Monitoring the vulcanization of rubber with ultrasound: Influence of material thickness and temperature

In consideration of the influence of the curing reaction on the material properties of elastomers, it is very important to control the vulcanization process properly.As shown previously [1], the effects of the vulcanization reaction of an elastomer can be monitored by use of ultrasonic sound waves. This technique has to a certain degree a high similarity to the standard curemeter test according to ISO 6502 but can be applied also inside a production tool.So far, the method was only applied to the vulcanization of one compound at a given temperature and sample thickness. This is now complemented with measurements on another compound system at different temperatures and sample thicknesses.The expected effects of temperature and thickness on the curing behaviour were found by use of the ultrasound online control.     

Influencing factors for static immersion tests of compatibility between elastomeric materials and lubricants

Material compatibility is an important factor to consider during the development of new lubricants and sealing materials. Static immersion tests provide a first idea about the compatibility between elastomeric materials and lubricants. For the same material combination, significant deviations among the test results of different laboratories have been reported. In order to identify the relevant factors affecting the results, a systematic investigation was carried out. Reproducibility tests show that the estimation of the compatibility is compromised due to the deviations that appear for the change of the mechanical properties of the considered reference elastomers. The influences of the closure of the test apparatus and the volume ratio, as well as the development of the aging process, were investigated. The results showed that, in order to differentiate between elastomer-lubricant material combinations, standard test durations of approximately 1008 h are preferred. Volume ratios of 64 and 80 and small variations of the vessel closure did not lead to significant deviations of the results. In contrast, tests with open and closed vessels showed significant deviations for the hardness and mechanical properties of the elastomeric materials.     

The use of nano- and micro-instrumented indentation tests to evaluate viscoelastic behavior of poly(vinylidene fluoride) (PVDF)

Nano-load (n-IIT) and micro-load (μ-IIT) instrumented indentation tests (IITs) were used to characterize elastic modulus and hardness in a semicrystalline polymer. The tests were conducted with loading rates ranging from 4.9 to 317 mN.min⁻¹ for n-IIT and from 300 to 10000 mN.min⁻¹ for μ-IIT. A decrease in the elastic modulus was observed as the load rate increased for the n-IIT process, and the elastic modulus increased as the load rate increased for the μ-IIT process. This behavior was explained by two-flow volume control under the indenter and the corresponding shear stress, which can influence the state of stress. The effect of holding time on the elastic modulus and hardness was also investigated for μ-IIT. E decreased with increasing holding time up to 30 s and became constant from there on. Hardness, however, decreased for all holding times evaluated. The steady state creep was only reached after 90 s, which is significantly higher than the time for elastic modulus stabilization.     

Characterization by ultra-micro indentation of an oxidized epoxy polymer: Correlation with the predictions of a kinetic model of oxidation

This study aims to understand better the influence of thermo-oxidation on the degradation of an epoxy resin used as the matrix of aeronautical composite laminates. Neat epoxy resin plates have been aged at 150 °C, under vacuum and ambient air. Using an instrumented ultra-micro indentation device, modifications of mechanical properties due to oxidation at elevated temperature are characterized through the measurements of elastic indentation modulus, Vickers hardness and indentation creep. By using a kinetic model of oxidation developed specifically for this epoxy resin, the local values of oxidation product concentration are calculated and correlated to experimental indentation measurements.     

Hybrid cyanate ester resin-based nanocomposites: Increased indentation size effect due to anomalous composition of micron subsurface layer

Hardness of materials increases with decreasing indentation depth from macro-to nano-scales, which is known as the indentation size effect (ISE). This effect has been associated with indenter shape, frictional forces, dislocation models and other features. We show an anomalously high ISE for a 1-μm subsurface layer in the hybrid nanocomposites based on densely cross-linked Cyanate Ester Resins (CER) containing functionalized 3-D POSS or 2-D MMT nanoparticles (NP). This effect disappears after mechanical stripping of the surface layer. Energy dispersive X-ray (EDX) spectral analysis shows that this anomaly was caused by increased content of NP (Si and Al elements), by 2.5 times, in the 1-μm subsurface layer. The hardness of the 1-μm subsurface layer in these brittle nanocomposites is due to its peculiar composition, and must be taken into account when considering mechanical strength and frictional properties.     

HPLC Analysis of complex mixtures of triglycerides using gradient elutions and an ultraviolet detector

Summary A method of reserved-phase HPLC analysis for mixtures of triglycerides (TG's) that provides good resolution at acceptable analysis times for high-ECN TG's has been developed. An elution gradient of methyltert-butyl-ether (MTBE) in acetonitrile (ACN) was used with an ultraviolet detector operated at 215nm. The effect of the proportion of MTBE in the mobile phase, gradient time, temperature and sample solvent on TG retention and resolution was studied. Linear relationships were derived between the logarithm of the capacity factor (log k'), and the logarithm of selectivity (log α) and the above-mentioned chromatographic factors. The conditions selected were: an elution gradient of from 23 to 30% MTBE, an elution gradient time of 25 minutes, and a temperature of 30°C, which provided good resolution of soybean oil TG's in less than 30 minutes.     

Heat shrinkability of electron-beam-modified thermoplastic elastomeric films from blends of ethylene-vinylacetate copolymer and polyethylene

The heat shrinkability of electron-beam-irradiated thermoplastic elastomeric films from blends of ethylene–vinylacetate copolymer (EVA) and low-density polyethylene (LDPE) has been investigated in this paper. The effects of temperature, time and extent of stretching and shrinkage temperature and time have been reported. Based on the above data, the optimized conditions in terms of high heat shrinkage and low amnesia rating have been evaluated. Influence of radiation doses (0–500 kGy), multifunctional sensitizer levels (ditrimethylol propane tetraacrylate, DTMPTA), and blend proportions on heat shrinkability has been explained with the help of gel fraction and X-ray data. With the increase in radiation dose, gel fraction increases, which in turn gives rise to low values of heat shrinkage and amnesia rating. At a constant radiation dose and blend ratio, percent heat shrinkage is found to decrease with increase in DTMPTA level. Gel content increases with the increase in EVA content of the blend at a constant radiation dose and monomer level, giving rise to decrease in heat shrinkability. Heat shrinkage increases with the increase in percent crystallinity, although the amnesia rating follows the reverse trend.     

Computer-aided optimization of stepwise gradient and multiple-development thin-layer chromatography

Summary Equations for the final R_F values for mobile phase gradient development and gradient multiple development are presented. Computer simulations of gradient development for both modes of thin-layer chromatography are discussed for the preliminary optimization of gradient programs.     

Inkjet printing of laminin gradient to investigate endothelial cellular alignment

To investigate the influence of the protein surface-density gradient on endothelial cell alignment, a novel approach for the fabrication of a laminin gradient on gold-coated substrates has been developed in this study. Our approach involves programmed inkjet printing of an alkanethiol (11-mercaptoundecanoic acid, C₁₀COOH, MUA) gradient onto gold-coated substrates, followed by backfilling with 11-mercapto-1-undecanol (C₁₁OH, MUD). The –COOH moieties were activated and then covalently linked with laminin. This treatment led to a surface-density gradient of laminin. Contact angle measurement, X-ray photoelectron spectroscopy (XPS) and fluorescence microscopy were employed to characterize the self-assembled monolayers (SAMs) and protein gradient, respectively. Results proved the feasibility of the fabrication of a protein gradient by using the inkjet printing technique. The self-assembled monolayer gradients displayed a high packing density, as indicated by dynamic contact angle measurement. More importantly, the gradient slope was easily tunable over a significant distance from 20 to 30 mm. The laminin gradient was clearly visible by fluorescence microscopy observation. Endothelial cells cultured on the surface-density gradient of laminin demonstrated a strong alignment tendency in parallel to the gradient. The higher the laminin density the more cells were observed. The result indicates that cell attachment is dependent on the surface density of laminin. This work broadens our methodology to investigate chemical stimuli-induced cell directional alignment. It is potentially important for understanding cell alignment/ingrowth behavior for angiogenesis and implant technology including tissue-engineered structures.     

Separation of proteins with a molecular mass difference of 2 kDa utilizing preparative double-inverted gradient polyacrylamide gel electrophoresis under nonreducing conditions: application to the isolation of 24 kDa human growth hormone

A method for separating proteins with a molecular mass difference of 2 kDa using SDS-PAGE under nonreducing conditions is presented. A sample mixture containing several human growth hormone (hGH) isoforms was initially separated on a weak anion-exchange column. Fractions rich in 24 kDa hGH as determined by analytical SDS-PAGE were pooled and further separated by cation-exchange chromatography. The fractions pooled from the cation-exchange chromatography contained two hGH isoforms with a 2 kDa molecular mass difference according to SDS-PAGE analysis, 22 and 24 kDa hGH. The 22 and 24 kDa hGH were separated using continuous-elution preparative double-inverted gradient PAGE (PDG-PAGE) under nonreducing conditions. The preparative electrophoresis gel was composed of three stacked tubular polyacrylamide matrices, a 4% stacking gel, a 13-18% linear gradient gel, and a 15-10% linear inverted gradient gel. Fractions containing purified 24 kDa hGH were pooled and Western blot analysis displayed immunoreactivity to antihGH antibodies. PDG-PAGE provides researchers with an electrophoretic technique to preparatively purify proteins under nonreducing conditions with molecular mass differences of 2 kDa.     

Turbidimetric titration and gradient high-performance liquid chromatography of polystyrene samples

Summary Polystyrene samples of narrow molecular-weight distribution have been eluted according to their molecular weight from columns packed with bare silica Si50, phenyl, or C18 bonded phase by gradients of methanol and tetrahydrofuran (THF) or ofiso-octane and THF. Among the six combinations investigated,iso-octane/THF with a silica column formed a proper normal-phase system whereas methanol/THF with a C18 column formed a proper reversed-phase system. The combinations of C18 column andiso-octane/THF or of Si50 column and methanol/THF gradient did not correspond to the approved polarity rules in high-performance liquid chromatography but were nevertheless effective in separating polystyrene mixtures by molecular weight. Methanol andiso-octane are nonsolvents for polystyrene whereas THF is a solvent. The solubility of polystyrene as a function of molecular weight and concentration was determined by means of turbidimetric titration of solutions in THF with the nonsolvents used in the gradients. The solubility and elution characteristics were almost identical on C18 columns or in methanol/THF combinations. The elution from phenyl bonded phase and Si50 columns usingiso-octane/THF gradients required more THF than the solubility experiments. Information is also given on the occurrence of multimodal elution patterns.     

The kinetic plot method applied to gradient chromatography: Theoretical framework and experimental validation

The kinetic plot method, originally developed for isocratic separations, was extended to the practically much more relevant case of gradient elution separations. A set of explicit as well as implicit data transformation expressions has been established. These expressions can readily be implemented in any calculation spread-sheet program, and allow to directly turn any experimental data set representing the relation between the separation efficiency and the flow rate measured on a single column into the kinetic performance limit curve of the tested separation medium. Since the kinetic performance limit curve is based on an extrapolation to columns with a different length, it should be realized that the curve is only valid under the assumption that the gradient time and the delay time (if any) are adapted such that the analytes are subjected to the same relative mobile phase history when the column length is changed. Both experimental and numerical data are presented to corroborate the fact that the kinetic performance limit curves that are obtained using the proposed expressions are indeed independent of the column length the experimental data were collected in. Deviations might arise if excessive viscous heating occurs in columns with a pronounced non-adiabatic thermal behaviour.     

Influence of moisture and CO2 on the material behavior of thermoplastic elastomers for beer bottle closures

Metal closures with a polymer-based sealing for beer bottles have been known since the late nineteen-sixties. However, to what extent which parameter of the polymer sealing gasket plays a role in closure performance to keep the beer quality to a highest level possible is not fully understood. For this purpose three thermoplastic elastomer (TPE) liners were investigated by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectrometry, dynamical mechanical analysis (DMA), scanning electron microscopy (SEM) and surface roughness in order to understand the effect of relative humidity (RH) and carbon dioxide (CO₂) on their physical and chemical properties as well as their macromolecular structures. The TPEs’ viscoelasticity in the frequency domain under different isothermal conditions was evaluated. RH and CO₂ were effective for changes in thermomechanical and surface properties. Liner C was found to have lower seal performance attributed to its relatively higher crystallinity, stiffness, weaker bond structure and rougher surface. Vibrations can lead to seal leakage in Liner B due to its higher damping behavior during production and transportation. Liner A outperforms others due to stabile behavior within the operational temperature range.     

Response surface methodology to optimize gradient ion chromatographic separation of inorganic anions and organic acids in tobacco leaves

The separation optimization of nine organic and inorganic anions in tobacco leaves using gradient ion chromatography by response surface methodology was investigated.In order to achieve this goal the usefulness of the chromatographic response function(CRF) for the evaluation of the two different chromatographic performance goals(resolution and analysis time) was tested. The experiments were performed according to a Box-Behnken design response surface experimental design.     

Theoretical study of preparative chromatography using closed-loop recycling with an initial gradient

Complementing classical isocratic elution, several more sophisticated operating modes have been proposed and are applied in preparative chromatography in order to improve performance. One such approach is gradient elution, where the solvent strength is altered by varying the fraction of a modifier added to the mobile phase to enhance selectivity and to achieve faster elution. Another useful technique is closed-loop recycling, allowing better peak resolution and increased yields. This study focuses on a modified new scheme which incorporates the advantages of both gradient elution and closed-loop recycling for the separation of a ternary mixture where the intermediately eluting component is the target. A parametric study was carried out using typical adsorption isotherm parameters to elucidate the effects of varying loading factors and parameters specific to the two basic operational modes on production rates and yields. A comparison was also made between the proposed scheme and conventional techniques. It was found that the studied scheme could exploit increased column loadings and offers significantly higher production rates.     

The application of gradient saturation in unidimensional planar chromatography of polar lipids. Part 1: Presentation of the HPTLC system

A thermostated HPTLC system was assembled to test the influence of gradient chamber saturation upon resolution of polar lipids chromatographed on boric acid-impregnated silica gel plates. Inexpensive mixtures of nonacidic (14 component peaks) and acidic (8 component peaks) lipids were prepared from soya lecithin by a simplified procedure involving DEAE-Sephadex liquid column chromatography to monitor performance of the system using a readily available reference material. The system requires ca. 5 min chamber presaturation after which the polar lipids are chromatographed in less than 30 min. Retention times (t_R) as well as peak area % values of all component peaks and measured by scanning photodensitometry to demonstrate the influence of different modes of chamber saturation upon the resolution of polar lopids.     

Physicochemical features and antioxidant activity of polysaccharides from Herba Patriniae by gradient ethanol precipitation

Four polysaccharides BJP50, BJP60, BJP70 and BJP80 (total named BJPs) were separated from Herba Patriniae via gradient ethanol precipitation with 50%, 60%, 70% and 80%, respectively. After decolorization and deproteinization, their physicochemical features and antioxidant activities were investigated. The results showed that the total sugar content of BJPs accounted above 50% but no protein contained, while BJP50 and BJP60 contained a small amount of uronic acid. GC analysis indicated that BJPs were mainly composed of galactose, arabinose, glucose, mannose, xylose and rhamnose. From BJP50 to BJP80, the types of monosaccharides and the content of arabinose, glucose and mannose increased but that of galactose and rhamnose decreased, and their molecular weights gradually reduced from 2.3 × 10⁶ to 4.5 × 10³. BJPs had a good thermal stability with the order of BJP80 > BJP70 > BJP60 > BJP50. The vitro bioactivity assay showed that BJP80 and BJP70 exhibited stronger scavenging capacities for DPPH, ABTS and hydroxyl free radicals than that of BJP60 and BJP50. As the concentration reached 4 mg/mL, the scavenging capacities of BJP80 and BJP70 on DPPH and hydroxyl free radical were up to 92% and 95%, respectively, and their antioxidant activities gradually approached to the positive control.     

A new approach to the prediction of RP-HPLC retention times in gradient elution from isocratic data

Summary A general chromatographic model has been set up starting from a set of equations based on the concept of the velocity of a solute along the column. The composition of the mobile phase is taken into account solely as a numerical factor entering into suitable equations and totally independent of the chemical-properties of the constituents. A few isocratic experimental runs are necessary as input data, and subsequently a small amount of computational effort is sufficient to make predictions of retention times under gradient elution conditions for solutes of whatever chemical structure. The prediction errors are dependent on the steepness of the linear gradient chosen but are, in any case, acceptably low.     

Continuous and gradient TLC development of 4-dimethylaminoazobenzene-4′-sulfonyl amino acid derivatives

Thin-layer gradient elution chromatography is a convenient technique for separation of very complicated mixtures; in some cases the gradient technique can be replaced by continuous development. The two techniques are compared in practice by using them to separate a mixture of dabsyl derivatives of amino acids.