Evonik’s biopolyamides, marketed under the name Vestamid Terra form the matrix of the ne composite. Two polyamide grades of the Vestamid Terra product families form the polymer matrix: Terra HS and Terra DS. The latter is a 100% bio-based PA10.10 that offers properties between standard shorter-chain PA6 and high-performance, long-chain PA12; the former is a PA6.10 containing approximately 60% renewable raw materials. Both are produced, partially or wholly, from the non-edible castor oil plant and both compounds are available as glass fiber-reinforced grades with glass content of 30-65%.
Adding reinforcing glass fibers improves the mechanical properties of the polymer, providing greater strength, heat resistance, and reduced mold shrinkage.
But in the case of bio-based polymers, the use of glass as a reinforcement system implies a lowering of the bio-content, reducing the ecological advantage. And, while natural fibers can offer advantages such as low cost, low energy consumption, non-abrasiveness, safety in handling, low density, and a potentially higher volume fraction, their use is associated with problems such as low thermal stability, low resistance to moisture, and seasonal quality variations. Moreover, according to Evonik, which has tested various natural fibers such as basalt, aspen wood, cellulose, and bamboo, the use of natural fibers tends to result in significant deterioration of reinforcing potential, as well as odor problems in the end product.
Evonik has now opted for the middle road. In this newest product, commercially available chopped rayon fibers form the reinforcing fiber substrate. Rayon is also known as regenerated cellulose fiber, man-made fiber, or viscose fiber. Although derived from a natural source, these fibers are not classified as natural fibers because they require extensive processing to become the finished product. Regenerated cellulose fibers are obtained entirely from wood residues (dissolving pulp), and are therefore based on renewable raw materials. The overall bio-content is thus high, varying from 67-100%.
Compared with glass-fiber reinforced systems, the combination of viscose fibers and polymer matrix offers a significantly improved carbon balance. As an example, CO2 savings for a viscose fiber system of PA1010 with a fiber content of 30% are 57% higher than for a 30% glass fiber-reinforced PA66. Moreover, viscose reinforcing fibers have a significantly lower density than mineral fibers: depending on fiber content, bio-polyamides reinforced with viscose fibers offer a weight reduction of up to 10%, for the same reinforcing performance.
It’s an extremely interesting development. But does this higher biocontent automatically make for a “green” product? The rayon used by Evonik is produced via an energy-intensive process—known as the viscose process—that makes use of carbon disulfide and sulfuric acid. The use of carbon disulfide is typically highly dispersant; 50% of what is used is released into the air. Environmental exposure to carbon disulfide can have adverse health effects in humans. True, technologies have been developed that are gradually lowering the environmental impact of rayon production. Integrated production facilities make use of renewable energy from production steps; processing occurs in a closed‐loop system in which process chemicals are recovered and reused; fiber is sourced from Forest Stewardship Council (FSC) certified forests. Yet, unfortunately not all rayon facilities operate in this way.
On the other hand, glass production and glass fiber production are both energy intensive processes depending mainly on fossil fuels. It has been calculated that for each kilogram of glass melted, 1 kilogram of carbon dioxide (CO2) is emitted into the atmosphere, in addition to nitrogen oxide and dioxide (NOx), sulphur dioxide (SO2), chloride, fluoride, volatile organic compounds (VOC), and particles. That’s also something to think about.
Rayon does offer end-of-life benefits: whereas glass costs energy to incinerate, cellulosic fibers, according to global viscose manufacturer Lenzing, “burn readily with a heat of combustion of 15 kJ/g,” or will biodegrade into water and carbon dioxide after six weeks in a static aerated compost pile.
"With this product development, we want to further support the unrestricted expansion of bio-based products in technically demanding applications for our customers," says Dr. Benjamin Brehmer, business manager biopolymers at Evonik.
What’s more, it’s likely that this is only the beginning for Evonik: at the Bio-based Plastics and Composites conference earlier this year, Brehmer extensively discussed the ongoing developments in natural reinforcement for bioplastics, noting that Evonik had “a long list of ideas and trials in the future,” which included the “testing of newer fibers and fiber alternatives, which are more stable and offer even higher mechanical strength, e.g. lignin-based carbon fibers.”