ASTM D 3596

Summary of ASTM D3596

Scope:
This practice provides a quantitative method to determine the number of gels (fisheyes) remaining in flexible vinyl compounds made from general-purpose PVC resins under specified processing conditions.

Principle:
PVC resin is mixed with plasticizer (DOP) and calcium stearate, then milled on a two-roll mill at controlled temperature. A small amount of carbon black dispersion is added for contrast. The resulting sheet is press-polished, and gel particles (fisheyes) are counted under magnification using a bottom-lighted viewing box.

Significance and Use:
Gels (fisheyes) are hard, unfused resin particles that fail to melt during processing.
Their presence indicates incomplete fusion or contamination and negatively affects product quality.
This test helps evaluate resin quality and process uniformity.

Apparatus Required:

• Laboratory mixer and two-roll mill
• Hydraulic press with heating and cooling
• Magnifying comparator and viewing box
• Pyrometer, balance, timer, and standard measuring tools

Materials Used:

• PVC resin
• DOP (Dioctyl Phthalate) plasticizer
• Calcium stearate
• Carbon black dispersion

Procedure (summary):

1. Mix PVC resin, DOP, and calcium stearate.
2. Mill the blend at 154 ± 3 °C to form a uniform sheet (0.64 mm thick).
3. Add carbon black dispersion during milling.
4. Press-polish the sheet at 163 °C to 0.5 mm thickness.
5. Inspect the sheet under a lighted magnifier.
6. Count and classify gels by size:
   • A: ≥ 0.5 mm
   • B: ≥ 0.4 mm
   • C: 0.3–0.4 mm

Report Includes:

• Gel count per unit area
• Mill specifications and sheet thickness
• Any deviations from standard procedure

Precision:
Interlaboratory tests showed repeatable precision for gel size classifications (A ±1, B ±5, C ±12).
No bias statement is possible since there is no absolute reference method.

Keywords:
PVC resin, fisheyes, gels, quality control.