On 30 November, Helena Halonen defended her doctoral thesis “Structural changes during cellulose composite processing”. Her work demonstrates two methods for creating a new composite material based only on cellulose fibres.

In a composite material, the reinforcing agent and the matrix must adhere well at the interface; any effect can reduce the strengthening effect. Using components composed of the same material, as in all-cellulose composites, the adhesion at the interface is expected to be improved. In addition, this construction would improve the recyclability of the material.

In her thesis, Helena Halonen presents a production method that entails more advantages including low cost, simplicity and relatively predictability. The method involves the use of existing raw materials, i.e. pulps that are already available from the pulp mills. Biocomposites based on chemical pulp or dissolving pulp are processed via compression moulding, a commonly used technique found in nearly all industrial sectors. It would therefore be relatively easy to implement the method in an industrial installation.

Structural changes during processing have been studied as well as the relation between the organisation of cellulose fibrils in the fibre cell wall and the mechanical properties of the end material. A higher press temperature yielded increases in the fibril aggregation, higher stiffness and better water resistance. The experiments show that the optimum press temperature would be 170 °C in order to achieve fast drying without the material being broken down thermally, while still having excellent mechanical properties.

The thesis also describes a method for producing biocomposites based on bacterial cellulose fibril aggregates from Gluconacetobacter xylinus processed through a biosynthesis. The formation of cellulose fibrils was affected by the addition of hydroxyetylcellulose (HEC) which resulted in a biocomposite with remarkable mechanical properties including improved strength, modulus and toughness. The HEC coating improved the fibril dispersion and prevented fractures.

“Helena has demonstrated the importance of fundamental understanding of linking process induced modifications in the fibre cell wall structure to the final product properties,” says Assoc. Prof. Tommy Iversen who has been Helena's supervisor at Innventia.

“Helena’s doctoral work is an important contribution to the ‘renewable materials’ area and shows a great potential for wood as a raw material base,” says Catharina Ottestam, Director of business area Packaging Solutions at Innventia.