CHAPTER - 01
(BASICS OF THERMOSET MATERIALS)
We Use Plastics........From Morning to Evening........🤔🤔
BASICS OF THERMOSET
Thermosetting plastics are made up from long chains of molecules that are cross-linked. They have a very rigid structure. Once heated, thermosetting plastics can be molded, shaped and pressed into shapes. Once set they cannot be reheated since they are permanently set.
Examples of Thermosetting Polymers:
- Phenol Formaldehyde (PF)
- Urea Formaldehyde (UF)
- Melamine Formaldehyde (MF)
- Polyesters
- Epoxy Resin
PHENOL FORMALDEHYDE
Phenol formaldehyde was the first commercial synthetic resins. PFs are synthetic polymers obtained by the reaction of phenol (an aromatic alcohol derived from benzene) or substituted phenol with formaldehyde (a reactive gas derived from methane). It is used as the basis for Bakelite.
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Structure of Phenol Formaldehyde |
Manufacturing of PF-
In industrial practice, there are two basic methods for making the polymer into useful resins.
- In one method, an excess of formaldehyde is reacted with phenol in the presence of a base catalyst in water solution to yield a low-molecular-weight prepolymer called a resole. The resole, frequently in liquid form or solution, can be cured to a solid thermosetting network polymer.
- The other method involves reacting formaldehyde with an excess of phenol, using an acid catalyst. This process produces a solid prepolymer called a novolac (or novolak), which resembles the final polymer except that it is of much lower molecular weight and is still thermoplastic.
RESOLE:
- Base-catalyzed phenol-formaldehyde resins are made with a formaldehyde to phenol ratio of greater than one (usually around 1.5). These resins are called resoles. Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on the resin to be formed, and are then heated. The first part of the reaction, at around 70 °C, forms a thick reddish-brown tacky material, which is rich in hydroxymethyl and benzylic ether groups.
- Hydroxymethyl phenols will crosslink on heating to around 120 °C to form methylene and methyl ether bridges through the elimination of water molecules. At this point the resin is a 3-dimensional network, which is typical of polymerized phenolic resins.
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| Structure of Resole |
NOALAC:
- Novolac are phenol-formaldehyde resins with a formaldehyde to phenol molar ratio of less than one. In place of phenol itself, they are often produced from cresols (methyl phenols). The polymerization is brought to completion using acid-catalysis such as sulfuric acid, oxalic acid, hydrochloric acid and rarely, sulfonic acids. The phenolic units are mainly linked by methylene and/or ether groups. The molecular weights are in the low thousands, corresponding to about 10–20 phenol units. Obtained polymer is thermoplastic and require a curing agent or hardener to form a thermoset. Hexamethylenetetramine is a hardener added to crosslink novolac. At a temperature greater than 90 °C, it forms methylene and dimethylene amino bridges. Resoles can also be used as a curing agent (hardener) for novolac resins. In either case, the curing agent is a source of formaldehyde which provides bridges between novolac chains, eventually completely crosslinking the system.
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| Structure of Novalac |
Properties of Phenol-Formaldehyde Resin:
- High Heat Resistance: PF resins exhibit excellent heat resistance and can withstand high temperatures without significant degradation. This property makes them suitable for applications requiring thermal stability.
- Mechanical Strength: They have high mechanical strength, including good tensile, compressive, and impact strengths. This property makes PF resins ideal for structural applications.
- Dimensional Stability: PF resins have low shrinkage and excellent dimensional stability over a wide range of temperatures, which is important for applications where precise dimensions are critical.
- Chemical Resistance: They are resistant to many chemicals, oils, and solvents, making them suitable for use in harsh environments.
- Electrical Insulation: PF resins possess good electrical insulation properties, which make them suitable for electrical and electronic applications.
Applications of Phenol-Formaldehyde Resin:
- Wood Adhesives: PF resins are commonly used as adhesives in the production of plywood, particleboard, and fiberboard. They provide strong bonds and improve the water resistance and durability of wood products.
- Molding Compounds: PF resins are used in compression molding and transfer molding processes to produce molded articles such as electrical components, automotive parts, and household appliances.
- Coatings and Laminates: PF resins are used as binders in surface coatings, laminates, and varnishes to provide durability, scratch resistance, and moisture resistance.
- Foundry Binders: In foundry applications, PF resins are used as binders for sand cores and molds due to their excellent strength and heat resistance.
- Electrical Insulation: PF resins are used in the manufacture of electrical insulation materials, including laminates, adhesives, and potting compounds.