The Development of Low-Energy Electron Beam Technology for Automotive Interior Plastic Coating Applications

By Anthony Carignano, business development and strategic marketing specialist, PCT Engineered Systems, LLC

Introduction

Electron beam technology has many potential uses in automotive applications. From curing carbon fiber composite coatings to polymer grafting, over the past decade, with the integration of compact 280mm and 400mm sealed tube electron beam and X-ray lamps, low-energy electron beam technology has expanded into a variety of new application areas. This article addresses the potential use and benefits of electron beam technology for curing coatings on interior automotive flat and multidimensional plastics parts. Developments in the usage of low-energy electron beam for polymeric crosslinking and grafting to enable compliance with automotive lightweighting standards¹ will be reviewed in a later article.

What is Low-Energy Electron Beam Curing Technology?

Low-energy electron beam (defined as operating with an accelerating voltage of less than 300 kV) is a form of ionizing radiation. Electrons produced by electron beam equipment are accelerated with sufficient energy to break chemical bonds. The accelerated electrons directly ionize polymers without the need for photoinitiators. Another advantage of electron beam is its very consistent output. This includes uniform energy deposition though the thickness of a coating, ink or adhesive, as well as across the width of a web. Well-established industrial electron beam applications range from coatings on metal coil to adhesives for tapes where electron beam curing takes place irrespective of color, density or pigment loading.

In addition, electron beam surface curing technology is not limited to 100 percent solid acrylate functional resin systems. Organic solvent- and water-based branched and block copolymers used in pressure-sensitive adhesives also will crosslink with electron beam. In certain waterborne acrylate functional applications, it has equally been demonstrated that inks and coatings with total solids contents below 50 percent can effectively be dried by electron beam energy alone or in combination with optional thermal drying.²

The energy emitted from an electron beam unit is directly delivered to the surface of the coating and causes chemical changes that convert a liquid into a fully cured coating film. Compared to thermal drying and curing methods, electron beam requires a smaller footprint and reduces elapsed cure times for coatings. Electron beam eliminates the need for cooling lines, solvent incinerators and explosion-proof rooms.

Ultraviolet curing uses non-ionizing energy, which requires the use of photoinitiators (or at least some photo-reactive moiety incorporated into an oligomer backbone) to initiate the curing polymerization process. Most ultraviolet-type formulations cure by free-radical polymerization in which ambient oxygen can reduce the efficiency of the curing reaction, often requiring greater UV energy to achieve effective cure. Electron beam curing is characterized by the dose (energy per unit mass measured in kilograys) deposited in the substrate. Ultraviolet curing is characterized by exposure (maximum instantaneous UV energy delivered to the surface per unit area, expressed in joules or millijoules per square centimeter J/cm² or mJ/cm²) and energy density (radiant energy arriving at a surface per unit area).

Examples of Energy-curable Chemistry Found in Automotive Plastic Coating Applications

Ultraviolet-cured hard coats for polycarbonate exterior vehicle lenses and reflectors have been commercially available for more than 20 years and currently represent over 80 percent of coatings used for automotive forward lighting applications. UV-cured hard coats are typically applied to lenses by spray or curtain coating. These materials must be applied at a uniform thickness and cured uniformly to prevent the coating from changing the optical properties of the lens. UV-curable materials also are applied to automotive reflector bodies to prime and level the plastic prior to metallization of the reflector. The UV-curable primer is either dip-, spray- or curtain-applied. UV-curable hard coats and primers typically contain organic solvents to reduce viscosity and assist with flow-out. Aside from improvements in coating performance and quality, when compared with conventional thermal drying systems, UV curing advantages for hard coat lens and reflector bodies include:

• reduced use of organic solvents and volatile organic compounds (VOC);
• reduced costs in capital equipment investment, maintenance and energy used;
• reduced footprint; and
• reduced product spoilage.

Leading UV hard coat suppliers to the automotive industry include Red Spot Fujikura, Akzo Nobel, Manikewicz, Peter-Lacke and Lankwitzer. Most UV hard-coating systems are based on aliphatic urethane acrylate chemistry diluted with monomers and organic solvents. In primer applications, lower cost acrylated polyester-epoxy blends are used.

In Europe, the move toward UV is based on performance: scratch and abrasion resistance, along with low-gloss mar resistance, energy savings and environmental sustainability. To comply with increasingly stringent volatile organic compound (VOC) requirements, certain European coating suppliers, such as Lankwitzer, now offer solvent-free 100 percent solid sprayable acrylate functional UV systems for automotive applications. They require nitrogen inerting for reactivity and to mitigate VOCs.

For thicker and hard-to-cure pigmented coating, dual cure systems have been developed combining UV with another (usually thermal) cure technology to ensure good cure at the bottom of the thick pigmented coating. However, the penetration of UV energy through coating films can become a significant challenge when curing through opaque materials. In addition, curing three-dimensional objects with UV technology can be hindered by the fact that object surfaces are subjected to the varied energy density and irradiance of UV as a result of uneven distances and exposures to UV lamps. UV curing is “line-of-sight,” meaning that every point on a surface must be exposed to UV energy to cure the coating. Hidden and shadow areas will remain uncured (wet). Leading UV lamp manufacturers have developed novel integrated three-dimensional UV curing systems for automotive plastic applications to provide improvements in cure consistency. Today, UV curing is being done successfully on a variety of complex shapes, from plastic molded parts such as mobile phone cases to golf balls and fiberglass composite parts.

Electron Beam Curing for Automotive Plastic Applications

Electron beam curing technology has been explored for automotive surface curing applications since the 1960s. Documented in a 2011 IAEA report on Industrial Radiation Processing with Electron Beams and X-Rays: “To capitalize on the opportunity for electron beam coatings at Ford, PPG set-up the Radiation Polymer Company (RPC), which subsequently became an independent low-energy electron beam company, RPC Industries.” In the 1980s, Bill Burlant, Ph.D. at the Ford Motor Company, showed that electron beam-cured coatings on profiled and three-dimensional metal and plastic components could produce outputs well over 750 times the speed of conventional drying techniques. Unfortunately, interest at Ford never developed into a sustained commercial practice.

Today, flash-off and cooling lines still are bottlenecks for thermally cured conventional interior automotive plastic coating systems. Heat must be gradually applied for crosslinking to occur without coating film deformations. Cooling lines are used to remove residual solvents from the coating film post cure. For most automotive plastic applications, the drying process for most coatings typically is limited to temperatures <200°F, which again further limits throughput. For example, Acryonitrile Butadiene Styrene (ABS), which is commonly used for laser ablatable interior automotive trim applications, can be used in processes between -20 and 80°C (-4 and 176°F), and its mechanical properties vary with temperature. For processing temperature above 140°F, ABS becomes too difficult to coat. It is for this reason that room temperatire curing is optimal for heat-sensitive plastics, such as ABS. The data in Table 1 suggest that an electron beam curing process would increase energy productivity over hour by three times while dramatically reducing process time and emissions. Electron beam VOC emissions are based on the usage of near 100% solid formulations.

Over the past decade, a number of studies have compared the energy demands to thermally drying solventborne and waterborne coatings vs. electron beam-cured coatings. Table 2 summarizes data from Berejka’s 2009 IAEA presentation, titled “Prospects and Challenges for the Industrial Use of Electron Beam Accelerators,” and calculates the energy to dry/cure one gram of dried coating. For a water-borne coating (at 40 percent solids), the energy required to dry and cure the coating (independent of other energy required to heat the substrate) is 3390J/g. On the same basis, the equivalent energy required for a solvent-borne coating (40 percent solids in toluene) is 555J/g. This is compared to an electron beam-cured coating (100 percent solids, solvent-free) using a dose of 30kGy (30J/g). Converting Joules to kWh, the total kWh cost to dry 1 Mil (25.4 microns) of dry applied coating is almost twenty times higher for solvent-based versus electron beam-cured 100 percent solid formulations.

Electron Beam Provides Solutions for Automotive Lightweighting

The photo at right shows polycarbonate (PC) and ABS/PC (1x1x1 inch) buttons and a center console trim plate support used to demonstrate proof of concept for the ability of electron beam to cure multi-dimensional parts. A black high-gloss mono-coat system containing approximately 78 percent total solids from Akzo Nobel was used for electron beam cure testing. All parts were sprayed to a total wet thickness of from 20-30 microns and allowed to flash dry for 10-15 minutes before being cured in one pass with a COMET 200 kV EBLab Unit. All flat parts were cured at 110 kV with a dose of 30 kGy at a line speed of 15m/min.³) Buttons and trim plate supports were cured at a dose of 40 kGy at 3 meters per minute to ensure good electron beam exposure to the sides of the parts. Tests were later repeated at EBLab Unit with maximum line speeds of 30 meters per minute. Upon electron beam cure, all parts exhibited high gloss, hardness, scratch resistance and good flexibility after +30 MEK double rubs. The same electron beam cure behavior test work completed with high-gloss piano black coating systems from a second coating supplier using a polyester acrylate-based system demonstrated similar performance properties. All coupons and multidimensional part samples were cured in an electron beam tray, which accommodates shapes up to 8.5W x 11L x 2H inches.

What is the Potential for Electron Beam Curing Technology in Interior Automotive Plastic Coating Applications?

Energy-curable coating chemistry and electron beam curing processes have the proven potential to drastically reduce VOCs and energy consumption, along with manufacturing physical and environmental footprints, while significantly increasing line speeds. Both the chemistry and the electron beam curing process for coatings are well known and well positioned to offer novel solutions for interior automotive plastic coating applications as vehicle lightweighting grows as a standard requisite within the transportation industry. Compared with conventional drying and ultraviolet curing methods, electron beam offers the unique advantage of delivering room-temperature, color-blind curing for both inline and multidimensional substrates within greatly reduced footprints. However, electron beam also has certain penetration limitations that could be overcome through hybridization with X-ray within the same integrated curing system. X-ray has about 5 to 10 times the penetration of electrons. In summary, the availability and proven integratibility of compact 280mm and 400mm sealed tube electron beam and x-ray lamps should be considered as an enabling opportunity for the acceleration of new processes designed for compliance with automotive lightweighting standards.

Anthony Carignano is business development and strategic marketing specialist at PCT Engineered Systems, LLC. With more than 20 years of experience developing and growing new markets and applications in the specialty chemicals industry, his recent accomplishments include leading PCT’s efforts to establish a fully operational ebeam laboratory unit, which is being used for instructional and technical proof of concept purposes at Georgia Power’s Customer Resource Center in Dunwoody, Georgia. Carignano is an active member of RadTech International North America and other coating and ink-related trade organizations. He has spoken globally on issues ranging from chemicals of concern to bio-renewable building block chemistries for consumer product packaging applications. Contact Carignano at awcarignano@teampct.com.

Acknowledgements

The author would like to acknowledge Jim Kostecki of Akzo Nobel, Paul Uglum of Delphi Automotive and Steve Lapin, Ph.D. of PCT Engineered Systems, LLC for their contributions to this article. Acknowledgement is equally made to Georgia Power’s CRC as a partner for the promotion of low-energy electron beam technology.

Resources

1. According to recent MarketsandMarkets[1] and McKinsey[1] reports, “demand for automotive plastics in light-duty cars and trucks is expected to reach USD 40.1 Billion by 2020 and will grow by a CAGR of 8.3 percent in total volume between 2015 and 2020.” Aside from increased car production, the high growth rate in demand for automotive plastics in the United States is attributed to initiatives by automobile manufacturers towards the lightweighting of cars to reach compliance with new greenhouse gas emission (GHG) reduction and fuel economy standards. Developed jointly by the Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA), “the standards are projected to result in an average industry fleet wide level of 163 grams/mile of carbon dioxide (CO2) in model year 2025, which is equivalent to 54.5 miles per gallon (mpg) if achieved exclusively through fuel economy improvements.”
2. According to Lapin and Classon in “Electron Beam Curing Properties of Water Based Monomer and Polymer Materials,” to be presented at RadTech 2016, electron beam can be used as both a drying and curing mechanism for waterborne polyurethane acrylate dispersions.
3. One kilogray (kGy) is equal to one joule per gram. Electron beam penetration is determined by the accelerating voltage, distance from the electron beam lamp window to the materials and mass density of the material. Depending on the coating chemistry, the typical dose needed for curing applications can range from about 15-50 kGy with nitrogen inerting to less than 200 ppm oxygen.

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