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A development of natural rubber extraction from Ericameria Nauseosa (rabbitbrush)
AdvisorMiller, Glenn C.
Environmental Sciences and Health
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Natural rubber, primarily obtained from the Para rubber tree (Hevea brasiliensis),is a critical resource and has important properties, including elasticity, resistance toabrasion and impact resilience (e.g. road damage). Natural rubber also dissipates heatefficiently and is malleable at low temperature. However, the rubber tree only grows intropical areas, and has to be imported to the United States for the many uses that it has.Thus, extensive interest exists in finding a domestic native rubber source as a supplementto importing natural rubber. Ericameria nauseosa (rubber rabbitbrush, or rabbitbrush) isconsidered a potential rubber-producing plant which is able to thrive in many nontropicalenvironments. This plant has excellent drought tolerance and a high adaptabilityto a wide range of soil conditions allowing rabbitbrush to grow in widely distributedregions of North America. In the present study on rabbitbrush, seed germinationcharacteristics, extraction methods for rubber recovery, soil characteristics, andcharacterization of rabbitbrush rubber quality were examined.In the germination examination for rabbitbrush, we found that: (1) Rabbitbrushgermination percentage was about 20% in a moist and warm environment. Coldstratification can shorten the germination period but had no significant improvement onthe germination quantity; (2) No significant difference of germination quantity wasobserved on different soil substrates; (3) In this test, we found that the soil moistureshould be maintained at about 35%, equivalent to a 20 ml/day water treatment, tomaximize the germination percentage. (4) Rabbitbrush seeds can be germinated in thedark, but the germination quantity (14%) is reduced by about 30% compared togermination in light. (5) Temperature is a key factor for rabbitbrush germination. Theseeds did not germinate at temperatures below 7 ℃.For the rubber extraction study, an extraction method was developed optimizingsafety, efficiency and cost. Several extraction methods were examined in this study. Thefollowing conclusions can be made. (1) Although rabbitbrush bark has a higher rubbercontent than other tissues (branches, wood with bark removed), extraction of the wholeplant (without leaves) is efficient and requires less labor than extracting the separate partsof the plant. (2) Large scale extractions and accelerated solvent extraction (ASE) appearto have a higher extraction efficiency than Soxhlet extraction. Temperature and pressureare two important factors for improving rubber recovery. However, the ASE result waspotentially overestimated because many chemicals present in rabbitbrush could beremoved under the aggressive extraction conditions (140 ℃ and 1500 psi), and theextraction yields are based on gravimetric measurements. (3) A single hexane extractionremoves higher amounts of rubber, including smaller rubber molecules. Sequentialextraction using acetone first and hexane second is considered as a “purification” processbecause acetone can remove the less desirable lower molecular weight rubber compoundsfrom rabbitbrush, leaving the higher quality rubber to be extracted in the hexane extract.(4) We collected rabbitbrush samples from several locations and performed extractionsusing a Soxhlet apparatus. We found that rubber yield might not be only related torabbitbrush taxonomy, but also potentially affected by the growing conditions of therabbitbrush, presumably the soil properties of its habitat. Rabbitbrush growing in thesaline-alkaline soil tended to have higher rubber yield. Ericameria nauseosa subsp.nauseosa (gray form) var. oreophila contained the highest rubber yield. (5) The loss ofrubber content (rubber degradation) was observed with storage time, and approximately50% was lost in each of the four storage methods examined, probably by oxidation. Thetests included a cold temperature, room temperature and a limited oxygen availability.Nearly all of the loss of rubber from the milled rabbitbrush samples occurred during thefirst month of storage.Molecular weight is an important parameter for determining rubber quality. GelPermeation Chromatography (GPC) was used to determine the molecular weight ofrabbitbrush rubber in three sets of studies where the variables were: (1) rabbitbrushrubber from plants grown in different habitats. (2) rabbitbrush rubber resulting fromdifferent extraction methods. (3) rabbitbrush rubber extracted from samples stored underdifferent conditions and times. The conclusions are: (1) The molecular weight ofrabbitbrush rubber is lower (2-4 * 105 Da) than from the rubber tree (H. brasiliensis)rubber (generally more than 10-30 * 105 Da). (2) The molecular weight varied withrabbitbrush taxonomy. Ericameria nauseosa subsp. nauseosa (gray form) var. oreophilahad higher rubber molecular weight than samples examined from other varieties. (3)Extraction methods (sequential extraction versus a single hexane extraction) affected themolecular weight of recovered rubber extracted from various rabbitbrush samples.Rubber recovered by sequential extraction method had a higher molecular weight than asingle hexane extraction method. Rabbitbrush rubber polydispersity index (PDI,calculated by the weight average molecular weight divided by the number averagemolecular weight, Mw/Mn) is large (from 2.0-8.0) which indicates a low homogeneity ofrubber molecules. (4) The molecular weight of rubber decreased from 4.2 * 105 to 0.8 *105 Da after the first month of storage in all four of the storage conditions. Thus, thequantity and quality of rubber extracted rabbitbrush was reduced substantially when themilled samples were stored, particularly during the first month.Although these conclusions are helpful to develop rabbitbrush as a potentialindustrial crop, some questions were identified for further research:(1) Rabbitbrush is widely distributed in North America and could survive inmany environments including the relatively harsh conditions encountered in semiaridregions. However, the germination percentage in this study was not high (about 20% inmoist and warm conditions). Additional research is required to determine the appropriateconditions for improved rabbitbrush seed germination.(2) As a perennial shrub, the rubber content increases with age. Previous studieshave indicated that the rubber content is low until rabbitbrush grows to a mature age (5-8years old). In this study, all the rabbitbrush samples were collected in a naturalenvironment, and the age of those rabbitbrush plants was not known, but the plants werepresumed mature. Cultivated stands of rabbitbrush could be grown to examine therelationship between rubber content and rabbitbrush age, but this test was not possible inthe time frame of this project.(3) In addition to natural rubber, rabbitbrush also contains other chemicals,including resins, phenols, carboxylic acids, etc. Some of these may be utilized for biofuelproduction and the chemical industry. Therefore, rabbitbrush could be more valuable asan industrial crop if we can separate and utilize these compounds in the future.(4) Rabbitbrush (Ericameria nauseosa) has two subspecies and 22 varieties, andmost of them can grow in different environments. However, additional studies arerequired which focus on the relationship between rubber content and rabbitbrush taxa andthe growing environment.(5) The molecular weight and thus the quality of rabbitbrush rubber is dependenton the rubber producing mechanism. In this study, the rabbitbrush rubber Mw (weightaverage of molecular weight) is about 0.1 - 0.6 million Da which is about 10 times lowerthan from the rubber tree rubber (Mw: about 1 - 3 million). Therefore, we need to betterunderstand the mechanism of rubber production in rabbitbrush to maximize the molecularweight/quality of the rubber obtained.