Glass Transition Temperature vs Melting Point
Glass transition temperature (Tg) is simply described as the temperature where if a polymer is used below this it will behave as a brittle glassy substance, but above this temperature the polymer will be more pliable and flexible. This definition is a bit vague but its application to polymer science and powder coating is without doubt of the utmost importance. Glass transition temperature effect a wide range of properties of a powder coating, everything from heat resistance, mechanical flexibility, impact and chemical resistance and even how the powder is used for subsequent modifications such as metallic bonding.
At the molecular level Tg is a temperature range where molecules have sufficient kinetic energy to afford a certain degree of molecular movement. Below the Tg an amorphous substance is glass and above the same substance exists in a soft and rubbery state. This increased flexibility is a result of increased molecular motion due to heat energy. More specifically the molecules of a solid don’t move from position A to position B, but rather portions of the molecules move and rotate like the arms and legs of a person standing in one location-this is referred to as long-range segmental motion.
Temperature is a measure of the random kinetic energy of the molecules in a substance. Polymers are brittle below their Tg and soft and pliable above their Tg because above their Tg polymer molecules have sufficient heat energy/kinetic energy to accommodate externally applied forces like shear. Below their Tg polymers are rigid because the molecules are stiff and have insufficient kinetic energy to move in response to externally applied forces. This is why plastics are more prone to cracking in the winter when left outside. Some polymers are used below their Tg and some are used above their Tg. Powder coatings are commonly used above their Tg. The range of Tg for powder coatings range but are typically in the 55 to 70°C range although specialty resins and coatings exist that can dramatically increase these values. (Note that the Tg of a substance is not an absolute value as multiple methods exist to determine the Tg which will each yield differing values). The most common way to describe Tg is with differential scanning calorimetry and measuring the energy released during heating after a standard cooling protocol. As heat is applied at a constant rate to the sample the temperature also increases at a constant rate, however at the Tg the rate of temperature increase changes with respect to heat. Another way of saying this is that the specific heat capacity of the substance will change at the Tg.
The difference between melting point and glass transition temperature is that only crystalline solids undergo melting and only amorphous solids undergo glass transition. At the molecular level beyond the Tg of an amorphous substance the polymer molecules are still pretty stationary but experience increased molecular motion in position. Melting however is a phase transition where a solid becomes a liquid and there is freedom of movement among the molecules such that the bulk substance will flow with reduced viscosity and assume the shape of its container. Melting is classified in modern terms as a first-order phase transition with an associated enthalpy of fusion. This enthalpy, or latent-heat, means that for a constant supply of heat the temperature of a crystalline substance will rise to its melting point where the temperature will remain constant for a period as the solid changes phase to a liquid.
Effects of Glass Transition Temperature on Powder Coating
Identifying general trends with Tg and the resulting properties of a powder coating can be a little like comparing apples with oranges. In order to change the powder coating’s Tg the chemistry has to be altered and so controlled experimentation is difficult.
Powder coating formulators are concerned with glass transition temperatures since it dictates many properties of the final coating. Resins with higher Tg values are typically considered to be more durable than coatings with a lower Tg. Increased Tg values are also often associated with increased impact resistance, chemical resistance, and heat resistance of the final coating. Increasing a powder coating’s Tg will typically result from increased polymer chain length and/or decreased polymer mobility. Powder coatings with higher Tg will show better performance at elevated operating temperatures such as in high temperature pipelines where fusion bonded epoxy coatings can easily exceed 100°C.
Standard powder coatings which undergo bonding with metallic pigments are modified at or around their Tg. This practice is done to ensure that the bonded metallic powder is actually a “bonded powder” where the metallic pigment is attached to the powder coating molecules. Bonded powders are in opposition to a dry blend powder where metallic pigments are simply mixed with a powder to give a heterogeneous mixture of metallic pigments in a powder coating matrix which can lead to inconsistency in coating finish.
How to Alter Glass Transition Temperature: Chemical Structure and Additives
Polymer Tg is increased for decreased polymer chain mobility. Simply put if a polymer chain has a large amount of freedom and mobility it will have a lower Tg than a more rigid polymer. This should not be a surprise since a more flexible polymer should take less heat energy to begin moving. Altering the Tg of an existing powder coating is not easily accomplished, and in fact increasing a Tg is practically impossible without completely altering the chemistry of the coating itself. Lowering a powder coating Tg is however within the realm of possible.
Plastic chemistry depends upon the use of additives known as plasticizers . It was previously believed that plasticizers work by positioning between polymer chains and spacing these molecules further apart leading to increased flexibility. This mechanism previously described whereby plasticizers increase the free volume within a polymer network does not account for all of their attributes but is none the less interesting. Common plasticizers in the powder coating industry are typically derivatives of the phthalic and trimellitic acid.