Coating Reduces Wear of PET Preforms
Wear of PET preforms occurs primarily through friction during transport. In addition to visible quality defects in the finished bottles, such wear regularly causes considerable interruptions to the filling process. To get to grips with the problem, two solutions have crystallized over the years. The first solution, which was to incorporate additives in the plastic, brought measurable improvements but not to the desired extent. The second solution, involving post-coating, often achieved a significant reduction in wear but nearly always affected taste and was unsuitable for use in contact with food. Now, the Tunap Group of Wolfratshausen, Germany, has broken new ground with the development of Tunform PO lubricant and coating agent.
To eliminate all legal uncertainties in relation to the specified measurements and migration limits (as per Article 8 of the Plastics Directive), the company dispensed with all the raw materials commonly used so far for the production of food lubricants – despite FDA or other approvals. Marking a first in the industry, the raw materials used are all registered as food additives. All tests for compliance with migration limits have been passed.
Tribological tests in the translatory oscillation tester as per DIN 51834 unexpectedly revealed significant differences. The coated plastic achieved a friction coefficient (min/max) of 0.038/0.039, which was far lower than the values for the untreated plastic (0.130/0.317). Both the wear diameter and wear depth of the untreated test specimens exceeded the measuring range. So comparison with the coated test specimens was not possible. Neither was calculation of the wear factor, because these parameters are also used in calculating this factor.
The product is preferably applied in the form of an emulsion. The optimum concentration depends on the required protection and generally lies between 10 and 25 %. Uniform application is ensured by spraying. But application with a brush, roller or by dipping is also possible. A pleasing additional cost benefit was also revealed in the large-scale trials. On account of the excellent slip properties of the coated preforms, the achievable bulk density in the transport containers increased, so that up to 30 % more preforms could be carried for the same capacity. At a time of continually rising transport costs, this is a remarkable cost saving.
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