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Thermosetting polymer

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Left: individual linear polymer chains
Right: Polymer chains which have been cross linked to give a rigid 3D thermoset polymer

A thermosetting polymer, resin, or plastic, often called a thermoset, is a polymer that is irreversibly hardened by curing from a soft solid or viscous liquid prepolymer^[1] Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalystcross-linking between polymer chains to produce an infusible and insoluble

The starting material for making thermosets is usually malleable or liquid prior to curing, and is often designed to be moldedadhesivethermoplastic

Contents

• 1 Chemical process
• 2 Properties
• 3 Fiber-reinforced materials
• 4 Examples
• 5 See also
• 6 References

Chemical process[edit]

Curing a thermosetting resin transforms it into a plastic, or elastomer (rubber) by crosslinking

Acrylic resins, polyesters and vinyl esters with unsaturated sites at the ends or on the backbone are generally linked by copolymerisation with unsaturated monomer diluents, with cure initiated by free radicals generated from ionizing radiation or by the photolytic or thermal decomposition of a radical initiator – the intensity of crosslinking is influenced by the degree of backbone unsaturation in the prepolymer;^[2]

Epoxy functional resins can be homo-polymerized with anionic or cationic catalysts and heat, or copolymerised through nucleophilic addition reactions with multifunctional crosslinking agents which are also known as curing agents or hardeners. As reaction proceeds, larger and larger molecules are formed and highly branched crosslinked structures develop, the rate of cure being influenced by the physical form and functionality of epoxy resins and curing agents^[3] – elevated temperature postcuring induces secondary crosslinking of backbone hydroxyl functionality which condense to form ether bonds;

Polyurethanes form when isocyanate resins and prepolymers are combined with low- or high-molecular weight polyols, with strict stochiometric ratios being essential to control nucleophilic addition polymerisation – the degree of crosslinking and resulting physical type (elastomer or plastic) is adjusted from the molecular weight and functionality of isocyanate resins, prepolymers, and the exact combinations of diols, triols and polyols selected, with the rate of reaction being strongly influenced by catalysts and inhibitors; polyureas form virtually instantaneously when isocyanate resins are combined with long-chain amine functional polyether or polyester resins and short-chain diamine extenders – the amine-isocyanate nucleophilic addition reaction does not require catalysts. Polyureas also form when isocyanate resins come into contact with moisture;^[4]

Phenolic, amino and furan resins all cured by polycondensation involving the release of water and heat, with cure initiation and polymerisation exotherm control influenced by curing temperature, catalyst selection or loading and processing method or pressure – the degree of pre-polymerisation and level of residual hydroxymethyl content in the resins determine the crosslink density.^[5]

Benzoxazines^[6]

Thermosetting polymer mixtures based on thermosetting resin monomers and pre-polymers can be formulated and applied and processed in a variety of ways to create distinctive cured properties that cannot be achieved with thermoplastic polymers or inorganic materials.^[7][8] Application/process uses and methods for thermosets include protective coating, seamless flooring, civil engineering construction grouts for jointing and injection, mortars, foundry sands, adhesives, sealants, castings, potting, electrical insulation, encapsulation, 3D printing, solid foams, wet lay-up laminating, pultrusion, gelcoats, filament winding, pre-pregs

• Reactive injection moulding (used for objects such as milk bottle crates)
• Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables)
• Compression molding (used to shape SMC and BMC thermosetting plastics)
• Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)

Properties[edit]

Thermosetting plastics are generally stronger than thermoplastictemperature^[9] They normally decompose before melting.

Hard, plastic thermosets may undergo permanent or plastic deformation under load. Elastomers, which are soft and springy or rubbery and can be deformed and revert to their original shape on loading release.

Conventional thermoset plastics or elastomers cannot be melted^[10] New developments involving thermoset epoxy resins which on controlled and contained heating form crosslinked networks permit repeatedly reshaping, like silica glass by reversible covalent bond exchange reactions on reheating above the glass transition temperature.^[11] There are also thermoset polyurethanes shown to have transient properties and which can thus be reprocessed or recycled.^[12]

Fiber-reinforced materials[edit]

When compounded with fibers, thermosetting resins form fiber-reinforced polymer^[13] and as site-applied, cured and finished composite repair^[14][15] and protection materials. When used as the binder for aggregates and other solid fillers, they form particulate-reinforced polymer composites, which are used for factory-applied protective coating or component manufacture, and for site-applied and cured construction, or maintenance

Examples[edit]

• Polyester resin
• Polyurethanes
• Polyurea/polyurethane
• Vulcanized rubber
• Bakelite, a phenolformaldehyde
• Duroplast
• Urea-formaldehyde foam used in plywood
• Melamine resin^[16]
• Diallyl-phthalate (DAP) used in high temperature and mil-spec electrical connectors and other components. Usually glass filled.
• Epoxy resin ^[17] used as the matrix component in many fiber reinforced plasticsgraphite-reinforced plastic; casting; electronics encapsulation^[18]; construction; protective coatings; adhesives; sealing and joining.
• Epoxy novolac resins used for printed circuit boards, electrical encapsulation, adhesives and coatings for metal.
• Benzoxazines
• Polyimides and Bismaleimides
• Cyanate esters
• Mold or mold runners (the black plastic part in integrated circuits or semiconductors).
• Furan resins used in the manufacture of sustainable biocomposite construction,^[19] cements, adhesives, coatings and casting/foundry resins.
• Silicone
• Thiolyte
• Vinyl ester

See also[edit]

• Fusion bonded epoxy coating
• Thermoset polymer matrix
• Vulcanization

References[edit]

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "thermosetting polymerdoi:10.1351/goldbook.TT07168
2. ^ Unsaturated Polyester Technology, ed. P.F. Bruins, Gordon and Breach, New York, 1976
3. ^ Chemistry and Technology of Epoxy Resins, ed. B. Ellis, Springer Netherlands, 1993, ISBN 978-94-010-5302-0
4. ^ Polyurethane Handbook, ed. G Oertel, Hanser, Munich, Germany, 2nd edition, 1994, ISBN 1569901570, ISBN 978-1569901571
5. ^ Reactive Polymers Fundamentals and Applications: A Concise Guide to Industrial Polymers (Plastics Design Library), William Andrew Inc., 2nd edition, 2013, ISBN 978-1455731497
6. ^ http://polymerdatabase.com/polymer%20classes/Polybenzoxazine%20type.html
7. ^ Concise Encyclopedia of Polymer Science and Engineering, ed. J.I. Kroschwitz, Wiley, New York, 1990, ISBN 0-471-5 1253-2
8. ^ Industrial Polymer Applications: Essential Chemistry and Technology, Royal Society of Chemistry, UK, 1st edition, 2016, ISBN 978-1782628149
9. ^ S.H. Goodman, H. Dodiuk-Kenig, ed. (2013). Handbook of Thermoset Plastics (3rd ed.). USA: William Andrew. ISBN 978-1-4557-3107-7
10. ^ The Open University (UK), 2000. T838 Design and Manufacture with Polymers: Introduction to Polymers, page 9. Milton Keynes: The Open University
11. ^ D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Science, 2011, 334, 965-968], doi:10.1126/science.1212648
12. ^ Fortman, David J.; Jacob P. Brutman; Christopher J. Cramer; Marc A. Hillmyer; William R. Dichtel (2015). "Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers". Journal of the American Chemical Society. doi:10.1021/jacs.5b08084
13. ^ Polymer Matrix Composites: Materials Usage, Design, and Analysis, SAE International, 2012, ISBN 978-0-7680-7813-8
14. ^ PCC-2 Repair of Pressure Equipment and Piping, American Society of Mechanical Engineers, 2015, ISBN 978-0-7918-6959-8
15. ^ ISO 24817 Composite Repairs for Pipework: Qualification and Design, Installation, Testing and Inspection, 2015, ICS: 75.180.20
16. ^ Roberto C. Dante, Diego A. Santamaría and Jesús Martín Gil (2009). "Crosslinking and thermal stability of thermosets based on novolak and melamine". Journal of Applied Polymer Science. 114 (6): 4059–4065. doi:10.1002/app.31114
17. ^ Guzman, Enrique; Cugnoni, Joël; Gmür, Thomas (2014). "Multi-Factorial Models of a Carbon Fibre/Epoxy Composite Subjected to Accelerated Environmental Ageing". Composite Structures. 111 (4): 179–192. doi:10.1016/j.compstruct.2013.12.028
18. ^ Kulkarni, Romit; Wappler, Peter; Soltani, Mahdi; Haybat, Mehmet; Guenther, Thomas; Groezinger, Tobias; Zimmermann, André (1 February 2019). "An Assessment of Thermoset Injection Molding for Thin-Walled Conformal Encapsulation of Board-Level Electronic Packages". Journal of Manufacturing and Materials Processing. 3 (1): 18. doi:10.3390/jmmp3010018
19. ^ T Malaba, J Wang, Journal of Composites, vol. 2015, Article ID 707151, 8 pages, 2015. doi:10.1155/2015/707151

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Categories:

• Polymer chemistry
• Thermosetting plastics