| 1. |
- Wikant, A. W., et al.
(författare)
-
Suboptimal light curing and direct exposure to water of two contemporary composites: degree of conversion, sorption, solubility, and Knoop hardness
- 2020
-
Ingår i: European Journal of Oral Sciences. - : Wiley. - 0909-8836 .- 1600-0722. ; 128:4, s. 345-353
-
Tidskriftsartikel (refereegranskat)abstract
- The water sorption and solubility of two polymer resin-based dental composite materials were assessed in order to evaluate the effects of immediate post-cure water exposure on the water sensitivity of the composites. Each material was tested with two different light curing setups. The radiant exposure of the two curing setups differed by a factor of 5. After exposure to water and subsequent drying, the Knoop surface hardness was measured. The change in the degree of conversion in both water and air storage medium within the first 24 h after curing was monitored by Raman spectroscopy. No significant differences in the degree of conversion were detected 24 h after curing. Samples exposed to the lower irradiation dose showed higher solubility and a lower surface hardness than the samples exposed to the higher irradiation dose. Early exposure to water did not cause detectable differences in the ongoing polymerization process. Increase in radiant exposure reduced the fraction of unbound constituents and increased the crosslink density, thereby reducing the plasticity of the material.
|
|
| 2. |
- Nilsson, Fritjof, et al.
(författare)
-
Simulating the effective electric conductivity of polymer composites with high aspect ratio fillers
- 2016
-
Ingår i: Composites Science And Technology. - : Elsevier. - 0266-3538 .- 1879-1050. ; 132, s. 16-23
-
Tidskriftsartikel (refereegranskat)abstract
- Three simulation models have been developed for predicting the electrical conductivity and the electrical percolation threshold of field-grading polymer composites intended for high voltage applications. The three models are based on finite element modelling (FEM), percolation threshold modelling (PTM) and electrical networks modelling (ENM). A Monte Carlo algorithm was used to construct the geometries, with either soft-core (overlapping) or hard-core/soft-shell (non-overlapping) fibres. Conductivity measurements on carbon-fibre/PMMA composites with well-defined fibre aspect ratios were used for experimental validation. The average fibre orientations were calculated from scanning electron micrographs. The soft-core PTM model with experimental fibre orientations and without adjustable parameters gave accurate (R-2 = 0.984) predictions of the electrical percolation threshold as a function of aspect ratio. The corresponding soft-core ENM model, with close-contact conductivity calculated with FEM, resulted in good conductivity predictions for the longest fibres, still without the use of any adjustable parameters. The hard-core/soft-shell versions of the models, using the shell thickness as an adjustable parameter, gave similar but slightly poorer results.
|
|
| 3. |
- Akhlaghi, Shahin
(författare)
-
Degradation of acrylonitrile butadiene rubber and fluoroelastomers in rapeseed biodiesel and hydrogenated vegetable oil
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biodiesel and hydrotreated vegetable oil (HVO) are currently viewed by the transportation sector as the most viable alternative fuels to replace petroleum-based fuels. The use of biodiesel has, however, been limited by the deteriorative effect of biodiesel on rubber parts in automobile fuel systems. This work therefore aimed at investigating the degradation of acrylonitrile butadiene rubber (NBR) and fluoroelastomers (FKM) on exposure to biodiesel and HVO at different temperatures and oxygen concentrations in an automated ageing equipment and a high-pressure autoclave. The oxidation of biodiesel at 80 °C was promoted by an increase in the oxygen partial pressure, resulting in the formation of larger amounts of hydroperoxides and acids in the fuel. The fatty acid methyl esters of the biodiesel oxidized less at 150 °C on autoclave aging, because the termination reactions between alkyl and alkylperoxyl radicals dominated over the initiation reactions. HVO consists of saturated hydrocarbons, and remained intact during the exposure. The NBR absorbed a large amount of biodiesel due to fuel-driven internal cavitation in the rubber, and the uptake increased with increasing oxygen partial pressure due to the increase in concentration of oxidation products of the biodiesel. The absence of a tan δ peak (dynamical mechanical measurements) of the bound rubber and the appearance of carbon black particles devoid of rubber suggested that the cavitation was caused by the detachment of bound rubber from particle surfaces. A significant decrease in the strain-at-break and in the Payne-effect amplitude of NBR exposed to biodiesel was explained as being due to the damage caused by biodiesel to the rubber-carbon-black network. During the high-temperature autoclave ageing, the NBR swelled less in biodiesel, and showed a small decrease in the strain-at-break due to the cleavage of rubber chains. The degradation of NBR in the absence of carbon black was due only to biodiesel-promoted oxidative crosslinking. The zinc cations released by the dissolution of zinc oxide particles in biodiesel promoted reduction reactions in the acrylonitrile part of the NBR. Heat-treated star-shaped ZnO particles dissolved more slowly in biodiesel than the commercial ZnO nanoparticles due to the elimination of inter-particle porosity by heat treatment. The fuel sorption was hindered in HVO-exposed NBR by the steric constraints of the bulky HVO molecules. The extensibility of NBR decreased only slightly after exposure to HVO, due to the migration of plasticizer from the rubber. The bisphenol-cured FKM co- and terpolymer swelled more than the peroxide-cured GFLT-type FKM in biodiesel due to the chain cleavage caused by the attack of biodiesel on the double bonds formed during the bisphenol curing. The FKM rubbers absorbed biodiesel faster, and to a greater extent, with increasing oxygen concentration. It is suggested that the extensive biodiesel uptake and the decrease in the strain-at-break and Young’s modulus of the FKM terpolymer was due to dehydrofluorination of the rubber by the coordination complexes of biodiesel and magnesium oxide and calcium hydroxide particles. An increase in the CH2-concentration of the extracted FKM rubbers suggested that biodiesel was grafted onto the FKM at the unsaturated sites resulting from dehydrofluorination.
|
|
| 4. |
- Akhlaghi, Shahin, et al.
(författare)
-
Degradation of fluoroelastomers in rapeseed biodiesel at different oxygen concentrations
- 2017
-
Ingår i: Polymer degradation and stability. - : Elsevier. - 0141-3910 .- 1873-2321. ; 136, s. 10-19
-
Tidskriftsartikel (refereegranskat)abstract
- The degradation of fluoroelastomers (FKM) based on different monomers, additives and curing systems was studied after exposure to rapeseed biodiesel at 100 °C and different oxygen partial pressures. The sorption of fuel in the carbon black-filled FKM terpolymer was promoted by the fuel-driven cavitation in the rubber. The bisphenol-cured rubbers swelled more in biodiesel than the peroxide-cured FKM, presumably due to the chain cleavage caused by the attack of biodiesel on the double bonds formed during the bisphenol curing. With any of the selected types of monomer, the FKM rubbers absorbed biodiesel faster and to a greater extent with increasing oxygen partial pressure due to the increase in concentration of the oxidation products of biodiesel. Water-assisted complexation of biodiesel on magnesium oxide and calcium hydroxide particles led to dehydrofluorination of FKM, resulting in an extensive fuel uptake and a decrease in the strain-at-break and the Young's modulus of the rubbers. An increase in the CH2-concentration determined by infrared spectroscopy, and the appearance of biodiesel flakes in scanning electron micrographs of the extracted rubbers, were explained as being due to the presence of insoluble biodiesel grafted onto FKM on the unsaturated sites resulting from dehydrofluorination. The extensibility of the GFLT-type FKM was the least affected on exposure to biodiesel because this rubber contained less unsaturation and metal oxide/hydroxide particles.
|
|
| 5. |
- Akhlaghi, Shahin, et al.
(författare)
-
Deterioration of acrylonitrile butadiene rubber in rapeseed biodiesel
- 2015
-
Ingår i: Polymer degradation and stability. - : Elsevier BV. - 0141-3910 .- 1873-2321. ; 111, s. 211-222
-
Tidskriftsartikel (refereegranskat)abstract
- The deterioration of acrylonitrile butadiene rubber (NBR) exposed to rapeseed biodiesel at 90 degrees C was studied. The oxidation of biodiesel and NBR during ageing was monitored by H-1 NMR and infrared spectroscopy, HPLC and titration methods. The oxidation of biodiesel was impeded in the presence of NBR, but promoted in biodiesel-exposed rubber. This was explained as being due to the migration of stabilizer from the rubber to biodiesel, the diffusion of dissolved oxygen from biodiesel into NBR and the absorption of oxidation precursors of biodiesel by the rubber. The resemblance between the anomalous sorption kinetics of biodiesel in NBR and the equilibrium benzene uptake by the aged rubbers revealed that biodiesel caused a network defect in NBR, resulting in a gradual increase in the equilibrium swelling. The cleavage of crosslinks was implausible since the Young's modulus of the rubber at low strains, disregarding an initial decrease, increased with increasing exposure time. The appearance of 'naked' carbon black particles in the scanning electron micrographs of the aged rubbers and a drastic decrease in the strain-at-break of NBR after exposure to biodiesel suggests that internal cavitation was caused by the attack of biodiesel on the acrylonitrile units of NBR.
|
|
| 6. |
- Akhlaghi, Shahin, et al.
(författare)
-
Deterioration of automotive rubbers in liquid biofuels : A review
- 2015
-
Ingår i: Renewable & sustainable energy reviews. - : Elsevier BV. - 1364-0321 .- 1879-0690. ; 43, s. 1238-1248
-
Forskningsöversikt (refereegranskat)abstract
- Concerns over the fast depletion of fossil fuels, environmental issues and stringent legislation associated with petroleum-based fuels have triggered a shift to bio-based fuels, as an alternative to meet the growing energy demand in the transportation sector. However, since conventional automobile fuel systems are adapted to petroleum-based fuels, switching to biofuels causes a severe deterioration in the performance of currently used rubber components. The degradation of the rubber materials in biofuels is complicated by the presence of different additives in biofuels and rubber compounds, by oxidation of biofuels and by the effects of thermomechanical loadings in the engine. This paper presents a comprehensive review of the effects of different types of biofuels, particularly biodiesel and bioethanol, on the physical, mechanical, morphological and thermal properties of elastomers under different exposure conditions. In addition, the literature data available on the variation of rubbers' resistance to biofuels with the changes in their monomer type and composition, cure system and additives content was also studied. The review essentially focuses on the compatibility of biofuels with acrylonitrile butadiene rubber, fluoroelastomers, polychloroprene rubber and silicon rubber, as the most commonly used automotive rubbers coming into contact with fuels during their service. The knowledge summarized in this study can help to develop a guideline on the selection of rubber for automotive parts designed to withstand biofuels.
|
|
| 7. |
- Akhlaghi, Shahin, et al.
(författare)
-
Effects of ageing conditions on degradation of acrylonitrile butadiene rubber filled with heat-treated ZnO star-shaped particles in rapeseed biodiesel
- 2017
-
Ingår i: Polymer degradation and stability. - : Elsevier. - 0141-3910 .- 1873-2321.
-
Tidskriftsartikel (refereegranskat)abstract
- The degradation of acrylonitrile butadiene rubber (NBR) after exposure to biodiesel at different oxygen partial pressures in an automated ageing equipment at 80 °C, and in a high-pressure autoclave at 150 °C was studied. The oxidation of biodiesel was promoted by an increase in oxygen concentration, resulting in a larger uptake of fuel in the rubber due to internal cavitation, a greater decrease in the strain-at-break of NBR due to the coalescence of cavity, and a faster increase in the crosslinking density and carbonyl index due to the promotion of the oxidation of NBR. During the high-temperature autoclave ageing, less fuel was absorbed in the rubber, because the formation of hydroperoxides and acids was impeded. The extensibility of NBR aged in the autoclave decreased only slightly due to the cleavage of rubber chains by the biodiesel attack. The degradation of NBR in the absence of carbon black was explained as being due to oxidative crosslinking. The dissolution of ZnO crystals in the acidic components of biodiesel was retarded by removing the inter-particle porosity and surface defects through heat treating star-shaped ZnO particles. The rubber containing heat-treated ZnO particles swelled less in biodiesel than a NBR filled with commercial ZnO nanoparticles, and showed a smaller decrease in the strain-at-break and less oxidative crosslinking.
|
|
| 8. |
- Alipour, Nazanin, et al.
(författare)
-
Structure and properties of polyethylene-based and EVOH-based multilayered films with layer thicknesses of 150 nm and greater
- 2015
-
Ingår i: European Polymer Journal. - : Elsevier BV. - 0014-3057 .- 1873-1945. ; 64, s. 36-51
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the structure and properties of two multilayered systems where polymers in adjacent layers were either miscible or immiscible. The miscible system consisted of 2, 17, 18, 24 and nominally 288 layers of alternating low-density (LDPE) and low-density/linear-low density (mPE) polyethylene layers with observed thicknesses ranging from 150 nm to 20 urn. The immiscible system consisted of 5 and 19 layer films with a combination of poly(ethylene-co-vinyl alcohol) (EVOH) (thickness: 9 and 1 gm, respectively), LDPE (17 and 7 gm) and a polyethylene adhesive (3 and 1 gm). The purpose of the multi-layering was to increase the crack growth resistance and, in the EVOH-based system, to decrease the oxygen transmission rate. Indeed, the crack growth resistance, as measured on tensile-tested notched films, increased with increasing number of layers. The thinnest polyethylene and polyethylene adhesive layers showed a clear ductile failure when fractured even in liquid nitrogen. Simultaneous synchrotron wide-angle/small-angle X-ray scattering and tensile testing indicated no new deformation features with changes in the layer thickness. The oxygen permeability was the same in the 5- and 19-layer EVOH-based films, but the uptake of n-hexane was strongly reduced in the 19-layer films, demonstrating the effective protective role of the EVOH layers. The n-hexane desorption data of the 2-layer LDPE/mPE film was successfully modeled using the diffusivities and solubilities of the single layers. Crystallization was slower and more confined in the films with thinner layers. The interlayer mixing in the melt (measured by isothermal crystallization from melts of initially layered polyethylene-based systems) was, as expected, significantly faster in the 24- and 288-multilayer films than in the 2-layer film.
|
|
| 9. |
- Alipour, Nazanin, et al.
(författare)
-
VOC-Induced Flexing of Single and Multilayer Polyethylene Films As Gas Sensors
- 2016
-
Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 8:15, s. 9946-9953
-
Tidskriftsartikel (refereegranskat)abstract
- The differential swelling and bending of multilayer polymeric films due to the dissimilar uptake of volatile organic compounds (VOCs; n-hexane, limonene) in the different layers was studied. Motions of thin polyethylene films triggered by the penetrant were investigated to learn more about how their deformation is related to VOC absorption. Single layers of metallocene or low-density polyethylene, and multilayers (2-288-layers) of these in alternating positions were considered. Single-, 24-, and 288 layer films displayed no motion when uniformly subjected to VOCs, but they could display simple curving modes when only one side of the film was wetted with a liquid VOC. Two-layer films displayed simple bending when uniformly subjected to VOCs due to the different swelling in the two layers, but when the VOC was applied to only one side of the film, more complex modes of motion as well as dynamic oscillations were observed (e.g., constant amplitude wagging at 2 Hz for ca. 50 s until all the VOC had evaporated). Diffusion modeling was used to study the transport behavior of VOCs inside the films and the different bending modes. Finally a prototype VOC sensor was developed, where the reproducible curving of the two-layer film was calibrated with n-hexane. The sensor is simple, cost-efficient, and nondestructive and requires no electricity.
|
|
| 10. |
- Amanizadeh, Farhad, et al.
(författare)
-
Starve fed emulsion copolymerization of vinyl acetate and 1-hexene at ambient pressure
- 2014
-
Ingår i: Polymer international. - : Wiley. - 0959-8103 .- 1097-0126. ; 63:10, s. 1850-1855
-
Tidskriftsartikel (refereegranskat)abstract
- A novel emulsion copolymer of vinyl acetate (VAc) and 1-hexene was synthesized at ambient pressure. The feeding technique, initiation system and reaction time of the copolymerization were optimized based on molecular characteristics such as the weight contribution of 1-hexene in the copolymer chains and glass transition temperature (T-g) as well as on bulk properties like minimum film-formation temperature (MFFT) and solid content. According to nuclear magnetic resonance spectroscopy and differential scanning calorimetry results, the combination of starve feeding and redox initiation, within a reaction time of 4h, effectively led to the copolymerization at ambient pressure between highly reactive polar VAc monomers and non-polar 1-hexene monomers of low reactivity. The copolymer showed a lower T-g and MFFT, and a reasonable solid content compared to the poly(vinyl acetate) (PVAc) homopolymer. The consumption rate, hydrolysis of acetate groups and chain transfer reactions during the polymerization were followed using infrared spectroscopy. Based on the results, the undesirable reactions between the VAc blocks were hindered by the neighbouring 1-hexene molecules. Tensile testing revealed an improvement in the toughness and elongation at break of VAc-1-hexene films compared to PVAc films.
|
|
