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Selection and application of high performance phenolic resin for abrasive cutting disc manufacturing(1)

1. Introduction of abrasive cutting disc

Phenolic resins have been used for nearly a hundred years. Because phenolic resin raw materials are easy to obtain, the price is low, the production process and equipment are simple, and the products have excellent mechanical properties, heat resistance, cold resistance, electrical extinction, dimensional stability, molding processing, flame retardancy and low smoke. 

Therefore, it has become an indispensable material in the industrial sector, and is widely used in various fields such as bonded abrasives, coated abrasives, friction materials, refractory materials, and bakelite powder, fireworks, and casting.

The phenolic resin is a polymer compound obtained by polycondensation of a phenol compound or an aldehyde compound as a raw material under the action of a catalyst, and a phenol resin which is polycondensed with phenol and formaldehyde is most important.

Phenolic resins are roughly classified into two types: thermosetting and thermoplastic. The thermosetting resin is synthesized by reacting phenol with an excess amount of formaldehyde under alkaline conditions; the thermoplastic resin is synthesized by reacting phenol with a small amount of formaldehyde under acidic conditions. The factors affecting the synthesis of phenolic resin and determining the properties of the resin are: chemical structure of the raw material and monomer functionality, molar ratio of phenolic aldehyde, properties of the catalyst and pH of the reaction medium.

Thermosetting resins have reactive functional groups that cure under both heat and acid action. This automatic reaction precisely explains why the viscosity of the thermosetting resin increases during storage and the gel speed increases. Since the automatic reaction is inherent in the thermosetting resin, the reaction rate is doubled every 10 °C increase in temperature. Therefore, the thermosetting resin must be stored under low temperature conditions in order to maximize its shelf life. The thermoplastic resin needs to be added with a curing agent to crosslink. The most commonly used curing agent for thermoplastic resins is hexamethylenetetramine (commonly known as urotropine), and the resin that has been crosslinked and cured contains a part of nitrogen, which is derived from urotropine. 

The phenolic resin forms a three-dimensional network structure from the A stage to the B stage and the C stage to form a solidification. Both the linear resin and the resin having a small molecular weight are melted, so the resin at this time is referred to as an A-stage resin. When the resin is hardened, it is in the gel phase, phase B. At this stage, the resin swollen nitrogen can still be dissolved by the solvent, and this is the C stage.

With the development of the industry, higher requirements have been placed on high performance materials, such as higher decomposition temperature, better wear resistance, sufficient toughness and strength. Since the phenolic resin has a structural weakness: the phenolic hydroxyl group and the subunit are easily oxidized, so heat resistance is affected.

Ordinary phenolic resins can be used stably for a long period of time at temperatures below 200 ℃, but they change significantly beyond 200 ℃. From 300 ℃ to 360 ℃ into the thermal decomposition stage, to 600 ℃ -900 ℃ release of CO, CO 2, H 2 O, phenol and other substances. Moreover, ordinary phenolic resin releases water molecules when it is cured, which is brittle and has poor toughness, which limits its development in high-performance materials. Therefore, it is necessary to improve the phenolic resin to improve its toughness and heat resistance. 

The main ways to improve phenolic resin are:

1) Add external toughening substances such as natural rubber, nitrile rubber, styrene-butadiene rubber and thermoplastic resin to the phenolic resin.

2) Adding an internal toughening substance to the phenolic resin, such as etherifying the phenolic hydroxyl group, introducing a long methylene chain and other flexible groups between the phenolic cores.

3) Improve the brittleness with reinforcing materials such as glass fiber, glass cloth and asbestos.

Other improved methods include: etherification of phenolic hydroxyl groups of phenolic resins, esterification, chelation of heavy metals, or addition of curing agent, strict molding conditions or post-cure conditions, or introduction of rigid structures such as imine rings or triazine rings. . Although these methods improve the heat resistance of the resin, the toughness is lowered. Therefore, it is currently difficult to simultaneously improve both the toughness of the resin and the heat resistance thereof.

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