PETpla.net Insider 09 / 2016

CAPPING / CLOSURES 45 PET planet insider Vol. 17 tact the neck finish to form a seal when fully torqued. This requires a certain softness on the part of the cap material. PP’s stiffness pre- vents this from happening.  The unscrewing torque is an impor- tant measure for any applica-tion as consumers with low hand muscle power must be able to open the package easily. With a growing per- centage of elderly people in all indus- trialized societies, unscrewing torque will gain even more importance in the coming years. Because of its stiffness, use of PP often results in unaccep- table unscrewing torques when one-piece designs are used. This is because of the relatively large sur- face area of the cap that comes in contact with the bottle neck and the mate- rial’s “unwillingness” to give. Typical acceptable values for unscrewing tor- ques are 10–18 in lb. Because of its high heat resistance, PP shines in hot-fill applications. Here two-piece designs are customary to circumvent PP’s incompatibility with one-piece closures. Devel- opments are under way to use one-piece HDPE clo- sures with a modified seal for hot-filled bottles, which brand owners consider for cost reduction. 4.5.2 High- density Polyethylene HDPE is the most widely used material for closure production. It is also the simplest polymer in terms of chemical structure. This “linear” poly-ethylene can be combined to a length that is appropriate for the applica-tion. Its relative soft- ness allows the construction of a variety of one-piece designs. Slightly better heat transfer characteristics allow for faster cycle times than those for PP (Figs 4.33 and 4.34, page 46). We do more. Contiform AseptBloc #GermanBlingBling #ContiformAseptBloc Figure 4.33 HDPE consists of long chains or repeating CH2 units