Thermal & Mechanical Properties of Coatings

By Clifford K. Schoff, Schoff Associates

I recently was asked to comment on the thermal and mechanical properties of coatings and resins. Let us begin with thermal properties, which include the glass transition (T_g ), softening temperature (T_soft), thermal expansion coefficients, heat stability and, for powder coatings, the melting point (T_m). The T_g is the temperature region where a polymer or coating changes from being glassy and hard to being rubbery and soft as its temperature is increased. For polymers, the T_g is an indicator of flexibility and influences the viscosity of solutions and, therefore, of solventborne paints. High T_g resins give higher viscosities than those with lower T_g values at equal solids. High T_g latexes tend to give high minimum film forming temperatures and formulations usually need to be plasticized for good film formation. For cured coatings, the T_g  also is an indicator of flexibility or lack of it. Coatings with high T_g values may be brittle and prone to cracking or chipping. Those with low T_g values may be soft and easily marred and scratched and/or prone to dirt pick-up.

Techniques for T_g measurement include differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA or DMA). A DSC instrument measures temperature or heat input difference as a material and a reference are heated side by side. A plot of heat flow versus temperature changes slope in the T_g  region. Besides T_g values, the instrument can be used for measuring heat capacity and melting point (T_m ), for monitoring polymerization reactions, and for characterizing the unblocking temperature of blocked catalysts and blocked crosslinkers. ASTM D3417, D3418, and D7426 describe DSC measurements. DMTA testing subjects the free film specimen to cyclic motion (twisting, push-pull, or wiggling) as it is heated. The resulting response provides information on elastic and viscous (damping) characteristics, the latter showing a peak in the T_g region. See ASTM D4065, D4440, and D5279 for DMTA methods.

The T_soft is the region where a material softens as it is being heated while under some kind of stress (bending, indentation, etc.). The softening point is not the same as the T_g , but usually is close to it in value. Since a softening point is indicative of behavior under load, it often is more useful for predicting performance than is the T_g . A device commonly used to measure T_soft values of coatings and also coefficients of thermal expansion is a thermal mechanical analyzer (TMA), which has a loaded vertical rod with an indenting tip for softening points and a flat tip for expansion coefficients. The rod is placed at a right angle on the specimen, which usually is heated by a built-in furnace. A transducer and microprocessor sense the position of the probe and temperature of the specimen. The resultant plots of indentation versus temperature or time can be used to measure indentation hardness, elastic modulus, and creep as well as softening points and thermal expansion coefficients. Testing can be done on free films, paint chips, or coatings on substrates, including specimens cut from panels or field failure parts. ASTM D696, D831, and D1545 show TMA methods.

Coatings or components in them may degrade at high bake or use temperatures. Others may retain solvents or moisture. These can be evaluated by applying thermogravimetric analysis (TGA), a technique for following weight change of materials on heating. The basic instrument is a balance with a pan loaded with the specimen, an oven for heating the specimen, and a thermocouple to measure the temperature. TGA may be used to characterize a number of material properties, including coating composition (volatiles, nonvolatiles, and ash), coating and component stability or volatility, and the amount of retained solvent or moisture. Processes such as drying, baking, curing, and degradation may be monitored and a combination of DSC and TGA can be used to follow the unblocking of blocked crosslinkers and catalysts. TGA methods may be found in ASTM E1131 and ISO 11358.
See JCT Coatings Tech, 5 (6), 64; (7), 60; (9), 64; (10), 44, (all 2008) and 6 (1), 60 (2009) for more information on thermal analysis techniques.

Mechanical properties include DMTA-measured storage and loss moduli and crosslink density. Stress-strain measurements on free films provide information on tensile strength, elongation and elastic modulus (ASTM D2370). The properties measured by both types of instruments can be related to flexibility, toughness, tendency to crack, and abrasion resistance of coatings. Testing of films cured at different temperatures can show whether high bakes help or hurt properties or cause degradation. Hardness can be thought of as a mechanical property as well as a physical one. There are several kinds of hardness and each one is related to a different material property and involves a different test. The most precise methods are indentation (Pfund, Tukon, Knoop, all ASTM D1474) and the more recent nanoindentation technique. Damping methods include the Sward Rocker (ASTM D2134) and the Persoz and König Pendulums (ASTM D4366 and ISO 1522). Other methods are scratch and pencil hardness. The latter probably is the most common hardness test, but has the worst precision and is of questionable use.

See Nichols, M. and Hill, L.W., Mechanical Properties of Coatings, 2nd Ed., ACA, Washington, D.C., 2010 and JCT CoatingsTech, 4 (8), 64 (2007) and 5 (10), 44: (11), 92 (2008) for greater detail on mechanical properties.