Process to improve the adhesive properties of poly-olefin products

Abstract: The adhesive properties of polyolefin products are improved by submitting said products to a surface modification process which includes the following steps: A) application of a layer of one or more photosensitive materials selected from: 1) chloroparaffin containing 5 to 80 wt% of chlorine and having an average molecular weight greater than 200; 2) vinyl chloride homopolymers and copolymers, containing 25 to 80 wt% of chlorine and K values between 40 and 60; 3) vinylidene chloride homopolymers and copolymers; B) surface exposure to ultraviolet radiations. The product treated in these manners are ready for painting or coating with various materials.

Description: This invention refers to a process of surface modification of polyolefin products designed to improve the adhesive properties of the product, to coating materials and in particular to paints. It is already known to use materials, like paints, for coating polyolefin products. For instance, US-A-2 961 346 describes polyethylene structures coated with compositions containing chlorinated paraffins. Coatings of sheets of regenerated cellulose or polyethylene are also disclosed in US-A-3 076 723. Said sheets are treated with an aqueous dispersion comprising a vinylidene chloride copolymer and a dispersing compound capable of absorbing U.V. light such as alkali metal and ammonium salts of sulfonic acids. The U.V. light-absorbing compound is a mean for measuring coating thickness. A coating is also described in US-A-3 551 235. The compositions used for coating contain halogenated hydrocarbons as photoinitiators; these photoinitiators are capable of initiating polymerization of free radical polymerizable monomers contained in the compositions applied to polyethylene structures. It is also well-known that the adhesiveness of coating materials to polyolefin products is not usually good, hence one needs to modify the surface with suitable treatments. In fact, the polyolefin materials do not contain polar groups for a physical adhesion and have an elevated degree of inertia to chemical reactions with other polymers or monomers as well as with solvents. Various solutions are already known in the business to obtain the adhesion of various materials to the surface of polyolefins without considerable impact to the chemical and mechanical properties of the polyolefins themselves. Significant examples of broad application processes are the surface flame treatment with an eventual application of primers, or the plasma treatment or corona discharge, or the application of oxidizing solutions on the surface, like for instance K 2 Cr 2 O 7 in H 2 SO 4 solutions, or the use of photoactivators or photosensitive materials. For instance in the processes for coating exemplified in US-A-4 121 006 the surface of polyolefin films is subjected to the corona discharge treatment; subsequently the films are coated with an undercoat layer of a mixture of a vinylidene chloride polymer and a chlorinated polypropylene, and a top coat layer of a vinylidene chloride copolymer. In the US-A-3 764 370 it is described in particular the process for the surface modification of polyolefin products which include the application of a diluted solution of a photoactivator especially selected among the multiple rings ketones and hydrocarbons, the drying, and the exposure to ultraviolet radiations; specific examples of compounds of the kind mentioned above are benzophenone and methyl ethyl ketone. However, the adhesion of paints and various coating materials to surfaces treated by this method are not satisfactory. As described in US-A-2 873 241, certain chlorinated hydrocarbons like methylene chloride, trichloroethylene, and chloroform can also be used in this type of surface treatment, but the results as far as adhesion of coating materials to the polyolefins is concerned are also very inferior. According to another procedure, described in US-A-4 303 697, the surface of polyolefin products is prepared for painting with a treatment which includes the coating with a thin layer of a photosensitive material consisting of chlorinated polypropylene, containing from about 5 to 25 wt% of chlorine and having an average numeral molecular weight between 5,000 and 50,000, and consequent exposure to ultraviolet radiations. Whereas in US-A-4 129 676 the surface of polyolefin products is prepared for coating with a polyvinyl film by a treatment which includes the coating with a layer of a primer composition containing a chlorinated hydrocarbon and resin material, such as a chlorinated paraffin wax. Each one of these solutions is satisfactory for some applicatins, but not for all, because the different composition of the various polyolefins in some cases, or the form of the products in other, can be critical factors for the efficiency or uniformity of the treatment. These problems are particularly critical in case of products which require a high degree of adhesion when they are submitted to thermal shocks and exposed to high levels of humidity. Therefore, there is a definite need for a process which will give the greatest possible reliability and versatility in this concern. This object can be obtained with high levels of adhesion for paints and other materials to polyolefin products having a complex shape, by using a process of surface modification comprising the following operations: A) application on the product surface of a layer of one or more photosensitive materials applied as a diluted solution from 0.5 to 5 weight-% in aromatic or aliphatic hydrocarbon solvents or tetrahydrofurane, provided that said aromatic or aliphatic hydrocarbons are not chlorinated, said one or more photosensitive materials consisting only of a material selected among: 1) chloroparaffins containing 5 to 80 wt% of chlorine and having an average molecular weight greater than 200; 2) polyvinyl chloride, optionally postchlorinated, and vinyl chloride copolymers, both types pf polymeric materials having a chlorine content between 25 and 80 wt%, and K values between 40 and 60; and 3) vinylidene chloride homopolymers and copolymers; and B) Exposure of the surface to ultraviolet radiations. Excellent results can be obtained with regard to the surface adhesion of paints and other materials on polyolefin products by using the above mentioned process. The polyolefins suitable of use in manufacturing products include the crystalline polyolefins such as polypropylene, polyethylene, partially crystalline copolymers of propylene with ethylene, butene or other alpha olefins, or with dienes, the latter cross-linked if necessary, and amorphous olefinic polymers such as copolymers of ethylene with propylene or other alpha olefins containing, if needed, small amounts of dienes. The polypropylene is preferably constituted of macromolecules having prevailingly isotactic structure. Specifically, the polypropylene with high isotactic index can be a homopolymer of a compositions obtained by sequential polymerization of the propylene, and mixtures of propylene and ethylene or other olefins, with stereospecific catalysts. Such polymers can be mixed with each other or, as alloys, with other polymers; can contain various types of mineral fillers such as talc, calcium carbonate, micas; reinforcing charges, like glass fibers, mineral, metallic or polymeric fibers, and dyes. Photosensitive materials are applied according to known methods; they are used in diluted solution, from 0.5 to 5 wt%, particularly from 1 to 2 wt% in the above solvents, also mixed with each other. Said solutions may be applied to the surface to be treated by spraying, for example, with a micronized jet or a spray gun, or by immersing the product directly in the solutions. If necessary the surface to be treated may be washed, with water and detergents or organic solvents, and dried before the application of the photosensitive material. Generally the thickness of the layer of photosensitive material is less than 1-2 µm ; preferably between 0.2 - 0.4 µm. Examples of chloroparaffins, among the photosensitive materials, which can be used according to the present invention, are some product known by the following trade names: Cloparin® (produced by Caffaro), Cereclor® (ICI Ltd.), Chlorowax (Diamong Shamrock Corporation). As previously stated, such chloroparaffins contain 5 to 80 wt% of chlorine, preferably 30 to 70%, and their average molecular weight is greater than 200, preferably between 400 and 2000. Specific examples of chloroparaffins which gave good results, are the Cloparin® S70, which has an average molecular weight of 1035, and a chlorine titer of about 70%, the Cereclor® 70 and 70L, for which are given respectfully the average empirical formulas of C 24 H 29 Cl 21 and C 12 H 15 Cl 11 , and the Chlorowax® 70S. Examples of vinyl chloride copolymers are the copolymers with vinyl acetate or other monomers, such as methyl methacrylate and vinyl alcohol. The vinyl chloride homopolymers and copolymers may also be used in mixtures with small quantities (up to 10% by wt for instance) of other polymers, such as methacrylate/butadiene/styrene copolymers. Examples of vinylidene chloride copolymers are the vinylidene chloride/acrylonitrile copolymers. Specific examples of vinylidene chloride homopolymers and copolymers with vinyl chloride or acrylonitrile are the products known as Saran®, by Dow Chemical Company. Before submitting the above mentioned photosentitive materials to ultraviolet radiations, the excess solvents used for the application of such materials as a diluted solution, have to be removed by spontaneous evaporation at room temperature, or forced evaporation with ventilation and/or heating. The products treated in such manner are then submitted to ultraviolet radiations using lamps having an emission band preferably between 200 and 400 nm, like for instance the mercury vapors lamps. The surface treated with photosensitive materials must receive, through ultraviolet radiations, enough energy to have a high degree of photochemical conversion of the photosensitive material, without triggering perceptible degradation phenomena of the underlying polyolefin. The optimum radiation dose depends essentially on the type and quantity of the photosensitive material applied to the surface of the product which has to be treated. As an example, when the photosensitive material is applied with a 2% wt solution, the optimum radiation dose may be between 0.2 and 5 J/cm 2 . The products treated in such manner can easily be painted, glued or coated with various materials. The process described above produces, in the polyolefin products, a surface modification characterized, among other things, by a noticeable stability at normal environmental conditions and may be washed with nonaggressive solvents. The painting and application of coating materials, threfore, may be done at a much later date. In the case of painting, there is also a high resistance to "blistering", or, in other words, to the formation of small bubbles on the paint film following a prolonged contact of the painted piece with water or steam. The following examples are given in order to demonstrate, but not limit, the present invention. EXAMPLES 1-9 Used as specimen are strips 20 cm long, 7 cm wide and 0.3 cm thick, obtained by injection molding at a temperature of 250°C, of Moplen® SP 179 polypropylene (examples 1-4) and Moplen® SP 151 polypropylene (examples 6-9) (shock-resistant polypropylenes modified with ethylene/propylene rubber, products of HIMONT Italia S.p.A.). The melt index values, flexural rigidity and Izod resiliance, are respectively 8 g/10 min, 1000 MPa and 500 J/m for the Moplen® SF 179, and 3.5 g/10 min, 750 MPa and 500 J/m for the Moplen® SP 151. The strips are immersed in a solution of Cloparin® 70 in xylene for 4 seconds. The strips are then taken out of the solution, dried at room temperature for about 10 minutes, and consequently irradiated perpendicularly to the surface with an ultraviole lamp located at a distance of 20 cm, making two or more passes at a speed of 3 meters per minute. A lamp made by Societa Aermeccanica Giardina I.S.T. type 200/l mod., 20 cm. long is used for the irradiation. The Cloparin® S 70 concentrations in xylene solutions and the number of passes under the ultraviolet lamp are shown in Table I for each example. After irradiation, the specimens are painted using polyacrylic bicomponent paints with isocyanic hardener. The following tests are made on the painted specimens: Paint adhesion The measure of adhesion of the paint is made by determining the maximum charge necessary to obtain the detaching of the paint from the polymer on a round area 2 cm. in diameter. For this purpose the base of a small steel cylinder is glued to the painted surface and an adequate tensile stress is applied in the direction of the axis of the cylinder. The material used for gluing the cylinder is the bicomponent epoxy resin "Acciaio Rapido" made by Bostick. Resistance to blistering By "Blistering" is meant the formation of small bubbles on a paint film caused by prolonged contact of the painted pieces with water (liquid or steam). The specimen is immersed in distilled water at 55 o C and the time it takes for the first blister to be seen with the naked eye is recorded. The test can be interrupted and the results considered positive after 200 hours. The resistance to blistering of the painted pieces is considered of maximum importance for the application on automotive and motorcycles exterior body parts. The results of the paint's adhesion and resistance to blistering tests are reproduced for each example in Table I. Ex No. Concentration Cloparin® S70 (% by wt) Number of passes Adhesion (kg) Blistering (hours) 1 1 2 165 > 200 2 1 4 220 > 200 3 2 2 190 >200 4 2 4 220 > 200 5 2 4 115 > 200 6 2 8 130 > 200 7 4 2 100 > 200 8 4 4 145 > 200 9 4 8 150 > 200 EXAMPLES 10 AND 11 The specimens will be of the same type as those for examples 1-4. Said specimens will be sumbitted to surface treatment through an application of photosensitive materials and consequently will be exposed to ultraviolet rays using the same operating methods of examples 1-9. The photosensitive materials used are polyvinyl chloride AC 404/D made by TPV of Argenta (Ferrara) (example 10) and the vinylidene chloride/acrylonitrile Saran® F 310 E made by Dow Chemical Company (example 11) in a 2% tetrahydrofurane solution. The specimen treated in this manner are painted with bicomponent polyacrylic paints with isocyanic hardener. The number of passes under the ultraviolet lamp, the results of the paint adhesion tests and the resistance to blistering are shown, for each example, in Table II. Ex. No. Number of passes Adhesion (kg) Blistering (hours) 10 4 110 > 200 11 2 190 > 200 Comparison example 1 The same procedure of example 1 is followed using, as photosensitive material, the chlorinated polyolefin CP-313-3, marketed by Eastman Chemical International Company. The results of the adhesion and the blister resistance tests are respectively 65 kg and 50 hours. EXAMPLES 12-16 The specimens used are of the same type as per example 1, they are treated with photosensitive material and submitted to ultraviolet radiations according to methods described in example 1, with the difference that the specimen are kept still under the lamp for an exposure period of a few seconds. In this case, therefore, the quantity of energy received by the specimen through ultravioled radiation can be estimated based on the exposure times instead of the number of passes under the lamp. The photosensitive materials are used in a xylene solution. The specimens treated in this manner are then coated, by spraying with a spray gun, with bicomponent polyurethane adhesive produced by the National Starch and Chemical GmbH, which consist of a polyurethane base and an isocyanic hardener. Adhesion tests of the adhesive to the polyolefin are made using the same method described in examples 1-9 for the paint adhesion tests. In this case the metallic cylinder is glued to the adhesive layer, still using the epoxy resin "Acciaio Rapido" made by Bostick. These tests are made 24 hours after spraying and annealing in a hygrostatic cell at 90°C for 24 hrs. In Table III are shown, for each example, the type and concentration in solution of the photosensitive materials used, the time of exposure to ultraviolet radiations, the adhesive used, with weight ratio between the polyurethane base and isocyanic hardener specified in parenthesis, as well as the results of the adhesion tests. In the case of the adhesion tests 24 hrs after spraying, the maximum charge necessary to have the separation of the adhesive from the polyolefin must be considered greater than the values reported in Table III, because at such charge values is when the separation between the metallic cylinder and the layer of adhesive occurs. EXAMPLE 17 A specimen of the same type as the one in example 1 is used. This specimen is treated with a 2% Saran® F 310 E xylene solution, and exposed to ultraviolet radiations with the same operative method as per example 15. The specimen thus treated is spray coated with the polyurethane adhesive Sestriere, manufactured by Societa Sestriere di Torino. On the adhesive layer is applied, by heat compression, a multilayered PVC film 2 mm in diameter, previously heated at 80°C. After 24 hours of conditioning at room temperature a right angle peeling test is made applying traction to the PVC film at a speed of one cm per minute on a separation front of 5 cm. The cohesive breakage of the PVC film is detected at the unit charge of 17 daN/cm (17 kg/cm), without separation of the film from the support.

Claims: A process for surface modification of polyolefin products comprising the following steps: A) application on the product surface of a layer of one or more photosensitive materials applied as a diluted solution from 0.5 to 5 weight-% in aromatic or aliphatic hydrocarbon solvents or tetrahydrofurane, provided that said aromatic or aliphatic hydrocarbons are not chlorinated, said one or more photosensitive materials consisting only of a material selected among: 1) chloroparaffins containing 5 to 80 wt% of chlorine and having an average molecular weight greater than 200; 2) polyvinyl chloride, optionally postchlorinated, and vinyl chloride copolymers, both types of polymeric materials having a chlorine content between 25 and 80 wt%, and K values between 40 and 60; and 3) vinylidene chloride homopolymers and copolymers; and B) exposure of the surface to ultraviolet radiations. The process of claim 1 wherein the polyolefins in the products are selected among polypropylene with high isotactic index, polyethylene, partially crystalline propylene copolymers with ethylene, butene and other alpha olefins or with dienes, amorphous propylene copolymers with ethylene, and, optionally, with smaller proportions of a diene, and their mixtures. The process of claim 1 wherein the product surface is washed and dried before applying the photosensitive materials. The process of claim 1 wherein the exposure to ultraviolet radiations is done with a lamp having an emission band between 200 and 400 nm. The process of claim 1 wherein the chloroparaffins have a molecular weight between 400 and 2000. The process of claim 6 wherein the chloroparaffins have a chlorine weight content of about 70%. The process of claim 1 wherein the vinyl chloride copolymers are selected among vinyl chloride/vinyl acetate, vinyl chloride/methyl methacrylate, vinyl chloride/vinyl alcohol copolymers. The process of claim 1 wherein the vinyl chloride homopolymers and copolymers contain up to 10% wt of methacrylate/butadiene/styrene copolymer. The process of claim 1 wherein a vinylidene chloride/acrylonitrile copolymer is used as the vinyledene chloride copolymer.

Inventors: CECCHIN GIULIANO, POLATO FELICE, CECCHIN, GIULIANO, POLATO, FELICE
Applicants: MONTELL NORTH AMERICA INC, MONTELL NORTH AMERICA INC.
Application year: 1989
Main classification: C08J7
IPC codes: C08J7