Nucleation and Crystallization of PA6 Composites Prepared by T-RTM: Effects of Carbon and Glass Fiber Loading.
ABSTRACT: Thermoplastic resin transfer molding (T-RTM) is attracting much attention due to the need for recyclable alternatives to thermoset materials. In this work, we have prepared polyamide-6 (PA6) and PA6/fiber composites by T-RTM of caprolactam. Glass and carbon fibers were employed in a fixed amount of 60 and 47 wt.%, respectively. Neat PA6 and PA6 matrices (of PA6-GF and PA6-CF) of approximately 200 kg/mol were obtained with conversion ratios exceeding 95%. Both carbon fibers (CF) and glass fibers (GF) were able to nucleate PA6, with efficiencies of 44% and 26%, respectively. The ? crystal polymorph of PA6 was present in all samples. The lamellar spacing, lamellar thickness and crystallinity degree did not show significant variations in the samples with or without fibers as result of the slow cooling process applied during T-RTM. The overall isothermal crystallization rate decreased in the order: PA6-CF > PA6-GF > neat PA6, as a consequence of the different nucleation efficiencies. The overall crystallization kinetics data were successfully described by the Avrami equation. The lamellar stack morphology observed by atomic force microscopy (AFM) is consistent with 2D superstructural aggregates (n = 2) for all samples. Finally, the reinforcement effect of fibers was larger than one order of magnitude in the values of elastic modulus and tensile strength.
Project description:The effects of 1-vinyl-3-butyl imidazole chloride (VBIM) on the structure and properties of Polyamide 6 (PA6) were investigated systematically. It was found that PA6/VBIM blends were homogeneous without phase separation. The glass transition temperature (Tg) of PA6 increased with small VBIM loadings followed by the decreasing in Tg with further increasing the amount of VBIM. The crystallization temperature decreased with the addition of VBIM because of the strong interactions between VBIM and the PA6 matrix, as well as the dilution effect when large amounts of VBIM was introduced to the matrix. According to rheological testing, small amounts of VBIM enhanced the storage modulus and melt viscosity of PA6. Tensile tests also show an increase in strength and modulus at relatively low loadings of VBIM. The strength of PA6 with only 1 wt % VBIM improved by 108% compared to that of neat PA6. Fourier transform infrared (FTIR) investigations revealed that the ions of VBIM preferred to form hydrogen bonds with amide groups in PA6. Therefore, VBIM acts as physical connection point for the neighboring PA6 molecular chains. The specific interactions between VBIM and PA6 account not only for the enhanced melt viscosity of PA6, but also for the improved mechanical properties. Moreover, outstanding antistatic property was also observed. The surface resistivity of the sample with 1 wt % VBIM was 1.50 × 1010 Ω/sq, which means good electric dissipation property.
Project description:The crystallization behavior of novel polyamide 6 (PA6) copolyamides with different amounts of bis (2-aminoethyl) adipamide/adipic acid (BAEA/AA) segment was investigated. The wide-angle X-ray diffraction (WAXD) results showed that as the amount of BAEA/AA segment increased to 10 mole%, the crystalline forms of all PA6 copolyamide were transferred from the stable ?-form to the unstable ?-form because of the complex polymer structure. According to studies of crystallization kinetics, the Avrami exponent (n) values for all copolyamide samples ranged from 1.43 to 3.67 under isothermal conditions, implying that the crystallization is involved in the two- to three-dimensional growth at a high temperature of isothermal condition. The copolyamides provided a slower crystallization rate and higher crystallization activation energy (?Ea) than neat PA6. Polyamide containing 10 mole% of BEAE/AA content exhibited a unique crystallization behavior in the coexistence of the ? and ? forms. These results deepen our understanding of the relationship between BAEA/AA content, crystal structure, and its crystallization behavior in low-melting PA6, and they make these types of copolyamides useful for their practical application.
Project description:An aromatic macromolecular polyimide (API) was synthesized and characterized, and used as a synergistic charring flame retardant in glass fiber reinforced polyamide 6 (GF/PA6). API and aluminum diethylphosphinate (ADP) exhibited better flame inhibition behavior and synergistic charring flame retardant behavior compared with ADP alone. The 5%API/7%ADP/GF/PA6 sample achieved the lower peak value of the heat release rate (pk-HRR) at 497 kW/m² and produced higher residue yields of 36.1 wt.%, verifying that API and ADP have an outstanding synergistic effect on the barrier effect. The API/ADP system facilitated the formation of a carbonaceous, phosphorus and aluminum-containing compact char layer with increased barrier effect. FTIR spectra of the residue and real-time TGA-FTIR analysis on the evolved gases from PA6 composites revealed that API interacted with ADP/PA6 and locked in more P⁻O⁻C and P⁻O⁻Ar content, which is the main mechanism for improving flame inhibition and charring ability. In addition, the API/ADP system improved the mechanical properties and corrosion resistance of GF/PA6 composites compared to ADP alone.
Project description:This paper presents an investigation into the behavior and performance of blends of Nylon 6 (PA6), polypropylene (PP), and poly(lactic acid) (PLA), compatibilized with maleic anhydride-grafted PP (PP-g-MA). The mechanical performance of ternary PA6/PP/PLA blends was superior to that of binary PA6/PP blends because of the addition of PLA. Through blending with PLA, the tensile and flexural strength and modulus were enhanced, maintaining performance similar to that of neat PA6. Tensile performance was further enhanced through reactive compatibilization of the blends with PP-g-MA due to the improved homogeneity of the materials. Impact behavior of the blends was found to be highly dependent on morphology, and the toughening behavior was observed at certain blending ratios. In PA6/PP blends, fractionated crystallization behavior was investigated through differential scanning calorimetry, in which both PA6 and PP droplets were crystallized at supercooled states. This effect was highly influenced by the presence of the compatibilizing agent and its effect on the morphology of the dispersed phase. As the droplet size of the dispersed phase was decreased to submicron levels, the efficiency of heterogeneous nucleation was limited. Crystallization of PLA in the blend was poor, but PP-g-MA was found to have an impact on its rate of crystallization.
Project description:It is known that the percolation threshold of polyamide 6 (PA6)/multiwalled carbon nanotube (MWCNT) composites is higher than that of PA66/MWCNT composites under the same mixing conditions and melt viscosity. A series of blends of PA6 and PA66 containing 1 wt % MWCNTs have been prepared to investigate this phenomenon. At contents up to 20 wt % PA66, the blends were not electrically conductive. The electrical resistivity dropped to 10⁸ Ohm∙cm for PA66/PA6 30/70 blends. The resistivity was 10⁵ Ohm∙cm at higher PA66 contents. Differential scanning calorimetry was used to investigate the thermal behavior of blends. The glass transition temperature was almost constant for all blend compositions, indicating that the amorphous phases are miscible. The MWCNT addition influenced the crystallization of PA66 much more than the PA6 crystallization. A heterogeneous crystallization of the polyamide in PA66/PA6 blends took place, and the MWCNTs were mainly localized in the earlier crystallizing PA66 phase. Thus, the formation of the nanotube network and thus the electrical volume resistivity of the PA6/PA66 blends with 1 wt % MWCNTs is significantly influenced by the crystallization behavior. In PA66/PA6 blends up to 60 wt %, the more expensive PA66 can be replaced by the cheaper PA6 while retaining its electrical properties.
Project description:Studies of the production of fiber-forming polyamide 6 (PA6)/graphene composite material and melt-spun textile fibers are scarce, but research to date reveals that achieving the high dispersion state of graphene is the main challenge to nanocomposite production. Considering the significant progress made in the industrial mass production of graphene nanoplatelets (GnPs), this study explored the feasibility of production of PA6/GnPs composite fibers using the commercially available few-layer GnPs. To this aim, the GnPs were pre-dispersed in molten ?-caprolactam at concentrations equal to 1 and 2 wt %, and incorporated into the PA6 matrix by the in situ water-catalyzed ring-opening polymerization of ?-caprolactam, which was followed by melt spinning. The results showed that the incorporated GnPs did not markedly influence the melting temperature of PA6 but affected the crystallization temperature, fiber bulk structure, crystallinity, and mechanical properties. Furthermore, GnPs increased the PA6 complex viscosity, which resulted in the need to adjust the parameters of melt spinning to enable continuous filament production. Although the incorporation of GnPs did not provide a reinforcing effect of PA6 fibers and reduced fiber tensile properties, the thermal stability of the PA6 fiber increased. The increased melt viscosity and graphene anti-dripping properties postponed melt dripping in the vertical flame spread test, which consequently prolonged burning within the samples.
Project description:In the present study, lithium chloride (LiCl) was utilized as a modifier to reduce the melting point of polyamide 6 (PA6), and then 15 wt % microcrystalline cellulose (MCC) was compounded with low melting point PA6/high-density polyethylene (HDPE) by hot pressing. Crystallization analysis revealed that as little as 3 wt % LiCl transformed the crystallographic forms of PA6 from semi-crystalline to an amorphous state (melting point: 220 °C to none), which sharply reduced the processing temperature of the composites. LiCl improved the mechanical properties of the composites, as evidenced by the fact that the impact strength of the composites was increased by 90%. HDPE increased the impact strength of PA6/MCC composites. In addition, morphological analysis revealed that incorporation of LiCl and maleic anhydride grafted high-density polyethylene (MAPE) improved the interfacial adhesion. LiCl increased the glass transition temperature of the composites (the maximum is 72.6 °C).
Project description:Glass fibers (GF) are the reinforcement agent most used in polypropylene (PP) based composites, as they have good balance between properties and costs. However, their final properties are mainly determined by the strength and stability of the polymer-fiber interphase. Fibers do not act as an effective reinforcing material when the adhesion is weak. Also, the adhesion between phases can be easily degraded in aggressive environmental conditions such as high temperatures and/or elevated moisture, and by the stress fields to which the material may be exposed. Many efforts have been done to improve polymer-glass fiber adhesion by compatibility enhancement. The most used techniques include modifications in glass surface, polymer matrix and/or both. However, the results obtained do not show a good costs/properties improvement relationship. The aim of this work is to perform an accurate analysis regarding methods for GF/PP adhesion improvement and to propose a new route based on PP in-situ polymerization onto fibers. This route involves the modification of fibers with an aluminum alkyl and hydroxy-α-olefin and from there to enable the growth of the PP chains using direct metallocenic copolymerization. The adhesion improvements were further proved by fragmentation test, as well as by mechanical properties measurements. The strength and toughness increases three times and the interfacial strength duplicates in PP/GF composites prepared with in-situ polymerized fibers.
Project description:During the "slit die extrusion-hot stretching" process, highly oriented polyamide 6 (PA6) dispersed phase was produced and retained in the polypropylene (PP) matrix directly. By adjusting the stretching forces, the PA6 spherical phase evolved into the ellipsoid, rod-like microfibril with a decreasing average diameter; then, the PA6 microfibrils broke. Moreover, the effects of the PA6 phases formed in the process of the microfibrillation on PP's crystallization behaviors were studied systematically. As the stretching forces increased, the crystallization ability and orientation degree of PP crystals improved significantly. Differential scanning calorimetry and polarizing optical microscopy confirmed the formation of PP spherulite, fan-shaped lamellae and a transcrystalline layer under the induction of the PA6 phases with different morphology. In the PP/PA6 microfibrilar composites (MFCs), PP crystals showed smaller average size, more crystals and stronger interface adhesion due to more excellent heterogeneous nucleation ability of the PA6 microfibrils, which made contributions to the improvement of the melt elasticity responses and oxygen barrier properties of the PP/PA6 polymer blends.
Project description:Transparent nanoceramics embedded with highly dense crystalline domains are promising for applications in missile guidance, infrared night vision, and laser and nuclear radiation detection. Unfortunately, current nanoceramics are strictly constrained by the stringent construction procedures such as super-high pressure and containerless processing. Here, a pressureless crystallization engineering strategy in glass for elaboration of transparent nanoceramics and fibers is proposed and experimentally demonstrated. By intentional creation of a sharp contrast between nucleation and growth rates, the crystal growth rate during glass crystallization can be significantly suppressed. Importantly, this unique phase-transition habit enables the achievement of transparent nanoceramics and even smooth fibers with extremely tiny crystalline size (≈20 nm) and high crystallinity (≈97%) under atmospheric pressure. This allows the generation of an attractive nonlinear optical response such as dynamic optical filtering and luminescence in the mid-infrared waveband of 4300-4950 nm. These findings highlight that the strategy to switch the phase-transition habit of glass into the unconventional crystallization regime may provide new opportunities for the creation of next-generation nanoceramics and fibers.