Abstract:
Method for applying an aqueous benzyl alcohol paint strippling composition to the surface of a high strength steel substrate or of an assembly containing high strength steel components, such as steel fasteners on the surface of aircraft assemblies, while minimizing the effect of hydrogen-induced embrittlement, hydrogen-induced cracking and corrosion normally caused by aqueous benzyl alcohol. The invention comprises applying a liquid silane to the surface of the steel substrate or components immediately before and/or as an ingredient of a novel aqueous benzyl alcohol composition of the present invention which contains from about 1% to 10% by volume of the liquid silane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to novel benzyl alcohol compositions for stripping coatings such as paint coatings from high strength steel surfaces such as cadmium-plated steel, or from assemblies containing high strength steel components such as aircraft surfaces containing high strength steel fasteners, while minimizing the effects of corrosion or hydrogen-induced embrittlement normally caused by the action of benzyl alcohol formulations on such steel surfaces. 
     2. State of the Art 
     Water-based benzyl alcohol paint stripping compositions have been developed as substitutes for methylene chloride-based paint strippers which were highly effective for their intended purpose but have been found to be hazardous air pollutants and potential human carcinogens due to the presence of methylene chloride. Reference is made to U.S. Pat. Nos. 4,732,695 and 5,454,985 for their disclosure of non-hazardous, low cost, effective aqueous paint stripping compositions based upon benzyl alcohol, and the advantages of such compositions over prior-known organic solvent compositions. 
     While such aqueous benzyl alcohol stripping compositions are very effective for their intended purposes, it has been found that they can cause hydrogen embrittlement and hydrogen-induced cracking of high strength steel such as but not limited to, AISI 4340 and 300M, and Aer Met® 100 alloy, which can be cadmium plated or ion vapor deposited with aluminum, or otherwise coated. It is well known that structural steel, normally designed to be heat-treated above 180 ksi, can be subject to hydrogen induced embrittlement, hydrogen induced cracking as well as corrosion induced by the environment, particularly after treatment with conventional benzyl alcohol paint stripping compositions. 
     Aqueous aromatic acid compositions are also known for removing flux/contamination from printed circuit boards, and reference is made to U.S. Pat. No. 5,431,739 which discloses such compositions containing various additives including corrosion inhibitors for the solder and other materials on the printed circuit boards. 
     SUMMARY OF THE INVENTION 
     The present invention relates to methods and compositions for stripping coatings such as paints from high strength steel substrates and components. The technique involves applying to the surface of the coating or steel, before and/or as a component of an applied aqueous benzyl alcohol stripping composition, a liquid silane surface treating agent. It has been found that the liquid silane, whether pre-applied or applied as an ingredient of the aqueous benzyl alcohol stripping composition, forms a barrier layer on the surface of the metal substrate or component which protects high strength steel against hydrogen-induced embrittlement, hydrogen-induced cracking and corrosion normally caused by the aqueous benzyl alcohol stripping compositions. 
    
    
     DETAILED DESCRIPTION 
     The novel method of the present invention involves protecting the surface of high strength metals against hydrogen-induced embrittlement, cracking and/or corrosion while stripping coatings such as paint therefrom by means of an aqueous benzyl alcohol stripping composition, comprising the step of (a) applying a layer of a liquid silane compound over the surface of the coating to be stripped, prior to the application of a layer of a conventional aqueous benzyl alcohol stripping composition or of a layer of a novel aqueous benzyl alcohol composition containing from about 1% to about 10% by volume of the silane compound, or (b) applying a layer of novel aqueous benzyl alcohol stripping composition of the present invention containing from about 1% to about 10% by volume of the silane compound directly to the surface of the coating or paint layer to be removed from the surface of the high strength steel covered thereby. 
     In either case, the silane compound penetrates or migrates to the interface of the metal and the paint, with the stripping composition, to form a protective silane layer on the metal surface during or after the paint layer is swollen and separated from the metal surface by the benzyl alcohol/water stripping composition. 
     The novel aqueous benzyl alcohol stripping compositions of the present invention comprise conventional aqueous benzyl alcohol compositions to which is added from about 1% to about 10% by volume, preferably 5-10%, of a liquid, silane compound. Such compositions generally contain between about 30% to about 60% by weight of benzyl alcohol, preferably 35-50%, from 1% up to about 10% by weight, preferably 5-10%, of an alkali such as an alkali metal hydroxide, an alkaline earth metal hydroxide or ammonium hydroxide, from 3% up to about 15% by weight, preferably 5-15%, of one or more wetting or coupling agents which have an affinity for both water and for benzyl alcohol to render mixtures thereof miscible or co-soluble, such as boric acid-diethanolamine reaction products and long chain alkyl-substituted aryl compounds such as phenol, and the balance water, preferably greater than about 45% by weight. 
     The critical silane additive to the present stripping compositions preferably comprises a hydrophilic hydroxyl- or lower alkoxyl-terminated silane compound, such as 3-glycidoxy-propyltrimethoxy silane, which is commercially-available under the trade designation Dow Corning Z-6040. This is an organofunctional hydrophilic silane which hydrolyzes to form silanol groups capable of bonding with inorganic surfaces such as metal and having an affinity for organic materials. Also suitable is a water-soluble composition available under the trademark Rain-X® which is a 9% solution of a mixture of siloxane silanes in water-miscible alcohols, essentially ethyl alcohol. Hydrophobic silane compounds are also suitable. 
     The essential novelty of the present methods and the novel stripping compositions of the present invention is that they protect high strength steel substrates or components against hydrogen embrittlement, cracking and corrosion normally induced by aqueous benzyl alcohol paint stripping formulations without significantly reducing the effectiveness of the stripping composition for its intended purpose, namely, the environmentally-compliant, low or non-hazardous, low cost, rapid and thorough stripping of coatings, particularly paint coatings, from such steel substrates or components including structural assemblies containing high strength steel, such as aircraft components and assemblies. 
     The following comparative testing was conducted on identical unpainted specimens or coupons of cadmium-plated high strength steel designated &#34;Cad Plated IAW QQ-P-416F Type 1, Class 2, baked within 30 minutes of plating at 375° F. for 100 hours, dichromate sealed&#34;, as required by military test procedures. The coupon material is AISI 4340 per MIL-S-5000E, configured according to ASTM F519-93 Type 1a. The steel was heat treated and tempered according to MIL-H-6875H to a hardness of HRC 51-54. The purpose of the testing was to evaluate the hydrogen embrittling effect of aqueous benzyl alcohol paint stripping compositions A and B which are identical except that composition B has added thereto 5% by volume of Dow Z-6040 silane. 
     All testing was conducted in accordance with Grumman Technology Services Procedure Number QSL 26906-95 dated Nov. 21, 1995 titled &#34;Procedure For Embrittlement Testing of Plating Processes and Aircraft Maintenance Chemicals&#34;, under a test load of 75% of the original notched tensile strength. 
     In the initial testing, nine of the control coupons were treated, in the absence of any silane, with the following aqueous benzyl alcohol paint stripping composition A: 
     
         ______________________________________COMPOSITION AIngredients      % by weight (approx.)______________________________________Benzyl alcohol    40  Ammonium hydroxide &lt;5  Linear alkyl aryl hydrocarbon &lt;5  Boric acid-diethanolamine &lt;5  Water &gt;45______________________________________ 
    
     The treated coupons are then subjected to the embrittlement testing under the load of 7071 pounds for a plurality of hours, with the following results: 
     
         ______________________________________Coupon   Test Duration                Results   Static Residual______________________________________1        169     hours   failure none  2 169 hours failure none  3 211.7 hours no failure data not determined  4 23.9 hours failure none  5 115 hours failure none  6 114 hours failure none  7 214.5 hours no failure data not determined  8 11.0 hours failure none  9 30.8 hours failure none______________________________________ 
    
     Next, three of the coupons 10-12, were treated with the following silane-containing aqueous benzyl alcohol paint stripping composition B 
     
         ______________________________________Composition BIngredients      % by weight (approx.)______________________________________Benzyl alcohol    40  Ammonium hydroxide &lt;5  Linear alkyl aryl hydrocarbon &lt;5  Boric acid-diethanolamine &lt;5  Water &gt;45  Plus 5 vol. % Dow Z-6040______________________________________ 
    
     Another three of the coupons 13-15 were first soaked for 15 minutes in a naphtha solution containing 5% by volume of the Dow Z-6040 silane, and then treated with the stripping composition B. 
     Next, the coupons 10-15 were subjected to the same embrittlement testing as the control coupons 1-9, with the following results: 
     
         ______________________________________Coupon Test Duration              Results       Static Residual______________________________________10    207.8 hours  no failure    7800 pounds  11 207.8 hours no failure 6640 pounds  12 207.8 hours failure-(pass 200 hrs.) None  13 213.2 hours no failure 8820 pounds  14 213.2 hours no failure 8830 pounds  15 213.2 hours no failure 8810 pounds______________________________________ 
    
     As is clear from a comparison of the test results, the coupons 10-15 treated with the novel aqueous benzyl alcohol stripping compositions of the present invention have substantially greater strength, as evaluated by sustained load (creep) testing, compared to the control coupons 1-9. 
     In subsequent testing, three of the control coupons 16-18 were treated with the following commercially-available benzyl alcohol paint stripping composition C, and three coupons 19-21 were treated with the same composition to which was added 5% by volume of 3-glycidoxypropyl trimethoxy silane (Z-6040). 
     
         ______________________________________Composition C Ingredients       % by weight______________________________________Benzyl alcohol      30-40  Ethanolamine  5-10  2-Ethylhexyl Mercopto Acetate 1-5  Petroleum hydrocarbon 1-5  Water balance______________________________________ 
    
     The treated coupons are then subjected to the embrittlement testing under the load of 7071 pounds for a plurality of hours, with the following results: 
     
         ______________________________________ Test                  Static Residual                                % original Notch  Coupon Duration  Results (lbs) Strength______________________________________16    200    hrs    No failure                       7920     84  17 196.5 hrs Failure -- NA  18 200 hrs No failure 7900 83.8  19 260.7 hrs Failure -- NA  20 308.7 hrs No failure 8490 91.8  21 308.7 hrs No failure 8430 91.2______________________________________ 
    
     In subsequent testing three of the control coupons 22-24 were treated with the following commercially-available benzyl alcohol paint stripping composition D, and six coupons were treated with the same composition to which was added 5% by volume of 3-glycidoxypropyl trimethoxy silane (Z-6040) for coupons 25-27 and 5% by volume of siloxane silane (Rain-X®) for coupons 28-30. 
     
         ______________________________________Composition DIngredients     % by weight (approx.)______________________________________Benzyl alcohol   30-40  Ethanolamine  5-10  2-Ethylhexyl hydrocarbon 1-5  Petroleum hydrocarbon 1-5  Ammonium hydroxide 1-5  Sodium nitrate 1-3______________________________________ 
    
     The treated coupons are then subjected to the embrittlement testing under the load of 7071 pounds for a plurality of hours, with the following results: 
     
         ______________________________________ Test                  Static Residual                                % original Notch  Coupon Duration  Results (lbs) Strength______________________________________22    181.7  hrs    Failure --       NA  23 181.7 hrs Failure -- NA  24 181.7 hrs Failure -- NA  25 307.2 hrs No failure 8700 94.1  26 307.2 hrs No failure 8700 93.4  27 307.2 hrs No failure 8700 90.8  28 406 hrs No failure 8570 92.7  29 406 hrs No failure 8590 92.9  30 406 hrs No failure 8600 93.0______________________________________ 
    
     In further testing, three of the control coupons 31-33 were treated with the following commercially-available benzyl alcohol paint stripping composition E, and three of the coupons 34-36 were treated with the same composition to which was added 5% by volume of 3-glycidoxypropyl trimethoxy silane (Z-6040). 
     
         ______________________________________Composition E    Ingredients         % by weight (approx.)______________________________________Benzyl alcohol      30-60  Ammonia 1-10  Linear Alkylated Aryl Hydrocarbon 1-10  Phenyl methyl ether (Anisole) 5-20  Water 24-45______________________________________ 
    
     The treated coupons are then subjected to the embrittlement testing under the load of 7071 pounds for a plurality of hours, with the following results: 
     
         ______________________________________ Test                  Static Residual                                % original Notch  Coupon Duration  Results (lbs) Strength______________________________________31    200    hrs    No failure                       8770     94.8  32 196.5 hrs Failure None NA  33 196.5 hrs Failure None NA  34 329 hrs No failure 8150 88.1  35 329 hrs No failure 8150 88.1  36 329 hrs No failure 8170 92.7______________________________________ 
    
     Many variations of the present invention will suggest themselves to those skilled in this art in light of the above, detailed description. All such obvious variations are within the full intended scope of the appended claims.