Care and Repair of Advanced Composites ~ ~ Materials

2.3.2 Carbon Fiber Finishes The reactivity of carbon fiber toward both electrolytic and thermal oxidation varies withthe temperature of the original carbonization. The least reactive surfaces are those thatare most crystalline (i.e., those that were exposed to the highest temperatures duringformation).

It should be understood that the continuous carbon fiber produced for sale has been sized(i.e., given a protective surface coating) (See Ref. 2.11.) because raw fiber straight from acarbonizing furnace is likely to pick up surface charge in rubbing contact with guides,rollers, etc., and is then awkward to handle. The electrostatic charges are clearly s.eenonpassing a comb through a bundle of the tows, because the fibers fly apart. Winding orunwinding from a spool, passage through guides, weaving, and other textile processes leadto snagging, breakages, and fine floating particles, which can short electrical machinery.

In ascending order of susceptibility to damage, weaving is more abrasive than simple fila-ment winding; braiding machines inflict the most severe abrasion. The fiber intended forbraiding needs a heavier coating of size.

2.3.3 Aramid Fiber Finishes

Many different polymer solutions have been tried, but the general consensus is that a low-molecular-weight epoxy resin (or a blend of liquid and solid for braiding) is th~most satis-factory. This is normally applied without added hardener.

In addition to a size, carbon fiber may require a surface tre,trnent to improve adhesion tothe matrix. Surface treatment is essentially an oxidative process. The idea is not simply toclean the surface, but also to l?roducean etching or pitt4t~ pf fhe surface and to providereactive oxide sites by chemical reaction.

Thereis a needfor a goodfinishforaramidsto improveadhesionto epoxyresinsfor struc-tural applications and where long-term durability is required. Normally, any sizing used isremoved by a scouring process involving passage of the fibers through a bath containingchemicalagents,followedbydrying,beforebondingwithepoxyresins(CourtauldsCode001 finish) or equivalent define the process. In the same way as with other fibers, differentsurface finishes are recommended to suit the chosen matrix resin, and it is necessary toensure that the correct one is specified. In the case of polyester and vinyl ester resins, aproprietary[mish(CourtauldsCode286)is recommendedfor Kevlar. However,theperfor-mance of a [mish for aramids is limited because, above a certain bond strength to the fibers,they will disbond by fibrillation. For this reason, the bond strength cannot usefully beincreased beyond this level, but to reach it would be helpful. A finish that could ensuredurability of the resin/fiber bond would be useful.

An early wet oxidation, rust introduced by AERE Harwell, was sodium hypochlorite solu-tion. The "short-beam" shear test, a good measure of interlaminar adhesion, demonstrateda useful increase; however, the tensile strength and the impact strength both decreasedconsiderably. When continuous carbon fiber became available, the hypochlorite methodwas abandoned in favor of the electrolytic process (Ref. 2.12). ' .

Because carbon fiber is a good conductor of electricity, a section of tow passing between apair of separated conducting rollers can be made into the anode of an electrolytic cell. Anumber of anions are theoretically available (for example, sulfate, phosphate, chloride, andpermanganate) which on discharge at the anode produce nascen~oxygen, an effective oxi-dizing agent. In practice, salts are chosen to yield the minimum permanent residue.

2.3.4 Sizing and Finish for New TYpesof Fibers and Fabrics

Each new type of fiber and hence fabric produced may require its own specific surfacefinish. Manufacturers' data sheets should be consulted to ensure that the correct finish is

specified for the resin matrix to J:>eused in each case.

2.4 Matrix (Resin)Systems

Resins fall into two general types: thermoplastics or thermosets. Both types are discussedseparately and in more detail in this section.

By varying temperature, concentrations of electrolyte, current density, and speed throughthe bath, surface attack on the fiber can be accurately controlled. Moreover, the speed isconvenient to interpose the electrolytic process between the last carbonization furnace andthe sizing bath. Another method of surface treatment which leaves no residue is thermal

oxidation. It is usually carried out at 300 to 450°C (572 to 842°F) in air. Atmospheres ofother gases have been used to speed the process.

2.4.1 ThermoplasticResinsThese are already high-molecular-weight, strong solids. They soften on heating, and oncooling they regain their original mechanical properties.

The most common thermoplastics are: ..

NylonPolycarbonate (PC)Polyimide (Condensation Type) (PI)

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Care and Repair of Advanced Composites ~. Polyetherimide (PEl)

Polysulfone (PS)Polyethersulfone (PES)Polyphenylenesulfide (PPS)Polyetheretherketon~ (PEEK).

Several resins from this list are beginning to be used as composite matrices. These include:

Polyimide (PI), used as a matrix material for composites and as an adhesive, a fiber,and a film

Polyethersulfone (PES), made by ICI as VictrexPolyphenylenesulfide (PPS), made by Phillips Petroleum as RytonPolyetheretherketone (PEEK), made by ICI

Thermoplastics can be remelted many times and for this reason are said to be easy to repair.However, because they have high glass transition temperatures (Tg) (a desirable property),they also have high melting points. This makes repair a problem because the temperaturesinvolved are dangerous near fuel tanks and would seriously damage the heat treatmentcondition of aluminum alloys. With suitable "corona discharge" surface treatments, repairwith epoxy systems is possible in some cases. Repair of thermoplastic parts requires newtechniques specially developed for the purpose. These are needed now for those partsalready in service. See ~oeing B.747-400 SRM and papers by Kinloch in Ref. 2.13.

2.4.2 Thermosetting Resins

Thermosetting resins harden or cure by a process of chemical cross-linking, whereby resinsof low molecular weight and good solubility grow into products of high molecular weightand limited solubility. Cross-linking is an irreversible process, and these resins can nevermelt. They may become softened to a considerable extent and will eventually char andbum, but they cannot melt. Many systems will cure at, or slightly below, room temperatureand emit some exothermic heat. Exothermic heat is a real problem in some cases, whichlimits the amount that can be mixed at anyone time. Other systems require heat toapproximately 120 or 180°C (250 or 350°F) to cause a cure. Verylow temperatures willslow or prevent chemical reaction and cure. In all cases, the speed of cure increases if heatis applied.

Thermosetting resins are the most commonly used today, although thermoplastics areslowly entering service in aircnut and other applications. The most common thermosettingmaterials are the followmg:

- .Phenol-formaldehydeMelamine-formaldehydeResorcinol-formaldehyde

58

~ Materials

I,

I

II

i

'. Urea-formaldehydePolyesterVinyl esterEpoxyAcrylicPolyurethaneSiliconePolyimides (PMR type) (PI)Bismaleimides (BMI)Cyanate ester

BMI and cyanate ester are more difficult to process than the others and are reserved forhigh-temperature applications.

Phenolic Resin Systems: This title covers a range of products. Each subtype consists of arange of materials tailored to suit specific applications.

Modified Phenolic Resin Pre-Pregs: Phenolic resins used in pre-pregs provide excel-lent fire performance properties to composites, which on burning produce low smokeemission and low evolution of toxic gases. Hence, they receive extensive use in inte-rior passenger cabin composite components as a matrix resin. They are used in ships,railway carriages, and aircraft, and they have good heat resistance up to 120°C (250°F).Phenolic pre-preg resins can be used with a range of fibers-primarily carbon, glass,and aramid. Fibredux 917 is an example of this type. It is an ideal facing material forlightweight honeycomb sandwich panels for use in interior furnishings in passengertransport systems.

Phenol-Formaldehyde: Some forms are used as wood adhesives and for plywood,particle board, pulp board, and hardboard manufacture, and for the impregnation ofpaper to make decorative laminates. Others are used for the bonding of grit to formabrasive grinding wheels, abrasive paper, and cloths, and for the treatment of filterpaper for oil filters.

Polyvinyl Formal Phenolics: These were the first metal-to-metal bonding strncturaladhesives known as Redux 775. Although first developed in 1943, they have beenproven to be more durable than epoxies in service and are used today. Some aircraftwith Redux 775 bonding continue to be in service after more than 30 years.

Resorcinol-Phenol-Formaldehyde: These are used for bonding a wide range of mate-rials to porous substrates, e.g., brick, concrete, unglazed porcehiin, expanded plasticssuch as ebonite, polystyrene, polyurethane and PVC, industrial and decorative lami-nates, leather, cork, linoleum, nylon; natural and synthetic rubbers but not silicone, andsheet metals to wood.

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Care and Repairof Advanced Composites ~ ~ Materials

Melamine Formaldehyde: These are used as laminating resins for the impregnation ofpaper for decorative laminates.

Urea-Formaldehyde: These are used for bonding wood, in the manufacture of plywood,and the bonding of veneers. Some versions are made for bonding decorative andnondecorative laminatetl plastic sheets such as Paxolin, Thfnol, Formica, and others.

polyurethane: Polyurethane resins can be rigid or flexible, with poor strength and fireperformance. They are used infrequently in modem aircraft. Made in a variety of types,they can be thermosetting two-component systems, single component using atmosphericmoisture to initiate the cure, thermoplastic, or solvent-based, depending on the formulation.polyurethane resins are not very water resistant, but this can be improved by the use ofprimers, especially on metal surfaces. They are flexible and tough with a range of applica-tions in the shoe-making industry, bonding windscreens in motor vehicles, and bondinglarge panels in coaches, buses, and caravans.

Silicone: Silicone resins are sealants rather than adhesives, but they can be useful when aflexible adhesive of no great strength is required. Cure initiation is by surface moisture forthe one-part types. When large-area bonding with a flexible adhesive is necessary, thentwo-part versions are available which must be mixed in the same way as other two-partsystems. They remain flexible down to very low temperatures (-60°C [-76°FD and can beused up to 200°C (392°F) or more in some cases.

Polyimides: Polymerization of monomer reactants (PMR) types are thermosetting, whereascondensation polyimides are thermoplastic. They are fairly weak as a basic resin, but theyoffer high-temperature performance (greater than 200°C [392°FD and good fire-smokeperformance. They can be blended with epoxy resins to toughen them.

Bismaleimides: Commonly known as BMIs, these are a special type of polyimide, pre-pared from maleic anhydride and diamines, preferably aromatic diamines. They are easierto process than polyimides and have processing characteristics similar to epoxies. Postcure for a short time is usually required to achieve full conversion and/or optimum proper-ties. A major advantage of BMIs over other polyimides is that their cure and post-curereaction mechanisms do not involve condensation products; therefore, no volatiles areproduced. BMIs can be problematic because they tend to be brittle as a result of their highcross-link density. This is inevitable because this characteristic enables them to retain highjoint strength at high temperatures. However, they can be made less brittle by the incorpo-ration of polymeric modifiers such as rubbers. These modifiers have low glass transitiontemperatures (Tg) and consequently reduce the Tg of the toughened resin, which in turnmay reduce the shear strength of joints at high temperatures. Modifications of BMIs withepoxy resins can improve peel strength without loss of high-temperature shear strength aslong as the epoxy is carefully chosen. Fillers can be used to adjust flow characteristics toimprove processing. Blowing agents can be incorporated to produce foaming adhesives,and minor changes in co-cure chemistry can be made to achieve compatibility with particu-lar substrates and control speed of cure. Redux 326 from Ciba is an example of a recentdevelopment in this field. See Ref. 2.14.

Polyester: These two- or three-part systems offer good environmental resistance and aregood to 150°C (300°F). They are used as wet resin and pre-preg. Although easily fireretarded, they emit considerable smoke when burning; consequently, their use in aircraft isdiminishing. Polyester resins are not as strong as epoxy. They can be dangerous if incor-rectly mixed (i.e., risk of explosion); therefore, the manufacturer's instructions must befollowed carefully. Polyester resins are widely used in boat building because of their lowcost. Tho basic types of polyester are orthophthalic and isophthalic. Isophthalic resinshave better mechanical properties and chemical resistance and are used as a gel coat orbarrier coat because of their greater resistance .towater permeation.

Vinyl Ester: These resins are increasing in importance in boat building, and, althoughmore expensive, their high quality has resulted in their consumption being almost equal topolyesters at the time of this writing. They offer good impact and fatigue resistance, and

. they make a good permeation barrier to resist blistering in marine laminates. Vmyl estersfall between polyesters and epoxies in both cost and performance.

Epoxy: Epoxy resins are very strong and provide good environmental resistance. They canhave high-temperature resistance (greater than 2OQ°C [392°F]) and are used as wet resin,

pre-preg, or film adhesive layers. Epoxy resins will usually bum readily, but additives canre(iuce this considerably. They also emit a significant amount of smoke when burning.Epoxy resins are the most commonly used system in aerospace, although their durabilityhas not been d~veloped to the level of the phenolic Redux 775.

Acrylic: These are finding considerable use in automobile applications and many otherareas, and are closely related to the anaerobic type commonly used for thread locking andthe bonding of bearing housings and bushes. Their low modulus and high elongation tofailure render them tough and impact resistant. Tho types are made: the resin plus activa-tor type in which the resin is applied to one surface and the activator is applied to the other,and the two-part type in which the activator is mixed with the resin before use. Both typesachieve good performance from a room-temperature cure. Another version is the ultravio-let curing acrylic for bonding glass and clear or translucent plastics, which contains photo-initiators in the resin to trigger the curing reaction. A further type is the epoxy cationicversion for opaque substrates. In this system, ultraviolet light is used to initiate the cure,which proceeds slowly enough to allow a short assembly time.

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Care and Repair of Advanced Composites ~ ~ Materials

Cyanate Ester: These resins are coming into use on the Eurofighter and in other high-perfonnance applications because of their improved properties at high temperature.Although they can be made with Tg values in the range 200 to 300°C (392 to 570°F), theyare expensive. These resins are difficult to use for repair purposes because of their highcuring temperatures, but cyanate ester composites can be repaired with epoxies if servicetemperatures are not ttJohigh. Cyanate ester resins form trifunctional highly cross-linkedand high-temperature stable polymeric networks (B.2.5). However, to ensure chemicalstability in harsh environments, blending with epoxies seems desirable. Unfortunately, thisreduces Tgand fracture toughness; therefore, careful compromise is necessary. Cyanateesters offer good adhesion to many substrates including metals, plastics, ceramics, andhigh-temperature thermoplastics such as PEEK. A significant amount of research remainsto be done on these materials, and B.2.5 should be consulted for more detail. If repair isneeded, it will be essential to consult SRMs and material data sheets for curing proceduresand safety requirements. I

Further Details on Thermosetting Resins: Most adhesives and resins used in currentcomposite structure manufacture and repair are thermosetting resins (i.e., they change dur-ing the cure stage from a liquid to a hard, brittle solid by a chemical cross-linking process,which irreversibly changes the polymer chains that make up the structure of the resin).

. After curing, the resin canno~revert to its previous liquid form. The cure stage can beachieved by either application of heat to a one-part system, which already contains a hard-ener that begins to react only as it approaches the cure temperature, or by a chemical reac-tion resulting from the mixing of a resin and hardener compoJlent (two-part system) to forma room-temperature curing resin. Adhesives are available in the form of two-part resin-hardener pastes and liquids or as pre-made film adhesives in which the resin-hardenermixture has already been cast into a thin film that is ready for curing by the application ofheat. The adhesives/resin types most commonly used in aerospace structures manufactureand repiPr include:

2.4.3 properties Required of Matrix Resins and Adhesives

2.4.3.1 Physical and Chemical Properties

The resins used to bind the fibers in composites and structural adhesives have many thingsin common, although their mechanical properties must be different. See Refs. 2.5 and 2.15.The common characteristiCsrequired are:

Must have good wettability to the fibers or surface to be bonded (substrate) anddevelop good adhesion on cure

Should not emit volatiles of any cure products during or after cure if they cause corro-sion or other ill effects, such as crazing of plastics

Should have a simple cure cycle process

Should have a good ambient-temperature storage life

Must be tolerant of imperfect processing

Should be tolerant of small inaccuracies in mix ratio (two-part systems)

Should not shrink during cure

Should have excellent retention of room-temperature properties when exposed toextremes of temperature and humidity

Must have a low water absorption rate and a low saturation water content

Should have a pH of the resin itself, and of any leachable extracts, that will not encour-age corrosion of the metal surface being bonded. Epoxy Resins: Epoxy resins are the most widely used resin types for aerospace adhe-

sives and composite applications. They range in type from two-part, room-temperaturecuring pastes to hot-cure film adhesives capable of operating up to 150°C (300°F) andfor long periods of time in aero engine applications.

Phenolic Resins: Phenolic resins were the first resin types used for aerospace adhesiveand composite applications and continue to be used extensively as adhesives for metal-to-metal bonding and as matrix resins for aircraft passenger-cabin furnishing panelswhere the low smoke and toxic gas emission characteristics of phenolic resins are ad-vantageous. The key problems with phenolic resins are that they produce water as aproduct of cure and require cure temperatures of l25°C (257°P) to 150°C (3000P) toachieve a cure.

Should not have any toxic hazards in either the uncured form or during decompositionin the event of fire (e.g., in an aircraft passenger cabin)

Must have a pot life (work life) suitable for the job in hand

. Should have a Tg (dry) and a Tg (wet) as high as possible and high enough, ideally, toallow the repair of significant areas of parts manufactured from hot-cured pre-pregswhen cured at least 25°C (45°P) below the original cure temperature of the part andpreferably 50°C (90°F) lower

Should be supplied with a viscosity appropriate foreach end use

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