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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)



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)



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)



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)



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)



The reaction of propylene &chlorine & hypochlorous acidto form epichlorhydrin:

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Epoxy resin (contd)


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)


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)


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)


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)


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)


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)


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Epoxy resin (contd)


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Epoxy resin (contd)


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Epoxy resin (contd)


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)


Effect of molar ratio on molecular weight of epoxy resins

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Epoxy resin (contd)


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)


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)


Their structuress:

BDMA

DMAMP TDMAMP

TriethanolamineN-n-butylimidazole

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Epoxy resin (contd)


Polyfunctional amines

Diethylenetriamine (DTA)

Triethylenetetramine (TET) m-Phenylenediamine (MPD)

4,4-diaminodiphenylmethane (DDM)

4,4-diaminodiphenylsulphone (DDS)

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Epoxy resin (contd)


Their structures:

DTA TET

MPDDDM

DDS

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Epoxy resin (contd)


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)


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)


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)



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Epoxy resin (contd)



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)


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)


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.

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