Syntheses of functional polyesters with tunable glass transition temperatures

Abstract

Polyesters are important polymers for many applications and one of them is powder coatings. They meet the required properties of powder coating resins. Furthermore, the ease of altering physical properties of polyesters opens a huge area to be studied. One of the important properties of polyesters is their glass transition temperature (Tg) which is the temperature that polymers change from rubbery to brittle structure. Tg can be affected by many factors related to the structure of polymers. Number average molecular weight (Mn) is one of the factors that has a great effect on it. It is also a critical feature for powder coating applications since powder coating resins undergo curing reactions in which crosslinking between chains occur and the coating takes its final form by applying heat. Therefore, the curing temperature is highly dependent on the Tg of powder coating resin. It is challenging to control Tg values of polyesters that are used as powder coating resins since it changes curing temperature. In order to study the effect of molecular structure, end group and molecular weight on Tg, various polyesters were synthesized with hydroxyl and carboxyl end groups. Polyesters with the same molecular weight but different end groups showed different Tg values. Higher Tg value was observed in carboxyl terminated polyesters probably because of hydrogen bonding between chains. On the other hand, changing one of the monomers to less sterically hindered one caused a big decrease on Tg. From the polyesters synthesized before, poly(ester-anhydride)s were tried to be obtained by using different anhydrides. Thus, it was achieved to increase Mn and Tg by increasing number of functional groups. Increasing Tg by this way, it was aimed to keep curing temperature low since these functional groups can also form crosslinks during curing reactions. Some reactions were successful with the formation of anhydride linkages, others ended up with the formation of anhydrides and chain scission at the same time. Formation of anhydride linkages between chains increased the molecular weight and Tg as expected.