As we covered in the post yesterday, the glass transition temperature (Tg) of a polymer adhesive is actually a range of temperatures during which the solid polymer material experiences some substantial changes in its physical properties (transitions from glassy to rubbery solid). Because the Tg of a polymer is a range of temperatures, determining the Tg is not exactly straightforward.
Three testing methods exist and this post discusses the benefits and drawbacks of each. Understanding how and why each of these testing methods work also provides some insight to exactly what is occuring at the glass transition temperature.
Other polymer properties worth checking out:
- Comparing Shore A And Shore D Hardness Values
- Coefficient Of Thermal Expansion And Thermal Stress Management
- Glass Transition Temperature Of Polymer Adhesives
Differential Scanning Calorimetry (DSC)
The basic concept of this method is that as a polymer transitions from a glassy to a rubbery state, one of the physical properties of the material that substantially changes is its heat capacity - the amount of heat it takes to raise the temperature of the material by one degree.
A DSC gradually adds more and more heat to a sample of a polymer material and at the same time measures how much the material changes its temperature as a result. When the polymer reaches the lower end of its glass transition temperature its heat capacity will spike up and the DSC will use this information to ultimately estimate the glass transition temperature of the material (Tg is point B on the graph below).
"Tgdscenglish" by Original PD work by Afluegel, SVG by User A1 (talk) - I created this work entirely by myself.. Licensed under CC BY-SA 3.0 via Wikipedia - https://en.wikipedia.org/wiki/File:Tgdscenglish.svg#/media/File:Tgdscenglish.svg
Estimating the Tg of a material using a DSC is a cost effective method, however, it is not always accurate. Many materials will only have a small increase in heat capacity, making the transition hard to identify.
Thermo Mechanical Analysis (TMA)
TMA estimates the glass transition temperature of a material by tracking its coefficient of thermal expansion (CTE). In many cases the CTE of a polymer material will increase 3-5x as it transitions from the glassy to the rubbery state.
Similar to DSC testing, TMA involves slowly adding heat to a sample material and simulataneously tracking changes in physical properties. In the case of TMA, the physical dimensions of the material are tracked and the Tg is identified as the temperature where there is a dramatic change in these dimensions.
TMA testing is slightly more involved than DSC testing, however, this method generally yields more accurate results.
Dynamic Mechanical Analysis (DMA)
One of the most immediate and obvious differences between a glassy and a rubbery material is of course the stiffness. DMA testing uses this fundmental physical differentiator as a guide and tracks the stiffness of a polymeric material over a range of temperatures in order to determine its Tg. The graph below plots stiffness vs. temperature and points out
"Rubber plateau" by Original by Philip Poppe, svg version by Booyabazooka - Own work. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Rubber_plateau.svg#/media/File:Rubber_plateau.svg
This method is extremely accurate and sensitive, however, is also very time consuming and costly compared to the other two methods described.