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Technological Advances in Polymeric and Composite Materials

A. M. Lovelace, Member AIME

Introduction One area of engineering utilization of materials in which the require-

ments are especially rigorous and demanding is that of aerospace systems, including aircraft, helicopters, missiles, and space satellites. Materials used in these systems must reliably perform many critical functions in a broad range of adverse environments. Some of the most critical functions are performed by polymeric nonmetallic materials such as elastomers (rubber), coatings, fibrous materials, and reinforced plastics. These types of polymeric materials have some common characteristics that make them attractive to aerospace system designers: light weight, corrosion resistance, and relative ease and low cost of processing and fabrication. Each type also has its own special characteristics that enable it to perform certain required functions better than other competing materials can (Table 1).

The desirable properties of polymeric materials, which in some applications make them the only practical candidates, are partly offset by their principal limitation, which is degradation by exposure to high temperature.

TABLE 14HARACTERISTICS OF POLYMERIC MATERIALS

Type of Material, Special Characteristics Engineering Function

Elastomer Compliant Recovers after deformation

Coating Forms continuous film Adheres to substrates

Fibrous textile Drapable material Resists wear ,

Strong May be used for diverse

physical constructions

Reinforced plastic Has high strength to density Has hiah modulus to densitv Some Gpes possess good -

dielectric properties Is readily fabricable

Seals, O-rings, gaskets

Protection of substrates from adverse environments ,

Clothing Decelerators

Primary aerospace structures

Jet engine components Radomes Fairings, wing tips,

control surfaces

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Although aerospace systems must operate in a broad range of environments, common to many of them is that they, or certain of their components, must be able to operate at elevated temperature. This requirement has been the motivating influence behind much of the aerospace community's research on polymeric materials. Much of this paper, therefore, will deal with high- temperature-resistant polymeric materials. Where high-temperature requirements are not a limiting factor - such as in structural parts of some aircraft - nonmetallic materials in the form of fiber-reinforced plastic composites are in many cases proving to be lighter in weight and therefore more efficient than conventional metal structures. We shall discuss these and other examples of the selection and use of nonmetallic materials in preference to other competing materials.

With regard to "futuristic materials", it is likely that many materials being used and developed now for aerospace applications will later be used at industrial, commercial and consumer levels when higher volume production brings lower costs. Which is to say - to corrupt a familiar bromide - "what is past in polymeric materials for aerospace applications is prologue for industrial, commercial and consumer applications".

Elastomers Some of the more important aerospace applications of elastomers are

as follows:

Fluid Containment Applications

O-rings, gaskets, valve seats Integral fuel tank sealants Flexible bladder cells for fuel Hose

Special Applications

Aircraft tires Potting compounds for electrical components Electrical wire insulation Electrical connectors Oxygen masks Materials for damping vibration Rain erosion protective boots for radomes Seals for windshields and transparent enclosures (canopies)

Since one of the most critical applications is fluid containment, the discussion in this paper will be limited to that type of use.

Seals Perhaps the most important elastomer item in aircraft is the O-ring

seal. Modem jet aircraft use literally thousands of seals to insure reliable operation of the many airframe hydraulic and pneumatic systems and the jet engine fuel and lubrication systems. Existing problems with seals are

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very complex and anticipated problems associated with high Mach number almost defy solution, considering that seals face the combined hostile environments of fluids, temperature and pressure. In most current aircraft hydraulic, fuel and lubrication systems, Buna-N (nitrile rubber) seals are used. Although such seals have extremely long life (thousands of hours) at temperatures below 225OF, they are limited to about 500 hours of reliable service at 275OF, especially for dynamic sealing, where high strength and low compression set are required. It must be remembered that these seals have to perform over a range of -65O to 27S°F and at pressures up to 2,000 psi.

Seals fabricated from state-of-the-art fluorocarbon (e.g., Fluorel and Viton) have proved serviceable at temperatures up to 400°F for approximately 1,000 hours. A major drawback of these elastomers is that they are limited at low temperature to about O°F for reliable performance; however, there are reported cases where fluorocarbon seals have been effective at significantly lower temperatures. Epichlorohydrin elastomers (e.g., Hydrin, Herclor) have inherently good low-temperature properties and are currently being developed and evaluated for -65O to 275OF service. Seals and other elastomeric components fabricated from fluorosilicone rubbers are being used on a limited basis in current aircraft. These materials have good fluid resistance and broad temperature range capability (-65O to 450°F). However, before they can be used more extensively their tensile strength and abrasion resistance will have to be improved, especially for dynamic seals.

A word about future seal materials and the environment they will face is now in order. First, the environment to be created by high-Mach-number aircraft is expected to require seals with an operational capability of - 65O to 700°F and the ability to withstand pressures up to 5,000 psi. Needless to say, the seals will also have to be resistant to advanced fuels, hydraulic fluids, and lubricants; and naturally we would like to have good retention of physical properties at high temperatures (a critical and often conflicting requirement when -65O service is required) and after exposure to these temperatures for several thousand hours. To meet this challenge at least for a few years, improved fluorocarbon compositions offer promise for use up to 500°F, provided service to only about -20°F is required. Improved and new fluorosilicone polymers are being developed for use over a range of -65' to 500°F, or perhaps even 550°F. Further down the road, heterocyclic polymers such as triazines or polyimides, silicon-nitrogen, and modified silicone-carboranes offer promise for future seal applications. Silicone- carborances (Dexsil) are currently the most advanced of these types, but unfortunately, they are not currently resistant to fuels or hydraulic fluids and have only marginal physical properties. In addition to new elastomers, new concepts in reinforcement, stabilization, and crosslinking of elastomers for high temperature use are being developed.

Integral Fuel Tank Sealants Integral fuel tank sealants are used to render airframe structural cav-

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ities (for example, wings) fluid tight at modest pressures. Most modem aircraft designs do not employ discrete fuel tanks as such, but convert cav- ities within the vehicle, particularly wings, to integral tanks by using fuel resistant sealants to coat or caulk structural seams, joints, fasteners, etc. The lower schematic in Fig. 1 shows how a filleting sealant is used to seal the skin to spar cap joint in a representative wing structure. A ill and drain elastomeric topcoat is also often used to insure complete coverage of the interior of the tank. Currently used aircraft integral fuel tank sealants are based on polysulfides (Thiokol). These sealants have excellent fuel resistance, adhesion, good low-temperature properties, high solids, and can be cured at room temperature. The principal problem with polysulfides is that their long-term service is limited to temperatures below 200°F. This will not be close to what is required on high-Mach-number aircraft where aerodynamic heating will increase temperature requirements to about 600°F. The requirements for advanced integral fuel tank sealants are listed below :

1. They must adhere to aircraft metals. 2. They must be resistant to advanced jet fuels. 3. They must tolerate wide ranges in temperature (-65O to 600°F).

\ FUEL STOWAGE AREA

FUEL SIDE

Fig. 1 Use of elastomeric sealants in joints of integral fuel-tank wing structures.

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4. Their service life must be at least 3,000 hours. 5. They should be noncorrosive to aircraft metals (especially titanium). 6. They must cure at room temperature. 7. Their solids content should be high (100 percent preferred). 8. They must be easy to process and apply. We are a long way from meeting these requirements, but significant

progress has been made and development efforts are continuing. Hydro- fluorocrabon sealants that have survived laboratory tests for several thousand hours at 500°F in fuel vapor appear to be promising for near-term use. They are, however, not 100 percent solids, and during early work were corrosive (caused stress-cracking) to sensitive titanium alloys. Recent work indicates that the corrosion problem is being solved. Fluorosilicone sealants also show very good promise for use over a broad temperature range ( -6S0 to 450°F). They have good fuel resistance and do not appear to be corrosive to titanium at temperatures up to 450°F. However, adhesion to titanium, and physical properties such as tensile and tear strength need improvement.

Liquid Propallant Rocket Systems The functions of elastomers in liquid propellant rocket systems are not

obvious because the elastomers are not visible externally. Nonetheless, all liquid rocket systems have O-ring seals, gaskets, and valve seats to prevent leakage and to control flow of propellants. Positive expulsion bladders or diaphragms are used for mechanical expulsion of propellants under zero gravity conditions. These bladders fit in metal tanks, and gas pressure between the tank and bladder walls insures a positive exodus of contained propellants upon demand. A bladder made of carboxy nitroso rubber for test purposes is shown in Fig. 2. Rubber bladders are preferred over compatible plastics or metals for such applications because they have multi- cycle capability and do not tear when partially collapsed.

The development of seals, valve seats, and expulsion bladders for liquid propellant systems presents a most formidable problem because most common elastomers degrade, ignite, or detonate when placed in contact with highly energetic oxidizers such as nitrogen tetroxide and fuels such as mixed hydrazines. The problem is complicated because elastomeric components must be capable of long-term exposure to these reactive propellants over a range of -6S0 to 200°F with little or no change in their physical properties.

With regard to requirements and problem areas for elastomers used in contact with liquid propellants, there are two distinct areas, one pertaining to the fuel, principally hydrazine, and the other pertaining to the oxidizer, principally nitrogen tetroxide (N2O4).

Elastomers for Use with Liquid Rocket Fuels Resin-cured butyl rubber has excellent resistance to hydrazine fuels

and limited resistance to N204 oxidizer. Its use with hydrazine monopropellant has, however, been limited because it promotes decomposition of the

A. M. LOVELACE 167

fuel, creating undesirable pressure buildup in the system. Thus, butyl hasbeen used largely in O-ring applications where little surface area is exposedto hydrazine. Butyl has found considerable use in expulsion bladders formixed hydrazine fuels in bipropellant hypergolic systems because it has lowpermeability to both propellant and pressurizing gases such as helium ornitrogen. More recently, specially formulated ethylene propylene rubber(EPR) compounds have been used with hydrazine fuels on current systems.This material not only is compatible with hydrazines, but also causes lessdecomposition of the monopropellant fuel. The latter property has already

1'1 I I' I 1 I' I ':.1 1 I. I I' I ' II I I I·1 2 3- .;4. 5 6............__.--.._""--- ----'-Fig. 2 Prototype carboxy nitroso bladder for use with nitrogen tetroxide.


led to the use of EPR in positive expulsion bladders. With compounding modifications to eliminate fuel decomposition problems and reduce permeability to pressurizing gases, EPR, or perhaps ethylene propylene ter- polymers (EPT) has the greatest potential for use with fuel systems during the next decade.

Elastomers for Use with Liquid Oxidizers Development of elastomeric materials for use with advanced oxidizers

such as NzO, has been a particularly vexing problem. Until very recently, the best available elastomer was post-cured, resin-cured butyl. This material has been used for seals where only short-term exposure to N204 was required. It is useful for about 30 days at room temperature, 7 days at 100°F, or 1 hour at 165OF. Obviously, this limited compatibility has precluded the use of butyl in Nz04 expulsion bladders. The recent development of carboxy nitroso rubber (CNR) has provided a dramatic break- through in propellant resistant elastomer technology. This new perfluori- nated elastomer, when properly processed, is essentially unaffected after more than 1 year in N20, at 165OF. It is also very resistant to inhibited red fuming nitric acid, hydrogen peroxide, and liquid oxygen (good impact resistance). However, CNR is not resistant to hydrazine fuels. Because the need for an elastomeric material with prolonged resistance to N204 was so great, CNR valve seats and O-rings were quickly developed and are already being used on some advanced bipropellant propulsion systems.

A word of warning to potential CNR materials fabricators is in order at this time. To have a CNR compound that is resistant to NzO4, the reinforcing material and curing system must also be resistant to NZO4. (This is true for any elastomer-liquid propellant system.) For CNR, silica is recom- mended as a reinforcing material and chromium tritluoroacetate as the curing agent. As with any new material, certain problems associated with CNR remain to be solved. For example, CNR was evaluated as prototype expulsion bladders for NzO4 (Fig. 2). However, it was found that these CNR expulsion bladders had excessive permeability to N204 and pressurizing gases. Improvement in compression-set resistance is also needed. Effort is under way to correct these problems as well as to create modified nitroso systems for use with oxidizers even more energetic than N204 - such as fluorine-containing oxidizers - for advanced liquid propulsion systems.

Coatings For aerospace systems, protective coatings are used for many purposes,

including corrosion control, thermal control, erosion resistance, chemical resistance, nuclear thermal flash resistance, camouflage, marking, static dis- charge, anti-reflection and abrasion resistance. In the area of thermal control, coatings are primarily used for aircraft, missile, and spacecraft heat balance. Since there are so many uses for coatings, this discussion will be confined to those protective coatings required for thermal and erosive en- vironmental extremes encountered in aeronautics.

A. M. LOVELACE 169

High Temperature Polymeric Coatings As the speed of aircraft approaches and even exceeds Mach 3, the need

for improved thermally stable polymeric coatings becomes more and more apparent. Coatings will be required for camouflage, marking, corrosion control and other uses on future high-speed aircraft. As a result, the coatings for these various functions must also be capable of withstanding temperatures resulting from aerodynamic heating at high speeds (up to l,OOO°F from Mach 4 flight). In addition, these coatings must be serviceable for long periods ( 1,000 hours), tough, resistant to abrasion, weathering, and fluid, and simple and economical to apply (room temperature cure).

Currently, a variety of coatings are used for aircraft finishing and protection. These include nitrocellulose lacquers, alkyd enamels, vinyls, epoxies, polyurethanes and acrylic lacquers and various modifications of all of them. All things considered, the acrylic nitrocellulose paints are good currently available coating systems for general aircraft surface fishing; however, their resistance to high temperatures (above 200°F) is limited. With regard to thermal capabilities, the acrylics, epoxies and polyurethanes can be used at temperatures up to 350°F.

For future high-speed (Mach 3) aircraft in the next 5 to 7 years, it is expected that silicones will play a predominant role. For example, a new silicone coating has recently been developed by the Air Force that shows outstanding properties for long-term operation (up to 10,000 hours) at 600°F and short-term use at temperatures up to 700°F. This system has been extensively evaluated on stainless steel and titanium alloy substrates utilizing a specially developed pigmented silicone primer. The coating has successfully withstood prolonged exposure to Florida weathering and, based on laboratory tests, shows good resistance to fluids. It has performed satis- factorily on the XB-70 aircraft.

To meet the higher temperature requirements of higher-speed aircraft (Mach 4 and greater) in the more distant future, polyimides, silazanes, and perhaps inorganic polymeric coatings such as silica or phosphates formed in situ will probably be employed. Of these, the polyimide coatings have shown the most potential thus far, based on laboratory investigations. They show high oxidative stability at high temperature and are readily ap- plicable. They show relatively good stability at 700°F, based on isother- mal TGA (in air). Since the thermally stable polymeric coatings such as polyimides require sophisticated solvent systems or high-temperature curing if applied by conventional spray techniques, new application techniques such as flame and plasma spraying are being investigated. In recent development work, pigmented polyimide and polyaryloxysilane coatings have been successfully plasma sprayed onto metal substrates. These laboratory- applied coatings have been fairly glossy and hard, have adhered well, and have demonstrated good thermal stability.

Erosion Resistant Coatings .- . Because of higher speeds of aircraft, the need for all-weather capability,


and the necessity of operating at low altitudes, the erosion of materials on high-speed aircraft and missiles subjected to a rain or sand environment is an ever-increasing problem. Extended flight through these extreme erosive environments can cause catastrophic failure of major components and, in some cases, to the entire aircraft or missile system. Coatings are required that will protect radome and leading-edge surfaces of aeronautical systems of both subsonic and supersonic speeds. For supersonic systems, these coatings must be capable of withstanding high temperatures (up to 800°F for extended periods and 2,000°F for short times) caused by aerodynamic heating. For radomes (radar antenna housings) the coatings must be radar transparent and prevent static charge buildup on the surface (antistatic). Also, the coatings should be easy to apply and repair and be resistant to weathering and aircraft fluids and fuels. For applications other than radomes (e.g., aircraft leading edges, helicopter rotor blades, turbine engine compressor blades), radar transmission is not a requirement and com- pletely different classes of coatings may be considered. Also, for these latter applications, the substrate materials may be either metallic or nonmetallic, whereas radomes are constructed of dielectric reinforced plastics (plastics reinforced with a dielectric material such as glass or silica fibers).

On operational military aircraft, the coating currently used to protect radomes is neoprene. This type of coating is considered inadequate to meet the requirements of high-performance military aircraft, especially those capable of flying at supersonic speeds (even though they encounter only rain or sand at subsonic speeds). The neoprene coating that has been in use for more than 15 years was the best available elastomeric rain erosion resistant coating during that era. However, a new polyurethane coating has been developed by the Air Force that offers as much as an eightfold increase in protection over the neoprene coating. This new coating has been extensively evaluated in several rotating propeller test facilities under simulated rainfall conditions, at the rocket sled track at Holloman AFB, and under actual flight conditions on several operational aircraft. In all cases, the superiority of this coating compared with other polymeric (elastomeric) coatings has been clearly demonstrated. In addition to having improved erosion resistance, the polyurethane is easier to apply and repair and offers a slightly higher temperature capability than neoprene. In addition to being used for protecting aircraft and missile radomes, the polyurethane coating is expected to be used on aircraft leading edges, helicopter rotor blades and on advanced composite engine fan or compressor blades that operate at subsonic speeds. For the protection of aircraft and missile radomes at high supersonic speeds, such as up to Mach 3, either high-temperature polymeric coatings or ceramic coatings offer possibilities, depending on the velocity (and surface temperatures caused by aerodynamic heating) of the system. For very high-speed systems, such as Mach 3 and above, either bulk ceramics or thin, dense, adherent ceramic coatings over reinforced plastic materials are candidate materials. The ceramic protective coatings can be applied by flame or plasma spraying or can be preformed using slip casting techniques

A M. LOVELACE 171

and bonded to the plastic radome. Alumina, silica, titania and combina- tions of these may be used. Ceramic coatings for protecting radomes from erosion at supersonic speeds are being developed. However, the coatings developed to date still do not meet the requirements of future systems for supersonic erosion resistance. Some problems that must be solved are the mismatch of modulus of elasticity and coefficient of thermal expansion of coating and substrate, as well as thermal shock resistance and adhesion of the coatings.

For supersonic (and in some cases subsonic) erosion resistance where radar transmission is not a factor, nickel electroplated coatings are prime candidates. For example, leading edges of glass-fiber reinforced plastic helicopter rotor blades and aircraft propeller blades must be protected from erosion by rain and sand. Also, leading edges of aircraft structures made of "advanced composites" (boron and graphite fiber reinforced plastic) must be protected from erosion. Based on pioneering work conducted by the Air Force, electroplated nickel coatings have received increased at- tention throughout the aerospace industry and have already been applied. Based on extensive evaluations of many Merent coatings and coating types using rotating arm and supersonic sled facilities, the electroplated nickel coating has shown the best performance to date for protecting reinforced plastic materials from both subsonic and supersonic rain erosion damage.

Textile-Type Fibrous Materials Aerospace uses of textile-type fibrous materials primarily involve dece

lerators and clothing. Some other uses are in armor and as reinforcements for elastomers and plastics.

In addition to the well known parachutes for aircraft crew members, there are a number of other types of parachutes and decelerators that are used currently and that will be required for future Air Force systems.

For some special applications requiring deployment of the decelerator at supersonic speeds, temperatures of 600° to 1,500°F are encountered. Even parachutes that are used in subsonic aircraft may be exposed to heat and flames from a burning aircraft as the crew members egress from the aircraft. Therefore, common requirements for fibers to be used in decelerators are strength at high temperature and nontlammability.

The most widely used fiber for all types of decelerators is nylon. This fiber has the desired physical characteristics of high tenacity, high elonga- tion, and good energy absorption. However, nylon is limited in that it melts at 482OF and loses approximately 50 percent of its strength at 350°F. In high speed decelerators where aerodynamic heating occurs, nylon does not offer suflicient thermal resistance, and in personnel parachutes exposed to an aircraft fire, nylon can melt and fuse together, causing a malfunction and loss of life. Nomex, a commercially available polymeric fiber chemically related to nylon, has a greater thermal resistance than conventional nylon. Nomex retains more than 50 percent of its original strength at 550°F and does not melt, but chars at about 800°F. However, because of the methods


of solution dyeing presently being used, it is subject to severe fading and loss of strength upon exposure to sunlight. For this reason, nylon parachute packs and harnesses in non-ejection-seat aircraft will not be. replaced with Nomex until additional studies are made of this problem and more data are acquired on the sunlight resistance of Nomex.

Another fiber that does not melt on contact with fire and that offers thermal resistance exceeding that of Nomex is polybenzimidazole (PBI). At the melting point of nylon (482OF), PBI fiber has a tenacity of 4 gm;f denier, or better than 80 percent of its room temperature tenacity. As the temperature is increased, the tenacity of PBI decreases, but at 750°F the fiber still retains about 2 gm/denier. As with Nomex, the room temperature strength of PBI is less than that of nylon, so webbing and tapes of PBI must be made heavier to achieve breaking strengths equal to nylon. PBI is not commercially available, and its present cost ($200/lb) restricts its use to prototype components.

In addition to this need for textile-type fibers that have high strength retention at high temperature, there is also a need for nonflammable textile fibers for various aircraft and spacecraft uses.

Following are some uses of nontlammable fibrous materials i n aircraft: . . .

Aircraft Furnishings Crewmen Flight Accessories

Headlining fabrics Flight clothing Parachute harness Seat cushion cover fabrics Underwear Parachute pack Equipment cover Coveralls Cushion cover Emergency escape Flight jackets Carpeting Gloves

Boots Anti-G suits Exposure suits

For spacecraft, nonflammable fibrous materials are used in such ways as the following:

Personnel Accessories

Constant-wear garments Couch and cushion covers Pressure suits Equipment covers Pressure-suit bladder netting Restraint harnesses Life vests and rafts Litter nets and bags

Fasterners - Velcro Oxygen hoses

. . .

For fibrous materials and other materials used inside crew compart- ments of aircraft and spacecraft, there are also requirements limiting -the degree and amount of toxicity of decomposition products upon exposure to flames or high temperatures. Other requirements for flight crew clothing are comfort and- effective thermal insulation in case of exposure to

A. M. LOVELACE 173

flame. Fabrics made of PBI fiber are nonflammable in air in standard laboratory tests and have a high comfort rating in a garment; therefore they are excellent for flight suits for aircraft crew personnel (Fig. 3). Tests have been conducted of PBI fabric flight suits under simulated aircraft fuel fire conditions in which mannekins clothed in PBI flight suits were dragged through burning JP-4 fuel. Similar tests were made with Nomex and cotton flight suits. The PBI suits were superior from the standpoint of both non- flammability and protection from high temperatures.

Fiber Reinforced Plastic Composites New Fiber Reinforcements

In recent years, fiber-reinforced plastic composites have become well known to the general public through consumer items such as plastic boats and automobile bodies. Plastics reinforced with glass fiber have also been used in aircraft and other aerospace vehicles for radomes and for secondary structural applications such as wingtips and fairings.

Although plastics reinforced with glass fiber are considered materials for consumer use, they are generally not as structurally efficient as alumi- num for high performance primary aircraft structures such as wings, fuselage, and empennage. The principal structural deficiency of glass-fiber- reinforced plastic for the primary structures is its low modulus of elasticity.

During the last 5 to 10 years, however, two new types of fiber reinforcements for plastics have been developed that have about five to six times the modulus of elasticity of glass fibers and are equally strong. These are boron fibers and graphite fibers.

During the last 6 years, the Air Force has been experimenting on plastics reinforced with boron fiber to construct prototypes for aerospace vehicles. These include primary structures for aircraft and structural components of jet engines, and spacecraft. Some of these structures and structural components made of boron-reinforced plastics are listed below.

Aircraft Applications Helicopter Applications

Horizontal stabilizer Rotor blades Fuselage section Rudder Jet Engines

Wing box Compressor blades Wing skins Compressor hub Wing leading edge slat Landing gear doors

A photograph of a full-scale F-111 horizontal stabilizer having skins made of boron-fiber-reinforced plastic is shown in Fig. 4. A representative aircraft wing box, also incorporating boron-fiber-reinforced plastic is shown schematically in Fig. 5.

The development work on boron-fiber-reinforced plastic prototype structures was started earlier than that on the graphite-fiber plastic struc-

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Fig. 3 Flight suit made of PBI fabric.

A. M. LOVELACE 175

tures. Considerable engineering data and design data have already beenobtained on boron-tiber-reinforced plastics. The results and test data havebeen so encouraging that these plastics have been selected for use in thehorizontal stabilizer of two U. S. fighter aircraft now being developed.

Prototype aircraft structural parts made of graphite-fiber-reinforcedplastic are also being developed rapidly. A prototype fuselage structuremade of this kind of plastic has already been fabricated and tested. Fig. 6shows such a structure. This particular one was 27 percent lighter than analuminum one designed for the same ultimate load capability. It was asimple "idealized" design and did not incorporate all the engineeringdetails such as access doors and attachment points required for a real-life,flyable structure. However, real-life prototype structures, such as wings,fuselages and empennages, are being designed and built of graphite tiberreinforced plastic.

The extent of use of plastics reinforced with boron and graphite fiberin future aerospace vehicles and for nonaerospace applications will probably depend more on cost than on technical limitations or deficiencies ofthe materials. Boron fibers now cost about $250jlb and graphite fibersabout $100 to $300jlb. The production cost and selling price of both

Fig. 4 Prototype F-Ill horizontal stabilizer made with boron fiber-epoxy composite skins.

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boron and graphite fibers could potentially be reduced by almost half ifcurrent research on alternative raw materials and on processes to producethe fibers is successful, and if the demand for the fibers increases, thusleading to more economical, high-volume production.

New Matrix MaterialsEven though the high structural performance of plastics reinforced

with boron and graphite fiber is attributable primarily to the propertiesof the fibers, the properties of the plastic matrix materials are alsoimportant and sometime critical. Some of these mechanical properties arefiber-matrix bond strength, tensile strength, modulus of elasticity, andelongation toughness. In addition, the matrix and the composite mustbe resistant to outdoor exposure and flight conditions all over the world.The matrix materials used most extensively to date with boron andgraphite fibers have been epoxy resins. These resins and the compositesmade with them have demonstrated outstanding structural performance.

Fig. 5 Prototype wing box structure incorporating boron epoxy composite components.

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However, one of the inherent limitations of epoxy resins in structuralcomposites is their incapacity to withstand high temperatures; they arelimited to about 300° to 350°F. Since the surface temperature of supersonic aircraft operating at Mach 3, for example, can exceed 600°F,matrix materials with higher temperature capabilities are required. Hightemperature matrix materials are required not only for use with boronand graphite fibers in structural applications, but also for use with glassand silica fibers in radome applications.

One of the general approaches pursued by the Air Force for the pastseveral years to develop plastic matrices resistant to high temperatures isbased on polyaromatic heterocyclic polymers. These are polymers thathave a rigid "backbone" primarily of atoms arranged in closed ringsrather than in chains, and whose carbon atoms are either in rings orconnected to rings. These factors contribute to improved thermal stability.One of the first laminating resins having the polyaromatic heterocyclicstructure was polybenzimidazole (PBI). Other laminating resins in this

Fig. 6 Prototype fuselage structure made of graphite fiber epoxy composite.

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Polybenzimidazole (PSI)

Polyimide (PI)

Polyb~nzothiazole (PST)

Fig. 7 Molecular structures of new high-temperature-resistant laminating resins.

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generic class are polybenzothiazole (PBT) and polyimide (PI). Representative molecular structures for each of these types of resins are shownin Fig. 7.

PBI resin has been used in a development program to fabricate theprototype radome structure shown in Fig. 8. Although PBI laminateshave excellent mechanical properties for short times (10 minutes) at veryhigh temperatures (1,200°F), they do degrade during long time exposure

Fig. 8 Prototype radome made with PBI laminating resin.


(1,000 hours) at somewhat lower temperatures (600°F). At 600°F for thousands of hours of exposure the polyimides are the best of the three types of resin.

Laminating resins having even higher temperature capability than the three described above have been synthesized in the laboratory. The problem in utilizing these newly synthesized resins in reinforced plastics is that they lack the flow and solubility characteristics required to im- pregnate fibers and to permit consolidation into a composite of low void content.

The near-term emphasis on laminating resins is on striking a reason- able balance between high temperature capability and fabricability. In this endeavor, the polyaromatic heterocyclic backbone would still be used but modified to achieve lower-temperature and lower-pressure curing and no release of volatiles during cure.

Summary Even where metals are used as the major material of construction in

aerospace systems, polymeric materials are still required in a supporting role, such as for seals or protective coatings. The performance of these supporting materials in many cases establishes limits on the performance of the entire system. The Air Force has a continuing program on the development of improved polymeric materials that will increase the capabilities of Air Force systems; as an added benefit they are expected to upgrade the performance of these materials for civilian use also.

The engineering applications of polymer-based materials are being broadened based on improvements of these materials in two major cate- gories. One is the extension of the environments - particularly, high temperatures - to which the materials may be exposed and still perform their required function. The other is the use of these materials as composites in structural applications that were previously confined almost solely to metals; this increased use is based on the fact that the structural efficiency of the composites is superior to that of metals.

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