Storage of elastomers

Correctly stored elastomer products retain their properties almost unchanged over the course of years. If the storage conditions are neglected, even high-quality seals, O-rings, profiles and moulded parts may already be damaged before installation – through hardening, softening, permanent deformation or crack formation on the surface. The following guidelines apply to pure rubber products as well as to composite components made of elastomers and other materials, and are based on the recommendations of the German Standards Committee and on DIN 7716 – the central standard for the storage, cleaning and maintenance of rubber products.

Temperature in the storage of elastomers

The optimum storage temperature is +15 °C and must not permanently exceed +25 °C. Excessively high temperatures accelerate the ageing of elastomers and change the physical properties of the material – recovery capability, hardness and compression set can deteriorate while still in storage.

As regards lower temperatures: temperatures below –10 °C are not, in principle, harmful to most rubber products, but they can lead to considerable stiffening. Strongly cooled components must be brought to a temperature above +20 °C for a sufficiently long time – at least several hours – before they are fitted or tested. For products made of CR (chloroprene rubber) a minimum storage temperature of +12 °C applies, since this material has a greater tendency to crystallise in the cold.

Radiators in the storage room must be shielded – the minimum distance between the heat source and the stored goods is 1 m.

Humidity and ventilation

The storage room should be cool, dry and moderately ventilated. The relative humidity should not exceed 65 %. Condensation on the components must be avoided under all circumstances. Draughts accelerate the oxidation of the surfaces and are therefore likewise undesirable – components with a large surface-to-volume ratio (e.g. thin profiles or rubberised fabrics) are particularly sensitive to this.

Light and ozone

Direct sunlight and artificial light with a high UV content damage elastomers through photo-oxidation and the associated ozone formation. Storage windows should therefore be provided with a red or orange protective coating – on no account blue, since blue light transmits UV radiation. Ordinary incandescent lamps are suitable for room lighting. Fluorescent tubes, mercury-vapour lamps and other UV-emitting light sources are to be avoided.

Ozone is particularly aggressive towards elastomers with double bonds in the polymer backbone – including NBR and natural rubber – and causes characteristic ozone cracks. No ozone-generating equipment of any kind may therefore be operated in the storage room: no electric motors, no spark or discharge sources, no photochemically active light sources. Combustion gases and vapours that can lead to ozone formation through photochemical processes must be extracted.

Storage without stress and deformation

Elastomer products must be stored free of stress – without tension, compression or any other mechanical deformation. Sustained stresses promote permanent deformations and crack formation, even when temperature and humidity are optimal. Certain metals – in particular copper and manganese – have a catalytic effect on the oxidative breakdown of rubber. Products made of elastomers must therefore not be stored in direct contact with these metals. Even a separating layer of paper or polyethylene is entirely sufficient as protection.

Different compounds – also recognisable by their different colours – must not be stored in mutual contact, since plasticisers and other compound constituents can migrate and trigger changes in the material. Packaging films containing plasticisers are unsuitable for the same reason. Approved packaging materials are coated kraft paper, aluminium foil or opaque PE film with a wall thickness of at least 0.075 mm.

Solvents and chemicals

No solvents, fuels, lubricants, acids, disinfectants or similar chemicals may be kept in the storage room for elastomer products. Even small quantities of volatile solvents in the room air can attack the surface of seals and profiles and permanently impair the chemical resistance of the material. Rubber solutions and solvent-based adhesives are to be stored in a separate, separately ventilated room in compliance with the relevant regulations for flammable liquids.

Cleaning and care

Elastomer products can be cleaned with soap and warm water. After cleaning, the components must be dried completely immediately – with a soft, dry cloth or by brief storage at approximately +23 °C in a dry environment. Radiators, hairdryers or other heat sources must not be used for drying.

After prolonged storage of 6 to 8 months, cleaning with a 1.5% sodium carbonate solution can be worthwhile. The cleaning residues must then be rinsed off thoroughly with water. Solvents such as trichloroethylene or carbon tetrachloride, as well as hydrocarbons, are unsuitable for cleaning elastomers. Sharp-edged tools, wire brushes and emery paper are likewise prohibited.

Storage life and standards

Elastomer products should remain in storage for as short a time as possible. When placing new goods into storage, strict separation from older stocks already present must be ensured – first-in-first-out applies here without restriction.

Under optimum storage conditions in accordance with DIN 7716, standard elastomers such as NBR, EPDM or CR can be stored for 5 to 7 years without any problem. High-performance materials such as FKM or silicone (VMQ) achieve storage times of up to 10 years if the storage conditions are consistently observed. After these periods have elapsed, the components should be subjected to a visual inspection and – depending on the application – a functional test before use.

For storage times exceeding 6 months, the requirements of these guidelines apply as binding. For shorter storage times, they are to be applied accordingly, provided that function and appearance are not impaired.

For aerospace applications, the permissible storage times in accordance with DIN 9088 apply. If defined storage times are exceeded, the supplier decides, after inspection, whether the products can continue to be used or must be discarded.

Source: German Standards Committee. The most recent edition of the standard sheet in A4 standard format is authoritative.