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?
Engineering textbooks contain hundreds of pages and charts. Many listing the properties of engineering materials. They are typically 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 . . . blah, blah, blah." Some go further in their classification of engineering materials. They include metals, ceramics, plastics, composites, and more.
But, what does it mean when we refer to an engineering material?
Merriam-Webster defines engineering as “the design and manufacture of complex products.”
By this definition, an engineering material would provide an open book for engineers and scientists. Based on the complex problems they are trying to solve, they could write their own properties.
This begs the question: "Do materials like this actually exist in today’s world?"
Throughout history, scientists, engineers, and metallurgists have worked hard. They've developed new materials with properties to solve specific sets of problems.
The time and expense necessary for this type of research and development are not justifiable for every product. But, what would it mean if engineers could design products and then customize a material to fit the needs of their application?
What is a polyurethane elastomer? It’s not a metal or a ceramic, but it’s also not exactly a plastic or composite. Urethanes fall into the category of materials called rubber. But they’re not quite rubber either.
Urethanes fill the gap between rubber and plastic. They’re harder than rubber which contributes to their outstanding
This accounts for their outstanding impact resistance and
The material properties of urethane are best when the elastomer formulation falls within the hardness range from about 60 Shore A to 70 Shore D.
Other hardnesses are achievable. But, certain material properties will suffer.
Elastomer formulations that fit this sweet spot can survive very large strains. Under certain circumstances, urethane can withstand strains as high as 50%. With little or no permanent plastic deformation. This trait alone is unheard of with most of the other materials available to a part designer. One would never expect a metal component to undergo 50% strain and recover to its original shape.
As already mentioned, polyurethane is a tough material. It has outstanding cut and
One of the most helpful characteristics of urethane is its resistance to abrasion. Polyurethane elastomers will outlast metal,
Another useful property within the control of the urethane molder is an elastomer’s resilience. Merriam-Webster defines resilience as: “the ability of something to return to its original shape after it has been pulled, stretched, pressed, bent, etc.”
A good way to visualize urethane resilience is to visualize the shock absorber and spring on an automobile.
The shock absorber absorbs energy. While a spring stores energy when it’s compressed and then returns the energy back to the system. The springs prevent the passengers from bouncing around as the car travels down the road. The spring and shock absorber work together to provide a comfortable ride.
A polyurethane elastomer behaves the same way as an automobile shock absorber and spring. When it’s compressed or stretched, a certain amount of energy is absorbed -- like a shock absorber. And a certain amount of energy is stored -- much like a steel coil spring. The spring to shock absorber tendencies of a urethane part is adjustable through chemistry.
For example, if the need is a resilient material that will absorb very little energy. The urethane molder has the ability to provide a part that will return up to 80% of the energy applied. But, if the need is for a low resilience material designed to absorb energy (such as a vibration isolation pad). The urethane molder can adjust the chemistry to meet that need with ease.
Another very unique and useful characteristic of urethane elastomers has to do with how it’s processed. In the raw state, urethanes are liquid. After mixing the liquid prepolymer with a liquid curative, the part is left to cure or solidify.
To get the best properties, the curing process takes place in the presence of heat. Heat helps drive the chemical reaction that forms the solid end product.
Starting with a liquid in the raw state has several benefits.
First, a liquid raw material makes customization not only possible but quite easy. You can adjust your final product to meet the requirements of a specific application. To do this, alter the ratio of prepolymer to curative or substitute one curative for another. For example, for a low compression set, the molder can reduce the amount of curative. In contrast, for better cut and tear resistance, the molder can increase the ratio of curative used.
Second, a liquid is pourable into molds at atmospheric pressure. This reduces the complexity and cost of tooling. As a liquid, the material will fill out thick and thin cross-sections in a mold. With polyurethane injection molding, the part designer can be more liberal in their designs. Requiring less attention to keeping a uniform cross-section thickness throughout the part.
The last processing benefit of starting with a liquid is the most important. The ability to bond urethane to other materials. With a bond strength that exceeds the strength of the urethane itself. Most often, this bond is between urethane and metal.
A bonded metal-urethane composite provides a great advantage to part designers. They can enjoy the best of both materials to solve complex problems.
For example, let's consider a bonded metal-urethane engine mount. It utilizes the rigidity of the metal to attach the mount to other metal components such as the engine and chassis. But, the compliance of the urethane allows slight independent movements. Compensating for opposing forces and strains within the vehicle.
Continuous Advancements in Cast Custom Polyurethane Expertise and Equipment
Bring Gallagher even the trickiest requirements of your most important components. Time after time, we turn them into success. Our 100,000-square-foot, ISO-certified facility in Gurnee, Illinois will meet your needs. We are always reinvesting in our people and equipment to meet your demands. New machinery, new processes, new materials, and more training to meet your requirements. It's part of Gallagher's culture of committment to you. Partner with Gallagher as your chosen polyurethane manufacturer . . . where you can be confident that you’ll always have your toughest demands – delivered.
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