How to Identify Plastic Types: Resin Codes, Float Test, Lab Checks

Mixing polymers is one of the most common causes of rejected lots in the recycled plastic trade. A small amount of the wrong material can ruin a batch: PVC in PET, PET in PP, or a filled grade mixed into an unfilled one. Whether you are buying or selling, being able to identify plastic types quickly is a basic skill.

This guide covers practical methods, from markings on the product to simple density checks and lab tests, and explains why lab testing is a better choice than the traditional burn test.

Start with resin identification codes

Many plastic products carry a resin identification code: a number inside a triangle of arrows, often moulded into the base. The codes are:

Code	Polymer	Common items
1	PET	Beverage bottles, food trays
2	HDPE	Milk and detergent bottles, crates, pipes
3	PVC	Pipes, cable insulation, blister packs
4	LDPE	Film, carry bags, squeeze bottles
5	PP	Woven sacks, containers, caps, furniture
6	PS	Disposable cups, packaging, foam
7	Other	Polycarbonate, nylon, ABS, multilayer and others

Codes are helpful but not always present or accurate, and code 7 covers many different materials. Treat them as a first clue. Read more about resin identification codes.

Use the float test and density

Density is one of the most reliable quick checks. Drop a small, clean, bubble-free piece of plastic into plain water:

• Floats: polyolefins such as PP (about 0.90 to 0.91 g/cm3), LDPE (about 0.91 to 0.94) and HDPE (about 0.94 to 0.97).
• Sinks: most other common plastics, including PET (about 1.38), PVC, ABS, PC and nylon.

The float test cannot tell PP from PE, and fillers, foaming or trapped air can change the result. A mineral-filled PP may sink, for example. Recyclers use the same principle at scale in float-sink separation. For a more precise answer, a lab can measure density accurately.

Look and feel

With experience, you can narrow down many plastics by appearance and handling:

• PP is light and slightly waxy; woven PP sacks have a characteristic flat-tape structure.
• HDPE feels waxy, is stiffer than LDPE and is usually opaque.
• LDPE film is soft and stretches noticeably before tearing.
• PET bottles are clear or tinted, rigid and crinkle when squeezed.
• GPPS is clear and brittle, and polystyrene items often make a metallic sound when tapped or dropped.
• ABS is rigid with a glossy finish and is common in appliance and electronic housings.

Look and feel are useful for sorting, but they are not proof. Similar-looking materials are easy to confuse, especially after grinding, painting or colouring. Knowing the source of the scrap helps a great deal: a lot of crate grindings from a single moulder is far easier to identify than mixed household waste. Sellers should record the source of each lot, and buyers should ask for it.

Why to avoid burn tests

Burning a sample and judging the flame, smoke and smell is a traditional method, but it has real drawbacks. Burning plastics releases harmful fumes, and some, such as PVC, can release corrosive and toxic gases. Results also depend on additives and colourants, and different people read the same flame differently.

For safety and reliability, use lab tests instead. If burn testing is ever done, it should only be by trained people with proper ventilation and protective equipment, and never as the basis for a commercial decision.

Lab tests that give a clear answer

For buying and selling decisions, these tests are more dependable:

• FTIR spectroscopy: identifies the polymer from its infrared spectrum and is widely used by labs and larger recyclers.
• DSC (differential scanning calorimetry): measures melting behaviour and can help distinguish PP from PE and detect blends.
• Density measurement: more accurate than a simple water float test.
• MFI: not an identification test as such, but confirms whether a material matches the expected grade. See MFI.
• Ash content: shows the level of fillers and inorganic contamination.

Many testing labs offer these services, and testing a sample is far cheaper than dealing with a rejected truckload.

Trade with clarity on PlasticSetu

Clear polymer identification makes every quotation more reliable. Buyers can get quotes against a defined specification, and suppliers can list your materials with polymer, form and test values so the sourcing desk can match them accurately.