Key Tin Catalysts

Although there are many types of tin catalysts, only a few are widely used in practice. The most important ones are dibutyltin dilaurate (DBTL) and stannous octoate.

(1) Dibutyltin Dilaurate

Dibutyltin dilaurate (DBTL), also known as dibutyltin bis(lauroylate), was the first organotin catalyst introduced and remains significant in foam production today.

-Chemical Formula: (C₄H₉)₂Sn(OCOC₁₁H₂₃)₂

-Molecular Weight: 631.56

-Physical Properties:

Appearance: Pale yellow transparent liquid

Melting point: ≤ -10°C

Density (25°C): 1.025–1.065 g/cm³

Tin content: 18.6 ± 0.6%

Color (Gardner method): ≤ 8

Viscosity (25°C): 40–50 mPa·s

Refractive index: 1.479 ± 0.009

Solubility: Soluble in most plasticizers and solvents, insoluble in water

-Production Process:

DBTL is synthesized through a complex process. Initially, butyl iodide reacts with metallic tin in the presence of a small amount of magnesium catalyst and butanol as the solvent to produce butyltin iodide. This is hydrolyzed to dibutyltin oxide, which then undergoes esterification with lauric acid to yield DBTL.

-Toxicity:

DBTL is highly toxic, and appropriate safety measures should be taken during handling.

(2) Stannous Octoate

Stannous octoate, also known as stannous 2-ethylhexanoate, is primarily used in the production of soft polyurethane foams.

-Chemical Formula: [C₄H₉CH(C₂H₅)COO]₂Sn

-Molecular Weight: 405.1

-Physical Properties:

Appearance: Pale yellow liquid

Tin content: 28 ± 0.5%

Stannous content (as a percentage of total tin): ≥ 96%

Density: 1.25 ± 0.02 g/cm³

Refractive index: 1.495 ± 0.005

Viscosity (20°C): ≤ 500 mPa·s

Solubility: Soluble in polyols and most organic solvents, insoluble in alcohol and water

-Production Process:

2-Ethylhexanoic acid reacts with sodium hydroxide to produce sodium 2-ethylhexanoate. This then undergoes metathesis with stannous chloride in an inert solvent to yield stannous octoate. The molar ratio of 2-ethylhexanoic acid, sodium hydroxide, and dihydrate stannous chloride is 1:0.516:0.5. Adding a small amount of antioxidant 264 during the reaction enhances the tin content and stability of the product.

-Storage and Application:

Stannous octoate can be stored for at least 12 months in a sealed container in a cool, dry place. It is non-toxic and non-corrosive, making it suitable for producing medical supplies.

This catalyst is commonly used in low-density, flexible foam production, where the foam must have uniform cell density, 97% open cells, as well as strength and elasticity. Production efficiency is high, and product quality must meet stringent requirements to avoid significant economic losses. Small-scale tests are necessary to determine the optimal "tin usage range" for each formulation.

(3) Other Organotin Catalysts

The catalytic activity of organotin compounds can be influenced by steric hindrance, which decreases with rising temperatures. Replacing small substituents with bulkier ones can enhance stability, hydrolytic resistance, and delay catalytic activity.

Examples:

Dioctyltin compounds provide delayed catalytic effects compared to dibutyltin.

Dialkyltin maleates and thioglycolates enhance hydrolytic stability.

Tin-thiolates with large groups, such as dioctyltin sulfates, offer both high stability and delayed catalytic activity.