GB2340434A - Promoting adhesion on the surface of a sheet material - Google Patents

Abstract

A method of promoting adhesion on the surface of a web of sheet material comprises subjecting the web to two surface treatment stages in series comprising: a first, heating, stage in which the surface of the sheet material is passed through a gas flame; followed immediately after the first stage by a second, discharge, stage in which the surface of the web following the heating stage is subjected to at least one electrical corona discharge. Typically the first treatment stage is spaced from the second treatment stage along the path of the web by a distance comparable with the width of the web. Apparatus for undertaking the method comprises a heating stage H having a gas jet nozzle heater 27 and heat supporting roll 24 and a discharge stage D having corona discharge electrodes 31 and a discharge support roll 30. The sheet material can be a plastic film or a metallic foil.

Description

2340434 PROMOTION OF ADHESION This invention relates to the promotion of

adhesion particularly in relation to film and film like materials including foils. Hereafter the term 'film' is used to cover all such materials. Film is widely used for, or in relation to, packaging for a wide range of products in a number of industries. For this purpose there is an increasing pressure to use contact materials such as water based inks and adhesives on the film. The use of these contact materials is desirable, as against oil or solvent based inks and adhesives, to accord with current views on reducing, if not avoiding, environmental problems once the combined material is disposed of as refuse.

The present invention is concerned with achieving improved surface adhesion of a film than has heretofore been readily achieved and in addition providing for the resulting improved surface adhesion to be maintained at a high level for an extended period while being stored.

Adhesion between surfaces is apparent when roughness provides a mechanical key for an adhesive layer between surfaces. When surfaces are smooth then adhesion still appears to occur readily with some materials but not with others. It is generally accepted that a strong bond can be made between two polar materials. A bond of a lower strength arises if one or both linked materials are non-polar.

Adhesive or ink is often applied to a surface in the form of a liquid. The contact angle of a droplet of liquid on a flat surface is a function of the energy levels of the liquid and the solid forming the surface. Diagram 1 (and Diagram 2) shows a droplet 11 (21) of a liquid on a surface 12 (22) of a sheet material 13 (23). Contact angle 0 (EY) is the angle between tangent T, (T') of the droplet 11, (21) at the surface 12, (22) and the surface 12, (22) itself.

Diagram 1 shows that with a relatively high level of energy in the liquid making up the droplet 11 the contact angle 0 is relatively large and low wetting of the surface 12 occurs.

Diagram 2 shows a condition where there is a relatively low level of energy in the liquid making up the droplet 21 with the result that the contact angle 0' is relatively small and greater wetting occurs. The contact angle will further decrease as the energy levels of the 2 greater wetting occurs. The contact angle will further decrease as the energy levels of the sheet material and that of the droplet approach one another. When the energy levels are the same complete wetting has occurred.

The relationship between the work of adhesion (Wj of a liquid relative to a solid surface is given by the combined Young-Dupre equation:

WA = + COS 0) Where: -4 is the surface tension of the liquid and 0 is the contact angle of the liquid on the surface.

Surface tension in a liquid arises from unbalaiced molecular forces at or near the surface. If the surface tension is higher than that of the surface energy.9f the material on which it rests then the liquid tends to form a tighter droplet rather thir spread out. Surface tension of a liquid is normally measured in energy units (dynes/cm or megaNewtons/ meter). The dyne level of a material is referred to as its'surface energy'.

Polyolefins gvfierally have low energy levels. Polypropylene-has -a lower surface tension of 29 dynes/CM. Nylon has a high surface tension of 46 dynes/cm. The various solvents range from 18 to 29 dynes/cm.

Water has a surface tension of 72 dynes/on. Water molecules will tend to cling to themselves rather to a low energy weakly attractive surfaces such as polyethylene. Solvents, in contrast to water, readily spread and are attracted to a plastics film surface having a surface energy of 30 dynes/on or more. Up to now the spreading of solvent, as distinct from water based, inks on plastic filrr has not been a problem. Even with good liquid transfer and spreading good adhesion for water based inks will not necessarily occur as water based materials do not wet a surface so readily.

The most common method of measuring surface tension of the surface of a substrate is by the use of wetting, or 'dyne', solutions. The ASTM specification D2578 defines measurement of surface tension of various Polyoleffiis substrates using testing solution in

3 surface energy of the substrate, the contact angle will decrease as improved wetting occurs. The substrate 'surface energy' is taken to be the lowest value of the dyne solution that instantaneously wets the substrate and does not form droplets within two seconds.

In order to achieve good adhesion when printing or applying an adhesive it appears that the surface energy of the polymer sheet material should be at least 10 dynes/cm. higher than the surface tension of the ink applied to it. The difference appears to enable the ink to wet out uniformly over the surface of the substrate and not to reticulate (that is to say form droplets). Thus the contact angle of the liquid is smaller when the surface energy of the substrate is sufficiently higher than the surface tension of the ink.

It will be apparent that if the surface energy of a plastic substrate can be raised to a higher level than heretofore then adhesion of adhesives and inks can be improved. In addition if the surface tension of the plastic substrate at or near its higher level can be maintained in a stored product for an extended period before the product is printed then the useful shelf life of the plastic substrate is extended.

According to a first aspect of the present invention there is provided a method of promoting adhesion on the surface of a web of film material comprising subjecting the web to two surface treatment stages in series made up of a first, heating, stage in which the surface of the sheet material is passed through a gas flame; followed immediately after the first stage by a second, discharge, stage in which the surface of the web following the heating stage is subjected to at least one electrical corona discharge. Typically the first treatment stage is spaced from the second treatment stage along the path of the web by a distance comparable with the width of the web.

According to a first preferred version of the first aspect of the present invention there is provided the further step during or after the heating stage of subjecting the part of the web heated during the first Stage to cooling prior to the second stage. Typically the cooling is undertaken by a flow of water.

According to a second preferred version of the first aspect of the present invention or of the first preferred version thereof including the further stages of extracting gaseous 4 combustion products following the heating stage and of gaseous electrical discharge generated products following the discharge stage.

According to a second aspect of the present invention there is provided apparatus for promoting adhesion on the surface of a sheet material in the form of a web of the material comprising in sequence: a heating stage comprising a heating support roll juxtaposed with a gas burner extending the width of the web along the roll; at discharge stage comprising a one or more discharge support rolls; the or each discharge support roll being juxtaposed with a corona discharge electrode extending the width of the web along the juxtaposed roll. Typically the heating support roll is spaced from the, or the first, discharge support roll by a distance along the web comparable to the width of the web According to a first preferred version of the second aspect of the present invention the apparatus comprises a unitary structure serving to locate as a unit: the heating stage; the discharge stage; inlet web path guide means providing for incoming web material to be presented to the heating support roll of the heating stage; web path guide means downstream of the heating stage providing for material in the web which has been subjected to heating in the heating stage to pass from the heating stage to the discharge stage; and outlet web path guide means downstream of the discharge stage providing for material in the web which has been subjected to electrical discharge in the discharge stage to pass from the apparatus.

According to a second preferred version of the second aspect of the present invention or of the first preferred version thereof the heating support roll incorporates, or is associated with, a cooling system to enable the bulk temperature of web material in passing through the heating stage to be kept below a maximum. Typically the cooling system provides for a supply of cooling water to be fed into, passed through and discharged from, the interior of the heating support roll; the heating support roller being adapted to provide for good heat transfer between the heating support roller and web material in contact with the heating support roller.

According to a third preferred version of the second aspect of the present invention or of any preceding preferred version thereof there is provided gas extraction means in the vicinity of the heating stage and/or discharge stage providing for the extraction of combustion products from the vicinity of the heating stage and/or discharge products from the discharge stage.

According to a fourth preferred version of the second aspect of the present invention or of any preceding preferred version thereof the apparatus is provided with a motor for driving the heating support roll. Typically the there are provided rotation sensing means for the heating support roller and for the, or each, discharge support roller and the motor is controlled in dependence on the outputs of said rotation sensing means.

According to a third aspect of the present invention there is provided a web of plastics material that has been subjected to a method of promoting adhesion according to the method of the first aspect of the present invention or any preferred version thereof.

According to a fourth aspect of the present invention there is provided a web of material subjected to treatment in apparatus according to the second aspect of the present invention or any preferred version thereof.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawing of a unit for promoting adhesion of which the sole figure is a vertical side section of a web treating unit.

The Figure shows a unit 11 made with a left side frame 12 to a right side frame member (not shown) linked by upper cross members 13, 14 and lower cross members 15, 16.

Left side frame 12 has front colurnn 17, rear colun-un 18 joined by way of top beam 19, intermediate beam 20 and bottom beam 21. The right hand frame is formed of components mirroring these items making up the left side frame 12.

6 The left side frame 12 along with the unshown right side frame serve to support a sequence of rollers providing for the passage of a web W of material through the unit 11. This provides for the material in the web W to be subjected to a two stage surface treatment so as to enhance adhesion such as of printing ink to the web material or adhesion between layers of material including material from the web W.

HEATING STAGE Web W is fed into the unit 11 by way of a transit roll 21. From the roll 21 the web passes upwardly to first unit roll 22 from which it enters first, heating, stage H performed by way of the unit 11. The heating is undertaken by means of heating support roll 24 which is of hollow construction with an annulus 25 through which cooling water can be passed when the unit 11 is in operation. The heat support roll 24 is belt driven by way of electric motor 26.

The surface of the web W on arriving at the heating support roll 24 is subjected to heating along the width of the web W by way of a heater 27 which has a series of gas jet nozzles (typically nozzle 27A) which series extends for the full width of the web W from a gas supply pipe 27B. The pipe 27B is supplied by way of a main 28 which can be isolated by way of a solenoid operated stop cock 28B. The relative positioning of the heater 27 and the web W in passing over the roll 24 to provide for uniform heating of outer surface of the web W in passing around the roller 24. In an alternative version the gas heating means can be duplicated to provide for the opposite side of the web W to that side which is treated by the gas flame on roll 24.

The temperature of the web is regulated by the passage of water through annulus 25 of heat support roll 24 in a manner providing for effective heat transfer between web and roll surface. The extent of wrap around of the web on the roll 24 can be varied to provided for the most such effective heat transfer for a given application. As shown the wrap around covers about two thirds of the periphery of the roll 24. This can be adjusted if necessary by horizontal displacement of the first unit roll 97.

7 Having passed around the required extent of the periphery of the roll 24 the web W passes down to the second, discharge, stage D provided by the unit 11.

The stage D involves the use of a discharge support roll 30 and an array of discharge electrodes 31 in a housing 32 with a longitudinal axis D' parallel to, and extending the full width of, the discharge support roll 30. The electrodes 31 are maintained at an appropriately high voltage to provide for the entire passing surface of the web W to be subject to the necessary discharge.

The distance measured along the web W from the heat support roll 24 to discharge support roll 30 is comparable with the width of the web W measured along the roll 24. In this way the amount of time between the two treatment stages is kept short.

From stage D the now fully treated web W passes down and around the outlet roll 33 and then passes horizontally out of the back of unit 11 to the next stage of the production process involving the web W.

The two stage treatment of material in the web W can leads to the generation of combustion products during the first stage and of ozone during the second stage. An extractor hood 35 is consequently provided in the upper part of the unit 11 which coupled to an extractor duct 36 to which an extractor fan is coupled. for the extraction of such products by means of extraction fans in a known manner. In the event the generation of ozone is likely to prove problematic then it can be eliminated by the use of known ozone destruction units.

The unit 11 provides for the close juxtaposition of a heating process and a corona treatment process for the material of web W in a single pass through the unit 11.

In using a unit similar to that described above a number of benefits have been found.

Some material in the form of film is known to pose a challenge when subject to heating or discharge treatment. By combining the treatment in a unit according to the present invention a marked improvement has been achieved.

8 TREATMENT OF FILMS It has been found that treatment by means of the present invention of a white polypropylene film can results in substantial improvement in adhesion quality.. By way of example by treating a sample of the fihn in a unit similar to that described in the exemplary embodiment an adhesion level some 34% greater was achieved than was possible form the use of a single gas heating stage. Applying the two stage process of the present invention resulted in an improvement of 64% over a single treatment discharge stage Treatment to improve a PVC plastic having a high plasticiser content has hitherto been regarded as virtually impossible. However the use of the two stage process of the present invention on a test sample of such a material resulted in the achievement of an adhesion level of 45 dynes/cm.

TREATMENT OF FOILS The presence of rolling mill residuals on film in the form of an aluminium foil has an adverse affect on surface energy. Typically such residues have been found on examination to cause the surface energy of the foil to drop from a dyne level of 73 dyne / cm to an level of less than 35 dynes / cm. This latter figure is not acceptable in any converting application. Until recently it was assumed that the use of a corona discharge or of a flame treatment could enable the aluminium to gain a dyne level in the vicinity of 73 dynes/ cm. Research has revealed that in many cases however this assumption is not correct. To achieve a practical solution it was decided to increase the adhesion level at the edges of a foil, as distinct from the central region, where the degrading in the energy level was most prominent. The use of corona discharge on its own or gas heating on its own failed to increase adhesion at the edges of the foil. However by subjecting the material under test to a two stage treatment described in relation to the exemplary embodiment a 10% improvement in adhesion at the edge of the foil was achieved which is a substantial improvement over what has been to be the case in previous practice.

9 DEcAY RATE The decay in energy level in many extrusion or converting applications can be overcome to some extent by increasing the intensity of the treatment. However increase in intensity can cause problems such as pin holing. The largest factor in causing high (that is to say rapid) decay rates apart from environmental conditions is the presence of slip agent in the material making up the web. Treatment of materials with high content of slip agent during an extrusion process can bum away the slip agent. However after a few days further slip agent blooms to the surface causing the surface energy level to fall. In a converting process this can be overcome by boost treating the web material on line prior to printing, laminating or coating. However up to now materials with high quantity of added slip material which has not been treated during an extrusion process has been regarded as impossible to boost treat.

Apart from an immediately usable increase in energy level for a subsequent printing or adhesion stage following a convention heating or discharge stage there would be an additional advantage for the increased energy level to be maintained in material to be stored following a treatment stage rather than being treated with adhesive or inks following a convention treatment stage. This is particularly true for material hitherto known to exhibit high decay rates following convention treatment.

It has been found that following treatment by the two stage process of the present invention it has proved possible to maintain their pre- treated dyne level for extended periods of time. In particular by way of example a self-adhesive label stock on being treated with a process of the present invention retained an energy level in excess of 60 dyne/cm. after being stored for two months from when it was produced. This would be ample for a subsequent printing or adhesion application without a need for a further processing or refreshing step.

Claims (16)

Claims

1 A method of promoting adhesion on the surface of a web of sheet material comprising subjecting the web to a combination of two stage surface treatment in series comprising: a first, heating, stage in which the surface of the sheet material is passed through a gas flame; followed immediately after the first stage by a second, discharge, stage in which the surface of the web following the heating stage is subjected to at least one electrical corona discharge.

2 A method as claimed in Claim 1 wherein the first treatment stage is spaced from the second treatment stage along the path of the web by a distance comparable with the width of the web.

3 A method as claimed in any preceding claim wherein during or after the first stage heating the web there is provided the further step of controlling the level of heating by means of cooling prior to the second stage.

4 A method as claimed in Claim 3 wherein the cooling is undertaken by a flow of water.

5 A method as claimed in any preceding claim including the further stages of extracting gaseous combustion products following the heating stage and/ or the extraction of gaseous electrical discharge generated products following the discharge stage.

6 Apparatus for promoting adhesion on the surface of a sheet material in the form of a web of the material comprising in sequence: a heating stage comprising a temperature controlled heating support roll juxtaposed with a gas burner extending the width of the web along the roll; a discharge stage comprising a one or more discharge support rolls; the or each discharge support roll being juxtaposed with a corona discharge electrode extending the width of the web along the juxtaposed roll.

11

7 Apparatus as claimed in Claim 6 wherein the heating support roll is spaced from the, or the first, discharge support roll by a distance along the web comparable to the width of the web

8 Apparatus as claimed in Claim 6 or Claim 7 comprising a unitary structure locating:

the heating stage; the discharge stage; inlet web path guide means providing for incoming web material to be presented to the heating support roll of the heating stage; web path guide means downstream of the heating stage providing for material in the web which has been subjected to heating in the heating stage to pass from the heating stage to the discharge stage; and outlet web path guide means downstream of the discharge stage providing for material in the web which has been subjected to electrical discharge in the discharge stage to pass from the apparatus.

9 Apparatus as claimed in Claim 6, 7 or 8 wherein the heating support roll incorporates, or is associated with, a cooling system to enable the bulk temperature of web material in passing through the heating stage to be kept below a maximum.

Apparatus as claimed in Claim 9 wherein the cooling system provides for a supply of cooling water to be fed into, passed through and discharged from, the interior of the heating support roll; the heating support roller being adapted to provide for good heat transfer between the heating support roller and web material in contact with the heating support roller.

11 Apparatus as claimed in any of preceding claims 6 to 10 provided with gas extraction means in the vicinity of the heating stage providing for the extraction of combustion products from the vicinity of the heating stage.

12 Apparatus as claimed in any of preceding claims 6 to 11 provided with a motor for driving the heating support roll.

12

13 Apparatus as claimed in Claim 12 provided with rotation sensing means for the heating support roller and for the, or each, discharge support roller.

14 Apparatus as hereinbefore described with reference to the accompanying drawing.

15 A web of plastics material having been subjected to a method of promoting adhesion as claimed in claims 1 to 5.

16 A web of plastic material having been subjected to treatment in apparatus as claimed in claims 6 to 14.