Thermosetting systems, processing

Mechanism and Kinetics of the Cure Process in Tetraglycidyl-methane Dianiline-Diaminodiphenyl Sulphone Thermoset System... 

In the field of high thermomechanical performance polymers, linear and thermosetting systems offer complementary properties. Among the thermosetting materials, BMIs and BNIs have been extensively studied and are now commercially available. In this chapter, firstly the main preparation and characterization methods are reviewed, and then the chemistry of the polymerization processes is discussed for both families. For the BMIs, due to the electrophilic character of their double bond, different polymerization pathways have been published, which is not the case for BNIs. Special attention has been paid to thermal polymerization which has already been used in industrial achievements however, on the other hand, the structure of these materials has been considered for the purpose of establishing relationships between processability, stability and thermomechanical properties. 

Solid epoxy adhesive formulations can be processed to either a thermoplastic or a thermoset state. Solid epoxy resins of exceptionally high molecular weight (e.g., phenoxy) can be used without any degree of cure as a hot-melt type of adhesive. However, fully crosslinked, thermoset systems are generally employed in structural applications. 

The work described herein relates primarily to lamination and bonding processes. However, the techniques are generic to most forms of thermoset resin processing. In the discussion which follows many of the resin systems contain glycidyl amines. The bulk of the epoxy formulations used in the aerospace industry today are based on tetraglycidylmethylenedianiline, I (TGMDA) and with diaminodi phenylsulfone, II (DOS). Systems based on... 

Cross-Linking. A thermoset system is produced when a polymer is cross-linked under static conditions, as in a compression mold. This is the basis of the production of vulcanized articles or cross-linked polyethylene pipe and wire insulation. If the same polymer is lightly cross-linked while it is being sheared in the molten state, however, it will remain thermoplastic. If it is more heavily cross-linked during this process, the final product may contain significant quantities of gel particles, but the whole mass will still be tractable. 

Isothermal steady time tests are used to determine the gel point of a thermoset system as the point at which the shear viscosity tends towards infinity. In these tests the viscosity is measured as a function of time at a constant shear rate. This method has the following major disadvantages. Firstly, the infinite viscosity can never be measured due to equipment limitations and thus the gel time must be obtained by extrapolation. Secondly, shear flow may destroy or delay network formation. Finally, gelation may be confused with vitrification or phase separation since both these processes lead to an infinite viscosity (St John et al., 1993). However, some work by Matejka (1991) and Halley et al. (1994) has shown that extrapolation to zero values of reciprocal viscosity or normal stress (i.e. extrapolation to infinite viscosity and normal stress) can be used with some success. 

The prepregging process is basically similar to that of a thermoset system . The set up is shown in Figs. 1 and 2. The impregnation of the fibre yarns is performed in a small resin chamber which contains the ABS solution of a... 

Unsaturated polyester thermoset systems use styrene as a reactive dilnent (sometimes along with substituted styrenes such as vinyl styrene or methyl methacrylate). Some of the styrene evaporates in the process (the emission factor is estimated at about 1-3% of the styrene for composites and a little higher for continuous lamination) and poses a threat as a hazardous air pollutant. The time-weighted average (TWA) for styrene is 50 ppm (or 213 mg/m ). In 1996 the Occupational Safety and Health Administration (OSHA) endorsed a proposal by the styrene industry to voluntarily adopt the 50-ppm exposure limit. 

Solution coating n. Any coating process employing a solvent solution of a resin, as opposed to a dispersion, hot-melt or uncured thermosetting system. 

Effective use of a thermosetting system requires prediction of the cure kinetics of the system [17] to consistently obtain the maximum and also to predict the flow behaviour of the curing resin, in particular to precisely locate when the sol-gel transition occurs. This is because the polymer can be easily shaped or processed only before the gel point, where it can still flow and can be easily formed with stresses applied relaxed to zero thereafter. Accurate knowledge of the gel point would therefore allow estimation of the optimum temperature and time for which the sample should be heated before being allowed to set the mould. The gel point can also be used to determine the activation energy for the cure reaction of the system [18]. 

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