1. Introduction

2. What is castable polyurethane ?

3. What are the advantages of castable polyurethanes versus other materials ?

4. Are there any drawbacks to castable polyurethanes ?

5. How do I know which castable polyurethane to use for my application ?

6. Can I cast my own polyurethanes products ?

7. If I don't cast my own polyurethane products, how do I choose a good processor ?

8. References

1. Introduction

The information provided within this document was designed to serve two purposes. One, to provide design engineers with a basic outline of this truly versatile engineering material. Second, to satisfy the many questions I have been asked over the years about polyurethanes by the general public. This text will not discuss all types of polyurethanes such as; foams, sealants coatings or injection-molded. I will only cover the portion of the industry that deals with solid cast polyurethane products. The majority of these products are never seen by general public. This is because they are used mostly in industrial type applications. Please remember that this article was not written as a technical manual. Although, it could turn into one.

Castable polyurethane ideas and processes vary from one manufacturer to the other. That does not mean that any are wrong or right. This simply means that the goal of each manufacturer should be to provide the highest quality product attainable. The information contained within this document is based solely upon the hands-on experience of a single individual within the polyurethane industry. Statements and processes found in this and later versions of this article are based on the authors' hands-on experience and cannot be responsible for ideas and processes used by a variety of manufacturers for which are unknown by the author.

Any ideas or processes which fall into this particular category of materials is welcome for inclusion within this FAQ. Please feel free to send me information that you would like to see included. It would be very helpful if you would use a standard text editor format so that I may easily cut and paste the information you send into the body of the faq.

If you would like to see more information on a topic then post a request to the newsgroup. Someone will probably come forward with some type information that will helpful to you.

YOUR INPUT IS NEEDED.

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2. What is castable polyurethane ?
"Plastic rubber". Castable polyurethanes take on both of these characteristics. At one end polyurethanes can be processed to form a compound as soft as bubble-gum. At the other end they can be as hard as a soft metal. The wide range of hardnesses make them an extremely versatile material unlike any other. This makes useful in a variety of applications. The typical rubber compound cannot surpass the hardness range of castable urethanes without becoming brittle. The typical plastic compound cannot give you the elasticity without tears or breaks.

Technically speaking; the base materials used to form polyurethane compounds are a by-product of the oil refining process. The end product cast by the processor conists of two basic ingredients; a "prepolymer" and "curative".

2a. PREPOLYEMRS
These "prepolymers" are prepared by reacting an isocyanate and a polyol of various types. Practically all commercial grade prepolymers available are based on two different isocyanates; TDI (toluenediisocyanate) and MDI (methylenebisdiphenyl diisocyanate). Both of these isocyanates offer different properties to the prepolymer and requires varying types of processing systems. The other reactant within the prepolymer; the polyol, is available in three basic types: PTMEG (polytetramethylene ether glycol), PPG (polypropylene ether glycol) and polyester. There are other isocyanates and polyols that may be used to manufacture prepolymers. We will touch on these in a later version of this FAQ. This does give the castable polyurethane processor six basic types of prepolymers to manufacture with.

2b. CURATIVES
The prepolymer is then blended with a diamine, diol or triol "curative". As with the prepolymers, the type of curative used can effect the overall physical properties of the final product. The most commonly used diamine curative is methylene-bis-orthochloroaniline (MOCA); primarily for use with TDI based prepolymers. The main diol curatives, 1,4 Butanediol (BDO) and Hydroquinone Di-(beta-hydroxyethel)-Ether (HQEE) are primarily used with MDI based prepolymers. Triols can be used in combination with diols in MDI systems or in TDI systems to offer a wide variety of physical properties.

2c. ADDITIVES
Additives can also be incorporated into a compound by the processor. The most commomnly used additive of course is a pigment. Other additives could include plasticizers, fillers, UV and hydorlysis stabilizers.

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3. What are the advantages of castable polyurethanes versus other materials ?
Typically, castable polyurethanes are used for their high performance properties such as; abrasion resistance, tear resistance and load bearing capabilities. These extraordinary properties provide a cost effective alternative for many industrial applications by lessening the down time in process operations. Tooling costs are also a major factor. Since polyurethanes are a cast material, tooling can be quite inexpensive compared to the massive structures needed for rubber compression or plastic injection molding.

3a. ADVANTAGES VS. RUBBER
The chief advantage of castable polyurethanes over rubber is abrasion resistance. The combination of a high tear resistance and a high modulus gives polyurethanes this advantage. Other advantages would include ozone resistance and a higher duromemter range.

Castable polyurethanes can also be pigmented with a variety of colors. This gives the end user the advantage of color coding or possibly matching the companies logo.

One of the nicest advantages of polyurethanes over rubber from the processing end is the fact that polyurethanes can be poured instead of compression molded. This eliminates the need for heavy equipment such as presses and large steel molds.

3b. ADVANTAGES VS. METAL
Mosts metals are much heavier than polyurethanes. This makes urethanes much easier to handle and cheaper to operate equipment. Noise levels are minimized since polyurethanes have a tendency to absorb it.

Polyurethanes can outwear metals in a lot of places, especially where parts are subject to abrasive materials. Their resistance to corrosion makes them an ideal replacement for metals as well.

Probably the biggest advantage polyurethanes have over metals is that they can be manufactured much cheaper. Casting metal parts is quite a bit more expensive than casting polyurethanes. Welding and machining of metal parts are time also time consuming and expensive.

3c. ADVANTAGES VS. PLASTIC
As with the previous materials mentioned the chief advantage of polyurethanes over plastics is abrasion resistance. This is due to the fact that polyurethanes have the elastomeric properties that plastics lack. This leads to another advantage of urethanes over plastics; elastomeric memory. Polyurethanes will, in most cases, return to their original shape when they have been stretched. This brings us to another advantage. Since polyurethanes are elastomeric they are brittle not plastics. Even in the higher durometer ranges polyurethanes remain elastomeric.

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4. Are there any drawbacks to castable polyurethanes ?

4a. HEAT
As with any engineering material polyurethanes do have their drawbacks. The biggest problem for urethanes is heat. Polyurethanes, I have found through experience, can operate at a continuous temperature of about 160 degrees F. and intermittently at about 180 degrees F.

4b. MOISTURE
Another drawback is moisture. Polyurethanes can operate for years in the presence of moisture but, when the combination of moisture and heat occur the length of operation time is reduced dramatically.

4c. CHEMICALS
Certain chemicals can limit the use of polyurethanes as well. Strong acids and bases, aromatic solvents and some esters can have a detrimental effect on polyurethanes.

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5. How do I know which castable polyurethane to use for my application?

The most important consideration when choosing polyurethanes for a particular application would be, of course, the physical properties. There are a wide variety of prepolymer and curative systems available. Each with their own set of physical properties. Choosing a casting system can be a difficult job. Typically, that is why most processors will choose more than one material type to test for a new application. We will discuss some of the more general characteristics of these properties below.

5a. ESTERS VS. ETHERS
Esters, by in large, are best suited for applications that require a higher tensile or tear strength. Although tensile strength is usually not a determining factor when choosing a urethane compound it is good to note that they excel in this area. Ester compounds are the choice for resistance to oil and heat-aging. Where abrasion resistance is involved esters have a tendency to wear better in applications that involve sliding abrasion.

Ethers, on the other hand, perform better where impingement abrasion is a factor. Ethers are the choice for applications where heat buildup is a consideration. Since the ethers have a higher resilience this gives them the capability of not taking on as much heat in dynamic applications. Finally, ethers fair much better in moist enviroments than esters.

5b. TDI VS. MDI
Both TDI and MDI compounds have similarities to them. It is useful to note, however, that MDI Ethers can perform much better in applications that demand a high resilience, hydrolysis resistance and low temperature properties. TDI compounds do withstand the higher temperatures better and have the best compression set properties.

The second factor to note when choosing a polyurethane compound is the ease of processing. TDI systems are the easiest of both systems to process. Perhaps 80% or more the processors use the TDI system for this reason.

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6. Can I cast my own polyurethane products?

Yes, you can cast your own polyurethane products. There are many things to consider before taking on such a task. Most people think that it's easy. You mix Part A with Part B, throw in a little pigment and pour it in the mold. That is not the case.

All things considered, polyurethane processing is as much an "art" as it is a process. Proper material and mold temperatures, degassing of the raw materials, heating-aging of raw materials and mixing are just a few the items needed to control before attempting to manufacture a part with optimum physical properties. We will touch on some basic processing techniques in later versions of this FAQ.

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7. If I don't cast my own polyurethane products, how do I chose a good processor ?

There is probably a castable polyurethane processor in every major metropolitan area of the world. All of these with a varied degree of expertise and capability. Your best choice is a processor with some years of hands-on type experience. A product seen during the course of the daily production routine is a good sign that the processor knows what he is doing. Not a product that has been chosen as a "display item". If time permits, you can also obtain later versions of this FAQ. They will become more technical as time passes. Any additional information can be obtained from the author of this FAQ at the addresses listed in this document.

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8. References

What Polyurethane? Where? Selecting the Right Polyurethane for Various Applications
by Dr. Ronald W. Fuest, Uniroyal Chemical Company

Engineering Properties of Castable Polyurethane Elastomers
Air Products and Chemicals, Inc.

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If you have any suggestions, queries or corrections please email me at: danwebb@elastech.com with comments and corrections. and I will do my best to answer it. I hope this document is helpful to new people and experienced people alike and answers some queries that they may have.

Copyright Information

This document is Copyright © 1995-2005 by Daniel Webb, Sr. Any redistribution for profit, or in altered content/format is prohibited without the permission of the author. Any other distribution must include this copyright notice. Comments can be e-mailed to danwebb@elastech.com. Permission is given for this document to be uploaded to any anonymous FTP servers.

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