How to prepare hydrophilic polyurethane foam?--Prepared by adding water-absorbent resin

Hydrophilic polyurethane foam has strong affinity for water and high water absorption capacity. It is used in sponges, carpets or fabric linings, hygiene products, sanitary materials, absorbents for civil engineering and waterproofing, soil water retainers for agriculture and gardening, plant cultivation, and oil-water separators.

There are three main preparation methods for water-absorbent polyurethane foam:
(1) Adding water-absorbent resin into the PU foaming formulation to enhance hydrophilicity and absorption. Common resins include: polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydrogel, methyl methacrylate-styrene copolymer, etc.

(2) Introducing hydrophilic segments or groups into the polyether or polyester molecular chain, such as ethylene oxide or triethanolamine, and adjusting their ratios to control absorption.

(3) Dipping method, where PU foam is immersed in aqueous solution of hydrophilic resin and dried to achieve water absorption.

1. Preparation by Adding Water-Absorbent Resin
Water-absorbent resin can be purchased or synthesized, used directly or to prepare prepolymer with diisocyanate. One-step foaming is also applicable.

(1) Preparation of Water-Absorbent PU Soft Foam
This foam maintains good water absorption even under load.

Preparation of Starch-Acrylonitrile Copolymer Gel

 Process

Under nitrogen, stir starch and water at 90°C for 1h to form colloid. Cool to 20°C, add acrylonitrile and CAN solution, react at 20–30°C for 2h (starch:AN = 1:4). Filter, wash, then react with calcium hydroxide at 90–95°C for 3h. Adjust pH to 7–7.5 with HCl solution, dry at 80°C to get gel.

Preparation of Water-Absorbent Polyurethane Soft Foam

(2) Preparation of Water-Retaining PU Resin
Used in hygiene materials, civil engineering, agriculture, horticulture, and oil-water separation. Prepared by reacting polyisocyanate with active hydrogen compounds (including hydrophilic resins), catalyst, foaming agent, chain extenders, and additives.

Preparation of Highly Absorbent Resin with Hydroxyl Groups

Process
(1) Add cyclohexane and surfactant under nitrogen, heat to 75°C.
(2) Neutralize acrylic acid in caustic soda solution, add K₂S₂O₈, dissolve, bubble nitrogen.
(3) Drip into step (1) over 1h, co-polymerize and dehydrate, yielding resin with 17% moisture.
(4) Add HEMA, crosslinker (EGDGE + water), react at 75°C for 1h, vacuum remove solvent, dry at 80–100°C to get the resin.

Hydrophilic PU foam can be made via prepolymer or one-step method. Applications include soil retention, forestry, dews prevention, etc.

Raw Materials
Polyols: Hydroxyl-terminated polyesters, polyethylene glycol, polypropylene glycol, EO/PO block copolymers.
Isocyanates: TDI, MDI, HDI, IPDI, etc.

The water-absorbing resin adopts a polystyrene-hydrophilic segment-polystyrene structural configuration, with the average molecular weight of both segments preferably exceeding 3000. The weight ratio of the hydrophobic segment to the hydrophilic segment ranges from 5–95 wt% to 95–5 wt%.

Examples of water-absorbing materials include:

(1) Sodium thiolate of polystyrene-polybutadiene-polystyrene copolymer (15% polystyrene content), absorbing 420 times its own weight;

(2) Sodium thiolate of polystyrene-polybutadiene-polystyrene block copolymer (30% polystyrene content), absorbing 150 times its own weight;

(3) Polystyrene-polyisoprene-polystyrene block copolymer (13% polystyrene content), absorbing 250 times its own weight;

(4) Sodium hydroxytitanate of polystyrene-block copolymer (4 parts methyl methacrylate to 1 part ethyl methacrylate)-polystyrene block copolymer (26% polystyrene content), absorbing 50 times its own weight.

Formulation and Performance
Formula (1)

Production Process

Mixing time:10s, Post-aging temperature:100℃,Post-ripening time:1 hour

Performance:fine, Appearance:good, Water absorption speed:10 seconds，Water retention:85%, Durability:78%

Formula (2)

Prepolymer preparation

Prepolymer made by reacting polyether (MW 3000, EO 60%) with TDI at 90°C under N₂, followed by cooling and TDI addition.

Test Method Notes:

Absorption time: Time for a 2×2×1 cm foam sample to sink.

Water retention: Pressed under 0.4 kg/cm² for 1 min on filter paper.

Durability: Soaked-dried 10 cycles; water retained ratio.

Additional Preparation Method