A Comparison of the Environmental Attributes of Thermoplastic vs. Thermoset Composites

Transcription

1 A Comparison of the Environmental Attributes of Thermoplastic vs. Thermoset Composites Michael Favaloro, Celanese 84 Dixie Highway, Florence, KY 4142 USA 1. Abstract The demand for composite materials in aerospace is stronger than ever, due mainly to the savings in fuel cost for lightweight structures. The vast majority of composites used in aerospace are thermoset based. Increased usage of continuous reinforced thermoplastic composites for aerospace are demonstrating that the environmental advantages of thermoplastic vs. thermoset are substantial. These advantages include a substantially reduced energy usage for processing, reduced scrap in processing, reduced waste stream, improved recyclability, and reduced use of volatile organic compounds. Furthermore, composites based on thermoplastic matrix resins are recognized for the improved toughness they offer. Due to this inherent advantage, current developments show promise for designing further weight reduction over conventional materials. A thin fuselage for a single aisle aircraft made with thermoplastic composite may potentially offer improved toughness and substantial weight savings as compared to a thermoset counterpart. others. For the most part, however, the bulk of exterior aerospace components in Europe are based on thermoset composites. In the US, there is not yet one thermoplastic based composite in large commercial aircraft exterior structures. The current focus of the commercial aerospace composites industry is on thermosets for a number of reasons. Thermoset composites have been successfully manufactured for aerospace since the 196 s, and the knowledge database is very mature. Substantial investment has been expended in the US for man rated thermoset composite structural components. This investment includes design tools, material property databases, capital equipment, employee training, test methodologies, a mature material value chain and so on. Conversion to alternate materials and processes would require substantial reinvestment and requalification costs. KEY WORDS: Applications Aerospace, Characterization, High Temperature Composite Materials/Structures, Flame Spread/Flammability/Smoke/Toxicity, Resins/ Materials Polyetheretherketone (PEEK), Resins/Materials Polyetherketoneketone (PEKK), Resins/Materials Polyphenylene Sulfide (PPS) The 45-year-old aerospace thermoset composites industry has matured to a point where new process technology innovations that result in parts with reduced cost are unlikely, with some notable exceptions. The industry has greatly increased usage of composites to achieve cost savings resulting from reduced fuel consumption, but structural component cost is not going down. Thermoplastic composites offer several methods to reduce part costs as compared to their thermoset equivalent. Furthermore, thermoplastic composites can be considered as a more sustainable approach for the industry. This paper compares environmental attributes of both thermoset and thermoplastic based composite materials used in the aerospace industry. 2. Background 3. Thermoplastic Vs. Thermoset The aerospace industry has increased usage of composites structural components for large aircraft, mainly because of weight reduction and resultant fuel savings. To date, the vast majorityof composite materials for aerospace are thermoset based, especially in the US. In Europe, thermoplastics composites have been used on the Airbus A34 and A38 for several years. Some parts on these vehicles made from thermoplastic composites include the fixed wing leading edge, keel beams and Cost The advantages of thermoplastics vs. thermoset composites can be broken into four categories:cost, process, performance and environmental. Some advantages fit multiple categories. In terms of cost, Figure 1 provides a relative comparison of the costs of carbon based composites. The first column shows comparable material (prepreg) and process cost for an epoxy based composite, and the other three columns compare various This paper will outline the specific advantages of thermoplastic vs. thermoset composites for aerospace. A comparison of the environmental effects for similar parts made with thermoplastic and thermoset composites will be performed. Page 1

2 Figure 1 Cost Comparison of Conventional Versus Candidate Composite System 1 Epoxy Processing Material 8 System Cost in % Figure 2 Typical Composite Process Temperature Cycles Temperature thermoplastic composites (1). PEEK is a relatively expensive thermoplastic, and is more costly in prepreg form than most epoxies. However, the processing cost for a PEEK composite in production is far less than the processing cost for an epoxy. The net cost for a carbon/peek composite, as a result, is less than a conventional carbon/epoxy composite. The lower cost polyphenylene sulfide base composite offers a total part cost of less than half that of a conventional carbon/epoxy Time Conventional PEEK PEI PPS Thermoplastics, for the most part, exhibit raw material costs that are less than epoxies and other aerospace based thermoset polymers. Since thermosets must be maintained at elevated temperature for relatively long times as compared to thermoplastics, the process energy and associated cost for thermosets are higher. Thermoplastics can be re-formed by application of heat, so repair and rework procedures for thermoplastics are less costly. With the rapid fabrications processes described below, capital equipment, facilities cost, floor space and environmental control requirements for thermoplastic composites are less. Process: A thermoset material must be heated until cure, or crosslinking, of the polymer occurs. The typical heating cycle for an aerospace grade epoxy can range from 1 to 4 hours. By comparison, thermoplastics are melt processable. They are heated to a temperature above their melting point, formed into the desired shape, and then cooled until they re-solidify. Process temperature for a thermoplastic can be in the order of seconds or minutes, depending on the thickness of the part. As a result, the cycle time for thermoplastic composites is much less than for thermoset composites. A comparison of thermoset and thermoplastic process cycles is shown in Figure 2. Page 2 The cycles assume a match metal mold and a press with heated platens. For the thermoset part, the part is in the tool for heat up, and remains in the tool while the resin is cured. For the thermoplastic part, the tool is heated up to temperature, then the tool opens and the part is thermoformed to shape. The tool is opened and the part is removed and replaced with new material. As shown the ramp-up temperatures for both materials are the same. As a thermoplastic part is pulled from the tool, the tool temperature is reduced until the press closes and the next part is heated up. The figure shows a cycle time of 6 minutes for the thermoplastic part, so the thermoforming process results in 1 parts for every one thermoset part. This is just an example for generic comparison of the two materials. Actual cycle times are dependent upon epoxy cure temperature, thermoplastic melt temperature, part thickness, and other factors. There are other process advantages for thermoplastic composites. Thermoset prepreg typically has a usable shelf life of 6 months and must be refrigerated in a freezer. Production control requirements dictate how much material can be used within a given time period, and, combined with minimal material purchases, can result in scrapped material and additional part cost. Thermoplastic prepreg requires no refrigeration, resulting in less equipment cost, less floor space and less energy cost, as well as less production control. If a minimum material purchase is required, the inert material can be stored on a shelf almost indefinitely without loss of use. Automated fiber placement and automated tape laying processes are

3 Process step reductions can be achieved with thermoplastic composites by using fusion bonding to connect integral stiffener components to aero skin structures (3). With this process, adhesives and mechanical fasteners can be minimized or eliminated. The net result is a reduction in process cost and part weight. amount of energy required for cure is proportional to the area under the time/ temperature cure cycle, shown in Figure 3. Figure 3 Typical Composite Process Temperature Cycles with Energy Shown Epoxy Temperature becoming increasingly used for both thermoplastic and thermoset composite materials. Both materials share the process advantages of reduced layup time, reduced scrap, increased placement accuracy and reproducibility, and in-place consolidation. With thermoplastics there is an added process advantage, since in-placement consolidation can eliminate the need for an autoclave or press cycle subsequent to fiber placement (2). Performance Thermoplastic matrix composites offer some very specific performance improvements over thermosets. Thermoplastics are typically tougher than thermosets, since the long polymer chains of a thermoplastic are tougher than a cross linked thermoset backbone. As a result, thermoplasticcomposites are more damage tolerant and better in fatigue than thermosets. Since thermoplastics solidify as they cool down, and thermosets solidify as they heat up and are cured, thermoplastics generally exhibit less residual stress in molded parts. Green Reduced Energy Requirements A relative comparison of energy requirements for thermoplastic vs. thermoset composites can be made by considering the cure/ consolidation cycles of each as shown in Figure 2. Since thermoplastics are not cured, but melted and re-solidified to shape, the figure shows that multiple thermoplastic parts can be consolidated in the same time that one thermoset part is cured. With some basic assumptions, we can compare the energy savings of a thermoplastic vs. a thermoset composite part. A realistic approach would be to assume the same part would be made, but with either a thermoset or thermoplastic matrix. The matrix selection for a simple part could allow for use of the same match metal tool with the same heated press. The same number of heating elements is used to bring the press up to temperature, so the same amount of power is used to reach the same temperature for either material. One can safely ignore negligible differences in heat capacity as well as a minor thermoset exothermic reaction during cure. Since the power required to reach temperature is directly proportional to the temperature of the press, the Epoxy Cure Energy Requirements 2 4 Time 6 8 The first thermoplastic part to be consolidated, if it were loaded in the tool at the start of the cycle, would require a similar amount of energy to reach cure/melt temperature as the thermoset part. Subsequent parts could be consolidated with less energy by using automation techniques to shuttle parts in and out of the press. The net result is an energy requirement that is substantially less for thermoplastics than the thermoset counterpart (Figure 4). With the tools attached to the platens of the press, one part can be thermal formed in a matter of minutes. Heat losses will occur when the platens are opened and closed regularly to insert cold preforms and remove molded parts. For other composite manufacturing processes, energy reduction opportunities for thermoplastics can be greater. Consider the autoclave energy requirements for curing a large carbon/epoxy fuselage structure. Placement of the fuselage reinforcement can be performed with automated tape placement equipment for either carbon/ epoxy or carbon/thermoplastic composites. However, in-situ consolidation of the thermoplastic would eliminate the entire autoclave step. The consolidation energy requirement for automated tape placement varies with the type of heat, but in each case, is far less than the thermoset equivalent.another energy savings for thermoplastics materials is the fact that refrigeration is not required for storing of thermoplastic prepreg materials. Page 3

4 Figure 4 Typical and TS Composite Process Temperature Cycles with Energy Startup Energy Energy per Part Temperature Epoxy 2 Epoxy Cure Energy Requirements 4 Time 6 8 Reduced Scrap and Reusability An industry standard for manufacture of complex thermoset composite parts is to purchase two pounds of prepreg for every pound of final part weight. As fabric based prepreg is cut and kit, residual uncured prepreg scrap must, in many cases, be handled as hazardous waste, or, in best cases, as more landfill. The final parts offer little in the form of reuse and recyclability. Reject parts that cannot be reworked for use ultimately end up in the scrap heap. For thermoplastic prepreg, any scrap material can be pelletized and converted to material for injection molding or extrusion processes. Also, since thermoplastics can be re-formed after heating, rejected parts have an improved chance of being reworked, repaired or redirected to a lowerperformance spectrum product. For those parts that cannot be repaired, the option still exists for conversion to pellets for recycling in other non-continuous fiber composites or blends (2). Reduced VOCs and Toxic Decomposition Products A drive past a composite boat manufacturing facility on a hot summer day provides an olfactory reminder of environmental concerns with the process. Composite facilities that use a wet layup process can have a distinguishable odor of volatile organic compounds. Large conventional composite process facilities, such as boatbuilding plants, have historically exhausted a substantial amount of volatile organic compounds into the atmosphere. Typically the resins used are diluted with a volatile solvent for ease of processing. Additionally the curing mechanisms for many thermoset materials result Page 4 in generation of VOC s as by-products of cure. Thermoset based prepreg typically contains solvent and uncured thermosets that will release organic compounds with cure. As a result, VOC exposure is a consideration in MSDS s, and composite facilities must maintain sufficient air quality equipment to maintain the health and safety of their personnel. In thermoplastic processing, occasional trace VOC s are generated as the polymers are heated; however, there are no solvents in the polymer waiting to be outgassed, and there are no cure mechanisms that release VOC s as by-products. Consequently, the amount of VOC s encountered in thermoplastics processing are insubstantial as compared to thermosets. Halogenated additives are typically used in thermosets, and in many thermoplastic materials to reduce flammability. However, the resultant toxic smoke is resulting in reconsideration of this approach, across industry lines. For composite materials used in aircraft interiors, flame, smoke and toxicity (FST) requirements of the materials must be equally considered. The materials must not be able to support flame. Smoke resulting from adjacent flame must be minimized, and the smoke must be relatively free of toxic decomposition by-products. Structural thermoplastic composites such as those made with polyphenylene sulfide (PPS) as a matrix can exceed FST requirements without the use of halogenated flame suppressants (4). 4. Candidate Thermoplastics Three thermoplastic matrices are commonly used for manufacture of aerospace grade composites. Poly ether ether ketone (PEEK) has extensive commercially available data, and substantial flight history for exterior load bearing structures. It is considered the baseline for aerospace grade thermoplastic composites, and has been considered and applied in the industry for over twenty years. There are several suppliers of PEEK resin in the industry. Polyetherimide (PEI) is manufactured by Sabic Innovative Plastics, formerly GE Plastics. PEI composites are used in numerous aircraft structures, including Airbus floor panels, Gulfstream rudder ribs, pressure bulkheads, and more. Inadequate chemical resistance of PEI when exposed to aerospace fluids, such as Skydrol, as well as high water absorption, inhibit use of PEI composites for many aerospace applications. Polyphenylene Sulfide (PPS) is manufactured in a high quality linear form by Fortron Industries, a joint venture owned by Celanese and Kureha Corporation.

5 This polymer is the lowest cost of the three thermoplastics discussed in this paper. Successful application of PPS composites in aircraft include the undercarriage door for the Fokker 5, fixed wing leading edges for the Airbus A34 and A38, keel beams and others. PPS is a semicrystalline thermoplastic like PEEK; however, the resin cost for PPS is less than PEEK by an order of magnitude. PPS resin cost is also less than half that of PEI cost. Figure 6 Keel Beam The fixed wing leading edge, shown in Figure 5, is made from PPS prepreg manufactured by TenCate. The prepreg layers are thermally stamped to the desired shape, machined, and welded in place. These parts were originally developed and used in production for the Airbus A34 aircraft. They are now being used on the A38 (5). Another application using PPS composites is the keel beam, as shown in Figure 6. The substitution of metal components with PPS resulted in more than 2% by weight reduction for these parts (5). Figure 5 Fixed Wing Leading Edge 11. Summary Thermoplastic composites appear to offer substantial environmental advantages over thermoset based composites. Improvements in reduced energy requirements, reduced scrap stream, improved recyclability, improved rework, and reduced VOC s and hazardous additives can all be achieved by converting to use of thermoplastic based composites. Furthermore, improvements in reduced part cost, increased part toughness, reduced capital cost and reduced facility requirements can also be achieved by considering thermoplastic composites over thermosets. 12. References 1) Favaloro, M. (28), Continuous Reinforced Thermoplastic Composites for Aerospace Applications, SAMPE 28 Fall Technical Conference, Memphis, TN 2) Favaloro, M. and Hauber, D. (27). Process and Design Considerations for the Automated Fiber Placement Process, SAMPE 27 Fall Technical Conference, Cincinnati, OH. 3) Red, Chris, Automated Manufacturing the Boom in Advanced Composites, presented at the Society of Manufacturing Engineers Composites Manufacturing 27 Conference, Salt Lake City, UT, April 27, pages 8-9 One projected use of PPS composites is for aircraft seat frames. PPS meets the interior FST requirements, and is by far the cheapest of the candidate thermoplastics to meet these needs. Replacement of existing aluminum and glass/ epoxy seat frames will offer a substantial weight reduction for this application. 4) Improved Fire- and Smoke Resistant Materials for Commercial Aircraft Interiors A Proceedings, Committee on Fire- and Smoke- Resistant Materials for Commercial Aircraft Interiors, pages 93 to 114, National Academy Press, Washington, DC, ) Fortron PPS for Thermoplastics Composites, CELANESE Page 5

6 ENGINEERED MATERIALS celanese.com/engineered-materials World -Class Engineered Materials Contact Information Celanex thermoplastic polyester (PBT) Americas 84 Dixie Highway, Florence, KY 4142 USA Hostaform and Celcon acetal copolymer (POM) Celstran, Compel and Factor long fiber reinforced thermoplastic (LFRT) Celstran continuous fiber reinforced thermoplastic (CFR-) Fortron polyphenylene sulfide (PPS) GUR ultra-high molecular weight polyethylene (UHMW-PE) Impet thermoplastic polyester (PET) Riteflex thermoplastic polyester elastomer (C-ET) Thermx polycyclohexylene-dimethylene terephthalate (PCT) Vandar thermoplastic polyester alloy (PBT) Vectra and Zenite liquid crystal polymer (LCP) Product Information Service t: t: Customer Service t: t: e: Europe Am Unisys-Park 1, Sulzbach, Germany Product Information Service t: +() t: +49-() e: Asia 456 Jinke Road, Zhang Jiang Hi Tech Park Shanghai 2123 PRC Customer Service t: f: e: Copyright 213 Celanese or its affiliates. All rights reserved. This publication was printed on 19 September 213 based on Celanese s present state of knowledge, and Celanese undertakes no obligation to update it. Because conditions of product use are outside Celanese s control, Celanese makes no warranties, express or implied, and assumes no liability in connection with any use of this information. Nothing herein is intended as a license to operate under or a recommendation to infringe any patents. PPS-23_FortronPPSComparisonAerospaceWP_AM_913