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标题: Coating failures [打印本页]

作者: sunny 时间: 2009-1-6 15:00
标题: Coating failures
本帖最后由 sunny 于 2009-1-6 15:01 编辑

Coating failures may appear during application, at the stage of curing or after a certain period of service life. Statistics show that as much as 95% of all coating failures is a result of poor surface preparation and application.
Below you will find some examples of common coating failures and the reason why they occur. Please note that there may be many reasons for a coating failure and in some cases it requires a lot of experience to find the exact cause.

Sagging

Sagging occurs when: Paint is applied in excess of the DFT specified ;Too much thinner has been added to the paint ;The gun is held too close to the surface.
Sags are recognised as "curtains" on the painted surfaces. If the wet film thickness is too high, excessive sagging can result in pools of paints forming on horizontal surfaces or in corners. After curing, the paint may crack all the way to the substrate in such areas and reveal unprotected steel.
If sagging is noticed at the spraying stage, it should be brushed out while the paint film is still wet. Repairs after drying consists of abrasion (sanding) and re-coating.

作者: sunny 时间: 2009-1-6 15:02
Pinholes and pores

Using the wrong spraying technique, such as excessive air pressure, excessive film thickness, strong wind (too good ventilation) and too long application distance may cause craters, pinholes and pores. If noticeable on the paint film, check the spraying equipment to ensure that the air pressure and nozzle size is correct. Pinholes in a paint film can also result from overspray. On excessive film thickness air will be entrapped in the paint. The escaping air will create pinholes. The consequence being that pinpoint rusting occurs, followed by undercutting of the coating around the pinholes.
Repairs consist of grinding and re-coating the area using appropriate coats to seal the defects and build up the coating to the correct DFT. If the coating is still uncured, brush out and apply the additional coat.

Blistering

This is one of the most common types of failure related to the adhesion of the paint. Sometimes the blisters are dry and sometimes filled with liquid. The blisters can be both large and small, often shaped as hemispheres. The size usually depends on the degree of adhesion to the substrate, or between the coats, and the internal pressure of the gas or the liquid inside the blister.
Blistering can be caused by a number of different conditions:

Soluble salts contaminating the substrate or contaminating the surface between coats. No coatings are 100% water proof. The moisture vapour passing through the coating can dissolve salt into a concentrated solution. Pressure in the high concentration liquid will cause blisters. This phenomen is called osmosis.
Contamination of the surface (e.g. oils, waxes, dust, etc.) will not allow proper adhesion of the coating. The moisture vapour tends to be concentrated in these areas of low adhesion. In this case, the blisters are so-called "dry" blisters.
Poor or inadequate solvent release from the coating. Entrapped solvents can increase the water absorption and moisture vapour transmission of the coating and lead to blistering. Solvent odour is usually connected with retained solvents. If the blistering is widespread on a construction: reblast and wash before a new system is applied. For local areas: blast or carry out other mechanical cleaning before recoating.

Lifting

Lifting is a raising of the undercoat. It is caused by a stronger solvent in the topcoat attacking the previously applied film. The result is a wrinkled surface. A typical example is a topcoat containing xylene, on top of an alkyd-based primer containing white spirit. The xylene in the topcoat will dissolve the primer.
Blast cleaning and reapplication of the paint is necessary to repair the damaged surface.

Delamination/peeling

Loss of adhesion to the substrate or between coats of paint is delamination or peeling.
The causes are:

Unsatisfactory surface preparation
Incompatible primer or undercoat
Substrate or intercoat contamination
Excessive cure time between coats

The way to repair the surface, is to remove paint down to sound paint or to the substrate, and recoat.

Orange peel

Finely pebbled or dimpled surface texture with an appearance similar to the skin of an orange.
Caused by:

Improper atomisation due to low air pressure
Spraying too close to the surface
Rapid solvent evaporation

Orange peel is mainly a cosmetic defect; sand down to smooth surface and repaint if necessary.

作者: sunny 时间: 2009-1-6 15:27
本帖最后由 sunny 于 2009-1-6 15:29 编辑

涂层失败
产品应用中涂层失效可能发生在固化阶段或一定使用时期后。统计显示，多达95 ％的涂层的失败是由于表面处理及应用的不足。
接下来，你会看到一些通常发生的涂层缺陷案例及其产生原因。请注意，涂层失败可能是由许多原因造成的，在某些情况下，需要大量的经验才能找到找到确切原因。
流挂
发生在：
•DFT涂料干膜厚度过了
•太多稀释剂添加到油漆中
• 喷枪太接近表面。

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