Designation: D – 01Standard Test Method for Evaluating Adhesion by Knife1 This standard is issued under the fixed desig. From ASTM D Standard Test Method for Evaluating Adhesion by Knife. Section 3 | Summary of Test Method. Adhesion is determined by making an “ X”. Buy ASTM D Standard Test Method for Evaluating Adhesion by Knife from SAI Global.
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ASTM D – 18 Standard Test Method for Evaluating Adhesion by Knife
Adhesion testing may be a requirement of a coating specification, or may be used for coating system performance qualification in a laboratory. Adhesion testing is also a valuable indicator for determining the integrity of coating systems that have been in service for extended periods of time, may require maintenance, and overcoating is a strategy being considered; and is frequently used during coating failure investigations. Irrespective of the application of the test, there are standard test methods procedures for conducting adhesion testing that should be followed to ensure consistency, especially when performing comparative analyses.
This article discusses tape and knife adhesion test methods performed according to standardized ASTM International. Tensile pull-off adhesion is the subject of an article by Melissa Swogger that is also available on the KTA University site. While tape and knife adhesion tests are generally regarded as more subjective than their tensile pull-off adhesion test counter parts, the tape and knife adhesion tests can be much more revealing of the true adhesion properties of a coating system.
This is primarily due to the directional forces applied to the coating system during the tests.
Standard Test Method for Evaluating Adhesion by Knife
The shear tests are oftentimes more definitive because they better replicate the way in which coatings fail. That is, coatings generally do not disbond from a substrate or other coating as a result of forces that are exerted perpendicular to the surface. Undercutting and peeling can occur as a result of shear forces. The test was developed for assessing the adhesion of coating to steel, but can be used on other hard substrates.
The test has also been used successfully on softer substrates e. Both tests are performed by scribing the coating to the substrate with a sharp knife blade in a specific pattern, applying a pressure sensitive tape and then rapidly pulling the tape from the surface. When the coating is greater than 5-mils thick an X-cut with each leg approximately 1. When the coating is less than 5-mils thick, a cross-cut lattice pattern is created with either six or eleven cuts in each direction.
For coatings up to 2. Once the incisions are made, a pressure sensitive tape with adhesive properties conforming to the requirements of the standard; Figure 1 is applied over the incisions and pressed in place using a pencil eraser. After removal of the tape, the amount of coating removed from the substrate or underlying coating is rated.
It is important to evaluate the coated surface and not the back of the tape, since coating debris from the incisions is often removed by the tape. Adhesion is rated based on the scale provided in the ASTM standard. Table 1 provides the evaluation criteria for Method A; Table 2 provides the evaluation criteria for Method B.
The standard also contains a pictorial guide to aid in the rating of the cross-cut Method B.
However, the subsequent delamination is a result of shear forces. When appropriate, the nature and location of the separation is documented.
A cohesive separation is one that occurs within a coating layer; an adhesive separation is one that occurs between coating layers or between the coating and the substrate. Generally, adhesion ast, of 4 and 5 are considered aetm, adhesion values of 2 and 3 s6677 considered marginal and adhesion values of 0 and 1 are considered poor.
Precautions are included regarding the use of the test on coatings with a high cohesive strength that may appear to have worse adhesion than one that is brittle and fractures easily. In addition, the method is not to be used on overly thick coatings that cannot be cut to the substrate with a utility knife blade in one stroke.
The tip of a knife blade is then inserted into the intersect of the two incisions and used to attempt to lift the coating from the substrate or an underlying coating.
Adhesion is rated on an even number scale between 0 and 10, with 10 having the best adhesion and 0 the worst. A description of the adhesion criteria is included in Table 3. Generally, adhesion ratings of 8 and 10 are considered good, adhesion values of 4 aastm 6 are considered marginal, and adhesion values of 2 and 0 are considered poor. There are circumstances and situations that do not allow standard procedures and methods to provide ashm accurate representation of coating adhesion.
Variations in temperature and humidity can affect the efficacy of the method employed.
Asfm chalked paints typically show very good tape adhesion properties, since only the friable chalk layer is removed by the tape the weakest planeleaving the coating system intact. The adhesion by knife test may provide a more accurate picture astmm the actual adhesion characteristics. If the tape adhesion test is required, the chalking should be removed from the area prior to performing the test.
Adhesion testing conducted on acrylic elastomeric coatings applied to cement or stucco asm be evaluated using the tape adhesion test. Further, the results of any knife adhesion tests performed on these coatings must be carefully considered.
Never the less, the adhesion is oftentimes considered to be acceptable under these conditions. Adhesion tests that consistently reveal an adhesive break between coats or a cohesive break within a coat do not provide any information relative to the adhesion of the coating or coating system to the substrate. Knife adhesion tests may be used to assess the bond to the substrate when the tape adhesion test asrm revealed a break somewhere higher up in the coating system.
Simply stated, the ASTM standard test procedures have limitations that need to be considered when making judgements or decisions based wstm the test results. Recently, have seen hubs in service within one year showing heavy signs of spots, blisters, etc due to coatings applied at low temperatures at the construction stage.
Compliance concerns here maybe need further investigation before final releases. Its a major concern. That is a correct statement. Tensile adhesion tests exert a force perpendicular to the coating system. Tape and knife adhesion exert a prying type of force.
In many instances a coating system can have excellent tensile adhesion but poor shear adhesion. In order to be successful a coating system has to have both good tensile and shear adhesion.
Your email address will not be published. Leave this field empty. Inspection Instruments Pipeline Coatings — Volume 2: March 3, at April 7, at 6: August 7, at 8: Hi, You mention on this article that: It is correct this affirmation?