Thermosetting resins John Summerscales

Thermosets - outline of lecture phenol-formaldehyde (phenolic resin) epoxides (generally diglycidyl ethers) polyurethanes bismaleimides (BMI) unsaturated polyesters (UP or UPE) vinyl esters methacrylics

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

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

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

Phenolic resins first truly synthetic resins to be exploited. Butlerov (1859) formaldehyde polymers. Adolf Bayer (1872) phenols and aldehydes reacted 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

Phenolic resins can be broadly divided into three groups: resol(e) resins o typically 1 : phenol : formaldehyde novolac resins o typically 1 : 0.8 phenol : formaldehyde Friedel-Crafts polymers

Phenol (C 6 H 5 OH) naming reactive sites: ortho- OH para-/tere- meta- iso-

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

Phenolic resin: methylolation using Φ to represent phenol Φ + CH 2 O  Φ CH 2 OH Φ CH 2 OH + CH 2 O  Φ (CH 2 OH) 2 Φ (CH 2 OH) 2 + CH 2 O  Φ (CH 2 OH) 3

Resoles alkaline catalyst and excess formaldehyde methylene bridge formation may result in the release of water: HO~CH 2 ~Φ~CH 2 ~OH + HO~CH 2 ~Φ  HO~CH 2 ~Φ~CH 2 ~O~CH 2 ~Φ, or  HO~CH 2 ~Φ~CH 2 ~Φ~CH2~OH continued reaction to network o first product may lose formaldehyde: HO~CH 2 ~Φ~CH 2 ~O~CH 2 ~Φ  HO~CH 2 ~Φ~CH 2 ~Φ + CH 2 O

Novolacs acid catalyst and excess of phenol: Φ~CH 2 ~OH + Φ  Φ~CH 2 ~Φ + H 2 O further condensation and methylene (-CH 2 -)bridge formation results in fusible and soluble linear low MW polymers (novolacs): ~Φ~CH 2 ~Φ~CH 2 ~Φ~CH 2 ~Φ~CH 2 ~Φ~

Crosslinking novolacs I add paraform or further formaldehyde more usually add HMT hexamethylenetetramine: (CH 2 ) 6 N 4 2 (CH 3 ) 2 Φ  [(CH 3 ) 2 ΦCH 2 ] 2 NH 3 (CH 3 ) 2 Φ  [(CH 3 ) 2 ΦCH 2 ~] 3 N

Crosslinking novolacs II when the benzylamines are heated at °C in the presence of phenol, ammonia or methylamine are evolved: [(CH 3 ) 2 ΦCH 2 ] 2 NH  (CH 3 ) 2 ΦCH 2 Φ(CH 3 ) 2 + NH 3 [(CH 3 ) 2 ΦCH 2 ] 3 N  (CH 3 ) 2 ΦCH 2 Φ(CH 3 ) 2 + CH 3 NH 2

Phenolics Generally brittle due to moisture released during curing Exceptional FST properties when burning: o low spread of FLAME o low emission of SMOKE o low TOXICITY: only CO 2 and H 2 O released Key markets: o underground railways o mining o submarines

Epoxy resins

Epoxy (glycidyl) groups Epoxy Glycidyl CH 2 (O)CH-CH 2 - NB: bond angles of 60°, rather than 109°28´ of sp 3 hybrid: highly strained,  highly reactive CH 2 CHCH 2 O

Epoxy resin Epichlorohydrin: CH 2 (O)CH-CH 2 -O-Cl Bisphenol-A: HOΦ-C(CH 3 ) 2 -ΦOH  CH 2 (O)CH-CH 2 -OΦ-C(CH 3 ) 2 -ΦOH + HCl Di Glycidyl Ether of Bisphenol-A (DGEBA) CH 2 CH-CH 2 -O- CH 3 - C- CH 3 -O-CH 2 CH-CH 2 O O OH

DGEBA methylene (-CH 2 -) and ether (-O-) groups give free movement of atoms before cure: o less steric hindrance and higher reactivity for terminal rather than internal epoxy (oxirane) o terminal epoxy groups mean crosslink sites are free of mobile chain ends so higher T g achieved.

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

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

Gel time of epoxy resin

Post-cure full cure uses 100% of reactive sites but constrained movement of polymer chain may lead to incomplete cure: o lower glass transition temperature o lower resin density  fewer bonds/m 3  lower moduli and strengths o additional free volume  easier diffusion of chemicals  reduced durability to achieve optimum high-performance composites, post-cure at higher temp’ shortly after production: o unreacted sites may become inactive over time.

T g for high-performance epoxy DGEBA/DICY T g ~ ºC o di glycidyl ether of bis phenol A o aliphatic dicyandiamide TGDDM/DDS T g ~ ºC o tetra glycidyl-4,4‘-diamino diphenyl methane o 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

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)

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)

Polyurethanes primarily for RIM processes

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

Unsaturated polyesters

Curing of polyester resin -A-B-A-B-A-B- plus styrene ( ΦCH=CH 2 ) reactive diluent where B block contains unsaturation (double bonds) in backbone of polymer chain, leads to 3D network

Curing of polyester resin Unsaturated polyester and styrene both have double bonds. Polyester chain top and bottom, with 3 styrene molecules between. A broken bond = two free radicals. Assume all double bonds break. Five double bonds are now ten free radicals Free radicals pair-up to make new bonds Molecules move closer together and rotate to form cross-link. Net shrinkage of system results. Note two remaining reactive sites.

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

Summary (key polymers) Polymer n CuringProperties Ph-F Condensation Low-cost, brittle, FST Epoxy CondensationRing-opening High-cost, High-performance UPE CondensationAddition Intermediate cost Balanced performance