Coatings Clinic: glass transition, minimum film forming temperature and softening point.
The [T.sub.g] of a polymer depends on chain stiffness, symmetry, and intermolecular forces. Block copolymers may have two glass transitions (occasionally more), a low one for flexibility and a higher one for structural integrity. The [T.sub.g] of a coating depends on the polymers in it, the degree of crosslinking, the level and type of pigmentation, the presence of plasticizers, and amount of retained solvent. The glass transition influences many properties, including solution viscosity, solvent release, drying speed, adhesion, hardness, impact resistance, toughness, tensile strength, and abrasion resistance.
The glass transition can be measured by a variety of techniques, including differential scanning calorimetry (DSC) where a change in baseline in a plot of heat flow versus temperature denotes the [T.sub.g] (see JCT CoatingsTech, 5 (7), 60 (2008) for information on DSC]. Dynamic mechanical analysis (DMA) can provide [T.sub.g] values via peaks in plots of an energy dissipation factor, tan [delta], versus temperature [see JCT CoatingsTech, 5 (10), 44 (2008) for information on DMA]. The volume coefficient of thermal expansion undergoes an abrupt increase at the [T.sub.g] on heating. Assuming that a coating has the same properties in all directions (is isotropic), linear thermal expansion, which is easily followed by thermal mechanical analysis (TMA), can be used instead of volume expansion to measure T [see JCT CoatingsTech, 5 (9), 64 (2008) for more information on TMA]. Values measured by different techniques and at different heating rates may differ considerably.
Another area where the glass transition temperature is very important is in the film formation of air dry coatings. If the [T.sub.g] is higher than the temperature at which the film is forming, the result will be a discontinuous and powdery layer. This is almost never a problem with solventborne coatings as films are sufficiently plasticized by the solvents. With waterborne coatings, poor films are a definite possibility. The temperature below which a coating will not form a continuous, cohesive film is called the minimum film forming temperature (MFFT). The MFFT is measured by drawing down a paint, latex, or resin film on a special temperature gradient bar, allowing the film to dry, observing the dividing line between clear, continuous film and opaque, fragmented pieces and reading the temperature at that point. Unfortunately, ASTM D 2354, "Minimum Film Formation Temperature (MFFT) of Emulsion Vehicles," was withdrawn in 2007, but this useful document is still available from ASTM. However, the measurement is tedious and the apparatus can be difficult to clean. When I was working in the lab, we preferred to measure the [T.sub.g] of the latex or dispersion instead. Unfortunately, the relationship between MFFT and [T.sub.g] is complex, so it usually is not possible to accurately predict the MFFT this way. However, the MFFT always is higher than the [T.sub.g] and there is a rule of thumb that MFFT [less than or equal to] ([T.sub.g] + 10C[degrees]), which works fairly well for coatings with glass transitions greater than 0[degrees]C. I once worked with a styrene-acrylic latex that when tested with different levels of coalescing solvent gave MFFT values about 4[degrees]C above the [T.sub.g].
The third member of our trio of parameters is the softening point ([T.sub.soft]), which is the temperature at which a material softens under load. It usually is measured by an indentation technique such as TMA or transition temperature microscopy (TTM) [see JCT CoatingsTech, 6 (4), 52 (2009) for information on TTM]. The softening point is not the same as the glass transition. It occurs at a lower temperature than the [T.sub.g] but usually is close to it. Some people call the beginning of the transition region by DSC or DMA the softening point, but this may or may not give a value similar to a true softening point, which itself is dependent on the load applied and the heating rate. Softening points of coatings can provide information regarding cure, formability (particularly of coil coatings), weathering effects, and solvent resistance ([T.sub.soft] before and after exposure to solvent). One great advantage of softening point measurements is that they can be done on coatings on substrates, including coupons cut from field specimens such as car hoods, washing machine parts, metal roof shingles, as well as from test panels.
"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the 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, 527 Plymouth Rd., Ste. 415, Plymouth Meeting, PA 19462; or email email@example.com.
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|Author:||Schoff, Clifford K.|
|Date:||Oct 1, 2009|
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