Adhesive formulations glass-transition temperature

Typical glass transition temperatures for adhesive resins are shown in Table 3.9. Note, however, that the Tg for epoxy adhesives can vary significantly with their formulation. Glass transition temperatures for several epoxy formulations are shown in Table 3.10. 

Perhaps, the earlier materials found to have a useful capacity for adhesive bonding underwater depended upon the use of a stoichiometric excess of water-scavenging polyamide hardener in an epoxide-based adhesive. This approach can lead to the production of effective joints in the short term, but formulations of this type, which are hydrophilic in the uncured state, are also likely to absorb significant amounts of water in the cured condition. It is a widely accepted view that the extent of joint weakening in susceptible joints, quite apart from the consequences of plasticization, is a function of the water-uptake characteristics of the adhesive (see Glass transition temperature). The consequence is therefore likely to be that such joints will show poor durability in the presence of water, when rapid uptake of water may lead to equally rapid degradation of both cohesive and interfacial properties (see Durability fundamentals). 

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. 

Perhaps the most significant property that is controlled by the degree of crosslinking is the glass transition temperature Tg. The importance of Tf in epoxy adhesive formulations is discussed next. 

Glycidyl amine epoxy resins are reaction products of aromatic amines and epichlorohy-drin. They have high modulus and high glass transition temperature. These resins find use in aerospace composites and high-temperature adhesive formulations. 

Free Radical Cure UV Adhesives. As with any adhesive, formulation variables are critical to the processing and performance characteristics. Variables such as oligomer selection, modifiers and additives, monomer structure, molecular weight, and glass transition temperature directly affect application and performance properties. 

It can be expected, then, that one of the major problems in adhesives technology is the development of adhesives that must withstand both elevated temperatures as well as periodic excursions to low temperatures. Several solutions have been developed. Certain adhesive systems, notably blends of epoxy resin with more elastic resins, have been formulated with a very broad glass transition temperature range or with multiple glass transitions at both high and low temperatures. These have found some success in the applications discussed in this chapter. 

Table 3 Influence of the plasticizer in the coating formulation on the force of adhesion and the glass transition temperature (Tg) of an acrylic resin copolymer (standard deviation in parentheses)...

Most paint formulations consist of disperse systems (solid in liquid dispersions) [2]. The disperse phase consists of primary pigment particles (organic or inorganic) which provide the opacity, colour and other optical effects these are usually in the submicron range. Other coarse particles (mostly inorganic) are used in primers and undercoats to seal the substrate and enhance adhesion of the top coat The continuous phase consists of a solution of polymer or resin which provides the basis for a continuous film that seals the surface and protects it from the outside environment Most modem paints contain latexes which are used as film formers. These latexes - which typically have a glass transition temperature (Tg) below... 

As was noted earlier, virtually any elastomeric material can be, and probably is currently, made into an adhesive or sealant. All elastomers are soluble in one or more organic solvents which allows the production of a solvent-based adhesive. Many elastomers are polymerized in an emulsion process, which yields a latex. And a number of elastomers have a low enough glass transition temperature to permit use as a hot-melt polymer, or as one ingredient in a hot melt formulation. Summary statements describing a few of these other polymers follow, but without the detail provided for the primary elastomers. 

PVC melts have a dry appearance, little strength, and lack of adhesion to a metal surface. Process aids are designed to change all these properties. Almost all rigid formulations and many flexible ones use acrylic process aids. Acrylic process aids are copolymers of methylmethacrylate with ethyl acrylate, butyl acrylate, or butyl methacrylate.Selection of monomers is important because it affects the glass transition temperature of copolymer, which should be lower than that of PVC to cause additive to melt below PVC process temperature and thus help to process it. 

In adhesive joints, for example with epoxy resin adhesives, internal stress in the adhesive layer may result from different coefficients of expansion in the glued materials, whereby their moduli of elasticity are important factors. The glass transition temperature, and thus the curing temperature, also play a role. The reaction shrinkage of the resin is another source of internal stress. Suitable formulations with added fillers, oligomers or copolymers are among the measures taken to reduce these influences. 

Other modifiers Although they are not common, additional ingredients can be added to polychloroprene adhesive formulations to improve specific properties. Plasticizers can decrease the glass transition temperature, influence crystallization tendency and reduce cost. Highly aromatic mineral oils can be used when reduction in crystallization rate is required. Stearic acid, in concentrations of 0.5-1 phr, improve processability and reduce mill sticking. Resorcinol type resins improve adhesion to textiles and metals. 

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