Boron Nitride Looks Promising As a Polyolefin Processing Aid.

* Boron nitride may well become the next processing aid of choice for polyolefin extrusion, blow molding, and blown film. A solid lubricant whose structure resembles graphite, boron nitride (BN) has been used as a nucleating agent in fluoropolymer foams for tubing and thermal insulation. It now looks promising as a processing aid for polyolefins and other TPs, where it may replace or supplement standard fluoroelastomer processing aids.

While most of the research on BN in polyolefins has centered on simple tubular extrusion and wire coating, work is now under way in blow molding, and blown film tests appear to be next on the agenda.

R&D progresses

In 1997, DuPont Co., Wilmington, Del., received a process patent on incorporating BN particles in polyolefins and fluoropolymers. Last December DuPont granted BN producer Carborundum Corp. of Amherst, N.Y., an exclusive license to use the technology in thermoplastics other than fluoropolymers. Carborundum is using DuPont's testing facilities to continue the R&D work in conjunction with the University of British Columbia's Dept. of Chemical Engineering in Vancouver.

Thomas Clere, technical manager at Carborundum, says the company aims to form joint development programs with resin producers and compounders. He also says work is in progress to get FDA food-contact approval. Carborundum has been working with several types of BN with differing particle sizes, morphology, and surface energy and is developing new grades. Clere co-authored a paper on this work at the recent SPE polyolefins 2000 RETEC in Houston.

He says research shows that BN can eliminate sharkskin and stick-slip melt fracture in PE extrusion, as do fluoroelastomer processing aids. But unlike fluoroelastomers, BN reportedly also shifts onset of gross melt fracture to much higher shear rates. What's more, BN functions at use levels of 500 ppm (0.05%), half the amount typical for fluoroelastomers. Clere says use cost is competitive with fluoroelastomers.

Studies on mLLDPE

In a study at the University of British Columbia, three types of BN were extruded with ExxonMobil's Exact 3128 metallocene plastomer and Exceed 116 metallocene LLDPE. Of the three BN types tested, Carborundum's grade CTF5, a highly pure, 5-10 micron powder, performed best. Level of dispersion played a crucial role in eliminating melt fracture, according to Prof. Savvas Hatzikiriakos. Blending BN powder with resin pellets produced little effect on melt fracture. However good results were obtained when BN powder was premixed with ground resin to make a 10% masterbatch.

In Exact 3128, smooth extrudates were obtained at shear rates above 920 [sec.sup.-1] This is well above the critical shear rate for the onset of gross melt fracture of unmodified resin, which is about 42 [sec.sup.-1] at 325 F. Optimal performance was obtained with lower BN content. The maximum shear rate of 926 [sec.sup.-1] was achieved with BN concentrations of 200 and 1000 ppm, whereas a concentration of 5000 ppm yielded a maximum shear rate of only 617 [sec.sup.-1].

Similar results were obtained for Exceed 116 mLLDPE. In neat resin, sharkskin appeared at about 100 [sec.sup.-1], followed by gross melt fracture at higher shear rates. Addition of 500 ppm BN eliminated sharkskin and shifted the onset of gross melt fracture to rates approaching 900 [sec.sup.-1].