How Tensile Strength Affects Packaging Performance

Quick answer: Tensile strength is how hard you can pull on a material before it breaks. For stretch film it’s measured by pulling a strip until it snaps and is given in megapascals (MPa), usually in two directions. It matters because pallet wrap is under tension from the moment it goes on. But it isn’t the whole story. A strong film can still tear at a nicked edge or a sharp corner, and how firmly a pallet is held depends as much on thickness, stretch and number of turns as on the raw strength of the plastic.

Tensile strength shows up on most stretch film datasheets, often next to a handful of other numbers. Here’s what it actually means, how it relates to thickness, and what it can and can’t tell you about how well a roll will hold your pallets.

What tensile strength means

Tensile strength is the pulling force a material can take before it breaks, divided by the size of its cross-section. That’s why it’s given as a pressure, in megapascals (MPa) or newtons per square millimetre, which are the same thing. Dividing by the cross-section means you can compare materials fairly, whatever their thickness.

Plastic film is made in a long continuous sheet, so its strength can differ along the roll and across it. Datasheets usually give two figures: MD (machine direction, along the roll) and TD (transverse direction, across it). For pallet wrap, the MD figure matters most, because that’s the direction you pull the film as you wrap.

How it’s tested

A strip of film is clamped at both ends in a testing machine, which pulls the ends apart at a steady speed until the strip breaks. The machine records the force at break and how far the strip stretched first. Common test methods for thin plastic film include ISO 527-3 and ASTM D882. When you compare two datasheets, check they used the same method, as different methods can give different figures for the same film.

The same test also gives elongation at break, which is how far the strip stretched before it snapped, as a percentage of its starting length. Stretch film often shows several hundred percent. That’s a lab figure, pulled slowly and evenly. You won’t get that much stretch wrapping a pallet by hand.

Strength and thickness

Because tensile strength is force per area, the force a strip of film can take goes up with its thickness. Two films made from the same plastic will have the same tensile strength, but the thicker one takes more force to break.

Our 23 micron film is about 35% thicker than our 17 micron film (23 divided by 17 is 1.35). If the plastic is the same, the thicker film takes roughly 35% more pull to break. That’s the main reason 23 micron is the better choice for heavy loads, sharp corners and long journeys.

Stretching the film also makes it thinner. So a film that’s stretched hard as it goes on is both holding the load more tightly and closer to its breaking point. That’s normal and it’s how stretch film is meant to work, but it’s why over-stretching on sharp corners leads to tears.

Why a strong film can still tear

Tensile strength is measured on a clean, undamaged strip. On a real pallet, film usually fails for other reasons:

Damaged edges. A dropped roll gets nicks along its edges. Each nick starts a tear as soon as the film is under tension.
Sharp corners. A sharp edge concentrates the force on one small spot and cuts the film. Edge boards spread the load.
Puncture. Pointed items poke through. This is measured separately, as puncture resistance.
Cold film. Film stored in a cold place is stiffer and tears more easily.
Too much tension. Pulling harder than the film can take, or a dispenser brake set too tight.

Our guide on why pallet wrap keeps tearing covers each of these and how to fix them.

What holds the load: containment force

Tensile strength tells you when the film breaks. What you actually want to know is how firmly it holds the load while it’s intact. That’s called containment force. It comes from how much the film was stretched, how thick it is, and how many layers there are. A strong film put on slack gives almost no containment force. A modest film put on with good, even tension and enough turns holds a load well. Our guide to load containment force goes into this in more detail.

Reading a stretch film datasheet

If you’re comparing films, these are the figures worth looking at:

tensile strength, MD and TD, in MPa
elongation at break, as a percentage
puncture resistance
tear resistance, often measured in two directions
thickness, in micron

Compare them only when the test method is the same. And check the simplest figure of all, the weight of the roll. A 400mm x 250m roll of 23 micron film should weigh about 2.43kg including the core, and a 17 micron roll about 1.87kg. A lighter roll means less film, whatever the datasheet says. Our guide on checking a roll’s real length and thickness shows how.

Which film for which load

For light, square, even loads kept indoors, 17 micron is usually strong enough. For pallets over about 500kg, sharp or uneven loads, and anything travelling a long way or stored outside, use 23 micron. Our 17 vs 23 micron guide goes into more detail, and our stretch film rolls come in both.

FAQs

What is tensile strength measured in?

Usually megapascals (MPa), which is the same as newtons per square millimetre. Older datasheets sometimes use psi.

Is a higher tensile strength always better?

Not on its own. Puncture and tear resistance, thickness and how the film is applied matter just as much in daily use.

Does thicker film have a higher tensile strength?

Not strictly. Tensile strength is per unit of area, so it doesn’t change with thickness. But thicker film takes more force to break, which is what matters on the pallet.

What do MD and TD mean?

MD is machine direction, along the length of the roll. TD is transverse direction, across the width. Pallet wrap is pulled mostly in the machine direction.

How Tensile Strength Affects Packaging Performance