Method for promoting adhesion between a backing and an adhesive composition

Process for producing adhesion promoter layers on a material in web form, characterized in that the adhesion promoter layers are applied to the web-form material by means of low-pressure plasma polymerization, the material in web form being guided continuously through a plasma zone in which there is a low-pressure plasma which is generated by electrical discharge, especially kHz, MHz or GHz discharge.

The invention relates to a process for producing adhesion promoter layers 
on a material in web form and to the use of the adhesion promoter layers, 
especially for adhesive tapes. 
It is known that adhesion promoter layers on materials in web form are 
employed for a large number of different applications. Examples are 
packaging materials, adhesive tapes or protective films, where adhesion 
promoter layers are intended in each case to provide sufficient strength 
of the laminate. 
In the case of adhesive tapes, this is often associated with particular 
problems since, in general, stringent requirements are set for the 
adhesion between adhesive composition and backing material. Detachment of 
the adhesive composition from the backing should occur neither during the 
removal of the adhesive tape from the roll (prior to use) nor when 
detaching the adhesive tape from whatever substrates (after use). 
In order to increase the adhesion of layers to materials in web form, 
especially that of adhesive compositions to backing materials in the case 
of adhesive tapes, various techniques are employed. 
These techniques are either processes for pretreating the materials in web 
form or for coating them with adhesion-promoting layers. Pretreatment 
processes employed include corona treatment, flame pretreatment, 
fluorination, or low-pressure plasma treatment. Adhesion-promoting layers, 
which are also referred to as primers, can be applied, for example, as a 
wet-chemical coating from solution (solvent or water), in which case 
subsequent drying and/or crosslinking is required. 
All processes for increasing the adhesion have specific disadvantages. 
Corona and flame pretreatment are in many cases not sufficiently effective 
to achieve adequate adhesion of subsequent coatings on the materials in 
web form. An example is the adhesion of acrylate-based adhesive 
compositions to polyolefin-based materials in web form. Similar comments 
apply to fluorination, which is also associated with high safety-related 
expense. 
A further disadvantage of flame pretreatment is the high thermal stress on 
the materials to be treated, making it impossible, or possible only under 
certain circumstances, to treat temperature-sensitive materials, in 
particular. 
By means of the low-pressure plasma pretreatment, which is carried out with 
non-polymerizing gases (e.g. noble gases, oxygen or nitrogen), it is 
generally possible to obtain better adhesions than by means of a flame or 
corona pretreatment. What are problematic, however, are the high costs for 
the necessary vacuum equipment, especially when treating materials in web 
form. In many cases, the increase in adhesion is, moreover, lower than in 
the case of wet-chemical primers, so that in the case of materials in web 
form the use of a plasma pretreatment is not rational. 
Low-pressure plasma polymerization has not so far been used for the 
industrial production of adhesion-promoting layers on materials in web 
form, although for fixed substrates, processes are already in existence 
for a large number of different applications. Examples are the coating of 
plastic bottles with permeation barrier layers, and the scratch-resistant 
coating of plastic surfaces. In the case of materials in web form, the use 
of low-pressure plasma polymerization has not generally been rational to 
date because the deposition rates are too low, resulting in a coating time 
of minutes or hours. For this reason, the production of coatings by means 
of low-pressure plasma polymerization in the case of materials in web 
form, and especially the production of adhesion promoter layers, is 
uneconomic. 
The use of primers applied by wet-chemical means generally entails high 
costs, since treating the web-form material with a primer implies a 
complete additional coating operation. Furthermore, some primers are 
classified as unacceptable from environmental and health standpoints, 
especially since solvents are necessary for the wet-chemical application 
of the primers. A further problem is the use of wet-chemical primers with 
rough materials in web form, since in such cases it is difficult to 
achieve a constant, uniform layer thickness. Moreover, 
temperature-sensitive materials can be coated with wet-chemical primers 
only under certain circumstances, since economic primer drying in 
conjunction with the processing of materials in web form normally 
necessitates drying temperature of at least 80.degree. C. 
A further problem arises when a certain layer, for example a layer of 
adhesive composition, cannot be made to adhere sufficiently to a certain 
web-form material by any of the known processes. 
The object of the invention is to avoid the disadvantages of the prior art 
or at least to lessen them. The object of the invention is, in particular, 
to achieve a marked increase in adhesion in the case of layers of adhesive 
composition on materials in web form, with the further requirements that 
the increase in adhesion must be stable in the long term, no solvents 
should be employed, and the process and the adhesion-promoting layers 
should be acceptable from the standpoints of both health and environment. 
This object is achieved by a process for producing adhesion promoter layers 
on materials in web form, as is characterized in more detail in the main 
claim. The subclaims relate particularly to advantageous embodiments of 
the process. The invention additionally relates to the use of the adhesion 
promoter layers, especially for adhesive tapes. 
The invention accordingly provides a process for producing adhesion 
promoter layers on a material in web form, characterized in that the 
adhesion promoter layers, which preferably are virtually or completely 
transparent, are applied to the web-form material by means of low-pressure 
plasma polymerization, the material in web form being guided continuously 
through a plasma zone in which there is a low-pressure plasma which is 
generated by electrical discharge, especially kHz, MHz or GHz discharge. 
Important process parameters which govern the process for depositing the 
adhesion-promoting layers, and hence control the layer properties, are the 
monomers or carrier gases or additional gases employed, the gas pressure 
or gas-mixture pressure during coating, and the electrical discharge 
employed for plasma excitation. Varying the process parameters serves to 
optimize and adapt the adhesion promoter layers to the technical boundary 
conditions in each application case. In particular it is possible, through 
an appropriate choice of process parameters, as exemplified in the 
Examples, to achieve a marked increase in the deposition rate in 
comparison to the prior art, resulting in coating times of less than 1 
second. 
Coating is preferably carried out at a gas pressure or gas-mixture pressure 
of from 10.sup.-3 to 20 mbar. 
It has also been found advantageous if pulsed electrical discharges are 
used to generate the plasma. 
In a further advantageous embodiment, the web-form material is conveyed 
through the coating zone at a rate of more than 0.1 m/min, in particular 
more than 50 m/min. 
In addition, the coating times are preferably shorter than one minute, in 
particular lower than one second. 
It is advantageous if the unwinding station of the web-form material, the 
winding station, and the plasma zone are located in a vacuum chamber 
(batch operation), or the web-form material is guided through the plasma 
zone by means of vacuum locks, which is termed air-to-air operation. 
Monomers used to form the low-pressure plasma are, in particular, saturated 
hydrocarbons having chain lengths from C.sub.1 to C.sub.6, especially 
methane, ethane or propane, and/or mono- or polyunsaturated hydrocarbons 
having chain lengths from C.sub.1 to C.sub.6, preferably acetylene or 
ethylene, and/or oxygen- or heteroelement-substituted compounds of the 
saturated or unsaturated hydrocarbons, such as ethylene oxide, for 
example. 
Carrier gases or additional gases employed are preferably non-polymerizable 
gases such as noble gases, oxygen, hydrogen, nitrogen or compounds, or gas 
mixtures. 
Additional gases and carrier gases are used in order to control layer 
deposition and, in particular, in order to increase the uniformity and 
stability of the plasma. 
Materials in web form that are employed are preferably polymer films, foam 
substrates, woven substrates, non-woven substrates or paper substrates. 
The use of the adhesion promoter layers produced by the process of the 
invention has been found to be particularly advantageous in the case of at 
least one-sided adhesive tapes, for promoting adhesion between backing and 
adhesive composition. 
Strong promotion of the adhesion is a particular feature of the joining of 
web-form materials and acrylate-based adhesive compositions. 
Various forms of electrical discharge can be utilized for plasma 
excitation, preferably kHz, MHz or GHz. The choice of form of excitation 
is governed by the boundary conditions of the process: for example, 
required coating rate or gas-mixture pressure during coating. 
A particular advantage of the process over the prior art is the possibility 
of controlling the process of layer deposition, and hence the 
adhesion-promoting effect of the coatings, by varying the process 
parameters. This allows for optimum adaptation of the layer properties to 
the particular application case. 
A further advantage of the novel process is the absence of solvent and the 
possibility of avoiding the use of substances unacceptable from a health 
or environmental standpoint. 
Yet another advantage of the process is the possibility of coating rough 
web-form materials uniformly with adhesion promoter layers. 
Furthermore, the thermal stress on the web-form materials is low because of 
the low-pressure plasma that is employed, so that temperature-sensitive 
materials in particular, such as polyethylene, polypropylene or foams, can 
be coated without damage. 
A further advantage is the high long-term stability of the novel adhesion 
promoter layers: because of the novel process, these layers are highly 
crosslinked and thermally stable. Moreover, they are insoluble in 
customary solvents, so that their use, especially for promoting adhesion 
between web-form materials and coatings applied by wet-chemical means, 
leads to very good results. 
The intention of the text below is to illustrate particularly advantageous 
embodiments of the process of the invention, without thereby wishing to 
impose any unnecessary restriction.

In accordance with FIG. 1, the web-form material 1 is conveyed from an 
unwinding station 2 through the plasma coating zone 3. Within the plasma 
coating zone 3, which is separated by partitions 8 from the remainder of 
the vacuum chamber 7, monomers are introduced by way of a monomer supply 
means 4. Plasma excitation, and hence the fragmentation of the monomers, 
takes place by way of a high-frequency alternating field which is applied 
between the electrodes 5 and 6. The electrode 5 is configured as an 
earthed cooling roll and hence serves at the same time for transportation 
of the web-form material 1. Finally, after coating, the web-form material 
1 is passed to a winding station 9. 
An alternative setup of the electrodes 10, 11 is shown in FIG. 2. In this 
figure, the two electrodes 10, 11 have a flat shape and the web-form 
material 1 is guided without contact through the electrode gap. 
The selection of the particular electrode setup depends on the specific 
application case. In the case of plasma excitation by means of GHz 
discharge, the electrodes are to be replaced by corresponding GHz input. 
Finally, the process of the invention is illustrated by way of example, 
again without wishing to impose any unnecessary restriction. 
EXAMPLE 1 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a polyester film (Hostaphan RN 25 film from Hoechst AG, Frankfurt), 
transparent, 500 mm wide and with a thickness of 25 .mu.m. Coating takes 
place in a unit corresponding to FIG. 1 with an acetylene flow of 500 sccm 
and an oxygen flow of 50 sccm at a process pressure of 0.5 mbar. The film 
is guided through the plasma zone (length 200 mm) at a rate of 100 m/min, 
giving a coating time of 0.12 sec with a layer thickness of 130 nm. Plasma 
excitation is by kHz discharge. Subsequently, a transparent acrylate 
adhesive composition (in-house polymer comprising 48% butyl acrylate, 48% 
ethylhexyl acrylate and 4% acrylic acid) is applied to the 
adhesion-promoting layer from solution (solvent acetone/petroleum spirit, 
adhesive application rate after drying: 40 g/m.sup.2). Adhesion between 
adhesive composition and backing film is examined by means of an anchorage 
test. For this purpose, a strip of adhesive bearing the adhesion promoter 
layer of the invention and having a width of 20 mm is bonded to a PVC 
sheet and rolled on (steel rollers, 80 mm diameter, 2 kg weight, rolled 
backwards and forwards five times over the adhesive strip at about 10 
m/min). Subsequently, the total assembly, comprising adhesive strip with 
adhesion promoter layer of the invention and PVC sheet, is stored for 3 
days at 40.degree. C. and an atmospheric humidity of less than 75%. 
Following storage, the adhesion between backing film and adhesive 
composition is tested by peeling off the test strip at a rate of 2400 
mm/min at angles of 180.degree. and 90.degree.. By virtue of the 
adhesion-promoting layer of the invention, the adhesion between backing 
film and adhesive composition is increased significantly and to a 
surprisingly high and permanent extent, so that there is cohesive 
splitting of the adhesive composition during the test. Adhesion between 
the adhesive composition and backing film is therefore much better than in 
the case of comparative adhesive films with which, instead of the adhesion 
promoter layer of the invention, wet-chemical primers or a corona or flame 
pretreatment were employed. In the case of the comparative adhesive 
strips, adhesive failure between backing film and adhesive composition was 
observed in all cases, and hence a markedly poorer adhesion between 
adhesive composition and backing film. 
EXAMPLE 2 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a polypropylene film, transparent, 500 mm wide and with a thickness of 
25 .mu.m. Coating takes place in a unit corresponding to FIG. 1 with an 
acetylene flow of 500 sccm at a process pressure of 3 mbar. The film is 
guided through the plasma zone (length 200 mm) at a rate of 50 m/min, 
giving a coating time of 0.24 sec with a layer thickness of 180 nm. Plasma 
excitation is by MHz discharge. Subsequently, a transparent acrylate 
adhesive composition (in-house polymer comprising 47% butyl acrylate, 47% 
ethylhexyl acrylate and 6% acrylic acid) is applied to the 
adhesion-promoting layer from solution (solvent acetone/petroleum spirit, 
adhesive application rate after drying: 40 g/m.sup.2). Adhesion between 
adhesive composition and backing film is examined by means of an anchorage 
test. For this purpose, strips of adhesive bearing the adhesion promoter 
layer of the invention and having a width of 20 mm are bonded to PVC 
sheets and rolled on (steel rollers, 80 mm diameter, 2 kg weight, rolled 
backwards and forwards five times over the adhesive strip at about 10 
m/min). Subsequently, the total assembly, comprising adhesive strip with 
adhesion promoter layer of the invention and PVC sheet, is stored for 3 
days at 40.degree. C. and an atmospheric humidity of 50% and 100% 
respectively. Following storage, the adhesion between backing film and 
adhesive composition is tested by peeling off the test strips at a rate of 
2400 mm/min at angles of 180.degree. and 90.degree.. By virtue of the 
adhesion-promoting layer of the invention, the adhesion between backing 
film and adhesive composition is increased significantly and to a 
surprisingly high and permanent extent, so that there is cohesive 
splitting of the adhesive composition during the test. Adhesion between 
the adhesive composition and backing film is therefore much better than in 
the case of comparative adhesive films with which, instead of the adhesion 
promoter layer of the invention, wet-chemical primers or a corona or flame 
pretreatment were employed. In the case of the comparative adhesive 
strips, adhesive failure between backing film and adhesive composition was 
observed in all cases, and hence a markedly poorer adhesion between 
adhesive composition and backing film. 
EXAMPLE 3 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a polypropylene film, transparent, 500 mm wide and with a thickness of 
25 .mu.m. Coating takes place in a unit corresponding to FIG. 1 with an 
ethylene flow of 1000 sccm at a process pressure of 0.5 mbar. The film is 
guided through the plasma zone (length 200 mm) at a rate of 20 m/min. 
Plasma excitation is by kHz discharge. Subsequently, a transparent 
acrylate adhesive composition (in-house polymer comprising 47% butyl 
acrylate, 47% ethylhexyl acrylate and 6% acrylic acid) is applied to the 
adhesion-promoting layer from solution (solvent acetone/petroleum spirit, 
adhesive application rate after drying: 20 g/m.sup.2). Adhesion between 
adhesive composition and backing film is examined by means of an anchorage 
test. For this purpose, strips of adhesive bearing the adhesion promoter 
layer of the invention and having a width of 20 mm are bonded to PVC 
sheets and rolled on (steel rollers, 80 mm diameter, 2 kg weight, rolled 
backwards and forwards five times over the adhesive strip at about 10 
m/min). Subsequently, the total assembly, comprising adhesive strip with 
adhesion promoter layer of the invention and PVC sheet, is stored for 3 
days at 40.degree. C. and an atmospheric humidity of 50% and 100% 
respectively. Following storage, the adhesion between backing film and 
adhesive composition is tested by peeling off the test strips at a rate of 
2400 mm/min at angles of 180.degree. and 90.degree.. By virtue of the 
adhesion-promoting layer of the invention, the adhesion between backing 
film and adhesive composition is increased significantly and to a 
surprisingly high and permanent extent, so that there is cohesive 
splitting of the adhesive composition during the est. Adhesion between the 
adhesive composition and backing film is therefore much better than in the 
case of comparative adhesive films with which, instead of the adhesion 
promoter layer of the invention, wet-chemical primers or a corona or flame 
pretreatment were employed. In the case of the comparative adhesive 
strips, adhesive failure between backing film and adhesive composition was 
observed in all cases, and hence a markedly poorer adhesion between 
adhesive composition and backing film. 
EXAMPLE 4 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a polypropylene film, transparent, 500 mm wide and with a thickness of 
35 .mu.m. Coating takes place in a unit corresponding to FIG. 1 with an 
acetylene flow of 500 sccm at a process pressure of 3 mbar. The film is 
guided through the plasma zone (length 200 mm) at a rate of 25 m/min, 
giving a coating time of 0.48 sec with a layer thickness of 750 nm. Plasma 
excitation is by pulsed MHz discharge at a pulse frequency of 10.sup.3 Hz 
and a duty factor of 0.3. Subsequently, a transparent acrylate adhesive 
composition (Primal PS 83 D, Rohm and Haas GmbH, Frankfurt) is applied to 
the adhesion-promoting layer from solution (solvent water, adhesive 
application rate after drying: 20 g/m.sup.2). Adhesion between adhesive 
composition and backing film is examined by means of an anchorage test. 
For this purpose, strips of adhesive bearing the adhesion promoter layer 
of the invention and having a width of 20 mm are bonded to PVC sheets and 
rolled on (steel rollers, 80 mm diameter, 2 kg weight, rolled backwards 
and forwards five times over the adhesive strip at about 10 m/min). 
Subsequently, the total assembly, comprising adhesive strip with adhesion 
promoter layer of the invention and PVC sheet, is stored for 3 days at 
40.degree. C. and an atmospheric humidity of 50% and 100% respectively. 
Following storage, the adhesion between backing film and adhesive 
composition is tested by peeling off the test strips at a rate of 2400 
mm/min at angles of 180.degree. and 90.degree.. By virtue of the 
adhesion-promoting layer of the invention, the adhesion between backing 
film and adhesive composition is increased significantly and to a 
surprisingly high and permanent extent, so that there is cohesive 
splitting of the adhesive composition during the test. Adhesion between 
the adhesive composition and backing film is therefore much better than in 
the case of comparative adhesive films with which, instead of the adhesion 
promoter layer of the invention, wet-chemical primers or a corona or flame 
pretreatment were employed. In the case of the comparative adhesive 
strips, adhesive failure between backing film and adhesive composition was 
observed in all cases, and hence a markedly poorer adhesion between 
adhesive composition and backing film. 
EXAMPLE 5 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a foam backing (Alveolit TEE 1000.8, Alveo AG, Lucerne, Switzerland), 
300 mm wide and with a thickness of 600 .mu.m. Coating takes place in a 
unit corresponding to FIG. 1 with an acetylene flow of 500 sccm and an 
argon flow of 50 sccm at a process pressure of 0.5 mbar. The film is 
guided through the plasma zone (length 200 mm) at a rate of 20 mm/min, 
giving a coating time of 0.6 sec. Plasma excitation is by kHz discharge. 
Subsequently, a transparent acrylate adhesive composition (in-house 
polymer comprising 47% butyl acrylate, 47% ethylhexyl acrylate and 6% 
acrylic acid) is applied to the adhesion-promoting layer from solution 
(solvent acetone/petroleum spirit, adhesive application rate after drying: 
50 g/m.sup.2). Adhesion between adhesive composition and backing film is 
examined by means of an anchorage test. For this purpose, an adhesive 
strip bearing the adhesion promoter layer of the invention (width 20 mm), 
as described in Example 1, laminated to the foam backing bearing the 
adhesion promoter layer of the invention and the adhesive composition 
described. Subsequently, the total assembly, is stored in a drying cabinet 
for 3 days at 40.degree. C. and an atmospheric humidity of 50%. Following 
storage, the adhesion between foam backing and adhesive composition is 
tested by peeling off the test strip. By virtue of the adhesion-promoting 
layer of the invention, the adhesion between backing film and adhesive 
composition is increased significantly and to a surprisingly high and 
permanent extent, so that there is cohesive splitting of the foam 
composition or detachment of the test strip. Adhesion between the adhesive 
composition and foam backing is therefore much better than in the case of 
comparative samples with which, instead of the adhesion promoter layer of 
the invention, a corona or flame pretreatment was employed. In the case of 
the comparative samples, adhesive failure between foam backing and 
adhesive composition was observed in all cases, and hence a markedly 
poorer adhesion. 
EXAMPLE 6 
An adhesion promoter layer is applied by low-pressure plasma polymerization 
to a polypropylene film, transparent, 500 mm wide and with a thickness of 
35 .mu.m. Coating takes place in a unit corresponding to FIG. 1 with an 
acetylene flow of 500 sccm at a process pressure of 3 mbar. The film is 
guided through the plasma zone (length 200 mm) at a rate of 100 m/min. 
Plasma excitation is by pulsed MHz discharge at a pulse frequency of 
10.sup.3 Hz and a duty factor of 0.3. Subsequently, a rubber adhesive 
composition (in-house polymer comprising 43% natural rubber, 3% Sillithin 
Z 86 white, 12% zinc oxide, 21% Escorez.RTM. 1202 (Exxon), 20% 
Escorez.RTM. 365 (Exxon), 0.4% AS MBI 2 PLV.RTM. (Bayer), 0.6% Sontal.RTM. 
(Bayer)) is applied to the adhesion-promoting layer from solution 
(adhesive application rate after drying: 20 g/m.sup.2). Adhesion between 
adhesive composition and backing film is examined by means of an anchorage 
test. For this purpose, strips of adhesive bearing the adhesion promoter 
layer of the invention and having a width of 20 mm are bonded to PVC 
sheets and rolled on (steel rollers, 80 mm diameter, 2 kg weight, rolled 
backwards and forwards five times over the adhesive strip at about 10 
m/min). Subsequently, the total assembly, comprising adhesive strip with 
adhesion promoter layer of the invention and PVC sheet, is stored for 3 
days at 40.degree. C. and an atmospheric humidity of 50%. Following 
storage, the adhesion between backing film and adhesive composition is 
tested by peeling off the test strips at a rate of 2400 mm/min at angles 
of 180.degree. and 90.degree.. By virtue of the adhesion-promoting layer 
of the invention, the adhesion between backing film and adhesive 
composition is increased significantly and to a surprisingly high and 
permanent extent, so that the test strips can be completely detached 
again. Adhesion between the adhesive composition and backing film is 
therefore much better than in the case of comparative adhesive films with 
which, instead of the adhesion promoter layer of the invention, 
wet-chemical primers or a corona or flame pretreatment were employed. In 
the case of the comparative adhesive strips, adhesive failure between 
backing film and adhesive composition was observed in all cases, and hence 
a markedly poorer adhesion between adhesive composition and backing film.