High-temperature curing

Adhesives. Because of exceUent adhesion to many substrates, epoxy resins are extensively used for high performance adhesives. These can be categorized into high temperature curing systems (soHds and Hquids) and room temperature curing systems (Hquids). 

Since the functionality available for curing most commercial PF resins is likely to consist mainly of o-hydroxymethyl units, it is obvious that these groups do condense al signihcant rates under the high temperature curing conditions that... 

Application of adhesive primer presents another challenge. Most adhesive primers require a high-temperature cure, which presents the potential for damaging existing bondlines. Infrared curing can be used to reduce the likelihood of problems. 

In order to achieve high-temperature cure possibilities, the use of Perkalink 900 is reported [86]. 

The present study reports the synthesis, characterization and thermal reactions of phenyl and carbomethoxy substituted norbornenyl imides. These substrates were designed to model the reactive end-caps of the PMR-15 resin and allow an assessment of the effect that conjugating substituents would have on the high temperature cure of such systems. The effect of these substituents on both monomer isomerization and polymerization is reported and a possible use of the phenyl substituent as a probe of polymer structure is suggested. 

Appropriate amounts of x y resin and liquified amine were mixed together and evacuated at 60 c in a vacuum oven for t i minutes to remove air bubbles. The reaction mixture was poured into a hot (85°C) aluminum mold vhich was prepared in advance by lic tly spraying mold release agent (ISi 515, Green Chem. Products Inc.) to the inner surfaces of the mold. Samples were cured two hours at 85 C followed by two hours at 150 c (standsuxl cure). In some cases samples were subjected to eui alternate, high temperature cure of two hours at 85 C and two hours at 175 C (HT cure). After curing, samples were stored in a desiccator until use. 

As can be seen, some larger fragments are seen in the sample held at elevated temperature, and sodium has segregated to the surface. Such segregation is very common in high temperature cured specimens, where sodium is often found at the failure surface in an adhesive failure mode. ISS/SIMS data from the adhesive side of a titanium-epoxy failure interface from a tensile test specimen are shown, in Figure 8. 

Postcuring at elevated temperatures after a room temperature cure is a common process in epoxy technology, and this can moderately increase the Tf in some systems.26 Such effects could be due to secondary reactions (irreversible) or to free volume effects (reversible). These effects could also be realized during the normal aging of the epoxy system in service. One should be careful, however, in assuming that a low-temperature cure followed by an elevated-temperature postcure (cure condition 1) will provide properties equivalent to only a high-temperature cure (cure condition 2). As explained in the previous section, the types... 

To develop the properties of an epoxy novolac to its fullest extent, a high-temperature cure is necessary. With room temperatures cures, the properties of the final product are similar to those of conventional DGEB A systems. The thermal stability of most epoxy novolac systems is affected markedly by the length of the cure cycle. 

DADS melts at 135°C and is employed stoichiometrically with DGEBA at 33.5 pph. Fortunately, it is relatively unreactive so it can be mixed with epoxy resin at elevated temperatures. It can also be used in epoxy solutions to provide an adhesive formulation for manufacturing supported or unsupported film with long shelf life. Because of the low reactivity of the system, DADS is generally employed at a concentration that is about 10 percent greater than stoichiometry, or an accelerator, such as BF3-MEA, is employed at about 0.5 to 2 pph. When DADS is mixed with liquid DGEBA resin, it provides a pot life of 3 h at 100°C and requires a rather extended high-temperature cure to achieve optimal physical properties. 

The high-temperature curing processes generally used... 

Table VII summarizes the plant data where this finish was applied to a 3 oz./sq, yd. printed polyester/cotton fabric. In the first and second trials (columns 1 and 2), the dryer temperature was maintained at 350°F. in order to overcome a heat set put into the fabric during a high temperature curing of the printed pigments. In the third trial (column 3) where the dryer was used only for drying the fabric, it was possible to operate the dryer at its lowest temperature and still increase range speed.

Epoxy resins produced by the reaction of bisphenol A and epichloro-hydrin are versatile polymers with several useful properties (subsection 2.2.2.1). However, one significant weakness is their brittle nature. Incorporation of plasticisers is not very useful. Dibutyl phthalate is an exception, showing good compatibility but offering only limited ability to flexibilise the resin. Moreover, plasticisers affect the mechanical properties and chemical resistance of the cured system. With polyurethanes it is possible to complement the flexibility of the epoxy system. Numerous attempts have been made to combine the two types to achieve beneficial modifications (Lee and Nivelle, 1967). These modifications proved successful under high-temperature cure but inferior results were obtained for ambient cures. 

Crosslinked rubbers follow the same description presented for thermoplastic compounds. A major problem with the use of organic pigments and dyes is that many decompose when high temperature curing agents are used. For these compounds, inorganic pigments are typically the only option. 

---