Coatings Clinic: thermal mechanical analysis.

This month the topic is thermal mechanical analysis (TMA), one technique in a long line of penetration and indentation tests that have been used for many years to characterize viscoelastic materials such as asphalt, rubber, plastics, and coatings. A TMA instrument used for coatings has an indenting tip (flat or hemispherical) that is attached to a vertical rod that can be loaded via weights or electronically. The indenter is placed on the specimen, which usually is heated, either isothermally or at a programmed rate, by a built-in furnace. A transducer to sense the position and movement of the probe is included along with a microprocessor for temperature control, data collection, and data reduction. The resultant plots of indentation versus temperature or time can be used to characterize coatings by measuring softening points, thermal expansion coefficients, indentation hardness, elastic modulus, and creep and its recovery. This can be done on free films, paint chips, or coatings on substrates, including specimens cut from panels or field failure parts.

Softening points (Figure 1) probably are the most common property measured by TMA. The softening point is not the same as the glass transition (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. In addition, most paint films are multi-layered and by trying different loadings, it often is possible to pick out the softening points of the different layers.

[FIGURE 1 OMITTED]

The softening point and degree of indentation can:

(1) show relative hardness and give an idea of flexibility.

(2) indicate degree of cure as shown in the coil coatings in Figure .1,

(3) signal solvent retention (high indentation) in a film, and

(4) indicate thai a coating may be difficult to repair or topcoat without adhesion problems due to the inability of topcoat solvents to soften or "bite in" to it (high very low indentation).

The latter two situations also may be evaluated by room temperature creep testing in which a load is applied and the indentation with time is measured (usually with a hemispherical tip) on the coating of interest. The testing of bite-in usually is done by applying a drop of the wet topcoat or repair paint or the solvent blend from the paint to the test specimen, then covering the spot with a small watch glass. After a given length of time {usually several minutes), the watch glass is removed, the paint or solvent is wiped off and the softening point or creep behavior of the treated specimen is measured (depending on the specimen size and the instrument used, this may require cutting small pieces from the test area).

Inability to withstand expansion and contraction often is blamed for cracking problems and adhesion failures of coalings. This has led to a theory that the coefficient of thermal expansion (CTE) of each of the layers in a coatings system along witii that of the substrate should be similar. CTE values can be measured by TMA by measuring the increase in film thickness (from die rise in die probe position) with increasing temperature while using a flat-tipped sensor and a tight load. An expansion plot is shown in Figure 2. The slope of the displacement-temperature plot (Ax/AT) divided by the initial film thickness gives the CTE for linear expansion, usually in cm/cm/[degrees]C. It will be noted that the slope below the in Figure 2 is lower than that above that transition. An experiment generating such a plot is one way of measuring the glass transition temperature.

[FIGURE 2 OMITTED]

Creep measurements can be used to measure indentation hardness in terms of force/unit indentation in a manner similar to Tukon, Pfund, and Vickers hardness. Elastic modulus in force/unit area also can be determined. Both measurements depend on die Hertz equation:

[H.sub.k] = 3 P/(4 [R.sup.[1/2]][h.sup.[3/2]]) = E/(1-[[mu].sup.2])

where [H.sub.k] is indentation hardness, R is the radius of the hemispherical indenter, P is the force on the indenter, h is the depth on indentation, E is the elastic modulus, and u is Poisson's ratio (usually approximated as 0.5). Warning: hardness and modulus results are dependent on film thickness, particularly below 50 urn (2 mils) and comparisons must be done on films of the same thickness.

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

By Clifford K. Schoff Schoff Associates