The molecular weight of the unsaturated polyester increases, the result is to increase the hardness, softening point, bending strength and tensile strength of the resin after curing. In order to control the molecular weight of the polyester, it is necessary to pay attention to a few points.
① Pay attention to increase the average molecular weight of the resin and avoid the generation of low molecular weight segments. To this end, the reaction conditions of the polyester should be well controlled to achieve the maximum degree of esterification in strict accordance with predetermined procedures. In order to prevent cross-linking between unsaturated molecular chains, stabilizers can be added appropriately, and a lower temperature and longer time reaction should be adopted in the process to increase the high molecular weight segment.
②In order to obtain a higher molecular weight, a two-step method can be used to synthesize the saturated acid ester to an acid value of about 50mg/g, and then add an unsaturated acid to react at a lower temperature to prevent crosslinking between double bonds . In this way, the molecular weight of the reaction is large, the viscosity of the product is higher than the one-step resin viscosity, and the strength after curing is also high.
③ The polyesterification reaction is carried out by mixing two polyesters with different acid values. For example, an acid value of about 20mg/g, an acid value of about 50mg/g, a shorter reaction time can produce a higher proportion of large molecular weight polyester.
④Adopt monocarboxylic acid or monohydroxy alcohol added to the polyester chain for timely termination, which can stop the growth of the chain to control the molecular weight.
In fact, the increase in molecular weight often requires an extension of the reaction time, sometimes the production efficiency decreases. Therefore, it is necessary to comprehensively consider the strength requirements and economic factors to determine the optimal molecular weight conditions.