KuZ researches innovative materials solutions

Recycling 3D printed and laser-sintered plastics, physically foamed recycled PET, plasma treatment enhancement of adhesion to polycarbonate and use of gluten as a toughening additive for polylactic acid (PLA) are among recent and current research projects at KuZ Kunststoff-Zentrum in Leipzig, Germany.

Announced at the end of August and running until June 2022, the "ReUp-3D-Printwaste" project uses chemical additives to improve additive manufacturing waste plastic quality. These include support structures and faulty printed parts in PLA from fused deposition modeling (FDM), and selective laser sintering (SLS) polyamide 12 powder. These "upcycled" materials save costs while benefiting the circular economy by use in additive processes from which they originate.

When added to PLA waste, chain extenders, hydrolysis stabilizers and antioxidants enable extrusion into new FDM filaments, and chain shortening and antioxidant additives in PA12 waste powder results in new SLS material. KuZ says it also focuses on combined SLS-FDM and FDM recycled materials.

Material costs take the largest share of additive process operating costs. Virgin PLA filaments cost 25-30 euros ($28-$34) per kilogram, virgin nylon 12 powder 80-120 euros ($92-$138) per kg. So there is plenty of material available for upcycling, as FDM generates 20-30 percent waste, SLS 50-70 percent.

In a "RecySchaum" recycled foam project, KuZ uses PET from post-consumer packaging waste as a basis for recycled PET thermoplastic foam injection molding lightweight automotive components. A recycled PET foam model achieved 40 percent weight saving over an equivalent real application reference part.

Recently concluded with Sandersdorf-Brehna, Germany-based contract compounder Compraxx GmbH, the "Gluplast" project involved wheat gluten as a naturally reoccurring additive in PLA to reduce inherent brittleness and improve low plastic deformability and impact strength, previously only achieved by blending PLA with conventional petroleum-based polymers such as thermoplastic polyurethane or ethylene copolymer.

Gluten forms a cross-linked elastomer phase at around 80° C (176° F), but adequate distribution of gluten needed to achieve sufficiently higher toughness through increased adhesion to the PLA matrix could not be obtained when gluten was compounded with PLA at 160° C (320° F) in a twin-screw extruder, due to insufficient gluten distribution in the PLA.

The 1.4 percent elongation at break and 1.5 megapascal impact strength were in fact lower than the respective 2.5 Mpa and 2 Mpa values for unmodified PLA (Natureworks Ingeo 6202D and Ingeo 3001D).

KuZ overcame this by diluting gluten with wheat flour to reduce the degree of cross-linking and/or chemical additives to temporary reduce cross-linking: sodium hydrogen sulfite, acetylsalicylic acid and L-cysteine proteinogenic amino acid, as respectively reducing agent, antioxidant and trapping reagent.

Addition of glycerin plasticizer and oxalic acid enhanced blendability through interphase adhesion enhancement and viscosity reduction. The various additives resulted in elongation at break rising to 30 percent and impact strength to 4 Mpa, well above the mentioned values for unmodified PLA.

"Maillard effect" browning of the modified PLA is related to reactions with proteins contained in gluten, but Compraxx concealed it with color masterbatches, widening potential applications, expected in household and office articles.

Bioflex 6514, a additive-modified PLA copolyester blend from Willich, Germany-based biodegradable plastics producer FKuR Kunststoff GmbH was also included in Gluplast project investigations, but KuZ has not indicated whether it too was modified further with gluten, or simply used as a reference to compare results with gluten modified PLA.