Microstructural characterization of SLS-PA12 specimens under dynamic tension/compression excitation

This paper describes the effect of dynamic tension/compression loading on selective laser sintered components in polyamide. To gain more insight in the fatigue phenomena, both thermal and microstructural studies were performed on the fracture surfaces of the test specimens. The presented micrographs, DSC curves, hysteresis loops and S/N-line facilitate an improved understanding of the fatigue properties of selective laser sintered materials, polyamide in particular. The results show that crack initiation starts from inclusions in the material caused by unfused powder particles. The inclusions give rise to the formation of semi-spherical depressions with raised edges. Ductile fatigue striations were noted on the sides of these depressions indicating the irreversible plastic deformation dominated by shear stress, which is typical for fatigue failure. Consequently, microstructural analysis indicates that the material density is a crucial factor influencing the fatigue life of SLS-PA12 components. The lower the density, the more unfused powder particles and the higher the chance of crack initiation.     

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.     

On the visco-elasto-plastic response of additively manufactured polyamide-12 (PA-12) through selective laser sintering

This work presents an in-depth study on the mechanical behavior of selective laser sintered (SLS) nylon (PA-12). The entire visco-elasto-plastic response is determined based on experimental data obtained through tensile, compression, shear and relaxation testing. In addition, ultrasonic non-destructive testing is proposed as an alternative to conventional testing for the derivation of the elastic properties of this material. An isotropic elastic behavior was observed, while a clear orthotropic and non-linear response was found for both the plastic curves and the relaxation behavior. Strength data suggests laser sintered PA-12 will fail in tension rather than in shear. The ultrasonic tests correspond well to conventional tensile data (at high rates), and represent a cost-effective alternative to extensive conventional tensile testing. The presented test data can potentially be used to derive a detailed material model suitable for modelling static, fatigue and impact applications using 3D printed PA-12.     

Mechanical properties of PA6/PA12 blend specimens prepared by selective laser sintering

The use of polymeric blends can increase the range of structures and properties of selective laser sintering (SLS) parts. This study investigates the processing of a binary polar system using polyamide 6 (PA6) and polyamide 12 (PA12) by SLS. The mixture composition and processing conditions, and their influence on the dynamic mechanical properties of the specimens manufactured were evaluated. The maximum tan δ values suggest that PA6 and PA12 have similar visco-dissipative behavior. The PA6/PA12 blends behavior varied according to the relaxation phenomena of the pure components, proportionally to the blend composition. The creep test showed that blends with a higher amount of PA6 had greater plastic deformation and less elastic recovery. In the fatigue test the 20/80 and 50/50 blends presented good fatigue resistance under the test conditions.     

Fracture analysis of annealed PVDF films

The effect of annealing 50 μm thick, extruded poly(vinylidene fluoride) films in form (-PVDF) was investigated by differential scanning calorimetry and mechanical testing on unnotched and double-edge notched tensile specimens. As to the fracture behavior, micromechanisms of cavitation, spherulite breakdown, fiber bundle structure formation and - to β-phase transformation were detected. The progressive structural evolution taking place during annealing affected the deformation and fracture behavior significantly. While the annealing-induced subtle distinctions in microstructure are slightly reflected by the determined mechanical and essential work of fracture (EWF) properties, a distinct differentiation was possible by analysis of the process zone. A correlation between characteristic values of the process zone at necking and the endothermic transitions due to secondary crystallization determined by differential scanning calorimetry (DSC) was established. Annealing of PVDF films facilitates the micromechanism of cavitation, which is presumably related to perfection of morphological superstructures (spherulites) and, thus, interspherulitic stress concentration and failure.     

Compressive-tensile fatigue behavior of cords/rubber composites

Cord/rubber composites are used to build complex structures which may be submitted to cyclic loads, sometimes leading to critical fatigue failure. The focus of this study is to investigate the cyclic compressive/tensile strain behavior of polyester, polyamide and hybrid polyaramid/polyamide cords. For that, the cords were embedded in rubber belts to be used in a specially designed rotating pulley equipment that allows monitoring and controlling of tensile force, frequency and strain level. All fatigue tests were performed using stress-control mode, and tensile residual strength of the cords was measured as a function of material type, number of cycles and compressive/tensile strain level. The results show that compressive and tensile cyclic strains decrease residual properties. Hybrid cords showed higher residual strength than polyester and polyamide cords when subject to high compressive strain or high number of cycles. Moreover, morphological evaluation indicated failure to be associated with microbuckling and extensive fibrillation.     

Mechanical response of three semi crystalline polymers under different stress states: Experimental investigation and modelling

The mechanical responses of high‐density polyethylene (HDPE), polypropylene (PP) and polyamide 6 (PA 6) were experimentally investigated for a wide range of stress states and strain rates. This was accomplished by testing numerous specimens with different geometries. The uniaxial compression of cylindrical unnotched specimens and the uniaxial tensile behaviour of dumbbell specimens at different strain rates, was determined. A series of biaxial loading tests (combined shear and tension/compression, pure shear, pure tension/compression) using a designed Arcan testing apparatus were also performed. Flat and cylindrical notched specimens with different curvature radii were additionally tested in order to explore a wider range of stress states. The Drucker‐Prager yield criterion was calibrated with a set of experimental data, for which analytical formulae for stresses are available, and then applied to predict the deformation behaviour under different stress states, prior to strain localization. The results of the numerical simulations show that the Drucker‐Prager model can capture the initial elastic range and the post‐elastic response very satisfactorily. For triaxial and biaxial stress states there is a good agreement, however some load‐displacement responses are only satisfactorily described. Deviations observed in the predicted and experimental results are very likely attributed to the third invariant stress tensor, which was not explored in the model calibration. The evolution of stress triaxiality and Lode angle parameters with equivalent plastic strain were extracted and analysed for several specimens. The results show a plastic yielding behaviour sensitive to the stress state, which can be attributed to different combinations of stress triaxialities and Lode angle parameters.     

Effect of surface orientation on the tribological properties of laser sintered polyamide 12

Experiments have been carried out to determine the tribological properties of laser sintered polyamide 12 (PA12). The effect of surface orientation on the wear properties was studied, which showed anisotropic behaviour. Laser sintered specimens were polished to achieve better surface finish for the wear tests. Tribological testing results showed that the wear resistance was greater and the coefficient of friction was smaller for the side surfaces as compared to the top surfaces. Experimental details and possible reasons for the results obtained are further discussed in the paper.     

Adsorption of polymers at the solution-solid interface. VII. Poly(acrylic acids), model compounds,and vapors on nylon fibers

Nylon fibers of various draw ratios have been used as the adsorbents in the adsorption of low molecular weight acids and of polyacrylic acids from solution. Polymer adsorption is lower than on a precipitated nylon and decreases with fiber orientation; however, propionic acid and glutaric acids do not show this dependence on nylon structure. The structural alterations consequent on fiber drawing also reduce swelling in hydroxylated liquids. The greater accessibility of the precipitated nylon is shown by vapor sorption and extends to nonhydroxylated vapors; however, the differences in behavior between polyamide powder and fiber cannot be fully attributed to the former possessing a porous structure, in the sense normally employed in surface chemistry.     

Microstructural origin of physical and mechanical properties of polyamide 12 processed by laser sintering

The microstructure and properties of plastic parts made by laser sintering are known to be affected by a number of process and material parameters, whose influences are not totally elucidated yet. This study addresses the influence of the energy supplied to the powder on the microstructure and on the physical properties of sintered parts. Two different polyamide 12 powders were used. The energy density, which is derived from the laser characteristics, was shown to have a great impact on the residual porosity and on the amount of nascent particles in parts.The study of the pore size and of the porosity distribution was carried out by X-ray tomography. It is shown that the particles size distribution and the crystallisation temperature are the key parameters in the porosity formation. Finally, the influence of the microstructure and crystalline features on the mechanical properties of sintered parts is discussed.     

纳米级金红石型二氧化钛与受阻胺光稳定剂协同改进ABS的抗老化性能研究

分别采用纳米级金红石型TiO2和受阻胺光稳定剂(HALS)对ABS进行改性,通过熔融共混法制备了ABS/纳米TiO2、ABS/纳米TiO2/HALS和ABS/HALS复合材料,采用GB/T16422.2~1999所述的塑料实验室光源暴露实验方法,用氙灯气候试验机对其进行28天人工加速老化,通过测定其老化过程中的无缺口和缺口试样冲击强度的变化来对其抗老化性能进行表征,同时利用光学显微镜和扫描电镜对其老化过程中的冲击断口形貌变化进行了研究.结果表明,纳米TiO2在ABS基体中分散良好,改性后的复合材料均具有较好的抗老化性能,纳米TiO2与HALS复合改性ABS对其耐候性的提高具有明显的协同作用,其老化后的抗冲击性能超过了单一使用HALS或纳米TiO2的改性效果,老化28天后纯ABS树脂试样的无缺口冲击强度保持率只有20%左右,添加2.0 wt%纳米TiO2后,老化28天后的ABS/纳米TiO2/HALS复合材料试样的保持率则提高到47.9%,为纯ABS树脂试样的2.5倍.     

Synthesis and polymerization of 1,5-dioxo-1-methyl-4-azaphosphepane

1,5-Dioxo-1-methyl-4-azaphosphepane was synthesized and polymeized in the presence of water, stannic chloride, or phosphoric acid. Polymers obtained by using these catalysts were shown to have lower molecular weights than nylon 6 samples prepared under identical reaction conditions. Thermal and other physical properties of the phosphorus-containing polyamide are reported and compared with those of nylon 6.     

Microinjection molding of polyamide 6

Processing polymers by microinjection molding (μIM) generate specific constraints upon the polymer melt such as high shear stress and rapid cooling, leaving a mark upon the microstructure of the micropart. The present work compares the morphology and structure of polyamide 6 samples produced by melt extrusion and μIM. The specimens obtained were analyzed by polarized light microscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction. α and γ crystalline forms were formed in polyamide 6 samples prepared by both methods. The γ form was dominant in the skin of the microinjection molded part, with larger contribution for these samples compared with extruded samples. The conditions used in μIM lead to considerable orientation at the skin region, decreasing toward the core, while the extruded samples showed almost no orientation. The overall degree of crystallinity of the microinjection molded part was lower compared with the extruded sample. Copyright © 2014 John Wiley & Sons, Ltd.     

Structure and mechanical behavior of nylon‐6 fibers filled with organic and mineral nanoparticles. I. Microstructure of spun and drawn fibers

The crystalline structure and fibrillar texture of nylon‐6 fibers filled with nanosized particles were investigated using wide‐angle and small‐angle X‐ray scattering. As‐spun fibers filled with organic nanoparticles consisting of aromatic polyamide‐like hyperbranched molecules with amine‐terminating groups exhibited strong modification of both the molecular orientation and the crystalline structure compared with that of unfilled spun fibers. Montmorillonite‐filled fibers mainly exhibited orientation improvement. The differences are discussed in terms of the rheological and nucleating effects during spinning. Drawing at 140 °C involves structural changes that resulted in the three kinds of fibers having a similar crystalline form and molecular orientation. In parallel, after significant strain‐induced changes, the microfibrillar texture of the various fibers displayed subtle differences at the ultimate stage of drawing. The changes in the fibril long period and fibril radius as a function of draw ratio are discussed in terms of the two sequential deformation processes of microfibril stretching and microfibril slipping. The occurrence of interfibrillar strain‐induced cavitation is discussed in relation to the nature of the interactions between the filler and the nylon‐6 matrix. And, finally, the mechanical properties are discussed in relation to the filler–matrix interaction. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3876–3892, 2004     

Revealing toughening mechanism for alternating multilayered polypropylene/poly(ethylene-co-octene) sheets

Alternating multilayered (16, 32, and 128 layers) polypropylene/poly(ethylene-co-octene) (PP/POE) sheets were prepared via multilayer co-extrusion. Impact (−20°C), tensile, and dynamic rheological tests were carried out on the prepared multilayered samples. The toughening mechanism for the multilayered samples was revealed via investigating their fracture surface morphology and analyzing the work of fracture. The results showed that the impact energy for the notched multilayered samples is mainly absorbed by the continuous thin ductile POE layers due to cavitation, whereas for the unnotched multilayered samples the continuous thin POE and PP layers both contributed significantly to the total work of fracture due to multiple crazing, cracking and distinct plastic deformation in both PP and POE layers, and thus significantly extended crack propagation paths. Hence, the multilayered samples possessed much higher unnotched impact strength than notched impact strength. Moreover, the multilayered samples exhibited slightly increased notched impact strength and obviously increased unnotched impact strength with increasing layer number. Interestingly, the multilayered samples exhibited lower notch sensitivity than the PP sample. In addition to significantly improved low temperature impact toughness, the multilayered samples maintained the strength and stiffness as well as having superior extensibility to that of the PP sample.     

Analysis of fatigue behaviour of notched specimens made of fibreglass reinforced polyamide by means of a cohesive model

Fatigue tests until fracture of five notched specimens of short glass fibre reinforced polyamide, with a humidity of 2.5%, were carried out at 23, 28, 33, 38 and 43 °C. A correlation between the environmental temperature and the number of cycles to failure was established. Three different regions of strain were observed, namely, transient, steady-state and pre-failure. Strain and strain-rate during the creep-fatigue process revealed the existence of non-temperature dependent critical strains between these regions. The fractographic study performed after failure suggested the suitability of describing the stress distribution in the remnant ligament through a cohesive model. This is based on the observed fact that, at the crack tip, the matrix and the fibres debond, giving rise to crazing mechanisms so that the ultimate bearing ability of the material relies on the matrix. The calculations involved in the cohesive model were based on the information provided by the fractographic study and the real-time measurements of the crack growth by means of an infrared thermographic camera. Based on this methodology, the cohesive stress of the material was estimated. In a last stage, the surface roughness of the samples was determined after being tested, revealing a reliable correlation between the roughness and the fracture micromechanisms undergone by the underlying material. This correlation makes it possible to use the surface roughness of an in-service component as a parameter to evaluate the level of microstructural damage that it has experienced.     

Study on the brill transition and melt crystallization of nylon 65: A polymer able to adopt a structure with two hydrogen-bonding directions

Real time temperature dependence of X-ray diffraction patterns and infrared spectra for nylon 65, a representative polymer of the even-odd nylon series, was studied. A particular structure based on the establishment of two hydrogen-bonding directions had previously been postulated for this polymer. Therefore, the determination of its temperature-induced transitions is a relevant topic. Results indicate that nylon 65 undergoes a reversible Brill transition at high temperature, leading to a pseudohexagonal chain axis projected unit cell. Furthermore, this polyamide shows a polymorphic transition around 100 °C which is not completely reversible on cooling.Crystallization of nylon 65 was also analyzed by simultaneous WAXD and SAXS synchrotron radiation experiments to determine the evolution of the degree of crystallinity and morphological parameters on cooling. Optical microscopy studies were also performed under isothermal and non-isothermal conditions to distinguish the different spherulitic morphologies. Results reveal that the optical properties of nylon 65 spherulites are different from those of conventional even-even nylon spherulites. Multiple melting peaks associated with lamellae of different thicknesses were observed in the calorimetric heating scan of melt-crystallized samples.     

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.     

Hydration in a new semiaromatic polyamide observed by humidity‐controlled dynamic viscoelastometry and X‐ray diffraction

Hydration in a new semiaromatic polyamide, named polyamide 9‐T (PA9‐T), a copolymer of terephthalic acid with n‐ and iso‐nonanediamines, is studied by dynamic viscoelastic analysis under controlled humidity conditions and wide‐angle X‐ray diffraction analysis in comparison with common polyamide nylon 6. The storage modulus of PA9‐T is retained at up to 60 °C with increasing humidity, then dropped with further increases in temperature past 70 °C. The decrease in mechanical properties at 70 °C due to moisture uptake is found to be substantially improved by annealing to develop molecular packing and/or crystallization. In contrast, the storage modulus of very highly crystallized (50% crystallinity) nylon 6 decreases markedly with humidity at low temperatures such as 20 °C. Thus, PA9‐T retains its mechanical properties in humid atmospheres at much higher temperatures than nylon 6. The crystalline X‐ray diffraction peaks for nylon 6 corresponding to (002) + (202) of the α form shift upon absorption of moisture, speculated to be due to the weakening of hydrogen bonds and the subsequent conformational disordering of the chains. Unlike nylon 6, the crystalline peaks of PA9‐T do not shift due to moisture uptake. This is considered to be attributable to that the long aliphatic chain in PA9‐T forms the large hydrophobic domain, rendering PA9‐T less hygroscopic than nylon 6. Additionally, strong hydrogen bonds formed by terephthalamide residues together with a strong stacking force of phenylene groups may also repel water, preventing moisture bind with the amide groups of PA9‐T crystals. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1640–1648, 2005     

剪切作用下马来酸酐接枝乙烯-辛烯共聚物和乙烯-丙烯酸丁酯-甲基丙烯酸缩水甘油酯共聚物对PA1010/PP共混物的增容作用比较

采用熔体共混的方法制备了两种增容剂增容的聚酰胺1010/聚丙烯(PA1010/PP)共混物,通过扫描电镜(SEM)、力学性能和差示扫描量热(DSC)测试,对动态保压注射成型(动态)和普通注射成型(静态)中增容剂POE-g-MAH(马来酸酐接枝乙烯-辛烯共聚物)和PTW(乙烯-丙烯酸丁酯-甲基丙烯酸缩水甘油酯共聚物)对PA1010/PP共混物的增容作用进行了比较研究.研究结果表明,普通注射成型中,PTW增容体系具有更小的分散相粒子,在DSC测试中出现两个结晶峰,即出现异相成核结晶和均相成核结晶,具有更好的拉伸和冲击性能,增容作用更佳.动态保压注射成型中施加剪切可以提高所有共混物的拉伸强度、拉伸模量和缺口冲击强度,PTW和POE-g-MAH两种增容剂增容体系冲击性能相近,但POE-g-MAH增容体系的分散相相区尺寸减小明显、分布均匀性显著增加,材料冲击强度增加幅度更大,表明剪切更有利于POE-g-MAH增容作用的进行.两种增容剂增容作用的不同源于它们化学组成的不同引起的材料形态差别.