【UP】Study on the curing mechanism of unsaturated polyester resin UPR

1． Introduction

The curing of unsaturatedpolyester resin​ (UPR) seems to be a very thorough study in theory and practice, but because the factors that affect the curing reaction are quite complex, and in UPR In various application fields, quality defects in products are almost always related to “curing” to a large extent. Therefore, it is necessary for us to conduct a more in-depth discussion on the curing of UPR.

(To discuss the curing of unsaturated polyester resin, you should first understand some concepts and definitions related to the curing of unsaturated polyester resin)

2. Concepts and definitions related to the curing of unsaturated polyester resin

2. 1 Definition of solidification

Under the action of light, heat or initiator, liquid UPR can form a three-way cross-linked insoluble and infusible body through the combination of the unsaturated double bonds in the linear polyester chain and the double bonds of the cross-linking monomer. structure. This process is called curing of the UPR.

2. 2 Curing agent

The curing of unsaturated polyester resin is a copolymerization reaction initiated by free radicals. How to start the reaction is the key to the problem. Once the monomer is triggered and generates free radicals, the molecular chain can grow rapidly to form a three-way cross-linked macromolecule.

The start of curing of unsaturated polyester resin is to first break the unsaturated C-C double bond. Due to the different energy required to break the chemical bond, for the C-C bond, its bond energy E=350kJ/mol, it takes 350 The temperature of -550℃ can trigger its cleavage.

Obviously, it is not practical to cure the resin at such high temperatures. Therefore, people have found substances that can decompose to produce free radicals at lower temperatures, which are organic peroxides. The O—O bonds of some organic peroxides can decompose at lower temperatures to produce free radicals. Some of these peroxides, which can decompose at 50-150°C, are of great use in curing resins. We can take advantage of this property of organic peroxides and select some of them as initiators, or curing agents, for the resin.

Definition of curing agent: Curing agent for unsaturated polyester resin is a peroxide that triggers resin cross-linking under the action of accelerator or other external conditions, also known as Initiator or catalyst.

The “catalyst” mentioned here is different from the “catalyst” in the traditional sense. In the traditional concept, the term “catalyst” is used to provide help for reactants. They promote the reaction without consuming themselves. In the UPR curing reaction, the peroxide must change its own structure before it can “catalyze” the reaction. Therefore, a more appropriate name for the peroxide used for UPR curing should be called “initiator” or “initiator”.

When it comes to peroxides, the two concepts we need to understand are active oxygen content and critical temperature. Among them, “active oxygen” or “active oxygen content” is a concept closely related to curing agents and often misunderstood.

Active oxygen content:Active oxygen content is simply the percentage of oxygen and the total weight of peroxide molecules in peroxide.

In terms of the concept itself, a peroxide with a lower molecular weight may have a relatively higher reactive oxygen species content. But this does not mean that peroxides with high levels of reactive oxygen species are more or faster active than peroxides with low levels of reactive oxygen species. (Because many of our application manufacturers use active oxygen content as an indicator to evaluate curing agents) In fact, active oxygen content is only a constant as a measure of the concentration and purity of any specific peroxide. It has been found that many peroxides with high reactive oxygen species are not suitable for use in curing resins because they decompose or “burn out” very quickly at standard curing temperatures, that is, they break down free radicals too quickly. quick. Since free radicals always have a strong tendency to combine with each other, when the free radicals are generated faster than they can be utilized by unsaturated double bonds, they will recombine or terminate the polymer chain, thus producing low molecular weight polymers. This results in incomplete curing. (A typical example is hydrogen peroxide).

Critical temperature: Simply put, the critical temperature is the lowest temperature at which peroxide decomposes in large quantities to produce free radicals. (This temperature is generally just an approximation. Free radicals are also released before this temperature, but the degree is different.)

We can classify peroxides into medium-temperature initiators or high-temperature initiators based on their critical temperatures. For pultrusion and compression molding, the working temperature is determined based on the critical temperature of the peroxide used. GeneralFor large-area spray molding, the cross-linking agent may be insufficient due to volatilization of the cross-linking agent, and the amount of accelerator and curing agent must be appropriately increased to shorten the gel time.

6. 5 The influence of fillers and other additives on the degree of resin curing

If other substances such as rubber, sulfur, copper and copper salts, phenol, phenolic resin, dust and carbon black are added to the resin, even a small amount can inhibit the polymerization reaction and sometimes even prevent the resin from curing at all. .

7． Evaluation of the degree of curing of unsaturated polyester resin

In the actual construction of FRP products, due to the different curing speeds of different batches of resin from different manufacturers, and the impact of many factors such as ambient temperature and humidity during construction, the time for the product to reach the optimal curing level will be different. The following introduces several common methods for indirectly judging the curing degree of FPR products.

①. On-site inspection

The surface of the FRP product should feel non-sticky when touched with your hands; dip a clean cotton ball in acetone and place it on the surface of the product to observe whether the cotton appears color; tap the product and listen to the sound, which should be crisp rather than blurry; there should be no scratches with coins.

②. Barcol hardness method

The Barcol hardness method uses a Barcol hardness tester to test the hardness value of molded materials. Generally, if the Barcol hardness reaches 40 to 50, it has reached an ideal degree of curing and can be put into use.

③. Rebound method

The rebound method is to drop a small steel ball from a certain height to the surface of the cured resin to be measured. Due to different degrees of curing, the rigidity of the resin is different, so the rebound heights are also different. The rebound height can indicate the degree of curing.

④. Measure the insoluble content in the molded product, that is, use acetone to extract the soluble components in the resin to obtain the insoluble content. This method is also applicable to epoxy or phenolic FRP and is currently the most commonly used method. The principle is that the resin is a linear molecule before curing and is easily soluble in organic solvents. After curing, it becomes a network structure and is difficult to dissolve in organic solvents. Organic solvents are used to extract the soluble components in the cured resin. The weight difference of the FRP sample before and after extraction is then calculated to indirectly illustrate the degree of curing.

8. The effect of resin curing degree on product performance

Theoretical and practical tests show that:

①. The higher the curing degree of the resin, the greater the bending strength and modulus;

②. The higher the curing degree of the resin, the better the corrosion resistance of the product;

③. The higher the curing degree of the resin, the better the heat resistance of the product;

④. The maximum values ​​of tensile strength and elongation at break appear when the resin curing degree is about 90%.

9． Conclusion

The curing of UPR is a complex chemical process that is affected by many factors. Therefore, both UPR manufacturers and application manufacturers should master some basic UPR curing knowledge in order to achieve safe production and improve production efficiency. Based on the characteristics of its own products, it effectively solves various problems that arise in actual production. I believe that as application manufacturers continue to gain a deeper understanding of resin curing knowledge, the quality of the products they produce will qualitatively improve