EFFECT OF MATURATION TIME ON THE STRUCTURE AND PROPERTIES OF NR

Rubber compounds are crosslinked by a sulfur, peroxide, or resole cure system. The sulfur vulcanization is the most popular method. In general, an accelerated sulfur cure system consists of elemental sulfur (S8), one or two cure accelerators, and cure activators. Crosslink density of a rubber vulcanizate determines the physical properties. By increasing the crosslink density, the modulus, hardness, resilience, and abrasion resistance increase, whereas the elongation at break, heat build-up, and stress relaxation decrease. Sulfur linkages are composed of monosulfide, disulfide, and polysulfides. Sulfur linkages, especially polysulfides, are dissociated by heating and this brings about decrease of the crosslink density. Curatives remained in a rubber vulcanizate make new crosslinks and this results in increase of the crosslink density. Crosslink density of a rubber vulcanizate is changed by thermal aging. In general, crosslink density of a sulfur-cured rubber vulcanizate increases with increase of the aging temperature. However, for a rubber vulcanziate with an elemental sulfur-free cure system, the crosslink density after thermal aging at high temperatures decreased. In the present work, we studied the thermal aging behaviors of the rubber vulcanizates with single and binary cure systems which have one and two cure accelerators, respectively. The single cure system had one cure accelerator of N-tert-butyl-2-benzothiazole sulfenamide (TBBS) and binary cure system had two cure accelerators of TBBS and 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)hexane (DBTH). Scheme 1 shows the chemical structures of TBBS and DBTH. The binary cure system has faster cure rate and better reversion resistance than the single cure system. The NR vulcanizates with different crosslink densities were prepared to investigate the influence of the initial crosslink density on the thermal aging behaviors. The aging temperatures (50-90 C) and aging times (4 and 8 days) were also varied to investigate the influence of the aging temperature and time on the thermal aging behaviors. Figures 1 and 2 show variations of the crosslink density changes by the thermal aging as a function of the aging temperature. The crosslink density change was calculated by dividing the difference in the crosslink densities of the vulcanizates after and before the thermal aging by the initial crosslink density; ΔXc (%) = 100 × (Xc aged – Xc )/Xc , where the Xc aged and Xc ini indicate the crosslink densities of the vulcanizates after and before the thermal aging, respectively. The crosslink densities after the thermal aging at 50-90 C on the whole increased. The increased crosslink density after

Chemical characterization of tequila maturation process and their connection with the physicochemical properties of the cask

This study compares the effect of sulfur and dicumyl peroxide (DCP) vulcanizing systems on the physical and mechanical properties of rubber compounds based on acrylonitrile butadiene rubber (NBR). NBR compounds cured by different amounts of DCP and NBR vulcanizates filled with various concentrations of carbon black (CB) and a constant amount of sulfur or DCP were prepared. The vulcanizates were characterized by tensile testing, dynamic mechanical thermal analysis (DMTA), and cross-link density determination. The tensile strength and Young's modulus were found to increase with the rising amount of DCP and CB, while elongation at break decreased. The samples vulcanized by the sulfur system and filled with CB show a substantial increase in tensile strength from 13.1 to 21.2 MPa. Higher storage modulus and glass transition temperature were observed with the increase in the amount of peroxide and filler, and consequently, the increase in cross-link density, indicating rigidity increase and lower molecular mobility. The changes in the physical and mechanical properties of the NBR vulcanizates were in correlation with the changes in solvent uptake and cross-link density.

Linear, 3-arm, and 6-arm star-shaped stereo diblock copolymers of l- and d-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via two-step ring-opening polymerization (ROP) with 1-dodechanol, glycerol, and d-sorbitol as the initiators, respectively. The chemical structure, nonisothermal and isothermal crystallization kinetics, crystalline structure, lamellar morphology, and mechanical thermal properties of PLLA-b-PDLAs with different macromolecular topologies were investigated. Compared to the high-molecular-weight (MW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends, PLLA-b-PDLAs exhibit faster crystallization upon cooling and isothermal melt crystallization; they crystallize exclusively in stereocomplex (sc) crystallites under all of the conditions investigated. This is attributable to the enhanced interactions between enantiomeric blocks linked covalently. Macromolecular topology influences the crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature upon cooling, melting temperature, degree of crystallinity, spherulitic growth rate, crystallite size, long period, and crystalline layer thickness of PLLA-b-PDLA decrease with increasing branching number because of the retarding effect of branching on the crystallization rate and crystallizability. Because of the formation of high-melting-point sc crystallites, both the linear and star-shaped PLLA-b-PDLAs exhibit better thermal resistance and higher storage moduli at high temperature than does homocrystalline PLLA.

Nanocellulose composites with enhanced mechanical and flame-retardant properties based on grafting of inorganic organic/multilayer core-shell matter - MSNs-TMSB/DA/TOCNF.

Degradable polymers are integral components in many biomedical polymer applications. The ability of these materials to decompose in situ has become a critical component for tissue engineering, allowing scaffolds to guide cell and tissue growth while facilitating gradual regeneration of native tissue. The objective of this work is to understand the role of prepolymer molecular weight and functionality of photocurable poly(caprolactone) (PCL) in determining reaction kinetics, mechanical properties, polymer degradation, biocompatibility, and suitability for stereolithography. PCL, a degradable polymer used in a number of biomedical applications, was functionalized with acrylate groups to enable photopolymerization and three-dimensional printing via stereolithography. PCL prepolymers with different molecular weights and functionalities were studied to understand the role of molecular structure in reaction kinetics, mechanical properties, and degradation rates. The mechanical properties of photocured PCL were dependent on cross-link density and directly related to the molecular weight and functionality of the prepolymers. High-molecular weight, low-functionality PCLDA prepolymers exhibited a lower modulus and a higher strain at break, while low-molecular weight, high-functionality PCLTA prepolymers exhibited a lower strain at break and a higher modulus. Additionally, degradation profiles of cross-linked PCL followed a similar trend, with low cross-link density leading to degradation times up to 2.5 times shorter than those of more highly cross-linked polymers. Furthermore, photopolymerized PCL showed biocompatibility both in vitro and in vivo, causing no observed detrimental effects on seeded murine-induced pluripotent stem cells or when implanted into pig retinas. Finally, the ability to create three-dimensional PCL structures is shown by fabrication of simple structures using digital light projection stereolithography. Low-molecular weight, high-functionality PCLTA prepolymers printed objects with feature sizes near the hardware resolution limit of 50 μm. This work lays the foundation for future work in fabricating microscale PCL structures for a wide range of tissue regeneration applications.

Performance enhancement of silica filled natural rubber nanocomposites using organic deep eutectic solvent

This study, uses two polyfunctional monomers (PFMs) namely ethylene glycol dimethacrylate (EGDMA) as a bifunctional monomer and trimethylolpropane triacrylate (TMPTA) as a trifunctional monomer were used as co-agents in irradiation crosslinking of Ethylene Propylene Diene Monomer Rubber (EPDM). The effect of concentration of each PFM and irradiation dose on the crosslinking density, gel content, swelling behavior in motor and brake oils, in addition to the mechanical and thermal stability properties of EPDM was investigated in detailed. The results showed a remarkable increase in the gel content, crosslinking density and mechanical properties as the concentration of PFMs increased from 1 to 5 phr (parts per hundred parts of rubber). The various blends of EPDM with the trifunctional monomer express the highest gel content and crosslinking density than those with the bifunctional monomer. The addition of 5 phr of TMPTA to EPDM causes a dramatic improvement in tensile strength (TS) of the prepared blend reached to 188% compared to neat EPDM at 50 kGy. At the same time, the maximum TS of the blend containing 5 phr of EGDMA achieved only 41% compared to neat EPDM at an irradiation dose of 100 kGy. The swelling of irradiated samples in brake oil revealed a stronger oil resistance than motor oil. For all irradiated samples, the oil uptake decreased with the irradiation dose up to 100 kGy. The EPDM samples containing 5 phr of TMPTA recorded the highest oil resistance at 100 kGy. The results also showed that the addition of PFMs and irradiation treatment of the various prepared blends improved the thermal stability of EPDM. Finally, neat EPDM and the blends containing 1 and 3 phr of EGDMA can be used as radiation dosimeters in the very high dose range (50–200 kGy).

Heat build-up (HBU) under deformation is a critical characteristic of rubber materials, and it significantly influences the performance and service safety of final products. In addition, the deformation modes of materials can vary considerably depending on the product and working conditions, resulting in a complex and sometimes unpredictable correlation between HBU and the viscoelasticity of materials. In this study, strain- and stress-controlled HBU experiments on rubber composites are conducted using a dynamic mechanical analyzer (DMA) equipped with a sample temperature measuring unit. During the HBU tests, dynamic mechanical properties are monitored simultaneously with HBU, ensuring a direct correlation between viscoelasticity and heat generation. The energy density of dissipation, Wdiss in the stress-strain hysteresis loop, is considered to be the source of heat generation and can be expressed through various rheological parameters. It is found that, in strain-controlled mode, the HBU of tested materials correlates linearly with the loss modulus, . In contrast, in stress-controlled mode, the HBU correlates linearly with the combined formats of rheological parameters, such as . These results suggest a new guideline for applying viscoelastic parameters, or their combinations, as evaluation indicators for the performance of rubber materials based on their working conditions.

Many naturally occurring color components, especially non-rubber components, are present in natural rubber (NR) latex, limiting some NR applications, particularly for light-colored NR products in high demand. Non-rubber components, like lipids and proteins, can be removed from NR to reduce its yellow color. This study looked at the factors that influence the yellow index (YI) in NR and NR latex discoloration techniques. Proteins were a crucial factor directly determining the YI of NR. Thus, increasing the number of centrifugation washings, speed, and time was used to remove proteins to minimize the YI of NR. Percentage of total solid content (% TSC) of field NR latex, the blend ratio between cream rubber and skim latex, or bottom fraction from fresh-field NR latex collected from centrifugation process, were also important factors. The addition of sodium metabisulphites as a polyphenol oxidase (PPO)-reducing agent reduced the YI significantly. The oxidative breakdown of endogenous non-rubber components at high temperatures was also confirmed to serve the increase in YI value due to air-drying conditions of NR.

Poly-vinyl alcohol (PVA) crosslinked by trisodium trimetaphosphate (STMP) and sodium hexametaphosphate (SHMP): Effect of molecular weight, pH and phosphorylating agent on length of spacing arms, crosslinking density and water interaction

A series of the solid emulsion gels with the oil volume fraction in the range of 0-50% were synthesized through a polycondensation reaction between activated p-nitrophenyl carbonate poly(ethylene glycol) and protein-stabilized oil-in-water emulsions. The resultant structures were investigated in terms of swelling behavior, composition, morphology, mechanical and skin hydration properties. Solid emulsions gels share the properties of both hydrogel and emulsion. Similar to the classical hydrogel, the SEG swells in water up to equilibrium swelling degree, which decreases as the oil volume fraction increases, and comprises immobilized drops of protein-stabilized oil. The impregnation of the oil phase is found to reduce tensile stiffness of the material, but improves material's extensibility. The mechanical properties of the constructs (Young moduli in the range of 9-15 kPa and the elongation at break of 120-220%) are interpreted according to the "rule of elasticity mixture" that considers the elasticity of the composite material to be a sum of the contributions from individual components, i.e. hydrogel and dispersed oil drops. An idealized model that takes into account the history of the material preparation has been proposed to explain the improved extensibility of the constructs. The results of the mechanical tests, equilibrium swelling, and the skin hydration effect of the solid emulsion gels in vivo are discussed from the perspective of the biomedical applications of the solid emulsion gels, in particular, for the transdermal delivery of hydrophilic and lipophilic drugs.

Influence of water type and maturation time on the properties of kaolinite–saponite peloids

A single-step method that produces percolated, dynamic covalent cross-links integrated into the PP homopolymer has not been previously demonstrated. Here, we synthesized covalent adaptable networks (CANs) from polypropylene (PP) homopolymers using 180 °C, radical-based, reactive processing with a free-radical initiator, dicumyl peroxide (DCP), and resonance-stabilized, aromatic disulfide cross-linkers, one methacrylate-based and another phenyl acrylate-based. Both cross-linkers yielded networks when reactively processed at 4 wt % with relatively high molecular weight (MW) PP (melt flow index (MFI) = 12) and 4 wt % DCP. The phenyl acrylate-based cross-linker also yielded PP networks at other studied DCP/cross-linker concentrations and with relatively low MW PP (MFI = 35). Notably, our highest cross-link density PP CAN exhibited full recovery of cross-link density after three reprocessing steps by compression molding; that PP CAN also exhibited full cross-link density recovery within experimental uncertainty after reprocessing by melt extrusion.

Calcium lignosulfonate (CaL) as biopolymer filler was incorporated into rubber matrices based on acrylonitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) in concentration scale ranging from 10 to 60 phr. The work was aimed at investigation of CaL content on crosslink density and mechanical properties of the composites. Subsequently, the composites were kept in hot air chamber for 7 days at 70 °C and 100 °C and the influence of thermooxidative ageing on the change of cross-link density and mechanical properties was investigated. Kinetics of thermal oxidation was studied by non-isothermal differential scanning calorimetry. The achieved results showed that the changes in modulus and elongation at break were in close connection with the changes in crosslink density before ageing as well as after ageing. When compared to the SBR based reference, the crosslink density of the composite with maximum CaL content decreased by about 62%, while the decrease in crosslink density of the corresponding materials based on NBR represented by about 39%. A higher percentage decrease in the crosslink density of composites based on SBR was reflected in a higher proportional decrease of modulus M300 (49%) and a higher increase of the elongation at break (more than 480%) of the corresponding composites in the dependence on CaL content when compared to those based on NBR. The tensile strength of composites based on SBR was found independent of the CaL content while a slight improvement in tensile behavior was recorded for NBR-based composites with higher CaL content (in more than 0,5 MPa for the maximally filled composites when compared to the reference). The experimental outputs revealed that CaL provided some stabilizing effect on the composites during thermooxidative ageing, though this effect was found to be low. Based upon the calculation of the induction periods of thermooxidation it can be concluded that the composites based on NBR exhibit a higher stability than those based on SBR.