Important Organic Isocyanates: Liquefied MDI

Pure MDI is solid at room temperature, making it inconvenient to use. During storage, 4,4-MDI tends to form dimers, leading to poor storage stability. Therefore, it must be heated to melt into a liquid before use. Repeated heating can affect the quality of MDI and complicate operations. As a result, polyurethane foams generally do not directly use MDI. Liquid MDI, developed in the 1970s, is a modified form of MDI that overcomes these drawbacks. It can be used to manufacture polyurethane integral skin molded products with special performance requirements, enhancing properties such as flame resistance. In foam production, MDI is typically used after being liquefied or prepolymerized.

Apart from making MDI liquid by increasing the proportion of 2,4'-MDI during production, it can also be mixed with TDI, crude MDI, etc., to create mixtures with higher MDI content. Some isocyanate manufacturers supply such isocyanate blends.

The most commonly used MDI liquefaction technology involves introducing carbamate or carbodiimide groups into 4,4'-MDI, which is widely available industrially, to obtain liquid MDI modifications. Depending on the modification method, different types of liquid MDI are produced. Specifically, there are four types:

1.Carbamate-modified MDI with a functionality of 2.0;

2.Carbodiimide-modified MDI with a functionality of 2.0;

3.Diazabutene-imide modified MDI with a functionality of 2.2;

4.Carbamate and diazabutene-imide modified MDI with a functionality of 2.1.

(1) Carbamate-modified MDI

Mixing MDI with a small amount of diol can produce carbamate-modified MDI. For example, by reacting MDI with polyether diol of molecular weight 600 in a feed ratio of NCO/OH molar ratio of 10:1, heating to 50-60℃, and stirring for 5 hours, the mixture liquefies.

(2) Carbodiimide-modified MDI

Carbodiimide modification is a crucial method for liquefying MDI to achieve easy-to-use and storage-stable liquid MDI. Typically, in the presence of trace organic phosphine catalysts (e.g., triethyl phosphate), pure MDI is heated to a certain temperature, causing its structure to change and form a liquid mixture containing some carbodiimide groups (-N==C==N-). A typical organic phosphine catalyst is 1-phenyl-3-methyl-1-phosphine oxide. After the reaction, the phosphine oxide catalyst must be removed from the mixture. Deactivating agents like Lewis acids, sulfonic esters, and phosphorus halides can be used to deactivate the catalyst. During cooling and storage of the carbodiimide-modified MDI solution, MDI's NCO adds to the carbodiimide group, forming diazabutene-imide (ketimine) with a triisocyanate functionality. Thus, the average functionality of this modified MDI is usually 2.15-2.20, with an NCO content of 28%-31%. It can be used for making soft foam. For low-functionality carbodiimide-modified MDI (functionality around 2 and without ketimine structure), specific substances can be added to inhibit the addition reaction between MDI and carbodiimide.

Example: 340g of diphenylmethane diisocyanate (melting point 37-41℃, containing 90% 4,4'-MDI and 10% 2,4'-MDI) is mixed with 5g of triethyl phosphate and heated to 200℃. After stirring for 25 minutes, the mixture is cooled to room temperature (25℃) and left for 48 hours. The small amount of solid is filtered out, resulting in a clear, transparent liquid isocyanate. No solids precipitate even after 8 weeks at 25℃.