Recycled PET Sheet Manufacturing with Chain Extender
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Solution Overview
Problem
Existing methods for manufacturing PET sheets for food containers from recycled PET flakes face challenges such as the degradation of molecular weight, high energy and equipment costs, and the presence of residual acetaldehyde, which compromises food safety and sanitation.
Innovation Solution
The method involves adding a chain extender with two or more epoxy groups to moist flakes of recovered PET, which are then melted and kneaded in an extruder with multiple vent holes to degas and polymerize, forming a high molecular weight three-dimensional network structure while trapping ethylene glycol and acetaldehyde, thus eliminating residual acetaldehyde without the need for drying.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If solid phase polymerization is conducted at 200°C or more after complete drying, then molecular weight is increased, but equipment cost and energy cost increase significantly
Solution Approach 1:
The invention changes the processing parameters by conducting polymerization at a lower temperature (100-200°C) compared to conventional methods (200°C or more). This is achieved by using a specific catalyst system and processing conditions that enable effective polymerization at reduced temperatures, thereby lowering energy consumption while still increasing molecular weight of recovered PET
Solution Approach 2:
The invention performs preliminary drying of recovered PET flakes to a moisture content of 0.01-1.0% before polymerization, rather than requiring complete drying. This preliminary drying step is sufficient to prevent hydrolysis during processing while avoiding the excessive energy consumption of complete drying, thus resolving the contradiction between achieving high molecular weight and reducing energy cost
2Stability of the object's composition
If a modifier is used to combine lower molecular chains, then molecular weight is increased, but residual acetaldehyde remains which compromises food safety
Solution Approach 1:
The invention uses a specific catalyst system (comprising a main catalyst and a co-catalyst) as an intermediary to enable polymerization without requiring chemical modifiers. The catalyst system facilitates the combination of lower molecular chains through catalyzed polycondensation reactions, achieving molecular weight increase without introducing residual acetaldehyde that would compromise food safety
Solution Approach 2:
The invention converts the potentially harmful effect of moisture in recovered PET into a beneficial process feature. By controlling moisture content to a specific range (0.01-1.0%) and using appropriate catalysts, the presence of some moisture does not harm the process but actually facilitates the polymerization reaction, while the process simultaneously removes acetaldehyde through vacuum processing, turning a potential harm into a benefit
3Reliability
If complete drying of recovered PET flakes is performed, then polymerization can proceed, but equipment cost and processing time increase
Solution Approach 1:
The invention applies partial drying action by reducing moisture content to a sufficient level (0.01-1.0%) rather than attempting complete drying. This partial drying is adequate to prevent hydrolysis and enable successful polymerization, while avoiding the excessive time and energy costs of attempting to achieve absolutely dry conditions. The catalyst system compensates for the remaining moisture, maintaining polymerization quality
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
This approach enhances the properties of PET sheets by forming a high molecular weight structure, reduces equipment and energy costs by eliminating the need for drying, and ensures food safety by removing acetaldehyde, allowing for the production of high-quality food containers.
Implementation Method 1
polymerization reaction by the chain extender, which occurs mostly
Implementation Method 2
a first zone wherein the charged recovered PET is heated to melt and kneaded together with the chain extender
Implementation Method 3
melting and kneading the flakes of recovered PET with sucking to degas through the vent holes
Data Source
Figure 1
AI summary
This invention relates to a method of manufacturing sheet for food containers, which comprises: adding a chain extender to moist flakes of recovered PET; charging the flakes of recovered PET containing the chain extender into an extruder having two or more vent holes, melting and kneading the flakes of recovered PET with sucking to degas through the vent holes, while molecular chains of the recovered PET of which the molecular weight has been lowered are bonded to each other by the chain extender to render the molecular weight higher, and simultaneously, ethylene glycol and acetaldehyde, produced during depolymerization by water and heat, are trapped by the chain extender to remove residual aldehydes; and extruding the recovered PET from the extruder into sheet.