Author: Tronserve admin
Thursday 29th July 2021 05:43 PM
Injection Molding Biopolymers: How to Process Renewable Resins
U.S. injection molders are still pretty green when it comes to processing the new crop of renewably sourced biopolymers. These biologically derived polymers made from PLA, PHA and starch-based resins are attracting growing market interest as materials with no ties to petrochemical-based thermoplastics.
These resins require some care in molding so as not to exceed their heat, shear, and hydrolytic stability. Materials suppliers have been developing new and enhanced grades with improved processability and end-use properties, which could help the material to advance into a wider range of applications. Additive suppliers have also come out with products to overcome processing and performance limitations.
“We have worked with the majority of the leading suppliers of biopolymers and have trialed the materials extensively,” says Michael McGee, director of technology at Nypro in Clinton, Mass. It’s not as simple as a drop-in substitution for a familiar material like PP. “The rheology, shrink rates, and venting requirements are different from material to material, but the differences are subtle so you have to understand product design, tool design, processing equipment and the parameters of your process. We have been amassing considerable knowledge about biodegradable and biobased materials. We understand more now about their drying, about gate design and location, runner channels, flow rates, venting, and molding.”
“Some say they are extremely difficult to process, but that view is starting to change,” says Dr. Jim Lunt, North American v.p. of sales and marketing for Tianan Biologic, a Chinese supplier of PHBV biopolymer (a type of PHA). “Many biopolymers seem tough to process because they have a small window between the melting point or processing temperature and the decomposition point. With biopolymers such as PHBV, a resin may melt at 310 F but degrade at 360 F, which is a fairly tight processing window. Too much heat can generate gels, black specs, or yellowing in your parts.” As a result, molders need to watch their melt temperature, screw speed, and injection speed—as well as proper drying, since these materials tend to be hygroscopic and moisture sensitive.
SUCCESSES IN THE FIELD
More and more molders are likely to be approaching this learning curve. Biopolymers established themselves first in film and sheet extrusion markets, but successful molded part applications are starting to prime the market for growth. The few current molders of bioresins are targeting mainly rigid packaging, disposable cutlery, medical parts, and consumer products.
Nypro says it sees interest in molded parts from biopolymers coming from its core markets in consumer packaging, electronics, telecom, and medical industries. “We are not trying to push the technology, the customers are pulling it,” says McGee. Nypro customers are interested in sustainability and biodegradability and want to know if these materials suit their products and markets.
“Demand is growing so quickly, we expect to see double-digit growth for biopolymer products every year for the next few years,” says McGee. Nypro has developed its own proprietary flow-simulation package to simulate molding behavior of biopolymers, including shrinkage and warpage.
Consumer products firm Design Ideas introduced its new EcoGen line of bath products produced from the Tianan PHBV materials that are compounded into a proprietary grade by PolyOne. Design Ideas has the products molded under contract in Asia, says Chris Hardy, design director. “EcoGen is our first opportunity to get into injection molded parts that are durable and 100% biodegradable,” he says.
As shown on the front cover, the products include a toothbrush holder, bath cup, large and small bath boxes, a pump dispenser, soap dish, and bath bin. Most parts are fairly small, the largest being the bath bin, weighing 500 g (1.1 lb). Meanwhile, EcoGen office desk accessories were launched in 2008. The next development will be larger parts such as storage bins for the kitchen and office, says Hardy.
Other injection molding applications for Tianan’s PHBV include dishes, office trays, toys, rulers, pencil sharpeners, cartridges, and plant pots.
PHA supplier Telles (a joint venture of Metabolix and Archer Daniels Midland) is focusing on agricultural applications, since its Mirel resin biodegrades in water and soil. An example is an agricultural stake designed to hold down sod. Mirel material is also being used in its first aquatic application. Bioverse Inc. in Pipestone, Minn., is producing a biodegradable version of its AquaSphere Pro pond and lake treatment system for golf courses.
Mirel can also be used in single-use consumer items, disposable razors, and packaging. Explained Bob Findlen, v.p. of marketing and sales, “Telles will target rigid applications, where PHA could replace ABS. There is R&D on injection molding parts for hand-held devices such as cell phones and PDAs, and office equipment like printers. There is already a small injection molded pipette tray for laboratory use, which is manufactured by Labcon North America in Petaluma, Calif.”
Salvador Ortega, market development manager for NatureWorks, the first and still largest supplier of PLA biopolymer, says a variety of parts are being injection molded, from waste baskets to cups and cosmetic items such as lipstick cases. Ortega says PLA can replace GPPS, HIPS, and ABS: “I think it can compete against styrenic parts and can replace PET and PP in some consumer goods. It shrinks like a styrenic and can use the same type of molds.”
NatureWorks also has customers using its Ingeo resin in injection stretch-blow molded bottles for water and dairy drinks. That raises the potential of a market for injection molded PLA preforms, which has motivated some tooling and equipment firms (see below).
“Molders are using a number of our injection molding grades,” says Thomas Black, v.p. for the Americas and global alliance manager for Plantic Technologies, an Australian firm that offers starch-based bioresins through DuPont Packaging & Industrial Polymers. “We are transferring our commercial successes in Australia to applications here in North America and in Europe. We are seeing strong interest.” A molder in Australia is producing seedling and planter pots using one of Plantic’s water-resistant grades. Another water-resistant grade is also used to injection mold containers for trapping and killing disease-carrying mosquitoes and their eggs. Now,
Plantic is exploring applications in meat packing, such as clips used to isolate animal entrails.
Injection molding applications for Cereplast Compostable PLA blends include dental products, toys, tools, tableware, packaging, mugs, and its own Nat-Ur line of disposable cutlery. Cereplast resins are also used in the Natures Plast line of spoons, drink stirrers, sandwich picks, flying discs, and other items from Harco Enterprises Ltd., Peterborough, Ont.
Teknor Apex Co., which recently obtained the license for thermoplastic starch (TPS) blends developed by Cerestech, is also targeting injection molding. “We anticipate applications like cutlery, containers, electronics, toys, and other consumer products,” says Edwin Tam, manager of new strategic initiatives. The license allows Teknor to make and sell globally alloys of TPS with PLA, PHA, biopolyesters, and PP. Teknor plans to roll out the first commercial grades at the NPE show in Chicago in June.
KNOW THEIR QUIRKS
“The biggest issues in injection molding these materials are heat, moisture, and degradation caused by excessive temperature, shear, or residence time,” says Stefano Facco, director of business development for Novamont, a supplier of thermoplastic starch-based resins. Processors face these issues with all resins to some extent, he adds, but a bit more so with biopolymers.
“We have found PLA unique in its processing requirements, says Bob Ameel, global business manager for hot-runner systems at D-M-E Co. Compared with standard resins like PS, he says, “PLA retains heat more, so longer cooling time is required. It tends not to flow well in thin walls over long distances. Adding more pressure to fill only increases shear, which can cause it to break down and become brittle.”
D-M-E has just commercialized its Eco-Smart standard hot-runner system for PLA and emerging biopolymers. It uses noncorrosive components to resist the acidic properties of PLA and its tendency to plate out acid residues on the walls of the molding system. It also has specially designed nozzle tips to minimize shear and provide high cooling capacity; low-pressure, low-shear channels; and a thermal profile to counteract PLA’s “temperature hypersensitivity.”
Bioresins are hygroscopic and must be dried or they will suffer a drop in molecular weight and melt viscosity, as well as increased potential for flashing and brittle parts, says NatureWorks’ Ortega. PLA and PHA are polyesters, and drying requirements are in the range of those for PET and PBT—i.e., more strict than for ABS, nylon, or PC.
“Moisture sensitivity and lack of heat resistance appear to be the biggest issues surrounding unmodified biopolymers,” says Marcel Dartée, biomaterials market development leader for PolyOne, which offers a range of bio-based modifiers designed to remedy some processing and performance limitations of biopolymers. Other firms like Arkema, Clariant Masterbatches, DuPont, and Rohm and Haas also offer additive solutions that address these issues (see Learn More box).
Dartée says some biomaterials process like a PE or PET. “After some initial adjustments, even an inexperienced molder should be able to process these materials consistently,” he adds. And even if not handled correctly, he says, “Unlike traditional resins, melt degradation of these materials isn’t likely to clog up the molding equipment.”
While bioresins have their own particular requirements in terms of drying and processing conditions, Telles applications development engineer Tom Pitzi says that does not always impose any special requirements on tool design. “You can use sub gates, fan gates, all standard gating geometries with Mirel.”
Bioresin suppliers say their materials process like traditional thermoplastics such as PC or ABS and can be run on conventional machines using general-purpose screws. They don’t recommend high-shear screws, such as a nylon screw, that can generate a lot of shear heating. Suppliers also warn that you cannot have hot spots in the machine. “A general rule of thumb is for shot volume to be 30% to 80% of barrel volume, much like a standard thermoplastic,” says Plantic’s Black.
These materials can be run on any type of injection press. Wittmann Battenfeld says its 5.5-ton Microsystem press is used to mold special PLA resins designed for medical implants. These “bio-sorbable” medical-grade materials reportedly process without degradation in the two-stage dosing and injection system, which minimizes melt residence time. The full inventory of melt is injected on each shot, notes David Purcell, injection molding sales manager. The Microsystem minimizes sprue size, which is significant for these very costly medical grades, which often do not allow use of regrind. It also offers complete integration of feeding, drying, molding, and packaging under cleanroom and dry-air conditions.
Stefan Bock, Netstal’s manager of application technology for PET systems in Switzerland, says its PET-Line preform presses have run PLA successfully using its standard PET screw. “The system has to be totally cleaned of PET resin because the processing temperature is lower for the biopolymer. It processes closer to PVC but is less difficult. Molders should hold process temperature to ±2° C,” says Bock. He notes that the hot-runner system may be modified to prevent leakage, and the mold should be run with water at around 24 C (75 F) to avoid plateout. PLA preforms up to 24 g have been molded in PET molds on its presses, says Bock.
Netstal isn’t alone in appreciating the potential of PLA preforms. As we reported in February 2008, Husky developed a 24-drop hot-runner system with NatureWorks, bottle maker Biota Brands of America, Inc. (Telluride, Colo.), and stretch-blow machine builder SIG Corpoplast to produce the world’s first compostable water bottle. Husky’s hot-runner system was used with its HyPet injection machine to mold the preforms for 0.5L to 1L bottles.
Timothy Womer, chief technical officer at Xaloy, says current-generation biopolymers “process a lot cleaner these days, so a specialized screw may not be required.” Molders might find that these resins process more like engineering thermoplastics than like PE, PP, or PS, he adds.
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. Additives are helping to overcome this limitation, says NatureWorks’ Ortega: “Nucleation technology is making it possible to improve both cycle times and heat resistance.”
Some sources have accused the biopolymers of a tendency to stick to metal surfaces in processing. But suppliers say sticking is mainly a concern when running high levels of amorphous (uncrystallized) reprocessed material. Adding mold release might help reduce chance of sticking to metal surfaces in the press or tool.
Today’s biopolymers are designed to process more easily. Many of the materials are “some form of copolymer, with the main aim being to increase the material’s operating window, primarily the temperature gap between crystallization and decomposition,” says Tianan’s Lunt. Use of copolymers also provides the opportunity to enhance or vary properties, such as rigidity or melt strength. “You can now widen the operating window to improve processability. Widening the window allows you to run the product on more conventional equipment.” That doesn’t mean these materials are as forgiving as PE or PS or that they can be run on ancient injection presses where some of the heaters aren’t working, he cautions.
If exposed to ambient air, these materials can absorb enough moisture in five minutes to defeat most of the benefits of drying,” warns Jamie Jamison, dryer product manager for the Conair Group. “They need to be properly handled at all stages 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,” says Jamison. Users can employ 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, warns Sonny Morneault, dryer product manager at Wittmann. His company made software modifications to adapt its Drymax E series dryers, which have special counter airflow and regeneration features, for the lower temperatures needed for bioresins. The updated dryers can maintain drying temperatures of 120 F. Wittmann dried a biopolymer with starting moisture content of 2400 ppm down to 250 ppm in 5 hr at around 158 F, says Morneault. Wittmann also suggests conveying the dried material to the throat of the injection press in small volumes in order to minimize chances of moisture regain.
Says Novamont’s Facco, “Starch-based materials must be purged out from the barrel by LDPE at the end of production to prevent excessive degradation.” Telles’ Pitzi also recommends purging Mirel PHA, and most sources do not recommend leaving bioresins in the machine at the end of the work day.
Autodesk’s Moldflow laboratory has been investigating the processing of biopolymers to optimize its simulation software for these materials, according to Russell Speight, senior manager and principle materials engineer at the lab in Melbourne, Australia.
At least eight companies offer or plan to offer injection moldable biopolymers. NatureWorks’ Ingeo PLA line includes three injection grades, though its newest—Ingeo 3251D—is a higher flow material that will replace the previous two Ingeo grades (3001D and 3051D). NatureWorks developed Ingeo 3251D for thin-wall applications. It has a melt flow of 70 to 85 g/cc at 410 F and 30 to 40 g/10 min at 374 F. Those compare with 10-25 g/cc and 10-30 g/cc, respectively, for the older injection grades. Mechanical properties are said to be virtually identical to the other grades. It can be molded with the same screws and molds used for PS, SAN, and ABS, though gating changes are required.
Ingeo 3251D is transparent and is aimed at consumer electronics, cosmetics packaging, housewares, toys, and custom molding. “We are initially targeting semi-durable parts (less than 3-yr life) where process heat requirements are no more than 370 to 410 F,” says NatureWorks’ Ortega. “We get good optics, high clarity, high gloss, high modulus, good toughness, and UV transparency with no UV stabilizer required. The material should be dried to less than 400 ppm moisture, with best moisture content being less than 100 ppm. It can be dried with desiccant, infrared, and wheel-type dryers at 80 C for 1-2 hr, or 3-4 hr if the bag was left open.”
NatureWorks recommends a general-purpose screw with a 3:1 compression ratio and 20:1 L/D to melt the material without excessive shear. Feed-throat temperature should be about 70 F, while recommended melt temperatures are 370 to 410 F. Screw speeds from 100 to 200 rpm should be used with 50 to 100 psi backpressure. Metering zone and nozzle should be at 370 to 400 F. Molds should be kept cool, around 75 F, and expect part shrinkage 0.004 in./in.
Novamont’s Mater-Bi starch-based biopolymer comes in an injection grade with MFI range from 6 to 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, says Facco. Melt temperature is 302 to 430 F. Molding of semi-crystalline biopolymers can be up to 50% slower than more commonly used semi-crystalline resins. Mold temperature of 65 F is typical.
Novamont’s Facco says any gate design can be used with Mater-Bi. Minimum gate size is normally 1 mm (full round) but can be greater or lower depending on the particular grade’s viscosity. Cold or hot runners are both suitable, notes Facco, though “the manifold must be free of dead spots and the nozzles of a free-flow type.” The company also recommends fast injection. Mater-Bi may require use of stainless-steel tooling. Molds are typically run at 68 to 104 F
Novamont can perform Moldflow simulation of mold filling. The company can develop process guidelines and can specify part wall thickness, pressure, sprue diameter, and use of hot or cold runner.
A biopolymer made from chemically modified, high-amylose industrial cornstarch by Plantic Technologies of Australia is offered here in five injection grades by DuPont 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 four or 12 weeks before biodegrading.
Low-compression screws (2.2 to 2.8) with a 20:1 L/D are recommended. “A screw you might use for flexible or rigid PVC will be fine,” says Plantic’s Black.
Mirel PHA (polyhydroxyalkanoate) resins from Telles are described as high-performance semi-crystalline polyesters engineered for high modulus. They are made by bacterial fermentation of cornstarch. The new P1003 injection molding grade will replace two other experimental grades (P1001, P1002). “It is a huge market opportunity for us. There is strong demand for it to be readily injection moldable in durable applications,” says Telles’ Pitzi.
Mirel should be dried to 1000 ppm (0.1%) or less. P1003 is processed using a reverse barrel-temperature profile ranging from 350 F in the rear to 320 F at the nozzle. “We are cooling down the melt as we go through the process,” says Pitzi. He recommends use of a single-flighted, general-purpose screw with 2:1 to 2.5:1 compression ratio. Backpressure can be as low as 50 psi. Screw speed should be 50 to 150 rpm. Mirel has a recommended melt temperature of 320 F, and it decomposes above 356 F so control of melt temperature is vital.
Molders should inject slowly at first—0.5 to 2 in./sec—then gradually increase the speed until 95 to 99% of the mold is filled. Injection pressures from 5000 to 13,000 psi have been found to work, says Pitzi. To avoid longer residence times, keep no more than three to four shots of material in the barrel, though two is better.
Mold temperatures should be 120 to 140 F to promote crystallization. “Instead of injecting into a cold tool at 70 F, we run the mold at 140 F. If you run it too far from the melt temperature you will waste time trying to get the heat out. Allow the material to crystallize first, then you cool it,” says Pitzi. Telles recommends that gate sizes be up to 80% of the part thickness. Most parts require 3 to 5 tons/sq in. of clamp force.
Pitzi says Telles has been working with the Autodesk Moldflow simulation software to develop a database. Mirel can be used with hot runners and valve gates. It can be purged with LDPE.
Tianan Biologic’s PHBV (polyhydroxybutyrate-valerate) is a bio-polyester produced via bacterial fermentation of plant starches. The Chinese company is said to be the world’s largest producer of PHBV, a member of the PHA family. 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 F. Users should keep feed throat temperature to no more than 275 F, compression section to 293 F, metering section to 311 F and adapter temperature to 322 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 says its TPS/PP blends can run in standard injection machines with no screw modification. Cereplast recommends low-shear screws with its starch/PP Hybrids. Cereplast recommends melt temperatures below 392 F. Teknor says its new blends can take barrel temperatures up to 400 F. Tam recommends slow early-stage injection, larger-diameter nozzles, and vented molds with cold slug wells and full-round rather than half-round runners.