Nonvolatile residue (NVR )—NVR is the quantity of molecular matter remaining after the filtration of a solvent containing contaminants and evaporation of the solvent at a specified temperature. More simply, NVR is a measure of contamination that will not evaporate. NVR is a residue of indistinct dimensions that can consist of high molecular weight organic moieties (e.g., hydrocarbons, silicones, alkyl phthalates, oligomers) or ionic species present in the wiper matrix. The composition of the residue will depend on the extractant solution used. Organic solvents such as alcohol will favor the extraction of organic residues, whereas water will be more efficient at extracting ionic constituents. Low-NVR wipers are specified for cleaning cleanrooms, so that the cleaning agents will not dissolve appreciable amounts of contaminants from the wiper to deposit on the wiped surface. Cleanroom wipers are commonly tested for NVR using 100 percent isopropyl alcohol (IPA) or deionized water as the extractant. An ASTM procedure and the RP4.3 document provide a comprehensive description of testing wipers for NVR. The RP4.3 document suggests either of two extraction condi- tions—boiling a quantity of wipers in the sol-vent of interest for five minutes or steeping the wipers in the solvent at ambient temperature for 10 minutes. After extraction, the solution is filtered and then evapo- rated. The weight of the residue remaining after evaporation is the NVR and is expressed either in g/m~2 or g/g of fabric. To avoid weighing errors, at least 50 mg of residue should be available after extraction of the fabric and evaporation of the solvent. This dictates the number of wipers that must be collectively extracted.
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