If one had to choose the most important physical parameter for an adhesive or sealant, it arguably would be glass transition temperature, Tg. Today's editorial reviews the glass transition temperatures and its affect on application and performance properties.
Polymeric materials have several fundamental physical properties that are important in adhesion. These properties are controlled by the chemical structure of the base polymer from which the adhesive or sealant is made and from the various formulating agents used to modify the base polymer.
The glass transition temperature, Tg, is actually a composite of several physical attributes. The Tg is a useful concept because it is a way of understanding the molecular motion that occurs in a polymeric system. The degree of molecular motion is of fundamental concern when considering adhesion, cohesion, and other properties of polymers.
Molecular freedom influences the behavior of all polymers. At low temperatures the polymers exist as solids in which the molecular segments vibrate rather gently and independently. As the temperature of a polymer is increased, a point is reached at which the molecule suddenly becomes more flexible and mobile. This increased flexibility occurs when the molecular vibrations become strong enough to shake the adjacent chain segments apart and allow molecules the freedom to slip by one another. The temperature required to cause this increase in molecular freedom is known as the glass transition temperature, Tg.
The Tg signifies a transition of the polymer from a glassy to a rubbery state. As the temperature of a polymer is raised further and further above its Tg, the effective distance between molecular segments is increased. This is evident by an increase in the slope of the polymer's specific volume as a function of temperature (See Figure 1). Glass transition temperature is generally measured by observing the variation of some thermodynamic property (e.g., measurement of specific volume by dilatometry) as a function of temperature. Note in Figure 1 that the slope of the volume versus temperature plot increases above the glass transition temperature.
The strength of a crosslinked adhesive at elevated temperatures is very much indicated by its Tg. The Tg should be above the upper use temperature of the adhesive or sealant for good bond strength and creep resistance. Conversely, the low temperature performance properties of a relatively high Tg resins are limited because of the material's relatively brittle properties.
Using a resin with lower Tg increases the molecular flexibility of the polymer, other things being equal such as crosslink density, fillers, etc. With lower Tg polymers, the molecules are more flexible and impact resistant at a given temperature. Polyurethane and silicone adhesives and sealants, which have good low temperature impact properties, illustrate this point. These materials, however, will become brittle once the service temperature finally falls below the Tg.
One of the requirements of a pressure sensitive adhesive is that it must undergo plastic flow on contact. (The other requirement is that it must wet the substrate surface.) A polymeric material can only be pressure sensitive (flowable under slight pressure) above its glass transition temperature. As a result, most materials considered pressure sensitive have a Tg below room temperature. Although this provides the desired pressure sensitive characteristic, pressure sensitive adhesives do not achieve high cohesive strengths because they are used above their Tg.
The formulator has several options in adjusting the Tg of the polymeric system used in adhesives or sealants. These options are primarily through (1) polymer selection or synthesis and (2) formulating with additives, such as tackifiers or plasticizers, that change the "packing density" of the molecules.
For a more detailed explanation of glass transition temperature in adhesives and sealants and its effect on performance properties, the reader is directed to a past SpecialChem4Adhesives article.1
Figure 1: The effect of temperature on the total volume of a polymer
Petrie, E.M., "The Importance of Glass Transition Temperature in Formulating Adhesives and Sealants", SpecicalChem4Adheisves, 2003.