ebb 427 (1) hazizan
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EBB 427 Application and Technology ofEngineering Polymers (Second Half)
Dr. Hazizan Md Akil
School of Materials and Mineral Resources EngineeringEngineering Campus, USM.
THERMOSETTING POLYMERS:Processing, Application and Future
Direction
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Outline of the Course (7 weeks)
The fundamentals of thermosetting
polymer processingCommercial thermosetting polymersApplications of thermosetting polymers
Recent development in thermosettingpolymer applicationsFuture outlook
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References
Handbook of Thermoset Plastics SecondEdition, by Sidney H. Goodman
Organic polymer chemistry, Saunders
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The fundamentals of thermosetting polymerprocessing
General characteristicsTypes of processing techniques
Processing parameters (Time, temperature,mass, shelf life, pot life)Crosslinking & curing mechanism (Kinetics,measurement)
Role of various additives (Diluent, catalysts)
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Commercial Resins (Trade Name)
Preparation, properties, application
Epoxy resin (Epon, DER etc)
Phenolic resin
Melamine formaldehyde Urea formaldehyde
Polyester resin
Unsaturated polyester
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Applications
Applications of Thermosetting polymers
Engineering components
Adhesives
Sealants Foams
Building and construction
Aerospace
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Recent Developments
Military and Defence
Engineering Foams Sandwich Composites
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General characteristics
Once shaped into a permanent form, usually with heatand pressure, a thermosetting plastic cannot be remeltedor reshaped because the basic polymeric component hasundergone an irreversible chemical change
The operation by which the raw material is converted to ahard, insoluble and infusible product is referred to as cure(or curing) and corresponds to the final step of thepolymerization reaction.
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General characteristics (contd)
A thermosetting material may be cured by the use ofheat, radiation, catalysts or a combination of these
The polymer component consists of molecules withpermanent cross-links between linear chains that form arigid three-dimensional network structure which cannotflow .
The tightly cross-linked structure of thermosettingpolymers immobilizes the molecules, providing hardness,strength at relatively high temperature, insolubility, good
heat and chemical resistance, and resistance to creep.
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General characteristics (contd)
Thermosetting materials are usually preferred forstructural applications because their strength is generallyhigher than that of thermoplastics and they do not have a
tendency to cold flow (creep) at room temperature
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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)
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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 andhardener used, each parameter will be discussed separately
for each system***
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Crosslinking
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Crosslinking (contd)
Most of the crosslinking reactions are initiated by free-radicals
Free redicals are generated through thermal or photodecomposition of peroxides, hydroperoxides, azo anddiazo compounds.
The use of initiator (hardener) is strongly dependent onthe types, fabrications and applications of the particularresin
***Since crosslinking agents and reactions are vary broadly withtypes of resin and hardener used, typical reaction and theirmechanism will again be discussed separately for each type ofresin***
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Epoxy resin
History/Introduction
Developed independently by Ciba AG (1943)
50% used for surface coating
Other applications include circuit boards, carbon fibrecomposites, electronic component encapsulations andadhesives
At present, 80-90% of commercial epoxy resins are
prepared by the reaction of Bisphenol A andEpichlorohydrin.
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Epoxy resin (contd)
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)
Bisphenol A
PhenolAcetone
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Epoxy resin (contd)
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 isobtained if additional phenol is present and the optimummolar ratio is 4:1 (phenol:acetone)
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Epoxy resin (contd)
Bisphenol A/Epichlorohydrin Epoxies
Raw materials (preparation of bisphenol A)
In a typical process, the phenol and acetone are mixedand warmed to 50C
Hydrogen chloride (catalyst) is passed into the mixture
for about 8 hours, during which period the temperature iskept below 70C to suppress the formation of isomericproduct
Biphenol A precipitates and is filtered off and washedwith toulene to remove unreacted phenol (which isrecovered). The product is then recrystallized fromaqueous ethanol
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Epoxy resin (contd)
Bisphenol A/Epichlorohydrin Epoxies
Raw materials (preparation of bisphenol A)
The formation of bisphenol A is thought to proceed asfollows:
Acetone
Phenol
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Raw materials (preparation of Epichlorhydrin)
A mixture of propylene and chlorine (4:1 molar) isheatedat about 500C and 0.2MPa (2 atm).
Free radical substitution reaction occurs at the doublebond and allyl chloride is the main product
The product is treated with pre-formed hypochlorous acid(formed in separate reactor by passing chlorine intowater) at about 30C to give the addition product,dichlorhydrin.
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Epoxy resin (contd)
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 giveepichlorhydrin.
The formation of epichlorhydrin is thought to proceed asfollows (next page):
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Raw materials (preparation of Epichlorhydrin)
The reaction of propylene &chlorine & hypochlorous acidto form epichlorhydrin:
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Epoxy resin (contd)
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 Aand epichlorhydrin (about 1:4) is heated to about 60Cwith stirring.
Solid sodium hydroxide (2 mole per mole bisphenol A) isadded slowly at such a rate that the reaction mixtureremains neutral
The reaction is exothermic and cooling is applied to keepthe temperature at 60C
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Resin Preparation
Excess of epichlorhydrin is then removed by distillationunder reduced pressure.
The residue consists of epoxy resin mixed with sodiumchloride.
The latter is filtered off and toulene is added to themixture in order to facilitate filtration
The toulene is removed by distillation under reducedpressure and then the resin is heated at 150C/0.6KPa toremove traces of volatile matter which may create bubbleformation.
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Resin Preparation
In the preparation of solid epoxy resins the aboveprocess is slightly modified.
A mixture of bisphenol A and epichlorhydrin (the molarratio of reactant used depends on the resin molecularweight required) is heated to 100C and aqueous sodiumhydroxide is added slowly with vigorous stirring.
When reaction is complete the agitator is stopped and ataffy (which is an emulsion of about 30% water in resin)
rises to the top of the reaction mixture.
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Resin Preparation
The lower layer of brine is removed; the resinous layer iscoagulated and washed with hot water.
The resin is heated at 150C under reduced pressure toremove water, clarified by passage through a filter andthen allowed to solidify.
Solvent can be added at the washing stage but whilst thisfacilitate washing and filtration it is very difficult to removesubsequently all traces of the solvent.
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Resin Preparation
Alternatively, solid epoxy resins may be prepared by atwo-step process in which a pre-formed liquid resin isheated with bisphenol A in the presence of a basiccatalyst to effect chain extension.
This method avoids the difficulty of washing sodiumchloride from highly viscous material
The reactions which are involved when epichlorhydrinreacts with phenol (ROH) in the presence of sodiumhydroxide are as follows:
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Resin Preparation
Reactions:
Formation of phenoxy anion
Reaction of epoxy group of epichlorhydrin with phenoxyanion
Elimination of chloride anion to form glycidyl ether
Reaction of epoxy group in glycidyl ether with phenoxy
anion Formation of hydroxyl group by protonation
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
E i ( td)
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Epoxy resin (contd)
E i ( td)
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Epoxy resin (contd)
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Effect of molar ratio on molecular weight of epoxy resins
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Crosslinking agents:
Most of the curing agent in common use can be classifiedinto three groups:
Tertiary amines
Polyfunctional amines
Acid anhydrides
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Tertiary Amines:
Benzyldimethylamine (BDMA)
2-(dimethylaminomethyl)phenol (DMAMP) 2,4,6-tris(dimethylaminomethyl)phenol (TDMAMP)
Triethanolamine
N-n-butylimidazole
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Their structuress:
BDMA
DMAMP TDMAMP
TriethanolamineN-n-butylimidazole
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Polyfunctional amines
Diethylenetriamine (DTA)
Triethylenetetramine (TET) m-Phenylenediamine (MPD)
4,4-diaminodiphenylmethane (DDM)
4,4-diaminodiphenylsulphone (DDS)
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Their structures:
DTA TET
MPDDDM
DDS
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Epoxy resin (contd)
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
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Epoxy resin (contd)
Bisphenol A/Epichlorhydrin Epoxies
Properties of crosslinked epoxies: (Table)
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Whilst the straight bisphenol A-epichlorhydrin epoxiesdescribed previously have found widespread use inapplications 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 andesterification
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Resin-modified epoxies
Blends with variety of other resin which contain reactivegroup
On curing, interaction occurs to give a cross-linkedcopolymer which exhibits characteristics of the twostraight resin
Examples are:
Phenol-formaldehyde resins
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Phenol-formaldehyde resins
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Phenol-formaldehyde resins
Most widely used is low molecularweight butylated resolswhich contain phenolic hydroxyl groups and etherifiedand unetherified methylol groups.
The epoxy resins used have a molecular weight of 3000 4000 and therefore contain secondary hydroxyl groups
At stoving temperature of 180C-200C, the phenolichydroxyl groups react with epoxy groups and both types
of methylol groups react with the hydroxyl group of theepoxy resin.
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Esterified epoxies
Epoxy with molecular weight of approximately 1400
Esterified with carboxylic acids through their hydroxyland epoxy groups:
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Epoxy resin (contd)
Modified Bisphenol A/Epichlorhydrin Epoxies:
Esterified epoxies
Catalysts such as alkali metal salts (sodium carbonate)are used to minimise etherification of epoxide groups byhydroxyl groups, since this reaction can lead to gelation.
May be carried out in either the absence or presence ofsolvent
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Epoxy resin (contd)
Other Epoxies:
Novolac epoxies
Polyglycol epoxies
Cyclic aliphatic epoxies
Acyclic aliphatic epoxies Glycidyl amine epoxies
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Epoxy resin (contd)
Novolac epoxies:
Low mw polymers consisting of phenolic nuclei linked inthe o- and p- positions by methylene groups
A typical commercial novolac epoxy resin has mw of 650and contains about 3.6 epoxy groups per molecule
Due to their multi-functionality, the novolac epoxy resinsgive, on curing, more tightly cross-linked products thanthe bisphenol A-based resins.
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Epoxy resin (contd)
Novolac eEpoxies:
This results in improved elevated temperatureperformance and chemical resistance.
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Epoxy resin (contd)
Polyglycol epoxies:
Linear polyglycols such as polypropylene glycol may beepoxidised through the terminal hydroxyl groups to givediglycidyl 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 otherepoxies.
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Epoxy resin (contd)
Halogenated epoxies:
Epoxies containing halogen may be prepared fromhalogenated hydroxyl compounds and resins areavailable based on tetrabromobisphenol A andtetrachlorobisphenol A.
The presence of halogen renders these resins flameretardant.
The ability of the resins to retard or extinguish burning isdue to the evolution of hydrogen halide upon
decomposition at elevated temperatures.
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Epoxy resin (contd)
Halogenated epoxies:
The brominated resins is more stable than thechlorinated resins but, once begun, the evolution ofhydrogen bromide is more rapid than that of hydrogenchlorode and the system is more effectively blanketed.
Brominated epoxy resins are generally used in blendswith other epoxy resins to confer flame retardance.