Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-11-29
2002-10-29
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S208400, C427S299000, C427S428010, C427S505000, C427S516000, C427S551000, C427S553000
Reexamination Certificate
active
06472025
ABSTRACT:
The invention concerns a method of producing a coating of solvent-free pressure-sensitive adhesive (PSA) systems on substrates, especially release-coated substrates, the operation of coating running by way of a roller. While the PSA system is on the roller, the applied adhesive film is crosslinked using electron beams (EB), UV or IR rays.
From the prior art it is known that in order to obtain PSA films possessing high shear resistance it is preferable to employ acrylic PSAs which are crosslinked by methods which include physical methods. In the case of EB and UV radiation, the desired product properties are adjusted by way of the radiation dose; in the case of IR radiation, by way of the product temperature and the residence time.
According to the methods known to date, PSA films are coated directly or indirectly onto release-coated or non-release-coated substrates with the aid, inter alia, of multiroller applicators, single-manifold, slot or multiple-manifold dies and are then subjected to the crosslinking operation.
For products comprising a coating to be crosslinked, which may be a PSA, for example, and a radiation-degradable backing, such as paper, cellulose wovens or nonwovens, and PP films, for example, it is possible to minimize the damage by optimizing the accelerating voltage. When this is done, the backing receives a significantly lower average dose than the coating, while the fall in dose in the coating remains within acceptable limits.
Relationships of this kind are described, inter alia, in EP 0 453 254 B (Yarosso et al.) and also in the paper accompanying a lecture given by Dr. Karmann at the 7th Munich Adhesives and Finishing seminar, 1982.
As a basis for the calculation there is used, for example, the following empirical formula, which was published by Neuhaus-Steinmetz at RadTech Europe, Mediterraneo 1993.
D
[
%
]
=
exp
{
-
(
18.8
*
X
(
U
B
)
1.57
-
0.7
)
2
}
1
+
(
9.7
*
X
(
U
B
)
1
57
)
15
where
D is the dose in %
UB is the accelerating voltage in kV
X is the irradiated weight per unit area in g/m
2
, consisting of the weight per unit area of the vacuum window, the air gap between vacuum window and product, and the depth in the product
A double-sided adhesive tape consisting of a backing with the adhesive compositions, intended for crosslinking, on both sides and of an antiadhesively treated release liner, when irradiated uniformly from one side with accelerated electrons, may receive a maximum dose of only about 20 to 40 kGy; otherwise, the release liner has to be replaced owing to the unacceptable deterioration of the mechanical and antiadhesive properties. The maximum permissible absorbed radiation dose is dependent on the type of the PSA and of the release coatings.
Here too, a reduction in the unwanted effects can be achieved, at appropriate layer thicknesses, by means of a skillful choice of the accelerating voltage if the radiation dose in the release liner has already fallen significantly. However, in this case it must be borne in mind that the PSA layer facing the release liner must still receive a radiation dose which is sufficient for crosslinking.
When a double-sided adhesive tape comprising a backing with PSA compositions on both sides and of an antiadhesively treated release liner is irradiated symmetrically from both sides, it receives the full radiation dose. The same applies to what are known as transfer tapes, where the PSA to be crosslinked is coated onto a release liner without a further backing.
It is clear from the above that the necessary complexity for the crosslinking of double-sided adhesive tapes with electron beams becomes considerable when the radiation doses required to crosslink the PSA layer are so high that the mechanical and antiadhesive properties of the release liner are adversely affected to an unacceptably high degree, since then the release liner must be replaced by a new, unexposed liner, by relining.
The crosslinking of adhesive compositions on temperature-sensitive backings, such as unplasticized PVC or PP spunbondeds, for example, requires thermal crosslinking in a tunnel or with IR lamps. UV crosslinking at higher doses necessitates greater technological complexity or lower running speeds.
If the increase in the attainable cohesiveness of the adhesive films is not to be achieved by way of a correspondingly high radiation dose, then the high molecular weight of the adhesive can be attained by means of suitable polymerization techniques selected in accordance with the prior art.
Acrylic PSAs prepared in this way, however, possess a high resilience, which makes coating by standard methods such as multiroller application or die coating more difficult.
WO 97/15722 describes a process used to bring about relaxation of the adhesive film directly following its emergence from a single or multiple-manifold die. The disadvantage of this process is that the free path which the adhesive film emerging from the die requires in order to relax increases with rising molecular weight. This in turn has the consequence that the adhesive film pulls itself in to a greater or lesser extent, relative to the film width originally dictated by the exit width of the dies (a phenomenon known as neck-in). The consequence is that, in order to achieve required adhesive film coating widths, there is a need for die widths which must be made wider, by some extent, than is the case with the present invention, since in this case lower molecular weights may be compensated by higher radiation doses.
It is an object of the present invention to provide a method of producing a solvent-free pressure-sensitive adhesive coating on substrates, especially release-coated substrates, which avoids the disadvantages of the prior art and which permits cost-effective manufacture and the use of radiation-chemical crosslinking, especially with accelerated electrons or UV radiation, without the need to accept damage to the backing and to the release liner.
This object is achieved by a method as specified in the main claim. Advantageous embodiments of the method are the subject of the dependent claims.
The invention accordingly describes a method of producing a coating of solvent-free pressure-sensitive adhesive systems on substrates, especially release-coated substrates, in which
the pressure-sensitive adhesive system is applied in one or more layers to a rotating roller by means of an adhesive applicator,
the pressure-sensitive adhesive system on the roller is crosslinked in an irradiating means by means of high-energy radiation, specifically using electron beams (EB), UV or IR rays, and
the roller is contacted with the substrate, so that the pressure-sensitive adhesive system is transferred from the roller to the substrate, and if desired is rolled up.
Typical irradiating means employed in the context of the inventive embodiment of the method are linear cathode systems, scanner systems or multiple longitudinal cathode systems, where the equipment in question comprises electron beam accelerators.
The acceleration voltages are situated in the range between 40 kV and 350 kV, preferably from 80 kV to 300 kV. The output doses range between 5 and 150 kGy, in particular from 20 to 90 kGy.
As UV crosslinking units it is possible in particular to employ two medium-pressure mercury lamps each with an output of 120 W/cm or one medium-pressure mercury lamp having an output of 240 W/cm. The doses set are preferably from 10 to 300 mJ/cm
2
.
The contacting of the substrate takes place in particular by way of a second roller. Substrates used include papers, films, nonwovens and release-coated materials such as release papers, films, and the like.
The second roller, also referred to as the contact roller, preferably has a rubber coating and is pressed against the roller with a linear pressure of preferably from 50 to 500 N/mm, in particular from 100 to 200 N/mm. The contact roller preferably has a Shore hardness (A) of 40-100, in particular a Shore hardness of 60-80 shore (A). The substrate is preferably brought into contact with the roller in such a way tha
Guldbrandsen Lars
Hirsch Ralf
Möller Dieter
Neuhaus-Steinmetz Hermann
Schulz Stefan
Norris & McLaughlin & Marcus
Pianalto Bernard
tesa AG
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