US2336266A - Airplane fabric and method of making same - Google Patents

Description

Patented Dec. 7, 1943 AIRPLANE FABRIC AND METHOD OF MAKING SAME Robert L. Lester, Newburgh, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application March27, 1942, Serial No. 436,476

7 Claims. (01. 117-7) This invention relates to coated fabrics, and more particularly to those designed for use in covering air frames of airplanes.

In the construction of aircraft, the practice has been to tailor an uncoated fabric to the air frame of an airplane and subsequently by brushing or spraying. or combination of both methods, to apply a solution of cellulose derivative coating in suflic ent quantity to tauten and protect the fabric- The uncoated fabric is sewn in the form of a sock and after turning it inside out it is stretched over the air frame and the open end is then closed. The sock is deliberately prepared undersize in order to make it fit more tightly. The application of a cellulose derivative solution to the fabric-covered air frame protects the fabric and effects shrinking of the fabric to produce the necessary tautness. Another reason for coating the fabric is to eliminate its porosity and to prevent the air from flowing through the fabric. In general. the practice is to apply the base coat of cellulose derivative called dope by hand brushing to force the dope into the interstices and also to permeate each individual thread thoroughly, the subsequent coats being ordinarily applied by spray application. The hand brushing and spray coating of the fabric is a bottleneck in the mass production of airplanes due to the fact that this is a long tedious step and each coat must dry before the subsequent coats are applied. Even the so-called "all metal" airplanes employ the fabric covered air frames for the ailerons, elevators and other con trol surfaces. The coating dope employed for .protecting and tautening the fabric covered air frame must contain a very low ratio of plasticizer to film forming material in order to obtain satisfactory tautening. Compositions containing a high ratio of plasticizer to film former when applied to a fabric tailored to an air frame do not result in suflicienttautening of the fabric resistance to the flow of air over the coated surface when the plane is in flight.

Heretofore there have been many attempts to apply the coating dope to thefabric by machine operation before it is attached to the air frame. It has also been proposed to apply only a portion of the dope to the fabric by machine coating and then apply to the predoped fabric after it is attached to the air frame, the balance of the required coating by hand brushing or spraying. In the prior-art attempts, in order that the partially coated fabric would shrink after it was attached to the air frame and subsequently wetted with additional spray coats, it was necessary to use very little or no plasticizer in the predoping composition. The unplasticized or slightly plasticized coating-rendered the fabric too. stifl for satisfactory tailoring to the air frame. When sufficient plasticizer was added to the coating to render the coated fabric sufficiently supple for satisfactory tailoring to the air frame, the coating would not shrink after it was attached to the air frame and treated with additional coats of the coating composition.

A primary object of this invention isto provide a predoped fabric suitable for covering air frames which eliminates a large portion of the time and labor consumed in doping and finishing the fabric by hand after it has been applied to the air frame.

A further object is to provide a predoped fabric which is sufficiently supple to permit satisfactory tailoring and application to an air frame The foregoing objects are accomplished by coating an airplane fabric, either water wet or dry. with a water-in-oil emulsion of a cellulose derivative solution having a very low ratio of softener to film forming material. The coating dope is made up, applied and dried in such a manner that the dry coating is deposited in the interstices in a porous condition but not into the individual threads and which leaves it capable of penetration by solvents therefor when additional coats of a cellulose derivative coating composition are superimposed over the dry coating.

The predoped fabric should preferably have a potential shrinkage of 5-15 per cent of the width of the fabric before coating. After the removal of the volatile ingredients of the first application of the coating composition, the predoped fabric is next tailored to the air frames and the balance of the necess'aryyamount of coating is applied by either spray or hand brushing applications.

The following specific examples are given by way of illustration and not limitation:

Example I Grade. A Flightex mercerized cotton sheeting,

36" wide, having a thread count between 80 and 84 in both the warp and filler directions and weighing approximately 4.0 ozs. per square yardis thoroughly wetted with water. The dry fabric absorbs approximately its own weight of water. While the fabric is in the wet condition the width shrinks to approximately 32 inches 1 or about elevent per cent of the original width.

The fabric is rolled up in the wet condition and transferred to a coating machine where it is coated by means of a doctor knife. A suitable coating apparatus for this purpose is disclosed by W. T. Anderson in U. 8. Patent No. 2,107,276. The predoping composition is a water-in-oil emulsion and is prepared in a well-known manner, the formula of which is as follows:

Percent by weight Cellulose nitr 14.4 Water 7 14.4 Dibutyl phthalaie--. 8.3 Castor ofl 3.3 Pigment 8.9 Methyl ethyl ketone 55.7

Sufilcient dope is applied to the wet fabric by means of the doctor knife application to deposit .5 to 1.5 ounces per square yard of non-volatile ingredients of the coating composition. The coating dope is forced into the interstices of the wet fabric and the water in the threads prevents any appreciable penetration of the coatingfdope into the individual threads. As the coated material moves'through a drying cham, ber, such as described by Anderson et. al. in

- U. S. Patent 2,107,275, .the methyl ethyl ketone' is volatilized first and the water present precipitates the cellulose nitrate at the point of contact with the water. The film is deposited in a porous condition and is readily re-dissolved when contacted with subsequent coats of a nitrocellulose solution. The precipitation of nitrocellulose at the point of contact with the wet threads is an important part of the present invention in that it allows the fabric interstices to be completely filled with the coating compov sition .without appreciably penetrating the individual threads. A composition containing a low ratio of softener to film forming material as in the above composition would result in stiffening the fabric if applied in the prior art manner and could not be satisfactorily tailored to an air frame. It is also an important part of this invention'that the fabric be dried under sufficient tension to pull the fabric back to its original 38" width. Tenter frame drying ipment is em- 2,ese,2oc I tion. After the material has been coated and dried as described above. it is subjected to calendering operation by passing between heated polished rolls under pressure. The predoped fabric may be stored in roll form indefinitely or may be immediately tailored and applied to an air frame.

After the .predoped fabric is properly attached to the air frame it is then given additional spray Sufiicient coats of the above composition are applied to bring the total weight of the dry cellulose derivative coating composition up to 4.0 ounces per square yard. The volatile solvents are allowed to evaporate after each successive spray coat. The spray coats which are applied after predoped fabric is attached to the air frame effect solution of the coating previously deposited and upon re-drying the coated fabric shrinks and produces the desired tautness.

Upon the re-solution of the porous coating in the interstices, the threads become permeated with the solution and when re-dried the coated fabric shrinks and becomes taut. Subsequent spray .coats, upon drying continue the tautening procedure until a satisfactory degree of tautnessand weight of coating have been reached. In general, the predoped fabric should have a potential'shrinkage of 3% to 15% of the original width of the fabric before predoping when the coating is wetted and re-dissolved by active solvents for the coating, and allowed to shrink without tension. This method of doping the fabric results in a very smooth surface without the necessity of sanding the coated surface which is a time-consuming operation. Further, the time and labor required for applying the spray coats to the predoped fabric is approximately 30 to 50 percent less than that required in former practice.

Emmple II A predoped fabric is also prepared in the same manner as described in Example I except that the predoping composition is applied to a dry fabric instead of a wet fabric. The amount of precipitation of the nitrocellulose is not as great when a dry fabric is used. However, the nitrocellulose is precipitated sufficiently by the water in the emulsion, as the solvent balance is upset.

by the evaporation of the methyl ethyl ketone at a rate greater than that of the water.

Example III A satisfactorypredoped fabric is also prepared in the same manner as described in Example I except that a dry fabric instead of a wet fabric is used and the volatile solvents are expelled from the coated fabric in the absence of I any tension in the direction of the width of the Played to rmtorethe coated fabric to its original 7 width during the drying of the coating composifabric. The nitrocellulose is precipitated by removing the methyl ethyl ketone at rates greater than that of the water. The width of the fabric shrinks from36" to 34'' during the drying operaasdaoo ation. The coated dried fabric is subjected to a tentering operation and is restored to its original 36" width. The coating being in a partially precipitated form is minutely ruptured throughout during the tentering or stretching operation,

which increases the suppleness of the coated fabric and makes the coating readily penetrable by organic solvents where subsequent coats of a cellulose derivative solution are sprayed or brushed over the dry coating.

Example IV Per cent by weight Cellulose aceto butyrate 15.75 Triphenyl phosphate 1.75 'Water 15.75 Methyl ethyl ketone 66.75

portant that the cellulose derivative film form ing material be deposited in the interstices of the fabric upon drying in a porous condition in order th: t it will be readily re-dissolved when contacted with additional coats of the finishing composition.

In the above examples the fabric shrinks when in the wet condition before coating from 36" width to 32" width or 11% of the original width. In the case where a dry fabric is coated and dried in the absence of tension in the direction of the width it shrinks from 36" widthto 34 width or 5.5% of the original width. If the fabric was coated and dried at 32" or 34" width without any further treatment, there would be no. appreciable shrinkage of the fabric after tailoring and application to an air frame when subsequent coats of the cellulose derivative composition were applied. For this reason it is important that the fabric be pulled back to its original 'width'simultaneously with the drying operation or after the coating has been completely dried.

In the examples of the predoping composition,

cellulose nitrate and cellulose acetobutyrate are disclosed. Cellulose acetopropionate may be used in place of cellulose acetobutyrate with equally satisfactory results. The term film forming cellulose derivative is used to cover these materials generically. Cellulose acetate and ethylceilulose can be used but is not preferred for airplanes due to their poor tautness retention.

It will be noted in the formulae of the predoping composition that the ratio of the total plasticizer components to the filmforming material varies between 31.6 parts of plasticizer to 68.4

parts film forming material and 10.0 parts of plasticizer to 90 parts of film forming material. It has been found that compositions containing a ratio of plasticizer to nitrocellulose higher than 40 parts of plasticizer to parts of film forms i. e., two-thirds as much plasticlzing agent as film forming cellulose derivative, do not have suflicient shrinking when re-dissolved and dried on a fabric base. When compositions containing such low ratios of plasticizer to cellulose derivative, film forming materials are deposited on a fabric and the interstices of a fabric are filled and the threads permeated with such a compo- I sition, the fabric is too still for tailoring and fitting to an air frame. However, by employing compositions with a low ratio of plasticizer to film forming material and following the coating procedure as outlined in the above examples, the permeation of the individual threads by the predoping composition is prevented and a supple and readily tailored precoated fabric is obtained which will shrink when the coating is re-dissolved on the fabric and allowed to dry under tension.

The coated fabric herein described is designed primarily for use in covering the air frames of airplanes and particularly the control elements such as ailerons, elevators, and rudders of the so-called all-metal planes. The predoped fabric is also used in the form of a tape to cover seams, rib stitching and other finishing purposes. In practice the tape is applied immediately after the application of a brush coat of dope to the area to be reinforced. It can also be used where coated fabrics are attached to frames and tautness is desired, such as large moving picture screens, including the translucent type of screen. It can also be used for drum heads, etc.

The primary advantage of the invention is the elimination of a large portion of the time and labor consumed in finishing and doping airplane fabrics according to the prior art methods. A further advantage is that the method herein described results in'a supple predoped fabric which may be easily and readily tailored to an air frame with no more difficulty than an uncoated fabric.

A still further advantage is the elimination of the sanding operation employed in the prior art methods of finishing and doping of airplane fabrics.

It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, and therefore, it is not intended to be limited except as indicated in the appended claims.

I claim:

1. A predoped fabric suitable for covering air frames and the like and having a potential shrinkage of at least 5% of its original width before doping, coir. ising a fabric b se having a water insoluble cellulose derivative coating composition containing a plasticizer in amount less than two-thirds of the cellulose derivative deposited in the interstices thereof without substantially permeating the individual threads, the portion of said coating contacting said fabric being in the form of fine discrete checks.

2. In the process of preparing a predoped air-.

plane fabric and the like having a potential shrinkage of 5% to 15%, the improvement which comprises applying an aqueous emulsion of a ric to its original width before coating to minutely fracture the dried coating. resulting in increased suppleness of the coated fabric.

3. In the process of preparing a supple coated fabric having a potential shrinkage of about 5% to of the width of the fabric before coating,- the improvement which comprises thoroughly wetting the fabric with water, allowing the fabric toshrink approximately 5% to 15% while wet, filling the interstices of the wet fabric with an aqueous emulsion of a water insoluble cellulose derivative solution in which water is the internal phase, without substantially permeating the individual threads, said emulsion containing a low ratio of plasticizer to cellulose derivative which normally would result in a non-supple coating, passing the water wet filled fabric through a drying chamber and simultaneously drying and stretching the coated fabric back to the original width before it was wetted to minutely fracture the dried coating and increase its suppleness, and thereafter calendering the dried coated fabric to obtain smoothness.

6. The process of claim 2 in which the emulsion has the following composition:

7. In the process of preparing airplane parts in which a supple coated fabric having a potential shrinkage of at least 5% upon the further application of airplane dope is made into a sock and fitted onto an airplane part, the improvement which comprises wetting a woven base fabric is present in amount between .5 and 1.5 oz. per

square yard of fabric.

with water, allowing the fabric to shrink approximately 5% to 15% of its original width and while wet filling the interstices of the wet fabric with a water-in-oil type emulsion of a water insoluble cellulose derivative without substantially permeating the individual threads, said emulsion containing less than two-thirds as much plasticizer as cellulose derivative, passing the wet coated fabric through a drying chamber and simultaneously drying and stretching the coated fabric back to its original dry width thereby producing a discontinuous coating, forming a sock from the said fabric and fitting it over an airplane part and thereafter applying a cellulose nitrate solution thereto the solvent of which is a solvent for the said cellulose derivative whereby the discontinuous coating of nitrocellulose is colloided and the fabric is shrunk tightly around the said air- 7 plane part.

ROBERT L. LESTER.