Ziegler-Natta and metallocene catalyzed isotactic polypropylene : a comprehensive investigation and comparison using crystallization kinetics, fiber spinning and thermal spunbonding

Author

Eric Bryan Bond

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Josoph E. Spruiell

Committee Members

Roberto Benson, Edward Clark, Paul Phillips, Bernhard Wunderlich

Abstract

Isotactic polypropylene (iPP) can be synthesized using, conventional heterogeneousZiegler-Natta (zriiPP) and homogenous metallocene catalysts (miPP). Materials catalyzed using the Ziegler-Natta catalysts typically have broad molecular weight distributions, high peak melting temperatures, a heterogeneous distribution of stereo defects, and significant portions of non crystallizable (atactic) material. Metallocene resins typically have a narrow molecular weight distribution, lower peak melting temperatures than zniPP resins, a more uniform distribution of stereo and regio defects, and very small amounts of atactic material.The purpose, of this work was to investigate and compare and contrast miPP resins tozniPP resins.The resins in this study were thoroughly characterized by cNMR and solutionfiuctionation to determine the number, type and distribution of defects. The resins were then studied under isothermal and nonisothermal quiescent crystallization conditions to determine the bulk and crystal growth kinetics, crystal structure, crystallinity and thermal properties. The resins were also melt spun into fibers to allow the effects of molecular weight md molecular weight distribution on as-spun filament properties to be determined.The as-spun fibers were then characterized to determine the crystalline and noncrystalline orientation functions, birefringence, density, thermal properties and tensile mechanical properties (elongation-to-break, modulus, tensile strength). In addition, on-linecrystallization studies were also conducted using the resins to determine the locations in the spinline where crystallization occurred. Selected resins were then used to study the effect of as-spun fiber properties on nonwoven fabric mechanical properties using a thermal point spunbonding process.The resins in this study were thoroughly characterized by cNMR and solutionfiuctionation to determine the number, type and distribution of defects. The resins were then studied under isothermal and nonisothermal quiescent crystallization conditions to determine the bulk and crystal growth kinetics, crystal structure, crystallinity and thermal properties. The resins were also melt spun into fibers to ^ow the effects of molecular weight and molecular weight distribution on as-spun filament properties to be determined.The as-spun fibers were then characterized to determine the crystalline and noncrystalline orientation functions, birefringence, density, thermal properties and tensile mechanical properties (elongation-to-break, modulus, tensile strength). In addition, on-linecrystallization studies were also conducted using the resins to determine the locations in the spinline where crystallization occurred. Selected resins were then used to study the effect of as-spun fiber properties on nonwoven fabric mechanical properties using a thermal point spunbonding process.The cNMR and xylene fractionation studies indicated the miPP resins had substantially more total defects in the crystallizable material than either of the zniPP resins in this study. The miPP resins also contained regio type polymerization defects, which were not present in the zniPP resins. The results also found that, in general, the miPP resins contained much smaller portions of atactic material, as determined by xylene fractionation. The totality of the stereoregularity results suggest the miPP resins have amore uniform defect distribution than the zniPP resins.Combined results from DSC, SAXS and WAXD indicated the miPP and zniPPresins have similar a-monoclinic equilibrium melting temperatures (T_m⁰), despite the differences in defect content, type and distribution.The presence of atactic material wasfound to lower the observed equilibrium melting temperature of a particular resin, whethermiPP or zniPP. The γ-crystal structure, observed in the miPP resins using WAXD andDSC, had a lower equilibrium melting temperature than the α-structure. The (T_m⁰) of the &alpha-monoclinic structure was found to be 186± 2°C, while the γ-structure T(T_m⁰) was found to be 178 ±2°C, when crystallized at atmospheric pressures. The miPP resins were found to melt lower temperatures than the zniPP resins, at similar crystal thicknesses, which is attributed to the ntiPP resins having significantly higher fold surface free energies when crystallized under isothermal conditions.The isothermal crystallization studies showed the miPP resins readily produce the γ-crystalstructure. The zniPP resins also produced small amounts of the γ-structure, at high crystallization temperatures. Defects were found to be excluded from the crystal under isothermal crystallization conditions. The defects excluded from the crystal core are thought to be rejected into the crystal fold surface region, increasing the fold surface free energy. SAXS studies indicate the lamellae fold surface of isothermally crystallized miPP resins might be rough, possessing a three-dimensional topology instead of a two dimensional structure. The T_m⁰ and fold surface free energy for each resin was determined from the Gibbs-Thompson equation. The Gibbs-Thomson equation normalizes to a two dimensional crystal fold surface, therefore the apparent fold surface free energy is higher in the miPP resins with a three-dimensional topology. These conclusions are supported by the non isothermal crystallization studies which showed that defects are incorporated into the crystal core and that the fold surface free energies of the non isothermally crystallized films using rruPP and zniPP resins are similar.For crystallization under isothermal crystallization conditions, the observed linear growth rates were found to be dependent upon defect content. Under non isothermal conditions, the growth rate was found to depend mostly on the molecular weight. Forresins with similar molecular weights, the number of defects was also found to be important under nonisothermal crystallization conditions. The nucleation density was found to have a strong effect on the overall bulk crystallization kinetics. The relative order of bulk crystallization rates for the resins in this study was found to be strongly determined by the relative nucleation density of a particular resin.Fiber spinning studies showed that the molecular weight and molecular weight distribution of an iPP resin is largely determined by the point of crystallization in thespinline, the crystallization temperature and as-spun filament properties. Increasing the molecular weight (also increasing the spinning speed) tended to increase the density and crystallization temperature, i.e. c^stallization in the spinline occurs closer to the spirmeret.Narrowing the molecular weight distribution and decreasing the molecular weight (also with increasing the spinning speed) tended to increase the noncrystalline and crystalline orientation function, birefringence and tensile strength (elongation-to-break was the inverse) for most the resins. The more narrow molecular weight distribution resins also delay crystallization to a distance further away from the spinneret and to lower crystallization temperatures. The as-spun fiber tensile modulus was found to increase as the spinning speed increased, a result of the birefringence and crystallinity increasing. The Observed differences in fiber spinning behavior between the miPP and zniPP resins are mainly attributed to the differences in molecular weight and its distribution.Studies on the mechanical properties of nonwoven fabrics made using the thermalpoint spunbonding process were found to be dependent on the fiber properties, when bonded at the optimum bonding temperature. , The optimum bonding temperature is the temperature in the bonding curve where the fabric mechanical properties are the best.Increasing the as-spun fiber non crystalline orientation function and birefringence increased the optimum bonding temperature. Increasing the fabric basis weight also increased the optimum bonding temperature. No significant differences in fabric properties between miPP and zniPP resins could be found that are not explained by differences in the as-spun fiber properties. The as-spun fiber properties were found to be different between the two. catalyst systems, a result of differences in their molecular weight and molecular weight distribution.

Recommended Citation

Bond, Eric Bryan, "Ziegler-Natta and metallocene catalyzed isotactic polypropylene : a comprehensive investigation and comparison using crystallization kinetics, fiber spinning and thermal spunbonding. " PhD diss., University of Tennessee, 1999.
https://trace.tennessee.edu/utk_graddiss/8765