Microporous PLA Articles Through Below-Melt Expansion
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Solution Overview
Problem
Conventional processes for forming polylactic acid (PLA) articles result in undesirable properties such as degradation, coloration, reduced tensile strength, and low molecular weight due to high melt viscosity, limiting their biodegradability and mechanical properties.
Innovation Solution
A process involving expansion of PLA polymer preforms at temperatures above the glass transition but below the melting point to create articles with beta crystal phase, nodes, and fibrils, achieving high tensile strength and modulus, and optionally incorporating fillers or coatings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional processes heat PLA above melt temperature to form articles, then the PLA can be processed and formed, but the PLA degrades, colors undesirably, and loses mechanical properties
Solution Approach 1:
The patent changes the temperature parameter from above-melt processing to below-melt processing (between Tg and Tm), fundamentally altering the processing conditions to avoid degradation while maintaining formability. This parameter change resolves the contradiction by enabling processing without the harmful effects of high-temperature melting.
Solution Approach 2:
The patent utilizes the phase transition behavior of PLA between its glass transition temperature and melting point, processing the material in a rubbery or leathery state rather than fully molten state. This phase transition approach allows forming without complete melting, avoiding degradation while achieving the desired article structure.
2Ease of manufacture
If conventional processes use low molecular weight PLA to reduce viscosity, then the material is easier to process, but the tensile strength and matrix modulus are reduced
Solution Approach 1:
The patent changes the temperature parameter to below-melt processing conditions, which fundamentally alters the viscosity characteristics. High molecular weight PLA exhibits different flow behavior at temperatures below its melting point, allowing it to be processed without requiring low molecular weight variants. This resolves the contradiction by enabling processing of high MW material through parameter change rather than material substitution.
3Reliability
If the PLA article is made porous to enhance biodegradability, then the surface area increases for degradation, but the mechanical integrity may be compromised
Solution Approach 1:
The patent deliberately creates a porous structure in the PLA article to enhance biodegradability through increased surface area. The porous morphology is achieved through the below-melt processing method, which allows formation of interconnected pores during the phase transition. This principle accepts reduced mechanical properties as a trade-off for enhanced biodegradability, which is appropriate for medical implant applications.
Solution Approach 2:
The patent creates different structural qualities within the article - the bulk material maintains high molecular weight for strength, while the porous structure provides degradation pathways. This local differentiation of quality (dense vs. porous regions) allows simultaneous optimization of both mechanical integrity and biodegradability for medical applications.
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 process produces biodegradable PLA articles with enhanced tensile strength and modulus, maintaining mechanical integrity and allowing for controlled porosity and densification.
Implementation Method 1
expanding a PLA polymer preform at a temperature above the glass transition temperature of the PLA polymer and below a melting temperature of the PLA polymer
Implementation Method 2
expanding a PLA polymer preform at a temperature above the glass transition temperature of the PLA polymer and below a melting temperature of the PLA polymer to create a porous PLA article having nodes and fibrils
Implementation Method 3
create articles with beta crystal phase, nodes, and fibrils
Data Source
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AI summary
PLA polymers that can be expanded into microporous articles having a node and fibril microstructure are provided. The fibrils contain PLA polymer chains oriented with the fibril axis. Additionally, the PLA polymers have an inherent viscosity greater than about 3.8 dL/g and a calculated molecular weight greater than about 150,000 g/mol. The PLA polymer article may be formed by bulk polymerization where the PLA bulk polymer is made into a preform that is subsequently expanded at temperatures above the glass transition temperature and below the melting point of the PLA polymer. In an alternate embodiment, a PLA polymer powder is lubricated, the lubricated polymer is subjected to pressure and compression to form a preform, and the preform is expanded to form a microporous article. Both the preform and the microporous article are formed at temperatures above the glass transition temperature and below the melting point of the PLA polymer.