Costas Tzoganakis

Chemical modification of polypropylene with peroxide/pentaerythritol triacrylate by reactive extrusion

To explore the possibility of producing branched polypropylene (PP) by a reactive extrusion (REX) process, isotactic PP was reacted with a polyfunctional monomer, pentaerythritol triacrylate (PETA), in the presence of 2,5-dimethyl-2,5(t-butylperoxy) hexane peroxide (Lupersol 101). Experiments were carried out in an intermeshing, corotating twin-screw extruder at 200°C using three concentrations of peroxide (200, 600, and 1000 ppm) and four concentrations of PETA (0.64, 1.8, 2.8, and 5.0%, by weight). Shear viscosity and MFI of the whole polymers was found to increase with PETA concentration and decrease with increasing the peroxide concentration at a given PETA concentration. The macrogel amount in the materials produced was determined in refluxing xylene using Soxhlet extraction and at PETA concentrations higher than 1.8wt % the macrogel content increased with increasing peroxide concentration. No macrogel was detected at low PETA concentrations (<0.64%) at all three peroxide levels, suggesting that low concentrations of PETA and peroxide should be used in order to minimize the formation of macrogels. The xylene soluble portions (sols) of the modified materials were characterized by FTIR and DSC. Generally, the relative intensities A1740/A841 in the FTIR spectra increased with increasing PETA incorporated into PP. Two melting peaks (Tm1 and Tm2) were observed in the DSC traces of some of the sols, and the crystallization temperatures (Tc) were higher than those of the virgin and degraded polypropylenes. The DSC behavior of the sols suggests that the modified PPs contain branched and/or lightly crosslinked chain structures.

Effects of supercritical CO2 on the viscosity and morphology of polymer blends

Supercritical carbon dioxide (CO2) was used in extrusion for the improvement of polymer blend properties. Various extruder configurations using a twin-screw extruder and a single-screw extruder were designed for investigating the effects of dissolved supercritical CO2 on the viscosity and morphological properties of polyethylene, polystyrene, and their blends. The viscosities of the polymer/CO2 and the blend/CO2 solutions were measured at various concentrations of CO2 and PE/PS blending ratios using a wedge die mounted on the twin-screw extruder. The effect of CO2 on the morphology of PE/PS blends was also investigated using a twin/single-screw tandem system. This system allowed for preferential dissolution of the CO2 into the matrix and/or dispersed polymer phase. By introducing devolatilization to the tandem system, the morphological behaviors of PE/PS blends were investigated on unfoamed filaments. In general, the mixing between two polymers was improved by the dissolution of CO2. The size reduction of the dispersed phase was explained using the viscosity ratio of the two polymers. Finally, the interface between foamed and unfoamed polymers was studied in a bilayer structure produced by using a special configuration of the tandem system.

Terminal functionalization of polypropylene via the Alder Ene reaction

Abstract A terminally anhydride functionalized polypropylene was synthesized via the Alder Ene reaction from a low molecular weight amorphous polypropylene. A Lewis acid, SnCl2·2H2O, was found to catalyse the reaction, thereby improving anhydride content in the polymer for reaction conditions comparable to those in an extrusion environment, i.e. elevated temperature and short residence time. The reaction was carried out at 230°C for 5 min in the presence of TEMPO, which acted as a free radical trap, preventing maleic anhydride from being grafted onto the backbone of the polypropylene. The product was characterized by several techniques, including FTi.r. and n.m.r., to provide evidence of the anhydride location on the chain. Several runs of varying catalyst concentration have shown that an optimum in anhydride incorporation onto the polypropylene exists. Examination of the system kinetics has shown that the overall order of the reaction remains second-order despite the presence of a catalyst.

Fractal analysis of the sharkskin phenomenon in polymer melt extrusion

A new approach has been adopted to study the sharkskin phenomenon and to characterize the extrudate surface roughness observed in polymer meltextrusion of a linear low‐density polyethylene through capillary dies. A previously proposed method, for the calculation of slip velocities as a function of both wall shear stress and pressure, was extended for polymer melts with a pressure‐dependent viscosity. In this work, the surface roughness of solidified extrudate samples was analyzed by means of digital image processing techniques. The fractal dimension of the extrudate surface was used as a quantitative measure of roughness and the effect of the wall shear stress on the severity of the sharkskin phenomenon was studied. The onset of surface roughness could be directly determined and it was found to lag behind the onset of slip in the die. The main advantage of the present method is the scale independence of the roughness measure. In addition, since only digital images of extrudates are required for the detection of surface roughness and due to short processing times involved, this method could potentially be employed in on‐line monitoring systems for the production of films with controlledroughness.

Rheological Characterization of Controlled-Rheology Polypropylenes Using Integral Constitutive Equations

Controlled-rheology polypropylene melts were prepared via molecular modification of a commercial polypropylene resin. A peroxide-initiated degradation was performed, resulting in materials with different molecular weight distributions. These resins were subjected to rheological characterization, and an integral constitutive equation of the K-BKZ type was used to study the effect of molecular weight characteristics on their rheological properties. Data for the linear viscoelastic spectrum and shear viscosities was used to obtain the model constants. The same constitutive equation has been used to predict the stress and Trouton ratios for simple shear and simple elongational flows, thus giving a quantitative assessment of the viscoelastic character of the melts. The results show the effect of the molecular modification on the rheological behavior of the melts. Polymers produced at higher peroxide concentrations exhibit reduced viscoelasticity manifested in less shear- and strain-thinning behavior. The present work clearly shows the potential of integral constitutive equations in fitting and interpreting experimental data and, thus, giving a much better understanding of the rheological behavior of commercial polymers. 0 1996 John Wiley & Sons, Inc.

Improvement in techniques for the determination of extensional rheological data from entrance flows: computational and experimental analysis

Abstract Flow through an abrupt contraction has been analyzed experimentally and theoretically through FEM simulations using the modified White–Metzner model. The results show that the ‘entrance viscosity, ηENT’ (entrance pressure drop divided by the shear rate) strongly depends on the shape of the steady extensional viscosity, the L/D ratio of the orifice die, and it can be properly described by the newly proposed model. An ‘effective entry length correction’ is proposed and successfully tested to improve the capability of the entrance techniques to exactly predict extensional viscosity at low elongational rates. It is demonstrated that the improvements proposed in this work help to more effectively evaluate extensional rheological data from capillary measurements.

Numerical Simulation of Polymer Coextrusion Flows

Abstract Non-isothermal calculation of flow history dependent visco-elastic stresses in a coextrusion die is performed using the 8-mode modified Leonov constitutive equation and the deformation rate field from finite element simulations. It is shown that a heuristic criterion based on the difference of normal stress differences across the layer interfaces in the merging area may be used to potentially detect the onset of interfacial instabilities. The developed technique can be a useful tool for coextrusion die design as well as for proper selection of materials and process conditions for coextrusion.

Control of a LDPE reactive extrusion process

Abstract A constrained model predictive control strategy is implemented to control the weight-average molecular weight and the high molecular weight end of the molecular weight distribution of a low-density polyethylene (LDPE) in a reactive extrusion process. To achieve simultaneous regulation of these properties, the amplitude and width of square wave-shaped pulses of a peroxide solution injected to the extruder are independently manipulated. The two polymer properties are inferred on-line from consistency and power-law index measurements obtained with an in-line wedge rheometer. Simulations and experimental results show that independent control of the polymer properties is feasible by this method, but only in a narrow range of operating conditions.

Alder Ene functionalization of polypropylene through reactive extrusion

A commercial isotactic polypropylene was degraded to increase its terminal vinylidene group concentration, and it was subsequently functionalized with maleic anhydride through the Alder Ene reaction at temperatures above 200°C in a co-rotating twin screw extruder. Characterization of the maleated product by Fourier transform infrared spectroscopy, 1H NMR, differential scanning calorimetry, and gel permeation chromatography showed the anhydride group to be terminally attached, and the degree of functionalization was determined by infrared analysis. Increased temperature and maleic anhydride concentration, as well as improved mixing in the extruder, were found to improve the extent of the reaction. The catalytic contribution of Lewis acid species was evaluated, and ruthenium chloride was found to increase the extent of the reaction by 16% in comparison with stannous chloride as a catalyst in the Alder Ene reaction.

Mixing analysis of reactive polymer flow in conveying elements of a co-rotating twin screw extruder

In this study, the finite element method was used to investigate the effects of screw speed, entering peroxide distribution, and pressure-to-drag flow ratio on the mixing characteristics of steady non-isothermal reactive flows in a forward conveying element of a self-wiping twin screw extruder. The reaction considered was the peroxide-initiated degradation of a commodity polypropylene resin. The predicted average degree-of-freedom profiles from the simulations largely conformed to expectations. The average flow efficiencies for all runs were found to remain at values close to that for two-dimensional flow, with fluctuations being observed in the channel intermeshing regions. No significant effect of either screw speed or peroxide distribution was found on the flow efficiencies; however, the pressure-to-drag flow ratio was found to have a significant influence.