NR Molecules Research Articles

Degradation during Mixing of Silica-Reinforced Natural Rubber Compounds.

The optimal mixing conditions for silica-filled NR compounds dictate the need to proceed at a high temperature, i.e., 150 °C, to achieve a sufficient degree of silanization. On the other hand, natural rubber is prone to degradation due to mechanical shear and thermal effects during mixing, particularly at long exposure times. The present work investigates NR rubber degradation during mixing in relation to prolonged silanization times. The Mooney viscosity and stress relaxation rates, bound rubber content, storage modulus (G'), and delta δ were investigated to indicate the changes in the elastic/viscous responses of NR molecules related to rubber degradation, molecular chain modifications, and premature crosslinking/interaction. In Gum NR (unfilled), an increase in the viscous response with increasing mixing times indicates a major chain scission that causes a decreased molecular weight and risen chain mobility. For silica-filled NR, an initial decrease in the Mooney viscosity with increasing silanization time is attributed to the chain scission first, but thereafter the effect of the degradation is counterbalanced by a sufficient silanization/coupling reaction which leads to leveling off of the viscous response. Finally, the higher viscous response due to degradation leads to the deterioration of the mechanical properties and rolling resistance performance of tire treads made from such silica-filled NR, particularly when the silanization time exceeds 495 s.

Review: Natural rubber – Improvement of properties

AbstractNatural rubber (NR) is valuable material of natural origination with growing demand worldwide. Thus, the methods for NR property improvement are continuously developed. The article will be focused on methods of chemical modification of NR and the use of improved traditional fillers such as carbon black (CB) and silica. Modification reactions include epoxidation, hydrogenation, and grafting vinyl monomers onto NR molecules. Some methods of surface modification of NR film were also reviewed. Thanks to modification the NR degree of unsaturatiton is decrease that leads to its enhanced aging resistance. Besides, grafted functional groups may impart new, never met before properties to NR. Application of modified traditional fillers obviously increase their reinforcement possibility, particularly mechanical. Also, cellulose as a potential reinforcing filler is considered due to its ability to meet requirement of environment protection while keep the mechanical properties of NR products at acceptable high level.

Influence of alkaline treatment and acetone extraction of natural rubber matrix on properties of carbon black filled natural rubber vulcanizates

In this study, air dried sheet (ADS) showed higher molecular weight, proteins, lipids and more gel content than the low protein natural rubber (LPNR) from alkaline treatment or acetone-extracted natural rubber (AENR). After removal of proteins and lipids, LPNR and AENR had shorter scorch and cure times among the rubber compounds observed. This is due to higher content of free fatty acids, glycerides and sodium salts of fatty acid that might act as cure activators with sulfur curing. Furthermore, a finer dispersion of CB was found in ADS, due to chemical interactions of CB with proteins and lipids at terminal ends of NR molecules. Also, high molecular weight and gel in ADS could induce a higher reinforcing index (α) resulting to superior mechanical, dynamic, thermo-mechanical, hardness and crosslink density. In contrast, CB agglomerates occurred in LPNR and AENR with low molecular weight and reduced proteins and lipids, causing inferior mechanical, dynamic, thermo-mechanical, along with loss of stiffness and crosslink density.

Structure and Properties of Natural Rubber/Zinc Disorbate Composite

The natural rubber/zinc disorbate composite was prepared by the in situ formation of zinc disorbate from zinc oxide and sorbic acid in natural rubber. The structure variations of fillers during mixing and vulcanization processes were studied by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The effects of zinc disorbate amount on processing performance and glass transition temperature (Tg) of compounds and the mechanical properties of vulcanizates were also determined. The XRD and FTIR analyses results indicate that the zinc disorbate is formed from the reaction of zinc oxide and sorbic acid during the mixing procedure followed by graft copolymerizing with NR molecules to form composite networks during vulcanization, which is initiated by dicumyl peroxide. Thus the mechanical properties of NR-based composite are increased significantly and Tg shifted towards to higher temperature.

Organoclay maleated natural rubber nanocomposite. Prediction of abrasion and mechanical properties by artificial neural network and adaptive neuro-fuzzy inference

Maleated natural rubber (MNR) was produced and employed in different amounts as compatibilizer in preparation of organoclay/natural rubber (OC/NR) nanocomposite. Oscillating disk rheometry, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, rheomechanical spectroscopy, dynamic mechanical analysis, tensile and abrasion tests were used to evaluate the prepared samples. The obtained results showed that the use of MNR caused an increase in the maximum torque of rheometry, curing rate index and crosslink density. XRD analysis indicated that using of MNR, especially in the amount of 8 parts, resulted in a greater intercalation of OC layers. SEM showed a good dispersion of the OC as well as higher interfacial interaction between NR molecules and OC. These lead to improvement of modulus, tensile strength and abrasion resistance. Then the obtained results were modeled by artificial intelligence techniques. Comparison made between experimental and predicted values, indicated that the modeling results were in good agreement with the corresponding experimental amounts.

Novel thermoplastic natural rubber based on thermoplastic polyurethane blends: influence of modified natural rubbers on properties of the blends

Three different forms of natural rubber: maleated natural rubber (MNR), epoxidized natural rubber (ENR) and natural rubber-graft-poly(methyl methacrylate) (NR-g-PMMA) were prepared. Degree of functional groups in rubber molecules was quantified using the integrated peak areas of 1H NMR. It was found that the modified rubbers with similar level of functionality had been successfully prepared. Thermoplastic natural rubber (TPNR) based on blending of thermoplastic polyurethane (TPU) and various forms of rubber were then prepared using melt blending method. The properties of the blends were studied and compared together in relation to different types of natural rubbers prepared (i.e., unmodified NR, MNR, ENR and NR-g-PMMA). It was found that the blends with modified NR exhibited superior stiffness, entropy effect and damping factor compared to other blends with unmodified NR. This is attributed to the chemical interaction between the functional groups of modified NR molecules and polar functional groups in TPU molecules which facilitated higher interfacial adhesion between both phases. The chemical interaction was verified by ATR-FTIR and TSSR techniques. It was also found that the MNR/TPU blend showed the highest tensile modulus, mechanical and elastic properties with smallest and finer grain dispersion of co-continuous phase compared to ENR/TPU, NR-g-PMMA/TPU and unmodified NR/TPU blends, respectively. This might be due to higher chemical interactions between MNR and TPU phases. Furthermore, the incorporation of rubber did reduce hardness (i.e., <60 Shore A) with improvement of elasticity of the blends compared with the original TPU (i.e., ~85 Shore A).

Identification of Binding Mode of a Platinum (II) Complex, (DIP), and Calf Thymus DNA

The Pt(II) complex, PtCl2(DIP) (DIP = chelating dinitrogen ligand: 4,7-diphenyl-1,10-phenanthroline), was synthesized and characterized by elemental analysis (CHN) and 1H NMR and UV-vis techniques. The binding of this complex to calf thymus DNA was investigated using various physicochemical methods such as spectrophotometric, circular dichroism, spectrofluorometric, melting temperature, and viscosimetric techniques. Upon addition of the complex, important changes were observed in the characteristic UV-Vis bands (hyperchromism) of calf thymus DNA (CT-DNA): increase in melting temperature, sharp increase in specific viscosity of DNA, and induced CD spectral changes. Also the fluorescence spectral characteristics and interaction of Pt complex with DNA have been studied. Pt bound to DNA showed a marked decrease in the fluorescence intensity. The results show that both the complex and the NR molecules can intercalate competitively into the DNA double-helix structure. The experimental results show that the mode of binding of the this complex to DNA is classical intercalation.

Interfacial adhesion and molecular diffusion in melt lamination of wood sawdust/ebonite NR and EPDM

AbstractAdhesion mechanisms and peel strengths of wood/ebonite NR‐EPDM laminates were investigated. Three different chemical coupling agents: namely; N‐(β aminoethyl)‐γ‐aminopropyl‐triethoxysilane (AAS), 3‐methacryloxypropyl trimethoxysilane (ACS), and Bis‐(3‐triethoxylpropyl) tetrasulfan (Si69) were introduced into the wood/NR composites to enhance an interaction between wood sawdust (SD) particles and NR molecules, and to improve the adhesion strength between the SD/NR and EPDM layers. The quantitative evidences were given to explain the changes in the adhesion or peel strengths of the SD/NR‐EPDM laminates through scanning electron microscopy with energy dispersive X‐ray analysis (SEM‐EDS). The experimental results indicated that the suitable cure time and cure temperature for SD/NR‐EPDM melt‐laminates were the tc90 of SD/NR composites and 140°C, respectively. The Si69 coupling agent was found to be the most effective coupling agent as compared with AAS and ACS coupling agents. The Si69 of 0.5 wt% was recommended for the optimizations of the tensile modulus of the SD/NR composites and the peel strength of the SD/NR‐EPDM laminates. The diffusion level between the SD/NR and EPDM layers could be quantitatively substantiated by determining the sulfur content transfer from the SD/NR layer to the EPDM layer. The diffusion and entanglement of molecular chains from the SD/NR to the EPDM layer initiated the co‐crosslinking reaction which played an important role on the changes in the interfacial strength in the SD/NR‐EPDM melt‐laminates. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Ab Initio Modeling of Defect Signatures in Infrared Reflection−Absorption Spectra of SAMs Exposing Methyl- and Hydrogen-Terminated Oligo(ethylene glycols)

Extensive ab initio modeling has been performed to explain quantitatively the apparent shapes of characteristic bands, which are systematically observed in the fingerprint region of infrared (IR) reflection-absorption (RA) spectra of oligo(ethylene glycol) (OEG)-terminated SAMs. The presence of defects was thoroughly examined by modeling the RA spectra using the DFT method BP86/6-31G* for all-helical and all-trans conformers of HS(CH2CH2O)nR (n = 5, 6, R = H, CH3) and HS(CH2)15CONH(CH2CH2O)6H molecules and for several defect-containing conformers. These data were then used to simulate RA spectra of SAMs with different content of defects and to compare them with experiments. It is shown that for SAMs of HS(CH2CH2O)nCH3 (n = 5, 6) the pronounced asymmetry of the dominating band can be attributed to the multimode nature of COC stretching vibrations of helical conformers combined with the contribution from few percent of all-trans conformers. Arguments are presented which disprove appreciable amounts of helical conformers with single trans and/or gauche defects. Much more complex combination of factors, which can come into play in the formation of the high-frequency shoulder of COC band, is exemplified by self-assemblies of OEG-terminated amide-bridged alkanethiolates. In particular, spectral signatures of defects with inverted OH terminus are compared with other contributions to the apparent shape of COC band formation. For this family of SAMs, the presence of about 10% of all-trans conformers gives a satisfactory quantitative agreement between the calculated RA spectra and experimental observations.

Dynamic vulcanization of natural rubber/high-density polyethylene blends: Effect of compatibilization, blend ratio and curing system

Thermoplastic vulcanizates (TPVs) based on NR/HDPE blends with various types of blend compatibilizers were prepared. It was found that TPVs with phenolic resin (i.e., SP-1045 and HRJ-10518) compatibilizers showed higher tensile strength, elongation at break and tendency to recover from prolonged extension than those of the TPV without a compatibilizer. This may be attributed to Chroman ring structures from the reaction of NR and phenolic molecules. The TPVs with modified phenolic resins (PhSP-PE and PhHRJ-PE) showed higher tensile strength and elongation at break than those of the TPVs with un-modified phenolic rein and without a compatibilizer. A reaction took place through trace quantities of un-saturation in the HDPE molecules and methylol groups in the phenolic molecules. Also, the remaining methylol groups in the phenolic molecules were capable of reacting with NR molecules and produced a linkage between NR and HDPE molecules. Therefore, it was found that the TPV with PhHRJ-PE gave the highest tensile strength and elongation at break. Increasing the content of the NR gave an increased trend of elongation at break but decreasing trends in permanent set (expressed as tension set), tensile strength and hardness. Various vulcanization systems were also used to prepare the TPV, with a mixed curing system showing the highest shear stress and tensile strength, whilst the sulphur curing system gave the lowest values, and the peroxide curing system exhibited intermediate values.

Rheological, thermal and morphological properties of maleated natural rubber and its reactive blending with poly(methyl methacrylate)

Maleated natural rubbers (MNRs) were prepared by blending natural rubber with maleic anhydride (MA) at 135 °C for 10 min. We found the IR absorption peaks at 1784 and 1854 cm −1 due to the C O symmetric and assymetric stretching vibration of the grafted MA on the NR molecules, respectively. Increasing trends of the grafted MA were observed with increasing concentrations of MA in the graft reaction. Increasing Mooney viscosity and shear viscosity with increasing levels of MA monomer were also observed. The levels of chemical interaction and chemical crosslinks between the polar groups in the MNR molecules might be the reasons for these increasing trends. Themogravimetry and dynamic mechanical analysis were used to characterize the MNRs. It was found that the decomposition temperature ( T d) and glass transition temperature ( T g) of the MNRs increased with increasing levels of monomeric MA. Reactive blending of MNRs and poly(methyl methacrylate) (PMMA) was later prepared. In reactive blends of MNR/PMMA, the shear stress and shear viscosity increased until reaching the maximum at a MNR content of 60 wt%. Increasing MNRs higher than this value caused a decrease of the shear stress and viscosity. It was therefore concluded that the MNR/PMMA blends are compatible blends according to the log-additive rule. SEM micrographs were also analyzed. We observed decreasing sizes of dispersed PMMA domains as the MA concentration increased. That is, increasing levels of MA caused increasing levels of chemical interaction between the MNR and PMMA phases. As a consequence, decreasing sizes of the PMMA domains dispersed in the MNR matrix were observed.

Synchronous fluorescence, UV–visible spectrophotometric, and voltammetric studies of the competitive interaction of bis(1,10-phenanthroline)copper(II) complex and neutral red with DNA

Constant wavelength synchronous fluorescence spectroscopy (CW–SFS), UV–visible absorption spectroscopy, and cyclic and differential pulse voltammetry were applied to investigate the competitive interaction of DNA with the bis(1,10-phenanthroline)copper(II) complex cation ([Cu(phen) 2] 2+) and a fluorescence probe, neutral red dye (NR), in a tris–hydrogen chloride buffer (pH 7.4). The results show that both the [Cu(phen) 2] 2+and the NR molecules can intercalate competitively into the DNA double-helix structure. The cyclic voltammetry method showed that both anodic and cathodic currents of [Cu(phen) 2] 2+ decreased on addition of the DNA and the intercalated [Cu(phen) 2] 2+–DNA complex formed ( β = (4.14 ± 0.24) × 10 3). CW–SFS measurements were facilitated by the use of the three-way resolution of the CW–SFS for NR, [Cu(phen) 2] 2+, and NR–DNA. The important constant wavelength (CW) interval, Δ λ, was shown to vary considerably when optimized (135, 58, and 98 nm for NR, NR–DNA, and [Cu(phen) 2] 2+, respectively). This approach clearly avoided the errors that otherwise would have arisen from the common assumption that Δ λ is constant. Furthermore, a chemometrics approach, parallel factor analysis (PARAFAC), was applied to resolve the measured three-way CW–SFS data, and the results provided simultaneously the concentration information for the three reaction components, NR, [Cu(phen) 2] 2+, and NR–DNA, for the system at each equilibrium point. The PARAFAC analysis indicated that the intercalation of the [Cu(phen) 2] 2+ molecule into the DNA proceeds by exchanging with the NR probe and can be attributed to two parallel reactions. Comprehensive information was readily obtained; the replacement of the intercalated NR commenced immediately on introduction of [Cu(phen) 2] 2+, approximately 50% of NR was replaced by [Cu(phen) 2] 2+ at a concentration of 0.45 × 10 −5 mol L −1, and nearly all of the NR was replaced at a [Cu(phen) 2] 2+ concentration of 2.50 × 10 −5 mol L −1. This work has the potential to improve extraction of information from the fluorescence intercalator displacement (FID) assay.

Rheological, mechanical and morphological properties of thermoplastic vulcanizates based on NR‐g‐PMMA/PMMA blends

AbstractGraft copolymer of natural rubber and poly(methyl methacrylate) (NR‐g‐PMMA) was prepared using semi‐batch emulsion polymerization technique via bipolar redox initiation system. It was found that the grafted PMMA increased with the increase of methyl methacrylate (MMA) concentration used in the graft copolymerization. The NR‐g‐PMMA was later used to prepare thermoplastic vulcanizates (TPVs) by blending with PMMA through dynamic vulcanization technique. Conventional vulcanization (CV) and efficient sulphur vulcanization (EV) systems were studied. It was found that the CV system provided polymer melt with lower shear stress and viscosity at a given shear rate. This causes ease of processability of the TPVs via extrusion and injection molding processes. Furthermore, the TPVs with the CV system showed higher ultimate tensile strength and elongation. The results correspond to the morphological properties of the TPVs. That is, finer dispersion of the small vulcanized rubber particles were observed in the PMMA matrix. Various blend ratios of the NR‐g‐PMMA/PMMA blends using various types of NR‐g‐PMMA (i.e. prepared using various percentage molar ratios of NR and MMA) were later studied via dynamic vulcanization by a conventional sulphur vulcanization system. It was found that increasing the level of PMMA caused increasing trend of the tensile strength and hardness properties but decreasing level of elongation properties. Increasing level of the grafted PMMA in NR molecules showed the same trend of mechanical properties as in the case of increasing concentration of PMMA used as a blend component. From morphological studies, two phase morphologies were observed with a continuous PMMA phase and dispersed elastomeric phase. It was also found that more finely dispersed elastomeric phase was obtained with increasing the grafted PMMA in the NR molecules. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Effects of Accelerator Type and Curing Temperature on Crosslink Distributions and Tensile Properties of Natural-Acrylic Rubber Blends

Abstract Effects of curing temperature and accelerator type on tensile properties of natural-acrylic rubber blends (NR:ACM) were investigated. Three different types of accelerators namely, MBT, ZDEC and DPG, were separately compounded with the rubber blends (50/50, % w/w) along with stearic acid, ZnO, sodium stearate, and sulfur (S) on a two-roll mill. The compounded blends were vulcanized to their optimum cure time in a hydraulic press at 150 and 170 °C. Morphology and tensile properties of the rubber blends were determined by using a scanning electron microscope and a universal testing machine, respectively. Crosslinking characteristics of both NR and ACM phases in the rubber blends were investigated by using a swollen state 1H-NMR and a differential swelling technique. From the tensile test, it was found that strength and strain of the blend vulcanized with the S/ZDEC curing system were remarkably lower than those of the blends vulcanized with the S/MBT and S/DPG systems. The superior tensile properties of the two latter blending systems were accompanied by greater chain mobility of both of the NR and ACM molecules, along with a lower volume fraction of rubber in swollen gel (Vr) of the both NR and ACM phases. These changes were discussed in the light of various possibilities including curative distribution, which were affected by the accelerator type. In term of the effects of curing temperature, it was found that Vr and tensile properties of the blends did not significantly change with the temperature.

The grafting of maleic anhydride onto natural rubber

Graft copolymers of natural rubber and maleic anhydride (NR -g-MA) or maleated natural rubber (MNR) were synthesized in a toluene solution. Benzoyl peroxide (BPO) was used to initiate the free radical graft copolymerization. Influence of monomer and initiator concentrations together with effect of the reaction temperature and reaction time were investigated. Quantities of the grafted anhydride (MA) were determined by titration of the carboxylic acid functions derived from the anhydride functions. Estimation of the grafted MA level was also performed using IR absorbance ratio of the peaks at 1780–1784 cm −1 plus 1854 cm −1 to 835 cm −1. We found that quantities of the grafted MA on NR molecules increase with increasing monomer and initiator concentrations. The increase in reaction time and reaction temperature also causes increasing levels of the grafted MA. We also found that the glass transition temperature of the MNR was higher than that of the pure NR. Furthermore, the T g values increase with increasing the monomer concentration used in the graft copolymerization.

Metabolic activators of spinach leaf nitrate reductase: Effects on enzymatic activity and dephosphorylation by endogenous phosphatases

Nitrate reductase (NR; EC 1.6.6.1) in spinach (Spinacia oleracea L.) leaves was inactivated in the dark and reactivated by light in vivo. When extracted from dark leaves, NR activity was lower and more strongly inhibited by Mg2+ relative to the enzyme extracted from leaves harvested in the light. When dark extracts were desalted at pH 6.5 and preincubated at 25° C prior to assay, enzyme activity (assayed either in the presence or absence of Mg2+) remained essentially constant, i.e. there was no spontaneous reactivation in vitro. However, addition of certain metabolites resulted in a time- and concentration-dependent activation of NR in vitro. Effective activators included inorganic phosphate (Pi), 5′-AMP, and certain of its derivatives such as FAD and pyridine nucleotides (both oxidized and reduced forms). All of the activators increased NR activity as assayed in the absence of Mg2+, whereas some activators (e.g. Pi, 5′-AMP and FAD) also reduced Mg2+ inhibition. The reduction of Mg2+ inhibition was also time-dependent and was almost completely prevented by a combination of okadaic acid plus KF, suggesting the involvement of dephosphorylation catalyzed by endogenous phosphatase(s). In contrast, the activation of NR (assayed minus Mg2+) was relatively insensitive to phosphatase inhibitors, indicating a different mechanism was involved. Compounds that were not effective activators of NR included sulfate, ribose-5-phosphate, adenosine 5′-monosulfate, coenzyme A, ADP and ATP. We postulate that NR can exist in at least two states that differ in enzymatic activity. The activators appear to interact with the NR molecule at a site distinct from the NADH active site, and induce a slow conformational change (hysteresis) that increases NR activity (assayed in the absence of Mg2+). Possibly as a result of the conformational change caused by certain activators, the regulatory phospho-seryl groups are more readily dephosphorylated by endogenous phosphatases, thereby reducing sensitivity to Mg2+ inhibition. Preliminary results suggest that light/dark transitions in vivo may alter the distribution of NR molecules between the low- and high-activity forms.

Imaging with reduced data : 38398 Hanstead, P.D. Ultrasonics International 87, Conference Proceedings, London (United Kingdom), 6–9 Jul. 1987. pp. 12–19. Butterworth and Co. (Publishers) Ltd., Guildford, Surrey (1987)

Maleated natural rubbers (MNRs) were prepared by blending natural rubber with maleic anhydride (MA) at 135 °C for 10 min. We found the IR absorption peaks at 1784 and 1854 cm−1 due to the CO symmetric and assymetric stretching vibration of the grafted MA on the NR molecules, respectively. Increasing trends of the grafted MA were observed with increasing concentrations of MA in the graft reaction. Increasing Mooney viscosity and shear viscosity with increasing levels of MA monomer were also observed. The levels of chemical interaction and chemical crosslinks between the polar groups in the MNR molecules might be the reasons for these increasing trends. Themogravimetry and dynamic mechanical analysis were used to characterize the MNRs. It was found that the decomposition temperature (Td) and glass transition temperature (Tg) of the MNRs increased with increasing levels of monomeric MA. Reactive blending of MNRs and poly(methyl methacrylate) (PMMA) was later prepared. In reactive blends of MNR/PMMA, the shear stress and shear viscosity increased until reaching the maximum at a MNR content of 60 wt%. Increasing MNRs higher than this value caused a decrease of the shear stress and viscosity. It was therefore concluded that the MNR/PMMA blends are compatible blends according to the log-additive rule. SEM micrographs were also analyzed. We observed decreasing sizes of dispersed PMMA domains as the MA concentration increased. That is, increasing levels of MA caused increasing levels of chemical interaction between the MNR and PMMA phases. As a consequence, decreasing sizes of the PMMA domains dispersed in the MNR matrix were observed.

Detection of the Molecular Orientation in Nonvulcanized and Vulcanized Natural Rubber Compounds by the Low-Temperature X-Ray Method

Abstract The low-temperature x-ray method can detect the orientation of NR molecules in both vulcanizate samples and nonvulcanizate samples of NR compounds. The presence of carbon black is important for causing the orientation. NR molecules orient in the direction of stretch-relaxation in the case of uniaxially fatigued vulcanizates or in the direction of shear in the case of nonvulcanizates prepared by various processing methods. The orientation of NR molecules in nonvulcanizates usually disappears when vulcanized, but a fairly large extent of molecular orientation remains, even after vulcanization, if there exist crosslinks in the oriented nonvulcanizate and the sample is prevented from shrinkage.

MNDO study of the E-Z isomerization mechanism of H2C=NR molecules (R = H, CH3, NH2, OH, F)

The geometry and E-Z isomerization mechanism were studied by the MNDO method for H2C=NR molecules with R = H, CH3, NH2, OH and F. All the molecules save H2C=NNH2 possess a plane of symmetry and isomerize by the inversion mechanism, whereas H2C=NNH2 has a nonsymmetrical geometry and the inversion-rotation mechanism is more favourable for its isomerization. Dependent considerably on the substituent at the nitrogen atom, the isomerization barrier height increases in order R = CH3, H, NH2, OH, F.

Tack and Green Strength of NR, SBR and NR/SBR Blends

Abstract 1. A simple NR compound has superior tack compared to an SBR stock because of its greater ability to flow under compressive load and its higher green strength. The difference becomes greater as surface roughness is increased or test rate decreased. Tack differences between the two stocks can be quantitatively explained without assuming differences in the interdiffusion rate of SBR and NR molecules. NR is an “ideal” material for developing high tack. It can be processed to a low viscosity and still maintain high green strength. Furthermore, the mechanism responsible for the high green strength (strain crystallization) is not active in the bond formation step, hence does not interfere with contact and interdiffusion, but rather develops upon stressing. SBR, on the other hand, depends on entanglements, which tend to inhibit flow, to achieve green strength. Thus, if the molecular weight of the SBR is raised to enhance green strength to the level of NR, the stock will flow very poorly. Likewise, if SBR molecular weight is lowered, flow can be improved substantially, but green strength will decrease rapidly. Either of the above conditions result in poor tack. 2. For smooth, shiny surfaces brought together, contact occurs rapidly and nearly completely for both the NR and SBR stocks. This results in a relative tack near unity and hence, in this case, the tack strength is really a measure of stock green strength, i.e., tack is green strength limited. Obviously, when this condition exists, the addition to the stock of plasticizers, tackifiers, oils or other additives to facilitate stock flow (but decrease green strength) will nontheless reduce tack. Here, tack can only be increased by enhancing stock green strength, preferable without hindering flow ability. 3. The relative tack of SBR and SBR-dominated blends with moderately rough surfaces is less than unity and increases with contact pressure. Thus, tack is contact limited. In this instance, relative tack can be improved by decreasing stock viscosity, however, absolute tack will only be increased if good green strength is maintained. This may be the mechanism for the action of commencal tackifiers. 4. The tack of NR/SBR blends passes through a maximum with composition at about 80% NR. This is readily understood by noting that the green strength also reaches a maximum at this composition. For NR/SBR blends containing 60% or more of NR, green strength plays the dominant role in controlling tack. 5. Uncured adhesion of SBR to NR is less than the tack of either pure component. This suggests that interdiffusion is required to obtain high tack.