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It must be understood that the mixing of the two components affects a chemical reaction between a blend of polyol chemicals and an isocyanate. The key variables that need to be understood are the affects of temperature, mix ratio, homogeneity of the polyol mixture, thoroughness of blending, and the effect of water.

The polyol and isocyanate react to form a polyurethane; other ingredients play a role in determining the properties of the final product but are not chemically reactive.

The rate of the chemical reaction between the isocyanate and the polyol is affected greatly by the temperature at which the reaction is carried out. Warmer temperatures excite the chemicals to react faster, and colder temperatures inhibit the reaction.

The temperature can be affected by the shipping and storage conditions for the drums as well as by heat that is added to the individual components while in their holding tanks or through the use of thermostatically controlled heated delivery hoses. Heat added to the urethane while it is curing will accelerate the cure, and will do so without affecting the “open” time.

It is also important to recognize that the reaction of polyurethane is exothermic; that is to say it generates heat as it cures. More heat is generated by thicker sections or greater masses of polyurethane than is generated by thin pours of polyurethane. This effect is self-perpetuating. The heat generated by exothermic reaction causes an increase in the rate of reaction, which increases the exotherm, and on and on until the reactants are consumed.

It is generally preferable in an assembly line to be able to add heat to the product while the urethane is curing so that parts of different sizes (containing different masses of polyurethane) will cure in the proper time frame. Temperatures of 110° - 150° F are generally recommended for post curing polyurethane; these temperatures being selected because they are enough to accelerate the reaction without being dangerous or uncomfortable.

The mix ratio is critical. The ratio prescribed provides for all the polyol ingredients to be chemically reacted with all the isocyanate ingredients. If an excess of polyol is placed into the mix, it will consume all of the isocyanate intended, and then unreacted polyol will remain floating in the urethane elastomer. As this polyol is reactive only with isocyanate, it will remain for all time as an unreacted component and will result in an elastomer that is lower durometer. If an excess of isocyanate is placed into the mix, it will consume all of the polyol, and the unreacted excess will remain floating in the urethane elastomer. Unlike the polyol, the isocyanate is reactive with other substances and it will react with the reacted urethane elastomer and with water that avails itself in the form of humidity. So while the cured elastomer that contains an excess of isocyanate will at first be low durometer due to the unreacted isocyanate floating throughout the matrix, the elastomer will eventually come to its full durometer (and above) when the isocyanate eventually reacts with the air-born water (or water present in the media) and with the elastomer itself.

It should be noted that product mixed with an excess of isocyanate will tend to foam or blister more and will be more brittle. Product mixed with an excess of polyol will have less chemical resistance and lessened adhesion.

A consequence of being off ratio in either direction is a reduced gel and cure rate. Any excess chemical that is present, but not contributing to the reaction, simply dilutes the reaction.

While the isocyanate is a mixture of very compatible liquids, the polyol component is a mixture of liquids and solids that tend to separate, much like a can of paint settles. The polyol component contains polyols, which are chemically reactive with isocyanate, and also contains pigments and other chemicals that are not reactive with the isocyanate. They are inert. The specified mix ratio is based upon the assumption that the polyol component is well mixed and a very specific quantity of reactive polyol is being brought into contact with a very specific quantity of isocyanate.

The inert pigments and other unreactive components are heavier than the polyols and are the ones that settle to the bottom of the drum. In the event that an unmixed drum is utilized, the materials that are pulled from the top of the drum are going to be rich in polyol and, at the proper mix ratio; the mix will contain an excess of polyol. When the bottom portion of the drum is tapped, the material is rich in pigments and other inerts, thereby yielding a base/iso mix that will be rich in isocyanate.

For the isocyanate and the polyol to be introduced to each other in precisely the proper reactive ratios, the polyol component must be homogenous.

In addition to making sure the individual components are homogenous and that they are brought together in the specified ratio, it is imperative that they be well mixed. The mixing head, whether it be a static mixing tube or a dynamic mixing head, must be capable of blending the components such that there are no swirls.

It is equally imperative that the metering pumps deliver the components into the mixing head at the same time, preventing the occurrence of a lead lag situation. Any component brought to the mixing head alone will end up as an unreacted puddle in the finished part, and will manifest itself potentially in a variety of process and product problems.

Water reacts with isocyanate and generates carbon dioxide gas which causes foaming. Water can be introduced into the system through contact of the polyol components to atmospheric humidity and through the introduction of wet media into the mixed polyurethane.Our Elastomer poly blends are packaged into drums only after they have been properly treated to remove all water. The drums are covered with dry nitrogen before they are sealed as a protection against moisture contamination.To protect the drums after they are unsealed, a Drierite® desiccant tube should be inserted into the vent bung or the drum should be covered with dry air or nitrogen. Dessicant tubes are available from the W.A. Hammond Drierite Co.Anything added to the polyurethane as part of the manufacturing process (such as filter media) should be dry.