Drying method

A method for drying a running web or belt-like support with an organic solvent-containing coating solution coated thereon is described, comprising introducing the support into a drying room and drying it mainly by hot air blown into the drying room in parallel to the running direction of the support in such a manner that it does not strike directly on the coated surface of the support, while conveying the support in an arch-like or straight form on rolls provided in the drying room. This drying method provides high quality coated-layers and increases producitivity. Further, this method is particularly useful in the preparation of magnetic recording material.

FIELD OF THE INVENTION 
The present invention relates to a method for drying materials coated with 
an organic solvent-containing coating composition. More particularly, it 
relates to a method for drying web or belt-like supports (hereinafter 
referred to as "supports") which have been coated with an organic solvent 
containing coating composition and are run continuously. 
BACKGROUND OF THE INVENTION 
Organic solvents are usually used in the preparation of magnetic recording 
materials, photographic light-sensitive materials, heat-sensitive sheets, 
etc. 
Various drying methods have been proposed and are now in practical use for 
the prevention of pollution, recovery of organic solvents and so forth. 
For example, there is a method employed for magnetic recording materials 
in which a continuously running support, e.g., a plastic film, is 
continuously coated with a coating composition prepared by dispersing 
magnetic powder, such as ferromagnetic iron oxide, ferromagnetic chromium 
dioxide, and ferromagnetic alloys, and a binder in an organic solvent. If 
desired, the magnetic recording material may be subjected to various 
treatments such as magnetic field orientation. Thereafter, the coated 
support is sent to a drying room where it is continuously dried by blowing 
hot air thereonto while conveying it in an arch-like or straight form on a 
number of rolls provided in the drying room. 
The various drying methods can be divided into two groups--i.e., a 
horizontal flow drying method and a vertical flow drying method. In 
accordance with the horizontal flow drying method, a coated support is 
conveyed in a drying room at a proper speed (as described hereinafter) and 
hot air is blown onto the coated surface of the support horizontally 
relative to the coated surface in either the same direction as or the 
opposite direction to the direction of the movement of the support. On the 
other hand, in the vertical flow drying method, hot air is blown 
vertically onto the coated surface of the support through holes or slits 
provided above a support-conveying unit in the drying room. 
In each drying method, the solvent is vaporized during the drying procedure 
and thus there is the danger of an explosion when the gas concentration in 
the drying room reaches a certain level. It is therefore necessary to 
maintain the gas concentration below the lower critical limit of the 
solvent gas explosion concentration--e.g., about 25 to 33% to prevent an 
explosion. Hence, it is preferrable to feed a larger amount of air to 
maintain the gas concentration below the critical limit. 
In accordance with the horizontal flow drying method, a good coated-surface 
can be obtained only when hot air is blown onto the support uniformly over 
the entire width of the support since, as described above, the hot air is 
blown directly onto the coated surface in either the same direction as or 
the opposite direction to the direction of the movement of the support. 
In the early stages of the drying procedure, the coated surface still 
possesses fluidity. Thus, the coated surface is readily influenced by 
conditions such as the air-blowing speed. When the air-blowing speed is 
increased, the support beings to flutter and it becomes impossible to keep 
the support in a stable running condition. 
In the vertical flow drying method, it is also necessary for the gas 
concentration to be maintained at a level below the lower critical limit 
of solvent gas explosion concentration (25 to 33%) for the sake of safety 
as previously described for the horizontal flow drying method. Further, it 
is desirable to increase the amount of air-feed (air-blowing speed). 
However, such an increase in the amount of air-feed leads to rapid solvent 
vaporization. 
On the other hand, in order to obtain a uniform coated-surface, it is 
preferable to lower the solvent vaporization speed and to decrease the 
air-blowing speed since this reduces the influences caused by air-blowing, 
e.g., fluttering of the web. Hence, it is difficult to satisfy the two 
requirements of maintaining a low level of solvent gas concentration and a 
low solvent vaporization speed at the same time. 
In accordance with conventional methods, it is difficult to produce a good 
coated-layer and increase productivity under safe conditions i.e., while 
preventing an explosion. The reasons for this are explained in detail 
below. 
The temperature of dry hot air is usually from 50.degree. to 120.degree. C. 
(the boiling point of the primary solvent used is 130.degree. C. or 
lower). Within this temperature range, it is suitable for the speed of air 
to be blown onto the coated layer--i.e., the air-blowing speed, to be 
maintained within the range of from 1 to 5 m/sec (the maximum value: 10 
m/sec). Further, it is required that the support-conveying speed be at 
least 40 m/min and the drying time be from 5 to 20 seconds. However, when 
the support-conveying speed is increased to, for example, 60 to 120 m/min 
(1 to 2 m/sec) or more in order to increase productivity, the amount of 
vaporization of the solvent increases. Therefore, if drying is performed 
without increasing the dry hot air-blowing speed (i.e., amount of dry hot 
air-feed), the solvent gas concentration increases. Hence, in performing 
drying by conventional methods, it is necessary to increase the hot 
air-blowing speed (i.e., amount of hot air-feed) to prevent an explosion. 
This makes it difficult to maintain the drying conditions required for 
producing a good coated-layer. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of drying a 
web-like support coated with an organic solvent-containing coating 
solution which is free from any danger of explosion due to an increased 
solvent gas concentration wherein the support is run at a speed of at 
least 60 m/min and wherein the method enables on to obtain a good 
coated-layer. 
The present invention relates to a drying method which comprises coating an 
organic solvent-containing coating solution on a running web-like support 
and sending the coated support into a drying room where the coated support 
is dried by blowing hot air thereonto while conveying it in an arch-like 
or straight form on rolls provided in the drying room, wherein the coated 
support is dried mainly by the hot air which is blown parallel to the 
running direction of the support and in such a manner that it does not 
strike directly on the coated support.

DETAILED DESCRIPTION OF THE INVENTION 
The drying method of the present invention is charactized in that a running 
support with an organic solvent-containing coating solution coated thereon 
is dried mainly by hot air which is blown from air-blowing openings 
designed to be broader than the width of the support and disposed of along 
the running direction of the support in such a manner that it does not 
strike directly on the support. 
The drying method of the present invention will hereinafter be explained 
with reference to the accompanying drawings wherein FIG. 1 illustrates an 
embodiment of the drying method of the invention, and FIG. 2 is a 
perspective view showing the cross-section of the embodiment of FIG. 1. 
Referring to FIG. 1, support 1 with an organic solvent-containing coating 
solution coated thereon is introduced into a drying room 4 and moves on 
rolls 2 at a constant speed. 
Primary organic solvents which are used in the preparation of such coating 
solutions have a boiling point of 130.degree. C. or lower. Examples 
include acetone, methyl ethyl ketone, methyl isobutyl ketone, toluene, 
tetrahydrofuran, methyl cellosolve, butyl acetate, ethyl acetate, 
methylene chloride, and ethylene chloride. The solvents can be used alone 
or in combination with each other. In addition, as sub-organic solvents 
having a boiling point of 130.degree. C. or higher--e.g., 
cyclohexanone--can be used. 
The drying method of the present invention is suitable particularly for use 
in the production of magnetic recording materials--i.e., it is 
particularly suitable for drying materials coated with an organic solvent 
dispersion containing magnetic powder as described above and a plastic 
binder. 
In the practice of drying, hot air is supplied from air-supplying duct 3. 
The thus-supplied air is blown out from slits or holes 6 provided in the 
frying room onto both sides of the support and is then discharged out of 
the system through discharging duct 5 along with the solvent gas which has 
vaporized from the coated layer. When the support-conveying speed is set 
at 60 to 120 m/min or more, it is nearly equal to an air-blowing out speed 
of 1 to 5 m/sec (the maximum speed: 10 m/sec). This permits the production 
of a uniformly dried coated surface. Furthermore, since the solvent gas 
which has vaporized from the coated surface is discharged by the supplied 
air, it is possible to maintain the solvent gas concentration below the 
critical limit of explosion. Moreover, since the hot air is not blown 
directly on the support, the fluttering of the support does not occur. 
This effect is very significant particularly in the early stages of drying 
because the coated layer still possesses fluidity. 
The drying method of the present invention enables one to obtain a coated 
layer of high quality compared with the conventional methods while 
increasing productivity thereof. 
Also, it is possible to provide slits or holes 7 orthogonal to the slits 6 
so as not to exert a bad influence on the coated layer. That is, near the 
air-supplying duct 3, slits or holes 7 are provided with a long distance 
to each other, while slits or holes 7 are provided with short distance to 
each other near the discharging duct 5. 
Although, in the above-described embodiment, air blowing openings are 
provided above the web-like support, such air blowing openings may also be 
provided below the web-like support. 
Furthermore, it is possible to employ the drying method of the present 
invention in combination with conventional drying methods depending on the 
type of solvent employed and differences in the drying conditions. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.