The short answer
Start with any label or lot number, then use quick shop tests to narrow it down: clarity and fibers, and a float test for density (PP and PE float in water; almost everything else sinks). For a reliable identification, send a small sample to a lab for FTIR spectroscopy, which identifies the polymer family, plus DSC for the melting point and an ash or TGA test for filler content. Burn tests are crude and give off toxic fumes; skip them if you can.
Every plant ends up with one: a gaylord with no label or a box of mixed pellets. Here is how to find out what it is, roughly in order of cost.
1. Look for paperwork first
Check the gaylord walls, the liner, the pallet tag and old receiving records for a grade name, lot number or ship date. A lot number can often be traced back through the supplier to a COA. If you find a material code, the material code decoder explains the abbreviations, and the resin glossary covers the terms.
2. Visual clues
- Water-clear pellets: PC, PMMA, GPPS, SAN or amorphous PET/PETG. Few other resins are glass-clear.
- Milky or translucent natural pellets: PP, PE or nylon. PE feels waxy, and LDPE marks easily with a fingernail; PP feels harder.
- Opaque white or ivory natural: often acetal, PBT or ABS.
- Glass fibers: cut a pellet and look at the cut face under a loupe; short fibers often stand proud. Glass-filled pellets feel rough and are heavy for their size.
- Pellet shape (cylinders vs. spheres) tells you how they were cut, not what they are.
Clear vs. opaque also hints at structure: amorphous resins can be clear; semi-crystalline resins are usually translucent or opaque.
3. Density and the float test
Drop a few pellets in water with a drop of dish soap (to break surface tension and release air bubbles). PP and PE float; PS and ABS sink slowly; nylon, PC, polyesters, acetal and PVC sink. To go further:
- Alcohol and water. Isopropyl alcohol is about 0.79 g/cm³. Mixed with water and checked with a hydrometer, it gives liquids between 0.79 and 1.00 that can separate PP from HDPE.
- Salt water. Saturated salt water is about 1.2 g/cm³. Nylon floats in it, acrylic and PC sit close to the line, and PBT, PET, PVC and acetal sink.
- Weigh in water. A lab balance with a density kit (ASTM D792 / ISO 1183) gives density to two decimals and is far better than float tests.
| Resin | Typical density, g/cm³ | In water (1.00) | Melting point (DSC) |
|---|---|---|---|
| PP | 0.90–0.91 | Floats | 160–165°C / 320–329°F |
| LDPE, LLDPE | 0.91–0.94 | Floats | 105–125°C / 221–257°F |
| HDPE | 0.94–0.97 | Floats | 125–135°C / 257–275°F |
| PS (GPPS, HIPS) | 1.03–1.05 | Sinks slowly | None (amorphous) |
| ABS | 1.03–1.07 | Sinks slowly | None (amorphous) |
| PA6 | 1.13 | Sinks | About 220°C / 428°F |
| PA66 | 1.14 | Sinks | 260–265°C / 500–509°F |
| PC/ABS | 1.10–1.20 | Sinks | None (amorphous) |
| PMMA (acrylic) | 1.17–1.19 | Sinks | None (amorphous) |
| PC | 1.20 | Sinks | None (amorphous) |
| TPU | 1.10–1.25 | Sinks | Broad, grade-dependent |
| PBT | 1.30–1.31 | Sinks | 220–230°C / 428–446°F |
| PET | 1.33–1.40 (amorphous to crystallized) | Sinks | 245–260°C / 473–500°F |
| Rigid PVC | 1.35–1.45 | Sinks | No sharp melting point |
| POM (acetal) | 1.41–1.42 | Sinks | About 165°C / 329°F copolymer; 175°C / 347°F homopolymer |
| PTFE | 2.13–2.20 | Sinks | About 327°C / 621°F |
| PA66 30% glass | 1.37–1.39 | Sinks | 260–265°C / 500–509°F |
| PBT 30% glass | 1.52–1.54 | Sinks | 220–230°C / 428–446°F |
Typical values for unfilled natural resin unless noted; grades vary. Glass, mineral, flame retardants and pigments raise density (talc-filled PP can sink in water), and foamed or voided pellets read low.
4. Melting point
A hot plate with a surface thermometer gives a rough softening range, enough to tell PE from PP with care. It gives off fumes, so ventilate. A DSC run is much better: it shows a sharp melting peak for semi-crystalline resins and only a glass transition step for amorphous ones. See glass transition vs. melting point for typical values.
5. The burn test, and why to be careful
Molders have used burn tests for decades: flame color, smoke, dripping, whether it keeps burning, and smell. As rough cues, PE and PP burn with a blue flame tipped with yellow and drip, with a candle-wax smell; PS and ABS give a sooty yellow flame; nylon smells like burnt hair; acetal burns with a nearly invisible blue flame and a sharp formaldehyde smell; PVC tends to self-extinguish with an acrid smell. Flame retardants, fillers and blends change all of these.
The fumes are toxic. PVC releases hydrogen chloride. Acetal releases formaldehyde. Nylon, ABS and other nitrogen-containing resins can release hydrogen cyanide among their combustion products, along with carbon monoxide from almost everything. Fluoropolymers such as PTFE give off fumes that cause polymer fume fever: never burn them. If you do a burn test, use one or two pellets, metal tweezers, outdoors or under a fume hood, a fire-safe surface, and do not breathe the smoke. Better yet, skip it and send a sample to a lab.
6. Lab methods
| Test | What it tells you | Reliability | Relative cost |
|---|---|---|---|
| Labels, paperwork, lot number | Often the exact grade | High, if the label belongs to the contents | Free |
| Visual (clarity, color, fibers, feel) | Narrows the family | Low | Free |
| Float test (water, alcohol mix, salt water) | Density band | Medium for single, unfilled resins; poor with fillers | Very low |
| Density by weighing in water (ASTM D792 / ISO 1183) | Density to about 0.01 g/cm³ | Medium to high as a screen | Low |
| Hot plate melt check | Rough softening or melting range | Low | Very low |
| Burn test | Flame, smoke, smell, dripping | Low; additives mislead it; toxic fumes | Very low cost, real safety risk |
| DSC | Melting point and glass transition | High for semi-crystalline resins | Moderate lab fee |
| FTIR spectroscopy | Polymer family, some additives | High for most single resins; harder with blends and heavily black-filled grades | Moderate lab fee |
| TGA | Filler, glass and volatile content | High for filler level | Moderate lab fee |
| Ash test (ASTM D5630 / ISO 3451) | Glass or mineral content by weight | High for filler level | Low to moderate |
| Melt flow (MFI) | Flow class within a family, once identified | High for grade matching, useless alone | Low to moderate |
Reliability for identifying the base resin; a full match to a specific grade also needs filler content, melt flow and often additive analysis.
FTIR is the standard answer. A lab compares the infrared spectrum to a reference library and names the polymer family. DSC then separates close relatives (nylon 6 vs. 66, PBT vs. PET, acetal copolymer vs. homopolymer), and TGA or an ash test gives the filler level. Together they usually tell you, for example, "PA66 with about 30% glass", which is enough to decide what to do with the box.
What to do next
- If you need it for production: identify it, then trial it. An identified family is not a qualified grade.
- If you want to sell it: identified material is worth more. Send us the details, with photos and anything you know. A polymer chemist can tell you which tests are worth doing and help arrange a lab check.
- If you need to replace it: send what you found (FTIR result, density, melting point, filler level) and we will quote a matching grade. We aim to send your quote within one to three business days.
Common questions
How can I tell what plastic pellets are?
Start with any label, paperwork or lot number. Then look at clarity, color and fibers, and do a float test for density. For a reliable answer, send a small sample to a lab for FTIR spectroscopy, which identifies the polymer family, and DSC, which gives the melting point.
How do you tell PP from PE pellets?
Both float in water. PP is about 0.90 g/cm³ and PE about 0.91–0.97, so a density measurement separates PP from HDPE, but not always from LDPE. A DSC melting point settles it: PP melts around 160–165°C / 320–329°F, HDPE around 125–135°C / 257–275°F.
What is the most reliable way to identify a plastic?
FTIR spectroscopy, compared against a reference library, identifies the polymer family in most cases. Pair it with DSC to separate close relatives such as nylon 6 and nylon 66, and with an ash or TGA test to measure filler content.
Is a burn test safe for identifying plastic?
It carries real risk. Burning plastic gives off toxic fumes, including hydrogen chloride from PVC, formaldehyde from acetal, and carbon monoxide and, for nitrogen-containing resins such as nylon and ABS, possibly hydrogen cyanide. If you do it at all, burn only a pellet or two outdoors or under a fume hood, and never burn fluoropolymers. Pellets that sink fast and measure about 2.1–2.2 g/cm³ may be PTFE; do not burn them.
Can a float test tell nylon 6 from nylon 66?
Not reliably. Their densities, about 1.13 and 1.14 g/cm³, are too close, and fillers and pigments shift both. A DSC melting point separates them easily: about 220°C / 428°F for nylon 6 and 260–265°C / 500–509°F for nylon 66.
How do I find out how much glass is in a resin?
Burn off the polymer in a muffle furnace and weigh what is left (an ash test, ASTM D5630 or ISO 3451), or run TGA. The residue percentage is the glass or mineral content.
Can unidentified resin be sold?
Sometimes, at a lower value than labeled material. Identifying it first, with FTIR, DSC and a filler test, usually raises what a buyer will pay.
Tell us what you found
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province