1. Field of the Invention
This invention relates to the manufacture of coated sheet material, such as, for example, photographic film or paper.
2. Description of the Prior Art
It is known to manufacture photographic film or paper by coating a continuous web of a transparent film formed of, for example, cellulose acetate or polyethylene terephthalate, or of paper, with materials in liquid form. As is known, the liquid materials may include gelatin and silver halides. After the liquid materials have been coated on the web they have to be dried so that the coated web can be wound up into a roll. It is known to chill the liquid materials prior to commencement of drying, in order to set them. By having the materials in set condition prior to commencement of drying, the drying may be conducted by directing large volumes of hot dry air at the coating. If the coating were to be still in liquid form when the drying is performed, the uniformity of thickness of the coating, achieved at the time of coating, would be destroyed. By setting the coating prior to drying, the uniformity of thickness is maintained through the drying stage.
Chilling has to be conducted in a manner which does not destroy the uniformity of thickness of the coated layer of liquid materials. This is achieved by withdrawing heat from the liquid materials through the web by passing the coated web across chilled rollers in contact with the non-coated side of the web. As is described in U.S. Pat. No. 4,231,164 issued to Eugene H. Barbee on Nov. 4, 1980, the web may be additionally chilled by chilled air directed at its non-coated side, which chilled air also contacts the rollers and serves to chill them. The web is kept in contact with the rollers and is caused to wrap around them for about 20.degree.-45.degree. by a pressure differential on the two surfaces of the coated web. The atmosphere impinging on the coated surface of the web, when the coated web is passing across the chilled rollers, is chilled but its velocity at all places must be so low that air which contacts the liquid coating does not disturb it.
It is desirable to chill the liquid coating rapidly, both to reduce the time during which the coating is still liquid on the web and to reduce the capital and running costs of the chilling zone. One way to increase the rate of chilling of the liquid coating is to lower the temperatures of the impinging atmosphere and chilled rollers right from the beginning of the chilling zone. However, a problem has been encountered when endeavoring to lower the temperatures at the beginning of the chilling zone. This problem is condensation. Atmosphere entering the chilling zone may have a dew point above the temperature in the chilling zone. For example, if the atmosphere outside the chilling zone has a dew point of 8.degree. to 10.degree. C. there will be condensation when such atmosphere meets -4.degree. C. air in the chilling zone. Some of the condensation will land on the coating and cause unacceptable blemishes in the finished product. Also, condensation will accumulate on conveyance means in the chilling zone and such accumulation will interfere with its proper function.
The problem is exacerbated as the dew point of atmosphere travelling with the coated web into the chilling zone increases. Recently there has been a proposal to form an enclosure around the coating station and to raise the pressure of the atmosphere in the enclosure to supra-atmospheric so that drafts and air-borne dust are excluded from the vicinity of the coating station. It is very undesirable to have drafts around the coating station, particularly, but not exclusively, when the coating is performed by the process known as curtain coating. Therefore, atmosphere in the recently proposed enclosure is changed, albeit very slowly by means of a slow downwards and uniform flow of air inside the enclosure. However, the rate of change of atmosphere in the enclosure is such that the dew point increases above that found outside the enclosure, because of moisture evaporation from the coating liquids in the enclosure. The air flow cannot be fast enough to keep the dew point in the enclosure below values ambient outside the enclosure.
Thus, the adoption of such an enclosure, while solving dust and draft related problems, does increase the likelihood of condensation in the upstream end of the chilling zone because the pressure in the enclosure is supra-atmospheric and because the dew point is higher than ambient. Of course, in fulfilling the desire to keep the duration between coating and setting to a minimum, it is desirable to locate the chilling zone as close as possible to the coating station, which further exacerbates the problem.
It is an object of the present invention to solve the problem of condensation in the upstream end of the chilling zone.