Introduction
Phenolic resin is the first industrial synthetic resin, which has a history of more than 100 years. It is widely used in molding plastics, insulating materials, coatings, wood bonding, etc. because of its easy to obtain raw materials, convenient synthesis and the performance of resin after curing. In recent years, with the improvement of people's requirements for safety, phenolic resin with the characteristics of flame-retardant, low smoke and low toxicity has attracted people's attention again, especially in the airport, railway station, school, hospital and other public building facilities, as well as the interior decoration materials of aircraft, etc.
Compared with unsaturated polyester resin, phenolic resin has low reactivity, and condensation water is released from curing reaction, so curing must be carried out under high temperature and high pressure. For a long time, it can only be used to impregnate reinforcement materials to make prepreg (cloth), and then used in molding process or winding process, which seriously limits its application in the field of composite materials. In order to overcome the inherent defects of phenolic resin, further improve the performance of phenolic resin, and meet the needs of high-tech development, a lot of research has been carried out on phenolic resin. Improving the toughness of phenolic resin, improving the mechanical and heat-resistant properties, and improving the process performance become the focus of research. In recent years, a series of new phenolic resins have been developed in China, such as boron modified phenolic resin, alkynyl modified phenolic resin, cyanate esterified phenolic resin and ring opening polymerization phenolic resin. It can be used in SMC / BMC, RTM, pultrusion, spray, hand paste and other composite molding processes. Based on the author's research work, this paper introduces the research progress of phenolic resin modification and the research and application of RTM, pultrusion and other phenolic composite molding technologies.
Study on the modification of 1-phenolic resin
1.1 polyvinyl acetal modified phenolic resin
The most widely used phenolic resin in industry is modified by polyvinyl acetal, which can improve the adhesion of the resin to the glass fiber, improve the brittleness of the phenolic resin, increase the mechanical strength of the composite, reduce the curing rate, and thus reduce the forming pressure. Phenolic resin used for modification is usually phenol formaldehyde resin synthesized with ammonia or magnesium oxide as catalyst. The polyvinyl acetals used for modification are generally butyraldehyde and methylacetaldehyde. When used, it is usually dissolved in alcohol as the solvent of resin. The resin synthesized by the reaction of acetal and phenolic hydroxymethyl is an excellent special ink carrier resin.
1.2 polyamide modified phenolic resin
The impact toughness and adhesion of phenolic resin modified by polyamide were improved. The polyamide used for modification is a kind of hydroxymethylated polyamide, which is modified by the reaction of hydroxymethyl or active hydrogen in the process of resin synthesis or curing. The thin-walled tubes such as fishing rods made of the resin have excellent mechanical properties.
1.3 epoxy modified phenolic resin
The composite made of thermosetting phenolic resin and bisphenol-A epoxy resin can have the advantages of both resins, improve their shortcomings, and achieve the purpose of modification. This kind of mixture has good adhesive property of epoxy resin, improves the brittleness of phenolic resin, has good heat resistance of phenolic resin, and improves the disadvantage of poor heat resistance of epoxy resin. This kind of modification is achieved by the chemical reaction of hydroxymethyl group in phenolic resin with hydroxyl group and epoxy group in epoxy resin, and the chemical reaction of hydroxy group in phenolic resin with epoxy group in epoxy resin, and finally crosslinking into complex body structure. It is the most widely used phenolic toughening method.
1.4 silicone modified phenolic resin
Silicone resin has excellent heat resistance and moisture resistance. The heat resistance and water resistance of phenolic resin can be improved by the reaction of organosilicon monomer with phenolic hydroxyl or hydroxymethyl in linear phenolic resin. Modified phenolic resin with different properties can be obtained by using different organosilicon monomers or their mixed monomers and phenolic resin, which has a wide range of selectivity. The composite prepared by modified phenolic resin with organosilicon can be used for a long time at 200-260 ℃, and can be used as an instantaneous high temperature resistant material, as an ablation material for rockets, missiles, etc.
1.5 boron modified phenolic resin
Due to the introduction of inorganic boron into the molecular structure of phenolic resin, the heat resistance, instantaneous high temperature resistance, ablation resistance and mechanical properties of boron modified phenolic resin are much better than those of ordinary phenolic resin. Most of them are used as excellent ablative materials in space technology fields such as rockets, missiles and space vehicles.
The most common way is to use boric acid to react with phenol to produce phenyl borate, and then react with polyformaldehyde or formaldehyde aqueous solution to form a phenolic resin containing boron. The carbon residue rate of boron phenolic resin is 70% at 900 ℃, and the decomposition peak temperature is 625 ℃. In addition, the b-o-bond is introduced into the boron phenolic molecular structure, which improves the toughness and mechanical properties. The three-way cross-linking structure containing boron in the solidified product makes its ablation resistance and neutron radiation resistance better than that of the general phenolic resin. The bending strength and shear strength of the carbon cloth boron phenolic laminate are 420 MPa and 39.7 MPa respectively, and the mass ablation rate of oxygen acetylene is only 0.0364 g / s, 20% lower than that of carbon / barium phenolic laminate [2]. The water resistance of BPA boron phenolic resin synthesized by formaldehyde aqueous solution method has been further improved. In the 1970s, Beijing Institute of glass and steel composite materials Co., Ltd. (Beijing 251 plant) and Hebei University successfully developed boron phenolic resin, but in recent years, it has been really mass production, and the annual output is about 20t.
1.6 rubber modified phenolic resin
It is an effective toughening method to add NBR to phenolic resin by blending. The rubber content is usually 2% - 10% of the resin quality, and the impact toughness can be increased by more than 100%. Because of their poor solubility, the reactive rubber modified phenolic resin can be synthesized by the reaction of carboxyl terminated or amino terminated NBR with phenolic hydroxymethyl. The resin can be widely used in aerospace and other fields.
1.7 alkynyl or allyl modified phenolic resin
Generally, linear phenolics are used as the parent material, and phenylacetylene, acetylene, propargyl, etc. are introduced into the phenoxy position or benzene ring. The curing is mainly realized by the polymerization of different functional groups, which changes the traditional way of phenolic condensation curing. The polymerization of acetylene group and propargyl group is relatively easy, while phenylacetylene group needs higher curing temperature. In addition to propargylic phenol formaldehyde resin part of the chain extension and has a higher molecular weight, the molecular weight of these polymers are lower. Compared with traditional thermosetting resins, these phenolic resins cured by addition polymerization have better thermal stability and higher carbon residue.
The synthesis of propargylized phenolic resin was studied in the Institute of chemistry, Chinese Academy of Sciences. The resin has good processability, and its viscosity is less than 400 ℃ at 100 ℃ MPA? S; the resin can be cured at 200-250 ℃; the heat resistance of the thermosetting compound is significantly improved than that of the traditional phenolic resin. DMA shows that the vitrification temperature of the thermosetting compound is as high as 370 ℃, while TGA shows that the initial thermal decomposition temperature of the thermosetting compound is above 400 ℃.
High temperature resistant resin for RTM molding can be prepared by copolymerization of bismaleimide and allylated phenolic resin (bman). The viscosity of the resin in 100 ℃ / 8 h & lt; 150 MPa? S, suitable for RTM molding process and molding process. DMA analysis showed that the turning point temperature of the modulus curve of the resin casting was above 390 ℃, and the glass transition temperature was & gt; 400 ℃. Quartz fiber / bman resin composite also has good high temperature resistance, which can be used at 350 ℃.
1.8 phenolic cyanate resin
Phenolic cyanate ester is a kind of phenolic resin derivative which is based on linear phenolic resin and whose phenolic hydroxyl is replaced by cyanate ester functional group. Under the action of heat and catalyst, tricyclization reaction occurs and high crosslinking density network structure macromolecule containing triazine ring is formed. The curing reaction is a self curing system, which has no volatile small molecules and low shrinkage. The resin has the processing properties of epoxy resin, the high temperature properties of bismaleimide and the flame retardant properties of phenolic resin. At the same time, the resin also has excellent dielectric properties. It is an excellent material for preparing high-speed digital and high-frequency printed circuit boards and high-power motor insulation accessories. It is also the best base material for manufacturers of high-performance wave permeable structural materials and high-performance structural composite materials for aerospace.
The phenolic resin cyanate ester resin was synthesized by improved phenol phenol cyanogen bromide method in Beijing Research Institute of FRP and Northwestern Polytechnical University. The structure and properties of the phenolic resin were characterized by IR, gel experiments and thermogravimetric analysis (TGA). Compared with the traditional phenol cyanogen bromide method, the improved phenol cyanogen bromide method has obtained a stable synthetic product. The gel time of the product is 6.5min at 200 degrees centigrade, and there is no smoke and blackening phenomenon when the gel is formed. The carbon residue rate of the cured resin at 63.6% degrees in nitrogen atmosphere is 63.6%. 637 institutes, East China University of science and technology and other units also carried out research work on this type of resin.
1.9 phenoxazine resin
Benzoxazine, an intermediate with heterocyclic structure, was synthesized from phenols, amines and formaldehyde. Under the action of heating and / or catalyst, benzoxazine intermediates can undergo ring opening polymerization to form a network structure containing nitrogen and similar to phenolic resin. Usually we call this new type of resin ring opening polymerization phenolic resin. The Benzoxazine Resin does not release small molecules during the curing process. In the process of ring opening polymerization, there is no release of low molecular weight, which improves the formability and processability of phenolic resin.
Since 1990, Sichuan University has carried out systematic and extensive research on the synthesis, performance, ring opening reaction mechanism, reaction kinetics, volume change during curing, computer molecular simulation, composite preparation, performance research and application of benzoxazine.
1.10 xylene modified phenolic resin
Xylene modified phenolic resin is the introduction of hydrophobic xylene ring into the molecular structure of phenolic resin, so the water resistance, alkali resistance, heat resistance and electrical insulation performance of the modified phenolic resin are improved.
1.11 diphenyl ether formaldehyde resin
Diphenyl ether formaldehyde resin is formed by condensation of diphenyl ether instead of phenol and formaldehyde. The glass fiber reinforced composite of diphenyl ether formaldehyde resin has excellent heat resistance and can be used as H-class insulation material. It also has good radiation resistance and low moisture absorption.
1.12 bismaleimide modified phenolic resin
Bismaleimide with excellent heat resistance is introduced into phenolic resin. Because of the hydrogen ion transfer addition reaction between the two, it can isolate or block part of phenolic hydroxyl, improve the thermal decomposition temperature of the modified resin, and improve the high temperature resistance of friction material.
Bismaleimide modified phenolic resin has outstanding heat resistance, the heat deformation temperature (HDT) is 273 ℃, the glass transition temperature (TG) is the output and the use amount increase rapidly. The main reasons why phenolic FRP can be widely used in foreign countries are as follows: first, this kind of product has its unique advantages in terms of performance; second, the production and research and development of phenolic FRP are relatively mature, almost involving various process methods. In contrast, the manufacture and application of phenolic FRP in China