Two-Stage Curing of Molded Polymeric Articles to Prevent Cracking
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Solution Overview
Problem
Higher cure temperatures required for more stable free-radical initiators in polymerization processes often result in defects such as surface and edge cracking in molded optical articles.
Innovation Solution
A method involving a free-radically polymerizable composition with a stable free-radical initiator, where the composition is initially partially cured in a mold and then further cured in an auxiliary chamber within a convection cure oven, allowing for controlled heat localization and reduced volatile loss, thereby minimizing defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a more stable free-radical initiator is used, then the reliability of the polymerization process is improved, but the cure temperature must be increased which causes surface and edge cracking in the molded articles
Solution Approach 1:
The curing process is divided into two distinct stages: a first cure cycle at an initial temperature to initiate polymerization, and a second cure cycle at an elevated temperature to complete polymerization. This segmentation allows the stable initiator to be activated without causing cracking, as the temperature increase is gradual and controlled rather than abrupt.
Solution Approach 2:
The first cure cycle performs preliminary polymerization at a moderate temperature before the second cure cycle completes the process at higher temperature. This preliminary action prepares the material for final curing without subjecting it to the full thermal stress that would cause cracking if applied all at once.
2Reliability
If a more stable free-radical initiator is used, then the reliability of the polymerization process is improved, but the cure temperature must be increased which results in severe defects in the final cured product
Solution Approach 1:
The curing process is divided into two distinct stages: a first cure cycle at an initial temperature to initiate polymerization, and a second cure cycle at an elevated temperature to complete polymerization. This segmentation allows the stable initiator to be activated without causing cracking, as the temperature increase is gradual and controlled rather than abrupt.
Solution Approach 2:
The first cure cycle performs preliminary polymerization at a moderate temperature before the second cure cycle completes the process at higher temperature. This preliminary action prepares the material for final curing without subjecting it to the full thermal stress that would cause cracking if applied all at once.
3Productivity
If the mold is placed directly into the cure oven for complete curing, then the productivity is improved, but the heat loss and volatile escape increase causing defects
Solution Approach 1:
The curing process is divided into two distinct stages: a first cure cycle at an initial temperature to initiate polymerization, and a second cure cycle at an elevated temperature to complete polymerization. This segmentation allows the stable initiator to be activated without causing cracking, as the temperature increase is gradual and controlled rather than abrupt.
Solution Approach 2:
The first cure cycle performs preliminary polymerization at a moderate temperature before the second cure cycle completes the process at higher temperature. This preliminary action prepares the material for final curing without subjecting it to the full thermal stress that would cause cracking if applied all at once.
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 method significantly reduces defects like surface and edge cracking in molded articles, improving the yield of commercially acceptable products by maintaining heat and retaining volatiles during the polymerization process.
Implementation Method 1
a free-radically polymerizable composition... subjecting the polymerizable composition within the mold cavity to a cure cycle sufficient to at least partially cure the polymerizable composition, thereby forming an at least partially cured polymeric article
Implementation Method 2
placing the mold of (c) into a cure oven, such as a convection cure oven... subjecting the polymerizable composition within the mold cavity to a cure cycle
Implementation Method 3
a convection cure oven... subjecting the polymerizable composition within the mold cavity to a cure cycle
Data Source
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AI summary
Provided is a method for preparing a polymeric article including introducing a polymerizable composition into a mold; sealing the composition within the mold (18) using a sealing means; placing the mold (18) into a convection cure oven (10); subjecting the polymerizable composition to a cure cycle sufficient to partially cure the composition; extracting the mold (18) from the convection cure oven (10); optionally, removing the sealing means from the mold; placing the mold (18) into an auxiliary curing chamber (16) which is placed into the convection cure oven (10), or placing the mold (18) into an auxiliary curing chamber (16) which is within the convection cure oven (10); subjecting the polymerizable composition to a further cure cycle to complete polymerization of the composition, thereby forming a cured polymeric article; and removing the cured polymeric article from the mold (18).