Urethane Basics | Cast Urethane Chemistry - Processing
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Introduction – 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.
Urethane Chemistry
The cast urethane technology at CUE 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”, “plasticisers”, 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.
Processing
Cast urethane technology is a very specific technology where reactive liquid chemicals called “prepolymers” and “curatives” are brought together and mixed under specific conditions (mainly mix ratio and temperature). Once mixed, the chemicals start to react with each other and, usually within a few minutes, start to form polymer chains long enough to bring about a liquid-to-solid transition. The time it takes for this change to begin to occur is known as the gel time. It is within this “gel time” that processors have to dispense or “cast” the material into the molds that dictate the resultant shape of the end product.
The material then remains in the “mold” a varied amount of time (from 5 minutes to 5 hours depending on several parameters) after they have solidified. This is done to allow the parts to develop enough “green strength” in order to de-mold the parts without causing damage. The parts at this stage are typically very fragile and must be handled with care. The chemical reaction that initially led to the liquid-solid phase change is still occurring all throughout the part on a molecular level. It is the uninterrupted continuation of this reaction that will help dictate the final properties of the material.
At this point in the process the part is transferred directly into an oven that is pre-set to a temperature that allows the part to continue this reaction for several hours. This portion of the process is known as “post-curing”. This post-curing process is a crucial phase of the process that can affect the physical properties of the part (especially the properties that allow for excellent dynamic results). After this initial post-curing operation at elevated temperatures the parts have developed approximately 95% of their final physical properties. Continual physical and mechanical property improvements will be seen, perhaps over months of ambient aging, as the urethane reaction continues and full cure is achieved.
For more information on polyurethane materials and C.U.E.'s polyurethane data chart click here.
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Urethane Basics
Urethane Benefits
Urethane Glossary - Terms
C.U.E., Inc.
11 Leonberg Road
Cranberry Township, PA 16066
Phone: 724-772-5225
Fax: 724--772-5280
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