How to Prepare Hydrophilic Polyurethane Foam? —By Introducing Hydrophilic Segments

Hydrophilic polyurethane foam can be prepared by introducing hydrophilic groups or segments, such as ethylene oxide (EO), into polyether or polyester polyols. When the proportion of hydrophilic segments reaches a certain level, hydrophilic and water-absorbent polyurethane foam can be synthesized using either a prepolymer or a one-shot process.

(1) Preparation of Water-Absorbent Polyurethane Foam

These foams are formed by foaming polyether polyols with more than 40% EO content (by weight), polyisocyanates, blowing agents, catalysts, and additives. The isocyanate index used typically ranges between 50–97%. The resulting foam is elastic, has excellent water absorption and retention, and exhibits anisotropic absorption and swelling, with high expansion ratios.

① Raw Materials

Polyols:

Poly(ethylene oxide) polyether polyols and EO/PO block copolymer polyols can be used. The initiators for polyols may include:

Diols: Ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butanediol, hexanediol

Triols: Glycerol, trimethylolpropane, trimethylolethane, hexanetriol, triethanolamine

Tetrols: Pentaerythritol

Mixtures of these initiators are also acceptable.

EO content must exceed 40% by weight, with molecular weights between 400–12,000.

Isocyanates:

TDI, MDI, NDI, and HDI can be used, with TDI and MDI being the most suitable in general.

Crosslinkers:

Diethylenetriamine, triethylamine, tetraethylenepentamine, glycerol, trimethylolpropane, EO-adducts of trimethylolpropane, ethylenediamine, trimethylamine, tetramethyl diamine, pentamethyl diamine, ethanolamine, diethanolamine, etc.

Foam Stabilizers:

Silicone stabilizers and various EO/PO adducts of fatty acids and sulfonated surfactants.

Catalysts:

Organometallics like dibutyltin dilaurate and tertiary amines like triethylenediamine.

Blowing Agents:

Low-boiling-point halogenated compounds and water.

Other Additives:

Pigments, preservatives, surface-active agents to increase absorption rate, calcium carbonate, etc.

② Formulations and Properties

Formula 1

Formula 2

Formula 3

Formula 4

③ Preparation Process

Water-absorbent flexible PU foam is made from polyether polyol (EO >40%), polyisocyanates, foaming agents, catalysts, and stabilizers. 5–50 wt% of inorganic fillers are added to the formulation. After foaming, the foam is post-cured at 80°C for 30 minutes, then compressed, followed by surfactant treatment to produce the final hydrophilic PU foam.

Materials:

Polyols:

Commercial triol polyethers with EO content 70–75%, Mn 4000–5000

Isocyanates:

TDI, MDI, IPDI

Crosslinkers:

Triethanolamine, diethanolamine

Catalysts:

Triethylenediamine, tetramethylguanidine, dibutyltin dilaurate

Inorganic Fillers:

Calcium carbonate (1.2, 0.7, 0.2 μm), silica (300 μm), kaolin (2 μm), bentonite (2 μm), talc (5–6 μm)

Surfactants:

Anionic: sodium/ammonium alkyl sulfate, sodium dodecylbenzenesulfonate

Nonionic: polyoxyethylene alkyl ethers, sorbitan esters

Cationic: dimethyl distearoyl ammonium chloride, stearamido propyl trimethylammonium chloride

Silicone surfactants and fluorosurfactants also applicable

Dosage: 0.1–10% by foam weight (optimal: 0.2–4.7%)

(2) Formulations and Performance

(A) One-Shot Method

Compression processing method

(B) Prepolymer Method

Prepolymer-1: Polyether (OH 35.6, EO 70%) 2000 parts + MDI 1892 parts, reacted at 60°C for 5 hours → NCO% = 15.2

Prepolymer-2: Polyether (same) + MDI 892 parts → NCO% = 9.9      

(3) One-Directional Water Absorbing Polyurethane Foam

Prepared using regular polyether polyols, isocyanates (e.g., PAPI, TDI or its prepolymer), foam stabilizers, catalysts, and other additives.
Key differences: High water content and low isocyanate index (0.1–0.8). Foam forms connected linear bubbles along the foaming direction only, creating directional water absorption properties, ideal for horticulture or floral applications.

For the preparation method by adding water-absorbent resin, click here to read.

For the preparation method by the impregnation method, click here to read.