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D5909-20 – Standard Test Method Technical Guide
ASTM D5909-20 describes the referee procedure for determining the drying time of oxidative-drying printing inks (commonly known as sheetfed inks) by evaluating squalene resistance. Squalene, an unsaturated aliphatic hydrocarbon (C₃₀H₅₀), imitates the action of human skin oil to test whether a printed ink film has fully polymerized. It serves as the definitive test when other less rigorous methods yield conflicting results.
📐 Scope and Significance
This test method is applicable to all paste inks that dry primarily by oxidation, regardless of the substrate on which they are printed. With appropriate changes in the test fluid, it may also be used with paste inks that dry by other mechanisms, such as heatset or ultraviolet light (Section 1.2). The method utilizes a modified rub tester and is specifically intended to serve as a “referee” procedure when laboratories cannot agree on their results from less rigorous test procedures (Section 1.3).
Inadequate setting or drying of sheetfed inks can cause blocking of stacked prints with subsequent loss of product. The standard highlights that primary causes of drying failure include the omission of metallic driers from the ink, improper ink formulation, unusual ink-substrate interactions, and the use of a fountain solution that is too acidic (Section 5.1).
📌 Important Distinction: The standard clearly differentiates between “setting” (a rapid rise in viscosity preventing transfer under light pressure) and “drying” (full chemical polymerization). Squalene resistance specifically targets the verification of complete drying, not just surface setting.
⚙️ Test Procedure and Equipment
The test core is a modified rub tester, consistent with Practice D5264. A test print is affixed to the base of the instrument. A removable one-half pound weight is fitted with a piece of rubber blanket (a composite structure of a rubber-like layer supported by fabric or foam) and covered with an absorbent towel to which a small amount of squalene oil has been added. The weighted, oil-impregnated towel rubs back and forth over the test print for a prescribed number of cycles.
After the test, the receptor towel is examined for evidence of ink transfer. The intensity of the stain can be assessed qualitatively against an agreed-upon standard or measured quantitatively with a colorimeter, spectrophotometer, or other reflectance-type device (Section 4.2). This provides objective, repeatable data.
| 🟦 Parameter | 📏 Specification |
|---|---|
| 🎯 Test Instrument | Modified Rub Tester (Sutherland type, per D526
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