Synthesis of Polyurethane Foam Prepolymers (Part One)

NCO-terminated prepolymers have limited use in special foam formulations. It is crucial to theoretically calculate the required amount of isocyanate to obtain a prepolymer with a specific NCO content.

The fundamental formulas for this calculation remain:

Ws = (EsWpI₂) / (Epρs)

Ws = (EsWpI₃) / (Epρs)

Ws = (EsWqI₂) / (Enρs)

(For more on calculating the isocyanate requirement for flexible polyurethane foam, refer to the article How to Calculate the Required Amount of Isocyanate for Flexible Polyurethane Foam?)

It is important to note that the NCO content refers to the mass fraction of unreacted NCO in the prepolymer. For example, in TDI (Toluene Diisocyanate), with an equivalent weight of 87 and NCO equivalent of 42, the NCO content Dtdi  is:

Dtdi = (42 / 87) × 100% = 48%

[Example]

To synthesize a semi-prepolymer using a polyether with a hydroxyl value of 490 and TDI at an isocyanate index of 4.5, how much of each component is needed, and what will the NCO content of the resulting prepolymer be?

First, calculate the equivalents:

Ep= 56100 / 490 = 114.5

Es = 174 / 2 = 87

Assuming 100 parts of polyether, the required TDI (Ws) is:

Ws = (Es × Wp × I₂) / Ep = (87 × 100 × 4.5) / 114.5 = 341.9

To calculate the NCO content of the prepolymer:

D = [(I₂ - 1) ×En / Wv] × 100%

Where:

En = 42 (equivalent of NCO)

Wv= mass of the prepolymer = 341.9 + 100 = 441.9

I₂ = 4.5

So:

D = [(4.5 - 1) × 42 / 441.9] × 100% ≈ 29%

As is well known, the required isocyanate for synthesizing a prepolymer with a specific NCO content depends on both the polyether amount and the target NCO level. The summarized formula is:

Ws = (Es × Wp / Ep) × [(100 × 42 + D × Ep) / (100 × 42 - D × Es)]       (Formula 11-13)

Where:

Ws = isocyanate amount

Wp = polyether/polyester amount

Es = isocyanate equivalent

Ep = polyether/polyester equivalent

D = NCO mass fraction in the prepolymer

[Example]

To react 100 parts of polyether (hydroxyl value 58 mgKOH/g) with TDI to synthesize a prepolymer with 6% NCO content, how much TDI is required?

Given:

Wp = 100; D = 6%; Es= 87; Ep= 56100 / 58 = 967

Then:

Ws = (87 × 100 / 967) × [(4200 + 6 × 967) / (4200 - 6 × 87)] = 24.5 parts