Lectures on composites Mikael Skrifvars, February 13-17, 2012 Contacts: MSK ARCADA

Introduction and definitions MSK ARCADA

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

Composite materials - the definition: 1.A combination of two or more materials, a bulk phase (matrix/resin) and a reinforcing phase (reinforcement), into a single material 2.Made by combining the components in a controlled way so that optimum properties are achieved 3.The resulted properties should be better compared to the properties for the resin and reinforcement separately

MSK ARCADA The matrix (resin) 1.binds the reinforcement together 2.transfers external loads to the reinforcement 3.protects the reinforcement from the environment 4.gives the composite product its shape, surface appearance, environmental tolerance and durability 5.can be metallic, ceramic or polymeric 6.the matrix is a continuous phase

MSK ARCADA The reinforcement 1.carries the structural load in the composite product 2.gives the product its macroscopic stiffness and strength 3.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 4.both fibrous and particulate reinforcements exists 5.the reinforcement is a dispersed, non-continuous phase

MSK ARCADA 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)

MSK ARCADA Why are composites special? 1.Composites are anisotropic; their properties vary significantly when measured in different directions 2.The external load is transferred from matrix to reinforcement, which enables the composite to withstand very high loads 3.In the manufacturing process the final article is made at the same time as the composite material itself is being processed

MSK ARCADA The effect of reinforcement configuration The configuration (form, fibre length and thickness, orientation, distribution, volume fraction) of the reinforcement will control the macroscopic properties of the composite Lay-up of different reinforcement types on each other, will make it possible to endlessly tailor the macroscopic properties of the composite

MSK ARCADA The laminate concept 1.Different materials placed in layers on each other, and bound together into a rigid structure 2.The laminate properties depend on its components, their amounts, their orientation, order of layers and manufacturing process 3.Any material combinations are possible

Composite markets in Europe MSK tonnes in 2009

General charcteristics for composites? MSK ARCADA

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

MSK ARCADA Reinforcement configurations DISCONTINIOUS CONTINIOUS RANDOM ORIENTATION ALIGNED ORIENTATION

MSK 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

Thermosets Thermoset polymers are especially usable in composites as the matrix MSK ARCADA

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 Low molecular weightInfinitely high molecular weight

MSK ARCADA 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 Gel time time

MSK ARCADA 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

MSK ARCADA 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

MSK ARCADA Effect of temperature and sample amount on crosslinking reaction Increasing temperature increases the crosslinking reaction rate → gel time decreases Increased mass decreases the ability to transfer the exothermic heat out of the system which increases crosslinking reaction rate → gel time decreases The ratio of the components affects also the reaction

MSK ARCADA 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

A, B and C stage MSK ARCADA A stage B stage C stage

MSK ARCADA One and two component thermosets A one-component thermoset is initiated by a catalytic (very small) amount of a second component, for example an unsaturated polyester cured with peroxide In a two component thermoset are two components mixed together, for example an epoxy resin cured with a hardener

Chemistry and resins MSK ARCADA

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 MSK ARCADA

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

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 MSK ARCADA

Crosslinking principles 1 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 The resin reacts directly with it self

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

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

MSK Some other definitions A curing (crosslinking) agent is a molecule participating in the crosslinking reaction, without the crosslinking reaction would not occur (synonyms: curative, hardener, catalysts, monomer, reactive diluent) The shelf life is the maximum storage time, can vary from a few hours to several years A cure schedule is the necessary sequence of time, temperature and pressure to obtain optimal crosslinking Post-curing is a treatment after the curing process to complete the crosslinking reaction

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 MSK ARCADA

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

MSK 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

MSK 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-%

MSK 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

Homolytic cleavage of bond gives two reactive species, radicals: A new bond is formed when two radicals meet: The reaction is called a free radical reaction MSK ARCADA

Free radical polymerisation Initiation Propagation Termination MSK ARCADA

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

MSK The formed crosslinked network

MSK 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

MSK Sailing yachts 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 Nautor, Finland

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

Vinyl esters are also cured by a free radical reaction Vinyl ester oligomer, % Styrene, % MSK ARCADA

MSK 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

MSK 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 m 3 storage tank Plastilon, Finland

MSK ARCADA 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

MSK ARCADA Redox initiatiors (1)R-OOH + Co 2+ → RO∙ + OH - + Co 3+ (2) R-OOH + Co 3+ → ROO∙ + H + + Co 2+ (3) RO∙ + Co 2+ → RO - + Co 3+ 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

MSK ARCADA 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∙ + N 2 The selection of the initiator depends on the activation temperature for the thermal decomposition

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

MSK ARCADA BPO can also be used at ambient temperatures together with amines which decompose the BPO to radicals

MSK ARCADA 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

Inhibitors – radical consumers MSK ARCADA

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

Epoxy resins chemistry A polar bond forming reaction MSK ARCADA

MSK Epoxy resins Contain more than one epoxy group situated terminally, cyclically, or internally, which can react in a crosslinking reaction with an other species The crosslinking can occur via a variety of mechanisms: Homopolymerisation with it self Heteropolymerisation with different curing agents with or without catalysts Alkaline and acid curing agents A wide range of resins are available!

Heterolytic cleavage gives two charged species: A new is bonded when the two species meet: The reaction is called a polar reaction MSK ARCADA

The epoxy group is very reactive towards nucleofiles (negative ions or negatively polarised atoms) MSK ARCADA

Many nucleofiles can react with the epoxy group: Amines Hydroxyions Carboxylic acids Acid anhydrides Phenols and more,…. Many possibilities! MSK ARCADA

MSK Preparation of epoxy resins

Epoxy resin; Diglycidyl ether of bisphenol A (DGEBA) MSK ARCADA

Curing agents for epoxy resins Catalytic curing agents: – Initiates the polymerisation of an epoxy resin – Activates other curing agents – Named catalyst – One component epoxy system Co-reactive curing agents: – Act as reactant in the reaction – Forms a part of the resulted network – Named hardener – Two component epoxy system – Most common type MSK ARCADA

US consumption of curing agents 2001 Curing agentConsumption, tonnesUS market share, % Amines Aliphatic amines Polyamines Amidoamines9 000 Cycloaliphatic amines7 000 Carboxylics Polycarboxylic polyesters Anhydrides Resole resins Amino formaldehydes4 500 Phenol formaldehyde4 500 Novolacs and other phenolics Polysulfides and polymercaptans Catalysts MSK ARCADA

Crosslinking of epoxy resins - homolytic reaction with itself Tertiary amine catalysed cure:

MSK ARCADA Co-reactive curing of epoxy resins Primary and secondary amines: Primary amine Secondary amine The hydroxyl group enhances reaction speed

Epoxy resins are cured by a polar reaction Diamine hardener + epoxy resin MSK ARCADA

Co-reactive curing of epoxy resins Reaction with anhydrides:

MSK ARCADA Co-reactive curing of epoxy resins Reaction with anhydrides, catalysed by tertiary amine

Epoxy resin formulation design Selection of epoxy resin and curing agent Selection of the amount of both components Selection of catalyst/accelerator (if needed) Selection of curing process and conditions Selection of post-curing process and conditions Selection of fillers and additives Selection of reinforcements MSK ARCADA

Relation between cured epoxy structure and properties The network structure will depend on the molar ratio of the epoxy resin and the hardener Can form two bonds Can form four bonds MSK ARCADA

Network structures depending on molar ratio MSK ARCADA

Network characteristics The properties for the cured epoxy resin depends on: Chemical structure between cross-link points, and at the cross-link Cross-link density = the molecular weight (length of molecular segment) between cross- links Curing process (temperature) MSK ARCADA

Effect of hardener - example Amine hardener: – Resistant for basic conditions – Oxidative sensitive Anhydride hardeners. – Resistant for oxidation – Sensitive for moisture under basic conditions MSK ARCADA

Additives in epoxy resins Diluents, added 2-20 wt-%, for the improval of fibre wetting and flow behaviour – Reactive: are incorporated in epoxy resin network – Nonreactive: solvents and plasticizers reduces viscosity Thixotropic agents – Controls the flow behaviour so it will not drip Fillers, inert, often inorganic materials, which are added to impart some specific properties, and to reduce costs MSK ARCADA

End Part 1 MSK ARCADA