General purpose thermosets

Thermosetting plastics are a type of polymer in which the resin undergoes chemical changes during processing, the molecular structure changes from a linear structure before processing to a network structure, and can no longer soften the flow after reheating after molding.
Thermosets differ from thermoplastics in many ways in terms of performance, such as high strength, good creep resistance, high heat resistance, high dimensional accuracy and arc resistance. Its disadvantage is that it is difficult to process, and the conventional processing methods are molding and lamination.
There are few varieties of general-purpose thermosetting plastics, and there are only four kinds of phenolic resins, amino resins, epoxy resins and unsaturated polyesters.
1. Phenolic resins and plastics
The polymer made by the polycondensation reaction of phenolic compounds and aldehyde compounds is called phenolic resin, among which phenolic resin with phenol and formaldehyde as raw materials is the most commonly used, and the English abbreviation is PF. Products made with phenolic resin as the main ingredient and adding other additives are called phenolic plastics.
Phenolic resin has developed rapidly due to its low price, abundant raw materials and unique properties, and currently ranks sixth in plastics, accounting for about 5%, and ranks first in thermosetting plastics.
Due to the shortcomings of brittleness and low mechanical strength, pure PF resin is rarely processed into products alone. Generally, phenolic plastic products are modified by adding a large number of fillers to the resin, and the different types of fillers have different properties and are used in different fields.
(1) Phenolic resin
1. Thermoplastic phenolic resin
Thermoplastic phenolic resins have a linear structure and are soluble and fusible. Pure thermoplastic phenolic resins do not cross-link and cure when heated, and can only be cured when an appropriate curing agent is added.
Thermoplastic phenolic resins are in powder form and are commonly used as raw materials for PF molding powders and foams.
2. Thermosetting phenolic resin
Thermosetting phenolic resin can be divided into three orders: A, B and C, with Class A PF being linear soluble fusible resin, B order PF being a semi-soluble fusible resin with a small amount of cross-linking, and Class C PF being cross-linked insoluble non-melting resin. Generally, most of the synthetic PF resins are controlled at the first or second order to ensure that they can flow during the processing of products. No reinforcing agent is required during processing, and it can be cured by heating, while PF resins in the C rank are difficult to process.
Thermosetting phenolic resins are powdered solids or emulsion-like liquids, which are commonly used as raw materials for lamination, foam and casting products.
3. Modified PF resin
Whether thermoplastic or thermosetting PF, there are many shortcomings, such as low heat resistance and oxidation resistance, brittleness, poor adhesion to glass fibers, easy water absorption, high frequency insulation and poor arc resistance, etc., which can be overcome by modification methods.
The specific modified varieties of PF are as follows:
(1) Polyvinyl acetal modified PF can improve the adhesion of PF to glass fiber, overcome its brittleness, and improve the mechanical strength of FRP products.
(2) Epoxy resin modified PF modified varieties have both the adhesion of epoxy resin and the heat resistance of PF to overcome the brittleness of PF.
(3) Silicone-modified PF can improve the brittleness, heat resistance, low temperature resistance, impact strength and toughness of PF, and is often used in aviation structural materials, heat resistance, insulating materials and melting materials.
(4) Boron-modified PF uses boric acid and phenol to generate borate phenol, and then reacts with formaldehyde to improve heat resistance, instantaneous high temperature resistance and mechanical strength, etc., and is used for brake parts and high-temperature resistant melting materials. For example, boron phenolic glass fiber composites have excellent high temperature resistance and ablative properties, and are ideal ablative materials in space technology such as rockets and missiles.
(5) Phosphorus-modified PF modified varieties have outstanding heat resistance and flame resistance.
(6) Blended PF specific blended varieties include nitrile rubber, thermoplastic elastomer, polyvinyl chloride and nylon. Others include aniline, xylene and melamine modifications.
(2) Processing of phenolic plastic products
1. Phenolic molding compound
Phenolic molding compound, also known as phenolic compression molding powder, is based on phenolic resin and composed of powdered fillers, curing agents and lubricants.
PF has poor processability and is difficult to recycle. In the early days, it could only be molded by pressing method, but later the injection molding method was developed to expand the range of applications. The temperature control of injection molding should be precise to absolutely prevent the material from curing in the barrel.
PF molding compounds are mainly used for electrical insulation, daily necessities, automotive appliances and instrument parts, etc., and the specific products include electrical switches, lamp heads, telephone shells, bottle caps, buttons, handles, household appliance parts, brake pads, etc.
2. Phenolic laminated products
Phenolic laminated products are made of PF resin as the adhesive, kraft paper, cotton cloth, asbestos cloth, glass silk cloth, etc. as the base material, and are cured into laminates, tubes or other shapes after heating and pressure treatment.
The advantages of phenolic laminated products are small relative density (1.3~1.4), small water absorption, mechanical properties such as tensile and compressive strength are larger than PF molded products, good electrical insulation, low thermal conductivity and low friction coefficient, can be arbitrarily machined, and the cloth base pressing plate is impact-resistant, bending-resistant, torsion-resistant and vibration-absorbing force, and can be used for larger vibration parts such as gears.
Phenolic laminates on different substrates have different properties and uses:
(1) The stability of the paper base pressing plate is not high to strong acid, and it is not alkali resistant, but it is resistant to mineral oil, and the insulation and heat resistance are E grade. It can be used to manufacture electrical insulation structural parts such as wiring boards, insulating washers, etc.
(2) Compared with the paper base plate, it has higher mechanical strength and oil resistance, and is often used in mechanical parts such as washers, bearings, bearings, pulleys and silent gears, as well as telecommunications, radio equipment and insulating materials with low requirements.
(3) Compared with other substrate laminates, the glass filament cloth base pressing plate has the advantages of heat resistance, mechanical strength, dielectric properties and good chemical stability, and its heat resistance temperature can reach more than 200 °C, which is a class B insulation and heat resistance, and is an important insulation material in the electrical industry, which is widely used in electrical, electrical and radio engineering. Due to the poor adhesion between pure PF and glass fiber, epoxy resin is generally used as an adhesive. In the PF plate, the amount of glass filament cloth base pressure plate is the largest, which can account for 40%~60%.
(4) Asbestos base pressure plate Its advantages are outstanding heat resistance and abrasion resistance, so it is mainly used for wear-resistant materials such as brake pads and clutches, as well as mechanical parts that require high mechanical strength and heat resistance.
(5) Super fiber laminate Laminates made of polyamide fiber, carbon fiber, graphite, whiskers, etc. as substrates have excellent heat resistance and can be used as ablation-resistant shells for missiles and spacecraft
(6) Laminated tubes and rods are made of winding floor paper, cotton cloth and glass cloth as the base material, phenolic emulsion as the adhesive, and hot rolled and baked. It is mainly used for electrical insulation structural parts.
(7) Copper laminate Copper foil is applied on one or both sides of paper or fiberglass laminate for making printed circuit boards.
3. Phenolic foam
Phenolic foam has low price and good heat resistance (up to 150°C, up to 200°C). PS foam is 70 °C, PU is 120 °C), light weight, good thermal insulation, high rigidity, good dimensional stability, flame retardant, no dripping combustion, low smoke and other advantages. Phenolic foam is mainly used for heat-resistant and insulating building materials, hydrophilic materials and life-saving materials for preserving and transporting flowers, etc.
4. Other phenolic plastic products
(1) Phenolic packaging material The traditional semiconductor packaging material is mainly epoxy resin (EP), but with the increasing requirements for insulation and thermal expansion, PF modified EP is used as the packaging material, which has the advantages of good fluidity, low thermal expansion coefficient and low water absorption.
(2) Phenolic fiber material is made of linear PF, which is cross-linked into thermosetting polymer after drawing, and there are two kinds of wire drawing methods: melt spinning method and wet spinning method.
The flame retardancy of phenolic fiber material is good, and the oxygen index is 36 for melt spinning method and 28~30 for wet spinning method. In the flame of 2500°C, it neither melts nor burns, and has good thermal insulation and abrasion resistance under ablative conditions. PF fiber materials can be used to make fireproof clothing, heat-insulating clothing, industrial fireproof boards, high-temperature sealing rings, brake pads, clutch friction plates, etc.
(3) Phenolic shell mold resin Phenolic shell mold resin is used for sand mold in the casting industry. About 70% of the shell resin is used for casting automobile engine shells.