US2476937A - Process for varnishing employing strippable protective coatings - Google Patents

Description

July 19, l949 PROCESS FOR VARNISHING EMPLOYING STRIPPABLE PROTECTIVE COATINGS FiledF'eb. 19, 1944 R. w. WHITE 2,476,937

Patented Jully 19, 1949 PR O CE S S FOR STRIPPABLE VARNISHIN G EMPLOYING PROTECTIVE COATINGS Robert W. White, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 19, 1944, Serial No. 523,081

2 Claims.

This invention relates to protective coatings of a temporary nature which are applied to metal surfaces, in particular, to protect the metal surfaces in various Ways, and the coatings being readily mechanically strippable. The strippable protective coatings of this invention are particularly advantageous for use in processes for protecting portions of metal surfaces of members which are to be dipped in hot varnish and baked, the strippable coatings and any overlying varnish being readily peeled off from such protected portions, while the untreated surfaces are coated with baked varnish.

In applying organic finishes to predetermined portions of the surface of members as in impregnating electrical apparatus with insulating varnishes in order to insulate the electrical conductors, it is frequently desirable to cover or to immerse the entire member or piece of apparatus in the coating composition or insulating varnish in order to secure a good penetration and covering. It has been a considerable problem heretofore to remove the organic nish or insulating varnish from those portions of the apparatus upon which such varnish or finish is not desired. A large amount of hand labor is frequently required to remove insulating varnish or paint from those portions of members upon which such composition is not desired. For instance, in order to remove insulating varnish from a motor frame which has been impregenated, most types of impregnating varnish must be scraped off When the frame is quite hot; otherwise, when cold, the varnish is so adherent that it would be extremely diiiicult to remove. As much as fteen hours of labor may be required to remove the undesired varnish present on the machined surfaces of a large motor frame.

The object of this invention is to provide for applying a protective varnish and baking resistant coating for use on metal surfaces, which coating is readily stripped.

A further object of the invention is to provide a process for applying to metal surfaces plasticized acyl esters of cellulose to form on the metal surfaces protective coatings which, after they have served their purpose, may be readily stripped off in sheets.

A still further object of the invention is to provide for applying on metal surfaces protective coatings having predetermined corrosion resistant properties, the coatings being readily removable by stripping,

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing, in which:

Figure 1 is a view in elevation, partly in section, of a member to which the coatings of the invention have been applied;

Fig. 2 is a vertical cross-section through a motor stator showing a further application of the invention;

Fig. 3 is a perspective view, partly in section, showing the application of the invention to conductor leads; and

Fig. 4 is a perspective a further application of vention. Y y

According to the present invention, it has been discovered that acyl cellulose esters when suitably plasticized may be applied to metal surfaces to produce thereon coatings or iilms of a tough resinous nature that will protect the applied surfaces from the action of the subsequently applied organic finishes either hot or cold and subsequent baking operations. While exhibiting outstanding protective properties, the plasticized cellulose esters are only weakly bonded to the metal surfaces, and when cut or slit in various places, the coatings or lms may be peeled off in sheets, thus removing any superimposed layers of organic finish. The bright uncorroded metal is uncovered by a simple stripping action.

The outstanding advantage of protective coa+- ings of plasticized acyl cellulose esters is apparent in hot vacuum or pressure impregnation. Other coatings that may be ample to prevent varnish adhesion during spray painting operations, for example, will be found to be ineffective under vacuum impregnation condition. Small pinholes and cracks will permit the entry of var nish below such coatings with resulting failure. Cellulose ester coatings give complete protection to metal even under vacuum impregnation of varnish.

The advantages of the cellulose esters for the practice of this invention reside in the unusual combination of properties present therein. Plas ticized cellulose esters bond only weakly to metal and this characteristic provides for easy mechanical stripping. However, the adherence of the plasticized cellulose ester lm to the metal has been found to be suicient to prevent cracking, aking, lifting at the edges or other mechanical failure. The cellulose esters also are poorly soluble in most of the common organic solvents that are employed in organic finishes such as inof a square coil showing the coatings of the in sulating varnishes or the like. The acyl cellulose esters form tough homogenous protective coatings which will withstand ordinary Shop abuse.

In practicing the invention, the acyl cellulose esters which have given best results are cellulose acetate, celluloseacetate butyrate, .and cellulose acetate propionate. y

Where protective coatings of a cellulose ester, such, for example, as cellulose acetate, are to be applied to a member to be subsequently dipped in hot varnish and baked, the cellulose acetate should carry from 50% to 115% Aof its-Weight ofr a compatible plasticizer. For usejto protect metal from varnishes to be applied and driedat room temperature as by sprayingpr.V brushing;`

about by weight of plasticizer in the cellulose acetate will be generally sufficient.A Examples of plasticizers which have been found satisfactory are triacetin, dimethyl phthalate, di-

ethyl phthalate. 4tripropioriins and p-toluene sul-- fonai'ri-idelvv The 'pla'sti'iz'er.should Y.bev selected on the-basis oi'prodicingfa tough lplastic film which will promote adhesion'to the metal Vsur faces, reduce celluloseester shrinkage, and will resist the action ofthe Vcommon finish solvents.

In'pro'duci'g' a suitable composition that may be applied to metaisurfacesin Vthe form of. a paint, itis' desirablethatlthe cellulose ester, such, for example, as cellulose acetate', be selected to provide a solution vof high solids content and low viscosity. Cellulose'acetatesofa grade suitable for laoquerus'e's are suggested as a `convenient form ofthe ester.. Y a viscosityY of frorn2` to 4'- seconds toa range of 150 to 250 seconds have been employed with successi. The aceticacidcontent is `not critical since cellulose acetates having 52 /2-% acetic acid equivalence have been employedgvvith as good results as' have .the trlac'etates.2 Similar considerations have been found.to-applywithfcellulose acetate butyrate.

Numeroussolvents are-available, as is well known to-those ,skilled inA the art, for the preparation of solutions of the cellulose esters. Acetone hasjcharacteristi'cl's as'asolverit for cellulose acetate that Yrenderit particularly desirable for the purposeof this invention. Thecost is low and coatings producedI therewith dry rapidly.- Methyl acetatdmethylene chlorideethanol,solutions,` and other solvent mixtures-are suitable for practicing the invention. ,A-typicat solution comprises 100 parts by yweightofacetone,: parts by weight of 2 to.4-se'cond viscosity, cellulose acetate, and 2G parts by weight,Y of dimethyli phthalate. This solution gave excellent results, in practice.

It will be apparent tothose skilled in the art that other resinous materia-1s may be admixed in minor amounts Withthe acylcellulose esters, that various otherplastlcizersfand small amounts of non-drying oilsmay beadded', and that many solvents may bemade useofxinpreparing the cellulose ester solutions. made only to the extent that adhesion to metal surfaces be not increased tothe extent that the cellulose acetate or` otherester cannot be peeled off readily.` Various `rosiri and abiotic acid compoundsand related derivatives, for instance, if added in amountsof up to improve the friable nature of the coatings-so -that they may be more readily removed. On theother hand plastics that have good metal adhesion properties such as cellulose nitrate vrmay not be tolerated in any appreciable' ,'quantity...

In olas't'icin'g` cellul'oSeactatesolutions, conto be quite However. cellulose acetates ofY Such additions should be sideration must be given to the physical shape of the metal surface to which the cellulose ester solution is to be applied. When cellulose acetate is applied to a portion of a large fiat metal sheet, the films must carry relatively large amounts of plasticizers, for example, 20%, to reduce undue shrinkage. For instance, anzunplasticizled cellu lose acetate film on' dryng .will break'lbose and fail to serve its intended purpose. Where the applied cellulose ester film can secure an anchor around edges of the metal, films with lesser amountsofplasticiz'e may hold adequately for many purposes; Coatings applied about shafts and small members where a continuous circumferential band or thelike is produced will adhere tenaciously due. tothe slight shrinkage of the ceilulose acetate asdryingpf the solvent takes place.

As stated' previously;Y about 5% of plasticizer based Von the weight of the cellulose ester is the minimum for use, and this is only satisfactory where the member is not exposedto temperatures above 35 C. .herel theV y l A cellulose ester isf applied*- is.- to `be treated in. hot varnish and baked. the minimum-piasticizep con.-`V

tent is from 50% andvhigher)v up to 115%.- VAt Y,

temperatures VofY Yabout 120 to 145'? C., thej best..

compositions for service contain y tov 115 parts mental to any cellulose ester -lmfand-itispreillustrated a motor rotor plasticized cellulose acetate-l5 -hasfbeenappl-ied to the shaft ends l2 insulating varnish and baked.V Y The coating-fit1 may be applied by brushing-.sprayingpr' dipping.

After the solution of cellulose ester is Vappiigedthe -v coating is dried to cause-the solvent vtobe re- 1 moved. A cellulose ester film 3 to 5 Vmils thiol; will be produced by'this procedure. The cellulose ester coating will be found to be suicie-ntly ad.- Y

herent for subsequent use.. The coating iii isY sufliciently tough and resilient to withstand theV Y ordinary abuse that will be met in the shop. The

entire rotor lf3 may be dippedan-y desiredv numn Y ber of times in hot varnish with or without-'vacu-4 um impregnation and baked at temperaturesof; to 140 C., as required in practice.:A YA coaty- Y ing of the insulating varnish: will'build upover; the protective cellulosel ester vcoating lil during,

these operations. After the rotorhasbeen other.-v

wise completed, the cellulose ester coating -i dmayfbe stripped by makingapairof oppositelydisg; posed axial cuts along the shaft.; The coating 1 may be peeled oi' in two pieces.,- It is not necese i sary to brush or-otherwise workon the shaft- 1 since the coating-strips completely and readily.

Another appiication of the invention iSv-illus'- trated in Fig. 2 of the drawing. The motorstator i6 provided with feet IB andassembledwith-thepole faces 2t is commonlyimpregnatedlwithan insulating varnish. AIt is -not desirable tohavethe insulating varnish present-oir the outside.v surface of the motor frame I6,the legsql, or on the pole faces 2t. The poles 20 must be'iitted-*with a very small clearance fto the rotorcooperating-s therewith and evena smalllayer of varnish on the pole faces may interfere 'withythe operation of the apparatus.

In applying the inventioneto thestator Itlfth'eoutside of fthe framel4 is coated Iwith assoluiv member` to .Which-the' to prevent the stickingof hot varnish thereto when the rotor is Vdipped `in j tion of plasticized cellulose acetate as described above. The legs I8 may be dipped or otherwise coated with the same solution, and the pole faces coated by brushing or spraying. After drying, an adherent coating I 9 of cellulose acetate will be present. The entire stator I6 may be dipped into the insulating varnish as required and baked. The cellulose acetate coating I may be readily and rapidly removed by making thin slits, after which the cellulose acetate may be stripped off in relatively large sheets. Only a fraction of a minute may be required in many cases to remove the cellulose acetate and the insulating varnish which has been superimposed therein during the previous operations. Clean metal is exposed by this operation.

Another advantageous application of the invention is illustrated in Fig. 3 of the drawing Where the conductor lead 24 of the conductor 2? previously provided with a tin coating 26 is provided with a plastlcized cellulose acetate coat ing l0 previous to varnish impregnation of the conductor or coil 22. The coating l0 may be out with a knife or other edged tool and peeled to expose a clean varnish-free surface. It is unneces-1 sary to re-tin the conductor lead 24 after stripping as heretofore required. Plasticized cellu lose acetate with rosin or rosin derivatives forms good protective coatings on leads since it flakes cif readily when cut. The ease of stripping the plas-= ticized cellulose esters of the present has rendered it possible to attain numerous manufacturing savings. For example, the square coil assembly 30 of Fig. 4 is provided with a supporting plate 32 at each end. It is necessary to drill and tap the supporting plate 32 before the coil is impregnated with an insulating composition. However, the insulating composition will lill the drilled and tapped holes, and the plates 32 must be subjected to redrilling and retapping to remove the composition therein. Though the second drilling and tapping operation is not diiiicult, it does take time and increases the cost of manufacture. The strippable protective composition of the present invention obviates this duplicated work. The coil 30, before being impregnated, has the screws 34 inserted into the drilled and tapped holes in the plate 32, and plasticized cellulose acetate is brushed over the plate and screws A coating I0 of the plasticized cellulose acetate is produced on drying. rEhe coil may now be safely impregnated Without the threads being affected. After the insulating varnish has been baked, the protective coating I9 may be peeled olf, generally in one motion, and the coil is ready for installation.

While the above examples are mainly directed toward use in the electrical industry, numerous applications in other industries are available for the easily strippable protective coating described above. The advantages of good protection and easy removal will render it desirable to make use of the invention in many instances where a ternporary protective coating is required.

suggestive of the advantages of the invention in places where the application of insulating varnishes is not involved is the application of the coating of plasticized cellulose ester as a temporary protective covering on machine metal surfaces during shipment, storage, and like applications. For corrodible metals, such as iron or steel, an anti-corrosion agent may be incorporated in the solution of cellulose ester. Thus, zinc chromate, lead chromate or zinc phosphate in a finely divided state may be distributed in the cellulose acetate solution which is then applied to the metal surfaces to be protected. Shafts of motors, machined surfaces of lathes, and other machine tools and similar surfaces may be coated with the plasticized cellulose ester to produce an adherent protective coating. The use of a chromate or phosphate compound will prevent corrosion of the underlying metal for prolonged periods of time. When it is desired to place the apparatus in condition for use, a few slits in the cellulose ester coating may be made, and the coating peeled off in large sheets to expose an uncorroded metal surface.

Since certain changes in carrying out the above process and certain modifications in the composition which embodies the invention may be made without departing from its scope, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the process of applying a permanent adherent coating of an insulating varnish to predetermined portions of a member having other portions witn metallic surfaces to be kept free of the varnish, the steps comprising applying to the metallic surfaces on said other portions a coating of a solution composed of a Volatile organic solvent and a solute consisting of from 10% to 30% of the weight of the solution of a cellulose ester selected from the group consisting of cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate, and a plasticizer for the cellulose ester in an amount of from 50% to 115% of the weight of the cellulose ester, the plasticizer selected from the group consisting of triacetin, dimethyl phthalate, diethyl phthalate, tripropionin and p-toluene sulfonamide, drying the applied coating to evaporate the organic solvent thereby leaving a tough solid film of plasticized cellulose ester on the metallic surfaces, applying a solution of the varnish to the member, the varnish overlapping the cellulose ester film without penetrating to the underlying metallic surfaces. baking the member with the applied varnish at temperatures of up to C. to cure and harden the varnish into a solid, adherent lm, and mechanically stripping from said protected metallic surfaces the film of cellulose ester and any varnish lm superimposed thereon.

2. In the process of applying a permanent adherent coating of a varnish to predetermined portions of a member having other portions with metallic surfaces to be kept free of the varnish, the steps comprising applying to the metallic surfaces on said other portions a coating of a solution composed of a volatile organic solvent and a solute consisting of from 10% to 30% of the Weight of the solution of a cellulose ester selected from the group consisting of cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate, and plasticizer for the cellulose ester in an amount of from 25% to 115% of the Weight of the cellulose ester, the plasticizer selected from the group consisting of triacetin, dimethyl phthalate, diethyl phthalate, tripropicnin and p-toluene sulfonamide, drying the applied coating to evaporate the organic solvent thereby leaving a tough solid film of plasticized cellulose ester on the metallic surfaces, applying a solution of the varnish to the member, the varnish overlapping the cellulose ester film Without penetrating to the underlying metallic surfaces, baking the member with the applied varnish at temperatures of up to 145 C. to cure and harden the varnish into a solid,

REFERENCES VCITED Thefollowing referefnlcee are of`1`iorl in the @le of 'thisJ patent:

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