By Yunyun Bi, Department of Paper and Bioprocess Engineering; Mark S. Driscoll, Department of Chemistry; Robert W. Meyer, Department of Paper and Bioprocess Engineering, SUNY ESF, Syracuse, NY and L. Scott Larsen, New York State Energy Research and Development (NYSERDA), Albany, NY
Abstract
Visible light curing of composite is predominantly used in dentistry. This paper introduces a novel industrial application of using blue (470nm) LEDs to photocure fiberglass-reinforced impact-resistant panels that are traditionally manufactured by heat curing. Photopolymerization takes place with blue light radiation from a customized LED array. Lab drop-weight impact tests demonstrated the light-cured panel withstood more than 90% of the test energy of the control panel.
Introduction
Fiberglass-reinforced impact-resistant panels (FRIRPs) play important roles in both civil and military applications. FRIRPs are usually made of woven fiberglass and a polymer matrix system, traditionally manufactured by thermal curing. Traditional thermal curing systems require a substantial amount of energy and time for controlled heating and cooling ramps during manufacturing cycles and inevitably emit Volatile Organic Compounds (VOCs) from the solvents used in the resin formulation.
The radiation curing process significantly minimizes those problems. Curing time required can be as short as fractions of a second in the case of clear acrylate composites. Energy consumption is greatly reduced because less energy is lost when compared with thermal curing. In addition, the formulations contain little or no solvent, making VOC emission negligible.
Visible light offers several significant advantages over other types of radiation sources – ultraviolet light (UV), electron beam (EB) and X-ray – including lower energy consumption, lower equipment cost and a nonhazardous operating environment. Along with these benefits came the need to develop a visible light curing unit and associated resin system to achieve desired physical and mechanical properties of the finished product, since visible light curing process is primarily used in restorative dentistry but rarely in industrial applications.
A series of preliminary experiments has been conducted to prove that a single or a row of three LED chips can cure a thin layer of epoxy acrylate-based resin within seconds, and 10 layers of woven fiberglass prepregs within minutes. An 8×10-inch LED array was later designed and characterized in irradiance distribution in the previous study. The purpose of this research is to develop the procedure for visible light curing of epoxy acrylate-based composite and to investigate the effect of curing time, concentration of photoinitiator, and different resin system on the mechanical properties.
Method
1. Materials
This study uses commercial E woven roving fiberglass with a density of 24 + 10% oz per square yard, provided by Armortex.
The resin system consists of an oligomer, a monomer, a photoinitiator and an amine synergist (Table 1).
The oligomer Bisphenol A diglycidyl ether acrylate forms the backbone of the polymer network. IBOA was used as a monofunctional acrylate monomer and reactive diluent, together with Bisphenol A diglycidyl ether, both donated by Rapid Cure Technologies. Camphorquinone has an absorbance peak in the visible light region at 468nm and is by far most widely used in biomedical applications1. The efficiency of camphorquinone alone is insufficient. In dental composite restoratives, it is frequently used with a tertiary amine co-initiator. Various studies2,3 suggest that a molar ratio of 1:2 (CQ/amine) achieves the best result.
2. Procedure
2.1 Preparation of Resin System
The resin formulation is mixed in a darkroom with yellow lighting and then heated in an oven at 40°C for 12 hours to accelerate the dissolution of camphorquinone.
2.2 Preparation of Specimen
Fiberglass is cut in 8.5-inch squares from continuous woven roving, and each layer is brushed with resin. The layers are stacked and then placed in a transparent plastic bag that does not absorb radiation. Each panel consists of 22 layers of fiberglass.
A two-piece Plexiglas 1-inch-thick mold fixture held the panel while an electric press applied 0.5 ton load. Three 0.5-inch-thick stoppers were placed in between the mold to determine the thickness of the panel. In this process, excess resin and air were squeezed out. The unpolymerized composite material was then irradiated with blue light for 5 or 10 minutes on each side.
3. Ballistic Test: Lab Drop-weight Screening Test
The drop impact tester simulates a speeding bullet by dropping a 250-lb weight on the specimen from a preselected height (Figure 1). The projectile is made from a 7/16- x 3-inch nondeforming hard steel bolt fixed in a grade 8 bolt and attached to the bottom of the weight. The impact tester lifts and drops the weight by electromagnetic control. The panel is fixed horizontally in a wooden frame holder with nuts and bolts in all four sides, as shown in Figure 1, with a 6- x 6-inch exposed area.
The impact tests are conducted with drop-weight energy of 800, 750 and 700 ft lb. The indicated energy is obtained by lifting the weight a required distance from the compression surface of the specimen. After 
the strike, each specimen is examined to determine the extent of penetration and delamination, as well as whether it passes or fails the test based on the criteria in Table 2.
Results: Lab Impact Test
Impact test results are shown in Table 3. The control panel passed at 750 ft lb, and the light-cured panel passed at 700 ft lb, lying within 10% of the control panel. The panel cured for 5 minutes on each side passed 700 ft lb as well, but presented the most severe delamination (Figure 2-c). The control panel, passing at 750 ft lb, had the least delamination (Figure 2-a). Figure 3 shows the front and back side of 10-minute light-cured panels after 750 ft lb and 700 ft lb strikes.
Discussion
Impact properties represent the capacity of a material to absorb and dissipate energy under low- or high-velocity impact. When the projectile hits the panel, the fibers under the projectile started to fail by compression. As the impact proceeds through the laminate, the compressive stress exerts pressure on the fibers in the surrounding area, causing compressive deformation. The fibers at the impact point are pushed forward by the projectile and eventually exit the panel after the bottom layer is broken by tension, or stop the projectile when all the kinetic energy is absorbed. Delamination or cracks usually occur in the experimental and control samples in either case. During the entire impact event, frictional resistance and heat generation are other energy-absorbing factors.
The light-cured panel passed the 700 ft lb drop-weight test, while the control panel passed 750 ft lb test. That could be attributed to two factors: resin loading and resin type, which significantly influence fiber-matrix interaction. At high levels of adhesion, the failure mode is brittle, and relatively little energy is absorbed. At low levels of adhesion, delamination may occur without significant fiber failures4. The control panels contain more than 30% thermal-cured polyester-based resin, while the light-cured panel is made of epoxy acrylate-based resin, with a loading of 20 to 25%. As can be seen from Figure 2, curing time also plays a significant role in impact response. Longer curing time indicates denser crosslink and more complete polymerization, resulting in better adhesion and, ultimately, less delamination.
It should be noted that even though the projectile has the same diameter as a 0.44 Magnum semi-wadcutter bullet – defined in Underwriters Laboratory 752 (UL 752) level 3 ballistic standards – the impact response is expected to be different because the lead-tipped bullet, when hitting the target, would absorb extra impact energy due to mushrooming effect, and its high velocity (1,350 to 1,485 ft/sec) may result in a different failure mode.
Conclusion
A 22-layered fiberglass-reinforced panel can be successfully cured by a blue LED array with irradiance ranging from 200 to 1,000 mW/cm2. Ten minutes of curing time on each side is preferred to 5 minutes because the incomplete cure caused more severe delamination. Lab impact tests demonstrated comparable results to those of control panels.
The effects of resin loading, resin type and concentration of photoinitiator on impact properties – as well as shooting test – are still ongoing and will be presented once the data are complete.
Acknowledgement
This work was financially supported by New York State Energy Research and Development Authority (NYSERDA). We would also like to thank Rapid Cure Technologies, Inc. and Armortex for donating raw materials and sample panels.
References
- Kamoun, E.A.; Winkel, A.; Eisenburger, M.; and Menzel, H. (2014). Carboxylated camphorquinone as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application. Arabian Journal of Chemistry.
- Schneider, L.F.J.; Cavalcante, L.M.; Consani, S.; and Ferracane, J.L. (2009). Effect of co-initiator ratio on the polymer properties of experimental resin composites formulated with camphorquinone and phenyl-propanedione. Dental Materials, 25(3), 369-375.
- Stansbury, J.W. (2000). Curing dental resins and composites by photopolymerization. Journal of Esthetic and Restorative Dentistry, 12(6), 300-308.
- Schwartz, M.M. (1997). Composite Materials. Volume 1: Properties, Non-destructive Testing and Repair.
