Gordana Marković

The effect of gamma radiation on the ageing of sulfur cured NR/CSM and NBR/CSM rubber blends reinforced by carbon black

In this work the effect of the γ-radiation dose on ageing of carbon black reinforced elastomeric materials was studied. The compounds based on natural rubber/chlorosulfonated rubber blend (NR/CSM) and butadiene acrylonitrile rubber/chlorosulfonated rubber blend (NBR/CSM) (50:50, w/w) with different loadings (0, 20, 40, 50, 60, 80 and 100 phr) of the filler with the average particle size of 40 nm were cured by sulfur. The obtained elastomeric composites were subjected to radiation doses (100, 200, 300 and 400 kGy) in the presence of oxygen. The changes of material mechanical properties were estimated after radiation accelerated ageing. By using Fourier transform infrared measurements (ATR-FTIR) it was assessed that after exposure to doses of 100 kGy alcohols, ethers, lactones, anhydrides, esters and carboxylic acids are formed in materials. The formation of shorter polyene sequences and aromatic rings in aged samples are assumed on the basis of the obtained spectra.

The Effect of Accelerators on Curing Characteristics and Properties of Natural Rubber/Chlorosulphonated Polyethylene Rubber Blend

The aim of this work was to determine the influence of accelerator type on curing behavior and properties of sulphur vulcanized rubber blend based on natural rubber (NR) and chlorosulphonated polyethylene (CSM). In NR/CSM compounds filled with carbon black three types of accelerator were used: N-cyclohexylbenzothiazylsulphenamide (CBS), tetramethylthiuram disulphide (TMTD), and 2-mercaptobenzothiazol (MBT). The curing characteristics were estimated using oscillating disk rheometer. The results revealed that the accelerator type not only affects the cure characteristics, but also has great influence on the mechanical properties of obtained elastomers. It was determined that the tensile strength of rubber blends cured in the presence of TMTD was relatively high.

Thermal stability of γ-irradiated chlorinated isobutylene–isoprene copolymer/chlorosulphonated polyethylene rubber blend/carbon black nanocomposites

In this study, the effect of γ-rays on the mechanical and thermal properties of polymer composites based on chlorinated isobutylene–isoprene rubber/chlorosulphonated polyethylene rubber blend with varying contents of carbon black (CB) filler was investigated. The samples were irradiated at ambient conditions with 100, 200 and 400 kGy radiation doses. Tensile strength and hardness are increasing with CB content and radiation dose is increasing, while elongation at break is decreasing. Loss of mass of 0.5, 10 and 30% was calculated for the samples from their respective thermogravimetric curves. The thermal stability of the nanocomposites was improved by both the degree of loading with filler and the cross-linking induced by γ-irradiation.

Characterization of composites based on chlorosulfonated polyethylene rubber/chlorinated natural rubber/waste rubber powder rubber blends

Chlorosulfonated polyethylene rubber (CSM) was blended with chlorinated natural rubber (CNR) with various formulations and blend ratios (CSM/CNR: 100/0; 75/25; 50/50; 25/75; 0/100) keeping the total waste rubber powder (WRP) content constant at 50 phr (parts per 100 rubber). Rheological, mechanical, dynamic mechanical and thermal aging properties as well as irradiation resistance were used as characterization of the blends. The amount of CNR in blends significantly affected the properties of the blends. The CSM/CNR/WRP rubber blend (50/50/50) possessed higher tensile strength compared with pure CSM and CNR rubber even after irradiation or thermal aging. Modulus, tensile strength and hardness of the blends appeared to increase, but elongation at break decreased progressively with increasing CNR content. These properties decreased in rubber blends after thermal aging. After irradiation, hardness, modulus and tensile strength increased up to 200 kGy and then decreased significantly for the blends with high CNR content, whereas no change in modulus was observed. CNR and CSM showed damping peaks at about 65 and −45°C, respectively, and these values correlate with the glass transition temperatures (T gs) of CNR and CSM, respectively. The shift in the T g values was observed after blending, suggesting an interfacial interaction between the two phases probably caused by the covulcanization in CSM/CNR blends.

Radiation stability of nanosilica-based urea–formaldehyde composite materials:

Three types of nanosilica-based urea–formaldehyde (UF) hybrid composite materials with a formaldehyde-to-urea (F/U) ratio of 0.8 were synthesized (UF with SiO2—Resin 1, UF + SiO2 + thiourea [TU]—Resin 2 and UF + SiO2 + thiosemicarbazide [TSC]—Resin 3). The thermal behavior of obtained materials was studied by nonisothermal thermogravimetric analysis (TGA), differential thermal gravimetry (DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) and supported by data from Fourier transform infrared spectroscopy (FTIR). Nanosilica-based UF hybrid composites have been irradiated (50 kGy) and after that their radiation stability was evaluated on the basis of thermal behavior. The percentage of free formaldehyde in all prepared samples was determined. DTG peaks of unmodified nanosilica-based UF resin (UF with SiO2) are shifted to a high temperature. The minimum percentage values of free formaldehyde (3%) for samples based on UF with SiO2 and UF + SiO2 + TSC after irradiation dose of 50 kGy are detected.

Properties of Vulcanized Polyisoprene Rubber Composites Filled with Opalized White Tuff and Precipitated Silica

Opalized white tuff (OWT) with 40 μm average particle size and 39.3 m2/g specific surface area has been introduced into polyisoprene rubber (NR). Their reinforcing effects were evaluated by comparisons with those from precipitated silica (PSi). The cure characteristic, apparent activation energy of cross-link (E ac) and reversion (E ar), and mechanical properties of a variety of composites based on these rubbers were studied. This was done using vulcanization techniques, mechanical testing, and scanning electron microscopy (SEM). The results showed that OWT can greatly improve the vulcanizing process by shortening the time of optimum cure (t c90) and the scorch time (t s2) of cross-linked rubber composites, which improves production efficiency and operational security. The rubber composites filled with 50 phr of OWT were found to have good mechanical and elastomeric properties. The tensile strengths of the NR/OWT composites are close to those of NR/PSi composites, but the tear strength and modulus are not as good as the corresponding properties of those containing precipitated silica. Morphology results revealed that the OWT is poorly dispersed in the rubber matrix. According to that, the lower interactions between OWT and polyisoprene rubber macromolecules are obtained, but similar mechanical properties of NR/OWT (100/50) rubber composites compared with NR/PSi (100/50) rubber composites are resulted.

Hybrid materials based on brominated copolymer isobutylene isoprene/chlorosulfonated polyethylene rubber blends reinforced by nano and micro silica

The effects of different particle size of silica (nano—primary particle size 15 nm and micro—primary particle size 28 μm) on the cure, mechanical, and swelling properties of hybrid materials based on brominated copolymer isobutylene isoprene/chlorosulfonated polyethylene rubber blend were studied. The enhancement in mechanical properties was supported by the data on the increased content of cross-link density obtained from swelling and stress–strain analyses. The interactions between rubber matrix and filler were investigated by Fourier transform infrared spectroscopy with attenuated total reflectance extension and scanning electron microscope. T g of reinforced blend shifted to −26°C and −30°C for nano and micro silica compounds, respectively.

Nanosilica and wood flour-modified urea–formaldehyde composites

In this study, the thermal behavior of modified urea–formaldehyde (UF) resin with nanosilica (nano-SiO2), wood flour (WF), and their mixture of SiO2/WF was investigated. Five modified UF hybrid composite materials with 0.8 F/U ratio with different filler were synthesized using the same procedure. The thermal behavior of materials was studied using nonisothermal thermogravimetric analysis, differential thermal gravimetry (DTG), and differential thermal analysis and supported by data from infrared spectroscopy. The shift of DTG peaks to a high temperature indicates the increase in thermal stability of modified UF resin with hybrid SiO2/WF fillers, which is confirmed by the data obtained from the Fourier transform infrared spectroscopic study. It was estimated that the UF/WF samples based on nano-SiO2 have better thermal stability.

Modeling of Non-Linear Viscoelastic Behavior of Filled Rubbers

The nonlinear viscoelastic behavior of the composites of rubber filled with carbon black, silica, carbon nanotube (CNT), clay and surface-modified nanosilica were studied. The behavior of carbon black-filled rubber is thoroughly analyzed with the intention of developing a constitutive model able to reproduce both static and dynamic material responses. Several nonlinear viscoelastic models have been examined thoroughly and for each of them advantages and disadvantages are highlighted. A series of experiments concerning both static and dynamic tests were performed aimed at measuring all the relevant nonlinear effects. Temperature and strain rate dependencies were investigated and discussed. The standard methodology was applied to perform both tensile and compressive quasi-static tests. Some shortcomings of this procedure, resulting in a unreliable stress-strain constitutive curve around the undeformed configuration, were identified. This lead to the design a non-standard cylindrical specimen able to bear both tensile and compressive loading. Consequently, the influence of the shape factor was removed and the same boundary conditions, in tension and compression, were applied. This allowed the stiffness around the undeformed configuration to be evaluated in detail. The quasi-static experimental results also allowed the influence of the Mullins effect on the quasi-static response to be investigated: during the loading cycles, there is a significant reduction in the stress at a given level of strain, which is a consequence of the internal material rearrangement, i.e., the Mullins effect. This damage phenomenon is sometimes reported to induce transverse isotropy in the material, which is usually assumed to be isotropic. The Payne effect becomes more pronounced at higher silica loading. The filler characteristics such as particle size, specific surface area, and the surface structural features were found to be the key parameters influencing the Payne effect. A nonlinear decrease in storage modulus with increasing strain was observed for unfilled compounds also. The results reveal that the mechanism includes the breakdown of different networks namely the filler-filler network, the weak polymer-filler network, the chemical network, and the entanglement network. The model of variable network density proposed by Maier and Goritz has been applied to explain the nonlinear behavior. The model fits well with the experimental results. The interaction between epoxidized elastomeric matrix and silica as filler was extremely improved, even in the presence of very low content of epoxy groups into the polymer chain.

The effect of γ-irradiation on thermal behavior of composites based on nanosilica and 4-chloro-3-nitro-2H-chromen- 2-one-modified urea–formaldehyde

Two types of nanosilica-based urea–formaldehyde (UF) hybrid materials with formaldehyde to urea (F/U) ratio (0.8) were synthesized without (UF + SiO2) and with coumarin (4-chloro-3-nitro-2H-chromen-2-one). Obtained composites have been irradiated (50 kGy) and their radiation stability was evaluated on the basis of thermal behavior before and after irradiation. The thermal properties of materials were studied by nonisothermal thermogravimetric analysis, differential thermal gravimetry, differential thermal analysis and differential scanning calorimetry supported by data from infrared spectroscopy. The free formaldehyde percentage was determined for all prepared samples. The shift of temperature values for selected mass loss to a high temperature indicates the increase in thermal stability of materials with coumarin after γ-irradiation. The free formaldehyde percentage was reduced after irradiation from 15% to 3% for sample without coumarin and from 7% to 4% for sample with coumarin. The asymmetric stretching vibrations of –N–CH2–N– absorption band and νas(Si–O) of siloxane or silicone in FTIR spectra of irradiated coumarin compound are shifted to higher wave number values and confirmed its higher thermal stability.