Coating Flexibility of Prepainted Sheet ASTM D4145
The ASTM D4145 method describes a T-bend test to evaluate flexibility and adhesive strength of coatings on precoated metal sheets. Coating flexibility is evaluated by coating cracks formed due to bending stresses. Adhesive tape pull-off test checks coating adhesion strength.
Precoated metal sheets can experience cracking and peeling of coatings, when subjected to rolling, bending or similar deformation during fabrication. The ASTM D4145 test evaluates flexibility and adhesiveness of organic coatings on precoated metallic sheets. The types of damage to coating evaluated by this test are cracking and loss of adhesion, caused by bend stresses. The metal substrate must be capable of withstanding the bending stresses without failure or excessive grain development. The test is useful in selecting the appropriate coating material for precoating on to metal substrates that will undergo deformation during fabrication. Inorganic coatings are also being evaluated in the industry, using ASTM D4145 methodology. The standard used are inch-pound units.
The test may be preceded if necessary, by UV, salt spray, or other accelerated ageing tests.The axis of bending, test temperature and any other user requirements are established prior to testing. The ASTM D4145 procedure involves folding the precoated sheet upon itself, several times, with the coated surface on the exterior, using a bending apparatus. A die may also be used. Each bend configuration is called a T-bend. A single 1800 bend is termed 0T, two bends termed 1T and so on till coating failure. After each bend the specimen is examined at low magnification, for coating cracks. The adhesion strength is evaluated by pressure sensitive adhesive tape which is placed on each bend and quickly removed. The tape is observed to assess any coating pick-off. The T-bend rating is the minimum number of thicknesses of metal around which the sample is bent till coating failure. If a die is used, the 1800 bend is around the die and the T-bend rating is the number of thicknesses of metal equivalent to the diameter of the die.
The coated specimen should be of at least 2 inches (50 mm) across the bend direction, with length sufficient to make the required number of bends. A 2 inch by 6 inch size is convenient. A triangle-shaped specimen is often used, as it leaves a portion of each bend exposed for later examination and for a permanent record.
The result of the T-bend test is reported as T-bend rating for the two cases of no paint fracture and no pick-off. The associated experimental set-up and material identification data are also reported.
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