Softeners make plastics pliable and are among the most sold chemicals worldwide. The market research institute Ceresana Research expects worldwide demand for softeners to increase to 7.6 million tons per year by 2018. Phthalate-based softeners are market leaders, but they are harmful to the environment and not approved for direct contact with foodstuffs. Legal provisions and increasing environmental awareness increasingly demand the use of phthalate-free softeners. Even though phthalate have been mostly replaced in petro-chemically based plastics, they are still contained in bio-based plastics such as the wood-based cellulose acetate (CA).

Fraunhofer UMSICHT is developing bio-based plastics. Biograde® is a foodstuff-conformant, CA-based material recipe for injection molding that was developed to tap into new areas of application for the plastics in the foodstuffs sector. To increase the current bio-based share of CA of approx. 50 percent and to improve the properties of the material, Fraunhofer UMSICHT is currently researching alternative bio-based softeners and other functional additives.

Cellulose acetate

CA can replace some conventional technical plastics that are manufactured from fossil resources. Comparable to polystyrene (PS), acrylonitrile-butadiene-styrene (ABS) or technical polypropylene (PP) types, CA features a very good heat resistance and a high modulus of elasticity. The surface feels pleasant, and the high density conveys a valuable impression. The processing into technical parts in an injection molding process is for the most part unproblematic and economical. In addition, the material displays a good potential for the manufacturing of foamed products.

Bio-based replacement for phthalates

Due to the chemical structure of CA consisting of glucose rings, the development of new softener systems for CA is complex. Not every softener is compatible with the bio-based plastics or displays the hoped for effect. In thermoplastic processing into an injection molded part, CA must be heated to become liquid so that it can be molded into a plastics part. The softener supports the melting. If its effect is insufficient, the material will burn. Another effect of non-suitable softeners is the quick embrittlement of the plastics parts. They break easily and have sharp edges at the fracture point.

Fraunhofer UMSICHT is investigating some known and some novel softener systems for CA. The focus of interest are, among others, the groups of citrate esters, phosphates and benzoates, since select softeners from these groups are non-toxic, biodegradable and between 50 to 100 percent bio-based. Furthermore, foodstuff conformity is assured in case of the citrate esters and benzoates.

Products made of CA can be found on the marketplace as household articles, IT products, and packaging. To be able to develop the optimal recipe for each application, the materials are subjected to comprehensive inspections and testing. Housing components (such as for keyboards, kitchen blenders or hair dryers) have to be dimensionally stable at elevated temperatures and extremely durable. In addition to the classic mechanical inspections with the tensile testing machine or the pendulum impact tester, injection-molded plastic components are stored for an extended period in climatic chambers and monitored for changes in the material's properties.

The flow properties of the material are also being examined to adjust the viscosity of the melt to the plastics engineering process. In particular in the case of the foaming process, the softeners used must be integrated into the CA so that they achieve the molding and flow properties for the foam extrusion and remain in the material during the foaming process and do not evaporate. No matter how manifold the challenges to material development and production engineering may be, the highly motivating objective is to develop sustainable plastics and to tap new areas of application for them.