GB1601529A - Provision of hydrophilic property to the surface of a hydrophobic polymer substrate - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE PROVISION OF HYDROPHILIC PROPERTY TO THE SURFACE OF A HYDROPHOBIC POLYMER SUBSTRATE (71) We, KANSAI PAINT CO., LTD., a Company of Japan, of 365, Kanzaki, Amagasaki-shi, Hyoga-ken, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a treating method for giving hydrophilic properties to the surfaces of hydrophobic polymer substrates. More particularly, the invention relates to a method in which a photochemical graft reaction occurs by applying actinic rays to a layer of a photopolymerizable composition which is in contact with the surface of the hydrophobic polymer substrate.

It is important to convert only the surface property of a polymer substrate without changing its mechanical and optical properties. With such treatment, the range of uses of the substrate can be widened and the usefulness thereof can be increased greatly. For example, since a polyolefin substrate has quite a nonpolar surface, there are several problems with such properties as adhesiveness, printability and the adaptability to take a coating finish. In order to overcome these problems, various surface treating methods have been tried.

These include corona discharge treatment, oxidation treatment, flame treatment, graft polymerization with radiant rays and the application of a surface coating. However, these methods are not always satisfactory for reasons of cost and because of other effects. In the above four methods (that is, corona discharge treatment, oxidation treatment, flame treatment and graft polymerization with radiant rays) the optical and mechanical properties of the polymer substrates are impared. Further, there are other disadvantages in that the effect of the corona discharge treatment is small and the homogeneity and durability of surfaces heated in this way, are not good. With regard to the radiant ray graft polymerization, even though high quality products can be expected, the operation of the treatment is complicated and the cost is high.The surface coating method isdefective not only in that the adhesiveness between the coating layer and the polymer substrate is poor but also in that the optical and mechanical properties of the treated substrate are lost because the double layered structure of the coating layer and the polymer substrate has different properties.

Whatever the form of polymer material it is necessary to produce active sites at or near the boundary, when the graft polymerization is done by the interface reaction of a solid polymer and a liquid. For this reason, it is proposed that the solidliquid interface reaction be carried out by the steps of prior compounding a photosensitizing agent into a polymer substrate, forming or moulding the substrate, immersing the formed substrate into a radically polymerizable monomer solution and then applying light rays through the liquid phase. In this method, however, there is a drawback with adding the photosensitizer during the forming step of the polymer substrate. In addition, there remains a danger of subsequent photodeterioration of the product since some unreacted photosensitizer remains in the products.

In order to eliminate the defects of the prior art described above, the following three conditions must be met: a) a photo-sensitizing system which is able to produce polymerization initiating active sites in polymer molecules should be used; b) the removal, after the reaction of the photosensitizer by using it outside the polymer substrate, should be made possible; and c) the concentration of active species at the interface between the polymer substrate and the liquid phase should be increased.

In the method which has been proposed by the same inventors at those of the present application (e.g. Japanese Published Unexamined Patent Application No.

14856W1977 (Published 9th December, 1977)), the polymer substrate property is converted by grafting, by the application of actinic rays, the radically polymerizable compound contained in the photopolymerizable composition which is in contact with the polymer substrate. When actinic rays are applied through the phase of photopolymerizable composition containing a photosensitizer, the light intensity I at the depth of X from the liquid surface is given as follows according to Lambert Beer's Law: I=IOe~(Ct in which Ia is the itensity of incident light at the liquid surface, E is the molar absorption coefficient of the photosensitizer at the wave length of applied light rays, and c is the concentration of the photosensitizer.

Since the value E iS constant, the quantity of light absorption in the whole system (lo-l) is increased with an increase of the concentration c: however, most of the absorption occurs in the vicinity of the liquid surface, so that the production of active sites at the interface between the polymer substrate surface and the liquid phase is not favourable. On the other hand, when the value of c is reduced, light transmission increases and the difference between the rate of active site production at the liquid surface and that at the polymer substrate-liquid interface is decreased; however, the total light absorption is also decreased so that the object cannot be attained.As described above, when the method for graft-polymerizing the radically polymerizable compound onto the surface of polymer substrate is put into practice by attacking the radical species or excited species in the triplet state which, are produced photochemically in the liquid phase, the conventional method of using actinic rays involves a theoretical contradiction.

In the abovementioned method that has been proposed by the same inventors as those of the present application, several defects have been found. For example, the rate of reaction is low during the application of actinic rays since the grafting reaction is retarded by the presence of surrounding oxygen, and, when wettability between the photopolymerizable composition and the polymer substrate is low, cissing occurs so that the layer of photopolymerizable composition becomes uneven. Further, when a photopolymerizable liquid composition which flow easily is applied to a polymer substrate, it is necessary to keep the treated polymer substrate horizontal during the irradiation, although even when the substrate is held horizontally dripping of the photopolymerizable composition cannot be avoided. The above method is therefore defective in its industrial workability.

The inventors of the present application have carried out extensive investigations in order to try to eliminate the abovedescribed disadvantages in the conventional methods. The present invention is the result of these investigations.

The present invention enables the provision of an improved method for giving hydrophilic property to the surface of a hydrophobic polymer substrate, which is entirely free from the abovedescribed disadvantages.

The present invention also enables the provision of a treating method for giving hydrophilic property to the surface of a hydrophobic polymer substrate which can be accomplished without impairing the mechanical and optical properties of the polymer substrate.

The present invention further enables the provision of the treating method mentioned above, which may be used to give an even covering to a large area of substrate but the procedure of which is quite simple and economical.

The present invention also enables the provision of an above-mentioned method which improves the adhesiveness, printability and adaptability to take a coating finish of the polymer substrate.

The present invention still further enables the provision of an above-mentioned treating method in which it is not necessary to compound the photosensitizer into the polymer substrate material.

According to a first apsect of the present invention, there is provided a method or giving hydrophilic property to a surface of a hydrophobic polymer substrate, the step of photografting by the application of actinic rays a radically polymerizable compound, which compound, when polymerized, is hydrophilic, to a surface of the substrate, the actinic rays being applied from a side of the substrate remote from the compound.

The radically polymerizable material is contained in a photopolymerizable composition which is in contact with the hydrophobic polymer substrate.

The viscosity of the photopolymerizable composition is prefereably in the range of from 10 to 1,000 centipoises before being applied to the hydrophobic polymer substrate.

A solid shielding material may be placed over the surface of the layer of the photopolymerizable composition which is in contact with the hydrophobic polymer substrate.

According to a second aspect of the present invention, there is provided a hydrophobic polymer substrate provided with a hydrophilic surface by the method of the first aspect.

When actinic rays are applied from the rear side of a transparent polymer substrate in a method according to the present invention, an increase in the concentration of photosensitizer causes an increase in light absorption at the polymer substrate-solution interface and an increase in the concentration of active species in the vicinity of the polymer substrate surface; the method of the present invention thus would appear to be quite significant industrially. It will be apparent that above principle can be applied to the case in which the reaction is initiated by the direct excitation of a monomer or a solvent.

As will be understood from the above theoretical considerations, the reaction conditions of the present method are favourable when the absorption by the light absorbing compounds that are effective in the formation of active sites is high. Since a monochromatic light source is not used in industrial actinic ray treatment, the absorption of a practical reaction system cannot readily be defined; however, it is desirable that the concentration C of a photosensitizer is set by the following equation: CXEmax > 1, in which Emax is the maximum absorption coefficient of the photosensitizer in the wavelength range of the applied light rays.

With an increase in the concentration of photosensitizer, the concentration of the active species at the polymer substratesolution interface is raised; thus. if the concentration of monomer is maintained at a constant level, the degree of polymerization of the produced graft polymer is lowered according to the theory of radical polymerization. The preferred upper limit of CXEmaX which will depend on the intended use of the treated substrate, can be determined as a function of the monomer concentration and the constant of elementary reaction.

The step of applying the photopolymerizable composition to the surface of the polymer substrate is made continuous with the step of applying the actinic rays by increasing the viscosity of the photopolymerizable composition so that workability is improved. Even when the method is worked in an ordinary coating line, the photopolymerizable composition does not flow, drip or scatter during the coating and irradiating steps so that the surroundings are not soiled; in addition, the coating irradiation can be done evenly. Polymer substrate surfaces can thus be improved cheaply.

As previously mentioned the preferred viscosity of these photopolymerizable composition is in the range of from 10 to 1,000 centipoises. If the viscosity of the composition is less than 10 centipoise, the composition easily flows off or scatters from the surface of the polymer substrate. If the viscosity of the photopolymerizable composition is higher than 1,000 centipoises, it becomes difficult to apply the composition to the polymer substrate evenly and continuously.

In a preferred method according to the present invention, the layer of photopolymerizable composition is interposed between the polymer substrate and a solid shielding material, thereby shutting off the incoming of oxygen from the outside and, at the same time, preventing the composition from flowing off. This may increase the workability of the method.

In a method in accordance with the present invention, the surfaces of the layer of photopolymerizable composition may be brought into contact with the polymer substrate and the solid shielding material by known methods. In one method, either the polymer substrate or the solid shielding material is coated with the photopolymerizable composition and the other of the above two is then brought into close contact with the coated layer. In another method, the photopolymerizable composition is placed between the polymer substrate and the solid shielding material and then the composition is pressed into a layer between the substrate and the shielding material. In yet another method, the photopolymerizable composition is poured or injected into the space between the layers of the polymer substrate and the solid shielding material.For example, when both the polymer substrate and the solid shielding material are films, the photopolymerizable composition is applied to one side surface of the polymer substrate or the solid shielding material by means of a roller coater, flow coater or a sprayer and, immediately after that, the other film is brought into close contact with the coated surface. This process may be carried out continuously; this is a virtue which enhances industrial productivity. It should be noted that any gas should not be in the layer of photopolymerizable composition.

The solid shielding material for use in a method according to the present invention should have a low oxygen permeability.

Many kinds of material may be used, but when the material is used repeatedly, the use of a material which is inert to the photograft reaction is desirable.

The thickness of the layer of photopolymerizable composition is determined depending on the purpose of treatment, the materials employed and the required productivity; however, it may be generally from 0.1 to 10,000 microns. If the thickness of the photopolymerizable composition is less than 0.1 micron, the treatment is not particularly effective since only a small quantity of radically polymerizable compound is grafted; in addiction, it becomes difficult to separate the solid shielding material from the polymer substrate after the actinic ray application.

On the other hand, when the thickness of the photopolymerizable compound layer exceeds 10,000 microns, the compound is liable to flow off. This defeats the object of the present invention and is not economic.

If a polymer substrate to be treated is used as the solid shielding material and the actinic rays are applied from both sides, both substrate surfaces may be treated simultaneously. This is desirable in practice.

The wavelengths of the actinic rays as the energy source of the graft reaction are preferably in the range of from 250 to 700 nm; more preferably, they are in a range which does not cause the polymer substrate to be deteriorated. Exemplified as suitable light sources are low pressure mercury lamps, high pressure mercury lamps, fluoroscent lamps, xenon lamps, carbon arc lamps, incandescent lamps and the sun.

The actinic rays are applied directly or indirectly through a transparent or partially transparent material such as a filter, glass plate or photographic film. The time of irradiation is generally within the range of 0.1 second to 24 hours. This time is varied depending on the degree of hydrophilic property of the treated surface and the kind of light source.

After the graft reaction with the application of actinic rays unreacted photopolymerizable composition and the remaining non-grafted polymer are generally removed from the surface of the polymer substrate. The removal of the composition and the polymer can be done by a known method such as mechanical removal or dissolution.

Suitable polymer substrates for use in a method according to the present invention are those which produce, on the main or side chains of polymer molecules, active sites which are able to initiate polymerization by radicals, ion radicals or excited species formed in the phase of the photopolymerizable composition. In view of this, a suitable polymer satisfies one of the following conditions: (a) the main or side chains of the polymer have carbon-carbon double bonds, or. (b) the main or side chains have hydrogen atoms which are susceptible to hydrogen pull reaction, that is, the polymer chains have carbon atoms each being bonded to one hydrogen atom, the dissociation energy of the bond between such a hydrogen atom and carbon- atom being not more than 80 Kcal/mol (335 kJ/mol).

Exemplified as substrate materials that meet condition (a) are: polybutadiene; polyisoprene; polypentadiene; double-or multiple-component copolymers which are produced by copolymerizing one diene compound such as butådiene, isoprene or pentadiene with comonomers such as styrene acrylic or methacrylic ester or acrylo- or methacrylonitrile; unsaturated polyesters; unsaturated polyepoxides; unsaturated polyamides; unsaturated acrylic resins; and internally cross-linked products of the above polymers.

Examples of substrate materials which meet condition (b) are: polystryene; polypropylene; polyvinyl chloride; polyvinyl carbazole; polyacrylonitrile; polyacrylic esters; polyvinyl acetate; copolymers of two or more monomers which are used for producing the above polymers; double- or multiple-component copolymers which are produced by using one or more of the above monomers and comonomers such as methacrylic ester, methacrylonitrile, butadiene and isoprene; internally cross-linked products of the above polymers; polyurethane; polyepoxide; polycarbonate; polyamide; polyester; polyacryl; and low density polyethylene. The polymer substrate may take its own shape and the oxygen permeability thereof should be low; however, there is no restriction on the configuration of the substrate. For reasons of workability, it is desirable that the curvature of the surface to be treated is low.

A suitable photopolymerizable composition for use in the method of the present invention is one which can be radically polymerized by the application of actinic rays and neither dissolves nor deforms the surface of polymer substrate.

More particularly, the photopolymerizable composition contains a radically polymerizable compound or compounds, and, if necessary, it contains photosensitizers, thickening agents, surface active agents, solvents and the like.

Exemplified as radically polymerizable compounds are maleic anhydride, acrylamide, methacrylamide, N-methyl acrylamide, N-methyl methacrylamide, Nethyl acrylamide, N-ethyl methacrylamide, N-propyl acrylamide, N-propyl meth acrylamide, N-butyl acrylamide, N-butyl methacrylamide, N-2hydroxyethyl acrylamide, N-2hydroxyethyl methacrylamide, N,Nmethylenehis(acrylamide), N,Nmethylenebis(methacrylamide), N-methylol acrylamide, acryl morpholine, methacryl morpholine, N-propyloxy acrylamide, N,Ndimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, diacetone acrylamide, acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, diethylene glycol monoacrylate, diethylene glycol monomethacrylate, triethylene glycol monoacrylate, triethylene glycol monomethacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate (mol. wt. of polyethylene glycol: not less than 170), 2hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3hydroxypropyl methacrylate, glycelol monoacrylate, glycelol monomethacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate (mol. wt. of polyethylene glycol: not less than 300), Nvinyl imidazole, vinylpyridine, Nvinylpiperidone, N-vinyl caprolactam, Nvinylpyrrolidone, 2-dimethylaminoethyl acrylate, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate, 2-diethylaminoethyl methacrylate, 2sulphoethyl acrylate, 2-sulphoethyl methacrylate, 3-sulphopropyl acrylate, 3sulphoprolyl methacrylate, p-styrene sulphonic acid, 2-phosphoric ethylene acrylate, 2-phosphoric ethylene methacrylate, 2 - phosphoric - 1 - chloromethylethylene acrylate and 2phosphoric - 1 - chloromethylethylene methacrylate. These compounds can be used solely or as a mixture of two or more.

In a method according to the present invention, well known photosensitizers can be used. For example, there are compounds that produce independently free radicals under the application of actinic rays such as benzoin ethers, azobisisobutyronitrile and thiuram compounds: those that produce free radicals by pulling active hydrogen atoms from other molecules such as benzophenone and acetophenone; and/or photo-redox compounds such as ferric chloride or dye-reduction compounds such as ascorbic acid. Of these photosensitizers those that produce free radicals by pulling active hydrogen atoms from other molecules are preferred.

In order to facilitate the removal of the unreacted photopolymerizable composition and the non-grafted polymer from the surface of polymer substrate, the reaction rate of the photopolymerizable composition may be controlled; however, it is more effective to use solvents in the photopolymerizable composition.

By means of a method of treatment in accordance with the present invention, surface defects of polymer substrates may be eliminated; in addition, the substrates may be more useful. For example, when the hydrophobic surface of a polyolefin is made hydrophilic by a method according to the present invention, the printability and adhesiveness thereof are greatly improved.

This may be of major, significance in industrial practice. As well as the above examples, the treated hydrophilic surfaces may have a further wide variety of uses.

The present invention will now be further described in detail with reference to several examples, which are by no means restrictive of the scope of the present invention.

Example I A photopolymerizable composition containing 2 mol/lit of acrylamide and 0.2 mol/lit of benzophenone in acetone was prepared. This composition was poured into a glass container with a bottom of Pyrex (Registered Trade Mark) glass plate with which a polypropylene (hereinafter referred to as "PP") film is in close contact; thus the PP film is in contact with the composition. After that, air in the glass container was replaced with nitrogen gas and the container was tightly closed. Then, 366 nm monochromatic light of 20 W/m2 intensity was applied through the bottom of the Pyrex glass plate and the PP film placed thereon. After the treatment, the film was rinsed with water and then with acetone, and it was further immersed in water for 24 hours or more.After drying, the infrared absorption (hereinafter referred to as "IRA") spectrum and the contact angle with water were measured, the results of which are shown in Table 1.

Example 2 A photopolymerizable composition containing 2 mol/lit of acrylamide and 0.05 mol/lit of benzophenone in acetone was prepared. A PP film was brought into contact with the above composition in a similar manner to Example I, and the air in the container was replaced with nitrogen gas. Then, in the open state, the film was irradiated for 10 minutes from the side that is not in contact with the composition by using a 3 KW ultra-high pressure mercury lamp placed at a 40 cm distance away. After the treatment, the film was rinsed with water and then with acetone, and the film was then immersed in water for more than 24 hours. After drying, the IRA spectrum and the contact angle with water were measured, the results of which are shown in Table 1.

Example 3 A photopolymerizable composition containing 2 mol/lit of acrylamide and 0.2 mol/lit of benzophenone in acetone was prepared. A low density polyethylene film was brought into contact with the above composition in a similar manner to Example 1. Air in the container was replaced with nitrogen gas and the container was closed tightly. The film was irradiated for 30 minutes, from the side which was not in contact with the treating solution, by using a 200 W medium-pressure mercury lamp placed at a 20 cm distance away. After the treatment, the film was rinsed with water and then with acetone, and the film was then immersed in water for more than 24 hours. After drying, the IRA spectrum and the contact angle with water were measured, the results of which are shown in Table 1.

Example 4 A photopolymerizable composition was prepared by dissolving 2 mol/lit of acrylic acid and 0.005 mol/lit of benzophenone in acetone. This composition was brought into contact with a PP film in a similar manner to Example 1 and the procedure just like that of Example 2 was then carried out. After that, the IRA spectrum and the contact angle with water were measured, the results of which are shown in Table 1.

Example 5 A photopolymerizable composition was prepared by dissolving 1.4 kg of 2hydroxyethyl acrylate, 0.3 kg of polyvinylpyrrolidone (average mol. wt: about 10,000), 20 g of benzophenone and 2 g of diethanol amine in 8.3 kg of methyl alcohol. The viscosity of this composition was 30 centipoise at 250 C. A rolled polyvinyl chloride film (trademark: "Vinyfoil" made by Mitsubishi Plastics Industries Limited) of 100 microns in thickness and 60 cm in width was applied with a 25 micron thick layer of the photopolymerizable composition at the rate of 10 m/min with a roller coater.

Continuously and at the same rate, the film was irradiated with actinic rays through the polyvinyl chloride film by using a 6 kW high-pressure mercury lamp that was placed at a 30 cm distance away. The photopolymerizable composition was uniformly applied to the polyvinyl chloride film and the composition did not drip from the film surface during the light irradiation.

After the irradiation, the film was rinsed with water. The contact angle with water of untreated film was 90+2 degrees, while that of the treated film was 42+4 degrees and the effect of the treatment was uniform all over the surface.

Example 6 A photopolymerizable composition was prepared by dissolving 1.4 kg of maleic anhydride into 8.2 kg of acetone. A 1,2polybutadiene film (trademark: "Neotop" made by Japan Synthetic Rubber Co., Ltd.) was brought into contact with the above photopolymerizable composition in a similar manner to Example 1. After that, in the open state, the film was irradiated for 30 minutes from the side which was not in contact with the treating composition by using a 100 W highpressure mercury lamp placed at a 40 cm distance away. After this treatment, the film was rinsed with water and then with acetone and was then immersed in water for more than 24 hours.

The film was then dried.

The contact angle with water of untreated film was 80+2 degrees, while that of the treated film was 57+4 degrees and the effect of the treatment was quite even.

Example 7 A photopolymerizable composition was prepared by dissolving 142 g of acrylamide and 18 g of benzophenone into 840 g of methyl alcohol. The photopolymerizable composition was placed on the surface of a polyvinyl chloride film (size: 200 mmx200 mmx0.05 mm, trademark: "Vinyfoil" made by Mitsubishi Plastics Industries Limited), then a polyester film (size: 200 mmx200 mmx0.l mm, trademark:FR-PET B 3030 made by Teijin Limited) was slowly put over the photopolymerizable composition. The thickness of the photopolymerizable composition became 3 to 6 microns. Actinic rays were applied from the side of the polyvinyl chloride film for certain time lengths (indicated in Table 1) by using a 400 W high-pressure mercury lamp placed at a 30 cm distance away. After the removal of the polyester film, the photo-grafted surface of the polyvinyl chloride film was rinsed with methyl alcohol and water. The contact angles with water of the treated surfaces were then measured, the results of which are shown in Table 1. The treated films were immersed in ethyl ether for 24 hours and IRA spectrums were measured. the results of which are also shown in Table 1.

Example 8 A photopolymerizable composition was prepared by dissolving 142 g of acrylamide, 18 g of benzophenone, and 20 g of polyacryloylethyl phosphate (average mol.

wt: about 10,000 in 820 g of acetone. The viscosity of this composition was 180 centipoises at 250C. A polypropylene film (thickness: 50 microns, width: 60 cm, trademark: "Polypro F 600" made by Mitsui Petrochemical Industries, Ltd.) was coated with a 50 micron thick layer of the above photopolymerizable composition with a roller coater. Another polypropylene film, the same as the above one, was brought into close contact with the coated surface. 5 pairs of 400 W high-pressure mercury lamps were placed side-by-side. For each pair, the mercury lamps were opposed at a 40 cm distance apart. Between these mercury lamps, the above layers of films were pass at the rates of 0.5 m/min and 0.2 m/min, and thereby irradiated with actinic rays. Then, the two sheets of films were continuously separated and the treated surfaces were continuously rinsed with water.The contact angles with water and the IRA spectrums of the treated film surfaces were measured, the results of which are shown in the following Table 1.

TABLE 1

(*2) Contact Angle/water Condition (*1) Test Number for Infrared Treated Untreated Irradiation Absorbance Film Film Example 1 -- 0.85 38 101" Example 2 0.21 41" 101" Example 3 -- 0.73 45" 96" Example 4 0.27(**) 69" 1010 6 min 1 0.68 332" 92" Example 7 30 min 1.3 30--32" 92" 0.5 m/min 0.43 36--38" 101" Exampie 8 0.2 m/min 1.2 1 36--38" 101" Comparative Example 1 -- Trace 81" 101" Notes: ('1) Infrared Absorbance: The absorption of the amide carbonyl group of acrylamide near 1663 cm-l was measured. (**). The absorption of carboxylic acid near 1720 cm was measured.

('2) Contact Angle: 10 ylit of distilled water were dropped on the film surface and, after 1 minute, the angle between the surface of a water droplet and the film surface was measured with a microscope (Precision contact angle meter made by Kyowa Kagaku K. K., Model CA-D).

It will be understood from the results of the tests in the above examples that the effects and advantages of the present invention are desirable.

Although the present invention has been described in connection with preferred examples thereof, many variations and modifications will now become apparent to those skilled in the art. The present invention is not limited by the specific disclosure herein, but only the appended

Claims (7)

claims. WHAT WE CLAIM IS:

1. In a method of giving hydrophilic property to a surface of a hydrophobic polymer substrate, the step of photografting by the application of actinic rays a radically polymerizable compound, which compound, when polymerized, is hydrophilic, to a surface of the substrate, the actinic rays being applied from aside of the substrate remote from the compound, the radically polymerizable compound being contained in a photopolymerizable composition, which is in contact with the hydrophobic polymer substrate.

2. A method according to Claim 1 wherein the viscosity of the photopolymerizable composition is in the range of from 10 to 1,000 centipoises before being applied to the hydrophobic polymer substrate.

3. A method according to Claim 1 or 2, wherein a solid shielding material is placed over the surface of the layer of the photopolymerizable composition which is in contact with the hydrophobic polymer substrate.

4. A method according to Claim 1, 2 or 3, wherein two sheets of hydrophobic polymer substrates are arranged in layers with the photopolymerizable composition in between the sheets.

5. A method according to any one of Claims 1 to 4, wherein transparent material is interposed between the source of actinic radiation and the rear of the polymer substrate.

6. A method of giving hydrophilic property to a surface of a hydrophobic polymer substrate, substantially as described in any of the foregoing Examples.

7. A hydrophobic polymer substrate provided with a hydrophilic surface by the method of any one of the preceding claims.