GB2214450A - Improvements in pressure-sensitive adhesive tapes - Google Patents

Abstract

A pressure-sensitive adhesive tape has a monoaxially oriented olefinic substrate (e.g. polypropylene or ethylene propylene/copolymer) and spaced transverse lines of perforations, preferably spaced aligned slits of 0.5 - 3mm width at intervals (e.g. 3-10 cms) along its length whereby it can be torn across at an exposed length. There is a balance of tensile strength and transverse tearability impossible to achieve using other substrates.

Description

IMPROVEMENTS IN PRESSURE-SENSITIVE ADHESIVE TAPES THIS INVENTION relates to pressure-sensitive adhesive tapes.

Pressure-sensitive tapes are produced for a variety of functional or decorative purposes. Two major forms of such tape are however produced in high volume namely the so-called general-purpose tape and the conventional "retail-and-office" tape. The largest sales volume is made up of "general-purpose" tape, with a biaxially oriented polyolefinic substrate, which is cheap, fairly strong and can tear arbitrarily on perforation. It is widely used for commercial packaging and sealing of parcels, etc. Also sold in large amounts is the slightly higher quality "retail-and-office" tape, based on transparent "Cellophane" (Registered Trade Mark) and used for temporary attachment, gift-wrapping, and the like.

Strapping tape is a more expensive specialist product, of high tensile strength, used commercially to bundle heavy parcels and packages together for transit and storage. Typically the substrate, or foil, upon which the pressure sensitive adhesive is applied, in order to provide the necessary tensile strength, is formed of an monoaxially oriented polyolefin, for example a polypropylene or an ethylene/copolymer oriented in the tape direction. This orientation is achieved by stretching and confers strength on the tape to make it resistant to transverse tear and provide a strong substrate for application of adhesive and subsequent use.

It is a commonplace with adhesive tapes that some expedient must be used for separating the desired length, whether a short length or whether as a long edge-closing length. Separation can be effected by a knife or scissors, but is often done by the provision of a serrated edge, against which the tape is pulled to produce perforations which then expand (as the tape pushes further over the teeth of the serrations) and finally meet at which time the end of the tape is severed. For example, it is possible to provide a heavy spool dispenser which carries a roll of tape in such a way that the free end passes over a plate with the serrated edge, being separated as desired from the serrated edge and leaving the sticky end-most region of the roll of tape temporarily attached to the plate for subsequent removal and use.It has also become known more recently, to provide a roll of tape with a sliding rigid polymer clip also presenting a plate surface terminating in a serrated edge, whereby, by suitable manipulation, the tape can be pulled from the roll over the plate and serrated edge and torn off against the edge in such length as may be desired.

The present invention is concerned to provide a pressure-sensitive adhesive tape which is pre-perforated at intervals, and can be torn off across the perforations after a required length of tape has been pulled from the roll.

This apparently simple concept is in fact extremely difficult to embody in practice. Thus, while pre-perforated tapes are known, it has not hitherto proved possible to manufacture acceptable pre-perforated tapes from thin films in conventional narrow widths, i.e. 12mum, l9mm or 25mm unless the film was exceptionally thick i.e. 70 microns or more,whether used as a single film or laminate. The cost and handling characteristics of such films in the given context are not suitable.They have been made from conventional cellophane materials for in an attempt to provide a "retail-and-office" tape.

If an unperforated roll of "retail-and-office"

tape, based on "Cellophane"l, is gripped by both thumb-and-finger pairs and twisted transversely, it will eventually tear. The tear is irregular and tends to pass variably more or less down the tape rather than straight across. Moreover, a significant amount of pressure has to be exerted to initiate tearing, and this means that the tape on either side of the tear becomes creased and wrinkled, and starts to adhere to itself in its creases.

We have found that if perforations are made across such tape then either the tensile strength of the tape is unduly decreased or perforations are so widely spaced as to have no guiding effect in transverse tearing. What happens in practice, therefore, is that such a tape, when unreeled will break at the first line of perforations it encounters. This is partly because only a proportion of the effective width of the tape is available to resist breakage, and partly because, in practice, any perforations will sooner or later so register with the width of tape as to nick the edge of the tape and give an immediate point of weakness for initiation of tearing. The upshot is that a perforated "retail-and-office" tape will not perform in a desirable fashion.

Similar, considerations apply to the biaxially polypropylene tape used for the "general purpose" product, which again cannot function if pre-perforated.

If on the other hand a length of strapping tape, based upon an monoaxially oriented polyolefin substrate is gripped between first fingers and thumbs of each hand and twisted, it does not tear. It merely twists and self adheres. For such a tape it is necessary to provide scissors or a sharp serrated edge for forming a set of progressively wider slits until the tape is severed.

We have now established however that contrary to expectations a pressure sensitive adhesive tape of the "strapping tape" type, (preferably less than 30mm wide and sop thick) and based upon a monoaxially oriented polyolefinic substrate, when provided at intervals with a generally transverse line of perforations, can thereafter be (a) unreeled from a roll without separation at the perforations but (b) twisted to tear cleanly along any line of perforation then exposed.

The invention as defined above is surprising in that it is based upon a "strapping tape" type of material.

Such material is a highly specialised product, and its essential nature is normally such as to prevent transverse tear. We have found, however, that when pre-perforated the tape still possesses enough residual strength in the longitudinal direction for use as a strapping tape (albeit with some diminution in tensile strength) or as a "retail-and-office" tape material, while providing clean transverse tear. No other combination of expedients provides such characteristics.

Preferably the line of perforations across the tape is a straight line, and most preferably it lies at right angles to the tape edge. The tape can be provided with such a line of perforations at regular or irregular intervals; we have found it preferable to provide the line of perforations at regular intervals ranging from say 3cm to 20cm, more particular from 6 to 10cm e.g.

about 7cm. This gives a module of tape, which, when: torn off, can be used in short lengths e.g. for temporary tacking purposes when packaging. It also provides in the usual way for lengths of tape e.g. up to tO or more centimetres long to be pulled off and used for edge or like seaming. Conventional widths of l2mm, l9mm and 25mum, and conventional single ply thicknesses of 40 > can be used.

It is also surprising that provision of perforations, which is necessarily such that occasionally a perforation will nick the edge of the tape, does not lead to undesired separations in such instances. In other words, it would appear that crack propagation across each space between adjacent perforations is essentially separate, and is not merely a continuous process initiated by a nick at the edge of the tape.

The spacing of the perforations can vary, but generally speaking it will be found desirable to have adjacent perforations commencing at spacing from 0.5 to 3mm. The shape of individual perforations is also variable, but broadly speaking it will be preferable to have elongate perforations or slits which, at least "as formed" are elongate and run along the line direction at their major axes. The total length of the slits can comprise between 30 and 70% of the width of the tape.

The man skilled in the art will thus recognise that by "perforation" is also included nrouletting" i.e. slit formation without excision of material.

Preferred materials for the tape substrate are are monoaxially oriented polypropylene or ethylene/propylene copolymer. Such films are described in U.K. patent 1 343 434 or 1 495 263. Commercially suitable films include polypropylene film Grade PPLC 40, supplied by Nowofol, Kunststoff Producte GmbH, 8227 Siegsdorf.

The conversion of these types of films to adhesive tapes is well known in the art. The tape structure normally comprises the base film, coated on one side1 optionally with a primer, and on the same side a normal pressure-sensitive adhesive. Optionally, the reverse side of the film can be release coated.

The film or tape can be clear transparent( and/or coloured. The tape additionally may be printed.

The longitudinal orientation is preferably such as to correspond with a degree of stretch of from 3 to 10 times the initial length, preferably about 4 to 7 lines e.g. 5 or 6 times.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 shows a typical line of perforations across a polypropylene or like tape in accordance with the invention, as formed, and Figure 2 shows the believed propagation of the slits on transverse tear.

In Figure 1 an example of a monoaxially oriented tape 1 of polypropylene, 19 mm wide and 40 > thick, coated with a conventional pressure sensitive adhesive coating (the nature of which per se forms no part of this invention) is provided with transverse lines 2 of slits 3, the lines being locums apart and the slits being produced by rouletting i.e. without excision of material, extending along the line and each being 0.9mm long with an 0.6 mm land between successive slits.

In Figure 2, such a tape is shown partly torn across, by usual dual finger-and-thumb grip at 4 and 5, where shown. While theory of operation is not intended to limit the scope of the invention, it appears that each slit successively propagates towards its next neighbour, which may by then have started to propagate back. There is not, however, a single initiated tear across the whole width.

We have found moreover, rather surprisingly, that in the context of wound adhesive tape, the slits do not individually open out to a "mouth" shape, as might be expected from the monoaxial orientation, but stay essentially closed. Thus, little or no adhesive through transfer to give a "ghost" dotted line, is encountered: the only transverse lines prominent visible are the tear line.

Claims (11)

Claims.

1. A pressure-sensitive adhesive tape based upon a monoaxially oriented polyolefinic substrate provided at intervals along its length with a general transverse line of perforations, whereby it may be unreeled from a roll without separation at the perforations but twisted to tear clearly along a line of perforations exposed on the unrolled length.

2. A pressure-sensitive adhesive tape as claimed in claim 1 with a straight line of perforations.

3. A pressure-sensitive adhesive tape as claimed in claim 2 in which the straight line of perforations extends at right angles to the edge of the tape.

4. A pressure-sensitive adhesive tape as claimed in any one preceding claim in which the lines of perforations recur at regular intervals of 3cm to 20cm.

5. A pressure-sensitive adhesive tape as claimed in claim 4 in which the intervals are from 6cm to locum.

6. A pressure-sensitive adhesive tape as claimed in any one of claims 1 to 5 in which successive perforations in each line are spaced from 0.5 to 3.0 mm apart.

7. A pressure-sensitive adhesive tape as claimed in any one preceding claim in which the perforations are shaped as slits extending along the line of perforations.

8. A pressure-sensitive adhesive tape as claimed in claim 7 in which the total length of the slits comprises from 30 to 70% of the length of the line of perforations.

9. A pressure-sensitive adhesive tape as claimed in any one preceding claim in which the tape substrate is a monoaxially oriented polypropylene or ethylene/propylene copolymer.

10. A pressure-sensitive adhesive tape as claimed in any one preceding claim in which the longitudinal orientation is produced by a degree of stretch of from 3 to 10 times the original length.

11. A pressure-sensitive adhesive tape as claimed in claim claim 10 in which the longitudinal orientation is produced by a degree of stretch of from 4 to 7 times the original length.