Storage & Elastomer Shelf Life of Compounds
Daemar over the years’ industry experience shows that the storage conditions of O-Rings have a major impact on its useful life compared to its generalized shelf-life. SAE International Aerospace Standard AS5316, a quality standard, is based upon both cumulative studies and industry input regarding practical storage limits and FIFO requirements, and it is the most widely referenced guide for information recording procedures, packaging, and storing of aerospace elastomeric seals/O-rings.
SAE AS5316 Recommended O-Ring Shelf life
The table below gives shelf-life recommendations of various elastomers as per the SAE AS5316 standards.
|Butyl Rubber, Isobutylene Isoprene||Unlimited|
|Ethylene Propylene, EPDM or EP||Unlimited|
|Highly Saturated Nitrile (HNBR or HSN)||15 Years|
|Nitrile (Buna-N or NBR)||15 Years|
|Polyacrylate (ACM)||15 years|
|Polyurethane (depending upon the type)||5 or 10 Years|
|Styrene Butadiene (Buna-S or SBR)||3 years|
How to store your Elastomer Seals and O-Rings?
Seals or O-rings stored in improper conditions when put into service, chances are there will be problems. Leaks, damage to equipment, or worse. It can be a big and expensive issue even if one seal/O-ring fails, the complete system can shut down, machinery could be damaged, or it can even cause injury to the workforce.
Proper storage methods are essential to meet the standard shelf-life period, as mentioned in the above table, and keep the O-Rings and seals intact. Following are some of the factors to keep in mind to properly store the elastomeric material:
- Stress & Strain: Elastomers must be stored in stress and strain-free condition. Avoid any kind of stretch or tension due to storage including caused by hanging or compression. It is recommended to store large inside diameter O-rings as at least three superimposed loops to avoid creasing or twisting.
- Humidity: Try to store the materials in moisture-proof bags. If storing in moisture-proof bags is not possible, then it must be stored in a dry contamination-free environment where the relative humidity is less than 75%. If the material being stored is polyurethane, then relative humidity should be less than 65%
- Light: Avoid direct sunlight and any artificial light having UV and other radiations.
- Radiations: All precautionary measures shall be taken to protect stored material and block all the sources of ionizing radiation likely to cause damage.
- Temperature: The recommended storage temperature is between 59°F (15°C) and 100°F (38°C). Keep the material stored away from a direct source of heating as it may increase the temperature to an inappropriate level.
- Ozone: Some electrical equipment, especially high voltage electrical equipment, generate ozone which can have a huge negative impact on the shelf-life of some elastomers. Exclude such equipment’s from the storage room. Combustion gases and organic vapour tend to produce ozone via photochemical processes. Hence, they must be excluded from storage rooms.
- Contact with Different Elastomers: Avoid contact between different elastomers and elastomeric seals/O-rings.
- FIFO: Rotate the elastomeric seal/O-rings stock on the FIFO (First In, First Out) principle.
- Contact with Liquid and Semi-Solid Materials: Elastomeric seals/O-rings should not come in contact with liquid or semi-solid materials or their vapours at any time during storage except when these materials are by design a fundamental part of the component or the manufacturer’s packaging. In case the elastomeric seals/O-rings are received coated with their operational media, they must be stored as it is.
- Contact with Dusting Powder: To avoid the sticking or blocking of elastomeric materials, dusting powders should be used for the packaging. Only use the minimum quantity necessary to prevent adhesion.
- Contact with Metals: Elastomeric seals shall not be stored in contact with certain metals (except when bonded to them), which are known to cause deleterious effects on elastomers. Some of these metals are Copper, Iron, and manganese.
- Elastomeric Seals bonded to Metal Parts: Metal bond seals/O-rings must be individually packed as their contact with the elastomeric element of other seals/O-rings will affect the shelf-life of the seals/O-rings. The preservative used on the metal element should be carefully selected as it may have an impact on the elastomeric element or the bond and it might result in the seal not comply with the product specification.
When does the Elastomer or O-Rings Shelf-life reach expiration?
The rubber industry uses old US military specs and current SAE Aerospace recommendations to calculate elastomer and O-Ring shelf-life in quarter-year units. So, the manufacturing cure date can occur anytime within 3 months, and the shelf-life expiration time does not start until after the first complete quarter. For example, if an O-Ring or elastomer was cured on May 10, 2017, and it would have its cure date established as 2Q17, the shelf-life expiration period won’t begin until July 1, 2017, that is until the start of the subsequent quarter month. Therefore, an O-Ring with 5 years shelf life would expire on July 1, 2022 (3Q22).
Even though it is said that shelf life begins with the manufactured date, and it can be maximized with proper packaging and if they are stored under proper environmental conditions, the rubber shelf life is not an exact science. It is very tough to attain proper storage conditions and meet all the required criteria under practical scenarios. Let us take the example of packaged food. Even though packaged food has a set expiration in the near future, it can get spoiled much earlier or even after the expiration date. There are many environmental factors underplay that can’t be controlled a lot of times. These expiration dates are set after the internal testing by the manufacturing company. Practically it is not possible to have the same conditions.
Guidelines for storage as per ISO 2230: Rubber products
There is some additional guidance on storage conditions to maximize the life of elastomeric parts that can be found in ISO 2230: Rubber products. These guidelines slightly differ from SAE- AS5316, and it mentions:
Suitable packaging material must be free from particles having a deteriorating effect on the elastomer. One must try to use heat-sealable opaque materials unless they are not practical to prevent distortion of the packaged material. Example of suitable materials includes polyethylene (PE)-coated Kraft paper, aluminum foil/paper, PE laminate and opaque PE film. Compounds containing plasticizers like PVC film are not recommended for packaging or direct contact with elastomer. PE is appropriate as a single wrapping as long as it is at least 0.075 mm thick. In environments where there is a significant risk of ingress of vapours or moisture, aluminum foil/paper, PE laminate or other similar means of protection must be utilized to secure material.
How SAE- AS5316 differ from ISO 2230 Storage Guideline?
- The storage temperature must be below 25°C, away from direct heating sources.
- When the temperature is below 15°C, elastomers can stiffen up and they must be handled with caution to prevent any damage. Careful heating to 30°C prior to installation is recommended if it is practical.
- Humidity: The relative humidity of the storage room must be less than 70%.
The ISO 2230 standard divides the maximum storage time into an initial storage period and an extended storage period which may be put into an application after representative samples of the stored products were inspected. Inspection must be done according to the relevant manufacturing specs.
On visual inspection, one must look for signs of permanent distortions (like flats or creases), mechanical damage (cuts, abraded areas, tears), change of surface condition such as hardening, softening or tackiness, or surface cracking when inspected under a magnification of 10X. Conducting tests can show if the pertinent performance attributes are within acceptable limits. Storage life gets influenced if the storage temperature is over or under 25°C. Storing at a temperature 10ºC higher will lessen the storage time by approximately 50%, and when stored at a temperature 10°C lower will increase the storage time by approximately 100%.