Water-based Paint Strippers - How they Work

 





Water Based Paint Strippers - How they Work
 
Paint strippers are the generic name given to chemical blends designed to remove paint and other finishes from a variety of substrates such as wood, metal, brick, concrete etc. Paint strippers can also be used for cleaning paint application tools such as brushes, rollers, spray guns.

Traditional paint strippers have primarily been based upon various solvent mixtures, the principal one being Dichloromethane. These products will strip a wide variety of paint types and generally have little impact upon the substrate over which the paint lies. Their biggest disadvantage lies in their toxicity. Dichloromethane itself is in the process of being severely restricted throughout the EU because of its inherent toxicity.

Over the last few years a lot of effort has gone into substituting “safe” solvents such as Dibasic esters and Dimethyl Sulfoxide for Dichloromethane with various levels of success. However, in general these materials are derived from petroleum, and hence are not renewable nor have a carbon neutral footprint.

The principle of water based paint stripping is entirely different to solvent based strippers. Water based strippers rely on an organic (solvent) component to penetrate the paint layer, and carry with it the water which normally carries the active stripping agent. When the water reaches the interface between paint and substrate, the active ingredient in the water breaks the bond between paint and substrate and the paint falls off. In general the paint is not affected by the stripper and simply falls off in large sheets.

This action of water based strippers has several advantages:

• the paint is not generally affected, hence harmful materials contained in the paint will not be released into the environment. This also means that the paint sludge is generally non hazardous
• since the stripper does not attack the paint, it is not depleted in the stripping process. This is particularly advantageous in a stripping bath, where very long bath lives can be achieved, dramatically improving process economics
• The paint substrate is generally much more impervious to water than it is to solvent and so very little penetration of the stripper into the substrate occurs. This makes cleaning the substrate after stripping often easier and quicker than if a solvent stripper is used

Other Components

The primary active ingredient in a water based stripper is contained in the water. It is generally an inorganic material that can work in several ways:

• At the paint/substrate interface it releases a gas which mechanically breaks the bond between the paint and substrate. Typical examples might be the release of carbon dioxide or oxygen e.g. by including ammonium carbonate or hydrogen peroxide in the formulations
• The bond between paint and substrate is chemically destroyed e.g. by a weak acid acting on a metal substrate, or by causing the substrate to preferentially bond to the active ingredient rather than the paint
• The paint at the interface can be hydrolysed and broken down thus reducing its ability to bond to the substrate

In general a water based stripper may use one or a combination of all these methods to break the paint/substrate bond.
Water based strippers are also much more sensitive to the level of acidity/alkalinity of the stripper – the pH. Additives are usually used to ensure the pH is maintained at its optimum value during use. These may include various buffers, such as sodium citrate, phosphates of various types etc.

In general water based strippers are not so sensitive to different paint types as solvent based ones. They do rely on an organic carrier to carry the stripper through the paint, but its purpose is only as a carrier. Examples of such might be benzyl alcohol, triethyl phosphate, dibasic esters.

In general the solvent components are not soluble in water, so the stripper must include some mechanism holding the solvent and water based components together. These can include a range of what are called co-solvents, such as various alcohols, glycols and esters. An alternative way to hold the water and solvent components together is to create emulsions using for example hydroxymethyl cellulose, or natural gums such as xanthan gum.

Surfactants are usually added to the formulations to increase wetting of the paint surface. This is particularly important for water based strippers to aid penetration of the paint. Anionic surfactants such as dodecyl benzene sulphonate or sodium xylene sulphonate are often used in formulations that are acidic, and non ionic surfactants such as alcohol ethoxylates or tall oil ethoxylates in formulations that are neutral or alkaline. In general the more surfactant in the formulation, the easier the stripper is to wash off, and the more suitable it is for use as a brush cleaner.

Strippers designed for use on vertical surfaces are generally thickened using e.g. cellulose based thickening agents such as hydoxypropyl or hydroxymethyl cellulose. However, these are not suitable for use in highly acidic or alkaline formulations, as they readily hydrolyse. In these instances natural gums such as xanthan gum, or inorganic clays such as fused silicaceous clays can be used.

In general water based strippers do not suffer from evaporation of the active ingredients to the same extent as solvent based strippers, and so do not generally need the addition of materials such as waxes to reduce evaporation.

Water based strippers suffer more from corrosion issues, especially when stripping steel, primarily because of the water in the stripper, and especially if the stripper is designed to work at low pH i.e. is acidic. These problems are ameliorated by inclusion of various corrosion inhibitors. Acidic strippers often use substituted thioureas, whilst alkaline ones can use amines or silicates.

Sometimes chelating agents are used in the stripper to counteract the effect dissolved metal ions can have both on stripping rates and stripper stability. Common chelating agents include EDTA, glutamates, phosphates.

Finally water based stripper formulations may include colourants and perfumes, the former often to give the stripper the look of being “environmentally friendly” and the latter to increase user acceptance.

Alternatives

Solvent based strippers are a chemical alternative. Heat guns, and various abrasive systems such as shot blasting are a viable mechanical alternative.

External Links

**To view the full, complete range of Biostrip's  water-based products, visit our website:  http://www.biostrip.co.uk