Progress in the Environmentally Friendly Flame Retarded Copper Clad Laminates of Epoxy Resin

  With the arrival of the highly information age, electronic components have entered a new stage of high integration and high reliability. Printed circuit board (PCB), the base plate necessary for installing them, has become an indispensable and important component of most electronic products. As the main substrate material in PCB manufacturing, copper clad laminate (CCL) plays three main functions: conductive, insulating and supporting. The performance, quality, manufacturability, cost and level of PCB depend on the substrate material to a large extent. At present, the output value of substrate materials in the world has reached tens of billions of dollars. The output of substrate materials in China is about 55 million square meters, with an output value of about 9 billion yuan.

  According to the types of adhesives used, CCL can be divided into phenolic resin, epoxy resin, polyester, polyimide and other types. According to the reinforcement materials, it can be divided into paper base, glass cloth base and composite base. Among them, glass cloth reinforced epoxy copper clad laminate, such as FR-4 and FR-5, has become the mainstream of printed circuit boards used in electronic products such as electronic computers, communication equipment, instruments and meters. It is speculated that epoxy CCL will be the first in the whole CCL in the future.

1. Significance of research and development

  Considering the flame retardancy of materials, copper clad laminate can be divided into two categories: flame retardant and non flame retardant, of which flame retardant is the dominant. Flame retardant substrate materials can be divided into flame retardant substrate containing bromine and antimony and green substrate without bromine and antimony, namely environment-friendly substrate.

  Since 1986, the development of halogenated flame retardant materials industry has encountered the problem of dioxin, that is, toxic polybrominated dibenzofuran (PBDF) and polybrominated dibenzodioxane are produced when halogenated flame retardants are pyrolyzed and burned at high temperature, which is a kind of substance that can damage the skin and internal organs, and has the effect of promoting malformation and carcinogenesis of the body. Therefore, halogenated flame retardant polymer materials failed to obtain the green environmental protection mark in Europe.

  Since the 1990s, the technology development of electronic products has increasingly required to adapt to the environment. Therefore, the development of environment-friendly substrate materials has become faster and faster. The use of halogen free flame retardant substrate materials has become an important direction for the greening of most plates. It is mainly reflected in: (1) Many countries or regions in the world (especially Europe) have formulated and promulgated many regulations on restricting or prohibiting the substrate materials containing halogenated flame retardants, and the relevant regulations will be enforced around 2004. However, China, as a major producer of substrate materials, is mainly halogenated flame retardant. (2) Some famous electronic product manufacturers in the world have started to use halogen free PCB.

  In the late 20th century, more and more attention has been paid to the research of environment-friendly flame retardant epoxy resin copper clad laminate. Because epoxy resin copper clad laminate, including paper based, glass cloth based, composite based and multilayer board, is the main variety of substrate materials, and most of them are non environment-friendly flame retardant. Take the FR4 sheet substrate material, which accounts for more than 95%, as an example. This is a kind of plate with bromine flame retardant. See Table 1 for its typical formula.

Tab.1 Typical Formulation of FR–4 Adhesive

  Therefore, it is of great environmental value and economic significance to develop non halogen flame retardant environment-friendly epoxy resin sheets to replace existing high halogen or low halogen type sheets.

2. Research and development approach

  The production of substrate materials generally includes three processes: resin paint manufacturing, semi-finished product impregnation and drying, and plate forming and pressing. The main materials include polymer resin, reinforcement materials and copper foil. Except for paper-based materials, reinforcing materials and copper foils are generally flame retardant and do not require flame retardant treatment. Therefore, the flame retardancy of epoxy resin is very important for substrate materials. The epoxy resin based substrate material resin paint includes epoxy resin, curing agent, curing accelerator and solvent. Epoxy resin as adhesive; The curing agent reacts with the epoxide group or hydroxyl group in the epoxy resin, thereby curing and crosslinking the epoxy resin; Accelerator is used to catalyze the curing of resin; Solvents are mainly used to dissolve epoxy resin and curing agent, which are removed during impregnation and pressing. The development of environment-friendly flame retardant epoxy CCL can only be started from the following three aspects: (1) develop halogen free flame retardant epoxy resin; (2) Adopt non halogen flame retardant curing agent; (3) Add non halogen flame retardant.

a) Study on Halogen free Flame Retardant Epoxy Resin

  According to the types of flame retardant elements, flame retardants can be divided into halogen series (bromine series and chlorine series), phosphorus series, nitrogen series, silicon series, antimony series, boron series, etc. Research and use of epoxy resin containing phosphorus, nitrogen, silicon and other flame retardant elements to replace the now widely used tetrabromobisphenol A epoxy resin is the main method to develop non halogen flame retardant epoxy resin.

i. Study on nitrogen-containing epoxy resin

  Nitrogen containing epoxy resin is the main variety used to replace brominated epoxy resin in the manufacture of flame retardant CCL, including glycidylamine epoxy resin and polyisocyanurate oxazolidone resin.

  Glycidylamine epoxy resin includes triglycidylamine cyanate with triazine as the core skeleton, p-aminophenol epoxy resin and diaminodiphenylmethane epoxy resin. Triglycidyl melamine cyanate is a white crystalline powder, with a relative molecular weight of 297.29, a melting point of 102~105 ℃, an epoxy value of 102~108 g/mol, three epoxy groups in the molecule, and a nitrogen content of up to 14% (mass fraction). Therefore, it is self extinguishing. It also has the characteristics of high cross-linking density, high temperature resistance and good arc resistance.

  Because the structure contains a large number of five - and six membered rings, such materials have excellent flame retardancy, heat resistance, dielectric resistance and mechanical strength. Its raw materials are mainly isocyanate and epoxy resin.

ii. Study on phosphorus containing epoxy resin

  The introduction of phosphorus into epoxy resin can make it have good heat resistance and flame retardancy. When polymer materials are heated in the air, they can be decomposed to produce volatile combustibles. When the concentration of combustibles and system temperature are high enough, combustion can occur. Flame retardant mechanism can be divided into gas phase flame retardant mechanism, condensed phase flame retardant mechanism and interrupted heat exchange flame retardant mechanism. The flame retardant mechanism of phosphorus is mainly condensation mechanism. When the material is heated, it generates phosphorus containing acid, which can catalyze the dehydration of the compound into carbon, reducing the mass loss rate of the material and the generation of combustibles, while most of the phosphorus remains in the carbon layer. The carbon layer formed on the surface of the material has good flame retardancy due to the following characteristics. First, the oxygen index of the carbon layer itself can be as high as 60%, and it is difficult to burn, heat insulated and oxygen isolated, which makes the combustion suffocate; Secondly, the carbon layer has poor thermal conductivity, which reduces the heat transferred to the substrate and slows down the thermal decomposition of the substrate; Third, phosphorus remaining in the carbon layer mostly exists in the form of viscous semi-solid substance, which forms a liquid film covering the carbon layer on the material surface, which can reduce the permeability of the carbon layer and protect the carbon layer from further oxidation. At present, there are mainly two kinds of phosphorus containing epoxy resin:

  They can be cured by amine or anhydride, or they can be cured by heating themselves. They also have the advantages of low viscosity and long service life.

iii. Study on silicon containing epoxy resin

Any silicone polymer with epoxy group can be used as silicone epoxy resin. They can be accessed by:

(1) Condensation reaction of diphenylpropane epoxy resin with low molecular weight polysilane containing methoxy, ethoxy and hydroxyl;

(2) Alkoxy dealcoholization reaction between epoxy propanol and polysilane;

(3) Addition reaction of propylene oxide allyl ether with active hydrogen in polysilane;

(4) Peroxide oxidizes unsaturated double bonds on polysilane;

(5) Diphenolpropane sodium salt, epichlorohydrin and alkyl halide containing

  Polysiloxane reaction and other methods are used to prepare # 665 silicone epoxy resin.

  Organic silicon epoxy resin has the advantages of both organic silicon and epoxy resin. It has excellent characteristics of flame retardancy, moisture resistance, water resistance, heat resistance, etc. It can be used in aerospace, aviation and other industries.

b) Study on Flame Retardant Curing Agent

  Epoxy resin itself is a thermoplastic polymer prepolymer, and pure epoxy resin has no use value. Only when curing agent is added to make it become three-dimensional reticular structure, it will show a series of excellent characteristics. Curing agent is also called hardener. Curing agent of epoxy resin includes amine, anhydride, polymer prepolymer, latent curing agent, etc. The flame retardancy of epoxy resin can be improved by adding curing agent containing flame retardant elements. At present, most of the flame retardant curing agents are compounds containing nitrogen, phosphorus or phosphorus nitrogen at the same time.

  Phosphoramides with the following structures are an important class of flame retardant curing agents, which contain both phosphorus and nitrogen. Due to the synergistic effect of phosphorus and nitrogen in flame retardants, these compounds can significantly improve the flame retardancy of materials. Table 2 shows the horizontal combustion test results of epoxy resin e51 system cured with phosphorus amine and non phosphorus amine curing agent.

Tab.2 Testing results of horizontal combustion of E–51 curing with or without phosphor hardeners

  The results in the table show that the flame retardancy of the resin system can be improved from Grade III to Grade II by the phosphorus amine curing agent.

c) Study on Preparation of Flame Retardant Epoxy Resin System by Adding Non Halogen Flame Retardant

  Flame retardant is used to improve the flame resistance of materials, that is, to prevent materials from being ignited and inhibit the spread of flame. Flame retardants can be divided into additive type and reactive type. 85% of flame retardants currently used are additive type. Adding flame retardant to epoxy resin system is a direct and effective method to improve its flame retardancy. Inorganic flame retardants such as Al (OH) 3, Mg (OH) 2 and red phosphorus and organic phosphorus flame retardants such as trimethyl phosphate, triphenyl ester and m-cresol are used for environment-friendly flame retardant epoxy resin copper clad laminate. And they are usually used at the same time to play a synergistic role. The epoxy resin copper-clad sheet was prepared from 270 parts of epoxy resin, 185 parts of nitrogen-containing epoxy resin, 100 parts of aluminum hydroxide, 60 parts of phosphate ester and 0.1 parts of 2-ethyl-4-methylimidazole used by Honda Xinxing. The flame retardant property reached V-0, and it has good peel strength and heat resistance. In addition, low smoke density flame retardant epoxy resin system can also be prepared by using Al (OH) 3 and red phosphorus. See Table 3 for the formula.

Tab.3 Fire-proofing EP system with low smoke density

3. Conclusion and prospect

  Environmental friendly flame retardant epoxy resin copper clad laminate was proposed very late, and the research in this field in China has just begun. Because the research and development of this kind of CCL involves flame retardant science, electronic science, material science and other disciplines, and needs their close cooperation. After China's entry into WTO, various laws and regulations in China will gradually be in line with international standards, and the requirements for flame retardancy and environment will be higher and higher. Therefore, the research and development of environment-friendly flame retardant CCL should be strengthened. The author believes that the following aspects should be strengthened. First of all, we should vigorously research and develop environment-friendly phosphorus, nitrogen and inorganic flame retardants suitable for electronic industry applications to overcome the shortcomings of the current common flame retardants, such as high content of impurities such as chloride ions, high mobility, and impact on electrical performance. At present, it is still an important, economical and effective method to change the flame retardancy of materials by adding flame retardants. Secondly, the fundamental way to solve the problem is to develop a new type of epoxy resin with intrinsic flame retardancy, which can overcome the problems such as mechanical and electrical performance degradation caused by adding flame retardants. However, cost may be the main contradiction. Third, develop and use flame retardant curing agents containing nitrogen, phosphorus, phosphorus nitrogen or polyimide and other modifiers, which can improve the flame retardancy of CCL and heat resistance of materials.