US3503773A

US3503773A - Process for the production of a casting layer for the manufacture of films and sheets

Info

Publication number: US3503773A
Application number: US610775A
Authority: US
Inventors: Johan Heinrich Bisschops; Eduard Marius Steffens; Andre Alfons Boulonne
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1966-01-21
Filing date: 1967-01-23
Publication date: 1970-03-31
Anticipated expiration: 1987-03-31
Also published as: DE1720051A1; NL6700796A; GB1124499A; FR1509410A; BE692914A

Description

United States Patent U.S. C]. 1175.3 4 Claims ABSTRACT OF THE DISCLOSURE A process for producing a casting layer for casting films of film-forming polymers wherein a known superficially hydrolyzed cellulose acetate layer applied to a metal support is treated with a solution in an organic solvent or a mixture of water and a water-miscible organic solvent of a Werner chromium complex salt to produce a thin coating of such salt on the layer surface, the coating being then heated to about 50 C. to firmly bond the coating to the layer surface.

This invention relates to the manufacture of films or foils from a film-forming solution, and is concerned in particular with the material employed for forming the high-gloss-surface, hereinafter called the casting layer, to which the film-forming solution is applied and from which the film is stripped off.

Films and foils can be prepared by casting a filmforming solution, e.g. a solution of cellulose triacetate or of another film-forming polymer, on a rotating metal drum or on an endless metal belt forming part of a film casting machine. After evaporation of the solvent or mixture of solvents the film formed adheres firmly to the casting surface so that it is difficult to detach it therefrom without deforming the film permanently.

To obviate adherence thereto the metal surface of the drum or endless belt is covered with a casting layer. The layer can only be prepared from compounds, which on drying obtain a very smooth high-gloss surface. Moreover, these compounds should be insoluble and even resistant to swelling in the organic solvents customarily used in the manufacture of the film-forming solution. At the same time the repellency of the casting layer towards the filmforming composition should be relatively high, so that the films formed can easily be stripped off without permanent deformation. When the compounds used to form the casting layer do not possess these properties by nature, it is sufiicient that the casting layer acquires the abovementioned properties by special treatments.

In this way the metal drums or endless belts of film casting machines are usually provided with a casting layer of cellulose acetate. When the metal drum or endless belt has been pretreated as generally known in the art, the cellulose acetate film adheres firmly to the drum or endless belt and dries with a high-gloss surface. In order to confer to the casting layer the desired repellency towards the film-forming solution that is to be applied thereto and to reduce its solubility and swelling tendency in organic solvents, the cellulose acetate layer is superficially hydrolysed with alkalis.

When casting a film-forming solution on the casting layer treated in this way, the film obtained after evaporation of the solvent or mixture of solvents is not of even thickness over the entire width of the film. Indeed, as a result of the evaporation of the solvents the film-forming Ice mass contracts towards the middle of the film so that finally a film with much thinner edges is obtained. The film tension at the moment of separation from the casting layer causes a more or less considerable stretching of the film, which stretching is much more important along the edges where the film is thinner. This may even result in a tearing of the film. The parts of the film remaining attached to the casting layer can easily damage the rather vulnerable casting layer so that the replacement of this layer may be necessary.

It has been found now that casting layers, which do not suffer from these disadvantages, can be prepared by applying an extremely thin layer of a Werner chromium complex salt to the hydrolysed cellulose acetate layer.

According to the invention a process is found for the production of a casting layer, which comprises coating a metal support with a solution of cellulose acetate, evaporating the solvent, superficially hydrolysing in a known manner the cellulose acetate layer formed, and applying thereto a solution in a mixture of water and of a watermiscible organic solvent of a Werner chromium complex salt in which a trivalent chromium atom is coordinated with a carboxylic acido group, whereby on the superficially hydrolysed cellulose acetate surface a coating having a thickness of 0.01 to 5, is obtained, and heating the coating to about C. in order to secure a fir-m bond between the Werner chromium complex salt layer and the hydrolysed cellulose acetate surface.

The .Werner chromium complex salts, hereinafter called chromium complex salts, useful in the invention include compounds in which a chromium atom is coordinated with a carboxylic acido group. The carboxylic acido group may derive from a monobasic organic acid, such as acrylic acid, methacrylic acid, stearic acid, palmitic acid and lauric acid whereby the best results are obtained with the long chain carboxylic acids. Carboxylic acido group may also derive from substituted carboxylic acids, such as p-aminobenzoic acid, cyanoacetic acid and mercaptoacetic acid, and from dicarboxylic acids such as maleic acid. Fluorinated aliphatic carboxylic acids may also be used in forming the chromium complex salts. The chromium complex salts may be prepared by reaction of the appropriate organic acid with chromium compounds by well known methods, for example as described in U.S. patent specification 2,273,040 filed Sept. 26, 1940 by K. Iler and issued Feb. 17, 1942.

In the process of the invention very good results are obtained with chromium complex salts wherein 1 to 5 chromium groups are present for every acid radical.

As solvents for the chromium complex salts may be used organic solvents, such as methanol, ethanol, isopropanol, acetone, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, and water.

Although Water is a good and cheap solvent for the chromium complex salts, it has some disadvantages to be used alone. Indeed, water is characterised by its very high surface tension. When water would be used as the sole solvent for the chromium complex salt, a uniform wetting of the superficially hydrolysed cellulose acetate layer would not be possible during the casting of the chromium complex salt solution. When drying the chromium complex salt layer, there would always be found areas where all of the chromium complex salt would be pulled away in consequence of the high surface tension of water and where consequently the superficially hydrolysed cellulose acetate layer would remain uncovered. In order to remedy this disadvantage a certain amount of a water-miscible organic solvent is added, e.g., methanol, ethanol, isopro' panol, acetone, ethylene glycol monomethyl ether, and ethylene glycol monoethyl ether. Good results are gained when at least 10% of organic solvent is present calculated on the total amount of water and organic solvent. A mixture comprising approximately 60% of water and 40% of organic solvent is preferred, however. The chromium complex salts are preferably applied in very low concentrations, even lower than 0.5 g. of chromium complex salt per liter of mixture of water and organic solvent. When solutions of much larger concentrations are applied to the superficially hydrolysed cellulose acetate layer, the high gloss surface of the latter disappears, and the coating of chromium complex salt, which remains after evaporation of the solvent or solvents, is not of even thickness over the entire width of the casting layer. Consequently, a very diluted solution of chromium complex salt is applied as an extremely thin layer to the superficially hydrolysed cellulose acetate surface. The drying time of such a thin layer is extremely short so that no transport of dry substance can occur under the influence of the surface tension of water, and the formation of casting lines and craters is avoided.

In this way the high-gloss of the hydrolysed cellulose acetate surface and the repellency characteristic of the thin chromium complex salt layer are combined. Optimal results are reached with layers of chromium complex salt having a thickness ranging between 0.01 and 5p.-

After application of the chromium complex salts to the hydrolysed cellulose acetate surface, the casting layer is heated for a short time to about 50 C. in order to secure a good bond between the hydrolysed cellulose acetate and the chromium complex salt layer. Heating to the temperature indicated for the lapse of time needed to dry the layer applied and eliminate the solvents by evaporation, may sufiice in most cases. In some instances, however, the heating time may be increased to one hour and even more.

The coating composition of chromium complex salt dissolved in a mixture of water and organic solvent can be applied to the superficially hydrolysed cellulose acetate surface according to any known coating system e.g. roll coating, dip coating, or knife coating. Care must be taken that the amount of solution applied is such that on drying a layer having a thickness of 0.01 to 5/L remains.

The amount of chromium complex salt applied per sur face unit of casting layer depends on the concentration of chromium complex salt in the solution and on the quantity of solution applied. Moreover, the concentration is dependent on the coating system. If e.g. an applicator roll is used, the amount of applied liquid is dependent on the speed of the roll and of the rotating drum or endless belt, on the distance between them, and on the number of coatings applied. The result is that the concentration of chromium complex salt should be adapted to the casting machine. When using a low rotation speed of the roll and of the casting machine and a distance of approximately 0.1 mm. between them, the concentration of chromium complex salt may even be lower than 1 g. per 1000 cc. of solvent mixture, and in this case the application can be carried out in two to four operations.

The repellency of the thin layer of chromium complex salt towards the film-forming solution is so strong that the film can be stripped with a higher solvent concentration from the casting machine without formation of stripping lines. This makes possible an increase of the casting speed.

The starting and the end of the coating procedure are reproduced visibly as a line on the casting layer. This line cannot be polished away owing to the extreme thinness of the chromium complex salt layer. In order to overcome this defect the coating procedure is started with a pure mixture of solvents. In the supply metering tank this pure mixture is replaced gradually by the chromium complex salt solution. At the end of the coating procedure i.e. after having applied the desired number of coatings, the same action is repeated but in reversed sense, so that finally a pure mixture of solvents is applied again. In doing so it is impossible to perceive a beginning or an end of the chromium complex salt layer. The replacement of the pure solvent mixture by chromium complex salt solution of increasing strength is carried out by means of a pump system. The pump removes pure solvent mixture and adds equal amounts of chromium complex salt solution. When the appropriate solvent concentration is reached the pump system is used to keep the solution homogeneous over the entire width of the coating tray. There are also provided means for keeping the level of the solution in the coating tray rigourously constant.

When the chromium complex salt casting layer is especially intended to avoid the tearing of the film edges and when enhancing of the repellency of the casting layer as a whole is not so much important, it may sufiice to apply to both sides of the hydrolysed cellulose acetate sur face a small strip of the chromium complex salt coating solution according to the invention.

The casting layer of the invention is suited for the manufacture of films from solutions in organic solvents of film-forming polymeric materials such as cellulose triace tate, polycarbonates, polyphenylene oxide and polyesters such as the copolyester of isophthalic acid, terephthalic acid and 2,2-bis(4-hydroxyphenyl)-propane. The sole condition is that the film-forming polymeric material must be soluble in an organic solvent at ordinary temperatures.

The following examples illustrate the present invention.

EXAMPLE 1 A casting layer of cellulose acetate is applied to the endless belt of a film casting machine and hydrolysed superfically according to known techniques.

A mixture of Water and ethanol (60:40) is then applied to the hydrolysed cellulose acetate surface with the aid of a roll coating machine. This pure mixture is gradually replaced in the supply metering tank by means of a pump system by a solution in the same mixture of water and ethanol (60:40) of a chromium complex salt wherein a chromium atom is co-ordinated with an acido-group derived from methacrylic acid. Of the methacrylato-chromic chloride obtained 1 g. is dissolved in 1000 cc. of the above indicated solvent mixture.

The roll coating machine is regulated such that approximately cc. of solution are coated per sq. m. The coated layer has a thickness of approximately l,u., is almost immediately dry; and shows no craters or lines.

After having provided the whole endless belt with a layer of methacrylatochromic chloride the solution of chromium complex salt is gradually replaced by the pump system with a pure mixture of water and ethanol. In doing so, no starting or end line of the applied chromium complex salt layer can be noticed.

The endless belt is then heated for 2 hours with an air-stream at 50 C. to bring about a good adhesion between the chromium complex salt layer and the superficially hydrolysed cellulose acetate layer.

If a cellulose triacetate solution known from the technique is applied to the thus formed casting layer and the solvents are partially evaporated, the film formed can be stripped much more easily without being torn or deformed.

EXAMPLE v2 The proces of Example 1 is repeated, with the proviso that in the chromium complex salt a chromium atom is co-ordinated with an acido group deriving from p-aminobenzoic acid. 0.5 g. of the chromium complex salt is dissolved in 1000 cc. of a mixture 20:80 of Water and ethanol.

A casting layer is obtained from which the cellulose triacetate film can be stripped off without harm or deformation.

EXAMPLE 3 The process of Example 1 is repeated. The chromium complex salt used contains a chromium atom co-ordinated with an acido group deriving from mercaptoacetic acid, i.e. mercaptoacetato chromic chloride. Of this complex salt 0.5 g. is dissolved in 1000 cc. of a mixture of water and ethanol (60:40).

The endless belt is heated for 20 mintues at about 50 C. with a hot air stream. The casting layer thus formed enables a cellulose triacetate film to be stripped ofi much faster and without being torn.

EXAMPLE 4 The process of Example 1 is repeated, but a mixture of water and ethanol (50:50) is used as the solvent. The chromium complex salt derives from cyanoacrylic acid, viz, a cyanoacrylato chromic chloride.

The coating system is regulated such that a layer having a thickness of approximately 2p. is obtained on drying. The layer is heated for 1 hour to 55 C. by means of a hot air current. The casting layer formed enables the cellulose triacetate film to be stripped off much faster and without being torn.

EXAMPLE 5 The process of Example 1 is repeated. The chromium complex salt derives from stearic acid, viz. stearatochromic chloride of this complex salt is dissolved in 1000 cc. of solvent mixture.

A layer of stearatochromic chloride of approximately 1, thickness is applied and this layer is heated for 1 hour at 50 C.

This casting layer enables the cellulose triacetate film to be stripped off much faster and without being torn.

EXAMPLE 6 The process of Example 1 is repeated. The chromium complex salt derives from palmitic acid. Of this complex salt 1 g. is dissolved in 1000 cc. of solvent mixture. Cellulose triacetate film can be easily stripped ofi without being torn at the edges.

EXAMPLE 7 The process of Example 6 is repeated but the palmitato chromic chloride is used in a concentration of 0.5 g. per 1000 cc. of mixture of water and ethanol (50:50). Curing of the applied layer occurs by heating at about 60 C. for 30 minutes.

The cellulose triacetate film which is applied on the casting layer so formed can be stripped off much faster without being torn.

EXAMPLE 8 A casting layer of cellulose acetate is applied to the endless belt of a film casting machine and hydrolysed superficially according to a known technique.

According to the method described in Example 1, a layer of about Lu. is applied to the hydrolysed cellulose acetate surface of a chromium complex salt wherein a chromium atom is co-ordinated with an acido group deriving from stearic acid. Hardening of the chromium complex salt layer is obtained by heating as in Example 1.

Onto this hardened chromium complex salt layer is coated a solution of a polycarbonate of 2,2-bis(4-hydroxyphenyD-propane in methylene chloride (50 g. of polymer per 100 cc. of solvent).

The films obtained can be taken from the machine when not using the above chromium complex salt layer.

6 EXAMPLE 9 Example 8 is repeated but after the hardening of the chromium complex salt layer another layer is applied thereto from a solution of polyphenylene oxide in chloroform g. of polymer per cc. of solvent). The film obtained does not stick to the casting layer and can be stripped off very easily.

EXAMPLE 10 Example 8 is repeated but after the hardening of the chromium complex salt layer another layer is applied thereto from a solution of the copolyester of isophthalic acid and terephthalic acid (75:25) with 2,2bis(4-hydroxyphenyl)-propane (25 g. of polymer per 100 cc. of solvent). Stripping off of the film obtained requires but a very small force.

We claim:

1. Process for the production of a casting layer for casting films of film-forming polymers, which comprises coating a metal support with a solution of cellulose acetate, evaporating the solvent, superficially hydrolyzing the cellulose acetate layer thus formed, apply thereto a solution in an organic solvent or in a mixture of water and of a Water-miscible organic solvent of a Werner chromium complex salt in which a trivalent chromium atom is co-ordinated with a carboxylic acido group to produce on the superficially hydrolyzed cellulose acetate surface coating of said chromium complex salt having a thickness of 0.01 to 5p, and heating the coating to about 50 C. in order to secure a firm bond between the chromium complex salt layer and the hydrolyzed cellulose acetate surface.

2. Process according to claim 1, wherein the Werner chromium complex salt is dissolved in an organic solvent taken from the group consisting of methanol, isopropanol, acetone, ethylene glycol monoethyl ether and ethylene glycol monomethyl ether.

3. Process according to claim 1, wherein the Werner chromium complex salt is dissolved in a mixture of water and of a water-miscible organic solvent taken from the group consisting of methanol, ethanol, isopropanol, acetone, ethylene glycol monoethyl ether and ethylene glycol monomethyl ether, said mixture comprising at least 10% of water-miscible organic solvent calculated on the total volume of water and organic solvent present.

4. Process according to claim 1, wherein 1 g. of chromium complex salt is present per 1000 cc. of solvent or solvent mixture.

References Cited UNITED STATES PATENTS 1,529,712 3/1925 Pfiifner 1l7-6O 2,726,164 2/1952 Eichrneier 1175.l 2,772,141 11/1956 Dunlap 1l75.1 3,039,887 6/1962 Schneider et al. 117-5.3

FOREIGN PATENTS 386,775 1/ 1933 Great Britain. 797,050 1/ 1958 Great Britain.

WILLIAM D. MARTIN, Primary Examiner RAYMOND M. SPEER, Assistant Examiner US. Cl. X.R.