electron beam processing

electron-beam curing













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composite curing



Electron-Beam Curing of Polymeric Composites

As An Enabling Technology

for Advanced Manufacturing


E-BEAM Services, Inc.

32 Melrich Road

Cranbury, NJ 08512



as presented at:


The International Composites Expo ’99 (ICE’99)


Cincinnati, OH


May 10-13, 1999

ABSTRACT

            Electron-beam (EB) composite curing of adhesives and resins is an emerging technology which has the potential to provide significant advantages for rapid manufacturing of a variety of composite structures and components for aerospace, automotive, and consumer applications. Traditional thermal curing methods necessitate long cure times, involve high energy consumption and volatile toxic by-products, create residual stresses in the materials, and require expensive tooling capable of withstanding high autoclave temperatures. E-beam curing is a fast non-thermal process which utilizes highly energetic electrons at controlled doses to polymerize and cross-link polymeric materials. EB curing of composites can take place at both ambient and sub-ambient temperatures. The advantages of this technology over the conventional autoclave curing process include significantly reduced curing time leading to order-of-magnitude improvements in throughput, ability to utilize low-cost tooling materials, reduced volatiles emissions, reduced energy consumption, and control over curing energy-absorption profile. EB cured materials have been shown to posses excellent mechanical properties, with low molded-in stresses, high glass transition temperatures (Tg) and low void content. These materials demonstrate excellent property retention after thermal or cryogenic cycling. This paper examines the recent applications of e-beam curing technology to several fabrication processes, including conventional pre-preg lay-up, resin transfer molding (RTM), vacuum-assisted resin transfer molding (VARTM), and filament winding. Developments in the area of EB-curable resin formulations and adhesives are also discussed.

INTRODUCTION

            Polymer matrix composite (PMC) materials have demonstrated clear-cut advantages in advanced performance at decreased weight over more conventional metallic materials in a number of demanding aerospace, automotive, infrastructure, and consumer applications [1,2]. A major cost issue in manufacturing  of PMC structures and parts is the conventional thermal cure process [3]. Additionally, the autoclave-based thermal cure processing requires long cure times, involves high energy consumption and volatile toxic by-products, creates residual stresses in the materials, and necessitates the use of expensive tooling capable of withstanding high autoclave temperatures [4]. It has been suggested that decreasing the manufacturing costs is a key step in increasing the overall usage of PMC’s [3,5].

            Electron-beam (EB) curing represents a significant technological advancement with respect to the thermal cure deficiencies mentioned above. EB curing is a rapid non-autoclave process which utilizes high-energy electrons generated by an industrial accelerator at controlled doses to initiate polymerization and crosslinking reactions in suitable material substrates [6]. A number of benefits have been identified for EB curing of PMC’s, as compared to thermal  curing [7].

            The purpose of this paper is to provide a brief introduction to EB processing, as well as to highlight selected recent applications of EB curing technology to several established composite fabrication processes.

ELECTRON BEAM PROCESSING

            Ionizing radiation is a distinct and efficient means of bringing about controlled beneficial changes in polymeric systems. These beneficial changes include increases in modulus, tensile and impact strength, hardness, deflection and service temperature, stress-crack resistance, abrasion resistance, creep and fatigue resistance, and barrier properties [8]. It has been observed that radiation processing requires a lower expenditure of energy than the conventional thermochemical processes [9].

In the case of electron-beam irradiation, highly energetic electrons strike at or near the carbon-hydrogen bonds in target molecules, and give up enough energy to the molecules to break some of the bonds, releasing hydrogen, and leaving the molecules with excited carbon atoms (free radicals). When this process occurs at two adjacent molecules or nearby sites, excited carbon atoms can release excitation energy forming a chemical bond, known as a cross-link, between them. The amount of electron beam radiation absorbed by the target is referred to as the dose, which is typically defined in terms of kiloGrays (where 1 kGy=1000 J/kg) or MegaRads (where 1 MRad=1,000,000 erg/g) [10]. The degree or efficiency of crosslinking depends on certain complex secondary chemistries involving, among others, polymer additives, including radiation promoters, initiators, stabilizers, etc.

Industrially, EB processing is performed using medium to high energy     (2.5 to 10 MeV), high power (50+ kW) commercially-available accelerators, which are equipped with a variety of material handling systems, and are capable of significant throughput [11]. A typical direct-current accelerator consists of the voltage generator, the  electron gun, the accelerator tube, the scan horn, and the control system [10]. This accelerator creates a beam of electrons approximately      1 inch in diameter and energizes it to near light speed. The beam passes through a scan horn, where a magnet scans it back and forth at ca. 200 Hz, creating a curtain of electrons 4-6 feet wide [10]. Target materials are passed under the scan horn using conveyors, carts, reel-to-reel equipment, or other specialized handling means. Worldwide, there are approximately 700-800 electron beam accelerators in industrial use today [12].

            With respect to autoclave processing, EB curing of  PMC’s is a considerably more energy efficient process. As illustrated in Table I, a commercially-available 50 kW accelerator delivering a cure dose of 100 kGy at 70% beam utilization uses roughly one-tenth of the energy of a typical autoclave or oven running a 4 hour thermal cure cycle [13].

TABLE I – RELATIVE ENERGY EFFICIENCY OF EB CURING

Equipment Type

Equipment Specification

Total Energy (kW-hr)

Capacity

(kg/hr)

Energy Efficiency

(kW-hr/kg)

Autoclave

12.2 m length

2.4 m diameter

480

273

1.76

Autoclave

15.2 m length

7.6 m diameter

7660

2730

2.81

10 MeV EB

50 kW

400

1800

0.22

EB-CURABLE RESINS

BACKGROUND

            Historically, a number of resin systems have been EB cured with mixed results. For example, oligomers containing acrylate and methacrylate groups, as well as those containing double bonds, such as dienes and vinyls have all been reported as being EB-curable. However, the performance of these materials has been characterized by low glass transition/service temperatures, low modulus, high moisture absorption, and high shrinkage upon curing [4].

            A 1992 European patent described a process for EB curing of bismaleimides at room temperature, and claimed glass transition temperatures (Tg) of over 300 oC [14]. Subsequently, a U.S. company had developed a series of acrylated epoxies and acrylated bismaleimides, which while demonstrating better properties than those of  the equivalent European materials, still failed to meet the end-user requirements [4]. Another patented curing system features certain highly reactive cycloaliphatic silicone epoxies which are EB-curable with the aid of certain cationic initiators, and have reported Tg of about 190 oC [16].

RECENT DEVELOPMENTS

           

A considerable materials development effort was undertaken from 1994 to 1997 as part of a Department of Energy (DOE) Defense Programs Cooperative Research and Development Agreement (CRADA), which involved the contributions from two  DOE national laboratories and ten industrial participants, including three advanced materials companies and three EB processing organizations. While the greater stated purpose of the CRADA was to advance general understanding and utilization of EB curing for the manufacture of PMC parts, its major achievement proved to be the successful development of cationic epoxy resin formulations suitable to EB curing that could also meet the demanding requirements of high-performance composite structures [4].

            The CRADA participants successfully developed and evaluated hundreds of new, toughened and non-toughened EB-curable epoxy resin systems. Carbon fiber composites manufactured using these systems were found to match favorably the performance of conventional thermally-cured composites and were typically characterized by low void contents, low water absorption values, high glass transition temperatures, and superior property retention upon cryogenic and thermal cycling [15]. The typical properties of selected EB-curable epoxy formulations are presented in Table II [4,7].

TABLE II – PROPERTIES OF SELECTED EB-CURABLE RESINS

Property

8H

10H

9H

3K

1K

Resin Shrinkage (%)

3.5

4.0

3.4

3.0

3.3

Tg (oC)

396

393

232

212

212

Water Uptake (%)

4.90

2.8

2.0

2.90

1.88

Density (kg/m3)

1260

1237

1225

1224

1228

APPLICATIONS

PRE-PREG LAYUP

            Several toughened EB-curable resin formulations have been evaluated for carbon fiber PMC applications. Table III summarizes the data collected for unidirectional test panels utilizing IM7-GP-12K carbon fiber unidirectional pre-preg which was fabricated by a melt process at ca. 70 oC. The panels were prepared using conventional lay-up techniques and then processed in multiple passes at a 10 MeV EB facility under vacuum bag pressure to total doses ranging from 150 to 250 kGy, at a dose rate of 50 kGy per pass [4]. Commercially available thermally-curable thermoplastic toughened aerospace epoxy resin 977-3 specifications are presented for comparison purposes.

Table III – COMPARATIVE PROPERTIES OF

EB-CURED AND THERMAL CURED UNIDIRECTIONAL LAMINATES

Resin System

977-3

8H

10H

9H

3K

Cure

Conditions

3 hrs. at

355 oF/85psi

250 kGy

150 kGy

150 kGy

150 kGy

Void Volume (%)

Not Reported

1.77

0.72

1.24

0.64

Tg (oC, Tan Delta)

190/240

396

392

232

212

0o Flexural Strength (MPa)

1765

1986

2006

1793

1765

0o Flexural Modulus (GPa)

150

196

163

163

154

0o Interlaminar Shear Strength (MPa)

127

77

79

79

89

FILAMENT WINDING

            Similarly, a number of epoxy resin-initiator formulations with low ambient viscosities have been evaluated for suitability in filament winding applications. IM7-GP-12K carbon fiber was used to produce hoop wound composite cylinders with a diameter of 15.24 cm and thickness of 0.3175 cm at room temperature. Two commercially available thermally-curable epoxy resins were used as controls. Representative specimens were also cryogenically and thermally cycled a total of three times as per the following regime: -194 oC for 30 min., ambient for 30 min, 121 oC for 30 min., ambient for 30 min. The comparative property data is presented in Table IV [4].

Table IV – COMPARATIVE PROPERTIES OF

EB-CURED AND THERMAL CURED FILAMENT WOUND LAMINATES

Resin System

ERL-2258

Tactix-123

34B

Cure

Conditions

3 hrs. @ 121 oC

3 hrs. @ 150 oC

4 hrs. @ 177 oC

3 hrs. @ 121 oC

3 hrs. @ 150 oC

4 hrs. @ 177 oC

150 kGy

Tg (oC, Tan Delta)

218

165

192

0o Tensile Strength (MPa)

1986

1538

2358

0o Tensile Strength Cycled (MPa)

2379

1565

2337

90o Flexural Strength (MPa)

68.3

57.2

70.3

90o Flexural Strength Cycled (GPa)

77.2

64.1

77.9

0o Interlaminar Shear Strength (MPa)

57.2

47.6

73.8

0o Interlaminar Shear Strength Cycled (MPa)

56.5

46.9

82.7

RTM/VARTM

            RTM can reduce costs as compared to labor-intensive hand lay-up. However, since it still requires high-temperature thermal curing, tooling for RTM is often made from expensive metal alloys [1]. Utilization of a non-thermal curing technology, such as EB, has recently been demonstrated to allow the use of inexpensive tooling and bagging, such as low cost thermoplastics, foams, wood, etc. [1]. A year ago, in separate papers presented at the 43rd International SAMPE Symposium, Vastava and Roylance, et al described efforts to combine EB curing and RTM/VARTM technologies in order to enable cost effective production of large assemblies and structures for the aerospace industry [1,3]. In both cases, the efforts were successful and served to clearly demonstrate that the use of EB curing to produce large military or aerospace components was not limited by any inherent material or process limitations.

In the case of VARTM of a carbon-fiber composite flat-braided intersection, the vacuum-bagged part was infiltrated immediately prior to EB exposure. Resin system 34B was used. The part was then cured utilizing a commercial 10 MeV accelerator to a total dose of 150 kGy in increments of 25 kGy per pass. The Tg of the finished part was determined to be 160 oC. It was estimated that the overall cost to manufacture a flat intersected braided demonstration part was nearly 100 times less utilizing inexpensive polyurethane foam tooling and EB curing, as compared to conventional thermal curing which necessitates the use of two-part metal tooling for closed mold RTM [1].

CONCLUSION

            EB curing of PMC’s is an emerging technology which has the potential to provide significant advantages for rapid and cost-effective manufacturing of a variety of structures and components for aerospace, automotive, and consumer applications. As detailed in the text above, considerable recent efforts have been expended to increase the understanding and utilization of EB curing for the production of PMC parts. A number of EB-curable resins have been developed and evaluated. Application of EB curing to a number of widely used fabrication methods has been attempted with a high degree of success, and at considerable economic advantage. Thus, it can be suggested that with continuing rapid advances in this enabling technology, we can expect to arrive in the future at a point when the use of EB curing would become more common than thermal curing, resulting in lower production costs and increased utilization of composites in general.

REFERENCES

1.                   Roylance, M.E., C.J. Janke, and J.M. Tuss; 43rd International SAMPE Symposium, 43:1660-1671 (1998).

2.                   Goodman, D.L. and C.A. Byrne; 43rd International SAMPE Symposium, 43:1691-1701 (1998).

3.                   Vastava, R.B., et al.; 43rd International SAMPE Symposium, 43:1681-1690 (1998).

4.                   Janke, C.J., et al.; May 1997. “Electron Beam Curing of Polymer Matrix Composites,“ Final CRADA Report, Lockheed Martin Energy Systems for the U.S. Department of Energy, Oak Ridge, TN.

5.                   Anonymous; High-Performance Composites, 6:41 (1998).

6.                   Singh, A. and C.B. Saunders; “Radiation Processing of Carbon Fiber-Acrylated Epoxy Composites” in Radiation Processing of Polymers, A. Singh and J. Silverman, eds. New York, NY: Oxford University Press (1992).

7.                   Saunders, C.B., V.J. Lopata, and W. Kremers; September 1997. “Electron Curing of Composite Structures for Space Applications” at Electron Beam Curing of Composites Workshop. Oak Ridge, TN.

8.                   Gehring, J and A. Zyball; Radiation Physics and Chemistry, 46:931-936 (1995).

9.                   Ivanov, V.S.; Radiation Chemistry of Polymers. Utrecht, The Netherlands: VSP BV (1992).

10.               Bly, J.H.; Electron Beam Processing. Yardley, PA: International Information Associates (1988).

11.               Minbiole, P.R.; Radiation Physics and Chemistry, 46:421-428 (1995).

12.               Machi, S. Radiation Physics and Chemistry, 46:399-410 (1995).

13.               Knobel, T.M.; September 1996. “Electron Beam Facilities Operation: Contract and Dedicated Composite Curing” at Electron Beam Curing of Composites Workshop. Oak Ridge, TN.

14.               Beziers, et al.; Eur. Pat. Appl. Publ. No. 499,542A1 (1992).

15.               Farmer, J.D., C.J. Janke, and V.J. Lopata; 43rd International SAMPE Symposium, 43:1639-1646 (1998).

16.               Crivello, J.V. U.S. Patent 5,260,349 (1993).


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