Before starting with the selection of the pump, let us first understand about the uses, types and properties of Isocyanates.
Isocyanates are a family of highly reactive, low molecular weight chemicals. Isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. So basically we will be focusing on diisocyanates.
Types and Uses of Diisocyanates:
Diisocyanates are manufactured for the production of polyurethanes, a class of polymers. Diisocyanates are a family of chemical building blocks mainly used to make polyurethane products, such as rigid and flexible foams, coatings, adhesives, sealants and elastomers etc.
There are two primary aromatic diisocyanates: toluene diisocyanate (TDI) and methylenediphenyl diisocyanate (MDI)
1. Toluene diisocyanate (TDI):
TDI is used primarily in the production of flexible foams. Toluene diisocyanate (TDI) is a chemical used in the production of polyurethanes, primarily for flexible foam applications including furniture, bedding and carpet underlay, as well as packaging applications. TDI is also used in the manufacture of coatings, sealants, adhesives and elastomers. In transportation applications, TDI helps produce lighter automobile parts, saving weight, which leads to improvements in fuel efficiency and thus energy conservation.
From a technical perspective, TDI is obtained by nitration of toluene. The hydrogenation of dinitrotoluene is then obtained to produce toluenediamine (TDA), which is in turn reacted with phosgene to form TDI.
2. Methylenediphenyl diisocyanate (MDI):
MDI, the second type of DII, comes in two forms: Pure MDI and polymeric MDI (PMDI).
Pure MDI is used in the production of a variety of polyurethane products like elastomers, sealants, adhesives and coatings.
PMDI-Polymeric MDI is a highly versatile product used to produce a wide variety of rigid, flexible, semi-rigid and polyisocyanurate and thermoset foams.
Methylenediphenyl diisocyanate (MDI) is a chemical used in the production of polyurethanes for many applications. MDI is used primarily in the production of rigid polyurethane foams used for insulation for your home or refrigerator, and many other uses. Insulation made with MDI can help consumers save on their heating and cooling costs and conserve energy. Some additional uses of MDI in polyurethanes include coatings, adhesives, sealants, and elastomers found in items such as paints, glues, and weather resistant materials. It is also used to make many types of footwear, sports and leisure products and to a much lesser extent, some specialty flexible foams. MDI can also be used as a binder for wood and to produce mold cores for the foundry industry.
From a technical perspective, MDI is obtained from the condensation of aniline with formaldehyde to produce methylene dianiline (MDA), which is in turn reacted with phosgene to form MDI.
Properties of Diisocyanates (TDI/MDI):
- Physical state : Liquid
- Color : Colorless to yellow
- Odor : Characteristic, pungent
- Viscosity, dynamic : Ranging from 1 cP to 500 cps (at 25 C) depending on the mixture of isomers
- Vapor pressure : 0.03 mm Hg (at 25 C)
- Specific Gravity: 1.2
Also below are few unique characteristic facts about TDI/MDI:
- TDI/MDI reacts with moisture, even atmospheric moisture, to form carbon dioxide gas.
- They when exposed to air gets solidify within few minutes.
- They are toxic and carcinogenic by nature
- Highly aggressive and reactive with chemicals
SELECTION OF THE GEAR PUMP FOR TDI/MDI APPLICATION:
Pump Body and Internal Parts:
Cast iron construction is usually acceptable. However, some process/ customers may require carbon steel or stainless steel.
Pump Seal:
As we know these chemicals are highly toxic, carcinogenic and even solidify when exposed to the moisture in the air. While traditionally MDI/TDI pumps used to immersed inside the DOP solvent to isolate from the atmospheric moisture and packed pumps worked as such. However, as the emission norms are getting stringent, double or tandem mechanical seal with seal plan became more popular. Viking has also developed Triple Lip Cartridge mechanical with universal seal pumps with good results. However the best suitable and most commonly applied configuration would be to use seal-less design pump i.e Magnetic driven gear pump.
As far as elastomers, are concerned usually Kalrez and PTFE is recommended.
Pump Speed:
As viscosity is not high for these chemicals, we can run the pump at higher speed/Motor speed depending on the flow rate required and NPSH available.
