Research

What are CNFs?

In recent years, demand for biodegradable, plant-based, composite, also called "green composites" or "biocomposites", is increasing in various sectors especially the automotive sector of industry. Various kinds of natural fibers have been used in green composites; such reinforcements can be classified into two categories of plant and animal fibers. Plant fibers may be produced from different parts of the plants, such as seed, leave, skin, bast, and fruit. Examples of such fibers are cotton, jute, kenaf, coir, and flax. Examples of animal fibers include silk, sinew, wool, catgut, angora, mohair and alpaca. Natural fibers have lower mechanical properties as compared to carbon and glass fibers. As a result, the current breed of two biocomposites suffers from the twin drawbacks of lower strength and fatigue properties compared with the carbon or glass-fiber based polymer composites.

Cellulose nanofibers (CNFs) (c.f. Fig-1), a new type of nanofibers made purely of cellulose molecules, have very good mechanical property compared to other natural fibers, and even carbon or glass fibers. Recent studies showed that the films or "nanopaper" of CNFs are the strongest man-made, cellulose-based materials. However, the full reinforcing potential of these materials has yet to be realized partly because of issues related to manufacturing processes. Recently, cellulose nanofibers have begun receiving serious consideration as potential reinforcement materials. The big problem with using CNF fibers was the energy requirements for breaking down cellulose fibers in nanofibers were prohibitively high until recently. Recent advances in chemical and mechanical technologies have drastically reduced the energy requirements for producing cellulose nanofibers.

For this project, CNFs are provided by FPL produced at their pilot facility (c.f Fig-2).

Fig-1: (a-left) Schematic of CNFs; (b-right) a transparent CNF film
(Photos courtesy: Dr. Ron Sabo, Forest Products Laboratory)

Fig-2: CNF production pilot plant located at Forest Products Laboratory, USDA
(Photos courtesy: Dr. Ron Sabo, Forest Products Laboratory)

Why do we Need to Hydrophobize CNFs

Each repeating unit of CNF has 3 hydroxyl groups which make CNF highly hydrophilic. This high degree of hydrophilicity makes CNF incompatible with hydrophobic polymers such as epoxy. Hence there is a critical need for hydrophization of CNFs. In this study, we aim to hydrophobize CNFs using silylation technology.

Structure-1: CNF backbone showing the position of hydroxyl groups
[Garner et al., Journal of Adhesion Science and Technology, 2008]

Description of Lyophilization Process

Lyophilization or freeze drying is a process in which water/solvent is removed from a product after it is frozen and placed under a vacuum, allowing the ice to change directly from solid to vapor without passing through a liquid phase. The process consists of three separate, unique, and interdependent processes; freezing, primary drying (sublimation), and secondary drying (desorption). (source: www.fda.gov)

In this project, we aim to develop random and unidirectional freeze-dried porous structures for subsequent fabrication of isotropic and anisotropic high-strength thermoset nanocomposites via liquid composite molding. The unidirectional porous structures will be developed using a unique directional freezing technique.

Directional Freezing Technique Developed at UW-Madison

Fig-3 shows the directional freezing technique developed at UW-Madison to fabricate unidirectional porous structures using liquid nitrogen. According to this method, a controlled freezing is implemented which allows for formation of ice crystals in the longitudinal direction. Fig-3 shows the formation of ice crystals at different exposure times. Once the ice-crystals are formed completely, the unidirectionally frozen sample is then run through the lyopholizer (Fig-4) for drying via sublimation.

Fig-3: Schematic of directional freezing

Fig-4: Laboratory scale freeze dryer located at the University of Wisconsin-Madison

Thermal-Differential Freezing Device

Though the above method provides uniform and aligned porous preforms for further composite fabrication, the method lacks in terms of obtaining varied sized pores. This is because of the temperature at which these are frozen i.e. liquid nitrogen (-196°). Since the directional freezing process depends heavily on the freezing temperature, it is critical to control the temperature to obtain preforms with different pore-morphologies. In addition, the directional freezing process also depends on solute-solvent concentration, direction of freezing, solvent type, etc. Hence it is essential that these parameters are also engineered to obtain optimized porous structures for efficient fabrication of anisotropic porous preforms.

Thus, at UW-Madison, Co-PIs Turng and Pilla aim to develop a thermally differentiable device for directional freezing of the CNF (Fig-5). The device design is shown in Fig-5. The device allows to control the freezing temperature from -196⁰C to 0⁰C could be obtained. Also, the thermal protective layers will leverage for regulated surface freezing.

Fig-5: Thermal differential freezing device to be built in this project

Scaled-Operations at FPL

Fig-6 shows an industrial scale lyopholizer located at FPL which will be used to fabricate large porous (random and unidirectional) preforms after fundamental process parameters are engineered at UW-Madison.

Fig-6: Industrial scale lyophilizer located at Forest Products Laboratory, USDA
(Photos courtesy: Dr. Ron Sabo, Forest Products Laboratory)