Side-feed coating method

Abstract

Method for uniform coating of a composition onto a carrier material in web form, through the use of a coating apparatus comprising at least one element A, that forms with a further element B a nip ahead of which the composition is held up, and that moves relative to the element B in such a way that a rotating body of composition is developed and the composition is conveyed into the nip; comprising introduction of the composition into the intake region of the nip, the application of the composition to a carrier material in web form in the nip or after the composition has travelled through the nip, and which limits the longitudinal extent E of the nip intake region fully fillable by the body of composition.

Claims

1. Method for coating a composition having a complex viscosity (plate/plate rheometer; 25 C.) of 0.1 Pas (100 rad) to 10.sup.6 Pas (0.1 rad) onto a carrier material in web form by means of a coating apparatus comprising at least one element A, that forms with a further element B a nip ahead of which the composition is held up, and that moves relative to the element B in such a way that a rotating body of composition is developed and the composition is conveyed into the nip, comprising the introduction of the composition via a supply means into the intake region of the nip, wherein every exit point of the supply means and every point at which the supplied composition comes into contact with the rotating body of composition is situated in or at a distance D from the plane that is orthogonal to the longitudinal extent of the nip and that limits the longitudinal extent E of the nip intake region fully fillable by the body of composition, where D has a maximum value D.sub.max which is 10% of the longitudinal extent E of the nip intake region fully fillable by the body of composition; and the application of the composition to a carrier material in web form in the nip or after the composition has travelled through the nip.

2. Method according to claim 1, wherein every exit point of the supply means is situated below the surface of the body of composition.

3. Method according to claim 1, wherein at both ends of the longitudinal extent of the nip there is in each case at least one exit of the supply means, where every exit point of these supply means is at a distance of no more than D.sub.max from the plane that is orthogonal to the longitudinal extent of the nip and that limits the longitudinal extent E of the nip intake region fully fillable by the body of composition.

4. Method according to claim 1, wherein the amount of composition introduced per unit time is not greater than the amount of composition taken off by the coating.

5. Method according to claim 1, wherein the element A of the coating apparatus is a rotating roll.

6. Method according to claim 1, wherein the coating apparatus is a roll calender, a multi-roll applicator, a doctor blade, a roll-over-roll applicator or a polishing stack.

7. Method according to claim 1, wherein the coating apparatus is a roll calender.

8. Method according to claim 1, wherein the method is a method for producing an adhesive tape and the composition is a pressure-sensitive adhesive.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 illustrated a coating apparatus which can be used with the method of the invention.

(2) FIG. 1 represents by way of example a coating apparatus which can be used in the method of the invention. The apparatus is a 3-roll calender applicator. The applicator comprises rolls 1 (corresponding to nip-forming element B), 2 (corresponding to nip-forming element A) and 3. The composition for coating is passed via the supply means 6 to the nip 4 between rolls 1 and 2. Ahead of the roll nip it is held up, and, because of the rotation of the roll 2 relative to the roll 1, a rotating body 8 of composition is developed, from which the composition is conveyed successively into and through the nip. The nip 4 intake region fully fillable by the body of composition is limited by the two side limiters 7. The respective inside face of the side limiters 7 is therefore situated in the plane that is orthogonal to the longitudinal extent of the nip 4 and that limits the longitudinal extent E of the nip 4 intake region fully fillable by the body of composition. The composition can be taken over by the roll 2 and applied in the roll nip 5 to a carrier material lying on the surface of the roll 3.

EXAMPLE

(3) The composition used for application was a reactive polyacrylate melt with a composition as follows:

(4) TABLE-US-00001 Acrylate copolymer (EHA/BA/AA 47.5/47.5/5 m %) 70 m % Tackifier resin (Dertophene T110; from DRT) 30 m % Crosslinker resin (Polypox R20; from UPPC AG) 0.6 m % Activator (Epikure 3223; from HEXION) 0.3 m %

(5) The coating material in web form that was used was a commercial glassine liner.

(6) Coating took place on a 3-roll L-shaped calender applicator which had the following settings (reference numerals corresponding to FIG. 1): Coating width: 500 mm, delimited laterally by form-fitting Teflon spatulas running on the roll surfaces of roll nip 1 Mass throughput: 51 kg/h Density of composition: 1.04 g/cm.sup.3 Web speed (roll 3): 17 m/min Coatweight: 100 g/m.sup.2 Surface speed of roll 1 0 m/min Surface speed of roll 2 3 m/min Surface speed of roll 3 17 m/min

(7) The surface temperatures of the calender rolls were conditioned so as to enable complete transfer of composition to the subsequent roll in the two calender nips 4 and 5. For this purpose, the required temperatures, according to composition rheology and relative roll speed, were around 40-160 C.

(8) The pipe of the composition supply line had an internal diameter of 16 mm and passed through one of the two side spatulas at a location whose depth in the intake region of the nip 4 was such that it lay beneath the surface of the body of composition that developed in operation.

(9) The composition pumped into the roll nip rotated in the direction of the quicker roll 2. In this arrangement, the composition underwent automatic distribution towards the other side spatula, and formed a conical body whose thicker end was located at the feed point. The body could be increased in size by making the feed of composition greater than the consumption by the coating procedure, and vice versa. The smallest possible setting for the body was such that it did not contact the side spatulas opposite the feed location, and instead ran out in a fine point. This location then marked the side edge of the coating on the running web.

(10) The volume of the body of composition was also determined by the relative speed of the two rolls: the greater this speed, the flatter the cone angle of the body, and therefore the smaller its volume became.

(11) The nip between roll 1 and roll 2 was set on the roll bearings in such a way that the originally higher coatweight on the feed side was exactly compensated.

(12) In order to verify the quality of the coating, the applied coatweight of the coated composition was measured continuously by a -emitter which traversed the entire web width. The findings were as follows:

(13) TABLE-US-00002 Target coatweight 100 g/m.sup.2

(14) Determination of standard deviation s by averaging from 3 successive transverse profiles, with 20 measurement points transverse to the web in each case:

(15) TABLE-US-00003 Conventional coating, variant X s = 2.3 g/m.sup.2 Conventional coating, variant Y s = 2.0 g/m.sup.2 Inventive coating: s = 1.1 g/m.sup.2.

(16) According to variant X, the conventional supply of composition to the calender at nip 4 took place by preliminary distribution of the composition by means of a slot die customary in film extrusion. This die produced a free-hanging curtain of composition that largely filled the nip 4. In this case, a cylindrical body of composition rotating towards the roll 2 was produced in nip 4, and so the curtain of composition was drawn towards the roll 2 and taken in between composition body and roll 2.

(17) According to variant Y, the conventional supply of composition into the calender at nip 4 took place by a pipe mounted centrally above the nip 4, in the form of a string of melt. As in variant X, a body of composition rotating towards the roll 2 and having a diamond-shaped longitudinal section was produced in nip 4, and so the string of composition was drawn towards the roll 2 and taken in between roll 2 and the composition body at the thickest point of said body.

(18) The example shows that the homogeneity of the coatweight over the web to be coated is improved with the method of the invention. A particularly even coatweight was achieved, which is manifested for example in the following advantages: 1. When the coated web was wound up into bales, there were no piston rings, the cause of which are locationally fixed thick points in the distribution of composition. Piston rings in the bale lead to local stretching of the carrier material, leading to poor flat lie of the web when it is unrolled. This in turn leads to problems with further processing, as a result of creasing, for example. 2. Furthermore, when webs with poor and locationally fixed distribution of composition are wound up, a particularly large quantity of entrained air becomes enclosed at thin locations between the plies, and may in turn cause optical defects. Bales produced in accordance with the invention have a virtually cylindrical shape and can therefore be wound without air inclusions; the unrolled web is planar and has a particularly good flat lie. 3. Another possible consequence of piston rings is that the bale evades the very high winding tension at the rings, and undergoes telescoping, so making it unusable for subsequent processing steps. The method of the invention allows the winding of dimensionally stable bales. 4. Thickness is also an important product quality in the application of coated webs, as for example in a preferred use as double-sided adhesive tape. The thickness is utilized, for example, in order to bond components fittingly in precisely defined joints. An inventively improved distribution of composition here allows narrower specification limits and hence greater product quality. 5. The weight of the coating material per unit area is normally a codeterminant of the desired properties of the coating. Particularly uniform coating is therefore a prerequisite for consistent product quality. In the case of pressure-sensitive adhesive tapes, the coatweight directly determines the adhesive properties, such as the bond strength. The method of the invention permits products having particularly consistent adhesive properties.