Curing of thermosets

Resin curing creates a three-dimensional, crosslinked molding compound. Even though the reaction mechanisms are different for the various thermosets, their curing reactions feature common aspects  

These characteristics have to be considered for processing. The processing method itself, e.g., temperature control, thus also controls properties and quality of the cured molding compound. Processing is essentially characterized by gelling and total resin curing. 

The point of conversion at which gelling occurs depends on the type of resin. The rule is that  

This can be explained by the fact that in addition reactions of EP or PF resins, curing takes place by functional groups reacting step-by-step according to their stoichiometric ratio. 

In resins that cure by polymerization, such as UP or VE resins, each molecule bears many double bonds (ten and more) and is thus very highly functional. Moreover, the radical chain reaches a very high chain length, thus linking many double bonds with each other and leading to reduced mobility even at a low conversion level [627]. 

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Dibutyltin and dioctyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as catalysts for the curing of room-temperature-vulcanized (RTV) sihcone elastomers to produce flexible siUcone mbbers used as sealing compounds, insulators, and in a wide variety of other appHcations. Diorganotin carboxylates also catalyze the curing of thermosetting siHcone resins, which are widely used in paper-release coatings. 

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

Nevertheless, with the development of new sensors, microdielectrometry constitutes a nondestructive and convenient method for monitoring in situ and in real time the cure of thermosets (Senturia et al., 1982 Kranbuehl et al., 1986). Great efforts have been made and research is still going on to establish relationships between (i) cure index and actual conversion of reactive groups, (ii) conductivity and viscosity, and iii) conductivity and glass transition temperature, in order to obtain models for smart processing. 

As a final example of modeling thermoset processing, the process called the shell molding or Croning process will be analyzed. This is just one of many processes found in a variety of industries (automobile, electronics, sports goods, furniture, etc.) that involve the cure of thermosetting polymers. The selected example is a typical process used in foundries. 

Important concepts in any analysis of the cure of thermosetting polymers are the possibility of reaching vitrification at particular locations in the part and the need to provide heat from an external source to devitrify the material and continue the cure. The analysis of vitrification cannot be avoided when the cure is performed at room temperature using different types of radiation (e.g., UV, EB, microwave) to initiate the polymerization. A temperature vs conversion transformation diagram (Chapter 5) may be useful in the design of cure cycles. 

Solid raw materials have to be transferred into a fluid or plastic state, which is, nearly always, attained by heating (in exceptional cases by solution). In the fluid state the material is shaped, after which the resulting shape is fixed by cooling (thermoplasts), or by a chemical reaction (curing of thermosets, vulcanisation of rubbers). 

Sheppard, Jr., N. F. Dielectric analysis of the cure of thermosetting epoxy/amine systems, Ph.D. Thesis, Massachusetts Institute of Technology, 1985, unpublished... 

Several researchers have described changes in the isothermal viscosity behavior during the cure of thermosetting resin systems with respect to time by the following expression ... 

An activated water based solution of magnesium chloride that provides rapid curing of thermosetting reactants. 

Wissbrun, K. etal. Model for correlation of rheological measurement with leveling and curing of thermosetting powder coating. J. Coat. Technol. 1976, 48, 42. 

Fig. 5 Schematic diagrams of effect of extent of reaction on viscosity and morphology in curing of thermosets. (A), Effect of extent of reaction on viscosity. (From Refs. " l) (B) Effect of extent of reaction on morphology. (From Refs. ° l)...

Li, M. Tucker, C.L. Optimal curing of thermoset matrix composites thermochemical and consolidation consideration. Polym. Compos. 2002, 23, 739-757. 

As previously mentioned many studies have been performed on microwave curing of thermosets, including polyesters, polyurethanes polyimides and epoxies. 

Relaxation processes during the curing (crosslinking) of networks have also been studied extensively, particularly with respect to the curing of thermosets... 

M.J. Richardson, Curing of thermosets, in Calorimetry and Thermal Analysis of Polymers (Ed. V. B.F. Mathot), Munich, 1994, Chap. 7. 

Vergnaud JM. and J. Bouzon. 1992. Cure of Thermosetting Resins. Springer Verlag, London. 

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