GB2061968A - Method for preventing bleeding of plasticizers on the surface of a shaped article of plasticized polyvinyl chloride resin - Google Patents

Abstract

In a method for preventing bleeding or exudation on the surface of a plasticizer contained in a shaped article of a plasticized polyvinyl chloride resin by treatment with low temperature plasma of an inorganic gas, more reproducibile and effective results are obtained by controlling the temperature T1 of the article in DEG C and the temperature T2 of the power-input electrode in DEG C to satisfy the following relationship: 3 DIVIDED 8T1-10</=T2</=100-5 DIVIDED 8 the temperature T1 being in the range from 0 DEG C to 80 DEG C.

Description

SPECIFICATION Method for preventing bleeding of plasticizers on the surface of a shaped article of plasticized polyvinyl chloride resin The present invention relates to a method for preventing bleeding of a plasticizer on the surface of a shaped article of a plasticized polyvinyl chloride resin.

As is well known, one of the major problems with shaped articles of a plasticized polyvinyl chloride resin is that the plasticizer contained therein gradually migrates toward and exudes on the surface of the shaped 'article in the long-run use of the article; this problem of bleeding or exudation is not limited to plasticizers but almost all kinds of additives cannot be free from like phenomena. In order to overcome this difficulty, in particular in articles of plasticized polyvinyl chloride resins, it has been proposed to subject the surface of such articles to treatment with lowtemperature plasma so as that a crosslinked layer is formed on the surface of the article which serves as a barrier layer for preventing migration and bleeding of the plasticizer on the surface.

Unfortunately, this low temperature plasma treatment cannot be widely undertaken in practice. This is partly because of the poor reproducibility and hence low reliability of the method in giving the expected results not only in batch-wise treatments but also in continuous treatments. This disadvantage has not yet been overcome even with the most careful control of several parameters thought to be influencing the results such as the pressure of the gaseous atmosphere, flow rate of the plasma gas, applied electric power and the like.

The method of the present invention, which has been established on the base of the inventors' discovery that the most important parameter for the reproducibility of the desired effect of the low temperature plasma treatment is the relationship between the temperature of the shaped article under treatment and the temperature of the power-input electrode, comprises placing a shaped article of a plasticized polyvinyl chloride resin in a plasma chamber of a plasma-generating apparatus having discharge electrodes built inside and exposing the surface of the shaped article to low temperature plasma of an inorganic gas under a pressure not exceeding 10 Torr while the temperature of the shaped article T1 is kept in the range from 0 "C to 80 "C and the temperature of the power-input electrode T2 in "C is controlled to maintain the following relationship with the temperature of the shaped article under treatment T1 in "C, 3 5 -8T1 - 10Th100--8t Preferred embodiments of the invention will now be described.

The method of the present invention is applicable to shaped articles of not only homopolymeric vinyl chloride resins but also copolymeric vinyl chloride-based resins containing a plasticizer in so far as the main component, say 50 % by weight or more, of the copolymeric resin is vinyl chloride. The copolymers suitable for use in the method include copolymers of vinyl chloride and vinyl acetate, copolymers of vinyl chloride and ethylene, copolymers of vinyl chloride and propylene, copolymers of vinyl chloride, ethylene and vinyl acetate, copolymers of vinyl chloride and acrylonitrile, copolymers of vinyl chloride and styrene, copolymers of vinyl chloride, styrene and butadiene and copolymers of vinyl chloride and vinylidene chloride as well as graft copolymers mainly composed of vinyl chloride.The resin may be a polymer blend of two or more resins selected from homopolymeric vinyl chloride resins and the above defined copolymeric resins.

Further, the vinyl chloride resins or copolymeric vinyl chloride resins may be blended with other kinds of resins to improve the mechanical and other properties of the article shaped therefrom. Suitable resins for such a purpose are exemplified by copolymers of ethylene and vinyl acetate, copolymers of acrylonitrile and butadiene, copolymers of styrene and acrylonitrile, copolymers of methyl methacrylate, styrene and butadiene, copolymers of acrylonitrile, styrene and butadiene, elastomeric copolymers of ethylene and propylene, elastomeric ternary copolymers of ethylene, propylene and a dienic monomer, polyamide resins, polymers of caprolactam, epoxy-modified polybutadienes, epoxy-modified polyols, organopolysiloxanes and the like.

A plasticized polyvinyl chloride resin is obtained by formulating one or more kinds of plasticizers in the resin. Suitable plasticizers are exemplified by esters of phthalic acid such as diethyl phthalate, di-2-ethylehexyl phthalate, dinonyl phthalate and the like, esters of aliphatic dibasic acids such as dioctyl adipate, diisodecyl adipate, dioctyl azelate and the like, esters of trimellitic acid such as trimethyl trimellitate, trioctyl trimellitate and the like and esters of fatty acids such as butyl oleate, methyl acetylricinolate, chlorinated methyl carboxylate and the like as well as ethylphthalylethylene glyclolate, diethyleneglycol dibenzoate, tributyl acetylcitrate and the like.

The polyvinyl chloride resin formulated with the above named plasticizers may contain other kinds of conventional additive ingredients such as stabilizers, lubricants, fillers, pigments, dyes, ultraviolet absorbers, anti-oxidants, cross-linking agents, crosslining accelerators, cationic, anionic, non-ionic and amphoteric surface active agents and the like.

The shape of the article to be subjected to the plasma treatment in accordance with the method of the invention is not important and inciudes films, sheets, tubes, hoses and any other irregular forms in so far as uniform exposure of the surface to low temperature plasma is ensured. Accordingly, the methods for shaping articles from the plasticized polyvinyl chloride resin compound may be conventional ones depending on the shape of the desired article including extrusion molding, injection molding, calendering, inflation, compression molding and the like among the molding techniques known in the art.

The thus obtained shaped article is then subjected to surface exposure to low temperature plasma in a plasma chamber of a plasma generating apparatus. The plasma generating apparatus should be provided with electrodes, i.e. a grounded electrode on which the shaped article to be treated is placed and a power-input electrode oppositely facing the grounded electrode, installed inside the plasma chamber.In the method of the invention, the relationship between the temperature of the shaped article under treatment T1 in "C and the temperature of the power-input electrode T2 in "C is very important and these temperatures must satisfy the following relationship: 3 5 -8T1 -10T2100--8t, the temperature T1 being kept in the range from 0 "Cto 80 C. When the above described temperature condition is not satisfied, the reproducibility of the results obtained by the plasma treatment is remarkably decreased.

The plasma generating apparatus used in the method is of the type provided with the electrodes installed inside the plasma chamber in contact with the inorganic plasma gas so that the inorganic gas under low pressure introduced into the plasma chamber is converted into low temperature plasma in the vicinity of the electrodes and acts on the surface of the shaped article placed in the plasma chamber so that a high-density crosslinked layer is formed on the surface of the article to achieve the desired effect of the treatment.

The form of the power-input electrode is not important and includes flat plate, wire netting, coil, rod and other forms of electrode but it is a desirable condition that the electrode is provided with a means for temperature control such as piping for passing a heating or cooling medium therethrough. It is of course desirable that the electrodes are made of a chemically stable material or coated with a chemically stable material such as glass, porcelain enamel and the like since the electrodes are always directly exposed to the plasma atmosphere.

Temperature control of the shaped article undertreatment may be carried out by various means. For example, the grounded electrode is provided with a temperature-controlling means similarto that in the power-input electrode inside a flattable-like electrode and the shaped article is placed on the electrode in as close contact therewith as possible so that the temperature of the shaped article is approximately the same as the temperature of the grounded electrode which is controlled by means of the heating or cooling medium. When the shaped article under treatment is a continuous length tubular body and to be treated continuously, it is convenient to pass a heating medium at a desired temperature through the tubular body.

The principle of generating low temperature plasma is well known in the art. For example, the plasma chamber of the apparatus is filled with an inorganic gas under a low pressure and a high-frequency, e.g.

13.56 M Hz, of 10 to 700 watts electric power is supplied between the electrodes. The time necessary for the treatment with low temperature plasma naturally depends on the applied voltage, electric power and other parameters but it is usually in the range from a few seconds to several tens of minutes. The frequency band for electric discharge generating low temperature plasma is not limited to the above mentioned high frequency but low frequency, microwaves and direct current may also be used.

The inorganic gas used to fill the plasma chamber under a low pressure is exemplified by helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon monoxide, carbon dioxide, hydrogen and chlorine as well as hydrogen chloride, sulfur dioxide, hydrogen sulfide and the like. These gases are used either singly or as a mixture of two or more.

The pressure of the above mentioned inorganic plasma gas in the plasma chamber is of course in the range where stable plasma discharge is readily obtained in the plasma chamber. The pressure is therefore limited to be inthe range from 0.01 to 10 Torr, preferablyfrom 0.01 to 1 Torr.

When the treatment with low temperature plasma is carried out in the above described manner, a high density crosslinked layer is formed on the surface of the article within a very short time to serve as a barrier layer against the migration and bleeding of the plasticizer contained in the article and the effectiveness of the,, treatment has good reproducibility so that the method of the present invention is applicable to large-scale treatment of shaped articles with high reliability.

The procedure of the method and the effectiveness of the method will now be described in further detail by way of examples.

Example 1 A plasticized polyvinyl chloride resin sheet of 0.5 mm thickness was prepared by milling a compounded mixture composed of 100 parts by weight of a homopolymeric polyvinyl choride resin (TK-1300, a product of Shin-Etsu Chemical Co., Japan), 50 parts by weight of dioctyl phthalate, 1.5 parts by weight of calcium stearate and 1.5 parts by weight of zinc stearate in a roller mill at 160 "C for 10 minutes followed by compression molding at 165 "C.

A piece of this sheet was placed on a table-like grounded electrode of metal in the plasma chamber of an apparatus for generating low temperature plasma and the pressure of the atmosphere in the plasma chamber was controlled at 1.0 Torr by passing carbon monoxide under reduced pressure. Both the grounded electrode and the power-input electrode were provided with piping for passing a heating medium there-through to control the temperature of the electrodes and the plasma treatment was undertaken with various combinations of the temperatures of the grounded electrode and the power-input electrode, utilizing these temperature controlling means. The temperature of the sample sheet was substantially the same as vthe grounded electrode on which the sample sheet was placed.

When the power-input electrode and the sample sheet had reached the desired temperatures, a high frequency electric power of 500 watts at 13.56 MHz was applied between the electrodes to generate low temperature plasma in the plasma chamber. The time of the plasma treatment was always 1 minute for each of the sample sheets.

The plasma-treated sample sheets obtained with various combinations of the temperatures of the power-input electrode and of the sample sheet were then examined for the amount of extractable plasticizer with n-hexane by the testing procedure described below to give the results set out in Table 1 to follow.

Testing method - plasticizer extraction with n-hexane: The sample sheet was placed on the bottom of a cylindrical extraction vessel of 100 ml capacity and 50 ml of n-hexane were introduced into the vessel so as that a 26cm2 area of the surface of the sample sheet was in contact with the solvent. After shaking the vessel for 2 hours at 37 "C, a portion of the solvent was taken and analyzed gas-chromatographically for the content of the plasticizer, dioctyl phthalate in this case, contained in the solvent.

TABLE 1 (plasticizer extraction, mg/26 cm2) Temperature of Temperature of power-input electrode T2, C sample sheet T1, "C -10 10 30 50 70 100 -5 83 83 37 49 64 86 0 8 0 0 0 0 7 40 48 5 0 0 3 77 80 81 40 1 5 69 109 85 71 85 78 83 92 111 Example 2 Twenty sample sheets were prepared in the same manner as in Example 1 and each of them was treated with low temperature plasma in the same apparatus as in Example 1 in a carbon monoxide atmosphere of 0.5 Torrfor 2 minutes with application of a high frequency power of 300 watts at 13.56 MHz. In the treatment of 10 of the sample sheets, both the temperature of the sample sheet and of the power-input electrode were controlled at 40 "C while the remaining 10 sample sheets were treated with no temperature control of the sample sheet and the electrode.

These plasma-treated sample sheets with or without temperature control were subjected to the measurement of plasticizer extraction in the same manner as described before. The amount of the extracted plasticizer was substantially zero from each of the sample sheets plasma-treated with temperature control while the amounts varied widely in the sample sheets plasma-treated without temperature control ranging from zero to 50 mg/26 cm2 with an average value and a standard deviation of 16.6 + 16.9 mg/26 cm2.

Claims (4)

1. A method for preventing bleeding of a plasticizer contained in a shaped article of a plasticized polyvinyl chloride resin on the surface thereof which comprises.

(a) placing the shaped article in a plasma chamber of a plasma generating apparatus provided with a grounded electrode and a power-input electrode inside the plasma chamber, and (b) exposing the surface of the shaped article to low temperature plasma of an inorganic gas generated in the plasma chamber under a pressure in the range from 0.01 to 10 Torr while the temperature of the shaped article T1 in "C and the temperature of the power-input electrode T2 in "C are controlled to satisfy the relationship.

3 5 8 T1 - 10 1008T1, the temperature T1 being in the range from 0 "C to 80 C.

2. The method as claimed in claim 1 wherein each of the grounded electrode and the power-input electrode is provided with a temperature controlling means.

3. The method as claimed in Claim 1 substantially as hereinbefore described in either Example.

4. A polyvinyl chloride resin when treated by a method as claimed in any preceding claim.