Curing is the heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. The Cure Date is the date that the rubber is fully molded in this process.
The maximum period of time from cure date to the date an elastomeric product is used as a component part in assemblies, sub-assemblies, and systems. During the shelf life time the stored elastomeric product is expected to retain its characteristics as originally specified.
The expected lifetime or acceptable period of use in service. It is the time that any manufactured item can be expected to be ‘serviceable’ or supported by its manufacturer. Since elastomers are used in many products, the service life would be dependent on the product manufactured. There are several primary factors that influence the service life of an elastomer in a finished product: heat, corrosion, wear and cracking.
As they age, rubber seals and molded products can undergo changes in physical properties and become unusable due to excessive hardening, softening, cracking, crazing, or other surface degradations. These changes may be the result of one particular factor or a combination of factors, such as the action of oxygen, ozone, light, heat, humidity, oils, water, or other solvents.
There are three important factors involved in determining the shelf life of rubber products: (1) the initial quality level of the rubber; (2) the quality assurance provisions of the specifications under which the product was procured; and (3) the age resistance of the rubber. The age resistance of the raw material may not always correspond to the age resistance of the finished product. These factors also assume that the product was packaged and stored in order to minimize deterioration due to such factors as temperature, humidity, ozone, sunlight, oils, solvents, corrosive liquids and fumes, insects, and rodents.
There are several standards for defining the shelf life of an elastomer. The standard used by the manufacturer is mostly dependent on where the product is manufactured or market in which the product will be used.
ASTM D1418 American Society of Testing & Materials: Standard Practice for Rubber Materials
DIN 7716 German National Standard: Requirements for Storage, Cleaning & Maintenance for Rubber Products
BS 4F68:2002 British Standards: Storage and Shelf Life of O-Ring Elastomeric Materials
ISO 2230 International Standards Organization: Rubber Products – Guide to Storage
DIN 9088 German Standard: Aerospace – Storage of Rubber Products
MIL-HDBK-695F Military Standard: Guidelines for Recommended Shelf Life of Rubber Compounds
SAE ARP5316E Society of Aerospace Engineers: Storage of Elastomer Seals and Seal Assemblies
A Durometer is used to determine the “grade” of hardness for a material sample. The Shore Durometer scale was defined by Albert Shore in the 1920’s and is the most common method of hardness measurement used in the USA.