How to Identify Recycled Polyester: The GB/T 39026 Lab Test Explained

More and more buyers ask us the same question: how do you identify recycled polyester once it has already been spun into yarn? A burn test won’t answer it, and neither will a microscope. Recycled and virgin PET are the same polymer, so they melt, smell and look alike. The real answer comes from the lab, through China’s national standard GB/T 39026-2020. This method uses high-performance liquid chromatography (HPLC) to read the small chemical traces that recycling leaves behind in the fiber.

The standard took effect on February 1, 2021. At release, it was described as filling a gap both in China and internationally, and it gives policymakers a technical basis for rules on recycled fiber. For anyone sourcing GRS yarn, it is one of the few tools that checks a recycled claim against the fiber itself instead of the paperwork. Below, we walk through how the method works, how to read its chromatograms, and how we handle recycled polyester verification on real orders.

What Counts as Recycled Polyester (rPET Fiber)

GB/T 39026 defines recycled PET fiber as polyethylene terephthalate fiber made from recovered waste PET polymer and waste PET textile materials. Put simply, recycled polyester starts life as waste. Typical feedstock includes bottle flakes, foamed PET scrap, waste filament, waste slurry from polymer plants and used textiles. Recyclers clean that material, melt it again and spin new fiber.

That second trip through the melt is the key point. Virgin polyester comes from fresh raw materials and goes through polymerization and spinning once. Recycled polyester carries extra thermal history, and sometimes material from a different PET grade, such as bottle resin. Those differences leave chemical fingerprints, even when the finished cone looks exactly like a virgin one.

Why You Can’t Identify Recycled Polyester by Look, Touch or Burn Test

In our QC routine, the burn test and the microscope do a good job of sorting fiber families. We explain how in our guide to fiber identification methods. But these checks stop at “this is polyester.” Recycled PET melts and shrinks near the flame, smells sweetish and leaves a hard black bead, just like virgin PET. Under the microscope, both show the same smooth, round or profiled shape.

So the difference has to come from chemistry, not from the eye or the nose. That is the job GB/T 39026 was written for.

The Principle Behind GB/T 39026: What the HPLC Test Measures

The whole method rests on one idea. Recycled and virgin polyester follow different production routes, and those routes change certain molecular features. Recycling shifts two things in particular: the heterogeneous units in the polymer chain, meaning segments that differ from the standard PET repeat unit, and the content and distribution of oligomers, the short chains that sit inside the fiber.

To identify recycled polyester through those shifts, the lab treats the sample under fixed conditions. It then runs the resulting solution on an HPLC and compares relative peak areas at specific retention times. Different peak area patterns point to different processing histories, and that is how the method separates recycled from virgin fiber. The result is qualitative: recycled or not recycled.

A 2025 study from two Portuguese textile technology centres gives a concrete example of such a marker. Bottle-grade PET often contains isophthalic acid units, and methanolysis converts them into dimethyl isophthalate, which shows up clearly on the chromatogram. The same peer-reviewed study in Materials also points out that GRS and RCS certification confirm the presence of recycled content but do not measure its exact amount.

Step by Step: How Labs Prepare Samples Under GB/T 39026

To identify recycled polyester reliably, the standard uses two separate sample treatments. Each one produces its own solution and its own chromatogram, and the lab reads both before it reaches a verdict.

Route 1: Methanolysis

Methanolysis breaks the polyester chain apart with methanol, which makes chain-level differences measurable.

1. Weigh the test sample and add the transesterification solution.
2. Hold the mixture at 220 ± 5 °C for 2 hours.
3. Cool it to room temperature, then filter.
4. Refrigerate the filtrate at 4–6 °C for 8 hours.
5. Filter again, let the solution return to room temperature and load it for HPLC analysis.

The study mentioned above describes this route in more detail: about 0.80 g of fiber, methanol with a zinc acetate catalyst, and a stainless-steel reaction tube. In practice, that means the lab needs very little yarn. One cone easily covers a few repeat runs.

Route 2: Swelling-Extraction

The second route doesn’t break the chain. Instead, it swells the polymer so that the small molecules trapped inside, mainly oligomers, can move out into the solvent.

1. Pre-dry the sample: weigh it, dry it at 105 °C for 30 minutes, then let it cool.
2. Weigh the dried sample and add the specified swelling solution.
3. Hold at 180 ± 5 °C for 90 minutes.
4. Take it out, cool it and centrifuge for 8 minutes.
5. Draw off the clear upper layer and filter it.
6. Refrigerate at 4–6 °C for 8 hours.
7. Filter once more, bring it to room temperature and run it on the HPLC.

Add up the hot and cold steps and you can see why a GB/T 39026 report never comes back the same day. Sample preparation alone takes more than ten hours per route, before any instrument time or waiting in the lab queue.

HPLC Detection and the Gradient Program

Both solutions run on the HPLC with gradient elution. In other words, the ratio of the two mobile phases changes during the run. Table 1 of the standard sets the program:

Time (min)	Mobile phase 1 (% by volume)	Mobile phase 2 (% by volume)
0.00	20.0	80.0
15.00	80.0	20.0
20.00	100.0	0
35.00	100.0	0

From 20 to 35 minutes, the column holds at 100% mobile phase 1. That hold covers the late peaks that appear around 21 minutes in the swelling-extraction run.

Reading the Chromatograms: Locating Peaks vs Characteristic Peaks

Each chromatogram carries two kinds of peaks. Locating peaks work as landmarks. They tell the analyst where they are on the time axis, so the right peaks can be found even when retention times drift slightly between runs. Characteristic peaks carry the actual signal, because their relative areas are what separate recycled from virgin polyester.

In the methanolysis run, peak 1 appears near 12.3 minutes, while peak 2 is the tall, dominant peak just before 16 minutes. The characteristic peaks, peak 3 and peak 4, sit on either side of it at about 15.3 and 17.0 minutes.

The swelling-extraction run looks busier. Here, peak 5 at about 21.25 minutes is the locating peak, and the characteristic peaks 6 and 7 appear at 20.81 and 15.46 minutes. Table 2 of the standard lists every position:

Chromatogram	Peak	Apex (min)	Integration window (min)
Methanolysis solution	Locating peak 1	12.33	12.11–12.55
Methanolysis solution	Locating peak 2	—	—
Methanolysis solution	Characteristic peak 3	15.33	15.21–15.45
Methanolysis solution	Characteristic peak 4	16.98	16.85–17.10
Swelling-extraction solution	Locating peak 5	21.25	21.10–21.40
Swelling-extraction solution	Characteristic peak 6	20.81	20.71–20.90
Swelling-extraction solution	Characteristic peak 7	15.46	15.35–15.56

Notice that Table 2 gives no apex time or integration window for peak 2. Each of the other six peaks has a narrow window, roughly 0.1 to 0.2 minutes on each side of the apex. A small retention shift is therefore enough to move a peak out of its window, which is why stable column conditions matter so much in this test.

Data Processing: How the Lab Reaches a Verdict

Recycling changes the content and distribution of heterogeneous chain units and oligomers, and those changes form the basis for telling recycled polyester from virgin polyester. This is the step where labs actually identify recycled polyester. The analyst integrates each peak inside its window, calculates the relative peak areas and compares them with the criteria set out in the standard. The report then states whether the sample is identified as recycled PET fiber.

When we review a report, we ask for more than the conclusion line. We want the chromatograms and the calculated values attached. If a result is ever challenged, those numbers are what a second lab can check.

What GB/T 39026 Can and Cannot Tell You

• Recycled or virgin: this is the question the method answers, based on chemical markers in the fiber.
• Recycled percentage: outside its scope, because the standard is a qualitative identification method.
• Origin and chain of custody: a chromatogram can’t show where the waste came from or who handled it.
• Blends, dyes and finishes: these can complicate the run, so declare the full composition before testing.

That last point matters for our own range. Our GRS certified recycled polyester color-spun yarn, for example, is a 90/10 blend of recycled polyester and cotton, so the lab needs to know about the cotton fraction from the start. Newer methods come with their own limits as well. A 2025 group standard from the China Chemical Fibers Association, T/CCFA 00014-2025, uses UV-visible spectroscopy. However, it only covers undyed and white recycled polyester made from bottles, and it excludes colored fiber.

For recycled content claims, documentation still carries most of the weight. Textile Exchange’s Global Recycled Standard verifies recycled material through third-party audits and chain-of-custody records. It also requires at least 50% recycled content for consumer-facing labels. A lab test and a transaction certificate answer different questions, and careful buyers ask for both.

How We Use Recycled Polyester Identification on Real Orders

From our factory view, the lab test is the last check, not the first. When a buyer asks us to identify recycled polyester in a yarn, or to prove our own, we start with the paperwork. The GRS transaction certificate, the lot number and the shade code all have to match on the PI, the packing list and the cone labels. If those don’t line up, no chromatogram will fix the problem.

For buyers who want a GB/T 39026 report, we pull cones from the bulk lot itself. Not the lab-dip cone, and not a bobbin from the sample room. The yarn that goes to the lab should be the same yarn that ran on our 18G sock machine for the trial knit, and the same lot that ships. We also seal a twin set of cones from that lot, so any retest uses identical material.

Honestly, the disputes we hear about on recycled yarn rarely involve fake fiber. They usually trace back to mixed lots, missing certificates, or a test run on a sample that never matched bulk. A clean sampling plan heads off most of that before it starts.

Checklist Before You Request a GB/T 39026 Test

• Choose a lab whose accreditation scope covers GB/T 39026-2020, and ask about turnaround up front.
• Send cones from the bulk lot, with the lot number and shade code written on the sample bag.
• Declare the full composition, including any cotton, spandex or functional additive.
• Request chromatograms and calculated peak area values in the report, not only the conclusion.
• Keep a retained sample from the same lot for any retest.
• File the report together with the GRS transaction certificate for that shipment.

Frequently Asked Questions

Can a burn test identify recycled polyester?

No. Recycled and virgin PET burn the same way. A burn test only confirms that the fiber is polyester.

Does GB/T 39026 give the recycled content percentage?

Not directly. It is a qualitative method that tells you whether the polyester is recycled or virgin, not how much recycled fiber a blend contains.

Is a GRS certificate enough without a lab test?

For most programs, GRS documentation is the main proof because it covers chain of custody. A lab test adds independent evidence from the fiber itself, which helps when a brand wants extra assurance or when someone questions a claim.

What is the most reliable way to identify recycled polyester yarn?

Combine two kinds of evidence. GRS chain-of-custody documents show where the material came from, and a GB/T 39026 lab test checks the fiber itself. Neither one replaces the other.

How long does a GB/T 39026 test take?

Each preparation route includes an 8-hour cold step plus heating time, so preparation alone runs past ten hours. Add instrument time and the lab’s queue, plan in days rather than hours, and confirm the timeline with the lab first.

Why Recycled Polyester Identification Matters More Every Year

Polyester consumption keeps rising, and environmental awareness keeps growing with it, so the industry pays more attention to recycling polyester waste. Turning that waste into recycled fiber can lower costs, cut oil consumption and improve economic returns. For the chemical fiber industry, it is an important part of long-term, sustainable development.

Growth also brings a new problem. As recycled polyester volumes increase, the industry looks harder at how far recycled and virgin polyester can replace each other. Their prices have also tended to move in the same direction. When the price gap narrows and the claims carry more weight, reliable ways to identify recycled polyester stop being a lab curiosity and become a normal part of sourcing.

Whether you need to identify recycled polyester in a new supplier’s yarn or document your own GRS program, start with the spec. Send us the yarn count, composition, end use and the documents your brand requires. We can match the GRS paperwork to the lot, prepare bulk-lot cones for a third-party GB/T 39026 test, and share trial-knit feedback before you commit to bulk. Talk to our team, and we will tell you clearly what we can verify, and what we can’t.