Plastics: Identification
How to sort and identify different types of plastic before recycling with SDS machines, from simple float tests to the Plastic Scanner app.
Why Do We Need To Identify Different Plastics?
Every plastic type has different properties, from melting temperature to flow behaviour to chemical resistance. If you mix the wrong types together in the Desktop Shredder V2 or Injection Mini V2, you get weak parts, contamination, or in the case of PVC, harmful fumes. Sorting before you process is not optional. It is the foundation of good recycled output.
The good news is that identification does not require a lab. A combination of simple hands-on tests (float, burn, break, smell) can narrow down most common plastics. And for fast, reliable results, the SDS Plastic Scanner app can identify a plastic type from a photo in seconds.
NEVER MIX PLASTICS!
Mixing different plastic types causes weak parts, accelerates microplastic breakdown, and can release harmful chemicals during processing. Always sort before you shred.
How To Identify Different Plastic Types
Resin Identification Code
Most plastic products are stamped with a resin identification code, a number from 1 to 7 inside a triangle of chasing arrows. This tells you exactly which plastic family the item belongs to. Check the base of containers, the inside of caps, or the back of panels. When the code is too small to read or missing entirely, the SDS Plastic Scanner app can identify the plastic from a photo instead.
Float Test
Every plastic has a specific density. By dropping a sample into liquids of known density (water, salt water, ethanol, glycerine), you can separate plastics into groups based on whether they float or sink. Water alone splits plastics into two camps: PP, LDPE, and HDPE float. Everything else sinks.
Burn Test
When a small sample is held to a flame, each plastic type produces a distinctive combination of flame colour, smoke, smell, and dripping behaviour. PP and PE burn with a clean yellow/blue flame and smell like candle wax. PS produces thick black smoke. PVC shows a green tinge at the base and releases a sharp, acrid chlorine smell. This test should always be done in a well ventilated area or under extraction.
Break Test
Bending or snapping a plastic sample reveals its mechanical character. PP bends and snaps cleanly. LDPE stretches and tears gradually. PS and PLA are brittle and shatter. Nylon flexes significantly before breaking. Stress whitening (a visible white mark at the bend point) is another clue, common in HDPE and PLA but absent in PS and nylon.
Smell Test
Heating a small sample with a lighter produces a characteristic smell for each plastic type. PP and PE smell like paraffin candle wax. PS has a sweet, fruity, slightly chemical scent. Nylon smells like burnt hair or wool. PVC produces a sharp, acrid, chlorine smell that is unmistakable. The smell test works best as a confirmation step alongside the burn test.
IR / App Identification
Infrared spectroscopy measures how a material absorbs light at different wavelengths, producing a unique fingerprint for each plastic type. Industrial recycling facilities use handheld NIR scanners for rapid sorting. For desktop recyclers, the SDS Plastic Scanner uses your smartphone camera and machine learning to identify plastic types from a photo, no special equipment needed. It is free to download and works on iOS and Android.
The float test is safe for any setting. The burn and smell tests involve heating plastic, which produces fumes. Always work in a well ventilated area or use fume extraction. Wear safety glasses and heat resistant gloves. Never heat PVC intentionally, it releases hydrochloric acid gas.
If you are working in a school or shared workshop, check your local health and safety requirements before conducting burn tests. For more on workshop safety, see Basic PPE.
Float Test & Density Reference
Different plastics have different densities, measured in grams per cubic centimetre (g/cm3). If you drop a plastic sample into a liquid, it will float if it is less dense than the liquid and sink if it is more dense. By testing with a sequence of liquids, you can narrow down exactly which plastic you have.
The simplest starting point is plain water (1.00 g/cm3). PP, LDPE, and HDPE all float in water. PS, ABS, nylon, PLA, PET, and PVC all sink. From there, you can use a second liquid to separate within each group.
| Material | Type | Density (g/cm3) | In water |
|---|---|---|---|
| Ethanol | Test liquid | 0.79 | |
| Isopropyl Alcohol | Test liquid | 0.79 | |
| PP (Polypropylene) | Plastic | 0.89 to 0.91 | FLOATS |
| LDPE (Low Density Polyethylene) | Plastic | 0.91 to 0.93 | FLOATS |
| Vegetable Oil | Test liquid | 0.92 | |
| HDPE (High Density Polyethylene) | Plastic | 0.94 to 0.97 | FLOATS |
| Water (dividing line) | Test liquid | 1.00 | |
| PS (Polystyrene) | Plastic | 1.04 to 1.05 | SINKS |
| ABS | Plastic | 1.04 to 1.08 | SINKS |
| Ethylene Glycol | Test liquid | 1.11 | |
| Saturated Salt Water | Test liquid | 1.12 | |
| Nylon (PA) | Plastic | 1.13 to 1.15 | SINKS |
| PMMA (Acrylic) | Plastic | 1.17 to 1.20 | SINKS |
| PLA | Plastic | 1.24 | SINKS |
| Glycerine | Test liquid | 1.26 | |
| PET | Plastic | 1.38 to 1.40 | SINKS |
| PVC | Plastic | 1.40 | SINKS |
Burn Test Reference
The burn test is a quick way to narrow down a plastic type using a lighter or small flame. Hold a small sample (about 1cm) to the edge of a flame and observe four things: flame colour, smoke, smell, and whether the plastic drips or self-extinguishes. Always do this in a ventilated space and never burn PVC intentionally.
| Plastic | Flame | Smoke | Smell | Drips? | Self extinguishes? |
|---|---|---|---|---|---|
| PP | Yellow top, blue base | Minimal, clear | Candle wax, sweet | Yes | No |
| PE (LDPE/HDPE) | Yellow top, blue base | Minimal, clear | Candle wax, sweet | Yes | No |
| PS | Yellow, sooty | Thick black | Sweet, fruity, chemical | Minimal | No |
| ABS | Yellow, sooty | Thick black | Burnt rubber, acrid | Minimal | No |
| Nylon | Blue with yellow tip | Minimal | Burnt wool/hair | Yes | Yes |
| PET | Yellow, sooty | Black | Sour, acrid | Slight | Partially |
| PVC | Yellow, green at base | Thick black | Sharp chlorine, acrid | Slight | Yes |
| PMMA | Blue, clean | Minimal | Fruity, floral | Minimal | Sometimes |
| PLA | Yellow | Light | Slightly sweet | Slight | Partially |
Safety: PVC produces hydrochloric acid gas when burned. If you see a green flash at the base of the flame or smell chlorine, stop immediately and ventilate the area. This is actually a useful identifier, but exposure should be minimised.
Break Test Reference
The break test tells you about a plastic's mechanical properties. Take a thin sample and try to bend it. How it responds, whether it flexes, snaps, stretches, or whitens, narrows down the type.
| Plastic | Flexibility | Breaking pattern | Stress whitening |
|---|---|---|---|
| PP | Flexible, bends easily | Snaps cleanly | Moderate |
| LDPE | Very flexible | Stretches and tears gradually | Minimal |
| HDPE | Moderately flexible | Snaps cleanly | Yes, noticeable |
| PS | Rigid, brittle | Shatters or snaps suddenly | Minimal |
| ABS | Moderately rigid | Snaps cleanly | Moderate |
| PET | Moderate | Snaps with some stretch | Moderate |
| Nylon | Very flexible | Stretches significantly | Minimal |
| PVC | Moderately flexible | Bends before breaking | Minimal |
| PLA | Brittle, rigid | Snaps cleanly | Yes, pronounced |
Tip: If you bend a sample and see a visible white line at the crease, you are likely looking at HDPE or PLA. If the sample bends without any colour change and does not break, it is probably nylon or LDPE.
Smell Test Reference
The smell test is best used alongside the burn test. When you hold a plastic to a flame, the vapour it produces has a characteristic odour. This is not a standalone method, but it can confirm what the other tests suggest.
PP and PE (LDPE, HDPE) both smell like paraffin candle wax when heated, sweet and waxy with no sharp edge. PS has a sweet, slightly fruity chemical smell. ABS smells like burnt rubber. Nylon smells distinctly like burnt hair or wool. PET produces a sour, slightly acidic smell. PVC is the most recognisable: a sharp, acrid, chlorine smell that is impossible to mistake for anything else. PLA has a mild, slightly sweet smell, much less intense than the others.
Quick Identification Summary
Use this table to cross-reference multiple test results and narrow down the plastic type.
| Plastic | Floats? | Flame | Smoke | Smell | Break |
|---|---|---|---|---|---|
| PP | Yes | Yellow/blue, drips | Clear | Candle wax | Snaps cleanly |
| LDPE | Yes | Yellow/blue, drips | Clear | Candle wax | Stretches, tears |
| HDPE | Yes | Yellow/blue, drips | Clear | Candle wax | Snaps, whitens |
| PS | No | Yellow, sooty | Black, thick | Sweet, fruity | Shatters |
| ABS | No | Yellow, sooty | Black, thick | Burnt rubber | Snaps cleanly |
| Nylon | No | Blue/yellow, drips | Clear | Burnt wool | Very flexible |
| PET | No | Yellow, sooty | Black | Sour, acrid | Snaps with stretch |
| PVC | No | Yellow/green | Black, thick | Chlorine, acrid | Bends, flexible |
| PLA | No | Yellow | Light | Slightly sweet | Snaps, whitens |
Why Results Sometimes Vary: Additives and Blends
Pure plastics behave predictably in these tests. But most real world products contain additives that change how the plastic performs. Manufacturers add plasticisers, UV stabilisers, impact modifiers, flame retardants, fillers, and colourants to tailor the material to a specific application. These change density, flexibility, burn behaviour, and smell.
For example, polypropylene in its pure form is rigid and snaps cleanly when bent. But a pickleball is made from PP with elastomer additives that make it softer and more flexible, closer to how HDPE behaves in a break test. If you tested one without knowing this, you might misidentify it.
Common examples where additives shift test results
PVC pipes vs PVC cling wrap. Same base polymer, different plasticiser levels. One is rigid, the other is soft and stretchy. The break test gives completely different results for each.
Flame retardant additives can change burn test results. A material that would normally continue burning may self extinguish instead.
Glass fibre or mineral fillers increase density, which can shift float test results. A filled PP part might sink in water when unfilled PP would float.
Carbon black (a common UV stabiliser) makes the burn test harder to read because it masks the flame colour.
This is why no single test is definitive. Cross-reference at least two methods, and if the results conflict, consider that additives may be the reason. The resin code (when present) or the SDS Plastic Scanner will provide the base polymer, regardless of additives.
Identify Plastic Without Any Tests
The manual tests above work well, but they take time and some require safety precautions. The Plastic Scanner is a free smartphone app that identifies plastic types from a photo. Point your camera at any plastic item and get a result in seconds.
SDS Plastic Scanner
No flames, no liquids, no special equipment. Take a photo and the app uses machine learning to identify the plastic type. It works on dark and coloured plastics where resin codes are often missing or unreadable.
- Instant identification from a photo
- Covers 10 plastic types
- Works on dark and coloured plastic
- Evolving AI database
- Classroom and workshop friendly
- Free on iOS and Android
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