Water-absorbent resin polymerization residual monomer reduction
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Solution Overview
Problem
Conventional methods for producing water-absorbent resins struggle to achieve a balance between high water-absorption performance, low residual monomer content, and suitable skin compatibility for absorbent articles, often resulting in increased crosslinking density and residual monomers.
Innovation Solution
A process involving reverse-phase suspension polymerization of water-soluble ethylenically unsaturated monomers in a hydrocarbon dispersion medium, using a combination of an azo-based compound and peroxide with specific ratios, along with an internal-crosslinking agent, followed by post-crosslinking, to produce a resin with enhanced water-retention and absorption capacities while minimizing residual monomers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If persulfate is used as polymerization initiator, then polymerization reaction is easily controlled, but self-crosslinking is promoted and crosslinking density increases, reducing water-absorption performance
Solution Approach 1:
The patent segments the crosslinking process into two distinct stages: (1) polymerization stage using persulfate as initiator to form the polymer backbone, and (2) post-crosslinking stage using a separate crosslinking agent to create controlled crosslinks. This segmentation allows the persulfate to perform its polymerization function without excessive self-crosslinking, while the post-crosslinking agent provides controlled crosslinking density to maintain water-absorption performance.
Solution Approach 2:
The patent performs preliminary polymerization using persulfate to establish the polymer structure before introducing the crosslinking agent. This preliminary action allows the polymer chains to form first, creating a matrix that can then be crosslinked in a controlled manner, preventing premature or excessive crosslinking that would reduce water-absorption capacity.
2Reliability
If azo-based compound is used as polymerization initiator, then self-crosslinking is reduced, but polymerization rate decreases and residual monomer content increases
Solution Approach 1:
The patent merges the advantages of two different initiators by using persulfate for polymerization (high polymerization rate) and then adding a crosslinking agent for crosslinking (controlled crosslinking density). This combination allows the system to achieve both high productivity during polymerization and proper crosslinking control, eliminating the need to choose between azo-based compounds (slow polymerization) or persulfate (excessive self-crosslinking).
3Reliability
If ratio of water-absorbent resin in absorbent material is increased, then water-absorption performance is enhanced, but residual monomers cause skin irritation
Solution Approach 1:
The patent performs preliminary polymerization with high conversion using persulfate as initiator, which achieves high polymerization rate and reduces residual monomer content before the resin is incorporated into absorbent materials. This preliminary action ensures that even when high ratios of resin are used in absorbent materials, the residual monomer content remains low enough to avoid skin irritation while maintaining water-absorption performance.
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 effectively reduces residual monomer content, enhances water-retention and absorption capacities, and improves skin compatibility, making the water-absorbent resin suitable for absorbent articles with superior performance.
Implementation Method 1
a step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of an internal-crosslinking agent, wherein the water-absorbent resin is obtained by performing reverse-phase suspension polymerization... while an azo-based compound and a peroxide are combined
Implementation Method 2
in the presence of an internal-crosslinking agent... a step of adding a post-crosslinking agent after the polymerization to carry out a post-crosslinking reaction
Implementation Method 3
the water-absorbent resin is obtained by performing reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium
Data Source
Figure 1
AI summary
The present invention relates to a process for producing a water-absorbent resin, wherein the water-absorbent resin has superior water-absorption performance including a high water-retention capacity, a high water-absorption capacity under a load and a high vertical diffusion water-absorption capacity, and has a decreased content of residual monomers, and wherein the water-absorbent resin is suitable for an absorbent article. The present invention also relates to a water-absorbent resin having specific performance and produced by the process. The present invention provides a process for producing a water-absorbent resin, characterized in that a reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer is carried out in a hydrocarbon dispersion medium comprising a dispersion stabilizer while an azo-based compound and a peroxide are combined in the presence of an internal-crosslinking agent, in that the following formulae are satisfied, 0.10≤B/A+B and 0.055≤B+9×C≤0.120 wherein A mol, B mol and C mol represent used amounts of the azo-based compound, the peroxide and the internal-crosslinking agent, respectively, relative to 100 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization, and in that a post-crosslinking reaction is carried out by adding a post-crosslinking agent after the polymerization.