The chemical name of bakelite is phenolic plastic, which is the first variety of plastics put into industrial production. It has high mechanical strength, good insulation, heat resistance, corrosion resistance, so it is commonly used in the manufacture of electrical materials, such as switches, lamp caps, earphones, telephone case, instrument case, etc. “bakelite” is named from this. Its appearance is of great significance to industrial development. Phenolic resin can be prepared by polycondensation of phenols and aldehydes in the presence of acidic or alkaline catalysts. Bakelite powder was obtained by mixing phenolic resin with sawdust, talcum powder, urotropine, stearic acid and pigment in mixer. Bakelite powder is heated and pressed in the mold to obtain thermosetting phenolic plastic products. The disadvantages of phenolic resin are poor mechanical properties, oil resistance and chemical corrosion resistance. In order to overcome the above defects, people have modified phenolic resin. Adding different fillers into phenolic resin can obtain modified phenolic plastics with different functions. For example, adding asbestos and mica in the ingredients can increase the mechanical properties of phenolic resin With its acid, alkali and wear resistance, it can be used as the material of chemical equipment and the accessories of motor and automobile; the hardness can be increased by adding glass fiber, which can be used as machine parts; the oil resistance and impact strength of the modified NBR are greatly improved; the mechanical strength and acid resistance can be improved after the modification with PVC.
Bakelite is non absorbent, non conductive, high temperature resistant and high strength. It is mostly used in electrical appliances. Like wood, it has high insulation and good plasticity, so it is called “bakelite”. Bakelite is a kind of powdered phenolic resin mixed with sawdust, asbestos or clay. It is extruded with a mold at high temperature. Phenolic resin is the first synthetic resin in the world.
Phenolic plastic (bakelite): the surface is hard, brittle and fragile, and there is a sound of board knocking. It is mostly opaque and dark (brown or black). It is not soft in hot water. It is an insulator. Its main component is phenolic resin.
When formaldehyde / phenol (molar ratio) is less than 1, thermoplastic products can be obtained, which is called thermoplastic phenolic resin, namely linear phenolic resin. It does not contain further condensation groups, and can be cured by adding curing agent and heating. Such as hexamethylene tetramine as curing agent, curing temperature 150 ℃, mixed with fillers made of molding powder, commonly known as bakelite powder. When the molar ratio of formaldehyde to phenol is greater than 1, the first step resin, namely thermosetting phenolic resin, is obtained under the catalysis of alkali. It can be dissolved in organic solvents. The first stage resin contains hydroxymethyl which can be further polycondensated, so it can be cured without adding curing agent. The second stage resin, also known as semi soluble phenolic resin, is insoluble and insoluble, but can be swelling and softening. Then, the insoluble phase C resin, also known as insoluble phenolic resin, was obtained by further reaction. Phase a resin can also self cure after long-term storage.
The curing forms of thermosetting phenolic resin can be divided into room temperature curing and thermal curing.
Non toxic curing agent NL, benzenesulfonyl chloride or petroleum sulfonic acid can be used for room temperature curing, but the latter two materials are toxic and irritant.
The main purpose of modification of phenolic resin is to improve its brittleness or other physical properties, improve its adhesion to fiber reinforced materials and improve the molding process conditions of composite materials. Modification is generally carried out through the following ways:
① Blocking phenolic hydroxyl groups. Phenolic hydroxyl group of phenolic resin does not participate in chemical reaction in resin manufacturing process. The phenolic hydroxyl left in the molecular chain of the resin is easy to absorb water, which makes the electrical properties, alkali resistance and mechanical properties of the cured products decrease. At the same time, phenolic hydroxyl groups are easy to form quinone or other structures under the action of heat or ultraviolet light, resulting in uneven color changes.
② Other components were introduced. In order to change the curing rate and reduce the water absorption of phenolic resin, the components which react with phenolic resin or have good compatibility with it are introduced to separate or surround hydroxyl groups. The advantages of the two polymer materials can be obtained by introducing other polymer components.
- Polyvinyl acetal modified phenolic resin is widely used in industry. It can improve the adhesive force of the resin to glass fiber, improve the brittleness of the phenolic resin, increase the mechanical strength of the composite, reduce the curing rate, so as to reduce the molding pressure. Phenolic resin used for modification is usually phenol formaldehyde resin synthesized with ammonia or magnesium oxide as catalyst. The modified polyvinyl acetal is a polymer with different proportions of hydroxyl, acetal and acetyl side chains. Its properties depend on: 1) the molecular weight of polyvinyl acetal; 2) the relative content of hydroxyl, acetyl and acetal groups in the molecular chain of polyvinyl 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 is reduced correspondingly, which slows down the curing rate of the resin and makes it possible to form under low pressure, but the heat resistance of the products is reduced.
- Polyamide modified phenolic resin the impact toughness and adhesiveness of the phenolic resin are improved, and the fluidity of the resin is improved, and the advantages of the phenolic resin are still maintained. Polyamide used for modification is a kind of hydroxymethyl polyamide, which is modified by the reaction of hydroxymethyl or active hydrogen to form chemical bond in the process of resin synthesis or resin curing.
- Epoxy modified phenolic resin composites made of 40% first-order thermosetting phenolic resin and 60% bisphenol propane type epoxy resin can have two kinds of composite materials
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