In construction quality management, film thickness management is a relatively important item. The so-called film thickness management is the process of controlling the paint film thickness during construction and detecting, analyzing, judging, quality feedback and processing the film thickness after construction . The purpose of film thickness management is to make the thickness of the coating of the object to be coated not only meet the thickness specified to ensure the protective performance, but also avoid the waste caused by over-thickness.
For the method of film thickness management, each industry has its own standards, such as the "Measurement Method for Ship Coating Film Thickness" (GB/T 3718) promulgated by CHINA Shipbuilding Corporation in 1994. Generally, in severe corrosive environments , the total dry film thickness is required to be above 200μm. On the coating design drawings, there are strict regulations on the thickness of each coating and the total dry film thickness, which should be implemented according to the requirements .
Film thickness management includes wet film thickness detection and dry film thickness detection: Wet film thickness is generally detected during the painting process, when each paint is painted, and detected in time. The dry film thickness can be detected during the construction process , but the total dry film thickness inspection is required after the construction is completed.
(1) Inspection of wet film thickness: Wet film thickness measurement is generally used by applicators to guide people to determine the prescribed amount of paint that needs to be applied to obtain expected dry film thickness. There are generally two types of wet Film Thickness Gauges: wet film thickness wheel gauge and wet film thickness comb gauge. For the detection method and technique of wet film thickness, please refer to the relevant content in Chapter 5. The advantage of thicknessusing . If the applicator knows the wet film thickness, when this data is multiplied by the volume percent , the dry film thickness can be estimated.
Dry film thickness (μm) / wet film thickness (μm) × coating solid content (volume %)
Actual amount of paint (Kg)/(1+a)×Theoretical coating amount of paint (Kg/㎡)×Painting area (㎡)
In the formula
a ——Paint consumption coefficient.
In actual construction, due to the uneven surface of the object to be coated, the consumption during the coating process, the influence of wind speed in environmental conditions, and the impact of different types of construction tools, the coating must have different degrees of loss, and the value of a is not a constant. , is determined by a combination of various factors. General high-pressure airless spraying flat surface. Value is 0.6a0.8; spray complex surfaces. The value is 0.8~1.0; hand-painted flat surface . The value is 0.3~0.4; manual painting complex surface. The value is 0.4~0.6.
Coating operators can use the wet film thickness meter to inspect, apply and adjust the wet film thickness at any time. During construction, the detection frequency of wet film thickness can be arbitrary. When spraying large and flat surfaces and skilled operation , the detection frequency can be smaller. In the case of complex structure and unskilled operation, the detection frequency can be higher. It is generally required to evenly distribute measurement points on every 10㎡ of coating surface, generally 5 measurement points are distributed in 1㎡, each measurement is three times, and the average value is measured to improve the reliability of wet film thickness.
In architectural coatings, the surface of reinforced concrete does not require or allow destructive tests, so the measurement of wet film thickness can be used to replace the measurement of dry film thickness.
In most cases, the measurement of wet film thickness is only an auxiliary means to ensure the dry film thickness. For inorganic zinc-rich coatings and some fast-volatile coatings, the ratio of dry/wet film varies greatly, so only the wet film thickness is used to estimate The dry film thickness may bring wrong results, and the dry film thickness shall prevail .
(2) Detection of dry film thickness: The importance of measuring dry film thickness is to ensure that the coating reaches the specified thickness and avoid premature failure of the coating due to inappropriate thickness. The measurement of the dry film thickness needs to be determined with a dry Film Thickness Gauge after the coating film is completely dry. The commonly used dry Film Thickness Gauges are: magnetic Thickness Gauge, fixed probe Thickness Gauge, eddy current gauge and destructive Thickness Gauge. Magnetic Thickness Gauges are widely used at present. Among them, there is a pen-type Thickness Gauge, which is very convenient for on-site inspection and not for precision inspection . When measuring, the magnetic probe at the tip of the pen needs to be in contact with and perpendicular to the coating surface. When the tension of the spring exceeds probe on the iron substrate, the magnetic probe at the tip of the pen is disconnected from the coating surface, and the reading at the moment of separation is dry film thickness.
When testing, the selection of measurement points should pay attention to the uniformity and representativeness of the distribution. For large-area flat surfaces, measure one point every 2㎡, measure each point three times, and calculate the arithmetic mean value. Welds, rivets and other parts that are really difficult to measure may not be measured, but in order to prevent the coating from being too thin, it should be painted by hand. For small areas or components, it is necessary to ensure that each side should have more than 3 detection points. The requirements for dry film thickness are generally as follows: the average value of all thickness measurement points should not be lower than 90% of the specified dry film thickness; the number of measurement points that do not reach the specified dry film thickness should not exceed 10% of the total number of measurement points. Those who do not reach the specified thickness should be dealt with as follows: the pass rate is lower than 80%, and a full recoat is required. The pass rate is 80%~90%, and local paint should be done according to the situation; welds and rivets need to be repainted with a paint.
The film thickness of many coatings exceeds the specified standard, which is generally not a problem, but it brings excessive loss of coatings and an increase in coating expenses. However, high film thickness cannot produce defects such as excessive sagging, wrinkling or cracking. Since the coating is too thick, it will affect the volatilization and complete drying of the solvent, as well as the curing of the next coating, so attention . When the film thickness exceeds 10% of the specified maximum dry film thickness, try to solve it.
