Polyurethane Technical Data and Design Guide
Polyurethane Technical Data and Design Guide Table of Contents
Polyurethanes are one type of a large family of elastic polymers called rubber. Unlike conventional rubber, polyurethanes start out as liquids and can be cast in low-pressure molds. Here at Gallagher, we also process thermoplastic urethanes that lend themselves well to injection molding.
What polyurethane should you be using?
The polyurethanes processed at Gallagher are typically two-component systems: a prepolymer and a curative. We also process compounds with three and sometimes four components to achieve specific properties. For our thermoset compounds, the prepolymers and the curatives are held in separate tanks under vacuum in our meter mix machines. When it’s time to make a part, the prepolymer and curative are mixed and dispensed into a mold. In the mold, a chemical reaction takes place which forms the solid elastomer. The thermoplastic urethanes we use for injection molding are different from the thermoset polyurethanes and are fully reacted prior to our processing operation.
The prepolymers used for urethane processing consist of isocyanate groups that react with either hydroxyl groups or amine groups in the curative. The resultant moiety from the isocyanate-hydroxyl reaction is called a urethane linkage; isocyanate-amine reaction forms what are called a urea linkage. As more and more isocyanate groups in the prepolymer link up with hydroxyl or amine groups in the curative, many urethane linkages are formed and this is where the term polyurethane comes from.
There are many combinations of prepolymers and curatives available which create unique properties in the cured urethane elastomer. Prepolymers are distinguished by the type of isocyanate and the type of polyol that is used to make them. The type of isocyanate used in the prepolymer is the main building block of the polyurethane compound. Here at Gallagher we typically use four different types of isocyanate: MDI, TDI, TODI, and PPDI. Other types are available and we can offer those for specialized applications. The major groups of polyol consist of Polyester, PTMEG Polyether, and Polycaprolactone, each type will produce an elastomer with unique properties.
What Are Polyurethane Elastomers?
Polyurethane elastomers (urethane elastomers) are one type of a large family of elastic polymers called rubber.
There are 14 types of rubber in general use. All of these polyurethane elastomers have been commercially successful, but they are all different in several ways. The chart below provides a quick initial screening guide of these polyurethane elastomers.
- Toluene Diisocyanate (TDI)
- Diphenylmethane Diisocyanate (MDI)
- Paraphenylene Diisocyanate (PPDI)
- Toluidine Diisocyanate (TODI)
- 1,5-Naphthylene Diisocyanate (NDI)
- PPG Polyether
- PTMEG Polyether
- 1,4-Butainediol (BD)
- Ethacure 300
While Gallagher Corporation has the technical expertise and equipment to process any of these materials, the combinations most typical to Gallagher include:
- TDI Polyester with hardness ranging from 70A to 75D
- MDI Polyester with hardness ranging from 85A to 95A
- TDI PTMEG Polyether with hardness ranging from 90A to 75D
- MDI PTMEG Polyether with hardness ranging from 60A to 95A
- TDI Polycaprolactone with hardness ranging from 60A to 55D
- NDI Polyester (Vulkollan®) with hardness ranging from 65A to 60D
TDI Polyester materials cured with MOCA produce tough elastomers that have outstanding abrasion resistance and excellent oil and aliphatic solvent resistance.
MDI Polyester materials cured with 1,4-Butainediol produce FDA wet and dry food grade urethanes which are tough, abrasion and tear resistant.
TDI PTMEG Polyether materials cured with MOCA have excellent low temperature flexibility, excellent dynamic properties are resistant to microbial attack and hydrolysis.
MDI PTMEG Polyether materials cured with 1,4-Butainediol have high resilience (even at low temperatures), great dynamic properties, outstanding hydrolysis resistance and improved resistance to impingement type abrasion. Some MDI Ethers can be formulated to be FDA wet and dry food approved.
TDI Polycaprolactone materials cured with MOCA have better hydrolysis resistance than other polyesters, while maintaining great resistance to abrasion. These materials also have great tear strength even at low hardness.
NDI Polyester cured with 1,4-Butanediol is trademarked with the name Vulkollan. Vulkollan is an ultra-high performance material that has the highest mechanical load bearing ability and is the best choice in highly dynamic applications.
What is an engineering material?
Countless numbers of engineering textbooks are filled with pages and pages of chart after chart listing the properties of engineering materials. Typically, the materials are classified as metals or non-metals. The verbiage used to describe each classification can take up several paragraphs, pages or chapters depending on how long the author wishes to think that he or she can hold the reader’s interest; “Metals are polycrystalline bodies which…” so on and so worth Some textbooks even break down the definition further to use three or four classifications to describe engineering materials including metals, ceramics, plastics, composites and more.
What does it really mean when we refer to an engineering material? The word Engineering, as defined by Merriam-Webster is as follows: “the design and manufacture of complex products.” One would think that by this definition, an engineering material would be one that’s an open book for engineers and scientists to write in their own properties based on the complex problems they’re trying to solve. This begs the question; do materials like this actually exist in today’s world? Certainly, throughout history, scientists, engineers, and metallurgists have worked hard to develop new materials with properties to solve very specific sets of problems. However, the time and expense necessary for this type of research and development work don’t justify their use for every new product that hits the market. What would it mean if engineers could design products and then customize a material to fit the needs of their application?
Enter polyurethane elastomers; the ultimate engineering material. What is a polyurethane elastomer? It’s certainly not a metal or a ceramic, but it’s also not exactly a plastic or composite. To be specific, urethanes fall into the category of materials called rubber, but they’re not entirely rubber either. Urethanes fill the gap between rubber and plastic meaning they’re harder than rubber (which contributes to their outstanding load bearing abilities), yet they’re softer and much more pliable than plastics (which accounts for their outstanding impact resistance and all around toughness). The material properties of urethane are best when the elastomer is formulated to fall in the hardness range from about 60 Shore A to 70 Shore D. While other hardnesses can be achieved, certain material properties will suffer. Elastomers that are formulated to fit this sweet spot can be expected to survive seemingly very large strains. Under certain circumstances urethane can withstand strains as high as 50% with little to no permanent plastic deformation. This trait alone is unheard of with most of the materials available to a part designer. One would never expect a metal component to undergo 50% strain and elastically recover to its original shape.
As already mentioned, polyurethane is an extremely tough material. Exhibiting outstanding cut and tear resistance, this trait correlates with many useful properties. One of the most advantageous characteristics of the material is a remarkable resistance to abrasion. In many cases, polyurethane elastomers will outlast: metal, plastic and rubber in highly abrasive environments. This characteristic alone has persuaded many OEMs to convert legacy components made from metals, plastics and rubber into polyurethane to increase the performance of their equipment offering. Furthermore, many aftermarket providers to the agriculture, construction, and automotive industries have built niche businesses by offering polyurethane components that directly replace other materials for the benefit of enhanced performance and longevity.
Another useful property and one that can easily be tweaked by the urethane molder is an elastomer’s resilience. Resilience as defined by Merriam-Webster is: “the ability of something to return to its original shape after it has been pulled, stretched, pressed, bent, etc.” A good way to visualize what’s happening when a urethane elastomer is strained is to think about the shock absorber and spring that can be found on an automobile. The spring is used to store energy when it’s compressed and then return the energy back to the system. The shock absorber absorbs energy such that the springs don’t have the passengers bouncing around uncontrollably as the car travels down the road. The spring and shock absorber work together in what provides the comfortable ride common to today’s cars. A polyurethane elastomer behaves the same way as the automobile shock absorber and spring. When it’s compressed or stretched a certain amount of energy is stored much like a steel coil spring, and a certain amount of energy is absorbed, like a shock absorber. A urethane processor has the power to adjust the spring to shock absorber tendencies of the urethane part through chemistry. For example, if a customer is looking for a highly resilient material that will absorb very little energy, the molder can provide a part that will return up to 80% of the energy applied to it that causes the strain. If a customer is looking for a very low resilience material for a part designed to absorb energy, such as a vibration isolation pad, it can be achieved simply by adjusting the chemistry of the urethane.
Another very unique and useful characteristic of urethane elastomers has nothing to do with its material properties, but instead has everything to do with how it’s processed. In the raw state, urethanes are liquid. They’re produced when a liquid Prepolymer is mixed with a liquid curative and left to cure or solidify. The best properties are obtainable when the curing process takes place in the presence of heat to help drive the chemical reaction that forms the solid end product. The fact that the material is liquid in the raw state is extremely beneficial for several reasons. The first reason that a liquid raw material is so beneficial goes back to the fact that this characteristic makes customization not only possible, but quite easy. By simply altering the ratio of Prepolymer to curative or substituting one curative for another, the final product can be tailored to meet the requirements of a specific application. For example, if a customer needs a urethane elastomer with very low compression set, the molder can reduce the amount of curative to achieve these desired traits. In contrast, if a customer requires a material with great cut and tear resistance, the molder can increase the ratio of curative that is used. A second and equally important characteristic of the liquid raw material has to do with the molds. The fact that the material is liquid and can be poured into molds at atmospheric pressure, greatly reduces the complexity and as a result, the cost of tooling. Furthermore, since the material is liquid it will fill out thick and thin cross-sections in a mold quite easily. In comparison to plastic injection molding, the part designer can be much more liberal in their designs with less attention paid to keeping a uniform cross-section thickness throughout the part.
Lastly, another processing advantage when working with molded polyurethane elastomers is perhaps the most important. That is the ability to bond urethane to other materials with a bond strength that will exceed the ultimate tensile strength of the urethane itself. The most common material to be bonded to urethanes is metal. A bonded metal-urethane composite offers a great advantage to part designers because they can utilize the best of both materials to solve a complex set of problems. For example, a bonded metal-urethane engine mount utilizes the rigidity of the metal to securely attach the mount to other metal components such as the engine and chassis. Yet it also utilizes the compliance of the urethane to allow slight independent movement which can compensate for opposing forces and strains within the vehicle.
Continuous Advancements in Cast Custom Polyurethane Expertise and Equipment
Bring us even the trickiest requirements of your most important components, and time after time, we turn them into success at our 100,000-square-foot, ISO-certified facility in Gurnee, Ill. Meanwhile, as your demands keep getting tougher, we’re always reinvesting in our business, constantly enhancing our expertise and continuously improving our operations with new machinery, new processes, and new materials to meet your evolving needs. It’s all part of a culture of commitment to your needs that you’ll find only when you partner with Gallagher as your chosen polyurethane manufacturer . . . where you can be confident that you’ll always have your toughest demands – delivered.
Ask about our Gallagher Quality Guarantee!
- Great products — On time
- Guaranteed to meet your specifications
- Performance will vary based on formulation and application.
- Contact us to find out what guarantee we can provide for your specific, custom product or application.