Melt index (melt flow rate) testing is the single most widely used quality-control measurement for recycled polyethylene, quantifying how easily post-consumer or post-industrial recyclate flows under standardized temperature and load conditions. Performed to ISO 1133-1 and ASTM D1238 at 190 °C with defined piston loads, the melt index of recycled PE reveals polymer degradation, contamination, and batch-to-batch consistency before the material enters extrusion, blow molding, or film production. This article explains the applicable standards, test samples, measured parameters, step-by-step procedures, application scenarios, and frequently asked questions related to recycled PE melt index determination.
Technical Overview
The melt index—formally the melt mass-flow rate (MFR), expressed in grams per 10 minutes—measures the mass of molten recycled polyethylene extruded through a standardized die in a capillary rheometer (extrusion plastometer) under a specified temperature and load. For polyethylene, the reference condition is 190 °C with a 2.16 kg load; low-flow HDPE grades are often evaluated at 190 °C with a 21.6 kg load. The governing standards are ISO 1133-1 and ASTM D1238, which are technically equivalent for Procedure A and Procedure B, along with the corresponding national standard GB/T 3682.1.
For recycled PE, melt index determination carries extra significance compared with virgin resin. Reprocessing introduces thermo-mechanical chain scission, which tends to raise the MFR, while partial crosslinking or oxidation may lower it. Contamination with polypropylene, fillers, or other polyolefins—common in post-consumer streams—shifts the flow value and increases scatter. As a result, the melt index serves as a fast, low-cost screening indicator of degradation level, contamination, and lot uniformity, and it directly informs process settings for extrusion, injection molding, blown film, and blow molding.
The melt mass-flow rate is calculated from timed extrudate segments as:
- MFR (g/10 min) = 600 × m / t, where m is the mass (g) of the extrudate cut in time interval t (seconds) and 600 s is the reference period of 10 minutes.
- The companion parameter MVR (melt volume-flow rate, cm³/10 min) is obtained from piston travel in Procedure B, using the melt density of PE at 190 °C (approximately 0.76 g/cm³) or a combined mass determination.
Testing Samples
Post-Consumer Recycled (PCR) HDPE
Derived from milk jugs, detergent bottles, and rigid packaging streams. This material typically contains residual caps (polypropylene), labels, and color pigments. Test results commonly range from below 1 g/10 min (blow-molding origin) to more than 8 g/10 min (injection-grade origin), and heterogeneity between bales makes representative sampling critical.
Post-Consumer Recycled LDPE / LLDPE Film
Derived from shopping bags, agricultural film, stretch wrap, and mixed packaging film. Film recyclate is frequently contaminated with polypropylene woven bags, inks, and EVA layers, so its melt index often shows wider lot-to-lot variation than bottle recyclate. Typical values lie between 0.5 and 4 g/10 min for film-origin materials.
Post-Industrial Recycled (PIR) PE
In-house regrind, purgings, skeletal film scrap, and off-spec virgin material from converters. Because the feedstock history is known, PIR recyclate usually yields narrower melt index distributions and is widely used in higher-specification blends with virgin PE.
Sample Forms and Preparation
- Accepted forms include pellets, granules, powder, film flakes, and shredded regrind.
- Recommended quantity: 300–500 g per submission to allow multiple charges and repeat determinations, drawn from several locations of the lot using quartering or riffling.
- Moisture control: dry flakes or pellets at 70–80 °C for 2–4 hours when moisture is suspected, since entrapped volatiles create bubbles in the extrudate and distort the measured mass.
- Do not test heavily contaminated, unpelletized, or wet material directly; oversize flakes should be reduced so the charge can be compacted uniformly in the barrel.
Testing Items
A typical recycled PE melt index test report includes the following items:
- Melt mass-flow rate (MFR) at 190 °C / 2.16 kg, in g/10 min, with individual values, mean, and scatter for at least two separate charges.
- High-load melt index at 190 °C / 21.6 kg for low-flow HDPE recyclate, where the 2.16 kg value is too small for precise measurement.
- Melt volume-flow rate (MVR) in cm³/10 min where Procedure B is specified.
- Flow rate ratio (FRR), for example MFR at 21.6 kg divided by MFR at 2.16 kg, as a sensitive indicator of shear-thinning behavior and contamination.
- Reporting details: standard and procedure used, test condition, preheat time, die/piston identification, sample description, drying history, and any anomalies such as bubbling, discoloration, or gels.
Testing Methods
Procedure A — Cut-and-Weigh Method
Procedure A of ISO 1133-1 (Procedure A of ASTM D1238) is the reference method for most recycled PE testing:
- Bring the plastometer to 190 °C, controlled within ±0.5 °C over the working zone; verify with a calibrated temperature sensor.
- Charge 3–8 g of sample into the preheated barrel within about one minute, compacting with the piston to eliminate air pockets.
- Preheat for the period prescribed by the standard (commonly about 5 minutes, adjusted according to flow rate), then apply the selected load (piston plus weight, e.g., 2.16 kg total).
- Allow the piston to descend to the measuring range; discard the first extruded segment, which contains air and degraded material.
- Cut successive extrudate segments at timed intervals chosen so each segment is roughly 10–20 mm long; collect at least three usable cuts and weigh each to 0.001 g.
- Calculate MFR = 600 × m / t for each cut; replicate values should agree within about 10–15 %, otherwise repeat with a fresh charge.
- Test a minimum of two independent charges and report the mean; for heterogeneous recyclate, three or more charges are recommended.
Procedure B — Piston-Displacement Method
Procedure B tracks piston travel with an optical or magnetic encoder and reports MVR directly without cutting and weighing. It is faster, produces a flow curve over the measurement window, and suits high-flow recyclates where cut timing is difficult. MFR is obtained from MVR using melt density, or by a combined measurement in which the extruded mass is collected once for calibration. For recycled PE containing volatiles or bubbles, Procedure A with careful drying is generally more reliable.
Special Considerations for Recycled PE
- Thermal degradation: for recyclates known to oxidize during the preheat, ISO 1133-1 permits testing under a nitrogen blanket to suppress degradation and obtain the true flow rate.
- Contamination screening: unexpectedly high or low MFR, excessive scatter between cuts, gels, or discolored extrudate indicate PP contamination, crosslinking, or mixed feedstock and should trigger further identification testing.
- Die and piston maintenance: clean the die bore (2.095 mm) and barrel after every charge; residue from filled or pigmented recyclate biases subsequent results.
- Sampling rigor: because recycled lots are less uniform than virgin resin, sample number and lot-representativeness matter more than instrument precision.
Applications
- Recyclate grading and trading: melt index is the primary specification parameter in purchase contracts and price negotiations for recycled PE pellets and flakes.
- Incoming inspection: converters verify supplier declarations before blending recyclate into virgin PE for blown film, bottles, pipes, crates, and injection-molded parts.
- Blend formulation: measured MFR of each stream allows engineers to set virgin-to-recycled ratios that keep the blend within the process window of the target application.
- Process setup: extrusion temperatures, screw speeds, and die pressures for film lines, blow molders, and injection molders are tuned to the melt index of the recyclate actually on the floor.
- Quality assurance for recycled-content claims: melt index data supports internal QC and documentation for recycled content certification programs and customer audits.
- Research and development: evaluating degradation across multiple reprocessing cycles, screening restabilization packages and compatibilizers, and qualifying new collection streams all rely on systematic melt index tracking in accredited third-party laboratories or in-house facilities.
FAQ
What is the standard test condition for recycled PE melt index?
Polyethylene is tested at 190 °C. The 2.16 kg load is the standard condition for most LDPE, LLDPE, and injection-grade HDPE recyclates; the 21.6 kg high-load condition is used when the 2.16 kg value falls below roughly 0.1–0.5 g/10 min, as with blow-molding-grade HDPE.
Why did my recycled PE show a higher melt index than the original virgin grade?
Thermo-oxidative and mechanical degradation during service life and reprocessing break polymer chains, lowering molecular weight and increasing flow. A moderate increase may simply reflect normal reprocessing history; a large jump or high scatter usually signals contamination, over-degradation, or a mixed feedstock that warrants further identification testing.
Procedure A or Procedure B — which should be used?
Procedure A (cut-and-weigh) is the reference method and the default for recyclates, especially when bubbles, volatiles, or inconsistent melting are possible. Procedure B is faster and better suited to high-flow materials or routine production monitoring with well-characterized, dry feedstock.
Can wet film flakes be tested directly?
No. Moisture generates steam bubbles in the melt, causing erratic extrudate masses and falsely scattered results. Dry the material at 70–80 °C for 2–4 hours, cool in a desiccator, and test immediately; if foaming persists, note it in the report.
How many measurements are needed for a reliable result?
At minimum, two independent charges with at least three timed cuts each. For heterogeneous post-consumer recyclate, three or more charges from a representative composite sample give a meaningful mean and range for the lot.
What acceptance tolerance is typical between replicate tests?
Cuts within a charge should agree within about 10–15 %, and the means of separate charges should generally agree within roughly 10 % for well-prepared recycled PE. Larger scatter points to sampling problems, moisture, or contamination rather than instrument error.
Is melt index alone sufficient to qualify recycled PE?
It is necessary but not sufficient. Melt index should be combined with density, tensile and impact properties, color, ash, and contamination identification (for example, checking for PP) to fully qualify recyclate for a given end use.