymers are usually used at temperatures below their Tg (Adams et al. 1997), except the rubbery adhesives such as 3532. It is unlikely, therefore, that temperature can be used as a device to enhance damping for structural adhesives. 

They are used as high-temperature structural adhesives since they become rubbery rather than melt at about 300°C. 

Ethylene has also been copolymerised with a number of non-olefinic monomers and of the copolymers produced those with vinyl acetate have so far proved the most significant commercially . The presence of vinyl acetate residues in the chain reduces the polymer regularity and hence by the vinyl acetate content the amount of crystallinity may be controlled. Copolymers based on 45% vinyl acetate are rubbery and may be vulcanised with peroxides. They are commercially available (Levapren). Copolymers with about 30% vinyl acetate residues (Elvax-Du Pont) are flexible resins soluble in toluene and benezene at room temperature and with a tensile strength of about lOOOlbf/in (6.9 MPa) and a density of about 0.95 g/cm. Their main uses are as wax additives and as adhesive ingredients. 

Similar to the tackifiers discussed earlier, plasticizers have a very dramatic softening effect on the rubbery plateau modulus of the PSA. For this reason, high levels of plasticizers have to be avoided to maintain good cohesive strength in the adhesive, especially at elevated temperatures. Indeed, if high cohesive strength is desired, the amount of plasticizer used in a PSA is typically kept to a minimum, if used at all. 

Poly(vinyl ethyl ether) is soluble in ethanol, acetone and benzene. It is a rubbery product which may be cross-linked by heating with dicumyl peroxide. Polyfvinyl isobutyl ether), has a glass transition temperature of —5°C. It has been used as an adhesive for upholstery, cellophane and adhesive tape. 

Plasticizers and flexibilizers are incorporated into an adhesive formulation to provide it with flexibility and/or elongation. Plasticizers may also reduce the melt viscosity of hot melt adhesives or lower the elastic modulus of a solidified adhesive. Similar to diluents, plasticizers are nonvolatile solvents for the base resin, and by being incorporated into the formulation, they separate the polymer chains and enable their deformation to be more easily accomplished. Plasticizers generally affect the viscoelastic properties of the base resin whereas diluents simply reduce the viscosity of the system. Whereas diluents result in brittle, hard adhesive systems, plasticizers result in increased flexibility and lower modulus. The temperature at which polymers exhibit rubbery properties (i.e., the glass transition temperature) can also be modified by incorporating plasticizers. 

Tiquid rubbers are polymeric products used primarily as adhesives, sealants, castable rubbers, and rocket propellant binders. The name liquid rubber comes from the properties of flowing at room temperature and curing to rubbery networks. These materials offer advantages over conventional elastomers in their ease of handling and processing as they can be readily pumped and mixed in low power (relative to normal rubber) equipment with resultant savings. 

This type of adhesive is generally useful in the temperature range where the material is either leathery or rubbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mostly uncompounded state. 

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