Nanocellulose material

Brief

This high level view provides a description of the fundamental material we offer; it does not provide a basis for application development.

Nanocellulose

Of the two forms of nanocellulose that get current technical press coverage - nano-fibrillated cellulose (NFC) and nano-crystalline cellulose (sometimes called nano-cellulose crystals NCC or crystalline nano-cellulose CNC) - the latter represent the lowest level of hierarchy for cellulose polymers, while the former constitutes the second level of hierarchy. Both represent pure cellulose, in different structural configurations.

We concentrate on NFC. NCC gets most of the press hype but it hasn’t proven extraordinarily useful. We’ll say no more about it here.

Varieties of NFC

We may derive from refined wood pulp, the most common feedstock, from cotton lint, from certain types of micro-organisms and from our sea friends the tunicates. Fibrils of different provenance are not entirely equivalent, and some authorities would object to using the term NFC for all of them.

Tunicate fibrils and, to a lesser extent, microbial fibrils occur naturally. They don’t have to be further refined, though in some cases they will benefit from being exposed to powerful shear forces to increase fibril uniformity. Wood pulp of course has no structural utility in its initial state, and it’s already highly processed.

Microbial cellulose.

Tunicate cellulose suspended in a gel

SEM images reveal significant difference among the different types of nano fibrils, and mechanical behavior differs as well. Very generally, microbial and tunicate cellulose formulations behave somewhat better for our purposes than those derived from wood pulp.

Choice of feedstock depends as much on price as mechanical attributes. Wood pulp derived NFC has steadily declined in price. Cellulose from algal sources may prove similarly cost effective if yields may be gotten up through genetic engineering – not an avenue we wish to pursue. Tunicate cellulose currently finds use as pig fodder.

Mechanical Properties

One can characterize structural materials in many ways, and a few numbers cannot capture their real world usefulness.

When we discuss light, strong materials, we generally look at tensile or pull strength and stiffness to mass ratios. You will find these measures routinely cited for fiberglass, Kevlar, and carbon fiber. You will seldom see data for compressive strength on these materials because they perform so poorly in this area. Among synthetic fibers, only basalt fiber and Magellan M5 perform well in compression.

Comparing materials

Tensile strength for wood pulp derived NFC and its kin varies from 140 mega Pascals to 400 mega Pascals. Spun Kevlar comes in at about 3620 mega Pascals. This does not provide an apples to apples comparison, because of the extremely light weight of NFC. In honeycombed configuration it can theoretically best Kevlar. Carbon fiber weighs in at 4000 mega Pascals, with the same apples-to-apples caveat.

Young’s modulus, a measure of stiffness, is about 15 giga Pascals for wood pulp derived cellulose and microbial derived cellulose versus 70 giga Pascals for carbon fiber and 6 giga Pascals for Kevlar, but the NFC from tunicates measures an astounding 150 giga Pascals. Other than diamonds and sapphires, no common material beats tunicate cellulose in this regard. Again, light weight of NFC makes a direct comparison difficult.

Tensile strength figures most importantly in skin-on-core constructions where incident compressive forces tend to translate in tension. A high degree of stiffness has more immediate use because it enables one to minimize flexure at very low weights. In a bicycle frame, for instance, flexure absorbs pedal power intended to drive the wheels, while excessive weight represents more work to be performed by the cyclist. The less work for a given velocity of travel, the better.

Other Aspects of Mechanical Performance

Because structural NFC has not yet gone beyond the earliest prototyping stages, we don’t have a large body of published measurements.

• We know that NFC can endure considerable elongation before breakage without apparent structural damage.

• We have no data on so-called “creep” wherein the material permanently elongates when exposed to protracted tensile forces.

• We know little about fatigue. What happens when you fold a sheet of the material 10,000 times? Spectra fiber won’t show effect in the slightest but everything else will quickly degrade.

• What about impact resistance? Fiberglass and carbon fiber are very poor in this respect. NFC has strong ballistic characteristics; in other words, highly impact resistant. Getting this kind of data will prove absolutely necessary to going forward with so-called ‘bulletproof’ applications.

• NFC can form a hard ceramic when treated with titanium oxide. If we treat the surface of the structure in this manner we would have the basis of a whole new class of artifacts—object suitable for serving as mechanical bearings and even cutting edges, without the brittleness of pure ceramic structures.

•  Materials known as cermets combine ceramics with metals, often in gradients where ceramic beads or fibrils distribute in a metal matrix in varying concentrations and distribution patterns. These have very interesting performance properties. Ceramic cellulose compositions could prove equally interesting.

•  Paper made of microbial cellulose has good self-damping, a valuable property in many structural applications, though we lack published data for pulp based formulations.

Thermal and Electrical Properties

• NFC based aerogels are already on the market and are superb thermal insulators as well as being lightweight. They are also stronger than other aerogels. We claim no intellectual property here.

• NFC cannot endure sustained high temperatures, a characteristic inherently shared by all cellulose formulations. As with wood, one can fireproof it by means of various application. At very high temperatures of several hundred degrees Celsius it will undergo pyrolysis and disintegrate. One can also sheath it with low cost basalt fibers to ameliorate such shortcomings.

• NFC, suitably prepared, exhibits properties of a semiconductor. This means that transistors, diodes, and capacitors can be built directly into the material just as is the case with a silicon wafer. One can imagine an HD video display the size of an entire stadium, or more modestly of walls in one’s home – just for starters. To our knowledge it constitutes one of only two structural semiconductors known. Silicon carbide, an extremely tough ceramic, can also serve as a semiconductor, but its cost restricts it to price insensitive applications.

• NFC exhibits piezoelectric and ferroelectric characteristics, meaning that it will bend upon the presence of an applied voltage and will store considerable energy as space charge capacitance. This makes it a smart structural material, something unheard of previously.

Whole more than the Sum of Parts

Recently many have researched use of woven split bamboo as a skin for composite construction.

Bamboo has seen very little use as yet in this application, but it has found favor with certain custom surfboard builders. Bamboo fibers are among the strongest in the plant kingdom, but their tensile strength is inferior to that of fiberglass. Even so the manufacturers report that the bamboo boards are much more durable and little subject to “dings”

This tells us, if it tells us anything, that we can’t always rely on a few metrics.

Isotropic/anisotropic

Some materials exhibit similar mechanical properties (isotropic) across all dimensions, while other do not. Many natural materials such as wood, ivory, bone, etc. are strongly directional (anisotropic), while manmade refined materials such as glass may have the opposite characteristics.

NFC has strong three dimensional isotropic linkages across fibrils, and these provide the source of its strength. Similarly, orientation of the fibrils provides the source of the material’s rigidity.

When numerous fibrils share the same axial alignment, the material will be stiff in the dimension where the alignment is present. In any other dimension the material will be highly flexible and compliant.

When we form structural elements such that we have alignment under precise control and variable millimeter by millimeter, we can then configure the material for all kinds of applications where we desire some combination of rigidity and resilience.

Porosity

One can endow NFC with any degree of porosity and can make it air breathing and water repellant at the same time.

Water Resistance

NFC in its native state is hydrophilic, that is, it attracts water, but the material is so dense and the surface so smooth that water drops penetrate poorly. Various chemical processes such as carboxymethylation as well as micro-texturing will render it completely water proof.

Light Admittance

Normally translucent, one can make NFC completely transparent. This makes it the only extremely strong, light, low cost structural glazing material known.

Recycling

Pure cellulose is completely nontoxic, and cellulose constructions disintegrate rapidly when deprived of protective coatings and broken up to expose their interiors.

© Cellulistics – Daniel Sweeney, PhD