METHOD FOR SEALING OF PLASTICS AGAINST GAS DIFFUSION AND AGAINST ABSORPTION OF WATER VAPOUR, AND A DEVICE ACCORDING TO THE METHOD

Abstract: Method for sealing plastics against gas diffusion and absorption of water vapour, whereby the plastic is brought into contact with a gaseous fluorine compound which is contained in an enclosed space, whereby the water molecules enclosed in pores between the macromolecules of the plastic are expelled and replaced by the gaseous fluorine compound, which condenses in the pores and seals the plastic against gas diffusion; and a device made according to the method, comprising a space enclosed by a plastic wall into which space the fluorine compound has been introduced.

Description: TITLE Method for sealing of plastics against gas diffusion and against absorption of water vapour, and a device according to the method. TECHNICAL FIELD The invention relates to a method for sealing plastics against gas diffusion and a device according to the method, by which an enclosed space is delimited from the surroundings by a layer of plastics. STATE OF THE ART Plastics are as a rule more or less permeable to gases. The level of permeability is decided by factors such as the type of polymer or copolymer in a layer, the thickness of a layer, and the possible gaseous pressure difference between the two sides of a layer and the possible difference in partial pressure of the gas on both sides of a layer. The type of gas acting on the plastics is also of great importance. The reasons for using a layer of plastics to delimit an enclosed space from the surroundings is often to enclose an object, in order to prevent it from being affected by the surrounding environment, or alternatively, to prevent the enclosed object from reaching the environment, such as the surrounding air. An example of such an enclosure is insulating windowpanes, in which an enclosed space between two panes must be kept hermetically sealed and in which at least one pane is made of a plastics material, such as acrylic plastic. If air from the surrounding atmosphere should diffuse into the space between the panes, water vapour will also be introduced. This may lead to the forming of condensation on the inner side of the panes, which is desirable to avoid. Sometimes a gas is enclosed between the panes in order to reduce the irradiation. A known gas for this purpose is argon. Diffusion is also possible in such cases, although in the reverse direction, that is, the heat absorbing gas is released and is replaced by a poorly insulating mixture of surrounding air and water vapour. A further important example is packaging for foods, medical preparations or other substances affected by ambient air or humidity. Other types of packaged goods are crusty pastries, which will lose their crust in contact with water vapour, or products, which will oxidise in contact with oxygen in the air. Such products are often packaged in an inert gas, such a nitrogen or carbon monoxide, whereby the gas may be replaced by air through diffusion. For products such as insulating windowpanes, attempts have been made to seal the plastic material by a coating of some solid substance. However, this only gives a limited and uncertain effect, and entails a complex procedure, which is particularly sensitive and may affect the transparency of the pane. In the case of disposable packaging for particularly sensitive products, it has been necessary to refrain from using plastic foil packaging. Plastic foil has been replaced by a metal foil, which must be laminated with a layer of plastics in order to become weldable. Such packaging will however be significantly more expensive than plastic packaging and cannot be made transparent for displaying the packaged merchandise. Further examples are packaging or containers for substances giving off odours and/or harmful gases. Examples of such are, on one hand, cosmetic, perfumed products and certain strongly smelling foodstuffs, and on the other hand, bags and other containers for malodorous refuse. When diffusion takes place through plastics, simultaneous absorption will take place as a step in the penetration. If plastic products are stored in ambient air, this will give rise to absorption of water vapour, which will be stored in pores in the plastic. This may cause unwanted effects if the plastics material is to be heat treated during processing, e.g. during bending or vacuum forming. DESCRIPTION OF THE INVENTION The purpose of the invention is to achieve an efficient and inexpensive method for sealing plastics against gas diffusion. The method according to the invention is particularly suited for use in arrangements having an enclosed space delimited from the surroundings by a layer of plastic material. Preferred embodiments of the invention are insulating windowpanes using plastic panes and packaging using sealing layers of plastic foil. According to a further aspect of the invention, the method is used for sealing pores in the macromolecular structure in order to prevent absorption of water. Using the method according to the first embodiment of the invention, the seal is achieved by using a gaseous carbon fluorine or hydrocarbon fluorine compound in the enclosed space. This will expel the water molecules absorbed into the plastic during storage in ambient air, and replace them by the gaseous fluorine compound. The result is a plastic impervious to water vapour. A simple form of the invention is to introduce said fluorine compound into the enclosed space before it is closed, whereby the sealing process takes place gradually. This method is particularly suitable for insulating windowpanes, as well as plastic foil packaging or plastic layers in the form of containers, where the enclosed product is not harmed by the fluorine compound. A further embodiment of the invention is to expose the plastics material to be sealed to the gaseous fluorine compound for a period of time, sufficient to expel at least the main part of the water molecules and replace them with the fluorine compound in condensed form. The plastic products can then be used for various purposes requiring prevention of gas diffusion and also where it is useful to remove moisture from the molecular structure of the plastic. PREFERRED EMBODIMENTS An insulating windowpane comprises a number of panes, usually two or three, mounted in a frame, with spaces formed in-between. By enclosing a layer of gas between the panes the capacity for heat and sound insulation is increased. The enclosed gas is often air, but other gases may be used to further increase the capacity for heat and/or sound insulation. One such gas is argon. A prerequisite for preventing an insulating windowpane from being coated with condensation within the gas-enclosed space, is that the frame must be made in such a way that the gas-enclosing space is hermetically sealed. Because the enclosed gas may contain a certain amount of water vapour, which is always the case when ambient air is used, a moisture absorbing substance is placed within the space. This will prevent the water vapour enclosed with the gas from forming condensation. Usually the individual panes in an insulating windowpane are made of glass, which is non-permeable to gas. However, in certain types of installations it is desirable to replace the glass with some other transparent material, in practice a transparent plastic material. As mentioned above, such materials have a certain permeability and only a limited assurance against penetration of water vapour into the gas-enclosed space can be achieved, even though the frame itself is a hermetic enclosure. From experience it is known that sooner or later moisture penetration, with related condensation phenomena, will occur in insulating windowpanes including plastic panes. This results in a reduction of transparency usually requiring the pane to be replaced, which shortens the useful life of the window. If the window contains a gas in order to improve the insulating capacity, the gas will gradually be replaced by air diffusing into the gas-enclosed space, while the gas will diffuse out into the atmosphere. If a moisture absorbing substance has been introduced into the space during manufacture, said substance can only absorb a certain amount of water and will only delay the course of events until condensation occurs. Insulating windowpanes sealed according to the invention, whereby the gas- enclosing space has been sealed using plastic panes, the plastic is sealed by introducing a carbon-fluorine or hydrocarbon-fluorine compound into the gas- enclosed space before the window is hermetically sealed. Said fluorine compound is gaseous at atmospheric pressure and will completely, or almost completely, replace the air or any other insulating gas present in the enclosed space in conventional designs. Water will virtually always be present in the pores between the polymer macromolecules of the plastic material. This absorption of water will always occur if the plastic is stored in ambient air without being subjected to any particular drying procedure. When the fluorine compound comes into contact with the plastic it will expel the water molecules by diffusing into the plastic and condense in said pores. Hereby the plastic is sealed so that water vapour cannot penetrate and create condensation problems. Sealing is also achieved against other gases such as oxygen in the air and such gases used in insulation windowpanes for improved insulation capacity, e.g. argon. The filling of the gas can be performed so that a sufficient amount of gaseous fluorine compound for sealing the plastic is introduced, whereby the remainder of the gaseous content of the space is made up of an insulating gas. Examples of plastics materials suitable for sealing using fluorine compounds are: acrylic plastic, such as polymethyl methacrylate, polycarbonate, polyethylene, polypropene, polyamide and polyurethane. Examples of preferable fluorine compounds are : carbon fluorines and hydrocarbon fluorines : CH 2 F 2 , CH 4 , C 2 F 6 and C 2 H 2 F 4 . An example of a minimum amount of gaseous fluorine compound required to seal an acrylic plastic surface: 0,02 g. of gaseous hydrocarbon fluorine compound at atmospheric pressure per 1 dm 2 of plastic pane (a gas enclosed space between two plastic panes will obviously require twice that amount). An alternative embodiment of the invention is that the plastic panes of an insulating windowpane are sealed before assembly. According to the invention this is achieved by placing the panes in a chamber into which the fluorine compound is introduced. After a certain amount of time the above- mentioned process involving expulsion of water vapour and condensation of fluorine compound in the pores of the plastic has taken place. The process can be controlled by adjusting the pressure and/or temperature inside the chamber. Possible choices of plastic materials and fluorine compounds have been stated above. Suitable pressures to be used in the chamber are from 0,5 to 2 bar and suitable temperatures are from 0 to 40 °C. Preferably, atmospheric pressure and room temperature can be used. It has been established that the fluorine compound intended to effect the seal might be included as a minor volume of the gas between the panes. The sealing process requires a certain amount per unit of surface area of a particular plastic material. If this amount per unit of surface area is present in the gas-enclosed space, a complete seal against diffusion is achieved. It is, of course, desirable to supplement this amount by a certain surplus, to ensure that the sealing process is carried out completely. The fact that at least in gaps of a certain width, a relatively large volume in the gas-enclosed space need not be filled completely with the fluorine compound may be used for the purpose of supplementing the contents with gases of value for the function of the insulating windowpane. Gases contributing to inCTeased heat and/or sound insulating properties are of primary interest. Some gases of this type, which improves the heat insulating properties, are argon, krypton, xenon and carbon dioxide. Improved sound insulation can be achieved by using gases such as argon combined with sulphur hexafluoride or alternatively argon combined with carbon or hydrocarbon fluorine compounds. As stated above, an insulated windowpane comprises, apart from the plastic panes, a frame holding the panes separated at a distance from each other, so that said enclosed space is formed. The window is usually assembled by first joining the frame and the plastic panes using a tacky, cohesive mass. The assembling process usually takes place in ambient air and the space will contain such air. For this reason, holes have been made in the frame so that the gas to be contained by the window can be introduced, while the air is expelled, whereafter the holes are sealed. Preferably the work should be carried out in a dry environment, so that a minimum amount of water vapour is retained in the space. It is however difficult to avoid inclusion of some water vapour, which is why a moisture absorbing substance is enclosed to take care of this limited amount. Even though the manufacture has been carried out according to the described method and a complete seal against diffusion has been achieved, it has been established that condensation phenomena have occurred due to presence of water vapour in the space. The cause of this problem is a chemical reaction between certain chemical components in the frame, mainly its sealing compound and the plastic material of the panes. During this reaction, free water has been formed, causing a gradual increase of the dew point, i.e. the temperature at which the enclosed water vapour will condense onto the pane at a drop in temperature. The phenomenon occurs when the chemical component in question contains NH 2 . Sealing compounds often contain additives, such as antioxidants, accelerators and wetting agents to improve tacking ability, which components are often based on phthalates and/or phenyl amines. The plastic material in the panes often contain carbonyl ions (C=0). This is shown in the following structural formulas for plastics commonly used for insulation windowpanes. POLYMETHYL METHACRYLATE: n C^C-^-OCH g -→-fCHs-C^ 0 C=0 OCH 3 POLYCARBONATE C\ o n HOO C O θH+n CI-C-CI- bisfβnol A kHg fosgen CH 3 0 7 When the sealing compound of the frame, containing NH2, and the plastic material containing carbonyl ions come into contact, the following reaction occurs: C=0 + NH 2 XX = C=NXX + H 2 0 Hence the H 2 from the sealing compound and the 0 from the plastic are combined to form free H 2 0. Apart from this unwanted effect, it also leads to a weakening of the plastic during the ongoing migratory process. Hence, a further aspect of the invention is that sealing the plastic against diffusion, using the described method for the manufacture of insulating windowpanes, must be combined with a choice of frame components which do not contain NH 2 -compounds, especially those components which will be in direct contact with the plastic. Compounds which can be used for this purpose are silicone and nitrogen-free MS-polymers. Example: For a windowpane having a gas filled enclosure with a gap width of 12 mm, and having two panes made of acrylic plastic, a minimum amount of 0,02 g of fluorine compound per dm 2 is required for each of the panes for complete sealing. The content of the gas-enclosed space is made up to atmospheric pressure using a gas such as argon, krypton or xenon, giving less gas convection in the space and hence better heat insulating properties. Using the said amounts means that the partial volume of fluorine compound at atmospheric pressure is approx. 15-20% of the total volume of the space. When choosing components for the frame the above instructions must be observed, that is the use of components including NH 2 -compounds must be avoided. Methods similar to those described for use in windowpanes can be used for plastic packaging and containers. An example of this is packaging for sealed enclosure of substances, or merchandise in types of packaging such as bags made from welded foils plastic. Further examples are boxes or vessels such as plastic bottles of a certain rigidity. Containers of this type may be manufactured by thermoforming, blow moulding, injection moulding or by assembling plastic sheets or other details. Plastic foil suitable for sealing using the method according to the invention: primarily polyethylene, polypropylene and polyamide plastics. 8 Plastics for containers suitable for sealing using the method according to the invention may be the same as those mentioned for insulating windowpanes. The pressure of the enclosed gaseous fluorine compound can, as a rule, not be significantly different from atmospheric pressure, while packaging of this kind can not withstand any major differences in pressure. An alternative to enclosing a gaseous fluorine compound in the packaging is to treat the plastic prior to the sealing of the package. As described above, the plastic is brought into contact with a fluorine compound in a chamber. The packaging may be prefabricated and ready for closing or manufactured using treated foils or sheets. However, it is necessary to make sure that the method of manufacture does not destroy the achieved sealing effect, e.g. when using heat treatments. It is also worth mentioning that during heat treatment, such as vacuum forming, of plastic sheets, it is advantageous for the plastic to be as free from water as possible. Otherwise the chances are that enclosed water will boil and burst the surface layer of the plastic sheet. Sealing the plastic through contact with a gaseous fluorine compound, as described above, has proved to be advantageous in this case. Hence, the content of the invention is that plastic which is to be sealed against gas diffusion, primarily water vapour and oxygen, is treated by means of a simple process, whereby the plastic material is placed in contact with a gaseous fluorine compound. This can be achieved by introducing and hermetically sealing the gaseous fluorine compound in an enclosed space inside the product of which the plastic is a part. The sealing process will thereby occur gradually through contact between the plastic and the fluorine compound. A further method, as described above, is to perform the sealing of the plastic material, in the shape of foils, sheets, or containers, by exposing it to the fluorine compound in a closed chamber before the product to be made up of said plastic is fabricated. By using this simple process, complicated coating processes can be avoided and the appearance or transparency of the plastic is not affected in any way. Plastic components, such as sheets and rods, treated by this method are advantageous for use in forming that involves heating. Since plastic treated by this method does not contain any absorbed water, its shaping properties are improved and it will be very pliable in contact with tool and moulds. It is usually not possible to achieve a good result when vacuum forming plastic sheets, unless water has been expelled beforehand. This has usually been done by using a relatively time consuming heating process, which can be eliminated by sealing the sheets using the method according to the invention. The device according to the invention shows an enclosed space delimited from the environment by a plastic layer, which space contains a gaseous fluorine compound.

Claims: 10 CLAIMS 1. Method for sealing plastics against gas diffusion and against absorption of water vapour, characterized in that the plastic is brought into contact with a gaseous fluorine compound such as a carbon fluorine or hydrocarbon fluorine compound with the gas enclosed in a sealed space, whereby the water molecules, which have been trapped in pores between the macromolecules of the plastic are expelled and replaced by the gaseous fluorine compound, which condensates in said pores and seals the plastic against diffusion. 2. Method according to claim 1, characterized in that, before the forming of the final product, the plastic is brought into contact with the fluorine compound in a closed chamber during a predetermined period of time, so that said expulsion of water molecules and condensation of the fluorine compound is achieved. 3. Method according to claim 1 , characterized in that the plastic is arranged to form a separating wall between an enclosed space and the environment, whereby the gaseous fluorine compound is introduced into the enclosed space before the space is closed, so that the expulsion of water molecules from the plastic wall and the condensation of the gaseous fluorine compound in the pores of the plastic begins, thereby sealing the space against the environment. 4. Method according to claim 3, characterized in that the enclosed space is arranged in a receptacle device such as a packaging which is designed to have at least part of its wrapping made out of plastic and that the gaseous fluorine compound is introduced into the packaging before closing. 5. Method according to claim 3, characterized in that the enclosed space is part of an insulating windowpane having one or more gaps between two or more transparent panes, whereby plastic, such as acrylic plastic, is used in at least one pane and that the gaseous fluorine compound is introduced into the gap, which forms said enclosed space. 11 6. Method according to claim 5, characterized in that said space between the panes is formed by arranging a frame, holding pairs of panes at a distance from each other, and that when using plastic panes containing carbonyl ions (C=0) such as polymethyl methacrylate and polycarbonate, it is ensured that components of the frame coming into contact with the plastic does not contain NH 2 , mainly to avoid the reaction between C=0 and NH 2 to form free H 2 0. 7. Method according to claims 5 or 6, characterized in that in addition to the fluorine compound, one or more gases suitable for increasing the heat and/or sound insulating properties of the insulating windowpane are introduced, while making sure that the amount of the gaseous fluorine compound is sufficient for achieving said sealing process. 8. Method according to claim 7, characterized in that the, or those, additional gases suitable for increasing the insulating properties to at least a main part comprises argon, krypton or xenon, or a mixture of these gases. 9. Device made according to the method according to one of the claims 3-8, characterized in that it comprises an enclosed space, separated from the environment by a plastic wall and that the gaseous fluorine compound present in the space is introduced before the space is closed. 10. Device according to claim 9, characterized in that it forms an insulating windowpane having one or more spaces containing gas between two or more panes of which at least one is made up of plastic and having said fluorine compound enclosed between the panes. 11. Device according to claim 10, characterized in that a frame is arranged between the panes in order to hold pairs of the panes at a distance from each other forming said spaces containing gas, which frame, when using plastic panes containing carbonyl ions (C=0), is arranged to have such components in contact with the plastic panes, which do not contain NH 2 , for example silicone and nitrogen free MS-polymers. 12 12. Device according to claim 9, characterized in that it forms a packaging for an object or a substance enclosed in a wrapping, which at least in part is made up of plastic, whereby said fluorine compound is enclosed with the object/substance.

Inventors: KARLSSON JAN, KARLSSON, JAN
Applicants: KARLSSON JAN, KARLSSON, JAN
Application year: 1999
Main classification: E06B3
IPC codes: C08J7, E06B3