Silica gel, commercial

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An amorphous material of Silica that has the ability to act as a humidity buffering agent. As a RH buffer, Silica gel absorbs moisture in humid environments and releases water in dry air. Once equilibrated to set moisture level, silica gel will maintain that humidity level in a tightly sealed environment. Silica gel performs best at room temperature (70-80F) and high humidity (60-90%). The pore size in silica gel is related to its ability to adsorb water and other pollutants. Silica gel Type A with approximately pore size of 2.5 nm is most often used for vapor-phase moisture. Silica gel saturated with water can be dried with heat (120 C for 2 hours) and used again as a desiccant. For re-use as a humidity buffer, it must be reconditioned at the required humidity.

Product Composition Pre-conditioned RH options Humidity for usage Comments/Forms Web page for more info
Arten Gel type A SiO2 0% 30% 40% Best for 0-40% range; avg value of MH* is 5 in range 8 mesh beads (2mm diameter) in moisture permeable bag APSNYC: Product page
Art Sorb 90% SiO2 10% LiCl 0% 40% 45% 50% 55% 60% 65% 70% Best from 40-70% Prosorb site says Art Sorb is best from 60-80%; beads, sheets, cassettes Fuji Silysia: Art Sorb
Moisture Pak cellulose, water, salts 62% +/- 3% in sealed container Same Food storage, tobacco, wood instruments, herbs; variable size packets Sorbent Systems: MoisturePak
Prosorb 97% SiO2 3% Al2O3 30% 35% 40% 45% 50% 55% 60% Optimum between 30-60% range Similar performance as Rhapid gel; beads, sachets, cassettes Sercalia: Prosorb
Rhapid gel Hybrid SiO2 0% 40% 45% 50% 60% Best for 0-65% range; avg value of MH* is 5 in range Flexible moisture-pearmeable sachets APSNYC: Products page
Silica gel E type A SiO2 Best for 0-30%
Silica gel M Macroporous SiO2 Best for 80-100% Prevents condensation at high RH
Silica gel orange type A SiO2 with either methyl violet or iron salts as a moisture indicator Best for 0-30% Moisture indicator: methyl violet changes from orange to green;iron salts change orange/brown to colorless; AGM: Blue vs orange
Grade 42 type A SiO2 with cobalt dichloride indicator Moisture indicator: changes blue to pink as RH increases from 15-45%
Grade 40 type A SiO2 may be mixed with 42

Moisture Reservoir Information

The moisture adsorbing properties of silica gels are affected by factors such as capillary pore size or the inclusion of hygroscopic salts, resulting in a wide range of performance. Thus, it is important to compare the buffering capacity of different types of silica gels to determine which has the best performance for a specific application. In 1977, Thomson described the specific moisture reservoir with the variable M where M represents the amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH. However for any specific silica gel type, M can vary widely due to the expected RH range and the hysteresis of the adsorption/desorption curve. In order to compensate, a new value called MH has been defined as the average amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH. This is determined by repeatedly cycling silica gel between adsorption and desorption within a specific RH range until a constant value is measured.

Silica Gel Comparison
Preservation Equipment

TABLE: Specific moisture reservoir (g/kg for a 1% RH change) at 20°C *

Moisture sorbent MH 50 ± 10% M 20 → 30% M 60 → 50% Recommended amounts
RH around 50% Keep RH low Keep RH high Depends on air flow and exhibit length
RHapid Gel 6.11 ± 2.16 4.48 ± 0.42 4.38 ± 1.40 2-4 kg/m3
PROSorb 5.42 ± 1.32 4.37 ± 0.10 4.25 ± 0.38 2-4 kg/m3
Art-Sorb 4.04 ± 0.80 2.84 ± 0.47 4.18 ± 0.49 4-8 kg/m3
Arten Gel 5.7 5.5 9-18 kg/m3
Regular density silica gel (clear, type A) 1.93 ± 0.44 5.48 ± 0.40 1.47 ± 0.16 9-18 kg/m3
Orange silica gel 1.16 ± 0.26 4.92 ± 0.59 0.94 ± 0.14 9-18 kg/m3
Bentonite clay (Desi Pak) 1.19 ± 0.07 2.25 ± 0.03 1.11 ± 0.02
Molecular sieves 4A (zeolite) 0.33 ± 0.02 0.47 ± 0.04 0.31 ± 0.01

Table from: J. Tetreault (2017) with additional information from S. Weintraub (2002)

Resources and Citations

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