What Is Resol Resin?
Resol resin is a type of phenolic resin.
It is obtained by reacting phenol and formaldehyde with an alkali catalyst and curing. There is also a type of resin called novolac. While resol resins use an alkali catalyst, novolac resins are reacted with an acid catalyst.
Also, while novolac is a thermoplastic resin, resol resins are thermosetting resins that cross-link and cure by heating.
Uses of Resol Resins
Today, resol resins are rarely molded as single items. They are generally processed into pellets and used as building materials. Examples include soundproofing and heat insulation materials, wall and ceiling panels, flooring materials, and architectural coatings. Resol resins are used because of their high heat resistance and fire resistance.
They are also compatible with a variety of base materials, so they are often used in combination with other base materials to take advantage of their respective properties. Generally, they are mixed with base materials to increase their strength before being molded. Phenolic resins, including resol resins, are used in large quantities in industrial products because of their high electrical insulation properties for their low cost.
When used as coatings for electronic components, they can reduce the risk of electric shock. As a molded product, it is sometimes used in switches and buttons of electrical appliances. In addition, it is sometimes used as a raw material for adhesives.
Adhesives containing resol resin are classified as phenolic resin-based adhesives and have superior water resistance and heat resistance compared to other resin-based adhesives. In this application, they are often used on their own.
Properties of Resol Resins
Resol resins have the following advantages:
- High Strength and Hardness
Resol resin is very hard, durable, and has excellent compressive, tensile, and shear strength. - High Heat Resistance
Resol resin has excellent heat resistance and is suitable for use at high temperatures. - High Chemical Resistance
Resol resin has excellent chemical resistance and is highly durable against chemicals, oil, and water. - High Flame Retardancy
Resol resins are flame retardant, highly durable against fire, and have excellent fire resistance. - High Adhesive Strength
Resol resins have high adhesive strength and are used as adhesives and paints. - Beautiful Tones.
Resol resins have beautiful colors and can produce glossy surfaces.
On the other hand, phenolic resin is brittle due to its hardness, making it vulnerable to impact. In addition, although they are highly flame retardant and have excellent fire and flame shielding properties, the smoke and toxic gases generated by heating and combustion can be harmful to the human body.
Other Information on Resol Resins
1. History of Resol Resins
Phenolic resins, including resol resins, are the oldest synthetic plastics and have a long history. 1907 saw the invention of Bakelite in the US, which was initially used for decorative purposes such as watches due to its high gloss.
Later, as a substitute for metal, it became popular for tableware and kitchenware in the 1930s, and in the 1940s it was used for grips and hand guards for firearms, electrical plugs and switches, and automotive parts. However, the use of phenolic resin as it is decreasing due to the introduction of various plastic materials that are cheaper, suitable for mass production, and have a certain level of functionality.
2. Manufacturing Process of Resol Resin
The production of resol resin proceeds in a two-step reaction. The initial polymerization with an alkali catalyst is the first step and then the cross-linking through heating. First, phenol and formaldehyde are reacted in the presence of an alkali catalyst. At this point, the cross-linking has not yet progressed, and the resin is in a fusible state. It also has many hydroxymethyl groups and can be dissolved in solvents.
Next, an oxidizing agent such as sulfuric acid or chromic acid is added to the resol and heated. In this reaction, cross-linking proceeds, resulting in an insoluble, insoluble resin. Care must be taken to avoid too much cross-linking by heating at this stage. If cross-linking proceeds too far, the resin becomes brittle.
Taking advantage of this two-step reaction, it is possible to use general processing methods for processing thermoplastic resins, such as injection molding in the first step and then heating to cross-link, even though it is a thermosetting resin that hardens when heated and does not return to its original form. Note that the properties of resol resins vary depending on the amount and type of alkali catalyst and the reaction conditions between phenol and formaldehyde.