| Compression Roll Drawing (CRD), a patented machine-direction orientation (MDO) technique used for years to make tough plastic strapping, can add cold impact strength to PP sheet as well. Parkinson Technologies in Woonsocket, R.I., licenses the technology from ITW Signode, Vernon Hills, Ill., for non-strapping applications and has built one commercial sheet system. CRD simultaneously stretches and compresses melt coming from a flat die in a “zero gap” between rollers turning at different speeds. This orients the molecules and compresses them at the same time, imparting unusual properties.
Semi-crystalline resins like PP can be oriented at low 2:1 or 3:1 draw ratios that aren’t usually possible in MDO systems. PP sheet naturally tends to develop thick and thin bands at less than 5:1 draw ratios. CRD also imparts a small amount of TD orientation, reducing splittiness and neck-in. Recent tests by Parkinson and Dow Chemical show that PP sheet with 2:1 CRD is much clearer and tougher at low temperatures. Haze is 3% with 3:1 CRD and 6% with 2:1 CRD, vs. 15% haze with no stretching. Sheet with 2:1 CRD maintains dart impact properties down to -20 F. Parkinson presented the CRD results at Plastics Technology’s Advances in Extrusion Conference in New Orleans in last month.
(401) 762-2100 • www.parkinsontechnologies.com |
