Composites Paper Cont Rein Aircraft

8/6/2019 Composites Paper Cont Rein Aircraft

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CONTINUOUS REINFORCED THERMOPLASTICCOMPOSITES FOR AIRCRAFT APPLICATIONS

Author:

Michael Favaloro, Ticona Engineering Polymers

1. ABSTRACT:

Thermoplastic composites offer substantial cost savings and improved performance as comparedto thermoset parts. This paper will provide a description of properties of high temperaturethermoplastic composites for aerospace applications. Matrix materials will include PPS, PEI andPEEK. A relative comparison of cost vs. performance will be provided. Limited descriptions of aerospace parts currently made in production and the manufacturing processes, will be provided.

KEYWORDS:

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)

2. BACKGROUND:

The composites industry is undergoing substantial growth in the aerospace industry, due mainly

to the fact that high performance light weight composite structures are more fuel efficient thantheir metallic predecessors. The current focus of the industry is on thermoset based composites,for a number of reasons. Thermoset composites have been used in the aerospace industry sincethe 1960’s, and the design and processing knowledge to exploit the materials has developedsubstantially. Major investments have been made in material property databases for thermosetcomposite man-rated aerospace applications. In many cases these databases are treated ascompany proprietary intellectual property, which makes it difficult to justify investment inalternate materials and processes. Industries associated with reinforcement that utilizesthermoset-compatible sizings are mature, including weaving and conversion into prepreg. Capitalequipment and support services, such as tooling and semi-automation methodologies, are wellestablished.

As expected with a maturing technology, major thermoset composite technology innovations andcost reduction opportunities are less frequent, with some notable exceptions. Aircraft designersare using more lightweight composite structures to reduce fuel consumption. However, theapplication of composites in aerospace is not being driven by reductions in costs for compositematerials or processing. The cost reductions are coming in the form of fuel savings and not partcosts.



One projected breakthrough in the composites industry cost equation is expected to involve largescale manufacture of continuous fiber thermoplastic composites. Thermoplastics offer improvedraw materials and processing costs, as well as improved functional performance. This paperdescribes cost and performance information on three aerospace quality thermoplastic polymersused in the composites industry.

3. THERMOPLASTIC VS. THERMOSET

A rational justification must be made for considering the investment into thermoplasticcomposites for aerospace applications. Thermoplastic matrix composites are melt processablepolymers. They are heated, melted or softened, reshaped, and then cooled to a final hardenedshaped. When compared to thermosets, which are cross-linked when heated and cannot be re-melted or re-formed, thermoplastics provide a more tailorable and more forgiving process.Thermoplastics can be more easily re-worked and repaired. Thermoplastic raw materials have anear infinite shelf life, as compared to thermoset prepreg, with a typical shelf life of ~ 6 months,requiring refrigeration, tight schedule control of material receipt and conversion to final form.

Without tight schedule control, scrap prepreg costs can become very high. Thermoplastics do notrequire refrigeration and storage costs are less. Thermoplastics are typically 4X tougher than themore toughened thermosets, resulting in more impact resistance and damage tolerance. Thethermoplastics considered in this paper are relatively insensitive to aircraft fluids and chemicalattack, and, with one exception, insensitive to moisture. Thermoplastics offer substantialreductions in flammability, smoke and toxicity performance as compared to thermosets, which isof major importance in man-rated aircraft.

When considering processing of the two families of materials, a major investment of capitalcurrently exists in the industry for thermoset composite processing. While some of the equipment,such as autoclaves, could also be used for thermoplastic processing the relatively slow andinefficient process times required for heat up and cool down of the autoclave would reduceprocess and cost efficiency options associated with thermoplastics. Thermoplastics are typicallyheated, formed and cooled rapidly as compared to thermosets, resulting in cost reduction. Theuse of thermoplastic processes would eliminate the need for bagging materials and labor. Withthe use of in-line consolidation techniques (Reference 1) kitting and debulking steps andequipment would also be eliminated. These techniques would also eliminate the need forautoclave processing, which would reduce capital cost, floor space requirements, and processingbottleneck issues. The green processing of thermoplastics vs. thermosets cannot be overstated.Thermoplastics can, by definition, be fully recycled. Little to no volatile organic compounds(VOC’s) are released during processing. Process scrap can also be reduced substantially withexisting fiber placement technology (Reference 2) .

4. CANDIDATE THERMOPLASTICS

Three aerospace grade thermoplastics are discussed in this paper. Poly ether ether ketone (PEEK)has extensive commercially available data. It is considered the baseline for aerospace gradethermoplastic composites, and has been considered and applied in the industry for over twentyyears. There are several suppliers of PEEK resin in the industry



C o n v e

n t i o n a l

P E E K

P E I

P P S

0

20

40

60

80

100

S y s t e m C

o s t i n %

FIGURE 1: Cost Comparison ofConventional and Candidate

Composites SystemsProcessing

Material

Polyetherimide (PEI) is manufactured by Sabic Innovative Plastics, formerly GE Plastics. PEIcomposites are used in numerous aircraft structures, including Airbus floor panels, Gulfstreamrudder ribs, pressure bulkheads, and many more.

Polyphenylene Sulfide (PPS) is manufactured in a high quality linear form by Fortron Industries,

a joint venture owned by Ticona, a business of Celanese Corporation, and Kureha Corporation.This polymer is the lowest cost of the three thermoplastics discussed in this paper. Successfulapplication of PPS composites in aircraft include the undercarriage door for the Fokker 50, fixedwing leading edges for the Airbus A340 and A380, keel beams and others.

A comparison of candidate resins is shown in Table 1. It must be noted that the use of Tg as anupper temperature use limit is not valid for thermoplastics. PEI exhibits creep initiation wellbelow its Tg, and as a result, has an upper temperature use limit that is similar to PPS.Furthermore, semi-crystalline thermoplastics such as PEEK and PPS are typically used underload for extended periods above their Tg.

TABLE 1: Candidate Resin Comparison

5. COST COMPARISON

As described above, thermoplastic composites may exhibitsubstantial cost savings as compared to relevant thermosets.Table 1 shows a relative comparison of conventionalthermoset composites to the three thermoplastics. Thiscomparison is highlighted in Figure 1. When consideringPEEK composites, the higher material cost is offset bylower cost conversion processing. The cost of PEI prepregis about half the cost of PEEK prepreg, but higherprocessing temperatures are needed, which could result inhigher processing cost. The net cost of PEI composites isstill lower than PEEK composites. The lowest cost optionis PPS, which has a prepreg cost of approx. 25% less thanPEI, and lower processing temperatures / processing cost.

Resin Tg (°C ) T m (°C ) RelativeResin Cost

RelativeProcessingCost

PEEK 143 343 2.7 1.2PEI 210 360 1.3 1.4PPS Linear 90 285 1 1



6. CHEMICAL RESISTANCE

Resistance to some typical solvents and fluids is shown in the following table. (References 3, 4,5, 6, 7)

TABLE 2: Chemical Resistance of Candidate ResinsPPS PEI PEEK

RT 100C RT 100C RT 100CSulfuric acid, <10% A A A C B B

Sulfuric acid, >10% A C A C C CEthanol A A A B A AEthylene Glycol A A A B A A

Acetone A A C C A AToluene A A C C A A

NOTES; A= no attack, B=slight attack, C=severe attack

PPS has excellent chemical resistance. It has no known solvents below 200C. PEEK also exhibitsvery good relative chemical resistance with some exceptions such as sulfuric acids. PEI exhibitsexcellent resistance to sulfuric acids, but is susceptible to attack at higher temperatures.

7. MOISTURE RESISTANCE

Water Absorption of the candidate materials is shown in Figure 2. (Reference 7)

PPS exhibits the lowest moisture absorption of the three materials. PEI absorbs a substantialamount of moisture, which can result in dimensional instability and in increased creep.



8. FLAME, SMOKE AND TOXICITY

Composite material candidates for aircraft interior applications must meet stringent requirements.They must survive vertical burn flame testing, where they are exposed to a direct flame for up to60 seconds, and rapidly self-extinguish. For this test, the number of airborne particles, number of burning particles and total burn lengths are measured. A minimum smoke density must beconsidered, to allow for ease of evacuation as necessary. Toxicity levels of the smoke must below for obvious reasons. A low heat release must also be considered, to prevent spread of theflame.

All three candidate materials exhibit excellent properties for use in aircraft interior applications(References 6, 7, 8). PEI and PPS are typically used in structural and non structural cabincomponents. PEEK is used sparingly because of the increased cost associated with the material.

9. MECHANICAL PROPERTIES

In continuous fiber reinforced composites, the load carrying fibers can be utilized to address thebulk of the mechanical property requirements of an application. The matrix contributes to longterm properties with toughness and long term creep resistance. Of particular concern withthermoplastic composites is creep resistance at elevated temperatures. In contrast to thermosetcomposite materials which exhibit decomposition at elevated temperatures, thermoplastics softenat elevated temperatures. An aircraft fuselage must survive temperature extremes withoutembrittlement or softening.

FIGURE 2: Moisture Absorption of Candidate Resins



In a recent study at Ticona, the candidate resins, along with PEKK, were examined for long termcreep resistance (Reference 9). The resins were converted to prepreg with AS4 carbon fiber byTenCate, Taunton MA., and made into test panels using an automated fabrication process byAutomated Dynamics, Schenectady, NY. Testing was performed using established differential

mechanical analysis methods as described in Reference 9.

Test data is shown in Figure 3 , PEEK and PEKK exhibit identical compliance curves, which compareswellwiththeTg of theresins.

Surprisingly, the creep compliance curves of PPS and PEI also match. In this case, the substantiallyhigher Tg of PEI does not result in improved creep performance at higher temperatures than PPS. C reepfor amorphous linear polymers is restrained only by the degree of entanglement of the largermolecules (Reference 10). Consequently, the creep compliance for the semi-crystalline PPS isidentical to that for the softer, amorphous PEI.

10. SOME PPS APPLICATIONS:

The fixed wing leading edge, shown in Figure 4, is made from PPS prepreg manufactured byTenCate. The prepreg layers are thermally stamped to the desired shape, machined, and weldedin place. These parts were originally developed and used in production for the Airbus A340aircraft. They are now being used on the A380 (Reference 7).

Another application using PPS composites is the keel beam, as shown in Figure 5. Thesubstitution of metal components with PPS resulted in more than 20% by weight reduction forthese parts (Reference 7).



FIGURE 4: Fixed Wing Leading Edge

FIGURE 5: Keel Beam

One projected use of PPS composites isfor aircraft seat frames. PPS meets theinterior FST requirements, and is by far the cheapest of the candidate thermoplastics to meetthese needs. Replacement of existing aluminum and glass/epoxy seat frames will offer asubstantial weight reduction for this application.

11. SUMMARY

The three candidate resins are finding increased use in commercial aerospace applications. Thelong term investment into the higher performance PEEK material has resulted in a good databaseand flight history of the material. PEI and PPS are both finding uses as the reduced cost of theseresins becomes obvious, and long term investments continue to be made in the materialdatabases and processes. As these resins receive more industry acceptance, the cost reductioncurves for all aerospace composite materials is expected to improve.



12. REFERENCES:

1)Favaloro, M., and Hauber, D., “ Process and Design Considerations for the Automated FiberPlacement Process”, ADC Acquisition Company, presented at the SAMPE 2007 fall technicalconference, Cincinnati, OH

2) Black, Sarah, “New Equipment Brings Tape Placement Within Reach of Smaller CompositesShops”, High Performance Composites Magazine, Nov, 2003

3) “Chemical Resistance Data”, CRAFTECH ® Industries, Inc., Hudson, NY ,www.craftechind.com

4) “Victrex ® Chemical Resistance Data”, www.victrex.com ,

5) ULTEM ® Chemical Resistance Chart, provided by Westlake Plastics Company

6) “Properties of Key Products for Aircraft Interiors” Architecture for the Sky , provided bySABIC Innovative Plastics

7) “Fortron PPS for Thermoplastics Composites”, TICONA Engineering Polymers,www.ticona.com

8) “ Improved Fire- and Smoke – Resistant Materials for Commercial Aircraft Interiors” AProceedings, Committee on Fire- and Smoke- Resistant Materials for Commercial AircraftInteriors, pages 93 to 114, National Academy Press, Washington, DC, 1995

9) Ramathal, H., and Favaloro, M., “A Comparison of Maximum Use Temperatures for HighPerformance Thermoplastic Composites”, presented at the SAMPE 2008 fall technicalconference, Memphis TN

10) Sepe, M. P. Dynamic Mechanical Analysis for Plastic Engineers, PDL Handbook Series,Plastic Design Library, 1998

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