Which is tougher at -20 °C in a PP vs PE case?

Answer: HDPE. Notched impact reads 10.5 kJ/m², and a 6.8 kg ball from 1.2 m dents the corner 7 mm where homo-PP splits 45 mm.

Is copolymer PP enough at -25 °C?

Answer: Barely. Brittleness sits at -28 °C, leaving 3 K of margin, so tighten the drop to 8.5 kg.

What does a 0.5 GPa stiffness gap cost?

Answer: Lid sag. A 520 mm × 360 mm lid sinks 2.8 mm in HDPE against 1.9 mm, widening to 1.2 mm after 24 hours of 3-high stacking.

Why does an HDPE base warp more?

Answer: Shrinkage direction. A 0.5% flow gap lifts a 600 mm edge 3.4 mm, against 1.6 mm for copolymer PP at 0.25%.

Can homo-PP handle short -10 °C runs?

Answer: Yes, with R12 corners, a 4 mm latch boss and a 1.0 m drop verified at -10 °C, holding cracks under 20 mm.

PP vs PE Cases: Cold and Stiffness

Q: Which resin suits a case holding detergent residue?

Answer: HDPE. Soaking 500 hours at 50 °C in 5% alkylbenzene sulphonate cracks the homo-PP latch boss three times and leaves HDPE intact.

Quick answer: At -20 °C a PP vs PE case splits 4.4 times on notched impact: homo-PP 2.4 kJ/m², HDPE 10.5, copolymer 18, while at 20 °C stiffness reverses.

Impact at -20 °C
Room-temperature stiffness
Shrinkage and warp
Chemical resistance
Selection criteria
First-article approval
FAQ

1. Impact at -20 °C

Strips conditioned 4 hours at -20 °C and notched to GB/T 1843 type 1eA read 2.4 kJ/m² for homo-PP, 10.5 for HDPE and 18 for copolymer PP; the faces confirm the order.

Slope matters more than the absolute figure. HDPE still yields at -20 °C with 320% elongation, homo-PP only crazes at 8%, and copolymer PP blunts the notch with a rubber phase to 45%.

A 6.8 kg tup from 1.2 m dents HDPE 7 mm, cracks copolymer PP 18 mm and splits homo-PP 45 mm into the latch boss, and an R12 corner only cuts that to 22 mm.

Brittleness temperatures run -75 °C, -28 °C and -8 °C, leaving homo-PP 12 K of margin under a -20 °C specification.

Property	HDPE	Homo-PP	Copolymer PP
Density g/cm³	0.95–0.96	0.90–0.91	0.90–0.91
Notched impact 23 °C, kJ/m²	14	6.5	48
Notched impact -20 °C, kJ/m²	10.5	2.4	18
Elongation at -20 °C, %	320	8	45
Brittleness temp, °C	-75	-8	-28
Flexural modulus, GPa	0.95–1.25	1.45–1.75	1.05–1.35

Copolymer PP loses 62% between the two impact rows, HDPE only 25%.

2. Room-temperature stiffness

Flexural modulus to GB/T 9341 spreads 0.8 GPa, from 0.95–1.25 for HDPE to 1.45–1.75 for homo-PP. On a 520 mm × 360 mm lid with 4 mm walls, a uniform 45 kg load for an hour sags HDPE 2.8 mm against 1.9 mm.

PP vs PE case handling area, wrapped red and black stacks on a pallet, a pallet truck alongside and two workers assembling at a table

At 40 °C, 25 kg for 24 hours deforms HDPE 3.6 mm and homo-PP 2.4 mm, recovering 68% and 82%.

Ribs at 2.5–3 times wall height recover 0.5 mm on PP and 0.9 mm on HDPE, at the cost of a sink groove near 0.1 mm that texture hides on PE and leaves a line on PP.

3. Shrinkage and warp

Shrinkage sets the cavity: 1.8%–3.2% for HDPE, 1.4%–2.0% for homo-PP and 1.2%–1.8% for copolymer PP, so one tool opens 2.2% for PE against 1.5% for PP.

Directional shrinkage drives warp: a 0.5% flow gap lifts a 600 mm base edge 3.4 mm, against 1.9 mm and 1.6 mm for the two PP grades. A base past 3 mm rocks 1.5 mm at the far corner and the gasket never seats.

Item	HDPE	Homo-PP	Copolymer PP
Shrinkage, %	1.8–3.2	1.4–2.0	1.2–1.8
Flow vs cross-flow, %	0.5	0.3	0.25
Warp on 600 mm edge, mm	3.4	1.9	1.6
Cavity allowance, %	2.2	1.7	1.5
Cooling time, s	45	38	40

Compensation means opening the cavity by measured shrinkage or evening out cooling: 30 °C to 45 °C adds 0.2% to PP shrinkage yet removes 0.4 mm of warp.

Write flatness as a number: on a 600 mm × 400 mm base, a 0.5 mm feeler must not enter beyond 20 mm.

4. Chemical resistance

A PP vs PE case does not rank alike across diesel, oil and detergent. After 72 hours in diesel at 40 °C, HDPE gains 2.1% in mass against 8.6% for homo-PP, a fourfold swelling gap.

PP vs PE case assembly line, grey-green bodies with black lids stacked six high at a bench, a wrapped pallet behind and workers assembling nearby

Alkalis reverse it: 30 days in 10% caustic at 40 °C adds 0.4% to homo-PP and 0.3% to HDPE.

Stress cracking opens the real gap. Soaking 500 hours at 50 °C in 5% alkylbenzene sulphonate cracks the homo-PP latch boss three times over 12 mm, leaves HDPE intact and copolymer PP with a single 4 mm crack.

Rinse water matters too, since PP hydrolyses above 80 °C and HDPE softens near 60 °C.

5. Selection criteria

Write down temperature, load and medium, and selection becomes a lookup.

At or below -25 °C: HDPE, brittleness -75 °C, 50 K of margin.
-25 °C to -15 °C: copolymer PP, if notched impact at -20 °C reaches 15 kJ/m².
-10 °C to 0 °C: homo-PP, with lid sag checked against 2 mm.
Load above 25 kg per case: latch boss to 4 mm whatever the resin.
Diesel or detergent residue: HDPE first, PP for short contact only.

When two criteria conflict, temperature wins. A sagging lid is a nuisance; a cold crack is total failure, and a 48 L case split at -20 °C costs three replacements before downtime is counted.

Cost belongs in the same pass: homo-PP runs 8%–12% cheaper than HDPE on density alone, while copolymer PP carries a 6% premium that shrinks to 3% at case level.

6. First-article approval

A test condition on the drawing beats a clause that says cold resistant: 4 hours at -20 °C, a 6.8 kg ball from 1.2 m within 30 seconds, no penetration, no latch release, no hinge offset. The same limits cover a PP vs PE case.

Run three first articles through the cold drop and pull one for a 24-hour immersion, then sample each 500-unit lot at three pieces, freezing one for 8 hours.

Keep the fracture faces: PP breaks mirror-smooth with radial lines, PE draws whitened strands, and a photographed pair exposes a substituted resin at a glance.

Conclusion

Choosing a PP vs PE case is a balance of three ledgers rather than a ranking of materials. Below -25 °C take HDPE and treat the -75 °C brittleness point as margin. From -25 °C to -15 °C take copolymer PP and hold the 18 kJ/m² line, accepting around 2.5 mm of lid sag under 45 kg. For cases that never see frost, use homo-PP to buy stiffness. Then check three numbers before talking price: lid sag under 2 mm, warp on a 600 mm edge under 1.5 mm, and no cracking at the latch boss after 500 hours in the medium, which is the test a loading dock runs every winter.

Minimum service temperature	First choice	Alternative	Control point
-40 °C to -30 °C	HDPE	—	Latch boss 4 mm, corner R12
-25 °C to -15 °C	Copolymer PP	HDPE	Impact at -20 °C ≥15 kJ/m²
-10 °C to 0 °C	Homo-PP	Copolymer PP	Cavity +1.6%, warp ≤1.5 mm
Above 0 °C	Homo-PP	Copolymer PP	Lid sag ≤2 mm

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.