Curing and Bonding of Composites using Electron Beam Processing

Curing and Bonding of Composites using Electron Beam Processing Daniel L. Goodman Science Research Laboratory / Electron Solutions Inc., Somerville, MA 02143

Giuseppe R. Palmese University of Delaware Center for Composite Materials, Newark, DE 19716 Drexel University Department of Chemical Engineering, Philadelphia, PA 19104

CONTENTS 1

2

3

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5 6 7 8

Introduction ................................................................................................... 1 1.1 Advantages of EB processing........................................................................... 1 1.2 Current limitations to EB processing ............................................................... 3 1.3 Background ..................................................................................................... 4 Aerospace composite fabrication using EB curing and bonding ..................................... 6 2.1 Filament wound rocket motors......................................................................... 6 2.2 EB-cured aircraft components.......................................................................... 9 2.3 Integrated aircraft structures........................................................................... 12 2.4 Other EB-cured components .......................................................................... 14 Automotive and vehicle composite demonstrations and potential................................. 15 3.1 An EB-cured automotive frame ..................................................................... 15 3.2 EB bonding of the Composite Concept Vehicle............................................. 17 3.3 Other vehicle applications.............................................................................. 18 EB-curable resin and composite development and current status ................................ 20 4.1 EB curable resin chemistry............................................................................. 20 4.1.1 Free radical polymerization ........................................................................ 20 4.1.2 Cationic polymerization.............................................................................. 22 4.1.3 Interpenetrating networks ........................................................................... 23 4.2 EB-curable resin and composite properties.................................................... 24 4.2.1 Prepreg resins ............................................................................................. 24 4.2.2 VARTM systems ........................................................................................ 26 4.3 EB-curable resins: further development......................................................... 29 EB-curable adhesive development and current status ................................................... 31 Equipment and facilities for EB curing and bonding ..................................................... 34 Conclusions and future directions .................................................................................... 38 References ................................................................................................. 39

Preprint of a chapter to be published in the Handbook of Polymer Blends and Composites, A. Kulshreshtha and C. Vasile, eds., Rapra Technology Ltd, publisher, Shropshire, UK

Curing and Bonding of Composites using Electron Beam Processing Daniel L. Goodman Science Research Laboratory / Electron Solutions Inc., Somerville, MA 02143

Giuseppe R. Palmese University of Delaware Center for Composite Materials, Newark, DE 19716 Drexel University Department of Chemical Engineering, Philadelphia, PA 19104

1 1.1

Introduction Advantages of EB processing Fabrication of fiber-reinforced polymer matrix composites incorporates assembly and curing

steps. Many different processes have been used for assembly, such as hand lay-up, resin transfer molding, filament winding, automated tape placement, etc. Although some resin matrices are designed to cure near room temperature, most composites are processed at elevated temperature, while pressure is applied for consolidation. Electron Beam (EB) curing of fiber-reinforced polymer composites is a promising new curing technology for fabricating aerospace and ground vehicle components. As a curing technology, it must be combined with a method of compaction such as tape or tow placement, hand-layup with hot debulk, vacuum assisted resin transfer molding, or pultrusion. The basic concepts of EB curing of composites and adhesives are shown in Figures 1 and 2. The process reduces the time required to cross-link the polymer matrix compared to conventional heat curing. This potential for high throughput is especially important for automotive fabrication. For aerospace applications, the principal advantages of EB curing are:

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Curing is done near room temperature, allowing the use of low cost, low temperature tooling such as wood, plaster or foam.

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Curing at low temperature can reduce residual thermal stresses.

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Co-bonding and co-curing operations with EB-curable adhesive allow fabrication of large integrated structure.

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EB-curable resins and adhesives have long shelf lives and can be stored at room temperature. They are typically one component and solvent-free.

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For large parts that are inconvenient or impossible to fit in an autoclave, EB processing may be the best low-temperature curing alternative. Using a portable electron beam system, one can bring the curing equipment to the part, rather than visa-versa.

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Recent cost comparisons of EB versus thermal fabrication have shown that EB processing can reduce costs by 10-40% for production of a variety of aerospace parts. These studies include both recurring and nonrecurring costs. Capital costs of EB curing systems (principally the electron accelerator and concrete radiation shielding) are similar to large autoclave costs.

Figure 1: Composite manufacturing processes suitable for EB curing include (a) filament winding, (b) pultrusion and (c) prepreg layup with vacuum bag. Other methods (not shown) include vacuum assisted resin transfer molding and automated tape placement. Parts are transferred via cart or conveyor to be cured under an EB scan horn. In-line or in-situ curing processes are under development for pultrusion and automated tape placement. This is shown in (b) and discussed in 2

Section 6. Figure reprinted from Ref [1] with permission of T. Walton, Aeroplas International Corp., Hollis, NH. Adhesive bonding coupled with autoclave curing is widely used in the aerospace industry to produce large composite parts with complex core structures. EB-curable adhesives have the potential to replace thermally cured aerospace adhesives in bonding large integrated structures in fewer steps, at reduced cost and without autoclave processing. The automotive industry uses adhesives to bond composite parts and structural assemblies. For ground vehicle fabrication, EB bonding offers high throughput and “command-cure” bonding near room temperature. EB-curable adhesives can be used to bond composites to metals substructure. The process avoiding de-bonding during the cool-down cycle that can occur due to differences in thermal expansion of the metal and composite.

Figure 2: High energy electrons are produced in the accelerator (sometimes known as the EB gun), transport through the air, and then penetrate deeply into materials. The depth of penetration is proportional to the energy, measured in millions of volts (MeV). The dose is a measure of deposited energy. Typical doses for curing composites or adhesives are in the range 50-200 kGy.

1.2

Current limitations to EB processing Although electron beam curing and bonding of composites is an active area of research, the

technology has not yet been widely adopted in industry. This is partially due to the conservative nature of the aerospace and automotive industries. Pre-qualification has begun as part of the industry-wide Composite Affordability Initiative (CAI) program, but there will be many years between this work and the use of the materials in production. This is because few new aircraft designs are planned, the design cycles are extremely long and the amount of composite on aircraft may actually be shrinking. In the 3

automotive industry, work done by research groups on new technologies, though promising, are not necessarily adopted for production. The reason that novel automotive concepts are not produced is often due to market forces, rather than to technical issues. In addition to overcoming the resistance to change of the risk-sensitive automotive and aircraft industries, EB researchers still faces significant technical challenges, especially to meet demanding aerospace requirements [2]. EB-cured composite properties have achieved those incorporating firstgeneration heat-curable resins, such as Cytec-Fiberite 3501-6. But composite properties, especially compressive properties that require good fracture toughness, are not yet as good as those containing toughened second-generation resins such as Hexcel 8552. Although preliminary material qualification has begun, the EB community does not have a reputation for "stable" materials with consistent properties. Suppliers and users have not developed the processing specifications and acceptance standards that are the hallmarks of a mature technology. EB-curable adhesives have made significant strides in the last few years. Their properties meet the requirements of automotive applications. The strength of the best EB-curable paste adhesive is close to that of industry-standard aerospace systems. But these adhesives cannot be used in the hottest or coldest aerospace environments, and are just beginning to be used on secondary structure. In some cases, aerospace companies find it more convenient to have their adhesive supplied as a film rather than as a paste.

However, EB-curable film adhesives do not currently possess the strength needed for any

aerospace application. There are several reasons why EB-curable composites and adhesives properties do not match those of state-of-the-art aerospace systems. One reason is that the resin chemistry (either free radical or methacrylated and cationic epoxy-based) produces different polymer structures than heat-curable systems. Another oft-cited reason for reduced composite properties is reduced fiber-matrix interface strength due to incompatibility between EB-curable resins and fiber finish. These issues and current research to improve material are discussed in Section 4.

1.3

Background Electron Beam composite curing and bonding is a subset of the field of radiation processing of

polymers. Electron Beam irradiation is a widely used commercial process. In the electronics industry, EB processing is used to crosslink wire and cable insulation, to produce heat-shrinkable tubing and to make polymer devices such as resettable fuses for automotive and portable electronics [3,4]. The Radtech organization, founded in 1988, promotes the technology of inks and coatings curing by ultraviolet light or EB. Radtech sponsors biannual conferences in the US and Europe [5]. Electron beam irradiation is also 4

used to sterilize medical products, increase the strength of supermarket plastic bags and crosslink rubber in tires. The French firm Aerospatiale was the first company to use electron beam composite curing in a production environment. Aerospatiale fabricated large filament-wound rocket motor cases and cured the resin with EB [6]. They reduced the time to cure a rocket motor from about a week to less than eight hours, while producing lower residual stresses in their part. This application is described in Section 2.1. Encouraged by this success, the US Defense Advanced Research Project Agency (DARPA) decided to fund work on the use of electron beam curing to lower the cost of manufacturing aircraft composites. The two largest DARPA-sponsored programs were the Affordable Polymer Composite Systems (APCS) program led by Northrop Grumman during 1994-1998 [7,8] and the Integrated Airframe Technology for Affordability (IATA) program led by Lockheed Martin from 1995-1997 [9]. Both programs produced demonstration aircraft parts using EB curing and bonding. The APCS program found significant cost savings for the use of EB in a variety of aircraft parts. The APCS program and its results are described in Section 2.2. The IATA program designers were encouraged to be forward-looking and futuristic. They asked how novel integrated designs using EB curing and bonding as an enabling technology could radically reduce fabrication costs. Their results are summarized in Section 2.3. The most recent large aerospace program to examine EB composite curing and bonding is the Composite Affordability Initiative (CAI), a cooperative program between the Air Force, Navy, Lockheed Martin Corporation, Boeing and Northrop Grumman. One of the CAI demonstration articles is an EBcured and bonded keel-duct interface. The part is based on an IATA design, similar to that shown in Section 2.3. CAI has also tested many of the commercially available EB-curable resins and adhesives and is creating benchmarks, comparing the systems to thermal systems that have flown or are planned to fly. The CAI data was not available for this review, but will soon be available from the companies participating in the testing [10]. A material selector CD with processing data is also planned for release. The high production rate achievable with EB processing is an important advantage for automotive production. As early as 1992, researchers at Chrysler were considering the possibility of a completely EBcured and bonded automotive body [11]. This concept is described in Section 3.1. They decided that such extensive use of an unproven technology was too large a technological leap. The Composite Concept Vehicle (CCV) was instead produced from resin injection molded thermoplastic [12]. One possibility for bonding this car is with EB-curable adhesive, as described in Section 3.2. Other applications of EB curing to ground vehicle structures are described in Section 3.3. EB-curable resins and adhesives for aerospace and automotive applications cure by either cationic or free-radical polymerization. Aerospace programs have primarily adopted cationic resins, because of 5

their higher use temperature and low shrinkage. Oak Ridge National Laboratory has coordinated the largest development effort in cationic resins and has licensed their materials to several commercial suppliers [13]. Free radical resins and adhesives have an advantage that they cure more quickly than cationics and are not inhibited by nucleophilic (alkaline) materials on fibers or surfaces [11]. Cationic, free-radical and interpenetrating network EB-curable resins for composite matrices are described in Section 4.1. Section 4.2 describes the current status and the requirements for EB-curable composites, primarily in the aerospace industries. Additional planned research to improve EB-curable resins are summarized in Section 4.3. The current status of EB-curable adhesive development is described in Section 5. Several new facilities devoted exclusively to EB composite curing and bonding have recently begun operation. The capabilities and cost of these facilities are described in Section 6. Section 7 contains our conclusions and expected future directions in EB composite curing and bonding.

2 2.1

Aerospace composite fabrication using EB curing and bonding Filament wound rocket motors The French company Aerospatiale has manufactured solid propellant rocket motors for more than

20 years. The cases were traditionally made from filament wound carbon fiber with heat-cured epoxy resin. Beginning in 1979, they looked for ways to increase production speed and to reduce thermal stresses during cure and cool-down. Researchers found that a combination of EB and X-ray curing reduced their curing time from four days to less than eight hours while keeping the structures near room temperature [6]. Because appropriate EB-curable resins did not exist, they developed an acrylated formulation with the low viscosity needed for filament winding [14]. Aerospatiale was the first to realize the need for new fiber sizings to improve the adhesion of EBcurable resins to carbon fibers. They developed specialized coupling agents that improved the fibermatrix interface by creating covalent bonds between the fiber and the resin [15]. The hydroxyl groups on the carbon fibers react with an isocyanate group on the sizing, and the hydrogen bonds on the sizing's acrylate group are free to crosslink with the resin during EB curing. This improved the burst strength of the system to exceed their heat curable baseline. Aerospatiale installed a 10 MeV, 20 kW electron accelerator to cure their rocket motors. In their dedicated concrete bunker, the accelerator could move linearly and the composite rotated to be able to cure parts up to four meters in diameter and 10 meters long. The system is shown in Figure 3.

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Figure 3: EB curing of a filament wound rocket motor at Aerospatiale. (Photo copyright 1997 Aerospatiale Matra Lanceurs, Cedex , France, used with permission.) Following Aerospatiale’s lead, several groups have recently used electron beam curing to fabricate filament wound aerospace components.

A US Army-sponsored program investigated the

feasibility of developing faster, more economical manufacturing techniques for filament wound composite motor cases for small diameter tactical rocket motors [19]. They produced thin-wall pressure vessels from a variety of EB-curable resins and demonstrated the use of low cost foam tooling. They achieved burst pressures that compare favorably with their baseline thermally cured pressure vessels. Figure 4(a) shows their pressure vessels during EB curing. Figure 4(b) shows one of their pressure vessels after burst testing. A group in Italy patented an in-situ curing process that combines layup and EB curing in one system and has developed a system containing a low energy electron beam accelerator coupled to a twoaxis filament winder for in-situ curing of pressure vessels [16, 17]. Their layer-by-layer curing process is applicable to a variety of composite production processes and is capable of producing parts of arbitrary size. Using their layer-by-layer process, the group was able to achieve interlaminar shear strengths equal to that obtained when curing their structures all at once using a high energy (10 MeV) electron accelerator. Using a similar process, a US group has coupled a low energy electron beam gun to an 7

automated tape placement machine for in-situ curing [18]. Both systems are shown and described in more detail in Section 6.

Figure 4: (a) Filament wound pressure vessels during EB curing. (b) After burst testing. Reprinted from Ref. [19] with permission of R. Foedinger, DE Technologies, King of Prussia, PA. A 1998 program at the NASA Marshall Space Flight Center (MSFC) examined the use of EB curing to correct manufacturing problems with their 60,000 lb.-thrust "FastTrack" rocket motor. They were experiencing difficulties due to a thermal expansion mismatch between the filament wound graphite and the silica-phenolic tapewrap at the throat of this one-piece composite nozzle. They showed that EB curing was a practical alternative, as long as the dose rate (and therefore heating due to EB dose and resin exotherm) was kept low. This part is shown in Figure 5.

Figure 5: Filament wound nozzle cured by EB near room temperatue to avoid cracking due to thermal expansion mismatch between the silica (center) and graphite overwrap. Photo courtesy of William McMahon, NASA Marshall Space Flight Center, Huntsville, AL.

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2.2

EB-cured aircraft components The DARPA-sponsored Affordable Polymer Composite Structures (APCS) program examined

many of the key issues facing the aerospace industry as it begins to adopt EB curing and bonding. The overall goal of the APCS program was to see if EB curing could significantly reduce the recurring and non-recurring cost of aerospace composites.

Northrop Grumman personnel evaluated EB-curable

materials, developed new processes, accessed EB cost benefits, and produced demonstration assemblies including a full-scale article. Although new material development was outside the program scope, resin suppliers produced novel systems to meet Northrop requirements. The EB-curable resins evaluated by Northrop included acrylated and methacrylated epoxies, cationic-curing epoxies, a modified bismaleimide, a modified epoxy novolac and a modified epoxy. These were compared with a heat-curable Cytek-Fiberite 3501-6 baseline. The cationic epoxy CAT-B resin had the best overall properties [8]. CAT-B demonstrated a dry glass transition temperature (Tg) of 180°C, close to the 190°C baseline. Composite lamina and laminate properties were measured on woven and unidirectional material forms. The CAT-B properties approached those of 3501-6 except in compression and shear strength. Northrop attributed this to poor resin-fiber adhesive, as indicated by dry fibers in the failed region of the specimens. They believe that a fiber sizing or a coupling agent compatible with the EB-curable resin can improve the interface. The APCS process development concentrated on ways to reduce porosity in prepreg composite laminates, which were initially high, in excess of 5%. Hot debulk prior to EB cure significantly reduced porosity, and application of external pressure during hot debulk increased the fiber volume in the laminates. For resin transfer molding (RTM) and vacuum-assisted RTM, they found that laminate porosity could be controlled by proper degassing of the resin before injection and by maintaining vacuum integrity throughout the infusion and cure cycle. Northrop Grumman combined a wide variety of fabrication processes with EB curing including automated fiber placement, RTM, VARTM and manual lay-up together with cocuring, cobonding and secondary adhesive bonding. They fabricated many parts including skins, covers, doors and various substructures. The culmination of the APCS program was the fabrication of a full-scale demonstration article. Northrop Grumman selected the aft center fuselage assembly of the F/A-18E/F fighter plane containing engine inlet ducts, frames, fuel cells, keels, dust-to-side skin webs and side skins. A wide range of part forms, resins and fabrication processes were used to demonstrate all the EB curing and bonding processes examined during the program. Materials and processes used included: •

Woven AS4/Cat-B prepreg and IM7 /RB-47 tape with hand layup.

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Tow placed AS4 Tape. 9

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Resin transfer molding of woven AS4 with VEB-2 and CAT-B resins.

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Selective area curing for later assembly to "wet" flanges.

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Co-bonding and EB adhesive bonding.

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Use of syncore for buckling resistance.

Northrop Grumman’s materials and assembly methods for the EB-cured fuselage assembly are shown in Figure 6. Figure 7 shows the assembly undergoing EB irradiation of adhesive bond lines during final curing steps. After assembly, the fit of the frames to the duct showed a maximum gap of less than 1 mm.

Figure 6: The aft center fuselage assembly of the F/A-18E/F fighter was selected by Northrop Grumman to demonstrate a variety of EB curing processes and materials. EB-curable materials used included CAT-B cationic epoxy prepreg and Loctite 334 adhesive. Figure courtesy of R. Vastava, Northrop-Grumman Co. El Segundo, CA. Cost studies constituted a significant portion of the APCS program. Northrop conducted cost trade studies for composite monolithic details, sandwich and integrally stiffened structures, and various assemblies. They found that the cost savings potential of EB processing varied with the structure, complexity and manufacturing process, and ranged from ~10% in case of simple details with direct process substitution to up to 60% for a complex assembly. The cost estimates included the cost of EB accelerator systems, buildings with the required features, part handling systems, operating, maintenance and repair. Typical EB facilities costs are described in Section 6. Capacity analysis for each facility and return on investment (ROI) analysis were also performed. The pay-back period naturally depends on the volume of composite parts processed. Northrop found a 2-4 year payback period for the assumptions of a single program using EB, a seven year 10

equipment life, an equipment acquisition value of $5M and a total of 54,000 to 90,000 lbs of composite processed. Typical ROI curves are shown in Figure 8.

Figure 7: Duct assembly undergoing EB curing as part of the APCS program. The EB gun (at right) is directed along the adhesive bondline to cure the duct splices.

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Figure 8: Return on investment as a function of savings for an electron beam aerospace composite processing facility. (Based on data collected by the APCS program [8].)

2.3

Integrated aircraft structures The DARPA-sponsored Integrated Airframe Technology for Affordability (IATA) program at

Lockheed Martin examined cost reduction from a design and manufacturing approach [20]. From the IATA perspective, the largest cost reductions come from advanced integrated designs that make use of advances in composite material and manufacturing process. A large portion of the IATA program focused on EB-curable composites and adhesives as a means to create large integrated structures. IATA selected for design review the wing/fuselage section of the JAST/ASTOVL aircraft. because it is the most costly portion of the airframe and the most structurally challenging for composites. The IATA program strategy reflects where production dollars are spent during manufacture of high performance aircraft. For prototypes or short production runs, cost is dominated by the tooling (up to 70% of final costs), which is related to the number and complexity of the parts required to produce the final structure. For larger production runs, cost is dominated by the recurring costs for fabrication and 12

assembly (up to 65% of total costs). The primary fabrication costs elements are material placement and processing, which can be 40-70% and 25-35% of fabrication costs, respectively, depending on the number of parts and their complexity. Unlike metallic parts that can be fit-up and assembled economically using mechanical fasteners, the mechanical assembly of composite structures is expensive, and is the largest contributor to scrapped parts. To reduce costs, the IATA program examined integration of parts for reduced part count and combinations of manufacturing processes including advanced fiber placement, EB-cured prepreg and resin transfer technologies (RTM, VARTM). They also examined co-processing technologies and unique methods for assembly that eliminates the extensive use of fasteners. Among the results of the program were many detailed designs, several demonstration manufacturing parts and preliminary material specifications for EB-curable resins and adhesives. One of the IATA fuselage section designs is shown in Figure 9. A demonstration part, shown in Figure 10, was fabricated to verify the concept design and provide a check on cost estimates.

Figure 9: An Integrated Airframe Technology for Affordability design utilizing EB curing and bonding to reduce costs. (a) The VARTM tri-resin keelson is EB-cured and rotated into place. (b) An integrated EB-cured tape-placed upper skin is lowered into place. EB-curable adhesive bond lines are also shown. IATA figures courtesy of D. Sidwell, Lockheed Martin Corp, Palmdale, CA.

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Figure 10: Demonstration EB-cured bulkhead section of JAST/ASTOVL airframe. (a) As displayed at the SAMPE Symposium in Long Beach CA, 1997. (b) The position of the structure, next to the inlet duct. IATA figures courtesy of D. Sidwell, Lockheed Martin Corp, Palmdale, CA.

2.4

Other EB-cured components A windshield frame and arch for the T-38 Talon is one of the few EB-cured aircraft components

that has flown. The T-38 Talon is the US Air Force's primary supersonic jet training aircraft. Two windshield assemblies were fabricated, substituting EB-cured materials for their thermally cured analogues [21]. As conventionally fabricated, most of the windshield frame is composed of 121°C-cured epoxy/6781 S-2 woven fiberglass prepreg, thin strips of 301 half-hardened stainless steel bonded together with high-strength epoxy film adhesive, and 6061 aluminum for structural attachment points. The windshield is rated to withstand a 1.8 kg bird impact at 740 km/hr. The windshield frame is made from four components that are assembled into one windshield frame and one windshield fairing. The four components are the fairing, innerskin, bulkhead doubler and outerskin/arch. The first three components are sufficiently thin (< 5mm) to be easily penetrated by electrons with energy of