Main Raw Materials for Reactive Polyurethane Adhesives

Category of Polyester Polyols

Core Raw Materials

Key Properties

Conventional Polyester Polyols

1. Dicarboxylic acids: Mainly adipic acid, with a small amount of sebacic acid, phthalic anhydride, terephthalic acid, succinic acid, etc. (two types can be used in combination)

2. Diols: Ethylene Glycol (EG), 1,2-Propylene Glycol (PG), 1,4-Butanediol (BDO), Diethylene Glycol (DEG), Neopentyl Glycol (NPG), 2-Methyl-1,3-Propanediol (MPD), 1,6-Hexanediol (HDO), etc. (one or more types can be used in combination) 

3. Branching regulator: A small amount of Trimethylolpropane (TMP), Glycerol

1. Products made from even-carbon diols (1,4-Butanediol, 1,6-Hexanediol): High crystallinity, which can endow adhesives with high initial tack strength 

2. Products made from diols with side groups (Neopentyl Glycol, etc.): Good flexibility and excellent hydrolysis resistance 

3. Aromatic polyester diols: Light yellow to brown-red transparent viscous liquid, high hydroxyl value, slightly low molecular weight, benzene ring-containing structure, high viscosity

Polycaprolactone Polyols (PCL)

1. Monomer: ε-caprolactone 

2. Initiator: Diols, triols, alcohol amines 

3. Catalysts: Tetrabutyl Titanate, Stannous Octoate, etc.

1. Basic properties: Low color (colorless and transparent), high purity (narrow molecular weight distribution), lower viscosity (including melt viscosity) than general polyester diols

2. Functionality: Determined by the functionality of the initiator 

3. Advantages of polyurethane products: Good crystallinity, better hydrolysis stability than adipic acid-based polyester diols (low acid value products have more excellent hydrolysis resistance), high tensile strength, good resistance to hydrocarbon solvents and chemicals, and certain biodegradability

Polycarbonate Diol (PCDL)

1. Initiator: Small molecule diols 

2. Reaction raw materials: Dimethyl Carbonate, Diphenyl Carbonate

Divided into two categories: 

1. Traditional type: High crystallinity, high performance, more widely used 

2. New low-crystallinity polyether carbonate diol: Still in the promotion and application stage

Polyisocyanate Grade

Core Physical Properties

Key Characteristics & Reactivity

Toluene Diisocyanate (TDI)

Colorless/light yellow low-viscosity transparent liquid with special pungent odor at ambient temperature; viscosity 3.2 mPa·s at 20℃; insoluble in water, soluble in organic solvents; white crystals precipitate when stored below 10℃; molecular weight 174.15, mass fraction of —NCO group approx. 48.2%

Low melting point, convenient to use as liquid at room temperature; relatively high vapor pressure, vapor is irritant and toxic; industrial product purity (total isomer content) ≥99.5% generally; indicators including acidity and hydrolyzable chlorine affect reactivity with polyols

Methylene Diphenyl Diisocyanate & Liquefied MDI (MDI Series)

Pure MDI (MDI-100, ≥99.5% 4,4'-MDI): an off-white to pale yellow solid at room temperature, melting point 39~43°C, soluble in most organic solvents; molecular weight 250.2, —NCO mass fraction 33.5%. Products with >25% 2,4'-isomer content are liquid at ambient temperature; optimal storage temperature 25~35℃. C-MDI: pale yellow liquid, —NCO mass fraction 28%~30%, average functionality slightly higher than 2.0

Vapor pressure far lower than TDI, extremely low volatility and nearly no odor at room temperature; 4,4'-MDI features higher reactivity and melting point than 2,4'-MDI; symmetric molecular structure delivers high mechanical strength to polyurethane products; pure MDI slowly forms infusible & insoluble dimers under low-temperature storage; repeated cooling-thawing accelerates dimer formation; prepolymers made from high 2,4'-MDI grades have lower viscosity

Isophorone Diisocyanate (IPDI)

Colorless/light yellow liquid at ambient temperature, fully miscible with organic solvents; molecular weight 222.3, —NCO mass fraction

Very low vapor pressure, almost odorless at room temperature; aliphatic diisocyanate with lower reactivity than aromatic isocyanates such as TDI and MDI; prepolymer synthesis requires reaction temperature of ~80~90°C; resulting polyurethane adhesives are colorless/pale and feature outstanding long-term anti-yellowing performance

Polymethylene Polyphenyl Isocyanate (PAPI / Crude MDI / Polymeric MDI / PMDI)

Brown to dark brown medium-low viscosity liquid at ambient temperature; viscosity of different grades ranges from ~100 to ~2000 mPa·s; relative density is approx. 1.238 at 20°C, freezing point is approx. 5°C. Standard polymeric MDI: average functionality ~2.7, viscosity 100~300 mPa·s, —NCO mass fraction ~31%~32%, contains about 50% MDI

Multiple rigid benzene rings in molecular chain, high average functionality; manufactured polyurethane articles have high hardness and excellent solvent resistance; grades differ greatly in 4,4'-MDI/2,4'-MDI ratio, functionality, and reactivity; most grades have functionality ~2.2~3.0, and modified grades have average functionality ~2.3~2.6.

Other Diisocyanates

HDI: colorless/light yellow low-viscosity liquid at ambient temperature, molecular weight 168.2; HMDI: colorless to pale yellow liquid at room temperature, may crystallize below 25°C

Most are aliphatic/cycloaliphatic diisocyanates with lower reactivity than aromatic diisocyanates (TDI, MDI, etc.). prepared polyurethane products feature anti-yellowing performance and high strength; generally, they have high cost and limited supply