Quantifying Residual Coning Oils in Quantitative Viscose Content Determination
Unextracted coning oils add false mass to insoluble fiber fractions during quantitative chemical dissolution, distorting declared viscose blend percentages.
Formic Acid Zinc Chloride Dissolution
Quantitative Analysis
Chemical separation of fiber mixtures utilizes selective solvents to dissolve one component while leaving the other intact for weighing. A procedure known as formic acid zinc chloride dissolution is used to separate cotton from certain regenerated cellulose fibers or to isolate nylon in complex blends. The test is complete when the residue is washed, dried, conditioned and weighed to a constant mass.
It stops being effective if the sample contains fibers that are both soluble in the reagent.
Solvent Selection
Aggressive chemicals are required to break the hydrogen bonds in a polymer chain without damaging the secondary fiber in the sample. In the formic acid zinc chloride dissolution process, the concentration of the acid and the temperature of the bath are strictly controlled to prevent the degradation of cotton. The reagent is specific to the dissolution of viscose, modal or lyocell.
Fiber Separation
Recovering the insoluble portion of the specimen allows the lab to calculate the original percentage of each fiber type. When formic acid zinc chloride dissolution is performed correctly, the zinc chloride acts as a swelling agent that assists the acid in penetrating the crystalline regions of the cellulose. It provides a more reliable result than using simple acids alone.
Analytical Constraint
Limitations of the method appear when the fabric contains highly cross-linked finishes or pigments that resist the solvent. Formic acid zinc chloride dissolution cannot be used if the cotton itself has been chemically modified to the point of being soluble in the reagent. It is the preferred method for verifying the label claims of intimate blends.