Project II and III – Wood Nanotechnology

Goal for project II – Processing Fundamentals

The goal is development of methods for control of molecular interactions and phase behavior in wet biopolymer and nanocellulose systems: solutions, colloids and fibrous gels. Associated phenomena are critical for the successful development of processes for new fibers, biocomposites and nanomaterials. Competences in physical chemistry, fluid mechanics and polymer science are combined with forest products research in multidisciplinary efforts.

Illustration of fluid induced alignment followed by the phase transition from liquid dispersion to gel, induced by electrolytes or acid.
Illustration of fluid induced alignment followed by the phase transition
from liquid dispersion to gel, induced by electrolytes or acid.

Goal for project III – Materials and Devices

The goal is nanostructural control for improved properties and performance of cellulose-based nanomaterials and devices. This includes molecular scale understanding to address moisture sensitivity and improve mechanical performance. Classical methods from materials science are combined with activities exemplified by molecular dynamics modeling, NMR spectroscopy, polymer synthesis and molecular biology. The wood cell wall is studied for basic understanding and as a source of inspiration. The bottom-up approach in biosynthesis results in structures, such as wood, which we are unable to replicate by synthetic methods.

SEM image of  a cellulose foam. Published with permission from Nicholas Tchang Cervin.
SEM image of a cellulose foam. Published with permission from Dr Nicholas Tchang Cervin.

Project outlines

In order to pave the way for radically new products, the present strategy is to focus on research areas with potential for substantial leaps in product performance or provision of new functions. Wood nanotechnology is such an area, with large potential to stimulate the development of high-performance forest products. The general goal of Projects 2 and 3 is to develop a competence platform in wood nanotechnology, as a basis for industrial development of nanostructured high-performance products. This includes processing methods for such materials and devices.

As the smallest dimension of a material component is decreased from 30 μm (wood fiber diameter) to 10 nm (nanocellulose), it becomes necessary to focus on the specific properties of the nanoparticle rather than on technologies or applications. This is very different from the dominating paradigm in classical ”forest products” research. The term Wood nanotechnology therefore refers to a broad range of research and industrial applications unified by the size of the nanoparticle (primarily cellulose) rather than by a specific technical goal. An important research question becomes ”What is specific to wood-based nanoparticles?”

Nanotechnology in general is an area of strategic interest. Large investments have been made through governmental and industrial funding of research and development. However, the technical progress in terms of nanomaterial applications is still fairly limited, particularly for large volume applications. For instance, the vision of strong composite bulk materials based on carbon nanotubes (CNT) has not been realized. Challenges include cost, but also great difficulties in processing of the desired material structures. Within WWSC, there is a growing realization that nanocellulose and water-based processing has strong advantages compared with the man-made inorganic nanoparticle technologies, which dominate current research. Nanocellulose (Cellulsoe nanofribrils (CNF) and cellulose nanocrystals (CNC)) has many unique characteristics:

• Very small size, 5-15nm diameter and 200nm-5μm length, and ”perfect” fibrous crystal structure. Forms stable suspensions in water, which facilitates ”green processing”.

• Very low cost, from renewable resources, already produced industrially in large volumes (Borregard, Stora Enso, Daicel). Biodegradable and recyclable.

• CNF nanopaper and nanocomposites are very strong and tough, probably due to small size, high instrinsic strength and good dispersion/paper-forming ability in water-based processing.

• Chemical modification of the nanocellulose surface is straight-forward, can be done already in wood pulp state and strongly influences properties (moisture sensitivity and new functionalities such as optical transparency)

• Nanocellulose can be carbonized, for use in device applications (CNT replacement), CNFis also readily miscible with inorganic particles, for instance to form fire-retardant, magnetic or conducting hybrid materials.

The first commercial applications of nanocellulose are underway and include rheology additives in the food industry, barrier coatings for packaging materials and strength additives in paper and paperboard materials. However, the full potential of the nanoscale component is not utilized in the first products. The applications in paper and board products rely on existing production technologies, and nanocellulose is used in the same way as paper chemicals or polymer coatings. Instead, results from WWSC labs show that the potential of nanocellulose in terms of product performance is much superior to pulp fiber or polymer products, provided the organisation and dispersion can be controlled.

Based on this insight, two major aims are proposed:

The first aim of the Wood Nanotechnology projects (Project 2 and 3) is to solve underpinning problems, which will contribute towards development of new processing technologies for fibers, foams, coatings, nanocomposites etc. The ”underpinning problems” are provided in the description of Project 2. Focus is on water-based colloidal processing, which is particularly interesting for nanotechnology. The reason is that this ”green processing route” may also solve the challenging problem of controlled nanocellulose dispersion in new materials. During the first phase, WWSC has demonstrated that cellulose nanomaterials have much superior properties compared with conventional wood fiber products. In addition, new functions have been obtained such as low thermal expansion, fire retardancy, optical transparency, magnetic properties and electrical conductivity. However, several challenges remain:

  • ”Better” cellulose nanofibers are needed (smaller diameter, more uniform dimensions, higher strength etc), and this can only be accomplished based on an improved understanding of CNFstructure.
  • An improved understanding of the physical properties of cellulose-based nanomaterials is needed. For instance, it is not always apparent why nanocellulose provides so much better properties to biocomposites or paper compared with wood fibers.

Fibre reinforced composites of ultraporous fibre structures and a PMMA matrix. Published with permission from Anna Sjöstedt.
Fibre reinforced composites of ultraporous fibre structures and a PMMA matrix. Published with permission from Dr Anna Sjöstedt.

The second aim is to develop the understanding of unique characteristics of nanoscale components, so that nanoscale structures can be designed and controlled. This will help to realize the potential of wood nanotechnology in terms of new materials with improved properties and new functionalities, as well as improved functional performance of devices, as is addressed in Project 3.

Important targets of more practical nature are to solve the moisture sensitivity problems and to develop methodologies for nanostructural control of mechanical properties. Nanocellulose networks are of key importance since they have many unique characteristics and also serve as the load-bearing “skeleton” in plant structures. Wood and the secondary plant cell wall are of great interest as a reference for mechanical behavior, and as inspiration for man-made nanocomposites. For example, polymer-coated core-shell CNF may be a practical route to control the dispersion of CNF, and this is also how microfibrils are separated from each other in the wood cell wall.
In Project 3, the ambition is also to develop the Device area with a focus on the use of nanocellulose in selected energy and sensor-related applications. The goal is to demonstrate outstanding performance in, for instance, paper-based Li-ion batteries, super capacitors and fuel cells. For clarification, a device is an object or ”piece of equipment” made for a special purpose. In contrast, materials exemplified by plastics, wood, nanopaper, biocomposites etc are substances which are used in devices since
they have useful properties.