Use of a resin powder composition for electrostatic coating

Abstract: Use of a resin powder composition for electrostatic coating, wherein said resin powder composition comprises (1) a thermosetting or thermoplastic resin and (2) an electric charge-increasing agent incorporated in the resin.

Description: The present invention relates to use of a resin powder composition for electrostatic coating. In recent years, an attention has been drawn to the usefulness of a powder coating material reflecting the technological progress in the electrostatic powder coating. A powder coating material is advantageous over a solution-type coating material in that it does not use a solvent and thus is free from pollution, the coated article can be used immediately after the application of the coating, and yet a coating layer having a desired thickness can be obtained by a single application treatment. Because of these advantages, there has been a rapid increase in its demand. In spite of the above-mentioned advantages, however, it still has a number of drawbacks which must be overcome. Presently available practical methods for electrostatic powder coating are classified into a corona application system, a triboelectrification system and a hybrid system. Every system employs an electrostatic principle. In the corona application system, a high level of applied voltage is required to give a sufficient electric charge to the powder of coating material, and accordingly, a high voltage generator and an electric shock preventing device are required. Even if such a device is provided, there still remains a possibility of danger such as electric shock to human body or a haphazard due to sparking. The triboelectrification system has drawbacks such that the apparatus is required to be of a large size in order to impart a sufficient electric charge to the powder of coating material, the particle size is required to be small and the particle size distribution is required to be sharp, and due to the cumbersome operation and a decrease in the yield of pulverized powder during the pulverization and classification steps, the production costs tend to be high. The hybrid system has the drawbacks of the above-mentioned two systems. Further, as drawbacks common to all the systems, there may be mentioned irregularities in the thickness or quality of the coated layer due to a non-uniformity in the distribution of the electric charge in the powder of coating material, a difficulty in the control of the thickness of the coated layer due to the difficulty in the control of the electric charge, and a limitation in the choice of the material or shape of the substrate to be coated. The charged powder of coating material is susceptible particularly to an influence of the temperature and humidity, and the coating environment is also limited. In order to form a satisfactory coating layer, an apparatus and environment wherein the temperature and humidity are specially controlled, are required. Furthermore, the most serious drawback common to conventional electrostatic powder coating materials is that the coating efficiency is low. The coating efficiency is about 75% at best according to the conventional coating materials presently available for practical industrial purposes. Further, after the electrostatic deposition, the deposited powder may happen to fall off due to attenuation of the electric charge during the curing step. Accordingly, such non-deposited powder or fallen-off powder will be wasted, or must be recovered by using a special device for recovery and reused by adding the recovered powder in small portions to a fresh powder, or reused by recycling the recovered powder to the resin powder preparation step. In the case of reuse, the apparatus and machines are required to be an exclusive line, whereby it is not easy to change the type or hue of the coating material resin. Thus, the installation cost of the recovery apparatus and the cumbersome operation and time required for the recovery or the change add to the total cost. EP-A-214448 discloses a thermosetting powder coating composition comprising an acrylic copolymer, an epoxy novolac resin and an epoxy resin, and also discloses that the above composition may further contain a pigment such as titanium dioxide. EP-A-214448 further discloses a quaternary ammonium salt used as a catalyst for increasing the cross-linking rate of a powder coating composition on page 7, lines 28 to 34, but the object of using the quaternary ammonium salt and the amount used are essentially different from those of the alkyl onium salt used in the present invention since the alkyl onium salt used in the present invention is to enhance a coating efficiency. EP-A-250183 discloses a coating powder composition characterized by containing crosslinked polymer particles and also discloses that the above composition may further contain a pigment such as titanium dioxide. However, the pigment disclosed in these European patent laid-open documents does not exhibit an effect achieved by such an electric charge-increasing agent as used in the present invention. The present inventors have conducted extensive research by paying an attention to the fact that the above drawbacks are caused by an electrostatic phenomenon. As a result, it has been found possible to substantially overcome the above drawbacks inherent to the coating by conventional electrostatic powders, by adding an electric charge-increasing agent to a resin powder. The present invention has been accomplished on the basis of this discovery. The present invention provides use of a resin powder composition for electrostatic coating as defined in claim 1, wherein said resin powder composition comprises (1) a thermosetting or thermoplastic resin and (2) an electric charge-increasing agent incorporated in the resin. Now, the present invention will be described in detail with reference to the preferred embodiments. The resin powder composition for electrostatic coating of the present invention comprises a thermosetting or thermoplastic resin and from 0.01 to 20% by weight of an electric charge-increasing agent. This composition may further contain a curing agent, a pigment, a metal powder, a filler, a flowability controlling agent, a plasticizer or a stabilizer. In the present invention, the thermosetting resin may be of a conventional type such as an epoxy resin, a polyester resin or an acrylic resin. Likewise, thermoplastic resin may be a vinyl chloride resin, a polyamide resin, a cellulose resin, a polyolefin resin, a polyethylene resin, a polyester resin or a nylon resin. These resins may be used alone or in combination as a mixture. Particularly preferred is a thermosetting resin. The electric charge-increasing agent as the essential component of the present invention may be a metal compound of an organic carboxylic acid or an alkyl onium salt, which is used as an electric charge controlling agent for electrophotography. These compounds may be used alone or in combination as a mixture of two or more. The metal compound of an organic carboxylic acid includes, for example, those disclosed in Japanese Examined Patent Publications No. 42752/1980 and No. 7384/1984 and Japanese Unexamined Patent Publications No. 124357/1982, No. 127937/1983, No. 88743/1984, No. 88745/1984, No. 141450/1986 and No. 69073/1986. The alkyl onium salt includes, for example, those disclosed in Japanese Unexamined Patent Publications No. 158932/1979, No. 11461/1981, No. 119364/1982, No. 267059/1986 and No. 53944/1987. The resin powder composition of the present invention may readily be prepared in accordance with a conventional method. For example, the binder resin and the electric charge-increasing agent may be heated, melted and kneaded by means of a conventional mixing machine such as a single screw or multi-screw extruder, a Banbury mixer or heat rolls, then cooled and pulverized at room temperature or under freezing to obtain a powder. As a method for addition, any optional method commonly employed for the preparation of a powder mixture such as a mixing method of a binder resin powder and a powder of an electric charge-increasing agent, may be employed for the preparation. The particle size of the resin powder for coating according to the present invention is preferably within a range of from 30 to 250 µm. The resin powder coating composition of the present invention may further contain in addition to the above components, a hardener, a pigment, a metal powder, a filler, a flowability controlling agent, a plasticizer, a stabilizer and other additives, as the case requires. The resin coating powder of the present invention may be applied to substrates made of metals, ceramics, plastics, etc., by various electrostatic powder coating machines of a corona application system, a triboelectrification system or a hybrid system. Various primers may be applied to such substrates, or various other pretreatments may be applied to such substrates. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. In these Examples, "parts" means "parts by weight". EXAMPLE 1 91 Parts of a polyester resin (Viron GV-700, manufactured by Toyobo Co., Ltd.), 8 parts of pyromellitic dianhydride and 1 parts of Chromium complex compound (4,5-ditert-butyl-salcylate) as an electric charge-increasing agent, were pulverized and mixed by a high speed mixer, and the mixture was heat-melted and kneaded by an extruder, cooled, then pulverized and classified to obtain a resin powder composition having a particle size distribution of from 30 to 40 µm. As a Comparative Example, the same composition as above except that no electric charge-increasing agent was incorporated, was prepared in the same manner, and used as a comparative sample. Then, by using the powder samples obtained by the above method, Comparative Tests for the coating efficiency by a corona application system were conducted. The results are shown in Table 1. The test conditions for the comparative tests for the coating efficiency were as follows. Conditions for the Comparative Tests for coating Electrostatic powder coating apparatus: Product of Onoda Cement Co., Ltd. (G x 101) Discharge rate: 128.7 g/min. Substrate to be coated: Aluminum plate having a thickness of 3 mm, 30 cm x 30 cm Moving speed of the substrate: 2 m/min. Distance between the substrate and the forward end of the coating spraygun: 25 cm It is evident from the results in Table 1 that in the case of the Comparative Example wherein no electric charge-increasing agent is incorporated, the coating efficiency can be improved to some extent by increasing the applied voltage, but it still does not reach the level of the Example. Whereas, in the case of the Example of the present invention where the electric charge-increasing agent is incorporated, the coating efficiency was substantially 100%. After the coating, the sample of the Example was subjected to baking treatment at 200°C for 30 minutes, whereby a uniform strong excellent coating layer was obtained without falling off of the deposited powder up to the baking treatment. In the same manner as in Example 1, various resin powder compositions for electrostatic coating were prepared. The compositions and the results of the coating efficiency tests are shown as Examples 2 to 15 in Table 2, as compared with the corresponding Comparative Examples wherein no electric charge-increasing agent was incorporated. After the coating, the samples of Examples 2 to 15 were subjected to baking treatment in the same manner as in Example 1, uniform strong excellent coating layers were obtained without falling off of the deposited powders. Example 16: By using the resin powder composition having the same composition as in Example 1, comparative tests for the coating efficiency by a triboelectrification system were conducted by means of an electrostatic powder coating apparatus (BLE-400, manufactured by Toagosei Chemical Industries Co., Ltd.). The results are shown in Table 3. The test conditions for the comparative tests for the coating efficiency were the same as in Example 1 except for the discharge rate. It is evident from the results in Table 3 that in the case of the Comparative Example wherein no electric charge-increasing agent was incorporated, the coating efficiency did not reach the level of the Example. Whereas, in the case of the Example of the present invention wherein an electric charge-increasing agent was incorporated, the coating efficiency was 100%. After the coating, the sample was subjected to baking treatment at 200°C for 30 minutes, whereby a uniform strong excellent coating layer was obtained without falling off of the deposited powder up to the baking treatment step. In the case of the Comparative Example, falling off of the deposited powder was observed during the period up to the baking treatment step. Now, Examples 17 to 23 conducted by a triboelectrification system in the same manner as in Example 16 are shown in Table 4 together with the respective Comparative Examples. After the coating, the samples of Examples 17 to 23 were subjected to baking treatment in the same manner as in Example 16, whereby uniform strong excellent coating layers were obtained. Although the theoretical antistatic mechanism of the electric charge-increasing agent in the present invention has not yet been clearly understood, it is evident that as compared with the comparative composition containing no electric charge-increasing agent, the resin powder composition containing the electric charge-increasing agent can easily be charged with an electric charge to a level required for the resin powder by a corona application system even when the applied voltage is lower than the comparative composition, whereby the coating efficiency can be remarkably improved. Further, since the applied voltage can be low, the danger of the apparatus and handling can be avoided, and the operation can be extremely safe and can easily be conducted. In the triboelectrification system, the required level of electric charge can readily be reached in a very short period of time by triboelectrification treatment. This leads to a reduction of the size of the apparatus or to a reduction of the air required for triboelectrification, whereby the coating efficiency can be remarkably improved. In either system, the charged resin powder has excellent stability with no substantial change in the electric charge and with no substantial influence by the change in the temperature or humidity, by virtue of the incorporated electric charge-increasing agent, and no substantial attenuation of the electric charge is observed, whereby no falling off of the deposited powder is observed. Accordingly, the coating efficiency is as high as from 95 to 100%, and the penetrability is extremely good. This indicates that by using the composition of the present invention, no operation or apparatus for the recovery, or no special place where the temperature, humidity, etc. must be specially adjusted, is required, and a uniform coating can be formed without irregularity on a substrate of any shape. Further, by selecting the type and content of the electric charge-increasing agent, the type of the resin, the layer thickness and the layer quality can optionally be selected depending upon the particular purpose of the substrate to be coated. The simplification or reduction of the size of the coating installation or apparatus or equipments, the color change, etc. can easily be conducted, whereby the range of the application will be substantially broadened.

Claims: Use of a resin powder composition for electrostatic coating, wherein said resin powder composition comprises (1) a thermosetting or thermoplastic resin and (2) an electric charge-increasing agent incorporated in the resin, the electric charge-increasing agent being at least one member selected from the group consisting of a metal compound of an organic carboxylic acid and an alkyl onium salt, the thermosetting resin being an epoxy resin, a polyester resin or an acrylic resin, and the thermoplastic resin being a vinyl chloride resin, a polyamide resin, a cellulose resin, a polyolefin resin, a polyethylene resin, a polyester resin or a nylon resin. Use according to Claim 1, wherein the electric charge-increasing agent is incorporated in an amount of from 0.01 to 20 % by weight relative to the resin. Use according to Claim 1, wherein the resin powder composition has a particle size of from 30 to 250 µm. A method for improving the coating efficiency of a conventional resin powder composition for electrostatic coating comprising a conventional thermosetting or thermoplastic resin, characterized in that an electric charge-increasing agent, which is at least one member selected from the group consisting of a metal complex azo compound, an anthraquinone compound, an azine compound, a metal compound of an organic carboxylic acid and an alkyl onium salt, is incorporated in the resin. Method according to Claim 4, wherein the electric charge-increasing agent is incorporated in an amount of from 0.01 to 20 % by weight relative to the resin.

Inventors: NAGASAKA HIDEO, NIIMURA ISAO HODOGAYA CHEMICAL, NITA TOSHIYUKI, SUZUKA SUSUMU HODOGAYA CHEMICA, TAKEUCHI MANABU, NAGASAKA, HIDEO, NIIMURA, ISAO HODOGAYA CHEMICAL CO., LTD., NITA, TOSHIYUKI, SUZUKA, SUSUMU HODOGAYA CHEMICAL CO., LTD., TAKEUCHI, MANABU
Applicants: HODOGAYA CHEMICAL CO LTD, HODOGAYA CHEMICAL CO., LTD.
Application year: 1987
Main classification: C09D5
IPC codes: C09D5