Water-Absorbing Polymer Particles Surface Post-Crosslinking
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Solution Overview
Problem
Existing processes for producing water-absorbing polymer particles by suspension polymerization fail to achieve optimal absorption under pressure, permeability, and low extractables, with existing methods often decoupling absorption and centrifuge retention capacity.
Innovation Solution
A process involving suspension polymerization with thermal surface post-crosslinking, where the amount of crosslinking agent is chosen to achieve a centrifuge retention capacity of less than 37 g/g, and thermal surface post-crosslinking is carried out at 140 to 220°C, using a monomer solution containing ethylenically unsaturated acid-group-carrying monomers, crosslinkers, initiators, and optional water-soluble polymers, to produce particles with high absorption under pressure and permeability while minimizing extractables.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the degree of cross-linking is increased to improve gel strength, then the absorption capacity decreases
Solution Approach 1:
The patent applies surface post-crosslinking to create a crosslinked skin layer on the particle surface while keeping the inner core less crosslinked. This local differentiation allows the surface to provide mechanical strength and structural integrity, while the inner core maintains high water absorption capacity. The crosslinking density varies from surface to center, resolving the contradiction between overall gel strength and total absorption capacity.
Solution Approach 2:
The polymer particle is effectively segmented into two functional zones: a crosslinked surface layer and a less crosslinked inner core. This segmentation allows each region to optimize its function - the surface provides strength and the core provides absorption - thereby resolving the contradiction between gel strength and absorption capacity.
2Productivity
If surface post-crosslinking is performed to improve permeability and absorption under pressure, then the centrifuge retention capacity decreases
Solution Approach 1:
Surface post-crosslinking creates a differentiated structure where only the outer layer is crosslinked. This local crosslinking improves permeability and absorption under pressure by creating a porous skin structure, while the uncrosslinked or less crosslinked inner core maintains high centrifuge retention capacity. The spatial separation of functions resolves the contradiction between these two absorption parameters.
3Productivity
If thermal surface post-crosslinking is carried out at high temperatures to improve absorption properties, then hydrophobic solvent is expelled
Solution Approach 1:
The patent optimizes the thermal post-crosslinking parameters (temperature range of 80-200°C, time 1-24 hours) to achieve the desired crosslinking degree while minimizing solvent expulsion. By carefully controlling these parameters, the process balances the improvement of absorption properties against the loss of hydrophobic solvent, finding an optimal operating window that satisfies both requirements.
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 results in water-absorbing polymer particles with enhanced absorption under pressure, high permeability, and low extractables, achieving a centrifuge retention capacity of 20 to 36 g/g, absorption under 0.0 g/cm2 of 30 to 60 g/g, and permeability of at least 20×10−7 cm3/s/g, with less than 10% by weight extractables.
Implementation Method 1
polymerizing a monomer solution containing a) at least one ethylenically unsaturated, acid-group-carrying monomer
Implementation Method 2
the monomer solution being suspended in a hydrophobic organic solvent during the polymerization
Implementation Method 3
thermal surface post-crosslinking of the agglomerated polymer particles obtained using an organic surface post-crosslinker, characterized in that the thermal surface post-crosslinking is carried out at from 140 to 220°C
Implementation Method 4
water-absorbing polymer particles having a high absorption under a pressure of 0.0 g/cm2 (AUNL), a high absorption under a pressure of 49.2 g/cm2 cm2 (AUHL)
Data Source
AI summary
The invention relates to a method for producing water-absorbing polymer particles by suspension polymerization and thermal surface post-crosslinking, the agglomerated base polymer obtained by the suspension polymerization having a centrifuge retention capacity of less than 37 g/g and the thermal surface post-crosslinking proceeding at 140 to 220°C.