Advances in layered double hydroxide (LDH)-based elastomer composites

Fenton-like system of UV/Glucose-oxidase@Kaolin coupled with organic green rust: UV-enhanced enzyme activity and the mechanism of UV synergistic degradation of photosensitive pollutants

This study introduces the UV/glucose-oxidase@Kaolin (GOD@Kaolin) coupled organic green rust (OGR) system (UV/OGR/GOD@Kaolin) to investigate the promotion of glucose oxidase activity by UV light and its synergistic degradation mechanism for photosensitive pollutants, specifically targeting the efficient degradation of 4-chlorophenol (4-CP). The enzyme system demonstrates its ability to overcome drawbacks associated with traditional Fenton systems, including a narrow pH range and high localized concentration of H2O2, by gradually releasing hydrogen peroxide in situ within a neutral environment. In the presence of UV radiation under specific conditions, enhanced enzyme activity is observed, resulting in increased efficiency in pollutant removal. The gradual release of hydrogen peroxide plays a crucial role in preventing unwanted reactions among active substances. These unique features facilitate the generation of highly reactive species, such as Fe(IV)O, •OH, and •O2-, tailored to efficiently target the organic components of interest. Additionally, the system establishes a positive iron cycle, ensuring a sustained reactive capability throughout the degradation process. The results highlight the UV/OGR/GOD@Kaolin system as an effective and environmentally friendly approach for the degradation of 4-CP, and the resilience of the enzyme extends the system's applicability to a broader range of scenarios.

Enhancing Rubber Vulcanization Cure Kinetics: Lowering Vulcanization Temperature by Addition of MgO as Co-Cure Activator in ZnO-Based Cure Activator Systems

Vulcanization is a chemical modification of rubber that requires a considerable amount of thermal energy. To save thermal energy, the kinetics of rubber vulcanization should be improved. In this article, the curing properties of rubber vulcanization are thoroughly investigated using the moving die rheometer (MDR) technique. To enhance the kinetics in different stages of ZnO-based sulfur vulcanization systems, small amounts of MgO were added. The results revealed that the small amount of 1 to 2 phr (per hundred grams of rubber) of MgO in the controlled 5 phr ZnO-based curing systems can significantly improve the curing kinetics. For example, the optimum curing time of 1 phr MgO added to the 5 phr ZnO-containing semi-efficient vulcanization system at different temperatures was more than half that of the controlled 5 phr ZnO-only compound. While maintaining a similar rate of vulcanization, the vulcanization temperature can be reduced by up to 20 °C by using MgO as a co-cure activator, which exhibits similar or better rheometric mechanical properties compared to the controlled compounds. With the addition of MgO as a co-cure activator, the vulcanization reactions become very fast, enabling vulcanization to be completed, even at the boiling point of water (100 °C) with an affordable curing time (<1 h). By reducing the vulcanization temperature, the scorch safety time can be enhanced in the ZnO/MgO-based binary cure activator-containing vulcanizates. Overall, MgO could be a potential candidate as a co-cure activator with ZnO for the vulcanization of rubber, offering better economical and eco-friendly methods.

Synthesis and application of waste-based layered double hydroxide: A review

The synthesis of layered double hydroxide (LDH) from industrial wastes is a sustainable approach to aid circular economy and hazardous material disposal. In this review, the researches on the synthesis and application of waste-based LDH from 2010 to 2023 are summarized and discussed. At present, there are mainly four types of waste-based LDH produced from red mud, slag, fly ash and wastewater, with co-precipitation being the most typical synthesis method. Red mud is used as the trivalent metal source supplemented by chemical reagents or other types of waste as divalent metal source to produce red mud-based LDH. Slag can act as the sole metal source providing both divalent and trivalent metal sources for slag-based LDH. Fly ash was used either as the trivalent metal source or both divalent and trivalent metal sources to produce fly ash-based LDH. Wastewater-based LDH was typically synthesized by in-situ co-precipitation method to achieve the self-purification of wastewater. The impurities in waste-based LDH can act as a two-edged weapon. It may either hinder or promote the performance of waste-based LDH. The challenge in the synthesis of waste-based LDH lies in the efficient extraction of available metals. The future research prospects for waste-based LDH are suggested.

The Stability and Properties of Polystyrene/Kaolinite Nanocomposites during Synthesis via Emulsion Polymerization

The aim of this work was to study the stability and morphological properties of polystyrene latex containing kaolinite as a filler during the process of synthesis of nanocomposites viaemulsion polymerization. Nanocomposites with 1, 3, and 5 wt% of kaolinite were prepared. Latexes with 1 to 3 wt% of kaolinite were stable during the polymerization reaction. Hydrodynamic diameters of 93.68 and 82.11 nm were found for latexes with 1 and 3 wt% of kaolinite, respectively. The quantities of 1 to 3 wt% of kaolinite added during the reaction did not influence the reaction conversion curves or the number of particles. X-ray diffraction (XRD) and unconventional techniques of scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) showed the presence of exfoliated and intercalated structures of the kaolinite.

Strain-induced 3D-oriented crystallites in natural rubber/chitin nanofiber composites

Natural rubber (NR) composites containing bio-based chitin nanofibers (ChNFs) exhibit a wide range of mechanical properties - from rubber to plastic behavior - with increasing chitin contents. A constrained 3-dimensional network can be formed by mixing natural rubber latex and a modified zwitterionic rigid chitin counterpart. By inclusion of highly anisotropic chitin nanofibers (30 wt%), strain-induced NR crystallization occurs at a much lower strain of 50%. More intriguingly, 2D-WAXD results reveal that the strain-induced crystallization of NR/ChNFs composites show 3-dimensionally oriented crystallite formation behaving similar to "3D-single crystals orientation" when the content of ChNFs is over 5 wt%. It is suggested that not only c-axis (NR chains) orients along the stretching direction, but also the a- and b-axes deliberately arrange along the normal direction and transverse direction, respectively. Structure and morphology in 3-dimensional spaces after strain-induced crystallization of the NR/ChNFs30 composite are investigated in detail. Therefore, this study might pave a new way to enhance mechanical properties by incorporation of ChNFs, obtaining 3-dimensionally oriented crystallites of novel multifunctional NR/ChNFs composite with shape memory ability.

Rational construction and understanding the effect of metal cation substitution of three novel ternary Zn-Co-Ni-LDHs from 2D to 3D and its enhanced adsorption properties for MO

The layered double hydroxides (LDHs) have attracted attention in the water treatment field. In this paper, three novel ternary Zn-Co-Ni-LDH adsorbents were prepared successfully through rational construction from 2D to 3D using triethanolamine (TEA) as an alkali source and a structural controlling reagent by hydrothermal technique. Samples were characterized by the SEM, XRD, XPS, FTIR, BET, solid-state UV/vis spectra, and TG. Three Zn-Co-Ni-LDHs exhibited higher crystallinity and surface area which were beneficial to the adsorption for methyl orange (MO). The maximum adsorption capacity of three Zn-Co-Ni-LDH adsorbents can even reach as high as 1871.65 mg·g^-1, 1799.56 mg·g^-1, and 1646.44 mg·g^-1 for MO, respectively, which surpass those of most previously reported LDH-based adsorbents. The pseudo-second-order kinetic equation fitted the kinetic data of adsorption, while the equilibrium adsorption isotherm data followed the Langmuir model. The adsorption mechanism, electrochemical, and the antibacterial properties of three Zn-Co-Ni-LDHs were also discussed. This results not only demonstrates that three Zn-Co-Ni-LDHs are practical interest as an efficient adsorbent for the removal of MO from dye waste water, but also provides a strategy for the rational design through three ternary Zn-Co-Ni-LDHs from 2D to 3D.

Advances in Rubber Compounds Using ZnO and MgO as Co-Cure Activators

Zinc oxide performs as the best cure activator in sulfur-based vulcanization of rubber, but it is regarded as a highly toxic material for aquatic organisms. Hence, the toxic cure activator should be replaced by a non-toxic one. Still, there is no suitable alternative industrially. However, binary activators combining ZnO and another metal oxide such as MgO can largely reduce the level of ZnO with some improved benefits in the vulcanization of rubber as investigated in this research. Curing, mechanical, and thermal characteristics were investigated to find out the suitability of MgO in the vulcanization of rubber. Curing studies reveal that significant reductions in the optimum curing times are found by using MgO as a co-cure activator. Especially, the rate of vulcanization with conventional 5 phr (per hundred grams) ZnO can be enhanced by more than double, going from 0.3 Nm/min to 0.85 Nm/min by the use of a 3:2 ratio of MgO to ZnO cure activator system that should have high industrial importance. Mechanical and thermal properties investigations suggest that MgO as a co-cure activator used at 60% can provide 7.5% higher M100 (modulus at 100% strain) (0.58 MPa from 0.54 MPa), 20% higher tensile strength (23.7 MPa from 19.5 MPa), 15% higher elongation at break (1455% from 1270%), 68% higher fracture toughness (126 MJ/m³ from 75 MJ/m³), and comparable thermal stability than conventionally using 100 % ZnO. Especially, MgO as a co-cure activator could be very useful for improving the fracture toughness in rubber compounds compared to ZnO as a single-site curing activator. The significant improvements in the curing and mechanical properties suggest that MgO and ZnO undergo chemical interactions during vulcanization. Such rubber compounds can be useful in advanced tough and stretchable applications.

In-Situ Synthesis of Layered Double Hydroxide/Silica Aerogel Composite and Its Thermal Safety Characteristics

To adjust the thermal safety of hydrophobic silica aerogel, layered double hydroxide (LDH)/silica aerogel (SA) composites were prepared by an in-situ sol-gel process at ambient pressure. This study found the physical combination of SA and MgAl-LDH based on the FTIR spectra and phase composition of LDH/SA. The N2 sorption analysis confirms that the introduction of MgAl-LDH does not change the mesoporous attribution of LDH/SA significantly. With the increase in MgAl-LDH addictive content, the low density (0.12–0.13 g/cm3), low thermal conductivity (24.28–26.38 mW/m/K), and large specific surface area (730.7–903.7 m2g) of LDH/SA are still maintained, which can satisfy the requirements of thermal insulation. The TG-DSC analysis demonstrates that the endothermic effects and metal oxides formed during the MgAl-LDH decomposition are beneficial to the improvement of the thermal stability of LDH/SA composites. In addition, it was found that the gross calorific values of LDH/SA composites decrease with an increase in MgAl-LDH addictive content, all of which are lower than that of the pure SA. The research outcomes indicate that the thermal safety of LDH/SA composites is enhanced significantly by doping MgAl-LDH without impairing too many of the excellent properties, which benefits their expansion in the thermal insulation field.

Magnetic MnFe2O4/ZnFe-LDH for Enhanced Phosphate and Cr (VI) Removal from Water

Due to the layered structure and recycling characteristics, magnetic MnFe2O4/ZnFe-LDH was prepared by co-precipitation. In this study, we intensively explored MnFe2O4/ZnFe-LDH for water purification compared with the ZnFe-LDH. The morphological and structural characteristics of the obtained products were systematically characterized. MnFe2O4/ZnFe-LDH exhibits the maximum adsorption capacity of 94.52 mg/g for phosphate and 49.03 mg/g for Cr (VI), respectively, which is superior to that of the ZnFe-LDH, indicating that the magnetic MnFe2O4 effectively enhanced adsorption performance. Meanwhile, the mechanism of adsorption of phosphate and Cr (VI) was briefly studied, where the metal center ion in MnFe2O4/ZnFe-LDH and between layers of SO42- serves as capture sites for phosphate and Cr (VI) removal. Furthermore, the MnFe2O4/ZnFe-LDH can be used for magnetic recycled and still maintained excellent removal efficiency (70%) for phosphate and Cr (VI) after five adsorption-desorption cycles. This work could open a new vista of designing magnetic novel adsorbents in environmental remediation.

Nanodiamond—Carbon Black Hybrid Filler System for Demanding Applications of Natural Rubber—Butadiene Rubber Composite

The objective of the study was to investigate the effect of the partial replacement of carbon black (CB) by nanodiamonds (NDs) on the vulcanization, mechanical and dynamic properties of a natural rubber—butadiene rubber compound, a typical elastomer compound found in several applications (the tire and mining industry, for example). A studied hybrid filler system resulted in a 28% increase in tensile strength and 29% increase in 300% modulus at low ND loadings even though the total weight fraction of the filler system was kept constant at 25 parts per hundred rubber. The hybrid filler system improved dispersion of both fillers as was proven by scanning electron microscopy and the Payne effect study. In addition, the replacement of 2.5 and 5 phr CB by NDs resulted in 62% improvement in wear resistance. The DMA study showed that a certain ND-CB filler combination has a positive effect on tire properties such as wet grip and rolling resistance.

Active Corrosion Protection of Mg–Al Layered Double Hydroxide for Magnesium Alloys: A Short Review

Layered double hydroxides (LDHs) have been successfully applied in surface protection and functionalization of metal materials owing to their special structure, composition, controllability, anion exchange, and other excellent properties. Herein, this review focused on the most common LDH preparation methods on magnesium alloys in the past 10 years by summarizing about 70 research papers. At the same time, based on the corrosion protection mechanism of LDH materials, the performance of LDH films and LDH as filler on metal substrates was briefly introduced. The surface of LDH materials were chemically modified to enhance their compatibility with solvents, and their corrosion resistance function was developed as an additive. Finally, LDH-based composite coatings on Mg alloys by surface pretreatment and chemical modification were discussed.

Systematic Literature Review of the Effect of Layered Double Hydroxides on the Mechanical Properties of Rubber

Layered double hydroxides (LDHs) have attracted interest as reinforcing fillers in elastomers due to their ease of synthesis and customisability. A systematic review was performed on the effect of LDHs on the mechanical properties of elastomers using the Scopus database. Of the 61 articles relevant to the search criteria, the majority were published on polyurethane (PU) and nitrile butadiene rubber (NBR). Mg-Al LDH was used in most of the studies and Zn-Al LDH was used second most common. LDH can act as a reinforcing filler, typically increasing tensile strength even at low concentrations, so it could be used as an alternative to traditional reinforcing fillers for elastomers. LDH can also be made a functional filler by selecting the right metals and interlayer anions. It was found that Mg-Al LDH and Zn-Al LDH can both participate in crosslinking reactions and can replace MgO and ZnO, respectively. Less Zn ions are required for crosslinking when LDH is used than when ZnO is used, making LDH more environmentally friendly. Organic modification is usually required to improve compatibility with the elastomer matrix, especially in non-polar elastomers. It enables exfoliation of the LDH and intercalation of polymer chains into the LDH interlayer to occur. Organic modifiers can also be used to functionalise the LDH. Stearic acid used in crosslinking systems can be replaced by stearate anions from stearate-modified LDH.

Curing kinetics and ionic interactions in layered double hydroxides–nitrile rubber Mg–Al-LDHs–XNBR composites

The carboxylated butadiene–acrylonitrile XNBR composites containing layered double hydroxides (Mg–Al-LDHs) were prepared with the aim to investigate the effect of various Mg:Al ratio in the filler composition on the rheological behavior, kinetic of curing and viscoelastic properties of final material. Due to the presence of metal cations, LDHs can form ionic cross-links with carboxylic functional groups of XNBR nitrile rubber. The DSC analysis indicated lower ranges of curing temperatures as the Mg:Al ratio raised (90–184 °C for Pural MG63, 88–187 °C for Pural MG70). The apparent energy of activation Ea decreased with increasing Mg:Al ratio reaching the value of 111.46 kJˑmole−1 for XNBR composite containing Pural MG70. The rheological analysis revealed the presence of specific polymer–nanofillers and fillers–fillers interactions. The XNBR–LDHs rubbers exhibited two transitions corresponding to the glass temperature of elastomer Tg (in range of −13.08 to −13.60 °C) and the ionic transition temperature Tα′ (in range of 32.95–37.90 °C). The ionic transition temperature Tα′ was not observed for the nitrile rubber composite containing Pural MG5 with the lowest Mg:Al ratio. The formed ordered ionic phase significantly affected the viscoelastic behavior of XNBR composites at room and sub-zero temperatures leading to the increase in the storage modulus G′. Therefore, among the various possible applications of LDHs, their use in rubber technology offers the potential for production of environmentally friendly rubber products cured without any additional curatives.

Synthetic Iowaite Can Effectively Remove Inorganic Arsenic from Marine Extract

For the removal of arsenic from marine products, iowaite was prepared and investigated to determine the optimal adsorption process of arsenic. Different chemical forms of arsenic (As(III), As(V)) with varying concentrations (0.15, 1.5, 5, 10, 15, and 20 mg/L) under various conditions including pH (3, 5, 7, 9, 11) and contact time (1, 2, 5, 10, 15, 30, 60, 120, 180 min) were exposed to iowaite. Adsorption isotherms and metal ions kinetic modeling onto the adsorbent were determined based on Langmuir, Freundlich, first- and second-order kinetic models. The adsorption onto iowaite varied depending on the conditions. The adsorption rates of standard solution, As(III) and As(V) exceeded 95% under proper conditions, while high complexity was noted with marine samples. As(III) and As(V) from Mactra veneriformis extraction all decreased when exposed to iowaite. The inclusion morphology and interconversion of organic arsenic limit adsorption. Iowaite can be efficiently used for inorganic arsenic removal from wastewater and different marine food products, which maybe other adsorbent or further performance of iowaite needs to be investigated for organic arsenic.

Influence of Fillers and Ionic Liquids on the Crosslinking and Performance of Natural Rubber Biocomposites

This work concerns the effect of fillers and ionic liquids on the cure characteristics of natural rubber (NR) compounds, as well as the mechanical and thermal properties of the vulcanizates. Three types of white filler were applied, such as cellulose, nanosized silica and hydrotalcite, to modify the performance of NR composites. Additionally, ionic liquids (ILs) with bromide anion and different cations, i.e., 1-butyl-3-methylimidazolium (Bmi) and 1-butyl-3-methylpyrrolidinium (Bmpyr), were used to improve the cure characteristics of NR compounds and functional properties of the vulcanizates. The type of filler and the structure of ILs were proved to affect the rheometric properties and cure characteristics of NR compounds as well as the performance of the NR vulcanizates. Owing to the adsorption of curatives onto the surface, silica reduced the activity of the crosslinking system, prolonging the optimal vulcanization time of NR compounds and reducing the crosslinking degree of the elastomer. However, silica-filled NR exhibited the highest thermal stability. Hydrotalcite increased the crosslink density and, consequently, the mechanical properties of the vulcanizates, but deteriorated their thermal stability. ILs beneficially influenced the cure characteristics of NR compounds, as well as the crosslink density and mechanical performance of the vulcanizates, particularly those filled with silica. Cellulose did not significantly affect the vulcanization of NR compounds and crosslink density of the vulcanizates compared to the unfilled elastomer, but deteriorated their tensile strength. On the other hand, cellulose improved the thermal stability and did not considerably alter the damping properties of the vulcanizates.

Drug Delivery (Nano)Platforms for Oral and Dental Applications: Tissue Regeneration, Infection Control, and Cancer Management

The oral cavity and oropharynx are complex environments that are susceptible to physical, chemical, and microbiological insults. They are also common sites for pathological and cancerous changes. The effectiveness of conventional locally-administered medications against diseases affecting these oral milieus may be compromised by constant salivary flow. For systemically-administered medications, drug resistance and adverse side-effects are issues that need to be resolved. New strategies for drug delivery have been investigated over the last decade to overcome these obstacles. Synthesis of nanoparticle-containing agents that promote healing represents a quantum leap in ensuring safe, efficient drug delivery to the affected tissues. Micro/nanoencapsulants with unique structures and properties function as more favorable drug-release platforms than conventional treatment approaches. The present review provides an overview of newly-developed nanocarriers and discusses their potential applications and limitations in various fields of dentistry and oral medicine.

Renewable lignin-based surfactant modified layered double hydroxide and its application in polypropylene as flame retardant and smoke suppression

A novel and environmentally friendly lignin-based surfactant sodium lignosulfonate (SLS) modified layered double hydroxide (LDH) flame retardant (LDH-LS) was fabricated via co-precipitation method, and subsequently incorporated into polypropylene (PP) matrix to obtain the PP and LDH-LS composites (PP/LDH-LS) by melt blending method. The XRD, FT-IR and XPS results indicated that SLS had successfully modified LDH by adsorbing on the surface of the LDH nanosheet. The WCA and SEM results revealed that the hydrophobic property of LDH-LS had been evidently improved, and it displayed a more homogeneous dispersion than virgin LDH in the PP matrix. Furthermore, cone calorimetry tests (CCT) illustrated that the peak heat release rate (PHRR), total heat release (THR), and total smoke release (TSR) of PP/LDH-LS composites exhibited declines of 62.9%, 25.1%, and 43.3% compared with those of Neat PP, respectively. Besides, the PP/LDH-LS achieved a LOI value of 29.4% and a UL-94 V-0 rating, whereas the PP/LDH showed only a LOI value of 25.2% and a UL-94 V-2 rating at 20 wt% loading. These improvements of flame retardant properties can be attributed to that the well-dispersed LDH-LS and synergistic flame retardancy between LDH and SLS.

A Review on LDH-Smart Functionalization of Anodic Films of Mg Alloys

This review presents an overview of the recent developments in the synthesis of layered double hydroxide (LDH) on the anodized films of Mg alloys prepared by either conventional anodizing or plasma electrolytic oxidation (PEO) and the applications of the formed composite ceramics as smart chloride traps in corrosive environments. In this work, the main fabrication approaches including co-precipitation, in situ hydrothermal, and an anion exchange reaction are outlined. The unique structure of LDH nanocontainers enables them to intercalate several corrosion inhibitors and release them when required under the action of corrosion-relevant triggers. The influences of different variables, such as type of cations, the concentration of salts, pH, and temperature, immersion time during the formation of LDH/anodic film composites, on the electrochemical response are also highlighted. The correlation between the dissolution rate of PEO coating and the growth rate of the LDH film was discussed. The challenges and future development strategies of LDH/anodic films are also highlighted in terms of industrial applications of these materials.

Applications of porous frameworks in solid-phase microextraction

Porous frameworks are a term of attracting solid materials assembled by interconnection of molecules and ions. These trendy materials due to high chemical and thermal stability, well-defined pore size and structure, and high effective surface area gained attention to employ as extraction phase in sample pretreatment methods before analytical analysis. Solid-phase microextraction is an important subclass of sample preparation technique that up to now different configurations of this method have been introduced to get adaptable with different environments and analytical instruments. In this review, theoretical aspect and different modes of solid-phase microextraction method are investigated. Different classes of porous frameworks and their applications as extraction phase in the proposed microextraction method are evaluated. Types and features of supporting substrates and coating procedures of porous frameworks on them are reviewed. At the end, the prospective and the challenges ahead in this field are discussed.

Biodegradation of Poly (Butylene Succinate) (PBS)/Stearate Modified Magnesium-Aluminium Layered Double Hydroxide Composites under Marine Conditions Prepared via Melt Compounding

In the present work, polybutylene succinate (PBS)/stearate modified magnesium-aluminium layered double hydroxide (St-Mg-Al LDH) composites were prepared via melt processing and the effect of different loadings of St-Mg-Al LDH on the degradation behaviour of PBS under marine conditions was investigated. The morphological, mechanical and thermal characteristics of the composites were studied using different characterisation techniques. Optical imaging and scanning electron microscopy revealed that the incorporation of St-Mg-Al LDH accelerates the degradation of PBS along with the activity of microorganisms adhered to the composite films. PBS/St-Mg-Al LDH composites are found to have lower thermal degradation temperatures than those of pure PBS. The decrease in thermal stability is correlated with the degradation of PBS due to the catalytic action Mg and Al present in LDH. Tensile and DMA analysis revealed that the addition of St-Mg-Al LDH did not have a significant impact on the mechanical properties of PBS.

Novel Nanocomposites Based on Bacterial Polyester/LDH-SDS Clay for Stem Cells Delivery in Modern Wound Healing Management

Nanocomposite materials based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and modified mineral clay layered double hydroxides (LDH-SDS) were explored as novel nanostructured materials for potential tissue engineering applications. The mineral clay inorganic phase was modified with an anionic long-chain structure of carbon atoms, such as sodium dodecyl sulfate, in order to increase the compatibility between the two phases. The melt intercalation method used for nanocomposite fabrication ensures a good dispersion of the modified LDH-SDS within the polymer matrix without using a toxic solvent (chloroform). The nanocomposites were found to have an intercalated/exfoliated structure with an enhanced Young modulus and increased stiffness. This could allow them to be considered for autologous stem cells dressings in the view of efficient wound healing applications.

Novel fluoride rechargeable dental composites containing MgAl and CaAl layered double hydroxide (LDH)

OBJECTIVE

This study aims to incorporate 2:1 MgAl and 2:1 CaAl layered double hydroxides (LDHs) in experimental dental-composites to render them fluoride rechargeable. The effect of LDH on fluoride absorption and release, and their physico-mechanical properties are investigated.

METHODS

2:1 CaAl and 2:1 MgAl LDH-composite discs prepared with 0, 10 and 30wt% LDH were charged with fluoride (48h) and transferred to deionized water (DW)/artificial saliva (AS). Fluoride release/re-release was measured every 24h (ion-selective electrodes) with DW/AS replaced daily, and samples re-charged (5min) with fluoride every 2 days. Five absorption-release cycles were conducted over 10 days. CaAl and MgAl LDH rod-shaped specimens (dry and hydrated; 0, 10 and 30wt%) were studied for flexural strength and modulus. CaAl and MgAl LDH-composite discs (0, 10, 30 and 45wt% LDH) were prepared to study water uptake (over 7 weeks), water desorption (3 weeks), diffusion coefficients, solubility and cation release (ICP-OES).

RESULTS

CaAl LDH and MgAl LDH-composites significantly increased the amount of fluoride released in both media (P<0.05). In AS, the mean release after every recharge was greater for MgAl LDH-composites compared to CaAl LDH-composites (P<0.05). After every recharge, the fluoride release was greater than the previous release cycle (P<0.05) for all LDH-composites. Physico-mechanical properties of the LDH-composites demonstrated similar values to those reported in literature. The solubility and cation release showed a linear increase with LDH loading.

SIGNIFICANCE

LDH-composites repeatedly absorbed/released fluoride and maintained desired physico-mechanical properties. A sustained low-level fluoride release with LDH-composites could lead to a potential breakthrough in preventing early stage carious-lesions.

Novel synthesis of Ni/Fe layered double hydroxides using urea and glycerol and their enhanced adsorption behavior for Cr(VI) removal

Novel modified Ni/Fe layered double hydroxides with different morphology of spherical - like shape were fabricated via using urea as a ligand and glycerol (Ni/Fe LDH/GL) with Ni:Fe molar ratios of 2:1 by the simplest co -precipitation method. Also, for comparison purposes, Ni/Fe LDH was synthesized to be used as a control one. A suggested interpretation for the morphology change was also given. The materials were characterized by X-ray diffraction (XRD), The Fourier transform infrared (FT - IR) spectroscopy, field emission scanning electron microscopy (FESEM), EDX for elemental analysis, high resolution transmission electron microscopy (HRTEM), Brunauer, Emmett, and Teller (BET) equation, particle size distributions and Zeta potential measurements. In addition, the synthesized materials were used as adsorbents for removal of potassium dichromate from aqueous solutions under various experimental conditions. The adsorption of Cr (VI) was strongly pH dependant and the pH_PZC was studied. Kinetic studies were evaluated through different models including, pseudo first and second orders, mixed 1, 2 orders, intra particle diffusion and Avrami models. For adsorption isotherms, two-parameter models (Langmuir, Freundlich and Temkin) and three parameter models (Sips, Langmuir-Freundlich and Tooth) were investigated showing maximum adsorption capacity of 50.43 mg/g and 136.05 mg/g for Ni/Fe LDH and Ni/Fe LDH/GL, respectively. Also, the effect of temperature was investigated at (23, 35, 45, 55 °C) and the thermodynamic parameters (∆H°, ∆S° and ∆G°) were calculated showing exothermic and spontaneous adsorption process. The effect of coexisting anions (Cl^-, SO₄^2- and HPO₄^2-) and humic acid at different concentrations on the removal efficiency of dichromate ions was investigated. Chemical stability and recyclability of these adsorbents were also studied. The intermolecular hydrogen bonds formation between dichromate ion, urea, glycerol, LDH was explored by Monte Carlo simulation This study suggested that the modified Ni/Fe LDH/GL materials were promising nanoadsorbents for efficient potassium dichromate removal.

Layered Double Hydroxide Fluoride Release in Dental Applications: A Systematic Review

This systematic review appraises studies conducted with layered double hydroxides (LDHs) for fluoride release in dentistry. LDH has been used as antacids, water purification in removing excess fluoride in drinking water and drug delivery. It has great potential for controlled fluoride release in dentistry, e.g., varnishes, fissure sealants and muco-adhesive strips, etc. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement was followed with two reviewers performing a literature search using four databases: PubMed, Web of Science, Science Direct and Ovid Medline with no date restrictions. Studies including any LDH for ion/drug release in dentistry were included, while assessing the application of LDH and the value of the methodology, e.g., ion release protocol and the LDH production process. Results: A total of 258 articles were identified and four met the inclusion criteria. Based on two in vitro studies and one clinical study, LDH was previously studied in dental materials, such as dental composites and buccal muco-adhesive strips for fluoride release, with the latter studied in a clinical environment. The fourth study analysed LDH powder alone (without being incorporated into dental materials). It demonstrated fluoride release and the uptake of volatile sulphur compounds (VSC), which may reduce halitosis (malodour). Conclusion: LDHs incorporated in dental materials have been previously evaluated for fluoride release and proven to be clinically safe. LDHs have the potential to sustain a controlled release of fluoride (or other cariostatic ions) in the oral environment to prevent caries. However, further analyses of LDH compositions, and clinical research investigating any other cariostatic effects, are required.

Effect of Divalent Metals on the UV-Shielding Properties of MII/MgAl Layered Double Hydroxides

Both the particle size and compositions have a strong influence on the UV-shielding performance of layered double hydroxides (LDHs), and they will interact with each other. To investigate the effects of divalent metal ions on the UV-shielding properties of layered double hydroxides (LDHs), M^II/MgAl-CO₃ LDHs (M = Mg, Co, Ni, Cu, or Zn) with the same primary and secondary particle size have been prepared and their UV-shielding performance have been studied in this work. The UV-vis spectra show that the ZnMgAl-LDHs exhibit the highest absorbance under the ultraviolet B and ultraviolet A rays among these LDH samples, but in the ultraviolet C region, the CuMgAl-LDHs show the highest absorbance and this result is in good accordance with the UV-shielding performance, which was examined by protecting the photocatalytic degradation of rhodamine B aqueous solution. Moreover, under UV rays, PP/ZnMgAl-LDH films show excellent resistance to UV aging, which can be attributed to the strong inhibition of ZnMgAl-LDHs during the production of free radicals in polypropene, as has been confirmed by electron spin resonance results.

Preparation of LDHs Based on Bittern and Its Flame Retardant Properties in EVA/LDHs Composites

Bittern, as a byproduct of salt manufacture, is abundant in China. The researches and developments for seawater bittern have mainly focused on the reuse of magnesium, calcium, lithium, and boron. However, the utilization rate is less than 20%. The large amount of unused bittern has become a challenge that attracts much attention in academic and industry areas. In this paper, three kinds of layered double hydroxides (LDHs) were synthesized from bittern using a coprecipitation method and characterized by X-ray diffraction (XRD). The XRD results showed that the three kinds of LDHs(MgAl-LDHs, MgFe-LDHs and MgAlFe-LDHs) were successfully synthesized. Then, the flame retardant properties and thermal properties of the three LDHs in ethylene vinyl acetate (EVA)/LDHs composites had been tested by cone calorimeter test (CCT), limiting oxygen index (LOI), smoke density test (SDT), and thermogravimetry-Fourier transform infrared spectrometry (TG-IR). The CCT results showed that the heat release rate (HRR) of all three kinds of EVA/LDHs composites significantly decreased compared with that of pure EVA, and the EVA/MgAl-LDHs composites had the lowest PHRR value of 222.65 kW/m2. The LOI results showed that EVA/MgAl-LDHs composites had the highest LOI value of 29.8%. The SDT results indicated that MgAl-LDHs were beneficial to smoke suppression. TG-IR results showed that EVA/MgAl-LDHs composites had a better thermal stability.

Design, fabrication and anti-aging behavior of a multifunctional inorganic–organic hybrid stabilizer derived from co-intercalated layered double hydroxides for polypropylene

The concomitant presence of HALS/DBHP co-intercalated within the LDH structure strongly improved the anti-aging performance of PP, which opens the pathway for potential future research of high-performance PP associated with hybrid fillers.

Aluminum-Magnesium Hydroxycarbonate/Azo Dye Hybrids as Novel Multifunctional Colorants for Elastomer Composites

This study presents the preparation and characterization of new organic-inorganic pigments based on aluminum-magnesium hydroxycarbonate (LH) and azo dyes. Solvent resistance studies, XRD, SEM, and TGA confirmed the successful formation of hybrid pigments, which were characterized in terms of their physicochemical properties. The new hybrid pigments were applied in acrylonitrile-butadiene (NBR) and ethylene-propylene (EPM) rubber composites and cured with sulfur and peroxide curing systems, respectively. The mechanical properties, dispersion quality, and flame-retardant properties of the NBR/hybrid and EPM/hybrid pigment composites were determined by dynamic mechanical analysis (DMA), SEM, and microscale combustion calorimetry (MCC). Complex experimental investigations revealed that the layered nature of hybrid pigments could improve the barrier ability and flame retardancy of elastomer composites. In comparison to unmodified aluminum-magnesium hydroxycarbonate, the modified LH dye structures contributed to significantly decrease the heat release rate and the total heat release of the NBR and EPM composites, offering a new approach to imparting low flammability to elastomer materials.

Co-intercalated layered double hydroxides as thermal and photo-oxidation stabilizers for polypropylene

An elegant and efficient approach consisting in the co-intercalation of stabilizing molecular anions is described here. The thermal stabilizer calcium diethyl bis[[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]phosphonate] (Irganox 1425, MP-Ca) and a photo-oxidation stabilizer (hindered amine light stabilizer, HALS) are co-intercalated into the interlayer regions of layered double hydroxides (LDH) in a one-step coprecipitation. These hybrid organic-inorganic materials are successively dispersed in polypropylene to form H _n M _{n '}-Ca₂Al/PP composite films (with H = HALS and M = MP) through a solvent casting method. The corresponding crystalline structure, chemical composition, morphology as well as the resistance against thermal aging and photo-oxidation are carefully investigated by various techniques. The results show that the powdered H _n M _{n '}-Ca₂Al-LDHs hybrid materials have a much higher thermal stability than MP-Ca and HALS before intercalation. In addition, the H _n M _{n '}-Ca₂Al/PP composites exhibit a higher overall resistance against thermal degradation and photo-oxidation compared to LDHs intercalated with only HALS or MP. This underlines the benefit of the co-intercalation. The co-intercalated LDH materials pave a new way in designing and fabricating high-performance multifunctional additives for polymers.