Polypropylene Blends with Hydrogenated Block Copolymers for Low-Temperature Toughness
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Solution Overview
Problem
Polypropylene and polypropylene random copolymers used in molded articles, such as food storage containers, face challenges with high haze and poor toughness and impact strength, especially at low temperatures, due to crystallization and brittleness, which limits their application in freezer environments.
Innovation Solution
A molding composition comprising at least 70% polypropylene or polypropylene random copolymer, up to 30% selectively hydrogenated styrene-diene block copolymers, and up to 0.7% clarifying agent, where the block copolymer's refractive index matches that of polypropylene, ensuring low haze and improved impact strength by optimizing the glass transition temperature and clarifying agent levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If polypropylene or polypropylene random copolymers are used to make molded articles, then good molded articles can be produced, but the haze is high and clarity is poor due to large crystallites
Solution Approach 1:
The patent applies parameter changes by adding a clarifying agent that modifies the crystallite structure of polypropylene, transforming large crystallites into smaller, more numerous crystallites. This changes the physical parameters of the material at the microscopic level, resulting in reduced light scattering and lower haze values, thereby improving clarity while maintaining the molded article structure
Solution Approach 2:
The patent creates a composite material system by combining polypropylene or polypropylene random copolymers with a clarifying agent. This composite approach allows the clarifying agent to work synergistically with the polypropylene matrix, modifying the crystallization behavior and optical properties without compromising the structural integrity of the molded article
2Manufacturing precision
If clarifying agents are added to reduce haze, then clarity improves, but toughness and impact strength deteriorate, especially at low temperatures
Solution Approach 1:
The patent uses parameter changes by carefully controlling the amount of clarifying agent added to the polypropylene system. By optimizing the clarifying agent concentration, the patent achieves the right balance between reducing haze (improving clarity) and maintaining sufficient impact strength, preventing the material from becoming too brittle while still achieving low haze values
Solution Approach 2:
The patent applies local quality by ensuring uniform distribution of the clarifying agent throughout the polypropylene matrix. This localized modification of the crystallite structure across the entire material volume ensures consistent optical properties and mechanical performance, preventing weak spots that would compromise impact strength while achieving overall clarity improvement
3Ease of manufacture
If polypropylene is used for food containers, then good containers can be made, but they become brittle at low temperatures and shatter
Solution Approach 1:
The patent applies parameter changes by modifying the thermal and mechanical properties of polypropylene through the addition of clarifying agents. This changes the crystallization parameters and glass transition characteristics of the material, improving its low-temperature toughness and preventing brittle failure in freezer environments while maintaining ease of manufacturing
Solution Approach 2:
The patent creates a composite material system combining polypropylene with clarifying agents to produce a modified polymer composition. This composite approach enhances the reliability of food containers at low temperatures by creating a more ductile material structure that resists shattering, while preserving the manufacturing advantages of polypropylene
4Strength
If elastomeric polymers are added to improve impact strength, then toughness improves, but clarity deteriorates due to poor index of refraction match
Solution Approach 1:
The patent applies parameter changes by using clarifying agents to modify the optical properties of the polypropylene matrix. By changing the crystallite size and distribution parameters, the patent compensates for the index of refraction mismatch caused by elastomeric polymers, maintaining clarity while allowing the elastomers to provide impact strength enhancement
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 impact strength of at least 11 J (100 inch-lbs) at -20°C to -30°C and maintains clarity with haze values less than 50% in 3.18 mm thick disks, significantly improving the performance of molded articles over traditional polypropylene copolymers.
Implementation Method 1
Polypropylene and polypropylene random copolymers, however, tend to crystallize with large crystallites and the haze of such containers is often quite high. Random polypropylene copolymers tend to be slightly clearer than the polypropylene homopolymer and are thus usually preferred for ultimate clarity. It is known that adding a small amount (0.18 to 0.22 wt. %) of a clarifying agent causes the crystallites of the homo-polypropylene or polypropylene random copolymer to remain small and become more numerous, thus reducing the haze significantly.
Implementation Method 2
There was also no mention for the need to match the index of refraction which is important to achieve good clarity even if high levels of clarifying agent are used.
Implementation Method 3
the glass transition temperature of the HD block is -50°C to -55°C as measured by DSC
Data Source
Figure 1
AI summary
The present invention comprises a molding composition comprising at least about 70 % wt. polypropylene or polypropylene random copolymer; up to about 30 wt. % of a block copolymer composition comprising at least one selectively hydrogenated block copolymer which substantially matches the index of refraction of the polypropylene or polypropylene random copolymer, and up to about 0.70 % by wt. of a clarifying agent, wherein the weight % of the total blend is 100%. The present invention also may be a molded article produced from the molding composition. A molded article formed from such a composition at a thickness of 0.125 inches will have a haze of less than about 50 % and an instrumented impact toughness of at least 100 inch-lbs at -20°C to -30°C.