SILICA GEL OR ACTIVATED CLAY?
Both are desiccants and will adsorb moisture vapour whilst remaining dry to touch. The surface area and structure of the desiccant particles will influence the rate of moisture adsorption. The amount of moisture adsorbed will depend on the quality of desiccant employed. At 25° C and above the critical humidity zone of 50% RH (i.e. for prevention of corrosion or mould formation) silica gel will outperform typical desiccant clays by 35% in moisture adsorbency.
Colour is added to white silica gel to give a visual indicator that the desiccant has adsorbed moisture and may have become inactive. If the desiccant is dried out again the indicator will revert to its original colour. Traditionally the indicator used was “blue” changing to “pink”. In January 2000 new UK CHIPS legislation made special labelling necessary for all products containing more than 1 per cent cobalt. Subsequently a new food-safe indicator was adopted – Orange to Cream. One of the prime considerations in choosing this particular indicator was that the colour changes at the same rate as the old cobalt blue to pink thereby giving an equally effective indicator. In order that the indicator can be easily observed, small SG indicator sachets have a clear, polyester “window” on one side of the pack.
The Standard for silica gel is BS.2540. In addition, BS.1133 gives a formula for calculating the amount of silica gel required. Full copies are obtainable from BSI Sales, Milton Keynes, MK14 6LE, or contact us for further information. Our larger bags conform and can be supplied printed to Def. Stan 81-68.
(German Standard DIN 55473). This may be specified instead of “weight” of desiccant. A desiccant unit is the quantity of desiccant which, in equilibrium with air at 23°C ± 2°C adsorbs water vapour as follows:
Minimum 3.0g at 20% RH
Minimum 6.0g at 40% RH
25grm of silica gel used in SG sachet production is equal to 1 Desiccant Unit. Other desiccant materials, such as activated clays, will need more than 25 grm.