Method of Cleaning Epoxy Articles

This invention relates to a method of cleaning articles that contain an epoxy resin. In particular, it relates to a method of removing undesirable synthetic and natural resins from those articles without dissolving the epoxy resin.

In the electronics industry, circuit boards are used that are made from epoxy resins. When components are mounted on the boards, rosins used as solder fluxes contaminate the surfaces of the board. Other synthetic and natural resins are used as adhesives, and they also end up as undesirable substances on the board. In addition, the board can be contaminated by human contact and by particles and gases from the air. In order to preserve the electrical properties of the board, it is necessary to remove these contaminants without damaging the board itself. This is accomplished by cleaning the board with a solvent that readily dissolves most synthetic and natural resins, but does not dissolve or attack the epoxy resin from which the board is constructed.

Until recently, trichlorotrifluoroethane was used for this purpose because it is a good solvent for most resins but does not attack epoxy resins. However, trichlorotrifluoroethane persists in the atmosphere and has been implicated in the depletion of the ozone layer. For this reason, its use has been restricted and it may be prohibited entirely within a few years. Efforts are being made to identify other solvents with the same desirable properties but which will not affect the ozone layer.

I have discovered that certain chlorinated benzotrifluoride compounds are, like trichlorotrifluoroethane, good solvents for many natural and synthetic resins, yet are very poor solvents for epoxies. This was an unexpected discovery because aromatic compounds are usually very good solvents and chlorinated aromatic compounds are especially good solvents. Thus, it was surprising to find that these compounds do not dissolve epoxy resins.

However, unlike trichlorotrifluoroethane, the chlorinated benzotrifluoride compounds of this invention are not expected to attack the ozone layer. Because about two years are required for a solvent released on the ground to reach the ozone layer in the stratosphere, and the solvents of this invention have an atmospheric life in the troposphere of only about sixty days, they should not be ozone depleters.

The solvents used in the process of this invention have the general formula: ##STR2## where n is 1 or 2. Monochlorobenzotrifluoride compounds (n=1) are preferred for applications requiring high solvent volatility, and dichlorobenzotrifluoride compounds (n=2) are preferred for applications requiring a high flash point. A high volatility is desirable when it is desirable to rapidly evaporate the solvent after it is used. The flash point is the temperature above which combustion over a liquid can occur; a high flash point is desirable in applications where flammability may be a problem.

Of the three monochlorobenzotrifluoride isomers, ortho chlorobenzotrifluoride and parachlorobenzotrifluoride (PCBTF) are preferred because the ortho isomer can be easily made from orthochlorotoluene and the para isomer is already commercially available. All the isomers can be obtained by chlorinating benzotrifluoride and separating them from the resulting mixture. Of the seven dichlorobenzotrifluoride (DCBTF) isomers, 3,4-dichlorobenzotrifluoride (3,4-DCBTF) is preferred because it is already a commercial product and 2,5-dichlorobenzotrifluoride is preferred because it can readily be obtained by chlorinating orthochlorobenzotrifluoride. The other dichlorobenzotrifluoride isomers can be obtained by chlorinating benzotrifluoride or a monochlorobenzotrifluoride.

The solvents can be used in a pure form or they can be mixed with various optional components to preserve or enhance their properties. For example, about 0.01 to about 1.0 wt % (based on total composition weight) of a stabilizer can be included with the solvent to trap decomposition products that may catalyze the decomposition of the solvent. Stabilizers include compounds such as epoxides, glycidyl ethers, and diepoxides. The preferred stabilizer is allyl glycidyl ether because it is available and its boiling point is close to the boiling point of the solvents.

About 0.0005 to about 0.1 wt % of an antioxidant can be included in the solvent to act as a free radical inhibitor by scavenging free radicals such as monoatomic oxygen. Examples of antioxidants include hindered phenols such as thymol, hydroquinone monomethyl ether, and tertiary amyl phenol. Other examples of antioxidants include nitrogen ring compounds such as N-methyl pyrolle. Hindered phenols are preferred due to their low cost and availability.

About 0.001 to about 0.1 wt % of a corrosion inhibitor can be included in the solvent to prevent it from attacking various metals with which it comes into contact. Examples of corrosion inhibitors include acetates, glycol ethers, imidazoles, and amines, such as ethyl morpholine and methyl morpholine. The preferred corrosion inhibitor is ethyl morpholine because of its good performance and availability.

The addition of about 0.5 to about 10.0 wt % of an oxygenated solvent such as an alcohol, ketone, glycol or glycol ether to the chlorobenzotrifluoride solvent may be desirable to improve the cleaning of ionic contaminants from solder fluxes. Examples of useful oxygenated solvents include amyl alcohol, butyl alcohol, hexyl alcohol, amyl acetate, butyl acetate, propylene glycol mono methyl ether, propylene glycol ether acetate, and/or propylene glycol methyl ether acetate. The selection of a specific oxygenated solvent is based on cleaning performance, recoverability with the chlorobenzotrifluoride solvent, and cost.

The chlorobenzotrifluorides can also be combined with water and an anionic surfactant to form an emulsion or a semi-aqueous cleaner. These cleaners offer solvent power for a specific cleaning job, but are less flammable and less expensive because they contain water. Useful surfactants include a C.sub.10 to C.sub.14 sulfated or sulfonated anionic surfactant such as sodium dodecyl benzene sulfonate, ammonium salts of a sulfonated C.sub.10 to C.sub.14 alcohol-ethylene oxide condensation product, and the ammonium salt of the sulfated condensation product of ethylene oxide and dodecanol. A preferred composition is about 0.5 to about 10 wt % of the chlorobenzotrifluoride, about 0.5 to about 10 wt % anionic surfactant, and the balance water. See U.S. Pat. No. 3,835,070, herein incorporated by reference, for additional details.

Any article that contains an epoxy resin and is contaminated with a synthetic or natural resin can be cleaned using the process of this invention. Such articles include printed circuit boards, magnetic tape, coaxial wire, copier equipment, office machines, electronic components and equipment, glass products, instruments and meters, jewelry, metals, medical equipment, and plastic and rubber products.

In the cleaning process of this invention, the contaminated article is contacted or washed with the chlorinated benzotrifluoride solvent. Washing is typically done for about 30 to about 60 seconds at room temperature. One part can be washed by itself or many parts can be washed together. The amount of solvent used depends upon the shape of the parts and the number of parts and the type and amount of contaminant. The contaminants dissolve in the solvent and the solution of the contaminants, carrying entrained soil particles, is separated from the article. Recovery of the solvent from the solution is accomplished by evaporating the solvent and condensing its vapors. In practice, an article to be cleaned is washed in stages with the used solvent from the later stages being reused in the earlier stages. The solvent used in the earliest stage is recovered and is recycled back to the last stage. Because the solvents of this invention are expensive and are combustible, they are preferably used in a closed system and are recovered after use. Such a closed system would include a treatment chamber where the parts to be cleaned are washed with the solvent, and a separate solvent recovery chamber where the solvent is evaporated to recover it. Evaporation can be accomplished by heating, such as by infrared light, or by vacuum, or a combination of heat and vacuum. The solvent vapors are passed to a condensor where the solvent is condensed back to a liquid. The cleaning of articles can be accomplished in either a batch or a continuous process.

The following example further illustrates this invention.

Using ASTM Test D1545-89, the solubility of various resins in a variety of solvents was determined. The kinematic viscosity (in centipoises) was also determined at the solubility limit. The resins tested were those commonly used as industrial adhesives. The solvents tested were PCBTF and 3,4-DCBTF, which are within the scope of this invention, and a blend of 20 vol % PCBTF--50 vol % orthochlorotoluene (OCT)--30 vol % perchloroethylene (PCE) ("20--50--30"), a blend of 50 vol % PCBTF--50 vol % OCT ("50--50--0"), a blend of 30 vol % PCBTF--20 vol % OCT--50 vol % perchloroethylene ("30--20--50"), and OCT. The following table gives the wt % solubility (the number in parenthesis is the viscosity).