US 2372982 A
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April 3, 1 A. B. RICHARDS ET AL v PROTECTIVE COATING Filed March 29, 1943 B Richards A.M1'ckev T. ORei11 INVEN 0R5, BY (flg l Q6 46? I Patented Apr. 3, i945 PROTECTIVE COATING 7 Alvin B. Richards, John A. Mickey, and James illy, Dear-born, Mlch., alsignors to Ford Motor Company, Dear-born, Mich, a corporation of Delaware Application March 29, 1943, Serial No. 480,994
(Cl. 117-B) 1 Claim.
This invention relates to masking and protective coatings and more particularly to a transparent protective coating particularly applicable to various transparent materials such as plastics, glass and the like.
An object of this invention is to devise a protective coating for the plastic materials used in the windows and closures in aircraft and elsewhere to prevent abrasion or scarring of the surface during construction and to ailord a readily removable masking means permitting painting or other treatment of adjacent structure without permanently affecting the protected material. Another object is to devise a protective medium highly resistant to abrasion which will not affect the material protected, is readily removable, is not affected by commonly used paints or other coatings and is transparent to permit full lighting through the material protected at all steps of the construction. Another object is to devise a protective coating which can be readily applied to develop a thin, tough, even, impervious skin over the material to be protected, which will resist deterioration under extreme conditions of heat and moisture and which, on
' removal, can be reworked for reuse.
Present aircraft construction requires the use of large transparent enclosures as in the bombardier's quarters in the nose, the various gunner's turrets or emplacements, the pilot's enclotics and safety, these are generally formed of a 4 methylmethacrylate resin or the like, which has good transparency and which may readily be formed to the various shapes required. ,One outstandingnisadvantage of the resins available, however, is that as compared to glass they have a very soft surface which is readily abraded and scarred by the ordinary handling encountered in assembling these pieces and in the further construction work on the plane. These resins, moreover, are subject to chemical reaction with the solvents of some of the paints used on the adjacent metal structure and they must be fully masked oil before any paint is applied, for, while the dried paint may be removed from them, it
will be found that the surface is largely deteriorated due to chemical decomposition.
The resin, as usually furnished, is protected by a cemented paper coating'or by a hard, brittle and opaque coating materia1 which has very little adhesion and which does not usually survive an unpacking operation. After the resin has been formed to the desired shape, it is sometimes covered with several sprayed thicknesses of latex. It may then be placed in its. frame and the coating reinforced by-a heavy, brown paper covering cemented along the edges of the formed which may be reworked to part. This masking remains on until the final assembly operations on the ship, after which it can be removed. With this procedure it is difficult to obtain a close adhesion with the paper, particularly in the more complicated surfaces,
and neither it nor the latex has great resistance to abrasion. Moreover, because of fitting difllculties, additional masking means must usually -be used around the edges adjacent to the struc-- tures to be painted to be sure that complete masking is maintained. Certain other masking compounds are available, but they, too, suffer from characteristics of brittleness, poor adhesion, nontransparency or poor resistance to abrasion.
An advantage of the present invention lsthat a protective coating is devised which is transparent, abrasion resistant, tough, elastic, adherent, paint resistant and which does not affect the plastic materials usually used in this type of work. As used here the word adherent" does not signify a bonding between the coating and the material-quite the contrary, for the coating must peel away from the material readily. Rather, this signifies that the coating will remain in close contact with the surface of the material'under all normal conditions. Another advantage is that a masking material is obtained which may be applied by dipping or, if
desired, by spraying, resulting in a thin, uniform continuous coating without formation of flow holes or surface irregularities. Another advantage is that a protective jcoating is obtained which is easily removable when desired and reapplication, thus considerably reducing the cost of the material as used. Another advantage is that the transparency of the material treated is not affected so that adequate light is transmitted through it at all stages during assembly operations and yet, by suitable coloring, clear indication is given at all times whether or not the protective coating is still in place and is covering the surface. Another advantage is that. theprotective coating may be applied immediately after the forming step and will remain throughout the assembly of the formed material in its frame, the assembly of the frame in the airframe structure and. thereafter. through all the succeeding assembly stages without further treatment. Further, it
will protect the plastiesurface against abrasion I vention consists in the arrangement, construction and combination of the various parts of the improved material, described in the specification,
claimed in the claim and illustrated in the accompanying drawing, in which:
normally located in the forward portion of an aircraft and comprising a framework ll supporting transparent individual panes II which aregenerally formed of an acrylic resin. The
' view shows the removal of a protective coating mounted and applied according to this invention, It representing the portion of the coating already removed and H the portion still adhering to the panel. 4
Figure 2 shows a means by which the removal of the protective coating maybe started which employs the placing of a thread i5 against the panel surface before coating and,thereafter, when it is desired to remove the coating, pulling the thread in the manner shown to initially break the protective coating l6 and raise it from the surface I'l preliminary to complete removal. Other means may be used for thispurpose, such as the inclusion of a small paper tab or the like placed against the coating while still wet, allowing it to dry in place and thereafter using it to break the film preliminary to removal.
The protective coating comprises, essentially, an aqueous solution of a polyvinyl alcohol with the addition of a wetting agent and a, plasticizer. A particular composition which is discovered by the desired properties comprises substantially the following:
Polyvimrl alcohol .grams -60 Water cubic centimeters..- 400 AerosoP' grams 0.5
Alcohol (as vehicle for "Aerosol) cubic centimeters- 5 Glycerine (plasticizer) ..per cent... 2
The components are mixed to form an aqueous solution and the panels to be protected are dipped into it. A thin coat of uniform thickness '5 formed on the panel which is then passed through a drying chamber in which air' circulated at room temperatures will thoroughly dry and set the material in about 10 minutes. It may then be handled or piled in practically any manner without fear of abrasion. The coating is sufilciently tough and adherent so that even if breaks inthe protecting costing do occur adjacent the frame during mounting operation, the coating will not pull away from the panel but will remain closely attached to it. As a further convenience, the emulmay be coloredby the addition of coloring matter such as a methyl orange dye to gives.
ready indication of whether or not a particular panel has been treated with the protective coatins. as the uncolored film is sufiiciently transpareat as to make it difllcult to detect its presence. The "Aerosol referred to is the trade name of The composition set forth above evidently has been found satisfactory for conditions pertaining in this locality. However, in the event that a more heatand water-resistant film was required, as for subassemblies to be shipped or stored under tropical conditions with the protective coatin still in place, the film may be hardened and its water resistance increased by the addition of such material as formaldehyde, chromium compounds, diba sic acids, copper-ammonium compounds, zinc-ammonium compounds and the like. It is thus possible to obtain a protective coating to withstand practically any atmospheric conditions which is transparent and adherent but readily removable from the object protected, abrasionresistant and tough. This material, moreover, is not limited in use to the protection of transparent plastics but may be applied by dip, brush or spray in connection with paint masking of other structures. In this field it finds particular application, for instance, in masking of! the colored identifying symbols used in aircraft hydraulic and electrical hookups which heretofore have required tedious paper masking preliminary to the painting step. Moreover, because of the transparency,
the indicia are visible at every stage.
To remove the film, after all assembly operations have been completed, it may either be ripped with the rip cord shown in Figure 2 or pulled away by a tab as explained in the specifications or one corner may be moistened sufilciently to permit removal. The film peeled ofl may be dissolved in water, the paint and foreign matter contained on it filtered off, and filtrate recompounded and reused. The dried film has practically complete resistance to gasoline, any of the solvents in the paints generally used, alcohols, ethyl acetate and most any other substance to which it might normally be exposed. It possesses a high degree of elasticity and adhesion and is sumciently hard so that most abrasive actions will not destroy it. The crazing" of resin sheets manifested by the formation of a great number of small surface cracks impairing the transparency of the sheet which is caused by chemical interaction between some masking agents used and a stressed resin sheet is avoided, since the present compound is not reactive. Usually this erasing had to be carefully buffed out and the whole sheet repolished.
Some changes made he made in the combina-.
tion of the various components of the improved material without departing from the spirit of the a well-known commercial wetting agent which comprises a, dioctyl ester of sodium sulphosuccinic acid, but there are numerous other commercial agents available based on a sulphonic acid base which may be used in its stead.
invention. It is the intention to cover by the claim, such changes as may reasonably be included within the scope thereof.
The invention claimed is:
A method of temporary surface protection of plastic sheets characterized in having a normally relatively soft surface which comprises the steps of coating said sheet with a thin and uniform film of polyvinyl alcohol, said alcohol being supplied in an aqueous solution having low surface tension, drying said solution in place thereon to form a thin transparent, adherent, resistant to abrasion, hard film, and peeling said film from said sheet when said protection is no longer required.
ALVIN B. RICHARDS. JOHN A. MICKEY. JAMES T. O'REJLLY.