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Coatings Clinic: glass transition, minimum film forming temperature and softening point.

Not long ago, I was asked about the relationship between the three parameters in the title of this article. The glass transition ([T.sub.g]) of a polymer or coating is the temperature at which the material goes from being hard and glassy to soft and flexible as it is heated and the reverse as it is cooled. In the glassy state, large scale molecular motion does not occur. The glass transition corresponds to the onset of liquid-like motion of longer segments of polymer chains with heating. Although a glass transition is quoted as specific temperature, many of the properties used to measure it go through the change over a range of several to tens of degrees, depending on the molecular weight distribution and other factors.

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 pigmentation, name for the coloring matter found in certain plant and animal cells and for the color produced thereby. Pigmentation occurs in nearly all living organisms. , 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 Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature.  (DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. ) where a change in baseline in a plot of heat flow versus temperature denotes the [T.sub.g] (see JCT JCT Junction
JCT Jerusalem College of Technology
JCT Joint Contracts Tribunal (UK build contracts governing body)
JCT Journal of Coatings Technology
JCT John Christner Trucking
JCT Journal of Curriculum Theorizing
 CoatingsTech, 5 (7), 60 (2008) for information on DSC]. Dynamic mechanical analysis (DMA (1) (Digital Media Adapter) See digital media hub.

(2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases.
) 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 coefficient of thermal expansion,
n See expansion, thermal coefficient.
 undergoes an abrupt increase at the [T.sub.g] on heating. Assuming that a coating has the same properties in all directions (is isotropic Refers to properties that do not differ no matter which direction is measured. For example, an isotropic antenna radiates almost the same power in all directions. In practice, antennas cannot be 100% isotropic. ), linear thermal expansion, which is easily followed by thermal mechanical analysis (TMA TMA Turnaround Management Association
TMA Texas Medical Association
TMA Transportation Management Association
TMA Training and Management Assistance (a component of OHRD, which is a component of OWR)
TMA Tooling & Manufacturing Association
), 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 The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state).  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 pow·der·y  
adj.
1. Composed of or similar to powder.

2. Dusted or covered with or as if with powder.

3. Easily made into powder; friable.

Adj. 1.
 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 MFFT Minimum Film Forming Temperature (polymer temperature transition testing instrumentation) ). 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 ASTM
abbr.
American Society for Testing and Materials
 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 in·den·ta·tion
n.
A notch, a pit, or a depression.
 technique such as TMA or transition temperature microscopy (TTM TTM

Trailing 12 months. Often used with Earnings Per Share.
) [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 publications@coatingstech.org.
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Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Oct 1, 2009
Words:913
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