Super absorbent polymer

· Sodium polyacrylate: the most common SAP, accounting for more than 90% of all types of SAP. The chemical formula is

[−CH2−CH(CO2 Na )−]n)

· Potassium polyacrylate: a SAP designed for agricultural use, which can be used as a soil water retainer. The chemical formula is

[−CH2−CH(CO2K)−]n

· SAP grafted products have special uses, such as:

Starch grafted sodium polyacrylate for natural degradation

Bentonite grafted potassium polyacrylate can significantly extend the service life

The surface crosslinker of the absorbent resin is a key chemical used to form a crosslinked structure on the surface of the resin particles, aiming to improve its mechanical strength, compression resistance, water retention capacity and reduce soluble matter. The following is a detailed description of the surface crosslinker:

The role of surface crosslinking

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- Enhance gel strength: Through surface crosslinking, a dense outer network is formed to prevent the gel from collapsing after water absorption.

- Improve compression resistance: Reduce water release under pressure (such as squeezing in sanitary products). - Reduce soluble matter: reduce the dissolution of unreacted polymer monomers or oligomers and improve safety.

- Adjust water absorption rate: the surface cross-linking layer can control the water penetration rate to meet different application requirements.

Common types of surface cross-linking agents

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Multifunctional compounds

- Epoxy: such as ethylene glycol diglycidyl ether (EGDGE) and glycerol polyglycidyl ether, react with the carboxylic acid group of the resin through the epoxy group.

- Polyol: propylene glycol and pentaerythritol, cross-link with the carboxylic acid esterification of the resin through the hydroxyl group.

- Polyisocyanate: such as hexamethylene diisocyanate (HDI), reacts with hydroxyl or amino groups to form a polyurethane cross-linking structure.

Ionic cross-linking agents

- Metal ions (such as Al³⁺, Ca²⁺, Fe³⁺): form ionic cross-linking by chelating the carboxylic acid groups on the surface of the resin.

Other new crosslinking agents

- Silane coupling agents (such as KH-560) and nanoparticles (silicon dioxide, clay) enhance mechanical properties.

- Bio-based crosslinking agents (such as citric acid, tannic acid) are used in environmental protection or medical fields.

Surface crosslinking process

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1. Coating: Dissolve the crosslinking agent in water or organic solvent and spray it on the surface of the resin particles.

2. Reaction conditions:

- Temperature: Usually heating (80–200°C) is required to promote the reaction.

- Catalyst: Alkaline catalysts (NaOH, ammonia) or Lewis acids (AlCl₃) accelerate the reaction.

3. Post-treatment: Wash to remove unreacted crosslinking agent and dry to obtain the finished product.

Application areas

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- Hygiene products: diapers, sanitary napkins (require high pressure resistance and low back infiltration).

- Agriculture: soil water retention agent (requires salt resistance and slow release of water absorption).

- Medical: wound dressing (requires biocompatible crosslinkers such as citric acid).

- Industrial: cable water-blocking materials, concrete internal curing.

Key influencing factors

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- Crosslinker concentration: Too high a concentration may result in a hard surface and reduced water absorption.

- Reaction time and temperature: affect the depth and uniformity of crosslinking.

- Resin structure: carboxylic acid group density and particle size distribution affect the crosslinking effect.

Research progress

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- Environmentally friendly crosslinkers: reduce the use of organic solvents and develop water-soluble or solvent-free systems.

- Nanocomposite crosslinking: introduce nanoclay or graphene to improve mechanical properties.

- Dynamic crosslinking network: achieve self-healing function based on reversible bonds (such as hydrogen bonds and coordination bonds).

Precautions

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- Biosafety: non-toxic crosslinkers (such as citric acid and EDC) should be selected for medical use.

- Residue control: ensure that the crosslinker reacts completely to avoid the migration of residual monomers.

- Cost and process: In industrial production, it is necessary to balance performance and cost (e.g., epoxy crosslinking agents are more expensive).