Polyurethane vs. PET Security Film: Why the Usual Spec-Sheet Numbers Point the Wrong Way

The short answer

Almost every security window film on the market is made of polyester (PET). 3M™ Scotchshield™ S2400 is made of polyurethane. On the two numbers most spec sheets lead with — tensile strength and break strength — polyester is far “stronger.” On the two numbers that govern what a film does when glass is struck — elongation and tear resistance — polyurethane leads by a wide margin: about 4X the elongation (560% vs 130%) and two-thirds more tear resistance (24.6 vs 14.8 lbf·in) than a 22-mil polyester security film, by 3M’s own published bulletin. A forced-entry event loads a film in exactly those two ways, which is why Epic specifies S2400 for entries and vestibules and installs it anchored to the frame with the Epic Method.

The numbers, side by side (3M published data)

3M’s technical bulletin on S2400 physical properties compares it directly with a 22-mil polyester (PET) security film. These are 3M’s figures, not Epic’s.

Source: 3M™ Scotchshield™ Security Window Film S2400 — Physical Properties Technical Bulletin, Revision A, March 2024; 3M S2400 product page (“its polyurethane construction provides greater tear strength and elongation than PET resin films of similar thickness”). Figures as published; test methods as stated by 3M.

Why the first two rows mislead

Tensile and break strength come from the same test: clamp a strip of film, pull it slowly in one direction, record the peak force before it snaps. It is a fine way to compare two polyesters. It is a poor way to predict a forced-entry event, for three reasons.

• The load is a point, not a pull. A bat, a hammer or a boot concentrates force on a few square inches of film that is bonded to shattered glass. Nothing is pulling the film evenly from the ends.
• The load is fast and repeated. D882 pulls at inches per minute. An impact arrives in milliseconds and then arrives again. What matters is how much energy the film can take per blow, not the single peak force it can resist once.
• The film is already notched. The moment the glass breaks, the film is in contact with thousands of sharp edges. Every one is a potential tear point. Peak tensile strength says nothing about whether a tear starts there and how far it runs.

What actually decides it: energy, not force

A material’s ability to absorb a blow is the area under its stress–strain curve — force multiplied by how far it stretches before it fails. PET has a tall, short curve: very high peak stress, little stretch. Polyurethane has a lower, much longer curve. The area under the long curve is larger, so the film absorbs more of the blow by deforming instead of passing it straight to the nearest notch. That is the whole argument in one picture.

Tear resistance is the second half of the story. A high-tensile film that tears easily is a strong film with a weak point everywhere the glass touches it. 3M’s Graves figure — 24.6 lbf·in for S2400 against 14.8 for the 22-mil PET film in its bulletin — is the number that says how hard it is to start that tear and keep it going. It is also why Epic cuts S2400 by machine in the shop: a nick in a hand-cut edge is a tear point before the film ever meets a window. That is the reason Epic Precision exists.

How to compare films properly

1. Ask what the film is made of. Polyester or polyurethane. If the answer is a brand name, ask again.
2. Read the right rows. Elongation at break (ASTM D882) and Graves tear (ASTM D1004). Treat tensile and break strength as background, not the headline.
3. Insist on assembly tests. Film alone is never what an intruder meets. Ask for the test of the complete assembly — glass, film, attachment, frame — and the standard it was run to. If the answer is ASTM F3561, read why that standard was written for a different product before you weigh it.
4. Ask how it is attached. The most tear-resistant film in the world does nothing if the whole filmed pane is pushed out of the frame in one piece. The attachment is half the product; the Epic Method is how Epic installs it.
5. Ask how it is cut. A notched edge is a tear point. Shop-cut, machine-cut film arrives with clean edges; film trimmed by hand on site does not.

Where PET still belongs

None of this makes polyester a bad film. 3M’s Ultra series — micro-layered PET — is an excellent product, and Epic installs it where it is the right answer: upper floors, safety-glazing and seismic applications, storefronts where smash-and-grab is the threat, and budgets that cannot reach S2400 across every opening. The point of this page is narrower: when the question is a person with a tool at a school entry, the film property that matters is how it fails, and polyurethane fails better.