|Measuring the Hardness of Paints Using Pencils
by Paul Guevin, Jr.
The two most frequently used test methods in the coatings industry measure surface hardness and adhesion to a given substrate. These test methods have been used by the coatings industry since the early 20th century. The test methods currently accepted by industry for these two tests are ASTM D 3363, Test Method for Film Hardness by Pencil Test, and ASTM D 3359, Test Methods for Measuring Adhesion by Tape Test.
Paints need to adhere to the surface to which they are applied. The test methods in D 3359 determine just how well the organic coating is bonded to the substrate. Method A of D 3359 involves using an X cut and tape test to measure adhesion. This is typically used in the field or for coatings having a film thickness greater than five mils (127 µm). Method B involves using a cutting tool to manually make a grid composed of up to 100 squares and then the tape test to evaluate adhesion. This is typically used in the laboratory and on thin films less than five mils thick (127 µm).
Likewise, paints need to be hard enough to protect a surface, yet not so soft that they could dent and be tacky. Hardness is a measure of the cure of the film or how the coating has softened when in contact with various liquids. When an organic coating cures, it does so in either of two processes. It can allow the solvent used in the formulation to evaporate, thereby leaving a hard film. Lacquer is an example of a coating that cures by this process. The second process by which paints cure is when the coating crosslinks. Crosslinking is a chemical process where the polymer portion of the paint formulation reacts to form a dry, hard and, typically, solvent-resistant film. Test Method D 3363 is an easy and inexpensive technique to determine how hard or soft an organic coating is.
All paint manufacturing companies list the hardness of their products in terms of pencil hardness in their technical literature. This lets the end-user know if the hardness is appropriate for the end-use application.
Pencil hardness is a popular test in the paint industry. It is used to determine scratch (mar) and/or gouge hardness of coatings. Its popularity is due to two of its basic strengths; it is inexpensive and it is quick.
The coatings industry performed pencil hardness tests long before there was an ASTM test method. The use of pencils to measure the hardness of organic coatings and paints goes far back in history. The Paul N. Gardner Companys collection of back editions of The Gardner-Sward Handbook shows that the first mention of pencil hardness testing of paint occurred in 1925. At that time, the Dixon Crucible Company offered 17 different hardness drawing pencils with which to measure the hardness of paints and coatings.
In 1923, the Scientific Section of the National Paint, Varnish and Lacquer Association (NPVLA), now the NPCA (National Paint and Coatings Association), published the first known test method on pencil hardness. (1) Like any test method, this one was shortly improved upon and published. (2) The improvements consisted of redefining the shape of the pencil, the pressure on the pencil, and the angle at which the pencil was moved.
Later, the federal government established quantitative requirements for their paints in the form of federal specifications. TT-E-529, Semi-Gloss Alkyd Coatings, required a minimum pencil hardness of 2B. Since ASTM Test Method D 3363 hadnt yet been issued, they wrote their own test method as follows:
4.4.26 Pencil hardness. Prepare a film as specified in 4.4.22. Strip the wood from the lead of a standard drawing pencil for a distance of 1/4 inch. With a rotatory motion, square the point of the exposed lead against No. 400 carbide abrasive paper. Hold the pencil at approximately 45° and push forward against the film using a pressure just short of breaking the lead. Clean the mark with a soap or art gum eraser. Hold the plate at an oblique angle in strong light and examine for marring. Determine the softest pencil that will mar the film. Express the film hardness as the next softest pencil and check for compliance with table III.
Since there was a need for an industry accepted standard test method, Committee D01 undertook the task to developed a method and ASTM D 3363 was adopted in 1974. At that time, the test method was used to measure the cure of coil coatings and paints. In 1992, D 3363 replaced the abovementioned federal test method.
Hardness, as we understand it here, is the capacity of a given surface to resist scratching, marring or gouging. When expressing the measurement of pencil hardness, we do so with values that range from 6B (softest) to 9H (hardest). Test Method D 3363 goes to 6H. This property is verifiable and can be an indication of results from objects being placed on various painted surfaces that can mar the coating. ASTM D 3363, Test Method for Film Hardness by Pencil Test, is now the appropriate test method for this property.
Today, D 3363 describes a procedure for a rapid, inexpensive determination of the film hardness of an organic coating on a substrate in terms of using leads or pencil leads of known hardness. A coated panel is placed on a firm horizontal surface. The pencil is held firmly against the film at a 45° angle (point away from the operator) and pushed away from the operator in a 0.256-in. (6.5-mm) stroke. The process is started with the hardest pencil and continued down the scale of hardness to either of two end points; one, the pencil that will not cut into or gouge the film (pencil hardness), or two, the pencil that will not scratch the film (scratch hardness).
This test method is especially useful in developmental work and in production control testing in a single laboratory. It should be recognized that the results obtained may vary between different operators and laboratories. Every effort should be made to standardize the hardness of the lead used and a technique followed. If used as a basis for purchase agreements, this test method will achieve maximum precision if a given set of referee pencils are agreed upon between the purchaser and the seller. Obviously, the results may be inconsistent from one operator to another because we all know different people will push a pencil at different pressures each time.
Subcommittee D01.23 realizes there are shortcomings of this standard. The most overlooked necessity of test method D 3363 is the requirement to report the make and grade of lead or pencil used. We realize there is lack of hardness standards within the pencil industry. //
(1) Wm. H. Wilkinson, A Method for the Determination of the Comparative Hardness of Varnish Films, Scientific Section Circular 184, 302 (June, 1923).
(2) H. A. Gardner and H. C. Parks, Hardness of Varnish and Other Films, Scientific Section Circular 228 (March, 1925).
Copyright 2002, ASTM