Solvent cleaning of articles

Abstract: Low molecular weight, fluorine-containing ethers of boiling point 20°C to 120°C are used in solvent cleaning applications.

Description: This invention relates to solvent cleaning applications in which contaminated articles such as, for example, metals, textiles, glass, plastics, electronic components and printed circuit boards are cleaned using a solvent or solvent vapour and more particularly to the use of low molecular weight ethers as solvents in solvent cleaning applications. Solvent cleaning applications wherein contaminated articles are immersed in or washed with halogenated hydrocarbon solvents and/or the vapours thereof are well known and are in common use. Applications involving several stages of immersion, rinsing and drying are common and it is well known to use the solvent at ambient temperature (often accompanied by ultrasonic agitation) or at an elevated temperature up to the boiling point of the solvent. Examples of solvents used in these cleaning processes are 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,1-trichloroethane, trichloroethylene, perchloroethylene and methylene chloride. These solvents are used alone or in mixtures with cosolvents such as aliphatic alcohols or other low molecular weight, polar additives and depending to some extent upon the articles to be cleaned are often stabilised against degradation induced by light, heat and the presence of metals. In the known common solvent cleaning applications and especially in those applications where the solvent is used at an elevated temperature, there is a tendency for solvent vapour to be lost from the cleaning system into the atmosphere. Further losses may occur in loading and unloading the solvents into cleaning plant and in recovering used solvent by distillation. Whilst care is usually exercised to minimize losses of solvent into the atmosphere, for instance by improved plant design and vapour recovery systems, the expense of totally preventing losses is exhorbitant and most practical cleaning applications result in some loss of solvent vapour into the atmosphere. Until recently, the use of the common cleaning solvents has been regarded as safe practice in that the solvents are stable, of low toxicity, non-flammable materials believed to be environmentally benign. However recent evidence suggests that some at least of the common solvents may have a long-term deleterious effect on the stratosphere, the so-called ozone layer, so that a replacement solvent is seen to be desirable. According to the invention there is provided the use in solvent cleaning applications of solvents comprising low molecular weight fluorine-containing ethers of boiling point in the range of 20°C to 120°C and containing at least 3 carbon atoms and further containing one or more hydrogen atoms. The ether has a boiling point in the range 20°C to 120°C, preferably from 25°C to 85°C, such that it may be used in conventional and existing cleaning equipment. For any particular cleaning application, an ether may be selected having a boiling point close to that of the solvent the ether is replacing. The ethers can be obtained by reaction of a halogenated aliphatic olefin with an optionally halogenated aliphatic alcohol in known manner and thus contain at least three carbon atoms in the molecule. Usually the ether will contain not more than five carbon atoms although it may contain six or more carbon atoms providing its boiling point is below 120°C. The ether contains at least one and will usually contain two or more fluorine atoms but will not be perfluorinated. In addition to fluorine atoms, the ether may contain chlorine atoms, bromine atoms and hydrogen atoms. Ethers containing chlorine and/or hydrogen may contain one or two chlorine atoms and/or one or two hydrogen atoms. Examples of alcohols which may be used to produce the ethers are methanol, ethanol, propanol and butanol and halogenated derivatives thereof. Alkenes which may be used include tetrafluoroethylene, hexafluoropropene, chlorotrifluoroethylene and the chlorofluoropropenes and hydrogen-containing analogues of these compounds for example trifluoroethylene and chlorodifluoroethylene. Examples of ethers which may be used, and their boiling points, include the following:- Boiling Point (°C) 1,1-difluoroethyl methyl ether 47 1,1,2,2-tetrafluoroethyl methyl ether 36.5 1-chloro-1,2,2-trifluoroethyl methyl ether 70.6 1-1-dichloro-2,2-difluoroethyl methyl ether 104.8 1-chloro-2,2-difluoroethyl methyl ether 27.5 1,1,1,2,3,3-hexafluoropropyl methyl ether 54.5 1,1-difluoroethyl ethyl ether 65 1,1,2,2-tetrafluoroethyl ethyl ether 56 1-chloro-1,2,2-trifluoroethyl ethyl ether 82 1,1,1,2,3,3-hexafluoropropyl ethyl ether 64.5 1,1,2,2-tetrafluoroethyl n-propyl ether 71.7 1-chloro-1,2,2-trifluoroethyl n-propyl ether 109 1,1,1,2,3,3-hexafluoropropyl n-propyl ether 92 1-chloro-1,2,2-trifluoroethyl isopropyl ether 100 1-chloro-2,2-difluoroethyl isopropyl ether 53 1,1,1,2,3,3-hexafluoropropyl isopropyl ether 76 1,1,2,2-tetrafluoroethyl n-butyl ether 49 1,1,1,2,3,3-hexafluoropropyl n-butyl ether 108 1,2,2-trifluoroethyl 1,1,1-trifluoroethyl ether 75 1,1,2,2-tetrafluoroethyl 1,1-difluoroethyl ether 77 di(1,1-difluoroethyl) ether 103 1-chloro-1,2,2-trifluoroethyl 1,1-difluoroethyl ether 102 1,1,2,2-tetrafluoroethyl 1,1-di(trifluoromethyl)methyl ether 85 Mixtures of ethers, including azeotropic mixtures, may be used if desired as may mixtures of an ether with one or more cosolvents. The same cosolvents may be used as are used with the principal solvents in known cleaning applications and in particular polar compounds such as alcohols are preferred cosolvents. Cleaning compositions comprising the ether and a cosolvent, notably a lower alkanol cosolvent, are used according to another embodiment of the invention. Azeotropic mixtures of ethers and alcohols represent preferred solvent compositions used in the process of the invention. Lower aliphatic alcohols containing 1 to 4 carbon atoms are useful in such mixtures. The ethers used according to the invention tend to be more stable than the commonly-used solvents and generally will not require stabilisation against degradation. However, stabilisers may be added if desired or if required for particularly onerous cleaning applications and the stabilisers used in the common solvents may be employed, notably nitroalkanes and epoxides. The ethers may be used as replacements for the solvent(s) used in any of the known cleaning applications and have the advantage of being generally more stable towards aluminium than the solvents they replace. The ethers may be used to replace part of the solvent(s) used in known cleaning applications. The invention is illustrated by the following examples. EXAMPLE 1 This Example illustrates the use of 2-chloro-1,1,2-trifluoroethyl methyl ether in cleaning flux residues from copper-coated boards. A known weight of solder cream was applied to test boards (5 cm x 7 cm) cut from copper-coated FR4 (epoxy/glass fibre laminate) board and the cream was reflowed in a MICRO™ VPS soldering unit. The solder cream used was a 62% tin/38% lead solder available as Multicore™ PRAB 3. 2-chloro-1,1,2-trifluoroethyl methyl ether (boiling point 65°C at 839.9 x 10 2 Nm -2 (630mm Hg)) was boiled in a beaker fitted with an upper cooling coil through which cold water was circulated to create a boiling liquid phase and a vapour phase and the contaminated board was dipped into the boiling liquid for 60 seconds and then held in the vapour for 30 seconds. Residual ionic contamination of the test board, expressed as mg sodium chloride per square centimetre was determined using a Protonique Contaminometer. The ionic contamination of an unwashed test board was determined and the % removal of ionic contamination was calculated. 61% of the ionic flux residues were removed from the test boards. The ether was heated to 190°C and the vapour pressures above the ether were determined over the range 50 - 190°C. A slight increase in vapour pressure was observed at approximately 120°C but there was no visible evidence of solvent breakdown at this temperature. In a Comparative Test, using 1,1,2-trichloro-1,2,2-trifluoroethane as the solvent, 45% of the ionic flux residues were removed. EXAMPLE 2-3 These Examples illustrate the use of mixtures of 2-chloro-1,1,2-trifluoroethyl methyl ether and methanol for cleaning flux residues from copper coated printed circuit boards. In Example 2, a mixture of 2-chloro-1,1,2-trifluoroethyl methyl ether and methanol was boiled until a constant boiling mixture was obtained. This azeotrope contained 18.5% by weight of methanol and boiled at 56.8°C at normal pressure. The azeotropic mixture was used to remove ionic residues from the test boards as described in Example 1. 66.9% of the ionic resudes were removed. In Example 3, the procedure of Example 2 was repeated except that a mixture of the ether (95% by weight) and methanol (5% by weight) was used instead of the azeotropic mixture. 65.1% of the ionic residues were removed. EXAMPLE 4 This Example demonstrates the stability of 2-chloro-1,1,2- trifluoroethyl methyl ether in the presence of aluminium. The ether was refluxed in contact with aluminium for 48 hours. The aluminium test piece was partly immersed in the liquid and partly in the vapour above the liquid. In the test, no increase in chloride ion or fluoride ion was observed in the liquid phase and the GC trace of the solvent after the test showed no change. There was no significant weight change in the metal test piece which emerged from the test clean and bright with no evidence of corrosion. The results demonstrate that the ether has high stability in the presence of aluminium and is suitable for use in aluminium cleaning applications. Stabilisers may be added to inhibit the build up of acidity in the ether when it is used to clean metals. EXAMPLE 5 This Example illustrates the use of tetrafluoroethyl methyl ether in cleaning flux residues from copper-coated boards. Tetrafluoroethyl methyl ether, boiling point 33-35°C (839.9 x 10 2 Nm -2 ) (630 mm Hg) and density (25°C) 1.28g/ml, was used to clean flux residues from copper coated boards as described in Example 1. 62% of the ionic flux residues were removed. EXAMPLE 6-8 These Examples illustrate certain azeotropic mixtures suitable for use in the process according to the present invention. Tetrafluoroethyl methyl ether forms an azeotrope with methanol containing 4% by weight methanol and boiling at 34.5°C. The ether forms an azeotrope with 1,1,2-trichloro-1,2,2 -trifluoroethane containing 39.5% by weight of the haloethane and boiling at 34.9°C. The ether forms a ternary azeotrope with 1,1,2-trichloro- 1,2,2-trifluoroethane and methanol containing 41% by weight of the haloethane and 3% by weight of methanol and boiling at 34.5°C. EXAMPLE 9 This Example illustrates the use of a ternary azeotropic mixture in the process according to the present invention. The ternary azeotropic mixture of tetrafluoroethyl methyl ether, 1,1,2-trichloro-1,2,2-trifluoroethane and methanol prepared in Example 8 was used to remove solder flux residues from circuit boards by the procedure described in Example 1. 48.2% of the ionic flux residues were removed. EXAMPLE 10-11 These Examples illustrate further azeotropic mixtures for use in the process according to the present invention. Tetrafluoroethyl ethyl ether, boiling point 56°C and density 1.21 g/ml, forms an azeotrope with methanol containing 10.6% by weight of methanol and boiling at 48.6°C. The ether forms an azeotrope with ethanol containing 38.5% by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and boiling at 46.3°C.

Claims: A process for cleaning contaminated articles which comprises contacting the contaminated articles with a solvent composition comprising a low molecular weight fluorine-containing ether having a boiling point in the range of 20°C to 120°C and containing at least 3 carbon atoms and further containing one or more hydrogen atoms, or the vapour thereof or both. A process according to claim 1 wherein said low molecular weight fluorine-containing ether contains not more than 5 carbon atoms. A process according to any of claims 1 or 2 wherein said solvent composition further contains a cosolvent. A process according to claim 3 wherein said cosolvent is a polar compound. A process according to claim 4 wherein said cosolvent is an alcohol. A process according to claim 5 wherein said cosolvent is a lower aliphatic alkanol having 1 to 4 carbon atoms. A process according to claim 1 wherein said solvent composition is in the form of an azeotrope, said azeotrope being selected from the group consisting of a binary azeotrope of tetrafluoroethyl methyl ether and methanol, a binary azeotrope of tetrafluoroethyl methyl ether and 1,1,2-trichloro-1,2,2-trifluoroethane, a ternary azeotrope of tetrafluoroethyl methyl ether, methanol and 1,1,2-trichloro-1,2,2-trifluoroethane, a binary azeotrope of tetrafluoroethyl ethyl ether and methanol and a binary azeotrope of tetrafluoroethyl ethyl ether and 1,1,2-trichloro-1,2,2-trichloroethane. A process according to claim 1 wherein said solvent composition comprises a mixture of said low molecular weight fluorine-containing ether. A process according to claim 1 wherein said contaminated articles are printed circuit boards contaminated with flux residue. A process according to any of the previous claims wherein said solvent composition further includes a stabiliser. A process according to claim 10 wherein said stabiliser is an epoxide or a nitroalkane. A process according to claim 1 wherein said low molecular weight fluorine-containing ether is obtainable from the reaction of a halogenated aliphatic olefin with an optionally halogenated aliphatic alcohol. A process according to claim 12 wherein said optionally halogenated aliphatic alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol and halogenated derivatives of these compounds. A process according to claim 12 or 13 wherein said halogenated aliphatic olefin is selected from the group consisting of tetrafluoroethylene, hexafluoropropene, chlorotrifluoroethylene, chlorofluoropropenes and hydrogen-containing analogues of these compounds.

Inventors: MCBETH DAVID GEORGE, WINTERTON NEIL, MCBETH, DAVID GEORGE, WINTERTON, NEIL
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1991
Main classification: C11D7
IPC codes: B08B3, C23G5, C11D7