US2320473A - Casting sheet material - Google Patents

Description

June 1, 1943. w J. H. ROONEY ET AL 2,320,473

CASTING SHEET MATERIAL Filed Feb. 12, 1941 INVENTORS. J. H. ROONEY- P. R- HAWTIN.

BY D-C- MACPHERSON.

ATTORNEYS material.

Patente dJune l, 1943 CASTING SHEET MATERIAL James Henry Rooney, Philip Richard Hawtin, and Daniel Carswell Macpherson, Spondon, near Derby, England, assignors to Celanese Corporation of America, a corporation ol Dclaware Application February 12, 1941, Serial No. 378,632

in Great Britain March 6, 1940 9 Claims.

This invention relates to sheets, especially sheets of cellulose acetate or like film-forming Thick sheets of cellulose acetate, i. e., sheets of thickness 0.01" or higher, have previously been made by forming a block of the cellulose acetate together with a plasticiser, cutting sheets of the required thickness from this block, and seasoning, straightening, and polishing these sheets. The present invention provides a simpler, more rapid, and cheaper method of producing such sheet materials. The process of the invention has the further important advantage that within certain limits the size of the sheets produced can be readily varied. In the block method, on the other hand, for economic working, the size 'of sheet produced depends on the size of block press available, and the capital outlay involved in increasing either the output of the plant or the-size of the sheets is very great. 7

We have now found that the advantages referred to above can be obtained by building up the sheets from a large number of successive thin layers formed by the evaporation method on a support travelling through a heated evaporative atmosphere, stripping" the product from said support while it still contains sufficient volatile solvent to be stripped without damage.

cutting sheets of the required. size from the product, seasoning these sheets, i. e., removing casting box, it has changed to the solid state. It there receives a further coating of dope, and this process is repeated until a sheet of the desired thickness has been built up.

Care must be taken to avoid bubble formation within the sheets. Exclusion of dust from the film-forming band and surrounding atmosphere, and the use of high quality dope are essential precautions. In addition, it is of considerable advantage to pretreat the dope so as to removeany dissolved gases, and to limit the speed of the band and the temperature of the e'vaporative atmosphere. V

With regard to the pre-treatment of the dope, useful results are obtained by maintaining the dope for severalhours at a temperature of about 40-45 C. under a reduced pressure which, however, need not be low enough to cause the dope to boil at this temperature.- Another useful method of pretreatment is to make up the dope with an excess of acetone over that finally required and to boil off this excess under reduced pressure immediately before use. The excess of weight of the dope after the boiling, or may be the bulk of the residual volatile solvent by slow evaporation, and straightening them if necessary. The cutting step may not, of course, be

necessary when'large sheets are required, and for some purposes, e. g. whenthe sheets are to be subsequently moulded, straightening is unnecessary.

acetate in acetone containing the desired content of plasticiser, flows from a casting box onto the surface of the band to form a layer thereon of controlled thickness, e. g., 0.001" to 0.002 or 0.003", which is carried by the band through a above 15%, e. g., up to 40% or even higher. Treatment of the dope under reduced pressure .at ordinary temperature, on' the other hand,

production of a sheet some 30 layers thick can be avoided by maintaining the temperature at 40-45 C., and a sheet of 50 layers be built up under the, same conditions, then bubble formation may occur towards the end of the process but may be avoided either by carrying out the heated atmosphere, preferably enclosed to form a drying'tunnel, wherein the greater part of the acetone is evaporated. Thus by the time the first layer of dope is brought round again to the .whole process at a lower temperature, e. g., about 5 0 lower, or by lowering the temperature during the process, e. g., after the deposition of about 30'layers. Similarly, it is of advantage to work throughout at a temperature some 5 C. lower still when building up sheets of 50-70 layers, or to lower the temperature progressively during the operation to such lower value. Further reductions of temperature of the same order may be made with advantage after about the th and th layers when building up still thicker sheets. i

The speed of the band may also with advantage be reduced as the thickness of the sheet in tion creases. Thus, for instance, in building up a sheet of 50-100 layers, starting with a band speed of about 15 ft. per minute, and reducing the temperature by stages from about 45 C. to about 35 C., the speed may be reduced progressively to about ft. per minute or less, during the process.

It will be understood that the speed and temperature may be maintained throughout the 2,320,473 L minute, The sheet is then stripped from the band while still containing between and by weight ofacetone. After stripping, the sheet is cut into portions of the desired size. These are seasoned in a warm atmosphere until substantially the whole 'of the residual acetone has ness. From the point of view of obtaining maximum production, it is of advantage to work so that at any moment the temperature and speed are just low enough to avoid bubble formation.

The composite sheet material is, as indicated above, removed from the band while still containing enough volatile solvent to be stripped without damage, and is then cut into sheets of the desired size, seasoned and straightened if necessary, The volatile solvent content at the time of stripping from the band may be between 15 and and is preferably in the neighbourhood of 22%, e. g., 20-25%. Seasoning may be effected by supporting the sheets in a warm or hot atmosphere. They may be straightened before this step for-convenience, but will usually require straightening after seasoning even if flat before. This straightening or flattening can be effected in a platen press if one of suitable size is available, or by passing the sheet material in a. straight line through the nips of several pairs of rolls on to supporting and conveying rolls. The press or the straightening rolls may be heated but the rolls should not be heated to such a temperature that the sheet material becomes limp. With, a view to minimising distortion during seasoning, seasoning may be effected while the sheets are tensioned in two directions at right-angles e. g., by resilient tensioning members. The seasoned material may be polished if desired by any suitable means, e. g., by means of chamois leather. I

The following example illustrates the inven- Example A dope of the following composition by weight is prepared:

instead of dimethyl phthalate, other suitable plasticisers or mixtures of plasticisers, e. g., a mixture of three parts of dimethyl phthalate to one of triphenyl phosphate, may be employed.

not essential, however. film-forming support need not be a band. It

been removed and are then straightened under pressure and polished. 1

Instead of cellulose acetate, cellulose acetatebutyrate or cellulose acetate-propionate can be used.

that may be used for carrying out the invention. the film or sheet I is formed by extrusion of a cellulose derivative in a volatile solvent medium through a shaping device 2. The film is formed on a continuous nickel casting band 3 supported on two rollers-4 and 5. The'film I travels (in a clock-wise direction as illustrated) with the casting band 3 repeatedly under the extrusion device .2 and at each revolution of the band 3 a layer of, dope is deposited. The whole apparatus is enclosed in a casing 6 provided with a partition I extending across its width. Evaporative atmosphere enters the casing by means of the inlet pipe v8, a valve 9 being provided to permit control of the rate of flow thereof, and issues from the'casing at the exit pipe l0, whence it may be led to a solvent recovery plant. To permit access for stripping the resultant'sheet, there is provided for the casing 6, a door ll hinged at I2 and provided with the latch 13.

The invention has been described with reference to forming the layers of dope directly on the polished surface of a metal band. This is Thus, the travellin may, for instance, be a drum; Moreover, the

surface of the band or drum may be provided' with a surfacing composition adapted to facili tate removal of the sheet material. surfacing composition may, for example, be of gelatin, pectin, or other film-forming animal or vegetable product; or of an artificial fllm-form ing substance such as a cellulose ether which does not adhere to the cellulose acetate or. other film-forming base of the sheet material. Soaps can also be used to form the casting surface, as described in U. S, application S. No. 375,952, filed January 25, 1941.-

To remove any dissolved air from the dope,

this is boiled under reduced pressure until 10 to 15% of the acetone has been removed.

Thedope is extruded from a casting box on to a .continuous, metal, film-forming band which runs for the greater part of its length in'a drying tunnel provided with controllable heating means. The casting box is set to deposit a layer of dope of thickness between 0.0015" and 0.0025" at each revolution of the band. During the deposition of layers of dope the temperature of the drying tunnel is progressively reduced from 45 C. to 35 C. and the speed of the band is reduced from 14 ft. per minute to 10 ft. per

Although of special importance in connection with the production of thick sheets of cellulose acetate, the process of the invention can be employed with other film-forming substances. e. g., other simple organic esters of cellulose, such as cellulose propionate and cellulose butyrate;

mixed esters of cellulose such as cellulose acetatepropionate, cellulose acetate-butyrate, cellulose acetate-stearate, and cellulose nitrate-acetate; ethers of cellulose such'as ethyl cellulose and benzyl cellulose and ethyl-hexyl cellulose; ether-esters of cellulose such as ethyl cellulose acetate; and film-forming synthetic resins, for

instance, polyvinyl acetate, polyvinyl chloride. polymethyl methacrylate, the polyvinyl ethers and ether-esters, co-polymers of two or more unsaturated compounds such as vinyl alcohol or its derivatives, and the polystyrene synthetic resins. The invention is, in fact, generally ap-'- plicable to the production of sheets which can be built up from thin layers of film-forming substance deposited by the evaporative method. It

is not essential that successive layers should be Referring to the accompanying drawin wherein there is shown one form of apparatus Such a v e. g. sheets of thickness from 0.01"

. removing the bulk formed of the same film-forming substance provided that they bond together satisfactorily.

The invention is of most importance in forming sheets of thickness from about 0.10" upwards, but, as indicated above. it can be used with advantage in forming much thinner sheets,

upwards. Having described our invention, sire to secure by Letters Patent is:

1. Process for the production of sheets of thickness at least 0.01" from a solution of film-forming substance in a volatile solvent, which comprises building up the sheet on a support travelling through a heated evaporative atmosphere, by the successive deposition of layers or said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere as the number of layers increase, stripping the built-up sheet from the support, and

of the volatile solvent remaining in said sheet.

2. Process for the production of sheets of thickness at least 0.01" from a solution of a filmwhat we deby the successive deposition of layers of said solution, a substantial proportion of the solvent iniforming substance in avolatile solvent, which 0 comprises building up -the sheet on a support travelling through a heated evaporative atmosphere, by the successive deposition of layers of said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere, and the speed of said support as the number .of layers increases, stripping the built-up sheet from the support, and removing the bulk oi'the volatile solvent remaining in saidsheet. r

3. Process for the production of sheets of thickness at least 0.01" from a solution oi a filmforming substance in avolatile solvent, which comprises building up the sheet on a support travelling through a heated evaporative atmosphere, by the successive deposition of layers of said solution, tially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature ,of said atmosphere as the number of layers increases, stripping the built-up sheet from the support while it still contains between 15 and 25% .of volatile solvent, and removing the bulk of the volatile solvent remaining in said sheet.

a substantial proportion of the solvent inia tially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere as the number of layers increases, stripping the built-up sheet from the band while itstill contains between 15 and 25% of volatile solvent, and removing the bulk of the volatile solvent'remaining in said sheet.

6. Process for the production of sheets of thickness at least 0.01" from a solution of a filmforming substance in a volatile-solvent, which comprises building up the sheet on a band travelling through a heated evaporative. atmosphere,

by the successive deposition of layers of said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere, and the speed of said band as the number of layers increases, stripping the builtup sheet' from the band whileit still contains between 15 and 25% of' volatile solvent, and removing the bulk of the volatile solvent remaining insaid sheet.

7. Process for the production of sheets of thickness at least 0.01" from a solution of an organic derivative of cellulose in a volatile solvent, which comprises building up the sheet on a support travelling through a heated evaporative atmosphere, by the successive deposition of layers of said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere as the number of layers increases, stripping the built-up sheet from the support, and removing the bulk of the volatile solvent remaining in said sheet.

8. Process for the production of sheets of thicknessat least 0.01" from a solution of an organic .derivative of cellulose in a volatile solvent, which comprises building up the sheet on a support travelling through a heated evaporative atmosphere, by the successive deposition of layers of said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere, and the speed of said support as the number of layers increases, stripping the 4. Process for the production of sheets of thickness at'least 0.01" from a solution or a film.-

it still contains between 15 and 25% ofrvolatile solvent, and removing the bulk of the volatile solvent remaining in said sheet.

5. Process for the production of sheets of thickness at least 0.01" from a solution of a fllmforming substance in a volatile solvent, which comprises building up the sheet/on a band travelling through a heated evaporative atmosphere,

built-up sheet from the support, and removing .he bulk of the volatile solvent remaining in said sheet.

9. Process for the production f sheets of thickness at least 0.01".from a solo ionof an organic derivative of cellulose in a volatile solvent, which comprises building up the sheet on a band travelling through a heated evaporative atmosphere, by the successive deposition of layers of said solution, a substantial proportion of the solvent initially present in each layer being removed by evaporation before deposition of the next layer, progressively reducing the temperature of said atmosphere, and the speed of said band as the number of layers increases, stripping the builtup sheet from the band while it still contains between 15 and 25% of volatile solvent, and removing the bulk of the volatile solvent remainin in said sheet.

JAMES HENRY RODNEY. PHILIP RICHARD HAWTIN. DANIH-I CARSWELL MACPHERSON.

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