ASC: Forest fibres can bring added value

Cellulosic nanofibres have the potential to enhance the performance of a range of adhesives and sealants as well as improve the profitability of forestry operations

Part of the mission of the US Department of Agriculture’s Forest Service is to establish and maintain resilient forests throughout the country. Often this work involves mechanical thinning of forests to remove woody biomass.

FPL aims to add value to forest production Copyright: Rex Features

Researchers at the Forest Service’s Forest Products Laboratory (FPL) assist in this effort by identifying higher-value products derived from wood that have potential for widespread application. Such new products can also greatly assist privately owned forests that are planted and grown for resale yet face a declining market due in part to reduced paper usage.

One such new wood-based product under research at FPL is cellulose nanocrystals (CNCs), which are actually nanoscale fibres with tremendous strength that can enhance the properties of adhesives, sealants, composites and coatings.

HIGH STRENGTH FIBRES
The annual global production of cellulose totals more than 1 trillion tonnes, according to Greg Schueneman, a supervisory materials research engineer at FPL. Approximately 20% of that material comprises cellulose nanocrystals, which are nanofibres with a width and thickness of about 5nm each and a length of 200nm. Most notably, isolated CNCs have a modulus and tensile strength similar to those of aramid fibres such as Kevlar.

CNCs are obtained by first separating cellulose from hemicelluloses and lignin using processes commonly applied for wood pulp manufacturing. The cellulose consists of ordered and disordered domains, which can be separated using a variety of methods.

In the FPL pilot plant, the cellulose is heated at 45°C with concentrated (64%) sulphuric acid in order to hydrolyse the disordered cellulose. The cellulose nanocrystals (the ordered domain) are subsequently isolated and concentrated as a 10wt% aqueous dispersion that can be used directly or freeze dried to provide a film or cake of CNCs that can be redispersed when needed.

The strength of CNCs has attracted significant attention, and Schueneman, along with other colleagues at the FPL, university researchers and many industrial partners, is exploring their potential applications. For instance, when a 10wt% aqueous CNC solution is used instead of water for the emulsion-based formulation of epoxy composites, products containing up to 20wt% CNCs (and possible higher – research is still ongoing) are obtained.

Presently there is no evidence to indicate that the unmodified CNCs react directly with the epoxy resin, but Schueneman has demonstrated that the fibrous nanocrystals form a percolated network in which neighbouring fibres interact with one another via hydrogen bonding of the hydroxyl groups of the cellulose.

The network results in a greater reinforcement effect than is observed for nanoparticles incorporated into epoxy composites. The composites have higher modulus values than the original epoxy, even at higher temperatures.

Thus, even at the glass transition temperature of the pure epoxy, 65-70% of its modulus at room temperature is maintained due to the addition of the CNCs. Such an enhancement of the high-temperature modulus of epoxies should expand their potential use in more demanding applications.

Epoxy films can also be formed with added CNCs that are attractive as protective coatings for industrial, marine and infrastructure applications. Interestingly, these coatings are transparent, because the refractive index of the CNCs matches that of the epoxy resin.

Transparent protective coatings that can withstand abrasion are particularly attractive for metal protection and for coating of plastics where it is desirable that the plastic remain transparent.

Cellulose nanofibres can add strength

LOW THERMAL EXPANSION
It should also be noted, according to Schueneman, that the cellulose nanocrystals have a very low coefficient of thermal expansion: <10 compared to 100-200ppm/°C for typical polymers. Therefore, CNCs can be used to lower the coefficient of thermal expansion of the overall composite or help match the coefficient of thermal expansion of a substrate to ensure longer durability – an issue that is particularly challenging for adhesives used in the consumer electronics industry.

Researchers at FPL are also investigating the use of CNCs in neat epoxy, acrylic, and poly-urethane adhesives (at around 10wt%). For these applications, however, it is necessary to modify the surfaces of the CNCs in order to convert the hydrophilic hydroxyl groups to other hydrophobic substituents so that a uniform and stable dispersion is created.