Measure the Hardness of Rubber Gasket Material

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.

Is PTFE Safe?

Some PTFE material can be used for gaskets and seals in the food and medical equipment industries, and some cannot. Here’s why you might want to use PTFE, and the difference between material that is and is not FDA approved.

PTFE as a Sealing Material

Polytetrafluoroethylene, a.k.a. PTFE, also sold under the tradename Teflon®, has properties that make it an excellent choice for many sealing applications.

  • Extremely inert chemically – fluorine and nitric acid are its only vulnerabilities
  • Soft, so it conforms to uneven sealing surfaces
  • Remains plastic down to -400°F and is still usable at 500°F (-240 to 260°C)
  • Slippery surface assists fastener tightening

PTFE is sold in both sheet form and as expanded PTFE. Expanded, it’s softer and more compliant than the sheet version.

In high bolt load applications PTFE suffers from creep. This is reduced by adding fillers. Glass, carbon and graphite are the most commonly used. They increase strength without significantly impairing the other properties.

Safe and UnSafe PTFE

Pure or virgin PTFE is completely inert so can safely be used in applications where it will come into contact with food, pharmaceuticals or healthcare products. In fact it’s so safe the FDA classes it as “Generally Recognized As Safe” (GRAS). (The relevant regulation is 21CFR177.1550.) This means it’s suitable for gaskets, seals and washers on mixers, storage vessels, kettles and similar equipment.

Filled PTFE does not meet the GRAS criteria and is therefore not FDA approved for food and medical applications. Despite their higher strength, these grades of PTFE must not be used in places where they could be ingested or otherwise taken into the human body.

Expert Advice on Gasket Materials

When choosing a gasket material for food processing or medical equipment applications, a PTFE gasket can be a good choice. However, only the virgin grades are considered safe, and these are prone to creep under high bolt loads. Filled PTFE offers more strength but doesn’t come in FDA-approved grades.

Gasket material selection can be complicated. For expert advice on materials, chemical compatibility and FDA-approved materials, contact us and speak to a product specialist.

Will Acetone Damage Rubber Seals?

When choosing rubber gasket material it’s important to consider chemical compatibility. What’s sometimes overlooked though is that compatibility relates to more than just the fluid being sealed. Cleaning agents and solvents that will come into contact with the sealing material must also be considered.

One widely used cleaner/solvent is acetone. While compatible with some types of rubber, it reacts negatively with others. Here’s what to consider.

Acetone Exposure

Although perhaps best known as a nail polish remover, acetone has many other uses. It’s a key ingredient in lacquers used in the automotive and furniture industries, it’s used in some textile manufacturing, and it’s an industrial cleaner, particularly in the printed circuit board sector.

Chemists classify acetone as a ketone. It’s generally harmless to humans, although is very flammable. However, it will damage some types of rubber.

Rubber Takes Many Forms

In its original sense, “rubber” refers to an elastic material made from the sap of the rubber tree. Today though it’s often used as a general term for almost any elastomeric material.

The closest materials to natural rubber are SBR or red rubber, and NBR, also known as nitrile and Buna-N. (These were originally developed as synthetic forms of natural rubber.) Then there are other elastomers like neoprene, EPDM, silicon and FKM/Viton® that behave in similar, often superior, ways to natural rubber. All these are sometimes lumped under the heading of “rubber”. This causes a problem because they react differently to chemicals like acetone.

Positive and Negative Compatibility

The good news is that acetone won’t react with or degrade EPDM. Unfortunately it reacts negatively with many other rubber-like gasket materials.

With natural rubber, SBR and neoprene, the degradation is minimal: brief exposure is unlikely to cause any problems. However, in NBR and FKM/Viton it causes swelling which can quickly lead to failure of the seal. For this reason these types of rubber are best kept away from acetone.

If in Doubt, Ask an Expert

Material compatibility is an important part of gasket material selection. If you have any concerns, contact us and speak with a specialist.

What is Food Grade Silicone?

Silicone is an excellent material for gasket applications. It resists most chemicals, stays flexible over a wide temperature range, has good elongation and doesn’t take much of a compression set. If used where it could come into contact with foodstuffs or beverage products though, it must be food-grade.

This isn’t just a recommendation. If you’re making or handling food the Food and Drug Administration (FDA) mandates the use of materials from their approved list. Here’s what to know about food-grade silicone.

Silicone Basics

Silicone, (notice the letter ‘e’ that distinguishes it from silicon,) is produced by heating silica, (also known as silicon dioxide or SiO2,) with carbon. This produces polymer chains that can be processed into liquids and elastomeric solids and gels.

Solid silicone gasket material is sold in sheet form in thicknesses from 1/32” up ¼”. Important points for gasket applications are:

  • Temperature range: -67⁰F to 450⁰F (-55⁰C to 230⁰C)
  • Durometer: 30 to 80 Shore A
  • Resists UV light and ozone
  • Resists most chemicals, except for chlorine, methane and acetates

Why and When to Ask For “Food-Grade”

FDA regulation 21 CFR 177.2600 is a list of materials considered suitable for use with food. Silicone is on the list, but unless you specify food-grade silicone you’ll almost certainly get material that isn’t suitable for use around food products.

Silicone that isn’t food-grade contains additives, mostly colorings, that could contaminate product or make it taste off in some way. Food-grade silicone is white, and for this reason is sometimes called “white silicone”.

Food-grade silicone gaskets should be used in food processing and handling equipment. Storage vessels, kettles, mixers and even freezer doors are all good applications. Be sure to check what cleaning chemicals will be used, as those containing chlorine could damage the silicone.

Hennig Gasket for Food-Grade Material

People involved with food understand the importance of avoiding contamination. No one wants to make someone sick, which is why kitchens and food processors should always use food grade-materials. Stainless steel is ubiquitous for hardware, but when it comes to gaskets, food-grade silicone is often the way to go.

Does Cork Absorb Water?

Used on its own or blended with rubber, cork makes an excellent gasket material. It’s flexible over a wide temperature range and very compressible. It resists oil, it’s fire-resistant and it doesn’t creep. Why then, do some gasket and gasket material buyers express concern when presented with it?

The answer is, in some quarters there’s a perception that cork absorbs water.  Does cork absorb water is a frequently asked question.  If this were true, wine bottles would leak when laid horizontally, but it’s not entirely incorrect either. For an explanation, let’s delve into the properties of this natural material.

The Structure of Cork

Cork grows on trees, and like all biological materials, is composed of cells. As a paper from North Carolina State University (NCSU) noted in 2015, the cells in cork contain air, so it can be considered a closed cell foam. (“The rationale behind cork properties: A review of structure and chemistryPereira, H. (2015).)

This similarity to a sponge material is probably the origin of the belief that it absorbs water, but there’s a bit more to it. The NCSU paper notes that, “The porosity coefficients of cork range from below 2% to over 15%”. Or in other words, it will absorb a little water. This occurs at the cells that have been cut open on the surface and also through defects in the cell structures.

Countering this though, the cell walls are made mostly of a substance called suberin. Naturally hydrophobic, this repels water, which works against absorption.

So, as a conclusion, yes, cork can absorb water, but the amount is very small. It’s not a sponge.

Relevance to Gasket Materials

Natural cork, often described as composition cork, can be used for gaskets. More often, it’s blended with a synthetic rubber like neoprene, nitrile or silicone. This increases strength and flexibility while enhancing oil resistance.

Thanks to high compressibility, cork-rubber blend gasket material works well on uneven surfaces and in low pressure applications. It’s available in thicknesses from 1/32” to ¼”, can be die, water jet or laser cut to shape, and no, it won’t absorb water – to any significant degree.

Is Graphite Metallic or Nonmetallic?

Graphite is something of a wonder material for gasket applications. Not much will attack it and it seals over a very wide temperature range. Graphite gasket material is sold mostly in sheet form and has a dark gray, silver-ish color that looks a lot like lead. This is one reason people sometimes assume it’s a metal. The other reason is that graphite conducts electricity, which is something associated with metals.

In this blog we’ll set the record straight.

What You Should Know About Graphite

Graphite is a form of carbon. Carbon atoms can arrange themselves in several ways, which determines its properties. In graphite the atoms are arranged in layers. These can slide over one another, which is what makes graphite slippery, but at the same time, the layers are surprisingly strong.

Carbon is found on the top right side of the periodic table, between boron and nitrogen. This tells us it’s not a metal, because those elements are grouped in the center of the table.

Useful Properties for Gaskets

Graphite is soft and conforms to match uneven surfaces, a key requirement for a gasket material. It also retains its properties over a temperature range from -400°F to 950°F in air (and higher in non-oxidizing atmospheres.)

Another requirement for gaskets is chemical compatibility, and here graphite excels. It resists almost everything except for a few highly oxidizing chemicals.

Graphite Reinforcement

One weakness of graphite is a tendency to creep under load. This is remedied by mounting it to a stainless steel carrier via little hooks or tangs stamped into the metal. Alternatively, it’s often bonded to stainless foil for increased creep resistance.

Not for Food and Beverage

Unfortunately, graphite is unsuitable for applications where it would come into contact with food or drink. This is because carbon particles can detach and enter into the media. PTFE is usually an acceptable alternative.

Graphite for Gasket Applications

Graphite, particularly when reinforced with stainless steel, makes an excellent gasket material for many applications. Hennig Gasket carries a wide range of types, widths and thicknesses. Contact us to learn more.

How to Measure Bolt Circle Diameter

Two styles of gasket are used for pipe flanges: full face gaskets and ring gaskets. Full face gaskets need holes for the flange bolts to pass through. When you order this type of gasket the manufacturer needs to know the diameter of the bolt circle. Here we’ll explain what that is and how to measure it. First though, an explanation of the two gasket styles.

Gasket Styles

A full face gasket has roughly the same outside diameter as the flanges being joined. The inside diameter is of course equal to the bore of the pipe. Flange bolt holes in the gasket simplify assembly as the bolts hold the gasket in place and stop it intruding into the flow. The downside is that all the bolts must come out when installing a new gasket.

In contrast, a ring face gasket sits inside the bolts and can be replaced without completely disassembling the joint. They also have a smaller outside diameter, which saves material. They are used principally on raised face flanges where the sealing surfaces are inside the bolt hole diameter.

A full face gasket can be used with raised face flange. Likewise, ring gaskets can be used on flat face flanges, providing the impact of the reduced gasket width is considered.

Bolt Circle Diameter Measurement

The bolt circle is defined as a circle running through the centers of all the bolt holes. As these points are in space they can’t be measured directly. However, there is way.

The bolt holes will all be the same diameter and arranged in diametrically opposite pairs. Looking at the flange end-on, measure from the left edge of the leftmost hole to the left edge of the hole diametrically opposite. (Or measure from right edge to right edge.)

To eliminate any measurement error, repeat on a number of other hole pairs and average the result. This is the number you’ll give your gasket manufacturer, along with inner and outer diameters, number of bolt holes, and the bolt hole diameter. With this information they’ll cut the gasket to the size you need.

What is a Mechanical Seal?

Like gaskets, a mechanical seal stops fluids leaking into places they are not supposed to be. The difference is that gaskets are for static applications like pipe flanges while mechanical seals prevent leaks from around rotating shafts. Hennig Gasket & Seals is in the business of cutting gaskets to shape and supplying gasket material, but we do offer a mechanical seal repair service. Here’s a primer on mechanical seals for those who’d like to know more.

The Shaft-Sealing Challenge

Pumps, compressors, and similar rotating machinery are built around a rotating shaft that passes into or through a housing. Bearings hold the shaft in place but don’t stop the fluid from escaping. Historically, this was done with gland packing material. This looks like rope and is packed around the shaft to prevent leaks.

Gland packing has several limitations:

  • Wears out, requiring adjustment and eventually, replacement
  • Can wear the shaft
  • Creates friction for the motor to overcome

Modern gland materials incorporate PTFE, graphite or other friction-reducing compounds, but the risk of leaks remains. As awareness of the environmental and health and safety consequences has grown, many machinery manufacturers have replaced packing with mechanical seals.

Mechanical Seal Basics

In principle, mechanical seals are straightforward. They comprise one element fixed to the shaft and a second element on the housing or body. These are arranged with surfaces that are perpendicular to the shaft axis and are brought together so the rotating shaft element rubs against the fixed housing element.

This perpendicular configuration accommodates runout between shaft and housing. A small gap – less than 0.0001″ – between the two contacting surfaces stops particles getting into the fluid but can allow a degree of leakage. The alternative is to put combinations of low friction materials like graphite and ceramic or carbide in contact. This drives leakage rates almost to zero. For additional sealing, many modern mechanical seals spring-load one element to push it against the other.

Contact Hennig

Hennig Gasket & Seals are experts in all types of gasket and sealing applications. Contact us for gasket materials or mechanical seal rebuilds.

Soft Rubber Gasket Material

Uneven sealing surfaces need a soft rubber gasket material that deforms to fill the space. Covers and enclosure doors are examples where the closed gap often varies. A soft material will seal better than one that’s firmer.

Gasket material softness is measured by durometer and expressed in terms of Shore A and D. Understanding this will help when it’s time to order soft rubber gasket material.

Shore Hardness Scales and Durometer

Rubber and plastic firmness is measured by pushing a hard point into the material and measuring the indentation. The measurement system used is the Shore hardness scale, named after its inventor, Albert Shore.

Shore’s indentation device is called a durometer. This is why rubber hardness is sometimes spoken of in terms of durometer, or “duro.”

As the hardness or rubbers and plastics varies widely, Shore defined several scales but for most practical purposes Shore A and Shore D are enough.

Shore A and D Values

Shore A covers softer materials, Shore D those that are firmer. Both scales run from 0 to 100. They overlap, so rather than specify the top end of the A scale it’s more usual to use the middle of the D scale.

Here are typical Shore values for common materials:

  • Rubber band – 20 Shore A
  • Pencil eraser – 55 Shore A
  • Shoe soles – 70 Shore A
  • Leather belt – 80 Shore A.
  • Golf ball – 50 Shore D
  • Shopping cart wheels – 60 Shore D.

Shore Values and Soft Rubber Gasket Material

The softest material generally used for gasket applications is 30 duro, or Shore A, neoprene. Slightly firmer neoprene, nitrile and EPDM are all available in 40 duro / Shore A.

Foam is usually softer than solid material, as the air pockets provide additional compressability. Open cell foam is softer than closed cell in the same material, but will let fluids through.

Advice on Soft Rubber Gasket Material

A soft, closed cell rubber material is better for sealing uneven surfaces like those on doors and covers. The firmness of these is usually specified in Shore A. Ask us for help in choosing the best material for your application.

Food Grade Gasket Materials – the Basics to Know

To avoid contamination, food and beverage handling and processing equipment is generally fabricated from materials like stainless steel. Gaskets can’t be avoided though, and some will come into contact with the products being manufactured. When this is the case it’s essential they are made from gasket material considered food grade.

The FDA publishes a long list of substances that can safely be included in elastomeric (rubber-like) gaskets: it’s in 21 CFR 177.2600. However, relating that to common gasket materials is no easy task. Asking a gasket material supplier for their expert advice and recommendations is a better approach. Here’s what you should know.

Food-Grade Material Choices

Commonly used gasket materials are also available in grades considered food-safe. That means material choice is dictated by what the application needs. Only once temperature, environment, media and pressure, (summarized by the acronym TEMP,) are known do you need to add the “food grade” requirement.

Silicone is often the first choice when food grade is required. It’s an excellent gasket material with a wide temperature range, low compression set, good strength and elasticity and resistance to many chemicals. In some applications though it may be overkill. Nitrile, neoprene, EPDM, Gylon®, Viton® and PTFE are alternatives that may perform better while possibly saving money.

Not Necessarily White

An absence of color-adding substances mean food-grade gasket materials are usually white. However, there are alternatives. Food-grade EPDM for example is available in black, and food-grade silicone may be translucent or other colors.

The bottom line is, don’t assume your food-grade gasket material must be white. If you need something else, ask us.

Contact is the Important Point

Consider food-grade material mandatory for gaskets coming into contact with food. Elsewhere in a food or beverage facility though, you can use other materials. To avoid confusion, a plant may decide to use only food-grade material everywhere, or implement appropriate controls to prevent misuse.

Seek Specialist Advice

If you’re responsible for food handling and processing equipment you need to know about food grade gasket materials. You could spend a lot of time on research, or you could come to us instead.