CA2154623A1 - Process for the plasma pretreatment of polyolefin films - Google Patents

Abstract

The invention relates to a process for the produc-tion of highly polar, scratch-resistant at least monoaxi-ally stretched polyolefin films by plasma pretreatment, characterized in that titanium is used as the cathode material, the applied voltage is in the range from 400 to 800 V and preferably in the range from 500 to 700 V, the voltage being an a.c. voltage or a pulsating d.c. vol-tage, and in that the treatment of the film is carried out under pressures of 0.01 to 0.5 mbar and preferably 0.03 to 0.42 mbar in an oxygen atmosphere or in an atmosphere containing non-metal oxides and mixtures thereof with noble gases.

Description

215~623 -A PROCESS FOR THE PI~SI~ PRETR~T?~s~NT OF POLYOLEFIN FILMS

This invention relates to a process for the plasma pretreatment of polyolefin films, more particularly scratch-resistant BOPP films (biaxially oriented poly-propylene films), by the low-pressure method.
Numerous publications on the low-pressure plasma are available in the literature. DE-PS 997 093 describes a process for the treatment of polypropylene in an electri-cal discharge under a pressure of up to 1.3 mbar in order to improve the dyeability of the polymer in a subsequent graft polymerization reaction.
US-A-374,091 describes a process for the treatment of films in an atmosphere of, for example, acrylic acid and nitrogen. An interval between electrode and polymer of 0.25 to 3.2 mm is mentioned as a process parameter.
The energy density is of the order of 0.016 Ws/cm2.
US-A-3,686,018 describes a process for the pretreat-ment of organic substrates in a low-pressure plasma and subsequent treatment of the pretreated substrate in an oxidizing gas atmosphere to reduce the permeation of gas through a metal layer subsequently applied by vapor deposition. The pretreatment is carried out under a pressure of 0.013 to 1.3 mbar with a distance of 50 mm between the electrode and the substrate.
EP 0 436 918 describes the plasma pretreatment of polyolefin films in which certain parameters, such as energy density, partial pressure, distance and tempera-ture have to be observed.
The present invention relates to a process for the production of scratch-resistant, at least monoaxially stretched polyolefin films by plasma pretreatment, characterized in that titanium is used as the cathode material, the applied voltage is in the range from 400 to 800 V and preferably in the range from 500 to 700 V, the ww s 4 o 6 - Forei gn countri es voltage being an a.c. voltage or a pulsating d.c. vol-tage, and in that the treatment of the film is carried out under pressures of 0.03 to 0.42 mbar in an oxygen atmosphere and/or in an atmosphere containing non-metal oxides.
The adhesion of polyolefin films to adhesives (solvent-containing and water-based dispersion adhesives) and to water-based printing inks is considerably improved by the pretreatment according to the invention.
The polyolefins used for this purpose may be single-layer or multilayer films. An isotactic polypropylene with an n-heptane-soluble component of 2% to 6% by weight is preferably used for stretched (more particularly biaxially stretched) polyolefin films.
The polypropylene has a density of 0.9 to 0.91 g/cm3 and a melt index of 2 to 5 g/10 mins. (230C/21.6 N, DIN
53735). The polymer is thermoplastically extruded through a flat film die onto a cooling roller where it solidifies, is reheated via heating rollers and is stretched by a factor of 5 in the machine direction. The longitudinally stretched film is then transversely stretched by a factor of 10 in a stretching frame at a temperature of 160C. Finally, after transverse stretch-ing, the film is fixed, i.e. is kept in the stretching frame for 0.5 to 10 seconds at a temperature 5 to 10C
below the stretching temperature.
Multilayer sealable films are also eminently suit-able for plasma pretreatment. Multilayer sealable films are generally symmetrical in structure (ABA structure), the B layer being the polypropylene described above. The A layers with a thickness of around 1 ~m consist of statistical copolymers, of propylene and ethylene units with an ethylene content of 4% by weight and a propylene content of 96% by weight or of copolymers of propylene and 1-butene units or of copolymers of ethylene and 1-WW s406 --butene units or of mixtures thereof, the propylene making up more than 80% by weight of the mixture.
opaque multilayer films may also be subjected to the pretreatment. The base layer may consist, for example, of preferably 6 to 14% by weight of an additive incom-patible with the polypropylene such as calcium carbonate, silicone dioxide and/or titanium dioxide as inorganic material and polyamide 6, polybutylene terephthalate and polytetrafluoroethylene or mixtures thereof as organic material. Incompatible in this context means that the organic additives differ in their melting point and/or in their stretchability from the polypropylene so that, providing the biaxial orientation of the multilayer films is carried out under suitable conditions, the polymer matrix breaks up and vacuoles are formed. The formation of vacuoles in the films is responsible for their opaque character.
The multilayer films treated in accordance with the invention may carry a heat lamination layer of low-melting polymers on one side. By application of heat andpressure, a film such as this may first be heat-laminated to other substrates, subsequently bonded on the side pretreated in accordance with the invention and printed.
The layers of the films may be provided with typical additives and auxiliaries such as, for example, lubri-cants, antiblocking agents and antistatic agents in the usual quantities.
The films suitable for a plasma pretreatment may be produced by standard processes, such as lamination, coating or melt (co)extrusion. They are preferably stretched at least monoaxially and preferably biaxially.
In multilayer films, the polypropylene base film layer should preferably have a thickness of 12 ~m to 50 ~m while the heat-sealable layers should preferably have a thickness of 0.8 to 2 ~m and, more preferably, of the W~ 5406 21~623 order of 1 ~m.
The polyolefin, preferably polypropylene, film is preferably wound onto a roll and is delivered via a cooling roller to the winding roller, the arrangement being accommodated in a vacuum chamber.
The cathode(s) are arranged around the cooling roller, the process gas (oxygen) being introduced in this zone.
The plasma is energized by an electrical field with rectified a.c. voltage. A voltage in the range from 300 to 2,000 V can be adjusted. The polyolefin film is situated at a certain distance from the cathodes, prefer-ably of the order of 20 to 100 mm.
The cathode may be a simple cathode or even a magnetron cathode. Typical process pressures are in the range from 0.01 to 0.5 mbar. The process gases used are preferably oxygen, ozone or gases containing non-metal oxides such as, for example, 2~ H2O2, H2O, N2O and NO2 and mixtures thereof with noble gases, such as He, Ne, Ar, Kr and Xe.
It has surprisingly been found that a plasma pre-treatment is only appropriate in a very narrow process parameter range. If this parameter range is not observed in the plasma pretreatment of BOPP films, firm adhesion to adhesives may still be achieved, but only at the ex-pense of high sensitivity of the film surface to scratch-es.
The following process parameters are of importance in the plasma pretreatment: applied voltage, current flow, rate of travel of the film web, cathode area, pressure prevailing during the plasma pretreatment and the type of gas used.
The effect of the plasma pretreatment on the BOPP
films was assessed on the basis of the following cri-teria. Bondability of the film with commercial adhe-21$~623 sives, printability with commercial printing inks andsensitivity of the film surface to mechanical stressing and, hence, loss of optical film quality. (The plasma pretreatment should not affect sensitivity to mechanical stressing, for example sliding of the surface on various parts of the processing machine and stacking of the finished packs or rubbing of the packs against one another. If the film loses its optical properties, such as opacity and gloss, through mechanical stressing, it is unsuitable for finishing.) A very small working range in which the plasma pretreatment can be effectively used for BOPP films has surprisingly been found among the large number of para-meters mentioned above and their possible variations.
The following parameters are critical to particular-ly effective plasma pretreatment: the applied voltage must be in the range from 500 to 700 V. The energy density, which is calculated as the input of electrical energy from the width of the cathode and the rate of travel of the film:
Pspec. = U I = [V A] = [Ws]
L v [m.m/s] [m2]
should preferably be in the range from 0.1 to 1.5 Ws/cm2.
Accordingly, the following values go into the calculation of the energy density: applied voltage U, current flow I, cathode width L and film speed v.
Low energy density values do not lead to any im-provement in the adhesion of adhesives or printing inks to the polyolefin films. On the other hand, an energy density above 1.5 Ws/cm2 is generally unfavorable be-cause, in this case, the surface of the polyolefin film is irreversibly damaged by breaking of the bond between the olefinic chain members in a layer near the surface, Ww 5406 215~623 so that a boundary layer with substantially oligomeric chain members that causes poor adhesion is formed. If the plasma pretreatment is carried out at higher volt-ages, good adhesion to the adhesive can still be achiev-ed at relatively low energy densities, but only at theexpense of an increase in the sensitivity of the film surface to mechanical stressing. This sensitivity means that the surface of the film can easily be scratched, resulting in a distinct reduction in gloss and in an increase in opacity.
The sensitivity of the film surface can be quanti-fied by measuring the difference in opacity before and after mechanical stressing.
In addition, it is important that the temperatures of the film during the treatment should not exceed 30C
and should preferably be in the range from 2C to 10C
because otherwise adhesion in a laminate subsequently produced remains poor.
After the film has been pretreated in accordance with the invention, it is removed from the vacuum ar-rangement and can be further processed to a laminate.
This may be done, for example, by laminating the film onto paper and cardboard by bonding or by applying a metal layer or a layer of organic oxides, for example by vapor deposition. Laminates such as these are used inter alia in the packaging industry.
In addition, the film pretreated in accordance with the invention is eminently suitable for printing with difficult printing inks, for example water-based printing inks. Water-based inks are being increasingly used in the packaging field for ecological reasons.
In one particularly preferred embodiment, biaxially stretched polypropylene films corona-pretreated on one side are pretreated in accordance with the invention on the non-corona-pretreated side. That side of the film 21S~623 which has been pretreated in accordance with the inven-tion is fully laminated to the front of the paperboard.
The paperboard/BOPP film laminate is made up into a folding box, the side of the film which has been pre-treated in accordance with the invention being bonded tothe back of the paperboard. By virtue of the high receptivity if the side pretreated in accordance with the invention to adhesives, the laminate is particularly suitable for processing in high-speed folding box glueing machines.
The test methods used are described in the follow-ing.
The adhesion tests for this film are carried out as follows:
Adhesive (Adhesin A 7250, a dispersion adhesive manufactured by Henkel KGaA) is applied by applicator to the surface of the film pretreated in accordance with the invention. The applicator is square, consists of VA
steel and has a 4 mm bore through which the adhesive passes onto the film. The thickness of the adhesive film is controlled through the 0.1 mm gap width. After a short open time, a pulp paperboard (product of Igesund, paper type: Inwercoate 250 g/m2) is placed with its back on the glue seam in the absence of pressure. The bond is established by pressing with two glass plates for 30 seconds under a pressure of 10 g/cm2.
The test specimens are then stored for 24 h at room temperature in the absence of pressure and subsequently separated. Adhesion is evaluated from the appearance of the fracture pattern. The film and the pulp paperboard are separated by peeling off the film at an angle of soo to the paperboard.
If pulp paperboard or parts thereof adhere to more than 95% of the film surface coated with adhesive-after the separation process, adhesion is assessed as very W~ 5406 ,, 215~62~

good. The strength of the bond between a film and paperboard is thus greater than the strength of the paperboard.
The following evaluation scales are used:

Percentage of surface Evaluation Coding coated with adhesive > 95% Very good ++
> 75% Good +
> 20% Moderate 0 < 20% Indequate The sensitivity of a transparent film to scratching is measured as follows:
The surface of the film is mechanically treated with silicon carbide powder under specific conditions. The silicon carbide particles (type Makrokorn~ E 120) drop onto the surface of the film from a certain height at a certain angle. The difference in the opacity values tas determined in accordance with ASTM 1003) before and after the treatment is a measure of sensitivity to scratching.
The following evaluation scales are used:

Difference in Evaluation Coding used opacity [%]
0 S 2 Very good ++

2 ~ 4 Good +
4 < 10 Moderate 0 > 10 Indequate In another test, the films are printed with water--soluble ink on the side pretreated in accordance with the invention in a Rotova printing machine. After 24 h, anchoring of the ink is tested with an adhesive tape (for example Beiersdorf Tesafilm No. 133). The adhesive tape is applied to the printed film. Anchoring of the ink can be evaluated after the adhesive tape has been peeled off the film. As in the adhesion test, the adhesive tape is peeled off perpendicularly of the film.
If very little or no ink is found on the adhesive tape after it has been peeled off the printed film, ink adhesion to the film is very good. If the ink adheres almost completely to the adhesive tape, ink adhesion is inadequate. In this case, evaluation is based on the ink-covered area of the adhesive tape.
The following evaluation scales are used:

Percentage of surface Evaluation Coding with ink on test tape 20 > 20% Inadequate 20 - 10% Average 0 10 - 5% Good +
5 - 0% Very good ++
The invention is illustrated by the following Examples.

Example 1 A 15 ~m thick biaxially stretched polypropylene film is subjected to a plasma pretreatment on one side.
In accordance with its requirement profile for lamination, the film is internally finished with anti-blocking agent (synthetic silica), antistatic agents (ethoxylated fatty acid amides) and with lubricant ~W 5406 (erucic acid amide).
After introduction of the film, the vacuum arrange-ment is evacuated to a pressure of 10-5 mbar. Oxygen 2 is then introduced in the vicinity of the cathode so that the required pressure of 0.05 mbar is established. After ignition of the plasma field, a luminous area with a dark space can be seen between the film and the cathode. The plasma pretreatment was carried out at adjusted voltages (rectified a.c. voltage) of 700 V and an energy density PspeC. of 0.3 Ws/cm2.
The results are set out in the Table.

Example 2 A 20 ~m thick co-extruded, biaxially stretched film of isotactic polypropylene with two heat-sealable pro-pylene/ethylene copolymers was subjected to a plasma pre-treatment. The heat-sealable layers are 1 ~m thick and consist of around 4% by weight of statistically dis-tributed ethylene in 96% by weight of propylene. The layers contain the additives important to further proces-sing, such as antiblocking agent (silicon dioxide), lub-ricant (erucic acid amide) and antistatic agent (ethox-ylated fatty acid amide). The pretreatment data are taken over from Example 1, see also the Table.
The slightly higher sensitivity to scratching observed in this Example is not attributed to the plasma pretreatment, but instead to the higher overall sensi-tivity of this sealing layer. Through its 4% ethylene component, this sealing layer has higher sensitivity than pure homopolypropylene for example.

Comparison Examples Films obtainable from Wolff Walsrode AG under the following names were used as Comparison Examples 1 and 2:

Comparison Example 1 WalothenX O 15 SZ one-layer BOPP film (non-sealable) corona pre-treated on both sides.

Comparison Example 2 Walothen~ C 20 SE three-layer BOPP film (sealable on both sides) corona pre-treated on one side.

Comparison Example 3 The film of Example 1 was subjected to a plasma pretreatment at a voltage of 300 V and an energy density of 0.3 Ws/cm2.

Comparison Example 4 The film of Example 1 was subjected to a plasma pretreatment at a voltage of 1,500 V but at the same energy density of 0.3 Ws/cm2 and under otherwise the same conditions.

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Claims (6)

1. A process for producing a highly polar scratch-resistant at least monoaxially stretched polyolefin film by plasma pretreatment which process comprises treating the film under a pressure of 0.01 to 0.5 mbar in an oxygen atmosphere or in an atmosphere containing non-metal oxides or mixtures thereof with a noble gas with a titanium cathode at an applied voltage of 400 to 800 volts as an a.c. voltage or a pulsating d.c. voltage.

2. A process according to claim 1, wherein the pressure is 0.03 to 0.42 mbar and the voltage is 500 to 700 volts.

3. A process according to claim 1, wherein the polyolefin film is extruded, at least monoaxially stretched and consist of one or more layers and of the same or different polyolefins.

4. A process according to claim 1, wherein the films are heat-sealable, biaxially stretched polypropylene films.

5. A process according to claim 1, wherein pretreatment is applied to a biaxially stretched polypropylene film provided on a non-pretreated side with a heat-lamination layer which can be bonded to cardboard and paper by application of heat and pressure, the thickness of the heat lamination layer being 3 to 30 µm.

6. The use of a film produced by a process claimed in any of claims 1 to 5 for production of packaging material.