Polyurethane / Urethane Chemistry & Processing Basics
Introduction of Urethane Basics
Cast polyurethane technology at its essence is a duality of a very simple concept and extremely complex chemistry.
The difference between the two views of this technology can make a very dramatic difference in the end result.
At its simplest, cast polyurethane technology consists of bringing together two or more reactive chemicals in a liquid state and mixing them together at the appropriate ratio. This action causes a chemical reaction to occur which results in the chemicals phasing or turning into a solid shape. This of course, is what allows cast urethane materials to take on almost any shape or form imaginable. Unfortunately, this simple conceptual approach to processing urethanes can cause consumers of the end product to be very unhappy with the results if their chosen processor does not understand the underlying complexities of the chemistries, materials and processes.
On the other hand, if the processor understands the complexities involved with all aspects of the materials and processes, the parts made with this technology can be used with great success across a wide spectrum of applications. It is ultimately the understanding of these complexities that makes urethanes one of the most versatile materials in the world.
For instance, in some applications polyurethane parts may be subjected to a continuous oscillating force and resulting displacements (a perfect scenario for fatigue failure). It is a fact that the flex fatigue life of certain urethane materials can be affected by a factor of up to 10,000 times with a plus or minus 10 % change in the ratio of the mixed components. Without a doubt this could spell disaster if the processors are not well equipped and do not have the knowledge or controls in place to control the process.
A broad knowledge of material chemistries and the process variables as stated above is key to the success of any part in any given application. If given the chance to work on your application, rest assured that C.U.E. Incorporated will take into account all of the complexities, and further, have the knowledge and state-of-the-art equipment necessary to control the important underlying variables associated with this technology.
Please keep this in mind as you read through our Urethane Basics Section. The blue highlighted words will automatically link you to a Glossary of Terms page. Please feel free to glance at this page if you would like a deeper explanation of any term or lingo.
The cast urethane technology at C.U.E. Inc. is based primarily on two component “thermosetting”, “prepolymer” chemistry. Hundreds, if not thousands, of urethane compounds can be derived from this chemistry and it is widely viewed as the chemistry of choice when materials having demanding properties are required. Below is just a brief explanation of the chemicals, processes and calculations used to manufacture these tough and durable cast polyurethanes.
The first step in prepolymer chemistry is to produce a precursor to the final product by reacting a polyol component with an excess of an isocyanate component. This results in what is known as an isocyanate-terminated prepolymer. The ratio of the reactive groups of the polyol and isocyanate are closely measured during this process and the resultant product has an excess of reactive isocyanate left over for processors like C.U.E. Inc. to complete the reaction. These reactive isocyanate groups that remain are measured as “% NCO”. This % NCO is used in “Stoichiometry” calculations by processors to determine the correct amount of curative needed to complete the reaction and produce high quality polyurethane materials. The specific type of polyol, isocyanate, and curative used, combined with the processors abilities, determine the end properties of the finished product. The most common polyols include “Polyether (PPG, PTMEG), Polyester, and Polycaprolacotne types. Isocyanate terminated prepolymers are commercially available in “Aromatic (TDI, MDI, PPDI)” or “Aliphatic” versions. Curative components are typically short-chain glycols or diamines such 1,4-butanediol and MOCA.
Prepolymers are typically manufactured in large reactors by major chemical companies like Bayer, Uniroyal, Air Products, Dow, and BASF among several others. Urethane processors like C.U.E. Inc. purchase these prepolymers and complete the chemical reactions using the proper amounts of “curatives”, “additives”, “flame retardants”, “plasticizers”, and often a “catalyst” to produce the end product polyurethane.
Once a formulation for a specific application has been determined the correct prepolymer is processed with the appropriate curative package. The curative package is typically determined by several factors including the end use of the material and the prepolymer that is chosen.
The term stoichiometry refers to the calculations used by processors like C.U.E. Inc. to manufacture polyurethanes. These calculations determine the correct amount of reactive components to be used in the manufacturing process. The ratio of the number of reactive groups remaining in the prepolymer to the number of reactive groups present in the curative is known as the “index”. In general, the number of reactive groups present in the two components must be nearly equal in order to produce an end-product which possesses acceptable physical and mechanical properties. Typically when reacting polyurethane components the index is maintained at just over 1.00 or slightly isocyanate rich. However, depending on the desired end results, the index of reactive parts may be run anywhere within a range of 85-130. Small changes in index can create wide variations in final properties.
There are many facets to the chemistry involved with processing urethanes. C.U.E. Inc. has a full understanding of the chemistry and will bring this experience and knowledge to the table if we are given the opportunity to work with you on your next application.
For more information about C.U.E.'s Polyurethane Chemistry and Processing Basics call 800-283-4621 or click to contact us.