Method of cleaning membrane filter

DETAILED DESCRIPTION Typical cataphoretic paints which have been found to foul the membranes are those which have acidic pH's, i.
e.
, less than 7, in which the resinous vehicle of the paint contains amine salt groups or quaternary ammonium salt groups.
Resinous vehicles containing amine salt groups are described in U.
S.
Pat.
Nos.
3,799,854; 3,984,299, both to Jerabek; and 3,947,338 to Jerabek, Marchetti and Zwack.
Quaternary ammonium salt group-containing resins useful in electrodeposition paints are described in U.
S.
Pat.
Nos.
3,962,165 and 3,937,679 to Bosso and Wismer.
Besides the resinous vehicle, the cataphoretic paints also contain lead.
The lead is usually added in the form of pigments such as lead silicate, lead oxide, lead carbonate and lead chromate but may also be added in the form of soluble salts such as described in U.
S.
patent application Ser.
No.
785,154, filed Apr.
6, 1977, to Zwack and Jerabek.
Cataphoretic paints also contain other pigments, co-solvents, surfactants and optional ingredients such as plasticizers and stabilizers such as are described in the aforementioned patents.
As mentioned above, the types of membranes which are fouled in the process of the invention are ultrafiltration membranes.
The use of ultrafiltration for treating an electrodeposition bath is described in U.
S.
Pat.
No.
3,663,405 to Christenson et al.
Ultrafiltration involves forcing the paint under a pressure gradient against a suitable membrane.
The membrane passes low molecular weight species such as water and organic co-solvents and low molecular weight impurities which accumulate in the bath such as metal pretreatment chemicals such as chromates and phosphates and other ions such as chloride and sulfate and low molecular weight resinous fragments while retaining higher molecular weight vehicle resins and pigments.
Ultrafiltration operates at a pressure of about 10 to 150 psi and usually from about 25 to 75 psi.
The basic ultrafiltration process is relatively simple.
The paint to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in the cell, in contact with an appropriate ultrafiltration membrane suitably supported