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D2839-16 – Standard Test Method Technical Guide
📌 Scope and Significance of D2839-16
This standard practice provides a method for preparing polyethylene samples specifically for density determination using the density-gradient technique outlined in Test Method D1505. The sample preparation relies on producing a consistent strand via an extrusion plastometer under Condition 190/2.16 (Melt Index) as defined in Test Method D1238. The values stated in SI units are to be regarded as the standard.
The practice is exceptionally valuable for quality control in resin manufacturing environments where rapid, reproducible comparative density values are critical for process monitoring. However, it is explicitly noted that this practice is not necessarily recommended for establishing resin specifications in buyer-seller contracts. The density of polyethylene is highly dependent on thermal history, and results from this strand method are known to differ from those obtained by compression molding per Practice D4703, Annex A1. Always consult the material specification, as its parameters take precedence over the defaults in this practice.
🧪 Sample Preparation and Apparatus
To create a test specimen, the polyethylene sample is extruded in an extrusion plastometer according to Test Method D1238 at Condition 190/2.16 to produce a melt index strand. The required apparatus includes the plastometer, a hot plate to boil water, and 250-mL low-form beakers with watch-glass covers. The boiling water step is essential for wetting the cut strands and removing air bubbles, preparing them for accurate placement in the density gradient column specified in D1505.
| 🔧 Key Parameter | 📐 Specification |
|---|---|
| Sample Type | Polyethylene (any form suitable for D1238) |
| Extrusion Condition | 190/2.16 (Temperature 190°C, Load 2.16 kg) |
| Produced Specimen | Melt Index Strand |
| Density Test Method | D1505 (Density-Gradient Technique) |
📊 Referenced Standards and Metrological Comparison
This practice relies on three key ASTM standards. The extrusion plastometer and conditions are governed by D1238. The actual measurement of density is performed using D1505, which houses the precision data for this preparation method. The alternative preparation method is governed by D4703.
| 📂 Standard | 📝 Role in D2839 |
|---|---|
| D1238 | Defines the Melt Index test and extrusion plastometer used to create the strand. |
| D1505 | Governs the density gradient column used to measure the density of the strand. |
| D4703 | Provides an alternative sample prep method (compression molding) which yields different density levels. |
✅ Quality Control Application: D2839 is specifically recommended for quality control purposes where fast, reproducible comparative results are needed. The simplicity of the strand preparation minimizes variables compared to molding.
⚠️ Critical Note on Density Variability: The density of a polyethylene sample is highly dependent on its thermal history. The level of density obtained by this strand preparation practice differs from that obtained by other methods, such as the compression molding technique in D4703. Density values are therefore not directly interchangeable between these methods.
❓ Frequently Asked Questions
🔍 How is a “Melt Index Strand” defined in this standard?
It is the extrudate produced specifically when polyethylene is extruded in accordance with Test Method D1238, Condition 190/2.16. This strand is the test specimen used for the density determination.
📌 When should I use this practice instead of D4703 compression molding?
Use this practice when a fast, comparative density measurement is needed for internal quality control, especially in a resin manufacturing facility. For official specifications or sales contracts, D4703 or the specific material specification is generally preferred, as the density levels differ between the two preparation methods.
⚡ What happens if my material specification has different conditioning requirements?
According to Section 4.3, any specimen preparation, conditioning, or testing parameters covered in the material specification take precedence over the default conditions mentioned in this practice. If no material specification exists, the default conditions of D2839 apply.
💡 Where can I find the precision data for this method?
The precision data on densities measured using this sample preparation practice is not listed within D2839 itself. Instead, it is explicitly stated in Note 2 of the standard that the precision data is listed in Test Method D1505.