Safety of the UV/EB Curing Process

Transcription

1 Safety of the UV/EB Curing Process

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3 Abstract The coatings and paint industries strive to provide high technology coatings while reducing volatile organic compounds (V.O.C.s) and energy consumption to produce a finished coating. A one hundred percent reactive coating chemistry, eliminating V.O.C.s and improving the energy/ finished product ratio has grown in the 1970 s, 1980 s and into the 1990 s. The ultraviolet (UV) light and electron beam (EB) curing industry is one of the fastest growing technologies in the coatings industry. Improved handling properties of this three-decade- old technology will be addressed in this paper. Reduced volatility, higher flash points, one hundred percent reactivity and equipment friendliness are the strengths of this technology. Introduction Ultraviolet light and electron beam curable coatings are formulated with reactive solvents, oligomers and additives to provide color, gloss, and leveling properties. The reaction mechanism is free-radical polymerization based on acrylates, methacrylates, vinyl and allyl compounds. The ultraviolet curing is initiated by a photo-catalyst absorbing distinct light energies. The catalyst forms free radicals which in turn initiate polymerization. The free radical polymerization provides cure-in- place polymer networks. Conventional curing methods depend on a predetermined chemistry based on a single polymeric backbone or a two-part system. The unsaturated carbon compounds in UV/EB curing may be changed in ratio quickly by each formulator. Novel cure-in-place polymers are uniquely developed. Electron Beam curing does not require a photo-catalyst for free radical initiation. Electrons are excited and produce initiation of the acrylates, methacrylates, vinyls and allyl chemistries incorporated into the formulations. Polymerization continues as in ultraviolet light free radical curing methods. The low-viscosity reactive monomers provide application viscosity, adhesion, leveling, and physical properties. Conventional coatings depend on solvents such as toluene, methylethyl ketone, acetates or even water. These solvents/diluents must be removed with hot forced air ovens or infrared heaters. This process allows the coating resin to dry or crosslink to form a coating. Conventional coatings may be based on isocyanates, epoxies or polymeric resins used in solvent-based, high solids or emulsion application systems. Ultraviolet light and electron beam cured coatings are based on similar backbone structures. Isocyanates and urethane prepolymers are end-capped with hydroxyl acrylates and methacrylates. Epoxy resins are reacted with unsaturated acids to provide acrylate and methacrylate terminated oligomers. Polyesters and acrylic polymers are similarly terminated for free radical polymerizations. This reduces the health and safety hazards involved with the use of these chemicals as pre-polymers. Additional advantages of ultraviolet and electron beam curing is the reduction of manufacturing space and energy. Solvent-based and water-containing coatings require ovens, air flow, solvent recovery units and/or incinerators. Free radical curing equipment which initiates ultraviolet light or electron beams are smaller and use less energy. Discussions Monomers Reactive monomers provide solvent-like properties and adjust viscosities for easy application. The molecular weights are generally greater than two hundred making the coating very low in volatility. The monomers themselves have low vapor pressure with flash points greater than ninety three degrees Centigrade which minimizes the risk of fire hazards. Most monomers are classified as non- hazardous under the Department of Transportation regulations. Monomer functionality, backbone structure, polarity, surface tension, viscosity and solubility provide adhesion, leveling, surface wetting, gloss and reactivity. These chemicals require standard safety and handling equipment. Impervious gloves and protective eye wear are normally required for monomer handling at room temperature. Acrylate, methacrylate, vinyl and allyl products used in coating and ink formulations will not dry, harden or cure with time. The ongoing liquid nature of these products requires those who come in contact with them to practice standard chemical hygiene. The reactive monomers in UV/EB instantly crosslink to provide a cured, crosslinked coating with no volatile organic compounds. 3

4 Materials that come in contact with the skin should be removed with copious amounts of mild soap and lukewarm or cool water. Cool water will prevent opening of skin pores. This will minimize the surface area of contact and reduce the chemical contact time. Skin should be monitored for reddening or chemical burns. Mild hand soap should be used to prevent abrading the skin or rubbing the chemicals into pores during cleansing. A soap with aloe, vitamin E or other suitable skin oils helps prevent the skin from drying or cracking. This further protects the skin in the event of additional chemical contact. Eye wear with protective side shield are a basic requirement in all chemical industries. The best kept areas in terms of cleanliness cannot prevent equipment failures or spills from unsuspected sources. If the eye is exposed to chemicals, flush with copious amounts of water and seek medical attention. Consult Material Safety Data Sheets (MSDS) for additional actions. It is always a good idea to have the MSDS accompany the person receiving medical attention. General cleanliness with UV/EB curing chemicals is important. These materials do not harden, polymerize or cure when left on lab tops, manufacturing equipment or door knobs. It is for the safety of everyone to maintain a clean work area when using any chemical. Oligomers/Resins Urethane Acrylates They are among the most versatile backbones in the UV/EB field. Molecular weights may be changed from only a few hundred to over ten thousand. Typical urethane acrylate molecular weights are eight hundred to five thousand. A urethane pre-polymer made of a polyol end-capped with a diisocyanate is used in conventional solvent- based and two-component coatings. These same pre-polymers may be temporarily terminated with reactive ingredients for emulsification which will then volatilize and use the isocyanate reactive chemistry. The urethane pre-polymers in UV/EB curing are permanently end-capped with unsaturated hydroxyl chemistry. The isocyanate and hydroxy group form a non-volatile urethane. The unsaturated group, being an acrylate, methacrylate, vinyl or allyl, is used for the free radical polymerization. The amount of free isocyanate or free hydroxy unsaturation in urethane acrylates are typically undetectable. Isocyanates have considerable toxicological effects at low concentrations. The isocyanate free urethane acrylates are low skin and eye irritants in comparison to their isocyanate terminated counterparts. The urethane acrylates are generally viscous and require elevated temperatures or dilution with previously discussed reactive monomers to reduce the viscosity. The safety considerations are based on handling urethane acrylates at elevated temperatures. There is little possibility of volatile free isocyanates due to the nature of urethane acrylate manufacturing. Protective gloves and eye wear are, of course, required with any handling of chemicals to prevent personnel exposure. Epoxy-based Oligomers/Resins These chemicals are the largest volume class in UV/ EB industries. A bisphenol-a-epoxy resin is reacted with acrylic acid or methacrylic acid to provide unsaturated terminal reactive groups. The versatility of the backbone structure provides a wide range of chemical coating properties. The epoxy-based oligomers may be viscous and would have the same handling precautions as the heated urethane acrylates. The oligomers and resins are diluted with reactive monomers to adjust the application and handling viscosities. Safety and handling precautions are generally required based on the diluent. Impervious gloves and safety wear should be worn. The acrylic acid/epoxy reaction to make Bisphenol- A- diacrylate destroys any free ingredients such as epichlorohydrin used to make the bisphenol-a-epoxy starting raw material. Emerging UV Curable Technology During the last fifteen years, an important chemistry has grown providing adhesion and flexibility - particularly for the metal, plastics and polyolefin film industry. This chemistry involves the use of UV light initiated, acid catalyzed epoxy and vinyl ether curing. This mechanism requires a photo catalyst to absorb light and form a Lewis acid. The Lewis acid initiates the reaction of epoxy and vinyl ethers to undergo additional polymerizarion. The mechanism continues until the reactive ingredients are consumed. 4

5 The epoxy and vinyl ethers are generally greater than two hundred molecular weight. The volatility, flash points and shipping regulations are very similar to the free radical chemistries discussed previously in the article. Standard safety and handling precautions are required with these materials. Impervious chemical gloves and protective eye wear should be worn during handling. This chemistry does not harden, polymerize or cure without exposure to light. These products should be cleaned off lab benches, manufacturing equipment and door knobs. This protects the personnel working with the materials and those who may work in these areas at a later time. Manufacturing/Converting All manufacturers of raw materials and suppliers of formulated products must provide complete Material Safety Data Sheets and technical support for in-plant or laboratory use of these products. Please consult directly with your supplier for detailed information on recommended Safety & Handling. The manufacturing and equipment for UV/EB curing provides additional safety. The solvent-free chemistry provides an explosion-proof environment. This is enhanced with greater than ninety four degree centigrade flash points and low vapor pressures of the chemicals. UV Equipment This equipment is supplied as simple, single lamp curing units on fixed speed conveyors to units designed for two, four, or eight or more lamps with variable power sources for speed control and varying total input energy. The curing units come equipped with automatic shut-downs and cooling alarms. The ultraviolet light curing equipment is basically an artificial source for sunlight. Cylindrical or spherical vacuum bulbs generally contain a very small quantity of mercury. The metal is vaporized by an electrode passing current through the metallic vapor or by an instantaneous microwave energy source. The vaporized metal emits light and infrared heat. The light energies are directed in a single direction by use of elliptical or semi-circular, polished reflectors. The infrared energy is removed by high velocity air and/or water cooling of the reflectors. The ultraviolet light emits strongly in the UV A and UV B ranges similar to the sun. Personnel are protected from these light waves through equipment designed to prevent opening of the equipment with the light source energized. Personnel working around UV equipment should wear UV absorbing protective eye wear because stray light may reflect off of the cured substrate. Maintaining skin moisture with the use of barrier/ moisturizing creams and skin conditioning soaps is strongly recommended. The infrared energy and ultraviolet light energy may impart heat to the coating material. This heat generally is very low resulting in only a small rise in temperature. The heat from the chemical reaction is a further source of energy to elevate temperatures. The only time the heat is a factor during manufacturing is during a web break when the material would stay under the light system. Accidents, although rare, are stopped quickly by shutting off the lamps. Only the material under the lamps is immediately damaged. There are no explosions, fire balls or large incinerations. The monomer s flash point will not cause a fire. The heat of combustion of the substrate or coating material may cause a fire. Electron beam provides accelerated electrons impinging on a coating surface or penetrating into a substrate. A tungsten electrode is heated until electrons are emitted from the metal. The electrons are accelerated through a vacuum with the use of increasing magnetic fields. The additional radiation emitted during the process is completely shielded so there is no operator exposure. The high energy curing provides additional advantages of lower cost formulations and economical converting. Cure-in-place technology is further enhanced due to the polymer crosslinking of composites. Saturated wood, paper and impregnated inorganics are crosslinked immediately with energized electrons. Summary The ultraviolet light and electron beam curing technologies are growing industries. One hundred percent reactive inks, coatings and composites produce high quality finished products for the consumer market. Manufactured items may be tested, packaged and shipped in the same day. There are no pollutants given off into the atmosphere, nor is high energy required for the curing or drying of the inks/coatings. This growing technology allows the efficient manufacturing of coatings, inks, and adhesives, etc. with energy efficiency, no V.O.C.s, improved health and safety considerations and a cure-in-place technology. 5

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