Thermosetting resins

1. Thermosetting resins John Summerscales

2. Thermosets - outline of lecture phenol-formaldehyde (phenolic resin) epoxides (generally diglycidyl ethers) polyurethanes bismaleimides (BMI) unsaturated polyesters (UP or UPE) vinyl esters methacrylics vitrimers ( covalent adaptable networks: CAN )

3. Thermosets • generally supplied as a liquid • cross-linked (cured) by chemicals (and heat) • heat reduces the instantaneous viscosity • heat increases the rate of cure • cure decreases the viscosity over time • product is a 3D molecular network • whereas a thermoplastic is usually a 2D chain

4. Total Flow (------) Progress of cure Ease of Flow Temperature

5. Stages of cure • A-stage: soluble and fusible • aka. resol in phenolics • B-stage: may be swollen but not dissolved by a variety of solvents • aka resitol in phenolics • C-stage: rigid, hard, insoluble, infusible • aka resit in phenolics

6. Phenolic resins • first truly synthetic resins exploited. • Butlerov (1859) formaldehyde polymers. • Adolf Bayer (1872) phenols and aldehydes react to form resinous substances. • Arthur Smith (1879) first British Patent (16274) for phenol-aldehyde resins as an ebonite substitute in electrical insulation. • Baekeland (1907) controlled and modified the reaction to produce useful products

7. Phenolic resins • can be broadly divided into three groups:

8. Phenol (C6H5OH) • naming reactive sites: OH ortho- meta- para-/tere- iso-

9. Phenolic resins • Phenol ...and... formaldehyde sites react first OH H O C H

10. Phenolic resin: methylolation • using Φ to represent phenol • Φ + CH2O Φ CH2OH • Φ CH2OH + CH2O Φ (CH2OH)2 • Φ (CH2OH)2 + CH2O Φ (CH2OH)3

11. Resoles • alkaline catalyst and excess formaldehyde • methylene bridge formationmay result in the release of water: • HO~CH2~Φ~CH2~OH + HO~CH2~Φ • HO~CH2~Φ~CH2~O~CH2~Φ,  or •  HO~CH2~Φ~CH2~Φ~CH2~OH • continued reaction to network • first product may lose formaldehyde: • HO~CH2~Φ~CH2~O~CH2~Φ  HO~CH2~Φ~CH2~Φ  +  CH2O

12. Novolacs • acid catalyst and excess of phenol: Φ~CH2~OH + ΦΦ~CH2~Φ + H2O • further condensationand methylene (-CH2-)bridge formation results in fusible and solublelinear low MW polymers (novolacs): ~Φ~CH2~Φ~CH2~Φ~CH2~Φ~CH2~Φ~

13. Crosslinking novolacs I • add paraform or further formaldehyde • more usually add HMT hexamethylenetetramine: (CH2)6N4 • 2 (CH3)2Φ [(CH3)2ΦCH2]2NH • 3 (CH3)2Φ[(CH3)2ΦCH2~]3N

14. Crosslinking novolacs II • when the benzylamines areheated at 180-190°Cin the presence of phenol,ammonia or methylamine are evolved: • [(CH3)2ΦCH2]2NH(CH3)2ΦCH2Φ(CH3)2 + NH3 • [(CH3)2ΦCH2]3N(CH3)2ΦCH2Φ(CH3)2 + CH3NH2

15. Phenolics • Generally brittledue to moisture released during curing • Exceptional FST properties when burning: • low spread of flame • low emission of smoke • low toxicity: only CO2 and H2O released • Key markets: • underground railways • mining • submarines

16. Epoxy resins

17. Epoxy (glycidyl) groups O CH2 CH CH2 CH2(O)CH-CH2- Epoxy Glycidyl NB: bond angles of 60°, rather than 109°28´ of sp3 hybrid: highly strained,  highly reactive

18. Epoxy resin O O CH3 -C- CH3 -O-CH2CH-CH2 CH2CH-CH2-O- • Epichlorohydrin: CH2(O)CH-CH2-O-Cl • Bisphenol-A: HOΦ-C(CH3)2-ΦOH  CH2(O)CH-CH2-OΦ-C(CH3)2-ΦOH + HCl Di Glycidyl Ether of Bisphenol-A (DGEBA)

19. DGEBA • methylene (-CH2-) and ether (-O-) groupsgive free movement of atoms before cure: • less steric hindrance and higher reactivity for terminal rather than internal epoxy (oxirane) • terminal epoxy groups mean crosslink sites are free of mobile chain endsso higher Tg achieved. https://www.sigmaaldrich.com/content/dam/sigma-aldrich/structure8/196/mfcd00080480.eps/_jcr_content/renditions/mfcd00080480-large.png

20. Cure of epoxy resins • Reactive site is the 3-atom epoxy ring • which may yield an hydroxyl group • Curing agents include: • amines • amides • carboxylic acids • anhydrides: 2 carboxylic acids with water removed

21. Epoxy cure temperatures • low temperature • ambient to 60°C • medium temperature • up to 120°C • high temperature • up to 180°C • pot-life: time from mixing to 1500 mPas • fibres stick to the brush during lamination • only tow surfaces are wetted.

22. Gel time of epoxy resin

23. Post-cure • full cure uses 100% of reactive sites • but constrained movement of polymer chain may lead to incomplete cure: • lower glass transition temperature • lower resin density • fewer bonds/m3 lower moduli and strengths • additional free volume • easier diffusion of chemicals  reduced durability • to achieve optimum high-performance composites, post-cure at higher temp’ shortly after production: • unreacted sites may become inactive over time.

24. Tg for high-performance epoxy • DGEBA/DICY Tg ~180-190ºC • di glycidyl ether of bis phenol A • aliphatic dicyandiamide • TGDDM/DDS Tg ~240-260ºC • tetra glycidyl-4,4‘-diamino diphenyl methane • aromatic diamino diphenyl sulphone • advantages: • low cure reactivity … long storage times • strength retention after time at temperature • disadvantages • low strain to failure • high moisture absorption • poor hot/wet performance

25. Epoxy (vs polyester) resin • outstanding adhesion • excellent static and fatigue strengths • outstanding corrosion protection • excellent chemical resistance • excellent weather resistance • very low shrinkage on curing • good toughness • good heat resistance from AB Strong “Fundamentals of Composites Manufacturing” (1989)

26. Epoxy (vs polyester) resin • medium to high cost • relatively difficult to handle • potential toxicity of uncured material • poor appearance after weathering from AB Strong “Fundamentals of Composites Manufacturing” (1989)

27. Polyurethanes primarily for RIM processes

28. Bismaleimide O C N C O imide group Note: delocalisation across • benzene ring • both C=O groups • p-orbital on N  stiff molecule

29. Unsaturated polyesters

30. Curing of polyester resin - A - B - A - B - A - B - | | | S S S | | | - A - B - A - B - A - B - | | | S S S | | | - A - B - A - B - A - B - -A-B-A-B-A-B- where B block contains unsaturation (double bonds) in backbone of polymer chain, leading to a 3D network plus styrene (S:ΦCH=CH2 ) reactive diluent

31. Curing of polyester resin

32. Vinyl esters • epoxy backbone • addition-type curing Methacrylics • methylmethacrylate instead of styrene as reactive diluent

33. Summary (key polymers) and finally, vitrimers …

34. Vitrimers/CAN • new class of materials from ~2010 • covalent adaptable networks (CAN) • cross-link exchange mechanisms • bulk synthetic organic polymers, processable like glass or metal • radical generation triggered by • heat: nanoparticles + electromagnetic heating, • light: photoinitiator or photothermal dyes • pH: acidity or basicity of solvent. • claims for easy reprocessing, recycling or repair • self-healing/mendable, triggerable shape change, stress reduction • Tv = topology freezing transition temperature • Tv may occur above or below Tg (but latter of minimal interest)

35. Vitrimers viscoelastic liquid (1012 Pa s) Tv topology freezing transition temperature viscoelastic solid rubbery (tough/viscoelastic) Tg glass transition temperature glassy (brittle /elastic) HOT COLD

36. Summary of thermosetting resins lecture phenol-formaldehyde (phenolic resin) epoxides (generally diglycidyl ethers) polyurethanes bismaleimides (BMI) unsaturated polyesters (UP or UPE) vinyl esters methacrylics vitrimers ( covalent adaptable networks: CAN )