The Charm of PLA

At Inderøy Techno-workshop, the standard plastic used on our 6 x 3D-printers is Polylactic Acid = PLA. The weblog post explains why.

The name Polylactic Acid is actually a misnomer. It does not comply with IUPAC (The International Union of Pure and Applied Chemistry) standard nomenclature (naming standards for chemicals), and is potentially ambiguous/ confusing: PLA is not a polyacid (polyelectrolyte), but a polyester.

What distinguishes PLA from other thermoplastics/ -polymers is that it is made from plant-based renewable feedstocks. PLA’s list of raw materials include cassava, corn/ maise, sugar beet, sugarcane, potatoes and similar products. What is interesting, from an Inderøy perspective, is that the municipality has a potato processing plant that was established in 1844.

Despite its natural origins, PLA offers properties similar to other thermoplastics used industrially. One of the reasons for selecting PLA as a standard product, is that the workshop wants participants to reflect over their choice of materials, and to choose those that are least damaging to the environment and living things, including themselves and other human beans, as some people call them. PLA has less negative impact than most other plastics, so people can use it with a good conscience. If workshop participants want to switch to a different plastic, they will have to defend their choice!

There are several methods used to manufacture PLA plastic. Those interested in the fine details of PLA engineering should consult Lee Tin Sin, A. R. Rahmat, W. A. W. A. Rahman, Polylactic Acid: PLA Biopolymer Technology and Applications, (2012) ISBN: 9781437744590

PLA is a thermoplastic, which means it can be melted and reshaped without significant degradation of its mechanical properties. Thus, it is easy to recycle.

PLA is biodegradable. Microorganisms transform it into natural components, such as water and carbon dioxide. The speed of the transformation is strongly dependent on temperature and humidity. At the Inderøy Techno-workshop, we will ensure that PLA is properly recycled. There is a special bin, clearly marked with PLA – and not with resin code 7 used to identify “other” plastics. The plastic recycling resin codes 1 to 6 are used for petroleum based plastics.

One of the projects this writer wants to prioritize in 2020 is to work with Innherred Renovation, the local waste recycling company, to examine the feasibility of processing PLA locally to avoid excessive transportation costs, and to give the workshop a source of raw material to make coils of PLA filament – yet another project, scheduled for 2021. Disposal (as distinct from recycling) involves heating PLA to about 60°C and exposing it to special microbes, that will digest and decompose it within three months. If these conditions are not met, PLA can take between 100 and 1 000 years to decompose.

Because PLA is derived from renewable resources, and is not petroleum-based, it offers many positive characteristics for manufacturers. It is almost carbon neutral. The raw material it is made (plants) from absorbs carbon. When oxygenated or heated, it does not release toxic fumes. Yet there is a down side. With the world’s population raising, at least temporarily until towards the end of this century, there are concerns about using agricultural land for the production of non-food crops, such as bioplastics. In addition, raw materials for PLA typically use transgenic plants, plants that have genes inserted into them that are derived from another species.

Other challenges include agriculture based on monocultures; a lack of long-term testing; mixing/ contaminating PLA with petroleum based plastics (PLA plastic is brittle unless it is mixed with some petroleum based polymers.); decomposition of food storage PLA plastics during production, packaging, transportation, selling and consumption phases. There are also strength and crystallinity deficiencies.

PLA plastic is recognized as safe by the United States Food and Drug Administration. Its non-toxicity allows it to be used safely in all food packaging and many medical applications including implants. These can be biodegraded in the body over time, if PLA is in its solid form. There are some ventilation issues. Fumes emitted by PLA are claimed to be harmless, however, there are suggestions that the release of nanoparticles can potentially pose a health threat. At Inderøy Techno-workshop, extractors will be fitted to our 3D-printers, with both HEPA and active charcoal filters.

Physical characterics of PLA that are important to users are its mechanical, rheological (flow) and thermal (heat) properties. The makeitfrom.com database is a convenient site to get basic information abouta number of materials. Here are the results for PLA.

PLA has good mechanical properties, that are often better than many petroleum based plastics such as polypropylene (PP), polystyrene (PS) and polyurethane (PU). It’s Young’s modulus, ability to tolerate elongation under tension or compression, is ~3.5 GPa, in contrast to 0.1 GPa for rubber and 200 GPa for steel. Its tensile yield strength, the force needed to pull something, is ~50 MPa. Its flexural strength, the stress needed to start plastic deformation, is ~80 MPa, All of these are at the low end compared to other thermal plastics.

Rheology is the study of materials with both solid and fluid characteristics. PLA is a pseudoplastic, non-Newtonian fluid. Non-Newtonian means that its viscosity (resistance to flow) changes depending on the stress that it is subjected to. PLA is a shear-thinning material, which means that the viscosity decreases with applied stress.

PLA’s thermal properties depend on its molecular weight. It is classified as a semi-crystalline polymer, with a glass transition temperature at ~55°C and melting temperature at ~180°C. These are low compared to other thermoplastics such as ABS. PLA can burn. This means that heat/ and smoke detectors are necessary, if 3D-printers are to be used without people present.

Processing PLA requires humidity and temperature control to avoid unnecessary degradation.

Some sources recommend storing PLA in its original package at ambient temperatures but drying it before use, because of its hydroscopic tendencies.

The main usage of PLA at the techno-workshop will be 3D printing with filament. In addition, PLA can be extruded. While heat is needed to allow PLA to flow under pressure, more specific processes are needed to pump, mix and pressurize PLA. Related to this is injection molding, for small-series production. The main challenge is making inexpensive molds. Injection molding for PLA production is limited, because of its slow crystallization rate, compared to other thermoplastics.

Other processes include injection stretch blow molding, cast film and sheet and thermoforming.

Bioplastics such as PLA have a large economic potential, allowing job creation opportunities, especially in rural areas, such as Inderøy. There are estimates that the European bioplastics industry will provide 300 000 skilled jobs by 2030, up from an estimated 30 000 in 2020. Thus one of the key tasks of the Techno-workshop is to encourage young people to develop business ideas based on the use of PLA.

PLA is biocompatible, it can be used in the human body with minimum risk of inflammation and infection. It has been used to produce biomedical products for drug delivery systems and bone fixation, including plates, screws, surgical structures and meshes. These can dissolve inside the body show over a period of between three months and two years. that it posses great promise in solving problems such as tissue loss and organ failure

There are efforts in the textile industry to replace non-renewable polyester textiles with PLA. Advantages include breathability, lower weight, and recyclability.

The cosmetics industry facing a consumer backlash for using petroleum based plastic products, has sought more sustainable solutions using PLA.

While there were hopes that PLA could be used for structural applications in the construction industry, the same characteristics that made it useful in biomedical applications, detracted from its use as foam for insulation, fiber for carpets and more generally in furnishings.