Understanding the Glass Transition Temperature

November 10, 2015 02:00 PM
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Read the specifications for any Mallard Creek Polymer product and you'll see a number for its glass transition temperature. Written as “Tg,” this helps users anticipate how the latex polymer will behave at it's service temperature. In this blog post we'll explain some of the science behind Tg and the implications for product performance.

Glass transition science

Polymers are composed of long, randomly entangled, chains of molecules. At low temperatures these chains are locked together and unable to move. The polymer feels hard and is quite brittle. Attempt to stretch or bend it and it cracks.

Add energy, in the form of heat, and the molecular chains become more mobile. If heating continues, enough chains become mobile to make the polymer behave differently. Rather than being hard, it becomes softer and more pliable, more like a rubber than something that is rigid.

The temperature at which this happens is the “Glass Transition Temperature.” Strictly speaking, it's not a single temperature, like the melting point of a crystalline material, but a range. As with melting though, the Tg is signified by a change in heat capacity. In other words, it absorbs heat but rather than getting hotter the material goes through this change of state.

Tg is determined in a differential scanning calorimeter or DSC. This measures the temperature of a sample as it's heated. Crystalline materials show an abrupt change as they change state, (and also a change in volume,) but for a polymer the change is more gradual. By convention, the Tg is reported as the midpoint of this transition.

As a side note, the Tg of glass is typically in the range 500 to 600 C, depending on composition. This is why, at room temperature it's hard and brittle, yet on heating in a furnace it becomes soft enough to mold into an infinite range of shapes.

Setting the Tg

At MCP, polymer Tg can range from - 80 to +100 C. However, this can be manipulated by varying the polymer composition. For example, in a styrene-butadiene latex emulsion, the styrene has a Tg of +100 C while that of butadiene is -85 C. Changing the ratio of styrene to butadiene will change the Tg.

Why vary Tg?

Every application needs something different. In an adhesive it's often desirable to have a Tg as low as -40 C. This ensures the adhesive stays pliable and tacky at room temperatures, and even if used in the freezer. Paints and coatings generally need a higher Tg because high hardness and the lack of tack are requirements at the service temperatures, meaning the the coating will be more rigid and less susceptible to dirt pick-up.

For some coatings, such as automotive underbody finishes, toughness is more important than flexibility. In this case, a higher Tg polymer, (meaning that it's strong at it's service temperature,) will be desirable.

Another consideration in coating formulation is film forming ability of the polymer, usually measured by Minimum Film Formation Temperature (MFFT). The impact of Tg on these properties is complex. In general, the increased mobility of the molecular chains within a low Tg polymer helps ensure proper film formation, although this acts in contrast to the need for hardness.

Consult a specialist

The Tg of a latex polymer indicates how it will perform at it's the wide range of temperatures the product is exposed to. The polymer emulsion should be selected with both this and the performance requirements in mind. For most application of water based emulsion polymers, this is a complex task due to the significant temperature swings of the polymers life cycle. To help resolve this challenge, it's best to discuss your application and ideas with the specialists at Mallard Creek Polymers.

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