The paper used as the filtration medium for automotive type filters has been traditionally treated with phenolic resole type resins. This has been done to improve the paper's strength properties and allow it to be pleated in an accordian-like shape and to hold this shape when the paper composite is cured. The standard phenolic resin used to treat automotive filter paper had relatively low mol ratios, on the order of 1.0/1 to 1.3/1 formaldehyde to phenol. This has been necessary so that good final paper properties, especially flexibility, could be achieved. Higher mol ratio resins tend to result in brittle paper on curing.
The traditional method of making an automotive filter has been for a paper maker to treat a base filter sheet with an alcohol solution of these phenolic resole resins. The treated sheet was passed through an oven to drive off the solvent and make a so-called B-stage sheet. This sheet was then shipped to the filter maker where it was pleated and put through an oven to further cure the sheet and to hold the shape of the pleats.
With the onset of greater concern for environmental quality, the filter manufacturers have requested paper suppliers to supply a formaldehyde-free impregnated sheet that holds its pleat, and meets all of the requirements for oil filters.
This invention is directed to oil filters and oil filter media impregnated with a water-based latex containing polymerized vinyl chloride. Such oil filters are manufactured with only a single cure and do not impair the environment by emitting volatile solvents.