This section concentrates on the principal environmental concern of the printing industry, VOC emissions. You will learn what VOCs are, what causes VOC emissions, how VOCs can be treated, and what techniques can be used to reduce the level of VOC emissions in the printing processes.
Volatile organic compounds
VOCs or volatile organic compounds can be defined as any organic compounds which react with nitrogen oxide (NO2) to form ozone in the presence of sunlight. Many chemicals used in the printing industry are VOCs.
Ozone appears in two separate locations in the atmosphere: in the higher altitudes, ozone layer protects earth by absorbing UV radiation from sun rays. In lower atmosphere, ozone is a harmful gas causing lung irritation to humans.
There are four main sources of VOC in the printing process:
- isopropyl alcohol (IPA, also known as isopropanol), used as an additive in the offset dampening solution)
- ink components (alcohols, aliphatics, ketones, toluene, etc.)
- cleaning solvents (acetone, chlorinated solvents, kerosene, methanol, naphtha, etc.)
- adhesives (ammonia, IPA, toluene, etc.)
VOC emissions take place as the compounds evaporate. This happens, for example, when inks are dried using hot air, or when IPA evaporates at the printing unit or from the printed sheets.
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VOC treatment techniques
The traditional ways of dealing with high and concentrated VOC emissions are to recover the evaporated solvent by filtering or distillation, or to incinerate it in an afterburner.
Solvent recovery is a well-established method in publication rotogravure. Toluene is used as the ink solvent, and it evaporates in the dryers of each printing unit. Toluene can be recovered using carbon filters.
In heatset offset and packaging printing applications, ink solvent or mixture of solvents cannot be recovered. In these cases, the waste gases are incinerated. The VOCs are in essence burned. The heat energy is usually recovered and used e.g. to heat the production facilities.
VOC treatment is difficult to organise, if the emission sources are spread over a wide area. This is the case in sheetfed offset printing, where IPA evaporates in and around the printing units, from the printed sheets.
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Reduction of VOC emissions
There are many ways to prevent the VOC emissions. Usually, these require changes in raw material types or production process.
Solvent based inks can be changed to solvent-free or less solvent containing. Examples:
- petroleum-based heatset offset inks are replaced with vegetable oil based inks, which have 80 % less VOCs
- UV- or electron beam curable inks can be used after some process changes (the equipment is expensive, though)
- water-based inks can replace solvent-based inks in some flexographic and gravure applications (however, water-based inks have some process limitations)
Other ink-related reduction methods include the following:
- keeping ink containers closed to eliminate evaporation
- regulating ink temperature to reduce solvent evaporation
- reducing ink consumption through process improvements
Normal lithographic printing process needs dampening solution, where IPA is the best additive. Changing to waterless offset (also known as dry offset) eliminates the VOC emissions caused by IPA.
Waterless offset requires a large initial investment on printing presses, which makes it less attractive. It also requires strict process control; e.g. ink temperature must be regulated during printing.
Reducing or replacing IPA
IPA is an excellent additive to the dampening solution on offset presses. However, it causes VOC emissions, it's vapours cause respiratory problems, and it is highly flammable.
There are two ways of reducing VOC emissions from the dampening systems: IPA can be replaced with alcohol substitutes, or the degree of evaporation can be reduced.
Changing IPA to glycol- or glycol-ether based alternatives can reduce VOC emissions by 90 %. However, some changes to the printing press or the process may be required.
The evaporation of IPA can be reduced by refrigerating the dampening solution. This could reduce the VOC emissions by more than 40 %. Stabilizing the temperature of the fountain solution usually also improves printing quality.
Cleaning solvents are used to clean printing plates, blankets, equipment etc. Many of the common solvents are petroleum-based: toluene, xylene, methanol, MEK (methyl ethyl ketone), glycol ethers, TCA (trichloroethane) etc. VOC content of these solvents is generally over 60 %.
Alternative cleaning agents include the following:
- Water-based cleaning solvents. Water replaces some of the solvent. VOC reduction 20 - 50 %.
- Vegetable oil based blanket washes. These have an average of 5 % VOCs. However, using vegetable oil based washes requires extra effort, leave on the blanket an oily film which needs to be removed, and have a longer drying time.
- Terpene cleaners. Produced from wood and citrus. Also these require extra effort in washing and leave blankets oily.
The amount of washing solvents can be decreased by using automatic blanket washing devices. There are many different versions available. Generally, these use less solvent than manual washing, and can work with low-VOC solvents.
Further measures to limit VOC emissions from the cleaning stages include the following:
- training of operators to minimise the use of solvents
- use proper solvent dispensers to eliminate over-use of solvents
- ensure that all containers and receptables can be and are closed tightly
- plan the production schedule and process so that washing needs are minimised
VOC emissions are the most important environmental concern for the printing industry.
There are many ways of reducing VOC emissions. However, many of them require changes in operational practices, production planning and equipment. Printers, in cooperation with materials suppliers, should actively seek ways to improve its processes to minimise and eliminate this environmental problem.
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