Preprocess

Q1.
What kind of pretreatment methods are available for the application of resin coatings? Also, please advise me of the range of applications, the advantages and the disadvantages of each pretreatment method.
A1.
No. Pretreatment Method Range of Application, Advantages and Disadvantages
1 Solvent Wiping Method The surface of the object to be coated is wiped with a cloth that has been dipped in a solvent. This is an easy method of pretreatment, well suited for small production lots.
2 Solvent Vapor Cleaning Method This method initially involves the heating and evaporation of trichloroethane or freon type solvents. The object to be coated is then left exposed to the solvent vapor atmosphere for a period of time. Although this method is easy to perform and results in a thorough cleaning, it is no longer used, due to recent regulations against pollution.
3 Alkali or Acid Cleaning Method The spraying of alkali or acid agents can be used to clean objects before coating, as is commonly done to clean metal parts. Several rinse processes must be performed after cleaning to remove all of the sprayed chemical cleaning agent. Moreover, water drainage and drying facilities are required subsequent to the rinsing processes. This method requires larger facilities than those used for other pretreatment methods, thus resulting in greater operating costs.
4 Plasma Treatment Method Plasma treatment must be performed within a vacuum chamber, using batch processing. Large costs are associated with the facilities required.
5 Corona Discharge Treatment Method In this method, electrons are forced to collide with the surface of the object to be coated. Can only be utilized for objects with flat surfaces. Large costs are associated with the facilities required.
6 Flame Treatment In this method, the surface of the object to be coated is burned using a short flame burner to condition the surface. The challenge is to ensure safety during the treatment process.

<Recent Trends in Pretreatment Methods>

A variety of pretreatment methods have been developed for use with resin coatings, as it was previously quite difficult to achieve good adhesion of polyolefin resin coating materials. However, in recent years primer coat materials that possess outstanding adhesion have been developed, thus reducing the importance of the pretreatment process.

(Reply from: Parker Engineering Co., Ltd.)

Coatings

Q2.
Is it possible to utilize the electrostatic coating process to apply resin coating materials? If it is possible, what specific procedures are used?
A2.

In general, resins are electrical insulators. Therefore, some kind of treatment must be performed to allow the conduction of electricity, prior to performing electrostatic coating.

  1. Application of electrically conductive fluid to the resin surface.
    Electrically conductive fluid is a particular kind of surfactant. As it is a quick drying solvent, moisture within the air will adhere to the surface, reducing the electrical resistance. However, this method requires careful implementation, as it requires an additional process and is also limited by the ambient humidity, the time available and the particular type of resin.
  2. Application of electrically conductive primer to the resin surface.
    Although this method also requires one additional process, it possesses two special features that make it widely applicable to the coating of vehicle parts: a large electrostatic effect and enhanced product quality due to the high adhesion. Either air spray or airless spray coating methods should be used, as the electrically conductive primer contains powdered carbon.
  3. The coating operation should be started from the jig and should proceed carefully to other parts, one by one.
    This method does not increase the number of coating processes. However, it is necessary to ensure that the coating operation is always started from the jig (metal: object to be coated is grounded). The reason for starting at the jig is because the solvent contained within the coating material does conduct a certain amount of electricity, thus some degree of electrostatic effect will occur.

(Reply from: Asahi Sunac Corporation)

Drying

Q3.
Please advise me of the precautions that must be observed during the drying process.
A3.

First of all, the drying method to use for a coating object (made of resin) should be selected with consideration for the object's shape, size, material (type of resin), coating method and the manner in which it is conveyed. It is recommended that a YES / NO style of questionnaire, containing these factors, be prepared and filled out, as it will be of great help in eventually determining the optimal drying method. The upper temperature limit and drying time are determined by the particular properties of the resin itself. It is important to utilize a coating system in which these conditions can be precisely controlled.

(Reply from: Takubo Engineering Co., Ltd.)