Plastics — recognising them, and knowing what they take

01

The tests

Five tests, in the order they should be done: from the most harmless to the most dangerous. The first three are nearly always enough.

1 · The water test

Put a piece in a glass of water and tap the bubbles off. If it floats it is a polyolefin — PP or PE — or expanded foam. No other common plastic has a density below 1.0. A single test divides the plastic world in two and takes ten seconds.

The salt water test narrows it further. Dissolve table salt in water until no more will go in — about 360 g per litre, which gives a density of 1.20. What floats there is PS, ABS, ASA, nylon or acrylic. What sinks is PVC, PET, PETG, POM, PTFE or PLA. Polycarbonate and TPU sit so close to the solution's own density that they can hang suspended — no clear answer is an answer too.

water 1.00 g/cm³ · saturated brine ≈ 1.20 g/cm³

2 · The acetone drop

One drop of acetone on a hidden surface, thirty seconds, wipe it off and feel with a fingernail.

Softens and goes tacky → ABS, ASA or polystyrene. Acetone dissolves them.
Goes dull or whitens → PMMA, polycarbonate or PETG. Attacked but not dissolved.
Nothing happens → PP, PE, POM, nylon, PET or PTFE. Chemically resistant.

The test is almost as informative as the water test and is therefore the second one you do. On a visible surface it does permanent damage — choose the inside or an edge.

3 · The bend test

Bend a thin part until it folds.

Whitens at the fold and bends without breaking → PP, PE or nylon. The whitening is the crystal structure orienting itself.
Breaks brittle with a sharp fracture → polystyrene, acrylic, PLA or PET.
Bends toughly without whitening and will not break → polycarbonate, POM or PVC.

A hinge moulded into a part that survives being folded thousands of times is always polypropylene. No other cheap plastic manages it.

4 · The boiling water test

One minute in boiling water, out with tweezers, and press with a fingernail while the piece is still hot. Nothing more dangerous than a kettle.

Unaffected → PP, ABS, ASA, nylon, polycarbonate, POM or PTFE.
Softens or distorts → PE, polystyrene, PLA, PETG, PVC or PET.

This is the only simple test that separates PP from PE — PP takes boiling water, PE does not. It also separates polycarbonate from PETG, a confusion that is otherwise easy to make.

5 · The burn test

Do not do this on unknown plastic without reading on

PVC gives off hydrogen chloride, which becomes hydrochloric acid in the airways.
POM gives off formaldehyde.
PTFE does not burn but decomposes, giving off perfluoroisobutene among other things — one of the most toxic gases there is.

Use a very small piece, outdoors, downwind of you, and smell by wafting air towards your nose — not by holding your nose over it. If the water test and the acetone test have already answered: skip this one.

If you burn it anyway
Behaviour	Smell	Plas
Drips burning, blue flame with a yellow tip	Candle wax	PP or PE
Heavy soot and black smoke	Sweetish, like gas	PS, ABS or ASA
Burns cleanly with a crackling flame	Sweet, fruity	PMMA
Soots, goes out when the flame is removed	Phenolic, sweet-sour	Polycarbonate
Goes out at once, hard to light	Sharply acidic, pungent	PVC
Burns with an almost invisible flame	Pungent, irritates the eyes	POM
Drips and draws threads	Burnt hair or celery	Nylon
Drips, moderate soot	Sweetish, caramel	PLA or PET
Does not burn at all	—	PTFE. Stop immediately.

The simplest test of all

Look for the recycling mark. The triangle with a number, on the bottom of packaging and moulded parts, says it outright: 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other. Technical parts are often marked with the abbreviation written out instead, as in >ABS< or >PA66-GF30<. Look before you start burning things.

02

Polyolefins

The two commonest plastics in the world, and the two that nothing sticks to.

PP Polypropylene · recycling code 5 · food boxes, chemical cans, living hinges

0.90 g/cm³

Density0.90

WaterFloats

Service temp−20 to +100 °C

Surface energy29 mN/m

ChemistryTakes almost anything

BondableNo

Recognised by

Floats, whitens at a fold, unmoved by acetone, and often has a living hinge — a thin fold bent again and again without breaking. Feels slightly waxy. Takes the dishwasher and boiling water, unlike PE.

Good and bad

Chemically almost untouchable. Acid, alkali, solvent, oil — nothing bites. That is why every chemical can is PP or PE.
Takes 100 °C and is the only cheap plastic that survives a dishwasher and an autoclave.
Cannot be bonded without primer or flame treatment. See Bonding.
Degraded by UV if it is not stabilised. White PP outdoors turns to chalk in a few years.
Welds excellently with hot air and PP rod. That is the right method, not adhesive.

FloatsChemical-resistantLiving hingeNot bondable

PE Polyethylene · code 2 rigid (HDPE), code 4 soft (LDPE) · cans, bags, chopping boards

0.92–0.96 g/cm³

Density0.92–0.96

WaterFloats

Service temp−50 to +80 °C

Surface energy31 mN/m

FrictionVery low

BondableNo

Told apart from PP by being softer, more slippery and less heat-resistant. A PE part deforms in a dishwasher. HDPE is stiff and matt, LDPE soft and flexible. UHMWPE is the same polymer with very long chains: extremely wear-resistant and used for sliding surfaces.

Chemically as resistant as PP and just as impossible to bond. Welding is the method, and PE welds even more easily than PP.

FloatsSlipperyLow heat resistanceNot bondable

03

Styrenics

The ones acetone dissolves, which is both their greatest weakness and the best way to join them.

ABS Acrylonitrile butadiene styrene · Lego, housings, car interiors, 3D printing

1.04 g/cm³

Density1.04

Salt waterFloats

Service temp−20 to +80 °C

AcetoneDissolves

Impact strengthGood

UVPoor, yellows

Recognised by

Sinks in water but floats in salt water, softens with acetone, and has a characteristic dull note when tapped — unlike polystyrene, which rings. Matt to semi-gloss surface.

Acetone is the tool

Solvent welding gives a joint stronger than any adhesive: brush acetone on both surfaces, press together, wait. The molecular chains interlace and the joint becomes one piece of material. Vapour polishing in acetone vapour gives 3D prints a glossy, sealed surface.

Yellows

The butadiene is degraded by UV and heat, and the brominated flame retardants contribute. That is why old computer cases go yellow. It can be reversed temporarily with retrobright, but the yellowing comes back.

Acetone-weldedImpact-toughYellows

PS and EPS Polystyrene · code 6 · CD cases, disposable cutlery, foam

1.05 g/cm³

Density1.05

SoundRings metallically

FractureBrittle, sharp

AcetoneDissolves easily

LimoneneDissolves

Service tempMax 70 °C

Brittle, ringing and transparent in its pure form. Breaks with a sharp, keen fracture. Dissolved by both acetone and d-limonene — foam melts into a small lump in limonene, which is a classic demonstration of how little polymer there is in it.

HIPS is the same plastic with rubber mixed in: matt, tough, and used as support material in 3D printing because limonene dissolves it and leaves the model alone.

RingsBrittleDissolved by limonene

ASA Acrylonitrile styrene acrylate · ABS for outdoor use

1.07 g/cm³

Density1.07

UVVery good

AcetoneDissolves

Service temp−20 to +85 °C

Used inVehicles, signs

PriceSomewhat higher than ABS

ABS with the butadiene replaced by acrylate, which removes the UV-sensitive part. In practice ABS that does not yellow. Found in mirror housings, roof rails and outdoor signage.

Important when identifying: it behaves exactly like ABS in every home test. If the part is an exterior vehicle component and looks unaffected after years in the sun, it is almost certainly ASA. Anything else is almost always ABS.

UV-resistantDoes not yellowVehicles

04

Transparent

Three plastics that look alike and behave entirely differently. Confusing them is the commonest and most expensive mistake in the whole document.

PMMA Acrylic · Perspex · polymethyl methacrylate

1.19 g/cm³

Density1.19

Light transmission92 %

Impact strengthLow, brittle

Scratch resistanceGood

UVExcellent

Service tempMax 80 °C

Clearer than glass, harder than polycarbonate, and brittle. Hit acrylic and it cracks with sharp edges. It can be laser cut with a perfectly polished edge, which polycarbonate cannot.

Told apart from polycarbonate by the sound — acrylic rings, PC thuds — and by a thin piece of acrylic snapping brittle while PC bends. In the burn test acrylic burns cleanly with a crackling flame and a sweet smell.

Joined invisibly

Dichloromethane is drawn into the joint by capillary action and welds acrylic into an almost invisible seam. It is the professional method and beats every adhesive. Dichloromethane is volatile, narcotic and a suspected carcinogen — outdoors, or under extraction.

ClearestBrittleUV-resistantLaser cut

Polycarbonate PC · Lexan, Makrolon · visors, machine guards, LED lenses

1.20 g/cm³

Density1.20

Impact strengthExtreme

Scratch resistancePoor

Service temp−100 to +120 °C

UVYellows uncoated

AlkaliDestroyed

Practically unbreakable — a PC visor takes a blow that would shatter acrylic into fragments. The price is that it scratches from almost nothing and yellows in sunlight unless it is UV-coated.

Stress cracking

Polycarbonate crazes into a fine network of cracks on contact with isopropanol, acetone, many cleaning agents and even some adhesives — particularly on parts with internal stress from moulding. The cracks appear hours or days later, not immediately, which is why people rarely work out what caused them. Clean with mild washing-up liquid and water, nothing else.

Alkali breaks PC down chemically by hydrolysing the carbonate bond. Caustic soda, soda ash and strongly alkaline cleaners make the material brittle and cloudy. See the material table in Chemicals.

UnbreakableScratchesStress crackingNot alkali

PETG Glycol-modified polyester · guards, 3D printing, packaging

1.27 g/cm³

Density1.27

Impact strengthGood

Service tempMax 70 °C

ChemistryGood, not acetone

FormingEasily thermoformed

Food contactApproved

Halfway between acrylic and polycarbonate: tougher than acrylic, cheaper and more chemically resistant than PC, but takes the least heat of the three. It sinks in saturated brine, which separates it from the other two.

It thermoforms at a low temperature and without pre-drying, which makes it the easiest transparent plastic to work at home.

ToughCheapThermoformsNot heat

05

Engineering plastics

The ones that replace metal in mechanisms. Strong, dimensionally stable — and the three hardest to bond of all.

Nylon PA6, PA66, PA12 · polyamide · gears, cable ties, plain bearings

1.13 g/cm³

Density1.13–1.15

Wear resistanceExcellent

Service temp−40 to +100 °C

Moisture uptakeUp to 3 %

AcidAttacked

Smell when burntBurnt hair

Nylon absorbs water, and that changes everything

A nylon part takes up several per cent moisture from the air and grows in size while doing so — by up to a couple of per cent. Stiffness falls and toughness rises. That means a gear can stop fitting after a damp summer, that a freshly printed part creeps, and that nylon filament has to be dried before printing. It also explains why adhesives so often let go: the moisture comes out of the material and breaks the joint from the inside.

In exchange, nylon is the best cheap material for plain bearings, gears and wear parts. Self-lubricating, tough and hard-wearing. Glass-filled grades (PA66-GF30) are stiffer and more dimensionally stable, at the cost of toughness.

Hard-wearingAbsorbs moistureGearsHard to bond

POM Acetal · Delrin · precision parts, valves, sliding surfaces

1.41 g/cm³

Density1.41

Dimensional stabilityExcellent

Moisture uptakeAlmost none

FrictionVery low

AcidDestroyed

BondableNo

The best plastic to turn and mill: it cuts like brass, holds size, and comes out smooth without finishing. Unlike nylon it takes up no moisture, which makes it superior wherever the dimension matters.

Notoriously hard to bond. The surface energy is 37 mN/m and the surface is chemically inert. Expect a mechanical fixing, a thread or a moulded-in insert instead.

Acid and heat

POM is degraded by acids and by prolonged heat, and decomposes to formaldehyde — pungent and irritating to the eyes. Do not mill with a blunt tool that heats the material, and do not burn it indoors.

Dimensionally stableMachines bestNot acidNot bondable

PTFE PTFE · Teflon · thread tape, gaskets, sliding surfaces, cable insulation

2.2 g/cm³

Density2.2 — heaviest

Service temp−200 to +260 °C

FrictionLowest of all solids

ChemistryPractically inert

Surface energy18 mN/m

CreepsUnder load

Recognised immediately: the heaviest of all common plastics, waxy and slippery, milky white, and soft to press with a fingernail. Sinks like a stone. Takes anything chemically.

Overheating

Above roughly 260 °C PTFE starts to decompose and gives off gases that cause metal fume fever, and above 350 °C genuinely toxic ones. Never in a frying pan left dry on the heat, and never in a laser cutter.

It creeps under constant load, which makes it unsuitable as a load-bearing part. As a gasket the creep is an advantage instead — it conforms to the surface.

HeaviestSlipperiestInertCreeps

06

PVC and PET

Two heavy plastics that sink in salt water, and both with a quirk worth knowing.

PVC Polyvinyl chloride · code 3 · pipe, flooring, cable, imitation leather

1.38 g/cm³

Density1.38 rigid

FireSelf-extinguishing

Service tempMax 60 °C

ChemistryGood against acid and alkali

AcetoneSwells

Joined withPipe cement, THF-based

The only common plastic that goes out by itself when the flame is removed, thanks to the chlorine. The smell when heated is sharply acidic and unmistakable. Rigid PVC is pipe and profile, flexible PVC is hose, flooring and imitation leather.

The plasticiser migrates

Flexible PVC contains up to 40 % plasticiser, which slowly migrates out. That makes the material stiff and brittle over the years, breaks adhesive joints from within, and attacks other plastics it lies against — a PVC trim against an acrylic sheet gives clouding and cracks. Never lay flexible PVC directly against acrylic or polycarbonate.

Self-extinguishingPipe cementPlasticiser

PET Polyethylene terephthalate · code 1 · bottles, film, polyester fibre

1.38 g/cm³

Density1.38

Gas barrierExcellent

Service tempMax 60 °C

ChemistryGood, not alkali

ShrinksWith heat

Food contactApproved

The same density as PVC but easily separated: PET burns with soot and a sweet smell, PVC goes out and smells acidic. PET is also optically clear and stiff in thin sections.

It shrinks strongly with heat, because bottles are blow-moulded with oriented chains that want to go back. A PET bottle in boiling water pulls itself into a small, thick shape — the same principle as shrink sleeve.

BottlesShrinksGas barrier

07

From the 3D printer

Two more that are common at home but rare in the trade, and that differ from everything else.

PLA Polylactide · polyester from corn starch

1.24 g/cm³

Density1.24

Softens at55–60 °C

StiffnessHigh

Impact strengthLow

AlkaliDegraded

SmellSweetish

55 degrees is the whole problem

PLA softens just above 55 °C. A part in a car in summer, in a sunlit window, next to a power supply or in a dishwasher deforms. This is the single commonest reason a 3D printed part fails in real use rather than on the desk.

In exchange PLA is the stiffest of the printing plastics, shrinks least, smells pleasant and prints without trouble on any machine. Degraded by alkali — a PLA part in a soda bath is destroyed.

StiffEasy to printNot heat

TPU Thermoplastic polyurethane · the rubber-like one

1.10–1.25 g/cm³

Density1.10–1.25

HardnessShore 60A–95A

Elongation400–600 %

AbrasionExcellent

HydrolysisPolyester TPU degrades

Service temp−30 to +80 °C

Rubber that can be printed and injection moulded. More wear-resistant than most real rubbers and resistant to oil, which is why it is used for wheels, seals and cable sleeving.

Polyester-based TPU hydrolyses in damp heat and goes tacky and crumbly after a few years. It is the same degradation that ruins tool grips and old headphone pads. Polyether-based TPU does not have the problem.

ElasticAbrasion-resistantHydrolysis

08

Chemical resistance

What happens when the plastic meets what is in the chemical cupboard. Valid at room temperature and short contact — heat and time move everything to the left.

Key

✓ resists · ! short contact, rinse immediately · ✗ attacked, dissolved or cracks

What the table cannot tell you

A tick is a simplification of something that is in reality a surface with three dimensions: concentration, temperature and contact time. The same plastic that takes an agent dilute and cold for a minute can be destroyed by it concentrated, warm and overnight. The table holds for room temperature and brief contact, and nothing else.

Two more things that shift everything: fillers and plasticisers change the resistance noticeably, so a glass-filled grade does not behave like the pure polymer, and internal stresses from moulding decide whether a solvent causes crazing or not — the same plastic, the same agent, different outcome on two parts.

If it must not go wrong: test on a hidden part of your actual component, with your actual agent, for at least as long as the real contact will last. Or read the manufacturer's resistance chart, which gives concentration and temperature instead of a tick.

Plastic against chemical
Plas	Weak acid	Strong acid	Alkali and soda	IPA	Acetone	White spirit	Limonene	UV outdoors
PP	✓	✓	✓	✓	✓	!	!	✗
PE and HDPE	✓	✓	✓	✓	✓	!	!	✗
PS	✓	!	✓	!	✗	✗	✗	✗
ABS	✓	!	✓	!	✗	!	!	✗
ASA	✓	!	✓	!	✗	!	!	✓
PMMA, acrylic	✓	!	!	✗	✗	!	✗	✓
Polycarbonate	✓	!	✗	✗	✗	!	✗	!
PETG	✓	!	!	✓	✗	✓	!	!
PET	✓	!	✗	✓	!	✓	✓	!
PVC, rigid	✓	✓	✓	✓	✗	!	✗	!
Nylon, PA	✗	✗	✓	✓	✓	✓	✓	!
POM	✗	✗	!	✓	✓	✓	✓	!
PTFE	✓	✓	✓	✓	✓	✓	✓	✓
PLA	!	✗	✗	✓	!	✓	!	!
TPU	!	✗	!	✓	✗	✓	!	!

Four patterns

Polyolefins and PTFE take everything. That is why chemical cans are PE and gaskets are PTFE. The price is that nothing sticks to them either.
Polyesters are broken down by alkali. PET, PETG, PLA and polycarbonate all have ester or carbonate bonds that alkali hydrolyses. Never caustic soda or soda ash on those four.
Nylon and POM are the victims of acid. Contrary to most other plastics they take alkali but not acid.
Stress cracking is not chemical degradation. Polycarbonate in isopropanol does not dissolve — it cracks because the solvent penetrates where the material is already under internal stress. A stress-free part can survive the same contact unharmed.

09

Machining plastic

Joining, cutting, threading. The method follows from the plastic — and for half the list adhesive is the wrong answer.

Joining
Plas	Best method	Also works	Does not work
PP and PE	Hot air welding with filler rod	Primer + cyanoacrylate, methacrylate adhesive, screw	All ordinary adhesives
ABS and ASA	Solvent welding with acetone	Cyanoacrylate, epoxy, MS	Silicone
PS	Solvent welding, model cement	Cyanoacrylate	Limonene, which dissolves it
PMMA	Dichloromethane, capillary joint	UV adhesive, two-component acrylic	Cyanoacrylate, which blooms
Polycarbonate	Two-component PU or epoxy	UV adhesive, MS	Cyanoacrylate and IPA — cracks
PETG	Epoxy	Cyanoacrylate, UV adhesive	Acetone
PVC, rigid	Pipe cement, THF-based	Cyanoacrylate, epoxy	Silicone
Nylon	Dry in an oven, then PU or epoxy	Cyanoacrylate on an abraded surface	Anything on damp nylon
POM	Mechanical fixing or threading	Primer + cyanoacrylate	Essentially every adhesive
PTFE	Mechanical fixing	Etching with sodium naphthalenide	Everything else
PLA	Cyanoacrylate	Epoxy, PLA slurry in dichloromethane	Heat above 55 °C
TPU	Cyanoacrylate or PU adhesive	MS polymer	Rigid adhesives, which crack off

Cutting and milling

Heat is the enemy. Molten plastic sticks to the tool and burns into the cut. High feed and low spindle speed, the opposite of metal. Sharp tools, and clear the chips away.
Acrylic wants sharp, positive tools and cracks at the exit unless it is supported underneath. Drill with a bit reground to zero rake, otherwise it grabs and splits the sheet.
Polycarbonate must not be laser cut — the edge goes brown and it gives off corrosive smoke. Acrylic, on the other hand, laser cuts with a perfectly polished edge.
PVC must never be laser cut. The chlorine becomes hydrogen chloride, which destroys the machine and your airways.
POM is the best plastic to turn. It cuts cleanly, holds size and needs no finishing.

Threads and fixings

Threading directly into plastic works well in POM, nylon, PVC and ABS. Use a coarse thread and a long engagement, at least two thread diameters.
Brass threaded inserts heat-set with a soldering iron are the right solution in 3D printed parts. Considerably stronger than a printed thread and they can be re-tightened.
A plastic screw in plastic with a coarse, self-tapping profile is made for the job and holds better than a machine screw in a drilled hole.
Expect creep. A plastic part under constant clamping load gives way over time and the joint loosens. Use a metal spacer through the part so the clamping load goes through metal, not plastic.

10

Common mistakes

Arranged by what happened. Most of them come back to the wrong plastic being chosen, or the right plastic meeting something it does not take.

When the plastic did not do as it should
Symptom	Likely cause	Remedy
A fine network of cracks a few days after cleaning	Stress cracking in polycarbonate or acrylic	Cannot be repaired. Clean PC with washing-up liquid and water, never spirit or acetone.
The part deformed in the car in summer	PLA softens at 55 °C	Reprint it in PETG, ABS or ASA.
A transparent cover went cloudy and brittle	Alkaline cleaner on polycarbonate	A neutral agent. See the chemical table.
The adhesive let go although the joint looked perfect	PP, PE or POM — the surface energy is too low	Primer, welding or a mechanical fixing. See Bonding.
The nylon gear stopped fitting	Moisture uptake has made the part grow	Design with clearance, or change to POM, which is dimensionally stable.
White plastic outdoors turned to chalk	UV degradation of unstabilised PP or PE	A UV-stabilised grade, or paint it. ASA instead of ABS.
The tool grip went sticky	Polyester TPU or flexible PVC that has hydrolysed	Wipe it with IPA and rewrap it. The material does not come back.
The acrylic sheet cracked at the drilled hole	Wrong drill geometry and no support underneath	Zero rake, a backing block underneath, low feed at the breakthrough.
The laser-cut edge went brown and smelled acidic	PVC or polycarbonate in the laser	Stop immediately, ventilate. PVC destroys the machine internally.
The screw in the plastic worked loose over time	Creep under constant clamping load	A metal spacer sleeve, or a threaded insert.
A white plastic part yellowed indoors	ABS with brominated flame retardants	Retrobright temporarily. The yellowing comes back.
The joint to flexible PVC broke from within	Plasticiser that migrated into the bond line	An elastic adhesive intended for flexible PVC, or a mechanical fixing.
The part went dull from the cleaning agent	Acetone or spirit on acrylic	Polish it out with cutting compound. Acrylic can be saved, PC cannot.
A freshly printed nylon part shrank and warped	The filament was damp	Dry the filament at 70 °C for six hours before printing.
The foam was dissolved by the cleaning agent	D-limonene dissolves polystyrene	No remedy. Keep citrus-based agents away from PS.

Which plastic is it?

The tests

1 · The water test

2 · The acetone drop

3 · The bend test

4 · The boiling water test

5 · The burn test

The simplest test of all

Polyolefins

PP Polypropylene · recycling code 5 · food boxes, chemical cans, living hinges

Recognised by

Good and bad

PE Polyethylene · code 2 rigid (HDPE), code 4 soft (LDPE) · cans, bags, chopping boards

Styrenics

ABS Acrylonitrile butadiene styrene · Lego, housings, car interiors, 3D printing

Recognised by

Acetone is the tool

PS and EPS Polystyrene · code 6 · CD cases, disposable cutlery, foam

ASA Acrylonitrile styrene acrylate · ABS for outdoor use

Transparent

PMMA Acrylic · Perspex · polymethyl methacrylate

Polycarbonate PC · Lexan, Makrolon · visors, machine guards, LED lenses

PETG Glycol-modified polyester · guards, 3D printing, packaging

Engineering plastics

Nylon PA6, PA66, PA12 · polyamide · gears, cable ties, plain bearings

POM Acetal · Delrin · precision parts, valves, sliding surfaces

PTFE PTFE · Teflon · thread tape, gaskets, sliding surfaces, cable insulation

PVC and PET

PVC Polyvinyl chloride · code 3 · pipe, flooring, cable, imitation leather

PET Polyethylene terephthalate · code 1 · bottles, film, polyester fibre

From the 3D printer

PLA Polylactide · polyester from corn starch

TPU Thermoplastic polyurethane · the rubber-like one

Chemical resistance

Four patterns

Machining plastic

Cutting and milling

Threads and fixings

Common mistakes