Plate mounting tape

A plate mounting tape with a self-adhesive composition on both sides of a polyolefin substrate, in particular PE foam film substrate having low variation in thickness and having a compressive stress value of 10-80, in particular 10-50 N/cm.sup.2 under 50% compressive deformation.

The invention relates to a plate mounting tape, as used in the prior art
 with a foam substrate and adhesive composition on both sides.
 Mounting tapes with foam film substrate have been known and commercially
 available for some time. However, as before, the production of mounting
 tapes with low variation in thickness is a fundamental problem, in
 particular in the case of thickness tolerances of less than .+-.15%. Small
 deviations in the thickness from the desired value as well as small
 overall thicknesses are desired or necessary in many application areas for
 double-sided foam adhesive tapes, in particular also in the printing
 industry. In the case of the flexographic printing process, it is for
 example advantageous for mounting the polymer printing plate onto the
 impression cylinder by means of a double-sided adhesive tape if the
 adhesive tape has a suitable compressive stress value and small variations
 in thickness. The printing quality can namely be increased considerably in
 this way. However, after completion of the printing operation, the
 adhesive tape must allow itself to be detached again both from the
 impression cylinder and from the plate, leaving behind as little residue
 as possible, which presupposes an adequate tearing strength.
 Until now, for this application area use has been made of adhesive tapes
 based on surface-finished polyurethane foams (Messrs Mayser, Lindenberg)
 or split or ground polyethylene foams or polyethylene vinyl acetate
 copolymer foams (Messrs Alveo, Lucerne). In both cases, to obtain adequate
 tearing strength together with the low compressive stress value necessary
 at the same time, reinforcement with a PVC, polyester or similar film is
 necessary. This additional production step and use of material not only
 makes the product considerably more expensive, but has an adverse effect
 on the compressive stress value, since part of the maximum thickness
 available has to be taken up by an incompressible film and a laminating
 adhesive. The PE-based foam films used do admittedly have the advantage
 over the above-mentioned PU-based films that they are available with
 substantially lower compressive stress values. The closed-cell PE films
 are also superior to the open-cell PU films with respect to the
 deterioration in recovery behaviour due to fatigue of the material after
 numerous compressing and relieving cycles. However, a major problem is
 represented by the fact that PE foam films are commercially available only
 in relatively large thicknesses (&gt;500 .mu.m) and by the considerable
 variations in thickness. These rule out use of the unmachined films for a
 broad range of the application areas mentioned above. The laborious
 splitting or grinding of the foam film does offer a solution to this.
 However, this causes the material to lose even more of its tearing
 strength and the closed-cell nature of the foam is also reduced; the
 advantage mentioned is partly lost again.
 The object of the invention was to provide a remedy for this, in particular
 to provide a plate mounting tape which has the advantages of a closed-cell
 polyolefin substrate, in particular a PE foam film substrate, and at the
 same time exhibits a small overall thickness and variation in thickness,
 as required by the printing industry.
 Accordingly, the invention relates to a plate mounting tape as
 characterised more precisely herein.
 The foam film as substrate material advantageously consists of polyolefin,
 in particular of polyethylene or a polyethylene copolymer, for example
 with vinyl acetate. The substrate film has in this case a thickness of
 200-1000 .mu.m, in particular 200-500 .mu.m, a density of 50-300
 kg/m.sup.3, a compressive stress value of 10-80 N/cm.sup.2 under 50%
 compressive deformation (as specified in DIN 53 577) and preferably such a
 value of 10-50 N/cm.sup.2. In particular, the adhesive tape according to
 the invention is characterised by a small thickness and also variation in
 thickness, which is advantageously up to no more than .+-.12%, in
 particular 5-10%.
 According to the invention, a monoaxially stretched polyolefin foam film,
 in particular a PE or PE copolymer foam film, is used advantageously as
 substrate material for such an adhesive tape. This stretching operation
 makes it possible in the first place to obtain suitable substrate
 materials having a thickness of less than 500 .mu.m, but substrates of up
 to 1000 .mu.m are also suitable. For example, a 350 .mu.m thick film can
 be obtained from a 500 .mu.m thick PE/EVA foam film having a density of
 200 kg/m.sup.3 with a stretching ratio of 1:2.
 In addition, the thickness tolerance can be advantageously reduced to less
 than .+-.10% by an in-line thickness measurement of the foam and
 corresponding control of the stretching ratio, in particular with a
 longitudinal stretching ratio of 1.5-4, preferably 2-2.5. Since thicker
 foam films can also be calibrated in this way, many new application areas
 for adhesive tapes are opened up for the first time on account of the
 relatively inexpensive foam, in particular also for the application
 mentioned above in flexographic printing.
 A particularly favourable effect for the final product is the tearing
 strength of the material, increased by up to 30% due to stretching, since
 this dispenses with the otherwise necessary reinforcement with a film. A
 double-sided adhesive tape produced from the stretched material can, for
 example, even in thicknesses of less than 500 .mu.m with a density of 200
 kg/m.sup.3 be removed again without tearing off or being torn into, even
 with adhesive forces of 5 N/cm.
 A particular advantage in setting a desired thickness by stretching of the
 foam film instead of splitting or grinding is also that in this way there
 is a gain in material in the form of a larger useful surface area, whereas
 in the case of the other methods part of the material used ends up as
 scrap.
 In addition to these advantages, it has surprisingly been found that,
 although the density remains virtually unchanged in stretching, the
 compressive stress value of the materials decreases by up to 30%. This
 effect represents a particular advantage for use in the adhesive bonding
 of printing plates.

EXAMPLE 1
 A 550.+-.20% thick, crosslinked PE foam film (TE 500.5 of Messrs Alveo,
 containing vinyl acetate in the polymer) was monoaxially stretched
 longitudinally at room temperature. The stretching rate was 10 m/min, the
 stretching gap 5 cm. The thickness was measured continuously during
 stretching. The stretching ratio was 1:2 to 1:2.5. A material having a
 thickness of 380 .mu.m.+-.10% was obtained. The compressive stress value
 (50% compressive deformation) decreased from 38 to 30 N/cm.sup.2, the
 tearing strength increased from 18 to 23 N/cm, the elongation at tear
 decreased from 300% to 100%. The material lost 6% in width, but yielded a
 25%-increased running length.
 After corona pretreatment on both sides of the foam, 30 g/m.sup.2 of a
 polyacrylate adhesive composition (German Patent Specification 1,569,898,
 Example 2) was applied to each side by the transfer method (spreading of
 the solvent-containing adhesive composition onto release paper, drying at
 80.degree. C., laminating onto the foam, removal of one of the release
 papers).
 The finished product had an overall thickness of 44 .mu.m.+-.10%, the
 adhesive strength of the product covered on one side by a 23 .mu.m-thick
 polyester film (Hostaphan, Messrs Hoechst) was 4.8 N/cm (AFERA 4001 P 11).
 Consequently, when used for plate adhesive bonding, an excellent printed
 image was achieved.