METHOD OF ACTIVATING SURFACE OF SHAPED BODY FORMED OF SYNTHETIC ORGANIC POLYMER

Abstract: A shaped body formed of a synthetic organic polymer is surface-treated for increasing the surface energy by a method including the steps of contacting the surface with a UV ray-­ absorbing organic liquid at a temperature and for a period of time sufficient to swell the surface; and irradiating UV rays having wave lengths in the range of 1800-2100 Å on the swollen surface.

Description: This invention relates to a method for the treatment of a surface of a shaped body formed of a synthetic organic polymer for the activation of the surface. Since synthetic organic polymers, such as polyolefins which consist only of saturated hydrocarbons and which are hydrophobic and highly crystalline in nature, generally have a low surface energy, the surfaces of shaped bodies such as sheets and fibers of such polymers are hard to coat, print, bond or dye. Therefore, it is necessary to pretreat such shaped bodies and to increase the surface energy thereof prior to coating, printing, bonding, dyeing or the like surface processing. A variety of surface pretreating techniques have been hitherto proposed. For example, as pretreating methods for coating, printing and bonding, there are known a sand blast treatment, a treatment with a solvent, a treatment with a chromic acid mixture, a flame treatment, a corona discharge treatment, a plasma treatment, a functional groups-introducing treatment and a photo-grafting treatment. These conventional methods, however, are not entirely satisfactory for the reasons as set forth below. The sand blast treatment method is one in which granules of a grinding material are caused to impinge on a surface of a shaped body to be treated for roughing same. The granular grinding material causes fouling of the working environment and the product. The surface of the treated product is, therefore, required to be washed with water. Further, there are caused problems that the treatment makes the surface opaque and that the grinding material once cut into the surface cannot be removed. The solvent treatment includes exposing, for a short period of time, a shaped body to vapors of a super-heated, halogenated hydrocarbon so that an amorphous portion of the surface of the shaped body is etched by swelling. It is, however, necessary to provide over the treated surface a primer layer containing chlorinated propylene prior to coating. In addition, unless the primer coat is provided immediately after the treatment, the treatment soon becomes ineffective. The treatment sometimes causes distortion of the shaped body. The treated surface should be heated for the removal of the solvent remaining after the treatment. Additionally, this method is dangerous because of the necessity to handle vapors of a halogenated hydrocarbon which also tends to cause deterioration of the apparatus. The treatment with chromic acid mixture includes heating the mixture (containing 75 parts of potassium bichromate, 120 parts of water and 1500 parts of concentrated sulfuric acid) to about 100 °C, and immersing a shaped body to be treated in the heated mixture for about 5 min. This method requires high costs for the treatment of the waste chromic acid mixture. The flame treatment includes exposing the surface of a shaped body to be treated to an oxidizing flame (1000-2500 °C) produced by the combustion of a gas containing an excess air. The surface is liable to be distorted or melted by the heat. The corona discharge treatment includes passing a film or film-like body to be treated through a space between an electrode and a metal roller while applying a high electric voltage therebetween. This method is not applicable to other shaped bodies than those of a film-like form. The plasma treatment includes exposing the surface of a plastic body to a low temperature plasma so that the surface undergoes a chemical change by the action of dissociated oxygen and ultraviolet rays. Plasma of oxygen or air is used. This method is disadvantageous because of the necessity of an expensive apparatus. The functional groups-introducing method includes irradiating UV rays on the surface of a shaped body in an atmosphere of chlorine, and then treating the irradiated surface with an alkali. This method poses a problem of handling very dangerous chlorine gas. The photo-grafting method includes admixing benzophenone with polypropylene powder, forming the admixture into a film, and photo-treating the surface of the film in the presence of acrylamide in an oxygen-free atmosphere for photochemically graft-copolymerizing the acrylamide with the polypropylene. This method requires complicated steps and is economically disadvantageous. As surface pretreatment methods for dyeing polypropylene fibers which are typical examples of fibers of an inactive polymeric material, there are proposed the following methods: (1) a method in which the surface is oxidized with nitrogen dioxide; (2) a method in which the surface is chlorinated and, if desired, then treated with an amine; (3) a method in which the surface is sulfonated with fuming sulfuric acid and, if desired, then treated with an amine; (4) a method in which the surface is chlorosulfonated by UV irradiation in the atmosphere of sulfur dioxide gas and chlorine gas and is then treated with an amine; (5) a method in which the surface is heated after depositing thereon sulfolauric acid or sulfosalicylic acid and is then treated with an amine; (6) a method in which the surface is treated with an alkylamine, pyrrole or naphthylamine; (7) a method in which vinylpyridine or isopropenylpyridine is grafted on the surface; (8) a method in which vinyl chloroacetate or vinyl sulfonate is grafted on the surface and thereafter the surface is treated with an amine; (9) a method in which the surface is treated with an emulsion containing as a major component a chlorosulfonated polyolefin; and (10) a method in which the surface is impregnated with tetrachlorosilicon. These methods, however, are not industrially actually applied because they cannot be performed safely, because the treated surface cannot be sufficiently satisfactorily dyed and because they are not economically fully acceptable. GB-A-1414975 describes a method for improving the adhesion of coatings to surfaces of polymers including polyolefins and polyolefin copolymers, in which the polymer surface is pre-treated with UV radiation in the presence of a liquid "photosensitizer". Halogenated hydrocarbons such as methylene chloride, trichloroethylene and chloroform are mentioned as being possible photosensitizers in this pre-treatment process but are indicated to result in less strong bonding than before or multiple hydrocarbon ring photosensitizers which therefore are preferred. Chemical Abstracts, 105 (1986), 134929e discloses an adhesion-improving pre-treatment method for polymers in which the surface of the polymer, eg polypropylene, is first contacted with a chlorine-containing hydrocarbon, eg trichloroethylene, and then is irradiated with UV light. The chlorine-containing hydrocarbon may be in either gaseous or liquid form. The present invention provides improvements in a method for activating the surface of a shaped body of a polyolefin which, like the method of Chemical Abstracts, 105 (1986), 134929e, involves contacting the body with a chlorine-containing hydrocarbon and then irradiating the body with UV radiation. More particularly, the present invention provides a method of treating a surface of a shaped body formed of a synthetic organic polymer selected from polyolefins, copolymers thereof and mixtures thereof, comprising the steps of: (a) heating the surface to a first temperature of 30°C or more, and then (b) contacting said surface in an oxygen-containing atmosphere with a liquid chlorinated aliphatic hydrocarbon which is or which includes tetrachloroethylene at a second temperature and for a period of time sufficient to swell said surface, said second temperature being 10°C to 80°C lower than said first temperature, and (c) irradiating said swollen surface with UV rays having wave lengths in the range of 1800-2100 Å, thereby increasing the surface energy of said surface of the shaped body. The method of the present invention will now be described in more detail. Illustrative of suitable polyolefinic polymers which can be treated by the method of this invention are as follows: (1) Polypropylene; (2) Copolymers or mixtures of two or more of polypropylene, polyethylene, ethylene-propylene rubber; (3) Ethylene-propylene rubber; (4) Copolymers or mixtures of ethylene-propylene rubber with another polymer or polymers; (5) Polyethylene; (6) Copolymers or mixtures of polyethylene with another polymer or polymers; and (7) Blends of at least one of the above polymers (1)-(6) with a filler and/or an additive. Any shaped body of the polyolefinic polymer may be used in the present invention. The shaped body may be, for example, a film, sheet, fiber, plate, rod, pipe or block. In this invention, a surface to be activated of the shaped body is treated with a liquid chlorinated hydrocarbon which consists of or at least includes tetrachloroethylene at a temperature and for a period of time sufficient to swell the surface with the liquid. It is a particular feature of this invention that the swelling step is conducted by first heating the surface of the shaped body to be activated, preferably to 50°C or more, and then contacting the heated surface with the liquid chlorinated hydrocarbon maintained at a temperature lower by 10-80°C than that of the heated surface by any suitable cooling means. By this expedient, vaporization of the liquid chlorinated hydrocarbon, which acts as a UV ray-absorbing liquid, can be minimized without lowering the swelling efficiency. That is, when the surface of the shaped body which has been heated to a high temperature, say above the boiling point of the liquid chlorinated hydrocarbon, is contacted with the liquid chlorinated hydrocarbon, the liquid in contact with the heated surface can be heated to a high temperature and can swell the surface with the simultaneous generation of vapors thereof. The vapors are, however, immediately condensed upon contact with the liquid chlorinated hydrocarbon which is present abundantly and maintained well below the boiling point thereof. The heating of the surface may be effected by any known means such as an infrared lamp, an electric heater, a microwave generator, a high frequency generator or an oven. The contact of the surface of the shaped body to be activated with the chlorinated hydrocarbon liquid may be advantageously effected by immersion. The contact with the heated surface of the polymer body is performed for a period of time sufficient to swell the surface of the shaped body. More particularly, the contact is carried out preferably for a period of time from 1 second to 20 minutes, more preferably for a period of time from 5 seconds to 5 minutes. By this contact with the heated polymer surface the organic liquid penetrates into amorphous portions constituting part of the surface of the shaped body, thereby to swell the surface. If desired, the contact may be carried out while applying an ultrasonic wave to the surface of the shaped body to improve the swellability of the surface. The shaped body whose heated surface has been thus swollen by contact with the liquid chlorinated hydrocarbon is then subjected to a UV irradiation treatment in an oxygen-containing atmosphere so that the surface of the shaped body is activated and is converted to a state suitable for e.g. dyeing, printing, coating or bonding. Since the liquid chlorinated hydrocarbon incorporated into the swollen surface of the shaped body decreases in amount with time by evaporation, the UV irradiation is desired to be performed before lapse of a long period of time, preferably within 1 minute after the completion of the swelling treatment. As a source of the UV rays, there may advantageously be used a low pressure or a high pressure mercury lamp having an envelope made of a synthetic quartz glass and capable of emitting UV rays with a wave length of 1800-2100 Å. It is preferred that the wave length of the UV rays to be irradiated on the swollen surface be concentrated to 1849 Å. It is also preferable to use a UV source generating UV rays with a high intensity. The UV irradiation may be performed at a temperature of 30-60°C. In accordance with this invention, a liquid chlorinated hydrocarbon consisting of or containing tetrachloroethylene is used as the UV ray-absorbing liquid. Such a liquid has excellent surface activating properties, which are considered to be attained according to the following mechanism. When a shaped body formed of a polyolefin and having a surface swollen with tetrachloroethylene is subjected to UV irradiation in an oxygen-containing atmosphere, the tetrachloroethylene can generate chlorine radicals which abstract the hydrogen atoms from the polyolefin to leave radicals on the polyolefin. Oxygen is then reacted with the radicals on the polyolefin to form oxygen-containing groups which are responsible for the increased surface energy of the resulting polyolefin shaped body. The shaped body which has undergone the foregoing UV treatment may then be subjected, as such or after the removal of static electricity, to a printing, coating, bonding or any other desired treatment. The method for the treatment of a surface of a shaped body according to the present invention can provide the following advantages: 1) The treated surface gives a smooth, beautiful and strong coated, printed or dyed layer; 2) The method does not require priming; 3) The method is simple, economical and does not require a long treatment time and, therefore, it is suited for industrial applications; 4) The method may be carried out by the use of a simple and inexpensive apparatus; 5) No dangerous gases such as chlorine gas are used; 6) The method does not cause deformation or melt of the surface to be treated. 7) The method is applicable to any shaped bodies such as films, fibers, rods, plates; and 8) The method may be performed without generation of vapors of UV ray-absorbing organic liquid. The following example illustrates the present invention. Example A plate formed polypropylene was heated by means of an infrared lamp to 90°C and maintained at that temperature for 30 seconds. Then the plate was immediately immersed in an organic solvent as shown in Table 4 for 30 seconds. The solvent was maintained at 22°C. After being allowed to stand at room temperature for 30 seconds, the plate was subjected to a UV irradiation treatment conducted by means of a low pressure mercury lamp (200 W) having a synthetic quartz glass envelope in the air for 1 minute with the lamp being maintained at a distance of 1.5 cm from the surface of the plate. An acrylic resin paint containing a polyacrylic ester as a main ingredient dispersed in a ketone-base solvent was sprayed over the thus treated surface about 30 min after the completion of the UV irradiation to give a uniform beautiful coat. The coat was dried in the air at room temperature for about 24 hours and tested for its adhesive strength. The test was carried out by cutting the coated layer on the polypropylene plate into small squares each having a 2 mm side, then pressingly applying an adhesive tape (Trade Mark: MITSUBISHI UNICELLOPHANE TAPE 18), and peeling off the tape. The number of the cut squares remaining on the polypropylene plate was counted. The adhesion strength was calculated as a percentage of the number of the remaining squares on the basis of the original number of the squares before the stripping. The results are shown in Table 1. Solvent Adhesive strength (%) Benzene 0 CCl₄ 42 Tetrachloroethylene/CCl₄ (1:1) 73 Tetrachloroethylene 89

Claims: A method of treating a surface of a shaped body formed of a synthetic organic polymer selected from polyolefins, copolymers thereof and mixtures thereof, comprising the steps of: (a) heating the surface to a first temperature of 30°C or more, and then (b) contacting said surface in an oxygen-containing atmosphere with a liquid chlorinated aliphatic hydrocarbon which is or which includes tetrachloroethylene at a second temperature and for a period of time sufficient to swell said surface, said second temperature being 10°C to 80°C lower than said first temperature, and (c) irradiating said swollen surface with UV rays having wave lengths in the range of 1800-2100 Å, thereby increasing the surface energy of said surface of the shaped body. A method as set forth in Claim 1, wherein said organic polymer is a resin selected from (a) polypropylene; (b) copolymers or mixtures of two or more of polypropylene, polyethylene and ethylene-propylene rubber; (c) ethylene-propylene rubber; (d) copolymers or mixtures of ethylene-propylene rubber with another polymer or polymers; (e) polyethylene; (f) copolymers or mixtures of polyethylene with another polymer or polymers; and (g) blends of at least one of the above polymers (a) - (f) with a filler and/or an additive. A method as set forth in Claim 1 or Claim 2, wherein a majority of said UV rays to be irradiated on the swollen surface has a wavelength of 1849 Å. A method as set forth in any preceding claim, wherein said irradiation is with a low pressure or high pressure mercury lamp having an envelope made of a synthetic quartz glass and capable of emitting UV rays with a wave length of 1800 to 2100 Å. A method as set forth in any preceding claim, wherein step (b) is performed for a period of 1 second to 20 minutes. A method as set forth in any preceding claim, wherein said surface is heated in step (a) to 50°C or more.

Inventors: KATOH, KOICHIROH, Katoh, Koichiroh, c/o Industrial Products Res.Inst
Applicants: DIRECTOR-GENERAL OF THE AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 1988
Main classification: C08J7
IPC codes: C08J7, B29C71, H05K3, B29C35, B29C59