Important Organic Isocyanates: Polymeric Methylene Diphenyl Diisocyanate (PAPI)

Polymeric methylene diphenyl diisocyanate, abbreviated as PAPI, also known as crude MDI or polymeric MDI, is extensively used in the manufacture of rigid polyurethane and polyisocyanurate foams. It is also partially used in semi-rigid polyurethane foams and molded flexible foams.

PAPI is essentially a mixture containing various functionalities of methylene diphenyl diisocyanates. Typically, MDI accounts for about 50% of the total mixture. The remainder consists of oligomeric isocyanates with functionalities of three or more.

In the domestic polyurethane foam industry, especially in the rigid polyurethane foam sector, some refer to PAPI simply as MDI in material purchasing and production, which newcomers should note.

The production method of PAPI (crude MDI) is the same as MDI, with a slightly lower molar ratio of aniline to hydrochloric acid. The synthesis of PAPI, like MDI, is carried out in two steps. First, aniline and formaldehyde solution (formalin) condense under the action of Lewis acid to form a polyamine mixture containing a certain amount of diamines. This polyamine compound is then phosgenated to obtain PAPI. During the synthesis of polyamines, the diamine content must be controlled at around 50%. This is generally achieved by adjusting the input amounts of aniline and formaldehyde to maintain a specific molar ratio. Experimentally, when the molar ratio of aniline to formaldehyde is 1.75, the resulting polyamine compound contains about 50% diamines. Therefore, the molar ratio of aniline to formaldehyde generally varies within the range of 1.6-2.0. The other operational conditions for synthesizing polyamines are similar to those for MDA.

For the production process of MDI and PAPI by the co-production method, the required polyamines must contain 60% to 75% diamines. This is also achieved by changing the molar ratio of aniline to formaldehyde. When the molar ratio of aniline to formaldehyde is 4/1.4, the synthesized polyamines contain about 75% diamines. The phosgenation process conditions for polyamines are similar to those for MDA, so they are not detailed here.

Typically, the co-production process of MDI and PAPI is adopted. Currently, the global production of isocyanates is moving towards large-scale, continuous, and automated processes. Foreign MDI and PAPI co-production plants have reached production capacities of up to 90,000 tons per year.

PAPI contains 40% to 50% pure MDI, with the remainder being polyisocyanates of functionalities ranging from three to six and a small amount of high molecular weight polyisocyanates.

Based on differences in product viscosity and the average functionality of NCO groups, PAPI can be divided into various grades. The differences between various PAPI products mainly lie in the proportions of 4,4'-MDI, 2,4’-MDI, and different functionalities of methylene diphenyl polyisocyanates, resulting in varying average functionalities and reactivities. Based on viscosity, PAPI can be classified into high viscosity, standard grade, and low viscosity PAPI. Standard grade and low viscosity polymeric MDI are used only in flexible foams, while high viscosity and ordinary grade PAPI are used in rigid foams. The average functionality of standard grade PAPI is about 2.7, with a viscosity of 100-300 mPa·s, and an NCO mass fraction of about 31%-32%, containing approximately 50% MDI by mass, most of which is 4,4’-MDI. This type of polymeric MDI is extensively used in non-water-blown self-skinning flexible foams and semi-rigid foams, as well as in cold-cure high-resilience foams when mixed with TDI and liquid MDI. Lower viscosity implies lower functionality, for example, the average functionality of low viscosity PAPI is generally between 2.5 and 2.6, mainly used in high-density flexible foams and self-skinning foams.

MDI series products include PAPI and modified MDI, with vapor pressures much lower than TDI, making MDI safer to use than TDI. Additionally, MDI has a higher overall reactivity than TDI, reflected in the foam process performance as faster curing, lower molding temperatures, shorter demolding times, and better foam properties. MDI-based flexible foams are characterized by high hardness and good load-bearing properties. Therefore, PAPI and modified MDI are widely used in rapid reaction injection molding (RIM) products and high load-bearing automotive cushions. There is a trend abroad towards using MDI systems instead of TDI systems in molded flexible foams.