Mixing LDPE and PP. Types of polypropylene.

Q. I am extruding sheet film using a single-screw extruder with a 36-in. adjustable-lip die. I am extruding polypropylene with a 3.2 MFR index mixed with Foamazol 72 chemical foaming agent from Bergen International. The sheet is cut into strips or ribbons that are drawn out through an oven at a 7:1 draw ratio. Recently we had to switch from a lead-based colorant to a heavy-metal-free colorant. Since changing the color we can no longer consistently stretch the ribbons without getting undrawn sections. The sheet seems to be very brittle and has no elasticity. Should we change color concentrate or is there an easier solution? The colorant masterbatch is a high-melt-flow LDPE and is loaded via a separate feed hopper. The resin is gravity-fed into the throat of the extruder. At the base of the resin hopper the color is augered in and voltage controlled. The Foamazol is loaded the same way.   

— Northeast sheet extruder

A. LDPE and PP will not mix. You need a PP-based colorant.

Jim Frankland, Frankland Plastics Consulting LLC
     (724) 651-9196 • jim.frankland@comcast.net

Q. What are the various types of polypropylene, and might any display compatibility problems with proteins such as enzymes? 

— East Coast medical OEM

A. There are many types of PP, ranging from homopolymer to various copolymers and terpolymers. Homopolymer comes in isotactic, atactic, and syndiotactic forms, which relate to the molecular structure. Isotactic PP is semicrystalline, melts at 165 C, and is what most packaging films are made from (with co- and terpolymer surfaces). Atactic PP is amorphous, not crystalline, and is what many hot glues are made from.

I believe there is some biocompatibility with PP but am not well versed in its use for medical applications aside from medical devices. It has a low surface energy, so it would not be wetted by proteins unless surface treated. It will not be thermodynamically compatible with the proteins and will not mix spontaneously with them. However, it should be possible to use anhydride-grafted PP tie-resin grades to bond to proteins. This technology is used in producing PP/nylon coextruded films for formable meat packaging. The tie resin is placed between the PP and nylon to bond them. So depending on what you are trying to accomplish, it may be possible to design a PP structure that can be made compatible with the proteins. I don’t know if they could be made to bind the enzymes; this would have to be determined experimentally. In the normal state I don’t think PP would bind any enzymes.

Eldridge M. Mount III, Emmount Technologies, LLC
     (585) 223-3996 • emmount-technologies.com