GB2053236A - Preventing plasticiser bleeding in shaped articles of vinyl chloride-based resins - Google Patents

Abstract

Shaped articles of a vinyl chloride-based resin containing 0.1 to 10 p.h.r. of anti-oxidant are exposed to a low temperature plasma of a gas having substantially no polymerisability in the plasma condition. A strong barrier layer is formed, suppressing oxidative degradation of the polymer and preventing surface bleeding of plasticiser.

Description

SPECIFICATION Preparing shaped articles of vinyl chloride-based resins Vinyl chloride-based resins are important thermo-plastic materials, owing to their excellent and very versatile properties and their low cost relative to many otherthermoplastics. They are therefore widely employed, in a variety of fields, as shaped articles. The rigidity or flexibility of shaped articles of vinyl chloride-based resins can be adequately controlled by formulating the resin with a plasticiser. Such plasticised resins can give flexible shaped articles, e.g. films, sheets, synthetic leathers, tubes, hoses, bags or coating materials, for use as, for example, mechanical instruments, packaging for foodstuffs, materials for agricultural use or building materials.

It is very common that vinyl chloride-based resins are formulated with various kinds of additive ingredients such as flame retard ants, ultraviolet absorbers or lubricants, according to the particular needs for the improvements of the workability of the resin composition as well as the properties of the articles shaped with the resin composition.

One of the most serious problems in these articles shaped with the vinyl chloride-based resin composition formulated with the additive ingredients, typically, a plasticizer, is that the plasticizer contained in the shaped article may sometimes migrate toward the surface of the article and exude on the surface of the article, in the long run resulting in inferior properties of the plasticized vinyl chloride-based resin articles. This phenomenon is usually called "bleeding", prevention of which is one of the most important problems difficult to solve in the technology of synthetic resin processing.

Bleeding of the plasticizer is undesirable not only due to the deterioration of the properties of the shaped articles but also due to the transfer of the plasticizer exuded on the surface of the article to the surface of another body in contact with the shaped article of the plasticized vinyl chloride-based resin composition. Along with the loss in the surface appearance of the articles, bleeding ortransferofthe plasticizer is especially undesirable when the shaped article is used in a medical instrument or in contact with foods because physiological inertness of plasticizers to the human body is not established in general, so that the use of plasticized vinyl chloridebased resin compositions in these fields is largely limited. Similar problems are involved in the use of shaped articles containing the other additive ingredients.

Various attempts have been made to solve the problem of surface bleeding. The methods hitherto proposed for the purpose include treatment or irradiation of the surface of the article with ionizing radiation, electron beams or ultraviolet light as well as with high voltage, high frequency electric discharge and chemical treatment. These methods are to some extent effective to improve the heat resistance and anti-solvent resistance and to impart affinity to water, anti-static effect and printability with a printing ink to the surface but relatively ineffective in preventing the bleeding of plasticizers and the like with, instead, adverse effects on the useful properties inherently possessed by the vinyl chloride-based resins.

For example, irradiation with ionizing radiation or high energy electron beams may produce crosslinking between the polymer molecules not only in the superficial layer but also in the subsurface layer of the shaped article due to the excessively high energy of the radiation, so that the flexibility, the most characteristic feature of the plasticized vinyl chloride resin articles, is largely lost. Treatment with ultra-violet light is undesirable by reason of coloring in the surface due to the degradation of the polymer molecules in the surface layer of the shaped article.

Chemical treatment may cause surface erosion of the shaped article and poor adhesion or durability of the films provided on the surface of the article by such chemical treatment.

It has now been found that, when an article shaped with a vinyl chloride-based resin composition containing a specified amount of an anti-oxidant is sub jected to exposure to lowtemperature plasma, a barrier layer which can effectively prevent bleeding of the plasticizer is formed on the surface, presumably by cross-linking between the polymer molecules.

Further, the undesirable oxidative degradation of the polymer molecules, which may occur under the influence of the low temperature plasma, can be reduced to a negligible extent.

According to the present invention, a method for preparing a shaped article of a vinyl chloride-based resin comprises (a) blending 100 parts by weight of vinyl chloride-based resin with from 0.1 to 10 parts by weight of an anti-oxidant to form a resin composition; (b) shaping the resin composition to form an article; and (c) exposing the shaped article to low temperature plasma of a gas having substantially no polymerisability in the plasma condition, under a pressure between 0.001 Torrand 10Torr.

'Low temperature plasma' is an established technical term, as an antonym of 'high temperature plasma'. Low temperature plasma is a gaseous atmosphere full of exited or electrically charged species produced by electric discharge through a low pressure gas where the temperature of the gaseous atmosphere as a whole is not excessively high in comparison with the ambient temperature, irrespective of the energies of the individual excited or charged species distributed in a wide range.

The vinyl chloride-based resin which is used in the method of the invention may be a homopolymeric vinyl chloride resin or a resin which is a copolymer of vinyl chloride, of which the main (monomer) component, say, 50% by weight or more, is vinyl chloride, with one or more copolymerisable monomers exemplified by vinyl esters such as vinyl acetate, vinyl ethers such as vinyl ethyl ether, acrylic acid and esters thereof, methacrylic acid and esters thereof, maleic acid and esters and its anhydride, fumaric acid and esters thereof, aromatic vinyl compounds such as styrene, vinylidene halides such as vinylidene chloride, acrylonitrile, methacrylonitrile and olefins such as ethylene or propylene.

The anti-oxidant which is used as an essential ingredient in the resin composition for the shaped article, accelerates the cross-link formation on the surface layer by the exposure to low temperature plasma while suppressing the undesirable oxidative degradation so that a very effective barrier layer is formed on the surface for preventing bleeding of the plasticizer and other additive ingredients contained in the shaped article of the resin composition.

Further, the anti-oxidant is effective in reducing the necessary amount of the plasticizer, increasing the flexibility of the shaped article, improving the surface smoothness of the shaped article and imparting good appearance to the shaped article.

Examples of anti-oxidants suitable for use in the present invention are as follows: 1) Anti-oxidants belonging to the class of phenolic compounds such as alkylene or alkylidene bisphenols, e.g. 2,2' - bis(4 - hydroxyphenyl) propane, tris(2 - methyl - 4 - hydroxy - 5 - tert - butyl - phenyl) butane and hindered polyphenols with high molecular weights; substituted phenols, e.g. 3 - methyl - 4 isopropylphenol, 2,6 - di -tert - butyl hydroxytoluene, 2,6 - di - tert - butyl -p - cresol and hindered phenols with high molecular weights; and thio - bis alkylated phenols, e.g. 4,4' - thio - bis(6 - tert - butyl) m - cresol and 4,4' - thio - bis(3 - methyl - 6 - tert butylphenol).

2) Anti-oxidants belonging to the class of thiopropionic acid esters such as dilauryl thiopropionate and distearyl thiodipropionate.

3) Anti- oxidants belonging to the class of organic phosphites such as tricresyl phosphite, tri(nonylphenyl) phosphite and tri(isooctyl) phosphite.

Among the above named anti-oxidants, the phenolic compounds are most preferred.

The amount of the anti-oxidant in the resin composition is from 0.1 to 10 parts by weight per 100 parts by weight of the vinyl chloride-based resin.

When the amount of the anti-oxidant is smallerthan 0.1 part by weight, the beneficial effectto be obtained by the addition of the anti-oxidant cannot be exhibited while an excessive amount over 10 parts by weight cannot give any additional effect despite the increased cost with, instead, undesirable bleeding or blooming of the anti-oxidantperse on the surface during storage of the shaped article.

The shaped articles of the vinyl chloride-based resin to be manufactured by the inventive method contain at least one kind of additive ingredient other than the above-mentioned anti-oxidants.

For example, plasticizers are formulated in the resin composition when flexibility or reduced hardness is desired for the shaped article, as exemplified by esters of phthalic acid such as dioctyl phthalate or butylbenzyl phthalate, esters or aliphatic dibasic acids such as dioctyl adipate or dibutyl sebacate, glycol esters such as esters of pentaerythritol or diethyleneglycol dibenzoate, esters of fatty acids such as methyl acetylricinolate, esters of phosphoric acid such astricresyl phosphate ortriphenyl phosphate, epoxidated oils such as epoxidated soybean oil or epoxidated linseed oil, esters of citric acid such as acetyl tributyl citrate or acetyl trioctyl citrate, trialkyl trimellitate, tetra - n - octyl pyromellitate and polypropylene adipate as well as polyesters and plasticizers having diversified chemical structures.

Other kinds of additive ingredients for improving the properties such as heat resistance, lubricity and stability include stabilizers such as metal salts of carboxylic acids, e.g. calcium stearate, barium stearate and cadmium stearate, lead compounds, e.g.

tribasic lead sulfate and di - basic lead phosphite and organotin compounds, e.g. dibutyltin dilaurate, di - n - octyltin maleate and di - n - octyltin mercaptide; lubricants such as higher fatty acids and esters thereof, e.g. butyl stearate, fatty acid am ides, e.g.

ethylene bis - stearoamide, and polyethylene waxes; and other kinds of additive ingredients conventionally used for formulating vinyl chloride-based resin compositions for shaping articles including fillers, ultraviolet absorbers, anti-static agents, anti-fogging agents, pigments, dyes and cross-linking aids.

It is further optional that the vinyl chloride-based resin used in the inventive method is a polymer blend, in particular with a polymeric rubbery elastomer such as 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, polyamide resins, polymers of caprolactam, epoxy-modified polybutadiene resins and polyurethane elastomers, though in a limited amount not exceeding 50 parts by weight per 100 parts by weight of the vinyl chloride-based resin.

The vinyl chloride-based resin composition obtained by blending the above-described ingredients is fabricated into shaped articles. The method for shaping the resin composition into articles may be a conventional, such as extrusion moulding, injection moulding, calendering, inflation method or compression moulding, according to the desired shape of the article. The shapes of the articles are not particularly limitative in so far as the subsequent treatment with low temperature plasma can be effected uniformlyoverthe surface of the shaped article.

The next step is exposure of the surface of the shaped article to low temperature plasma of a gas having no polymerizability in the plasma condition under a pressure in the range from 0.001 Torrto 10 Torr. The low temperature plasma is readily generated in the gaseous atmosphere of the abovespecified pressure by applying a high frequency, e.g.

13.56 MHz, electric power of 10 to 500 watts to the electrodes effecting electric discharge through the atmosphere. Satisfactory results can be obtained either by the electrode discharge or by the electrodeless discharge. The optimum time for the plasma treatment may differ widely depending on the applied voltage but it is usually in the range from a few seconds to several tens of minutes.

The frequency band for the electric discharge is not limited to the above-mentioned high frequency region but may range from direct current to low fre quency to microwave regions. The mode of the electric discharge is also not limitative including, in addition to the glow discharge, corona discharge, spark discharge and silent discharge. Exterior electrodes and inside electrodes as well as coiled electrodes may be used as the discharge electrode connected to the power supply by capacitive coupling or by inductive coupling. In any case, however, it is a require ment that the surface of the shaped article is never subject to thermal denaturation by the heat evolved in the electric discharge.

The term "a gas having no polymerizability in the plasma condition" means a gas from which products with high molecular weights are not formed when low temperature plasma is generated in the low pressure atmosphere of the gas. Suitable gases are mostly inorganic, exemplified by helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon monoxide, carbon dioxide, hydrogen, chlorine and hydrogen chloride. These gases are used either singly or as a mixture of two kinds or more. The pressure of the gaseous atmosphere in the plasma generating chamber is in the range from 0.001 Torrto 10 Torr or, preferably, from 0.01 Torrto 1 Torr in order to obtain stable plasma discharge.

When the shaped article of the vinyl chloridebased resin composition is subjected to exposure to the low temperature plasma in the conditions as described above, a barrier layer, which is effective in preventing bleeding of the plasticizer and other additive ingredients on the surface of the shaped article, is formed on the surface with high efficiency and with suppression of the undesirable oxidative degradation of the vinyl chloride-based resin by the influence of the plasma. Owing to the adequately controlled cross-linking density, the barrier layer retains flexibility with excellent heat-sealability and durability and the shaped articles thus treated have good durability in the mechanical strengths such as tensile strength, anti-scratch resistance and impact strength, especially at low temperatures.Furthermore, the plasma-treated shaped articles have superior surface properties such as affinity to water, decreased stain, anti-oil resistance and anti-chemical resistance.

Following are Examples which illustrate the method of the present invention in further detail but do not limit the scope of the invention. Examples 1 and 5 are comparative. In the following Examples, the effectiveness of the method, i.e. decrease in the amounts of the plasticizer bleeding on the surface of the shaped articles, was evaluated by the following procedure: a piece of a sheet of the test specimen was placed on the bottom of a cylindrical extraction vessel of 100 ml capacity to expose 26 cm2 area of the plasma-treated surface which was brought into contact with 50 ml of n - hexane introduced into the vessel followed by shaking for 2 hours at 37 C with subsequent gas chromatographic analysis of the n hexane extract to determine the amount of the plasticizer extracted from the test piece into the solvent.

Examples 1 to 4 Four kinds of resin composition were prepared each by uniformly roll milling a blend composed of 100 parts by weight of a homopolymericvinyl chloride resin (TK-1300, a product by 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 with 0, 0.2, 1.0 or 5.0 parts by weight of bisphenol A as an antioxidant at 1 60 C for 10 minutes. The resin composition was shaped into a sheet of 0.5 mm thickness by compression moulding at 165 C.

Each of the thus-prepared sheets was placed in a plasma generating chamber and the surface of the sheet was exposed to low temperature plasma generated by the electric discharge with application of a high frequency electric power of 150 watts at a frequency of 13.56 MHz while the gaseous atmosphere inside the chamber was maintained at a pressure of 0.4 Torr by passing carbon monoxide gas under a reduced pressure. The time of the plasma treatment was 1,2,5 or 10 minutes.

The thus plasma-treated sheets were subjected to the extraction test with n - hexane in the procedure above described to give the results of the extracted amounts of dioctyl phthalate in mg as set out in Table 1 below.

Table 1 (Extracted amount of dioctyl phthalate, mg)

Example 1 2 3 4 Amount ofanti-oxidant (parts by weight) : 0 0.2 1.0 5.0 Exposure time 1 130 | 104 91 91 to plasma 2 i 52 26 13.0 15.6 (minutes) 5 13 1.04 0.52 0.52 10 1.04 0.52 0.52 0.52 Examples 5 to 8 Resin compositions were prepared in the same manneras in Example 1,from 100 parts byweightof the same polyvinyl chloride resin as used in Example 1, 50 parts by weight of dioctyl phthalate, 1.5 parts by weight of calcium stearate and 1.5 parts by weight of zinc stearate with 0, 0.2, 1.0 or 5.0 parts by weight of 2,6 - di - tert - butyl -p - cresol as an anti-oxidant. The resin compositions were shaped into sheets of 0.5 mm thickness in the same manner as in Example 1.

The plasma treatment of these sheets was carried out also in the same manner as in Example 1 except that the pressure of the carbon monoxide atmosphere was decreased to 0.15 Torr while the high frequency power supply was increased to 500 watts.

The length of time of the plasma treatment was 1, 2, 5 or 10 minutes.

The results of the extraction test of the plasticizer with n - hexane carried out in the procedure as described before were as set out in Table 2 below.

Table 2 (Extracted amount of dioctyl phthalate, mg)

Example | 5 6 7 8 Amount of antiwxldant (parts by weight) 0 0.2 1.0 5.0 Exposure time 1 52 39 26 26 to plasma (minutes) 2 13 2.6 1.3 1.3 5 10.4 1.04 0.52 0.52 10 0.78 0.52 0.52 0.52

Claims (4)

1. A method for preparing a shaped article which comprises (a) blending 100 parts by weight of a vinyl chloride-based resin with from 0.1 to 10 parts by weight of an anti-oxidant to form a resin composition; (b) shaping the resin composition to form an article; and (c) exposing the shaped article to low temperature plasma of a gas having substantially no polymerisability in the plasma condition under a pressure of from 0.001 to 10 Torr.

2. A method according to claim 1 wherein the anti-oxidant is a phenolic compound.

3. A method according to claim 1 or claim 2 wherein the pressure of the gas is from 0.01 to 1 Torr.

4. A method according to claim 1 substantially as described in any of Examples 2,3,4,6,7 and 8.