Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2002-08-23
2004-01-13
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S207100, C427S208400, C427S243000, C427S385500, C427S428010, C427S496000, C427S505000, C427S508000, C427S514000, C427S516000, C427S551000, C427S558000, C427S559000, C427S561000, C427S566000
Reexamination Certificate
active
06677000
ABSTRACT:
The invention relates to a process for irradiating material by means of electron beams and/or UV rays, the material receiving a first irradiation and being transferred to a substrate, after which a second irradiation is carried out. In particular, said material comprises an adhesive tape.
Within process engineering, radiation crosslinking has found broad application, especially, for instance, in the crosslinking of adhesives, which can then be coated onto carrier materials such as polymer films, wovens or nonwovens.
With acrylic pressure sensitive adhesives, for example, crosslinking is needed in order to produce outstanding adhesive properties. With rubbers as well, the adhesive properties are improved by crosslinking.
In the production of adhesive tapes, the use of radiation-chemical crosslinking by UV radiation or electron beams (EB) possesses particular advantages as compared with the chemical/thermal crosslinking techniques.
Radiation-chemical crosslinking is also used advantageously in the production of double-sided adhesive tapes.
The depthwise distribution of the absorbed radiation dose in a product irradiated with accelerated electrons is known for any given acceleration voltage. A variety of authors have developed empirical functions for this (for example Heger, beta-gamma 1, 20, 1990). At RadTech Europe, Mediterraneo 1993, Neuhaus-Steinmetz published the following empirical function:
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 %,
U
B
is the acceleration voltage in kV, and
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.
If the high weight per unit area of a product means that the maximum acceleration voltage of the electron beam unit is not enough for sufficiently uniform irradiation, the literature describes the possibility of irradiating from both sides, setting the same acceleration voltage and radiation dose for both sides.
For products which are composed of a coating to be crosslinked, which may for example be a pressure sensitive adhesive, and a radiation-degradable carrier, such as paper, woven or nonwoven cellulose or OPP film, for example, the damage can be minimized by optimizing the acceleration voltage. In this case the carrier receives a much lower average dose than the coating, while the dose reduction 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 text accompanying a lecture given by Dr. Karmann at the 7th Munich Adhesives and Finishing Seminar, 1982.
In UV crosslinking, the depth of penetration of the UV radiation is dependent on the formula and on the UV wavelengths at which the photoinitiator, which is necessary for crosslinking, becomes excited. In the case of UV-crosslinkable adhesive tapes it is advantageous if the photoinitiator is not “swimming” freely in the composition but is instead attached chemically to the polymer chains of the adhesive. This attachment typically occurs as early as during polymerization, since otherwise photoinitiators that are still present after crosslinking act as lubricants and lower the cohesion.
Moreover, it is advantageous to use photoinitiators which do not react to sunlight and do not react to the light from fluorescent lamps, since otherwise this may cause uncontrolled subsequent crosslinking. It follows from this that it is advantageous to use only photoinitiators which are sensitive at wavelengths <300 nm.
In the case of the UV polymerization of adhesive tapes, this is of little or no importance, since the aim in UV polymerization is to achieve as complete as possible a conversion of the monomers to polymers. A subsequent reaction would primarily bind in residual monomers, which would be more advantageous than detrimental. Accordingly, the use of photoinitiators which are sensitive at UV wavelengths between 300 and 400 nm is favored. Where these photoinitiators are used for polymerizing straight acrylic adhesives, it is possible to carry out successful polymerization of film thicknesses of significantly more than 1000 &mgr;m.
For adhesive tapes with shearing-resistant UV-crosslinking adhesives, the photoinitiators used at present are primarily those having their principal sensitivity in a range from 250 to 260 nm. On UV irradiation, in the absence of any secondary effects, the intensity of the radiation generally decreases exponentially with the depth. In straight acrylic adhesives, at a wavelength of 250 nm the UV intensity is typically only half of its surface level at a depth of about 25 to 35 nm. Accordingly, for film thicknesses of 60 to 90 &mgr;m at the latest, severe impairments in the adhesive properties are to be expected, since in that case either the side facing away from the UV lamps will be crosslinked hardly at all or the side facing toward the UV lamps will be heavily overcrosslinked.
In many double-sided adhesive tapes featuring acrylic adhesives, the adhesive films have thicknesses of around 100 &mgr;m or more. At such thicknesses, UV crosslinking with wavelengths around 250 to 260 nm is already highly problematic for the reasons described above. Consequently, such products are typically crosslinked thermally or using electron beams. In order to optimize the adhesive properties, resins are often mixed into the adhesives. In the wavelength range of interest, however, they absorb the UV radiation to a very great extent, so that producing a double-sided adhesive tape with adhesive films each of 50 &mgr;m presents problems, and it is also necessary to bear in mind a very low UV absorption when selecting the resins.
DE 199 05 934 A1 disclosed a process for producing a coating of solvent-free pressure sensitive adhesive systems on substrates, especially release-coated substrates, in which
a) the pressure sensitive adhesive system is applied in one or more layers by means of an adhesive applicator to a rotating roll,
b) the pressure sensitive adhesive system present on the roll is crosslinked in an irradiation means by high-energy radiation, specifically by means of electron beams (EB), UV or IR rays, and
c) the roll is contacted with the substrate so that the pressure sensitive adhesive system is transferred from the roll to the substrate and, where appropriate, is rolled up.
Typical irradiation means employed when configuring the process include linear cathode systems, scanner systems, or multiple longitudinal cathode systems, where electron beam accelerators are concerned.
The acceleration voltages lie within the range between 40 kV and 350 kV, preferably from 80 kV to 300 kV. The output doses range between 5 to 150 kGy, in particular from 20 to 90 kGy. The contacting of the substrate takes place in particular by way of a second roll. Substrates used include papers, films, nonwovens, and release-coated materials such as release papers, films, and the like.
DE 199 05 935 A1 discloses a process for producing a coating of solvent-free pressure sensitive adhesive systems on substrates, especially release-coated substrates, in which
a fluid film is applied by means of a fluid applicator to a rotating roll,
the pressure sensitive adhesive system is applied in one or more layers by means of an adhesive applicator to the fluid film, so that the fluid film is situated between roll and pressure sensitive adhesive system, and
the roll is contacted with the substrate, so that the pressure sensitive adhesive system is transferred from the roll to the substrate (release-coated or otherwise).
Contacting of the substrate takes place in particuar by way of a second roll. Substrates used include papers, films, nonwovens, and release-coated materials such as release papers, films, and the like. The second roll, also referred to as a contact roll, is preferably provided with a rubber coating and is pressed against the roll with a linear pressure of preferably from 50 to 500 N/mm, in
Hirsch Ralf
Neuhaus-Steinmetz Hermann
Norris & McLaughlin & Marcus
Pianalto Bernard
tesa Akteingesellschaft
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