Classes of Polymeric Materials Chapter 3 Thermosets Professor Joe Greene CSU CHICO 1 Thermosetting Resins thermosets Introduction Thermoplastics are supplied as pellets powders or granules and do not undergo a chemical reaction Thermoplastics have large molecular weights long molecules The high viscosities are reduced by high temperatures Thermoset resins are supplied as liquid chemicals low MW and low viscosity and undergo a chemical reaction that features polymerization and crosslinking Liquid chemicals have short chains that polymerized into long chains and high molecular weights and high viscosity The chains are crosslinked attached to each other to make a stiff molecule Rubbers involve cross linking of already polymerized molecules to stiffen the molecules together in Vulcanization Heat is needed to cause polymerization to build MW and to cause stiffening of molecule through cross linking Heat reduces the viscosity of the chemicals until the reaction occurs and then causes the viscosity to get very large during crosslinking 2 Thermosetting Resins thermosets Types of thermosets Temperature activated Catalyst activated Mixing activated Temperature activated Fig 3 84 All thermosets require heat to undergo chemical reaction Lower temperature thermosets room temperature cure react to a more rubbery polymer that gets stiffer upon additional heat Pot life time that it takes for the thermosets to react to a solid after mixed Gel time time it takes for two liquid thermoset polymers that are mixed to form a gel or skin and stop flowing Several thermosets are supplied as powder or granular form Heat reduces the viscosity and melts the polymer to allow it to flow mold Additional heat triggers a chemical reaction which forms a cross linked 3D Common heat activated polymers Formaldehyde FOR phenoplasts PF amnioplasts UF polyester vinyl ester alkyd allyl furan some epoxies and polyimides 3 Thermosetting Resins thermosets Catalyst activated Fig 3 85 Some thermosets supplied as stable liquid form Small amount of liquid catalyst is added which starts a chemical reaction and leads to formation of 3D structure Chemical type and amount of catalyst controls the extent of reaction and the speed of polymerization Many systems can set at room temperature Useful for casting resins and for glass fiber reinforced composites Common polymer is unsaturated polyester resin UPR 4 Thermosetting Resins thermosets Mixing activated systems Fig 3 86 Some thermosets supplied as two stable liquids When the two are added together a chemical reaction starts and forms a 3D structure Ratio of the two chemicals and temperature controls the extent of reaction and the speed of polymerization Many systems can set at room temperature Useful for casting resins and for glass fiber reinforced composites Common polymers are polyurethane and epoxies Polyurethane can be mixed at high speeds in a Reaction Injection Molding RIM process 5 Commercial Thermosets Formaldehyde Systems Functional Groups H O C H Formaldehyde plus one of the three hydrogen containing chemicals to form a 3D molecular network Phenol Melamine or Urea HNH O N CN HN C NH H C C H H H N N N Condensation reaction involvingH the oxygenHand two hydrogens from two OH different molecules Phenol Urea or formaldehyde One stage systems with resols Two stage systems with novolacs prepolymers or precursers Usually have large amounts of filler e g wood flour cellulose fibers and minerals Supplied as powder or granual form or pills compacted preforms Molding temperatures 125 C 200 C and molding pressures of 2000 to 8 000 psi for compression molding and 18 000 psi for injection molding 6 Commercial Thermosets Formaldehyde Systems Functional Groups Phenoplasts phenolics are based on phenol and formaldehyde and were one of the first commercial polymers Bakelite and were used for billiard balls Used with other materials to act as a binder adhesive coatings surface treatments etc Applications Temperature resistant insulating parts for appliances handles knobs electrical components connectors distributor caps and bottle closures Abrasive binder for grinding wheels and brakes Decorative laminates counter tops or table tops Fire resistant rigid foams 7 Commercial Thermosets Formaldehyde Systems Functional Groups Aminoplasts amino resins are based on urea and formaldehyde or melamine and formaldehyde Can be made translucent or in light colors for aesthetics Urea formaldehyde resins are used for many of the same applications as phenolics if have color requirements Castable foam system is used for home insulation Melamine formaldehyde resins are based on melamine and formaldehyde Noted for their excellent water resistance Used for dishwater safe dinner ware which can be decorated with molded in paper overlays Form the surface layer for decorative laminates Formica Used as an adhesives for water resistant plywood 8 Commercial Thermosets Furan Systems H H C C C C O H H Feature a ring structure which can be opened cleaved to yield polymeric molecules which have 3 D molecular networks Combined with fomaldehyde related thermosets Used as binder for sand and foundry work or abrasive particles in grinding wheels Used as adhesives and matrix for reinforced plastics where corrosion resistance is important Allyl systems Pg 171 Manufacture involves the reaction of a monofunctional unsaturated alcohol allyl alcohol AA with a difunctional acid Ester linkages are formed though not a polymer 2 unsaturated C C per monomer permits formation of 3 D molecule with the use of catalysts and elevated temperatures DAP diallylphthalate is most common allyl monomer Thermoplastic pre polymers are available that are cured with little shrinkage Applications include high performance molding compounds for electrical 9 Commercial Thermosets Alkyd Systems Alkyd comes from alcohol alk and acid yd Reaction of difunctional alcohol and difunctional acids or anhydrides forms a polyester which is what alkyd is Used as coatings paints coatings varnishes Unsaturated Polyesters Thermoset reaction between a difunctional acid or anhydride and a difunctional alcohol glycol At least some of the acid or anhydride features double bonds between adjacent carbon atoms for unsaturation Characteristic ester linkages are formed hence the name Polyester 10 Polyester Chemistry Unsaturated Polyesters Thermoset reaction between a difunctional acid or anhydride and a difunctional alcohol glycol O C6H4 COOH 2 CH2 2 OH 2 terephthalic acid PET