The chemical name of bakelite is phenolic plastics, which is the first variety of plastics put into industrial production. Because of its high mechanical strength, good insulation, heat resistance and corrosion resistance, it is often used in the manufacture of electrical materials, such as switches, lamp caps, earphones, telephone cases, instrument cases, etc.
Phenolic resin can be prepared by condensation polymerization of phenols and aldehydes with acid or basic catalyst. Bakelite powder was obtained by mixing phenolic resin with sawdust, talc (filler), urotropine (curing agent), stearic acid (lubricant), pigment, etc. and heating in a mixer. Thermosetting phenolic resin products were obtained by heating and pressing bakelite powder in mold. The disadvantages of phenolic resin are poor mechanical properties, oil resistance and chemical corrosion resistance,
People have modified phenolic resin, adding different fillers in phenolic resin can get modified phenolic plastics with different functions. For example, adding asbestos and mica in ingredients can increase its acid resistance, alkali resistance and wear resistance, which can be used as materials for chemical equipment and accessories for motor and automobile; adding glass fiber can increase hardness, which can be used as machine parts, etc.; using nitrile rubber After modification, the oil resistance and impact strength are greatly improved; after modification with PVC, the mechanical strength and acid resistance are improved.
The characteristics of bakelite are non absorbent, non-conductive, high temperature resistant and high strength. It is often used in electrical appliances. Because of its high insulation and good plasticity, it is called “bakelite”. Bakelite is made of powdered phenolic resin, mixed with sawdust, asbestos or clay, and then pressed out in a mold at high temperature. Among them, phenolic resin is the first synthetic resin in the world.
Phenolic plastic (bakelite): the surface is hard, brittle and fragile. There is a sound of wood when knocking. It is mostly opaque and dark (brown or black). It is not soft in hot water. It is an insulator, and its main component is phenolic resin.
When the formaldehyde / phenol (molar ratio) is less than 1, the thermoplastic product can be obtained, which is called thermoplastic phenolic resin, that is, linear phenolic resin. It does not contain further condensation groups, and can be cured only by adding curing agent and heating. Such as hexamethylene tetramine as curing agent, curing temperature 150 ℃, mixed with filler made of molding powder, commonly known as bakelite powder. When the formaldehyde / phenol (molar ratio) is greater than 1, the thermosetting phenolic resin, which is soluble in organic solvents, can be obtained first under the catalysis of alkali. The resin in stage a contains hydroxymethyl which can be further condensed, so it can be cured without adding curing agent. The resin in stage B, also known as semi soluble phenolic resin, can be obtained under the reaction of heating, which is insoluble and insoluble, but can swell and soften. Further reaction can obtain insoluble and insoluble structure c resin, also known as insoluble phenolic resin. The resin at stage a can also self cure after long-term storage.
The curing forms of thermosetting phenolic resin can be divided into room temperature curing and heat curing.
Non toxic room temperature curing agent NL, benzene sulfonyl chloride or petroleum sulfonic acid can be used for room temperature curing, but the latter two materials are more toxic and irritating.
The main purpose of phenolic resin modification is to improve its brittleness or other physical properties, improve its adhesion to fiber reinforced materials and improve the molding process conditions of composites. Generally, the modification is carried out by the following ways:
① Blocking phenol hydroxyl. Phenolic hydroxyl group of phenolic resin does not take part in chemical reaction in the process of resin manufacturing. The phenolic hydroxyl left in the molecular chain of the resin is easy to absorb water, which reduces the electrical properties, alkali resistance and mechanical properties of the cured products. At the same time, phenolic hydroxyl groups are easy to form quinones or other structures under the action of heat or ultraviolet light, resulting in uneven color changes.
② Introduce other components. In order to change the curing speed and reduce the water absorption, the components which have chemical reaction with phenolic resin or good compatibility with phenolic resin are introduced to separate or surround hydroxyl groups. The introduction of other polymer components can combine the advantages of the two polymer materials.
- Polyvinyl acetal modified phenolic resin is widely used in the industry. It can improve the adhesion of resin to glass fiber, improve the brittleness of phenolic resin, increase the mechanical strength of composite materials, reduce the curing rate, and reduce the molding pressure. The modified phenolic resin is usually synthesized with ammonia or magnesium oxide as catalyst. Polyvinyl acetal used as modification is a polymer with different proportion of hydroxyl, acetal and acetyl side chains
① The molecular weight of polyvinyl acetal;
② The relative content of hydroxyl group, acetyl group and acetal group in the molecular chain of polyvinyl alcohol acetal;
③ The chemical structure of the aldehyde used. Due to the addition of polyvinyl acetal, the concentration of phenolic resin in the resin mixture decreases correspondingly, which slows down the curing rate of the resin and makes low-pressure molding possible, but the heat resistance of the product decreases.
- Polyamide modified phenolic resin the impact toughness and adhesiveness of phenolic resin were improved by polyamide modified phenolic resin, and the fluidity of the resin was improved, while the advantages of phenolic resin were still maintained. Polyamide used for modification is a kind of hydroxymethyl polyamide, which can be modified by the reaction of hydroxymethyl or active hydrogen in the process of resin synthesis or curing.
- Epoxy modified phenolic resin is a composite made of 40% first-order thermosetting phenolic resin and 60% diphenolpropane epoxy resin, which can have the advantages of both resins and improve their disadvantages, so as to achieve the purpose of modification. The mixture has excellent adhesion of epoxy resin, which improves the brittleness of phenolic resin. Meanwhile, it has excellent heat resistance of phenolic resin, which improves the disadvantage of poor heat resistance of epoxy resin. This 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 phenolic hydroxyl group in phenolic resin with epoxy group in epoxy resin, and finally cross-linking into a complex body structure.
- Silicone modified phenolic resin has excellent heat resistance and moisture resistance. The heat resistance and water resistance of phenolic resin can be improved by reaction of phenolic hydroxyl or hydroxymethyl groups in linear phenolic resin with silicone monomer.
Modified phenolic vinegar resins with different properties can be obtained by modifying phenolic resin with different organosilicon monomers or their mixed monomers, which have wide selectivity. The composite prepared by silicone modified phenolic resin can work at 200 ~ 260 ℃ for a long time, and can be used as instantaneous high temperature resistant material for rocket, missile and other ablative materials.
- Boron modified phenolic resin has better heat resistance, instantaneous high temperature resistance and mechanical properties than phenolic resin due to the introduction of inorganic boron into the molecular structure of phenolic resin. The heat resistance, instantaneous high temperature resistance and ablation resistance of boron modified phenolic resin are much better than those of common phenolic resin. They are widely used as excellent ablative materials in space technology fields such as rockets, missiles and space vehicles.
- Xylene modified phenolic resin xylene modified phenolic resin is the introduction of hydrophobic xylene ring into the molecular structure of phenolic resin,
The water resistance, alkali resistance, heat resistance and electrical insulation properties of the modified phenolic resin were improved.
- It can also be used as insulation material of diphenyl formaldehyde and diphenyl ether.