Small Rubber Particle Research Articles

Proteomic Landscape of the Mature Roots in a Rubber-Producing Grass Taraxacum Kok-saghyz.

The rubber grass Taraxacum kok-saghyz (TKS) contains large amounts of natural rubber (cis-1,4-polyisoprene) in its enlarged roots and it is an alternative crop source of natural rubber. Natural rubber biosynthesis (NRB) and storage in the mature roots of TKS is a cascade process involving many genes, proteins and their cofactors. The TKS genome has just been annotated and many NRB-related genes have been determined. However, there is limited knowledge about the protein regulation mechanism for NRB in TKS roots. We identified 371 protein species from the mature roots of TKS by combining two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS). Meanwhile, a large-scale shotgun analysis of proteins in TKS roots at the enlargement stage was performed, and 3545 individual proteins were determined. Subsequently, all identified proteins from 2-DE gel and shotgun MS in TKS roots were subject to gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses and most proteins were involved in carbon metabolic process with catalytic activity in membrane-bounded organelles, followed by proteins with binding ability, transportation and phenylpropanoid biosynthesis activities. Fifty-eight NRB-related proteins, including eight small rubber particle protein (SRPP) and two rubber elongation factor(REF) members, were identified from the TKS roots, and these proteins were involved in both mevalonate acid (MVA) and methylerythritol phosphate (MEP) pathways. To our best knowledge, it is the first high-resolution draft proteome map of the mature TKS roots. Our proteomics of TKS roots revealed both MVA and MEP pathways are important for NRB, and SRPP might be more important than REF for NRB in TKS roots. These findings would not only deepen our understanding of the TKS root proteome, but also provide new evidence on the roles of these NRB-related proteins in the mature TKS roots.

Effects of rubber elongation factor and small rubber particle protein from rubber-producing plants on lipid metabolism in Saccharomyces cerevisiae

The common proteins rubber elongation factor (REF) and small rubber particle protein (SRPP) are associated with Hevea brasiliensis (Hb) rubber particles. They are involved in the stability of rubber particles and natural rubber biosynthesis. Recently, we cloned cDNAs encoding REF and SRPP from laticifers of Ficus carica (Fc). In the present study, we overexpressed REF/SRPPs (HbREF, FcREF, and FcSRPP) in Saccharomyces cerevisiae in anticipation of future rubber biosynthesis in recombinant yeast. The proteins were localized in the endoplasmic reticulum and lipid droplets (LDs), and affected LD morphology. Furthermore, their overexpression resulted in an accumulation of neutral lipids and a decrease in yeast cell size. This suggests that REF/SRPPs affect lipid metabolism and lead to a decline in the phospholipid content of yeast. We also found that expression of these proteins induced accumulation of steryl esters and triacylglycerols in yeast. This suggests that the coexpression of REF/SRPPs with key enzymes for the biosynthesis of target lipids in yeast is a promising way of increasing production of important lipids like triacylglycerols and terpenes, and that a protein complex consisting of cis-prenyltransferase (CPT), CPT-like protein, and REF/SRPPs for rubber biosynthesis could be reconstituted on yeast lipid droplets.

A myelocytomatosis transcription factor from Hevea brasiliensis positively regulates the expression of the small rubber particle protein gene

Small rubber particle protein (SRPP) is a major rubber particle-associated protein in latex from rubber tree (Hevea brasiliensis Muell. Arg.), and it implicated in the natural rubber biosynthesis. However, the regulatory mechanism of H. brasiliensis SRPP gene (HbSRPP) is incompletely understood. In this study, a myelocytomatosis (MYC) transcription factor (designated as HbMYC2b) regulating the HbSRPP expression was identified. HbMYC2b possesses a 2037 bp open reading frame that encodes 681 amino acids. The HbMYC2b protein has a basic helix-loop-helix (bHLH)-leucine zipper domain, and a bHLH-MYC-N domain. The highest expression of HbMYC2b is observed in the bark followed by the latex, flowers, and leaves. HbMYC2b significantly increases the activity of the HbSRPP promoter, suggesting that HbMYC2b activates this promoter. HbMYC2b may be a positive regulator of HbSRPP in rubber tree.

The Effect of Poly(ethylene-octene) Grafted Maleic Anhydride Elastomer (POEgMA) and Liquid Epoxidised Natural Rubber (LENR) on Mechanical Properties of Poly(hydroxyl-3-butyrate) (PHB)

Poly(hydroxyl-3-butyrate) (PHB) has a great potential in many applications due to its biodegradable and biocompatible properties. However, PHB is considered as a stiff material since it has high brittleness and narrow processing windows which limit the use of PHB. In order to improve the brittleness of PHB, poly(ethylene-octene) grafted maleic anhydride elastomer (POEgMA) and liquid epoxidized natural rubber (LENR) were incorporated into PHB using internal mixer. The blends were melt blended at temperature of 165 °C and rotor speed of 60 rpm for 8 min. The concentration of POEgMA and LENR were varied from 5, 10, 15 and 20 wt%. The mechanical and morphological properties were determined using tensile testing, Izod impact testing and scanning electron microscopy (SEM). Addition of 5 wt% POEgMA decreased the elongation at break and impact strength of PHB. The tensile strength and tensile modulus showed no significant changes with increasing amount of elastomers added to the blends. SEM micrograph revealed smaller rubber particles in the PHB/LENR system in comparison to the PHB/POEgMA blends at the same rubber concentration. The impact strength of PHB/LENR blends decreased significantly with the addition of elastomer. Thus based on this observation, it is suggested that the presence of LENR improves the plasticizing effect of the blends and not acting as an impact modifier.

Viscoelastic and mechanical properties of large- and small-particle natural rubber before and after vulcanization

Insights into viscoelastic and mechanical properties of large- and small-particle natural rubber (NR) were gained for developing new rubber products. NR latex was first subjected to centrifuging to separate large rubber particles (LRP) and small rubber particles (SRP). The LRP and SRP solid rubbers were prepared to investigate their molecular weight characteristics and mechanical and viscoelastic properties. Before vulcanization, the LRP rubber was found to be better in both tensile strength and elasticity, as a result of natural branch-point networks formed by aggregation of phospholipid groups at the terminating end (α end) of the LRP rubber chains. After vulcanization, the SRP vulcanizate was, on the other hand, found to perform better in all respects. This could be explained by the accelerating effect from the proteins bound to the initiating end (ω end) of the SRP rubber chains in the crosslinking process during vulcanization.

Purification and characterization of small and large rubber particles from Hevea brasiliensis.

Natural rubber (NR) is synthesized by the rubber transferase (RTase) on rubber particles (RPs) in latex. Due to the heterogeneity of the RPs in latex, it is difficult to precisely characterize the RTase activity. In this study, we separated the RPs of Hevea brasiliensis with different particle size distributions, via stepwise centrifugations. Analyses of protein compositions and size distributions of NR in the RPs suggest that RPs in Hevea latex can be categorized into two distinct subclasses, the larger RPs (termed 1kRP, 2kRP, and 8kRP) and the smaller RPs (termed 20kRP and 50kRP). Precise enzymatic assays using the RPs revealed that 50kRP showed the highest RTase activity, whereas the larger RPs, which had been regarded to have quite low activity, also exhibited a comparable activity to the smaller RPs. Immunological detections of cis-prenyltransferases in the RPs showed that the abundance of these enzymes correlates with the extent of RTase activity.

Jasmonate signalling in the regulation of rubber biosynthesis in laticifer cells of rubber tree, Hevea brasiliensis.

Rubber trees are the world's major source of natural rubber. Rubber-containing latex is obtained from the laticifer cells of the rubber tree (Hevea brasiliensis) via regular tapping. Rubber biosynthesis is a typical isoprenoid metabolic process in the laticifer cells; however, little is known about the positive feedback regulation caused by the loss of latex that occurs through tapping. In this study, we demonstrate the crucial role of jasmonate signalling in this feedback regulation. The endogenous levels of jasmonate, the expression levels of rubber biosynthesis-related genes, and the efficiency of in vitro rubber biosynthesis were found to be significantly higher in laticifer cells of regularly tapped trees than those of virgin (i.e. untapped) trees. Application of methyl jasmonate had similar effects to latex harvesting in up-regulating the rubber biosynthesis-related genes and enhancing rubber biosynthesis. The specific jasmonate signalling module in laticifer cells was identified as COI1-JAZ3-MYC2. Its activation was associated with enhanced rubber biosynthesis via up-regulation of the expression of a farnesyl pyrophosphate synthase gene and a small rubber particle protein gene. The increase in the corresponding proteins, especially that of farnesyl pyrophosphate synthase, probably contributes to the increased efficiency of rubber biosynthesis. To our knowledge, this is the first study to reveal a jasmonate signalling pathway in the regulation of rubber biosynthesis in laticifer cells. The identification of the specific jasmonate signalling module in the laticifer cells of the rubber tree may provide a basis for genetic improvement of rubber yield potential.

Regulation of Natural Rubber Biosynthesis by Proteins Associated with Rubber Particles

Natural rubber, cis-1,4-polyisoprene, is an essential raw material used in thousands of products, many of which are absolutely necessary for medical purposes. Natural rubber is obtained from latex, an aqueous emulsion present in the laticiferous vessels of the natural rubber-producing plants. Hevea brasiliensis (the Brazilian rubber tree) currently is the only commercially important source of natural rubber. H. brasiliensis crops have very little genetic variability, leaving rubber plantations at risk of serious pathogenic attacks. In addition, repeated exposure to residual proteins in latex products derived from H. brasiliensis have led to serious and widespread allergic (type I) hypersensitivity. Therefore, identification of alternative sources of natural rubber is a very important biotechnological task. Potentially, Russian dandelion (Taraxacum kok-saghyz) may be such an alternative because significant amounts of natural rubber are produced in its root system. However, H. brasiliensis is a more efficient producer of natural rubber than T. kok-saghyz. Thus, improvement of rubber biosynthesis in plants is a first-priority problem of modern biotechnology. In this review, we describe proteins that may increase the concentration of natural rubber in laticiferous vessels of T. kok-saghyz and its close relative Taraxacum brevicorniculatum, when overexpressed in the plants. These proteins, cis-prenyltransferases, rubber transferase activator, and small rubber particle proteins, are directly involved in synthesis of the polyisoprene chain. We also analyze the effects of their expression levels on the production of natural rubber in vivo.

Small rubber particle proteins from Taraxacum brevicorniculatum promote stress tolerance and influence the size and distribution of lipid droplets and artificial poly(cis-1,4-isoprene) bodies.

Natural rubber biosynthesis occurs on rubber particles, i.e. organelles resembling small lipid droplets localized in the laticifers of latex-containing plant species, such as Hevea brasiliensis and Taraxacum brevicorniculatum. The latter expresses five small rubber particle protein (SRPP) isoforms named TbSRPP1-5, the most abundant proteins in rubber particles. These proteins maintain particle stability and are therefore necessary for rubber biosynthesis. TbSRPP1-5 were transiently expressed in Nicotiana benthamiana protoplasts and the proteins were found to be localized on lipid droplets and in the endoplasmic reticulum, with TbSRPP1 and TbSRPP3 also present in the cytosol. Bimolecular fluorescence complementation confirmed pairwise interactions between all proteins except TbSRPP2. The corresponding genes showed diverse expression profiles in young T. brevicorniculatum plants exposed to abiotic stress, and all except TbSRPP4 and TbSRPP5 were upregulated. Young Arabidopsis thaliana plants that overexpressed TbSRPP2 and TbSRPP3 tolerated drought stress better than wild-type plants. Furthermore, we used rubber particle extracts and standards to investigate the affinity of the TbSRPPs for different phospholipids, revealing a preference for negatively charged head groups and 18:2/16:0 fatty acid chains. This finding may explain the effect of TbSRPP3-5 on the dispersity of artificial poly(cis-1,4-isoprene) bodies and on the lipid droplet distribution we observed in N. benthamiana leaves. Our data provide insight into the assembly of TbSRPPs on rubber particles, their role in rubber particle structure, and the link between rubber biosynthesis and lipid droplet-associated stress responses, suggesting that SRPPs form the basis of evolutionarily conserved intracellular complexes in plants.

Cloning and Aggregation Characterization of Rubber Elongation Factor and Small Rubber Particle Protein from Ficus carica

Rubber elongation factor (REF) and small rubber particle protein (SRPP) are major latex proteins harvested from Hevea brasiliensis (the rubber tree; HbREF and HbSRPP, respectively). Their amino acid sequences exhibit high homology with each other. In the present study, we cloned two cDNAs encoding REF/SRPP-family proteins (FcREF/SRPP-1 and -2) from the laticifers of Ficus carica (fig tree). The amino acid sequences of these proteins showed high homology not only with each other but also with HbREF and HbSRPP. Recombinant FcREF/SRPP-1 and -2 were expressed in E. coli, and their aggregation properties were examined using a Congo red binding assay, agarose gel electrophoresis, and transmission electron microscopy. FcREF/SRPP-1 formed fibrils when incubated in PBS, and grew to micrometer-sized amorphous aggregates that precipitated rapidly. These aggregation properties of FcREF/SRPP-1 are quite similar to those of HbREF, although the growth rate and size of FcREF/SRPP-1 aggregates were inferior to those of HbREF. FcREF/SRPP-2 also formed aggregates during the incubation, but they did not precipitate, as has been reported for HbSRPP. Our results suggest that FcREF/SRPP-1 and -2 correspond to HbREF and HbSRPP, respectively. These aggregation properties could provide useful benchmarks for classifying REF/SRPP-family proteins as REF or SRPP.

Elucidation of rubber biosynthesis and accumulation in the rubber producing shrub, guayule (Parthenium argentatum Gray).

Guayule biosynthesizes and accumulates rubber particles predominantly in epithelial cells in the parenchyma tissue, and this biosynthesis and accumulation is accompanied by remodeling of the roles of epithelial cells. The mechanism underlying the biosynthesis and accumulation of large quantities of rubber particles and resin in the parenchyma tissue of the stem bark of guayule (Parthenium argentatum Gray) remained unanswered up to now. Here, we focused on rubber particle biosynthesis and accumulation in guayule and performed histochemical analyses using a lipophilic fluorescent dye specific for lipids and spectral confocal laser scanning microscopy. Unmixing images were constructed based on specific spectra of cis-polyisoprene and resin and showed that guayule accumulates a large amount of resin in the resin canals in parenchyma tissue and in pith. Interestingly, the fluorescence signals of rubber were predominantly detected in a specific single layer of epithelial cells around the resin canals. These epithelial cells accumulated large rubber particles and essentially no resin. Immunoblotting and immunostaining of guayule homologue of small rubber particle proteins (GHS), which contributes to the biosynthesis of rubber in guayule, showed that GHS is one of several small rubber particle proteins and is localized around rubber particles in epithelial cells. De novo sequencing of the rubber particle proteins showed the presence of all known organelle proteins, suggesting that epithelial cells biosynthesize rubber particles, followed by remodeling of the cells for the accumulation of rubber particles with subsequent decomposition of the organelles. These results indicate that epithelial cells around resin canals are bifunctional cells dedicated to the biosynthesis and accumulation of rubber particles.

Validation of rheological responses for morphological evaluation of incompatible polyamide 12/thermoplastic elastomer blends filled with nanoclay

The interrelationship between viscoelastic responses and multiphasic morphology in compatibilized PA 12/nanoclay/SBS nanocomposites has been investigated. Different effects of material parameters including nanoclay, SBS rubber and compatibilizer contents on the rheology and morphology correlations have been evaluated. The findings showed that the solid like and shear thinning behavior along with relaxation time and Casson yield stress intensify at higher contents of nanoclay and SBS rubber. By increasing the SBS level, smaller rubber particles with narrower size distribution were obtained that was in agreement with the rheological properties. Nonetheless, the incorporation of more nanoclay effectively resulted in finer matrix/dispersed morphology just in the presence of larger compatibilizer contents. Although m-SEBS influenced the biphasic morphology similar to common compatibilizers, the non-terminal behavior of the nanocomposites weakened at higher m-SEBS levels due to the stronger compatibilizer effect on the chain confinement in nanoclay galleries than the one on the PA12/SBS rubber interfacial adhesion.

Size effect of tire rubber particles on tribological properties of brake friction materials

Size effect of rubber particles on the friction and wear of brake friction materials was investigated since their morphology is often connected to friction instability. Friction materials were fabricated with recycled rubber particles of two different sizes, approximately 75 and 450µm, and their frictional performance was examined with a reduced-scale brake dynamometer. The results showed that the specimen with small rubber particles showed higher contact stiffness than that with large rubber particles. However, the small rubber particles in the friction materials increased the friction level and friction instability, accompanied by a higher wear rate, which were attributed to the larger high-pressure contact plateaus produced at the sliding interface. This study indicated that the frictional instability of brake friction materials is strongly affected by the contact plateaus and suggested that friction instability can be reduced by regulating the recycled rubber particle size.

Subcellular localization and interactions among rubber particle proteins from Hevea brasiliensis.

Natural rubber (polyisoprene) from the rubber tree Hevea brasiliensis is synthesized by specialized cells called laticifers. It is not clear how rubber particles arise, although one hypothesis is that they derive from the endoplasmic reticulum (ER) membrane. Here we cloned the genes encoding four key proteins found in association with rubber particles and studied their intracellular localization by transient expression in Nicotiana benthamiana leaves. We show that, while the cis-prenyltransferase (CPT), responsible for the synthesis of long polyisoprene chains, is a soluble, cytosolic protein, other rubber particle proteins such as rubber elongation factor (REF), small rubber particle protein (SRPP) and Hevea rubber transferase 1-REF bridging protein (HRBP) are associated with the endoplasmic reticulum (ER). We also show that SRPP can recruit CPT to the ER and that interaction of CPT with HRBP leads to both proteins relocating to the plasma membrane. We discuss these results in the context of the biogenesis of rubber particles.

Comparative Proteomics of Rubber Latex Revealed Multiple Protein Species of REF/SRPP Family Respond Diversely to Ethylene Stimulation among Different Rubber Tree Clones.

Rubber elongation factor (REF) and small rubber particle protein (SRPP) are two key factors for natural rubber biosynthesis. To further understand the roles of these proteins in rubber formation, six different genes for latex abundant REF or SRPP proteins, including REF138,175,258 and SRPP117,204,243, were characterized from Hevea brasiliensis Reyan (RY) 7-33-97. Sequence analysis showed that REFs have a variable and long N-terminal, whereas SRPPs have a variable and long C-terminal beyond the REF domain, and REF258 has a β subunit of ATPase in its N-terminal. Through two-dimensional electrophoresis (2-DE), each REF/SRPP protein was separated into multiple protein spots on 2-DE gels, indicating they have multiple protein species. The abundance of REF/SRPP proteins was compared between ethylene and control treatments or among rubber tree clones with different levels of latex productivity by analyzing 2-DE gels. The total abundance of each REF/SRPP protein decreased or changed a little upon ethylene stimulation, whereas the abundance of multiple protein species of the same REF/SRPP changed diversely. Among the three rubber tree clones, the abundance of the protein species also differed significantly. Especially, two protein species of REF175 or REF258 were ethylene-responsive only in the high latex productivity clone RY 8-79 instead of in RY 7-33-97 and PR 107. Some individual protein species were positively related to ethylene stimulation and latex productivity. These results suggested that the specific protein species could be more important than others for rubber production and post-translational modifications might play important roles in rubber biosynthesis.

Identification and subcellular localization analysis of two rubber elongation factor isoforms on Hevea brasiliensis rubber particles

Rubber elongation factor (REF) is the most abundant protein found on the rubber particles or latex from Hevea brasiliensis (the Para rubber tree) and is considered to play important roles in natural rubber (cis-polyisoprene) biosynthesis. 16 BAC (benzyldimethyl-n-hexadecylammonium chloride)/SDS-PAGE separations and mass spectrometric identification had revealed that two REF isoforms shared similar amino acid sequences and common C-terminal sequences. In this study, the gene sequences encoding these two REF isoforms (one is 23.6 kDa in size with 222 amino acid residues and the other is 27.3 kDa in size with 258 amino acid residues) were obtained. Their proteins were relatively enriched by sequential extraction of the rubber particle proteins and separated by 16 BAC/SDS-PAGE. The localization of these isoforms on the surfaces of rubber particles was further verified by western blotting and immunogold electron microscopy, which demonstrated that these two REF isoforms are mainly located on the surfaces of larger rubber particles and that they bind more tightly to rubber particles than the most abundant REF and SRPP (small rubber particle protein).

Rubber particle proteins REF1 and SRPP1 interact differently with native lipids extracted from Hevea brasiliensis latex

Rubber particle membranes from the Hevea latex contain predominantly two proteins, REF1 and SRPP1 involved in poly(cis-1,4-isoprene) synthesis or rubber quality. The repartition of both proteins on the small or large rubber particles seems to differ, but their role in the irreversible coagulation of the rubber particle is still unknown. In this study we highlighted the different modes of interactions of both recombinant proteins with different classes of lipids extracted from Hevea brasiliensis latex, and defined as phospholipids (PL), glycolipids (GL) and neutral lipids (NL).We combined two biophysical methods, polarization modulated-infrared reflection adsorption spectroscopy (PM-IRRAS) and ellipsometry to elucidate their interactions with monolayers of each class of lipids. REF1 and SRPP1 interactions with native lipids are clearly different; SRPP1 interacts mostly in surface with PL, GL or NL, without modification of its structure. In contrast REF1 inserts deeply in the lipid monolayers with all lipid classes. With NL, REF1 is even able to switch from α-helice conformation to β-sheet structure, as in its aggregated form (amyloid form). Interaction between REF1 and NL may therefore have a specific role in the irreversible coagulation of rubber particles.

HbMADS4, a MADS-box Transcription Factor from Hevea brasiliensis, Negatively Regulates HbSRPP.

In plants MADS-box transcription factors (TFs) play important roles in growth and development. However, no plant MADS-box gene has been identified to have a function related to secondary metabolites regulation. Here, a MADS-box TF gene, designated as HbMADS4, was isolated from Hevea brasiliensis by the yeast one-hybrid experiment to screen the latex cDNA library using the promoter of the gene encoding H. brasiliensis small rubber particle protein (HbSRPP) as bait. HbMADS4 was 984-bp containing 633-bp open reading frame encoding a deduced protein of 230 amino acid residues with a typical conserved MADS-box motif at the N terminus. HbMADS4 was preferentially expressed in the latex, but little expression was detected in the leaves, flowers, and roots. Its expression was inducible by methyl jasmonate and ethylene. Furthermore, transient over-expression and over-expression of HbMADS4 in transgenic tobacco plants significantly suppressed the activity of the HbSRP promoter. Altogether, it is proposed that HbMADS4 is a negative regulator of HbSRPP which participates in the biosynthesis of natural rubber.

Influence of fibers partially coated with rubber from tire recycling as aggregate on the acoustical properties of rubberized concrete

The aim of this study is to analyze the main parameters that influence the acoustic properties of lightweight concrete containing high volume of recycled rubber aggregates. To that end, the sound absorption and the sound insulation on rubberized concrete are tested under different frequencies. Concrete specimens were designed with 0–100% substitution of its coarse aggregate by two different rubber aggregates from recycled tires, which is up to a 60% of the overall concrete volume fraction. Moreover, to evaluate the influence of the external texture and the direct contact of rubber, concrete panels with different surface finishes (rough and smooth face) were casted. Crumb rubber (CR) and fibers partially coated with crumb rubber (FCR) have been studied for sound barriers. FCR are composed of steel and plastic fibers mixed with small rubber particles from recycled tires and obtained during the granulation process, before the complete rubber separation. The results of this research indicate that the combination of steel and textile fibers contaminated with rubber powder increase sound absorption when compared to ordinary or rubberized concrete. The addition of CR or FCR reduces concrete density and increases its open porosity. However, concrete with large VF of FCR presents an even larger volume of open pores than concrete with CR admixtures, which increases the sound absorption. In addition, the results obtained for sound insulation in high frequencies are also improved with CR and FCR, even when the concrete becomes lighter, because damping of CR and FCR concrete is enhanced. As a consequence, high VF of FCR (80–100%) can be incorporated to concrete for non-structural uses to increase sound absorption.

Investigation on the mechanical behavior of polyester-scrap tire composites

An investigation of the mechanical properties of unsaturated polyester composites containing rubber waste particles is presented. The main objective of this work was to formulate and develop polyester-rubber composite as an insulating material with acceptable mechanical performance. To determine the effect of the rubber particles ratio on the compressive and tensile strengths and moduli of a polyester composite 8 different concentrations (i.e., 5%, 10%, 15%, 20%, 25%, 30%, 35% and 40% by volume) were considered. The experimental results revealed that mixing rubber particles with the polyester matrix decreased both the compressive and tensile strengths and moduli of the composite. As the rubber content increases, and due to its low hydrophilicity towards polyester resin, the degree of crosslinking between the rubber particles and polyester is reduced causing a noticeable reduction in the composite strengths. On the other side for all investigated rubber contents, the larger the particle size of rubber, the lower the composites strength and modulus of elasticity. This is due to the difficulty of dispersion of the large particles into the matrix resulting non homogenous distribution of the particles and less interfacial contact with the matrix and in turn degradation in the mechanical behavior. However, smaller rubber particles have more specific surface area that could improve the interfacial contact with the polyester matrix. The SEM micrographs indicated that voids within the composite matrix increased with the content and size of rubber particles. Though adding rubber particles reduces the mechanical properties, the prepared composites demonstrate superior mechanical tensile strengths (22.8–1.25MPa) and compressive strength (79.5–8.42MPa) if compared with the currently used insulating materials and comparable to some construction materials like the stone masonry.