ABS plastic can be identified through a combination of markings, physical tests, and visual characteristics. If the part has a recycling symbol, that’s the fastest route. If not, a simple acetone test or water float test can confirm ABS in under a minute. Here’s how to work through each method.
Check for a Recycling Code First
The easiest way to identify ABS is to look for a molded or stamped recycling symbol on the part itself. ABS carries the resin identification code 9, displayed as the letters “ABS” inside a triangle of arrows with the number 9 beneath it. This marking is typically found on the inside or underside of molded parts, often near a seam or in a recessed area where it won’t affect the product’s appearance.
Not all ABS parts carry this code. Many manufacturers skip it, especially on smaller components, decorative parts, or items not intended for consumer recycling streams. If you don’t see a marking, move on to the physical and chemical tests below.
Know What ABS Looks and Feels Like
In its natural, unpigmented state, ABS is an opaque ivory color with a smooth, slightly glossy surface. It has a high degree of surface quality, which is why manufacturers favor it for visible consumer products. The material feels hard and rigid but not brittle the way polystyrene does. If you tap it against a hard surface, it produces a dull, solid sound rather than a hollow ring.
ABS shows up in a wide range of everyday products, and knowing where it’s commonly used can help you make an educated guess before testing. LEGO bricks are the classic example. Computer keyboards, printer housings, vacuum cleaner cases, power tool enclosures, plastic musical instruments like recorders and clarinets, and gardening hand tools are all typically made from ABS. If the object you’re examining falls into one of these categories, ABS is a strong bet.
When ABS breaks, it tends to fracture with a rough, matte surface rather than snapping cleanly. It doesn’t turn white along a bend or crease line the way polypropylene does. That white-stress-mark test is actually a quick way to rule polypropylene in or out when you’re unsure which of the two you’re dealing with.
The Acetone Test
This is the most reliable DIY test for confirming ABS. Place a small drop of acetone (standard nail polish remover works, as long as it’s pure acetone and not an acetone-free formula) on an inconspicuous area of the plastic. Use the tip of a knife or toothpick to gently work the acetone into the surface for 10 to 15 seconds.
If the surface softens, gets tacky, or begins to dissolve, you’re looking at ABS. Acetone aggressively attacks ABS but leaves polypropylene, polyethylene, and nylon completely unaffected. PVC will also soften slightly in acetone, but PVC is noticeably more flexible and heavier than ABS, making the two fairly easy to tell apart even before you test.
If you have methyl ethyl ketone (MEK) or methylene chloride available, those solvents work even more decisively on ABS. But for most people, acetone from the drugstore is all you need.
The Water Float Test
ABS has a specific gravity of 1.03 to 1.07, which means it’s denser than water and will sink. Cut or break a small piece and drop it into a glass of room-temperature water. If it sinks, it could be ABS (along with several other plastics like PVC, PET, and polycarbonate). If it floats, it’s not ABS. Polypropylene and polyethylene both float, with specific gravities below 1.0, so the water test is a fast way to eliminate those two from consideration.
The water test works best as a process of elimination rather than a definitive confirmation. Combine it with the acetone test for a more certain answer: sinks in water plus dissolves in acetone equals ABS.
The Burn Test
Burning a small sample can identify ABS, though it should be done outdoors or in a well-ventilated area since ABS releases toxic fumes. Using a lighter or match on a small sliver of the material, watch for these characteristics:
- Flame color: Yellow with blue edges.
- Smoke: Heavy black smoke that leaves soot particles in the air.
- Smell: A sharp, acrid odor, sometimes described as similar to burning rubber or a harsh chemical smell.
- Burn behavior: ABS burns slowly and continues to burn after you remove the flame source. It does not drip while burning initially, though it may drip as it continues.
Compare this to polypropylene, which burns with a blue flame tipped in yellow and gives off a sweeter smell once the flame source is removed. Polypropylene also drips readily. These differences are distinct enough to tell the two apart with practice.
How ABS Responds to Heat
ABS is an amorphous plastic, meaning it doesn’t have a sharp, defined melting point the way crystalline plastics do. Instead, it softens gradually. Its glass transition temperature, the point where it shifts from rigid to rubbery, is around 105°C (221°F). It reaches a workable melt state around 220 to 230°C (428 to 446°F), which is why it’s a popular material for 3D printing at those nozzle temperatures.
If you’re testing with a heat gun or soldering iron, ABS will begin to soften and deform well before other high-temperature plastics like polycarbonate or nylon. It won’t melt into a clean liquid pool. Instead, it gets progressively gummy and pliable, which is consistent with its amorphous structure.
Ruling Out the Most Common Look-Alikes
The plastics most frequently confused with ABS are polypropylene, high-impact polystyrene, and PVC. Here’s how to separate them quickly.
Polypropylene floats in water, turns white when you bend it sharply, and is unaffected by acetone. ABS sinks, doesn’t whiten at bends, and dissolves in acetone. These two are easy to distinguish.
High-impact polystyrene (HIPS) looks and feels similar to ABS and also dissolves in acetone. The burn test helps here: HIPS produces a dense, sooty black smoke like ABS, but the flame is more purely yellow without the blue edges, and the smell is more of a sweet, styrene-like odor rather than ABS’s harsh, acrid scent. HIPS is also slightly more brittle and prone to cracking under stress.
PVC is heavier and more flexible than ABS in most formulations. It produces a green-tinged flame and gives off a distinctly harsh, chlorine-like smell when burned, very different from ABS. PVC also self-extinguishes when you remove the flame, while ABS keeps burning.
For professional-grade certainty, infrared spectroscopy or Raman spectroscopy can distinguish ABS from any other plastic by detecting the aromatic ring structures in styrene and acrylonitrile that are unique to ABS. But for home workshop purposes, the combination of a water sink test, an acetone wipe, and a quick burn test will give you a reliable identification.