Changes In Crosslink Density Research Articles

MECHANOBIOLOGICAL STUDY OF TENOCYTES CULTURED ON MICROGROOVED SUBSTRATES

Orthopedic injuries, particularly those involving tendons and ligaments, are some of the most commonly treated musculoskeletal ailments, but are associated with high costs and poor outcomes. A significant barrier in the design of biomaterials for tendon tissue engineering is the rapid de-differentiation observed for primary tenocytes once removed from the tendon body. Herein, we evaluate the use of an anisotropic collagen-glycosaminoglycan (CG) scaffold as a tendon regeneration platform. We report the effects of structural properties of the scaffold (pore size, collagen fiber crosslinking density) on resultant tenocyte bioactivity, viability, and gene expression. In doing so we address a standing hypothesis that scaffold anisotropy and strut flexural rigidity (stiffness) co-regulate long-term maintenance of a tenocyte phenotype. We report changes in equine tenocyte specific gene expression profiles and bioactivity across a homologous series of anisotropic collagen scaffolds with defined changes in pore size and crosslinking density. Anisotropic scaffolds with higher crosslinking densities and smaller pore sizes were more able to resist cell-mediated contraction forces, promote increased tenocyte metabolic activity, and maintain and increase expression of tenogenic gene expression profiles. These results suggest that control over scaffold strut flexural rigidity via crosslinking and porosity provides an ideal framework to resolve structure-function maps relating the influence of scaffold anisotropy, stiffness, and nutrient biotransport on tenocyte-mediated scaffold remodeling and long-term phenotype maintenance.

EPDM, IIR, 그리고 BIIR 내후성 고무 복합체의 열노화 거동

EPDM, IIR, and BIIR composites were thermally aged and the crosslink density changes were investigated. Crosslink densities of the EPDM composite increased with increasing the aging time and temperature, whereas those of IIR and BIIR composites for long-term aging at high temperatures tended to decrease. Activation energies for the crosslink density changes of the EPDM composite were higher than those of the BIIR one. The experimental results were explained with the number of allylic hydrogens, activation of the zinc complex, the steric hindrance effect, and oxidation of rubber chain.

노화 매질과 충진 시스템이 천연고무 복합체의 회복 거동에 미치는 영향

Difference in recovery behaviors from the circular deformation of natural rubber (NR) composites aged in air and distilled water, respectively were investigated. Recoveries of the samples aged in air were larger than those of the samples aged in distilled water. Recovery rates of the samples reinforced with filler were faster than those of the unreinforced ones. Recovery rates of the carbon black-filled samples were faster than those of the silica-filled ones. Difference in the recovery behaviors according to the aging media can be explained by the crosslinking density changes and the annealing effect.

Lifetime prediction and thermal aging behaviors of SBR and NBR composites using crosslink density changes

SBR and NBR composites were thermally aged and the crosslink density changes were investigated. The crosslink density changes of the SBR composite after thermal aging were larger than those of the NBR one. This could be explained by the allylic proton contents and the role of phenyl or nitrile group as a ligand in a zinc complex. The activation energies of the SBR composite were slightly lower than those of the NBR one. The lifetime to reach 50 and 100% increase of crosslink density of the NBR sample was shorter than that of the SBR one.

Sonochemical Synthesis of Mineral Nanoparticles and Its Applications in Epoxy Nanocomposites

Investigation of thermal and morphological behavior of epoxy nanocomposites system was evaluated with mineral nanofiller. Meanwhile, epoxy resin and nanomaterial (CaCO3) was synthesized under controlled ultrasound cavitation technique. During synthesis of epoxy resin nanomaterial (2 to 10 mass % loading) was added under controlled ultrasound. The intense collapse effect was arising due to cavitation, which enhances the polymerization process and uniformity in the mixing of nanoparticles. The effect of nanomaterial on thermal behaviors like curing time and thermal stability, and attachment of functional groups using FTIR were studied. Addition of nanomaterial accelerates the rate of curing of epoxy resin during composite formation with proper dispersion of nanomaterials under controlled ultrasonic waves (20 kHz). Addition of nanomaterial showed remarkable changes in cross-linking density. Thermal stability of epoxy nanocomposites found to be improved compared to neat epoxy resin. Increasing amount of loading of nanomaterials showed enhancement in mechanical as well as thermal properties of epoxy nanocomposites. It was demonstrated that ultrasonic synthesis is a unique and ultimate way for preparation of epoxy resin with nano-addition, due to controlled ultrasonic waves formation of long-term stable polymerization occurs, which consists of polymer/inorganic nanoparticles composite.

Eco-friendly elastomeric composites containing Sencha and Gun Powder green tea extracts

Abstract A permanent increase in the demand for polymers is closely related to the issue of their degradation. Typically, aromatic amines or phenol derivatives are used to protect materials against ageing. In our studies, we propose to use natural pro-ecological substances, such as polyphenols extracted from green tea leaves, to protect elastomers against ageing. Several extracts from Sencha and Gun Powder green tea leaves were incorporated into ethylene-propylene rubbers (EPM, Dutral CO-054). The vulcanizates of EPM samples containing the anti-oxidants under investigation were then subjected to UV radiations and climatic and thermal ageing processes. The changes in deformation energy, colour, and crosslinking density of the EPM vulcanizates were then measured before and after each ageing process. Based on the investigations performed, it is has been found that both the Sencha and Gun Powder green tea extracts display anti-oxidative properties that can protect EPM rubbers against the action of climatic factors.

Influence of Nano-Inorganic Particles on Properties of Epoxy Nanocomposites

Solution polymerization of Bisphenol-A and Epichlorohydrine gives epoxy resin. Spray pyrolysis technique was found to be promising method for synthesis of nanomaterials like CaCO3 of different sizes (10, 15, and 18 nm). The nanomaterial (2 to 10 mass % loading) was added at the time of resin formation. Mechanical stirring as well as an ultrasonication technique were used for uniform distribution of nanomaterials inside the resin. The effect of nanomaterials on thermal behaviors like curing time and glass transition temperature (Tg) were studied. Addition of nanomaterials accelerates the rate of curing of epoxy resin during composite formation. Moreover, addition of nanomaterial doesn't show any consequent change in Tg of epoxy composite but cross-linking density changes linearly. The rheological parameters like viscosity and torque were recorded on a Brookfield viscometer and correlated with M = CSα and τ = κ(γ)n.

Derivatives of flavonoides as anti-ageing substances in elastomers

A permanent increase of demand for polymers is closely connected with the issue of degradation. Nowadays aromatic amines or phenol derivatives are used to protect materials against ageing. In our studies, we want to propose the use of natural pro-ecological substances such as flavonoides to protect elastomers against ageing. Several flavonoides (hesperidin, hydroxymorin, hydroxyrutin, xanthone, flavon) were incorporated into ethylene-propylene rubbers (EPM, Dutral CO-054) and then the vulcanizates of EPM containing the anti-oxidants under investigation were subjected to UV, climatic and thermal ageing processes. The changes in deformation energy, color and cross-linking density of the EPM vulcanizates were then measured before and after each ageing process. Their carbonyl index was also determined. Based on the test results obtained, it has been found that the best protection is shown by hesperidin.

Chlorine Effect on Thermal Aging Behaviors of BR and CR Composites

Chloroprene is a chlorine substituent of 1,3-butadiene. Butadiene rubber (BR) and chloroprene rubber (CR) composites were thermally aged at 60, 70, 80, and 90 o C for 2 - 185 days in a convection oven and changes of the crosslink densities by the accelerated thermal aging were investigated. The crosslink densities increased as the aging time elapsed and as the aging temperature became higher. Degrees of the crosslink density changes of the BR composite were on the whole larger than those of the CR one except the short-term thermal aging at 60 and 70 o C. The crosslink densities abnormally increased after themal aging at high temperatures for a long time. Activation energies for the crosslink density changes of the rubber composites tended to increase with increase of the aging time and the variation showed a local minimum. The activation energies of the CR composite were lower than those of the BR one. The experimental results were explained with a role of ligand of chlorine atom of CR in a zinc complex, steric hindrance by chlorine atom of CR, and oxidation of rubber chain.

Influence of filler and cure systems on thermal aging resistance of natural rubber vulcanizates under strained condition

Aging and decomposition of polymer based articles are influenced by various environmental factors. Hence, it is important to research the various factors precisely to interpret the aging mechanism. This study deals with the tensile set properties of natural rubber (NR) vulcanizates with different reinforcing and cure systems after thermal aging. The tensile set increased with increasing both the aging temperature and aging period. Thermal aging resistance and tension set was enhanced by increasing the cure accelerator content in the sample. The carbon black-filled NR vulcanizates showed better thermal aging resistance than the silica-filled ones. It was found that silane coupling agent enhanced the thermal aging resistance of the silica-filled NR vulcanizates. The experimental results could be explained by the cross-link density change and also from the consumption of the antidegradants due to thermal aging. The permanent tension set and cross-link density were increased as the aging temperature and aging time were increased. The filler and cure systems were found to influence on the surface morphologies of the thermally aged samples. The cross-link density change was a key factor of the changes of tension set. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Useful Lifetime Prediction of Rubber Components Using Accelerated Testing

Rubber material properties, and useful lifetime prediction and evaluation are very important in the design procedure to assure the safety and reliability of rubber components. We have been interested for many years in predicting the lifetimes of rubber materials in oxygen-containing environments using a combination of accelerated aging tests and extrapolation models. In this paper, we investigate the heat aging effects on the material properties, and useful lifetime prediction of ethylene-propylene diene M-class rubber (EPDM), and acrylonitrile-butadiene rubber (NBR) for refrigerator components. The experimental results sssay that the crosslink density changes are varied with compositions of rubber vulcanization, and characteristics of heat aging behaviors of rubber vulcanizates are described well using the activation energy change with aging time. The stress and strain curves were obtained from the results of the simple tension test for specimens. To predict the useful lifetime of EPDM, and NBR, the mechanical property changes were determined under accelerated aging conditions. Rubber was aged at temperatures ranging from 50°C to 100°C for times ranging from 1 to 180 days. We present a general approach for more confidently correlating accelerated aging results with aging under service conditions using the Arrhenius methodology. By using the compression set test, several useful lifetime prediction equations for rubber material are proposed.

Analysis of thermally aged adhesion specimen between precured and uncured rubber sheets

Retreading of tire is closely related with adhesion between precured and uncured rubber sheets. Adhesion specimens between precured and uncured rubber sheets of BR and NR were prepared and thermally aged at 90 °C for 3 days. Organic materials remained in the adhesion sheets were extracted with THF and analyzed using gas chromatography/mass spectrometry (GC/MS). Elemental sulfur (S 8), mercaptobenzothiazole (MBT), antidegradants, and fatty acids were detected as major organic materials remained in the adhesion sheets. Amounts of the organic materials were reduced by the thermal aging and varied with the adhesion specimens of BR/BR, NR/NR, BR/NR, and NR/BR as the precured and uncured rubber sheets. Decrements of the S 8, MBT, and fatty acids in the NR sheet were larger than in the BR one due to the faster cure characteristics of NR. Changes of the adhesion forces and crosslink densities by the thermal aging were measured. Adhesion force of only the precured BR/uncured NR specimen by the thermal aging was enhanced whereas those of the other specimens were reduced. This was explained with the crosslink density changes and migration of the organic materials through the interface of the adhesion specimen.

Strain effect on recovery behaviors from circular deformation of natural rubber vulcanizate

AbstractThe recovery behaviors of vulcanized natural rubber from circular deformation were studied by varying sample length to investigate the influence of the degree of applied strain to the sample. The stress and strain varied uniformly across the thickness of the sample, and the degree of applied stress and strain to the sample decreased with increasing the sample length. The linear sample was made into a circular form and thermally aged in a convection oven. The recovery was measured after removing from the circular deformation. The recovery increased by decreasing the sample length. The difference in recoveries of the samples with different lengths reduced at high temperature. The short sample had higher heat‐resisting property for recovery than the long sample. The experimental results were explained by the rearrangement and stabilization of the deformed rubber chains as well as the crosslink density change. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Comparison of recovery behaviors of thermally aged SBR composite from compressed and circular deformations

Recovery behaviors of the SBR composite from the compressed and circular deformations after thermal aging were compared. Instantaneous recovery and target recovery time to describe the sealing properties of a sealant were obtained from the recovery variation with the measuring time. For the compressed deformation, the recovery decreased at the initial measurement time and then increased. Since the recovery from the compressed deformation showed a local minimum and the minimum point varied in accordance to the thermal aging conditions, the linearity of the recovery variation was very low. For the circular deformation, the recovery continuously increased as the measurement time elapsed, the linearity was very high, and the instantaneous recovery of less than 0.1 s could be obtained from the extrapolation method. The circular deformation method was found to be suitable for study of recovery behaviors of a rubber article. The recovery behaviors varied with the thermal aging conditions and the difference was explained with the crosslink density change by the thermal aging.

Synthesis and Characterization of Core-Shell Acrylate Based Latex and Study of Its Reactive Blends

Techniques in resin blending are simple and efficient method for improving the properties of polymers, and have been used widely in polymer modification field. However, polymer latex blends such as the combination of latexes, especially the latexes with water-soluble polymers, were rarely reported. Here, we report a core-shell composite latex synthesized using methyl methacrylate (MMA), butyl acrylate (BA), 2-ethylhexyl acrylate (EHA) and glycidyl methacrylate (GMA) as monomers and ammonium persulfate and sodium bisulfite redox system as the initiator. Two stages seeded semi-continuous emulsion polymerization were employed for constructing a core-shell structure with P(MMA-co-BA) component as the core and P(EHA-co-GMA) component as the shell. Results of Transmission Electron Microscopy (TEM) and Dynamics Light Scattering (DLS) tests confirmed that the particles obtained are indeed possessing a desired core-shell structural character. Stable reactive latex blends were prepared by adding the latex with waterborne melamine-formaldehyde resin (MF) or urea-formaldehyde resin (UF). It was found that the glass transition temperature, the mechanical strength and the hygroscopic property of films cast from the latex blends present marked enhancements under higher thermal treatment temperature. It was revealed that the physical properties of chemically reactive latexes with core-shell structure could be altered via the change of crosslinking density both from the addition of crosslinkers and the thermal treatment.

Surface UV aging of elastomers investigated with microscopic resolution by single-sided NMR

Depth profiles taken from the surface of UV irradiated natural rubber sheets have been measured with microscopic resolution using a Profile NMR-MOUSE ®. An NMR observable related to the sum of the spin echoes in the Carr–Purcell–Meiboom–Gill pulse sequence was used to characterize the cross-link density changes produced by the action of UV radiation in each sheet. The aging process was investigated as function of irradiation time and penetration depth. An exponential attenuation law with a space dependent absorption coefficient describes the change in the NMR observable with penetration depth. An Avrami model is used to describe the dependence of the absorption coefficient on the aging time. The method can be applied to investigate the effect of various aging agents on the surfaces of elastomers.

Relationship Among Thermal Ageing Degradation, Dynamic Properties, Cure Systems, and Antioxidants in Natural Rubber Vulcanisates

The present work aims to study the relationship among the thermal ageing stability, dynamic properties, cure systems, and antioxidants in natural rubber (NR) vulcanisates. Thermal degradation behavior of NR vulcanisates has been investigated and correlated to the changes in cross-link density, tensile and dynamic mechanical properties. The results obtained show that thermal ageing properties of NR vulcanisates depend strongly on cross-link density, which changes during thermal oxidative ageing or the so-called postcuring effect. In addition, the increases in ageing temperature and time lead dominantly to the postcuring and linkages scission phenomena in vulcanisates cured with CV and EV systems, respectively. With increasing ageing temperature, the tensile strength shows sharp drop at ageing temperature higher than 70°C and 100°C for the specimens cured with CV and EV systems, respectively. The sharp drop of tensile strength of vulcanisates cured with CV system is attributed to the too high cross-link density, which is caused by the postcuring effect. In the case of the vulcanisates cured with EV system, the linkage scission causes the sharp drop of tensile strength. The addition of amine-based antioxidant appears to improve ageing properties. However, the excessive antioxidant reduces tensile properties via a decrease in cross-link density.

Surface modification study of low energy electron beam irradiated polycarbonate film

The effect of low energy electron beam irradiation on polycarbonate (PC) film has been studied here. The PC film of thickness 20 μm was exposed by 10 keV electron beam with 100 nA/cm 2 current density. The irradiated film was characterized by mean of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and residual gas analyzer (RGA). Formation of unsaturated bonds and partial graphitization of the surface layer are measured by XPS. Results of the AFM imaging shows electron implantation induce changes in surface morphology of the polymer film. The residual gas analyzer (RGA) spectrum of PC is recorded in situ during irradiation. The results show the change in cross-linking density of the polymer at the top surface.

Accelerated Thermal Aging Behaviors of EPDM and NBR Vulcanizates

The crosslink densities of rubber vulcanizates cured by a sulfur accelerator system are changed by thermal aging. Change of the crosslink density occurs by formation of new crosslinks and dissociation of existing crosslinks. Crosslink density change of a rubber vulcanizate with a high sulfur content after thermal aging was larger than that with a low sulfur content. Permanent deformation of a rubber vulcanizate results from the crosslink density change by thermal aging under the deformed state. Strain applied to a rubber vulcanizate affects the change of crosslink density. The crosslink density of the more strained vulcanizate was changed by thermal aging more than those of the less strained one. Ethylene-propylene diene rubber (EPDM) is a weatherresistant rubber and has slow cure characteristics of long induction time and slow cure rate. Acrylonitrile-butadiene rubber (NBR) is oil-resistant rubber and has fast cure characteristics of short induction time and fast cure rate. EPDM and NBR have different chemical properties each other. EPDM is nonpolar and has slow cure characteristics, whereas NBR is polar and has fast cure characteristics. In this work, we studied changes of crosslink densities of EPDM and NBR vulcanizates by thermal aging at high temperatures of 70-100 C for long time of 180 days. Levels of the crosslink density change and the activation energies of the vulcanizates were also compared. Thermal aging behaviors depending on the aging time were explained with the activation energy variation. Figures 1 and 2 show variations of the crosslink density changes of the EPDM and NBR vulcanizates with the aging time, respectively. The crosslink density change was obtained by dividing the difference in the crosslink densities after and before the thermal aging by the crosslink density of the initial sample. The crosslink density change increases as the aging time elapses. Level of the crosslink density change becomes larger by increasing the aging temperature. For the EPDM vulcanizate, the crosslink density change increases notably during the initial period and then the increment becomes smaller. The crosslink densities increase by 10.0, 17.6, and 22.6% after the thermal aging at 70, 85, and 100 C for 30 days, respectively, and increase by 17.3, 27.9, and 42.5% after the thermal aging for 180 days, respectively. The initial 30 day increments (0 to 30 days) are 10.0, 17.6, and 22.6% for 70, 85, and 100 C, respectively, while the final 30 day increments (150 to 180 days) are 0.2, 0.4, and 1.7%, respectively. For the NBR vulcanizate, the crosslink density change increases notably with increase of the aging time as shown in Figure 2. Especially, the crosslink density change at 100 Figure 1. Variation of the crosslink density change of the EPDM vulcanizate by the thermal aging as a function of the aging time. Squares, circles, and triangles indicate the aging temperatures of 70, 85, and 100 oC, respectively.

Thermal Aging Behaviors of Rubber Vulcanizates Cured with Single and Binary Cure Systems

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