/baodian/ 1004.htm
Source: Author: Date of Release: 2009- 10- 16
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Phenolic resin has excellent high temperature resistance, thermal ablation resistance, high carbon residue rate, dimensional stability and processability, and is widely used in construction, military equipment, aerospace and other fields. However, when PF is used in aerospace and other environments that require very high material properties, its comprehensive properties, especially brittleness, flame retardancy, oxidation resistance and thermal stability, need to be improved to ensure the normal work of aerospace and spacecraft. Boron-modified PF is to introduce boron into PF, that is, some hydrogen atoms in phenolic hydroxyl groups in PF are replaced by boron atoms. Because the bond energy of B-O (774.04kJ/mol) is higher than that of C-C (334.72kJ/m0 1), the heat resistance and ablation resistance of boron modified PF are much higher than that of ordinary PF. In addition, the B-O bond has good flexibility, so the brittleness of boron modified PF is reduced, and the mechanical properties are improved. It is often used as an adhesive to improve the comprehensive properties of materials. BPF has higher heat resistance, instantaneous high temperature resistance and mechanical properties than ordinary PF, and is often used as an excellent ablation resistant material in space technology fields such as rockets, missiles and spacecraft. In this paper, various methods of boron modified phenolic resin in recent years are reviewed, and the properties and applications of boron modified phenolic resin are briefly introduced.
Mechanism of improving thermal properties of 1 BPF
It is generally believed that the improvement of antioxidant properties of BPF includes both chemical and physical effects. (1) chemical action. Boride reacts with PF chemically, that is, benzyl hydroxyl reacts with boride in batches to form boron ester bonds, which reduces the number of ether bonds in PF molecules. Because the bond energy of B-O bond is much greater than that of ether bond, BPF will crack at higher temperature, thus improving the heat resistance of PF. In addition, due to the existence of boron, BPF can improve the structure of coke in the cracking process, that is, form glassy carbon with dense structure, which can effectively prevent oxygen from entering the resin, thus inhibiting the further combustion of the resin, thus improving the flame retardant performance of the resin. ② Body movements. With the addition of boric acid and boron compounds, a dense glass structure layer can be formed on the surface of PF at high temperature. It is preliminarily considered that this glassy substance is boron oxide. This glassy structure layer can effectively exclude oxygen and other gases from entering the material, and at the same time prevent the resin from further burning, so the high temperature stability and oxidation resistance of PF are obviously improved. The mechanism of BPF improving oxidation resistance is shown in formula (1): Boron compounds such as boric acid have low melting points, and when heated slowly to about 170t, boric acid loses water to generate unstable boric acid; When the temperature rises to about 270℃, boric acid continues to lose water to generate stable boron oxide; When the temperature is higher than 325℃, boron oxide transforms into a dense glassy structure, thus preventing oxygen from entering the resin. Therefore, the oxidation resistance of the resin is improved.
Synthesis method of 2 BPF
The synthesis methods of chemically modified BPF are mainly divided into two categories: ① solid-state synthesis, that is, borate is synthesized first, and then reacted with paraformaldehyde to obtain BPF, as shown in Formula (2) and Formula (3); (2) aqueous solution method, that is, phenol reacts with formaldehyde aqueous solution to generate salicyl alcohol, and then reacts with boric acid to prepare BPF, as shown in Formula (4).
The thermal stability of PF can be further improved, because chemical bonds can be formed by chemical action, and a stable six-membered ring structure can be formed after curing of BPF. Therefore, introducing boron into the molecular structure of PF through chemical modification has become an important method to improve the antioxidant performance of PF. Therefore, this paper mainly studies the synthesis method of boron chemically modified phenolic resin.
2. Solid-state Synthesis of1BPF
Solid-state synthesis is the most important method to synthesize BPF. Hirohatap et al reported a method for preparing BPF by solid-state synthesis. Firstly, phenol reacts with boron oxide at 300℃ to produce triphenylboron. Triphenylboron reacts with paraformaldehyde at 150℃ to generate BPF；; . Then the product was heat-treated at 80℃ and 65438 000℃ for 24h, respectively, to obtain yellow solid. The experimental results show that monosubstituted, disubstituted and trisubstituted boron substituted benzene mixtures can be prepared according to the different ratios of boron oxide and phenol. With the increase of boron oxide content, the esterification degree of phenolic aldehyde increases, and the boron content in borate ester increases, so the amount of paraformaldehyde required decreases accordingly. With the increase of heat treatment temperature, the bending strength decreases gradually, but the bending modulus is almost unaffected by boron content and heat treatment temperature. Increasing the temperature or prolonging the time is beneficial to the curing of BPF, because compared with phenol, the benzene ring in triphenylboron has high activity only in ortho position and low activity in para position, so its reaction rate with paraformaldehyde is low, so it is necessary to extend the time and increase the temperature to promote the smooth progress of the reaction. In addition, the related properties of BPF, modified PF and common PF are also discussed. The results show that BPF has the highest oxygen index, but its thermal transition temperature is lower, which is between bromine PF and chlorine PF. The thermal-oxidative stability of cured BPF is obviously better than that of common PF and halogenated PF.
Preparation of 2.2 bpf by aqueous solution method
Aqueous solution method is another method to prepare BPF. A series of thermal decomposition kinetics and heat resistance were synthesized by aqueous solution method. Phenol reacts with formaldehyde solution under alkaline conditions to generate phenol alcohol; Adding boric acid after vacuum distillation to remove water, reacting at 65438 000℃ for 40-60 minutes, and then slowly dehydrating to obtain boron modified phenolic resin. Gao~ 16qsl and others compared the esterification activities of phenolic hydroxyl group and benzyl hydroxyl group: when boric acid reacts with benzyl alcohol and phenol respectively, the conversion rate of boric acid/benzyl alcohol is 50%, while that of boric acid/phenol is only 4%, and most boric acid will precipitate after stopping stirring, indicating that the reaction activity of benzyl hydroxyl group is much higher than that of phenolic hydroxyl group. Therefore, it is considered that the structure of BPF produced by esterification is formula (5) rather than formula (6). In addition, the results of thermal decomposition kinetics show that the heat resistance of BPF is better than that of common PF. The heat resistance of BPF increases with the increase of boron content. Under the same conditions, the resin with high boron content has small thermal weight loss and low thermal decomposition rate constant. For example, at 590℃, the thermal decomposition rate constant of BPF (when boron content is 0.8%) is 8.02x 104s- 1, and that of common PF is 9.2 1x 104s when boron content is 0.3%. Obviously, the addition of boron can effectively improve the thermal stability of the resin.
Due to the low reactivity of borides such as boric acid with hydroxymethyl, the boron content in the synthesized BPF is correspondingly low (
Martin et al. used modified boride to prepare BPF. Firstly, boric acid reacts with catechol to synthesize boron alkoxide which is soluble in dioxane; Alcohol boron was added to the primary PF dissolved in dioxane to react for 48 hours, and then water and dioxane were removed under reduced pressure to obtain orange BPF. The experimental results show that because alcohol-based boron is easily soluble in organic solvents, alcohol-based boron has higher reactivity than boric acid and boride, that is, it is easier to react with resin. The boron content of BPF prepared by modified boron is higher (3.8%), the glass transition temperature (yR) is increased by 1 1.4%, the oxygen index is increased by 50%, the thermal-oxygen stability is significantly improved, and the carbon residue rate at 600℃ is over 20%. In addition, with the increase of boron content, the thermal-oxidative stability of modified BPF is obviously improved, but the initial decomposition temperature of BPF is 2765438 0℃, which is because modified BPF contains more small molecules.
Performance and application of 3 BPF
Because boron is introduced into the molecular structure of PF, BPF has better heat resistance, instantaneous high temperature resistance, mechanical properties, high temperature thermal stability, higher oxygen index and carbon residue rate than ordinary PF. When PF is modified with boric acid, boric acid will react with phenolic hydroxyl to form a flexible B-O bond, which reduces the brittleness of PF and improves the mechanical properties of the resin. In addition, because the bond energy of B-O bond (774.04U/m0 1) is higher than that of C-C bond (334,72kJ/M01), the heat resistance and ablation resistance of boron-modified PF cured product (boron-containing three-dimensional cross-linked network structure) are much higher than that of ordinary PF. In addition, because the hydrogen atom of phenolic hydroxyl group is replaced by boron atom, the water resistance of BPF is improved. At the same time, because phenolic hydroxyl groups participate in the reaction, the content of free phenolic hydroxyl groups is reduced, so that BPF will not release a lot of toxic gases during pyrolysis (that is, it is different from halogen modified PF).
Table 1 and Table 2 list the thermal properties of BPF and the mechanical properties of BPF composites respectively. From Table 1 and Table 2, it can be seen that the performance of polyacrylate interior wall humidity control coating is studied compared with ordinary PF.