Structure-property relationship curing temperature

University of Maryland. He is currently Associate Director of Research at Loctite. His main research interests are the structure-property relationships of new materials and the study of new, ambient temperature, cure systems. 

Recently, because high-performance epoxy resin is strict in its requirements, polyfunctional epoxy has been offered in practical fields. In particular, epoxy novolac resin (ENR) is largely used as electronic encapsulation material because of its well-known thermal resistance properties. Because the strucmres of ENR exert a significant influence on the properties of the cured resins, it is necessary to understand their structure-property relationship. Despite various advantages, epoxy needs modifications to overcome some crucial disadvantages like limited solubility in polar solvents, higher cost of bisphenol A-based epoxy and maximum service temperature of only about 100 °C. 

Dynamic mechanical analysis is the most widely used technique for the investigation of mechanical properties and the structure-property relationships in polymeric materials. The dynamic mechanical results expressed as storage modulus ( ), loss modulus ( ") and loss tangent (tan S) in the function of temperature demonstrate for example the phase composition, phase transition with glass transition temperature and the structural relaxation processes. The phase segregation in the cured UPRs with an increase in styrene concentration and the dependence of glass transition temperature of UPRs... 

Solvent absorption measurement has been shown to be a sensitive and useful test method in the manufacture of epoxy powder coatings. A test method was defined and the effects of time and temperature of immersion described. It was shown that solvent absorption is a measure of raw material properties (EEW of the epoxy resin, and CTBN elastomer type and concentration), the homogeneity of the extrudate, as well as the state of cure. The information obtained from solvent absorption measurements has proven to be extremely important not only in quality control analysis but also in providing an insight into the structure function relationships in epoxy resin chemistry. 

A fundamental property that determines the state of a reacting system is its extent of cure or chemical conversion (a). Several papers have shown that there is a unique relationship between the glass-transition temperature (Tg) and a that is independent of cure temperature and thermal history. This may imply that molecular structures of materials cured with different histories are the same or that the changes in molecular structure do not affect Tg. There are generally accepted to be two approaches to modelling glass-transition-conversion relationships, namely thermodynamic and viscoelastic approaches. These are summarized in Table 3.8. 

These studies pointed out that a comprehensive model of thermoset cure for stress calculations must account for a large number of processing influences and material properties. Mass transfer (22.), chemical kinetics, network structure formation, and material property development are essential ingredients. Induced strains must be accurately calculated, as must stress relaxation. Properties dependencies on temperature are significant and must be accounted for, as must the inter-relationship between reaction kinetics and diffusion. 

The final physical properties of thermoset polymers depend primarily on the network structure that is developed during cure. Development of improved thermosets has been hampered by the lack of quantitative relationships between polymer variables and final physical properties. The development of a mathematical relationship between formulation and final cure properties is a formidable task requiring detailed characterization of the polymer components, an understanding of the cure chemistry and a model of the cure kinetics, determination of cure process variables (air temperature, heat transfer etc.), a relationship between cure chemistry and network structure, and the existence of a network structure parameter that correlates with physical properties. The lack of availability of easy-to-use network structure models which are applicable to the complex crosslinking systems typical of "real-world" thermosets makes it difficult to develop such correlations. 

Studies of the dependence of EEC properties obtained as above on the formation method showed that the precipitated finely dispersed rubber that is not bonded to the epoxy matrix does not endow it with high mechanical characteristics. When reactive oligomers are applied with high-temperature cure, there is a sharp enhancement of the modifying efficiency, accounting for the more intensive separation of the phases in the system and for the increase of the yield of the epoxy rubber copolymer. Thus, the structure and properties of epoxy resins modified by rubbers are mainly determined by the mode of system curing, but the quantitative relationships between curing mode and final properties have not been sufficiently studied. 

This result is consistent with the study of Barral et al. that showed the relationship between the structure and the mechanical properties of the epoxy matrix as a function of the degree of cure [38]. It explained that etherification reactions are important in the highest temperature treatment of epoxy resin and influenced the mechanical properties of the matrix. The higher activation energy for a polymer matrix cured at high temperature is attributed to the higher crosslink density of the network, which diminishes with the availability of molecular sized holes in the polymer structure. 

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