Lectures on composites

1. Lectures on composites Mikael Skrifvars, February 13-17, 2012 Contacts: mikael.skrifvars@hb.se

2. Introduction and definitions

3. Composites Resin/matrix + reinforcement + interphase = > single material with better properties than the resin and reinforcement separately

4. Composite materials- the definition: • A combination of two or more materials, a bulk phase(matrix/resin) and a reinforcing phase (reinforcement), into a single material • Made by combining the components in a controlled way so that optimum properties are achieved • The resulted properties should be better compared to the properties for the resin and reinforcement separately

5. The matrix (resin) • binds the reinforcement together • transfers external loads to the reinforcement • protects the reinforcement from the environment • gives the composite product its shape, surface appearance, environmental tolerance and durability • can be metallic, ceramic or polymeric • the matrix is a continuous phase

6. The reinforcement • carries the structural load in the composite product • gives the product its macroscopic stiffness and strength • can be: • inorganic such as glass, carbon or basalt • polymeric such as aramid fibres (Kevlar™) • of natural origin, such as wood, flax, hemp or sisal fibres • both fibrous and particulate reinforcements exists • the reinforcement is a dispersed, non-continuous phase

7. Polymer composite materials • The matrix is a polymeric material (commonly called resin) • The polymer is either a thermoset or a thermoplastic Synonyms: • Reinforced plastics • Fibre reinforced plastics (FRP) • Glass fibre reinforced plastics (GRP)

8. Why are composites special? • Composites are anisotropic; their properties vary significantly when measured in different directions • The external load is transferred from matrix to reinforcement, which enables the composite to withstand very high loads • In the manufacturing process the final article is made at the same time as the composite material itself is being processed

9. The laminate concept • Different materials placed in layers on each other, and bound together into a rigid structure • The laminate properties depend on its components, their amounts, their orientation, order of layers and manufacturing process • Any material combinations are possible

10. Composite markets in Europe 815 000 tonnes in 2009

11. General charcteristics for composites?

12. General characteristics for composites • Very good mechanical properties • Low density which gives enhanced properties when considering the weight of the material • Possibility to orient the structural strength (anisotropy) • Large design freedom in the manufacturing process • Large structures can be manufactured relatively easily in one piece • Insertion of other components can be integrated in one manufacturing step • Good chemical and corrosion resistance • Many manufacturing methods can be used for composite products • More expensive compared to traditional materials • Temperature tolerance can be a problem for some composite materials • More demanding to use in engineering than metals due to different design demands

13. Reinforcement configurations RANDOM ORIENTATION ALIGNED ORIENTATION DISCONTINIOUS CONTINIOUS

14. The interface is an important factor in composites Poor adhesion between the matrix and the reinforcement will cause composite failure due to bad load transfer

15. Thermosets Thermoset polymers are especially usable in composites as the matrix

16. Low molecular weight Infinitely high molecular weight Crosslinking in thermosets • Covalent crosslinks are formed between the polymer chains • Formed polymer network can be considered as one gigantic molecule • The obtained molecular weight is infinitely high

17. The curing process • The process to cause a thermoset to crosslink is called curing • Can be followed by the viscosity • At the gel point the viscosity starts to increase, and a macroscopic gel can be seen • At the end a solid material is obtained time Gel time

18. The crosslinking reaction • Exothermic reaction due to the chemical reactions involved • An exotherm peak temperature can be detected, the temperature depends on used curing system, mass of material and type of resin

19. Initiation of the crosslinking • The curing process must be initiated by the end-user prior to the processing by addition of an initiator, by heating or by radiation • An initiator is a component which is added in a small amount, and starts the crosslinking reaction • An accelerator (synonyms: activator, co-initiator) is a component which activates the initiator • Postcuring is necessary for complete reaction

20. A, B and C stage • The crosslinking reaction can be defined as three stages: A stage: the crosslinking reaction is just initiated B stage: the crosslinking reaction is just below the gel point C stage: the final, fully crosslinked state • A significant slow-down of the crosslinking reaction at the B-stage can be done by refrigeration or by freezing • Phenol-formaldehyde resoles and epoxy prepregs are examples

21. A, B and C stage B stage Cstage A stage

22. Chemistry and resins

23. Introduction to resin chemistry • A thermoset resin requires functional groups which can undergo chemical reactions so that covalent chemical bonds are formed • The covalent bonds are permanent and cannot be broken reversibely • The chemical reaction is called curing • The result of the curing reaction is most often a molecular network

24. Thermosetting resins • Unsaturated polyesters (UPE) • Vinyl esters (VE) • Epoxy resins • Phenol-formaldehyde resins (PF) • Polyurethanes (PUR) • Polyimide thermosets Both liquid and solid thermosets exists!

25. Thermoset resins - requirements • Must have proper viscosity depending on processing method • Must impregnate the reinforcement well (at processing temperature) • The curing must be possible to adjust depending on process method • The crosslinking reaction should proceed well towards completion • The resin should not be toxic or harmful for the environment when properly used

26. Crosslinking principles 1 The resin reacts directly with it self Formation of covalent bonds directly between linear polymer chains Examples: electron beam crosslinking of polyethylene homopolymerisation of epoxy resins with the aid of a catalyst

27. Crosslinking principles 2 The resin and a small molecule reacts with each other Formation of covalent bonds via a participating molecules (crosslinking agent, reactive diluent, monomer). Examples: unsaturated polyesters, vinyl esters, epoxy resins

28. Crosslinking principles 3 A step wise reaction of a bifunctionalcompound and a trifunctional compound leading to bigger and bigger network Reaction of a multifunctional monomer mixtures The reaction is stopped (at the B-stage) before final crosslinking Examples: phenol-formaldehyde resins, epoxy resins

29. Chemical reactions between organic molecules • In unsaturated polyesters and vinyl esters new bonds are formed between carbon atoms • In epoxy resins bonds are also formed between carbon atoms and oxygen respective nitrogen atoms

30. Unsaturated polyesters (UPE) and vinyl esters (VE) chemistry Free radical cross-linking mechanism

31. Unsaturated polyester resins • The most common thermoset resin for composites • Low cost resin which can be processed by many methods to obtain products with optimal properties • Mostly reinforced with glass fibres • The resin is a low molecular weight polyester oligomer which is diluted in a reactive solvent (monomer) most commonly styrene

32. Synthesis in two steps: • Step 1: Condensation of unsaturated and aromatic dicarboxylic anhydrides with difunctional alcohols (diols/glycols) which gives a low molecular weight polyester oligomer • Step 2: The oligomer is diluted in a reactive solvent (monomer), most commonly styrene, which gives a viscous resin • The styrene content is normally 35 – 45 weight-%

33. Processing to composite • The polyester resin is impregnated into glass fibre reinforcements, and cured to obtain a rigid materials • The curing involves the formation of a crosslinks between the polyester and the styrene, and a 3-dimensional infinitely large network structure is obtained • The crosslinking reaction is a free radical reaction, which is initiated chemically, thermally or by radiation

34. Homolytic cleavage of bond gives two reactive species, radicals: A new bond is formedwhentworadicalsmeet: The reaction is called a freeradicalreaction

35. Free radical polymerisation Initiation Propagation Termination

36. Styrene 35 – 45 wt-% Polyester oligomer 55 – 65 wt-% Unsaturated polyesters are cured by a free radical reaction

37. The formed crosslinked network

38. Leisure boats • Very important in Scandinavia (1/3 of composite production) • Hand and spray lay up and vacuum injection • Unsaturated polyesters and glassfibres • Work environment problems due to styrene emission when doing hand and spray lay up

39. Sailing yachts Nautor, Finland • Unsaturated polyester, vinyl ester and epoxy • Glass fibres, carbon fibres • Sandwich structure • Hull, deck, hatches, rudder shafts • Use of advanced composites is increasing • Hand lay up, spray lay up, vacuum injection Swan 48 feet

40. Futuro - concept house from 1968 in GRP Design by Matti Suuronen, Finland

41. Vinyl esters are also cured by a free radical reaction Vinyl ester oligomer, 55-65 % Styrene, 35-55 %

42. Vinyl ester resins - properties • Similar properties as for epoxy resins, due to similar prepolymer structure • Very good chemical and hydrolytic resistance • No ester linkages in the prepolymer • Methacrylate end group protects the terminal ester linkage • Good toughness and greater tensile elongation than UPE resins • Good thermal properties • Hydroxyl groups gives good wetting and adhesion for glass fibres • Highly reactive resins due to the terminal reactive group

43. Plastilon, Finland Process equipment Piping, ducts, stacks, tanks, absorbers and reactors • Corrosion resistance • Mechanical reliability in various severe chemical conditions • Low maintenance requirements • Chemical industry, pulp & paper mills, power plants, oil platforms, fertilizer plants 500 m3 storage tank

44. Organic initiators • Can decompose to form radicals by cleavage of a covalent bond X-Y → X∙+ Y∙ • The formed radical will initiate free radical chain reactions • The decomposition is induced by heat, radiation or by a oxidation-reduction (redox) chemical reaction

45. Redox initiatiors • R-OOH + Co2+→ RO∙ + OH- + Co3+ (2) R-OOH + Co3+→ ROO∙ + H+ + Co2+ (3) RO∙ + Co2+→ RO- +Co3+ • Mainly added to the resin as Cobalt octoate or cobalt naphtenate • Typical amount 1 – 2 weight-% • Too high amount will retard the radical formation (eq 3) • Most common peroxide is methyl ethyl ketone peroxide

46. Thermal initiators • Organic compounds which decompose in the temperature range 50 to 150 ºC • Organix peroxides and organic diazocompounds are the most common RO-OR → RO∙ + RO∙ R-N=N-R → R∙ + R∙ + N2 • The selection of the initiator depends on the activation temperature for the thermal decomposition

47. Benzoyl peroxide (BPO) is the most common thermal initiator: Curing temperature: 90 to 130 ºC

48. Photochemical initiation • The radicals are produced by UV or visible light irridation of a photoreactive compounds • Two mechanisms: • The compound is excited by energy absorption and subsequent decomposition to radicals • The compound undergoes excitation and interacts with a second compound to form radicals • Photochemical initiation is easy to control by turning on/off the light source or by directing the light source on the object to be cured • Main use in printing coatings and dental resins

49. Inhibitors – radical consumers

50. Inhibitor effect Degree of cure inhibering No inhibition With inhibitor Undercured resin retardation Time Gel time is prolonged