University of Limerick
College of Engineering
Department of Material Science and Technology
Wood Technology
The comparison of water based finishes with reactive finishes with respect to properties, uses and way of application
Limerick, November 2007
Content
1Introduction. 1
2Curing. 1
2.1Curing of water-based finishes. 1
2.2Curing of reactive finishes. 2
3The emission of volatile organic compounds (VOCs)3
4Properties, ways of application, and uses. 3
4.1Properties of water-based finishes. 3
4.2Properties of reactive finishes. 5
4.2.1Properties of conversion finishes. 5
4.2.2Properties of Varnishes (including polyurethane)5
5Conclusion. 6
References. 7
Table of figures
Fig. 1 Cured coalescing finish. 2
Fig. 2 Reactive finish. 2
1 Introduction
Wood finishes are applied on the wood surface for different reasons. Firstly, finishes keep the wood clean, finishes seal the porous surface, making it less sensitive to soiling and easier to sanitise. Secondly, coatings stabilise the wood by reducing the water-vapour exchange. Furthermore, wood finishes are used as decoration, providing the wood surface a particular appearance (Flexner 1999).
According to utilisation (e.g. staircases) and required properties (e.g. high scuff-resistance) of the completed wood product different finishes can be chosen for application. The range of wood finishes is very high. However, this work will only focus on two important finishes in the forest products industry: water-based and reactive finishes.
A water-based finish is a solvent-based finish, usually acrylic or polyurethane, that is dispersed in water. The acrylic and polyurethane are manufactured in tiny cured droplets, which are than dispersed in water. On the other hand reactive finishes (e.g. varnish, oil/varnish blend etc.) comprises thinners which are mineral spirits and naphtha (Flexner 1999).
This essay will compare water-based finishes with reactive finishes with respect to properties, ways of application and uses. In addition, it will analyse the emission of volatile organic compounds (VOCs) of these finishes.
2 Curing
2.1 Curing of water-based finishes
Most of the used water-based finishes are coalescing finishes which are made up of tiny dispersions (or droplets) of a cured reactive finish emulsified in water (thinner) and a slow evaporating solvent (usually glycol ether). After applying, water evaporates, the droplets pack together and the glycol ether solvent softens the outer molecules of the tiny dispersions, causing them to extend outward and become interlocked with the outer molecules of other droplets.
The droplets are held together (see Fig.1). However, they are not crosslinked and therefore in the case of contact of the cured coat with solvents, like alcohol or lacquer thinner, the finish reconverts into solution (Flexner 1999).
Flexner (1999) adds that each of the droplets of water-based finishes is resistant to scratching, redissolving and penetration, but in the finished surface the droplets join together like evaporative finishes, without crosslinking. Crosslinking makes a film far more resistant to heat, scratches, solvents, acids, and alkalis, and less penetrable.
Furthermore crosslinked molecules are difficult to break apart and there is less opportunity of water and water vapour penetration. For this reasons water-based finishes can easily be softened or dissolved by heat and a number of solvents and chemicals. And, in addition, they can be penetrated by liquids and gases.
Another type of water-based finishes is the two component water-based finish. These finishes contain a special composition of film forming agents. Curing occurs due to crosslinking reactions of the master batch and hardener, which is based on isocyanates, as well as due to reactions of the hardener with water. Two component water-based film finishes have an improved resistance, especially mechanical, and they are applicable for durable surfaces (e.g. floor coverings and kitchen fronts) (DFIU 2002).
Fig. 1 Cured coalescing finish
Source: Flexner (1999)
2.2 Curing of reactive finishes
Reactive finishes like linseed oil, oil/varnish blend, wiping varnish, varnish, polyurethane etc. comprises thinners which are mineral spirits and naphtha. When they cure they change chemically. After application the thinner evaporates, the resin molecules come closer together and a chemical reaction occurs, they crosslink. Furthermore, this strong connection can not be broken down by reapplying the thinner.
This type of chemical reaction is often referred as crosslinking or polymerisation (see Fig. 2) (Flexner 1999).
Reactive finishes fall into two categories:
· Catalyst curing finishes or conversion finishes cure by reacting when a chemical catalyst is introduced. Conversion finishes include conversion varnish, catalysed lacquer, epoxy finish, moisture curing polyurethane, two-part polyurethane, polyester finish and ultraviolet-curing finish.
· Finishes that cure by reacting with oxygen e.g. varnish (Linseed oil and tung oil also cure by reacting with oxygen, but they are penetrating, not film finishes).
The problem is that a thin skin is developed by curing the top of the finish and this barrier inhibits the oxygen to penetrate into the finish and slows down curing For this reason the applied coats should be keep thin (Flexner 1999).
Fig. 2 Reactive finish
Source: Flexner (1999)
3 The emission of volatile organic compounds (VOCs)
The emission of volatile organic compounds (VOCs) e.g. caused by solvent-based wood finishes to the atmosphere has become an environmentally issue (Giddings et al. 1991). VOCs are organic compounds that readily evaporate, and they contain pure hydrocarbons, partially oxidised hydrocarbons and organic compounds containing chlorine, sulphur, or nitrogen.
VOCs emissions contribute to air toxicity, urban smog, and Earth’s greenhouse effect and so they may be a cause of global warming. They are widely used as fuels (e.g. gasoline), in the production of plastics and as paint thinners and solvents (Encyclopaedia Britannica 2007). The emitted VOCs react under the pressure of solar radiation photochemically with nitrogen oxides in the atmosphere.
For this reason the use of solvent-based finishes is becoming more and more restricted, and therefore coat manufactures are trying to tackle this problem by reducing the solvent content of traditional products or by creating new alternative environmentally sound finishes like water-based finishes. Giddings et al. (1991) adds that water-based finishes have water instead of an organic compound as the principle solvent or diluent.
And the advantages of these finishes are the reduced levels of air pollution and the lower toxicity and flammability hazards. Furthermore smaller expenditures are necessary for the storage and drying of water-based finishes, compared to conventional materials (DFIU 2002). Culix points out:
(Culix cited in DFIU 2002)
4 Properties, ways of application, and uses
4.1 Properties of water-based finishes
Water-based finishes are very tough nonyellowing finishes with a high scuff resistance. The very little solvent content of water-based finishes seems to be the most important advantage of these finishes. Flexner (1999) summarizes the pros of the low solvent content following:
There is therefore less solvent to evaporate into the atmosphere and cause pollution, less solvent to fuel and fire, and less solvent to breath. ( .). Water-based finishes do not contain enough solvent to burn in their liquid state, and they are much less toxic than varnish, lacquer, or conversion finish.
In addition, water-based finishes are very easy to cleanup. For example, after applying and before curing of the finish, a brush or spray gun is easy to clean by simply washing the equipment in water. Even the finish has cured solvents like lacquer thinner, acetone, tolvene, and xylene can be used to clean the application tools. The disadvantages of these finishes are that they are less resistance to heat, solvents, acids, or alkalis, and that they provide only a weak barrier against water penetration and water-vapour exchange, compared to polyurethane (Flexner 1999).
Giddings et al. (1991) adds that the main disadvantages of water-based finishes are the wetting out problem, the raising of the grain, the greater sensitivity to contamination and to variations in humidity and temperature, and the potential pollution of water courses if spilled.
Flexner (1999) sums up that the hardener increases the cured film´s resistance to heat, solvents, acids, alkalis, and water and water-vapourexchange. The problem of two component water-based finishes is that the hardener tends to be very toxic, negating one of the primary rationales for using water base in the first place.
Uses of water-based finishes
As a stain:
· wipe-on positive stain for softwood varieties
· Water-thinnable wax stain for polished, brushed, chip-carved, roughed, or planed softwood varieties
· Water-thinnable softwood stain specially designed for polished, brushed, chip-carved, roughed, or planed softwood varieties and providing a distinctive, positive stain finish
· Etc.
As a film finish for:
· Nearly all types of furniture application (e.g. kitchen, stool, table etc., one or two component).
· Windows (intermediate coating , thick-coat glace finish, top coat)
· Floor or parquet sealant (one or two component)
· Exterior wood construction (wood preservative coating, glaze finish etc.)
· Etc. (Adler 2003)
Two component water-based finishes have an improved mechanical resistance, therefore they are primarily used for floor coverings or kitchen fronts (DFIU 2002).
Application of water-based finishes
Water-based finishes can be applied like other finishes by brush, spray gun, or cloth (Flexner 1999). The main problem of applying water-based coatings seems to be the water itself. Jewitt (2000) argues that water-based finishes tend to raise the grain of the surface. Methods to overcome that problem are the use of a sealer coat of dewaxed shellac, which prevents the next coat of water-based finish from raising, sponging the wood with distilled water to preraise the grain and the application of one or two light coats of sanding sealer, sanding between coats when dry (Jewitt 2000).
The curing time of water-based finishes varies much more in different weather conditions than the curing time of other finishes, because water is far more weather-sensitive than solvents (Flexner 1999). Jewitt (2000) confirms that water-based finishes are the most temperature sensitive of all finishes and the application in hot, cold or humid conditions can be problematic.
Water-based finishes do not flow out and cover the surface as mineral spirits or oil would, because water or water-based coatings have a high surface tension compared to solvent-based finishes. Therefore, substances which are reducing the surface tension, like soap or ammonia can be added to the finish to improve the flow-out of water (Flexner 1999).
The water-based finish takes several weeks to achieve the maximum bond after applying. When there is only a short time between the applications of different coats, no sanding is required, because the solvent softens the outer molecules or the droplets in the existing coat, so they interlock with the outer molecules of droplets in the new coat. However, after a day, the solvent in the new coat may not adequately soften the existing coat, for this reason the surface should be scratched with sandpaper or synthetic steel wool in order to achieve a mechanical bond (Flexner 1999).
4.2 Properties of reactive finishes
Because of the crosslinking during the curing process, reactive finishes are far more resistant to heat, scratches, solvents, acids and alkalis and less penetrable. The strong bonds produce a more durable film. On the other hand, the finish is difficult to repair, rub out, or strip and only a few solvents are able to dissolve the finish for removal (Flexner 1999).
The problem is that conversion finishes are non-decorative simple coatings and are, compared to nitrocellulose lacquer, difficult to apply. The disadvantages of conversion finishes are the short pot life, the limited application time, the poor rubbing properties, the extremely irritating formaldehyde emissions while curing and the high use of toxic, flammable, air-polluting solvents.
Because of their high solvent content the use of conversion lacquers has become restricted (Flexner 1999).
Uses of conversion finishes
· Heavy-duty furniture (school furniture, sanitary furniture etc)
· Indoor stairs and staircases
· High gloss furniture
· Etc.(Adler 2003)
The cured finish in for example a spray gung can not be removed by solvents, therefore the applying equipment can only be cleaned by abrading the finish off and this process may ruin the gun (Flexner 1999).
4.2.2 Properties of Varnishes (including polyurethane)
Varnish (including polyurethane) is the most durable of the commonly available finishes. The finishes have an excellent resistant to heat, wear, solvents, acids, and alkalis and they provide a barrier against water and water-vapour penetration. And furthermore, they are cheap. The disadvantages of varnishes are the difficult application and the yellowing over time (Flexner 1999).
Uses of varnishes
· Standard furniture (often used by amateurs)(Adler 2003).