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Biopolymers require special care for processing during injection molding |
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Biologically derived polymers made from PLA, PHA and starch-based resins are attracting growing market interest as alternatives to petroleum based polymers. Initially biopolymers penetrated the film and sheet extrusion markets. Recent developments have encouraged successful application in molded products such as rigid packaging, disposable cutlery, medical parts and consumer products. Demand for biopolymers is expected to show double-digit growth every year.
Processors have recently started adopting biopolymers in injection molding process that require some care so as not to exceed their heat, shear and hydrolytic stability. Use of biopolymers is not mere substitution for a conventional polymer, as the rheology, shrink rates and venting requirements differ. Their usage requires understanding of product design, tool design, processing equipment and the parameters of the process. Many biopolymers seem tough to process because the window between the melting point (processing temperature) and the decomposition point is very narrow. Too much heat can generate gels, black specs or yellowing. These materials tend to be hygroscopic and moisture sensitive, making it mandatory for molders to watch melt temperature, screw speed, and injection speed and proper drying. Compared with standard resins, PLA retains heat more, requiring longer cooling time. It tends not to flow well in thin walls over long distances, and if additional pressure is added to fill, it increases shear, which can cause it to break down and become brittle. Biopolymers are hygroscopic and must be dried to avoid a drop in molecular weight, melt viscosity, as well as increased potential for flashing and brittle parts. Since PLA and PHA are polyesters, drying requirements are stricter than for ABS, nylon or Polycarbonate. The issues of moisture sensitivity and lack of heat resistance appear to be the biggest issues surrounding unmodified biopolymers and are being addressed by bio-based modifiers designed to remedy some processing and performance limitations of biopolymers.
Biopolymer suppliers say their materials process like traditional thermoplastics such as PC or ABS and can be run on conventional machines using general-purpose screws. High-shear screws such as a nylon screw are not recommended as they can generate a lot of shear heating. Hot spots are recommended to be avoided in the machine - shot volume should be 30-80% of barrel volume, much like a standard thermoplastic. Most biopolymers are semi-crystalline, but they can tend be relatively slow to crystallize or set up in the mold, even though they have relatively low melting temperatures. Nucleation technology is making it possible to improve both cycle times and heat resistance. Biopolymers are also reported to have a tendency to stick to metal surfaces in processing. This is refuted by suppliers who claim that sticking is mainly a concern when running high levels of amorphous (uncrystallized) reprocessed material, and can be reduced by adding mold release. If exposed to ambient air, these materials can absorb enough moisture in five minutes to defeat most of the benefits of drying. Hence proper handling at all stages is required to minimize moisture regain. If drying temperature is too high, the material may soften and agglomerate in the drying hopper. If it is too low, it will not dry as readily. It is recommended to use hopper agitation, fluid-bed crystallizers or infrared crystallizing and drying units. Typical twin-bed desiccant dryers are not able to maintain the low temperatures required by these biopolymers, owing to temperature spikes after regeneration. It is recommended to purge biopolymers from the machine at the end of the work day.
Biopolymers face several processing and performance limitations that are being addressed by the resin as well as additive producers. New and enhanced grades with improved processability and end-use properties are being developed to make the material suitable for a wider range of applications. Eight producers offer or plan to offer injection moldable biopolymers:
NatureWorks� Ingeo 3251D is a higher flow material that will replace the previous two Ingeo grades. It is transparent and is aimed at consumer electronics, cosmetics packaging, houseware, toys and custom molding. It offers good optics, high clarity, high gloss, high modulus, good toughness and UV transparency with no requirement of UV stabilizer. Another grade has been developed for thin-wall applications with melt flow of 70-85 g/cc at 410 F and 30-40 g/10 min at 374 F. It can be molded with the same screws and molds used for PS, SAN, and ABS, though gating changes are required.
Novamont�s Mater-Bi starch-based biopolymer for injection grade comes with MFI range from 6-30 g/10 min. It should be processed with a constant-taper, single-flighted screw having a 2.5:1 compression ratio and 25:1 L/D. A standard check ring can be used, along with medium to high injection speed. Melt temperature is 302-430 degree F. Molding of semi-crystalline biopolymers can be up to 50% slower than more commonly used semi-crystalline resins. Mold temperature of 65 degree F is typical. Any gate design can be used, cold or hot runners are both suitable and fast injection is recommended.
DuPont is offering a biopolymer made from chemically modified, high-amylose industrial cornstarch by Plantic Technologies of Australia in five injection grades under the Biomax TPS (thermoplastic starch) brand. There are general-purpose, high-flow and water-resistant grades, as well as an �engineering� grade for thick parts requiring high stiffness. Two water-resistant grades withstand water exposure for up to 4 or 12 weeks before biodegrading. Low-compression screws (2.2 to 2.8) with a 20:1 L/D are recommended.
Mirel PHA (polyhydroxyalkanoate) resins from Telles are high-performance semi-crystalline polyesters engineered for high modulus. They are made by bacterial fermentation of cornstarch. It is recommended to be dried to 1000 ppm (0.1%) or less. P1003 is processed using a reverse barrel-temperature profile ranging from 350 degree F in the rear to 320 F at the nozzle. Use of a single-flighted, general-purpose screw with 2:1 to 2.5:1 compression ratio is recommeded. Backpressure can be as low as 50 psi. Screw speed should be 50 to 150 rpm. Mirel has a recommended melt temperature of 320 degree F, and it decomposes above 356 F so control of melt temperature is vital.
Tianan Biologic�s PHBV (polyhydroxybutyrate-valerate) is a bio-polyester produced via bacterial fermentation of plant starches. Its Enmat PHBV material is approved for food contact in Europe and is approved by the Biodegradable Products Institute (BPI), N.Y.C., for composting. Tianan offers Enmat Y1000P injection grade powder and pellets and EnMat Y5010P pelletized blend of PHBV and BASF�s Ecoflex biodegradable (but not bio-based) resin. Compounder PolyOne formulates an injection grade based on the Tianan material blended with its biodegradeable additives. PHBV should be dried to 250 ppm moisture. Tianan recommends a melt temperature of 338 to 347 degree F. Users should keep feed throat temperature to no more than 275 F, compression section to 293 degree F, metering section to 311 degree F and adapter temperature to 322 degree F. Tianan says shrinkage of its PHVB is similar to that of PLA.
Cereplast supplies a Compostables line composed of PLA blends and a new �Hybrid� line of starch reactively blended with PP. Cereplast offers standard and higher flow (35 MFR) Compostable injection grades, as well as one recommended for cutlery, plates, and bowls and one with higher flexibility for freezer applications. Processing recommendations for new Compostable 1001 grade are shown in the accompanying table.
Teknor Apex�s TPS/PP blends can run in standard injection machines with no screw modification.
(Reference : Plasticstechnology)
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