Controlled-rheology Polypropylene Research Articles

Reactive extrusion of polypropylene: production of controlled-rheology polypropylene (CRPP) by peroxide-promoted degradation

Results of experimental studies of free-radical-induced degradation of polypropylene (PP) in the melt phase are presented. Experiments were carried out in a co-rotating twin-screw extruder. The initiator, dicumyl peroxide (DCP) was used as a radical generator. Concentration of the peroxide was in the range 0.02–0.6 %wt. Melt flow index, mechanical, thermal and rheological properties were measured to see the effect of initiator concentration, temperature (200 and 230 °C) and screw rpm (30 and 70 rpm). Initiator concentration was the most significant variable. Melt flow index increased with initiator concentration. Reaction temperature and screw rpm had no significant effect on the melt flow index of extrudate. This was attributed to the minimum residence time in the extruder being sufficient for all degradation reactions to be completed. Degraded resins had more Newtonian behavior than the original resin. Viscosity reduced with peroxide concentration. Melting point of degraded resin remained relatively constant at different levels of peroxide but the crystalline fraction increased slightly. No significant change of the mechanical properties was seen up to 0.1 %wt of peroxide. Beyond 0.1 %wt elongation at break and impact strength reduced considerably.

Melt fracture behavior of polypropylene‐type resins with narrow molecular weight distribution. II. Suppression of sharkskin by addition of adhesive resins

AbstractStanding on a hypothesis that the sharkskin of a polymer with a narrow molecular weight distribution at extrusion processing originates from a stick‐slip of the polymer at the die wall, the suppression of the sharkskin was tried by means of suppressing the slip by the addition of adhesives. To polypropylene (PP)‐type resins with narrow molecular weight distributions such as a PP‐type thermoplastic elastomer, PER and a controlled rheology PP were added small amounts of adhesives such as maleated PP, maleated PER, reactive polyolefin oligomers, ethylene/ethylacrylate/maleic anhydride (MAH) copolymer, ethylene/vinyl acetate copolymer, and styrene/MAH copolymer, and their melt fracture behaviors at capillary extrusion were observed. It was found that the sharkskin of the PP‐type resins with narrow molecular weight distributions was suppressed by the addition of the adhesive resins with good adhesion to metal. The suppressive effect of the sharkskin was generally the more remarkable by the higher loading of the adhesives with the higher MAH content. This is the direction of increasing adhesion. From this fact, it was assumed that the sharkskin of the PP‐type resins with narrow molecular weight distribution does not originate from a periodic growth and relaxation of tensile stress at the extrudate surface but from a stick‐slip at the die wall. Based on this mechanism, it may be said that the sharkskin can be suppressed by both ways of directions of promoting and suppressing the slip at the die wall. The former way is the previously known method, and the latter way is the method proposed in the present study. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2120–2127, 2002

A study of extrudate distortion in controlled‐rheology polypropylenes

AbstractThe melt fracture characteristics of controlled‐rheology polypropylenes (CRPP) were studied by means of capillary rheometry experiments. CRPPs were produced through reactive extrusion of a commodity polypropylene resin using various peroxide concentrations. These materials exhibited lower molecular weights and narrower molecular weight distributions than those of the starting commodity resin. The CRPP materials studied were found to exhibit only gross melt fracture. At extremely high shear rates and relatively low temperatures, a sigmoidal flexure was observed in the flow curve of certain CRPPs. Generally, it was found that the severity of melt fracture decreased with increasing shear rate for a given material and temperature and in some cases, the extrudates exhibited completely smooth surfaces. Also, the severity of surface distortions was reduced when high L/D dies were employed at a given shear rate. The critical shear stress for the melt fracture onset was found to increase with decreasing molecular weight and polydispersity, and correlations have been developed between the critical stress values and the polymer polydispersity and shear compliance.

Reactive Extrusion of Polypropylene with Pulsed Peroxide Addition: Process and Control Aspects

Controlled-rheology polypropylenes (CRPPs) have been produced by using pulsed peroxide addition during reactive extrusion of a commodity resin. It has been shown that, in contrast with processes involving continuous peroxide addition, it is possible to independently vary the weight-average molecular weight (Mw) and polydispersity (PDI). The effects of peroxide pulse characteristics on the product molecular weight characteristics have been investigated using the rheological properties of the CRPPs produced, and it has been found that the pulse width, amplitude, and period provide extra degrees of freedom for controlling Mw and PDI. A simple kinetic model has been employed for process simulation, and model predictions have been used to rationalize the control options through a relative gain array (RGA) analysis of the process. The results from this analysis have shown that such reactive extrusion operations are highly nonlinear and that multivariable control strategies would be required for large operatin...

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.

Determination and correction of peak broadening in size exclusion chromatography of controlled rheology polypropylene

AbstractThe peak broadening in size exclusion chromatography of seven commercial polystyrene (PS) standards with narrow molar mass distribution (MMD) and of 6–9 polypropylene (PP) fractions (M̄w/M̄n = 1.34–2.10) obtained by direct extraction was determined in four different column sets. The dependence of the Gaussian peak broadening parameter, σ2, on the peak elution volume was different for PS and PP. For a commercial grade of controlled rheology PP (CR‐PP) (Daplen PT55) with M̄w/M̄n < 3 it was shown that disregard of the peak broadening effect leads to considerable errors in the MMD at low and higher molar mass and in the polydispersity parameter M̄w/M̄n. The same lot was studied in an Austrian National Research program for its physical propeties in processing and application and in a round‐robin test of IUPAC working party IV.2.2. on molecular characterization of commercial polymers. © 1994 John Wiley & Sons, Inc.

Controlled Rheology Polypropylenes. An Improved Model with Experimental Validation for the Single Screw Extruder Process

The polypropylene produced in most industrial polymerization processes is a resin which has excellent mechanical properties but broad molecular weight distribution and high molecular weight tails which makes the processability of the material difficult. A process called controlled rheology is being employed to solve this problem. It consists of the addition of organic peroxides in a post-reaction extrusion process to partially degrade the polymer modifying its molecular weight distribution. The objective of this work is to study experimentally and theoretically the process of controlled rheology of polypropylene. The experiments were carried out in a single screw extruder at different temperatures, peroxide concentrations and screw speeds. The modifications induced by the peroxide on the rheological properties of the melt were evaluated measuring the shear viscosity in a Mechanical Spectrometer. High temperature size exclusion chromatography was used to establish the molecular weight distributions of the resulting resins. The model presented here includes all the basic reactions that have been proposed to take place during the scission and recombination of the polymer chains. This represents an improvement over the simplified models published in the literature. The kinetic constants and the initiation efficiency employed in the model were estimated by minimizing the differences between calculated and measured average molecular weights. These parameters were then used in the model at other experimental conditions. Good agreement was found between the measured and calculated values in all cases.

Melting behaviour of controlled rheology polypropylene

The melting behaviour of reactor-made isotactic polypropylene (PP) and various controlled rheology polypropylene (CR-PP) samples crystallized under both isothermal and non-isothermal conditions were studied by differential scanning calorimetry and the crystal structure of isothermally crystallized samples at 120°C was investigated using wide angle X-ray diffraction. Multiple endotherms were observed dependent on the molecular weight of the samples, heating rate and cooling rate, and method of crystallization. Recrystallization or reorganization occurred more easily for low molecular weight samples and also for fast cooled samples during the reheating and the melting process. The isothermally crystallized CR-PPs produced by peroxide controlled degradation showed a small melting endotherm peak at ∼143°C which is probably connected to the β form crystal, while unreacted PP showed the α form crystal only. The small endotherm peak was more prominent in one CR-PP sample of intermediate molecular weight. Attempts were made to explain this behaviour through the observed maximum in the half-time of crystallization versus molecular weight plot.