Tuesday, 17 November 2020 08:53

O-rings - Why the details matter

The quality of a manual isolation valve is heavily influenced by the quality of its rubber elastomer (o-ring) componentry, therefore ensuring that the rubber elastomer is appropriate for the intended application is essential for optimal performance. Which is why insisting on only the highest quality elastomers is a key point of difference for Zetco Valves and a good example of why it matters to get the details right.

History of Rubber Elastomers

Latex being collected from tree
Latex being collected from tree
Image: Abhilash Jacob from Pixabay

The earliest rubbers were obtained from latex, a milky fluid produced by a wide range of plants as a defence against insect attack. Still harvested today, latex is an emulsion of organic isoprene polymer microparticles dispersed in water. Sensitivity to temperature changes meant that early natural rubbers were limited in their utility and were mainly used for applications such as waterproofing.

In 1839 Charles Goodyear applied the vulcanisation process to natural rubber, producing much more stable material that could be used for industrial belts and bands, tyres, hoses, shoe soles and other items requiring strength, flexibility and resilience.

The burgeoning popularity of bicycles with pneumatic tyres in the 1890s lead to an increased demand for rubber. In 1909, the first synthetic rubber Isoprene was developed by Fritz Hofmann at the Bayer lab in Elberfield, Germany. The first synthetic rubber with high resistance to heat and chemicals, Neoprene, was developed by DuPont™ in 1930 based on earlier work of Julius Nieuwland, a professor of chemistry at the University of Notre Dame.

In 2017 the production of rubber surpassed the 25 million metric tons per year mark and synthetic rubber represented approximately 53% of that total volume.

Physical Properties of Rubber Elastomers

While the terms elastomer and rubber are often used interchangeably, there are some basic differences between the two:

Elastomer, from ‘elastic polymer’ relates to the physical properties of viscoelastic polymers that recover their shape after deformation.

Rubber refers to a material that must undergo vulcanisation or some other curing process to result in a useful product.

Rubber elastomers are polymers that have high elasticity, low Young’s modulus (an objective measurement of ‘stiffness’) and a high yield strain when compared with other materials.

Youngs modulus chart
Source: University of Cambridge – Department of engineering

Synthetic rubbers may be classified into three broad groups:

Dienes: polymers consisting of molecules possessing two double bonds which enable cross-linkage (usually by vulcanisation) that imparts elasticity to the compound. Examples include polyisoprene, polybutadiene, and polychloroprene.

Non-diene: polymers consisting of molecules with no double bonds in their structure, requiring methods other than vulcanisation to achieve cross-linking. Typical examples include butyl rubber, silicone rubber, spandex and fluoro-elastomers.

Thermoplastic elastomers: co-polymers consisting of a mix of rigid (usually plastic) and soft (rubber) polymers that imparts both thermoplastic and elastomeric properties to the compound. A range of different physical properties can be achieved as the constituent polymers can ‘microphase separate’ and form periodic nanostructures within the material. SBS (Styrene Butadiene Styrene) block copolymer, SIS (Styrene Isoprene Styrene) rubber and some urethanes are examples of these compounds.

The choice of moulding process used to form rubber elastomers can be influenced by factors such as the size and shape of the component, the tolerances required, the quantity to be produced, the end-use of the elastomer and the raw material costs. Manufacturing is generally carried out in one of three ways:

  • injection moulding
  • transfer moulding
  • compression moulding

Selection of Rubber Elastomers for Valves

O-rings in different sizes
Rubber elastomers in different
types and shapes
Source: National Oil Seal

Rubber elastomers are used in Zetco’s valves and fittings to provide sealing between internal components and between the valves and appliances or piping systems. As with all materials, selecting the right elastomer for the application requires consideration of many factors such as the mechanical and physical performance requirements, potential exposure to chemicals, operating temperature range, service life, manufacturability of parts and raw material and manufacturing costs.

The most common types of elastomer used in Zetco valves are:

FKM which is a vinylidene fluoride synthetic rubber, originally developed by DuPont™ and commonly known as Viton™. O-rings and gaskets made of FKM exhibit resistance to a broad range of chemicals combined with very good high temperature properties. FKM materials are not generally recommended for exposure to hot water, steam, polar solvents, low molecular weight esters and ethers, glycol-based brake fluids, or hot hydrofluoric/chlorosulfuric acids.

FFKM, known as perfluoro elastomer or Kalrez® is a synthetic rubber also developed by DuPont™. O-rings and gaskets made of FFKM deliver outstanding performance in aggressive process environments, where resistance to acids, amines, plasma, ultrapure de-ionized water or strong bases is extremely important.

EPDM, also known as ethylene propylene is a synthetic rubber. O-rings and gaskets made of EPDM show excellent resistance to environmental factors such as ozone, UV and general weathering but are not recommended for applications with petroleum oils, mineral oil, diester lubricants or fuel exposure.

NBR, Buna N or nitrile is a synthetic rubber. O-rings and gaskets made of NBR display excellent resistance to petroleum-based oils, fuels, water, alcohols, silicone greases and hydraulic fluids. It also has a good balance of desirable working properties such as low compression set, high abrasion resistance and high tensile strength.

HNBR, known as hydrogenated nitrile butadiene is a synthetic rubber. O-rings and gaskets made of HNBR are known for their physical strength and retention of properties after long-term exposure to heat, oil and chemicals.

Temperature Capabilities of Rubber
Source: DuPont™ Performance Elastomers

Potential causes for rubber elastomer failure

The outcomes of a rubber elastomer failure are generally expensive and include:

  • loss of professional reputation
  • the costs associated with property damage caused by leakage or incorrect sealing
  • costs related with the recall of defective products
  • costs related with scrapping or reworking of defective products

The two most common reasons for failure of rubber elastomers are human causes and service-related failures.

Human causes of elastomer failure
The most common human cause of elastomer failure, at an estimated 45%, is related to mis-selection of materials which may result in leakages or improper sealing of system components. Poor product design is also accountable for about 20% of the recorded failures of rubber elastomers. The design stage should consider carefully:

  • the service conditions of the final product
  • the expected working lifetime of the product
  • the type of application, whether dynamic or static
  • the expected service stresses and strains
  • limits of the rubber’s physical properties
  • the potential for exposure to damaging materials or substances

Human causes of rubber failure chart
Source: Smithers laboratories

Service-related causes of rubber elastomer failure
Chemical aggression is among the most common causes of service-related failures, and for this reason the rubber elastomer must be selected with consideration of the chemical properties of the contact media. Factors such as chemical concentration, operating temperature and pressure may all affect interactions between rubber elastomers and chemicals.

Ozone is responsible for about 21% of service-related failures for rubber elastomers. Ozone exposure is usually referred to as dry rotting and may result in cracking along the exposed, stretched or stressed face. Ozone is a gas made of molecules of three oxygen atoms and can be found everywhere, even in normal air where its concentration can reach levels of 0.01 ppm. The potential effects of ozone should be one of the first design considerations that engineers should consider when working with rubber elastomers.

Service-related causes of rubber failure chart
Source: Smithers laboratories

Zetco Valves

Zetco Valves have been serving the Australian and global market for over 50 years, and we understand how critical reliable quality is to any business operation. Low quality valves and incorrect material selection can result in impactful failures across systems and equipment, and result in high business losses due to delays and costly replacements.

Zetco offers an extensive range of valves designed to meet the specific requirements of our customers, considering all the factors required to achieve optimal performance across a wide range of applications.

To ensure Zetco valves meet the most exacting standards we select only the highest quality materials and undertake rigorous quality testing at all stages of the manufacturing process. With several global patents and ISO 9001 certification, you can depend and rely on Zetco’s quality and ingenuity, knowing we’ve worked hard to get all the details right.

 

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