PET (polyethylene terephthalate) and PP (polypropylene) are both food-grade thermoplastics used for cups, but they behave very differently:

• PET: crystal-clear, stiff, excellent gas/barrier properties (great for cold, carbonated drinks and clear presentation). Widely recycled as resin code. 
• PP: lighter, more flexible, higher service heat tolerance and commonly used for hot-drink cups or microwaveable containers; marked and is less universally recycled (but recycling is improving).

1. Chemistry & basic structure

• PET (polyester): polymer of terephthalic acid (or dimethyl terephthalate) and ethylene glycol. It’s a polyester (contains ester linkages), and is typically semi-crystalline (can be amorphous if quenched). That chemistry gives PET high stiffness, clarity and reasonably good chemical resistance except to strong alkalis.
• PP (polyolefin): polymer made from propylene monomers; a non-polar polyolefin with a hydrocarbon backbone. PP is intrinsically more flexible/impact-resistant and chemically robust to many acids/alkalis and fats.

(Short-form use: PET = “clear polyester”; PP = “tough polyolefin”.)

2. Key physical properties

• Density/feel: PP ≈ 0.90–0.91 g/cm³ (lightweight). PET ≈ ~1.3 g/cm³ (feels heavier/denser for same wall thickness). That’s why a PET cup often feels more “solid” than a thin PP cup of the same size.
• Melting/thermal behaviour: PP melting (homopolymer) ≈ 160–165 °C (copolymers a bit lower). PET melting peak is higher (measured crystalline melt ≈ ~240–255 °C), but PET’s glass transition (Tg) is only ~70–80 °C, which means PET softens and loses rigidity well below its crystalline melt and therefore is not automatically suitable for hot liquids unless it’s specially processed (heat-set).
• Mechanical: PET = stiff, high tensile modulus, brittle at thin sections; PP = more flexible, tougher/impact-resistant.
• Why the Tg vs melt matters: PET can melt at very high temperatures but it becomes soft and deformable above ~70–80 °C — so a standard PET cold-cup will distort or creep at hot-drink temperatures unless it’s heat-set during manufacture. Heat-set PET (used for hot-fill bottles) is a special process/grade that raises usable hot-fill temperature.

3. Optical & barrier performance (why PET is used for cold drinks)

• Clarity/gloss: PET is naturally crystal-clear (glass-like) when processed as an amorphous sheet, making it ideal for the display of colorful beverages (smooth, glossy look). PP is naturally translucent/opaque unless specialty “clarified” grades/processing are used.
• Gas/barrier: PET offers better oxygen and CO₂ barrier properties than PP, which helps preserve carbonation and aroma in bottled drinks. That’s a major reason PET is the default for soft-drink and cold-cup applications. PP typically has higher oxygen permeability and is often used in multi-layer structures when barrier function is needed.

4. Thermal use & food-safety practicalities

• PET cups (typical, non-heat-set): intended for cold or ambient beverages. Avoid hot liquids (>~60–70 °C) because PET will soften, deform, and — under heat + time — there is evidence some trace catalysts/contaminants (e.g., antimony used in PET manufacture) can migrate at elevated temperatures. For hot filling, you must specify heat-set/hot-fill PET — those bottles/cups are specially processed and rated for higher fill temps (often up to ~85 °C depending on design).
• PP cups: widely used for hot foods/drinks (yogurt cups, hot-food tubs) and many PP containers are explicitly marketed as microwave-safe because PP does not absorb microwave energy and has a higher softening/usable temperature range for reheating. Still, “microwave-safe” is a manufacturer-level designation and migration under intense heating remains an area of active study.

Bottom line for consumers:

• Use PET Cold Cups for cold, carbonated, and chilled beverages — they look great and keep fizz better.
• Use PP cups for hot drinks or for reheating in the microwave (but check the manufacturer’s microwave-safe label).

5. Chemical resistance & flavour/odor

• PP has excellent resistance to most acids, alkalis and oils — that’s why it’s popular for containers that contact fatty or strongly flavored foods.
• PET resists many foodstuffs but is susceptible to hydrolysis by strong alkalis and can be affected over time by certain solvents; nevertheless, PET is widely used for juices, water and sodas because of its low sorption and inert taste profile when used as intended.

6. Manufacturing methods & appearance options

• PET cups are commonly thermoformed from clear PET sheet (fast, cheap for disposable clear cups) or produced as thermoformed rigid items; PET offers an extremely good surface finish for printing/branding.
• PP cups are produced by thermoforming for disposables and by injection molding for thicker/reusable cups — injection molding enables double walls, textured grips, and sturdier reusable designs.

7. Recycling & environmental notes

• Resin codes: PET = #1 (PET, PETE); PP = #5. PET bottle take-back and recycling infrastructure is far more widespread globally, so PET bottles/cups are more commonly accepted in curbside streams. PP recycling access is improving but is less universal (and capture rates remain lower).
• Recycling rates: PET bottle recycling has historically been much higher than average plastic packaging (reports show U.S. PET bottle collection rates in the high-20s to low-30s percent range in recent years and rising in some markets), while overall plastic packaging recycling remains low — so “recyclable” does not always mean it will get recycled locally.
• Biodegradability: neither PP nor PET is meaningfully biodegradable in natural environments (research into microbial degradation exists but is not a practical disposal pathway today). If sustainability is important, emphasize reuse, refill programs, or design for recycling and local collection.

8. Health & regulatory points

Both PET and PP have many food-contact authorizations and are commonly used in food packaging. Regulatory oversight (e.g., FDA in the U.S., EU rules) governs permitted uses and migration limits. However, chemical migration is situation-dependent — it depends on resin grade, additives, temperature, contact time, and the food matrix.

• Specific concern with PET: antimony (used as a catalyst in many PET processes) can be detected in liquid stored in PET and its migration increases with temperature and storage time — generally levels are low under normal use, but heating PET (boiling, microwaving, leaving in a hot car) increases migration risks. That’s another reason to avoid hot liquids in standard PET cups/bottles.
• Specific notes on PP: PP typically does not contain BPA and is widely considered safe for repeated food contact; still, like any plastic, PP may contain additives (stabilizers, slip agents, clarifiers) that can migrate under extreme conditions.

Practical consumer messaging: avoid categorical health claims. Instead, say: “Food-safe grade PET; intended for cold/ambient use. For hot drinks or reheating, use PP or heat-resistant packaging. Follow manufacturer's microwave guidance.”

9. When to choose PET vs PP

Use PET when you want:

• Crystal-clear look to show the product (iced coffees, smoothies, bubble tea, sodas).
• Better carbonation/CO₂ retention (sparkling beverages).
• Good recyclability (if local PET recycling exists).

Use PP when you want:

• Hot/beverage or microwaveable capability (coffee, hot soup, reheating).
• A more flexible/tough cup less prone to cracking on impact.
• Products that contact fatty or strongly acidic foods where PP’s chemical resistance helps.

FAQ snippet:

“Q: Can I microwave a PET cup? — A: No. Standard PET cups are intended for cold or room-temperature beverages; heating increases the risk of deformation and may increase migration of trace substances. Use PP or other designated microwave-safe containers for reheating.”

10. Short troubleshooting & comparison table (quick copy)

Our PET clear cups are made from food-grade polyethylene terephthalate, offering glass-like clarity and excellent retention of carbonation — ideal for chilled and carbonated beverages. PET provides a premium visual finish and is widely recyclable where PET collection programs exist; however, standard PET cups are intended for cold/room-temperature use and are not recommended for hot liquids or microwave reheating unless a heat-set PET grade specifically rated for hot-fill is used. For hot drinks or microwaveable containers, polypropylene is generally the better option due to higher practical service temperatures and impact resistance.