Acrylonitrile Butadiene Styrene Research Articles

Toward improving the compatibility of the polypropylene (PP)/acrylonitrile–butadiene–styrene (ABS) blends through the incorporation of SEP and SEBS copolymers

Toward improving the compatibility of the polypropylene (PP)/acrylonitrile–butadiene–styrene (ABS) blends through the incorporation of SEP and SEBS copolymers

Development of a phantom for assessing the precision of setup in skin mark-less surface-guided radiotherapy.

Surface-guided radiotherapy (SGRT) is adopted by several institutions; however, reports on the phantoms used to assess the precision of the SGRT setup are limited. The purpose of this study was to develop a phantom to verify the accuracy of the irradiation position during skin mark-less SGRT. An acrylonitrile butadiene styrene (ABS) plastic cube phantom with a diameter of 150mm on each side containing a dummy target of 15mm and two types of body surface-shaped phantoms (breast/face shape) that could be attached to the cube phantom were fabricated. Films can be inserted on four sides of the cubic phantom (left, right, anterior and posterior), and the center of radiation can be calculated by irradiating the dummy target with orthogonal MV beams. Three types of SGRT using a VOXELAN-HEV600M (Electronics Research&Development Corporation, Okayama, Japan) were evaluated using this phantom: (i) SGRTCT-a SGRT set-up based solely on a computed tomography (CT)-reference image. (ii) SGRTCT + CBCT-a method where cone beam computed tomography (CBCT) matching was performed after SGRTCT. (iii) SGRTScan-a resetup technique using a scan reference image obtained after completing the (ii) step. Both the breast and face phantoms were recognized in the SGRT system without problems. SGRTScan ensure precision within 1mm/1° for breast and face verification, respectively. All SGRT methods showed comparable rotational accuracies with no significant disparities. The developed phantom was useful for verifying the accuracy of skin mark-less SGRT position matching. The SGRTScan demonstrated the feasibility of achieving skin-mark less SGRT with high accuracy, with deviations of less than 1mm. Additional research is necessary to evaluate the suitability of the developed phantoms for use in various facilities and systems. This phantom could be used for postal surveys in the future.

Design and build of a portable apparatus for measuring lace tension

Laces have traditionally been used to secure the foot, limit slippage, enhance fit, and prevent injury across different types of footwear. Quantitatively assessing the merits and effectiveness of laces is technically challenging due to the lack of portable instrumentation that can measure lace tension reliably. Therefore, the purpose of the study was to design and build a portable apparatus to quantify lace tension to be used on footwear in both laboratory and real-world environments. The apparatus was designed to meet three major design criteria: (i) portable, (ii) able to accommodate different types of footwear, lace materials, and lacing patterns, and (iii) able to measure lace tension while the footwear is secured on the foot. As a result of the design process, the apparatus consisted of a base, fabricated from High Density Polyethylene (HDPE), and a frame, made from aluminum and 3D printed Acrylonitrile Butadiene Styrene (ABS). A measurement system was affixed to the frame and consisted of a lever, a non-deformable cable with a hook, a load cell, a caliper, and a microcontroller to measure the force and change in length of laces when a force was applied. The total height and weight of the apparatus was 25.5 cm and 6.35 kg, respectively. A reliability analysis was conducted using three different types of laces (waxed, non-waxed, and carbon fiber blend) and revealed a high internal consistency within lace types with alpha values of 0.95, 0.81, 0.91, respectively. The interclass reliability coefficient across lace types revealed an alpha value of 0.84. As a result of the design, build, and reliability analysis, the apparatus was able to provide reliable measurements of lace tension while satisfying the design criteria. It is envisioned that the apparatus can be used for ongoing investigations across different types of footwear and different types of laces.

Effect of acetone treatment and copper oxide coating on the mechanical and wear properties of 3D-printed acrylonitrile butadiene styrene structures

ABSTRACT 3D-printed ABS samples are treated with acetone and coated with copper oxide (CuO) for varying durations to understand how these treatment processes affect their mechanical and tribological properties. Tensile strength, flexural strength, coefficient of friction, and wear of ABS were analyzed under various pre-treatment and coating conditions and it is proved that acetone treatment and CuO coating can be effectively used to tune such properties to meet desired specifications. For 3D-printed ABS structures, CuO coating significantly enhances their tensile strength without acetone treatment. Although acetone treatment reduces their tensile strength as demonstrated, CuO coating can increase it again which means their tensile strength can be tuned by using the two processes. Furthermore, both acetone treatment and CuO coating improve their flexural strength. Additionally, acetone treatment and CuO coating both decrease their coefficient of friction. FESEM and EDS analyses were conducted on the structures to analyze and verify their wear properties.

Sustainable strategies towards better utilization of synthetic polymers.

The abstract provides an overview of a study focused on analyzing diverse strategies to achieve sustainable utilization of synthetic polymers through effective waste management. The escalating global consumption of synthetic polymers has precipitated a concerning increase in plastic waste and environmental degradation. To address this challenge, novel materials with specified application goals, such as engineered plastic, have been developed and are intended for recycling and reuse. Despite the reuse and recycling, when plastic gets disposed into the environment, the degradation properties of plastics render a direct disposal hazard, posing a significant environmental threat. To mitigate these issues, the concept of replacing specific monomers of engineered synthetic plastics with bio-alternatives or blending them with other polymers to enhance sustainability and environmental compatibility has emerged. In this study, Acrylonitrile Butadiene Styrene (ABS) plastic is the focal material, and three distinct investigations were conducted. First, replacing ABS plastic's butadiene monomer with natural rubber was explored for its properties and environmental impact. Second, ABS plastic was blended with virgin, recycled, and bio-alternatives of PET (polyethylene terephthalate) and PVC (polyvinyl chloride) polymers. Lastly, recycled ABS blended with recycled PET and PVC was analyzed for mechanical properties. Comparative assessments of these blends were made based on mechanical properties, carbon emissions, and cost-effectiveness. The study determined that the r-ABS/r-PVC (recycled) blend exhibited the most favorable characteristics for practical application.

Preparation and performance evaluation of microencapsulated acid used for gel breaking of fracturing fluid in low-temperature reservoirs

In low-temperature reservoirs, the generation of free radicals by oxidizing breakers is challenging, and the alkaline fracturing fluid generally inhibits enzyme’s activity, making it difficult to break down the fracturing fluid. Previous studies have demonstrated that acids can consume borate ions and disrupt crosslinking nodes even at low temperatures, thereby efficiently breaking gels. Thus, microencapsulated oxalic acid (OA) was prepared via a coacervation method, using ethyl cellulose (EC) or acrylonitrile–butadiene–styrene (ABS) as shell materials with varying initiator viscosities and core–shell ratios. An optimal microcapsule MC[OA/ABS-2000-1:2] was determined based on the coating effect, productivity, encapsulation rate, and release performance. The test results of FT-IR and TGA together proved that the optimal microcapsule was composed of 24.4 and 74.8 % of OA and ABS, respectively. Scanning electron microscopy and laser particle size analysis were employed to characterize the optimal microcapsule with an average particle size of 402.8 μm, which exhibited a high degree of sphericity. Because of their porous shells, these microcapsules exhibited sustained slow-release characteristics. Evaluations conducted at temperatures of 30 and 50 ℃, simulating the tight sandstone reservoir conditions in the Ordos basin, revealed that delayed gel breaking could be achieved within approximately 2.5 h at low temperatures by adding only 0.25 % of the optimal microcapsule. Furthermore, increasing the temperature and dosage of microcapsules resulted in shorter breaking times. Additionally, a synergistic breaking effect was observed between the enzymes and the prepared microencapsulated acid. Thus, by optimizing the microcapsule dosage based on the reservoir temperature and fracturing construction pumping time, employing the two breakers together can achieve delayed and complete gel breaking at the desired time. In conclusion, the microencapsulated acid prepared in this study has significant potential as a gel breaker for fracturing fluids in low-temperature reservoirs.

Design of melting parameters for safety airbag labels based on hot air welding technology: CFD simulation and experimental validation

This paper focuses on researching the various factors that influence the material melting rate during the hot air melting process. To achieve this purpose, the paper proposes a melting model for Acrylonitrile-Butadiene-Styrene (ABS) plastic under hot air heating based on Computational Fluid Dynamics (CFD). Orthogonal experiments were conducted based on the parameter range of each factor obtained in the pre-experiment. The results showed that the outlet distance, hot air velocity, hot air temperature, and their interaction items are important factors that affect the melting rate of ABS materials. Based on the simulation results, a multiple nonlinear regression model was constructed to predict the melting rate under different parameters. Comparing the simulation values with the predicted values of the multiple nonlinear regression model, the error between the two was no >10%, indicating the effectiveness of the prediction model. The research results can help guide the hot air welding melting process for different materials and characteristics, and provide a certain reference value for the development of hot air welding technology.

Impact of infill density on morphology and mechanical properties of 3D printed ABS/CF-ABS composites using design of experiments

Metal Extrusion (MEX) is a leading 3D printing technology for polymers, enabling intricate designs and personalized products in various applications. The current study evaluate how infill density affects the tensile, flexural, compressive, Izod impact and fracture behaviour of Acrylonitrile Butadiene Styrene (ABS) and Carbon Fiber Reinforced-Acrylonitrile Butadiene Styrene (CF-ABS) specimens manufactured using the MEX method. Different infill densities of 20, 40, 60 and 80 % are used in the production of honeycomb infill pattern samples for investigating the mechanical as well as fracture behaviour of MEX ABS/CF-ABS components. The experimental runs of fabricated composites were tested using a digital Izod impact tester and servo-controlled hydraulic universal testing machine, following ASTM standard procedures. The experimental findings show that CF-ABS specimens with an 80 % infill density and honeycomb fill pattern showed significant improvements in tensile strength, modulus, yield strength and elongation. The flexural strength (64.74 %), flexural modulus (209.15 %), compressive strength (125.21 %), compressive modulus (108.34 %) and impact strength (38.91 %) of these specimens are comparable to those of 3D printed ABS specimens and other infill densities. The research shows that precise management of processing variables can greatly improve the mechanical properties of 3D-printed ABS samples, providing valuable insights for a range of applications.

Microplastics and Nano-plastics Contamination in Foods: Current Understanding of the Health Impact on Human and Potential Solutions

Plastics released into the environment can be degraded by physical erosion, biodegradation, photocatalytic activity, and oxidation, resulting in smaller plastics particles. Microplastics (MPs) are plastic particles relatively smaller than 5 mm in size. Furthermore, microplastics with particle sizes less than 1000 or 100 nm are known as nano-plastics (NPs). The presence and effect of MPs and NPs in human body has not been adequately studies, thus we aim to explain the origins of MPs and NPs adept, carefully explore the pathways by which MPs and NPs enter the body system and highlight the impact of MPs and NPs on human health. Major examples include Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyamide (PA), polystyrene (PS), polyethylene terephthalate (PET), Acrylonitrile-Butadiene-Styrene (ABS), and polymethyl methacrylate (PMMA). MPs and NPs were found to have two sources (primary and secondary). The primary sources are materials purposely produced to suit household and industrial uses, such as exfoliants for skin care products, construction, and packaging materials. Secondary sources originated from the decomposition/degradation of large plastic products over time. Plastics were found in plants (fruits and vegetables), animals (fish, crab, shrimp, oysters, and mussels), water (taps, sachet, and bottled water), salt, sugar, and honey. Plastics' impact on the human ecosystem is getting increasingly severe, and it is imperative that proper attention be paid to this issue. The vast number of MPs and NPs available can influence the lives of the populace in each geographic area. In this review article, we identified three routes through which MPs and NPs gain entrance into the human body: oral ingestion, cutaneous (skin contact), and inhalation. Furthermore, we investigated and summarized the impacts of MPs and NPs on human health. The most impacted organs in the body included the lungs, blood, kidney, brain, ovary, testes, and intestines. In this review, we offered a viable solution that includes the use of biodegradable polymers, increased usage of eco-friendly biotechnology and engineering solutions, and the implementation of regulatory measures. In future, we intend to investigate the bioaccumulation and effect of MPs/NPs on human health.

Optimisation of injection process parameters to minimise mould deflection: design of experiment

ABSTRACT This research aimed to estimate the mould deflection by installing the strain gauges on the modified injection mould, which was analysed by the analysis of variance (ANOVA) method. Also, studied the correlation of the maximum mould deflection, thickness and defect of the moulded part. The optimisation of the injection parameters to minimise the mould deflection and defect was examined. There are seven processing factors with three levels of melting temperature, mould temperature, injection pressure, holding pressure, injection speed, holding time and cooling time, which are designed by using Taguchi’s experimental design. The parameter factors are statistically significant with P-values less than 0.05. The levels of those processing parameters are compared by Tukey pairwise comparisons using the Tukey method and 95% confidence. The optimum processing conditions attaining the lowest value of the maximum mould deflection of polypropylene (PP) and Acrylonitrile butadiene styrene (ABS) parts were investigated. The results showed that the mould deflection shows a good relation to those of thickness and flash defect; however, the occurrence of the short shot part presented a weak relation.

An Antimicrobial Copper-Plastic Composite Coating: Characterization and In Situ Study in a Hospital Environment.

A method has been proposed for creating an operationally durable copper coating with antimicrobial properties for the buttons of electrical switches based on the gas dynamic spray deposition of copper on acrylonitrile butadiene styrene (ABS) plastic. It is shown that during the coating process, a polymer film is formed on top of the copper layer. Comparative in situ studies of microbial contamination have shown that the copper-coated buttons have a significant antimicrobial effect compared to standard buttons. Analysis of swabs over a 22-week study in a hospital environment showed that the frequency of contamination for a copper-coated button with various microorganisms was 2.7 times lower than that of a control button. The presented results allow us to consider the developed copper coating for plastic switches an effective alternative method in the fight against healthcare-associated infections.

High-temperature energy storage with a new tri-layers polymer composites via hybrid assembly engineering

Dielectric film capacitors are fundamental components in advanced electrical fields such as smart grids and hybrid electric vehicle. The commercial film capacitors made by biaxially oriented polypropylene (BOPP) have high energy efficiency, but low energy density of only 2.0-3.0 J/cm3, while the inferior thermal stability restricts their high temperature applications. In this work, hybrid assembly engineering is proposed to design composite films with a new polymer of poly(acrylonitrile butadiene styrene) (ABS) as the matrix, boron nitride nanosheets (BNNS) and Na0.5Bi0.5TiO3-Sr0.7Bi0.2TiO3 (NBT-SBT) as two different fillers to improve high-temperature performance. The optimized composites SBS (NBT-SBT/ABS composites layer in the outside and BNNS/ABS composites layer in the middle) exhibit excellent high temperature energy storage characteristics, and its underlying mechanism is also understood by phase-field simulations. In particular, the maximum energy density at 120 °C can reach 15.0 J/cm3 at 575 MV/m, which is 8 times that of BOPP, while the efficiency is maintained at 89 %, far exceeding the performance of BOPP (<70 % at 120 °C). Together with their excellent cycling reliability (106 cycles) and thermal stability, this strategy shows a great potential for high-temperature and high-power energy storage capacitors.

Fatigue response of multiscale extrusion-based additively manufactured acrylonitrile butadiene styrene-graphene nanoplatelets composites

This study investigates the effect of varying concentrations of graphene nanoplatelets (GNPs) on the mechanical and fatigue characteristics of acrylonitrile butadiene styrene (ABS)/GNPs nanocomposites, both in filament form and as 3D-printed parts with different raster orientations. Quasi-static and cyclic loading tests were performed on specimens containing 0.1 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% GNPs. The results indicated that the addition of 0.1 wt% GNPs to the ABS polymer matrix increased the ultimate tensile strength (UTS) of the filament by 18 %. Addition of 1.0 wt% and 2.0 wt% GNPs to the matrix improved the Young's modulus of the filament by up to about 35 % but did not enhance the UTS. The filament containing 0.5 wt% GNPs demonstrated the highest fatigue life, even higher than that of the standard samples fabricated through injection molding technique. This was due to its high static strength, Young's modulus, and ductility. However, when the ABS/GNPs nanocomposites were 3D-printed, the addition of GNPs had no major impact on their fatigue life, especially in the higher load levels. Inadequate adhesion and reduced interlayer bonding strength resulted in a detrimental impact on fatigue life of 3D-printed objects. The extrudate-swell phenomenon seems to significantly affect this reduction in fatigue life. To address this, larger nozzle diameter (0.6 mm) was used in fabricating 3D-printed samples. Results indicated that samples with less distortion and smaller extrudate-swell had higher fatigue life. It highlights the importance of considering GNP concentration and printing parameters for optimal fatigue properties in ABS/GNPs composites.

The Effects of Printing Temperature on the Mechanical Properties of 3D-Printed Acrylonitrile Butadiene Styrene

The Effects of Printing Temperature on the Mechanical Properties of 3D-Printed Acrylonitrile Butadiene Styrene

Mechanical Properties and Performance of 3D-Printed Acrylonitrile Butadiene Styrene Reinforced with Carbon, Glass and Basalt Short Fibers.

This paper presents the results of experimental investigation of the mechanical characteristics of 3D-printed acrylonitrile butadiene styrene (ABS) and its modifications reinforced with different types of short-fiber fillers: carbon, glass, and basalt. Elastic modulus, tensile and bending strength, as well as fracture toughness were determined in series of mechanical tests for samples produced with different manufacturing parameters, such as nozzle diameter and infill angle. It was found that the use of ABS filament reinforced with the short fibers can significantly improve the mechanical properties of 3D-printed devices when the infill angle is oriented along the vector of the applied load. In such a case, the elastic modulus and tensile strength can be increased by more than 1.7 and 1.5 times, respectively. The use of a larger nozzle diameter led to the growth of tensile strength by an average of 12.5%. When the macroscopic load is applied along the normal to the printed layers, the addition of short fibers does not give much gain in mechanical properties compared to pure ABS, which was confirmed by both standard tensile and fracture toughness tests. The surface of the fractured samples was examined using scanning electronic microscopy, which allowed us to make conclusions on the type of defects as well as on the level of adhesion between the polymeric matrix and different types of short fibers.

Unlocking the nanoparticle emission potential: a study of varied filaments in 3D printing.

This study investigates nanoparticle emission during 3D printing processes, assessing various filament materials' impact on air quality. Commonly used 3D printers, including both filament and resin-based types, were examined. The study's scope encompasses diverse filament materials like ABS (acrylonitrile butadiene styrene), PLA (polylactic acid), PETG (polyethylene terephthalate glycol), ASA (acrylonitrile styrene acrylate), TPU (thermoplastic polyurethane), PP (polypropylene), nylon, and wood-based variants, alongside three types of resins. The research delves into the relationship between the type of material and nanoparticle emissions, emphasizing temperature's pivotal role. Measurement instruments were employed for nanoparticle quantification, including an engine exhaust particle sizer spectrometer, condensation particle counter, and nanozen dust counters. Notably, results reveal substantial variations in nanoparticle emissions among different filament materials, with ASA, TPU, PP, and ABS showing considerably elevated emission levels and characteristic particle size distribution patterns. The findings prompt practical recommendations for reducing nanoparticle exposure, emphasizing printer confinement, material selection, and adequate ventilation. This study offers insights into potential health risks associated with 3D printing emissions and provides a basis for adopting preventive measures.

Evaluation of the Viability of 3D Printing in Recycling Polymers.

The increased use of plastics in industrial and agricultural applications has led to high levels of pollution worldwide and is a significant challenge. To address this plastic pollution, conventional methods such as landfills and incineration are used, leading to further challenges such as the generation of greenhouse gas emissions. Therefore, increasing interest has been directed to identifying alternative methods to dispose of plastic waste from agriculture. The novelty of the current research arose from the lack of critical reviews on how 3-Dimensional (3D) printing was adopted for recycling plastics, its application in the production of agricultural plastics, and its specific benefits, disadvantages, and limitations in recycling plastics. The review paper offers novel insights regarding the application of 3D printing methods including Fused Particle Fabrication (FPF), Hot Melt Extrusion (HME), and Fused Deposition Modelling (FDM) to make filaments from plastics. However, the methods were adopted in local recycling setups where only small quantities of the raw materials were considered. Data was collected using a systematic review involving 39 studies. Findings showed that the application of the 3D printing methods led to the generation of agricultural plastics such as Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS), Polyethylene Terephthalate (PET), and High-Density Polyethylene (HDPE), which were found to have properties comparable to those of virgin plastic, suggesting the viability of 3D printing in managing plastic pollution. However, limitations were also associated with the 3D printing methods; 3D-printed plastics deteriorated rapidly under Ultraviolet (UV) light and are non-biodegradable, posing further risks of plastic pollution. However, UV stabilization helps reduce plastic deterioration, thus increasing longevity and reducing disposal. Future directions emphasize identifying methods to reduce the deterioration of 3D-printed agricultural plastics and increasing their longevity in addition to UV stability.

Interrelations between Printing Patterns and Residual Stress in Fused Deposition Modelling for the 4D Printing of Acrylonitrile Butadiene Styrene and Wood–Plastic Composites

Four dimensional printing enables the advanced manufacturing of smart objects that can morph and adapt shape over time in response to stimuli such as heat. This study presents a single-material 4D printing workflow which explores the residual stress and anisotropy arising from the fused deposition modelling (FDM) printing process to create heat-triggered self-morphing objects. In particular, the study first investigates the effect of printing patterns on the residual stress of FDM-printed acrylonitrile butadiene styrene (ABS) products. Through finite element analysis, the raster angle of printing patterns was identified as the key parameter influencing the distribution of residual stresses. Experimental investigations further reveal that the non-uniform distribution of residual stress results in the anisotropic thermal deformation of printed materials. Thus, through the design of printing patterns, FDM-printed materials can be programmed with desired built-in residual stresses and anisotropic behaviours for initiating and controlling the transformation of 4D-printed objects. Using the proposed approach, any desktop FDM printers can be turned into 4D printers to create smart objects that can self-morph into target geometries. A series of 4D printing prototypes manufactured from conventional ABS 3D printing feedstock are tested to illustrate the use and reliability of this new workflow. Additionally, the custom-made wood–plastic composite (WPC) feedstocks are explored in this study to demonstrate the transposability of the 4D printing approach.

Designing Centrifugal Membrane Filters with uniform-pressure for UF/NF/RO separations

A novel Ortho-Centrifugal Membrane Filter (O-CMF) was designed and assessed to tackle common issues in conventional laboratory centrifugal membrane filters, e.g. non-uniform transmembrane pressure and concentration polarization, and to ease membrane replacement. The O-CMF, manufactured by additive 3D printing in acrylonitrile butadiene styrene, with membrane area of 0.95 cm2, was designed to process a maximum liquid sample of 9 mL. It was tested with nanofiltration of aqueous solutions of NaCl, MgSO4, and lactose (concentrations of 1–20 g/L) at transmembrane pressures of 6–24 bar. Complementary Computational Fluid Dynamics (CFD) simulations, employing a custom solver from the open-source OpenFOAM-2306 c++ toolbox, were also run to seek insight of the O-CMF main features.The permeation experiments displayed minimal impact of concentration polarization. CFD simulations showed that concentration polarization is nearly uniform and that concentrate chamber is homogenized by centrifugal convection. The CFD permeate flux predictions, based on the osmotic pressure model, closely matched experimental data, and a correlation for predicting concentration polarization was also developed.The O-CMF device represents a substantial breakthrough in the technology of centrifugal membrane filters, consistently maintaining uniform transmembrane pressure and effectively minimizing concentration polarization. The device was evaluated using nanofiltration, but its pressure range is versatile enough to cover conditions suitable for both ultrafiltration and reverse osmosis operations. This adaptability extends its application across a variety of laboratory experiments, including sample concentration, hydraulic permeability tests, and membrane screening.

Material recycling of acrylonitrile butadiene styrene (ABS) from wiring devices using mechanical recycling

Plastics play a significant role in various sectors by offering advantages such as durability, cost-effectiveness, and lightweight properties. However, the extensive production and consumption of plastics create significant environmental threats, leading to high carbon footprints, global warming, and the accumulation of hard-to-recycle waste. This study explores innovative recycling technologies, focusing on the circular economy and zero waste strategies. In line with this scope, the study aims to conduct research on the reuse of coated plastic materials in production, specifically selecting ABS materials that are difficult to recycle. In the study, wiring devices were prepared using 70% raw ABS (Acrylonitrile Butadiene Styrene) material and 30% recycled ABS (rABS) material, with this cycle being repeated three times. Experiments have been conducted to evaluate the suitability and performance of recycled ABS material for mass production. The results have revealed a slight decrease in tensile strength and impact strength compared to raw ABS material, yet significant mechanical performance has been maintained, thus demonstrating the viability of rABS in preserving product quality throughout successive recycling processes.