Do you understand the cross-linking of thermoset plastics?

The actual cross-linking reaction is difficult to reach 100%, the main reasons are:

The cross-linking reaction is a process in which the thermosetting resin molecules develop in three dimensions and gradually form a giant network structure. As the process progresses, there is less or even less chance of contact between the reactive groups that are not acting, or between the reactive sites and the crosslinker.

Sometimes the reaction system contains a gas reaction product, which prevents the reaction from proceeding. In the plastic molding industry, the term cross-linking is often replaced by words such as hardening and ripening. The so-called "hardening is good" or "hardening is complete" does not mean the completeness of cross-linking, but refers to the development of cross-linking to the most appropriate degree, to achieve the best realm with the physical and mechanical properties of the product. . Obviously, the degree of crosslinking is not more than 100%, but the degree of hardening is ok. Generally speaking, the degree of hardening is more than 100%, which is "overcooked", and vice versa is "undercooked".

The type of hardening varies depending on the type of resin, and it plays a decisive role in the storage period of the thermosetting plastic and the time required for molding. Insufficient hard-setting plastic products, in which there are often more soluble low molecular substances, and because the molecular binding is not strong enough, the performance of the product is lost. For example, mechanical strength, heat resistance, chemical resistance, electrical insulation, etc.; thermal expansion, post-shrinkage, internal stress, creep of force, etc.; surface lack of gloss; prone to warpage, etc. . When the hardening is insufficient, the product may be cracked in some cases, and the performance described above may be further deteriorated. There is also a significant increase in water absorption. Cracks indicate that the mold or molding conditions used are not suitable, or that the amount of resin and filler in the plastic is not cracked.

Excessively hardened or overcooked plastic products have many shortcomings in performance. For example, the mechanical strength is not high, brittle, discolored, and dense vesicles appear on the surface. It is obvious that coking and cracking generated in excessive hardening or overcooking are also included. Over-cooking of products is usually caused by improper molding. It must be pointed out that overcooked and undercooked phenomena sometimes occur on the same product. The main reason for this may be that the molding temperature is too high, the temperature of the upper and lower molds is different, and the product is too large or too thick.

There are many ways to identify the degree of hardening. Commonly used physical methods are: thermal hardness test after stripping, boiling water test method, extraction method, density method, conductivity test method.

There are several kinds of plastics used for molding in the industry, such as powders, pellets, solutions and dispersions. No matter which material is used, it is generally not a simple polymer, and various additives are added more or less. The purpose of adding auxiliaries is to improve the plastic processing technology and improve certain characteristics of plastic products. Although it is used in a small amount and has a great effect, it has become an important component of the plastics industry. For the needs of the molding process, polymers and auxiliaries are formulated into powders or granules, and they are also formulated into solutions or dispersions. The methods of completing the formulation are mostly by mixing so that they form a homogeneous composite.