Microporous Polymer Products
Precipitated and fumed silicas and silica gels are used to make microporous polymer articles, notably for use as battery separators and polymer-based papers.
Separators for lead acid batteries are a significant but frequently overlooked application for precipitated silicas. The function of the separator is to prevent short circuiting by providing a barrier between the cathode and anode of the battery. At the same time, it must be permeable and allow ions to move freely between the electrodes and must also be robust and mechanically stable.
Separators for lead acid batteries can be made from various polymers, such as polyethylene (PE), polyvinylchloride (PVC), resins, or rubber, but polyethylene is the most common polymer used, due to its superior puncture resistance, flexibility, ability to be folded and sealed, oxidation resistance, and low acid displacement.
Ultrahigh molecular weight polyethylene (UHMW-PE) is preferred and is compounded with the precipitated silica, processing oil, and selected additives, such as lubricants, wetting agents, and carbon black. The compound is extruded as a sheet, and then the oil is solvent extracted to leave a microporous sheet where the pores are interconnected to generate a pathway.
The primary function of silica is to help generate the porosity. It also remains in the sheet making it hydrophilic and wettable by the sulfuric acid. The silica for battery separators needs to be tailor made for optimum performance.
A related application is in synthetic (polymer) papers. Synthetic paper is a print material made from plastic rather than wood pulp and mainly used in applications where the ability to perform in demanding environments is required. In its production, a precipitated silica is aggregated in a matrix with polyolefin to produce a smooth but highly porous printable surface. As with battery separators, the polymer is usually UHMW-PE, and a processing oil is used to aid compounding and extruding and is then solvent extracted.
Rheology Control Additives
This important application is common to both precipitated and fumed silicas and is covered later (section “Rheology Control Agents (Applicable to Both Fumed and Precipitated Silicas)”)
It is highly probable that development of precipitated silicas to better meet the varied and complex needs of the tire industry will continue to be a key activity. In particular we can expect to see more use in replacement and truck tires than is the present case. We may also see better uptake of silane pre-coated products, again principally for the tire industry, but this is by no means certain.