Comparison of nanoparticle generation by two plasma techniques: Dielectric barrier discharge and spark discharge

Dielectric barrier discharge (DBD) and spark discharge, two versatile atmospheric pressure plasma-based techniques, have been employed to generate nanoparticles. This study compares the characteristics of metal nanoparticles generated by a DBD reactor and a spark discharge generator with argon as the working gas. The gas temperature in the discharge region of the DBD reactor remained near room temperature, while that of the spark reactor varied from 470 to 1120 K and generally increased with increasing applied voltage amplitude in the range of 2–10 kV and driving frequency in the range of 1–10 kHz. Comparing to spark-generated nanoparticles under the same voltage, frequency, and flow rate, DBD-generated nanoparticles have smaller sizes, better monodispersity, and lower number concentrations. The number concentration of DBD-generated particles decreases significantly under high working voltage and frequency, while the number concentration of spark-generated particles increases with increasing working voltage. Under continuous operations over several hours, the DBD reactor has better temporal stability in generating nanoparticles than the spark generator.

© 2017 American Association for Aerosol Research     

Comparative Testing and Evaluation of Nine Different Air Samplers: End-to-End Sampling Efficiencies as Specific Performance Measurements for Bioaerosol Applications

Accurate exposure assessments are needed to evaluate health hazards caused by airborne microorganisms and require air samplers that efficiently capture representative samples. This highlights the need for samplers with well-defined performance characteristics. While generic aerosol performance measurements are fundamental to evaluate/compare samplers, the added complexity caused by the diversity of microorganisms, especially in combination with cultivation-based analysis methods, may render such measurements inadequate to assess suitability for bioaerosols. Specific performance measurements that take into account the end-to-end sampling process, targeted bioaerosol and analysis method could help guide selection of air samplers.

Nine different samplers (impactors/impingers/cyclones/ electrostatic precipitators/filtration samplers) were subjected to comparative performance testing in this work. Their end-to-end cultivation-based biological sampling efficiencies (BSEs) and PCR-/microscopy-based physical sampling efficiencies (PSEs) relative to a reference sampler (BioSampler) were determined for gram-negative and gram-positive vegetative bacteria, bacterial spores, and viruses.

Significant differences were revealed among the samplers and shown to depend on the bioaerosol's stress–sensitivity and particle size. Samplers employing dry collection had lower BSEs for stress-sensitive bioaerosols than wet collection methods, while nonfilter-based samplers showed reduced PSEs for 1 μm compared to 4 μm bioaerosols. Several samplers were shown to underestimate bioaerosol concentration levels relative to the BioSampler due to having lower sampling efficiencies, although they generally obtained samples that were more concentrated due to having higher concentration factors.

Our work may help increase user awareness about important performance criteria for bioaerosol sampling, which could contribute to methodological harmonization/standardization and result in more reliable exposure assessments for airborne pathogens and other bioaerosols of interest.

Copyright 2014 American Association for Aerosol Research     

Effect of desensitizers on the microleakage of previously restored Class V resin composite restorations

Objectives: The aim of this in vitro study was to evaluate the effect of different desensitizers’ application on the microleakage of previously restored Class V composite resin restorations.

Materials and methods: Class V cavities were prepared on the buccal surfaces of 40 extracted human third molars. Forty box-shaped cavities were divided into four groups, based on the desensitizers used (n = 10). All teeth were restored with the same bonding agent and composite material. No desensitizer was applied in the control group. In the experimental groups, BisBlock, Gluma and Universal bonding agents were the desensitizers. The desensitizers were applied after completion of composite restorations according to manufacturers’ instructions. All specimens were then thermocycled at 5–55 °C, with a 10-s dwell time for 500 cycles. The samples were then immersed in 0.5% methylene blue dye for 24 h, sectioned into two equal halves, evaluated for microleakage using a stereomicroscope at 30× magnification and scored on a scale of 0–3. The data were analysed using the Kruskal–Wallis test at the significance level p < 0.05.

Results: There were no significant differences in microleakage after desensitizer application (p > 0.05). However, based on the obtained numerical values in our study, while the BisBlock and bonding groups showed lower microleakage at the occlusal margin, BisBlock, Gluma and bonding group showed lower microleakage at the gingival margin compared to the control group.

Conclusions: The application of desensitizers as a post-treatment option could be considered an advisable procedure to minimize microleakage.     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

Prototype e-cigarette and the capillary aerosol generator (CAG) comparison and qualification for use in subchronic inhalation exposure testing

Our objective was to evaluate the suitability of using a capillary aerosol generator (CAG) instead of using e-cigarette devices in 90-day or longer inhalation studies. Aerosol characteristics for both the CAG (which uses heat to produce a condensation aerosol) and e-cigarette generators have been previously reported, but a side-by-side comparison with the identical formulation has not been reported. Aerosols from both devices were analyzed immediately after generation for chemicals in the formulation (propylene glycol [PG], glycerin, water, and nicotine), selected carbonyls (acetaldehyde, acrolein, and formaldehyde) by ultra-performance liquid chromatography with ultraviolet detection (UPLC-UV), and a chemical fingerprint analysis using gas chromatography-mass spectroscopy (GC-MS). Aerosol capture methods for chemical analysis included Cambridge filter pads or two impingers in series each containing solution to trap and stabilize selected carbonyl compounds. Particle size distribution (cascade impactor) and exposure port uniformity (gravimetric) was measured in four rodent inhalation exposure chambers under inhalation study conditions. The aerosol of both generators contained the same known and unknown chemicals. Similar levels of compounds in the formula except for PG were detected in the aerosol of both generators. CAG produced more consistent particulate aerosol than e-cigarette generator and had lower levels of carbonyls primarily due to lower levels of formaldehyde. Exposure port concentrations were consistent and closer to target values with the CAG compared to the e-cigarette aerosol generator. CAG was easier to operate on a daily basis although more difficult to maintain because it required daily cleaning compared to single-use e-cigarettes. CAG was determined to be suitable for use in 90-day or longer inhalation studies.

Copyright © 2016 American Association for Aerosol Research     

A New Clean Air Delivery Rate Test Applied to Five Portable Indoor Air Cleaners

Air pollution has been recognised as one of the major risk factors for the global burden of disease. In modern society the majority of the exposure occurs indoors where people spend most of their time. Indoor air quality may be improved with portable air cleaners utilizing various cleaning techniques, such as filtration, electrostatic precipitation, and ionization. The objective of this study was to quantify air cleaner particle removal by particle size resolved clean air delivery rates (CADR). This was obtained by utilizing particle concentration measurements and indoor aerosol modeling. Our test protocol was applied to five air cleaners designed for household and office use. For particles with diameters above 100 nm and at the chosen settings, the CADR was around 40 m³/h for an ion generator, around 70 m³/h for an electrostatic precipitator, and ranging from 100 to almost 300 m³/h for the three filter-based air cleaners. Similar performances were obtained for ultrafine particles, except for the ion generator that performed better in this size range.

Copyright 2014 American Association for Aerosol Research     

Recent Advances in Flame Retardancy of Polymeric Materials (Materials,Applications, Industry Developments,Markets)

The Fifth Annual Conference on Flame Retardancy was held May 24–26, 1994, at the Ramada Plaza Hotel, Stamford, Connecticut, USA. The conference was organized by Business Communications Company, Inc., Norwalk, Connecticut (Company President, Mr. Louis Naturman; Conference Coordinator, Mrs. Sharon D. Faust). New materials (polymers, blends, composites), their applications, industry developments, and markets were considered. Specifically, the most important topics were:

Introduction of new technological achievements and development in the field of flame retardancy (FR)

Review of the current state of science and technology in FR

Review of applications and markets for FR products

Presentation of recent developments in local and global standardization and in testing technology

Discussion of toxicity and environmental issues

Provision of a unique opportunity for newcomers to FR research technology and marketing to become acquainted with the FR field in all its aspects

Discussion of halogen-based and non-halogen-based flame retardant chemicals, syngergism, intumescence, FR mechanisms, modeling, flame parameters, inherently FR polymers, and polymer blends     

Do phosphoric acid esters affect the immediate enamel bond strengths?

Purpose: To investigate the effects of phosphoric acid esters (PAEs)-containing primers on the micro-tensile bond strengths (MTBS) of etched enamel, the micro-morphologies of the resin–enamel interfaces, and the enamel surfaces.

Materials and Methods: Thirty-three bovine incisors were used in this study. After the labial enamel surfaces were highly polished, they were etched, water sprayed, and air-dried. Afterward, the enamel surfaces were treated with or without (control) one of three PAEs-containing primers (40%MDP, Ivoclar Vivadent; Resulcin AquaPrime A + B, DMG; Xeno V, Dentsply), water sprayed, and air-dried. Subsequently, an adhesive Heliobond (Ivoclar Vivadent) was applied and resin composite (Core Build-up, Bisco) was placed incrementally. They were prepared into multiple beams of about 1 × 1 × 8 mm for MTBS tests. The enamel surfaces and resin–enamel interfaces were analyzed by SEM and TEM. The data were analyzed by two-way ANOVA and LSD test.

Results: Using PAEs-containing primers could increase the enamel MTBS (p < 0.05). The primer MDP produced higher MTBS than the primer Resulcin AquaPrime A + B and Xeno V. The SEM findings revealed the primed enamel surfaces were covered with a thin or glue-like layer of monomer-calcium salts of PAEs and the residual enamel crystallites, and various micro-porosities were detected within the hybrid layers. The TEM findings revealed the hydroxyapatite crystallites were tightly covered by the adhesive, the adhesive mixed with resin composite, or sparse irregular enamel crystallites.

Conclusion: The application of PAEs-containing primers on the etched enamel substrate could significantly increase the immediate enamel bond strengths. However, this effect was dependent on the individual PAEs-containing primer used.     

Effect of different application techniques on microshear bond strength of self-etch adhesives to dentin

Objectives: To investigate the effect of different self-etch adhesive systems application techniques: active or passive in a single or double layer on adhesive–dentin microshear bond strength.

Methods: Occlusal surfaces of 48 extracted human molars were ground to expose flat superficial dentin surfaces. Specimens were randomly divided into two main groups according to the tested self-etch adhesive system either: One-step self-etch (Adper^TM easy-one) or two-step self-etch (Adper^TM SE Plus). Each adhesive system was applied on the prepared dentin surfaces followed one of these techniques: (1) Passive application of a single layer, (2) Active application of single layer, (3) Passive application of double adhesive layer (with light curing in between), and (4) Active application of double adhesive layers. Resin composite was packed inside micro-tubes fixed on the bonded dentin surfaces and light cured for 40 s. All specimens were stored in artificial saliva either for 24 h or 3 months before testing. Microshear bond strength test was employed using a universal testing machine at a crosshead speed of 0.5 mm/min.

Results: Adper^TM SE Plus showed higher significant microshear bond strength in compared with Adper^TM easy-one. For both adhesive systems active application showed higher significant microshear bond strength to dentin than passive application. Double application of adhesive systems showed lower microshear bond strength than single application.

Conclusion: Active application of self-etch adhesives could improve the dentin microshear bond strength. Double application with curing in between the layers did not improve the bond strength to the tested adhesive.     

Controlling Blown Polyethylene Film Optical Properties

The conventional blown process imparts an inherent haze to the product. The percentage of haze varies with certain process variables:

1. Surface irregularities caused by melt flow phenomena

2. Crystallization behavior

3. Melt drawing phenomena in certain types of polyethylene     

Modeling of nonstationary processes during separation of multicomponent isotope mixtures

The article presents a mathematical model for calculation of nonstationary hydraulic and separation processes in a gas centrifuge (GC) cascade for separation of multicomponent isotope mixtures. The model has been applied to calculate the parameters of nonstationary processes in a GC cascade for separation of krypton, germanium and tungsten isotopes. As a result, the specifics of the excess holdup distribution along the cascade stages has been identified, and variations of the isotope concentrations in a nonstationary process have been revealed. The data obtained show that the proposed mathematical model is able to adequately describe nonstationary hydraulic processes in GC cascades for separation of multicomponent isotope mixtures.

Highlights:

Mathematical model of cascade for separation of multicomponent isotope mixture has been developed.

The model verification has been done.

The isotope transient regularities into cascade during nonstationary processes has been identified.     

Measurement of the Nanoparticles Distribution in Flat and Pleated Filters During Clogging

It is currently admitted that for each filtration process using pleated filters, at least three steps can be distinguished: depth and surface filtration, which are common to flat filters, and surface reduction. This step is caused by inefficient filling of the pleat due to the filter geometry. For combustion aerosol, it has been proved that this third step strongly depends on the filtration velocity resulting in an increase of the resistance when air flow decreases. This observation leads one to think that Brownian diffusion, higher for low velocities, could influence the clogging dynamic of a pleated filter.

In this article, a protocol derived from the dust cake preparation method published by Schmidt is developed. The aim of this study is to measure the aerosol penetration inside a filter media as well as in a pleat using a scanning electronic microscope and energy dispersive X-ray spectroscopy elementary detection. This method has also been extended to the study of pleated filters to measure the particle distribution inside the pleat. Filters were loaded with nanoparticles in order to evaluate the specificity of the diffusional regime on the clogging of pleated HEPA filters. For pleated filters, two filtration velocities were investigated: 2.5 and 0.2 cm/s.

Copyright 2014 American Association for Aerosol Research     

Conceptual model-based design and environmental evaluation of waste solvent technologies: Application to the separation of the mixture acetone-water

In this paper, two waste solvent technologies are presented as alternatives to the disposal of spent acetone-water mixtures.

In the first alternative, a batch rectifier is used to concentrate the waste in order to obtain a distillate with a higher calorific value, which is then sent to off-site incineration either in a cement kiln or in a conventional waste solvent incinerator. The second alternative is a hybrid process composed by a batch rectifier and a pervaporation unit that processes in batchwise mode the first cut from the distillation task to obtain a dehydrated solvent. Here, four scenarios are considered, comprising two kinds of membrane materials and two different vacuum systems.

For each alternative, the conceptual design was carried out with the aid of conceptual models of the unit operations involved. Quasi-optimal values for design and operation variables were used as input data to perform an economical and an environmental assessment of each alternative. The economic analysis suggests that the hybrid process is the best alternative given that the replacement cost of fresh solvent (about 850 U$S/ton) is considered as a credit value.

From the environmental analysis with life cycle assessment, two main conclusions can be drawn: i) the use of the distillate as an alternative fuel in a cement kiln leads to a reduction in emissions that is relevant for the categories related to human health and ecosystem quality; and ii) in terms of resource depletion, the hybrid process distillation/pervaporation with the ceramic membrane HybSi (Pervatech) shows the lowest impact due to the solvent recovery.     

Evaluation of the shear bond strength of two resin cements on different CAD/CAM materials

Objective: This study investigated the bond strength of two resin cements (Panavia F 2.0 and Multilink N) to different CAD/CAM materials: resin nanoceramic (RNC; Lava Ultimate), hybrid ceramic (HC; Vita Enamic), zirconia-reinforced lithium silicate ceramic (ZLDC; Vita Suprinity), and lithium disilicate glass-ceramic (LDG; IPS e.max CAD HT).

Material and methods: CAD/CAM blocks of 2-mm thickness were sectioned with a slow-speed diamond-saw sectioning machine. The slabs were then embedded in autopolymerizing acrylic resin (n = 12), and resin cements were applied to the surface of the specimens. All specimens were stored in water for 24 h and subjected to 5000 thermal cycles. Bond strength was measured by means of the shear bond strength test. The data were statistically analyzed by two-way ANOVA and Tukey LSD post hoc tests.

Results: The results of the two-way ANOVA test indicated that the bond strength values varied significantly depending on the CAD/CAM restorative materials, resin cements, and interaction of these variables (p < 0.05). The RNC group showed the highest bond strength for Panavia F 2.0 (p < 0.05); there were no significant differences among other CAD/CAM materials for Panavia F 2.0 (p > 0.05). The LDG group showed the highest bond strength for Multilink N, it was followed by the ZLDC group (p < 0.05); there were no significant differences between RNC and HC groups (p > 0.05).

Conclusions: Choosing resin cements for restorations should be done carefully because bond strength values vary significantly depending on the resin cement and CAD/CAM restorative material.     

The Use of Reverse Osmosis in the Removal of PAHs from Municipal Landfill Leachate

Polycyclic aromatic hydrocarbons (PAHs) are relatively well-known organic pollutants and due to their carcinogenic and mutagenic properties their presence in the environment still attracts a lot of attention.

According to literature reports and own research, PAHs presence in wastewaters is common. It was confirmed that PAHs are the components of municipal landfill leachate. Membrane techniques are one of the most interesting ways of removing PAHs from leachate.

The purpose of this article is to monitor PAHs concentration changes during the membrane (reverse osmosis - RO) leachate treatment processes. In the first stage of testing leachates were filtrated on the sand bed (pre-filtration). After the pre-filtration they were directed to the membrane module for the main filtration.

Sixteen PAHs listed by EPA were analyzed. The results with information on PAHs concentration in leachate samples were presented using HPLC with fluorescence detection (FLD). The changes in PAHs concentration were determined in leachate samples before and after pre-filtration as well as after RO. The decrease of PAHs concentration in the samples was observed after these processes. The total concentration of 16 PAHs in raw municipal landfill leachates amounted to 23.64–26.95 μg/L. The research confirmed the high efficiency in removal of PAHs while using a reverse osmosis (59–72%). Including the pre-filtration, the overall level of removed PAHs reached 81–86%. The average PAHs concentration after pre-filtration and RO was in the 4.46–4.99 μg/L range. The municipal landfill leachate with a high concentration of PAHs should be cleaned before it is discharged into the environment.     

Effect of monolithic zirconia on the degree of conversion of two resin cements analyzed by FT-IR/ATR spectroscopy

Objective: This study aimed to evaluate the degree of conversion (DC) of two different resin cements polymerized under the monolithic zirconia specimens in different thicknesses and colors.

Material and methods: Partially stabilized monolithic zirconia blocks (inCoris TZI) were cut into three different thicknesses (0.5, 1.0, and 2.0 mm) and the specimens were divided into four color groups (A1, A2, A3, and A4). The light transmittance of each specimen was measured. Panavia F 2.0 or Variolink N resin cement was applied into teflon mold and irradiated using the light emitting diode curing unit for 20 s under monolithic zirconia specimen (n = 10). The resin cement specimens were stored at room temperature under dry conditions. The DC of each specimen was measured by Fourier transform infrared-attenuated total reflection (FT-IR/ATR) spectroscopy after the 1st and 10th day. Data were analyzed with two-way analysis of variance (ANOVA), two-way repeated measures ANOVA, three-way repeated measures ANOVA, and the Tukey least significant difference (LSD) tests (α = 0.05).

Results: The light-cure resin cement groups showed higher DC than the dual-cure resin cement groups (p < 0.05). The DC of both resin cements reduced with an increase in the thickness and darkening of the color of monolithic zirconia specimens. There was a statistically meaningful increase in the 10th-day values for dual-cure resin cement (p < 0.05), whereas there were no significant differences between the 1st- and 10th-day values for light-cure resin cement (p > 0.05).

Conclusion: The use of light-cure resin cement can be suggested for the luting of monolithic zirconia restorations.     

Residual monomer release from orthodontic adhesives cured with different light sources

Introduction: The aim of this study was to evaluate the residual monomer release from orthodontic adhesives cured with light-emitting diode (LED) and halogen light sources.

Methods: Seven hundred and twenty stainless steel brackets were divided into 3 groups according to the adhesive system used (Transbond XT light-cure adhesive [TXT], Transbond LR capsule [LR], and Light Bond light-cure adhesive paste [LB]), and each group was divided into 2 subgroups according to light-curing procedure (LED or halogen). Brackets were bonded with adhesives onto tooth buccal surfaces and polymerized. Each specimen contained 24 brackets that simulated the oral environment (n = 5). The specimens were immersed in a 75% ethanol/water solution at 37 °C for 10 min, 1 h, 1 d, 7 d, 14 d, and 30 d, respectively. Eluted monomers (Bis-GMA, UDMA, and TEGDMA) were detected using HPLC.

Results: There was residual monomer release at all time periods, and the highest amount of release was observed cumulatively on the 30th day. The cumulative Bis-GMA released from adhesives was not different (p > 0.05). The cumulative TEGDMA released from adhesives was statistically different (p < 0.05). There was no statistical difference between QTH and LED light-curing units for each adhesive (p > 0.05).

Conclusions: The release of residual monomers stays at a high level for a long time after polymerization. The total leaching of residual monomers from the Light Bond light-cure sealant resin plus Light Bond light-cure adhesive paste was higher than that of other materials for both curing units. Different curing units (LED or QTH) did not affect the monomer release from the orthodontic adhesives.     

NaOCl degradation of one-step one-bottle self-etch adhesives bonded to bovine dentine

Objective: The aim of the present study was to assess the influence of an in vitro aging regime (NaOCl storage) on dentine microtensile bond strength (μTBS) of five one-step one-bottle self-etch adhesives in comparison with bond strength of two-step self-etch adhesive.

Material and methods: Flattened dentine surfaces from 30 bovine incisors were bonded with five one-step one-bottle self-etch adhesives (iBond, Clearfil S3 Bond, AdheSE One F, G-Bond, Optibond all-in-one) and one two-step self-etching adhesive (Clearfil SE Bond). Composite buildups were done with microhybrid resin composite. Bonded samples were sectioned into resin–dentine sticks of 0.8 mm² cross section. Randomly selected 20 sticks were tested directly by microtensile bond strength testing machine, whereas another randomly selected 20 sticks were tested after being stored in solution of 10% NaOCl for 5 h. Data were analyzed by two-way analysis of variance (ANOVA) and Tukey’s honest significant difference (HSD) tests (p < 0.05).

Results: All adhesives exhibited similar dentine bond strength with exception of G-Bond. After NaOCl storage, μTBS reduced in all groups significantly. The lowest μTBS were found for G-Bond. Rest of one-step self-etch adhesives presented similar μTBS with two-step self-etch adhesive.

Conclusion: Interfaces of resin–dentine bonding are susceptible to NaOCl degradation. The amount of the destruction depends on adhesive system. NaOCl degradation of the nonresin encapsulated collagen fibers might decrease long-term stability of resin bonding with dentine.