Determination Of Crosslink Density Research Articles

Effects of Peroxide and Sulfur Curing Systems on Physical and Mechanical Properties of Nitrile Rubber Composites: A Comparative Study.

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.

Measurement of cross link densities of prevulcanized natural rubber latex and latex products using low-cost near infrared spectrometer

The objective of this project is to use the four models to predict the crosslink densities based on reference method of PRM 300% using portable low-cost visible and shortwave near-infrared (Vis/SW-NIR) spectrometer across wavelengths of 350-1100 nm. The degrees of crosslink reflect on the properties of the prevulcanized (PV) latices and latex products produced and hence useful for quality control in the process of production. The four models were optimized using partial least squares regression (PLSR) that the spectra were collected from PV, PV50, thin and thick films. The use of thin and thick films was to emulate the latex products that used the PV latices. The results showed that there were almost no difference in the PV and PV50 models by the coefficient of determination (R2) and root mean square error of calibration (RMSEC) of 0.70 and 9.04 × 104 N/m2 and 0.75 and 8.59 × 104 N/m2, respectively. Among the four models, thin and thick film models had poor results indicated by the coefficient of determination (R2) and root mean square error of calibration (RMSEC) of 0.02 and 17.31 × 104 N/m2 and 0.05 and 16.63 × 104 N/m2, respectively. Based on these results, only the models of prevulcanized (PV) latices could be used for quality assessment. Hence the Vis/SW-NIR spectrometer could be a cheap alternative for crosslink density determination of PV latices and would be a beneficial tool for monitoring the process of vulcanization in the rubber industry.

Influence of e-Beam Irradiation on the Physicochemical Properties of Poly(polyol Succinate-co-Butylene Succinate) Ester Elastomers.

The purpose of this research was synthesis and electron beam modification of novel ester elastomers consisting of sugar alcohol–succinic acid block and butylene glycol–succinic acid block. Four different alditols were used in the synthesis—sorbitol, erythritol, xylitol, and glycerol. The materials were irradiated with doses of 50, 100, and 150 kGy in order to determine which dose is the most beneficial. As expected, irradiation of the materials has led to the cross-link density becoming higher and improvement of the mechanical properties. Additionally, the materials were also sterilized in the process. The great advantage of elastomers described in the paper is the fact that they do not need chemical cross-linking agents or sensitizers in order to undergo radiation modification. The following tests were performed on cross-linked poly(polyol succinate-co-butylene succinate) elastomers: quasi-static tensile test, determination of cross-link density, differential scanning calorimetry (DSC), dynamic thermomechanical analysis (DMTA), wettability (water contact angle), and Fourier transform infrared spectroscopy (FTIR). In order to confirm successful synthesis, prepolymers were analyzed by nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR).

Innovative γ rays irradiated styrene butadiene rubber/reclaimed waste tire rubber blends: a comparative study using mechano-chemical and microwave devulcanizing methods

Abstract Waste tire rubber was comparatively devulcanized by using two-roll mill mechano-chemical and microwave techniques at room temperature. The former technique was performed utilizing tetramethylthiuram disulfide and mercaptobenzothiazole disulfide. The developed devulcanized elastomer was characterized by scanning electron microscopy, chemical soluble fraction indication, and cross-link density determination. The blend was mixed in two roll mills by replacing a portion of virgin styrene-butadiene rubber (SBR) in a common formulation with the devulcanized waste rubber (DWR) product at various ratios, namely 10, 20 and 50 wt%. The morphological micrographs confirmed marked improvement in compatibility between both rubbery materials. The tensile strength and elastic modulus examinations of the fabricated blends ensured successful substitution of the virgin SBR with DWR. The abrasion resistance of SBR proved unaffected by blending with DWR. The compounded blends were subjected to γ rays at different radiation doses elevated up to 200 kGy and comparatively mechanically investigated.

Direct Determination of Cross-Link Density and Its Correlation with the Elastic Modulus of a Gel with Slidable Cross-Links.

The universal relationship between the elastic modulus and the cross-link density of a conventional rubber/gel has been demonstrated experimentally to be inapplicable to gels with slidable cross-links. Herein, we describe the synthesis of slide-ring (SR) gel networks devoid of intramolecular cross-links by the cross-coupling of two differently functionalized polyrotaxanes. The cross-link density was determined from the characteristic UV absorption attributed to the asymmetric cross-linked moiety. The cross-link density was shown to correlate considerably more weakly with the Young's modulus than conventional gels and rubbers that follow a universal proportional dependence. In addition, even at a similar cross-link density, the modulus appeared to be lower due to a lower density of cyclic components along the threading chain, i.e., the "coverage", though the data were limited in the narrow cross-link density range. These results might suggest a considerably lower contribution from the conformational entropy of chains associated with sliding through the cross-links and the counteracting entropy attributed to ring arrangement, though effects of the different persistence length due to the coverage difference could affect the modulus.

Synthesis and Investigation on Viscoelastic Properties of Urethane Acrylate- Polyaniline

The aim of this research is to investigate the viscoelastic behavior and determination of cross-link density of thin conductive coatings of urethane acrylate-polyaniline. For this purpose, polyaniline and urethane acrylate resin were synthesized. Nanocomposites with different percentages of polyaniline (1, 3, and 5%) were prepared and cured by ultraviolet irradiation. The viscoelastic properties of prepared thin films were evaluated by dynamic mechanical thermal analysis under stresses with a certain frequency at different temperatures. Also, cross-link density of nanocomposites was measured after ultraviolet curing. The results indicated that the glass transition temperature decreases and the cross-link density increases with the amount of polyaniline.

Revisiting Segmental Order: A Simplified Approach for Sulfur-Cured Rubbers Considering Junction Fluctuations and Entanglements

The use of modern multiple-quantum proton NMR experiments for the determination of cross-link density requires precise knowledge of several model-dependent physical and structural quantities, like the dipolar static frequency or the definition of vector segmental order. In this paper, different models for describing segmental order of the polymer backbone, based on different assumptions about the contribution of cross-links and entanglements, are critically reviewed and applied for the analysis of unfilled natural rubber samples with average mass between cross-links and entanglements determined from network tube model fittings of stress–strain data. A recent theoretical model developed by Lang and Sommer, which allows for the consideration of junction fluctuations, is adapted for the analysis of NMR experimental results. After having verified the correlation between the calculations with this enhanced theoretical treatment and the residual dipolar coupling from multiple-quantum NMR experiments carried out...

Improvement of elastomer swelling measurement for sulfur cross-link density determination

Due to growing demand from both environmental and industrial sectors the extent of rubber recycling is constantly increasing. Recycled rubber ranges from slabs of thermochemical reclaim, resembling virgin rubber, to fine rubber powders of diverse surface morphologies. Cross-Link Density (CLD) changes may characterize the effectiveness of rubber processing, but their measurements are often problematic. Most difficulties arise during determination of the volume fraction of polymer in the swollen rubber gel, also known as the Vr parameter (comparable to swell ratio), which closely relates to CLD. Vacuum, blotting and centrifugation were used to remove a swelling solvent (benzene) from the surface of the swollen rubber matrix. Centrifugation was more accurate for evaluating Vr of morphologically different types of rubber. Mechanical shear was used to produce a porous rubber sheet, while addition of acetamide and NaCl generated powder of devulcanized rubber. These samples were tested along with tire buffings and SBR sheets to show that extraction duration and time, needed to establish the swelling equilibrium between rubber and benzene, could be reduced from 72 h to less than 24 h in the case of porous or powdered rubbers. While compressed rubber might need longer durations, the overall time consumption for the CLD analysis could be shortened significantly, which helps in evaluating recycled rubber more thoroughly.

Epoxidized natural rubber and hydrotalcite compounds: rheological and thermal characterization

Abstract Epoxidized natural rubber (ENR) and synthetic non-modified hydrotalcite (HT) compounds were prepared and evaluated. Natural rubber (NR) was epoxidized with 20.6% of epoxy groups from a chemical modification of the latex. A sulfur-based curing system formulation with accelerators was used. The amounts of HT in the ENR-HT compositions was varied between 0, 2, 3 and 5 phr. All compositions were evaluated as to cure parameters, rheological properties, thermal resistance and crosslink density. The results showed that the mineral filler does not have a significant influence on the cure parameters. Different methods of crosslink density determination were used (swelling at equilibrium and elastic modulus). The results turn out to be equivalent and rise as the amount of filler is increased. The best results were found for the 5 phr hydrotalcite compound (ENR-HT5).

Rubber mixing process and its relationship with bound rubber and crosslink density

This research studied the relationship between bound rubber and crosslink density based on rubber mixing process. Bound rubber was obtained after natural rubber was masticated and mixed with rubber chemicals and filler while crosslink density was collected after rubber compound was vulcanized. Four methods are used and each method refers to four ways of incorporating carbon black during mixing. The first method, after rubber was masticated for 5 minutes, the addition of rubber chemicals and filler was done simultaneously. Rubber was masticated for 1 minute and continued mixing of rubber chemicals and filler where mixing was different from first method. This was the second method. The third method was the same as the second method but the filler used N 660 while in the second method N 330. The last method is not the same as the first and second, the rubber is only masticated for 3 minutes and then mixed with filler and followed by rubber chemicals sequentially. The results showed that bound rubber and crosslink density were influenced by mixing and mastication process. Bound rubber dropped and crosslink density was relatively stable in the first three mixing methods for increasing carbon black at the beginning of the mixing process. Bound rubber and crosslink density stated opposite results in the fourth mixing method. The higher the bound rubber the lower the crosslink density. Without regard to mixing methods, there is a non-linear relationship between bound rubber formation and crosslink density determination

Rubber composites cured with sulphur and peroxide and incorporated with strontium ferrite

In the present work, rubber magnetic composites were prepared by incorporation of strontium ferrite into rubber matrices based on natural rubber (NR) and acrylonitrile–butadiene rubber (NBR). Sulphur, peroxide and mixed sulphur/peroxide curing systems were introduced in cross-linking of rubber matrices. The aim was to investigate the influence of curing system composition on physical–mechanical, thermo-physical and magnetic properties of prepared rubber composites and the curing process. Then, the determination of cross-link density and the structure of cross-links were under consideration. The achieved results showed that all investigated parameters were changed depending on the composition of curing system, but also on the type of rubber matrix. While the tensile strength of composites based on NR increased with increasing amount of sulphur in mixed curing systems, in case of composites based on NBR, the highest value of tensile strength reached the value of the sample cured with equivalent ratio of sulphur and peroxide. On the other hand, thermo-physical and magnetic characteristics were found not to be dependent on the curing system composition. The results revealed that not only the composition of curing system, but also the type of rubber matrix plays an important role when preparing the final materials.

Antioxidative behavior of a novel samarium complex in styrene-butadiene rubber/silica composites

The focus of this study was on the thermo-oxidative aging resistance of a new rare earth complex, samarium lysine dithiocarbamate (Sm-LDC), in the styrene-butadiene rubber (SBR)/silica composites. The anti-aging behavior of SBR/silica composites with Sm-LDC, as well as several common commercial antioxidants, was systematically investigated by oxidation induction time (OIT), mechanical testing, crosslink density determination, Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), and thermo-gravimetric analysis (TGA). The results demonstrated that Sm-LDC significantly improve the thermo-oxidative stability of SBR/silica composites and protect the composites against the thermo-oxidative aging more effectively than the widely used antioxidant 4010NA and antioxidant MB in rubber industry. It could be attributed to the synergistic effect between dithiocarboxyl groups and samarium ions in Sm-LDC, which can high-efficiently decompose the hydroperoxide and scavenge the oxy radicals, respectively. Besides, the results of whiteness and yellowness index (Y.I.) analysis indicated that Sm-LDC can hardly cause discoloration to the SBR/silica composites and is better than the recognized light-colored antioxidant MB in the aspect of color. This research might open up new opportunities for preparing highly thermo-oxidative aging-resistant rubber composites without color contamination.

Rubber Composites Based on Polar Elastomers with Incorporated Modified and Unmodified Magnetic Filler

Rubber magnetic composites were prepared by incorporation of unmodified and surface modified strontium ferrite into rubber matrices based on NBR and NBR/PVC. Strontium ferrite was dosed to the rubber matrices in concentration scale ranging from 0 to 100 phr. The main goal was to investigate the influence of the type of ferrite on the curing process, physical-mechanical and magnetic properties of composites. The mutual interactions between the filler and rubber matrices were investigated by determination of cross-link density and SEM analysis. The incorporation of magnetic fillers leads to the increase of cross-link density and remanent magnetic induction of composites. Moreover, the improvement of physical-mechanical properties was achieved in dependence on the content of magnetic fillers. Surface modification of ferrite contributed to the enhancement of adhesion on the interphase filler-rubber. It can be stated that ferrite exhibits reinforcing effect in the composite materials and this reinforcing behavior was emphasized with the increase in polarity of the rubber matrix.

Reinforcing styrene butadiene rubber with lignin-novolac epoxy resin networks

In this study, lignin-novolac epoxy resin networks were fabricated in the styrene butadiene rubber (SBR) matrix by combination of latex compounding and melt mixing. Firstly, SBR/lignin compounds were co-coagulated by SBR latex and lignin aqueous solution. Then the novolac epoxy resin (F51) was added in the SBR/lignin compounds by melt com- pounding method. F51 was directly cured by lignin via the ring-opening reaction of epoxy groups of F51 and OH groups (or COOH groups) of lignin during the curing process of rubber compounds, as was particularly evident from Fourier trans- form infrared spectroscopy (FTIR) studies and maximum torque of the curing analysis. The existence of lignin-F51 net- works were also detected by scanning electron microscope (SEM) and dynamic mechanical analysis (DMA). The structure of the SBR/lignin/F51 was also characterized by rubber process analyzer (RPA), thermogravimetric analysis (TGA) and determination of crosslinking density. Due to rigid lignin-F51 networks achieved in SBR/lignin/F51 composites, it was found that the hardness, modulus, tear strength, crosslinking density, the temperature of 5 and 10% weight-loss were signif- icantly enhanced with the loading of F51.

Studies on the Mechanical Properties of Carbon Black/Halloysite Nanotube Hybrid Fillers in Nitrile Rubber Nanocomposites

Carbon black (CB)-Halloysite nanotubes(HNT) hybrid composites based on Acrylonitrile Butadiene rubber(NBR) were prepared. The Carbon black loading was kept fixed at 60 phr and Halloysite nanotube loading was varied from 0 to 10 parts per hundred rubber(phr). The nanocomposites were prepared by mixing using a two-roll open mill. The effect of CB-HNT hybrid fillers on the curing behaviour, mechanical properties and morphology of the NBR nanocomposites was investigated. The cure characteristics showed that the scorch time(ts2) and cure time (t 90) increased with the HNT loading. The maximum torque(MH) exhibited an increasing trend from 0 to 6 phr . The tensile properties which exhibited an increasing trend until an optimum loading of 6phr were mainly influenced by the NBR-CB-HNT interaction and the intercalation of rubber and CB into the tubules. These results were well supported by the XRD studies and cross link density determinations.

Fabrication of partially exfoliated and disordered intercalated cloisite epoxy nanocomposites via in situ polymerization: Mechanical, dynamic mechanical, thermal and barrier properties

Clay minerals, especially montmorillonite (Mt), have emerged as a subject of enormous scientific interest due to their unique atomic structure, high cation exchange capacity, high aspect ratio and large surface area. Clay polymer nanocomposites have established a unique position among technologically important materials because of their extensive and potential applications. Herein we have fabricated cloisite epoxy nanocomposites (CPNs) from diglycidyl ether of bisphenol A (DGEBA) epoxy and modified nanoclay, cloisite 93A using triethylenetetramine (TETA) as a curing agent. The major objective was to examine the static and dynamic mechanical properties and the performance of these CPNs for barrier resistance and thermal stability. Determination of cross link density (CLD) of CPNs by Flory–Rehner equation and activation energy by Coats–Redfern procedure were also a focus of the present study. The cloisite layer exfoliation in CPNs was monitored by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and further corroborated using transmission electron microscopy (TEM). Static mechanical properties of CPN were characterized in terms of tensile and flexural properties. Moreover, dynamic mechanical and thermal properties were evaluated by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) respectively. XRD and TEM revealed that the cloisite distribution in the epoxy matrix is of the partially exfoliated kind at lower level loadings and intercalated at higher loadings. The enhancement of mechanical properties of CPNs was to the level of 15% increase in tensile strength and 23% in flexural strength at very low cloisite loadings. The interfacial interactions brought about by cloisite 93A also benefited the thermomechanical properties to a large extent in the form of improved glass transition temperature and an impressive enhancement in storage modulus. Results based on equilibrium swelling experiments revealed that a maximum of 61% increase in barrier performance was achieved by the addition of 5 phr (per hundred gram of resin) cloisite. CLD measurements demonstrated an 11% increase in CLD at 1 phr cloisite loading. TGA results revealed that cloisite can impart greater thermal stability to pure epoxy. All the properties were correlated with the dispersion state of the cloisite in the epoxy resin matrix.

Determination of Crosslink Density and Network Structure of NR Vulcanizates by Means of TSSR

Unfilled vulcanizates based on natural rubber (NR) were investigated by temperature scanning stress relaxation (TSSR) measurements. Different sulfur based cure systems, i.e. conventional (CV), semi-efficient (SEV) and efficient (EV) vulcanization system, were used to prepare the vulcanizates. It was found that sulfur/accelerator - ratio has a strong impact on the shape of the relaxation spectrum, deduced from TSSR measurements. By deconvolution of the relaxation spectra, peak separation was performed and 3 different peaks were found in case of CV - cured samples. In contrast, only a single peak was found, in case of the EV-cured sample. After thiolamine treatment the shape of the relaxation spectra altered significantly in case of the CV-cured sample whereas the spectra of the SEV-and EV-cured sample exhibited only slight differences. Additionally, the crosslink density of the samples decreased after thiolamine treatment. This is due to selective cleavage of polysulfidic crosslinks. It has been concluded, that the significant peak in the relaxation spectrum at about 120 °C can be attributed to the cleavage of polysulfidic crosslinks. Furthermore, a linear relationship between the percentage of polysulfidic crosslinks and the sulfur/accelerator - ratio is assumed.

Determination of crosslinking density of hydrogels prepared from microcrystalline cellulose

AbstractHydrogels with four different plant‐based microcrystalline cellulose concentrations were prepared using the self‐assembly technique. The interaction parameter between cellulose and water was determined by the classical Flory‐Huggins theory, and was found to be around 0.44 with weak concentration dependence. The crosslinking density in these hydrogels was measured by both the Mooney‐Rivlin equation and the Flory‐Rehner theory. Reasonable consistency was found between the two methods albeit results from the Flory‐Rehner theory were slightly higher due to the contribution from the physical crosslinks. The crosslinking density values for all four hydrogels determined from both methods were found to range from 19 to 56 mol/m3. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Understanding the Regeneration of EPDM Rubber Crumbs from Used Tyres

In this work, EPDM rubber crumbs from used tyres have been treated in batch and continuous processes in order to produce a regenerated material. For this purpose, the effect of EPDM particle size, oil type, and concentration as well as processing time, temperature, and mixing intensity were controlled for treatment in an internal batch mixer and a twin-screw extruder. The degree of regeneration was quantified via a modification of the ASTM D6814 method based on crosslink density determination from swelling experiments. The results showed that crosslink density reduction can occur even without any addition of a chemical agent. It is also shown that some conditions can lead to an increase in crosslink density. Physical models are proposed to account for both increase and decrease in crosslink density with processing conditions.

Reinforcement of nitrile rubber by in situ formed zinc disorbate

Zinc disorbate (ZDS) was in situ formed by the reaction between sorbic acid (SA) and zinc oxide (ZnO) in nitrile rubber (NBR). The effects of SA amount on the curing characteristics, crosslink density and mechanical properties of per- oxide-cured NBR were studied. The results showed that ZDS was generated mainly during the rubber vulcanization, rather than the open mill compounding phase. The results from the crosslink density determinations showed that the formation of ZDS significantly increased the ionic bond content in the vulcanizates. In addition, the formation of ZDS greatly enhanced the mechanical properties of NBR vulcanizates. The modulus, tensile strength, tear strength and hardness were found to be increased with the loading of ZDS. Preheating the compounds before compression moulding was beneficial to the forma- tion of ZDS, and consequently the increases in mechanical properties. At 40 parts per hundred rubber (phr) of SA and 16 phr ZnO, five to six folds of tensile strength and tear strength of the neat NBR vulcanizate were achieved. Transmission electron microscopy (TEM) results confirmed the nano-dispersion structure of the polymerized ZDS in the NBR matrix.