THERMOSETTING POLYMERS: Processing, Application and Future Direction EBB 427 Application and Technology of Engineering Polymers (Second Half) THERMOSETTING POLYMERS: Processing, Application and Future Direction Dr. Hazizan Md Akil School of Materials and Mineral Resources Engineering Engineering Campus, USM.

Outline of the Course (7 weeks) The fundamentals of thermosetting polymer processing Commercial thermosetting polymers Applications of thermosetting polymers Recent development in thermosetting polymer applications Future outlook

References “Handbook of Thermoset Plastics” Second Edition, by Sidney H. Goodman Organic polymer chemistry, Saunders

The fundamentals of thermosetting polymer processing General characteristics Types of processing techniques Processing parameters (Time, temperature, mass, shelf life, pot life) Crosslinking & curing mechanism (Kinetics, measurement) Role of various additives (Diluent, catalysts)

Commercial Resins (Trade Name) Preparation, properties, application Epoxy resin (Epon, DER etc) Phenolic resin Melamine formaldehyde Urea formaldehyde Polyester resin Unsaturated polyester

Applications of Thermosetting polymers Engineering components Adhesives Sealants Foams Building and construction Aerospace

Military and Defence Engineering Foams Sandwich Composites Recent Developments Military and Defence Engineering Foams Sandwich Composites

General characteristics Once shaped into a permanent form, usually with heat and pressure, a thermosetting plastic cannot be remelted or reshaped because the basic polymeric component has undergone an irreversible chemical change The operation by which the raw material is converted to a hard, insoluble and infusible product is referred to as cure (or curing) and corresponds to the final step of the polymerization reaction.

General characteristics (cont’d) A thermosetting material may be cured by the use of heat, radiation, catalysts or a combination of these The polymer component consists of molecules with permanent cross-links between linear chains that form a rigid three-dimensional network structure which cannot flow . The tightly cross-linked structure of thermosetting polymers immobilizes the molecules, providing hardness, strength at relatively high temperature, insolubility, good heat and chemical resistance, and resistance to creep.

General characteristics (cont’d) Thermosetting materials are usually preferred for structural applications because their strength is generally higher than that of thermoplastics and they do not have a tendency to cold flow (creep) at room temperature

Types of processing technique Hand lay-up (Liquid resin) Vacuum Bagging (Liquid resin) Autoclave (Laminated/prepreg) Vacuum oven (Liquid resin) Vacuum Infiltration (Liquid) Spray forming (Foam/coating) Transfer/compression moulding Vacuum forming (Prepreg)

Processing parameters Time (Duration of cure) Temperature (Room/Elevated) Staging (Pre-curing/post-curing) Pressure (Atmospheric/Pressurised) ***Since these parameters vary broadly with types of resin and hardener used, each parameter will be discussed separately for each system***

Crosslinking

Crosslinking (cont’d) Most of the crosslinking reactions are initiated by free-radicals Free redicals are generated through thermal or photo decomposition of peroxides, hydroperoxides, azo and diazo compounds. The use of initiator (hardener) is strongly dependent on the types, fabrications and applications of the particular resin ***Since crosslinking agents and reactions are vary broadly with types of resin and hardener used, typical reaction and their mechanism will again be discussed separately for each type of resin***

Epoxy resin History/Introduction Developed independently by Ciba AG (1943) 50% used for surface coating Other applications include circuit boards, carbon fibre composites, electronic component encapsulations and adhesives At present, 80-90% of commercial epoxy resins are prepared by the reaction of Bisphenol A and Epichlorohydrin.

Epoxy resin (cont’d) Bisphenol A/Epichlorohydrin Epoxies Raw materials (preparation of bisphenol A) Bisphenol A is so called since it is formed from phenol (2 mole) and acetone (1 mole) Acetone Phenol Bisphenol A

Epoxy resin (cont’d) Bisphenol A/Epichlorohydrin Epoxies Raw materials (preparation of bisphenol A) Theoretically, the reaction requires the molar ratio of reactants to be 2:1 but improved yield of bisphenol A is obtained if additional phenol is present and the optimum molar ratio is 4:1 (phenol:acetone)

Epoxy resin (cont’d) Bisphenol A/Epichlorohydrin Epoxies Raw materials (preparation of bisphenol A) In a typical process, the phenol and acetone are mixed and warmed to 50°C Hydrogen chloride (catalyst) is passed into the mixture for about 8 hours, during which period the temperature is kept below 70°C to suppress the formation of isomeric product Biphenol A precipitates and is filtered off and washed with toulene to remove unreacted phenol (which is recovered). The product is then recrystallized from aqueous ethanol

Epoxy resin (cont’d) Bisphenol A/Epichlorohydrin Epoxies Raw materials (preparation of bisphenol A) The formation of bisphenol A is thought to proceed as follows: Phenol Acetone

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Raw materials (preparation of Epichlorhydrin) A mixture of propylene and chlorine (4:1 molar) isheated at about 500°C and 0.2MPa (2 atm). Free radical substitution reaction occurs at the double bond and allyl chloride is the main product The product is treated with pre-formed hypochlorous acid (formed in separate reactor by passing chlorine into water) at about 30°C to give the addition product, dichlorhydrin.

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Raw materials (preparation of Epichlorhydrin) The reaction mixture separates into two layers. The aqueous layer is removed to leave dichlorhydrin which is them strirred with a lime slurry to give epichlorhydrin. The formation of epichlorhydrin is thought to proceed as follows (next page):

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Raw materials (preparation of Epichlorhydrin) The reaction of propylene &chlorine & hypochlorous acid to form epichlorhydrin:

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation In a typical process for preparation of a liquid epoxy resin (can also exist in solid form), a mixture of bisphenol A and epichlorhydrin (about 1:4) is heated to about 60°C with stirring. Solid sodium hydroxide (2 mole per mole bisphenol A) is added slowly at such a rate that the reaction mixture remains neutral The reaction is exothermic and cooling is applied to keep the temperature at 60°C

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation Excess of epichlorhydrin is then removed by distillation under reduced pressure. The residue consists of epoxy resin mixed with sodium chloride. The latter is filtered off and toulene is added to the mixture in order to facilitate filtration The toulene is removed by distillation under reduced pressure and then the resin is heated at 150°C/0.6KPa to remove traces of volatile matter which may create bubble formation.

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation In the preparation of solid epoxy resins the above process is slightly modified. A mixture of bisphenol A and epichlorhydrin (the molar ratio of reactant used depends on the resin molecular weight required) is heated to 100°C and aqueous sodium hydroxide is added slowly with vigorous stirring. When reaction is complete the agitator is stopped and a “taffy” (which is an emulsion of about 30% water in resin) rises to the top of the reaction mixture.

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation The lower layer of brine is removed; the resinous layer is coagulated and washed with hot water. The resin is heated at 150°C under reduced pressure to remove water, clarified by passage through a filter and then allowed to solidify. Solvent can be added at the washing stage but whilst this facilitate washing and filtration it is very difficult to remove subsequently all traces of the solvent.

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation Alternatively, solid epoxy resins may be prepared by a two-step process in which a pre-formed liquid resin is heated with bisphenol A in the presence of a basic catalyst to effect chain extension. This method avoids the difficulty of washing sodium chloride from highly viscous material The reactions which are involved when epichlorhydrin reacts with phenol (ROH) in the presence of sodium hydroxide are as follows:

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Resin Preparation Reactions: Formation of phenoxy anion Reaction of epoxy group of epichlorhydrin with phenoxy anion Elimination of chloride anion to form glycidyl ether Reaction of epoxy group in glycidyl ether with phenoxy anion Formation of hydroxyl group by protonation

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies

Epoxy resin (cont’d)

Epoxy resin (cont’d)

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Effect of molar ratio on molecular weight of epoxy resins

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Crosslinking agents: Most of the curing agent in common use can be classified into three groups: Tertiary amines Polyfunctional amines Acid anhydrides

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Tertiary Amines: Benzyldimethylamine (BDMA) 2-(dimethylaminomethyl)phenol (DMAMP) 2,4,6-tris(dimethylaminomethyl)phenol (TDMAMP) Triethanolamine N-n-butylimidazole

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Their structuress: TDMAMP DMAMP BDMA Triethanolamine N-n-butylimidazole

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Polyfunctional amines Diethylenetriamine (DTA) Triethylenetetramine (TET) m-Phenylenediamine (MPD) 4,4’-diaminodiphenylmethane (DDM) 4,4’-diaminodiphenylsulphone (DDS)

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Their structures: DTA TET DDM MPD DDS

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Acid anhydrides: Both mono- and dianhydrides are used: Maleic anhydride (MA) Dodecenylsuccinic anhydride (DDSA) Hexahydrophthalic (HPA) Phthalicanhydride (PA) Pyromellitic dianhydride (PMDA) Nadic methylanhydride (NMA) Chlorendic (HET) anhydride

Epoxy resin (cont’d) Bisphenol A/Epichlorhydrin Epoxies Properties of crosslinked epoxies: (Table)

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Whilst the straight bisphenol A-epichlorhydrin epoxies described previously have found widespread use in applications such as adhesives, castings, encapsulations, composites and laminates they are used to a relatively small extent in surface coatings. In this important field, mainly modified bisphenol A epichlorhydrin epoxied are used. Two principal types of modification are commercially practised, namely combination with other resins and esterification

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Resin-modified epoxies Blends with variety of other resin which contain reactive group On curing, interaction occurs to give a cross-linked copolymer which exhibits characteristics of the two straight resin Examples are: Phenol-formaldehyde resins

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Phenol-formaldehyde resins

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Phenol-formaldehyde resins Most widely used is low molecularweight butylated resols which contain phenolic hydroxyl groups and etherified and unetherified methylol groups. The epoxy resins used have a molecular weight of 3000 – 4000 and therefore contain secondary hydroxyl groups At stoving temperature of 180°C-200°C, the phenolic hydroxyl groups react with epoxy groups and both types of methylol groups react with the hydroxyl group of the epoxy resin.

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Esterified epoxies Epoxy with molecular weight of approximately 1400 Esterified with carboxylic acids through their hydroxyl and epoxy groups:

Epoxy resin (cont’d) Modified Bisphenol A/Epichlorhydrin Epoxies: Esterified epoxies Catalysts such as alkali metal salts (sodium carbonate) are used to minimise etherification of epoxide groups by hydroxyl groups, since this reaction can lead to gelation. May be carried out in either the absence or presence of solvent

Epoxy resin (cont’d) Other Epoxies: Novolac epoxies Polyglycol epoxies Cyclic aliphatic epoxies Acyclic aliphatic epoxies Glycidyl amine epoxies

Epoxy resin (cont’d) Novolac epoxies: Low mw polymers consisting of phenolic nuclei linked in the o- and p- positions by methylene groups A typical commercial novolac epoxy resin has mw of 650 and contains about 3.6 epoxy groups per molecule Due to their multi-functionality, the novolac epoxy resins give, on curing, more tightly cross-linked products than the bisphenol A-based resins.

Epoxy resin (cont’d) Novolac eEpoxies: This results in improved elevated temperature performance and chemical resistance.

Epoxy resin (cont’d) Polyglycol epoxies: Linear polyglycols such as polypropylene glycol may be epoxidised through the terminal hydroxyl groups to give diglycidyl ethers Commercial products are available where n varies from 1 to 6. When used alonr these resins cure to soft product of low strength so they are normally used in blends with other epoxies.

Epoxy resin (cont’d) Halogenated epoxies: Epoxies containing halogen may be prepared from halogenated hydroxyl compounds and resins are available based on tetrabromobisphenol A and tetrachlorobisphenol A. The presence of halogen renders these resins flame retardant. The ability of the resins to retard or extinguish burning is due to the evolution of hydrogen halide upon decomposition at elevated temperatures.

Epoxy resin (cont’d) Halogenated epoxies: The brominated resins is more stable than the chlorinated resins but, once begun, the evolution of hydrogen bromide is more rapid than that of hydrogen chlorode and the system is more effectively blanketed. Brominated epoxy resins are generally used in blends with other epoxy resins to confer flame retardance.