WO1999010185A1

WO1999010185A1 - Label sheets for thermal transfer imaging

Info

Publication number: WO1999010185A1
Application number: PCT/GB1998/002528
Authority: WO
Inventors: Christopher Paul Richardson; Ian Stephenson; Stuart Williamson
Original assignee: Imperial Chemical Industries Plc
Priority date: 1997-08-22
Filing date: 1998-08-21
Publication date: 1999-03-04
Also published as: GB9717829D0

Abstract

A label sheet for thermal transfer imaging comprises a backing sheet and at least one label peelably mounted on the backing sheet, the label having a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side a layer of pressure-sensitive adhesive by which it is peelably held too the backing sheet with a peel strength less than the bond strength securing it to the substrate, and wherein the 180° peel strength as measured on sample strips 18 mm wide at a peel rate of 10 mm/s, is greater than 11 g.

Description

TITLE: LABEL SHEETS FOR THERMAL TRANSFER IMAGING

The invention relates to labels having a pressure-sensitive adhesive layer on the reverse side, which can be peeled from a backing sheet and applied to a new object, and in particular to labels suitable for thermal transfer imaging.

Peelable labels having a base with an imageable surface on one side and a pressure- sensitive adhesive layer on the other are used for many purposes, and are a familiar commodity which provides a convenient means for applying an image to one or a series of objects. Labels printed in various different ways, eg by typing, gravure and photocopying, are known and useful. Typical images applied include bar-codes or written information such as an address or identification name, or may include a picture or design as a logo or greetings label, for example. The image may be applied to the imageable surface by the end user, or the labels may contain preformed images at the point of sale, depending on the application.

Each label itself consists essentially of a substrate with an imageable surface on one side and a pressure-sensitive adhesive layer on the other. They are sold as label sheets having a protective backing sheet covering the adhesive layer and from which the label may be readily peeled. Each label sheet may have just a single label, but more usually the sheet will have a plurality of labels arranged in a rectangular array, each label being separately peelable from the backing sheet. Alternatively the sheet may be in the form of a ribbon with the plurality of labels extending end to end along it.

In each case the label is peelably held to the backing sheet by the adhesive layer with a peel strength less than the bond strength securing the latter to the substrate. Thus, on peeling the label from the backing sheet, the adhesive layer is retained on the label to enable it to provide the adhesion to the object to which the label is applied. To achieve such peelability from the backing sheet, it is usually necessary to provide the backing sheet with a release treatment, generally by application of a release layer of a composition containing a polydimethylsiloxane modified with a polymerisable group, but some residual adhesion is necessary to retain the label on the backing sheet prior to use. The peelability from this layer is therefore tailored by the addition of a controlled release additive, and sufficient additive is generally added to increase the 180° peel strength to about 2 to 6g, as measured on sample strips 18mm wide at a peel rate of lOmm/s.

Thermal transfer imaging is a printing process in which one or more thermally transferable dyes are caused to transfer from selected areas of a dye-donor sheet to a receiver pressed against it and thus form an image. Using a dye-donor sheet comprising a thin substrate supporting a dyecoat containing one or more uniformly spread dyes, printing is effected by heating selected discrete areas of the dye-donor sheet while the dyecoat is pressed against a dye-receptive layer of a receiver sheet, thereby causing dye to transfer to corresponding areas of the receiver. Normally this is carried out in a printer having a thermal head with tiny separately-controllable heating elements, against which the dyesheet and receiver sheet are pressed together under several atmospheres pressure by a platen roller. The shape of the image transferred is determined by the number and locations of the discrete areas heated, and full colour prints can be produced by printing with different coloured dyecoats sequentially in like manner.

It has been proposed to take advantage of the photograph-like qualities possible with thermal transfer printing, and to form images by this printing technique in peelable labels adapted by provision of a dye-receptive layer. To this end, label sheets have been placed on the market, with the labels consisting essentially of a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side the layer of pressure sensitive adhesive by which it is peelably held to the backing sheet. These adapted label sheets are then printed using a thermal head printer as though they were normal thermal transfer receiver sheets. However, we have found these to be unreliable in that the printed labels may not all remain securely on the backing sheet after printing. Blocking of the printer by wayward labels may also occur.

According to a first aspect of the present invention, a label sheet for thermal transfer imaging comprises a backing sheet and at least one label peelably mounted on the backing sheet, the label having a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side a layer of pressure sensitive adhesive by which it is peelably held to the backing sheet with a peel strength less than the bond strength securing it to the substrate, wherein the 180° peel strength as measured on sample strips 18mm wide at a peel rate of lOmm/s, is greater than llg.

We have found that the increasing the peel strength to this level above what has previously been used commercially, we have obtained a very significant improvement in the suitability of the label sheets for thermal transfer printing using thermal print heads. However, this may not be sufficient for total security in all printers, and we particularly prefer to use a label sheet wherein the peel strength is at least 15g, for example within the range 15 to 20g, and most preferably greater than 20g.

When the peel strength approaches the strength of the label substrate, attempts at peeling the label off the baking sheet become increasingly likely just to cause tearing of the label instead. The value for this gives the upper practical limit for the peel strength in any system. For a substrate formed from a voided polyester (such as the 50μm Melines™ 329 referred to in the Examples hereinafter), such practical upper limit is about 38g. However, although these may be secure in the printer, such label sheets are not very user friendly for manual peeling because of the large effort required to peel them off after printing. A preferred upper limit for more user friendly patches is about 30g.

The substrate may be opaque or transparent, as required for its end use, and for most applications we prefer to select the substrate material and any coatings to be applied to it, according to the same parameters that we would consider for a normal thermal transfer print. Thus the substrates particularly suited are microvoided polymer sheets formed of polyester or polyolefin compositions, and laminates of these with paper or other polymer sheets. However, because labels do not need to be self supporting and indeed are generally better for not being too stiff so that they can more readily take up the shape of the object to which they are to be applied, the thickness of the substrate though not critical is preferably less than would be optimum for free-standing receiver sheets. Thus a preferred thickness for microvoided substrate sheets of polyester or polyolefin is from about 50μm and 75«m, although we have used lOOμm thick substrates for other labels: these compare with free-standing receiver substrates of typically 125«m or 150μm for sheet-fed receivers.

The backing sheet can be used to make up the thickness of the label sheet to that of a normal receiver sheet and to give it greater stiffness, thereby to improve the feed through the printer. Thus when using a fairly limp 50 m microvoided polyester substrate for the label, a lOOμm microvoided polyester backing layer of about 75μm to 125μm thick is particularly suitable for sheet fed label sheets, but a thinner backing layer of about 30μm to 50«m is preferred for roll-fed label sheets. Sheets of other materials, such as cellulosic paper, synthetic paper or non-voided polymer film, can also be used as the backing layer, although too stiff a backing sheet may reduce the compliance of the label sheet as a whole, and hence adversely affect the quality of the image when duly printed. In view of the greater peel strength required for the present invention, it may be possible to achieve sufficient peelability using a non-stick polymer for the backing sheet, eg a fluorinated polymer. However, by using a backing sheet comprising a sheet base coated with a release layer positioned between the base and the adhesive layer of the label, in conventional manner, the peelability can be tuned more readily to achieve the tighter than normal release according to the invention.

Label sheets are made with various compositions for the pressure sensitive adhesive and for the release treatment for the backing sheet against which it operates, and we have not found any compositions which cause problems with the thermal transfer printing process provided the peel strength is sufficient according to the invention. The adhesive can be applied either directly to the substrate, or to the release-coated backing sheet prior to its subsequent lamination to the substrate.

For use on polyester substrates, we have found acrylic copolymer adhesives, formulated as aqueous solutions, to be effective. These may contain various additives common in such compositions, such as for example defoamer, tackifier, wet-out agent, viscosity modifier, flow additive and adhesion promoter. There is a wide variety of commercially available release compositions which can be used with such acrylic adhesives, including thermal catalysed systems. However, we prefer to use UV-cured systems for the present thermal transfer applications, and have used two different UV-cured silicone systems to particularly good effect. One is a cationic system and the other a free radical system. Both systems use a silicone pre-polymer of polydimethylsiloxane, but are differently modified according to the initiator: ie by the addition of epoxy groups for the cationic system and an acrylic group for the free radical system. In the former, the photoinitiators were selected from sulphonium and iodium compounds, which when exposed to UV light release strong acids to open the epoxy rings and start the polymerisation of the silicone. However, unless these systems are kept free from acid and metal contaminants, polymerisation of the modified siloxanes may be jeopardised, so generally we prefer to use the free radical system.

For conversion from a roll to individual sheets containing several labels, this can be carried out using a rotary cutter or a flat bed cutter, both being techniques known in the art. For labels to be used for thermal transfer printing according to the invention, we prefer to use a flat bed cutter, as giving generally a cleaner cut more appropriate for the quality of image achievable by the thermal transfer.

A preferred label sheet is one wherein at least one of the label and backing sheet contains an antistatic layer.

According to a further aspect of the present invention, a method of imaging a label sheet comprising a backing sheet and at least one label peelably mounted on the backing sheet, by thermal transfer imaging using a thermal head printer, is characterised by the 180° peel strength of the label with respect to the backing sheet as measured on sample strips 18mm wide at a peel rate of lOmm/s, being greater than llg.

According to another aspect there is provided a label sheet for thermal transfer imaging comprising a backing sheet and at least one label peelably mounted on the backing sheet, the label having a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side a layer of pressure-sensitive adhesive by which it is peelably held to the backing sheet with a peel strength less than the bond strength securing it to the substrate, wherein the 7V_ ° peel strength as measured on sample strips 18mm wide at a peel rate of 0.5mm/s is greater than 20g, preferably greater than

25g.

According to another aspect there is provided a method of imaging a label sheet comprising a backing sheet and at least one label peelably mounted on the backing sheet, by thermal transfer imaging using a thermal head printer, wherein the 7V_ ⁰ peel strength of the label with respect to the backing sheet as measured on sample strips 18mm wide at a peel rate of 0.5mm/s, is greater than 20g, preferably greater than 25g.

The invention is illustrated by reference to the following specific Examples and the accompanying drawing which is a diagrammatic side view illustrating a 1VA peel test.

EXAMPLE 1

Thin receiver sheets were prepared by coating one side of a 50μm thick voided polyester sheet (Melinex™ 329 sheet) as substrate, with a dye-receptive layer composition set out in the table below, then heated on-line to dry and cure the composition. The substrate was subsequently coated on the other side with a layer of an R-Type adhesive from Precision Identification Products, formulated for strong adhesion to the polyester substrate and based on an aqueous solution of acrylic polymer. This dried to provide a pressure-sensitive adhesive laver.

Separately a backing sheet was prepared by coating one side of a lOOμm thick white voided polyester sheet (Melinex™ 335) with a back coat composition as set out in the table below, and then a release layer was coated onto the other side. The release system employed was the Goldschmit system, using free radical polymerisation of an acrylic modified polydimethyl siloxane, and the controlled release additive was added in an amount to give a peel force of about llg (the actual value achieved in the finished label sheet was measured later when evaluating the product, and is given in the table of results hereinafter).

The separately prepared backing sheet and receiver sheet were then brought together to form a laminate, with the adhesive layer adhering to the release layer of the backing sheet. The laminate thus produced was in the form of a 125cm wide roll, which was passed through slitters to form smaller pancake rolls prior to conversion into A6 sheets. Conversion was carried on a flat bed die cutter, to cut through just the receiver sheet and thereby form the individual labels still adhered to the backing sheet, and then through a guillotine to chop the roll into A6 sheets, each containing separately peelable labels. The feel of these sheets was similar to a standard receiver sheet based on a voided polyester substrate of 150μm thickness, perhaps slightly stiffer. Evaluation:

Several label sheets from each batch thus formed were evaluated by printing them in normal manner, as though they were standard receiver sheets without a laminated backing sheet, and examining the printed sheets for any peeling of the labels from the backing sheet which might have occurred during their passage through the printer. The printer used was a standard Sony UP 1000 printer. Some lifting of a portion of an occasional label was observed, but all images were undistorted and all labels had remained on the backing sheet after their passage through the printer. Even the partially lifting labels did not appear to be about to come loose. In the table of results hereinafter this is classed as satisfactory.

A 180° peel test was also carried out on unprinted sample strips 18mm wide from the same batch, using an Instron 6021 mechanical tester, at a peel rate of lOmm/s. Four samples were peel-tested in this manner, and the average value recorded in the table of results hereinafter.

EXAMPLE 2

A receiver sheet was prepared as in Example 1. The backing sheet was based on a lOOμm thick clear polyester sheet (Melinex™ O grade), and this was coated with the same back coat composition as that used in Example 1. The Goldschmit release system was also again used, but the amount of controlled release additive was increased with the intention of giving a slightly tighter release than before. The resulting laminate was converted into A6 sheets and evaluated as described in Example 1.

Evaluation:

Examination of the printed label sheets showed no indication of any tendency for the labels to strip from the backing sheet during printing. This has been recorded as excellent in the table of results hereinafter.

COMPARATIVE EXAMPLE 1'

A receiver sheet was prepared as in Example 1. This was laminated onto standard glassine paper release liner as the backing sheet. The resulting laminate was converted into A6 sheets and evaluated as described in Example 1.

Evaluation

Labels were lifted from the backing sheet during printing, to the extent that the printer actually jammed during the printing of one sample. This would be totally unsatisfactory as product for every day use in thermal head printers.

COMPARATIVE EXAMPLE 2'

This was a commercial label sheet currently on sale for thermal transfer printing using a thermal head printer of the Sony UP1000 format.

Evaluation

Although none of those tested actually caused the printer to stop during the tests, there was considerable lifting of the labels during printing, to the extent that we were expecting the printer to stall. Clearly these were better than those using standard glassine release liner, but were still much less satisfactory than the samples of Example 1. and appeared to be very unreliable.

The results obtained for all the Examples and Comparative Examples are collected together in the Table of results hereunder.

The 180° peel test involves measuring the force needed to peel the label sample from the backing sheet when a free end of the label sample is bent back through 180° with respect to the remaining area of the label sample adhering to the backing sheet. In reality, a user peeling a label from a backing sheet probably pulls off the label with a smaller bend angle than 180°, so the applicants have conducted a fresh set of tests at a bend angle of IVz degrees. This is illustrated diagrammatically in the accompanying drawing where a label sample 1 is shown being pulled off a backing sheet 2 at a peel angle θ of 7 _ ° . The peel force is applied by a flexible strip 3 which passes over a rotatable pulley 4, the peel force being measured by a load cell 5. The test uses the same 18mm sample width as the earlier test described but uses a peel rate of 0.5mm/sec. The test results for the same two examples as previously, for a peel angle of 7V_ ° , are:

The minimum peel strength for acceptability is 20g, but this is preferably greater than 25g.

The results were taken as the mean of four measurements. It will be appreciated that a label sheet should desirably pass both the 180° peel lest and the 7 _ ° peel test.

Claims

1. A label sheet for thermal transfer imaging comprising a backing sheet and at least one label peelably mounted on the backing sheet, the label having a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side a layer of pressure-sensitive adhesive by which it is peelably held to the backing sheet with a peel strength less than the bond strength securing it to the substrate, wherein the 180┬░ peel strength as measured on sample strips 18mm wide at a peel rate of lOmm/s, is greater than llg.

2. A label sheet as claimed in claim 1, wherein the peel strength is at least 15g.

3. A label sheet as claimed in claim 2, wherein the peel strength is within the range 15 to 20g.

4. A label sheet as claimed in claim 1, wherein the peel strength is greater than 20g.

5. A label sheet as claimed in claim 1, wherein the backing sheet comprises a sheet base coated with a release layer positioned between the base and the adhesive layer of the label.

6. A label sheet as claimed in claim 1, wherein at least one of the label and backing sheets contains an antistatic layer.

7. A label sheet as claimed in any one of claims 1 to 6, wherein the dye-receptive layer has one or more dyes diffused therein and forming an image.

8. A method of imaging a label sheet comprising a backing sheet and at least one label peelably mounted on the backing sheet, by thermal transfer imaging using a thermal head printer, wherein the 180┬░ peel strength of the label with respect to the backing sheet as measured on sample strips 18mm wide at a peel rate of lOmm/s, is greater than llg.

9. A label sheet for thermal transfer imaging comprising a backing sheet and at least one label peelably mounted on the backing sheet, the label having a thermal transfer receiver comprising a substrate having on one side a dye-receptive layer and having on the other side a layer of pressure-sensitive adhesive by which it is peelably held to the backing sheet with a peel strength less than the bond strength securing it to the substrate, wherein the 7¹/.⁰ peel strength as measured on sample strips 18mm wide at a peel rate of 0.5mm/s is greater than 20g.

10. A label sheet as claimed in claim 9, wherein the peel strength is at least 25g.

11. A label sheet according to claim 1 and 10, wherein the 180┬░ peel strength is greater than llg and the 7V_ ┬░ peel strength is greater than 20g.

12. A method of imaging a label sheet comprising a backing sheet and at least one label peelably mounted on the backing sheet, by thermal transfer imaging using a thermal head printer, wherein the 7V_ ┬░ peel strength of the label with respect to the backing sheet as measured on sample strips 18mm wide at a peel rate of 0.5mm/s, is greater than 20g.