The Investigation of Curing Reactions with IsoStep

Introduction 

Curing reactions play an important role in the manufacture of polymeric materials. Besides so-called prepregs and coatings, composites are worth mentioning here in particular. Composites are an important class of materials because of their mechanical properties and low weight. They usually consist of glass fiber or carbon fiber material that is held together by a cured resin. The resin and the fiber give the material its characteristic properties. Composite materials are often used in areas where safety is of major importance, e.g. in the construction of aircraft and automobiles. This means that the curing behavior of the resin must be precisely monitored and characterized.

Fully cured material is relatively hard and might even be unsuitable for some applications because of its brittleness. In this case, material that is not completely cured would be preferable. Incompletely cured material can, however, undergo changes over a long period of time, which in turn results in the material properties changing. An important criterion for the determination of the degree of cure of a material is the glass transition temperature. Material with a higher degree of cure has a higher glass transition temperature than material with a lower degree of cure. An accurate DSC measurement of the glass transition temperature is however difficult, or even impossible, if the curing reaction takes place at the same time because the exothermic reaction peak then overlaps the glass transition. Vitrification can also occur, particularly with resins that cure at higher temperatures. Here the resin changes from the liquid to the glassy state and the reaction rate decreases significantly.

Usually, one wants to characterize the material, and to determine the limiting conditions for the curing process and for the material’s practical use afterward. Typical questions are for example:

• What is the relationship between the glass transition temperature and the material properties? 
• Under what conditions does vitrification of the material occur?
• Under what conditions does incompletely cured material continue to cure during storage or use? 

Especially for applications where safety is involved, the manufacturing process must ensure that uniformly cured material is produced, and that the long-term stability of the material under defined conditions is guaranteed. In the following example, the curing behavior of a two-component epoxy resin consisting of the diglycidylether of bisphenol A (DGEBA) and diaminodiphenylmethane (DDM) as hardener or curing agent were investigated. If the reaction takes place slowly, the material has sufficient time to form more cross-links and the glass transition temperature shifts to higher and higher temperatures.

If the glass transition temperature is higher than the actual reaction temperature, vitrification can suddenly occur. The curing reaction in the now solid material slows down significantly [1]. This can, for example, lead to problems in thermally forming or molding equipment. If certain regions heat up too slowly, the temperature is not uniform throughout, and the curing process is incomplete. To determine whether a material vitrifies with DSC, the heat capacity curve has to be separated from the reaction peak. The measurements presented here were performed with the new IsoStep™ method and the results compared with those obtained from conventional DSC.

Measurement and results 

The samples were measured twice, once relatively slowly and once somewhat more rapidly. The segments had isothermal periods and heating times of 60 s and 30 s and incremental temperature changes of 0.5 K and 1 K respectively. The conventional DSC curve was measured at the same average heating rate. The results for the sample heated relatively rapidly are shown in Figure 1: the DSC curve has a glass transition at about -20 °C and a curing peak with a maximum at about 115 °C. The heat capacity curve increases gradually during the reaction with the greatest increase at the highest reaction rate (DSC peak). This is due to increased molecular movement at the cross-linking points [2]. Vitrification would become apparent through a sharp decrease in the heat capacity.

Conclusions 

The IsoStep™ method is an excellent technique for separating glass transitions and overlapping kinetic processes such as curing reactions, crystallization and vaporization processes. The investigation of the curing reactions of resin systems is particularly important. It enables the materials used to be characterized, the production conditions to be determined, and yields information on long-term behavior and aging. Polymer resins are nowadays used in so many important materials such as composites, coatings and prepregs.

_{The Investigation of Curing Reactions with IsoStep™ | Thermal Analysis Application No. UC 155  | Application published in METTLER TOLEDO Thermal Analysis UserCom 15}