My humble opinion on the technical development of

2022-08-11
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Introduction

vinyl ester resin is prepared from epoxy resin and methacrylic acid by ring opening addition chemical reaction. It retains the basic chain segment of epoxy resin and has the good process performance of unsaturated polyester resin. After curing under suitable conditions, it shows some special excellent properties. Therefore, it has developed rapidly since the 1960s. First, shell chemical launched the epocrgl brand, then Dow Chemical launched the derakane brand in 1966, followed by the hetron brand of Ashland chemical and the ripoxy brand of Showa polymer Co., Ltd. in Japan. Other foreign brands or manufacturers include AOC, interplastics, etc, However, China has also developed its own vinyl ester resin. At present, some existing bisphenol A epoxy vinyl ester resins in China have basically stayed in the modification means (such as isocyanate modification, fumaric acid modification, etc.) to improve the process and mechanical properties of vinyl ester resin due to the supply and cost of raw materials (mainly epoxy resin), but due to the limitations of its chemical structure, synthesis method, etc, It greatly restricts the performance and application of the resin, but on the contrary, the synthetic technology of vinyl resin of domestic and foreign manufacturers has developed rapidly, and various patents and other bulletins have been frequently seen, which has greatly promoted the enrichment of vinyl resin varieties and related applications. Here is a summary of the development of some vinyl resins at home and abroad

2. Synthesis and technological development of vinyl ester resin

2.1 development of low shrinkage vinyl resin

vinyl ester resin, as a category of unsaturated polyester resin, has high activity and fast curing reaction speed, resulting in a large curing shrinkage rate after curing of vinyl ester resin. Generally, unsaturated polyester resin (including conventional vinyl resin) has a large shrinkage when curing, which can reach a volume shrinkage of about%, With the improvement of technical requirements for high-performance resins at home and abroad, we hope to find some vinyl ester resins with low curing shrinkage, which is a technical breakthrough that many manufacturers at home and abroad are trying to seek

the mechanism of low shrinkage resin is relatively complex. In order to overcome the curing shrinkage of the resin, some manufacturers used to add low shrinkage additive (LPA) to achieve the purpose, but it has its application limitations, and more manufacturers try to solve this technical problem through resin synthesis method and sub design level, But now an ultra-low shrinkage vinyl ester resin newly developed by a domestic manufacturer basically solves this technical problem. The biggest feature of this resin is the ultra-low curing linear shrinkage. The test was carried out according to the standard "determination of linear shrinkage of epoxy casting resin" (hg/t). The test results are shown in Table 2.1, and a vinyl ester resin modified by fumaric acid was selected for comparison. At the same time, the test results (under ASTM standard) of the resin by the American composite engineering technology center of Michigan State University in the United States also show that the curing shrinkage of the resin is extremely low. The center selects the products of a famous vinyl resin supplier in the United States as the control, as shown in table 2.2. Table 2.1 resin linear shrinkage test results

Table 2.2 resin body shrinkage test results (provided by the United States)

because the ultra-low shrinkage epoxy vinyl ester resin has sufficient mechanical strength and stiffness, sufficient dimensional stability, heat cycle resistance and corrosion resistance, it better meets the requirements of high-quality FRP products. The introduction of this product has caused repercussions in FRP industry at home and abroad. FRP products are widely used in the development of FRP products in automobile, train, kitchen and bathroom, shipbuilding, modeling, casting and other industries. The main applications include: ① FRP mold; ② Large area FRP plate; ③ Polymer concrete; ④ Overall FRP production, etc; ⑤ Other bonding, etc

2.2 impact resistant vinyl ester resin:

vinyl ester is currently most used in corrosion-resistant occasions, but because vinyl resin has more secondary hydroxyl groups, it can improve the wettability and adhesion to glass fibers, and improve the mechanical strength of laminated products; In addition, it is crosslinked at both ends of the molecule, so the molecular chain can stretch under the action of stress to absorb external force or thermal shock, showing resistance to microcracking or cracking. Therefore, vinyl resins are used in some occasions requiring high mechanical properties and impact resistance, but they have been gradually eliminated by the market. Conventional vinyl resins still need to be improved in terms of impact resistance, especially some vinyl resins modified by Fuma acid. Because this type of resin has high curing crosslinking density and short molecular chain segments between crosslinking points, its impact resistance is poor. In the synthetic design of these resins, it is required that there are more ether bonds on the main chain of resin molecules, which can fully improve the impact resistance of the resin. Now there is another way, that is, through rubber modification, that is, carboxyl terminated nitrile butadiene rubber (CTBN) and nitrile butadiene rubber (BNR) are used to toughen methacrylic acid epoxy vinyl ester resin. At present, a lot of work has been done on the latter method at home and abroad, The elongation of natural rubber modified vinyl resin has been greatly improved, which can reach 12%

generally, the impact strength (without notch) of vinyl resin is not greater than 14.00 kj/m2, but now some newly developed impact resistant non rubber modified vinyl resin can reach more than 22 kj/m2, and rubber modified vinyl resin can reach 25kj/m2. In this way, these impact resistant vinyl resins can be well applied to the production of some high impact resistant FRP, such as sports sleds, sports helmets, etc

2.3 thickening vinyl ester resin

as a high-performance unsaturated resin, the thickening characteristics of vinyl resin have always been the research direction of various manufacturers, because the unique application characteristics of bmc/smc have been recognized by the majority of customers, especially with the application of bmc/smc in automotive parts, the thickened vinyl resin can withstand higher impact than the general unsaturated resin, It has good creep resistance and fatigue resistance. These parts include wheels, seats, heat sinks, gate plates to enhance the comprehensive development of children, engine valve sleeves, etc. Of course, thickened vinyl resin can be widely used in electrical insulation, the manufacture of industrial pump valves, golf heads, etc

as a thickening vinyl resin, naturally, the resin is required to have the following characteristics: ① good wettability with reinforcing materials and fillers; ② Initial low viscosity and rapid thickening characteristics; ③ Good mechanical properties, including toughness and fatigue resistance; ④ Long storage period; ⑤ Low curing exothermic peak and low styrene volatilization. In order to achieve the use effect, in the synthesis research of vinyl resin, the original more common method is to introduce acidic functional groups (carboxylic acids) into vinyl ester molecules, and then use these carboxyl groups and alkaline earth metal oxides (such as magnesium oxide, calcium oxide, etc.), but this method takes a long time to thicken, generally takes a few days, and is sensitive to water content. Now, another method has also been developed, that is, polyisocyanate and polyol react to produce the like structure, so as to achieve the rapid thickening of the resin. This method can be suitable for low-pressure molding. It has the characteristics of stable viscosity control, low requirements for temperature and humidity, long storage period, high interlayer bonding strength of the product, and low acid value resin with excess alcohol can also be used as thickener. The principle of this method is as follows:

2.4 high temperature resistant vinyl resin

the molecular skeleton of vinyl resin is epoxy resin. If phenolic epoxy resin is used as raw material, the synthesized novolac vinyl resin has good corrosion resistance, solvent resistance and high temperature resistance. We tested the phenolic epoxy vinyl ester resin of well-known manufacturers at home and abroad according to the relevant national standards of China, and the results show that, The heat deformation temperature (HDT) of these resins is between ℃, while the heat deformation temperature of phenolic epoxy vinyl resin of some domestic manufacturers is lower, which is lower than 125 ℃. However, in some industrial applications, higher requirements have just been put forward for the heat resistance of the resin. At present, the heat deformation temperature of high crosslinking density vinyl resin provided by a small number of domestic and foreign manufacturers can reach more than 150 ℃, and the molecular structure of this type of resin has been modified, The heat-resistant properties of the resin were optimized, and the styrene content was adjusted reasonably to meet the actual use requirements. Compared with the conventional phenolic epoxy vinyl resin, it has higher temperature resistance and can be used in the strong corrosive environment of 200 ℃ gas phase for a long time. At the same time, our application experience shows that this type of resin can withstand the temperature impact of 300 ℃ in minutes. This unique application is in insulation applications, which can completely reach the insulation grade above C. Table 2.3 and Table 2.4 list some high temperature mechanical properties of 898 high crosslinking density resin FRP. Table 2.3 retention rate of flexural strength of 898 resin FRP under continuous high temperature

note: the data in Table 2.3 represents the retention rate of flexural strength after continuous high temperature placement, and the flexural strength at 25 ℃ is 100%

this type of resin can be widely used in some high-temperature applications such as smelting and power desulfurization (FGD) equipment, such as cooling towers, chimneys and chemical pipelines. At the same time, this type of resin also has the characteristics of strong solvent resistance and strong oxidizing media. Table 2.4 high temperature mechanical properties of 898 resin FRP

2.5 photosensitive vinyl resin

because the unsaturated double bond in the vinyl resin is at the end of the molecular chain, due to its high activity, it is also equipped with molecular design, such as using epoxy resin with high epoxy value, using acrylic acid to replace methacrylic acid to synthesize vinyl resin, and adding photoinitiators (such as benzoquinone, benzoin ether, etc.) to absorb ultraviolet energy, And transfer it to the resin system to polymerize and cure the vinyl resin

this kind of resin can be used in printing, photosensitive ink, etc., as photosensitive coating in paint industry, and as photoresist film on PCB in radio industry. In addition, in the pultrusion process, if photosensitive vinyl resin is used, the pultrusion speed can be greatly improved. For example, in the pultrusion process of optical cable core, the speed can reach 10m/min

2.6 air drying

vinyl ester resin is the same as unsaturated polyester resin. When cured at room temperature, the product surface has sticky phenomenon, which brings inconvenience to application. The main reason is that oxygen in the air participates in the polymerization reaction on the surface of vinyl ester resin. To overcome this shortcoming, researchers have developed a variety of effective methods. One of them is to synthesize gas dry vinyl ester resin by inserting allyl ether (ch2=ch-ch2-o -) groups into the structure of vinyl ester resin. This kind of resin is suitable for making high-grade air dry gel coat, coating, sealing fabric, etc. according to the different needs of data manufacturers

it is worth noting that the content of allyl ether in the resin has a suitable value. If it is too small, the resin cannot absorb oxygen well, and if it is too large, the air dryness will also decrease due to the "automatic polymerization inhibition"

2.7 low styrene volatilization technology

vinyl resin generally contains about 35% styrene monomer, while the vapor pressure of styrene is low. Therefore, in hand paste molding and spray molding, the resin is laid on the open mold layer by layer, especially in spray molding, and a part of the resin is foggy. Therefore, styrene is continuously removed from the resin before the resin is fully cured

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