Multimodal Polyethylene Caps with Shear Thinning Flow
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Solution Overview
Problem
Conventional polyethylenes used for injection molding of caps and closures face a trade-off between flow properties and mechanical properties like stiffness and environmental stress crack resistance (ESCR), leading to reduced productivity and quality due to shrinkage issues and poor impact strength.
Innovation Solution
A polyethylene composition with a specific molecular weight distribution and shear thinning index relation, combined with a multimodal structure, enhances flow properties while maintaining excellent mechanical properties and ESCR, allowing for lower molding temperatures and reduced shrinkage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If polyethylene with narrow molecular weight distribution is used to achieve desired impact strength and stiffness, then mechanical properties are improved, but flow properties deteriorate
Solution Approach 1:
The invention segments the molecular weight distribution into multiple distinct peaks (bimodal or multimodal distribution), combining low molecular weight fractions for good flow properties with high molecular weight fractions for excellent mechanical properties including impact strength and stiffness. This segmentation allows the polymer to exhibit both good processability and mechanical performance simultaneously.
Solution Approach 2:
The invention changes the molecular weight distribution parameters from a narrow single peak to a broad multi-peaked distribution, specifically controlling the ratio and positions of different molecular weight fractions to optimize both flow characteristics (MFR2) and mechanical properties (impact strength, stiffness, ESCR).
2Productivity
If polyethylene with broader molecular weight distribution is used to improve flow properties, then flow properties are improved, but stiffness and impact properties deteriorate
Solution Approach 1:
The broad molecular weight distribution is segmented into specific discrete peaks at controlled positions and intensities, ensuring that low molecular weight components provide flow properties while high molecular weight components maintain stiffness and impact resistance, preventing the deterioration of mechanical properties.
Solution Approach 2:
The polymer composition acts as a composite at the molecular level, combining different molecular weight fractions (analogous to different material phases) where each fraction contributes specific properties: low MW for flow, high MW for mechanical strength, creating a synergistic composite material with balanced properties.
3Productivity
If polyethylene with lower average molecular weight is used to improve flow properties, then flow properties are improved, but impact strength and ESCR properties deteriorate
Solution Approach 1:
The molecular weight population is segmented into low MW fractions (for flow properties and lower processing temperature) and high MW fractions (for impact strength and ESCR), with the low MW segment having MFR2 ≥ 0.5 g/10min and the high MW segment providing structural integrity and environmental stress crack resistance.
Solution Approach 2:
The average molecular weight parameter is changed by introducing a bimodal/multimodal distribution where the weighted average may be lower for good flow, but the presence of high MW tail fractions ensures adequate impact strength and ESCR, effectively decoupling the relationship between average MW and mechanical properties.
4Productivity
If the moulding polymer is heated to higher temperature to improve flow properties, then flow properties are improved, but cooling time and shrinkage increase
Solution Approach 1:
The flow properties are improved by changing the molecular weight distribution parameters (bimodal/multimodal with specific MFR2 ≥ 0.5 g/10min) rather than changing temperature, allowing processing at lower temperatures with consequently shorter cooling times and reduced shrinkage while maintaining adequate flow.
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 composition achieves superior flow properties, high stiffness, low creep, and improved ESCR, enabling increased productivity and complex mold usage with reduced polymer degradation, while maintaining good taste and odor properties for food applications.
Implementation Method 1
the shear thinning index SHI(1,100) and the log MFR2 of the composition satisfy the following relation: SHI(1,100)≧−10.58 log MFR2
Implementation Method 2
the mould, however, is at a lower temperature than the polymer melt therefore as the polymer melt cools, shrinkage tends to occur
Data Source
AI summary
The present invention relates to a multimodal polyethylene composition wherein (i) the composition has an MFR2 of 0.1 to 100 g/10 min, (ii) the shear thinning index SHI(1,100) and the log MFR2 of the composition satisfy the following relation: SHI(1,100)≧−10.58 log MFR2 [g/10 min]/(g/10 min)+12.94, and (iii) the composition has an environmental stress crack resistance ESCR of 10 h or more. Furthermore, the present invention relates to a process for the production of said composition, an injection moulded article, in particular a cap or closure, comprising said composition and to the use of said composition for the production of an injection moulded article.