Environment · water, sun, temperature
Plastic cores for moisture, outdoor storage, sun and frost
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Water: why plastic does not give way
Cardboard is hygroscopic. It reaches equilibrium with the air: at high relative humidity it absorbs water, swells and loses strength. According to Fiaptech technical documentation (a company with corporate ties to ROLL), a 3" × 7.5 mm cardboard core going from 8% to 10% moisture content loses about 20% of its strength. Wet, it loses its shape.
The plastics used for cores absorb negligible amounts of water: PP and HDPE practically none, rigid PVC fractions of a percentage point. Their stiffness does not depend on humidity. A plastic core in the rain, in a washed-down area or in a container with condensation stays as it was.
Where it really matters
- Outdoor storage or under open-sided shelters, typical for agricultural film, geosynthetics and nets.
- Wet areas and frequent washdowns: food, tissue mills, wet textile processing.
- Container export to tropical climates, with temperature swings and condensation (“container rain”).
- Cold stores and chilled areas, where condensation forms on the way out.
To be fair: some cardboard cores with water-repellent treatments, or composite cores, cope reasonably well with humidity, and silage film producers still recommend dry, sheltered storage. Plastic is chosen when water is the rule, not the exception.
Sun and UV
Ultraviolet radiation breaks polymer chains. The signs are a chalky surface, fading, micro-cracks and loss of toughness. Resistance depends on the polymer and its additives:
| Polymer | Without stabilisers | With protection |
|---|---|---|
| PP | the most sensitive: degrades within a few months | UV stabilisers (HALS) or dark pigments |
| HDPE | sensitive | with carbon black (about 2–2.5%) very resistant, like water pipes |
| Rigid PVC | fades and loses surface impact strength | stabilisers and titanium dioxide |
| ABS, PS | sensitive, turn yellow | outdoor use not recommended |
Qualitative ROLL Lab assessment based on polymer literature. For long exposure, ask the supplier for accelerated weathering data on the compound.
Cold: stiff but brittle
In the cold, plastics get stiffer, but some become brittle on impact:
- Homopolymer PP: glass transition around 0 °C; in frost a knock can split the core.
- Copolymer PP: far tougher, the usual choice for PP outdoors.
- HDPE: tough down to −30 °C and below; the safest choice for loading and unloading in winter.
- Rigid PVC: below freezing it becomes notch-sensitive; watch out for chipped ends.
Heat: the weak point
Outdoors, heat is more treacherous than cold. A dark surface in the sun gets much hotter than the air; the inside of a container standing in the sun can exceed 50–60 °C. As temperature rises, plastic loses stiffness and creeps faster: according to Fiaptech documentation, at 50 °C a virgin PVC core loses about 20% of the strength measured at 25 °C, and a recycled PVC core about 30%. Long cores with one side in the sun bend like a banana. If the roll stays in the sun, you need a wall margin or a sheet that gives shade.
Thermal expansion: millimetres that matter
Plastics expand with heat far more than the steel of the shafts. Typical coefficients: about 70–80 millionths per degree for rigid PVC, 100–150 for PP, 150–200 for HDPE, against about 12 for steel. A few examples (ROLL Lab analysis):
- a 3 m HDPE core going from 20 °C to −10 °C shortens by about 15 mm;
- the bore of a 3" HDPE core shrinks by about 0.4 mm over the same swing;
- a 2 m PVC core taken from 20 °C to 45 °C grows by about 4 mm.
On shafts with several cores side by side and on plain shafts with little play, these millimetres explain many seasonal problems. Letting cores acclimatise in the department before use reduces them.
Which polymer for outdoor use
| Situation | Advice |
|---|---|
| Sun, rain, frost, light or medium roll | black HDPE |
| Sun, heavy roll, temperate climate | PVC with a thicker wall or UV-stabilised copolymer PP, in the shade |
| Containers to hot climates | PP or PVC with wall margin; check at 50–60 °C |
| Covered wet area | PVC or PP |
| Winter loading and unloading with knocks | HDPE or copolymer PP; avoid homopolymer PP |
Use the calculator below, or open the PVC, PP or HDPE selector full page, with method and sources.
Which plastic for your core?Three questions on heat, food and priorities, and I’ll tell you which plastic to start from. Then you run the test, on the real roll.
Indicative ROLL Lab choice: 10 °C margin on the material’s continuous service temperature. Always check the compound data sheet.
ROLL Lab estimate, not a test certificate. Prepare your enquiry for the manufacturer →
Good storage practice
- Rest rolls on cradles or saddles, not on the bare core: concentrated weight ovalises warm plastic.
- Use plugs in the cores to protect the bore and the ends.
- Cover rolls with sheets that give shade, not with black film in contact.
- Rotate stock: cores stored in the sun for a long time lose toughness.
- Acclimatise cores and rolls before putting them on the machine after a cold or hot journey.
Frequently asked questions
Can plastic cores be stored outdoors?
As far as water goes, yes: they absorb no moisture and lose no strength. Sun and heat are the real limits: you need a UV-stabilised polymer, such as black HDPE, and a wall margin for summer temperatures.
Which plastic core should I choose for container export?
A PP or PVC core with a wall margin, checked at 50–60 °C, because the inside of a container in the sun can exceed those temperatures. Humidity and condensation, which damage cardboard, are not a problem for plastic.
Do plastic cores break in the cold?
Homopolymer PP becomes brittle around 0 °C and rigid PVC is impact-sensitive below freezing. HDPE and copolymer PP stay tough even at very low temperatures.
Why do plastic cores no longer fit the shaft well in winter?
Because of thermal expansion: a 3" HDPE core loses about 0.4 mm of bore from 20 to −10 °C, and a 3 m core shortens by about 15 mm. Let cores acclimatise in the department before use.
Further reading
HDPE cores
Virgin and recycled HDPE cores: toughness in the cold, UV resistance with carbon black, stiffness, creep, reuse, cutting and recycling with the polyolefins.
Materials
Plastic core materials compared: virgin and recycled PVC, plain and filled PP, HDPE, PS and ABS. Density, temperature, stiffness, creep and recycling.
Agricultural film
Plastic cores for agricultural film: greenhouse, mulch, silage and nets. Outdoor storage, sun, frost, mud, black HDPE, sizes, reuse and end of life.
Geosynthetics and membranes
Cores for geotextiles, geomembranes and building membranes: heavy, long rolls, bending, site storage, materials, walls and safe handling with lifting bars.
Plastic or cardboard
Plastic vs cardboard cores: cost per use, weight, moisture, dust, crush, heat, recycling, UK Plastic Packaging Tax and PPWR compared, with a calculator.
Sources
- Fiaptech technical documentation, Smart Book “Which core?” (a company with corporate ties to ROLL): cardboard moisture, temperature, sun effects
- SpecialChem, polymer guides (HDT and thermal behaviour)
- EN 12201-1, PE pipes for water (carbon black as UV stabilisation)
- Göweil, storing silage film properly, goeweil.com
- ROLL Lab analysis (thermal expansion, selection table)
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