SPI Silver Membrane Filters

Additional technical information about the membranes

The SPI Silver Membrane filters are so different from the more common polymer membranes that we thought it important to lay out some of the unique and unusual characteristics that make the silver system so attractive for so many users worldwide:

Chemical and Thermal Properties
SPI Silver membrane filters have the chemical inertness and high thermal stability of pure silver. They are resistant to alcohols, fuels and hydrocarbons, natural and synthetic oils, alkalies, cryogenics, photoresists , ether, propellants, oxidizers, halogenated hydrocarbons, esters, and most organics and acids. Under normal conditions, the silver membrane is attacked only by cyanide solutions and nitric and sulfuric acids.

The SPI membranes provide excellent filtration performance at temperatures from cryogenic up to 1022° F (550° C). They may be repeatedly autoclaved and can be sterilized with steam or hot air.

High Strength
In the manufacturing of the SPI silver membrane filters, the size and orientation of the silver particles are closely controlled and uniformly deposited in sheet form. The points of contact between the silver particles are then molecularly bonded into a homogeneous material that resists pore collapse even under conditions of high temperature and pressure. They can be used in low-intensity ultrasonic baths with no loss of integrity.

Absolute Retention
Careful control of the manufacturing process yields unusually precise pore size and uniformity. Performance tests indicate that absolute particle retention is achieved as the result of simple mechanical sieving by the filter structure. Thus, particle retention by the silver membrane is independent of random electrostatic charges or other extraneous, unpredictable molecular forces.
The surface of the silver membrane is flat and smooth, allowing surface capture. There is minimal background interference from the silver when using x-ray diffraction.

No Fiber Release - No Media Migration
Conventional tortuous pore membranes and other filters often shed fibers or parts of the filter material itself. With changing conditions of flow and pressure drop, pores may change in size allowing previously-filtered contaminants to migrate through the filter medium and to contaminate the filtrate. In the case of the silver membranes there are no fibers, and because the silver membrane is a strong, uniform, porous, monolithic structure where the silver particles are molecularly bonded to each other, there is no media migration.

High Flow Rates
The SPI Silver Membranes are very thin (0.002"/ 50 ľm) and this fact, when combined with a characteristically high void volume allows for exceptionally good flow rates (and low pressure drops).

Non-Absorptive and Non-Adsorptive
Many conventional membranes will absorb or adsorb constituents out of a solution and thereby alter analytical results. This is avoided with silver membranes due to the chemical resistance and intertness of pure silver and smoothness of the interstices within the membrane. Absorption and adsorption by silver membranes is virtually nonexistent.

Minimum Fluid Holdup
The thinness, characteristic non-adsorption and non-absorption and the smoothness of the interstices of the SPI Silver Membrane filters means retention of an insignificant quantity of filtrate.

Intrinsic Bacteriostatic Nature
The intrinsic bacteriostatic property of silver does not allow the growth of bacteria and other microorganisms.

Economical
Although the initial cost of the SPI Silver Membranes is higher than conventional "single use and discard" polymeric membranes, the silver is more economical in many applications because it can be used repeatedly by chemical cleaning, ignition cleaning, plasma etching and cleaning, or a combination of the three.

Cleaning and Reuse of SPI Silver Membrane Filters
The SPI Silver Membrane filters can be cleaned and reused repeatedly, in most instances. The membrane should be cleaned immediately after each use, and handled carefully to avoid membrane punctures or tears. Various cleaning procedures can be used, depending on the nature, type, and degree of contamination. There are potentially five effective cleaning methods, depending on circumstances, that can be used to clean silver membranes:

Ignition cleaning:
Placing the silver membrane filter in a laboratory muffle furnace for approximately 30 minutes will effectively remove organic contaminants from the membrane. Do not exceed the following temperatures:




Retention                              Maximum Temperature
Rating                                   
  ľm                                        °C / °F
--------------                             ----------
5.0                                         550/1022
3.0                                         400/752
1.2                                         350/662
0.8                                         300/572
0.45                                        300/572
0.2                                         250/482

One note of caution: If the contamination itself was pigmented, for example, with titanium dioxide pigment, then although the organics will be removed, the pigment particles will not be removed.

Chemical cleaning:
Immerse in either a strong alkaline solution, a solvent, or an acid except nitric acid, sulfuric acid, and cyanide solutions.

Combination cleaning:
A combination of chemical and ignition cleaning is usually used to completely regenerate the membrane. Immerse in a 10% concentration of hydrofluoric acid for ten minutes, followed by ignition cleaning in a muffle furnace, following the guidelines above mentioned. This allows reuse of the membrane up to ten times.

Ultrasonic cleaning:
Low-intensity ultrasonics can be used to clean the silver metal membrane. The cleaning intensity and time will depend on the degree and type of contamination encountered. Do not use high-intensity ultrasonics.

Plasma etching/cleaning
The use of an oxygen plasma etcher/asher has several advantages over the other methods described above. For one thing, many laboratories do not have high temperature muffle furnaces but do have available a table top plasma etcher. Also, for the removal of pure organics , since it is a room temperature process, the membranes can be regenerated virtually an unlimited number of times. When the membrane contamination includes metal oxides, one can use argon, but great care must be taken since the argon can also slowly etch the silver, effectively increasing the pore size. A low power plasma cleaner in general, will not be effective for the removal of the levels of contamination being discussed here. However, for certain types of critical filtration experiments, an oxygen plasma cleaning (with low power, less than 10 watts) or oxygen plasma etching (at 100 watts, but still room temperature) will alter in a positive way the surface characteristics and therefore the wetting of the surface by the liquid to be filtered.

Shelf Life
The shelf life of the SPI Silver Membrane filters is indefinite when they are kept in sealed packages. However, since the silver membrane is pure silver, it can become tarnished when exposed to some chemicals in the air. This is a cosmetic imperfection and the filtration properties are not affected in any way.
The most common tarnishing compounds will cause the membrane to turn black in the case of Ag₂S, and dark brown in the case of AgCl. AgCl is simple to remove, requiring only a rinse with an ammonia solution. Ag₂S is very stable, and is difficult to remove from the membrane without altering the structure. This it is important that the SPI Silver Membrane filters be kept in a sealed package as long as possible prior to use.
For really long term storage of the packages, after opening, we would recommend storage in some kind of an inexpensive vacuum desiccator cabinet.
The compounds formed on the SPI Silver Membrane filters with time should not be confused with the natural grayish-white appearance of the silver membrane filter surface. This appearance is due to the microporous structure of the membrane, which reflects light in a way that differs from polished silver.

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