The coating industry was established on the principle of mixing resins, pigments and other ingredients in solvents to form paints. When these paints are applied to a substrate and the solvent is removed by air drying or application of heat, they leave a solid coating with all the protective and decorative properties that are the reason for the coating in the first place.
However, this practice has faced increasing environmental regulations because the solvents that are vented into the atmosphere are volatile organic compounds (VOCs), which react with oxides of nitrogen to form ozone, the primary cause of smog. Starting with Rule 66, which was promulgated in California in 1966, there has been increasing legislative actions to reduce or eliminate these harmful VOCs. Since the passing of Rule 66, the federal government has enacted the Clean Air Act (CAA) of 1970 and the Clean Air Act Amendments (CAAA) of 1990. Although there has been progress in reducing VOC emissions, it has been slow, primarily due to the steady growth of our economy that keeps adding to the VOC loading.
The primary approach to the reduction of VOCs has been, and still is, the collection and destruction of evaporating solvents, also known as end-of-pipe or add-on technology. This type of technology is attractive to painting shops because it does not affect their finishing process.
However, there are problems with this pollution prevention technique. The VOC destruction equipment has high capital costs, and it is expensive to run, monitor, adjust and repair. The equipment consumes a large amount of energy as well, which contributes to other pollutants in the atmosphere.
Following the passage of the CAA, efforts were made to avoid generating pollutants to start with. This technique, known as "source reduction," was advocated by EPA in some areas. In the coatings field, VOC limits were set on the basic coating compositions by the early regulations derived from the CAA. In 1990, Congress passed the Pollution Prevention Act, mandating such an approach in all of EPA's activities. It was a really great idea! Rather than creating a VOC and then destroying it, why not go back and avoid using VOC-producing materials in the first place.
When Carol Browner became EPA administrator, she took the concept to a higher plane, insisting that pollution prevention, or P2, as it is known by workers in the field, be considered in all aspects of EPA's pollution control efforts.
These actions established a hierarchy of pollution control guidance:
- Pollution Prevention—pollution should be prevented or reduced at the source whenever feasible.
- Recycling—pollution that can't be prevented should be recycled in an environmentally safe manner whenever feasible.
- Treatment—pollution that can't be prevented or recycled should be treated in an environmentally safe manner whenever feasible.
- Disposal or Release—disposal or other release into the environment should be employed only as a last resort.
But nearly 10 years later, P2 is still not very well understood or used in the air emissions control arena. The reasons for this failure are widely debated but little understood. Some of the reasons include:
- Many of the pollution control laws are specific in dictating how regulations should be developed. The laws are based on end-of-pipe techniques, and EPA regulation writers must obey the letter of the law.
There has not been sufficient guidance on what P2 means to industrial operations.
Phases of Pollution Prevention
A coating shop will approach any suggestion for pollution control with the question, "What does this do to my process?" This is no small matter since development of a process that works well takes so much time, sweat and sometimes tears and is zealously protected by its practitioners. The phases of P2 were developed to answer this question.
Phase 1: The Low Hanging Fruit. Most P2 specialists know all about the low hanging fruit. It is important for the coating shop to know that this phase does not require that it alter its process at all. This phase concentrates on the peripheral aspects of the shop's responsibilities: housekeeping; purchasing; logistics; work rules; simple equipment changes; and management attention. The shop should realize that most of these approaches are cost-free or very inexpensive. And, carrying out many of these activities can result in substantial savings for the company.
The EPA recommends starting on P2 with an "environmental audit" that can pinpoint where substantial progress can be made in a given plant. There is a Pollution Prevention Audit Manual published by EPA that gives good guidance on such audits. I highly recommend this approach. Although some air pollution reduction can result from Phase I activities, the major sources of air pollution remain untouched. Therefore, one must progress to Phase II.
Phase II: Reduction of Process Pollutants. This phase calls for some modest process changes to partially remove remaining VOCs. High-solids and waterborne formulations can generally be used without equipment changes. Thus, capital costs are restrained. Conditions of the processing must be altered, however. This may result in the loss of coating properties, although over the years, resin manufacturers have been able to produce some very acceptable materials. Many industrial plants have made the transition to Phase II technologies already. This results in another reduction in air pollution, but still leaves much to be accounted for.
Phase III: Elimination of Process Pollutants. Though Phases I and II reduce the amount of air pollution, a fair amount of pollution remains. Furthermore, continued economic growth will add to the current numbers unless something is done.
The regulation writers are still bound by the laws on how they must approach their tasks. They are finding that collect-and-destroy technology has been improving, with reports of 95-99% destruction of VOCs. There are indications that, based upon this performance, the new regulations will call for 95% reduction of VOCs or better. This would seem to force the users of Phase II technologies to put in collect-and-destroy equipment.
Fortunately for them, this does not have to be their fate. There are Phase III technologies that easily meet the equivalent of 95% reduction and frequently are far below that level. This gives the industrial user an option of technologies to use. Coating shops will also find that these new technologies have other advantages that they have been loath to consider in the past.
Ultraviolet (UV) and electron beam (EB) coatings are Phase III technologies that perform as well or better than their solvent-borne counterparts without the emissions into the atmosphere. To eliminate the production of VOCs and cure these coatings, different compounds are required to formulate the coatings:
- Oligomers—The properties of any coating crosslinked by radiant energy are determined by the oligomers. Oligomers are moderately low molecular weight polymers, most of which are based on the acrylation of different structures. Acrylation imparts the unsaturation of the C=C (carbon=carbon) group to the ends of the oligomers.
- Monomers—Monomers are primarily used as diluents to lower the viscosity of the uncured material, facilitating application. They can be monofunctional, containing only one reactive group or unsaturation site, or multifunctional. Unsaturation allows them to react and become incorporated into the cured coating rather than volatilizing into the atmosphere as is common with conventional coatings. Multifunctional monomers, because they contain two or more reactive sites, form links between oligomer molecules and other monomers in the formulation.
- Photoinitiators—This ingredient absorbs light and is responsible for the production of free radicals or cations. Free radicals or cations are high energy species that induce crosslinking between the unsaturationsites of monomers, oligomers and polymers. Photoinitiators are not needed for EB cured coatings because the electrons are able to initiate crosslinking.
- Additives—The most common additives are stabilizers, which prevent gelation in storage and premature curing due to low levels of light exposure. Color pigments, dyes, defoamers, adhesion promoters, leveling agents, wetting agents and slipaids are examples of other additives.
Environmental Advantages. Because UV/EB coatings have negligible emissions, operators who switch to them can show a tremendous reduction in emissions, generally removing the operator from the "major source" category. As an example, the Coors Co., which makes more than 4 billion beer cans a year, switched its production entirely to UV coatings. It reduced emissions in the manufacture of each billion cans from 28.9 tons/yr to 1.677 tons/yr with a UV-cured acrylic enamel and then to 0.224 tons/yr with UV-cured epoxy enamel. In short, the company reduced its emissions more than 99% without the use of collect-and-destroy equipment.
By virtually eliminating the emission of VOCs, UV/EB coatings could provide numerous other environmental advantages as well:
- Minimal or no state and federal clean air operating permit requirements;
- Low or no clean air permit fees;
- No new compliance assurance monitoring equipment requirements;
- Technology that easily meets states' standards for "reasonably achievable control technology" and "lowest achievable emission rate" equipment requirements in ozone non-attainment areas;
- An equipment alternative for "best available control technology" for facilities located in ozone attainment areas; and
- A "maximum achievable control technology" candidate for reducing HAP emis- sions in selected industries.
In addition, the EPA is now putting the finishing touches on the establishment of a market for excess VOC reduction. Assume you are emitting 10 tons/yr of VOCs and regulations allow you only 5 tons/yr. You replace your present operation with a UV/EB coating system with virtually zero VOCs. You have reduced your emissions by 10 tons. You have used up 5 tons to meet the regulations. You can now sell the rights for the other 5 tons of VOC emissions to someone who has not put in the new technology and has no other economic way of meeting the standards. Other Advantages. UV/EB coatings provide advantages beyond their environmental friendliness. Some of these advantages include:
- Low-temperature curing—no heat is required for complete cure, allowing UV/EB coatings to be used on all heat-sensitive substrates.
- No curing ovens—elimination of curing ovens saves on the cost of fuel for heating such ovens and the floor space taken up by those ovens.
- High-speed curing—cure time is reduced from the 15 min or more for oven curing to just a few seconds, allowing for higher production rates and greater efficiency.
- Reduced capital costs—since there are no emissions from UV/EB coatings, it is not necessary to install collect-and-destroy equipment.
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