Cleaning and liquid contact with solids – Processes – Paints – varnishes – lacquers – or enamels – removal
Reexamination Certificate
1999-04-19
2002-02-19
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Paints, varnishes, lacquers, or enamels, removal
C134S026000, C134S028000, C134S029000, C134S004000, C510S201000, C510S202000, C510S206000, C510S207000
Reexamination Certificate
active
06348107
ABSTRACT:
This is a national stage application of PCT/GB97/02917 filed Oct. 22, 1997.
BACKGROUND OF THE INVENTION
The present invention relates to a method of stripping paint from a painted substrate, especially a metal substrate. The stripper comprises an aqueous phase and an organic phase. A wide range of organic solvents have been used in paint stripper formulations. The most successful solvents are chlorinated hydrocarbons; especially methylene chloride. For environmental reasons, however, it is desirable for the levels of organic solvents, especially chlorinated solvents, to be reduced. The present invention allows reduced levels of chlorinated solvents to be used or even allows the use of such solvents to be avoided all together.
It is known to use water as an activator compound in organic solvent based paint strippers. Such compositions tend to have low levels of water, for instance less than 10%.
Other paint stripper compositions comprising both water and organic solvents which are immiscible with water have been described. Generally emulsifiers are added in quantities such that an emulsion of one phase in the other phase is formed. Such emulsions are storage stable and are required not to separate before use. Alternatively cosolvents may be added to create a homogeneous single phase system. Some examples are described for instance in DE-A-3325166, GB 1114220, EP-A-0611214 and US-A-4619706.
SUMMARY OF THE INVENTION-DETAILED DESCRIPTION OF THE INVENTION
We have now surprisingly found that paint stripper compositions comprising immiscible (i.e. only partially soluble) organic solvent and water, but which are not in the form of an emulsion, can be used as paint strippers with very useful properties.
In a new method of stripping paint from a painted substrate surface according to the invention, the surface is contacted with the aqueous phase of a two-phase liquid stripper composition comprising an aqueous phase which comprises water, dissolved water-soluble activator compound and an organic solvent which has a solubility in water in the range 0.1 to 10% by weight at 20° C. the organic solvent being present in the aqueous phase at a concentration of about the saturation level (as at the ambient conditions for the method) and comprising also an organic phase comprising the said organic solvent, which organic phase is in interfacial contact with the aqueous phase.
The inventors believe that it is the aqueous phase of the stripper composition which provides the primary stripping function, that is which attacks the paint on the substrate surface, thereby enabling its removal. It has been found that some thermal or mechanical agitation may enhance stripping. This enhancement may be due to provision of some solvent contact with the substrate. The solvent phase primarily provides a reservoir for dissolved solvent in the aqueous phase. In view of the continuous interfacial contact, as solvent is dragged out of the aqueous phase by contact with paint surfaces, it is replenished from the solvent phase. The solvent thereby remains dissolved in the aqueous phase at saturation level.
It is preferred for organic solvents to be used which have a solubility in water at 20°C. in the range 0.5 to 6.0% by weight. The level of dissolved solvent in the aqueous phase during the paint removal method may be higher than the end of that range, since it is possible for the methods to be carried out under conditions at which the solubility of the solvent in water is higher. Thus where the method is carried out at raised temperatures, the level of organic solvent in the aqueous phase may be at least 50% higher than the saturation level at 20° C. for instance at least 100% higher or more.
The organic solvent may include chlorinated hydrocarbons, for instance in combination with other organic solvents. Thus the invention allows lower levels of chlorinated solvents to be used when a combination of such solvent and another organic solvent with which the chlorinated solvent is miscible is used. Preferably, however, the stripper composition is free of chlorinated organic solvents.
Suitable organic solvents are, for instance, benzyl alcohol, methylene chloride, dibasic ester, which is an oxygenated solvent composed of a mixture of methyldiesters of glutaric, succinic and adipic acids, or ethyl-3-ethoxy propionate. The solubility of these solvents in water are benzyl alcohol 3.3%, methylene chloride 1.32%, dibasic ester 5.6% and ethyl ethoxypropionate 2.9%.
The dissolved activator compound present in the aqueous phase of the composition used in the invention may be any compound which increases the paint stripper activity. Such compounds may be selected for the specific paint which is to be removed and/or the substrate from which the paint is to be removed, in order to provide optimum paint removal characteristics and/or minimise damage to the underlying substrate. The compound may often be a pH modifying compound. Such pH modifiers may be acids and considerable benefits have been observed with such acid activated systems. Acid activators are usually organic acids, for instance trichloroacetic, alkyl benzene sulphonic acid, benzoic or lactic acid. Preferably a pH modifying compound is an alkali. Suitable alkalis are, for instance, alkali metal hydroxides, alkaline earth metal hydroxides, silicates, for instance of alkali metals and organic basic compounds such as amines or ammonium compounds.
Another category of activator compound is an oxidising agent. Such oxidising agents are found to improve the performance of the aqueous phase. One example of an oxidising agent is hydrogen peroxide. Another example is permanganate, usually potassium permanganate.
The composition may, in addition to one of the above mentioned activator compounds, contain a secondary activator, such as an alcohol, for instance a C
1-4
alcohol, preferably methanol.
The paint stripper composition may optionally contain surfactants, generally in low amounts such that the surfactant does not lead to the formation of an emulsion between the immiscible aqueous and organic phases. The inclusion of such surfactants may improve wetting of the substrate surface. Suitable surfactants are, for instance, alkyl benzene sulphonic acids, and fluorosurfactants. The surfactant is, generally, present in an amount of less than 2% by weight, more preferably less than 1% by weight, based on the total stripper composition weight.
Where the activator compound is a pH modifier, the pH is preferably above 9, more preferably above 10, or below 5, preferably below 4.
The present invention is of particular value where the stripping method uses an immersion technique, in which the article from which paint is to be stripped is dipped into a bath containing the stripper composition. In such embodiments, the stripper composition is generally present as a two-phase system with top and bottom continuous layers. Preferably the bottom layer is the organic solvent layer, that is the organic solvent preferably has a higher density than water. In this embodiment, the article from which paint is to be stripped is immersed in the top aqueous phase only and not into the bottom organic phase. After immersion for a suitable period of time, the article is removed from the bath.
The bath is reused for stripping paint from further articles. Where the bath is reused, it is generally necessary to replenish the bath to maintain an appropriate mixture of ingredients in the top aqueous phase. It is found that both solvent and water are dragged out of the bath whilst in addition the activator compound and solvent are consumed. Replenishment of the bath is straightforward in the invention which is convenient for the user. Thus since the bottom organic phase provides a reservoir of solvent for dissolution into the top aqueous phase, as the volume of the top phase becomes lower, it can be replenished merely by adding water. The aqueous phase maintains its saturation level of solvent since solvent can pass from the organic phase into the aqueous phase across the interfacial layer. The level of activator co
Braley Joanna May
Lawlor Angela Mary
Whitton Colin Alfred
Andrus Sceales Starke & Sawall LLP
Chemetall PLC
El-Arini Zeinab
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