WO1998030646A1

WO1998030646A1 - Process for obtaining an improved adhesion between the surfaces of two polymer compositions

Info

Publication number: WO1998030646A1
Application number: PCT/NL1998/000006
Authority: WO
Inventors: António AVIDES MOREIRA; Arminda Maria Leite Magalhaes; Krijn Dijkstra
Original assignee: Dsm N.V.
Priority date: 1997-01-06
Filing date: 1998-01-06
Publication date: 1998-07-16
Also published as: AU5499098A

Abstract

The invention relates to a process for obtaining an improved adhesion between the surfaces of two polymer compositions, each polymer composition comprising at least one polymer, in which process the surface of at least one polymer composition is subjected to a plasma treatment using an electrical field, characterized in that: a) the surfaces of the two polymer compositions are subjected simultaneously to the plasma treatment; b) during the plasma treatment the two polymer compositions are in contact with each other and c) the electrical field is passed through the adjoining surfaces of the two polymer compositions. A 10-fold increase of the adhesion strength between the surfaces of the two polymer compositions is realized by a corona discharge treatment in air at atmospheric pressure. Also, the invention relates to a multilayered object, comprising at least two layers of a polymer composition with an improved adhesion between the layers. The polymers constituting the polymer compositions, are chosen from the group consisting of polyolefins, polyesters, copolyesters, polyetheresters, polyamides, polyetheramides, acrylonitrile-butadiene-styrene copolymers, cellophanes, polystyrenes, polyvinylchlorides, vinylidene chloride polymers, ethylene vinyl acetate copolymers, polyurethanes, polytetrafluoroethylene and blends thereof.

Description

PROCESS FOR OBTAINING AN IMPROVED ADHESION BETWEEN THE SURFACES OF TWO POLYMER COMPOSITIONS

The present invention relates to a process for obtaining an improved adhesion between the surfaces of two polymer compositions, each polymer composition comprising at least one polymer, in which process the surface of at least one polymer composition is subjected to a plasma treatment using an electrical field.

Such process is known from the Encyclopedia of Polymer Science and Engineering, Supplement Volume, in the chapter on Surface Modification, p. 674 and following, (1989) and the references mentioned therein. The most simple plasma treatment using an electrical field is the corona discharge treatment, for instance in air and for instance at atmospheric pressure. The corona discharge treatment is described in more detail in for instance T.appl. 15_.(8), 75-78 (1982) and Kunststoffberater JLO_., 31 (1987) and is in essence a high-frequency electric discharge between two electrodes, one of which is grounded, by which negative charges, i.e. electrons and ions, are transported with very high velocity, due to the imposed high field strength, to the positive electrode, which in general supports the polymer composition of which the surface is to be treated. By applying the corona discharge treatment to the surface of a polymer composition, the surface of the polymer composition becomes chemically modified. After the treatment of the surface of the polymer composition, the treated surface is contacted with the surface of a second polymer composition, optionally followed by hot or cold sealing of the resulting ultilayered object. The plasma treatment using an electrical field is often applied to the surface of a polymer composition, that either cannot be adhered directly to the surface of a second polymer composition or that shows a low adhesion when adhered to the surface of a second polymer composition by hot or cold sealing.

By direct adhesion is meant the adhesion of two surfaces without any intermediate layer acting as a glue.

The process according to the state of the art is frequently applied for the adhesion of the surface of polymer films, sheets or foams to the surface of other polymer substrates, for example polymer films and sheets of different thickness, woven and non-woven polymer compositions.

As a major drawback of the process according to the state of the art, the inventors found that, for instance, in the case of a polyolefin composition and a polyether block copolymer composition, no improvement of the adhesion could be obtained by a corona discharge treatment of the polyolefin surface before contacting it with the surface of a polyether block copolymer composition. Contrary to what was expected, even a decrease of the adhesion was observed.

As another drawback, the process in which the surface of one of the polymer compositions is plasma- treated using an electrical field and subsequently adhered to the surface of a second polymer composition, is quite complicated : it comprises at least subsequently steps for respectively (a) plasma treatment using an electrical field of the surface of one polymer composition, (b) joining the surfaces of the two polymer compositions together and (c) sealing the resulting multilayered object. Also, the adhesion obtained by the process according to the state of the art is sensible to variations in time between the plasma treatment step, the joining step and the sealing step. Furthermore, said process is prone to surface contamination, thereby worsening the adhesion. The object of the present invention is to provide a process for obtaining an improved adhesion between the surface of two polymer compositions, that is less complicated, involves fewer steps, is more reproducible and is less prone to contamination of the surface.

Very surprisingly, the inventors have found that the drawbacks of the process according to the state of the art can be overcome if : a) the surfaces of the two polymer compositions are subjected simultaneously to the plasma treatment using an electrical field, b) during the plasma treatment the surfaces of the two polymer compositions are in contact with each other and c) the electrical field passes through the adjoining surfaces of the two polymer compositions.

By contact is meant close physical contact between the surfaces of the two polymer compositions, however not excluding the situation where the contact area comprises a gap of microscopical dimensions, for instance 50 micrometer, nor the situation where the contact area comprises microscopical voids.

The process according to the invention is especially useful for adhering the surfaces of two polymer compositions that have a different chemical composition or cannot or can only insufficiently be adhered to each other using exclusively cold or hot heat sealing.

Preferably, the polymer composition with the highest charge conductivity should be closest to the grounded electrode of the corona discharge device.

Preferably, one of the polymer compositions has the form of a film or a sheet. By film is meant a flat section of material that is extremely thin in comparison to its other dimensions and has a nominal maximum thickness of 250 micrometer. By sheet is meant a flat section of material, equivalent to a film but with a thickness greater than 250 micrometer. Preferably, the sheet has a thickness smaller than 500 micrometer.

According to a preferred condition, the surfaces of both polymer compositions are contacted under pressure before, during or after the plasma treatment using an electrical field. As the plasma treatment is preferably performed as a continuous process, this pressure is very practically exerted by at least one cylinder roll. The amount of pressure is not critical and may vary from case to case. The amount of pressure may easily be determined by a skilled person in practice. Best results are obtained if the polymer compositions are contacted under pressure at least both before and after the plasma treatment using an electrical field.

The energy of the electrical field that is applied may vary over a wide range and depends inter alia on the thickness of the polymer compositions, frequency of the discharges, and duration of the treatment. The average skilled person can easily determine the optimum conditions from case to case.

The treatment is preferably limited to such a time and energy per unit of surface that no pinholes are made in either the polymer compositions.

The treatment can be carried out under enhanced, normal or reduced pressure.

Preferably, the treatment is carried out in the presence of a gas. The gas may be a single gas or a mixture, the constituent gasses chosen from inert gases, for example He, Ar or Ne, oxidizing gases, for example 0₂, 03, N₂, N0₂ and air, or reducing gases, for example H₂. Preferably, air or a gas mixture containing about 20 % 0₂ is used. Most preferably, air at normal pressure is used.

An advantage of the process according to the invention is that the mechanical properties of the polymer compositions remain intact during the plasma treatment . This allows the manufacture of a multilayered object possessing at least the properties of the individual constituent polymer compositions. The process according to the invention can easily be adopted to obtain a multilayered object of more than two layers of polymer compositions. For example, a number of films, sheets or other substrates can be adhered to each other by subjecting them to the process according to the invention.

The fact that the main advantage of the process according to the present invention is that the surfaces to be adhered do not need a pretreatment to improve adhesion, does not preclude the use of the process according to the invention for surfaces that have been pretreated using, for example, chemical or physical means .

Multilayered objects obtainable by the process of the present invention, the layers adhering directly to each other, show outstanding properties with respect to the adhesion between the layers that make up the objects. These multilayered objects form therefor another part of the present invention. Multilayered objects in which the layers are directly adhering to each other are known from Owens, J. Applied

Polymer Science, Vol 19, pp. 265-271 (1975) , in which a multilayered object consisting of two polyolefin (PE) films is described.

Multilayered objects in which at least one of the layers is a film or a sheet find use for example as packaging materials for food packaging, as a barrier film for water vapor or oxygen or as a film or a sheet having the property of being waterrepellant but of being permeable for water vapor. Such films and sheets find application in a wide range of fields, for instance in surgical and other protective garment, as surface modifiers for laminating substrates such as polyolefin panels used in appliances, automobiles, etc., in diapers, sports and rain garment and in the building industry, for instance in roof constructions. In these latter applications optimal use of the water vapor permeability can be made if the films, constituting the multilayered object are very thin. However, in that case mechanical strength of the single film is too low and use has to be made of an adjoining polymer composition as substrate that supports the film. Supporting substrates are for instance woven and non woven fibrous materials. For instance, in sports wear and rain coats the water repellant and water vapor permeable film is supported by a textile. The material of the textile, for instance a polyester, is in most cases good compatible with the material of the film, for instance a copolyetherester, and a permanent adhesion between the film and supporting material can be obtained by, for instance, a sealing process.

However, in many multilayered objects, in which the layers are adhering directly to each other, the adhesion between the surface of the adhering polymer compositions is insufficient.

Another object of the present invention is to make available such a multilayered object, comprising at least two polymer compositions with an improved adhesion, the compositions adhering directly to each other.

The inventors have found that such an multilayered object with an exceptional high adhesion between the surface of a polymer composition and the surface of a second polymer composition is obtainable with the process according to the invention. In case the two polymer compositions have a different chemical composition, the maximum peel energy, determined according to ASTM D1876-D2, is at least 20 J/m² and the average peel energy, being 50 % of the maximum peel energy, is at least 10 J/m². More preferably, the maximum peel energy is at least 30 J/m² and the average peel energy is at least 15 J/m².

In case the two polymer compositions have the same chemical composition, the maximum peel energy, determined according to ASTM D1876-D2, is at least 80 J/m².

According to the purpose of the multilayered object to be produced, the polymers constituting the polymer compositions, are chosen from the group of polyolefins, for example polyethylene and polypropylene, polyesters and copolyesters, for example polyethylene terephtalate and polybutadiene terep talate, polyetheresters, for example a polyetherester blockcopolyτner comprising polybutyleenterephtalate and polytetramethyleneoxide, polyamides, for example nylon-6, nylon-6,6 and nylon-4,6, polyetheramides, acrylonitrile-butadiene- styrene copolymers, cellophanes, polystyrenes, polyvinylchlorides, vinylidene chloride polymers, ethylene vinyl acetate copolymers, polyurethanes, polytetrafluorethylene and blends thereof . The polymer composition suitable to be used may have the form of a film or sheet, for example made by sheet extrusion, but may also be a substrate, for example a woven or non- woven composition.

The polymer compositions according to the invention may contain, if necessary, additives and fillers, for instance colorants, stabilizers, processing aids, flame retardant agents, talc, minerals and/or fibrous reinforcing compositions. The proportion of these additives should be chosen so that the polymer forms a continuous phase and the interface between the two polymer surfaces is mainly a polymer-polymer contact . Preferably, the polymer material contains less than 80 % by weight of one or more additives. However, in most applications the polymer compositions will contain only low amounts of additives.

The invention is now illustrated by some examples and comparative experiments. However, the invention is not limited thereto.

Experimental

Compositions

Films of Arnitel EM400^®, a polyetherester blockcopolymer, Shore D hardness of 40 and consisting essentially of polybutyleenterephtalate as hard segments and polytetramethyleneoxide as soft segments, supplied by DSM N.V. , The Netherlands, were made by sheet extrusion with a thickness of 12 μm, 25 μm, 50 μm and 100 μm. Films of polybutadiene terephtalate (PBT) and nylon-6 were made by sheet extrusion with a thickness of 30 μm.

Films of polyethylene (PE) and polypropylene

(PP) (standard homopolymer film grade) were made by sheet extrusion with a thickness of 50 μm (PE) and 25 μm (PP) .

Before the corona discharge treatment all films were dried for 2 hours at 80°C.

Plasma treatment

The corona discharge treatment was performed with a Softal generator type 3030 (maximum output voltage: 13 kV, output frequency: 16-30 kHz) in a standard configuration. The thickness of the film and the speed of the treated film or multilayered object were systematically varied. The input voltage (220 V) remained constant during the experiments on the maximal setting of the corona discharge device.

The corona discharge treatment was performed in two different ways. In a first method, according to the state of the art, the surface of a PP film was subjected to a corona discharge and subsequently the surface was hot sealed on an Arnitel film surface.

By the process according to the invention, a multilayered object, consisting of two films was formed by pressing the films together by the weight of a cylinder roll, after which the multilayered object was subjected to a corona discharge treatment. On this multilayered object no subsequent sealing (either hot or cold) was performed after the corona discharge treatment .

Sealing experiments (for PP/Arnitel multilayered objects only)

Hot sealing of the PP and Arnitel films was done on a HSG/ETK hot seal apparatus, supplied by Otto Brugger. The temperature of the two sealing plates was set at 200 °C for the Arnitel side and 100 °C at the PP side. The sealing pressure was kept constant at 0.8 MPa. The sealing time was varied between 10 and 99s. Also the time-gap between the corona discharge treatment of the surfaces and the sealing experiments was varied.

Adhesion measurements

The test chosen to determine the strength of adhesion between the two polymer compositions was the

T-Peel Test according to ASTM D1876-D2. The test was performed in a tensile apparatus ZWICK 1445 with a speed of 25 mm/min and a load cell of 5N. The values of the T-Peel test recorded are averaged over three samples. The dimensions of the multilayered objects were : 100 x 20 mm (length x width) and 125, 75, 50, 37 μm of thickness. Comparative Examples 1-10

In Table 1 the strength of adhesion between the surfaces of multilayered objects of PP/Arnitel is presented, in which only the PP-surface is subjected to a corona discharge treatment, after which the PP- surface is pressed and sealed on the Arnitel surface, using the method according to the state of the art. It can be seen that the strength of adhesion of PP/Arnitel even decreases, compared to the strength of adhesion of a PP film with an surface that was not subjected to a corona discharge treatment and that was hot sealed on an Arnitel film under similar conditions (Comparative Example 10) . Variations in the corona discharge treatment and sealing conditions do not essentially change the strength of adhesion between the two surfaces .

w c

%

H

C m

X m 10 m

H «^ 39

C P m 10

15

Table 1 : Comparative experiments of PP/Arnitel multilayered objects. Different conditions of the corona discharge treatment of PP and subsequent hot sealing on Arnitel and results from the T-Peel test according to ASTM D1876-D2 are given. G_c/raax : maximum peel energy during peel test ; G_c/av : average peel energy during peel test ; δ_n-1 -. standard deviation of the peel energy ; 'Time gap' is the time between the corona discharge treatment and the sealing step. In Comparative Example 10, no corona discharge treatment was performed.

Examples 1-7

When a multilayered object of PP/Arnitel is subjected to a corona discharge treatment according to the invention, surprisingly a dramatic (a factor 10) increase in the strength of adhesion is obtained, in comparison to adhesion values according to the state of the art. To our knowledge this effect was never reported before. This increase in the strength of adhesion of PP/Arnitel multilayered objects by the method according to our invention is only slightly dependent on the thickness of the film (see Table 2) . For the thinnest films tested, the strength of adhesion exceeds the yield strength of the polymer composition during the T-Peel test, thus giving cohesive failure instead of interfacial failure.

Table 2 : Effect of the Arnitel film thickness and film speed on the peel strength of a multilayered object of PP/Arnitel, obtained by the process according to the invention. G_craax : maximum peel energy during peel test ^Jc/av average peel energy during peel test . δ_r is the standard deviation of the peel energy.

* Cohesive failure in the Arnitel film instead of interfacial failure.

Examples 8-20

In Table 3, an excellent strength of adhesion is demonstrated for different combinations of polymer compositions. In the PE/PE multilayered object the value obtained with the process according to the invention, is even far superior to the value found in literature (77.8 J/m², see : Owens, J. Applied Polymer Science, Vol 19, pp. 265-271 (1975)), obtained by a corona discharge treatment of the polymer films before adhering the films together using a heat treatment.

Table 3 : Maximum peel energy (G_c/max, in J/m²) during peel test of a multilayered object of two polymer films of different or equal composition. Film speed : 380 mm/min .

(*) Cohesive failure in the film instead of interfacial failure ; maximum peel energy estimated on at least 300 J/m² ; n.d. : not determined

Claims

C L A I M S

1. Process for obtaining an improved adhesion between the surfaces of two polymer compositions, each polymer composition comprising at least one polymer, in which process the surface of at least one polymer composition is subjected to a plasma treatment using an electrical field, characterized in that a) the surfaces of the two polymer compositions are subjected simultaneously to the plasma treatment, b) during the plasma treatment the surfaces of the two polymer compositions are in contact with each other and c) the electrical field passes through the adjoining surfaces of the two polymer compositions.

2. Process according to claim 1, characterized in that the two polymer compositions have a different chemical composition or cannot or can only insufficiently be joined using exclusively cold or hot sealing.

3. Process according to anyone of claims 1-2 wherein at least one of the polymer compositions has the form of a film or a sheet.

4. Process according to any one of the claims 1-3 wherein the two polymer compositions are contacted under pressure at least both before and after the plasma treatment .

5. Process according to claim 4 wherein the pressure is exerted by at least one cylinder roll.

6. Multilayered object, obtained by a process according to anyone of claims 1-5.

7. Multilayered object comprising at least two layers of a different polymer composition, the layers directly adhering to each other, characterized in that the maximum peel energy, determined according to ASTM D1876-D2, is at least 20 J/m² and the average peel energy of at least 10 J/m².

8. Multilayered object according to claim 7 wherein the maximum peel energy is at least 30 J/m² and the average peel energy is at least 15 J/m².

9. Multilayered object comprising at least two layers of the same polymer composition, the layers directly adhering to each other, characterized in that the maximum peel energy, determined according to ASTM D1876-D2, is at least 80 J/m².

10. Multilayered object according to any one of the claims 6-9 wherein the polymers constituting the polymer compositions are chosen from the group consisting of polyolefins, polyesters, copolyesters, polyetheresters, polyamides, polyetheramides, acrylonitrile-butadiene-styrene copolymers, cellophanes, polystyrenes, polyvinylchlorides, vinylidene chloride polymers, ethylene vinyl acetate copolymers, polyurethanes, polytetrafluorethylene and blends thereof .