Effects of annealing (solid and melt) on the time evolution of polymorphic structure of PA6/silicate nanocomposites

The effects of annealing including solid-state annealing (190 °C) and melt-state annealing (230 and 250 °C) on the polymorphic behavior and thermal property of polyamide 6 (PA6)/layered-silicate nanocomposites (PA6LSN) have been investigated as a function of annealing time using Modulated Differential Scanning Calorimetry (MDSC) and wide angle X-ray diffraction. For comparison, thermal behavior and polymorphism of pure PA6 were also studied. It was demonstrated that PA6LSN and PA6 exhibited a similar polymorphic behavior when they were annealed in the solid state for different time duration. As the annealing temperature was elevated to 230 and 250 °C, significant differences in thermal behavior and polymorphism between PA6LSN and PA6 could be found. For example, the γ phase became the absolutely dominating for PA6LSN, while the α crystal was the most predominant phase in neat PA6. Moreover, a new endothermic peak is observed around 235 °C in all PA6LSN MDSC scans, which might be related to the melting of PA6 lamellae formed in the confined environment on the surface of the nano-silicate.     

Thermal history dependence of polymorphic transformation of polyamide 6/silicate nanocomposites

BACKGROUND: Although much research has been devoted to nanocomposites of polyamide 6 (PA6) with inorganic nanoparticles, most research has mainly focused on the effects of clay and annealing temperature on the crystallization behavior of the PA6 matrix, but the effect of annealing on the thermal behavior of PA6/clay nanocomposites remains an interesting subject which requires further investigation. This paper focuses on the effects of thermal history on the polymorphic transformation of PA6 and PA6/layered silicate nanocomposites (PA6LSN). RESULTS: It was found that PA6 and PA6LSN exhibit a similar polymorphic behavior when they were annealed at 190 °C. As the annealing temperature was elevated to 230 and 250 °C, significant differences in polymorphic transformation and thermal behavior between PA6 and PA6LSN could be observed. For example, the γ phase became the dominant crystalline phase for PA6LSN samples while the α crystal was still the predominant phase in neat PA6. Furthermore, a higher temperature endothermic peak observed at around 236 °C in all PA6LSN modulated differential scanning calorimetry scans was also found to be strongly dependent on the cooling rate and annealing temperature. CONCLUSION: The thermal history has a strong influence on the polymorphic transformation of PA6 and PA6LSN. For the first time, the effect of thermal history on the high‐temperature endothermic peak of PA6LSN was investigated and it was observed that this peak is also strongly dependent on the thermal history. Copyright © 2009 Society of Chemical Industry     

Effects of solid lubricants on friction and wear behaviors of polyamide 6

As solid lubricants, Polytetrafluoroethylene (PTFE), graphite, and ultra‐high molecular weight polyethylene (UHMWPE) can improve the tribological properties of PA6. The mechanical and the tribological properties of polyamide 6 (PA6) composites filled with solid lubricants were researched. The blended materials were injection molded to provide the test samples. Mechanical properties were studied in terms of the tensile strength and impact strength. Friction and wear experiments were run at a rotating speed of 1500 rpm and under loads of 40 and 160 N. The worn surfaces were examined using a scanning electron microscope. It was found that an improvement of tribological properties can be obtained by preparing PA6 composites, which was closely related to the varieties and the contents of solid lubricants added. At a load of 40 N, PTFE was the most effective to have reductions of both coefficient and mass wear rate, while at a load of 160 N, UHMWPE was the most helpful. The effects of Combination Solid Lubricants were also discussed. The results showed that synergistic reaction can be gained to modify the tribology capabilities of PA6. Moreover, the micrographs taken in the worn surface of PA6 composites revealed that adhesive wear, abrasive wear and fatigue wear occurred in this study. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The formation of fibrous structure of polyamide 6 induced by the three-step forming process and its prominent reinforcement in nitrile rubber

Nitrile rubber (NBR) blends with excellent performance have always been a hot research topic in petroleum field. Due to the excellent performance and compatibility of polyamide 6 (PA6), it provides an opportunity for the preparation of high-performance NBR/PA6 blends. In this article, NBR/PA6 blends were prepared by the three-step molding process. Experimentally, it was found that PA6 has a prominent reinforcement effect in NBR matrix. The variation of this mechanical property was investigated from different aspects of the crystal structure, crystallinities, phase morphology, and so on. It can be cleared that the formation of fibrous structure of PA6 phase is the main factor for reinforcement of the polymer blends. Meanwhile, the formation mechanism of the special phase structure induced by the three-step process is deeply expounded and its structural evolution schematic is established. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47472.     

Effects of processing history and annealing on polymorphic structure of nylon-6/montmorillonite nanocomposites

The effects of processing history and annealing treatment on the thermal property and polymorphic structure of nylon-6/clay nanocomposites (NCNs) have been investigated. The nanocomposites, including intercalated and exfoliated ones, were prepared by extruding nylon-6 (N6) with sodium montmorillonite (Na-MMT) or organo-montmorillonite (OMMT), respectively. DSC analysis revealed multiple melting endotherms in either the extruded or the injection-molded N6 and NCNs samples. It has been observed that a small exothermic peak around 195 °C just before the lower melting peak in the skin regions. We demonstrated that this sub-T_m transition was directly related to the processing-induced shear stresses. WAXD analysis was further performed to characterize the polymorphic structure of injection-molded N6 and NCNs before and after annealing. Annealing at a temperature (80 °C) above the T_g of N6 resulted in increase of the absolute content of γ-form in the skin regions and of the relative content of γ-form in the core regions of NCNs. In particular, annealing only leaded to increase the fraction of γ-form in the exfoliated N6/OMMT nanocomposites, which might be related to a confining effect of MMT platelet on the polymer chains mobility.     

侧吹风速度对高强聚酰胺6纤维结构和性能的影响

以相对黏度为2.8的聚己内酰胺(PA 6)切片为原料,采用熔融纺丝结合多级拉伸工艺制备高强PA6全拉伸丝(FDY);在其他工艺条件一定、侧吹风速度为0.22～0.42 m/s的条件下,研究侧吹风速度对高强PA6 FDY结构和性能的影响.结果表明:随侧吹风速度提高,高强PA6 FDY断裂强度变化不大而断裂伸长率有所下降,...     

A processing-induced clay dispersion and its effect on the structure and properties of polyamide 6

The nanostructures and morphologies of polyamide 6 (PA6)/organoclay nanocomposites prepared by melt compounding have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). A combination of XRD and TEM indicates that an exfoliated clay morphology dominates at low clay loadings (≤5 wt%) and a mixture of intercalated and exfoliated structures exists at high clay concentrations. It is worth noting, however, that optical microscopy (OM) even shows the presence of large clay agglomerates in samples with higher clay contents. OM images further present an overview of an uneven clay distribution due to the effect of injection molding. The crystalline structure of PA6 is greatly affected by this unevenness in the processing-induced clay dispersion, as evidenced by differential scanning calorimetry (DSC). The thermal dynamic and mechanical properties of PA6 and the nanocomposites have been investigated as a function of clay concentration. The tensile tests show that the degree of dispersion of the nanoclay within the polymer matrix plays a vital role in property improvement. Copyright © 2004 Society of Chemical Industry     

PA6/蒙脱土熔融插层复合材料结构与性能分析

通过熔融共混法插层复合制备了聚酰胺(PN)6／蒙脱土纳米复合材料，测试了力学性能并对不同蒙脱土含量的PA 6／蒙脱土纳米复合材料进行了对比。实验表明，通过熔融插层可使PN 6基体插层于蒙脱土中，所得到的复合材料的性能较PN 6有很大提高。蒙脱土特殊的层状结构使得利用熔融共混在机械力的作用下插层到纳米级复合材料成为可能。     

Effect of the structure on sorption and diffusion processes in polyamide 6, part 1: Activation energy and thermodynamic parameters of water desorption in oriented and unoriented polyamide 6

Effect of the structure on desorption of water in polyamide 6 is studied by differential scanning calorimetry and thermogravimetry. Oriented and unoriented polyamide 6 fibers annealed in water at 100°C for different times (2 s up to 1 h) are characterized by their structure and transport properties. It is found that water treatment leads to substantial change in the polymer structure (including phase transition even at room temperature). Activation energy and enthalpy of water desorption are determined, and their high values (E_a = 60–130 kJ/mol; ΔH_d = 950–1360 J/g) are attributed to the formation of strong sorbent–sorbat bonds. Both parameters are influensed substantially by the polymer structure (degree of crystallinity and orientation). Besides, it is found that the temperature of water desorption (T_d) is also very sensitive to the structure. The T_d‐values increase sharply with rise of degree of crystallinity, being always considerably lower for oriented fibers. This quite unexpected result is also confirmed and further interpreted in the next parts of the article. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

尼龙-6/蒙脱土纳米复合材料用POE-g-MAH改性及性能研究

制备了尼龙-6(PA6)/马来酸酐接枝乙烯-1-辛烯共聚物(POE-g-MAH)和PA6-蒙脱土纳米复合物(NCH)/POE-g-MAH两种复合材料,其脆韧转变点都是在POE-g-MAH质量分数为8%～10%。在脆韧转变点前,PA6/POE-g-MAH和NCH/POE-g-MAH的缺口冲击强度几乎相同;在脆韧转变点后,NCH/POE-g-MAH的冲击强度远高于PA6/POE-g-MAH。复合材料的拉伸强度都随POE-g-MAH的增加而线性下降,在相同POE-g-MAH含量时,NCH/POE-g-MAH的拉伸强度比PA6/POE-g-MAH的低4MPa左右。     

高相对粘度聚酰胺6冻胶的研究

采用阴离子淤浆聚合方法制备了聚酰胺6(PA6)冻胶,用扫描电镜、差示扫描量热和广角X射线衍射等手段对冻胶结构和性能进行了分析。探讨了PA6的相对粘度(η_r)、浓度和络合剂用量及不同溶剂对PA6冻胶的溶胶-冻胶转变点(T_f)的影响。结果发现:随冻胶中PA6的浓度、三氯化镓与PA6摩尔比的增加,T_f都会随之升高,呈较好的线性关系;η_r的增加使T_f升高,η_r小于12.5时,η_r和T_f呈线性关系,当η_r大于12.5时,则会发生偏离;冻胶表面有均匀的层状结构,有利于纺丝和高倍拉伸,络合剂三氯化镓能有效地将PA6分子间的氢键屏蔽,对冻胶的形成有很大作用。     

Processability of polysulfone improved by adding polyamide 6 and polysulfone‐polyamide 6 block copolymer

Although polysulfone (PSU) is a potential thermoplastic engineering plastic with high heat resistance, good dimensional stability and excellent mechanical properties, its poor processability has greatly restricted its application in electrical, aerospace, and medical fields. In this work, polyamide 6 (PA6) and PSU‐PA6 block copolymer (PSU‐b‐PA6) were used to improve the processibility and formability of PSU depending on their excellent fluidity and good compatibility between two components. Furthermore, the fluidity, thermal and mechanical properties of the blends were carefully investigated. It was found that, melt flow index of PSU could be increased above 10 times, and strength and toughness could be enhanced by 4–10% with the introduction of 10 wt % PA6 and PSU‐b‐PA6 without compromising the heat resistance of PSU obviously. The processing conditions of PSU could be improved while maintaining a decent comprehensive performance. Thus, the method has great potential for extending the applications of PSU. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41139.     

Effect of thermal annealing on the structure of Technora and Kevlar polyamide fibers

This paper describes X-ray diffraction analysis for the structural changes of Technora and Kevlar polyaramid fibers as a result of thermal annealing. Technora fibers are based on the aromatic copolyamide prepared by reaction of terephthaloyl chloride (TPA) with a 50/50 mole ratio of p-phenylene diamine (PPD) and 3,4′-diaminodiphenyl ether (DPE); Kevlar is the alternating copolymer prepared from TPA and DPE. X-ray fiber diagrams of Technora contain a series of non-periodic layer lines that are characteristic of a completely random comonomer sequence. The structure consists of ordered arrays of chain segments that have extended but sinuous conformations. By matching the profiles of the meridional peaks with those predicted for molecular models, the segment length in unannealed Technora is determined to be 300 ± 15 Å with a sinuosity g of 1.52 ± 0.05 %, where g is analogous to the paracrystallinity parameter for homopolymer structures. The segment length is similar to that for unannealed Kevlar 29 fiber (350 ± 10 Å) and the sinuosity is lower than that for thermally annealed Kevlar 149 (g = 1.69 ± 0.08 %), for which the crystallite size has increased to 440 ± 10 Å. On annealing Technora fibers at 300 °C for 48 hours, both the chain segment length and chain sinuosity remain essentially the same. In contrast, annealing Kevlar 29 under the same directions results in an increase in the crystallite size along the fiber direction to 410 ± 10 Å with g declining to 2.05 ± 0.06 %. Both polymers show evidence for improved perfection in the lateral packing as a result of annealing. The g values determined for Technora indicate that despite the random copolymer sequence, the chain conformation is straighter in the Kevlar homopolymer. This linearity is probably maximized during spinning and is not improved by annealing, whereas the ordering of the homopolymer increases due to crystal growth along all three axes.     

Side effects in the application of polyamide 6 barrier materials for fuel tanks

To reduce the pollution of air by minimizing evaporative emissions from fuel tanks, new plastic fuel tanks made of materials with excellent barrier properties have to be developed. Single‐layer polyamide 6 tanks are one option to meet the requested low‐hydrocarbon permeation rates for motorcycle vehicle tanks. Recently, some problems with respect to deposits in polyamide 6 tanks, blocked nozzles, tubing, and gasoline filters were observed. Thus, samples (precipitates) were taken from unused tanks after conditioning as well as of used tanks and filters after being in contact with gasoline for some time. By investigating the precipitates and deposits by means of infrared (IR) spectroscopy, the main constituents were identified to be cyclic caprolactam oligomers. Additional investigations on the extracted samples by mass spectroscopy allowed us to attribute specific features of the IR spectra to the individual cyclic oligomers (dimer, trimer, and tetramer). In addition, we could show that the crystalline precipitates and deposits in the fuel systems of used vehicles consist of mixtures of the cyclic dimer, trimer, tetramer, and even pentamer of caprolactam with varying contributions of the individual oligomers in dependence of the history of the part. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of clay on the morphology of blends of poly(propylene) and polyamide 6/clay nanocomposites

PP/NPA6 blends composed of poly(propylene) (PP) and polyamide 6/clay nanocomposites (NPA6) were prepared by twin‐screw extrusion and melt‐drawn into ribbons by a ribbon extrusion process. The influence of clay on the morphology of PP/NPA6 ribbons was investigated by means of field‐emission scanning electron microscopy and optical microscopy. The results show that at low clay content (3, 5 wt%), NPA6 exhibited continuous lamellar structure in PP as pristine PA6 did in a PP/PA6 blend, but at a higher clay content (10 wt%) only ellipsoids or elongated ellipsoids were observed. In order to explain the morphological difference, two factors, ie the compatibilization effect and melt rheology, have been taken into consideration. It has been found that both factors, and probably mainly the variation in melt rheology, were responsible for the morphological difference in the PP/NPA6 blends with different clay contents under the extensional flow field. Copyright © 2004 Society of Chemical Industry     

Study on dry spinning and structure of low mole ratio complex of calcium chloride‐polyamide 6

Polyamide 6 (PA 6) filaments with initial modulus around 48 GPa were produced by dry spinning from low mole ratio (MR) complex of calcium chloride and high molecular weight PA 6 (CaCl₂‐PA 6) in formic acid. From the results of XRD, DSC, FTIR, and SEM, the complexation of CaCl₂‐PA 6 in the MR range of 0.15–0.3 was efficient. The spinnability of the complex solution was excellent, which allowed a maximum draw ratio of 14.4 for as‐spun fibers. After decomplexing and annealing, the birefringence of drawn fiber could reach around 0.08. Porous structure was found in fibers spun from formic acid‐chloroform cosolvent but not observed by using pure formic acid. When MR of CaCl₂/PA 6 exceeded 0.3, some irregular particles formed on the fiber surface due to the recrystallization of CaCl₂. However, fibers with smooth surface could be obtained when the MR decreased to 0.15. During the process of decomplexing in ethyl alcohol, an axial shrinkage of drawn fibers in a relaxed state was observed. It turned out that this shrinkage could be avoided by decompexing the fibers under tension. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Physical properties of polyamide 6/H‐NBR blends and the effects of dynamic vulcanization on them

Polyamide 6 (PA 6) and hydrogenated nitrile rubber (H‐NBR) were blended with various blend ratios in a brabender plasticoder at 240°C/100 rpm. The processing characteristics with a mixing torque of the blends were investigated. The effect of the blend ratio on physical properties such as tensile strength, Young's modulus, elongation at break, permanent set, hardness, and swelling behavior of blends was analyzed. Most mechanical properties were found to decrease with an addition of H‐NBR. The morphology of the blends was observed, and the results show a two phase system where the component with high proportions exists as a continuous phase. A cocontinuous phase was observed in blend ratios of 50/50 and 40/60. Dynamic mechanical properties were observed to study a viscoelastic property of the blends. In addition, the effect of dynamic vulcanization with peroxide on physical properties was studied, and the influence of peroxide on PA 6 was also examined. It was found that the peroxide can have an effect on PA 6 as well as act as a crosslinker to H‐NBR. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Investigation on effects of aluminum and magnesium hypophosphites on flame retardancy and thermal degradation of polyamide 6

Two hypophosphites, aluminum hypophosphite (AlHP) and magnesium hypophosphite (MgHP), were applied to obtain flame retardant polyamide 6 (FR-PA6) composites. UL-94 and limiting oxygen index results indicated that AlHP contributed both good flame retardance and antidripping ability for PA6, while MgHP did not. Based on thermogravimetric analysis (TGA), AlHP and MgHP presented the different thermal degradation behavior. That is, the quick decomposition of AlHP took place at lower temperature than that of MgHP. AlHP promoted the early thermal degradation of PA6 and formed more char residue. The thermal decomposition mechanisms of AlHP and MgHP in nitrogen or air were suggested. Scanning electron microscope and X-ray photoelectron spectroscopy indicated that in the existence of AlHP, the morphological structures of char residue were more homogenous, and compact, and more char residue was formed. These results well illustrated the difference of the flame retardancy between AlHP and MgHP. Mechanical properties of PA6/AlHP and PA6/MgHP were also obtained. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Self‐assembly morphology evolution of the polyamide 6 (PA6) component in the PA6/polyethylene glycol system by in situ polymerization of ϵ‐caprolactam monomer

This paper describes the morphology evolution of polyamide 6 (PA6) components in the presence of polyethylene glycol (PEG) by self‐assembly via in situ anionic ring‐opening polymerization of ?‐caprolactam (CL) monomer and expands the study of morphology changes of PA6 in the PA6/PEG system. With a fixed mass ratio of [CL]/[PEG], it was found that by simply changing the reaction conditions the morphology of PA6 components could be changed from nano‐sized microspheres to regular 3D microsphere structured polyhedrons to 3D nano‐sized particle clusters to micro‐scale microspheres. The morphologies of the PA6 components were investigated by SEM and TEM analyses. The diameter range of the PA6 nano‐sized microspheres was controlled within 400 nm. The side length of an individual polyhedron could be effectively tuned from 10 µm to 100 µm. The diameter range of micrometer microspheres was about 5–8 µm. The results suggest that this strategy for self‐assembly can be applied to design materials with complex geometric structures. © 2018 Society of Chemical Industry     

Morphological and mechanical properties of polyamide 6/nanodiamond composites prepared by melt mixing: effect of surface functionality of nanodiamond

Surface chemistry of as‐received nanodiamond (ND) was first tailored by dry thermal oxidation to obtain carboxylated ND (ND‐COOH) and by wet chemistry to obtain ethylenediamine‐functionalized ND (ND‐EDA). Then, the surface‐functionalized ND particles were dispersed in polyamide 6 (PA6) using the melt‐mixing method. Transmission optical and scanning electron microscopies indicated a fine dispersion at low nanodiamond concentrations, e.g. 0.25 wt%, particularly with ND‐EDA. Differential scanning calorimetry revealed that ND‐EDA favoured the α‐phase crystal and enhanced the degree of crystallinity of PA6. Experimental data indicated that ND‐EDA had considerably improved tensile properties at low concentration of 0.25 wt% compared to ND‐COOH, which was correlated to the fine dispersion and stronger and thicker interphase in the case of ND‐EDA. It was also found that the toughness of PA6 was improved on incorporation of ND‐EDA due to development of microcracking and crazing. © 2016 Society of Chemical Industry