(346d) The Effect of Irradiation On the Processability of Lignin for Carbon Fiber Applications

Lignin is one of the most abundant macromolecules existing in the plant kingdom and the second major component of biomass. Lignin has gained interest as a source of fuel and new materials due to its renewability and its enormous amount available as a byproduct of the pulp and paper industry.  Utilization of technical lignin as high value added applications is limited currently. There are several factors restricting the use of lignin namely: a non-uniform structure unique chemical reactivity, and the presence of various organic and inorganic impurities. One main disadvantage is that when heated to form a liquid the lignin degrades.

One of the most interesting value-added products that can be made from lignin is carbon fiber for composite or adsorbent applications. Fibers for high performance composites are one of the most important advanced engineering materials produced today. Lightweight fatigue resistance materials possess high strength, high modulus and high stiffness. Carbon fiber composite products are routinely used in sports equipment, marine products, construction, aircraft and the automotive industry. The conversion of lignin into valuable products is one way to add value to an existing wood processing process, an enabling concept of the biorefinery.

Due to its technical and environmental advantages over traditional solvent-based treatments, the processing of polymers under ionizing radiations has known increasing industrial developments for the past 20 years, in particular in the field of synthetic coating cross-linking. The radiation based modification of biopolymers is also a feasible method either to produce material or to tailor their usage properties. From a molecular point of view, irradiation was shown to generate free radicals on polymers and then induce various reactions as scission or crosslinking depending upon both the environmental conditions and the radiation dosage.

The objective of this research was to develop from lignin a stable thermoplastic material with a reasonable temperature window for processing in the melt state.

To meet the objective, softwood kraft lignin was modified using irradiation at low doses with and without styrene present and characterized. Irradiation of kraft lignin alone with radiation shows an initial decrease in molecular weight due to chain scission up to about 10kGy followed by an increase in molecular weight due to crosslinking. NMR results indicate a decrease of about 15% in the OH content of the lignin with increased irradiation after 10 kGy.  Thermal properties such as Tg, free volume and Delta Cp follow accordingly.  Irradiation at very low dosages was determined to facilitate the grafting of styrene monomer to lignin, decreasing the OH content by 23%. This effect increased the hydrophobicity of the material, depressed the value of Tg, increased the Delta Cp, increased the mobility in the liquid state, and made the material more thermally stable relative to the lignin alone, thus improving its processability at high temperatures. Both the irradiation of lignin alone and the grafting of styrene to lignin increased the yield of mass during pyrolysis and the activation energy for mass loss relative to untreated lignin alone.  This work has demonstrated that the application of low dosages of irradiation is a promising method to attach functional molecules onto lignin for use in various applications.

This type of research is one of the first steps in modifying renewable lignin so that it is a more consistent and reliable precursor for material applications. Irradiation is a promising green tool in this respect and optimized/alternative methods to utilize irradiation to improve lignin as a precursor in a cost effective manner should be pursued.