Method of welding plastics

The invention relates to a method of welding plastics. More particularly, it relates to a welding method for obtaining what are called "peelable" welds. It also relates to plastic articles comprising such welds, in particular in the medical field.

The term "weld" is understood to mean a join between two elements, which become fastened together, the two elements being, at the join, directly in contact with each other. The present invention makes a distinction between a weld and a bonded joint, by means of which elements are joined together via a different substance from that constituting the elements. The welding of plastics generally involves heating them or the use of solvents, for example.

When plastic elements have been correctly welded, the usual aim is to have a high mechanical strength at the weld. Consequently, when it is attempted to break the weld by pulling on it, the welded elements are damaged before the desired break occurs. This may have drawbacks, for example in the packaging field, in which it is frequently sought to attain products that can be easily opened.

The expression "peelable weld between two elements" is understood to mean a weld that can be made to fail by simply pulling on it with the purpose of separating the two elements, without damaging them.

Such peelable welds are particularly useful between two plastic elements, mainly in film form, for example in the pharmaceutical packaging field, since they make it possible to produce packaging comprising several compartments in which different active substances are stored. At the moment of use, the weld separating the compartments is made to fail, allowing the active substances to be mixed together.

One solution for reducing the tensile strength of plastic welds is to make an imperfect weld, for example by heating them less. However, it turns out that such poor-quality welds are not reproducible, some of them being stronger than others.

A known means for obtaining reproducible peelable welds consists in using particular plastics, often consisting of a blend of various plastics, and in welding them under conditions which weld only one part of the various constituent plastics. However, such a means is expensive and complicated, as it requires an additional condition that restricts the choice of packaging materials.

The object of the invention is to provide a welding method for obtaining peelable welds simply and inexpensively.

Consequently, the invention relates to a method for producing a peelable weld between plastic elements, in which the elements are pressed against each other and heated in the weld area, and in which at least one of the elements comprises a layer of non-weldable material covering, at the moment when the elements are pressed together, only part of the weld area.

The expression "peelable weld between plastic elements" is understood to mean, as indicated above, a weld that can be made to fail by simply pulling on it with a view to separating the welded elements without damaging them. When a specimen of welded elements having such a peelable weld is subjected to a standardized tensile test carried out at a constant speed, it is advantageous to obtain a tensile curve (the curve showing the tensile force as a function of the elongation of the specimen, the tensile test being preferably carried out according to the ASTM F88 standard) exhibiting a plateau region. A plateau region of a tensile curve is a region exhibiting an approximately linear decrease in the tensile force as a function of the elongation, located after the maximum of said force. Such a plateau region is the signature of a delamination of the weld, which fails

progressively over the width as opposed to a sudden break. It has also been observed that peelable welds have a very small bead, or even advantageously no bead. The bead of a weld is an additional thickness of material located on the periphery of the region to which the pressure was applied during welding, due to creep of the softened plastic.

The plastic elements may have any useful form, determined by the function of the object that they constitute. Advantageously, they consist of sheets or films, possibly formed for example by thermoforming.

In the method according to the invention, the elements are pressed against each other and heated in the weld area (the weld area being the surface portion that is common to the elements and on which they are welded together). Conventionally, these operations are carried out in a welding machine that generally possesses jaws between which the elements to be welded are placed, the area of contact between the jaws and the elements defining the weld area. The jaws exert the pressure and may also supply the heat needed for the weld, for example by conduction. The welding machines are preferably provided with a system for setting the minimum gap between the jaws, in order to prevent excessive crushing of the material on either side of the weld area. Apart from this minimum value, the thickness of the weld depends on the pressure exerted and on the fluidity of the heated plastic, and therefore its temperature.

According to the invention, at least one of the welded elements includes a layer of non-weldable material covering, at the moment when the elements are pressed together, only part of the weld area. The term "non-weldable material" is understood to mean a material that prevents the diffusion of softened plastic from one element to the other and does not itself result in adhesion between the elements, under the temperature and pressure conditions used when welding according to the method. If such a material were to cover the entire weld area at

the moment when pressure is exerted, no weld would be produced and the elements would not adhere to each other.

The thickness of the layer of non-weldable material must be sufficient for welding to be effectively prevented. It is recommended that this layer have a thickness of at least 1 nm, advantageously 5 nm and preferably 10 nm. To avoid disturbing the geometry and the functions of the welded elements, it is also recommended that this thickness not exceed 500 nm, advantageously 100 nm and preferably 50 nm.

In a first variant of the invention, it is right from the formation of the layer of non- weldable material that the elements comprise the latter only over one part of the weld area. For example, it is possible to use a mask to protect part of the surface of the element corresponding to the weld area during formation of the layer. In a second, advantageous, variant, at the moment of formation of the layer, the entire surface of the element corresponding to the weld area is covered with a layer of non-weldable material. In this variant, when the plastic is heated and pressed, it creeps, by flowing, thereby damaging the layer of non-weldable material and making it disappear over part of the weld area. This makes it possible to achieve the objective of the invention. In this variant, it is particularly recommended that the layer of non-weldable material be not too thick.

The percentage of the weld area covered by the layer of non-weldable material, at the moment when the elements are pressed together, will determine, all other things being equal, the maximum force needed to separate the elements. One of the objectives of the invention is to reduce this force. Consequently, it is often recommended that the layer of non-weldable material cover at least 10%, preferably 20% and more preferably 25% of the weld area. Moreover, it is in general also recommended that it cover no more than 90%, preferably 75% and more preferably 50% of the weld area.

The layer of non-weldable material may be made of many organic or inorganic materials able to inhibit the welding of the constituent plastic of the elements to be welded together. For practical reasons, it is recommended that the weldable material have sufficient adhesion to the plastic used, where appropriate by a surface treatment or by the use of adhesives.

In a first advantageous implementation of the method according to the invention, the layer of non-weldable material comprises crosslinked polymeric organic material. A crosslinked polymeric organic material is a material comprising polymer chains forming a three-dimensional network. This three-dimensional network greatly reduces, or even eliminates, the capability of the material to flow, thereby making the crosslinked polymeric organic material non-weldable. This polymeric material may be different from the constituent plastic of the elements. However, it is preferable for it to be similar, the crosslinking being obtained directly on the surface of the elements. In this implementation, it is recommended that the crosslinking be not too extensive, in order to ensure that this non-weldable material covers, at the moment when the elements are pressed together, only part of the weld area.

In a second advantageous embodiment of the method according to the invention, the layer of non-weldable material comprises inorganic material. During welding, this inorganic material must prevent the plastic from diffusing across the weld area. The inorganic material may for example be a silicon compound, such as SiOx or SiNx, or carbon, the latter being preferred.

The layer of non-weldable material may be formed by any appropriate deposition treatment or surface treatment (vapour deposition, irradiation, plasma, etc.). The term "plasma" is understood to mean a random system formed from neutral particles and charged particles, created by ionizing a gas. To ionize the gas, energy must be supplied so as to strip electrons from the gas particles and thus

obtain a neutral overall system of ions, electrons and atoms. Excited molecules returning to their initial state result in the emission of characteristic electromagnetic radiation.

The energy needed to form a plasma may come from various sources. The most common source is incident electric field radiation. Depending on the frequency applied, the plasma is an AC (50 Hz) plasma, an audio frequency (kHz) plasma, a radiofrequency (MHz) plasma or a microwave (GHz) plasma. The plasma according to the invention, is a non-thermal ("cold") plasma. In thermal plasmas, the pressure of the gas is high enough to cause a large number of collisions between the various (neutral, excited, unexcited and ionized) particles, promoting energy transfer. This results in a plasma in thermodynamic equilibrium, all the particles of which have practically the same amount of energy. Non-thermal or cold plasmas are generated under reduced pressure for example down to 10 ^"4 Pa. Under these conditions, the mean free path is very long and substantial energy transfers between the particles can take place only by electron collision. A cold plasma is not in thermodynamic equilibrium. Macroscopically, it is at ambient temperature. However, it contains a number of high-energy particles, especially electrons. These high-energy electrons and the high-energy radiation arriving from the migration of electrons are capable of inducing chemical reactions at the surfaces exposed to the plasma treatment. However, there is no thermal load produced at this surface since the macroscopic temperature is matched to the ambient temperature. The treatment may be carried out, in batch mode or continuously, in a plasma chamber equipped on the inside with electrodes that generate in said chamber a cold plasma under reduced pressure by glow discharge, produced under the discharge potential between the electrodes. The gas from which the cold plasma is generated is in general chosen from helium, neon, argon, nitrogen, oxygen, air, nitrous oxide, nitrogen dioxide, carbon monoxide, carbon dioxide, sulphur dioxide, hydrogen sulphide, hydrogen, chlorine, fluorine, acetylene and hydrogen chloride. These gases may be used by

themselves or as mixtures. Among the gases mentioned above, argon, acetylene and carbon monoxide are particularly preferred. The plasma generator is not limited to those comprising a chamber equipped on the inside with electrodes, but may be of the type with external electrodes or with a coiled electrode. The electrodes may be connected to a high-frequency generator by capacitive or inductive coupling. The shape of the electrodes is not particularly critical. The power electrode and the earth electrode may have the same shape or different shapes, such as plates, rods, rings, cylinders, etc. It is frequently desirable for the inside walls of the plasma chamber to be made of metal, in order to provide the electrode, usually earth electrode, function. In any case, the cold plasma treatment is carried out in such a way that heat generated by the electrical discharge does not damage the surface of the plastic element that is subjected thereto.

In one particularly advantageous variant of the method according to the invention, the layer of non-weldable material is produced by cold plasma treatment. In a first implementation of this variant, the cold plasma treatment crosslinks the surface of the element to be welded. In this case, it is advantageous to use a plasma whose ionized gas is based on argon. This treatment also makes it possible, according to a second implementation, to deposit a carbon-containing non-weldable layer. In this case, an acetylene plasma has been successfully used.

The constituent plastic of the elements to be welded may be any weldable plastic. However, it is recommended that the material be thermoplastic, that is to say it becomes fluid when it is heated to a high enough temperature. Such thermoplastics are for example polyolefins, polyamides, polyvinyl chloride, etc.

The method according to the invention is particularly suitable for welding elements comprising polyvinyl chloride. Polyvinyl chloride may be chosen from vinyl chloride homopolymers or copolymers containing a large proportion (greater than 50% by weight) of this monomer. These copolymers are obtained by copolymerizing vinyl

chloride with one or more copolymerizable monomers such as the following: vinyl esters, for example vinyl acetate; vinyl ethers, for example ethyl vinyl ether; acrylic and methacrylic acids and their esters, for example methylacrylate; fumaric acid and its esters, for example ethyl fumarate; maleic acid, its anhydride and its esters, for example ethyl maleate; vinyl aromatic compounds, for example styrene; vinylidene halides, for example vinylidene chloride; acrylonitrile; methacrylonitrile; and olefins, for example ethylene. The polyvinyl chloride may also form part of a polymer blend, in particular a blend of vinyl chloride polymers with a synthetic elastomer such as the following: ethylene/vinyl acetate copolymers; acrylo- nitrile/butadiene copolymers; styrene/acrylonitrile copolymers, methyl methacrylate/styrene/butadiene copolymers; acrylonitrile/styrene/butadiene copolymers; polyamides; caprolactam polymers; ethylene/propylene/diene terpolymers; urethane elastomers; and polybutadienes modified by epoxy resins, the proportion of synthetic elastomer in the blend not however exceeding 50% by weight relative to the weight of vinyl chloride polymer. The elements to be welded may comprise different layers based on different polyvinyl chlorides. Each layer of the element also contains a conventional polyvinyl chloride plasticizer so as to give the sheet the desired flexibility. This plasticizer is in general chosen from the following: phthalic acid esters, such as for example dibutyl phthalate, dioctyl phthalate, diisodecyl phthalate and butylbenzyl phthalate; aliphatic diacid esters, such as for example dioctyl adipate and dibutyl sebacate; polyol esters, such as for example pentaerythritol esters, diethylene glycol dibenzoate and dipropylene glycol dibenzoate; fatty acid esters, such as for example methyl acetyl ricinoleate; phosphoric acid esters, such as for example tricresyl phosphate, triphenyl phosphate and trinonyl phosphate; epoxydized oils, such as for example epoxidized soybean oil and linseed oil; citric acid esters, such as for example acetyltrioctyl citrate and acetyltributyl citrate; or polyester-based plasticizers, such as for example trimethyl trimellitate, tetra-n-octyl pyromellitate and propylene glycol adipate. For accessibility and cost reasons, the plasticizer is preferably chosen from phthalic acid esters and aliphatic diacid esters.

In the method according to the invention, the heating of the elements pressed together may especially take place by conduction (thermal welding) or radiation (laser welding or high-frequency welding). The use of high-frequency electromagnetic radiation heating is recommended, in particular when the plastic is based on polyvinyl chloride. This is because, unlike in polyolefins (for example polyethylene or polypropylene), PVC, which contains C-Cl groups having dipoles that vibrate under the action of high-frequency radiation, can be easily welded by this technique, which is moreover generally used with such materials. In high- frequency welding, the elements to be welded are positioned between two electrodes, between which a high-frequency potential difference is applied. The voltage applied, the groups constituting a dipole start to vibrate. The material will thus be able to be heated up to its melting point. The combined action of the high frequency and the pressure exerted on the two elements results in entanglement of the macromolecular chains. This entanglement results in a weld. The high frequency is advantageously higher than 1 MHz ₁ preferably 10 MHz, frequencies between 20 and 30 MHz being most common.

A high-frequency welding machine comprises two very important parts, namely a high-frequency generator and a device for applying pressure to the elements to be welded, often consisting of the two electrodes. These two electrodes therefore have two functions. They serve to transfer the energy from the high-frequency power to the material, and to press the layers to be welded against each other. The shape of the electrodes may vary greatly. The constituent material of the electrodes must have good mechanical properties and good thermal conductivity and finally it must not cause sparks. In general, brass is used. The high-frequency generator must be capable of producing sufficient power, typically several thousands of watts. The main high-frequency welding parameters are the welding time, the inter-electrode distance and the power of the generator.

The method according to the invention makes it possible to obtain welded elements having a peelable weld, the strength of which can also be controlled. Consequently, the invention also relates to welded plastic elements, at least one of the elements comprising a layer of non-weldable material covering only part of a weld area, in order to obtain a peelable weld, which can be obtained by the method according to the invention.

In an advantageous variant of the elements according to the invention, these further include at least one non-peelable weld. To obtain this, it is advantageous to exert higher pressure on the weld area, so as to further damage the layer of non- weldable material.

The elements according to the invention are particularly useful in applications that require peelable and non-peelable welds to be present in the same product, such as packaging for pharmaceutical use, and in particular multi-compartment packaging.

The examples described below serve to illustrate the invention.

Example 1

Two polyvinyl chloride plastic strips 15 mm in width and 350 μm in thickness were welded together by means of a high-frequency welder of the COLPITT brand. Power of 1500 W was applied for 1 s. The weld thickness was about 500 μm. After being welded, this specimen was subjected to a tensile test according to the ASTM F88 standard. It was not possible to make the specimen fail at the weld, rather it was the specimen itself that was broken. The tensile force to break the specimen was 54 N. The weld was not peelable.

Example 2

The procedure was as in Example 1 , except that the polyvinyl chloride strips were subjected beforehand, over their entire surface, to a cold argon plasma treatment at a pressure of 100 mTorr and a power of 350 W, for 7 seconds, so as to crosslink the surface of the specimen. The specimen failed at the weld, without breaking. The tensile force needed was 25 N. The weld was peelable.

Example 3

The procedure was as in Example 2, but the treatment time was extended to 10 seconds. The tensile force needed to make the specimen fail at the weld was 12 N, measured on the plateau of the tensile curve.

Example 4

The procedure was as in Example 2, except that the plasma was based on acetylene, at a pressure of 95 mTorr, and the treatment was continued until a 20 nm layer of carbon was obtained on the surface of the specimen. The tensile force to make the specimen fail at the weld without breaking was 7 N. The weld was peelable.

Example 5

The procedure was as in Example 4, but the treatment time was extended so as to obtain a 40 nm carbon layer. The tensile force and the weld behaviour were substantially the same as those of Example 4.

Example 6

The procedure was as in Example 4, except that the heating power was increased to 3000 W and the weld thickness was about 530 μm. A weld failure force of 21 N, without the specimen breaking, was measured.

Example 7

In this example, the procedure was as in Example 6, but the thickness of the weld was reduced to 440 μm by applying greater pressure. The weld was no longer peelable and the specimen had a tensile strength of 45 N.