The Master ID Guide to Household Plastics (Codes 1–7 + At-Home Tests)
📑 Table of Contents
The Master ID Guide to Household Plastics (Codes 1–7 + At-Home Tests)
Section titled “The Master ID Guide to Household Plastics (Codes 1–7 + At-Home Tests)”Identifying plastics is a materials problem, not a marketing problem. The little triangle with a number inside is a resin identification code—a sorting hint for recyclers—not a guarantee of safety, purity, or heat resistance. Two items with the same number can behave very differently because real-world plastics are recipes: base polymer plus additives, fillers, colorants, and processing aids.
This pillar guide is built to help you identify plastics quickly, understand what the code really means, and then make a conservative, evidence-based safety decision for kitchen and household use—especially when an item is old, scratched, cloudy, or unmarked. If you’re trying to identify clear drinkware specifically, also see our deep dives on polycarbonate and our evidence-based explainer on BPA.
Think of me as your friendly neighborhood materials scientist: we’ll use simple observations and a few at‑home tests to narrow the field, explain why those tests work, and then choose the safest path forward.
Quick-Start: Identify a Plastic in 60 Seconds
Section titled “Quick-Start: Identify a Plastic in 60 Seconds”The 5-Step Household Plastic ID Method
Section titled “The 5-Step Household Plastic ID Method”- Check for a resin code (1–7) and any food-contact markings. Treat the code as a clue—not a promise—and assume “unknown additives” unless the item is from a reputable food brand.
- Haze check: Hazy or milky usually indicates a polyolefin (HDPE, LDPE, PP). Crystal-clear needs extra scrutiny.
- Sink or float in plain water: Strong floating points to PP or PE; sinking narrows it to PET, PVC, PS, acrylic, or PC.
- Tap (sound) test: A bell-like ring suggests stiff, glassy plastics (often PC). A dull thud suggests acrylic or styrenics.
- Flex test (gently): PP tends to bend and recover; PS tends to snap sharply.
Stop-here rule for food-contact items: If the plastic is old, heavily scratched, heat-exposed, or you cannot identify it confidently, retire it from hot, acidic, or fatty foods and replace with PP (code 5), HDPE (code 2), glass, or stainless steel.
Does the recycling number mean it’s safe? Not reliably. The code identifies the main resin family for sorting, not additives, recycled content, or heat rating. Use the code as a starting point, then confirm with context and tests.
Why Resin Codes Exist—and What They Do (and Don’t) Tell You
Section titled “Why Resin Codes Exist—and What They Do (and Don’t) Tell You”A Brief History of Codes 1–7
Section titled “A Brief History of Codes 1–7”The resin identification code system was introduced in 1988 by the Society of the Plastics Industry (SPI) to help material recovery facilities sort plastics efficiently. It was never intended as a consumer safety label or a chemical specification for food contact.
Today, the system is maintained through standards stewardship associated with ASTM. Municipal acceptance varies widely because recycling depends on local equipment, contamination limits, and bale value. That’s why the triangle can appear on items that are not accepted curbside.
The Modern Reality
Section titled “The Modern Reality”- A code identifies the dominant polymer backbone, not the formulation.
- Additives matter: plasticizers, UV stabilizers, antioxidants, slip agents, pigments, flame retardants, and fillers can dramatically change performance and aging.
- Code 7 is a catch‑all that includes polycarbonate, copolyesters, and bioplastics—this is where code-based assumptions fail most often.
- Even clearer cases (like how to identify PVC plastic (recycling code 3)) require context because flexibility and odor often signal plasticizers.
Why Codes Don’t Always Equal Safety (Especially in the Kitchen)
Section titled “Why Codes Don’t Always Equal Safety (Especially in the Kitchen)”A resin code doesn’t disclose additive choices, recycled content, or design intent for heat. Two containers labeled “5” (PP) can behave differently in a dishwasher if one includes clarifiers or recycled content that reduce oxidation stability. That’s why guides like identify HDPE plastic (recycling code 2) focus on use conditions, not just labels.
Common red flags you can detect without lab tools:
- Persistent chemical odor after washing
- Sticky or oily feel
- Unusual softness in vinyl-like items
- Chalking or powdery residue
- Rapid stress cracking at corners
If you see these on a food-contact item, treat it as end-of-life.
The At‑Home Identification Tests (and the Science Behind Them)
Section titled “The At‑Home Identification Tests (and the Science Behind Them)”1) Float/Sink Test (Water—and Oil, If Needed)
Section titled “1) Float/Sink Test (Water—and Oil, If Needed)”What to do: Place the item (or a small trimmed piece) in room-temperature water.
Why it works: Density.
- Water density ≈ 1.0 g/cm³
- PP and PE are less dense than water → float
- PET, PVC, PS, acrylic, PC are denser → sink
Oil variation: Vegetable oil (~0.91 g/cm³) can help distinguish PE (often floats) from PP (often hovers or sinks slowly).
Limitations: Fillers and trapped air can skew results.
2) Tap/Sound Test
Section titled “2) Tap/Sound Test”What to do: Gently tap with a spoon.
Why it works: Elastic modulus and damping.
- Glassy, stiff polymers (PC) transmit vibration → clearer ring
- More damped polymers (PS, acrylic) absorb vibration → duller sound
3) Flex/Recovery Test
Section titled “3) Flex/Recovery Test”What to do: Bend a thin edge slightly.
Why it works: Molecular mobility and crystallinity.
- PP has hinge-like chains → bends and recovers
- PS has bulky side groups → snaps
4) Visual Clarity and Haze
Section titled “4) Visual Clarity and Haze”What to do: Look through the plastic against text.
Why it works: Crystallinity scatters light.
- Polyolefins (PE, PP) are semi-crystalline → hazy
- Amorphous plastics (PET, PS, PC, acrylic) → clearer
5) Odor and Surface Feel
Section titled “5) Odor and Surface Feel”Why it works: Volatile additives and surface migration reveal formulation differences—especially in PVC and some PS items.
The Plastics, One by One: Codes 1–7 Explained
Section titled “The Plastics, One by One: Codes 1–7 Explained”Below is a deep dive into each resin: chemical name, properties, common items, and practical safety guidance.
Code 1 — PET or PETE
Section titled “Code 1 — PET or PETE”Chemical name: Polyethylene terephthalate
Acronym: PET, PETE
Physical Properties
Section titled “Physical Properties”- Clear to slightly blue-tinted
- Strong, lightweight
- Good gas barrier
- Softens with heat
Common Household Items
Section titled “Common Household Items”- Water and soda bottles
- Food jars (peanut butter, salad dressing)
- Some clamshell packaging
Safety Profile
Section titled “Safety Profile”- Cold food and beverages: Generally safe
- Heat: Not heat-safe; can warp and increase migration
- Dishwasher: No
- Reuse: Limited; scratches increase surface area
Neighborly advice: PET is a great single-use beverage plastic. Refill once or twice for cold water, then recycle.
Code 2 — HDPE
Section titled “Code 2 — HDPE”Chemical name: High-density polyethylene
Acronym: HDPE
Physical Properties
Section titled “Physical Properties”- Opaque or translucent
- Waxy feel
- Excellent chemical resistance
- Tough and impact-resistant
Common Household Items
Section titled “Common Household Items”- Milk jugs
- Cutting boards
- Detergent bottles
- Food storage containers
Safety Profile
Section titled “Safety Profile”- Food contact: Excellent
- Heat: Moderate; not for stovetop, okay for warm foods
- Dishwasher: Top rack generally fine
- Aging: Very stable
Bottom line: One of the safest all-around household plastics. Learn more in our HDPE care guide: identify HDPE plastic (recycling code 2).
Code 3 — PVC
Section titled “Code 3 — PVC”Chemical name: Polyvinyl chloride
Acronym: PVC
Physical Properties
Section titled “Physical Properties”- Can be rigid or very flexible
- Clear to opaque
- Often has noticeable odor if plasticized
Common Household Items
Section titled “Common Household Items”- Pipes, tubing
- Shower curtains
- Some clear food wrap
- Vinyl toys
Safety Profile
Section titled “Safety Profile”- Food contact: Depends on formulation
- Heat: Avoid
- Additives: Plasticizers can migrate
- Dishwasher: No
Red flags: Soft, oily feel; strong smell. For identification help, see identify PVC plastic (recycling code 3).
Code 4 — LDPE
Section titled “Code 4 — LDPE”Chemical name: Low-density polyethylene
Acronym: LDPE
Physical Properties
Section titled “Physical Properties”- Soft, flexible
- Translucent
- Low melting point
Common Household Items
Section titled “Common Household Items”- Plastic bags
- Squeeze bottles
- Bread bags
Safety Profile
Section titled “Safety Profile”- Food contact: Generally safe
- Heat: Not for hot liquids
- Dishwasher: No
Use it for: Cold storage and flexibility, not heat.
Code 5 — PP
Section titled “Code 5 — PP”Chemical name: Polypropylene
Acronym: PP
Physical Properties
Section titled “Physical Properties”- Translucent to opaque
- Fatigue-resistant (living hinges)
- Higher melting point than PE
Common Household Items
Section titled “Common Household Items”- Food storage containers
- Yogurt cups
- Bottle caps
- Microwave-safe containers
Safety Profile
Section titled “Safety Profile”- Food contact: Excellent
- Heat: Best of common plastics
- Dishwasher: Generally yes (top rack)
- Microwave: Often yes (check markings)
Gold standard for kitchens. If you’re replacing mystery plastics, choose PP.
Code 6 — PS
Section titled “Code 6 — PS”Chemical name: Polystyrene
Acronym: PS
Physical Properties
Section titled “Physical Properties”- Clear and brittle (rigid PS)
- Foamed (EPS) is lightweight and insulating
Common Household Items
Section titled “Common Household Items”- Disposable cutlery
- Foam cups and trays
- Clear rigid organizers
Safety Profile
Section titled “Safety Profile”- Food contact: Limited
- Heat: Avoid
- Breakage: Brittle; cracks easily
Neighborly note: Not a long-term kitchen plastic.
Code 7 — OTHER
Section titled “Code 7 — OTHER”Chemical name: Varies (PC, copolyesters, bioplastics)
Acronym: OTHER
Physical Properties
Section titled “Physical Properties”- Varies widely
- Can be crystal clear and very tough (PC)
- Or soft and biodegradable (PLA)
Common Household Items
Section titled “Common Household Items”- Reusable water bottles
- Baby products (newer BPA-free versions)
- Specialty containers
Safety Profile
Section titled “Safety Profile”- Food contact: Depends entirely on polymer
- Heat: Depends
- Dishwasher: Often not recommended
If it’s clear and tough, review our PC guide: polycarbonate and our BPA explainer.
Putting It All Together: A Conservative Safety Framework
Section titled “Putting It All Together: A Conservative Safety Framework”When in doubt:
- Cold, dry foods: HDPE, LDPE, PP
- Hot foods: PP, glass, stainless steel
- Microwave: PP only (labeled)
- Dishwasher: PP and HDPE (top rack)
When to retire plastic:
- Deep scratches
- Cloudiness after heat
- Persistent odor
- Cracks or stress whitening
For reducing overall exposure, see our guide on identify household plastics by type and code.
Final Takeaway
Section titled “Final Takeaway”Plastics are not villains or heroes—they’re materials with strengths and limits. The resin code is a useful clue, but your eyes, hands, and a little materials science will take you much further.
When safety matters, identify first, then choose the least risky use. And when in doubt, reach for PP, HDPE, glass, or steel. That’s the neighborly, science-backed way to care for your home—and the people in it.