Coatings Clinic: differential scanning calorimetry.

Last month, I introduced a number of thermal analysis techniques. This month, I am discussing one of them, differential scanning calorimetry (DSC), in more detail. A calorimeter is an apparatus that measures quantities of heat. A differential calorimeter is one that measures the temperature or heat of a material compared to a reference. A DSC instrument measures the difference in temperature as a material and a reference are heated side by side or the difference in the heat input necessary to keep the specimens and the reference at the same temperature as they are heated. The instrument can be used for measuring the thermal properties and thermal transitions of coatings or components, such as heat capacity, glass transition temperature ([T.sub.g]), crystallization temperature, melting point ([T.sub.m]) heat of fusion, and heat of reaction. DSC also can be used to monitor polymerization reactions and crosslinking processes and to characterize the unblocking temperature of blocked catalysts and blocked crosslinkers.

The measuring head of a DSC contains a compartment that has specimen and reference sensors located adjacent to each other. The usual practice is to place an empty aluminum pan and cover on the reference sensor. A 10-15 mg specimen of the test material in an equivalent covered aluminum pan is placed on the specimen sensor. The compartment cover is put into place, the heating is turned on at a programmed rate and a plot of heat flow versus temperature is produced (even if the instrument is measuring temperature difference) such as that shown in Figure 1. The figure describes a polymer that goes through a glass transition, [T.sub.g], around 60 [degrees]C (change in baseline due to change in heat capacity), partially crystallizes, [T.sub.crys], around 130[degrees]C (exothermic spike), then the crystalline portion melts over a range of temperature that peaks, [T.sub.m.] at approximately 215[degrees]C (broad endothermic peak). The areas under the peaks give the amount of heat given off or absorbed. Figure 1 is an example of a second heating scan on a given specimen, a strategy that often is useful because it gives a more distinctive [T.sub.g] and is more likely to pick up any crystallization behavior. In this case, the original specimen probably was cooled rapidly (quenched) after melting, so that it either did not recrystallize or only did so partially. Heating allowed it to crystallize and then it melted. Since low [T.sub.g] indicates relatively high resin flexibility, which in turn usually means low coatings viscosity at high solids, DSC often is used to characterize resins.

[FIGURE 1 OMITTED]

Following of polymerization reactions by DSC is a fairly simple process. The components are mixed and placed in the pan and it and the reference are heated. Most reactions give broad exotherms, but an autocatalytic (out of control) reaction that would be disastrous during resin manufacture shows up as a large, very sharp spike on the heat flow-temperature plot. The area under the peak gives the heat of the reaction. A number of publications recommend using DSC to follow cure. Crosslinking usually produces an exotherm, but it often is very broad, occurring over a wide range of temperatures, and is difficult to distinguish from the baseline, especially if solvent is being lost at the same time (an endothermic process). I have had very little success with monitoring cure by DSC except for powder coatings, which give excellent results. In addition, DSC instruments that are equipped with UV sources and configured so that the beam can be directed onto the specimen, work well for following UV cure. A newer type of DSC called modulated DSC should allow better monitoring of the cure of coatings during solvent evaporation. Modulated DSC separates kinetic effects from heat capacity effects. For example, it allows the measurement of [T.sub.g] when a reaction or molecular relaxation is going on and separates decomposition from crosslinking. It also tends to give more distinctive glass transitions, particular for copolymers or polymer blends.

Detailed DSC test procedures are given in the following ASTM methods:

--E 1356, "Assignment of Glass Transition Temperatures by DSC,"

--D 3418, "Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by DSC,"

--D 4419, "Transition Temperatures of Petroleum Waxes"; and

--D 7426, "DSC Procedure for Glass Transition of a Polymer or Elastic Compound."

"Coatings Clink" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We inane you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].

By Clifford K. SchoffSchoff Associates