Web coating method and apparatus

An array of at least three parallel rolls is provided. The rolls are constrained against each other to provide an elongated coating chamber through which passes a web entrained on at least one of the rolls. As it exits from the chamber, the coated web surface is wiped by another of the rolls, whose surface is entering the chamber at that point.

BACKGROUND OF THE INVENTION 
This invention relates to web coating, and particularly to a method and 
apparatus for applying a coating composition to a passing web or webs 
under controlled conditions which include close containment of the coating 
composition up to the point of application and wiping of the coated 
surface of the web by a countermoving surface as the web leaves the point 
of application. 
Various coating means of this general nature have previously been proposed, 
as in U.S. Pat. Nos. 3,235,400 and 4,294,872, but the present invention 
provides an alternative that is effective and overcomes disadvantages of 
prior proposals. 
The invention has various possible applications, and presently appears 
particularly useful for coating webs of paper or plastic with highly 
reactive solventless coating compositions, such as the catalyzed silicone 
compositions referred to in U.S Pat. No. 4,256,870. As discussed in that 
patent, such compositions are characterized by a very short pot life in 
the absence of the use of a cure inhibitor which delays cure until the 
temperature of the coating is elevated in a curing oven or the like. Such 
coatings are so reactive that no means of coating has been fully 
satisfactory without use of the inhibitor and a subsequent heat cure. 
Apparatus is available to accurately meter and quickly mix separately 
stored components of such coating compositions, particularly the base 
polymer, catalyst, and cross linker, but with the short pot life of such 
mixture in the absence of an inhibitor, there has been no way to apply the 
coating composition without use of the inhibitor and subsequent heat cure. 
The present invention provides such a means, or at least minimizes 
reliance on heat cure. The result is a means for reducing or possibly 
eliminating heat treatment with attendant savings in the cost of providing 
and operating some or all of the curing oven capacity which is presently 
used. 
Silicone coatings are generally used as release compositions to enable 
pressure-sensitive adhesive tapes or sheets or the like to be easily 
pulled from a protective liner or backing, or to be unwound in self-wound 
constructions, without degradation of the adhesive quality of the 
construction. Silicone coatings with no volatile components require light 
coating weights. The coatings must be reasonably uniform, and the 
combination of uniformity and light coating weight makes a demanding 
coating requirement. The present invention meets this requirement. 
Silicone coatings, and particularly highly reactive coatings, are mentioned 
because they exemplify demanding coating applications, one example being 
moisture cure silicones such as room temperature vulcanization type 
compounds. However, the invention also provides effective coating means 
for coatings other than silicone compositions, e.g., urethanes that are 
moisture-curing. Also, the invention may be used for coatings that are 
neither reactive nor light in coating weight. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a coating method and apparatus 
for carrying out the method are provided, wherein at least three parallel 
rolls are utilized. The rolls are constrained against each other to 
provide an elongated coating chamber through which passes a web entrained 
on at least one of the rolls. As it exits from the chamber, the coated web 
surface is wiped by another of the rolls, whose surface is entering the 
chamber at that point.

DETAILED DESCRIPTION 
Shown in FIG. 1 is an array 10 or rolls 12 mounted with their axes in 
parallel. Three webs 14 are entrained partially around the rolls 12, each 
web 14 being associated with its own roll 12. The rolls 12 are rotated, in 
the directions indicated by the arrows, by suitable power drives (not 
shown) or by the pulling tensions on the corresponding webs 14. 
The closely positioned rolls 12 and the webs 14 entrained thereon form an 
elongated chamber 16 surrounded by the array of rolls. Each of the rolls 
12 is rotated in a direction to move its associated passing web 14 into 
the chamber 16. It will be seen that adjacent pairs of roll surfaces move 
counter to each other, so that as each web 14 enters the chamber 16, it 
wipes against an adjacent web 14 which is exiting the chamber 16. The 
chamber 16 is filled with a coating composition (not shown) supplied from 
a suitable source or sources (not shown) at one or both ends of the 
chamber 16. Each web is bathed in the coating composition as the web 
passes within the chamber 16. The rolls 12 are biased against each other 
by the hydraulic pressure cylinders 18. An alternative arrangement is to 
clamp the axis of one roll and use only two pressure cylinders, which 
still allows all three rolls to move relatively in order to compensate for 
variations in web caliper. Other biasing means for the roll ends may be 
employed. For example, in an initial reduction to practice of the 
invention, end bearings for one roll were clamped into position on the 
frame and the end bearings for the other two rolls were urged toward the 
first by elastic bands encircling the roll end bearings. The coating was 
not of a highly reactive type, and excess coating was allowed to simply 
run out the ends of the chamber defined by the rolls. However, good 
coating action was observed. 
The arrangement shown in FIG. 2 is similar to that of FIG. 1, except that 
two small rolls 12a are provided, and the larger rolls 12 are fixed in 
adjusted positions to allow their corresponding webs 14 to barely pass 
each other in the chamber 16, which is thereby divided into two chambers. 
Only two webs 14 are coated in the installation shown in FIG. 2. 
In the installation shown in FIG. 3, four rolls 12 are provided, each 
having its own actuator 18 and each having its own web 14 entrained on it. 
Again, the chamber 16 is divided by the two closely positioned rolls. 
In FIG. 4, the arrangement is similar, except that the upper and lower 
rolls 12 are fixed and only the two lateral rolls 12 are provided with 
biasing means such as the cylinders 18. Also, the chamber 16 is not 
divided. 
FIG. 5 is similar, except that only two webs 14 are entrained, and the 
coated surfaces of the exiting webs are not wiped by the countermoving 
entering webs but, rather, by the countermoving surfaces of the respective 
upper and lower rolls 12. 
In FIG. 6, a web 14 is entrained over only one of the three rolls 12. The 
other two rolls are adjusted in position to just close the nip between 
them, and the third roll is biased by the cylinder 18. 
FIG. 7 is similar, except that a doctor blade or element 20 is provided to 
remove excess coating from the surface of the leftmost roll as it exits 
the chamber 16. Doctoring of excess coating and recirculation thereof, of 
course, is a common technique for coatings that are not extremely 
reactive. Obviously, the doctoring may be performed at other locations if 
indicated. 
It is preferred to close the ends of the elongated chamber 16, and this is 
required in the case of highly reactive coatings. Suitable end dams may be 
provided, such as the end dam 22 shown schematically in FIG. 8. To 
compensate for the clearances caused by the absence of the web near the 
ends of the rolls, nip seals or blocks 24 may be provided, as shown, on 
the external sides of the rolls. These are preferably yieldable nylon or 
the like to allow for a slight relative movement between the rolls. 
Similarly, an internal end seal or plug 26 may be provided, as indicated 
in FIG. 9. Such an element might be provided at each end of the roll array 
in combination with and mounted on the inner side of a roll dam, such as 
element 22. 
As shown in FIG. 10, a manifold or distributor pipe 28 may run along the 
elongated chamber 16 for distribution of the coating along the length of 
the chamber through small orifices as shown. The manifold 28 may pass 
through and be supported on a suitable end element such as the end dam 22 
indicated in FIG. 8. 
As shown in FIG. 11, filler means which may comprise a stationary filler 
element 30 may be provided to take up a substantial portion of the volume 
of the chamber 16. This increases the through-put velocity of the coating 
material, thereby reducing the resident time for, say, highly reactive 
coating compositions. As indicated in FIG. 11, the coating composition 
itself may be distributed through the element 30, for example, down a 
lengthwise central passage and then outwardly through radial passages 
which may, as shown, lead to the apices of the filler element 30. 
Another arrangement, shown in FIG. 12, employs an internal element 34 which 
provides both a distribution manifold and internal seal fins or doctor 
blades. The latter may be provided only at the ends of the chamber 16 to 
compensate for absence of the webs 14 near the roll ends. 
It should be understood from the above that references to relative biasing 
of the rolls include situations where a roll is fixed and another roll or 
rolls are biased toward it. References to rotating the roll, or, more 
fully, to rotating the roll to move the web in a specified direction, are 
intended to include arrangements where the roll is turned by pull on the 
web as well as those where the roll is turned by other means such as a 
drive that is independent of the web. 
The rolls referred to herein may be smooth or gravure, and may comprise 
materials commonly used for rolls such as steel or elastomer. Smooth rolls 
are presently preferred for highly reactive type coatings. 
It should be evident that this disclosure is by way of example and that 
various changes may be made by adding, modifying or eliminating details 
without departing from the fair scope of the teaching contained in this 
disclosure. The invention is therefore not limited to particular details 
of this disclosure except to the extent that the following claims are 
necessarily so limited.