In this study, bionanocomposite films based on poly(lactide) (PLA) plasticised with poly(ethylene glycol) (PEG) (7.5 wt%) and reinforced with various contents of nanofibrillated cellulose (NFC) (1, ...3, 5 wt%) were prepared. The hydrothermal degradation was investigated through immersion in several aqueous environments at temperatures of 8, 23, 58, and 70 °C as a function of time (7, 15, 30, 60, 90 days). The effect of water immersion on the physicochemical properties of the materials was assessed by monitoring the changes in the morphology, thermo-oxidative stability, thermal properties, and molar mass through field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC). The hydrothermal degradation behaviour was not critically affected regardless of the nanofibrillated cellulose content. All the materials revealed certain integrity towards water immersion and hydrolysis effects at low temperatures (8 and 23 °C). The low hydrothermal degradation may be an advantage for using these PLA biocomposites in contact with water at ambient temperatures and limited exposure times. On the other hand, immersion in water at higher temperatures above the glass transition (58 and 70 °C), leads to a drastic deterioration of the properties of these PLA-based materials, in particular to the reduction of the molar mass and the disintegration into small pieces. This hydrothermal degradation behaviour can be considered a feasible option for the waste management of PLA/PEG/NFC bionanocomposites by deposition in hot aqueous environments.
Spherical P-doped activated carbon (CSP) was obtained from poplar bark by microwave carbonization with phosphoric acid as the dopant and activating agent. Phosphorus bonded with four oxygen atoms to ...form a phosphate-like structure rather than directly bonding with C atoms in CSP; such groups can provide Lewis acid active sites with moderately strong acidity. The Lewis acid and total acid content increased with increasing phosphoric acid doping ratio. The use of phosphoric acid as an activating agent promotes the formation of a developed porous structure. CSP 1 (mass ratio of raw material to acid = 1 : 1) had a specific surface area of 1115 m 2 g −1 with developed micro–mesoporous structures, mainly micropores. Investigation of the activity of CSP in catalytic cellulose hydrolysis during hydrothermal degradation showed that Brønsted acid sites promote hydrolysis and dehydration of cellulose, Lewis acid sites promote subsequent reactions such as isomerization, rearrangement, and oligomerization, and the developed pores can provide reaction sites and facilitate reactant transportation. The optimal reaction conditions, i.e. , 1 h at 230 °C and 1 MPa with CSP 1 (0.4 g) as the catalyst, gave a cellulose conversion of >99.9%, with 42.30% selectivity for 1,2,4-benzenetriol. This study provides an effective method for preparing 1,2,4-benzenetriol from cellulose with biomass-based activated carbon as the catalyst, which enables green synthesis of 1,2,4-benzenetriol and promotes the use of biomass waste resources.
In this work, the effects of the microstructure on the hydrothermal degradation of yttria-stabilized tetragonal zirconia (Y-TZP) bioceramics and theoretical correlations of the zirconia grain size ...impact in simulations of body fluids were investigated. Samples of ZrO2 (3 mol.% Y2O3) powders were compacted and furtherly sintered for 4 h at different temperatures (1450, 1500, 1530 or 1600 °C), reaching nearly full density in microstructures with different grains sizes ranging from 0,35 ± 0,10 μm to 1,53 ± 0,34 µm. Tests of hydrothermal degradation were performed in autoclave (134 °C - 2 bar), varying the dwell time between 1 and 36 h. The growth of the ZrO2 grain size consistently affected the rate of degradation (represented by the transformed monoclinic m-ZrO2 content) of this ceramic: After 12 h of autoclaving, the monoclinic percentage quadrupled with an increase in the average grain size of 0.35 ± 0.1 μm to 1.53 ± 0.34μm. Using a theoretical correlation between the time of degradation in the autoclave and the respective time in the human body, it was possible to predict the content of the monoclinic phase present in the 3Y-TZP for up to 36 years of exposure in body fluids, and with that, mathematical equations that correlate grain size of ZrO2 with %monoclinic were proposed.
Objective: To evaluate the bond strength of different repair treatments for composite resin to aged Y-TZP ceramics. Material and Methods: Zirconia blocks were cut into smaller specimens, sintered ...according to manufacturer’s recommendations (final dimensions of 4×4×3 mm), and randomly allocated into nine groups (n=15) according to the surface treatment and presence/absence of aging of the substrate (subjected to low-temperature degradation - LTD), as follows: without LTD (Control: without treatment; TBS: tribochemical silica coating + silane + adhesive); with LTD (Control-LTD: without treatment; TBS-LTD: TBS with hydrothermal degradation; MoS-LTD: Monobond S + adhesive; MoP-LTD: Monobond Plus + adhesive; MZP-LTD: Metal/Zirconia Primer + adhesive; USB-LTD: Single Bond Universal; AP-LTD: Alloy primer + adhesive). LTD was simulated in an autoclave (134 °C, 2 bar, 5 h). The ceramic blocks were embedded in PVC cylinders with a self-curing acrylic resin; each surface treatment protocol was performed; a composite resin cylinder (Æ: 3.25 mm and height: 3 mm) was then build-up using split metallic matrices. All the specimens were aged (thermocycling + storage in water for 90 days) and subjected to the shear bond strength test using a universal testing machine (1 mm/min). The failure mode was classified into four types: adhesive, composite resin cohesive fracture, ceramic cohesive fracture, and mixed. The bond strength values were subjected to Mann–Whitney test. Results: Only air-abraded samples (TBS and TBS–LTD) survived thermocycling. More than 80% of the samples of the other groups presented pre-test failures. TBS groups presented higher values of bond strength (3.94) compared to TBS-LTD (0.96). The predominant type of failure for the surviving samples were adhesive. Conclusion: Air particle abrasion is mandatory to improve the bond strength of the Y-TZP substrate; an aged substrate presents an even more unfavorable scenario for adhesion.Keywords Dental prosthesis repair; Hydrothermal degradation; Zirconia; Shear bond strength; Sandblasting.
Fatigue is one of the most important properties to be considered in ceramic
dental implants due to cyclic mechanical stresses arising from the chewing
process. In this work, the fatigue behaviour of ...hydrothermally degraded
ZrO2-based ceramics stabilized with 3mol% Y2O3 (3Y-TZP) was studied in
4-point bending tests. Samples of 3Y-TZP were compacted (100MPa), sintered
at 1475 ?C for 2 h, polished and hydrothermally degraded in an autoclave as
described in the ISO-13356 standard. The samples were characterized by their
relative density, crystalline phase composition, microstructure and surface
roughness. The highly dense (>99.6%TD) sintered 3Y-TZP ceramics has only
tetragonal t-ZrO2 phase, even after hydrothermal ageing. Furthermore, the
ceramic materials presented a Vickers hardness of 12.7?0.2GPa, a fracture
toughness of 7.1?0.3MPa?m1/2 and a 4-point bending strength of 940.1?67MPa.
Based on the bending test results 5 different stress levels for the fatigue
tests were selected and conducted by cyclic 4-point bending obtaining the
S-N curve. Weibull statistics was used for the statistical analysis. The
fatigue tests indicate that the limit of fatigue resistance of this 3Y-TZP
ceramics is around 550MPa, i.e. higher than the limits established in the
ISO-13356 standard for the use of Y-TZP ceramics for the manufacture of
implants. The fatigue behaviour of the investigated 3Y-TZP ceramics was
related to the toughening mechanisms acting in Y-TZP ceramics, such as
transformation toughening related to t?m phase transformation and
microcracking.
A green closed-loop water-based process for chemical recycling of starch was demonstrated. The process starts from microwave-assisted controllable starch degradation to functional chemicals and ...continues to utilization of the formed degradation products, glucose and levulinic acid (LA), as starch plasticizers. The degradation process and the effect of process parameters on the formed degradation products and residues were carefully fingerprinted by multiple techniques, e.g., NMR, ESI-MS, FTIR imaging, UV–vis, and SEM. The degradation process took place in three steps, proceeding from rupture of starch granules and oligosaccharide formation to further hydrolysis to glucose. Finally, at well-defined time point rapid degradation of glucose to LA and formic acid (FA) was initiated through a 5-hydroxymethyl furfural (5-HMF) intermediate. 5-HMF also participated in carbonization reactions leading to formation of solid carbon spheres. Through selection of processing parameters, LA could be produced in high yield with 95% selectivity. The results demonstrated that microwave-assisted degradation of starch leads to well-defined controllable degradation products. These products are directly reusable as plasticizers for new starch products, which suggest great opportunities for biomass and starch recycling, opening the door toward potential industrial applications.
The purpose of this research is focused on the manufacture and characterization of a partially stabilized zirconia ceramic with 3 mol% of Yttria and doped with .5 and 1.5 mol% of Nb2O5 to analyze the ...influence of doping, with the purpose of improving the properties before hydrothermal degradation. In the first instance, the microwave sintering process was used for the consolidation of this material, then the physical and mechanical properties were characterized. Together, the results obtained by the conventional sintering process were compared. A low hydrothermal degradation study (LTD) is presented at low temperatures in which possible changes in the mechanical properties of the ceramic materials are analyzed and its influence on the phase transformation that zirconia may present is observed. The mechanical properties were evaluated through hardness, fracture toughness, and Young's modulus tests. Likewise, their density was analyzed, and microstructure was characterized by FESEM. It was found that the microwave‐sintered samples at 1200°C exhibited superior properties of toughness than even samples sintered by conventional methods at higher temperatures (1400°C). The sample of 3Y‐TZP with 1.5 mol% Nb2O5 sintered by microwave with <.2% of porosity achieved a maximum fracture toughness value around 40% higher than the dense monolithic 3Y‐TZP material.
•Alkaline pre-treatment was introduced to improve the degradation of cellulose.•The main products from the process were lactic acid, formic acid and the acetic acid.•The maximum yields of 39.54% for ...lactic acid, 21.85% for formic acid were achieved.
This paper presented a continuous process of an alkaline pre-treatment and hydrothermal reaction for cellulose degradation. Sodium hydrate solution was selected as the hydrating agent for soaking cotton cellulose at room temperature in which the cellulose was swollen by imbibition of alkaline liquid. After pre-treatment, superfluous alkaline liquid was separated by vacuum filtration and re-used. The treated cotton cellulose with sodium hydrate solution was used for the hydrothermal reaction in a low alkaline concentration for cellulose degradation. The main products from the combined cellulose soaking and degradation were lactic acid, formic acid and acetic acid. After various trials under different conditions and duration, the optimized conditions for the integrated pre-treatment and subsequent hydrothermal degradation processes have been developed. The maximum yields have been achieved for lactic acid (39.54%), formic acid (21.85%) and acetic acid (3.07%) using 3.0mol/L NaOH solution for the cellulose soaking followed by hydrothermal degradation at 483K for 30h. The mechanisms of the formation of lactic acid, formic acid and acetic acid were also investigated. This developed process has also been applied in the degradation of lignocellulosic biomass and similar yields to the cotton cellulose degradation were achieved.
The surface stability of yttria-doped tetragonal polycrystalline zirconia is critical for load-bearing biomedical applications. In this work, the small-scale mechanical behavior of this material is ...probed by employing the in situ micro-cantilever bending technique to near-surface regions. Micro-cantilevers are milled by the focused ion beam technique in the as-sintered condition as well as after hydrothermal degradation by water vapor and tested in order to investigate the effect of degradation on the local flexural response. Results demonstrate that the technique is reliable for assessing the mechanical properties of thin superficial layers and their dependence on orientation. In the non-degraded material, the flexural strength is surprisingly higher than in standard-size specimens and transformation-induced plasticity takes place during testing, inducing defects that become critical at the failure stress. The strength and stiffness of cantilevers obtained from the degraded surface are indeed much lower, and the magnitude of the effect clearly depends on their orientation with respect to the surface. These results are discussed in terms of the presence and spatial distribution of microcracks nucleated during hydrothermal degradation.
Zirconia (3Y-TZP) dental prostheses are widely used in clinical dentistry. However, the effect of ultrasonic scaling performed as a part of professional tooth cleaning on 3Y-TZP dental prostheses, ...especially in conjunction with low-temperature degradation (LTD), has not been fully investigated. The present study aimed to evaluate the influence of ultrasonic scaling and LTD on the surface properties of 3Y-TZP in relation to bacterial adhesion on the treated surface. 3Y-TZP specimens (4 × 4 × 2 mm) were polished and then subjected to autoclaving at 134°C for 100 h to induce LTD, followed by 10 rounds of ultrasonic scaling using a steel scaler tip for 1 min each. Surface roughness, crystalline structure, wettability, and hardness were analyzed by optical interferometry, X-ray diffraction analysis, contact angle measurement, and nano-indentation technique, respectively. Subsequently, bacterial adhesion onto the treated 3Y-TZP surface was evaluated using Streptococcus mitis and S. oralis. The results demonstrated that the combination of ultrasonic scaling and LTD significantly increased the Sa value (surface roughness parameter) of the polished 3Y-TZP surface from 1.6 nm to 117 nm. LTD affected the crystalline structure, causing phase transformation from the tetragonal to the monoclinic phase, and decreased both the contact angle and surface hardness. However, bacterial adhesion was not influenced by these changes in surface properties. The present study suggests that ultrasonic scaling may be acceptable for debridement of 3Y-TZP dental prostheses because it did not facilitate bacterial adhesion even in the combination with LTD, although it did cause slight roughening of the surface.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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