Glass transition temperature Tg

What is glass transition temperature?

One of the most important properties that define a polymer (and hence to an adhesive) is its glass transition temperature, the value of the glass transition temperature is directly related to the mechanical properties (strength, hardness, brittleness, elongation...) that the polymer (adhesive/plastic) presents at the temperature that is exposed, you've never seen in winter why some plastics become hard and brittle?, the explanation lies in its glass transition temperature.

Glass transition temperature (Tg) is defined as the temperature at which the mechanical properties of a plastic / adhesive radically changed due to the internal movement of the polymer chains that form the plastic / adhesive.

In the first part of the definition, the Tg identified as the temperature at which the mechanical properties of the adhesive change drastically, so the glass transition temperature indirectly define the temperature range in which it can be exposed or work the adhesive / plastic, so the Tg is also known as the working temperature and the knowledge of its value is of vital importance in the design phase.

In the second part of the definition, we identify the origin of the Tg for ease or restrict movements of the polymer chains that occurs inside the plastic / adhesive. When the adhesive or plastic is exposed to temperatures below its Tg, the movement of the polymer chains decreases doing the material acquires a hard and brittle behavior, at temperatures above the Tg polymer chains motion increases doing the material acquires an elastic behavior.

Depending on the requirements to fulfill may use an adhesive or other doing work at a higher Tg or lower, for example polyurethane adhesives 1-component curing by moisture have a Tg of -45 ° C, so in normal conditions behave as an elastomer, ie elastic behavior acquired and so this type of adhesive is used for elastic bonding design, if this same adhesive is placed in North pole at a temperature of -60 ° C would automatically to its glassy state where the adhesive would become stiff and brittle as glass, increasing its resistance to stresses but decreasing its elasticity.On the other hand we have rigid adhesive as epoxy, where its Tg is about 100 º C, so in normal conditions always work in its glassy state properties resulting in low elongation and high resistance to stress.

During the design and formulation of the adhesive there are chemical compounds capable of decreasing the glass transition temperature, these compounds are known as plasticizers and are formulated to separate the polymer chains that forms the adhesive, in order to increase the volume and thus facilitate the movement among them, so that greater movement decrease the glass transition temperature and produce more elastic behavior.

Methods and techniques for the identification of the Tg.

As we discussed above, the knowledge of the Tg value of an adhesive is a vital and necessary data information during the design phase of an adhesive bonding, imagine that we have designed a flexible bonding with an adhesive whose Tg is 5 º C, if the bonding is exposed at -3 ° C the adhesive itself will become rigid and hard getting itself to cause break against the forces to which it was initially designed.

Generally the value of the glass transition temperature is provided by the adhesive manufacturer and can be found in the technical data sheet, to know that value the manufacturer or user uses one of the following thermal analysis techniques:

In each of the above techniques an adhesive sample is subjected to a test which measures the amount of heat, dimensional variation or deformation of the sample as a function of temperature, i.e. the collection of data takes place when there is a change in the temperature at which the sample is subjected, so that we are able to represent the mechanical properties of the adhesive according to the temperature at which it is located, and thus are able to identify its glass transition temperature.

Finally it is necessary to indicate that the Tg only applies for thermoplastic and elastomers polymers, thermosets be invaluable due the movements of polymer chains are very restricted by its own configuration of highly crosslinked, for that reason is why thermoset adhesives always behave as rigid and hard materials, ie always behave as glassy materials.

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