PETpla.net Insider 09 / 2016

CAPPING / CLOSURES 44 PET planet insider Vol. 17 No. 09/16 www.petpla.net Different applications, neck finishes for various, part 8 Closure for PET bottles & Resins PETplanet insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottles, Preforms and Closures,” which was published by hbmedia. A newly revised version is reissued under the Elsevier imprint. 4.5.1.1 Chemical Properties PP has excellent stress-crack resistance and tolerates higher tem- peratures quite well but does not fare as well at lower temperatures. Compared with HDPE, it has a lower density, has a higher softening point as it does not melt below 130 °C, and is also relatively hard and stiff. Additives are applied to all commer- cially produced PP resins to protect the polymer during processing and to enhance end-use performance. Maxi- mum density is 0.915g/cm 3 (fig. 4.32). PP comes in three grades:  A general-purpose homopolymer.  A block copolymer with 5–15% ethylene, which has a much improved impact resistance extending to temperatures below -20 °C. Its toughness can be fur- ther enhanced by the addition of impact modifiers, traditionally elas- tomers in a blending process.  A random copolymer incorporat- ing comonomer units arranged randomly (as distinct from dis- crete blocks) along the PP long- chain molecule. Such polymers typically contain 1–7% ethylene and are selected where a lower melting point, more flexibility, and enhanced clarity are advanta- geous. Physical Properties Tensile Strengh 0.95 - 1.30 N/mm² Notched Impact Strengh 3.0 - 30.0 Kj/m² Thermal Coefficient of expansion 100 - 150 x 10 -6 Max Cont Use Temp 80 °C Density 0.905 g/cm³ Resistance to Chemicals Dilute Acid **** Dilute Alkalis **** Oil and Greases **variable Aliphatic Hydrocarbons * Aromatic Hydrocarbons * Halogenated Hydrocar- bons * Alcohols **** Key * poor ** moderate *** good **** very good Figure 4.32 PP properties at a glance PP’s high stiffness makes it unsuit- able for one-piece closure applica- tions. There are two reasons for this:  In one-piece designs (see chapter 4.2) small cap protuberances con-