How strong is Polycro really? Tensile strength, tear strength & puncture resistance explained

4 days ago

7 min read

Ultralight foil tent footprints have a clear appeal: they are extremely light, pack down small, and often cost significantly less than traditional fabric footprints. However, to truly assess the durability of such foils, a gut feeling ("feels robust") or a single anecdotal report is insufficient – measurable properties such as tensile strength, elongation, tear resistance, and puncture resistance, determined according to recognized standards, are crucial.

In this article, we look at what lies behind standards such as ASTM D882, DIN EN ISO 13937-2 and ASTM F1306, and how the published characteristics of window insulating films, brand footprints and specialized outdoor films such as XFOIL can actually be compared with them.

Mechanical properties: the basis for reliable comparisons

Before we talk about testing standards and suppliers, it is worth taking a look at the most important parameters used to describe the stability of films.

Tensile strength – Tensile strength describes the maximum tensile stress a material can withstand before breaking. It is measured as the force required per unit cross-sectional area, usually in N/mm² or MPa. This is relevant for foil groundsheets when they are under tension – for example, when tensioning a tarp or when a load is transferred across the foil.

Elongation at break – Elongation at break indicates how much a film can stretch before it tears. High values often indicate a tough material that doesn't fail brittlely but continues to flex a bit under load.

Stiffness / Modulus of Elasticity – The modulus of elasticity (often simply called "modulus") describes how much a material stretches under a given stress. A high modulus means a stiffer sheet, a low modulus a more flexible one. For groundsheets, this is particularly important for reasons of comfort and handling.

Tear strength describes how much force is required to propagate an existing tear. This is crucial, especially with very thin films: a small cut at the edge can either remain localized or, under tension, propagate along the entire length.

Puncture resistance – Puncture resistance indicates the force required to puncture the film with a defined test object. This is the standard equivalent of everyday questions such as: "How well does the film withstand sharp stones, pine needles, or thorny bushes?"

These parameters are determined in standardized test setups – and that's precisely where the standards come into play.

ASTM D882: Tensile test for thin plastic films

ASTM D882 is one of the most important standards when it comes to the tensile strength of thin plastic films. It is used in the packaging industry as well as for technical films.

What is being done?

Narrow strips are cut from the foil.
These strips are clamped into a tensile testing machine and pulled apart at a constant speed until they break.
Meanwhile, the machine records force and strain; tensile strength, elongation at break and the modulus of elasticity can be calculated from the curve.

Why is this relevant? For groundsheet membranes, the ASTM D882 tensile test is a key tool for objectively describing how much tensile strength a material can withstand and how it behaves before failure. Two membranes with similar basis weights can have very different values here – and therefore a significantly different safety margin in practice.

DIN EN ISO 13937-2: Tear resistance of fabrics (Trouser Method)

While ASTM D882 primarily addresses films and thin layers, DIN EN ISO 13937-2 focuses on the tear resistance of textiles, especially woven fabrics (e.g., tent floors made of nylon or polyester).

Test principle (Trouser test)

A rectangular piece with a central slit is made from the fabric, forming a "trouser leg" shape.
The two legs are pulled apart in a tractor; the crack continues from the cut.
The standard defines how the resulting force peaks are evaluated and summarized into an average tear propagation value.

Transferability to films – The method was developed for fabrics; it is only of limited use for films without a fiber structure. Nevertheless, it helps to understand the difference between film-based groundsheets and fabric footprints: Fabrics can partially "fiber up" differently at the onset of a tear and dissipate energy, while films tend to have a sharper tear front.

ASTM F1306: Puncture resistance of films and laminates (Slow Rate Penetration)

The third relevant standard concerns precisely the property that is often most important for a groundsheet from a user's perspective: puncture resistance. ASTM F1306 tests the so-called "slow rate penetration" resistance of flexible barrier films and laminates.

Test principle

Test pieces (usually 3 × 3 inches) are individually clamped from the foil into a clamping device.
A standardized, pointed test piece is moved at a constant speed against the clamped foil until it is completely pierced.
The maximum penetration force, the energy absorbed, and the distance traveled by the test specimen until penetration are measured.

Why doesn't this change the core problem? ASTM F1306 (and the related ASTM D1709 for the falling weight impact test) have long existed as recognized test methods for precisely this property. The real problem isn't the lack of a standard, but rather that very few suppliers of window insulation films or branded footprints actually publish a puncture resistance value determined according to ASTM F1306 or a comparable standard. A recognized test standard alone doesn't solve the comparison problem – what matters is whether a supplier actually applies it and discloses the value.

Key performance indicators in practice: what providers actually publish

Whether a film is sold as window insulation film, a branded film, or a specialized outdoor film says nothing about its properties – "Polycro" is simply a collective term for various types of film, not a material name with a fixed meaning. Anyone wanting to know where the term comes from and which types of film are currently sold under it can find detailed information in our article "Polycro, Polycryo, xfoil, Window Insulation Films and More..." . Here, however, we will focus on just one question: Which of the properties explained above are actually published by which supplier group?

1. Window insulating films ("DIY store Polycro")

A large proportion of the early "Polycro" setups were based on inexpensive window insulating films: transparent shrink film that is actually intended to seal windows in winter.

Characteristic of this group:

Area of application: short-term window insulation, non-permanent outdoor use.
Material: mostly simple polyolefin or PE films, not necessarily cross-linked; details are rarely found on the packaging.
Data situation: Standard data on tensile strength, elongation or puncture resistance are practically unavailable to end customers; what remains is weight (if specified at all) and subjective impressions from tour reports.

Such films work as a starting point, but are difficult to compare technically because the specifications are not published. Users who use two window films from different brands may experience very different levels of stability in practice, without knowing exactly why.

2. Brand footprints of Gossamer Gear & Six Moon Designs

One level above are the familiar "Polycro" footprints from Gossamer Gear and Six Moon Designs. Here it is documented that these are "multilayer, cross-linked polyolefin film," meaning cross-linked, multi-layered polyolefin films.

Gossamer Gear Polycryo Ground Cloth

Material description: "multilayer, cross-linked polyolefin film", also referred to by brand name ("PolyEarthylene") in some sources.
Typical size L: approx. 183 × 244 cm, weight around 98–104 g depending on the retailer.
Recommended use: as an ultralight groundsheet / tent footprint, reusable, with a focus on low weight.

Six Moon Designs Polycro Footprint

Material specification: "industrial grade cross-linked polyolefin", sometimes with the reference to "ultra strong and ultra light Polycro membrane".
Example Small-Footprint: approx. 244 × 102 cm, weight according to dealers about 44–51 g per sheet, film thickness approximately 0.75 mil (≈ 19 µm).
Intended use: as a groundsheet for solo shelters or 2-person tents, often sold in packs of 2.

It is important to note that both brands clearly state they use cross-linked polyolefin films. Subjective descriptions such as "extremely tough for its weight" or "amazingly puncture resistant" originate from marketing materials or dealer descriptions – not from published, standardized test reports according to ASTM D882, ISO 13937-2, or ASTM F1306. Publicly available standards data is not yet provided; therefore, true comparability remains limited.

3. XFOIL: cross-linked polyolefin film with documented data sheet

XFOIL also belongs to the group of cross-linked polyolefin films, but was designed from the outset as an outdoor material with clearly documented properties. Instead of relying on formulations like "feels stable," we publish concrete measurements – so that groundsheet options can at least be compared where other manufacturers also disclose their data.

Key figure (MD/CD)	Value
Tensile strength	approx. 135 / 125 N/mm²
Elongation at break	approx. 130 / 120 %
Tear strength (Elmendorf)	approx. 17.5 / 16.5 g
Foil thickness	19 µm
Construction	cross-linked, multilayer polyolefin film (POF)

(MD = machine direction, CD = transverse direction to production – depending on the manufacturing process, films cannot withstand exactly the same load in both directions, therefore tensile strength and elongation are usually given as a pair of values.)

What the comparison means in practice

One point that stands out across all three groups: While there is a recognized testing standard for puncture resistance – often the most important property from a user's perspective, as it determines whether the product will withstand stones, roots, and debris – there is no uniform practice according to which all manufacturers and suppliers actually measure and publish the results. Therefore, a puncture resistance value from one supplier cannot be directly compared with that of another, even if both values were determined reliably. This applies to window films as well as brand footprints and XFOIL.

However, what can be accurately compared once figures are available are tensile strength, elongation at break, and tear strength according to the same or comparable standards. This is precisely where the difference between the three groups lies – not necessarily in the actual mechanical quality of the material, but in whether and how this quality is documented.

Checklist: Critically assess information on groundsheet films

Is there a specific material name behind it (e.g., "cross-linked polyolefin") or just a collective term like "Polycro"?
Are tensile strength, elongation, or tear strength specified with numbers and testing standards – or only with advertising adjectives?
Is the film thickness specified, and in what unit (µm, mil)?
Was the puncture resistance specified – and if so, according to which test method (e.g., ASTM F1306 or D1709)? Without a standard specification, a comparison between suppliers is only of limited value.
Does the material information come from the manufacturer itself or only from a reseller's description?

Conclusion

For the three most important mechanical properties of a groundsheet film – tensile strength, tear strength, and puncture resistance – there are recognized testing standards such as ASTM D882, DIN EN ISO 13937-2, and ASTM F1306. The real problem with comparison lies not in the lack of such standards, but in the fact that there are sometimes significant differences between a window insulating film, a documented brand footprint, and an outdoor film with a complete technical data sheet – not necessarily in the material itself, but in the verifiability of its properties. Knowing the tensile strength, elongation, and tear resistance according to recognized standards allows for a genuine comparison of groundsheet options, instead of having to rely on "feels robust." The remaining gap – a uniformly published puncture resistance value across all manufacturers – affects the entire industry and remains a point to keep in mind when making comparisons.