Drying Bioabsorbable Resins
Drying is an important part of the process for any product made of hygroscopic (meaning affinity for moisture) thermoplastic. For medical implants made of bioabsorbable polymers, dryness is particularly critical. Inadequate drying can produce a variety of problematic results. These include: lack of tensile properties and impact resistance, as well as varying flow characteristics.
Bioresins, much like other hygroscopic thermoplastic resins, can suffer three types (or a combination of these three types) of degradation: thermal, mechanical or hydrolytic. In most thermoplastics these types of degradation occur chiefly during the molding process. With bioresins such as the PLA, PLG , and PGA families, hydrolytic degradation also occurs before and after the molding process.
An implantable device must decay or degrade in the body as part of the absorption process. Different materials and part designs have different rates of degradation in the body (where it is in a moist environment). The rate of degradation and retention of mechanical properties is affected in no small degree by the way the resin was dried and how the dried resin and finished part were handled.
If a bioresin grocery bag degrades quicker than it was designed to, the results can be the bottom falling out and groceries on the ground. If an implantable device degrades quicker (or slower) than designed to, the results can be harmful to the patient. The degradation process of the implant is key to resorption in the body.
Run of the mill dryers are generally not sufficient to control the moisture as well or reach the super-low moisture levels desired for absorbable implants. Many implant molders opt for vacuum dryers or compressed air with membrane dryers. Since most implants are small, vacuum ovens designed for lab use is another option for resin drying. In any case, the drying schedule and temperatures provided by the resin manufacturer must be strictly followed.
In many cases, the resins must be dried to less than 0.02% (200 ppm) and the resin and finished product must be maintained dry. This requirement mandates an inert gas such as nitrogen atmosphere in any non-vacuum dryer hopper, humidity controlled atmosphere in the cleanroom, vacuum packing with a desiccant and nitrogen, and refrigerated storage of the resin prior to drying.
It is not enough to strictly follow the drying and handling procedure, the resin dryness must be well tested, documented, and controlled. The dryness data is so important because it must be correlated with part degradation data to be able to predict implant device performance and absorption in the body. Lost weight or halogen type moisture analyzers are relatively economical devices but should be equipped with data acquisition and logging technology.
Drying bioabsorbable resins requires specialized knowledge, methods, and equipment, but is key to successful bioresin implant molding.
Biodegradable Plastics: The Green Dilemma – Part 2
In Part 1 , we discussed the history and sources of biodegradable and compostable plastics – what we call “green resins.” They are here; they are with us; and they come with unique challenges in both the mold building and injection processing of these resins. Most are still in the developmental stages.
Bio-resins are formulated to give end properties mimicking well known thermoplastic resins, and in some cases the processing techniques of a bio- resin emulate the conventional resin for which it is designed to mimic, but in many cases they are processed quite differently.
Most bio-resins thermally degrade quite readily. Many run at a melt temp around 350°F or less, and even if kept at the recommended melt temperature, can degrade in the barrel or a hot runner system over time. Generally, the molder wants between 50-75% of his barrel capacity for each shot, and barrel or hot runner residence time shouldn’t exceed two minutes by much.
A good processing method is to begin and end the molding with an easy flow PE resin. While the PE is going through the barrel, the heats can be lowered to the bio-resin range. The hot runner can be purged with the PE either through air shots or by running parts. When shutting down this procedure is also followed. The mold and machine, after being cleaned with Polyethylene, are shut down with the PE in them. In addition to avoiding thermally degrading the bio-resin, this technique mollifies the tendency of the bio-resins, particularly PLA, to be corrosive, especially when degraded.
A well designed hot runner system with a well placed thermocouple for each drop, as well as manifold and sprue bushing thermocouples is needed to process bio-resins successfully. The hot runner system capacity should not exceed two shots.
A well fit sliding ring-type, non-return valve and a general purpose screw with a 20:1 or 22:1 L/D ratio will nicely plasticize most any bio-resin. Shear will greatly affect most of the bio-resins, so gate size can’t be too small, and fast injection speeds should be reserved only for the thinner walled parts.
Bio-resins should be well dried prior to processing. Most should be dried to under 0.010% moisture. Either desiccant, or compressed air dryers can be used. Temps can be moderate to low when compared to many of the common thermoplastic engineering resins.
Always consult the resin maker’s guideline literature before processing any resin, especially bio-resins.
Mold construction must be tight to successfully mold bio-resins without flashing. Vents are important, and care must be taken when cutting them. The experience gained from building molds for Nylons and LCP can be applied to constructing a mold for bio-resins. Again, the resin’s corrosive nature must be taken into account when selecting steels for the injection mold. Matrix Tooling has experience in designing, constructing, and running precision injection molds for bio-resins.
Bio-resins seem to be the wave of the future, and though new and a bit delicate, can be injection molded successfully with a little knowledge, care, and common sense.
Senior Process Engineer (Older Molder)
Biodegradable Plastics: The Green Dilemma – Part 1
“Green” is all the rage now, and a major part of the green revolution is biodegradable and bio-compostable plastic resins. Our landfills are quickly filling up and a large portion of the contents are plastic items. These items remain in their state “forever.” Incineration isn’t an option due to air pollution and grinding the plastic only reduces the size but not the volume.
This problem is a product of the disposable or throwaway society that we now live in. I can still remember families having one TV set, telephone, refrigerator, and radio per household. When these malfunctioned, they were repaired. People had one fountain or ball point pen, and they were refilled when the ink supply was depleted. Nuts, bolts and most everything else you used came from a bulk barrel and were wrapped in old newspaper or put into a brown paper bag which was then used for your lunch. There wasn’t such a thing as blister or clamshell packaging. You took your soft drink bottles back for deposit, and your empty milk bottles were picked up by the milkman. We live in a convenience-first throwaway society and plastics, particularly plastics packaging, play a big part in that convenience.
Enter biodegradable and bio-compostable plastics. With these two “green” families of plastics we can have our throwaway convenience without overfilling our waste disposal sites. There also is the advantage of deriving these plastics from renewable plant resources rather than finite resources based on crude oil and natural gas.
The earliest plastics were bio-plastics that were derived from cellulose (wood). Billiard balls were the first use. Later Henry Ford used soybean derived resins in his early automobiles. The Yo -Yo is cellulosic and most of today’s screwdriver handles are of this renewable resource, non-petroleum based plastic. This plastic won’t biodegrade though, and when disposed of becomes a permanent part of the landscape.
During the Arab oil embargo of the 1970’s, corn starch was used to extend polyethylene due to the lack of petroleum feed stocks. The resultant packaging which included milk caps and pails was functional though the resin was problematic to run. The products broke down into smaller pieces but never completely degraded as only about 25% of the product was corn starch.
My first exposure to a truly biodegradable, and in this case, bio-absorbable was at Matrix back in 2002. The resin was PLA (Polylactic acid) . This was a corn based resin for a PLA encapsulated implantable radioactive “seed” for treating prostate cancer. This medical grade resin at the time of development was $1,500 per pound. Medical uses of PLA soon became a commonplace.
PLA continued to be developed, and commodity grades of the resin soon were made available allowing for prices to fall until it became a viable alternative to petrochemical resins.
Development of “green” plastics continued at a frenzied pace between 2003 and the present, and soon soy and milk (casein) based plastics reappeared and new resins came out based on the castor bean. Characteristics and properties of the new resins began to mimic the traditional resins that we are used to. Cutlery, plates, cups and clamshell packages for the fast food industry are becoming common. Writing instruments are being developed that are made from these resins, and even the pesky and environmentally ugly shopping bag as well as beverage bottles are being developed using biodegradable resins.
One developmental problem of these resins is combining the lasting properties of conventional resins with the biodegradable and compostable properties of the “green” resins. You want your pen to decompose in the landfill but not in your pocket. Your shopping bag must survive the trip home from the store; your plate or cup can’t leak all over you.
These bio-plastics, especially PLA have come under criticism though. Many people blame the rise in food costs largely on these bio plastics (and bio fuel). Defenders and makers of PLA say that they only use feed corn not consumer corn for PLA. Detractors say that the shortage of feed corn raises milk, egg, and chicken costs as well as red meat costs, and more land is diverted to feed corn for PLA and less to consumer corn.
New development in castor bean plastics makes use of this product which is poisonous to human beings. Soy based resins also take some of the pressure off of corn, though it puts the beneficial soy plant under pressure. Recent plastic resin and also fuel developed from biomass may prove the most promising development in “green” plastics and fuel. The corn kernels and soy beans can be used to feed the world’s people, directly or indirectly, and the once discarded biomass can be used to make the fuel and plastics we need to save the environment. Exciting new developments in this area are forthcoming.
At Matrix Plastic Products, we are involved in ongoing R&D projects involving “green” plastics. They require specialized mold building techniques and processing methods. It is exciting to be involved in projects that not only lessen our dependency on foreign oil, but are good for our fragile environment. I think the future will accentuate the positives and minimize the negatives of “green” plastics.
Senior Process Engineer (Older Molder)
Injection Molding Bioabsorbable Medical Implants
Medical grades of biodegradable/bioabsorbable PLA, PLGA, and PLG resins can sell for upwards of $1,500 USD per pound. This and the delicate nature of biopolymers can send shivers down the spine of any injection molder. Really though, the standard precautions and best practices of any high level molding operation familiar with running engineering-grade resins similarly apply to the processing of these bioabsorbable resins. Every aspect of the manufacturing process must be closely monitored, and proper procedures strictly adhered to.
Mold construction must take into account the rheology or flow characteristics of the resin , shrinkage , and venting requirements. Due to the relative infancy of the bio-materials there is little data available from the resin makers, so all mold construction should be steel safe. Selection of mold material should keep corrosion resistance in mind.
Cleanliness, of course, is paramount. There can be no contamination present in an implantable device. Purging an injection unit after a run with a different resin, no matter how thorough, is not enough. The screw must be pulled and all residue removed from the screw and barrel physically. Don’t use copper gauze (here we differ slightly from the standard molding procedures) for fear of fragmenting and heavy metal contamination. A plastic abrasive cloth would be better, and the unit should be wiped, dusted, and blown or vacuumed free of any minute debris after cleaning. Having a dedicated injection unit is a better option, with a dedicated molding machine, dryer, and loading system being the best case scenario.
The molding machine really is not specialized. Usually a general -purpose screw is fine with about a 3:1 compression ratio and 20:1 length over diameter ratio (again, a general – purpose unit). Clearances should be tight in the injection unit, and the check ring non-return valve should be in good shape. Bioresins are not very heat or shear stable, so sizing the barrel capacity of the machine to the intended shot size is very critical, and high compression screws are not advisable. Since the resins are so costly, any scrap will immediately affect the bottom line. Savvy molders start and stop the run with a “commodity” bioresin with similar flow characteristics.
PLA/PLGA/PLG for implantables, like most bioresins, are slow to give up their heat and also can be slow to obtain optimum crystallization, so cooling times and pack/hold times can be quite long which lead to longer cycle times. Here, corners can’t be cut by running a colder mold in quest of faster cycles because you risk endangering the end characteristics of the implant.
In addition to shear and heat degradation, bioabsorbable resins are susceptible to hydrolytic degradation. Careful drying is important, with drying in the cleanroom next to the molding machine being advisable. Minimum out-of-dryer/in-molding-machine hopper time is also essential. Like many engineering resins, bioabsorbable resins must be dried and maintained at or below 250ppm (0.025%) moisture. A moisture analyzer is mandatory.
It is important to fast-track the process building and optimization portion of any injection project for PLA implants, if only due to the resin costs. One tool for this is DOE, or Design of Experiments. Here again, process building with a commodity PLA resin and final tuning with the PLA implantable resin is advised.
The implantable device must be made, handled, and packaged under the strictest of cleanroom conditions. Packaging must protect the implantable and be hermetic to keep it from picking up ambient moisture. Vacuum packing with laminated foil pouches is an option, and inclusion of a desiccant in the master carton is also popular. Sealing options include vacuum and heat sealing. Finally, sterilization methods and their impact on product end characteristics must be cautiously considered.
Care must be taken with these expensive and fragile resins; but the discipline is good for a molder, and the results for following good methods and procedures can be quite rewarding.
Brent G. Borgerson
Senior Process Engineer (Older Molder)
The Three R’s of Injection Molding
Plastics have long been associated with environmental unfriendliness and wastefulness of crude oil and petroleum byproducts. The advent of bioplastics (biodegradable and biocompostable plastics) which are derived from renewable sources such as corn starch or vegetable oil is helping to improve the image of plastics among those concerned with the environment, carbon footprints, sustainability, and being “green.” Bioplastics are slowly but steadily being improved, and in some cases their abilities to process and end-use properties can mimic or even surpass those of traditional petroleum based materials.
Bioplastics, aside from being derived from renewable resources, have the advantage of not releasing harmful toxins during their production, processing or degradation. Many conventional plastics can release known or suspected carcinogens such as formaldehyde or benzene during production, processing or destruction.
Growing the sources for bioplastics also reduces carbon dioxide in our atmosphere. Since the production of conventional plastics produces so much CO2 the use of bioplastics in place of a conventional plastic has a cumulative effect, with the substitution of just one ton of bio for conventional plastic having the net effect of reducing multiple tons of CO2 in the atmosphere. This not only takes into account the production methods for each type of plastic, but also the photosynthesis process in growing biomass or raw material for bioplastics. Bioplastics show great promise in reducing both our industry’s carbon footprint and impact on rising global warming.
What can plastics processors do until bioplastics are perfected in properties and reduced enough in costs to truly compete on a large scale with conventional thermoplastics? This is where the 3 R’s apply in injection molding. The 3 R’s in molding don’t stand for “reading, riting and ‘rithmetic,” but rather: reduce, reuse, and recycle. At Matrix Tooling / Matrix Plastic Products, we have been molding with bioresins, including bioabsorbables for a number of years, but as responsible members of the environmental community, we also have been practicing the 3 R’s.
Reduce: Scrap (and resin usage) is reduced through cold runner and sprue size reduction where possible without affecting moldability. In many cases we have reduced sprues and runners to the prescribed percentage of regrind allowed in the product specification. Hot runners and hot sprue bushings also have been used wherever possible. We have also thinned out wall stocks on parts where the product integrity wouldn’t suffer.
Reuse: We reuse what regrind we can and have come up with applications to use up to 100% in-house regrind. We utilize returnable/reusable packaging where possible and where allowed by the customer. We also have a closed circuit water system to reduce consumption and also filter, monitor and analyze hydraulic oil to avoid indiscriminate unneeded oil changes.
Recycle: Where we can’t reuse in-house regrind, we try to find it a good home. We sell the regrind where possible or even give it away for free if it can be used but there isn’t a paying market. Packaging is recycled also. We even collect our soda pop cans!
Matrix is serious about being environmentally responsible, using bioresins, and abiding by the 3 R’s. It not only makes environmental sense, but favorably affects the bottom line.
Senior Process Engineer (Older Molder)