Polypropylene Melt Research Articles

Features of spinning of modified polypropylene fibres

Incorporation of polyhexamethyleneguanidine chloride, polyethylene glycol, sodium oleate, aerosils, carbon nanotubules, and Tetralin in the amount of 0.2-10% of the weight of the polymer in polypropylene melt in the spinning stage is proposed for improving the mechanical and hygienic properties of PP fibres. The rheological properties of modified polypropylene melts were investigated. The insignificant effect of the additives on the properties of the melt in shear and the sharp increase in spinnability (longitudinal deformability of the melt) was demonstrated. This makes it possible to manufacture thin PP fibres and thread. Polypropylene (PP) fibres are in second place among chemical fibres with respect to the production volume. This is due to the low cost and availability of the initial raw material, simplicity of the spinning technology, low power consumption, and valuable properties, such as the lowest density, highest specific volume and specific area, high wet and dry wear resistance, highest moisture transport rate due to the special capillary structure and hydrophoby of the fibre layers, negative charge, etc. [1-4]. Due to the above, the wide assortment of PP fibres allows using them both for manufacturing consumer goods and in industry. However, the hydrophoby of PP fibres, flow under load, poor dyeability, and relatively low initial modulus limit the areas of application [2]. Searching for new methods of modification is pressing for improving the properties of PP fibres. We investigated the possibility of manufacturing fibres by spinning from polypropylene melt modified with additives

Rheological behavior of fibre-filled polymer melts at low shear rate. Part II. Experimental investigation

In this study, rheological properties of carbon fibre-filled polypropylene and polyethylene melts at low shear rate were measured using a cone-plate rheometer. The melts with high fibre loading show complex shear flow depending on the fibre concentration, its length and length distribution, as well as on shear rate and matrix characteristics. The existence of yield stress and time-dependent or shear-strain-dependent properties are also discussed and compared with previous results. The shear flow in the narrow gap of cone-plate rheometer influences the fibre assembly. However, the effect of low shear rate on the structure of fibre assembly is negligible.

Analysis of the isothermal crystallization of polypropylene/wood flour composites

Isothermal crystallization and melting of isotactic polypropylene in binary polypropylene-wood flour composites has been studied by DSC, and the influence of an ethylene-methacrylic acid copolymer interfacial agent analyzed. Wood flour induces a slight nucleating effect, reducing the basal interfacial free energy of nucleation and increasing the overall crystallization rate. The interfacial agent generates a compatibilization phenomenon and an increase in the basal interfacial free energies with respect to the binary composites. In both binary and ternary systems the melting behaviour is a function of undercooling and is unaltered by the presence of either wood flour or the interfacial agent.

Rheology of 1‐butyl‐3‐methylimidazolium chloride cellulose solutions. I. Shear rheology

AbstractIn this article, shear rheology of solutions of different concentrations obtained by dissolution of cellulose in the ionic liquid (IL) solvent 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) was studied by measuring the complex viscosity and dynamic moduli at different temperatures. The obtained viscosity curves were compared with those of lyocell solutions and melt blowing grade polypropylene melts of different melt flow rates (MFR). Master curves were generated for complex viscosity and dynamic moduli by using Carreau and Cross viscosity models to fit experimental data. From the Arrhenius plots of the shift factors with respect to temperature, the activation energies for shear flow were determined. These varied between 18.99 and 24.09 kCal/mol, and were compared with values for lyocell solutions and different polymeric melts, such as polyolefins, polystyrene, and polycarbonate. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Raman spectroscopy for spinline crystallinity measurements. II. Validation of fundamental fiber‐spinning models

AbstractThe original Doufas–McHugh two‐phase microstructural/constitutive model for stress‐induced crystallization is expanded to polyolefin systems and validated for its predictive capability of online Raman crystallinity and spinline tension data for two Dow homopolymer polypropylene resins. The material parameters—inputs to the model—are obtained from laboratory‐scale material characterization data, that is, oscillatory dynamic shear, rheotens (melt extensional rheology), and differential scanning calorimetry data. The same set of two stress‐induced crystallization material/molecular parameters are capable of predicting the crystallinity profiles along the spinline and fiber tension very well overall for a variety of industrial fabrication conditions. The model is capable of predicting the freeze point, which is shown, for the first time, to correlate very well with the measured stick point (i.e., the point in the spinline at which the fiber bundle converts from a solid‐like state to a liquid‐like state and sticks to a solid object such as a glass rod). The model quantitatively captures the effects of the take‐up speed, throughput, and melt flow rate on the crystallization rate of polypropylene due to stress‐induced crystallization effects. This validated modeling approach has been used to guide fiber spinning for rapid product development. The original Doufas–McHugh stress‐induced crystallization model is shown to be numerically robust for the simulation of steady polypropylene melt spinning over a wide range of processing conditions without issues of discontinuities due to the onset of the two‐phase constitutive formulation downstream of the die face, at which crystallization more realistically begins. Because of the capturing of the physics of polypropylene fiber spinning and the very good model predictive power, the approximations of the original Doufas–McHugh model are asserted to be reasonable. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Preparation of polypropylene/silica composites by in-situ sol–gel processing using hyperbranched polyethoxysiloxane

Based on a volatile-free silica liquid precursor polymer—hyperbranched polyethoxysiloxane (PEOS), an industrial compatible in situ sol–gel process for the preparation of polymer/silica nanocomposites has been developed. It has been shown that in the presence of a catalyst water vapor induced a fast conversion of liquid PEOS to solid silica in polypropylene (PP) melt in a twin-screw microcompounder. Solid state NMR showed that the in situ conversion of PEOS proceeded to a large extent. With small amounts of PEOS this procedure yielded PP/silica composites with particle size less than 100 nm. The particle size increased with the PEOS amount blended with PP. Nevertheless, the particles were observed to be homogeneously dispersed within the polymer matrix. PP/silica composites prepared by in situ sol–gel technology showed improved thermal properties, but almost not affected mechanical properties in comparison with pure PP.

An in‐process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts

AbstractThe orientation and relaxation behaviors of a low‐density polyethylene melt and polypropylene melts with different melt indices undergoing a shear flow in a restricted channel were investigated by using ultrasound. A capillary rheometer was used to force the polymer melt through a slit die equipped with pressure, temperature, and ultrasound sensors, and the variation of ultrasound velocity traversing the melt was measured. Experimental results revealed that due to different mechanisms involved, the relaxations of orientation and disorientation processes show different dependences of ultrasound velocity on shear rate, temperature, and melt index. POLYM. ENG. SCI., 2008. Published 2008 Society of Plastics Engineers

X-ray Scattering of Vinyl Polyolefin Liquids and Random Copolymers: Theory and Experiment

Polymer reference interaction site model (PRISM) calculations and wide-angle X-ray experiments were performed on vinyl polyolefin melts of polyethylene (PE), isotactic polypropylene (iPP), and poly(1-butene) (P1B). PRISM calculations were also carried out on poly(1-hexene) (P1H). For PE and iPP melts a single main peak in the scattering pattern was observed near wave vectors k ∼ 1 A−1; for P1B and P1H we observed this main peak to split with the development of a prepeak that moves to lower wave vectors as the side chain size increased. By calculation of the various partial structure factors, we found that the prepeak formation is due to a combination of two effects: (1) an increase in the average distance between the chain backbones due to screening of the side chains and (2) increased intermolecular ordering of the pendant methyl groups as the side chains become longer. The scattering of random copolymers (PEP, PEB, PEH) of PE with iPP, P1B, and P1H, respectively, was also studied with PRISM theory as a ...

Experimental Study and Modeling of Flow Behavior and Orientation Kinetics of Layered Silicate/Polypropylene Nanocomposites in Start-up of Shear Flows

Abstract Effects of organoclay contents on the startup flow properties of layered nano-scale particles in the simple shear mode are investigated. The addition of small amounts of nanoclays to polypropylene melts was found to dramatically change the flow characteristics and creates stress overshoots at large shear rates. A rheological model, initially developed for studying the motion of a group of symmetric ellipsoid particles in viscoelastic fluids was used to describe the orientation state of the uniformly dispersed suspensions of layered silicate in polypropylene melts. The effects of shear, particle loadings, particle interactions, flow reversal and rest time after cession of shear are studied and discussed according to our experimental observations and model predictions. It is shown that another diffusion term in the governing equation for the particles can be used to predict the properties by applying the rest time which was found to change the orientation of particles and shifts it to more isotropic microstructures. The experimental results of the startup viscosity are reasonably well predicted by the model at the three shear rates tested.

Melting Mechanism of Thermoplastic Elastomers and Comparison to Polyolefin Thermoplastic Melting Studies in a Single Screw Extruder

Abstract An experimental investigation of the mechanisms of melting of various thermoplastics elastomers (TPEs) in single screw extruder was carried out under different operating conditions. The TPEs included three polypropylene vulcanized EPDM (TPV) thermoplastic elastomers and two styrene-hydrogenated butadiene block copolymer. The results were compared to new and earlier experimental investigations of the melting of polyethylene and polypropylene. We also characterized the shear viscosity of all polymer melts studied. The TPEs all exhibited yield values in shear flows. To characterize the melting process in a single screw extruder, we used a flow marker procedure which consists of introducing pigments with the neat pellets. This is a technique originally developed by Maddock (1959). At the steady state, we abruptly stopped the extruder operation, cooled the barrel and pushed the screw from the barrel. The polymer carcasses were stripped from the screws and slices cross-sectioned every turn from the hopper to die were photographed and evaluated, and the melting mechanism was determined. The results for the TPEs exhibit significant differences from the crystalline polyolefin.

A comparative study of dispersing a polyamide 6 into a polypropylene melt in a Buss Kneader, continuous mixer, and modular intermeshing corotating and counter‐rotating twin screw extruders

AbstractWe have made a study of the development of phase morphology of an immiscible blend(75/25)(polypropylene–polyamide‐6) for different types of continuous mixers including (i) Buss Kneader, (ii and iii) modular intermeshing corotating and counter‐rotating twin screw extruders, and (iv) NEX‐T Kobelco Continuous Mixer. Comparisons are made using different screw configurations for each machine. Generally, in comparison of the different machines, the intermeshing counter‐rotating twin screw extruder produced the finest dispersed morphology. Using a droplet breakup kinetic model, we interpreted the blend dispersed phase droplet breakdown rate and coalescence rate. In comparison with our earlier study of the continuous mixing of agglomerates of CaCO3 particles the polymer droplet breakup rate was smaller than that of the particle agglomerates and the coalescence rates of droplets were many times greater than the particle reagglomerates rates. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Fibril formation of 1,3:2,4‐Di(3,4‐dimethylbenzylidene) sorbitol in polymer melts

Abstract1,3:2,4‐Di 3,4‐dimethylbenzylidene) sorbitol (DMDBS) is a known nucleator and clarifier for polypropylene (PP). It is also known that in the useful concentration range of roughly 0.2–2%, it forms nanofibrillar structures within the PP melt. The kinetics of the DMDBS solidification process within the PP melt and formation of the ensuing nanofibrillar structure are studied by in‐situ small angle X‐ray scattering (SAXS) analysis. The dynamic lag of the fibrillar structure formation kinetics and its temperature dependence indicate a nucleation and growth mechanism, controlled by the rate of nucleation. Examination of the polystyrene (PS)/DMDBS system indicate structure formation only in compounds containing 2.5% DMDBS, as evident in the rheological behavior. Light microscopy reveals that the fibrillar cross‐section in this case is larger by an order‐of‐magnitude in comparison to the structure in PP. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Carbon nanotube dispersion and exfoliation in polypropylene and structure and properties of the resulting composites

Nitric acid treated single and multi wall carbon nanotubes (SWNT and MWNT) have been dispersed in polypropylene using maleic anhydride grafted polypropylene (MA-g-PP) and butanol/xylene solvent mixture. SWNT exfoliation was characterized by Raman and UV–vis–NIR spectroscopies. Evidence for hydrogen bonding between maleic anhydride grafted polypropylene and nitric acid treated nanotubes was obtained using infrared spectroscopy. Polypropylene/carbon nanotube composites were melt-spun into fibers. Dynamic mechanical studies show that for fibers containing 0.1wt% SWNT, storage modulus increased by 5GPa at −140°C and by about 1GPa at 100°C, suggesting temperature dependent interfacial strength. The crystallization behavior has been monitored using differential scanning calorimetry and optical microscopy. Control fibers exhibited 27% shrinkage at 160°C, while the shrinkage in the composite fibers was less than 5%. Fibers heat-treated to 170°C show very narrow polypropylene melting peak (peak width about 1°C).

Fast and Slow Dynamics of Isotactic Polypropylene Melts

We report a quasi-elastic neutron scattering (QENS) study of local dynamics in isotactic polypropylene (iPP) melts. QENS measurements were carried out on the spectrometer IRIS (ISIS, UK), using a series of selectively deuterated iPP samples. Analysis of the intermediate incoherent scattering functions was conducted using two different models: (a) a two-component model involving a fast process and conformational transitions and (b) a three-component model including methyl group reorientations. We show that consistent results between the selectively deuterated samples are only obtained when methyl group reorientations are explicitly taken into account. The results presented here are consistent with the simulated intermediate scattering functions of atactic polypropylene [Ahumada, O.; Theodorou, D. N.; Triolo, A.; Arrighi, V.; Karatasos, C.; Ryckaert, J.-P. Macromolecules 2002, 35, 7110] which revealed contributions from torsional transitions of the methyl groups and segmental motion, in addition to a fast process.

Oriented Crystallization in Polypropylene Fibers Induced by a Sorbitol-Based Nucleator

The role of 1,3:2,4-di(3,4-dimethylbenzylidene)sorbital (DMDBS) in inducing oriented crystallization during the melt spinning of polypropylene (PP) fibers has been investigated, regarding the tensile properties and crystal orientation (WAXS, SAXS) of the fibers. Better orientation and higher Young's modulus were obtained at a content of 0.4% (w/w) DMDBS, under conditions in which the DMDBS nanofibrils are fully developed in the PP melt. It is suggested the nanofibrils, oriented by the shear and elongational flow prior to PP crystallization, serve as nucleation centers for its subsequent crystallization.

Numerical Simulation of Flow Characteristics of Co-Rotating Intermeshing Four-Screw Extruder

The flow of polypropylene melt in new-style four-screw extruder was analyzed using the finite element method (FEM). The pressure and velocity fields in a flight element of co-rotating intermeshing four-screw extruders were calculated. A special study of flow in the central region was employed by analysis of the velocity, pressure and resident time distribution (RTD). The extrusion characteristics of the four-screw extruder were calculated and compared with a twin-screw extruder, `-' arrayed tri-screw extruder and triangle arrayed tri-screw extruder. The results show that the pressure and velocity gradient of the material in the four intermeshing regions is larger than that in the barrel channel. There is one circumfluence in central region which prolongs RTD in the central region and reduces the ability of pressure generation for the four-screw extruder. The material in the central region cannot stagnate. The mixing efficiency of the four-screw extruder is better than that of the twin-screw extruder and tri-screw extruder.

Rheological properties of grafted maleic anhydride based polypropylene composites filled with organoclay and glass fibers

AbstractExfoliated polypropylene/layered silicate nanocomposites and chopped glass fiber reinforced composites were prepared by a melt compounding process using maleic anhydride modified polypropylene (PP-g-MAH). The effect of fillers on morphological and rheological properties in melt mixing of polypropylene matrix with compatibilizer was investigated and compared with various measurements. It was observed that polypropylene/layered silicate nanocomposites exhibited remarkable reinforcement compared with the conventional composites filled with glass fibers which were dispersed at micrometer scale. The nanocomposites had larger storage modulus at low frequency region and outstanding strain hardening behavior than those of pure polypropylene or glass fiber reinforced composites. It was shown that glass fiber reinforced composites had lower elastic properties and steady state elongational viscosities than pure polypropylene melt. Contrary to glass fiber reinforced composites, it was confirmed that 3-dimensional network structure due to strong intermolecular bonding between polypropylene matrix and layered silicates affected particular rheological properties of nanocomposites.

Effect of Metallic Salts of Pimelic Acid and Crystallization Temperatures on the Formation of β Crystalline Form in Isotactic Poly(propylene)

The effect of crystallization temperatures (100–140°C) on the formation of β crystalline form in isotactic polypropylene nucleated by pimelic acid and sodium, magnesium, zinc, calcium, barium and aluminium salts of pimelic acid has been investigated. It is found that β crystalline form can be produced from polypropylene melt, isothermally crystallized at 120 or 130°C. Aluminium pimelate is a α nucleator which suppresses the production of β crystalline form in polypropylene. Pimelic acid and magnesium pimelate are weak β nucleators. Sodium pimelate is a moderate β nucleator. Zinc pimelate, calcium pimelate and barium pimelate are good β nucleators for polypropylene with calcium pimelate being the best.

An improved technique for exfoliating and dispersing nanoclay particles into polymer matrices using supercritical carbon dioxide

An environmentally benign process, which uses supercritical carbon dioxide (sc-CO2) as a processing aid, is developed in this work to help exfoliate and disperse nanoclay into the polymer matrices. The process relies on rapid expansion of the clay followed by direct injection into the extruder where the mixture is dispersed into the polymer melt. Results from the mechanical properties, rheological studies, and X-ray diffraction (XRD) show that this method represents a significant improvement relative to direct melt blending in single or twin-screw extruders or other methods using sc-CO2. The greatest mechanical property response was a result of directly injecting pre-mixed sc-CO2 and nanoclay into the polypropylene melt during extrusion. It was observed that for concentrations as high as 6.6wt% (limited only by present process capabilities), XRD peaks were eliminated, suggesting a high degree of exfoliation. Mechanical properties such as modulus increased by as much as 54%. The terminal region of the dynamic mechanical spectrum was similar to that of the base polymer, contrary to what is frequently reported in the literature.

The effect of thermal oxidative degradation of polymers on their viscoelastic response

A model is developed for thermal oxidative degradation of polymer melts and its effect on their viscoelastic response. Based on the theory of temporary heterogeneous networks, stress–strain relations are derived for the time-dependent behavior of a polymer melt at arbitrary three-dimensional deformations with small strains. Within the fragmentation–annihilation concept, kinetic equation is proposed for the distribution of strands with various lengths in a network, and its explicit solution is obtained. Material parameters in the constitutive equations are found by fitting experimental data in shear oscillation tests on a polypropylene melt annealed at elevated temperatures for various times. Good agreement is demonstrated between the observations and the results of numerical simulation.