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D1921-18 – Standard Test Method Technical Guide
📋 Scope and Overview
ASTM D1921-18 describes test methods for measuring particle size of plastic materials in powdered, granular, or pelleted forms using dry sieving. The lower measurement limit is approximately 38 µm (No. 400 sieve); for smaller particles, sedimentation methods are recommended. These tests are crucial for assessing lot-to-lot uniformity and for understanding handling and processing characteristics of polymers. The standard is under the jurisdiction of ASTM Committee D20 on Plastics.
⚙️ Test Methods and Procedures
Two test methods are specified. Test Method A uses multiple sieves to determine mean particle diameter and particle size distribution. Test Method B is an abbreviated version with a few sieves to determine percent passing or retained, ideal for pellets or cubes without normal size distribution. The procedure involves placing a dry mass on a series of sieves arranged by increasing fineness and dividing the mass into fractions by sieve openings.
| 🟦 Test Method | 📏 Purpose | 📐 Sieve Usage | 🎯 Applicability |
|---|---|---|---|
| Method A | Mean particle diameter and distribution | Multiple sieves spanning particle size | General plastic materials |
| Method B | Percent passing or retained | Few specific sieves | Pellets, cubes |
📊 Sieve Specifications and Interferences
The standard uses SI units for wire cloth openings and wire diameters, with sieves conforming to ASTM E11. Interferences include static charge, which requires antistat use, and sieve overloading, which causes blinding. Wavy or torn wires invalidate results.
⚠️ Interference: Static charge during sieving can cause coarse bias. Antistats are essential for accurate results. Avoid overloading sieves to prevent blinding.
💡 Best Practice: Always inspect sieves for conformity to E11 specifications. Use sieves without wavy or torn wires to ensure accurate particle separation.
❓ Frequently Asked Questions
🔍 What is the lower particle size limit? The lower limit is about 38 µm (No. 400 sieve). Sedimentation tests are recommended for smaller sizes.
💡 How do you handle static charge? Use an antistat to reduce charge buildup and prevent coarse bias in results.
⚡ What are the main differences between Method A and B? Method A is comprehensive for distribution; Method B is for specific percent passing/retained on key sieves.
📌 What forms of plastic are suitable? Powdered, granular, or pelleted forms are suitable for these test methods.