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By Peter Holl, Eberhard Föll, Polymer-Physik Tübingen- RadTech Europe Conference 1989
More severe environmental protection requirements having given way to stronger governmental restrictions for limiting the emission of noxious substances into the atmosphere (Technical Instructions! TI Air), determine the permissible limits for mass flows and concentrations of potential noxious substances in the waste gases of industrial lacquer coating equipment.
As a result, pressure is growing on converters using solvent based coating media to look for methods which are environmentally acceptable and which give off significantly less solvent or fission products into the atmosphere.
Here the field of chemistry has new groups of products to offer:
- Low—solvent Systems with a high content of solids.
- Coating materials containing water.
- Coatings with solid resin in powder form.
- Pre—polymerised 100 % solid systems.
A common feature of the first three methods is that heat is required for the formation of a dry film of coating. However, the amount of the heat transferred to the coated layer is very low.
Most of the heat is 'wasted' in the oven, in the thermal transfer, in heating up and cooling down the product.
The classical method of thermal drying requires convection (thermal currents) and conduction (thermal conduction) for the transfer of energy. Here material is necessary as a heat carrier.
With convection moving mass particles are responsible for transporting the energy.
With conduction the mass remains in the same place. The energy is transmitted from particle to particle by oscillations.
Interesting techniques, which do not include these disadvantages of thermal drying, are the various methods of radiation drying, in particular those involving ionising radiation such as UV (ultraviolet) and EBC (electron beam curing).
Here the energy is transferred 'without contact'. Energy transfer can take place in a vacuum or through a vacuum section. After irradiation, the cured material is absolutely dry and immediately can be treated further.
UV radiation can be used without problems for curing and vulcanization in those places that are accessible to the light, i.e. the layers the radiation must pass through, have to be transparent at the appropriate wavelengths so that sufficient UV power is available for cross—linking the coating material by irradiation after passing the path length corresponding to the layer thickness.
This means that care should be taken with pigmentation and application of strongly absorbing filling materials. The same is valid for thick layers and for applications where coating material penetrates the base material, e.g. paper. In this case the coating material is not cured completely. Unused photosensitizers and unsaturated monomers represent a certain risk.
This can be avoided by the use of accelerated electrons which represent both wave radiation and corpuscular radiation.
