- Email: [email protected]
X-ray curing of composite materials
Nuclear Instruments and Methods in Physics Research B 241 (2005) 847–849 www.elsevier.com/locate/nimb
X-ray curing of composite materials Anthony J. Berejka b
a,*
, M.R. Cleland
b,1
, R.A. Galloway
b,1
, O. Gregoire
b,1
a Ionicorp+, 4 Watch Way, Huntington, NY 11743, USA Radiation Dynamics, Incorporated, a subsidiary of Ion Beam Applications, 151 Heartland Boulevard, Edgewood, NY 11717, USA
Available online 2 September 2005
Abstract The development of high current electron beam (EB) accelerators makes it possible to consider X-ray processing for industrial applications. The well-known inefficiency in converting electron beams to X-rays still affords better overall process efficiency when compared with historic thermal processes. X-ray processing permits depth of penetration of ionizing radiation into a material and, when derived from high current accelerators, can yield process through-puts comparable to low powered EB devices. X-rays are generated at lower dose-rates which are controlled by equipment and process parameters. Two feasibility studies were conducted which illustrate the potential for X-ray processing: (1) the curing of a reactive monomeric impregnant in a thick cross-section block of wood and (2) the curing of the matrix binder in fiber reinforced composites while the composite material was still constrained within a metal mold used to form an article. The ability to control dose-rate and to penetrate thick materials, such as the walls of a metal mold, indicate that X-ray processing can be of significant industrial interest. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.90 Keywords: X-ray processing; Curing fiber reinforced composites; Wood impregnation
1. X-ray processing X-rays have at least 10 times the penetration of even the highest voltage industrially viable electron beams. Heretofore, X-ray processing has been
*
1
Corresponding author. Tel./fax: +1 631 549 8517. E-mail address: [email protected] (A.J. Berejka). Tel.: +1 631 254 6800, www.e-beam-rdi.com.
most often considered when using ionizing irradiation to eliminate biohazards, as in food irradiation and in medical device sterilization [1]. With the development of high current EB accelerators, X-ray conversion can now present production process potentials comparable to lower current EB units and at greater through-put than classic thermal–chemical systems. Table 1 illustrates the through-put potential for X-ray processing using several different accelerator types as sources.
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.188
848
A.J. Berejka et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 847–849
Table 1 X-ray processing through-put potential Source
10 MeV Linac 10 MeV Rhodotron 10 MeV Linac 20 kW X-ray mode 7 MeV Rhodotron 700 kW X-ray mode 5 MeV Dynamitron 300 kW X-ray mode
Composite penetration (cm)
Emitted power (kW)
Potential through-put (kg/h)
20 200
216 2160
30
3
22
28
70
770
24
24
260
2.5 2.5
tioned sequentially for 20 days at 24 °C at 78% relative humidity (RH), 20 days at 21 °C 49% RH and then 20 days at 21 °C 29% RH or 60 days total. Wood with the cured WPP impregnant exhibited 0.3% dimensional change when conditioned in the same way over the same extended period of time. The sequential lowering of humidity causes the wood to shrink. Such shrinkage leads to the disbondment of paints and coatings, poor outdoor weathering performance and the like.
3. Feasibility study 2: fiber reinforced composites 2. Feasibility study 1: wood impregnation The use of methyl–methacrylate and like monomers to impregnate wood and then cure the impregnated wood with either electron beam or c-irradiation from a cobalt-60 source has been well-known [2]. The monomers used have been found to enter only the lumens and not the cell wall of the wood. The State University of New York College of Environmental Science and Forestry has developed a proprietary monomer system, a wood polymer penetrant (WPP), which has been found to enter the cell walls of the wood. When cured within the wood, the WPP imparts significantly greater dimensional stability to the impregnated wood than any system seen so far. When using EB processing, the high dose-rate (600 kGy/min) and concurrent heat generated within test samples raised concerns over monomer volatility. Doses in excess of 100 kGy were needed to attain the required cure of the penetrant with EB. When using lower dose-rate (2 kGy/min) X-ray processing, cure was attained through a thick wood cross-section (a 16 cm block) with practically no loss of monomer or moisture content from the wood (<0.5% weight loss). The desired cure of the impregnant was attained at 25 kGy for single sided X-ray exposure. The lower dose-rate X-ray process allowed the polymerization of the impregnant to self-propagate and itself generate some exotherm, but significantly less than the thermal input from much higher dose curing using the higher dose-rate EB process. Untreated wood has 5% dimensional change when condi-
During the 1990Õs, there emerged considerable interest in the development of electron beam curing for use in the manufacture of fiber reinforced composites for military and aero-space applications [3]. Much effort has been put into the development of resin systems that would meet aero-space requirements and in methods of manufacture that would allow for the forming of parts in open space, thus permitting the use of EB with its limits of penetration. There has been limited but successful commercial use of EB cured composites in some high-performance aero-space applications. More recently, studies have been conducted to see if the results of these aero-space developments could be translated into broader based interests such as automotive component manufacture. A cooperative research and development agreement (CRADA) was undertaken by Oak Ridge National Laboratory (ORNL) to assess the feasibility of making a fiber reinforced auto hood using EB curing [4]. The second set of X-ray curing feasibility studies showed that radiation curable fiber reinforced composites could be laid up within a mold. With the higher penetration of X-rays, the matrix resin could be cured within a simulated mold, having a top surface of 2 cm of aluminum, which is too thick for even 10 MeV electron beam penetration. This approach simplifies the use of radiation curing for fiber reinforced composites by eliminating various cumbersome approaches of trying to shape materials in open space using diverse lay-up means. In contrast to the historic use of thermal curing, such molds can be greatly simplified in that
A.J. Berejka et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 847–849
Fig. 1. X-ray cured carbon fiber deep-drawn shape – cured under 2 cm aluminum block.
there is no need for careful placement of heating elements or concerns over heat transfer. The aero-space interests in EB cured fiber reinforced composites lead to the use of sophisticated matrix binders that cure via cationic mechanisms, mechanisms known to proceed in air without oxygen inhibition. Such cationic systems require very high cost initiators, but may also be faced with shelf-life limitations and possible activation by sunlight [4]. In using X-ray processing to cure a matrix resin within a mold, lower cost, commonly available acrylated oligomers, bis-phenol-A diacrylates or epoxy acrylates, which are used in many other industrial application areas, can be used without concern over any oxygen inhibition of free radical mechanisms. The mold itself precludes the oxygen. Initial X-ray curing tests produced complex and deep drawn shapes with very desirable smooth surfaces as imparted by the mold (Figs. 1 and 2). Multi-layer carbon fiber and fiberglass prepregs were used as lay-ups. In the aero-space EB curing work on fiber reinforced composites, doses of 150–250 kGy were found required by the matrix systems to attain useful cure [3,4]. Such doses at the high dose-rates of EB processing required multiple passes under the EB source to avoid overheating. Using the lower dose-rate X-ray processing, commercial epoxy acrylate matrix binders were found to cure in the range of 25–60 kGy. Solubility tests based on
849
Fig. 2. X-ray cured complex fiberglass shape – cured under 2 cm aluminum block.
methylene chloride extraction for 16 h at 23 °C indicate nearly complete cure at 60 kGy for X-ray cured samples of the bis-phenol-A diacrylate. Infra-red analysis to determine the disappearance of the double bond associated with acrylate polymerization at 810 cm 1, a widely used industry analytical technique used in radiation curing, is under-way. Because of the difficultly in interpreting dynamic mechanical analysis (DMA) data for very stiff materials and possible confusion of DMA results for such stiff materials with actual thermal decomposition, differential scanning calorimetry (DSC) tests are in progress to determine the glass transition temperatures, Tg, of these X-ray cured materials, per ASTM E-1356. While feasibility has been demonstrated, considerably more materials work and cured product testing remains to be done. X-ray curing of fiber reinforced composites within a metal mold looks most encouraging as a viable industrial process.
References [1] J. Meissner et al., Radiat. Phys. Chem. 57 (3–6) (2000) 647. [2] A.E. Witt, US Patent 4,568,564, February 4, 1986. [3] A.J. Berejka, C. Eberle, Radiat. Phys. Chem. 63 (3–6) (2002) 551. [4] C.J. Janke et al., Final Report for CRADA No. ORNL010616; ORNL/TM-2003/129.
X-ray curing of composite materials Anthony J. Berejka b
a,*
, M.R. Cleland
b,1
, R.A. Galloway
b,1
, O. Gregoire
b,1
a Ionicorp+, 4 Watch Way, Huntington, NY 11743, USA Radiation Dynamics, Incorporated, a subsidiary of Ion Beam Applications, 151 Heartland Boulevard, Edgewood, NY 11717, USA
Available online 2 September 2005
Abstract The development of high current electron beam (EB) accelerators makes it possible to consider X-ray processing for industrial applications. The well-known inefficiency in converting electron beams to X-rays still affords better overall process efficiency when compared with historic thermal processes. X-ray processing permits depth of penetration of ionizing radiation into a material and, when derived from high current accelerators, can yield process through-puts comparable to low powered EB devices. X-rays are generated at lower dose-rates which are controlled by equipment and process parameters. Two feasibility studies were conducted which illustrate the potential for X-ray processing: (1) the curing of a reactive monomeric impregnant in a thick cross-section block of wood and (2) the curing of the matrix binder in fiber reinforced composites while the composite material was still constrained within a metal mold used to form an article. The ability to control dose-rate and to penetrate thick materials, such as the walls of a metal mold, indicate that X-ray processing can be of significant industrial interest. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.90 Keywords: X-ray processing; Curing fiber reinforced composites; Wood impregnation
1. X-ray processing X-rays have at least 10 times the penetration of even the highest voltage industrially viable electron beams. Heretofore, X-ray processing has been
*
1
Corresponding author. Tel./fax: +1 631 549 8517. E-mail address: [email protected] (A.J. Berejka). Tel.: +1 631 254 6800, www.e-beam-rdi.com.
most often considered when using ionizing irradiation to eliminate biohazards, as in food irradiation and in medical device sterilization [1]. With the development of high current EB accelerators, X-ray conversion can now present production process potentials comparable to lower current EB units and at greater through-put than classic thermal–chemical systems. Table 1 illustrates the through-put potential for X-ray processing using several different accelerator types as sources.
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.188
848
A.J. Berejka et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 847–849
Table 1 X-ray processing through-put potential Source
10 MeV Linac 10 MeV Rhodotron 10 MeV Linac 20 kW X-ray mode 7 MeV Rhodotron 700 kW X-ray mode 5 MeV Dynamitron 300 kW X-ray mode
Composite penetration (cm)
Emitted power (kW)
Potential through-put (kg/h)
20 200
216 2160
30
3
22
28
70
770
24
24
260
2.5 2.5
tioned sequentially for 20 days at 24 °C at 78% relative humidity (RH), 20 days at 21 °C 49% RH and then 20 days at 21 °C 29% RH or 60 days total. Wood with the cured WPP impregnant exhibited 0.3% dimensional change when conditioned in the same way over the same extended period of time. The sequential lowering of humidity causes the wood to shrink. Such shrinkage leads to the disbondment of paints and coatings, poor outdoor weathering performance and the like.
3. Feasibility study 2: fiber reinforced composites 2. Feasibility study 1: wood impregnation The use of methyl–methacrylate and like monomers to impregnate wood and then cure the impregnated wood with either electron beam or c-irradiation from a cobalt-60 source has been well-known [2]. The monomers used have been found to enter only the lumens and not the cell wall of the wood. The State University of New York College of Environmental Science and Forestry has developed a proprietary monomer system, a wood polymer penetrant (WPP), which has been found to enter the cell walls of the wood. When cured within the wood, the WPP imparts significantly greater dimensional stability to the impregnated wood than any system seen so far. When using EB processing, the high dose-rate (600 kGy/min) and concurrent heat generated within test samples raised concerns over monomer volatility. Doses in excess of 100 kGy were needed to attain the required cure of the penetrant with EB. When using lower dose-rate (2 kGy/min) X-ray processing, cure was attained through a thick wood cross-section (a 16 cm block) with practically no loss of monomer or moisture content from the wood (<0.5% weight loss). The desired cure of the impregnant was attained at 25 kGy for single sided X-ray exposure. The lower dose-rate X-ray process allowed the polymerization of the impregnant to self-propagate and itself generate some exotherm, but significantly less than the thermal input from much higher dose curing using the higher dose-rate EB process. Untreated wood has 5% dimensional change when condi-
During the 1990Õs, there emerged considerable interest in the development of electron beam curing for use in the manufacture of fiber reinforced composites for military and aero-space applications [3]. Much effort has been put into the development of resin systems that would meet aero-space requirements and in methods of manufacture that would allow for the forming of parts in open space, thus permitting the use of EB with its limits of penetration. There has been limited but successful commercial use of EB cured composites in some high-performance aero-space applications. More recently, studies have been conducted to see if the results of these aero-space developments could be translated into broader based interests such as automotive component manufacture. A cooperative research and development agreement (CRADA) was undertaken by Oak Ridge National Laboratory (ORNL) to assess the feasibility of making a fiber reinforced auto hood using EB curing [4]. The second set of X-ray curing feasibility studies showed that radiation curable fiber reinforced composites could be laid up within a mold. With the higher penetration of X-rays, the matrix resin could be cured within a simulated mold, having a top surface of 2 cm of aluminum, which is too thick for even 10 MeV electron beam penetration. This approach simplifies the use of radiation curing for fiber reinforced composites by eliminating various cumbersome approaches of trying to shape materials in open space using diverse lay-up means. In contrast to the historic use of thermal curing, such molds can be greatly simplified in that
A.J. Berejka et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 847–849
Fig. 1. X-ray cured carbon fiber deep-drawn shape – cured under 2 cm aluminum block.
there is no need for careful placement of heating elements or concerns over heat transfer. The aero-space interests in EB cured fiber reinforced composites lead to the use of sophisticated matrix binders that cure via cationic mechanisms, mechanisms known to proceed in air without oxygen inhibition. Such cationic systems require very high cost initiators, but may also be faced with shelf-life limitations and possible activation by sunlight [4]. In using X-ray processing to cure a matrix resin within a mold, lower cost, commonly available acrylated oligomers, bis-phenol-A diacrylates or epoxy acrylates, which are used in many other industrial application areas, can be used without concern over any oxygen inhibition of free radical mechanisms. The mold itself precludes the oxygen. Initial X-ray curing tests produced complex and deep drawn shapes with very desirable smooth surfaces as imparted by the mold (Figs. 1 and 2). Multi-layer carbon fiber and fiberglass prepregs were used as lay-ups. In the aero-space EB curing work on fiber reinforced composites, doses of 150–250 kGy were found required by the matrix systems to attain useful cure [3,4]. Such doses at the high dose-rates of EB processing required multiple passes under the EB source to avoid overheating. Using the lower dose-rate X-ray processing, commercial epoxy acrylate matrix binders were found to cure in the range of 25–60 kGy. Solubility tests based on
849
Fig. 2. X-ray cured complex fiberglass shape – cured under 2 cm aluminum block.
methylene chloride extraction for 16 h at 23 °C indicate nearly complete cure at 60 kGy for X-ray cured samples of the bis-phenol-A diacrylate. Infra-red analysis to determine the disappearance of the double bond associated with acrylate polymerization at 810 cm 1, a widely used industry analytical technique used in radiation curing, is under-way. Because of the difficultly in interpreting dynamic mechanical analysis (DMA) data for very stiff materials and possible confusion of DMA results for such stiff materials with actual thermal decomposition, differential scanning calorimetry (DSC) tests are in progress to determine the glass transition temperatures, Tg, of these X-ray cured materials, per ASTM E-1356. While feasibility has been demonstrated, considerably more materials work and cured product testing remains to be done. X-ray curing of fiber reinforced composites within a metal mold looks most encouraging as a viable industrial process.
References [1] J. Meissner et al., Radiat. Phys. Chem. 57 (3–6) (2000) 647. [2] A.E. Witt, US Patent 4,568,564, February 4, 1986. [3] A.J. Berejka, C. Eberle, Radiat. Phys. Chem. 63 (3–6) (2002) 551. [4] C.J. Janke et al., Final Report for CRADA No. ORNL010616; ORNL/TM-2003/129.