Selective laser sintering of porcelain

Selective Laser Sintering (SLS) technique is used to fabricate 3D porcelain products with complex shapes. Commercial powder has been studied and optimized in terms of morphology and particle size distribution in order to get a perfect powder layerwise which remains the critical step of such a technique. The influence of laser energy density (through the laser power and scan speed) and hatching space have been investigated to determine the optimized parameters that allow the greater densification of this complex multi-materials composed of kaolinite, quartz and potassium feldspar. The laser-sintered porcelain products which exhibit about 60% of porosity have been post-treated at 1350 °C under vacuum or air to further improve densification.     

Selective laser flash sintering of 8-YSZ

Flash sintering of ceramics is characterized by rapid sintering during simultaneous application of electric field and heat. Previous studies of flash sintering have been conducted in furnace environments, where sample temperatures are approximately uniform. In this work, we use highly dynamic heating from a scanning laser to initiate flash sintering while simultaneously applying a DC electric field. Onset of flash sintering is determined by a measurable increase in current through the sample. Our results show that stage I and stage II flash sintering can be initiated by laser heating. At low-to-moderate combinations of laser energies and applied electric fields, measured current rises slightly when the laser is scanned completely across the specimen from the positive to the negative electrodes. Microstructures for these samples show that powder consolidation is minimal in this regime (stage I flash sintering), and thus the observed current is likely due to onset of neck growth between powder particles rather than densification. At higher laser energies and fields, current rises steeply and microstructures show significant consolidation (stage II flash sintering). The demonstration that flash sintering occurs when ceramic is heated by laser-scanning supports future utilization of selective laser flash sintering as an additive manufacturing process.     

Selective laser sintering of clay‐reinforced polyamide

An experimental investigation has been carried out to study the feasibility of processing blended powder of polyamide (PA) and organically modified nanoclay using selective laser sintering. The effect of nanoclay on the sintering parameters and mechanical properties of the sintered specimen have been studied. This article presents the details of preprocessing studies required and conducted to find suitability of the blended powder material to be processed on SLS machine. A suitable part bed temperature has also been found to avoid curling as well as to ensure the powder reusability. In this work, laser power, beam speed, scan spacing have been considered as influential operating process parameters and are explored using Taguchi's L₉ orthogonal array. Tensile specimens of PA and PA/clay composite have been fabricated as per ASTM D638 standard and tested for ultimate tensile strength, elongation at break, and Young's modulus using universal testing machine. Ultimate tensile strength and elongation at break are found to be decreased in case of PA/clay composite when compared with virgin polyamide. To understand the sintering insight and to explain the behavior of obtained mechanical properties, further investigations have been carried out using material characterization techniques like X‐ray diffraction and scanning electron microscopy. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Melting behavior and nonisothermal crystallization kinetics of polyamide 6/polyamide 66 molecular composites via in situ polymerization

The melting behavior and nonisothermal crystallization kinetics of pure polyamide 6 (PA 6) and its molecular composites with polyamide 66 (PA 66) were investigated with differential scanning calorimetry. The PA 6/PA 66 composites had one melting peak, whereas the coextruded PA 6/PA 66 blends had two melting peaks. With the addition of PA 66 to PA 6 via in situ anionic polymerization, the melting temperature, crystallization temperature, and crystallinity of PA 6 in the composites decreased. The half‐time of nonisothermal crystallization increased for a PA 6/PA 66 molecular composite containing 12 wt % PA 66, in comparison with that of pure PA 6. The commonly used Ozawa equation was used to fit the nonisothermal crystallization of pure PA 6 and its composites. The Ozawa exponent values in the primary stage were equal to 1.28–3.03 and 1.28–2.97 for PA 6 and its composite with 12 wt % PA 66, respectively, and this revealed that the mechanism of primary crystallization of PA 6 and PA 6/PA 66 was mainly heterogeneous nucleation and growth. All the results indicated that the incorporation of PA 66 into PA 6 at the molecular level retarded the crystallization of PA 6. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2172–2177, 2005     

Crystal structures and their effects on the properties of polyamide 12/clay and polyamide 6–polyamide 66/clay nanocomposites

Both polyamide 12 (PA 12)/clay and polyamide 6–polyamide 66 copolymer (PA 6/6,6)/clay nanocomposites were prepared by melt intercalation. The incorporation of 4–5 wt % modified clay largely increased the strength, modulus, heat distortion temperature (HDT), and permeation resistance to methanol of the polyamides but decreased the notched impact strength. Incorporation of the clay decreased the melt viscosities of both the PA 12 and PA 6/6,6 nanocomposites. Incorporation of the clay increased the crystallinity of PA 6/6,6 but had little effect on that of PA 12, which explained why the clay obviously increased the glass‐transition temperature of PA 6/6,6 but hardly had any effect on that of PA 12. The dispersion and orientation of both the clay and the polyamide crystals were studied with transmission electron microscopy, scanning electronic microscopy, and X‐ray diffraction. The clay was exfoliated into single layers in the nanocomposites, and the exfoliated clay layers had a preferred orientation parallel to the melt flow direction. Lamellar crystals but not spherulites were initiated on the exfoliated clay surfaces, which were much more compact and orderly than spherulites, and had the same orientation with that of the clay layers. The increase in the mechanical properties, HDT, and permeation resistance was attributed to the orientated exfoliated clay layers and the lamellar crystals. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4782–4794, 2006     

Properties and morphology of supertoughened polyamide 6 hybrid composites

In this study, supertoughened polyamide (PA) nanocomposites were prepared by the incorporation of epoxidized polyhedral oligomeric silsesquioxane (POSS) into the polyamide 6 (PA6)/methyl methacrylate–butadiene–styrene copolymer (MBS) blend via a melt‐blending method. The effect of POSS on the rheological properties, mechanical properties, water uptake, and morphology of the hybrid PA6 nanocomposites was studied. The results show that under impact loading, the hybrid PA6 composites exhibited significant improvements in both the crack initiation energy and the crack propagation energy. This hybrid composite showed supertough behavior. Meanwhile, the tensile strength and the water absorption resistance was also improved with the addition of epoxidized POSS. The capillary and torque rheological results indicated that the epoxidized POSS, which acted as nanoscale ball bearings, significantly decreased the melt viscosity of the matrices and facilitated the melting process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to study the microstructure–property relationships of the hybrid PA6 composites. The TEM results showed that the MBS particles were dispersed homogeneously in the PA6 matrix. The mean diameter of the MBS particles decreased, and the size distribution of the MBS particles narrowed down with the introduction of the epoxidized POSS and compatiblizer. The SEM micrographs indicated that the impact fracture surfaces of the PA composites showed morphological characteristics of supertough polymers because of the synergistic effect of the functionalized POSS and compatibilized MBS particles. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermal and flow property–morphology relationship of sugarcane bagasse fiber‐filled polyamide 6 blends

The structure–property relationship of sugarcane bagasse fiber‐filled polyamide 6 blends at different blend compositions has been investigated. Blends were prepared in the composition of wt % PA6/wt % bagasse as follows: 98/2, 95/5, and 90/10 for three fiber length ranges (<100, <250, and <500 μm) using a twin‐screw extruder. Thermal properties were evaluated by measuring the glass transition temperature T_g, enthalpy of fusion ΔH_f, crystallinity X_c and thermogravimetry, TG. Results showed that T_g of the composites changed with change in fiber loading and length. The X_c as well as ΔH_f of the blends reduced to almost half its value for the neat PA6. The thermogravimetric curves TG showed that the thermal stability of the composites was lower than that of the neat PA6. Rheological properties were studied as a function of fiber loading, fiber length, shear rate, and temperature. The viscosity of composites increased with increasing fiber loading and length at low shear rates but decreased below that of neat PA6 at high shear rates. It was also found to be temperature sensitive, and influenced by fiber lengths particularly at higher temperatures. The morphology of the blends was studied using a Leica laser scanning confocal microscopy at two different regions: at the wall, and the core. The micrographs of the blends showed that fibers present in the form of bundles were found at the wall of the extrudates and increased in volume with increase in both length and concentration, at the same temperature and shear stress. In the core region, there is laminar flow, presenting striation morphology, with the omnipresent bundles of fibers dispersed in the matrix. At higher shear rates, the bundles were pushed to the wall. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3744–3754, 2004     

In-situ graphene platelets formation and its suppression during reactive spark plasma sintering of boron carbide/titanium diboride composites

Boron carbide composites with 10 vol.% TiB₂ were prepared by reactive sintering of B₄C, TiO₂, and carbon black powder mixture at the temperature of 1800 °C, under a pressure of 70 MPa in a vacuum. The combined effects of electric current and in-situ reactions led to a significant overheating of the central part of the sample, while no overheating was observed for hot press and non-reactive SPS processes. A lower electrical resistivity of TiB₂ produced a significant Joule heating of boron carbide, leading to its partial decomposition to form gaseous boron and graphene platelets. Homogenous, fully dense and graphene-free samples were obtained when employing an insulating Al₂O₃ paper during reactive SPS. A short dwell time (30 s after a degassing step of 6 min) and the uniform distribution of fine TiB₂ grains were the main advantages of isolated SPS over the reactive hot press and SPS processes, respectively.     

Developing electrically conductive polypropylene/polyamide6/carbon black composites with microfibrillar morphology

We have established that the PP/PA6/CB composite with 3D microfibrillar conducting network can be prepared in situ using melt spinning process. CB particles preferably were localized at the interface between polypropylene as the matrix and PA6 microfibrils, which act as the conducting paths inside the matrix. The percolation threshold of the system reduced when aspect ratio of the conducting phase was increased by developing microfibrillar morphology. The effect of annealing process on the conductivity of PP/PA6/CB composite with co‐ continuous and microfibrillar morphologies was studied. It was observed that, annealing process forces CB particles towards the interface (2D space) of PP and PA6 co‐continuous phases, and percolation threshold and critical exponent of classical percolation theory will be decreased, while the conductivity of conducting composite with microfibrillar morphology was not affected considerably by annealing process at temperatures either higher or lower than the melting point of the PA6 microfibrils. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Enhanced dielectric properties of polyamide 11/multi‐walled carbon nanotubes composites

Composites with multi‐walled carbon nanotubes (MWNTs) involved in polyamide 11 (PA11) were prepared via a conventional melt blending method. The structure, morphology, crystallization behavior, electrical, and dielectric properties of composites were investigated. The results demonstrated that the dispersed uniformly MWNTs favored the formation of α crystal of PA11 when the composites were quenched from melt. The dielectric constant of composites was dependent on the electric field frequency and MWNTs content, and the highest value of dielectric constant was as high as 350 for the composite with 1.21 vol % MWNTs at 10³ Hz, accompanied by a low dielectric loss. The enhanced dielectric properties could be interpreted by the formation of abundant nanocapacitors within the composites and the interfacial polarization effect resulting from accumulation of charge carriers at the internal interfaces between MWNTs and PA11. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42642.     

Rheological,thermal and morphological properties of polyethylene terephthalate/polyamide 6/rice husk ash composites

This work deals with the rheological, morphological, and thermal properties of composites having poly(ethylene terephthalate) (PET), polyamide-6 (PA6), and their blends as matrices, and rice husk ash (RHA) as a filler. The study determines the effect of composition on the change in viscosity and rate of degradation during processing in a torque rheometer. Our data indicates that thermal stability and degradation during processing depend on matrix composition and filler concentration. SEM micrographs show both partial adhesion of the filler to the matrices and filler pullout. Optical microscopy shows particle agglomeration and that agglomerate size increased with filler content. FTIR investigates the shifting of absorption bands of PET/PA6 composite after the addition of RHA and attributes the selective dispersion of RHA to the formation of hydrogen bonds. Our data supports the idea that filler employed here is an option to develop polymer composites with improved properties.     

Oxazoline‐containing compatibilizers for polyamide/SAN and polyamide/ABS blends

Polyamide (PA) and acrylonitrile/butadiene/styrene copolymer (ABS) may appear as a mixture in the recycled plastic stream. The incompatibility of these blends results in a blend with poor mechanical properties. The aim of this work is to partially convert the nitrile groups of the acrylonitrile/styrene copolymer (SAN) into oxazoline groups by reaction with aminoethanol (AE). Such modified SAN (SAN‐m) can react with the amine or carboxylic acid end groups of PA, and therefore used as compatibilizers for blends of PA with ABS. SAN‐m was found to reduce the SAN‐domain size in the PA/SAN‐blends. The initial acrylonitrile content of SAN‐m had a strong influence on the degree of conversion into oxazoline groups and on the compatibilizing effect. Mechanical properties of SAN‐m compatibilized PA/ABS blends were investigated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 449–455, 2002     

Statistical evaluation of laser energy density effect on mechanical properties of polyamide parts manufactured by selective laser sintering

Selective laser sintering (SLS) is a rapid manufacturing technology that builds layer‐by‐layer solid object from particulate materials. Nowadays there are materials that are used to produce prototypes and end‐user parts. Powders might be made from metals, ceramics, polymers, and composites. The union or fusion of the particles is made by the energy provided by a heated environment and a laser beam. Parts are built based on data extracted from its CAD design. The process has many variables that directly affect the mechanical properties of the parts. One important and direct processing parameter is laser energy density. This work evaluated the effect of the variation of the energy density in the mechanical properties of a polymeric material by changing laser beam speed and average power. The analyzed variables were stress at 10% of elongation, flexural modulus, and density of the samples built with polyamide 2200 (PA2200‐EOSINT) using a CO₂ laser (10 W). Specimens obtained by combination of different laser powers (2.7, 3.4, and 4.1 W) and laser scan speeds (39.0, 44.5, and 50.0 mm/s) were submitted to flexural tests. Additionally, volumetric density was calculated with mass and physical dimensions of specimens, and micrograph were taken using scanning electron microscope to analyze the changes of the sintering degree. The results indicated that laser power had more influence over density and mechanical properties than scan speed. The microstructures presented good correlation with the statistical results. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

尼龙12/OMMT纳米复合粉末的制备及性能

利用十八烷基三甲基溴化铵(OTAB)对蒙脱土(MMT)进行有机改性,并通过溶液插层法制备尼龙12/有机蒙脱土(PA12/OMMT)纳米复合粉末。利用X射线衍射、傅立叶变换红外光谱、扫描电子显微镜等手段对改性后的MMT及PA12/OMMT纳米复合粉末的结构和微观形貌进行表征,并将复合粉末热压成型制成标准件,测试其力学性能和热性能。结果表明,经过有机改性,MMT的层间距由1.24 nm增加到了2.13 nm,且改性后的MMT能均匀地分散在PA12基体中,PA12/OMMT纳米复合粉末的成型件在拉伸强度、弯曲强度、冲击强度和热性能方面都优于纯PA12粉末。PA12/OMMT纳米复合粉末为选择性激光烧结技术(SLS)提供了一种性能良好的粉末材料。     

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ～90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.     

Flash sintering of alumina/reduced graphene oxide composites

The flash sintering behavior of Al₂O₃/reduced graphene oxide (rGO) composites was investigated. rGO was used as a composite component and a conductive additive. Under the electric fields of 250–400 V cm⁻¹, the flash event occurred at extremely low temperatures of 236–249 °C. The current density limit played a significant role in the degree of densification. A larger current density resulted in a higher density of the sample. However, current densities larger than 33.33 A cm⁻² resulted in broken samples because of the localization of high current density coupled with the formation of hot spots. Flash sintering at a furnace temperature of 800 °C, electric field of 300 V cm⁻¹ and current density limit of 33.33 A cm⁻² produced nearly completely dense Al₂O₃/rGO composites. In addition to the current limit, the furnace temperature is also a key parameter that controls the degree of densification to achieve “safe” flash sintering.     

Dissolution window in in situ polymerization preparation of polyamide single-polymer composites

Single-polymer composites (SPCs) are promising composite materials in which the reinforcement and matrix have the same chemical composition. In situ polymerization shows a broad application potential for producing SPCs due to its wide temperature window. However, the dissolution of fiber reinforcements in the liquid monomer destroys the oriented chain structure of the fibers and affects the properties of SPCs. In this study, the polymerization time was adjusted by varying the activator dosage. The influence of the dissolution of fibers on the properties and the structure of SPCs was investigated. The maximum tensile strength was achieved at the highest activator dosage. A longer duration was required for the polymerization due to the low-activator dosage, which led to the destruction of the oriented chain structure and the formation of a large amount of oligomers on account of the dissolution. Eventually, SPCs showed a low strength and a high deformation at break. In order to realize a good reinforcing effect, a sufficiently large dissolution window must be created by reducing the polymerization time. This study enhanced our understanding of the structure–property relationships of SPCs and provided a way to improve their mechanical characteristics.     

Elaboration and relationships between structure and rheological properties of microtalc or nanotalc/polyamide composites

This study focuses on the elaboration of nanocomposites processed by melt mixing of a polyamide 12 matrix and a hydrogel filled with synthetic talc particles. The systems are obtained by simultaneous mixing using either an internal mixer or a lab twin‐screw extruder. The structure and rheological properties of synthetic talc/polyamide composites are compared with those of natural talc/polyamide microcomposites and modified montmorillonite/polyamide nanocomposites. A multiscale structure, composed of numerous nanometric particles but also few micrometric aggregates, is obtained for synthetic talc/polyamide composites. In terms of processability, the lab twin‐screw extruder is more adequate than the internal mixer for the elaboration of synthetic talc/polyamide composites with relatively high filler volume fractions. For composites elaborated with the extruder, the percolation threshold, estimated from linear viscoelastic measurements, is close to 1, 6, and 11%, respectively, with modified montmorillonite, synthetic talc, and natural talc particles, in agreement with structural results. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42299.     

Solid-state shear milling method to prepare PA12/boron nitride thermal conductive composite powders and their selective laser sintering 3D-printing

The selective laser sintering (SLS) is one of the most important 3D-printing technologies. However, the challenges in SLS could be in the limited high material cost and single material performance. Development of high-performance and multifunctional copowders suitable for SLS is of great importance. Here, polyamide 12 (PA12)/boron nitride (BN) thermal conductive copowders suitable for SLS were successfully prepared through solid state shear milling (S³M) technology in combination with cryogenic pulverization technology. The particle size, morphology, grafting reaction between PA12 and BN, rheology behavior, and coalescence behavior of the obtained PA12/BN copowders were carefully investigated. The optimal amount of silica flow additive (0.5 wt %) was determined to achieve the good powder flowability. Under the optimum 3D-printing conditions, the fabrication of parts with high BN loading could be achieved. When BN content was at 40 wt %, the flexural strength could reach 10.6 MPa and the thermal conductivity could reach 0.55 W/m·k, 77% higher than that of pure PA12. After treated with phenolic epoxy resin, the tensile strength and flexural strength of the printed parts with 40 wt % BN loading could reach 14.2 and 25.6 MPa, which were 130 and 115% higher than those of the untreated 3D-printed parts, respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48766.     

Friction and wear properties of scrap tire/potassium hexatitanate whisker composites

As a way of solving the environmental problem of waste tires, we developed a new type of friction material made of scrap tire composites with potassium hexatitanate in which rubber formed a continuous phase. The tribological behaviors of the scrap tire rubber composites were investigated by a friction and wear tester under dry conditions. According to the results, the optimum amounts of potassium hexatitanate were 5 phr in terms of the friction and wear properties up to 200 °C. The specimen containing other ingredients showed 0.72 of friction coefficient and 1.03 of wear rate which are highly compatible to those of the commercial ‘Sonata’ motor brake pad when it contains 5, 20, 10, 20, 10 phr of potassium hexatitanate, phenol, cashew, barium sulfate, and copper, respectively.