Hardness Determinations of Various Polymer Coatings
Objectives: Students will
- test coating hardness by the pencil test.
- determine which coatings have the best and worst hardness property.
- compare hardness properties of coatings to other properties.
Applicable Science Concepts:
- Following testing proceedures
- Variable control
- Pressure control
- Coating samples in their original containers
- Glass slides
- Set of calibrated drawing leads (preferred) or equivalent calibrated wood pencils meeting the following scale of hardness:
|6B- ||5B- ||4B- ||3B- ||2B- ||B- ||HB- ||F- ||H- ||2H- ||3H- ||4H- ||5H- ||6H |
| || |
- Abrasive paper (grit No. 400)
Manufacturers provide instructions for the use of their products, and these instructions should be followed completely. Protective gloves and lab coats should be used when preparing and handling the glass slides with coatings, and safety glasses should be used at ALL times.
Taken from the ASTM Standard Test Method for Film Hardness by Pencil Test: ASTM D 3363 - 74 (1989) http://www.astm.org/cgi-bin/SoftCart.exe/BOOKSTORE/COMPS/CONTENTS/71.html?L+mystore+mhth9641.
- Each group will need one glass slide of each coating obtained from the teacher.
- A set of pencils with a hardness scale should be ready to use. Each group could obtain one of the pencils out of the set and test all of the different coatings with their pencil hardness. This will increase communication and sharing after the lab.
- Each of the lead pencils should have approximately 3/16 to 1/4 in. (5 to 6 mm) of wood removed from the point of each pencil. A draftsman-type mechanical sharpener should be used, if possible, to leave a smooth cylinder of lead.
- Holding the pencil at an angle of 90o (straight up and down) to the abrasive paper, rub the lead against the paper in only ONE direction until a flat, smooth and circular cross section is obtained, free of chips or nicks in the edge of the cross section.
- Place the coated panel on a level, firm, horizontal surface, such as the top of a lab table.
- Starting with the softest lead, hold the pencil against the film at a 45o angle (point away from the operator) and push away from the operator. Exert sufficient uniform pressure downward and forward so that one of two results occur; one, the pencil will cut or scratch the film, or two, the edge of the lead will crumble.
- Repeat the process up the hardness scale until a pencil is found that will scratch the coating or will cut through the film to the glass slide below. You can feel for scratches with your fingernail.
- You can do the scratch test, gouge test, or both tests. Scratch Hardness: at which hardness the coating is scratched. Gouge Hardness: at which hardness the coating is cut or gouged.
The one/two endpoints as follows:
- Gouge Hardness – The hardest pencil that will leave the film uncut for a stroke length of at least 1/8 in. (3 mm).
- Scratch Hardness – The hardest pencil that will not rupture or scratch the film.
- The make and grade of lead or pencil used
- Any deviation from standard conditions, including roughness in the finish.
Have groups of students share their data with other groups by putting their data either on the board or on an overhead. Students should discuss why hardness is important. Where would you use softer or harder coatings? Why? They can then compare the results of the hardness test to other physical characteristics they have already observed.
OUR TEST RESULTS (Scratch Hardness):
|A: Armorall Car Wax – none ||E: Epoxy – F |
|B: 100% Acrylic Latex – HB ||F: Krylon Spray Enamel – F |
|C: Oil Based High Gloss Enamel – none || G: Kilz Spray – none |
|D: Krylon Fusion – none ||H: Kilz in a Can – none |
Polymer Science Learning Center | Department of Polymer Science | University of Southern Mississippi