High Molecular Weight Rubber Research Articles

NOVEL METHOD OF BREAKING DOWN RUBBER MOLECULES USING PEPTIZER AND THE PROPERTIES OF RESULTANT NATURAL RUBBER SOLVENT-BASED ADHESIVE

The huge cost of energy associated with the breaking down of high molecular weight rubber during adhesive production has placed a demand for alternative processing technology. In this study, the use of benzoyl peroxide as a peptizer to lower the molecular weight of natural rubber has been reported. The properties of natural rubber-based solvent adhesive produced by mechanical mastication and peptizer were also compared. Natural rubber (NR) was masticated in a two-roll mill at 80oC for 30 mins and was processed into adhesive using xylene as solvent. Similarly, adhesives were formulated by chemical break down of the heavy natural rubber molecules using benzoyl peroxide as peptizer. The peptizer loading was varied from 0.2 – 1.0 parts per hundred rubber (phr) of rubber at 100oC in a water bath while other compounding additives were kept constants for all samples under study. The adhesives produced were subjected to the following test: viscosity, lap shear test, T-peel test and drying time test. Other properties studied are weathering resistance, heat resistance and effect of moisture on the adhesives bond on different substrates. The result shows that the viscosity of the adhesives increases with increasing peptizer loading and the masticated sample has a viscosity slightly lower than that of 0.8phr and 1.0phr peptizer loading. Furthermore, T-peel test shows that 0.4 phr peptizer loading has a better average T-peel strength for the bonds while the masticated sample had the least average T-peel strength when compared with all samples. The sample with 1.0phr peptizer loading had the least decrease in shear strength while the 0.2 peptizer loading had the maximum decrease in shear strength. Finally, the FT-IR spectra shows similarities in spectra but with difference in the intensity of the peaks, indicating the ability of the benzoyl peroxide to effectively oxidize the natural rubber as compared with the mechanical masticated sample. The peak of 1080 cm-1 in the peptized sample indicates a major difference with the spectra of mechanical masticated sample. Generally, the benzoyl peroxide effectively lowered the molecular weight of the natural rubber which is evident in the viscosity, reduced process time and ease of the adhesive formulation.

Physical Parameters of Polymer Composite Materials Created on the Basis of Low and High Molecular Weight Rubbers

ABSTRACT. The theoretical foundations of the structural and mechanical behavior of filled three-dimensional cross-linked elastomers are supplemented. Numerical experiments are carried out, based on the data of numerical experiments, three dimensional cross linked, filled with dispersed particles frost-resistant elastomers based on low and high molecular weight rubbers are developed. Theoretical and experimental data are compared and their good convergence is shown. Comparison of the physical parameters of the composites created by the authors on the basis of low and high molecular weight rubbers showed that the deformation characteristics of the composite based on high molecular weight rubbers are many times superior to composites based on low molecular weight, as well as their glass transition temperature is also very different. The created composites are recommended as a structural material in relation to the engineering problem of creating wear-resistant parts and components of road and air transport operated in a wide temperature range, including the Far North and the Arctic.

To the Calculation of the Compatibility of Oligomers with High Molecular Weight Rubbers

The article presents a modified theoretical calculation of the solubility parameter for oligomers and polymers, taking into account the influence of their molecular structure. The obtained values of the solubility parameter make it possible to predict thermodynamic compatibility for polymers and oligomers

Natural rubber biosynthesis in plants, the rubber transferase complex, and metabolic engineering progress and prospects.

SummaryNatural rubber (NR) is a nonfungible and valuable biopolymer, used to manufacture ~50 000 rubber products, including tires and medical gloves. Current production of NR is derived entirely from the para rubber tree (Hevea brasiliensis). The increasing demand for NR, coupled with limitations and vulnerability of H. brasiliensis production systems, has induced increasing interest among scientists and companies in potential alternative NR crops. Genetic/metabolic pathway engineering approaches, to generate NR‐enriched genotypes of alternative NR plants, are of great importance. However, although our knowledge of rubber biochemistry has significantly advanced, our current understanding of NR biosynthesis, the biosynthetic machinery and the molecular mechanisms involved remains incomplete. Two spatially separated metabolic pathways provide precursors for NR biosynthesis in plants and their genes and enzymes/complexes are quite well understood. In contrast, understanding of the proteins and genes involved in the final step(s)—the synthesis of the high molecular weight rubber polymer itself—is only now beginning to emerge. In this review, we provide a critical evaluation of recent research developments in NR biosynthesis, in vitro reconstitution, and the genetic and metabolic pathway engineering advances intended to improve NR content in plants, including H. brasiliensis, two other prospective alternative rubber crops, namely the rubber dandelion and guayule, and model species, such as lettuce. We describe a new model of the rubber transferase complex, which integrates these developments. In addition, we highlight the current challenges in NR biosynthesis research and future perspectives on metabolic pathway engineering of NR to speed alternative rubber crop commercial development.

Analysis of the first Taraxacum kok-saghyz transcriptome reveals potential rubber yield related SNPs

Taraxacum kok-saghyz (TK) is a potential alternative crop for natural rubber (NR) production, due to its high molecular weight rubber, short breeding cycle, and diverse environmental adaptation. However, improvements in rubber yield and agronomically relevant traits are still required before it can become a commercially-viable crop. An RNA-Seq based transcriptome was developed from a pool of roots from genotypes with high and low rubber yield. A total of 55,532 transcripts with lengths over 200 bp were de novo assembled. As many as 472 transcripts were significantly homologous to 49 out of 50 known plant putative rubber biosynthesis related genes. 158 transcripts were significantly differentially expressed between high rubber and low rubber genotypes. 21,036 SNPs were different in high and low rubber TK genotypes. Among these, 50 SNPs were found within 39 transcripts highly homologous to 49 publically-searched rubber biosynthesis related genes. 117 SNPs were located within 36 of the differentially expressed gene sequences. This comprehensive TK transcriptomic reference, and large set of SNPs including putative exonic markers associated with rubber related gene homologues and differentially expressed genes, provides a solid foundation for further genetic dissection of rubber related traits, comparative genomics and marker-assisted selection for the breeding of TK.

CRISPR/Cas9 genome editing of rubber producing dandelion Taraxacum kok-saghyz using Agrobacterium rhizogenes without selection

CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9) is a highly accessible genome editing tool. Here we demonstrate its potential for use in Taraxacum species. Taraxacum kok-saghyz (TK, Rubber Dandelion) is notable for its ability to produce high molecular weight rubber in its roots and promise as an alternative source of natural rubber. In order to accelerate the domestication of TK, we have established a simple strategy to deploy CRISPR/Cas9 in this species. A critical gene encoding fructan:fructan 1-fructosyltransferase (1-FFT), implicated in inulin biosynthesis was selected as the target, as inulin is an expected antagonist of rubber production. TK plantlets were inoculated with Agrobacterium rhizogenes harboring a plasmid encoding a Cas9 and sgRNA (single guide RNA) targeting TK 1-FFT. We were able to rapidly induce hairy roots harboring knockout alleles without the selection of stable, herbicide or antibiotic resistant transformants. Mutagenesis was affirmed by observing a loss of restriction sites within 1-FFT, followed by sequencing. Of 11 hairy root samples tested, 10 showed the presence of genome editing, with mutation rates as high as 88.9%, suggesting a high efficiency mutagenesis induced by CRISPR/Cas9 via A. rhizogenes-mediated transformation. Whole TK plants were regenerated from hairy roots harboring knockout alleles. The regenerated plants contained knockout alleles, with mutation rates as high as 80.0%. TK plants with edited genomes were obtained within 10 weeks. By omitting a selection step, it was possible to generate edited TK plants lacking stably transformed CRISPR elements, which may potentially reduce off-target mutagenesis. Application of high efficiency CRISPR/Cas9 genome editing will facilitate the domestication and commercialization of TK as a rubber producing crop, and may accelerate basic research on the regulation of rubber biosynthesis.

Rubber formation in plants: a mini-review

The ability of plants to synthesize rubber is based on a single enzyme, rubber transferase, which causes the cis-polymerization of isoprene units, as isopentenyl pyrophosphate, into long, single stranded hydrocarbons. These are assembled and accumulated in the cell as rubber particles which range in size from 0.01 to 15 μm and are bounded by an osmiophilic film. Parthenium argentatimi (guayule) and Hevea brasiliensis represent two species which accumulate large quantities of high molecular weight rubber by different intercellular routes. Rubber synthesis takes place in parenchyma cells in guayule and in specialized latex vessels in Hevea. Biochemical questions concerning rubber synthesis are discussed.

In vitro synthesis of high molecular weight rubber by Hevea small rubber particles

Hevea brasiliensis is one of few higher plants producing the commercial natural rubber used in many significant applications. The biosynthesis of high molecular weight rubber molecules by the higher plants has not been clarified yet. Here, the in vitro rubber biosynthesis was performed by using enzymatically active small rubber particles (SRP) from Hevea. The mechanism of the in vitro rubber synthesis was investigated by the molecular weight distribution (MWD). The highly purified SRP prepared by gel filtration and centrifugation in the presence of Triton((R)) X-100 showed the low isopentenyl diphosphate (IPP) incorporation for the chain extension mechanism of pre-existing rubber. The MWD of in vitro rubber elongated from the pre-existing rubber chains in SRP was analyzed for the first time in the case of H. brasiliensis by incubating without the addition of any initiator. The rubber transferase activity of 70% incorporation of the added IPP (w/w) was obtained when farnesyl diphosphate was present as the allylic diphosphate initiator. The in vitro synthesized rubber showed a typical bimodal MWD of high and low molecular weight fractions in GPC analysis, which was similar to that of the in vivo rubber with peaks at around 10(6) and 10(5) Da or lower. The reaction time independence and dependence of molecular weight of high and low molecular weight fractions, respectively, indicated that the high molecular weight rubber was synthesized from the chain extension of pre-existing rubber molecules whereas the lower one was from the chain elongation of rubber molecules newly synthesized from the added allylic substrates.

Identification and comparison of natural rubber from two Lactuca species

Renewed interest in the identification of alternative sources of natural rubber to Hevea brasiliensis has focused on the Compositae family. In our search for Compositae models for rubber synthesis, we extracted latex from stems of two lettuce species: Lactuca serriola, prickly lettuce, and Lactuca sativa cv. Salinas, crisphead lettuce. Both species contained cis-1,4-polyisoprene rubber in the dichloromethane-soluble portions of their latex, and sesquiterpene lactones in their acetone-soluble portions. The rubber from both species and their progeny had molecular weights in excess of 1,000,000 g/mol, and polydispersity values of 1.1. Rubber transferase activity was detected across a range of farnesyl diphosphate initiator concentrations, with decreased activity as initiator concentrations exceeded putative saturation. These results add lettuce to the short list of plant species that produce high molecular weight rubber in their latex. Due to the genomic and agronomic resources available in lettuce species, they provide the opportunity for further dissection of natural rubber biosynthesis in plants.

Relationship between Particle Size and Molecular Weight of Rubber from Hevea Brasiliensis

Abstract Rubber particles with different mean diameters obtained from fresh natural rubber latex (FL-latex) were separated by high-speed centrifugation. The small rubber particles (SRP), having mean diameter less than 250 nm, were found to be composed of both low and high number-average molecular-weight (Mn) rubber molecules; while larger rubber particles (LRP), with mean diameter larger than 250 nm, mainly consisted of low Mn rubber molecules. The content of longchain fatty acid ester are mainly due to functional groups such as phospholipids were less in SRP than that of LRP. This indicates clearly that LRP contains mainly rubber molecules which were terminated to form a functional terminal group containing fatty acid ester groups. The high molecular-weight rubber in SRP, containing lower amounts of ester groups, is presumed to be mainly linear molecules having no chain-end group to form branch-points. On the other hand, the low molecular-weight rubber molecules with high ester content in the LRP are presumably terminated with the functional group containing fatty acid esters to form long-chain branching. The molecular-weight of rubber in LRP increased with increasing particle sizes. This suggests that the LRP was derived from the aggregation and/or association of functional groups at ω- and α-terminal end groups.

A novel cDNA from Parthenium argentatum Gray enhances the rubber biosynthetic activity in vitro*

Natural rubber (cis-1,4-polyisoprene) is an isoprenoid compound produced exclusively in plants by the action of rubber transferase. Despite a keen interest in revealing the mechanisms of rubber chain elongation and chain length determination, the molecular nature of rubber transferase has not yet been identified. A recent report has revealed that a 24 kDa protein tightly associated with the small rubber particles of Hevea brasiliensis, therefore designated small rubber particle protein (SRPP), plays a positive role in rubber biosynthesis. Since guayule (Parthenium argentatum Gray) produces natural rubber similar in size to H. brasiliensis, it is of critical interest to investigate whether guayule contains a similar protein to the SRPP. A cDNA clone has been isolated in guayule that shares a sequence homology with the SRPP, thus designated guayule homologue of SRPP (GHS), and the catalytic function of the protein was characterized. Sequence analysis revealed that the GHS is highly homologous in several conserved regions to the SRPP (50% identity). In vitro functional analysis of the recombinant protein overexpressed in E. coli revealed that the GHS plays a positive role in isopentenyl diphosphate incorporation into high molecular weight rubbers as SRPP does. These results indicate that guayule and Hevea rubber trees contain a protein that is similar in its amino acid sequence and plays a role in isopentenyl diphosphate incorporation in vitro, implying that it contributes to the enhancement of rubber biosynthetic activity in rubber trees.

Effect of rubber on microcellular structures from high internal phase emulsion polymerization

A microcellular foam, which combines a rubber with the conventional formulation of styrene/divinylbenzene/ sorbitan monooleate/water system, was prepared using high internal phase emulsion (HIPE) polymerization. Although the open microcellular foam with low density from the conventional HIPE polymerization shows highly porous characteristics with fine, regular and isotropic structure, the one having much smaller cell size is desirable for various applications. In this study, a polybutadiene was introduced to reduce the cell size with comparable properties. Major interests were focused on the effects of rubber concentration and agitation speed on the cell sizes and compression properties. Scanning electron microscopy was used to observe the microcellular morphology and compression tests were conducted to evaluate the stress-strain behaviors. It was found that the cell size decreased as rubber concentration increased, reflecting a competition between the higher viscosity of continuous phase and the lower viscosity ratio of dispersed to continuous phases due to the addition of high molecular weight rubber into the oil phase of emulsion. A correlation for the average cell size depending on agitation speed was attempted and the result was quite satisfactory.

プレニル鎖延長酵素の遺伝子解明研究から見た天然ゴム生合成の分子解析

Linear prenyl diphosphates whose carbon chain length varies widely from geranyl diphosphate (C10) to natural rubber (C>105) are biosynthesized by the catalytic function of a group of enzymes called prenyltransferases. Prenyltransferases are classified in two major groups, trans- or (E)-prenyltransferases and cis- or (Z)-prenyltransferases. From the year of 1987, many genes encoding (E)-prenyltransferases have been cloned and characterized well. However, the structure and detailed mechanism of (Z)-prenyltransferase had been completely unknown until our identification of a gene encoding the undecaprenyl diphosphate synthase (UPS) from Micrococcus luteus B-P 26. Not only the primary structure but also the tertiary structure of the UPS is quite different from those of (E)-prenyltransferases. Multiple alignment of primary structures of (Z)-prenyltransferases identified from various organisms reveals that there are five highly conserved regions among (Z)-prenyltransferases. In order to elucidate the biosynthetic pathway of natural rubber which is composed of cis-1, 4-polyisoprene, we isolated and characterized two genes encoding (Z)-prenyltransferases from the latex in Hevea brasiliensis by using sequence information on the conserved regions of (Z)-prenyltransferases. In vitro rubber transferase assay using the recombinant gene product revealed that the enzyme catalyzed the formation of polyprenyl products with approximate sizes around 1×104 Da. Moreover, in the presence of washed bottom fraction from latex, the rubber transferase activity and the size of the major product was increased, suggesting a possible requirement of certain activation factors in the washed bottom fraction for the production of high molecular weight rubber.

Toughening of unsaturated polyester with rubber particles. Part II: Fracture behavior

AbstractFracture behavior of rubber‐modified unsaturated polyester is studied. Unsaturated polyester was modified by the addition of different rubbers; liquid rubbers (BFGoodrich Hycar) and high molecular weight rubbers. Values of stress‐intensity factor Klc and fracture energy Glc were determined using a double‐torsion test. Young's modulus was determined in flexural. Increases in toughness were recorded when there was adhesion in the particle‐matrix interface. The toughening mechanism was identified. Whitening was not observed. Crack propagation changed from stick‐slip to continuous manner when volume fraction of Hycar VTBN increased.

Toughening of unsaturated polyester with rubber particles. Part I: Morphological study

AbstractThe morphology of rubber‐modified unsaturated polyester is described. In order to modify the particle‐matrix interface, different rubbers were used; BFGoodrich Hycar liquid rubbers and high molecular weight rubbers. Blends of different compositions were prepared. The microstructure of the materials was investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The volume fraction and the particle size of the rubbers' second phase was determined by applying quantitative metallography. The volume fraction of the precipitated phase was higher than the amount of added rubber. Such blends show phase inversion above 35 wt% of rubber.

The Enzymatic Synthesis of Rubber Polymer in Parthenium argentatum Gray

Washed rubber particles isolated from stem homogenates of Parthenium argentatum Gray by ultracentrifugation and gel filtration on columns of LKB Ultrogel AcA34 contain rubber transferase which catalyzes the polymerization of isopentenyl pyrophosphate into rubber polymer. The polymerization reaction requires Mg(2+) isopentenyl pyrophosphate, and an allylic pyrophosphate. The K(m) values for Mg(2+), isopentenyl pyrophosphate, and dimethylallyl pyrophosphate were 5.2 x 10(-4) molar, 8.3 x 10(-5) molar, and 9.6 x 10(-5) molar, respectively. The molecular characteristics of the rubber polymer synthesized from [(14)C]isopentenyl pyrophosphate were examined by gel permeation chromatography on three linear columns of 1 x 10(6) to 500 Angstroms Ultrastyragel in a Waters 150C Gel Permeation Chromatograph. The peak molecular weight of the radioactive polymer increased from 70,000 in 15 minutes to 750,000 in 3 hours. The weight average molecular weight of the polymer synthesized over a 3 hour period was 1.17 x 10(6) compared to 1.49 x 10(6) for the natural rubber polymer extracted from the rubber particles. Over 90% of the in vitro formation of the rubber polymer was de novo from dimethylallyl pyrophosphate and isopentenyl pyrophosphate. Treatment of the washed rubber particles with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate solubilized the rubber transferase. The solubilized enzyme(s) catalyzed the polymerization of isopentenyl pyrophosphate into rubber polymer with a peak molecular weight of 1 x 10(5) after 3 hours of incubation with Mg(2+) and dimethylallyl pyrophosphate. The data support the conclusion that the soluble preparation of rubber transferase is capable of catalyzing the formation of a high molecular weight rubber polymer from an allylic pyrophosphate initiator and isopentenyl pyrophosphate monomer.

Variation in the Molecular Weight Distribution of Rubber from Cultivated Guayule

Abstract The molecular weight (MW) and the molecular weight distribution (MWD) of natural rubber (NR) are directly related to rubber quality. Although over 2000 species of plants are capable of producing rubber, only about a dozen are recognized as producing sufficient quantities of high quality, high MW rubber necessary for commercial use. Guayule, Pathenium argentatum Gray, can synthesize high MW rubber similar to Hevea and is actively being developed as a new source of NR for the arid regions of the world especially in the United States. Rubber of MW 105 or less is considered unsuitable for much commercial use. Unfortunately, most of the 2000 rubber synthesizing species are believed to produce rubber with a MW of approximately 5×104 and a unimodal MWD4. The MWD of Hevea, however, has a bimodal distribution exhibiting two MW peaks. The major peak exists at a MW of about 7×105 and extends to 106 and beyond. A minor peak or flattened shoulder is also frequently present which corresponds to a MW of 5×104. This bimodal MWD nearly always exists for Hevea, but the amplitude of the two peaks may vary due to variety, age, and tapping frequency. Such variation in the MWD can lead to differences in the average MW. In rubber obtained from native stands of guayule, reports indicate the existence of only a unimodal MWD. Little is known about the environmental or genetic factors influencing rubber MW in stands of cultivated guayule. Since general husbandry and genetic variation seem to affect Hevea rubber, it is not unreasonable to suggest that differences may also appear in guayule, as other work implied.

The Effect of Modifying Resins on the Viscoelastic Properties of Styrenic Block Copolymers

Abstract Viscoelastic data from dynamic measurements show that modifying resins produce complex changes in the properties of styrenic block copolymers. These property changes are generally consistent with compatibility concepts. Resins which are compatible with the midblock of these polymers, Resins AL-1, Al-2, and RE, will increase the tan δ peak temperature and reduce plateau modulus, as is observed with other elastomers. They will also reduce the temperature of entry into the terminal zone. This is believed to result from a reduction in the concentration of the endblock phase as well as dilution of the entanglement network at elevated temperatures. A resin which is incompatible with the midblock, Resin AR, has no effect on the tan δ peak temperature but does cause an increase in the plateau modulus of S-B-S and S-EB-S, as seen with typical high molecular weight rubbers. However, in the S-I-S polymers, Resin AR appears to be at least partially compatible at the low volume fractions examined. It does not increase the plateau modulus, but appears to reduce it slightly. The aromatic resin, Resin AR, has no effect on the G′, G″ crossover temperature of S-EB-S or S-I-S(20) at the concentration examined. However, it increases the temperature of the high diblock S-I-S(40) and lowers the temperature of S-B-S, suggesting at least partial compatibility at elevated temperatures. G′(tan δmin ) data are useful for estimating the compatibility of resins with styrenic block copolymers. Minimum and maximum values, calculated from rubber elasticity theory and filler effects, can serve as reference points for compatibility.

An HF-Nickel-R3Al Catalyst System for Producing High Cis-1,4-Polybutadiene

Abstract A relatively new, high cis-1,4-polybutadiene and a novel trialkylaluminumnickel-HF catalyst system have been described. This polybutadiene is unique in that it possesses high tack and has exceptionally good processing qualities; the carbon black-loaded and cured rubber possesses good static and dynamic properties. The catalyst system is very effective for producing high molecular weight rubber, suitable for oil extension, and the molecular weight can be controlled by adjusting 1) HF to R3Al ratio, 2) total catalyst concentration and/or 3) polymerization temperature.