EP0643996A1

EP0643996A1 - Coating process

Info

Publication number: EP0643996A1
Application number: EP94114613A
Authority: EP
Inventors: Koichi Tsutsui; Shannon Libke; Koichi Inoue
Original assignee: Nippon Paint Co Ltd
Current assignee: Nippon Paint Co Ltd
Priority date: 1993-09-17
Filing date: 1994-09-16
Publication date: 1995-03-22
Anticipated expiration: 2014-09-16
Also published as: EP0643996B1; DE69408925D1; DE69408925T2

Abstract

The object of the present invention is to provide an electrostatic coating method employing a powder coating which provides for good transfer efficiency with consequent reduction of powder recovery load and, hence, is useful for conservation of resources and management of wastes.

The present invention relates to a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method wherein the temperature of said substrates is brought to a level not below the glass transition temperature of the component resin of said powder coating or the glass transition temperature of the component resin of of said plastic or coated coat prior to application of said powder coating.

Description

FIELD OF THE INVENTION

The present invention relates to an coating process wherein a powder coating is applied by an electrostatic coating method on the surface of the substrates formed with plastic or coated coat.

BACKGROUND OF THE INVENTION

In the field of plastic products, surface-coated metal or surface-coated plastic products such as automotive parts, architectural materials and household appliance parts, many advances have been made in the technology for improving the durability of products by way of applying a coated coat on the surface of such products. In this manner, electrostatic methods employing powder coating have recently been used with preference for the prevention of environmental pollution and improvement of production efficiency in factory.
In such electrostatic coating methods, the progressive formation of a coating film from the powder coating on the substrate surface results in a buildup of static electricity and the resultant static repulsion causes local stripping of the powder coating. Moreover, the powder coating deposited on the surface is partially driven off by the ambient flows of air. Because of these field engineering factors, the transfer efficiency of powder coating has not been fully satisfactory to this day.
Moreover, when a powder coating is applied by electrostatic coating, it is inherently difficult to insure that the whole amount of the powder coating used is deposited on the substrate and, therefore, it is essential if only from cost considerations and for the prevention of pollution to establish a process for recovery and reuse of the undeposited powder coating.
Japanese Kokoku Publication Sho-51-43152 discloses a technique for top coating with a powder coating after formation of a base coat with a thermosetting resin coating, which comprises heating the substrate to reduce the content of the volatile component of the base coat to not more than 6 weight % to thereby improve the metallic tone and durability of the coat as well as transfer efficiency.
Japanese Kokai Publication Hei-2-194878 discloses a technique for coating plastic substrates wherein the prevention of popping is sought by heating the substrate to a temperature necessary for degassing prior to application of the coating.
The above techniques are all intended to improve the coating processes employing powder coatings but while the content of the volatile component of the base coat is limited to, say, 6 weight % or less on the one hand, improvement of the transfer efficiency is sought by utilizing the residual volatile matter on the other hand, with the result that the transfer efficiency is not actually improved so much as desired, so that the above-mentioned problems have remained virtually unsolved.
Moreover, when a plastic substrate is coated with a powder coating, the so-called "popping" occurs due to the surface porosity of the substrate. If the heating temperature is too high, the powder coating inevitably tends to undergo partial curing to detract from the appearance of products.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electrostatic coating method employing a powder coating which provides for good transfer efficiency with consequent reduction of powder recovery load and, hence, is useful for conservation of resources and management of wastes.
The gist of the present invention is that in a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method, the temperature of said substrates is brought to a level not below the glass transition temperature of the component resin of said powder coating or the glass transition temperature of the component resin of of said plastic or coated coat prior to application of said powder coating.
The present invention is now described in detail.

DETAILED DESCRIPTION OF THE INVENTION

The coating process according to the present invention can be applied to the surface of plastic. Moreover, the coating process according to the present invention can be applied to the surface of metal or plastic after formation of a coated coat as a base coat with electrodeposition coating, aqueous coating, solvent type coating or powder coating. Said metal may for example be an iron or other electrically conductive material, and particularly preferred are iron phosphate-treated steel and zinc phosphate-treated steel sheets. In this specification, said surface of plastic and said surface of metal or plastic after formation of a coated coat, which are the substrates for application of the coating process according to the present invention, are collectively referred to as 'the surface of the substrates formed with plastic or coated coat'.
There is no particular limitation on the powder coating that can be used in the practice of the present invention. Thus, coatings using a polyester resins, acrylic resins, epoxy resins or other resins as the vehicle can be mentioned as examples. The pigment and additives are incorporated in such a vehicle to provide a powder with a nonvolatile content of 100 %.
The particle size of the powder coating, in terms of bulk mean particle diameter, is 5 to 50 µm and preferably 8 to 40 µm. In case the mean diameter is 5 to 20 µm, the proportion of particles up to 5 µm in diameter is preferably not more than 25 weight %. In case the mean particle diameter is 20 to 50 µm, the standard deviation of the particle size distribution is preferably not greater than 20 µm.
The standard deviation of a particle size distribution is expressed by
${[Σ {(D - X) ² F } / Σ F ]}^{1/2}$
, where D is the diameter of each individual particle, X is the bulk mean particle diameter and X is Σ (D F)/Σ F , and F is the frequency of particles.
The type of plastic is not critical, either. Thus, the plastic may be thermoplastic or thermosetting and need not necessarily be reinforced. Thus, phenolic resins inclusive of phenol-cellulose versions, silicone resins, amino resins, polyurethanes, polystyrenes, polypropylene, thermoplastic acrylic resins, polyvinyl chloride, polyacrylonitrile, polybutadiene and acrylonitrile-butadiene copolymers can be mentioned as examples.
When a fiber is used for reinforcement, boron fiber and other fibers except glass fiber can be employed.
In the coating process of the present invention, the temperature of the substrate is brought to a level not below the glass transition temperature of the resin component of the powder coating or not below the glass transition temperature of the resin constituting the plastic or coated coat.
To bring the temperature of the substrate to such a level, the substrate can be preheated prior to application of the powder coating. When the undercoating process involves a baking step, the powder coating can be applied before cooling of the substrate after baking.
Generally speaking, the above-mentioned temperature of the substrate is preferably not higher than the upper limit of baking temperature for the powder coating, and when the surface of the substrate formed with coated coat is used, the upper limit of said temperature is preferably not higher than the upper limit of baking temperature for said coated coat and for the powder coating.
When the surface of the substrate is formed with plastic, the upper limit of said temperature is preferably lower than the deformation temperature of said plastic.
In a preferred embodiment of the present invention, the above-mentioned temperature is within the range of 40 to 140°C. When the substrate temperature is below 40 °C, the transfer efficiency is poor, while the use of any temperature over 140 °C may cause partial curing of the powder coating to detract from the appearance of the coated product.
In a preferred embodiment of the present invention, where the surface of the substrate formed with coated coat is used, the volatile content of the base coat is controlled to not more than 10 weight %. If this limit of 10 weight % is exceeded, foaming, surface roughness, and yellowing may occur to detract from the appearance of the final product. The preferred limit is 5 weight % or less.
In a still preferred embodiment of the present invention, where the coated coat of the surface of the substrate is one formed by electrodeposition or from an aqueous coating, the content of the volatile amine compound is to be controlled to not more than 2.0 weight %. If this limit of 2.0 weight% is exceeded, the appearance of the finished product will not be as good as desired. The preferred limit is 1.5 weight % and the still preferred limit is 1.2 weight %.
In accordance with the present invention, the surface of the substrates formed with plastic or coated coat is first brought to the above-mentioned temperature and then applied electrostatic coating method with the powder coating. The electrostatic coating method as such can be carried out in the conventional manner.
In the practice of the present invention, an electrically conductive particulate substance, such as graphite powder, can be incorporated in the plastic material to impart electric conductivity. A similar effect can be achieved by using an electrically conductive rein forcing fiber. If necessary, the plastic substrate can be coated with an electrically conductive primer or an electrically conductive wash solution to impart electric conductivity.

EXAMPLES

The following examples are intended to describe the present invention in further detail and should by no means be construed as defining the scope of the invention.

Reference Example 1

Preparation of an aqueous metallic coating

To 140 parts by weight of an aqueous acrylic resin with a hydroxyl value of 20 and an acid value of 58 (amine-neutralized, nonvolatile matter 50%) was added 30 parts by weight of Cymel 303 (methoxylated methylolmelamine, Mitsui Toatsu Chemicals, Inc.) followed by addition of 15 parts by weight of an aluminum pigment paste (AW-666, Asahi Chemical Industry Co., Ltd.). The mixture was stirred to provide an aqueous metallic coating.

Reference Example 2

Preparation of an acrylic powder coating

To 315 parts by weight of a glycidyl group-containing acrylic resin (glass transition temperature (Tg) : 52 °C) were added 80.5 parts by weight of decanedicarboxylic acid, 4 parts by weight of a surface conditioner and 2 parts by weight of benzoin. After melt-compounding, the composition was finely divided to provide an acrylic powder coating.

Reference Example 3

Preparation of a polyester powder coating

To 100 parts by weight of a carboxyl group-containing polyester resin (Tg : 63°C) were added 7.5 parts by weight of triglycidyl isocyanurate, 60 parts by weight of titanium dioxide, 0.4 part by weight of a surface conditioner and 1.1 parts by weight of benzoin and the composition was melt-compounded and finely divided to provide a polyester powder coating.

Example 1

An iron phosphate-treated steel sheet was electrocoated (Powertop, U Series, Nippon Paint Co., Ltd.) in a coating thickness of 20 µm and further intermediate-coated (OTOH Series, Nippon Paint Co., Ltd.) in a coating thickness of 35 µm. The intermediate-coated steel sheet was then coated with the aqueous metallic coating prepared in Reference Example 1 and baked at 80°C for 10 min., at 100°C for 10 min. and at 120 °C for 10 min. This coated sheet was brought to the respective temperatures shown in Table 1 and, then, coated electrostatically using the acrylic powder coating prepared in Reference Example 2 (coating conditions: applied voltage -80 kV, delivery rate 120 g/min.). The transfer efficiency was then evaluated for each sample. The results were shown in Table 1. [Table 1]

Temperature 25°C 60°C 100°C 120 °C

Transfer efficiency 51% 63% 78% 81%
The above results indicated that when the temperature of the substrate is brought to a level not below the Tg of the acrylic resin contained in the powder coating, an improvement in transfer efficiency can be realized.

Example 2

A plastic blank for automotive bumper use (Tg : 90°C, Mitsui Petrochemical Co., Ltd.) was treated with trichloroethane and coated with a conductive primer(RB-1140 CD primer, Nippon B Chemical Co., Ltd.). This plastic blank was brought to the respective temperatures shown in Table 2 and coated electrostatically using the acrylic powder coating prepared in Reference Example 2 under the same coating conditions as in Example 1. The transfer efficiency was then evaluated.
The results were shown in Table 2. [Table 2]

Temperature 25°C 60°C 100°C

Transfer efficiency 48% 60% 82%
The above results indicated that while bringing the substrate temperature to a level not below the Tg of the acrylic resin component of the powder coating results in an improved transfer efficiency, a more remarkable improvement in transfer efficiency can be realized by bringing the substrate temperature to a level not below the Tg of the plastic substrate.

Example 3

As in Example 1, an iron phosphate-treated steel sheet was electrocoated, then intermediate-coated and further coated with the aqueous metallic coating prepared in Reference Example 1. The coated sheet was preheated at 80°C for 5 min. and 10 min., at 90°C for 5 min., at 100°C for 5 min., at 130 °C for 5 min. and at 140°C for 5 min. Then, without cooling, the sheet was coated with the acrylic powder coating prepared in Reference Example 2 under the same conditions as in Example 1 and baked at 150 °C for 25 min. The content of the nonvolatile component (weight %) and volatile amine compound (weight % based on total nonvolatile matter) of the aqueous metallic coat after preheating were determined and the appearance was rated. The appearance was indicated by the value of NSIC measured by portable measuring instrument of sharpness of the reflection (Sugai Shikenki Co.). The results were shown in Table 3.

[Table 3]

Preheating conditions	80°C 5 min.	80°C 10 min.	90°C 5 min.	100°C 5 min.	130°C 5 min.	140°C 5 min.
Nonvolative matter (wt. %)	85	90	88	90	95	99
Volatile amine compound (wt. % based on total nonvolatile matter)	2.5	2.1	1.5	1.2	0.6	0.3
Appearance (NSIC)	58	70	71	73	74	75

It was apparent from Table 3 that a good finished appearance can be obtained by controlling the volatile matter content of the coated coat to not more than 10% or the amine content thereof to not more than 2% based on total nonvolatile matter.

Example 4

A zinc phosphate-treated steel sheet, 0.6 mm in thickness, was coated with the same intermediate coating as used in Example 1 in a coating thickness of 35 µm. This coated sheet was brought to the respective temperatures indicated in Table 4 and coated electrostatically with the polyester powder coating prepared in Reference Example 3 under the same conditions as in Example 1. The transfer efficiency was then evaluated.
The results are shown in Table 4. [Table 4]

Temperature 20°C 70°C 120°C

Transfer efficiency 58% 70% 81%

INDUSTRIAL APPLICABILITY

Since the transfer efficiency of powder coating is thus enhanced and the powder recovery load is as much reduced by preheating the substrate to a temperature not below the Tg of the resin contained in the powder coating in accordance with the present invention, the invention is of value for the prevention of pollution and management of wastes. Furthermore, in the production sequence of (steel sheet)-(primer)-(bake)-(cool)-(powder coat), the (cool) step can be omitted. In the coating process for automotive bodies, the (cool) step in the sequence of (aqueous base coat)-(preheat)-(cool)-(powder top clear coat) can be similarly omitted. Therefore, the present invention is of value for the conservation of labor and energy.

Claims

In a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method;
a coating process wherein the temperature of said substrates is brought to a level not below the glass transition temperature of the component resin of said powder coating prior to application of said powder coating.
In a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method;
a coating process wherein the temperature of said substrates is brought to a level not below the glass transition temperature of the component resin of said plastic or coated coat prior to application of said powder coating.
In a coating process applying a powder coating on the surface of the substrates formed with plastic or coated coat by electrostatic coating method;
a coating process wherein the temperature of said substrates is brought into a range of 40 to 140 °C prior to application of said powder coating.
The coating process according to claim 1, 2 or 3 wherein the content of the volatile component in said coated coat of the surface of the substrates is not higher than 10 weight %.
The coating process according to claim 1, 2, 3 or 4 wherein said coated coat of the surface of the substrates is an electrodeposited coat or aqueous coat, the content of the volatile amine compound is not more than 2.0 weight % based on total nonvolatile matter.