When you think of materials like rubber, plastic, and polyurethane, one of the unique properties that comes to mind is their ability to withstand compression. What you may not know is that polyurethane actually doesn’t compress at all- at least not in the way you might think. In today’s blog we’re dissecting the idea of polyurethane compression and setting the record straight on this common misnomer.
Compression Defined
To start, let’s define what compression really is. In elastomers, compression is a type of elastic deformation that results in a change in volume when pressure is applied to a part. Since it is an elastic form of deformation instead of a plastic one, the part will return to its original shape once the pressure is released. For example, if you have a soft eraser laying around, go ahead and press it down onto a hard surface like a desk. Notice how the eraser appears to get smaller when you apply pressure to it- then when you release that pressure, it goes back to looking more or less the same as before. You’ve just applied a compression force to the eraser; in turn, the eraser has undergone compression.
The Problem with Compressible Materials
If you were to keep applying and releasing that exact same force over and over again, the eraser would start to degrade with time. All rubber parts are the same in that sense. There’s a threshold to how much compression any given part can withstand before it can no longer return to its original shape. At that point, the part begins to undergo plastic deformation. What that means is that once that threshold is crossed, either through repeated use or by a singular high pressure impact in some cases, the part will never be the same again. This is why many rubber seals and rings in your car will fail with age. Over time, they become deformed as a result of repeated compression through regular use or sudden jolts. Eventually, they can no longer serve their purpose and must be replaced. In addition, compression can also be affected by other external stimuli like temperature, humidity, and chemical exposure. In extreme conditions, a part’s ability to withstand compression will vanish even faster as a result.
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Polyurethane: Not Your Average Elastomer
Like rubber, a majority of elastomers undergo compression when a compressive force is applied to their surface. There is one exception to this, though. At the beginning of this blog, we claimed that polyurethane does not compress at all- and it’s true.
If you’ve ever seen a polyurethane part under pressure, you may be scratching your head right about now. Polyurethane definitely gets squished under pressure, especially parts that are formulated with a softer durometer. So how can it be that you can apply polyurethane compression, but the material itself doesn’t compress? What seems to be polyurethane compression at a glance is actually a similar reaction to compressive force that holds one key difference. This is called deflection.
What Is Deflection?
Similar to compression, deflection is a form of elastic deformation that an object experiences when pressure is applied to it. The key difference is that items which undergo deflection do not experience any change in overall volume. So, rather than the volume of an object being compressed in response to a force, it will instead respond by displacing its mass to continue taking up the same amount of volume.
Think about it this way. Imagine that you have a soft sheet of rubber and a soft sheet of polyurethane. If you poke the rubber with your finger, you won’t notice any significant change to the surface of the sheet. However, if you were to poke the polyurethane sheet, there would be a noticeably raised area around your fingertip where the polyurethane will deflect in response to the pressure. When you remove your finger from the surface, the polyurethane sheet will go back to normal.
Since deflection is a response to a compressive force, it is commonly referred to as “polyurethane compression/deflection” or sometimes just “polyurethane compression”. When discussing the type of force applied to an item, it would be correct to call it “polyurethane compression”; but when talking about how the material responds to that force, “polyurethane deflection” would be the appropriate term. Now that we’ve gone over the difference between compression and deflection, let’s talk about why it matters.
The Advantage of Deflection
As we’ve already discussed, objects that undergo compression will eventually experience plastic deformation and fail as a result. While the same is true for polyurethane, the road to failure is often many lifetimes longer when compared to traditional rubber parts.
Throughout a polyurethane product’s life cycle, it can respond much more effectively to compressive forces in a way that promotes more effective performance over a longer period of time. This behavior makes urethane an excellent replacement for parts like rings, seals, bumpers, and couplings which may be subject to sudden or repeated impacts in daily use. Any application where an elastomeric part is meant to be load-bearing is an ideal use case for a polyurethane part.
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Key Considerations for Deflection
Deflection is a pretty sweet feature, but it is by no means perfect. Here are some considerations to make when you’re concerned about deflection in polyurethane parts.
When a compressive force is applied to a polyurethane part, it is going to hold onto the energy that it needs to eventually bounce back to its original shape when the pressure is released. In this case, polyurethane stores energy in the form of heat. Excess heat can have a negative impact on deflection, just as it does on compression in other elastomers, because of the way that heat inhibits elastic properties. When working with polyurethane parts in a load-bearing application, beware of heat-related issues that can potentially impact other properties.
Additionally, deflection can be affected by seemingly unrelated factors like the surface condition of the urethane part. Although it may seem irrelevant at first, the surface condition of a part has the potential to affect the shape factor of the item as it’s being compressed. If a part is bonded in place to a metal plate, the shape factor will be fixed. However, if the surfaces are not bonded but lubricated instead, the shape factor is much more likely to change as the item undergoes compression. When planning to use a polyurethane part in any application, it’s important to consider how it might deflect in relation to these factors along with the compressive force. If you want to take a deeper dive into the polyurethane compression/deflection formula, check out this page that explains each part of the equation more in-depth.
