Thermoset processing tests

The effects of cure on the viscosity of thermoset resins are monitored on the catalysed resin and are associated with the late injection, packing and curing cycles of a thermoset process. There are essentially two types of test methods, namely isothermal and non-isothermal tests, which are usually performed in dynamic or steady shear. 

Most of this book is concerned with the properties of the finished material, i.e., as it is used in products. However, another important area of polymer testing is concerned with the tests needed to predict and control the properties of relevance to the various stages of processing. Collectively, these can be termed processability tests, and the majority are measures of the flow properties of the polymer melt and also the curing characteristics of thermosetting materials. 

Fast pyrolysis of biomass provides a method for the production of phenolics that has the potential to replace at least 50% or more of the phenol in phenol-formaldehyde thermosetting resins. The gel tests indicate that the P/N fractions from pine sawdust pyrolysis with paraformaldehyde have shorter gel times than commercial plywood resins such as Cascophen 313, even without prepolymer formation. A novolak formulation has been prepared using 1 1 by volume of phenol and P/N fraction and about half of the amount of formaldehyde that would be used than if phenol alone were employed. Very promising resols have also been made with a similar substitution of the P/N fraction for phenol. Wood testing and resin formulation development are ongoing activities. The projected economics suggest that additional research and development of this process are fully warranted. 

The Arrhenius treatment has been applied to aging studies on rubber (13), to predict the life of a polyester-glass laminate (14), to predict product stability of a thermosetting resin alone and in combination with two additives (15), in permanence tests on paper (16), to multi-component systems in which the principal component is paper (17), and to study the influence of temperature on the relative contributions of the oxygen-independent and oxygen-dependent processes to the total rate of newsprint deterioration (18-20). 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

The purpose of this paper is to present a technique whereby manufacturing process dynamics for structural polymers can be accurately defined through efficient laboratory rheological characterization. Structural polymers, in this paper, refer principally to the thermosetting epoxides, phenolics and polyimides. This type of test pattern, however, is generally applicable to the production and utilization of most polymers. The engineering applications associated with these polymers involves primary and secondary aerospace articles. In this situation, failure to meet performance criteria could result in catastrophic loss of the vehicle and associated cargo. 

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. 

In BS 2782, Method 132B [175], the blistering propensity of thermosets is examined by heating a suitable molded test piece in a liquid bath at a rate of 120 C/hr. At a temperature some lOX below the expected blister temperature, a test piece is removed and visually examined for blisters. The process is repeated on separate test pieces at every 5 increment in temperature until the examined test piece shows signs of blistering. 

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