APPLYING ENVIRONMENTALLY FRIENDLY COATING SYSTEMS ON WOOD: A SIMPLE  ANALYSES ON THE VOC IN WORK PLACES (SOLVENT BASE VS WATER BASE)

By MM, published on Asia Pacific Coating Journal (APCJ)

Solvents are liquids that have the property to dissolve and keep in suspension the basic materials that constitute a paints, such as main polymers, binders, additives, pigments, without changing their chemical properties. Beside of this important function, solvents present also very aggressive chemical properties, which makes them harmful for humans and wild life. In every solvent-base paint or varnish, the species and the quantities of solvents are normally to be indicated on labels and on safety data sheets, to worn the end users about the toxicity of the products they are about to use, and to which they will be highly exposed. However, chromatographic tests (carried out at CATAS, one of the most important test and certification laboratory in EU for wood coating), have demonstrated in several occasions that some dangerous solvents were found in a paint, even though not specifically declared on labels neither on safety data sheets by manufacturer. Those tests can not be considered as indicative of a common behaviour. However can be used as a warning. In fact, since solvents represent the part of a paint that must completely evaporate (in form of VOC) in order to allow the formation of the desired film on a wooden surface, and since in a solvent base paint the solvents can counts up to 60%  or even 80%, the information about yheir toxicological characteristics are not irrelevant for applicators and the end-users.  This is the case for example of paints formulated for impregnation, such as primers and wood stains, which are highly reach in solvents of many different kinds.

Today new formulations are possible using new species of polymers that utilize water as the main carrier, in additions to very small quantity of co-solvents at 3 % to 8 % maximum. These products present virtually no smell, they drastically reduce the aerial dispersion of VOC and toxic materials in the work places and in the environment, leaving finished artefacts free of dangerous chemicals.  Those are the main reasons why the water based products are more and more used in those parts of the world where regulations and public awareness are limiting VOC emissions. Today also many Asian countries are taking effective action on VOC emissions control, introducing also “green labels” for low-VOC  products. Water base products are becoming more popular than what was in the past also thanks to the availability of new water base self-curing acrylic copolymers and new waterborne polyurethane dispersions. Although in interior applications (furniture) the water base products find still hard to compete with solvent base products, due to strong requests of extreme hardness and gloss, however in applications such as wood primers, wood stains, wood finishing for exterior protection, windows, doors, wooden façade, wooden frames, MDF panels, garden furniture, swimming pool furniture, resorts, the water base products are today progressively eroding solvent based product market with success.

Here in this article we present a very simple but effective way to estimate the aerial emission in work places of a paint process and then some simple considerations about costs.

In spite of its extreme simplicity, it can help applicators to more clues about the paint process they are about to use.

We use for this example the coating of a wooden panel for exterior. In general a good level of protection starts from the application of a minimum quantity of 80 gr/m2 (commonly up to 120 gr/m2).  We consider now to obtain the same wood coating on two panels, by using water base industrial spraying system and solvent base industrial spraying system. We can now reasonably assuming a solid content around 30% for both typologies of paints (in WB paints the carrier is water, in SB paints the carrier are aromatic solvents). This means that when we apply 100 gr of fresh paint on the wood, when evaporation is completed, only 30 gr remain on wood to form the desired coating film. Since for our paint process we need 80 gr of finishing on wood, theoretically, it is necessary to apply 266 gr of fresh product (80:30)x100. However, during a spraying process not all the paint sprayed reaches the wood surface. This phenomena is always taken in consideration by applicators, and it is called “overspray”. The efficiency of the spraying process is related  to the equipment used and the skills of the applicators, and it gives the necessary quantity of the overspray in order to get the desired quantity of product on wood. We consider now a typical efficiency coefficient, around 60% (however some industrial spraying equipment and techniques can reach efficiency higher than 80%). In our specific case it means that in order to get 266 gr of fresh paint on wood we need to spray as a minimum 443 gr. (266:60)x100. Normally, the entire quantity of paint is never sprayed on the wood surface in one single layer, but it is divided in multiple layers. For this simple example (80 gr) we can assume application in two steps, two layers of 40 gr. Thus, the quantity to be sprayed on wood each layer is 221 gr (443:2). As we discussed above, in wood paints every ingredient present inside which is not “solid”, it means it does not become  part of the coating film,  will evaporate in the environment. We can now estimate in numbers what this means for  the two paint process we are analysing.  Here in numbers:

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