Testing of Tinplate Lacquer
Testing of tinplate lacquers
With most tinplate containers having interior protective coatings, tests of lacquers and tinplate properties that may affect lacquer behaviour are becoming increasingly important.The control of lacquers themselves is beyond the scope of this handbook, here only those aspects will be discussed that are of particular relevance to the tinplate user.The wetting of tinplate sufaces by lacquers is a complex process, depending on the surface chemistry of the plate and the quantity and the composition of the oil and the chemistry of the lacquer. A number of tests have been developed to quantify the “surface energy“ of tinplate with respect to wetting by lacquers, including the use of a surface tension balance.
Tests of lacquer continuity
Many of the tests in use to reveal eyeholes and other gaps in the lacquer coating depend upon making the gaps visible by means of a suitable coloration, and then counting them. In this way local areas of dewetting or damage are immediately apparent. Acidified aqueous copper sulphate solutions are commonly used for this purpose. The use of sulphuric acid solutions (5 g copper sulphate crystals, 1.5 ml concentrated sulphuric acid in 1 litre of water) provides a firmly adherent deposit of copper in lacquer discontinuities after immersion of a test-piece in the solution for 30 minutes.Hydrochloric acid based solutions (200g copper sulphate crystals and 10 ml concentrated hydrochloric acid in 1 litre water), act more quickly, an immersion of 2-5 minutes proving sufficient, but the copper deposits do not adhere as firmly as in the case of sulphuric acid treatment. Another method is to immerse the test-piece in an acid tinplating solution with a tin anode and apply a current of about 0.1 A/ dm2 to the tested surface so as to deposit visible tufts of tin on discontinuities in the lacquer.
Whereas the methods described above provide no indication as to whether or not the discontinuity extends through the tin coating to the steel base, other methods provide separate information on exposed steel and tin areas. The sulphur dioxide test already described in the section on tinplate corrosion testing may be used for lacquered plate. It is particularly useful for revealing areas of exposed steel where lacquer protection has been diminished by drawing or deformation, but the test is too slow for routine control measures.
Another method of determining the area of exposed steel is the anodic porosity test. In one such test, a test cell of the kind used for general coulometric measurements is set up. The cell includes a calomel reference electrode and a platinum wire counter-electrode together with arrangements for holding the test piece. The electrolyte is 0.5M ammonium thiocyanate, maintained at constant temperature, preferably 25°C . C ut edges and any exposed connecting wire must be well protected by wax or otherwise excluded from contact with the solution. The test piece is connected in series with a direct current source, a microammeter and the counter-electrode.
The reference electrode with its tip near the test piece is connected to the test piece through a voltmeter with high input impedance. If there is no exposed steel, the voltmeter will give an unstable reading, but usually it will settle down to a potential of around -700 mV. C urrent is then passed and increased so as to displace the potential to a selected test value, which may conveniently be -500 mV. The current then passing is an index of the exposed steel area.
To estimate area of tin exposed by lacquer discontinuities, an anodic test may be made using a 10 g/litre sodium hydroxide solution as electrolyte. A fixed voltage is applied to the sample as anode and the current recorded. The value of the current provides a measure of the exposed tin area.
All these electrolytic lacquer discontinuity tests may be performed using the can whose internal coating is to be tested as the test cell. The can is made cathodic with respect to a central anode, the electrolyte typically being sodium chloride solution (10 g sodium chloride per litre of water). A fixed voltage, usually 5 V, is applied across the cell, the current flowing between the can and the carbon electrode being measured with a milli-ammeter. A commercial form of this test is available which allows measurements in 5 seconds, the value of the current flow permitted for satisfactory performance depending upon the particular requirements of the intended service.AC impedance is another technique that has been used by many major companies especially as a quality control method.
Lacquer adhesion tests
Loss of lacquer adhesion may take the form of detachment caused by deformation during canmaking or undermining of the coating by the corrosive action of the can contents.For external lacquers, undermining may be caused by exposure to steam, hot water or other sterilising media. Ideally, the method of test should reproduce service conditions as closely as possible, with suitable control and perhaps with some intensification. Thus, if can making machinery is available, the most effective test of resistance to canmaking operations is to stamp a few ends from the lacquered sheet and then to examine the coating for discontinuities.
For particular applications it may be worthwhile to immerse sample ends in some convenient product or solution to reveal any extension of damage that is produced. If canmaking machinery is not available, an impact test can be performed with laboratory apparatus. A test-piece of lacquered tinplate is bent round a radius of 5 mm and then deformed by a weight of 0.75 kg dropping from a height of about 500 mm so that the radius of curvature of the test sample varies continuously from 0 to 5 mm. The deformed test piece is then examined for lacquer discontinuities.
A simple test generally used to estimate the degree of lacquer adhesion is the Scotch-tape test. A length of 25 mm wide pressure-sensitive adhesive tape is firmly applied to a pattern of scratches penetrating the lacquer film and then pulled away with a sharp jerk carrying with it particles of lacquer from poorly adherent films.The scratch pattern should be made in repeatable form, close spacing of the scratches helping to give a more definite result. A steel comb with 20 teeth spaced 1 mm apart, used to make two sets of scratches at right angles to one another, gives a suitable pattern.
Alternatively, one of the scratch testers produced for laboratory testing of the resistance of paint coatings to penetration by a loaded needle may be adapted. Such equipment gives a reproducible pattern of parallel scratches, but the ballheaded needle usually specified for these devices must be replaced by a pointed one.
Such testers are commonly provided with an electrical circuit including a battery and small light bulb, the circuit being completed by the needle-testpiece contact in such a way that the bulb lights when the needle penetrates through the lacquer film to the electrically conductive tin coating. Using such equipment, a pattern of scratches can be made that extends through the lacquer film but does not damage the tin coating. The appearance of the test pieces, after the Scotch-tape has been pulled away, will give a rough guide to lacquer adhesion, but interpretation of the results is aided if they are compared with a set of reference standards.
To assess failure of adhesion during heat processing, Scotch-tape tests may be made after test samples have been exposed either to boiling water for 30 minutes or to steam in a pressure vessel. For the pressure vessel test, it is common practice to immerse part of the test piece in water and, after treatment for 30 minutes at 120°C , to inspect separately the parts of the lacquered surface exposed to water and steam.For coatings on crown corks to be used as closures to bottles, which pass through a cleansing and sterilising treatment, a test solution similar to that used for the treatment may be employed. A common form of test is to immerse the crown corks in sodium hydroxide solution (25g sodium hydroxide per litre water) for 30 minutes at 70°C and then inspect for signs of lacquer detachment.
To test the undermining resistance of internal lacquer coatings for food cans, the lacquered test piece may be immersed in reagents such as starch solution or citric acid solution and heat processed before subjection to a Scotch tape test. Alternatively, the lacquered test piece may be scratched, (as described previously for the Scotch tape test), before immersion in the test solution, so yielding a more positive result. For even faster results, an anodic current of 1030µA/cm2 may be applied to the scratched lacquered test piece while immersed in solution. A cathodic polarisation test is often employed in the Institute’s laboratories.