Blow-Molded Articles for Automobile Exterior Parts and the Process Therefor

The present application relies for priority upon the inventors' Japanese Patent Application No. 06-258204 filed Oct. 24, 1994, the content of which is herein incorporated by reference.

1. Field of the Invention

The present invention relates to blow-molded articles, especially automobile exterior parts, which comprise a base layer and a surface layer and which require no surface coating and the process for forming such parts. The invention is particularly suited for blow-molded automobile exterior parts which require a good surface gloss as well as high strength, such as spoilers, bumpers, side moldings, etc.

2. Description of Related Art

The present invention and the related art are described below with reference to a spoiler as a typical example of a blow-molded article for an automobile exterior, but the present invention should not be limited thereto.

The melt index (MI) herein referred to is the weight, in terms of Grams, of a thermoplastic resin material extruded in 10 minutes, as measured according to JIS K 6758.

Heretofore, blow-molded spoilers have been Generally formed as one layer 3-5 mm thick by using molding materials having a high MI and high rigidity, such as polyphenylene oxide (PPO), acrylonitrile-butadiene-styrene (ABS) resin, etc., to prevent the draw-down of parisons.

However, blow-molded articles formed from molding materials such as polyphenylene oxide (PPO), acrylonitrile-butadiene-styrene (ABS) resin, etc. show surface unevenness, and molded articles with a good design and a good gloss are difficult to obtain. For large-sized blow-molded articles such as spoilers, in particular, molding materials which have a low MI and high parison strength must be used in order to prevent draw-down. Marked surface unevenness consequently develops, which cannot be hidden merely by coating the molded articles with paint. This is presumably because these materials have a low MI and do not easily extend (or slip) in the molten state. Also these materials solidify rapidly when coming into contact with the cavity surface of a die and hence exhibit a poor die surface replicability. Accordingly, blow-molded articles such as spoilers, etc. generally require finishing by coating after a polishing step, and hence the manufacture through various processes.

In view of the above-described situations, an object of the present invention is to provide blow-molded automobile exterior parts which have good gloss and with substantially no surface unevenness, even without application of surface coating thereto.

According to the present invention, a blow-molded article is provided which comprises a base layer and a surface layer. The base layer is formed from a thermoplastic resin material having an MI of about 0.05-2.0 and a flexural modulus (modulus of elasticity in bending) of 1,470 MPa (15,000 kgf/cm.sup.2) or more. The surface layer is formed from a colored or transparent thermoplastic resin material having an MI of about 1-30, so long as it is higher than that of the base layer, and an excellent weather resistance. The two thermoplastic resin materials are made from resins of the same family.

FIG. 1 shows a transverse cross section of a spoiler, which is one example of a blow-molded article according to the present invention.

The present invention is described in more detail below with reference to FIG. 1.

(1) The blow-molded article of the present invention comprises a base layer 11 and a surface layer (skin layer) 13 as shown in FIG. 1.

(2) The base layer 11 is formed out of a thermoplastic resin material having an MI of about 0.05-2.0 and a flexural modulus (modulus of elasticity in bending) of at least 1,470 MPa (15,000 kgf/cm.sup.2).

The base layer 11 has the function of preventing the draw-down. When the MI is less than 0.05, molding is difficult to perform with regard to parison extrusion and blowing. When the MI is higher than 2.0, draw-down tends to occur.

The thickness of the base layer 11 is usually about 1-5 mm. When the thickness is less than about 1 mm, the strength required for automobile exterior parts can be hard to obtain. Further, the base layer cannot adequately prevent draw-down, though depending on the thickness of the surface layer. When the thickness is more than about 5 mm, the base layer is undesirably heavy.

Materials used for forming the base layer 11 are not particularly limited so long as they meet the above-mentioned mechanical characteristics. They may be at least one member selected from the group consisting of PPO, ABS resin, polypropylene (PP)-containing block copolymer, polybutylene terephthalate (PBT) and the like. They may be used alone or as a mixture of two or more thereof. When the surface layer is formed from a transparent material or a light-colored material, the base layer is desirably incorporated, according to necessity, with a coloring agent or a white coloring agent.

The material(s) used to form the base layer 11 is/are desirably incorporated with a reinforcing filler to obtain a necessary level of strength. Suitable reinforcing fillers vary depending on kind of resin used, but may be selected, for example, from talc, mica, glass (in the form of spheres or flakes), whisker, wollastonite, potassium titanate, and the like. Each filler may be used alone or in combination thereof. The amount of the reinforcing filler to be added is usually about 10-40 parts per hundred parts resin.

(3) The surface layer (skin layer) 13 is made from a colored or transparent thermoplastic resin material having an Mi of about 1-30, provided that its MI is higher than that of the base layer, and excellent weather resistance qualities. When the MI is less than about 1, surface unevenness tends to develop. When the MI is higher than about 30, a stable extrusion of the surface layer 13 is difficult to maintain and the surface of the molded article tends to become wavy.

Even if the resin material itself is relatively poor in weather resistance, this can be remedied by addition of age resistance such as ultraviolet absorbers, or the like. Coloring may be effected with a pigment or dye. When the surface layer is thin and surface hiding is required, this can be accomplished by adding more pigment.

The thickness of the surface layer 13 is usually about 40 .mu.m--3 mm. When the thickness is less than about 40 .mu.m, the surface layer can hardly fulfil its function of hiding surface unevenness and color of the base layer. When the thickness is more than about 3 mm, the surface layer tends to undergo draw-down at the time of parison extrusion. Moreover, since the surface layer material is generally more expensive than the base layer material, it leads to increased cost. Further, more material tends to be wasted during a color change.

The resin material for forming the surface layer 13 may suitably include PP-containing resins which are transparent and readily develop a good gloss, for example, highly crystalline PP-containing block copolymers, highly crystalline PP homopolymers, and PP-containing random copolymers. In particules, the PP-containing resins show a relatively low solidification speed after contacting the die and exhibit good die surface replicability. Further, polycarbonate (PC), PBT, poly(methyl methacrylate) (PMMA), acrylonitrile-ethylene propylene diene monomer-styrene polymer (AES) resin, and the like may be also used.

(4) The thermoplastic resin materials used to form the base and surface layers should be resins of the same family. The term "resins of the same family" used herein refers to a resin and/or a resin mixture(s) containing at least one common resin component, or a resin(s) containing at least one common monomer unit component. This is important for keeping the base layer and the surface layer closely adhered to each other to prevent the draw-down of the surface layer at the time of parison extrusion. It is also important for ensuring the integration of the base layer 11 and the surface layer 13 after molding to secure a high rigidity.

To securely maintain close adhesion of the base layer 11 to the surface layer 13, a blend with the surface layer material may be also used as the base layer material.

Table 1 shows examples of combinations of these resins according to the present invention.

(5) Hereunder, one example of the process for producing the molded automobile exterior parts described above is illustrated below.

First, the base layer (resin) material and the surface layer (resin) material are prepared and extruded with a multi-layer blow molding machine into a double-layer parison with the base layer inside and the surface layer outside. The conditions for this extrusion process are, for example, as follows:

Resin temperature: 175.degree.-200.degree. C.,

Die temperature: 180.degree.-210.degree. C.,

Extrusion speed: 3-60 m/min

The surface layer material has a relatively high MI but the base layer material has a relatively low MI, so the surface layer material is closely adhered to the base layer material and, hence, no draw-down develops. Further, since the surface layer material has a relatively high MI, die lines are substantially suppressed and a highly glossy surface can be easily obtained.

Then, blow molding is carried out by blowing air into the parison. The conditions of the blow molding are, for example, as follows:

Degree of mirror polishing of die cavity: #1000 or more,

Air vent hole: 30-70 mm in pitch, 0.3-1.0 mm in diameter,

Die temperature: 30.degree.-80.degree. C.,

Resin temperature during blowing: 180.degree.-230.degree. C.,

Blow air pressure: 2.5 kgf/cm.sup.2 (245 kPa)--9.0 kgf/cm.sup.2 (882 kPa)

Since the surface layer material has a high MI, the material flows well and exhibits a good die surface replicability. Particularly when a PP-containing resin is used, the material slowly solidifies after contacting the die cavity surface and exhibits superior die surface replicability, so that glossy products can be readily obtained.

The blow-molded automobile exterior parts thus produced have substantially no unevenness on their exteriors and can be used as finished products without further treatment. However, they may be also further flame-treated, for example, to improve the gloss, or treated with a clear coat to increase scratch resistance and to improve appearance.

The blow-molded automobile exterior parts according to the present invention have a base layer and a surface layer, the base layer being formed out of a thermoplastic resin material having an MI of about 0.05-2.0 and a flexural modulus (modulus of elasticity in bending) of 1470 MPa (15,000 kgf/cm.sup.2) or more. The surface layer is formed from a colored or transparent thermoplastic resin material having an MI of about 1-30 and higher than that of the base layer, and excellent weather resistance. The thermoplastic resins used to form the base and surface layers being resins of the same family, whereby the following actions and effects are obtained.

In extruding of double-layer parisons, since the surface layer material has a relatively high MI but the base layer material has a relatively low MI, the surface layer material closely adheres to the base layer material and hence substantially no draw-down develops. Further, since the surface layer material has a relatively high MI, virtually no die lines develops.

While blowing air into the parison, since the surface layer material has a relatively high MI, the material flows well and exhibits good die surface replicability.

Accordingly, the blow-molded automobile exterior parts according to the present invention can be readily obtained with substantially no surface unevenness and with good gloss, even without applying a surface coating, and they can be used as finished products without further treatment.

To confirm the effect of the present invention, two spoilers having a structure as shown in FIG. 1 were produced by blow-molding with the following materials and under the following conditions: In the both runs, an exemplary red spoiler with substantially no unevenness on the surface and an excellent gloss was obtained. The surface layer material 2 is slightly better in gloss but is slightly lower in scratch resistance than the surface layer material 1.

(1) Base layer material

PP-containing block copolymer (comonomer: ethylene 10%) containing 15 parts of talc per hundred parts of resin, MI: 0.3, flexural modulus (modulus of elasticity in bending: 27,000 kgf/cm.sup.2 (2,646 MPa)

(2) Surface layer material (two types)

1 Highly crystalline PP homopolymer, MI: 1.8, degree of crystallinity: 75%, Rockwell hardness: R 85 (JIS K 7110), glossiness: 87 (angle of incidence 60.degree., JIS Z 8741), haze: 7% (thickness 1 mm, ASTM D 1003)

2 PP-containing random copolymer (comonomer: ethylene 10%), MI: 3.7, Rockwell hardness: R 68 (23.degree. C.), glossiness: 92 (angle of incidence 60.degree., JIS Z 8741), haze: 12.1% (thickness 1 mm, ASTM D 1003)

(2) Molding conditions

Degree of mirror polishing of die cavity surface: #1500, air vent hole: 50 mm in pitch, 0.5 mm in diameter, die temperature: 40.degree. C., resin temperature in blowing: 200.degree. C., blow air pressure: 5 kgf/cm.sup.2 (490 kPa)