Adhesion Test for the wood finishing

Adhesion strength of the finishing layer is the adhesion between the finishing layer and the substrate underneath. Adhesion is one indicator of the finishing quality. The finishing layer is not only purposed to make the beauty color and appearance but must also give protection to the substrates underneath. No matter how good is the finishing looks, it is useless if it does not have good adhesion. Finishing with poor adhesion will easily come off and will lose its beauty.

What is an adhesion test?

The adhesion test is one of the important methods to evaluate the adhesion between the finishing layer and the substrate underneath. This test is one of the important things to ensure that the finishing has the required quality to make the product fulfill its function. The good adhesion means the finish is able to provide the protection needed for the wood. The adhesion test is one of the standard tests needed for finishing layers for many wood products, such as : furniture, wood panels, wood floors, etc.

Common Adhesion Test Methods in Wood Finishing

Actually, there is already a standard test for conducting adhesion tests in the wood finishing industry, the ASTM D 3359 for America and EN ISO 2409. 

Sample for test

Any adhesion test result will damage the finishing; therefore, it is not recommended to be carried out on one item. In general, adhesion tests are carried out on a sample panel that is considered to represent the finishing in the production process. For this reason, it is necessary to prepare one sample for this test. This sample is a panel of wood with the same color and process as the finishing process in production.

Tools to do the adhesion test

•  Knife (cutter) with a sharp tip

A cutter or knife is needed to make cuts on the finishing layer. This knife must be sharp and thin to produce thin cuts without lifting and damaging the area surrounding the incision.

• Tape with strong adhesive power

Tape is used to pull the finishing layer. This tape must have strong enough adhesive power so that it can represent the pulling power according to the required standard.

• Small brush with soft bristles

A soft brush or cloth is needed to clean the surface before it is taped. 

•  Ruler or template 

Metal plate is needed as cutting template to ensure that the scratches are in accordance with the standard lines

• Hatch knife, a special knife that can make standard scratches with one pull.

Instead of using a knife that will make a single cut, there is a hatch knife that can be used to make many cuts with 1 application. This tool can make the cross-cut with 1 stroke, but it will only work for the flat surface. It cannot be used to make the x-cut and do the cut for the round surfaces. 

Actually, there are several methods of this adhesion test: the cross-cut test, the X-cut test, the pull-off test, and the knife test. There is the explanation of each.

Cross Hatch Test (Cross-cut Test)

This test method is the most common and widely used method in the finishing industry. This test is relatively simple but can provide clear visual results and clear measurements.

Steps:

• Clean the finishing surface from dust and other dirt

The surface must be clean before it is attached and pulled. Clean the dust or any other dirt from the surface by using a soft brush or clean rag. 

• Cutting the finishing layer

Make a series of parallel scratches using a special test knife or cutter, then make other scratches perpendicular to form a checkered pattern (usually the lines made are 6 × 6 or 11 × 11 lines). Use a cutting tool to make 6 parallel lines with a distance between lines of 1 mm or 2 mm depending on the thickness of the layer. There are 2 types of cut sizes; for finishing with a thickness below 60 μm, the distance between cuts is 1 mm. While for finishing layers with a thickness above 60 μm, the distance between cuts is 2 mm.

Make scratches on the finishing layer perpendicular to the first cut so that small squares are formed with a size of 1 mm² or 4 mm² (cross-hatch). These small squares can be 100 pieces (for 11x11 lines) or 25 squares (for 6x6 line cuts)

The knife used must be really sharp so that it only scratches and cuts the parts that are hit by the knife. The direction of the knife must be perpendicular to the surface of the panel so that no finishing layer is lifted due to improper scratching.

The cutting pressure must be enough to touch the substrate but not too strong to damage the wood.

• Pulling with tape

Clean the surface with a brush or cloth so that it is free from dust or dirt that sticks. Then stick the tape to the crosshatch evenly. Press and rub the tape evenly over the entire surface so that an even adhesion is formed over the entire surface; wait for 1 minute to ensure perfect adhesion between the tape and the surface.

Pull the tape quickly perpendicular to the surface and see the results.

• Evaluation of Results

Observe the surface and the tape to see the test results; use a magnifying glass if necessary. Based on the number of boxes that are pulled or lost, the results of this test are classified from 0B (very poor adhesion) to 5B (perfect adhesion).

the standard adhesion test result

finish with good adhesion

finishing with poor adhesion

The other adhesion test method

Cross-cut is the most common adhesion test and is often used for finishing layers on wood and wood products. In addition to this test, there are several other types of tests that are carried out for products with special substrates or objects with special shapes.

• X cut

This X cut is in principle very similar to the cross cut, but the cuts made are only 2 lines that form a cross. This test is often carried out on objects with textured shapes, round shapes, or other shapes that do not allow testing with cross cuts with many cuts.

The test process itself is very similar to the cross-cut test. The surface of the finishing layer is cut with a sharp knife to form a cross. Then the cut part is then attached with tape and pulled as in a cross cut.

The test results are seen from the evaluation results on the finishing layer; if there is no paint layer that is pulled and attached to the tape, then the adhesion is said to be good. If the finishing layer is pulled and released, it means that the finishing adhesion is not good.

• Tape Pull Test

Similar to the cross-cut test but without a scratching process. This test is done by attaching adhesive tape to the finishing surface and then pulling it. This test is done for uneven or flexible surfaces such as: woven, braided, leather that does not allow cutting

• Knife Test (Slash Test)

This test is used to test the adhesive strength of finishing layers with high thickness, such as resin, polyester, or epoxy casting finish.

The finishing layer is cut using a sharp knife, then lifted until the finishing layer is released from underneath. The process of lifting the finishing layer is observed and viewed. Adhesion is considered good if the layer is lifted only on the part that is hit by the tip of the knife; conversely, finishing adhesion is considered bad if the finishing layer next to it is also lifted and peeled off. This test is indeed rather difficult to do and may not be able to provide clear, measurable results like in the cross-cut test.

Things to be considered in the adhesion test process

• This test will damage the goods

Instead of the pull-off test, the adhesion test will damage the finishing layer and goods. Tests with cuts will leave scratches that will damage the appearance of the finishing. Therefore, this test should be done on one sample and not on the product. This test is usually done on the color panel at the beginning of the finishing system manufacturing process. This test can also be done on one sample of the product that is usually made to ensure that the finishing system that is made is correct before being run in the production process in large quantities.

In the ongoing production process, the adhesion test can be done on several samples for each production batch. Several panels can be included in the finishing line process to be used as samples to undergo adhesion tests. The results of this test can be used as a representation of the quality of finishing in ongoing production.

• Make sure the finishing layer is dry

The adhesion test must be carried out when the finishing layer is completely dry so that it can represent the strength of the finishing layer correctly. Make sure that the finishing layer has passed the drying time according to the type of finishing used. For NC-type finishing, a drying time of 24 hours is needed to ensure that the finishing layer is completely dry. For PU-type finishing, a longer drying time is needed; for finishing that requires an oven, make sure the drying process has been carried out according to the product data sheet of the finishing material used.

•  Consider the shape of the goods

The adhesion test is carried out according to the shape of the goods. In general, adhesion tests are carried out using the cross-cut method on flat panels. The results of the adhesion test with this cross cut are considered to be able to represent the adhesion strength on other forms with the same finishing system. However, adhesion tests can also be carried out using other methods, such as x-cut or pull-off tape, for objects with complex shapes.

• Consider the tools and process

To get the accurate result, the test must be done correctly with the good tools. You need the sharp knife and the standard tape. The cutting is very essential; make sure you cut with a sharp knife in the proper way. The blunt knife will damage the finishing layer and leave a wide cut that makes the finishing damage. Make sure the knife is cutting perpendicularly on the surface; the oblique incision will lift the finishing layer and make the test inaccurate.