Study on Improving the Insulation Resistance of 3240 epoxy sheet after Immersion

1. Introduction

  3240 epoxy sheet is widely used in the motor, electrical and electronic industries, and is still the largest production of electrical laminated products in China. The performance of the current product basically meets the requirements, and its insulation resistance after immersion is only 10^{^8}Ω, which is a big gap compared with the insulation resistance after immersion in ISO standard 5 x10^{^10} Ω. Therefore, it cannot meet the requirements of industrial development at home and abroad. For example, switch plants need plates with immersion resistance of 5.0 x 10^{^10} Ω. Therefore, in order to meet the needs of users, it is an urgent task to improve the insulation resistance of 3240 epoxy phenolic glass cloth plates after immersion to make them meet the ISO standard.

  The adhesive of 3240 epoxy phenolic glass cloth board is composed of epoxy resin and ammonia catalyzed phenolic resin. There are many polar groups in the phenolic resin, which affects the electrical performance of the board, leading to a considerable number of 3240 boards being replaced by some new materials. For example, the lifting rods of Shenyang High Voltage Switchgear Factory were replaced by molded parts, Beijing High Voltage Switchgear Factory introduced 3B switch boards to replace them with SMC plastic boards, and some imported materials were also used in some home appliances and electronic industries. Under this circumstance, we conducted research on improving the immersion resistance of 3240 pressing plates with epoxy phenolic system large adhesive without raising the product cost. Good results have been achieved through experimental research. The immersion resistance of the laboratory sample has reached 5.0 × 10^10 Ω, which provides test data for actual production.

2. Factors Affecting the Immersion Resistance of Epoxy Phenolic Glass Cloth

  There are many factors that affect the immersion resistance of epoxy phenolic glass cloth board. The main reason is that the crosslink density of epoxy phenolic cured material is low, and the integrity of the laminate is poor. In addition, because epoxy phenolic cured material contains a large number of polar groups, it is easy to form hydrogen bonds with water molecules.

a) Reasons for the Structure of Phenolic Resin

  The phenolic resin used in 3240 Epoxy Board is mainly the Resol type phenolic resin synthesized with ammonia as catalyst. Among them, the primary and secondary amino groups can react with epoxy, while the tertiary amino groups are tertiary amines, which are accelerators of epoxy resin curing reaction and accelerate the reaction of phenol and methyl groups with epoxy groups. The curing reaction speed increases rapidly with the increase of temperature.

  In addition, the phenolic resin made by ammonia catalysis has large molecular weight dispersion, small molecular weight, and large free phenol content (15%~20%). Therefore, the glass cloth plate made by mixing it with epoxy resin has poor integrity, low electrical and mechanical properties, and the insulation resistance of the glass cloth plate after immersion in water is low because of the large methyl content in the phenolic resin.

b) Influence of Different Substrates on Insulation Resistance of Laminates after Immersion in Water

  The surface structure of the resin and the substrate glass cloth has a great impact on the integrity of the glass cloth board. If the resin and the glass cloth surface combine well, the integrity of the laminate is good, and vice versa. The coupling agent can not only interact with the surface of glass fiber, but also with epoxy resin, so the glass cloth treated with the coupling agent has a high immersion resistance.

c) The Influence of the Type and Quantity of Accelerator on the Insulation Resistance of Glass Cloth Laminates after Immersion

  The cross-linking density of phenolic cured epoxy resin is different due to the different types and quantities of accelerants. Keep the same gelation time. In the same epoxy phenolic system, due to the existence of accelerants, the resin has a better degree of impregnation of glass fiber during the pressing process. Under certain curing conditions, it can be cured more fully to improve the integrity of the laminate, thus having different effects on the properties of the board after immersion.

3. The Properties of Modified Phenolic Resin and the Factors Affecting the Immersion Resistance of Plate

a) Properties of Modified Phenolic Resin

  Two different compounds are used to participate in the synthesis reaction of phenolic resin to produce phenolic resin, A and B modified phenolic resin are respectively obtained, which are characterized by less polar groups and less free phenol content. See Table 1 for the technical indicators of phenolic resin.

Table 1 Branch index of modified phenolic resin

  The properties of the modified phenolic resin are related to the type, dosage and reaction conditions of the modifier. Through orthogonal design, a more reasonable formula and process are obtained, and the content of free phenol in the modified phenolic resin is low.

  The content of free phenol in the central control index of modified phenolic resin has great influence on the properties of the board. Free phenol is caused by incomplete phenol reaction. The mechanical and dielectric properties of the cured epoxy phenolic resin are significantly reduced due to its existence, so the content of free phenol in the phenolic resin is minimized. Reducing the content of free phenol can be solved by adjusting the ratio of reactants, prolonging the holding time, and increasing the end point recovery temperature.

  b) Effect of Phenolic Resin on Immersion Resistance of Plate

  It can be seen from Table 1 that the content of free phenol in phenolic resin obtained by introducing modifier A into phenolic resin is small. The two modified phenolic resins are respectively used for epoxy resin curing agent to prepare 56% solid glue solution, and the properties of the plates obtained by gluing and pressing are shown in Table 2.

  It can be seen from Table 2 that the immersion resistance of the board made of epoxy resin cured with modified phenolic resin A is high.

  c) Effect of Accelerator on the Immersion Resistance of Laminates

  The accelerant also has a great influence on the immersion resistance of the sheet. We choose three kinds of accelerants to make a contrast experiment on the immersion resistance of epoxy phenolic glass cloth board. There are three situations when using C accelerator, D accelerator and no accelerator. The results are shown in Table 3.

  Table 2 Effect of phenolic resin on immersion resistance of plates

Table 3 Effect of accelerant on sheet immersion resistance

  It can be seen from Table 3 that the presence or absence of accelerant has a great impact on the immersion resistance of epoxy phenolic glass cloth board, and different accelerants have different effects on the resin curing. The prominent feature of accelerator D is that it has the role of secondary amine in addition to the catalytic role of tertiary amine, so the dielectric property of the cured product obtained with accelerator D is high.

  d) Effect of Substrate on the Immersion Resistance of Laminates

  In order to further improve the immersion resistance of 3240 Epoxy Resin Sheet, it is better to use glass cloth with surface chemical treatment. The commonly used chemical treatment agent is Voran, but when epoxy resin is used as the adhesive, the use of a-aminopropyltriethoxysilane treatment agent can achieve better results, because the amino group in it can react with the epoxy group in the epoxy resin. Table 4 shows the influence of different substrates on the immersion resistance of epoxy phenolic glass cloth board.

  It can be seen from Table 4 that the immersion resistance of the plate made of KH550 glass cloth as the substrate is high.

Table 4 Effect of immersion of plate in water on the bottom material

  4. Conclusion

  To sum up, the immersion resistance of the plate made by curing epoxy resin with thermosetting phenolic resin modified by compound A, selecting D additive and KH550 glass cloth as the substrate can reach 5.0 x 10^{^10} Ω.