Amine: Basic

Amine is commonly used in manufacturing flexible foam.

Amine accelerates the blowing reaction between water and TDI as well as to a lesser extent, the gelling reaction between TDI and Polyol.

For this reason, some foam manufactures use combinations of various amine types in the attempt to balance the gelling and blowing reaction. This helps in some ways to control the foaming process.

Out of the three types of amine that exist, only tertiary amines are used as catalyst in the foaming process.

Primary and secondary amines have free hydrogen atoms that will react very fast with TDI. However, tertiary amine has no free hydrogen atom so that it serves (by the free electron pair on the nitrogen atom) as a catalyst rather than as a reagent.

STANNOUS OCTOATE

This is an organo-metallic catalyst. It is the catalyst that promotes the polymer forming (or gelatin) reaction. It is a viscous liquid, light yellow to brown in color, with an unpleasant smell. The chemical needs special handling because it is easily hydrolyzed and oxidized in the presence of water and amines. It is for this reason that the chemical must be tightly covered until it is required for immediate use. When exposed to air, the water vapor in the surrounding air is enough to react with and render it impotent. Generally, foamers refer to stannous octoate as a cross-linking agent because the chemical strongly influences the cohesion and hardening of foam.

Silicone Oil: Basics

Silicon oil used in foam formulations acts as a surfactant. In the foaming process the surfactant plays the following rolls:

• Emulsifies otherwise incompatible ingredients, brings about homogeneity of the chemical mix

• Lowers the bulk surface tension
• Enhances formulation of cells (and bubbles) during the initial mixing stage.

• Prevents cell collapse during the rising stage.

• Aids introduction of solids (e.g. calcium carbonate) into foam formulation.

It should be appreciated that for any given foam formulation, a minimum level of silicone oil is required to have a well structured foam.

Silicone oil is a light-colored and moderately viscous liquid.

MDI: Basics

MDI is the acronym of Methylene diphenyl Di-Isocyanate. This chemical product has in fact 3 isomers: 4, 4’ – MDI, 2, 4’ – MDI, and 2, 2’ – MDI.



MDI could be present in a polymeric form called Polymeric MDI, PMDI or MDI polymer. PMDI has more than just two aromatic cycles holding each an Isocyanates function; therefore PMDI functionality is larger than two whereas MDI monomers are strictly difunctional.

In general, pure MDI refers to a mixture where almost 98% is made up of the 4, 4’ – MDI isomer. The isolated 4,4’ – MDI isomer has a melting point about 40º C, which means that Pure MDI is a solid at ambient temperature.

Crude MDI, or technical grade MDI, refers to a mixture of the three types of isomers as well as the polymeric form. Crude MDI, however, is a brown liquid at ambient temperature.

In order to be handled easily, Pure MDI could be converted into a liquid by reacting 4, 4’ – MDI with a di-alcohol. This process will result into a derivative MDI product called Modified MDI or MDI Prepolymer.

In General, MDI is not used as often as TDI in the production of slab stock Flexible Polyurethane Foam. Even that figures show that MDI production in almost double than TDI, the main usage of MDI is in the molded Flexible Polyurethane Foam, Rigid Polyurethane, and CASE.

TDI : Basics

Toluene Di-Isocyanate: it is an aromatic compound having two Isocyanate functions on a toluene molecule.

Two isomers of TDI are used in the production of Polyurethane Foam: 2, 4 – TDI & 2, 6 – TDI. The difference between those two isomers is the position of the Isocyanate function on the carbon atom of the aromatic cycle.

TDI counts up to 34.1% of the Isocyanate market. In general both isomers are marketed in as a mixture 80 / 20 or 65 / 35. TDI 80 / 20 could be sometime referred to TDI – 80 or simply T-80 (This has nothing to do with the Russian Tank T -80 U ;)

TDI is a colorless liquid with low viscosity; however, if exposed to air for a period of time it can turn to pale yellow color. Density is about 1200 kg per cu.m.

Due to the fact that TDI has a relative high vapor pressure, it seems to be very irritating to handle with it without using proper safety measures.

Isocyanates : Basics

Isocyanates have the following active functions (- N = C = O).

Even that the Isocyanates function has irritant effects on humans and other living creatures, they should not be confused with Cyanate: anion (OCN-) derived form the poisonous Cyanic Acid HCNO.

Isocyanates are essential factor to launch the reaction of producing Polyurethanes. Their reaction with Polyol in the presence of catalysts is fast, exothermic and total.

Isocyanates are a generic name for a group of chemicals having the NCO functions. The most notable Isocyanates are TDI (Toluene Di-Isocyanate), MDI (Methyl diphenyl Di-Isocyanate), Hexamethylene Di-Isocyanate (HDI), and IsoPhorone DiIsocyanate (IPDI).

Other types of Isocyanates form just 1.2% of total market. TDI and MDI form together 95.4 % of the Isocyanates market.

The dual functionality of the Di-Isocyanates along with the dual or triple functionality of the Polyol ensures the formation of the long chain polyurethane polymer.

The reaction between each Isocyanate group (- N=C=O) and an alcohol function (-O – H) results in a formation of the Urethane function as shown below:


R1 – N = C = O + R2 – OH -----------------> R1 – N – C OO – R2

Polyol : Basics

As the name might indicate, Polyol is a macromolecule formed by polymerization of monomers. The final macromolecule has many alcohol functions (– O – H) considered to be the active sites.

Polyol could be formed by the polymerization of ethylene oxide monomers and / or propylene oxide monomers. The respective Polyol would be then called Polyether Polyol due to the ether function (– C – O – C –) that repeats within the molecule.

Polyol, on the other side could be called: Polyester Polyol, when monomers of di-alcohol and di-carboxylic acid react to form the macromolecules. The ester function (– COO –), then, could be found within the chain. This reaction results with elimination of water H2O.

The length of the chain, along with the number of chains that could be found in one macromolecule defines the physical characteristics of the resultant Polyol. The molecular weight of Polyol used in the production of Conventional Flexible Polyurethane Foam is usually between 3200 and 3600.

The functionality of Polyol refers to the number of free alcohol (– OH) site that could be found on the molecule. The theoretical number is generally 2 or 3 depending on the starting molecule where the chains are attached ending with 2 or 3 OH function at the end of the chains. Polyol, however, might have fractional functionalities instead of full digit figures. The main reason for the reduction of functionality is the presence of impurities that block the OH functions and then the average number of alcohol groups per molecule decreases.