PxT stands for pressure times temperature. It’s a factor used more and more to characterize the performance of gasket material. The advantage is that it addresses how elastomers like silicone, EPDM and neoprene lose strength at higher temperatures.

An Inverse Relationship

Specifications for elastomers call out temperature limits. What’s rarely appreciated is that these values may not be good in all applications. Unlike metals where hardness changes very little as temperature rises, elastomers soften. For example, neoprene sheet material may have a quoted upper limit of 250°F but at this temperature it’s strength will be far lower than at say 68°F.

In many sealing applications pressure isn’t a concern. If the neoprene gasket only seals against warm air, that reduction in strength may not affect performance. However, if it’s sealing a pipe flange in a steam system reduced strength could be a problem. Even though the temperature might not exceed the upper limit, sustained pressure could extrude the material out through the flange.

Pressure AND Temperature

PxT shows how a gasket material performs under varying pressures and temperatures. Here are two examples.

Specs for a neoprene gasket material show the upper-temperature limit as 250°F and the maximum pressure as 250 PSI. However, the maximum PxT factor for the material is only 20,000 (°F x PSI). If used in an application with a continuous operating temperature of 250°F peak pressure is just 80 PSI. (20,000/250 = 80)

A sheet of EDPM gasket material has a maximum temperature of 300°F, a pressure limit of 250 PSI and a PxT factor of 30,000. In this case, the material manufacturer is saying that if sealing against media with a pressure of 200 PSI, the temperature should not exceed 150°F. (30,000/200 = 150)

If In Doubt …

Every gasket material has an upper-temperature limit. It may fail if taken beyond that, but it could also fail at a lower temperature if the pressure is high. The best approach is always to consult a material specialist like those at Hennig Gasket.

The terms sponge and foam are used interchangeably but they’re not the same. Buy foam for a gasket application and you may find the joint leaks. Use sponge material for cushioning and you may not get the protection you expected. Sponge and foam both have cellular structures, but there are important differences between the two.

Different Processes

Producing foam material is similar to baking bread. A chemical reaction in the liquid mixture creates carbon dioxide gas that leaves a very open structure.

Sponge materials also use a chemical reaction to create gas bubbles, but these remain self-contained cells, each one isolated from its neighbors. Some manufacturers can control the size and distribution of these pores to produce very consistent material with highly predictable behavior.

Open and Closed

The foam production process leaves an open structure more like a mesh than a solid. The stiffness or rigidity of this structure depends on the polymer used – typically that’s PVC, polyethylene or polyurethane.

In contrast, in a sponge the pores influence material behavior. Unlike a foam material, when sponge is compressed the gas in each pore has nowhere to go. That results in strong compression set resistance and good compression recovery.

Sealing Properties

Foam and sponge materials both have the advantage of low density but behave very differently when asked to act as a barrier. The open cellular structure of foam material lets fluid pass through readily, even when compressed. Sponge however blocks the movement of gases and liquids. That makes sponge material a good choice in HVAC sealing applications.

Cutting sponge will open up the edge pores. This allows a limited amount of liquid retention but the body of the material still acts as a barrier.

Materials for Sponge Gaskets

Practically all elastomers can be manufactured with a closed cellular structure. Consequently, neoprene, EPDM, nitrile and silicone gasket material are all available as sponges. As with their solid variants, these can be purchased with varying levels of temperature resistance and strength. Talk to a material specialist at Hennig Gasket if you need more advice.

Not all joints hold the gasket in place. Electrical cabinet doors, food mixers and HVAC access panels are situations that often lack mechanical gasket retention features. In such cases the gasket must be adhesively bonded to the frame, cover or lid. Adhesives can be messy and awkward to use, and that’s why it’s worth asking about gasket material with a coating of pressure sensitive adhesive (PSA.)

The advantages of “peel and stick”

PSA is bonded to the gasket material before shapes are cut out. On the reverse side, (away from the gasket material) there’s a release film which is peeled off before the gasket is installed.

The advantages of PSA-coated gaskets are:

• Easier to install.
• Stay in place when the joint is opened.
• Facilitates “kiss-cutting”. This is when the die cutter goes all the way through the gasket material but stops short of the PSA release film. Kiss-cutting holds the individual gaskets together in a sheet or roll, making them easier to handle.

Material compatibility

Most gasket materials will take a PSA coating. For example, both neoprene and silicone gasket material can be ordered with PSA. The major exception is PTFE gasket material as this is naturally non-stick.

PSA cautions

A PSA used on gasket material becomes part of the joint and experiences the same temperatures, pressures and chemicals as the gasket itself. Accordingly, the PSA must be matched to the application. Silicone gaskets are a good example. As these are used for both high and low temperatures the PSA must be chosen appropriately. In some cases the available PSA’s will limit the service conditions.

PSA selection is of particular importance when installing food grade or FDA gaskets. For these applications the adhesive must also be food grade.

Replacing a PSA gasket can be time-consuming. This is because the old adhesive must be completely removed from the joint faces before the new gasket goes on.

Make life easier

Depending on the gasket material and the nature of the application, a PSA can simplify gasket installation. If “peel and stick” gaskets would simplify your next sealing job, talk to Hennig.

When selecting gasket material it’s important to look at the media being sealed. Some gaskets will not work with some chemicals. One example is the group known as ketones. Many gasket materials are not recommended for sealing these. If the product you are sealing contains ketones it’s important to know which gasket materials will work. Unless you’re a chemist it may not be obvious when a fluid contains or incorporates ketones. Here we’ll explain briefly how to recognize ketones and discuss which gasket materials to consider.

Ketone recognition

To chemists ketones, and the closely-related aldehydes, are a family of organic compounds containing a carbon-oxygen grouping. Products composed of ketones can sometimes be recognized by their name. The solvent acetone for example, is a ketone, as indicated by the suffix, “one.” Likewise, the term, “aldehyde” appears in many chemicals, such as formaldehyde.

As organic compounds, ketones and aldehydes occur frequently in nature and are generally liquid at room temperature. Examples include extracts of cinnamon bark, vanilla bean, lemongrass and the coriander herb. In addition, many fragrances gain their distinctive odor from various aldehydes.

In short, many food preparation processes, especially in baked goods manufacture, expose gaskets to ketones, as do a range of perfume and solvent processes.

Gasket materials susceptible to ketone attack

A long list of commonly used gasket materials are attacked by ketones and aldehydes. This includes:

• Nitrile rubber/NBR/Buna-N
• Neoprene
• Hypalon® (chemically similar to neoprene.)
• Silicon
• Fluoro-silicones
• Viton and other fluor-elsatomers

Gasket materials with some ketone resistance

Three materials stand out:

• Natural rubber (rarely used due to its inconsistency and poor temperature properties.)
• SBR/styrene butadiene rubber (not resistant to methyl ethyl ketone)
• EPDM (not resistant to methyl ethyl ketone)

Consider the media

When selecting gasket material it’s essential to consider the media being sealed. Chemical incompatibility will lead to the breakdown of the gasket material and failure of the seal. Ketones, and their related organic compounds, aldehydes, present particular challenges because they occur widely and few materials offer good resistance. If ketone exposure is possible SBR and EPDM gasket materials should be your first choice.

Low temperatures play havoc with elastomeric gasket materials, as NASA will testify. (Details of how seal failure caused the Challenger disaster are available on the NASA website.) The issue is that at low temperatures gasket materials like nitrile rubber and neoprene become stiffer and less able to fill gaps. The TR10 number, derived from ASTM D1392, shows the temperature at which this stiffening affects sealing performance. The problem for problem for people buying gasket material is knowing how low temperatures can go.

Global lows

Military Standard MIL_HDBK_310_1851 “Global Climatic Data For Developing Military Products”, tells equipment developers what conditions to design for. This notes that the lowest temperature ever recorded is -68°C (-90°F), in the USSR. Statistically, the lowest temperature to be expected in the coldest regions of the world is -69°C (-92°F).

Low temperatures in the USA

Weather.com tells us the lowest temperature experienced in the U.S. is -62°C (-80°F), in Prospect Creek Alaska. Closer to home, the lowest in the contiguous 48 is -57°C (-70°F), measured at Rogers Pass, Montana. For those in the Midwest, record lows in Illinois, Indiana and Ohio are in the -34°C (-30°F) range, while Michigan, Wisconsin and Iowa have records ranging from -43°C to -48°C (-45°F to -55°F).

Of course, this is without the effects of wind chill. Air flowing over surfaces takes heat away, making the temperature appear lower than it actually is. And the harder the wind blows, the greater the cooling effect.

Implications for gasket material selection

When selecting gasket material for outdoor applications it’s essential to determine the lowest possible temperature. Not doing so risks leaks during periods of extreme cold, which is never a good time to be replacing a failed gasket!

Most nitrile rubber, EPDM and neoprene gasket materials work down to around -46°C (-50°F). HNBR is limited to around -40°C (-40°F) while silicone will endure temperatures down to around -59°C (-75°F). (values vary for individual material grades.)

If a gasket might be exposed to low temperatures material should be selected to suit. Specialists at Hennig Gasket will be happy to advise.

Before NBR, SBR and Neoprene there was natural rubber. The original elastomeric material for seals and gaskets, natural rubber literally grows on trees. Although supplanted by modern synthetic rubber materials it remains a good choice for some applications.

The origins of rubber

The South American Pará tree produces a milky sap known as latex. In chemical terms this is composed largely of the polymer polyisoprene which is both elastic and waterproof. In 1839 Charles Goodyear discovered that heating this product with sulfur, (the process we call vulcanization,) made it even more elastic. Within a few years it was being used in countless applications, including for sealing.

Synthetic rubber

By the early 1900’s rubber production was firmly established in places like India, Malaysia and Singapore. The two World Wars saw the supply of natural rubber disrupted, and scientists set to work on synthesizing artificial versions.

The result was Styrene Butadiene rubber or SBR. While this didn’t perform quite as well as natural rubber, for many applications it was good enough. Shortly afterwards came other synthetic rubbers aiming to address the weaknesses of SBR.

Natural rubber pros and cons

With ASTM D-2000 designations of AA and BA, natural rubber (NR) is still widely used as a sealing and gasketing material. It works over a temperature range of -60 to +220°F, which is a lower upper limit than most synthetic rubbers. However, it does have very good compression set resistance and excellent resilience. It’s also highly water-resistant, abrasion resistant, and tolerant to alcohols, acids and alkalis.

Where NR doesn’t perform well is in resistance to petroleum-based products. It is also susceptible to ozone attack, making it unsuitable for use around high voltage electrical equipment. Somewhat more expensive than SBR, like any natural product, it can exhibit some batch-to-batch variation.

When the application demands

When a seal needs water and excellent compression set resistance, a natural rubber gasket may be a good choice. To learn more about how this oldest of elastomeric sealing materials, contact a specialist at Hennig Gasket.

Elastomeric gaskets, like those made from nitrile rubber and neoprene, work because they are flexible. Compressed in a flange, the material fills the gap between joint faces, regardless of how each side moves. When a joint gets cold the gap often grows wider, and the gasket material is expected to recover enough to maintain a good seal.

When elastomers become very cold they lose this ability. When choosing gasket material for an application that could see low temperatures it’s important to check it has sufficient low temperature flexibility. This is indicated by their TR10 number.

Low-temperature Behavior

Nitrile rubber and neoprene gaskets are flexible because they are made from entangled chains of molecules that move over one another. When temperatures drop, contraction limits the movement of these molecules and the material gets stiffer. Eventually, there’s no room for any movement and the material becomes brittle.

The “freezing” temperature of an elastomer is termed it’s Glass Transition Temperature (Tg). Strictly speaking, freezing happens over a range, and the brittle point is several degrees lower than the Tg. What matters for gaskets though is when the material loses any ability to recover.

The Temperature of Retraction

ASTM D1392 advises that “..retraction rate is believed to correlate with low-temperature flexibility of … rubbers.” Retraction is measured by first stretching and then freezing an elastomer. It’s then gradually warmed up, and as the molecular chains regain some freedom to move it starts to shrink back.

This recovery is measured against temperature and indicates when the elastomer transitions into rubber-like behavior. Experience shows this happens when it recovers by around 10% of it’s elongated length. The point at which this happens is the temperature of 10% retraction or TR10.

Gasket Material for Low-temperature Applications

If an elastomeric gasket will get cold, two things are needed: the lowest anticipated temperature, and the TR10 value for the candidate materials. If TR10 is not available use Tg as a substitute. For reliable in-service performance, ensure these values are lower than the worst conditions the gasket will experience.