Thermal and mechanical degradation during polymer extrusion processing

Polymer processing often activates material degradation, which affects to some extent the performance or the life cycle of the obtained products. Polymer degradation is usually evidenced by molecular weight variations as consequence of exposure to high processing temperature and to relevant mechanical stresses. The degrading effects of melt extrusion under different thermo‐mechanical conditions on polymethyl‐methacrylate and polystyrene are investigated. Materials were processed at different temperatures and rates in a single screw extruder and in a capillary rheometer. Processed and as‐received materials were analysed by Gel permeation chromatography, solution viscosity, and melt rheology. Evaluations of molecular weight and rheological behavior changes after processing, as compared with as‐received materials, evidenced the extent of degradation in the different conditions. Experimental analysis was accompanied by finite elements simulation of the polymer flow and heat transmission within the extruder. This allowed an estimation of the expected shear stresses and temperatures distributions, thus indicating potentially critical situations. It was found that the materials processed at the highest rates in extruder presented lowest molecular weight reductions. This was attributed to the shorter residence time in the extruder and to the possibility of wall slip phenomena, which likely reduce the actual shear stress and viscous dissipation in the fast processing conditions. POLYM. ENG. SCI., 47:1813–1819, 2007. © 2007 Society of Plastics Engineers     

Pattern formation by dewetting of polymer thin film

Strategies for the utilization of dewetting of polymer thin film to fabricate ordered patterns are reviewed. After a brief introduction to the polymer thin film dewetting theory, simulation results of pattern formation induced by physically and chemically patterned substrates, and physical confinement are then summarized. Experimental results including the mechanisms behind and the conditions for good quality of pattern formation based on the dewetting of polymer thin film induced by physical, chemical heterogeneous substrates, topographic structure on film surface, physical confinement and the movement of three-phase line are then discussed. A short introduction to the application of fabricated patterns is also discussed.     

Surface dynamics during latex film formation

Surface dynamics during latex film formation has been investigated theoretically and experimentally by atomic force microscopy. The peak-to-valley distance, y(t), of the latex particles in the surface plane of the latex film decayed exponentially with time during film formation. A theoretical relationship between y(t) and time, t, is given by y(t)=y(0) exp[−t/τ], where y(0) is the value of y(t) when t is zero. τ is a characteristic constant related to the nature of polymer, the particle radius, the surface diffusion coefficient and the temperature. The relationship between the surface diffusion coefficient, D_s, y(0), the radius of the latex particles, R, temperature, T, and τ is given approximately by D_s=1.2×10⁻²⁰y(0)²[2R−y(0)]²T/τ (cm²/s), where the units are manometers for y(0) and R, kelvin for temperature, and seconds for τ. By measuring the decay of y(t) with time, the surface diffusion coefficient can be obtained. The surface diffusion coefficient for a poly(methyl methacrylate-co-butylacrylate) (50:50) copolymer latex film was found to be A×10⁻¹³ cm²/s, A is temperature-dependent.     

The development of laminar morphology during extrusion of polymer blends

Studies of the microstructure and permeability of extruded ribbons of polypropylene (PP)/ethylene vinyl alcohol copolymer (EVOH) and polyethylene (PE)/polyamide-6 (PA-6) blends have shown that it is possible to control the flow-induced morphology to generate discontinuous overlapping platelets of EVOH or PA-6 dispersed phase in a PP or HDPE matrix phase. The effects of the following factors on morphology development and blend properties were considered: blending sequence, melt temperature, composition, compatibilizer level, die design, screw type, and cooling conditions. The impact properties and interfacial adhesion of laminar blends of PP and EVOH were improved without diminishing the barrier properties. The oxygen and toluene permeability of extruded samples with EVOH content of 25 vol% resembled values obtained with multilayer systems. Processing conditions had a major influence on the morphology of blends of high density polyethylene and polyamide-6 (HDPE/PA-6), and, under special processing conditions, laminar morphology was obtained in this system. The toluene permeability of extruded ribbons of HDPE/PA-6 blends was in the range obtained with multilayer systems.     

Microphotometric inline determination of polymer blend morphologies during extrusion processing

A novel inline method using microphotometric measuring principle is able to determine mean particle sizes of polymer blends directly during extrusion processing. This process‐micro‐photometer (PMP) is adapted on a special slit die at the extruder head. Refractive indices needed for particle size calculation have been measured by an inline refractometer for all the involved polymers at molten state. For investigation, blends have been made from polystyrene (PS), polypropylene (PP), polyamide (PA6), and polymethylmethacrylate (PMMA); and the resulting blend morphologies are found to be smaller than 1 μm in all cases. Sensor calculations show good correlation to microscopic reference measurements. Because of multiple scattering effects at larger particle number concentrations an exact particle size calculation is limited to low volume concentrations of that polymer blends because of the small blend particle sizes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 258–262, 2007     

Structure formation during polymer blend flows

The structure formation processes that occur during the flow of dilute blends of high density polyethylene (HDPE) or polypropylene (PP) In a linear low density polyethylene (LLDPE) carrier phase have been studied. Due to low surface tensions, high deformations of the dispersed minor phase can be induced under slow flow conditions leading to the formation of slender filaments. Measurements on a slit die, having a large, converging flow entrance region, demonstrate that the mechanism for filament formation is droplet bursting, yielding growing tails during shear flow, or, unsteady drop elongation during extensional flow. Tail growth can be modeled as the flow of a slightly tapering cylinder in a fluid of different viscosity, For dispersed to carrier phase viscosity ratios greater than unity, extensional flow occurs in the tail phase, which can induce oriented crystallization. For ratios less than unity, the flow is compressive, which. Inhibits crystallization. Drop deformation and crystallization in the converging flow entrance region is greatly enhanced by the extensional flow, and droplet growth can be described by a model assuming a time-dependent, planar, extensional flow field. Data for birefringence and melting points of as-crystallized fibers are also presented and discussed.     

高聚物的精密挤出

研究了影响高聚物精密挤出的工艺因素和装备因素，提出了高聚物精密挤出的概念，论述了实现高聚物精密挤出的可行性，分析了实现高聚物精密挤出过程的具体措施和手段。     

Shear stress at polymer/metal interface during melting in extrusion

Prediction of the screw horsepower requirement involves, among many others, the calculation of the shear stress (τ_s) between the solid polymer and the barrel surface during melting. Prediction of the solid bed down-channel velocity also requires the calculation of τ_s. However, the pseudoplastic nature and strong temperature dependence of melt viscosity make the mathematics of calculating τ_s extremely difficult. As a first step of developing a reasonable mathematical model for calculating τ_s, experimental measurements of τ_s were made over a wide range of metal temperature and sliding speed for five commercial polymers using molded, block samples. Although dependences of τ_s on metal temperature and sliding speed were found to have similar functionality to those of the dependences of melt viscosity on melt temperature and shear rate, this study showed that τ_s could not be expressed as a sole function of the melt rheological properties. Our subsequent study, to be reported in a follow up paper, will show that τ_s must be expressed as a function of the thermodynamic properties and melt density of the polymer as well as the melt rheological properties and the melting conditions.     

Salt film formation during corrosion of aluminum

High-purity aluminium rods with 5 mm² cross-section were imbedded in epoxy resin and corroded from one end. Depths of these artificial pits were 0–1 mm. With concentrated aluminum chloride solution, > 80% of saturation, and high anodic potentials, it was possible to obtain a continuous salt film on the corroding surface and electropolishing. Current density under these conditions is mass transport controlled and increases with convective flow. At low salt concentrations and potentials the convection induced by simultaneous hydrogen evolution prevents continuous salt film formation.     

Investigation of structure development in polyamide 11 and polyamide 12 tubular film extrusion

The processability of single bubble polyamide 11 and polyamide 12 films was investigated. The development of crystalline structure and chain orientation of polyamide 11 and polyamide 12 films in single bubble film blowing was studied by differential scanning calorimetry (DSC), wide angle X‐ray diffraction (WAXS), infrared (IR) spectroscopy, and birefringence. DSC measurements of both films revealed a spontaneous increase in glass transition temperature (T_g) and cold crystallization temperature (T_c) during aging at room conditions, with crystallinity and melting point remaining constant. Single bubble polyamide 11 and polyamide 12 films exhibited triclinic α and monoclinic γ crystals, respectively. The level of biaxial orientation was evaluated by calculating White‐Spruiell biaxial orientation factors with pole figure data. We were not able to produce biaxially oriented double bubble films with either polyamide 11 or polyamide 12.     

Pattern formation in a confined polymer film induced by a temperature gradient

We have studied a morphological instability of a double layer comprising the polymer film and air gap confined between the two plates set to different temperatures. The temperature gradient across the double layer causes the breakup of the polymer film into well-defined columnar, striped or spiral structures spanning the two plates. The pattern formation mechanisms have been discussed. The formed patterns can be transferred to produce PDMS stamp, a key element of soft lithography for future microfabrication.     

Fusion of particulate structure in polyvinyl chloride during powder extrusion

It has been recognized recently that suspension polyvinyl chloride granules consist of particles ca 1μ in diam which are linked together to form a three-dimensional, sponge-like lattice. This paper deals with the fusion process of this particulate structure of PVC resin during powder extrusion. Extrudates prepared under various extrusion conditions are investigated through the electron microscope using a methyl methacrylate (MMA) embedding method followed by ultrathin sectioning. The MMA embedding method has been proven to be an effective means of revealing the detailed structure of molded PVC. Results showed that the 1μ-particles were subjected to breakdown due to heat and shear into fibrils 100-300 Å in thickness, which were the final structure attainable by extrusion. Discussion was focused on the fibrillar structure especially in relation to the elasticity in plasticized PVC.     

核磁共振研究未知聚合物结构

利用超导核磁共振(NM R)通过测定某一未知聚合物的1H和13C谱,对该聚合物的化学位移进行了归属,确定了该聚合物的成分及其结构。     

Effect of molecular weight distribution on polymer diffusion during film formation of two-component high-/low-molecular weight latex particles

We describe fluorescence resonance energy transfer (FRET) studies of film formation by a new type of two-component latex particles. These particles consist of a miscible blend of two components that have a similar composition but very different molecular weights. In our approach, we used sequential seeded emulsion polymerization to generate (in situ) a fraction of oligomer in poly(butyl acrylate-co-methyl methacrylate) P(BA-MMA) seed particles that contained a relatively high molecular weight (high-M) dye-labeled polymer. In this way we could systematically change the molecular weight distribution of polymer inside the particles. We varied the amount and the molecular weight of the oligomers. For latex films cast from these two-component particles, we studied the diffusion rate of the high molecular weight polymer by FRET. These measurements revealed that oligomers promoted diffusion rate during latex film formation (oligoplasticization). We analyzed our diffusion data in terms of the Fujita–Doolittle free-volume model and showed that higher molecular weight oligomers are less efficient as plasticizers. In separate experiments, oligomers with similar molecular weights as those in the two-component particles were introduced via latex blending. We compared oligoplasticization in latex blends films with that in the two-component particles films. Finally, we investigated the rheological behavior of the two-component polymers with compositions adjusted to have a common T_g (2 °C). The higher the molecular weight of the oligomer, the more that had to be added to achieve T_g = 2 °C. All of the oligomers were much shorter than the entanglement length and act as diluents of the entanglements in the high-M polymer. We found that incorporating larger amounts of oligomers with a higher molecular weight resulted in a more pronounced drop in polymer viscosity, associated with the decrease in the entanglement density.     

Gas film formation time and gas film life time during electrochemical discharge phenomenon

The formation of a gas film around an electrode is at the origin of the electrochemical discharges in aqueous solutions and in molten salts. The study of the electrochemical discharge phenomenon from the current signal point of view is conducted to identify the gas film formations and the following discharges. This is performed using the wavelet analysis as a denoising tool with the discrete Meyer wavelet as base function. The proposed algorithm allows to measure experimentally the gas film life time and its necessary building time prior to each series of discharges. The accuracy of the algorithm is evaluated using generated signals and its application to real data is demonstrated. From the experimental data, it is concluded that the gas film is more stable at high terminal voltages whereas its formation time decreases with it. Electrochemical and thermal interpretations for the measured statistics are proposed.     

Cationic polymerization of 2-vinylthiophene - the polymer structure formation

Summary The cationic polymerization of 2-vinylthiophene (VT) has been studied in solution and on the surface of silica particles. For this purpose different cationically active initiators have been used, e.g. SnCl₄, CF₃SO₃H, and CF₃COOH in homogeneous solution as well as (4-CH₃OC₆H₄)₂CH-Cl and (C₆H₅)₃CCl in conjunction with silica nanoparticles in a slurry. For all experiments a high conversion of VT to oligomers with M_n= 1500 −2000 g/mol (M_w/M_n= 1.4) (Tg = 109 °C) and oligoVT/silica hybrid particles, respectively, have been observed. The different head and endgroups have been ascertained by means of MALDI TOF spectroscopy. Different NMR spectra analyses of the oligomers show that both the vinyl double bond and the five position of the thiophene ring are involved in the propagation reaction. Received: 23 April 2001/Revised version: 25 July 2001/Accepted: 25 July 2001     

Characterization of film formation in waterborne polymer lattices: a focus on turbidimetry

This short overview describes the characterization of film formation of waterborne polymer lattices, as primarily determined with the aid of UV–Vis turbidimetry. Phenomena occurring during the drying process are explained, and the mechanism of film formation is described. The quality of dried and aged films has been measured with the aid of three techniques: (a) immersion films in water and subsequent measurement of turbidity, (b) measurement of water vapor permeability, and (c) atomic force microscopy.     

Thermoplastic extrusion—the mechanism of the formation of extrudate structure and properties

Systems processed by thermoplastic extrusion can be regarded as heterophase polymer melts of incompatible water-plasticized biopolymers. In the process of thermoplastic extrusion, proteins and polysaccharides are melted at high pressure and temperature below the temperature region of their thermal decomposition. Dispersed particles of these systems can be deformed in flow. The mixed-melt anisotropic structure, formed in flow, is fixed by rapid conversion of the melt jet that lets the extruder die from a viscous state to a rubber-like state and then to a glassy state caused by cooling and drying. Incompatibility of proteins and polysaccharides in their water-plasticized melt mixtures impacts on structure formation and texturization during thermoplastic extrusion. Presented at the 20th ISF World Congress and 83rd AOCS Annual Meeting and Expo, May 10–14, 1992, Toronto, Ontario, Canada.