Bio-based polymeric materials are of great commercial interest to attain environmental sustainability. Natural rubber (cis-1,4 polyisoprene) (NR) is a commodity that is extensively used in industrial, consumer, and medical industries. Over 5,000 plants produce natural rubber, but over 90% of the world’s supply of natural rubber is extracted from one plant species: the Brazilian rubber tree, Hevea brasiliensis. Hevea natural rubber (NR) has a high molecular weight, and can be produced in yields sufficient to meet market demands. NR contains a high amount of allergic proteins that can cause severe allergic reactions. One alternative sources of NR can be derived from the shrub Parthenium argentatum, commonly known as guayule. Guayule natural rubber (GNR) has a high molecular weight and does not contain proteins associated with allergic reactions, rendering it circumallergenic. However, previous work has shown that GNR and NR have differences in various properties, preventing GNR from being a direct substitute for NR in many applications and giving it advantages in others. Therefore, the proposed work focuses on creating bio-based elastomeric materials by optimizing vulcanization chemistries, as well as creating composites using fillers derived from waste streams.
Vulcanization chemistries of circumallergenic GNR and hypoallergenic NR (made by removal of soluble proteins which reduce allergic potential) for thin film applications was optimized using the chemical accelerators diisopropyl xanthogen polysulphide (DIXP) and zinc diisononyl dithiocarbamate (ZDNC). DIXP and ZDNC do not induce Type IV allergic reactions such as contact dermatitis, a unique benefit in comparison to other rubber chemical accelerators. The thin films were manufactured from natural rubber latex using traditional coagulation dipping methods onto stainless steel formers. The effects of chemical accelerator concentration on mechanical, rheological, morphological, and thermal properties of circumallergenic GNR and hypoallergenic NR thin films were investigated. Many of the GNR and NR thin films possessed mechanical properties superior to ASTM standards for surgical gloves and condoms. Multivariate models of mechanical properties as a function of film thickness and chemical accelerator concentration were generated to quantify differences between GNR and NR thin films. Data analytic methods such as canonical correlation analysis further quantified differences in mechanical properties between GNR and NR thin films, with GNR having greater elongation at break than NR, but NR having a higher Young’s Modulus and strength at break than GNR vulcanized films. Thicker films for both NR and GNR showed an increase in Young’s Modulus and strength at break. Scanning electron microscopic (SEM) analysis of GNR and NR thin films showed that smooth thin films without void spaces or defects were created. The glass transition temperatures and thermal degradation curves of GNR and NR thin films were determined to quantify differences in vulcanization, with GNR having a lower glass transition temperature than NR vulcanized thin films. By comparing vulcanization chemistries of GNR and NR thin films, differences in thin film properties attributed to species origin can be quantified.
Commercial thin film products made from NR often contain fillers from non-renewable resources to improve mechanical properties and thermal stability. Fillers from agricultural and industrial waste streams were compounded into thin films, using traditional coagulation dipping methods. Fillers included guayule bark bagasse, carbon fly ash, and calcium carbonate derived from eggshells, utilized at various particle sizes and loadings. The chemical accelerators used in these composites include zinc diethyldithiocarbamate (ZDEC), diphenyl guanidine (DPG), and dipentamethylene thiuram polysulfide (DPTT), which are traditional chemical accelerators associated with increased contact dermatitis risk, but create vulcanized thin films with superior stability compared to DIXP and ZDNC accelerators. The vulcanization chemistries in these films were not optimized in order to determine the sole effect of fillers on GNR and NR thin film properties. In addition, NR latex with and without soluble protein was utilized to determine how protein content impacts film properties. Due to the increased allergic potential of these films in comparison to those manufactured in the first method, these films have applications as industrial coatings and should not be implemented in medical or consumer applications.
Reinforcement of NR and GNR compounds were achieved using fillers that were nano sized, especially at loadings below 2 parts per hundred rubber (phr) of carbon fly ash. Adding fillers to GNR typically caused increased elongation at break, whereas NR had a decreased elongation at break. The differences in bulk mechanical properties of NR and GNR compounds with fillers can be attributed to variances in the polymer-filler interaction; non-rubber components such as proteins and phospholipids vary between GNR and NR and can affect surface activity of a filler. Variances in bulk mechanical properties of GNR due to different fillers are attributed to properties of the filler, including particle structure, size, bulk density, alkalinity, and surface activity. Particles of larger sizes, such as 300 microns, can provide texture to NR and GNR thin films, which could be utilized for the commercialization of industrial non-slip surfaces. These results can assist in successful commercialization of GNR, and create more sustainable NR and GNR composites.