Plastic identification is a process of narrowing possibilities, not guessing from appearance. Clear acrylic and polycarbonate can look similar; polyethylene and polypropylene can feel similar; additives and fillers can change density, stiffness and colour.
For bonding, welding, machining, food contact or regulated service, supplier documentation is better evidence than a shop test. Unknown plastic should remain “unknown” when the consequence of error is high.
Quick answer
Start with moulded markings, drawings and purchase records. Then compare density, stiffness, scratch behaviour, edge appearance and known samples. Use only controlled non-destructive tests, and obtain laboratory or supplier confirmation for critical work.
What you need to know
- Recycling codes identify resin families for sorting; they do not state grade or performance.
- Acrylic often has a clearer polished edge, while polycarbonate is much more impact resistant.
- Polyethylene and polypropylene have low surface energy and are difficult to bond with ordinary adhesives.
- Plasticized PVC behaves differently from rigid PVC.
- Flame and hot-needle tests can create toxic fumes and are not recommended as casual identification methods.
Tools and materials
- Marking and documentation checklist
- Known comparison samples
- Accurate scale and water-displacement setup where appropriate
- Calipers and simple stiffness observations
- Polarized light source for stress observation in clear plastics
- Supplier or laboratory support for critical identification
Step-by-step method
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Record origin and service
Find invoices, drawings, mould marks and previous-use information. Note exposure to fuel, cleaners, heat and sunlight.
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Inspect markings
Look for resin identification codes, trade names, stamped grade and manufacturer. Photograph the complete mark before cutting.
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Observe physical clues
Compare transparency, edge colour, stiffness, waxy feel, scratch response and sound with known samples. No single clue is decisive.
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Estimate density cautiously
A clean unfilled sample’s float behaviour can separate some families, but bubbles, fillers and geometry can mislead. Use it only as supporting evidence.
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Check fabrication response
On scrap, observe drilling chips, sawing, solvent response or heat-forming only within safe documented procedures. Stop if fumes, cracking or damage occurs.
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Compare with known material
Run the same observation on labelled reference samples under identical conditions. Record both matching and conflicting evidence.
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Escalate critical decisions
For structural, food, medical, fire-rated or chemical service, obtain traceable documentation or analytical identification.
Common mistakes
- Identifying plastic by colour
- Assuming every clear sheet is acrylic
- Using a burn test indoors
- Treating recycling code as a grade specification
- Bonding before confirming low-surface-energy plastic
- Calling an unknown material “food safe”
Safety and limits
Do not burn unknown plastics. Combustion and thermal decomposition can release corrosive or toxic products.
Machine unknown material with appropriate guarding, chip control and ventilation. Stop work when history suggests chemical contamination.
Frequently asked questions
Can I tell acrylic from polycarbonate visually?
Sometimes edge and scratch clues help, but confirmation from markings or supplier records is safer, especially before structural or solvent-cement work.
What plastics float in water?
Many unfilled polyethylene and polypropylene grades have density below water, but fillers, trapped air and product shape can invalidate the test.
Why does adhesive peel from my plastic?
Polyethylene, polypropylene and fluoropolymers have low surface energy and often need specialized treatment and adhesive selection.
Does a recycling symbol identify the exact grade?
No. It identifies a broad resin family, not additives, molecular grade, reinforcement or regulatory suitability.
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This guide provides general educational information. Products, substrates, site conditions and regulations vary. Follow the current technical data sheet, package directions and safety data sheet for every product you use. Code-regulated, structural or safety-critical work may require a qualified professional.