Thermosetting resins glass transition temperature

The glass transition temperatures, specific for each thermosetting resin, are used to characterize cure kinetics. They can be measured by many techniques, of which the widely used are Differential Scanning Calorimetry (DSC) and Torsional Braid Analysis (TBA)... 

SiLK resin is a solution of low molecular weight, aromatic, thermosetting polymer. The polymer s molecular weight and solution concentration were tuned to enable precise and convenient deposition by spin coating, a technique universally used by the industry for the deposition of photoresist materials. After deposition on a wafer, the polymer is thermally cured to an insoluble film that has a high glass transition temperature. The polymer has good mechanical properties at process temperatures, which is required for the application, and it is also resistant to process chemicals. 

The resin processed at 200°C reaches 100% cure because the glass transition temperature of fully cured epoxy is 190°C, less than the processing temperature. On the other hand, the sample processed at 180°C reaches 97% cure and the one processed at 160°C only reaches 87% cure. Figures 2.26 and 2.27 also illustrate how the curing reaction is accelerated as the processing temperature is increased. The curing reaction of thermally cured thermoset... 

Knot et al. (51) converted soybean oil to several monomers for use in structural applications. They prepared rigid thermosetting resins by using free radical copolymerization of maleates with styrene. The maleates are obtained by glycerol trans-esterification of the soybean oil, followed by esterification with maleric anhydride. They also synthesized several TAG-based polymers and composites and compared their properties. It was found that the moduli and glass transition temperature (Tg) of the polymers varied and depended on the particular monomer and the resin composition. They proposed that the transition from glassy to rubbery behavior was extremely broad for these polymers as a result of the TAG molecules acting both as cross-linkers as well as plasticizers in the system. 

In the early 1960s, a new class of addition polymer and addition poly(imide)s was developed by the Rhone Poulenc company. The most important of these were /jw-maleimides (BMI) [12, 13], which could be crosslinked or copolymerized to form thermosets with outstanding thermal and chemical resistance. These materials cure without volatile by-products, thereby, minimizing the formation of voids, and have high glass transition temperatures and low moisture absorption. The major uses of this class of resin is in advanced composites and printed circuit boards. 

Developing biphasic materials in order to improve the fracture toughness of thermoset resins is now a common practice. Thermoplastics that have a high glass-transition temperature (Tg) are used as tougheners in preference to low-Tg elastomers because of their insignificant effect on the thermal and modulus properties. 

Crosslinked cyanate ester systems typically exhibit higher glass transition temperatures (T S 250°C), lower moisture absorption and lower dielectric constants than conventional epoxy thermosets. Hence by mixing dicyanates or their corresponding prepolymers with epoxy resins, and then co-curing, the desired level of property improvements can be achieved. Such hybrid thermosets have been used in printed circuit board manufacture. 

Table 3.2 also includes data for the advanced thermoplastic resins (PEEK) and for a thermosetting resin, an end-capped bismaleimide (BMI) called PMR-15. Moisture contents tend to be lower for these advanced materials [12,13]. One way to overcome the environmental sensitivity of epoxide resins is to employ these advanced resins, as demonstrated in Table 3.2. However, changing to other resin systems brings with it other concerns. For example, PEEK relies on crystallinity for its higher temperature performance. Its glass transition temperature is only 143°C and a change in modulus can be observed at that temperature. In addition, higher process temperatures are required both for high performance thermosets and thermoplastics. The consequent higher residual thermal stresses can off-set some of the advantages of a higher service temperature, in comparison with advanced epoxy resins.

The combination with fibres has proved difficult however. Often there are issues with compatibility between bio-resins and fibres (both natural and synthetic), which cause defects in the composite structure and ultimately poorer physical properties. Castor-oil polyurethane was compared with phenolic resins when infused over sisal fibres however, the phenolic resins showed better structural performance when compared with the castor oil-based material [52]. This is not always the case, as some improvements have been made. Soybean oil thermoset polymers were used in a glass/flax hybrid composite resulting in improved mechanical performance [73], Thermoset resins were produced from triglyceride oils with a wide range of properties (tensile modulus 1-2 GPa, glass transition temperature Tg 70-120 °C) and glass- and hemp- fibre composites were manufactured [74,75]. 

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