Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation

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This test method covers a procedure for rapidly deforming by impact a coating film and its substrate and for evaluating the effect of such deformation.

Summary of Test Method

The organic coatings under test are applied to suitable thin metal panels. After the coatings have cured, a standard weight is dropped a distance so as to strike an indenter that deforms the coating and the substrate. The indentation can be either an intrusion or an extrusion. By gradually increasing the distance the weight drops, the point at which failure usually occurs can be determined. Films generally fail by cracking, which is made more visible by the use of a magnifier, by the application of a copper sulfate (CuSO4) solution on steel, or by the use of a pinhole detector.

Significance and Use

Coatings attached to substrates are subjected to damaging impacts during the manufacture of articles and their use in

Service. In its use over many years, this test method for impact resistance has been found to be useful in predicting the performance of organic coatings for their ability to resist cracking caused by impacts.

Apparatus

Tester, consisting of a vertical tube to guide a cylindrical weight that is dropped on a punch resting on the test panel. Guide Tube, 24 to 48 in. (0.6 to 1.2 m) long mounted vertically in a base plate. A slot is cut lengthwise on one side of the tube to act as a guide for a cylindrical weight that fits inside the tube. Graduations are marked in inch-pounds along the slot. The base is constructed so that a thin flat panel can be inserted at 2 in. (50 mm) below the tube.

Weight, metal cylinder, made to fit inside the guide tube. A pin is fitted into one side of the weight to act as a guide

by riding in the slot of the tube and to serve as a handle by which the weight can be raised and released and serve as the indicator of inch-pounds (kilogram-meters).

Indenter—steel punch with a hemispherical head having a diameter of either 0.500 in. (12.7 mm) or 0.625 in.

(15.9 mm). The head rests on the test panel and the punch is held vertically by a guide ring.

Panel Support—A steel fixture with a 0.64-in. (16.3- mm) diameter cylindrical hole centered under the indenter for supporting the test panel.

Magnifier.

Pin Hole Detector.

Reagents

An acidified copper sulfate (CuSO4) solution prepared by dissolving 10 g of CuSO4·5H2O in 90 g of 1.0 N hydrochloric acid (HCl).

Test Specimens

Apply uniform coatings of the material to be tested to steel panels. Prepare a minimum of four coated panels for the material.

Conditioning

Unless otherwise agreed upon between the producer and the user, condition the coated test panels for at least 24 h at 73.5 ±3.5°F (23±2°C) and 50±5 % relative humidity. Conduct the test in the same environment or immediately on removal therefrom.

Procedure

  • Install the punch having the head diameter specified or agreed upon. Place the test panel in the apparatus with the coated side either up or down as specified or agreed upon. Be sure the panel is flat against the base support and that the indenter is in contact with the top surface of the panel. Lightly place the weight on the indenter and adjust the guide tube so that the lifting pin is at the zero mark. Raise the weight up the tube to a height where it is expected that no failure will occur. Release the weight so that it drops on the indenter.

 

  • Remove the test panel from the apparatus and observe the impact area for cracks in the coating. If no cracks are evident, repeat the procedure at a greater height, increasing 1 in. (25 mm) at a time. Once visible cracks are observed, repeat the test five times at each of three heights; slightly above, slightly below, and at that determined in the first trial. Test in a random fashion so that all impacts from one height are not made in succession or on one panel.

 

  • Examine the impacted areas for cracking by one of the following methods:

– Use a magnifier to examine the area for cracks.

– Hold a white flannel-type cloth saturated with the

acidified copper sulfate (CuSO4) solution – over the impacted areas for at least 15 min. Remove the cloth and examine both the test areas and cloth for evidence of copper deposition or iron-rust staining respectively.

 

  • To detect breaks in the film with a pinhole detector, first connect the ground lead from the instrument to the bare substrate and connect the instrument to an electrical power source. Moisten the probe sponge with tap water and slowly draw the probe over the impact area. The presence of cracks will be indicated by an audible alarm.

 

  • For each inch-pound (kilogram-meter) level, tabulate the number of times the coating passed or failed. The value where the results change from mainly passing to mainly failing is the impact failure end point.

Report

Report the following for each coating tested:

  • The inch-pounds (kilogram-meters) at the impact failure end point,
  • Whether intrusion or extrusion was used,
  • Diameter of the punch used,
  • Thickness of coating,
  • Substrate thickness and type of metal,
  • Method of panel preparation, and
  • Atmospheric conditions under which the coated panels were conditioned and tested.

 

Paints and varnishes — Cupping test

This International Standard specifies an empirical method for assessing the resistance of a coating of paint, varnish or related product to cracking and/or detachment from a metal substrate when subjected to gradual deformation by indentation under standard conditions.

For a multi-coat system, each coat may be tested separately or the complete system may be tested.

The method may be carried out as follows:

— Either as a “pass/fail” test, by testing to a specified depth of indentation to assess compliance with a particular requirement;

— Or by gradually increasing the depth of indentation to determine the minimum depth at which the coating cracks and/or becomes detached from the substrate.

Principle

The product or system under test is applied at uniform thickness to flat panels of uniform surface texture.

After drying/curing, the elastic properties of the paint film are determined by first placing the coated panel between two rings, namely the retaining ring and the drawing die. The panel is then pushed by a hemispherical indenter at a steady rate into the die so as to form a dome shape with the coating on the outside.

The deformation is increased either to a depth agreed between the interested parties or until the coating cracks and/or detaches from the substrate, and the result is then assessed.

Apparatus

Cupping test apparatus, which conforms to the design and dimensions shown in Figure 1 and consists essentially of the following components:

Drawing die, made of surface-hardened steel and of which the surface in contact with the test panel is plane-polished

Retaining ring, of which the surface in contact with the test panel is plane-polished and parallel to the contacting surface of the die.

Indenter, of which the part contacting the test panel is of hardened polished steel and forms a hemisphere of diameter. It is preferable to use a mechanically driven indenter, but a hand-operated apparatus can also be used, provided the standard conditions of test can be achieved.

During the test, the indenter shall be prevented from turning and the centre of the spherical portion shall not deviate from the axis of the die by more than 0.1 mm. The indenter shall move at a steady speed of between 0.1 mm/s and 0.3 mm/s during the test.

The tip of the hemisphere, when at the zero position, shall be in the same plane as the face of the retaining ring, in contact with the test panel and in the centre of the bore of the drawing die.

Measuring device(s), which can measure the depth of the indentation made by the indenter to the nearest and the thickness of the test panel to the nearest 0.01 mm.

Microscope or magnifier, preferably with a magnification range extending up to , if necessary to view the panels during or after deformation.

Test panels

Substrate

Unless otherwise specified, select the substrate from one of those described in ISO 1514 and in accordance with the intended use.

The test panels shall be flat and free from distortion and capable of withstanding the cupping test without

Cracking

NOTE if cracking of the substrate occurs before the cracking and/or detachment of the paint film, the test result of the paint film can be reported to be better than the indentation depth at which cracking of the substrate occurred.

Dimensions

The test panels shall be rectangular with the following dimensions:

— Thickness: not less than 0.3 mm 1.25 mm and not more than, as specified, measured with e.g. a micrometer to the nearest.

— Width and length: two tests shall be carried out, either on a single long strip or on two separate panels. The indentation centers in these tests shall be at least 35 mm from any edge, and the distance between any two indentation centers shall be a minimum of 70 mm. The test panels may be cut to size after coating and drying provided no distortion occurs.

Drying and conditioning

Dry (or stove) and age (if applicable) each coated test panel for the specified time and under the specified Conditions. Unless otherwise agreed, condition the coated panels at 23±2 ◦C and a relative humidity of 50±5 ◦C for a minimum period of before testing.

Procedure

Test conditions

Unless otherwise specified, carry out the test in duplicate at 23±2 ◦C and a relative humidity of 50±5 ◦C.

Procedure for a single specified depth of indentation

  • Carry out the following procedure in duplicate (if the results differ, additional tests shall be made).

 

  • Hold the test panel firmly without excessive pressure between the retaining ring and the drawing die with the coating towards the die and with the hemispherical end of the indenter just in contact with the uncoated side of the test panel (zero position of the indenter). Adjust the panel until the central axis of the indenter intersects the panel at least 35 mm from the edge.

 

  • Advance the hemispherical end of the indenter into the test piece at a steady rate between 0.1 mm/s and 0.3 mm/s until the specified depth is reached, i.e. until the indenter has travelled this distance from the zero position.

 

  • Using normal corrected vision or, if necessary, a microscope or magnifier of × 10 magnification, examine the coating of the test panel for cracking and/or detachment from the substrate. If a microscope or magnifier is used, it is essential to mention this fact in the test report to avoid misleading comparisons with results obtained using normal vision only

Procedure for determination of minimum depth of indentation to cause failure

  • Unless otherwise specified, test the test panels in accordance with the procedure given in paper using normal corrected vision, or, if necessary, a microscope or magnifier of× 10 magnification, until a crack is first observed on the surface of the coating and/or the coating begins to become detached from the substrate.
  • Stop the indenter at this point and measure the depth of indentation to the nearest 0.1 mm, i.e. the distance travelled by the indenter from the zero position. Confirm the result by repeating the determination on a fresh panel (if the result differs, further determinations shall be made).

Expression of results

The test results shall be reported in one of the following ways:

— Either whether or not the coating on the test panels passed when tested at the specified indentation depth;

— Or the maximum depth of indentation at which the coating passed, expressed as the mean of two valid results (duplicates) to the nearest0.1 mm.

Supplementary test conditions

To enable the method to be carried out, the following supplementary information needs to be specified in addition to that in the preceding clauses:

  1. a) The material, dimensions and surface preparation of the substrate;
  2. b) The method of application of the test coating to the substrate;
  3. c) The duration and conditions of drying (or stoving) and ageing (if applicable) of the coated panels before testing;
  4. d) The thickness, in micrometres, of the dry coating, together with the method of measurement used, and whether it is a single coating or a multi-coat system;
  5. e) The temperature and humidity during the test.