US2086729A - Optical screen and material used in the manufacture thereof - Google Patents

Description

w. H. MOSS July 1-3, 1937.

2,086,729 MATERIAL USED IN THE MANUFACTURE THEfiEoF SCREEN AND Aug. 10, 1934 Filed Patented July 13, 1937 UNITED STATES I OPTICAL SCREEN AND MATERIAL USED IN -THE MANUFACTURE THEREOF William Henry Moss, London, England, assignor to Celanese Corporation of America, a corporation of Delaware Application August 10,

1934, Serial n'a vsazse In Great Britain September 1, 1933 3 Claims.

This invention relates to optical screens, and in particular to translucent projection screens such as are used, for example, in cinematography and allied arts.

5 Translucent projection screens such as are used in cinematography, for example where'it is required to provide a back-ground for a scene being photographed, have hitherto been constructed of glass. The screens in question are often required to be of large dimensions and the manufacture of such screens from glass, particularly since the surface is required to be substantially plane, is attended by considerable expense. It is the primary object of the present 15 invention to avoid this and other disadvantages of the use of glass, among which may be mentioned liability to breakage and undesirable weight, by substituting for glass 9. more suitable material and modifying the construction of the screen accordingly. In the case of rigid or semirigid unbreakable glass substitutes, difl'iculty is encountered in ensuring that the surface shall be plane, particularly in view of the tendency of many of such materials to warp. The present invention contemplates a material which shall be sufliciently strong and flexible to be stretched so as to-provide a plane surface. that very satisfactory screens can be made from sheets of flexibletranslucent material comprising 0 a textile fabric, having its interstices filled with an artificial film-forming substance.

The present invention therefore comprises optical screens, the sheet material of which comprises a textile fabric having its interstices 35 filed with an artificial film-forming material.

The fabric forming the basis of the sheet material of the screen is preferably a plain woven one, but special means maybe adopted in weaving-to reinforce the fabric so as to enable it to withstand the tension necessary to stretch it so as to provide a plane surface. With this object in view the fabric may be reinforced by a strong. selvedge and by extra strong wefts at the ends. Stretching of the fabric may be facilitated by turning over each edge in the form of a hem into which can be inserted a suitable member, e. g. a metal rod or lath, to which the tension can be conveniently applied. The hems, instead of being continuous so that the laths have to be gripped at their ends", may have openings in them at intervals through which the laths may be gripped, e. g. by cords or wires attached to tensioning means. When the material of the fabric is thermoplastic, heat and pressure may 55 be applied to cause the yarns of the hem to co- I have foundalesce so that the laths are vertically embedded in the fabric. Instead of laths or rods, strong,

' cords or wires, having loops or eyes at intervals along their lengths may be worked into the fabric along each side to facilitate tensioning. 5 Again the fabric,if a thermoplastic one, may be consolidated under heat and pressure at the edges, so as to embed the cords in the material.

The fabric forming the basis of the sheet material preferably has'a basis ofcellulose acetate. 10 Otherthermoplastic filament-forming substances may, however, .be employed, among which .mention may be made of other organic esters of cellulose, for example cellulose formate,

propionate or butyrate, ethers of cellulose, for

example ethyl, methyl, benzyl cellulose, ether.- esters of cellulose, for example oxyethyl cellulose acetate, esters of cellulose containing inorganic radicles, for examplenitrocellulose and cellulose nitroacetate, and thermoplastic derivatives having a noncellulosic base, for example polymerized vinyl. acetate and other polymerized vinyl compounds. The yarns of celulose acetate or other thermoplastic material used in the manufacture of the fabric may be produced by a wet or dry spinning method. Since high tensile strength is of advantage for the materials used in the present invention, it is preferable to employ in the manufacture of the yarns base materials which are particularly adapted to produce yarns of high strength, and/or to subject the yarns. during or after their production to processes which enhance their tensile strength. Thus base materials of relatively high viscosity may be employed with advantage. case of cellulose acetate, the yarns from which the fabric is made may have a viscosity of the order of 30-, and may-with advantage have a considerably higher viscosity, for example 50 or even 100 ormore, these viscosities being arrived 40 at by comparison of the viscosity of a 6% solution of the' cellulose acetate in acetone with glycerine at the same temperature. In spinning the yarns, special methods may be adopted with aview to obtaining a high tensile strength. The spinning may, for example, be carried out by one of the processes of U. S. applications S; Nos. 402,785 filed 26th 'October, ,1929, 418,414 filed 3rd January, 1930, 437,423 filed 20th March, 1930, 469,622 filed 21st July, 1930, and 601,043 filed 24th March, 1932, in which a considerable drawdown is applied during spinning. Again, the yarns may, continuouslywith or after their production, for example by a dry spinning process, be stretched in the presence of stretch-assisting For example, in the to 200% or even 500% or more of their original length. Further, cellulose ester materials may be partially saponified and this process may be effected so as to enhance the tensile'strength of the materials. The yarns used in the manufacture of' the fabric may have a tensile strength of 2.5, 3 or-even 4 or more'grams per denier.

However, yarns of much lower tensile strength,

e. g. 1.5-2 grams per denier or even less may be used.

Although it is preferable to employ as the material for the base fabric a thermoplastic subproduced by any convenient process, but again due regard should be had to the desirability of obtaining materials of relatively high tensile strength. In the case of regenerated cellulosic materials of the viscose or cuprammonium type, it is of advantage in view of this object to treat the formed materials with, or even to form them in the presence of, concentrated mineral acids, for example sulphuric or hydrochloric acid, preferably while they are under tension. Such processes may be carried out so as not only to increase the strength of the materials but also to render them translucent or even transparent if they are not already so. This is a point of importance since whatever the nature of the fabric employed it should preferably be naturally translucent or transparent, or capable of being rendered so at some stagein the manufacture of the sheet material, for example by treatment with the film-forming substance employed to fill up the interstices in the fabric. Cellulose acetate and similar derivatives of cellulose may be prepared in a form sufficiently translucent for the present purpose without further treatment. Moreover, treatment of such materials with, for example, polymerized vinyl acetate, which may form the basisof the material used in filling the interstices in the fabric, may render them even transparent. Materials having a cellulosic base, however, may require some special treatment such as that, indicated above. The yarn may, before'or aft'e 1 -is made into fabric, be treated with a substance" adapted to decrease its opacity, e. g. an oil fat or wax, or a natural or synthetic resin. Such substances may even in some cases be introduced into the dope from which the yarn is spun. The last method is however of limited application since in many cases such substances are incompatible either with the filament-forming material itself or with other constituents of the spinning solution, and tend to become dispersed throughout the material and to decrease its lustre without achieving the result desired. A more generally useful method in the case of wet spun products is to introduce such substances into the coagulating bath or in wet or dry spinning processes to apply-them, for example in a bath or by means of padding rolls or a Wick, to the spun materials continuously with their production. On the other hand, particularly when the film-forming composition is adapted to decrease the opacity of the fabric, it may even be of advantage to subject this to processes adapted to increase the opacity or to apply agents to a relatively great extent, for example such processes in the manufacture of the yarns. Thus, for example, substances incompatible with the base of. the materials may be distributed throughout the filaments, for example by introduction into the spinning solution, or methods such as those outlined below may be used. Whatever method is employed, however, care should be taken that the strength of the filaments is not unduly .afiected. I

The fabric may be made from a fibre of natural origin, for example a cotton, linen or other vegetable fibre. In this case also the material may with advantage be subjected to the action of strong mineral acids and to other processes directed to decreasing the opacity of the material. Further, materials of vegetable origin may, if desired, be mercerized, preferably under tension, or subjected toother superficial treatments, e. g. immunization. v

The film-forming substances employed to fill up the interstices of the fabric are preferably thermoplastic substances such, for example, as those listed above in connection with the fabric. Excellent results have been obtained, particularly where the fabric was composed of cellulose acetate by the use of polymerized vinyl acetate as the film-forming substance. The substance employed may be of the same chemical nature as that from which the fabric is made, but it is of advantage, with a view to its application to the fabric without damage thereto, that it should have different solubility characteristics. For ex ample a fabric made of acetone-soluble cellulose acetate may readily be coated with a composition containing a cellulose acetate of different solubility characteristics dissolved, for instance, in ethyl acetate, without danger of the solvent attacking the fabric. Again, a cellulose acetate of high acetyl value, soluble in ethylene methylene ether may be used for the film-forming composition when the fabric comprises a cellulose acetate of lower acetyl value insoluble in ethylene methylene ether. It is of considerable advantage for the film-forming substance to be soluble in volatile solvents, so that it can be applied in solution in such solvents, for example by spraying, by extrusion on to the moving fabric in an evaporative atmosphere, by spreading, or by a bath treatment the solvent being removed by evaporation leaving a coherent coating of film-forming material between the interstices of the fabric.' A film-forming substance which is soluble in volatile solvents may even be applied to a fabric comprising a substance of the same solubility characteristics without damage, by spraying methods such'that the solvent evaporates very rapidly, so that the composition impinging on the materials contains no more solvent than is required to produce a coherent coating. The liquid composition containing the film-forming substance may contain plasticizers therefor which should preferably be compatible with the material of which the fabric is composed. A large number of plasticizers are available for use with polymerized vinyl esters or the like and/or cellulose acetate and like derivatives of cellulose. Among these mention may be made of sulphonamides, for example toluene ethyl sulphonamide and the isomeric xylene monomethyl sulphonamides, organic tartrates, 101' example dibutyl or dibenzyl tartrate, organic phthalates, for example diethyl or dibutyl phthalate, esters of polyhydric alcohols, for example triacetin, the phosphates of phenolic and substituted phenolic bodies, for example 'tri-rcsolcinol phosphate and the phosphates of halochlor-phenols with ketones including aliphatic and cyclic ketones, for example acetone cyclohexanone, methyl cyclohexanone, cyclopentanone, cycloheptanone and other ketones of the cyclo paraffin series. Corresponding with diphenylol propane other diphenylol or substituted diphenylol substitution products of methane hydro-carbons or of substituted methane hydrocarbons may be used, the substitution being either by alkyl or arylgroups, as for example in dipara-oxy-phenyl-diphenyl-methane (obtainable by condensation of benzophenone dichloride with phenol). Again the solid intermediate condensation products of furfural or be'nzaldehyde phenol and acetone.

or other aliphatic or cyclic aldehydes with acetone or other ketones including cyclic lgetcnes may be used, the term intermediate product being used to denote the more or less crystalline chemical compounds obtainable before the condensation is carried out sufficiently far to produce a wholly resinous product. Other compounds which may be employed are alkyl derivatives of phenol, for example tertiary butyl phenol. The amount of plasticizer employed will depend upon the natural flexibility of the substance forming* thebase of the film-forming composition. Proportions of 20, 30 and have been used with success where the base of the film-forming composition is a polymerized vinyl acetate and even higher proportions may be used provided the resulting coating is'not thereby rendered sticky. Further, the compositions may contain synthetic resins, for example the resin obtainable by condensation of formaldehyde with diphenylol propane or with the crude condensation product of Very useful compositions are those, for example, prepared with the aid of polymerized vinyl acetate, an equal br somewhat smaller amount down to half the quantity of diphenylol propane formaldehyde resin and a quantity of trioresyl phosphate about equal to the quantity of diphenylol propane formaldehyde resin. Similarly the resins formed by condensation of diphenylol propane with 'furfural or with aldehydes may be used, or the resinous condensation products of formaldehyde, furfural or other aldehydes with other condensation prodnets of ketones and phenols, including the condensation productsof phenols with the'ketones of the cyclo paraflin series, such as cyclo hexanone, cyclo .pentanone and the like. Again the resinous products obtainable from phenolic bodiesandacetone or other aliphatic ketones or the cyclo paraflin ketones such as cyclo hexanone may be used, and in addition the synthetic resins obtainable from furfural'or other aldehydes and acetone or other ketones. The com positions may, moreover, contain sulphonamide aldehyde resins, urea formaldehyde resins or the resins obtained 'by condensation of glycols, glyc erine and other polyhydric alcohols or derivatives thereof with phthalic acid, phthalic anhydride or other polybas'ic acids or anhydrides. The compositions may contain substances adapted to decrease the inflammability of the material, and the presence of such substance is of particular value where the film-forming base or theniate- V rial of which the fabric is made is a highly inflammable substance such as nitro-cellulose. The substance employed to decrease the inflamma- 'bility of the material may comprise a halogenated organic compound or a compound capable, on

the application of heat, of evolving a non-inflammable gas, for example an ammonium salt.

The non-inflammators used may, as in the case,

agents for used include polymerized acrylic esters and similar polymerized esters of unsaturated acids and alkyde resins, preferably modified by heating with higher fatty acids or glycerides thereof.

Although'it is preferable to employ for the film-forming substance to be applied to the fabric a thermoplastic substance which is soluble'in volatile organic solvents, non-thermoplastic'substances may be 'used. I may, for example, deposit cellulose upon the fabric from solutions containing cellulose or unstable cellulose derivatives, for example from viscose or cuprammoniurn solutions of cellulose. Such materials are preferably employed when the fabric forming the base of the sheet material itself consists of or contains cellulose or regenerated cellulose'threads. The viscose or cuprammonium cellulose may conveniently be applied to the fabric by extruding the solution on to the fabric while it is passing through a bath containing a suitable coagulating agent which must not, of course, be of such anature as to affect the fabric deleteriously. Dyes may be applied to the sheet material of the screens either for the purpose of obtaining 'coloured screens which are desired for certain purposes or for obtaining special effects such as a screen of increasing opacity towards the centre as referred to hereinafter. For coloured screens the dyestuff may be applied uniformly over the' sheet material or locally, depending upon the ef fect desired, and of course multi-colour effects may be produced in any desired design. The dyestuffs may be included in the film-forming composition applied directly to the fabric or may be included in. a special surfacing composition.

.The sheet material should preferably have a matt finish on one or both sides. This may be applied in various Ways, depending upon the nature of the material. A convenient method which is of general application is to apply to the coated fabric a further coating composition containingcellulose acetate or other, preferably thermoplastic, film-forming substance, preferably in solution in a volatile solvent or solvent mixture, and a finely divided colourless or white talc, and other white or colourless heavy metal oxides, hydroxides and silicates. Insoluble organic compounds of high melting point, such as are referred to in U. S. application S. No. 173,781 filed 7th August, 1930, can also be used. Instead of applying the finely divided materials in the form of an additional coating upon the already coated fabric, they may be incorporated in the film-forming composition with which the interstices of the fabric are filled. In this event, however, unless the thickness of the sheet is considerably greater than that of the fabric, there is some possibility of an uneven effectv being producedin the sheet material owing to those parts passing it between heated rolls or through a platen press, so as to flatten the warp and weft yarns and, if the treatment is applied after the application of the film-formingcomposition, in

addition distribute the coating composition containing the finely divided substance more evenly over the whole fabric. Instead of coating the material with a composition containing a finely divided material the matt effect may be produced in the case of material coated with cellu lose acetate or like thermoplastic or film-forming substances simply by subjection of the sheet to pressure, preferably at elevated temperatures. It may, for example-be passed between rolls which may be heated and/or suitably engraved. Again, where the coating comprises cellulose acetate or like substances, a matt effect may be produced by subjecting the material to the action of moist steam. A method of wider application, and capable in fact of use whatever the nature of the coating composition, is to subject the surface of the material to abrasion. A further method applicable when the material is soluble in organic solvents is to subject it to treatment with a liquid composition containing a solvent in such concentration as to pit the surface of the material. Again, the material may be coated v with a film-forming substance in form having a discontinuous internal structure, e. g. cellulose acetate deposited from an aqueous bath in the absence of swelling agents, salts or other substances which tend to assist the formation of a continuous gel structure.

For certain purposes it is desirable to have the centre portion of the screen more opaque than the peripheral portions. This efiect may be obtaincd by applying to the centre portion a heavicr' coating of the matt'surfacing material. A similar effect may be obtained by applying to the centre portion a clear coat containing a suitable dye.

The stretching of the sheet material to obtain a plane surface may be effected during its manufacture. The fabric may, for example, be clamped into a suitable frame while held under tension in two directions at right angles. The film-forming composition and/or any finishing composition desired may then be .applied to the material, preferably by spraying, while it is still stretched in the frame. Alternatively, the matcrial may be clamped in a frame under substantially no tension and the requisite tension may be obtained by the careful application of a shrinking agent, for example, when the fabric is of .cellulose acetate or the like, one of the substances referred to in U. S. application S. No. 611,240 filed 13th May, 1932. On the other hand, the stretching of the sheet material may take place as the last stage in the manufacture. Any convenient stretching means may be employed. The material may, for example, be held between jaws, the distance apart of which can be varied by means of screws, or attached to rollers by rotation of which tension can be applied, so as to stretch the fabric until it presents a plane surface.v Or, as indicated above, the edges of the fabric may be providedwith rods or laths for the application of the tension. Such rigid members may be held apart, for example, by

by means of heat and pressure.

means of cords or struts, particularly struts which are hinged so as to produce a toggle effect. When the fabric is provided along its edges with eyelets or loops, they may be attached to suit-:

able tension devices, forexample springs, rollers, rotatable. pegs or levers, by means, for instance, of cords, wires or the like.

The following is an example of the manufacture of a fabric suitable for use in the production of the screens of the invention.

Example spraying so as to give a weight of about 4 ounces per square yard of dry film, This has the effect of making the cellulose acetate almost transparent. The dope-used in coating has the following composition Vinyl acetate of medium viscosity ounces 4.5 Amyl acetate ccs 540 Butyl alcohol ccs 56 Toluene ccs 336 The fabric is dried, after which a dope of the following composition is sprayed on to one'or both sides:

The effect of this coating is to'give the material a semi-opaque translucent surface.

The accompanying drawing illustrates diagrammatically tensioned optical projection screens in accordance with the present invention.

In Fig. 1 the fabric 4 coated as above described and provided 6 in the reinforcement is held by means of-the eyelets 6 and the springs I tothe frame 8. In

Fig. 2 the fabric 4 has cords or wires 9 stitched or otherwise attached thereto. In the case of a thermoplastic fabric the cord or wire may, as previously described, be embedded in the material The cords or wires 9 are continued out-of the fabric as cords or wires ID for tensioning purposes. The fabric is of course coated as previously described. In Fig. 3 the fabric 4, provided with hems I I through which are passed with reinforced edges 5 and eyelets coated as previously described, is

stiff laths or rods II for holding the fabric taut.

As previously described the hems may have openings in them to enable the rods or laths to be gripped at intervals by cords or wires.

What I claim and desire to secure by Letters Patent is:-+

1. Translucent optical projection screens comprising a sheet of an organic derivative of cellulose textile fabric having its interstices filled with a film-forming material comprising a, polymeric vinyl compound and having at least one matt surface, and means for maintaining the said sheet under tension.

2. Translucent optical projection screens comprising a sheet of a cellulose acetate textile fabric having its interstices filled with polyiiierized vinyl acetate and having at least one matt surface, and

meai is for maintaining the said sheet under tension.

3. Translucent optical projection screens com prising a, sheet of a cellulose acetate textile fabric having its interstices filled with e polymeric vinyl compound, at least one surface oi the 'said sheet being coated with e cellulose derivative composition containing finely divided silica, and means ior maintaining the said sheet under tension,

WILHAM HENRY MOSS.

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