Photopolymers are ultraviolet (UV) light sensitive materials. Photopolymer plates used for flexographic printing plates are similar to rubber plates in that they are flexible and resilient. Photopolymer plates are either viscous liquids or solid sheets of appropriate thickness.
A film negative is used to produce conventional photopolymer plates. The clear areas on the film transmit ultraviolet radiation and the black areas absorb it. The films are contacted to the photopolymer plate under a vacuum cover sheet. The plate receives ultraviolet exposure through the film, polymerizing those parts of the plate exposed to the UV light. The exposed polymerized areas are unaffected by the subsequent solvent wash out process that removes the unexposed photopolymer. The exposed polymerized area form the relief structures that are the ink-carrying printing surfaces of the plate.
Water washable plates are prepared with almost the same procedure as that used for solvent washable plates. The biggest difference in process equipment between solvent washable plates and water washable plates is the washout unit, which is usually accompanied with a washout water treatment unit. Water washable plates largely reduce or eliminate many of the concerns of solvent washable plates, including emissions of VOCs; flammability because of lower flash point; hazardous waste; and influence on human health. The use of these more environmentally friendly water washable plates enhances corporate image with regulators, customers and the public. By switching to these types of plates, the need to purchase and install pollution control equipment may be reduced or avoided. Water washable plates come in two versions: sheet and liquid photopolymer.
Dry Thermal Photopolymer Plates
Dry thermal plate processing eliminates the use of solvents, reduces plate making time and improves plate quality. This system eliminates the need for conventional chemical solvent or aqueous washout.
Prior to developing the plate in the thermal processor, the plate material is back-exposed to establish the "floor" or bottom side of the plate and main exposed through the film negative, which sets the artwork that will be the printing surface, in a typical photopolymer exposing unit. Back exposure stabilizes the plate. The main exposure forms the relief image. The thermal processor accepts the plate and, using precisely controlled heat sources and a small amount of contact pressure with a non-woven (synthetic) web material, removes the unexposed portion of the relief layer. To achieve the desired relief of the plate, it is rotated multiple times through the processor. Each rotation removes more polymer and increases the relief. A completed plate is then ejected from the unit and is ready for post-exposing and finishing.
Post-exposing and finishing involves exposing the final plate to short-wave ultraviolet (UV-C) lamps to cure any remaining partially cured photopolymer material.
Best Management Practices & Pollution Prevention
Perchloroethylene (PERC, PCA), a hazardous air pollutant (HAP), was traditionally used as a solvent to wash photopolymer plates. The regulation of chlorinated solvents due to air contamination, health hazards and hazardous waste has promoted the introduction of perc alternative solvent (PAS) and water washable plates. Considering the hazardous nature of PERC, there is no reason why it should be used to process plates.
PASs are now being used by many plate makers. PASs can be used to develop almost any solvent washable photopolymer plate.
Squeegees help reduce "dragout" of plate processing solutions. This helps reduce chemical costs as well as minimizes the amount of waste solution that must be handled and disposed.
Solvent can be recycled, but its useable lifetime depends on the number and size of the plates and the amount of material removed. Once the solvent is spent, it can either be sent to an off-site distiller, or equipment can be purchased to distill the solvent on-site. Still bottoms generated from distillation are usually incinerated, but can be landfilled depending on local requirements. It is advised to run the appropriate tests to determine if waste products are classified as hazardous wastes. If they are determined to be hazardous wastes, then they may not be eligible for landfilling, at least not prior to further treatment; check state and local regulations. Information and test results should be available from the supplier to help with this determination.
Using the correct right size sheet or using the correct amount of liquid, based on negative film size, will minimize waste of unexposed photopolymer. If waste is still generated, save unused sheet strips as test plates for the resetting of exposure and washout conditions.
Filtered polymer, unexposed photopolymer plates and processed photopolymer plates are normally classified as non-hazardous wastes. Cured plates may be incinerated, or alternatively, sent to a landfill that is authorized to accept the material. They should be qualified to determine if they are hazardous wastes by exhibiting the characteristics of toxicity. Ask the plate supplier for details on whether their plate may be a hazardous waste in your state.
The main environmental advantage of liquid photopolymer plates (over sheet material) is that unexposed portions of the plate can be reclaimed either manually with a squeegee or automatically with an air knife, and reused. This is especially beneficial when a relatively large plate with minimal print area is required.
After the available unexposed liquid resin is recovered, the residual material is removed in an aqueous bath containing additives such as detergents, defoamers, stabilizers and water treatment agents. Spent washout solutions should be acceptable to most conventional POTWs that use typical biological treatment technology.
A washout water treatment unit is designed to filter the photopolymer from the water in order satisfy the requirements of most local POTWs. Analytical data of filtered washout water should be available from the plate supplier. Before discharging filtered washout water into a sanitary drain, the local sewer authority should be contacted to determine if such discharge is permissible. Sometimes, secondary treatment may be required to pass strict limits along with the need to obtain a permit.
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
Solvent Waste - Volatile Organic Compound (VOC) emission source and may be classified as a hazardous waste unless the material is recovered for reuse.
Waste water containing photopolymer residues may be regulated by the local, regional, or state waste water authority. The waste water may need to be filtered and other treatment may be necessary before it can meet the industrial discharge limits for suspended solids, pH, BOD, or COD at the local POTW.
A permit to discharge this waste water from platemaking operations may be required. Check with the local, regional, and state waste water authority. See the PNEAC Contacts and Links.
Hazardous Air Pollutant (HAP) Emissions from Perchloroethylene Solvent Use. An air emission source permit may be required. Contact your regional or state air quality authority for more information about specific thresholds and exemptions.
Health & Safety
Manufacturers recommend careful handling of the waste resin as it can act as a skin irritant.
Most discarded liquid resin systems are not regulated as hazardous wastes or as CWA priority pollutants.
Perchloroethylene Solvent is a carcinogen and should be replaced with an alternative product (alternative solvent or water washout systems).