Process for Producing Molding Product

1. Field of the Invention

This invention relates to a process for producing a molded product. More particularly, it relates to a process for producing a molded product of a good appearance having a good moldability, a good luster and a superior uniformity of a product appearance.

2. Description of the Related Art

The appearance of conventional hollow vessels and multi-layer sheets has been still insufficient to satisfy the needs of markets. Thus, improvement in the surface appearance, particularly, notable improvement in the luster and the uniformity of the surface have become a very serious problem. Polypropylene resins have a number of superior characteristics such as stiffness, impact strength, heat resistance, hygienic properties, resistance to steam permeability, moldability, etc.; hence the resins have been very often used for producing hollow vessels according to a blow molding, and multi-layer sheets according to a multi-layer molding. On the other hand, however, the resins have drawbacks of being inferior in the gas barrier properties against oxygen, nitrogen, CO.sub.2 gas, etc. Thus, for the use applications of food vessels for soft drinks, flavoring materials, edible oils, etc., hollow vessels and multi-layer sheets wherein polyolefin resins are used as the inner and outer layers and resins having good gas-barrier properties such as ethylene-polyvinyl alcohol copolymer, nylon, etc. are used as the intermediate layer, have been used in a large quantity.

As a countermeasure for solving such a problem, for example, there has been known a process of using random ethylene-propylene copolymer as the outermost surface layer of a multi-layer vessel to improve the surface luster, as disclosed in Japanese patent application laid-open No. Hei 2-215529, but in the case of hollow molding, since degassing of air in the mold is insufficient; hence the luster is insufficient and luster unevenness occurs. Thus, a problem has been raised that the commodity value cannot be sufficient and the use applications are limited.

The object of the present invention is to provide a process for easily producing a hollow vessel or a multi-layer sheet having notably improved the insufficient luster and luster unevenness which have been drawbacks of conventional molded products, and having an good appearance by which the commodity value has been notably enhanced, while keeping various superior properties alive.

The present inventors have made extensive research in order to obtain a hollow vessel and a sheet having overcome the drawbacks of conventional process and having a superior surface luster, and as a result, have found that when the outer surface of a specified multi-layer molded product is heat-treated into a molten state, the resulting product has a far superior appearance.

The present invention has the following aspects (1) to (5):

(1) In a process for producing a hollow or sheet product according to a hollow or sheet molding process, the improvement is in that said molded product is a multi-layer molded product having a surface layer (A) comprising a propylene random polymer of a melting point of 100.degree. to 150.degree. C. and having a thickness of 1 to 20% of the thickness of said molded product, and a substrate layer (B) comprising a thermoplastic resin of a melting point higher than that of said propylene random polymer, and said surface layer (A) is heat-treated into a molten state, followed by immediately being cooled to form a smooth surface.

(2) A process for producing a molded product according to item (1), wherein the difference between the melting point of the surface layer (A) and that of the substrate layer (B) is 10.degree. to 50.degree. C.

(3) A process for producing a molded product according to item (1), wherein the thermoplastic resin is selected from the group consisting of polypropylene, polyethylene, polyester and polyamide and the propylene random polymer is an ethylene-propylene random copolymer and/or an ethylene-propylene-butene-1 random terpolymer, each of a melting point of 120.degree. to 145.degree. C.

(4) A process for producing a molded product according to item (1), wherein the heat treatment is carried out according to hot-air heating, irradiation heating or flame heating.

(5) A process for producing a molded product according to item (5), wherein a hot air of a temperature higher by 10.degree. to 50.degree. C. than the melting point of said surface layer (A) of the molded product is blown onto the surface of the molded product for 25 to 120 seconds.

The propylene random polymer used in the surface layer (A) of the molded product of the present invention has a melting point of 100.degree. to 150.degree. C. preferably 120.degree. to 145.degree. C., and preferably a melt flow rate of 0.5 to 50 g/10 min at 230.degree. C. If the melting point of the propylene random polymer exceeds 150.degree. C., it is difficult to afford a desired temperature difference between the temperatures of the surface layer resin and the substrate layer resin and a sufficient luster cannot be obtained. If the melting point is lower than 100.degree. C., the molded product is difficult to release from the mold after molding, and the increase in the percentage of rejects and the reduction in the productivity are brought about; hence the aim of the present invention cannot be achieved.

The melting point referred to herein means the peak temperature on the endothermic curve measured by means of a scanning type differential calorimeter when the temperature of a sample (10 mg) is raised in introgen atmosphere at a rate of 200.degree. C./min. The peak temperature is appeared by the fusion of crystals of the sample.

The propylene random polymer is obtained by copolymerizing propylene as a main component with .alpha.-olefin such as ethylene, butene-1, etc. as comonomer components. The propylene random polymer is a known polymer and can be used as far as the melting point thereof is specified. In the case ethylene-propylene random copolymer, if the ethylene content exceeds 2% by weight, the resulting copolymer has a melting point of about 150.degree. C. or lower. Therefore the ethylene content is preferably 2 to 8% by weight. Further, the propylene random copolymer may be a random propylene terpolymer of propylene, ethylene and butene-1.

Further, the surface layer (A) may be of a blend of the propylene random polymer with an isotactic polypropylene or a crystalline propylene-ethylene copolymer containing ethylene which content is of 2% by weight or less, or a blend of the propylene random polymer with a linear low density polyethylene, a low density polyethylene or a high density polyethylene which content is in the range wherein the object of the present invention is not damaged. Further, various known additives such as an antioxidant, a weathering agent, an ultraviolet light abosrber, an antistatic agent, a coloring agent, filler, hollow filler, or an organic nucleating agent, an inorganic nucleating agent such as talc, etc., may be added to the above components. Further, various known thermoplastic resins such as hydrogenated petroleum resin, elastomers, etc. may be added to the above components in a range wherein the object of the present invention is not damaged.

The surface layer (A) containing the above-mentioned propylene random polymer has a surface layer thickness of 1 to 20% based upon the product thickness. If the surface layer thickness exceeds 20% of the product thickness, this has a bad influence upon the intrinsic properties of the product. To the contrary, if the thickness is less than 1% or less than 10 .mu.m, the surface layer is too thin so that the influence of the substrate layer (B) appears and hence the object of the good appearance of the present invention cannot be achieved. The thickness of the molded product is preferably in the range of 200 to 2,000 .mu.m, although it is not limited thereto as far as the object of the present invention is not damaged.

The difference between the melting point of the surface layer (A) and that of the substrate layer (B) is preferably 10.degree. to 50.degree. C., more preferably 20.degree. to 50.degree. C.

The substrate layer (B) of the present invention consists of a thermoplastic resin having a melting point higher than that of the propylene random polymer used for the surface layer (A), and the difference of its melting point from that of the surface layer (A) is preferably 10.degree. C. or higher, more preferably 20.degree. C. or higher. As the thermoplastic resin used for the substrate layer, polyolefins such as polypropylene, polyethylene, etc., polyesters such as polyethylene terephthalate, PBT, etc., and polyamides such as 6 nylon, 66 nylon, etc. are usable without any limitation.

It is also possible for the multi-layer molded product of the present invention to use besides the surface layer (A) and the substrate layer (B), an intermediate layer of a polyolefin resin, a polyolefin elastomer, a gas barrier resin, etc. or an adhesive layer for improving the adhesion between the surface layer and the substrate layer, each in a range wherein the aim of the present invention is not damaged.

The multi-layer molded product of the present invention can be obtained according to a multi-layer blow molding process employing several screws for extruding the respective components, or a similar multi-layer sheet molding process. Among these processes, a multi-layer direct blow molding process and a multi-layer T-die molding process are suitable.

The heat-treatment of the present invention refers to heat-treatment of only the outer surface of the multi-layer molded product into a molten state. As a kind of heating, hot gas heating, irradiation heating, flame heating, etc. are used. By cooling just after the heating, a smooth surface may be formed. It is preferred that only the specified surface layer (A) is rendered to heat-treatment into a molten state without deforming the substrate layer (B) . At high temperatures, the treatment can be carried out in a short time. For example, when the heat treatment is carried out with hot gas, a hot gas temperature higher by 10.degree. to 50.degree. C. than the melting point of the surface layer (A) of the molded product for 10 to 120 seconds, and particularly, when the melting point of the surface layer (A) is 126.degree. C. a hot gas temperature of 150.degree. C. for 20 to 35 seconds, and that of 170.degree. C. for 10 to 20 seconds, and further that of 190.degree. C. for 3 to 8 seconds, can be employed.

In the flame heating, a flame such as that of propane gas, etc. is used. The flame heating is carried out till the surface is visually brought into a molten state, and it is distinct from an instantaneous flame heating which has so far been carried out in printing, wherein the surface is not brought into a molten state. At to the flame heating, heating at 800.degree. C. to 1,200.degree. C. for 0.02 to 2 seconds is general, and at about 1,000.degree. C., heating for about 0.1 to 0.5 second is preferred.

As the other heating methods, heated oven type, continuously heating type using a belt conveyor, and irradiation heating using a far infrated rays heating, an infrared rays lamp, etc. can be employed. In the case of irradiation heating, heating is carried out up to a temperature at which the surface layer is brought into a molten state, when the temperature is detected by means of a surface thermometer or the molten state is observed visually. It is preferred for the heating to be carried out uniformly, so that products are rotated in an oven and temperature is adjusted precisely.

The present invention will be described in more detail by way of Examples and Comparative examples, but it should not be construed to be limited thereto.

The evaluation methods of the characteristics employed in Examples and Comparative examples were carried out according to the following methods:

(1) Melting point (abbreviated to m.p. in the examples):

A sample (10 mg) was raised in its temperature starting from room temperature, at a velocity of 20.degree. C./min., in nitrogen gas, by means of a scanning type differential calorimeter. The melting point is measured by the peak temperature (unit: .degree. C.) of the endothermic curve due to the fusion of a crystal.

(2) Melt flow rate at 230.degree. C. (abbreviated to MFR-1 in Table) was measured according to the testing method of 10 JIS K 7210 (1976) (230.degree. C., 2.16 Kgf).

(3) Percentage of luster was measured according to the testing method of JIS K 7105 (1981) (60.degree. mirror surface, percentage of luster) (unit: %).

A propylene random polymer of m.p. shown in Table 1 as a surface layer material was fed into an extruder for the surface layer, and a polypropylene resin (melt flow rate: 1.5, m.p.: 160.degree. C.) as an inner layer was fed into an extruder for the inner layer), followed by extruding a double layer molten parison at an extrusion diameter of 210.degree. C., and subjecting the parison to blow molding in a blow mold of a temperature of 30.degree. C., to obtain a bottle having an average thickness of 700 .mu.m at its shell part and an inside volume of 500 cc. The thickness of the outer layer was 100 .mu.m and that of the inner layer was 600 .mu.m. This multi-layer hollow vessel was heat-treated in an oven at 150.degree. C. and 170.degree. C., respectively for 25 sec. and 120 sec. and the characteristics of the thus treated products were compared with those of untreated product. The results are shown in Table 1.

As apparent from Table 1, it is seen that the multi-layer hollow vessels of Examples 1 to 4, obtained by the present invention have a uniform and good luster as compared with those of the products of Comparative examples 1 to 4.

A polypropylene resin having a melting point indicated in Table 2, as a surface layer, was fed to an extruder for a surface layer, and an ethylene-propylene block copolymer having a melt-flow rate of 0.7 g and a melting point of 163.degree. C., as a resin for an inner layer, was fed to an extruder for a substrate material, followed by molding a double layer sheet of 300 mm wide and 0.8 mm thick, at an extrusion temperature of 220.degree. C., using a cooling roll (polishing roll) at 40.degree. C. Any of the thickness of the surface layers have been made about 50 to 80 .mu.m. The double layer sheets were heat-treated in an oven under heat conditions indicated in Table 2, and the resulting characteristics are shown in Table 2.

As apparent from this Table, it is seen that melt-layer sheets of Examples 5 to 7 obtained by the heat treatment of the present invention have a uniform and good luster as compared with those of Comparative examples 4 and 5.

Melting point of propylene homopolymer or copolymer is 130.degree. to 165.degree. C. and its decomposition temperature is 330.degree. to 410.degree. C., depending upon the kind of copolymer type. Thus, herein using a propylene-ethylene random copolymer of a melting point of 145.degree. C., a hollow molded vessel was produced according to a conventional process for producing a hollow vessel, wherein a molten resin was expanded in a mold of a cylindrical shape by means of an air pressure or the like to subject it to close contact to the mold and at the same time to be cooled. The vessel had a percentage of luster of 33%. It was placed on a belt conveyor and heat-treated at a temperature of 950.degree. C. for 0.2 sec. using upper and lower burners of each 30 cm long provided on both of the belt conveyor and just thereafter allowed to cool down to room temperature. After cooling, the vessel was cut and its cross-section was observed by a microscope. As a result, a smooth surface layer was formed and the percentage of luster of the vessel was 66%.

Further, when the surface of the same vessel was subjected to instantaneous flame heating so as to form no molten state (Comparative example 6), the percentage of luster of the product was 36%.

On the other hand, a conventional vessel of a double layer structure composed of an outer layer of a low density polyethylene and an inner layer of a high density polyethylene was similarly molded (Comparative example 7). This vessel was not heat-treated. When the vessel was cut and its cross-section was observed by a microscope. As a result, the surface had projections and depressions, and there was no luster. The percentage of luster of the vessel was 17%.

While conventional hollow or sheet molded products are insufficient in the luster, inferior in the uniformity and cannot satisfy a requirement of a good appearance of molded products in the market. The molded products of the present invention are superior in the luster of the products as compared with conventional products, very uniform in the luster and superior in the appearance. Hence the products are very useful for food vessels and general industry.