On the difference in material structure and fatigue properties of nylon specimens produced by injection molding and selective laser sintering

This paper describes the influence of dynamic tension/compression loading on notched and unnotched nylon specimens fabricated by Injection Molding (IM) and Selective Laser Sintering (SLS). The main objective of this work is to analyze and describe the differences in material structure and fatigue properties of as-built nylon parts produced by IM and SLM from the same polyamide 12 powder. The differences in dimensional quality, density, surface roughness, crystal structure and crystallinity are systematically measured and linked to the mechanical fatigue properties. The fatigue properties of the unnotched SLS specimens are found to be equal to those of the unnotched IM specimens. The presence of pores in the sintered samples does not lead to rapid failure, and the microvoid coalescence failure mechanism is delayed. The notched specimens show more brittle failure and increased fatigue resistance which is caused by local notch-strengthening. The results enable improved understanding of the difference in material structure and fatigue behavior of selective laser sintered and injection molded polyamide.     

The effect of short glass fibers on the process behavior of polyamide 12 during selective laser beam melting

In additive manufacturing, polymer composites are used for setting tailored properties. Short glass fibers can be used as fillers for polyamide 12 for enhancing stiffness or tensile strength as well as for reducing shrinkage. In this paper, the effects of short glass fibers on polyamide 12 concerning powder properties, process behavior and part properties in laser beam melting of polymers (SLS) are investigated. It could be shown that by increasing the short glass fiber content powder properties as well as part properties are immensely affected. By adding glass fibers, powder properties, like flowability and diffuse reflection decrease. The isothermal crystallization changes resulting in a narrower processing window. Concerning mechanical properties, short glass fibers allow for a higher stiffness until a critical limit of filler concentration within this study is reached, after which the tensile strength decreases. The elongation of break decreases by rising the filler content.     

The effect of processing conditions on the mechanical properties of polyethylene produced by selective laser sintering

A study was performed to explore the feasibility of processing a polyethylene by selective laser sintering. The polyethylene powder was characterised by scanning electron microscopy, particle size analysis and differential scanning calorimetry. The laser sintering processing conditions, especially powder bed temperature, laser power and laser scan counts, were studied. Well defined tensile testing specimens of the polyethylene were produced successfully by double laser scanning. The effect of the thermal history during the laser sintering process on the mechanical properties of the laser sintered parts is discussed for the first time.     

The microstructural characterization of PA6/PA12 blend specimens fabricated by selective laser sintering

This study investigates the processing of blends of polyamide 6 (PA6) and polyamide 12 (PA12) by selective laser sintering (SLS) using a CO₂ laser. Powder properties of undiluted polymers, mixture composition, and processing parameters, as well as their influence on the microstructure of the specimens manufactured, were evaluated. Polyamides showed higher absorption of laser energy during the sintering of blend specimens, with subsequent thermal energy transfer to the melting of the polymeric phases. The structure of parts obtained by SLS is dependent on the process parameters and the characteristics of the powder material to be processed. The microstructures of PA6/PA12 blend specimens were heterogeneous, with co-continuous and disperse phases depending on the quantity of PA12. The porosity and crystallinity also changed as a function of the component proportions. The use of polymeric blends can increase the range of structures and properties of SLS parts.     

Effect of CO2 laser radiation on the surface properties of polyethylene terephthalate

The surfaces of polyethylene terephthalate (PET) obtained by irradiation with a CO₂ pulsed laser in air were studied. The complicated microstructures using various laser wavelengths were observed by scanning electron microscopy (SEM). The changes in chemical and physical properties of the irradiated PET surface were investigated by attenuated total reflectance infrared spectroscopy (ATR-FTIR) and contact angle measurements. ATR-IR spectrum showed that the crystallinity in the surface region decreased due to laser irradiation. The water drop contact angle also decreased with increasing of laser pulses. The density of peroxides formed on the irradiated PET surface were determined by iodide method.     

Structure-property relationships in polyamide 12/halloysite nanotube nanocomposites

Polyamide 12 (PA12) nanocomposites based on halloysite nanotubes (HNTs) were obtained using a batch internal mixer or a twin-screw mini-compounder. In order to analyze the influence of HNTs dispersion on nanocomposite properties, morphological analysis (SEM and TEM) was combined with rheological and thermo-mechanical experiments. The linear viscoelastic properties and the dynamic storage modulus were expectedly found to increase with increasing HNT loading. Higher enhancements were observed for PA12/HNTs nanocomposites obtained by twin-screw mini-compounding. This finding was related to the better degree of dispersion and alignment of the silicate nanotubes throughout the matrix. Thermal stability was also improved by the halloysite nanotubes presumably by an entrapment mechanism of the volatile products inside the hollow tubular structure. DSC measurements further highlighted a nucleation effect of HNTs on the nanocomposites. In view of these results, halloysite nanotubes are promising candidates in the field of PA nanocomposites for structural applications.     

激光烧结制备尼龙12/累托石纳米复合材料

将有机累托石与尼龙12粉末混合,采用激光烧结(SLS)技术制备了尼龙12累托石纳米复合材料,这是一种使纳米复合材料的制备与材料的成型同时进行的方法.利用X射线衍射(XRD)、红外光谱(FTIR)、扫描电镜(SEM)等手段对复合材料的结构进行了表征,并对其力学性能及热性能进行了研究.结果表明,尼龙12分子在激光烧结过程中插入到累托石层间,形成的复合材料在拉伸强度、弯曲强度、冲击强度等力学性能及热稳定性能方面均优于尼龙12烧结试样.     

Effect of section thickness on fatigue performance of laser sintered nylon 12

Laser Sintering offers manufacturers freedom of design, which enables creating parts with complex geometries. However, very little investigation has been made into the effects of geometry on mechanical properties of the parts. In the present study, Laser Sintered Nylon 12 parts with different section thickness are subjected to displacement controlled tension-tension and force-controlled fully reversed fatigue loading to investigate the effect of geometry on their fatigue behavior. Section thickness of the parts is shown to have no significant influence on the fatigue behavior under tension only loading. However, fatigue life of parts under fully reversed loading is shown to increase with section thickness.     

Effect of modified glass fiber on tribological performance of water-lubricated bearing

Glass fiber (GF) is becoming one of the most popular reinforcement materials used in many fields. However, it still has some disadvantages such as brittleness and poor wear resistance. In order to improve its tribological property, the GF was modified and use high-density polyethylene (HDPE) as the matrix material to verify it. In this work, friction tests of HDPE samples with seven different compositions of modified GF were carried out. Different working conditions were applied to determine the tribological properties and service life as used in water-lubricated bearings. Their friction coefficient, surface morphology, wear scar, wear quality and wear mechanism were investigated. The results showed that the modified GF could significantly improve the HDPE tribological properties. Moreover, the wear mechanism of pure HDPE was mainly adhesive wear and slight fatigue wear, and the composite material was mainly abrasive wear. The 30% GF composite material had the best wear resistance, its friction coefficient was reduced by 51.56% under 0.4 MPa and 500 r/min. The findings from this experiment will provide a new method for reducing friction and wear of water-lubricated bearings.     

Synthesis of water-soluble carbon nanotubes via surface initiated redox polymerization and their tribological properties as water-based lubricant additive

Polyacrylamide (PAM)-grafted multi-walled carbon nanotubes (MWCNTs-g-PAM) which are dispersable in water were prepared by the surface initiated redox polymerization of acrylamide using ceric ammonium nitrate (CAN) as initiator. They are soluble in polar solvents such as water, tetrahydrofuran and acetone. The chemical structure of the resulting product and the quantities of grafted polymer were determined by FT-IR, TGA. TEM, and FE-SEM observations indicated that the nanotubes were coated with a PAM layer, exhibiting core-shell nanostructures, with the PAM chains as a brush-like or hairy shell, and the MWCNTs as a hard backbone. Furthermore, the tribological properties of the prepared MWCNTs-g-PAM composites as an additive in water were evaluated with a four-ball machine. The results confirmed that the composites exhibit good anti-wear and friction reduction properties as well as load-carrying capacity. This was attributed to the possibility of the composites acting as nanometer sized tiny bearings during lubrication.     

Thermodynamic properties affect the molecular motion of novolac type phenolic resin blended with polyamide

The effect of association reaction length on the substantial increase of molecular motion as well as entropy (−TΔS_m) of phenolic-polyamide blends is investigated with the ¹³C solid-state NMR and DSC. The H-bonding strength by forming the phenolic-polyamide interaction is great enough to overcome the breaking off the self-association of phenolic. With respect to decreasing the association reaction, the polyamide resonance intensity of ¹³C solid-state NMR spectra is weakened due to the reduction of the cross-polarization efficiency at a high mobile sample. The glass transition temperature of phenolic-polyamide blend as well as T^H_1ρ value from NMR experiments is also decreased. The decreasing strength of H-bonding resulting from blending causes higher entropy (−TΔS_m) and higher molecular mobility of the phenolic-polyamide blends. Accordingly, the polyamide-66 possesses higher H-bonding force and exhibits more mobile role in this phenolic/polyamide blends family. It can be concluded that the molecular segmental motion and entropy are progressively decreased while increasing the inter-association force of the polyamide within the miscible window.     

Influence of surface roughness on tribological and mechanical properties of micro-milled and laser ablated poly (methyl methacrylate) PMMA organic glass

Poly (methyl methacrylate) organic glass is used in aircraft windshield application; these structures should have better fatigue and fracture resistance to yield good service life. The tendency towards achieving these properties is lost during manufacturing process. This study aims to determine the effect of grooving on PMMA Organic glass. The grooves are manufactured using two different processes namely Micro-Milling (MM) and Laser Ablation (LA). The tribological properties of laser ablated PMMA (LA-PMMA) and micro-milled PMMA (MM-PMMA) were studied using Pin-on-disc tribometer. The grooved surface roughness of both MM-PMMA and LA-PMMA samples has decreased with increase in wear time, whereas after reaching minimum roughness the coefficient of friction has increased; due to higher adhesion between polymer and sliding metal. The tensile properties of differently machined samples have not shown significant difference; whereas the fracture toughness values were higher with LA-PMMA samples. This effect indicated LA-PMMA had greater capacity to resist crack propagation compare to MM-PMMA samples. Similarly the fatigue endurance limit was found higher with LA-PMMA compared to MM-PMMA, due to better finish of LA-PMMA. Further, the microscopic analysis of laser grooved sample before and after fracture have also shown smoother surface and less conic shapes (fracture point) compare to MM-PMMA.     

Selective laser sintering of polymer blends: Bulk properties and process behavior

Physically mixed powderous polymer blends consisting of at least two different thermoplastic materials with complementary properties could allow the successful fabrication of components with tailored and graded properties. In this work, powderous polymer blends of the partially miscible and chemically reactive blend system PBT/PC were produced from wet grinded powders at different weight ratios of 90/10, 80/20, 70/30 and 60/40, respectively. The PBT/PC is used as a model system for a blend with a semi-crystalline and amorphous component, while being relevant for industrial use, such as automotive applications. Before the implementation into the selective laser sintering process (SLS), the bulk properties of the powders were analyzed. The quadratic monolayer test specimens were generated with different energy densities by variating the laser power. The specimens' geometrical and microstructural properties were studied. The investigations showed that an improvement of geometric properties in terms of layer development can be achieved by increasing the PC content and that it is possible to generate polymer blends with matrix and dispersed phase from PBT/PC blends.     

Material stability investigation of polyamide material before and after laser sintering

Abstract

Polymers are the long chain organic materials that are held together by directional covalent bonds. These kind of organic materials are either synthesized or naturally obtained and they have wide range of application in the medical field due to its physical properties, chemical properties and multifarious processing technique. For the past two decade, these polymers were used to produce variety of medical devices and implants by laser sintering. Since laser based additive manufacturing technique was a thermal process, there may be a change in the material property after sintering and which may affect the usage of the polymer in medical field. This work presented here aims to investigate the material property of biocompatible eos PA12 polymer powder and a laser sintered part. The Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDAX), Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD) and Differential Scanning Calorimeter (DSC) analysis were carried out to investigate the properties. The experimental investigation carried out in order to get insight into laser-material interaction and the corresponding results indicated that the laser energy influences the material properties of polyamide powder.     

Effect of surface modification of montmorillonite on the properties of aromatic polyamide/clay nanocomposites

The surface modification of montmorillonite clay was carried out through ion‐ exchange reaction using p‐phenylenediamine as a modifier. This modified clay was employed to prepare aromatic polyamide/organoclay nanocomposite materials. The dispersion behavior of clay was examined in the polyamide matrix. Polyamide chains were synthesized from 4‐aminophenyl sulfone and isophthaloyl chloride (IPC) in dimethylacetamide. These amide chains were suitably end‐capped with carbonyl chloride end groups to interact chemically with modified montmorillonite clay. The resulting nanocomposite films containing 2–20 wt% of organoclay were characterized by TEM, X‐ray diffraction (XRD), thin‐film tensile testing; thermogravimetric analysis (TGA), differential scanning calorimetric (DSC) and water absorption measurements. Mechanical testing revealed that modulus and strength improved up to 6 wt% organoclay loading while elongation and toughness of nanocomposites decreased with the addition of clay content in the matrix. Thermal decomposition temperatures of the nanocomposites were in the range 225–450 °C. These nanocomposites expressed increase in the glass‐transition temperature values relative to pure polyamide describing interfacial interactions among the phases. The percent water uptake of these composites reduced upon the addition of modified layered silicate depicting improved barrier properties. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of POSS on crystalline transitions and physical properties of polyamide 12

Polyamide 12/Trisilanolphenyl‐POSS (PA 12/POSS) composites were prepared via melt‐compounding. The effect of polyhedral oligomeric silsesquioxane (POSS) on crystalline structure and crystalline transition of PA 12 was investigated by wide‐angle X‐ray diffraction (WAXD) and real time fourier transform infrared spectroscopy (FTIR). WAXD results indicated that PA 12 crystallized into γ‐form as slowly cooling from melt and the presence of POSS did not influence the crystalline structure of PA 12. Both PA 12 and PA 12/POSS composites underwent Brill transitions when they were heated from room temperature to melt point. Real time FTIR patterns showed that an absorption band at 697 cm^?1 ascribed to Amide V (α) mode was emerged along with the disappearance of Amide VI (γ) band at 628 cm^?1 with the increase of the temperature for PA 12 and PA 12/POSS composites, which suggested that the γ‐form crystalline has transformed into α form. The Brill bands were identified and the transformed mechanism was discussed based on the real FTIR results. The addition of POSS enhanced the tensile strength and thermal stability of PA 12. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 121–129, 2009     

Study on hypochlorite degradation of aromatic polyamide reverse osmosis membrane

A serious limitation of most commercial polyamide reverse osmosis (RO) membranes is their sensitivity to chlorine attack. By studying the hypochlorite degradation of aromatic polyamide RO membrane, this work was to get some understandings in the prevention of membrane depreciation and develop membranes with improved chlorine resistance. Membrane performances, including water flux and salt rejection, were evaluated before and after hypochlorite exposure under different pH and concentration conditions. The results showed that chlorination destroyed hydrogen bonds in polyamide chains, causing a notable decline of membrane flux especially in acid environment; however, membrane performance was slightly improved after the treatment of alkaline hypochlorite solution for a certain time, which was probably due to the effect of amine groups in barrier layer. Based on the attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) characterizations and performance measurements, the results indicated that N-chlorination reaction of aromatic polyamide was also reversible, in other words, the N-chlorinated intermediate could be regenerated to initial amide with the alkaline treatment before ring-chlorination reaction. This conclusion provided several relative suggestions for membrane cleaning procedures. Finally, a method adopting surface coating was proposed to develop membranes with good chlorine resistance, and the preliminary results showed its potential for applications.     

Study of ionizing radiation on the properties of polyamide 6 with fiberglass reinforcement

The use of polymers reinforced with fiberglass is becoming more and more common in the switches for household industries. These compounds perform a good tension resistance to the impact and the humidity absorption being used at the present time and also are in the automobile industry in parts underneath the hood, especially in the radiator frames. The aim of this work is to study the effect of ionizing radiation on the properties of polyamide 6 with fiberglass reinforcement and undergone to different irradiation doses. Samples were prepared and irradiated on JOB 188 accelerator with an electron beam energy of 1.5 MeV in air with different doses and a dose rate of 27.99 kGy/h. Afterward, the properties of the non-irradiated and irradiated polyamide 6 with fiberglass reinforcement were evaluated.     

Effects of reprocessing on the structure and properties of polyamide 6 nanocomposites

PA6 based nanocomposites (NCs) were reprocessed by repeated injection moulding to find out whether reprocessing is possible in these materials by means of the observation of the changes in the structure and mechanical properties. The studied variables were (a) the number of cycles (1-5), (b) the origin of the NC: either laboratory mixed or commercial and (c) the processing temperature (230 °C and 270 °C). Neat PA6 was also reprocessed as a reference material. In spite of the colour change, the Young's modulus, the solid state characteristics and the dispersion level were preserved upon reprocessing. The lack of change of chemical nature observed by FTIR, and the observed decreases in viscosity indicated that the main effect of reprocessing was a decrease in the molecular weight. At 230 °C the decreases in viscosity were smaller after reprocessing, and almost no change was seen in the structural parameters and properties. The decrease in the molecular weight after reprocessing at 270 °C leads to lower ductility and mainly to a decrease in the ability of the nPA6 matrix to cold draw. However, no change of the interphase conditions or agglomeration of the OMMT was detected and the NCs remained clearly ductile; thus, revealing a lack of deterioration of the interface and the ability of the NCs for recycling.