When specifying gasket material, along with factors like strength, working temperature range and chemical resistance, it’s important to consider hardness. Hardness determines how well a material fits against uneven surfaces, with softer materials performing better.

The hardness of rubbers and other elastomeric materials is specified primarily in terms of durometer or Shore number. Here’s an introduction to this rubber hardness scale.

Measurement by Indentation

Hardness is generally measured by pressing a point into a sample of material. Measuring the size, depth, or both, of the resulting indentation indicates the hardness.

For materials softer than metals, hardness is measured with durometer. This uses a calibrated spring to push a conical foot into the material. The foot penetrates further into softer material with depth inversely proportional to hardness.

A procedure for durometer testing is given in ASTM D2250. This standard covers factors like test duration, material temperature, material thickness, and minimum distance of the indenter from an edge.

Rubber Hardness Scales

The readout from a durometer is a dimensionless number on a scale from 0 to 100. A 0 shows the indenter went through the material while a reading of 100 means it left no mark at all.

As the hardness of rubbers and other elastomers covers a wide range – think latex gloves to golf balls – hardness values are reported using one of three scales. Developed by Albert Shore in the 1920s, these scales are identified as 00, A and D.

Originally there were other scales, but these are the only ones used today, and only Shore A and D are relevant for gasket material. The scales overlap so for example, the hardness of a particular rubber could be either 75 Shore A or 50 Shore D.

Specifying Rubber Gasket Material Hardness

Softer rubbers and elastomeric materials are preferred for sealing applications, with Shore A the rubber hardness scale used most often. Common gasket materials like NBR, EPDM and silicone are produced in a wide range of hardnesses: most are available from 30 to 80 Shore A. If you need assistance with hardness specification, Hennig Gasket and Seals can help.

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 effects sealing performance. The problem for people buying gasket material is knowing how low temperatures can go and which low temperature elastomer is best.

Global Low Temperature

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.

If gasket material isn’t chosen to suit the fluid being sealed, problems are almost inevitable. One reason is that some fluids will make the gasket grow thicker. This is an effect called swell. It increases bolt loads and can lead to material extruding out of the joint. Almost every gasket material has a fluid that will make it swell to some degree.

Bad Combinations

To give one example, an Acrylonitrile Butadiene Rubber (NBR) gasket swells significantly when exposed to acetone or methyl ethyl ketone yet shows almost no growth in the presence of vegetable or mineral oils. Hydrocarbons and petroleum products are a particular problem because they will cause swelling in several widely used gasket materials. EPDM, Styrene Butadiene Rubber (SBR) and Neoprene gaskets are prime examples and should not be used to seal these fluids.

Information on susceptibility to oil swelling is given in the ASTM D2000 classifications for elastomeric materials. This was addressed in “Buna-N (Nitrile) Gaskets and Oil” and “ASTM and Gaskets.”  Rubber gasket material sheet properties are essential to know.

Some Exceptions Apply

There are times when a gasket installer might use swell to his advantage. This would be when it’s difficult to get the required level of compression. To give two examples:

• Thinner flanges not meeting ASME/ANSI standards may distort as bolts are torqued, resulting in a variable gap.
• Bolts may lack the thickness or strength to take the necessary loads.

Faced with these problems, choosing a gasket material prone to swelling can be the solution. When exposed to fluid in the pipe the inner region of the gasket will swell, increasing the loading achieved.

Buy the Right Gasket Material!

Harnessing the swell effect doesn’t just mean deliberating selecting the wrong material. This would swell unpredictably, possibly with catastrophic results. However, some gasket material manufacturers produce so-called “controlled swell” material. Often employing Styrene Butadiene Rubber (SBR) binders, these provide predictable growth. (“Controlled swell” material is available for fluids other than hydrocarbons, even water!)

Ask About the Material

If you have a hard-to-seal joint “controlled swell” gasket material might be worth considering. Discuss what’s available with the material specialists at Hennig Gasket & Seals.

Specifications for rubber or elastomeric gasket materials often reference an ASTM classification. For example, silicone gasket sheet material might be shown as “ISO/ASTM Designation FE” while material for a nitrile gasket could be BF. These references come from ASTM D2000, one of many standards addressing gasket design, gasket material and gasket classification. Buyers don’t have to know these standards, but understanding what they address helps when selecting material.

ASTM and their gasket standards

ASTM International develops voluntary consensus standards. These help manufacturers and buyers alike by standardizing aspects of design, testing and manufacture.

For gasket materials the first two standards to be aware of are F104 and D2000. F104 is a system for classifying non-metallic gasket materials. The idea is to simplify material and gasket selection by translating application needs into a six digit code. F104 covers asbestos, cork, cellulose, PTFE, graphite and other non-asbestos materials. Rubber and rubber-like materials are excluded from this system and come under D2000 instead.

Material properties like compressibility and tensile strength are covered under a range of other standards. For example, D2240 addresses testing of rubber hardness, (durometer,) while F36 describes compressibility and recovery and F37 covers sealability test methods.

Interpreting ASTM classifications

The D2000 standard does the same for vulcanized rubber as F104 does for non-metallic gasket materials, namely, it sets out a standard way of describing every type of material. A complete D2000 specification covers maximum temperature, swelling performance, hardness and tensile strength, plus optional characteristics such as fuel and water resistance.

Maximum temperature is defined as the temperature at which material performs degrades to a set level. This is indicated by letter where “A” means a maximum of 70°C and K is 300°C. Swelling performance is also shown by letter with B the highest.

These two letters are used to describe many rubber-like materials. A “FE” designation for silicone gasket material shows that it’s performance degrades only slightly at 200°C but under defined conditions it will swell by 60%. Likewise, a nitrile gasket designated “BF” has the same swell behavior but is only good to 100°C.

The hardness of elastomeric gasket materials is measured with a durometer. Knowing how this device is used helps in interpreting specifications and selecting gasket material.

Durometer Construction

Durometers come in two forms, analog and digital. Analog durometers look like the traditional stopwatch with a single hand that sweeps around the dial. This dial is mounted on a flat foot, from which protrudes a pin. The pin is spring-loaded, so when the foot is pressed against the gasket material the pin moves up into the body of the durometer. The harder the material, the more the pin moves into the body. Or to put it another way, softer materials let the pin press in deeper.

The dial is marked from zero to 100. These numbers have no units but are related to the spring load and the size and shape of the head of the pin, more properly called the ‘indenter.’

Shore Hardness

Spring strength and indenter geometry are specified in ASTM standard D2240. This fixes every aspect of rubber hardness testing, including the size of the ‘presser foot’, sample preparation, the duration for which the indentor is pressed into the material, and calculation and presentation of results.

Rubber and rubber-like materials can vary enormously in hardness, so ASTM D2240 defines a number of different scales. Each scale has its own indenter form and spring load. Gasket materials are typically measured on the Shore A scale. The ‘A’ indenter is a pin of 1.27mm (0.050”) diameter, tapered at 35 degrees to finish as a truncated cone with a flat area of 0.79mm (0.031”) diameter. At a reading of 100 (no indentation,) the spring force will be 8.05 Newtons.

Determining the Hardness Number

According to ASTM D2240, the test specimen should be at least 6.0mm (0.24”) thick. Hardness is calculated as the mean or median of five measurements taken at least 12.0mm (0.48”) from any edge.

A Comparative Measure

Being dimensionless, the Shore A number tells you little about the properties of an individual material. Its real value is as a standardized test method, allowing comparison of alternative materials for elastomeric gaskets.

The question of how hard a gasket should be comes up quite often. For an answer we need to look at what the gasket actually does.

Gasket function

The job of every gasket is to fill an uneven gap between two surfaces, forming a barrier that stops fluid moving to where it shouldn’t be. Larger gaps and more uneven surfaces need a softer gasket. For example, a gasket between two parallel machined pipe flanges can be hard, resisting loads as the joint faces are tightened together. In contrast, the gasket sealing an electrical enclosure needs to be softer and compress more because the enclosure door will tend to bend as it’s latched.

So a general rule is that a gasket should be as soft as possible in order to fill the gap between two surfaces. At the same time it must be strong enough to resist the lateral forces acting on it.

For elastomeric gasket materials two parameters define hardness: Shore hardness and compression force deflection (CFD.) Here’s what these two terms mean.

Shore hardness

Hardness in this context is a measure of how well a material resists a permanent indentation. The hardness of rubber and elastomeric materials is measured on a durometer and reported as a “Shore A” number. Very soft materials like a rubber band will be around 20, a pencil eraser is between 30 to 40, and car tires measure 60 to 70 Shore A.

Compression force deflection

CFD measures firmness and is defined in ASTM standard D1056 as the force needed to reduce the material in thickness by 25%. According to this standard materials are given a grade correlating to their firmness. Grade 0 material needs less than 2 psi to reduce its thickness by 25%, so is very soft. At the other end of the spectrum a grade 5 material needs at least 17 psi to achieve the same compression. A gasket material that compresses easily accommodates variation in the gap between two surfaces without needing more closing force than can be applied by the clamps or latches.