Paints and varnishes Coating materials and coating systems for exterior wood

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prENV 927-8 CEN/TC/139/WG2/N343d

Paints and varnishes – Coating materials and coating systems for exterior wood

Part 8: Pull-off test for the assessment of the wet adhesion

of exterior wood coatings


This European Standard has been prepared by Technical Committee CEN/TC 139 ”Paint and varnishes”, the secretariat of which is held by DIN.

This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by …………. (month, year), and conflicting national standards shall be withdrawn at the latest by …………….. (month, year)
EN 927 consists of the following parts under the general title ”Paints and varnishes – Coating systems for exterior wood”:
Part 1: Classification and selection

Part 2: Performance specification

Part 3: Natural weathering test

Part 4: Assessment of the water-vapour permeability

Part 5: Assessment of the liquid water permeability

Part 6: Exposure of wood coatings to artificial weathering using fluorescent UV and water

Part 7: Assessment of knot and tannin stain resistance

Part 8: Pull-off test for the assessment of the wet adhesion of exterior wood coatings

According to the CEN/CENELEC Internal Regulations, the national standards organisations of the following countries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.
This standard describes a method to assess the wet adhesion by a destructive pull-off test with a good reproducibility of individual scores.


Most of the currently used exterior acrylic coating systems are characterised by their ability of being penetrated by moisture both into and out of the wood. Normally, a good adhesion of these coating systems at a higher load of humidity cannot be achieved. In cases of an already damaged coating film or an increased presence of humidity (i.e. construction during winter) in the boundary to the wood surface, only a short-term exposure to water is needed to detach a coating film with an insufficient wet adhesion completely.

At present, the adhesion test is being performed according to the European Standard “Cross-cut test” ISO 2409; 1994. This standard defines a test method for the estimation of the resistance of a cross-cut to the substrate coating against detachment. The long-term use of this test method for exterior wood coatings showed that comparable and repeatable results could not reliably be guaranteed.
This new fixed method is suitable for the testing of exterior wood or wood based material coatings and describes a method for the testing of the wet adhesion of a single-layer or multilayer coating system consisting of paints, varnishes or similar products. In this process the force, which is necessary to detach or pull-off the coating vertically from the substrate is measured. Furthermore, the modulus of fracture of the coating is assessed

1 Scope

This part of EN 927 specifies a method for the assessment of the wet adhesion of coating systems for exterior wood by determining the pull-off strength of water-exposed coated wood panels.

The test result shall be stated in MPa and a letter code for the assessment of the modulus of fracture shall be used.

2 Normative references

EN 927-1 1996 Paints and varnishes – Coating materials and coating systems for exterior

Wood – Part 1: Classification and selection.
EN ISO 2808 1999 Paints and varnishes – Determination of film thickness (ISO 2808:1997).
ASTM D 4541 2002 Standard Test Method for Pull-Off Strength of Coatings Using Portable

Adhesion Testers.

DIN EN 24624 2002 Paints and varnishes – Pull-off test for adhesion.
EN ISO 2409 1994 Paints and varnishes – Cross-cut test.
ISO 554 1976 Standard atmospheres for conditioning and/or testing – Specifications

3 Apparatus and materials

3.1 Test apparatus
Mobile test apparatus as described in ASTM D 4541-02 or tension testing machines can be used for the performance of the test method outlined in section 6.
3.2 Test dollies
Unless otherwise specified, test dollies made of steel or aluminium with a diameter of 20 mm are used. These should have a sufficient thickness to pretend any deformation of the dollies during the performance of the test.

3.3 Adhesives

In order to achieve a failure of adhesion within the coating it is important that the adherent properties of the adhesive are better than those of the tested coating. In a prejudgment of the adhesives the suitability for this application has to be clarified. The adhesive has to be water resistant and shall not penetrate the coating. The curing period of the adhesive should not be longer than the scheduled cycle of the test.

Annotation: The adhesives Actilane® or Metallon® are suitable in most cases.

4 Test substrates

4.1 Substrates
The tests are performed using beech multiplex ply wood (15 mm; 9 to 11 layers; water resistant glued). The selected substrate should be free from damages, knots, cracks or resin pockets. Before it is processed the substrate has to be conditioned equably at (11 ± 2)% wood humidity.
4.2 Manufacturing of the substrate
The samples should be free from deformations with dimensions of 300 mm x 50 mm x 15 mm.

To achieve an optimal surface they have to be sanded evenly with grain size 220 before the application.

5 Manufacturing of coated test samples

5.1 Conditioning
Before the coating, the substrate has to be conditioned in an controlled environment at (20  2)°C and a relative humidity of (65  5)% until constant mass is achieved. Alternatively, a controlled environment of (23  2)°C and a relative humidity of (50  5)% can be applied.
5.2 Manufacturing of the test samples
Three test samples for each tested coating system are selected and sanded before the coating.

According to the manufacturer’s instructions, the coatings have to be applied on the front (convex) side of the substrate. The back side should remain uncoated in order to achieve a quick and even compensation of the wood humidity. The amount of coating being applied on each panel is recorded and the average value for the three test samples calculated. If possible, results should be stated in g/m² or, alternatively in µm liquid film thickness.

The coated test samples have to be dried and stored at the conditions specified under section 7 for the fixed period in the controlled environment.

6 Procedure

6.1 Testing conditions
The tests have to be performed in the controlled environment at (20  2)°C and a relative humidity of (65  5)% or, alternatively in the controlled environment of (23  2)°C and a relative humidity of (50  5)%.
6.2 Preparation of the samples
Seven dollies for each sample are coated with an appropriate adhesive according to the manufacturer’s instructions. A thin layer of adhesive shall be applied uniformly on the dollies to achieve a solid continuous and even bond, surplus adhesive has to be removed immediately if possible.

6.3 Order of the test samples
Seven dollies for each sample are adhered in a row with a distance of at least 15 mm to the edge of the sample and 25 mm to each other. The remaining 10 mm are intended for the determination of the coating thickness according to DIN EN ISO 2808.

The adhesive is applied evenly on an acetone cleaned dolly and fixed on the testing surface. The curing period has to be taken from the manufacturer’s instructions, if not available, a curing period of at least 24 hours has to be allowed.

After the curing, the coating is cut through 0,3 mm with an appropriate cutting device in a ring-like manner around the edges of the dollies. A ring width of at least 3 mm is recommended.

6.4 Testing procedure
The tests are performed after the conditioning and preparation of the samples on the seventh day after application of the coating.

0,5 ml of deionised water is deposited into the ring groove. After a penetration period of 2 hours the testing is carried out.

The testing device according to ASTM D4541-02 is placed vertically on the test surface. The tensile stress (pull-off resistance) determined by pumping or tearing is recorded in MPa and the modulus of fracture is stated as set in section 6.5.1.
6.5 Evaluation
Both the best and the worst result of the seven measured values of each sample are rejected. However, if the fracture occurs mainly within the adhesive, this value has to be rejected. Does the fracture occur mainly within the adhesive in more than two cases the testing has to be repeated completely.

The average value stated in MPa of the remaining 15 single values of all three samples and the standard deviation is determined. The recommended pull-off resistance should be above the threshold of 1,5 ( 0,15) MPa. After re-drying of the samples an optical assessment of the modulus of fracture is carried out (section 6.5.1).

6.5.1 Modulus of fracture
Useful for the description of the observed results by visual inspection is the following scheme according to DIN EN 24624. For the optical assessment all test results (also the rejected ones) and the modulus of fracture are examined.

A = cohesive fracture in the subsoil

A/B = adhesive fracture between subsoil and top coat

B = cohesive fracture within the top coat

B/C = adhesive fracture between top coat and intermediate coat

-/Y = adhesive fracture between primer and adhesive

Y = cohesive fracture within the adhesive

Y/Z = adhesive fracture between adhesive and dolly

7 Test report

The test report shall contain at least the following information:

  1. all details necessary to identify the product tested, including name and address of the manufacturer or supplier of the coating system tested, name or other identification marks of the coating system tested, description of the coating method, quantity and thickness of the applied coating;

  2. a reference to this standard and used test apparatus;

  3. name and address of the testing laboratory;

  4. identification number of the test report;

  5. any deviations from the test methods specified;

  6. technical specification or brand name of the applied adhesive;

  7. test results with standard deviations and date

  8. climatic environmental conditions.

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