Thermosetting Polymers

-- Highly cross- linked (network) THERMOSETS OR THERMOSETTING POLYMERS : A thermoset is a type of plastic that undergoes a chemical change during production that cannot be repeated or reversed -- Highly cross- linked (network) -Thermosetting polymers initially begin as linear chains -Heat or pressure – initiates the cross-linking process -Cross-linking results in permanent chemical bonding of the individual polymer chains to one another -Cross-linking is irreversible – thermosets can’t be recycled

Thermoset plastics become hard and rigid on heating- on cooling the properties are retained By step growth mechanism(condensation polymerization), the chemical reactions are enhanced at higher temperatures and irreversible Properties: -- possess good strength , stiffness and hardness - poor ductility, poor impact properties - behavior similar to brittle metal or ceramic

Thermosets with fuctional units :

Thermoset Properties

PHENOLICS : Uses: - Bakelite – common phenolic thermoset Condensation reaction between phenol and formaldehyde - initial linear phenolic resin process continues until a linear phenol-formaldehyde chain is formed A third location on each phenol ring provides a site for cross-linking with the adjacent chains Uses: Adhesives Coatings Moulded components for electrical or motor applications

AMINES : - Produced by combining urea or melamine monomers with formaldehyde Uses: Adhesives Laminates Moulding materials for cookware Electrical hardware like circuit breakers , switches

Melamine formaldehyde (MF) COMMON NAME: Formica or Melaware - Used in the production of plastic laminates because of its smooth surface and hygienic qualities. PROPERTIES waterproof odourless mark and scratch resistance USES worktops buttons electrical insulation

POLYESTERS: - Form chains from alcohol and acid molecules by condensation reaction giving water as by-product Uses: Used as molding or casting materials for a variety of electrical applications Decorative laminates Boats and other marine equipment As a matrix for composites such as fiberglass

Polyester resin COMMON NAME: Orel or Beetle PROPERTIES Stiff, hard and brittle Resilient as laminated GRP USES panels for car bodies and boat hulls

EPOXIES : - Formed from molecules containing a tight C-O-C ring - During polymerization, the C-O-C rings are opened – bonds are rearranged to join the molecules Uses : Adhesives Rigid molded parts for electrical applications Automotive components Circuit boards Sporting goods Matrix for high performance fiber-reinforced composite materials for aerospace applications

Epoxy resin (ER) PROPERTIES Good electrical insulator hard COMMON NAME: Araldite Many adhesives (glues) are thermosetting plastics It will bond most materials including woods and metals as well as some plastics PROPERTIES Good electrical insulator hard brittle unless reinforced

POLYIMIDES : Ring structure that contains nitrogen atom One special group – the bismaleimides (BMI) important in aircraft and aerospace industry Operates continuously at temperatures of 175 ˚C and do not decompose even upto 460 ˚C INERPENETRATING POLYMER NETWORKS : -Produced when linear thermoplastic chains are inter-twined through a thermosetting framework Eg . Nylon, polypropylene chains can penetrate into a cross-linked silicone thermoset

Differences Between Thermoplastic And Thermosetting Polymers S.No THERMOPLASTIC THERMOSETTING 1. 2. 3. 4. 5. Long, straight chained carbon compounds Formed by addition polymerization Low strength compared to thermosets Can be processed again and again Eg . Polyethylene Cross-linked compounds Formed by condensation(step growth) polymerization More strength compared to thermoplastics Cannot be processed again or recycled Eg . Bakelite, epoxy