(38b) A Study on Chemistry and Fractionation of Guayule Resin As a Source of Secondary Metabolities and Energy

Parthenium argentatum, commonly known as guayule, a perennial shrub native to the arid region of the U.S. southwest and north-central Mexico belonging to the Asteraceae family (over 20,000 species). It is an alternative source of high quality natural rubber identical to that from the rubber tree (Hevea brasiliensis) with applications in pharmaceutical, tire, and food industries. The natural rubber derived from Hevea trees contains a high amount of allergic protein called pro- hevein, while the guayule rubber does not have the proteins associated with natural rubber latex allergy. The physical properties of natural rubber products are strongly affected by non-rubber constituents (resin) of guayule plant The degradation of rubber is facilitated in presence of this non-rubber compounds (like linoleic and linolenic acids) and triglycerides, so the separation of resin is a crucial step in processing guayule for producing a high quality rubber. The amount of extracted resin is approximately as much as produced rubber (6-8 wt.% of total biomass), which contain a wide variety of secondary metabolites (sesquiterpene esters, triterpene alcohols, fatty acids, etc.). In order to make the production of rubber economically feasible, utilization of the by-products is needed and has the potential to reduce gross rubber production costs significantly. The main objective of this research is proposing a general picture of guayule resin using rapid and accurate analytical techniques (FT-ICR MS) to identify compounds of potential commercial value. Then based on the characterization information, a resin refinery model was proposed including the most efficient separation unit (like supercritical fluid extraction and microwave assisted extraction) for each group of compounds with possibility of begin scaled up and some alternative routes and scenarios. This model was designed based on two main scenario: (1) fresh harvested and (2) filed dried shrub. The yield and composition of major volatile compounds are strongly influenced by drying process. Therefore, the first scenario was developed based on those purposes that are aimed to extract the high quantity and quality of terpenoids (e.g. α- and β-pinene, caryophyllene, camphene, and etc). On the other hand, the leaves (green+dried) and flowers (peduncles+inflorescences), which on average contributed 32% and 6% to the total biomass respectively, are discarded prior to bulk rubber extraction process, while they are rich in resin (~8%), protein (14-22%), and waxes. Therefore, using the fresh shrub helps to extract more terpenes, protein, and waxes compared to field-dried feedstock. As guayule rubber is synthesized and stored not only in stem and root, but also in mesophyll cells of leaves, the presence of leaves, which contains 1.6% to 2.0% rubber, can significantly increase up to 28% of total rubber yield (assuming the average simultaneous extraction is 7 wt.%).

The second scenario was developed to address the difficulties in handling and storage of fresh shrub, fungal contamination, growing mold and bacteria in a wet environment, and the limitation in rate of extraction in some techniques (like SFE) due to the presence of water.

Although this model focuses on refining resin for value-added products, it has potential to be used as a pretreatment process for current rubber processing techniques, which results in reducing time, energy and solvent consumption.

This process model can serve as a starting point for the lifecycle assessment (LCA) and economic feasibility study (EFS) of processing any natural resin/ or resin-like feedstock.