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Linhas areas InteligentesCOMPOSITE FUNDAMENTALS -1

REPAIR BASIC-1

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WHAT IS MEANT BY THE WORD COMPOSITE?

In it simplest form, composite does exist, when any two ormore pieces of material are joined together by bonding andthe strength will be carried-over through a chemical ratherthen a mechanical bond.

Parts with uniform material are not described as a

composite.

For example: plastic foils plastic sheets acrylic plastic windows plastic hoses plastic pipes etc.

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HOW PLASTICS ARE CREATED?

In this drawing you can see a set of twins. They can actas a model for the synthesis of polymers. Each pair ofMickey Mouses in the row represents a molecule of one ofthe most important gaseous hydro-carbons in themanufacture of plastic, ethylene. In the second row youcan see how they have Iet go one pair of hands and

joined up with their neighbors to produce a Iong chain. Ina chemical reaction the Mickey Mouses are monomersand the chain would be a polymer.

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POLYMERIZATION

The polymerization process has been generally understoodsince about 1930. Polymerization is a chemical reaction,generally carried out in the presence of a catalyst, whichcombines small molecules (monomers), containing a doublebond, into Iong chain molecules. The double bond is opened

up thereby making valency bonds available for Iinking with itsneighbouring monomer molecule.

No by-products are produced.

The monomer molecules may, for example, be:

ethylene polymerising to polyethylene (PE)

styrene polymerising to polystyrene (PS)

vinylchloride poly-merising to polyvinyl-chloride (PVC)

Different unsaturated (ie. with a double bond) componentscan be polymerised together (copolymerised)

For example:

styrene/acrylonitrile copolymer (SAN)

acrylonitrile/butadiene/styrene copolymer (ABS)

In addition there are two other types of reactions used tomake polymers, or giant molecules.

POLYCONDENSATION

Polycondensation was used and partly under-stood, evenearlier. The most famous product was Bakelite, so named byBaekeland, the Belgian chemist, who made it commerciallysoon after 1910. Polycondensation is a chemical reactionbetween two similar or dissimilar basic units which have atIeast two functional groups. lt gives rise to the elimination ofsmall, low molecular weight by-products such as water,hydrochloric acid, etc.

The most important commercial polymers are made in thisway.

For example: phenol-formaldehyde (Bakelite) resins (typical thermosets)

polyamides (nylon)

polyester.

Production of polyester- and polyamid fibres is also done by

polycondensation.

The polyester fibre or the polyester resin is produced bycombining the two monomers glycol and adipin acid. Thefission product wiIl be water.

There is another way of producing polyester bypolycondensation of glycol and terephtal acid. The polyestermacromolecule has build in benzol rings.

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Plastics fali into three classification:

Thermoplastic

Thermoplastics are Polymers n which the molecules are heldtogether by weak secondary bonding forces soften when theyare heated and so they are called thermoplastics.Thermoplastic foils, sheets, plates can be formed into

different shapes by applying temperatures of about 100C -200C centigrade. This can be done over and over again.

This is a Iist of thermoplastics used in aircraft industry.

polyethylene PE

polyvinylchloride PVC

polystyrene PS

polypropylene PP

polycarbonate PC

polyamide PA

polyethersulphone PES

polyimide P1

polytetrafluorethylene PTFE

polymethylmethacryleat PMMA

acrylonitrilbutadienestyrene ABS

Thermosetting

Plastics which are not softened by heat are known asthermosets. The monomeric material of which thermosettingplastics are made are mixed together at 0w temperatures andthey are extruded, molded or pressed at higher temperatures.

They undergo a chemical polymerization (molecular change)when heated and they become hard. Reheating, however, willnot reverse the process as it does with thermoplastics, andonce cured they remain in the hardened shape.

Bonded structure, as we think of it in aircraft construction, isnormally a Iaminated composite using thermosetting resins.

There are six types of thermosetting resin currently in usewith reinforcement. The most common thermosetting resin isepoxy resin. The other thermosetting resins are: polyester,vinyl ester, phenolics, polyimides and bismaleimides.

Polyester resins

Are the cheapest, have good properties at normaltemperatures and are widely used for large components, likeboats, and volume production.

Vinyl ester resins

Have a composition, and hence a cost of properties, betweenthose of polyester and epoxy resins.

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Thermosetting (continuation)

Phenolic resins

Have lower mechanical properties but retain them to hightemperatures and on burning do not produce toxic smoke.

Polyimides

Can withstand the highest temperatures of any thermoset andthe latest version can be processed as easily as epoxy resins.

Bismaleimide resins

Are the latest development for aircraft and are designed towithstand the hot-wet conditions, in which epoxy losestrength, without incurring the extra cost of polyimides.

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Elastomer

The third group of plastic materiaIs is called elastomer. If themolecules in a polymer are Iinked together by a small numberof valency bonds so that a loose network is formed then arubber- Iike material is formed called an elastomer, and thiswill have more elastic properties than a simple thermoplastic.

For example:

silicon rubber

tires

seals

sealing compounds

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Kinds of adhesives

There are 4 different kinds of adhesives CONTACT ADHESIVES

SOLVENT ADHESIVES

DISPERSION ADHESIVES

RESIN ADHESIVES

CONTACT ADHESIVES

The atmospherical pressure will press two solids together, ifthere is no air between the two plane parallel surfaces.Thereason for this adhesive force is the atmospherical pressure.

The barometric pressure is 1 bar iON/cm2. For a contact areaof 100cm2 a tensile force of 1000 N is needed to pull the twosurfaces apart. Contact adhesives work the same way byusing the atmospherical pressure.

SOLVENT ADHESIVES

Solvent adhesives contain plastics or resins dissolved in asolvent. The solvent has to evaporate to cure the adhesive.

DISPERSION ADHESIVES

Dispersion adhesives contain resins dissolved in a watersolution, such as wood glue or wood paste.

RESIN ADHESIVESResin adhesives cure by chemical reaction. Containing two ormore components.

Types of load

Adhesive joints may be subjected to four basic types of loadillustrated below:

Most structural adhesives are formulated to have excellent

tensile and shear strength. To a much lesser degree they

exhibit peel and cleavage properties. In fact, the purpose of a

well-engineered joint is to give as little exposure as possibleto peel and cleavage stress.

The design should be such that most of the load is distributed

over the joints as a tensile or a shear load.

Adhesive Joint Design

Lap joints

Angle Joints

Rods and Tubes

Sheet Reinforcement

Importance of the thickness of the adhesive

The thickness of the adhesive is important to the shear

strength. By increasing the thickness of the adhesive the

shear strength of the bonding will be reduced.

The optimum bond line thickness is in the range of 0.1 -0.2

mm. In very thin bond lines there is a risk of incomplete filling

of the joint due to contact between high points on the joint

surfaces.

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FILM ADHESIVES

Film adhesives are similar to prepregs. They are used forbonding metal to metal, sandwich core to skin and GFRP,

AFRP and CFRP.

Adhesives are available as unsupported and supported films.

Supported films contain a knitted nylon carrier.

Film adhesives are supplied on a release paper backing, inrolls with polythene interleaving to protect the adhesive in

storage and when being handled; these coverings must be

removed before the adhesive can be used. Heavy rolls should

be stored on a horizontal mandrel passed through the tube

core on which the roll is wound. This will avoid the risk of local

thinning of the film under the full weight of the roll.The film adhesive has to be cured under a curing cycle, in

accordance with the Structural Repair Manual.

The film adhesive has to be stored under refrigeration at -

18C.

FOAMING ADHESIVE FILM

A foaming adhesive film is an adhesive in sheet form which

expands during the curing cycle, to fill gaps and adhere

strongly to all parts of the structure with which it comes into

Contact. It is mainly used for the repair of honeycombsandwich panels as a honeycomb core splice and as an

edge- filling.

The foaming adhesive is designed for use in conjunction withstructural adhesive films and prepregs. Therefore it has to becured under a curing cycle, in accordance with the StructuralRepair Manual.

Foaming adhesive films have a limited shelf life at roomtemperature, therefore they should be kept under refrigeration

at -18C.

PLASTIC FOAMS

Plastic foams are produced with either closed or open(interconnected) pores and as either preformed rigid sheets

or as fluids for injection inyo cavities. Most thermoplastics andsome thermoset resins can be foamed but the materialscommonly used for aircraft components are polyvinyl chloridePVC, and polymethacrylimide. Polystyrene was used forsome experimental aircrafts, and polyurethan is alsoavailable.The choice of the material has to be considered by the

performance of the foam in a fire, in terms of the fireresistance and the toxicity of the smoke. Also thecompressive strength and the density of the foam core has tobe considered.

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THIXOTROPIC AGENTS

Some plastic resins are extremely sensitive to temperaturechanges. At 15C they may be as thick as molasses, while at30C they may run like water. And since heat is used as acuring agent, These resins may tend to run off of any verticalor near vertical surface before they have a chance to cure. Inorder to eliminate this problem, we can add a thixotropicagent.

There are two of them commonly used in aircraftmaintenance: micro- balloons and aerosil.

MICRO- BALLOONS

Micro balloons are made of hollow phenolic balls with a rangeof diameter from about ten to three hundred microns. Underthe microscope, each micro- balloon is seen to be a perfectsphere.In applying this agent, epoxy or polyester resins are mixed asdirected, and the micro-balloons are gently folded in, usingcare not to beat or crush them. A paste of light consistency ismade up and trowled onto the surface where it cures into a

hard, light-weight filler. This can be filed or sanded to therequired contour.

AEROSIL

Aerosil is a very light, white powder, which is a product ofcombustion. Aerosil is used as a thixotropic agent. You canfind it in combination with resins, lacquers, and even in toothpaste and sauce you can find it.

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RESIN ADHESIVES

The resin ensures the cohesion of the composite material.Thermosetting and thermoplastic are the two main types ofresin used as matrix.

The selection of the applicable type of resin to be used forelement buitd-up or repair execution is based on the study ofvarious parameters.

Nevertheless, it must be recalled that most of the mechanicalperformances of a composite are given by the fibers and theirorientation.

THERMOSETTING RESINS

When mixed with the specified hardener and cured, thethermosetting resin sets in a given form. The hardening is notreversible.Epoxy and phenolic resin are the two main types of resinused for composite structures. Epoxy resin for externalcomponents due to their good mechanical properties.Phenolic resin for cabin furnishing due to their fire resistance

and low toxicity.

THERMOPLASTIC RESINS

When heated thermoplastic resin becomes plastic. Aftercooling, the resin sets and hardens in a given form. The

hardening is reversible. Thermoplastic resins are not currentlyused on composite structures.

POLYMER RESINS

The mechanical properties of plastics (polymers) can beimproved drastically by adding fibres but for a composite toperform well the fibres must be bonded together so that theyact as a team. The choice of polymer is important.

The main features of a good polymer are

It must have the correct mechanical properties

It must coat every single fibre and bond well to them

It must be fairly easy to use

Polymers are combined with the fibres by melting or by usinga liquid polymers (resins) that can be hardened (cured)Melting is used to produce injection moulded articles such as

bodies for electrical equipment or mass producedcomponents but requires expensive machinery and moulds.Fibre reinforced components can be fabricated by using aliquid polymer, usually referred to as a resin.They are cured by addition of a hardener or catalyst, byapplication of heat or by a combination of both.The four most commonly used resins in fibre reinforcedcomposites are

Polyester resin

Epoxy resin

Vinyl ester

Phenolic resinWe shall be using epoxy resin which accounts for the majorityof aircraft repairs.

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EPOXY RESINS (EPOXIDE RESINS)

Epoxy resins are used in high tech composites because theirmechanical properties are superior to those of polyesters.They are however much more expensive. They too areviscous liquids but cure by a different process which requiresmore accurate mixing.

RESINS AND HARDENERS

The amount of hardener added is critical because using thewrong amount would result in one of the following

Unreacted resin in the final cured resin

Unreacted hardener in the final cured resinBoth of these conditions would result in a weak product, andin extreme cases the epoxy would remain sticky.

RESINIHARDENER RATIOS

There are hundreds ofepoxy resin systems on the market andeach requires a different resin hardener ratio. Some are as

low as 10:1 and others as high as 1:1 (eg Araldite D1Yadhesive).The manufacturer will supply exact details of the mixingratios.

COLD AND HOT CURE

Many epoxy resins will cure at room temperature, but manyrequire a high temperature to cure properly. Even roomtemperature resins can be heated to speed up the curing timeand to improve the properties of the cured resin.

EPOXY HARDENERS

Polyesters are cured by a chain reaction. That means thatyou only need a small amount of catalyst to start the reactionand then it will continue by itself So the amount of catalystneeded to cure the resin is not critical.

Epoxides are cured by linking together the resin moleculesand hardener molecules. This means that exactly the rightnumber of hardener molecules must be present to get thebest properties from the cured resin.

So what is the correct number of hardener molecules?

It seems that there must be ohe active hardener group foreach epoxide group on the resin. Unfortunately epoxides arecomplicated molecules and it doesnt work out exactly like

that The only way to find out how much hardener to add to aresin is to ask the manufacturers and they always state thecorrect resin/hardener ratio on the data sheet for each resin.

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RESIN HARDENER RATIOS

Manufacturers normally state how many grams of hardenerhave to be added to 100g of resin. This figure is oftenreferred to as phi, or parts per hundred of resin.

For instance the data sheet might say:

Recommended resin/hardener ratio = 100 : 60 by weight

This means that 1 00g of epoxy resin requires 60g ofhardener for complete cure, If, however, you are measuringout the resin and hardener by volume, the ratio may changebecause of the different densities of the two materials. In thiscase the ratio may be 100 : 65 by volume.

Resin/hardener ratios vary immensely from as low as 10 phrto as high as 150 phr for normal use. Of course 100 phimeans that you mix equal proportions of resin and hardener just like the epoxy adhesives that you buy as a householdglue.

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HARDENER CALCULATIONS

Once you know the resin/hardener ratio for the system thatyou are using you must be able to work out the amount ofhardener that you will need for any given amount of resin.You will rarely be using exactly 100 g of resin.

CALCULATIONS KNOWING THE FINAL WEIGHT

If you are going to produce a laminate using,say, 35g ofwoven glass fibres you know that the final weigbt after theresin and hardener are mixed should be 35g. So how much ofthat 35g is resin, and how much is hardener?

Answer:

Resin/hardener ratio = 100:90

Weight of resin needed = (100/190) x 35 = 18.6g

Weight of hardener needed = (90/190) x 35 = 16.4g

The matrix have two main functions, to keep the fibers in

position, and to distribute and transmit the loads to the fibers.The fibers and the matrix thus have a complementary role.

The epoxy resin family is the only type of resin used forstructural parts on aircraft. Due to different processes anddifferent chemical compositions, the properties of epoxy resin

can fluctuate.

The main properties are:

Good mechanical properties Medium to good toughness Low to medium service temperature NOT Valid for interior (Airbus Standard)

Phenolic Resin

Properties: Poor to medium mechanical characteristics Low to medium toughness Medium to high service temperature

Valid for interior furnishingPhenolic resin presents lower performance than epoxy resinexcept for the service temperature.

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EPOXY PHENOLIC OTHERS

General Used for structural Not valid for interior Valid for interior

BMI can be blended withEpoxy.PEEK PEI are validfor injection.

Mechanical Properties GOOD Low to medium Under research

Toughness Medium to good Low to mediumBMI PEI,PEEKLow high

ServiceTemperature

Low to medium Medium to highBMI PEI,PEEKHigh Medium

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EPOXY RESINS CHARACTERISTICS

AdhesionEpoxies have high adhesion to metals, glass, and ceramics.they can be formulated to give mixes of low viscosity withimproved wetting, spreading, and penetrating action. Thevariety of functional groups also provides good affinitybetween metals and plastics. For example, epoxies are in usefor bonding of copper to phenolic laminate in printed circuits.

CohesionWhen the resin is properly cured, the cohesive strength withinthe glue line is so great, and adhesion of the epoxy to othermaterials so good, That failure under stress often occurs inone of the adherents rather than in the epoxy or at the

interface. This happens with glass and aluminium as well aswith weaker adherents such as concrete or wood.

100% SolidsUnlike the phenolics and some other resinous adhesives, theepoxies cure without releasing water or other condensationby-products. This makes it possible to bond the epoxies at

only contact pressures, or with no pressure at all. Also, sincethere is no water to remove and no volatile solvents, theepoxies are convenient for the assembly-line bonding ofimpervious surfaces such as metal or glass.

Resistance to Moisture and Solvents

Epoxies are insensitive to moisture. Their resistance tosolvents is also outstanding and accounts for their rapidadvance in the coatings field. They are effective barriers toheat and electric current.

Low ShrinkageThe epoxies cure with only a fraction of the shrinkage of vinyl-type adhesives such as the polyesters and acrylics;consequently less strain is build into the glue line, and thebond is stronger. Also, the epoxy do not pull away from theglass fibres as polyesters do. The shrinkage can be reducedto a fraction of 1% by incorporation of silica, aluminum, and

other inorganic fillers.

Can be ModifiedThe properties of epoxy adhesive can be changed byselection of base resin and curing agent, alloying the epoxywith another resin, or compounding with fillers.

Can be cured at Ambient TemperaturesAdhesives curing within 5 mm at room temperature or lowertemperatures can be formulated by selection of special curingagents.

Resistance to Wide Temperature Range

Epoxies can be formulated for continuous service in hightemperature environments (in excess of 500F).

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THE MAIN DISADVANTAGES OF EPOXIES ARE:

ToxicitySome epoxies and diluents are known to cause dermatitis.Some amina curing agents are toxic. Good housekeeping isthe best preventive measure. The cured epoxies are notdeleterious to health.

Low Pot and Shelf-life

Most two component adhesive formulations must be mixedshortly before use. Some film and tape adhesives must bestored at low temperature for extended life, partially offsettingtheir advantages of convenience and reliability.

Moderate to High Cost

Epoxies are not cheap; however, their cost in a thin bond lineis hardly a factor in the overall cost of the assembled productin most industrial applications.

The term epoxy resin usually refers to an intermediatemolecule which contains at least two reactive epoxy groups.The most common epoxies used in adhesives are derived

from bisphenol A and epichlorohydrin (bis-epiresins) and areusually cured with reactive hardener containing primaryand/or secondary amine groups.

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SURFACE WETTING AND ADSORBTION

Surface wetting is probably the most important thing forbonding.

-Factors with positive effect on surface wetting:

Removal of surface contamination Viscosity reduction through application of heat

Viscosity reduction by solvent or diluent addition Time Pressure

-Factors with negative effect on surface wetting:

Fingerprints left on adherent surfaces Residues not removed from the adherent surfaces Glue lines not properly aligned Working at too low temperatures Wrong kind of surface pretreatment

-Two important aspects for a durable adhesive joint are:

Mechanical hookingSurface wetting

Mechanical hooking, as illustrated below, takes into accountthat the adhesive must penetrate into cavities before cure,otherwise the entrapped air will reduce the strength of theadhesive joint.

The phenomenon of surface wetting is frequently expressedwith the contact angle that is formed between a droplet of aliquid (adhesive) and a solid (substrate).

The smaller the contact angle the more the liquid spreadsover the surface, and vice versa. A complete surface wettingshould be expressed by a contact angle of = 0.

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Pretreatments for bonding

When ever a bonding is to be carried out, the condition of thebond surfaces must be considered. They are likely to becontaminated with materials which would affect theperformance of the joint. Surface preparation will normally benecessary.Surfaces are prepared by one of the following pretreatmentprocedures listed below.

Degrease only. Degrease, abrade and remove loose particles Degrease and chemically pretreat.

Care must be taken to avoid contaminating the surfaces

during or after pretreatment. Contamination may be causedby fingerprints by cloth which are not perfectly clean bycontaminated abrasives by sub standard degreasing orchemical solutions by other work processes taking place inthe bonding area.

Particularly to be excluded are oil vapours from machinery,

paint and mould release agents from spraying operations.When the pretreatment has been carried out, it is goodpractice to bond the surfaces as soon as possible. Should thebonding operation be delayed, the surface properties may bepreserved by priming the bond surfaces immediately afterpretreatment. For normal work the removal of all traces of oiland grease from the surfaces to be bonded is essential.

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DREGREASING METHODS

Suspend in halocarbon solvent vapour in a vapourdegreasing unit. The unit may be include a compartment toenable initial washing in the liquid solvent.

Immerse successively in two tanks each containing thesame halocarbon solvent 1,1,1- trichloroethane. The first tankacts as a wash and the other as a rinse.

Brush or wipe the joint surfaces with a clean brush or clothsoaked in clean halocarbon solvent. Allow the solvent toevaporate completely from the joint surfaces. Certain plasticsand rubbers are attacked by trichioroethylene and 1,1,1-trichioroethane. These materials may be degreased with

isopropanol or detergent solution.

Scrub the joint surfaces in a solution of liquid detergent, or,for metal only, immerse with a suitable alkaline cleaner. Washwith hot clean water and allow to dry thoroughly by using hotair.

The principal halocarbon solvents are trichloroethylene and1,1,1- trichloroethane. Trichloroethylene is the dominanthalocarbon solvent for vapour degreasing.

1,1,1-trichioroethane is the standard halocarbon solvent forimmersion and manual degreasing.

Safety precautions must be observed where halocarbon arein use.

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THE WATER BREAK TEST

The water break test is a simple method of determine wetherthe surface to be bonded is clean. It is best suited to metals.Apply distilled water to the surface. If the water film does notbreak up into droplets, the surface may be free ofcontamination. Uniform wetting of the surface by distilledwater indicates that it will probably be likewise wetted byadhesive.

It must be born in mind that certain plastics, even when clean,may not be wetted by distilled water but will be wetted byadhesive.

ABRADING

Light abrasion of the surfaces gives a better key to theadhesive than does a high polish. Freshly abraded surfacesusually have a better affinity for the adhesive. Abrasiontreatment must be followed by a further treatment to ensurecomplete removal of loose particles.

There are three methods in use

Repeat the degreasing operation. Lightly brush with a clean soft brush. Blow with a clean, dry, filtered compressed air blast.

METAL SURFACES

Remove surface deposits preferable by blasting with sharpgrit. If a grit blast unit is not available or the metal is too thinto withstand blast treatment, then clean the joint surfaces withabrasive cloth or waterproof abrasive paper. Wetting theabrasive paper assists removal of contaminants and reducesdust. Dry if necessary and remove all loose particles.Surfaces coated with paint which does not offer high

adhesion should be stripped; otherwise the strength of thejoint will be limited.The surface preparation described above, i.e. degreasingalone or degreasing followed by abrasion and removal ofloose particles, is sufficient for most adhesive work. But toobtain maximum strength a chemical or electrolytic

pretreatment may be required. For example anodising ofaluminium alloys.

PLASTIC SURFACES

Remove the surface layer of plastic surfaces to ensureelimination of all traces of release agent. As with metals,

abrasion by grit blasting is in general a good method. Thealternative is to use abrasive cloth or paper. After abrasion,remove all loose particles.

Removal is best carried out by method 2 & 3 above. The useof degreasing liquids on certain plastics could affect the

surfaces to be bonded.

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LAMINATES

With wet lay up moulded laminates it may be possible todesign the laminating process so that one layer of peel ply isplaced at the surface to be bonded. The ply becomes part ofthe laminate on curing. Just prior to bonding, tear off the peelply and the surface is ready for bonding.

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