Broad MWD Polypropylene Resin for High Processing Rates
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Solution Overview
Problem
Polypropylene resins, particularly those produced by traditional Ziegler-Natta polymerization, exhibit low melt strength and impact resistance, making them unsuitable for high-processing-rate applications like injection molding, oriented films, and thermobond fibers, and are prone to gelling issues, limiting their use in films and other converting processes.
Innovation Solution
A polypropylene resin with a molecular weight distribution (MWD) greater than 5, a branching index of at least 0.95, and a melt strength greater than 20 cN, achieved through single-stage polymerization using a Ziegler-Natta catalyst system with a non-aromatic internal electron donor and a blend of external organosilicon electron donors, enhancing properties for blown film, thermoforming, and foaming applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional Ziegler-Natta polymerization is used to produce polypropylene with high stereoregularity, then high flexural modulus and crystallinity are achieved, but melt flow rate becomes too low for high-processing-rate applications
Solution Approach 1:
The patent changes the molecular weight distribution parameter from narrow (MWD 3-4.5) to broad (MWD greater than 5) by modifying the catalyst system. This allows the polymer to maintain high stereoregularity and flexural modulus while achieving sufficient melt flow rate for high-processing-rate applications through the presence of lower molecular weight chains that facilitate flow.
Solution Approach 2:
The patent creates a composite molecular structure within the polypropylene resin, combining high molecular weight chains (for strength and stiffness) with lower molecular weight chains (for processability). This composite approach at the molecular level resolves the contradiction between mechanical properties and processing characteristics.
2Temperature
If conventional polypropylene homopolymer is used, then high crystallinity and melting point are achieved, but impact resistance becomes insufficient for many uses
Solution Approach 1:
The patent modifies the molecular weight distribution parameter to be greater than 5, creating a broader distribution that includes lower molecular weight chains. These chains act as impact modifiers within the crystalline matrix, improving impact resistance while maintaining the high melting point and crystallinity provided by the stereoregular polymer structure.
3Stability of the object's composition
If polypropylene resin with narrow molecular weight distribution (MWD 3-4.5) is used, then high stereoregularity is maintained, but melt strength becomes too low for converting processes like blown film
Solution Approach 1:
The patent changes the molecular weight distribution from narrow (MWD 3-4.5) to broad (MWD greater than 5) while maintaining high stereoregularity. The broader MWD provides sufficient melt strength for converting processes like blown film extrusion, while the stereoregularity is preserved through the catalyst system design, achieving both compositional stability and processing capability.
4Strength
If highly crystalline polypropylene is used, then desirable flexural modulus is achieved, but processing rate becomes too slow for injection molding and oriented films
Solution Approach 1:
The patent broadens the molecular weight distribution to greater than 5, which provides a population of lower molecular weight chains that facilitate faster processing rates for injection molding and oriented film production. Simultaneously, the high stereoregularity maintained by the catalyst system ensures the development of sufficient crystallinity and flexural modulus in the final product.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting polypropylene resin demonstrates improved melt strength, stiffness, and impact resistance, enabling its use in high-processing-rate applications and multi-layer films, replacing HDPE in certain applications and improving processing efficiency and product quality.
Implementation Method 1
single-stage polymerization using a Ziegler-Natta catalyst system with a non-aromatic internal electron donor and a blend of external organosilicon electron donors
Data Source
AI summary
Disclosed herein are embodiments of films which comprise a polypropylene resin comprising at least 50 mol % propylene, an MWD (Mw/Mn) of greater than 5, and a branching index (g′) of at least 0.95.