On a film supplier’s data sheet, PET and PETG can look almost identical. Both are polyesters. Both are clear. Both shrink. Their density, tensile strength, and optical properties overlap. A purchasing spec that says “polyester shrink film” could mean either one.

On a seaming line, they are different materials. A Class 1 solvent that gives strong, clean seams on PETG will produce zero bond strength on PET. This is not a subtle difference in seam quality — it is a pass/fail boundary. Understanding why requires looking at what happens at the molecular level when solvent meets polymer.

Same Family, Different Structure

PET and PETG are both built from terephthalic acid, but they use different diols. Standard PET uses ethylene glycol — a small, symmetrical molecule that produces a polymer chain with regular, repeating geometry. That regularity matters. PET chains can fold and pack alongside each other into ordered crystalline domains, the same way uniform bricks stack into a wall.

PETG replaces a portion of the ethylene glycol with cyclohexanedimethanol (CHDM) — a bulky, six-membered ring that breaks the regularity of the chain. PETG chains cannot fold into ordered structures because the CHDM units interrupt the repeating pattern. The polymer stays amorphous: a disordered tangle of chains with no long-range crystalline order.

This single substitution — one comonomer — determines whether a seaming solvent can weld the film.

Why Amorphous Matters for Seaming

A solvent weld works by dissolving and swelling the polymer surface so that chains from both faces of the overlap can interdiffuse. The solvent must penetrate into the polymer network, push chains apart, and create enough mobility for them to migrate across the interface.

In amorphous PETG, the chains are already disordered. There are no crystalline barriers. A moderate-strength solvent — the kind found in Class 1 or Class 2 products — penetrates the surface, swells the chain network, and the weld proceeds normally. The solvent does not need to be particularly aggressive because the material offers no structural resistance to penetration.

In semicrystalline PET, the situation is fundamentally different. The crystalline domains are regions of tightly packed, ordered chains held together by regular intermolecular forces. These domains are dense, thermodynamically stable, and functionally impervious to general-purpose seaming solvents. The solvent may interact with the amorphous regions between and around the crystallites, but it cannot dissolve or swell the crystalline structure itself.

The result: the solvent wets the PET surface, and it may even soften the amorphous intercrystalline material to some degree, but there is not enough mobile polymer to form a meaningful welded joint. The seam separates cleanly because the chains never interdiffused in any structurally significant way.

The Practical Test

If you are unsure whether the boundary is your problem, here is a diagnostic that takes minutes.

Run both films through your seamer with identical parameters — same solvent, same flow rate, same line speed, same overlap. Pull a seam sample from each and try to peel it by hand.

If the PETG seam holds (tears the film rather than peeling the overlap) and the PET seam peels cleanly with little resistance, you have confirmed the crystallinity boundary. Your solvent is working. It is just working on a substrate it was designed for, and failing on one it was not.

This test rules out dosing errors, applicator problems, and line-speed mismatches in one step. If both films fail, your problem is upstream of the substrate — check flow rate and applicator condition. If both films succeed, you are not actually running PET (confirm with the film supplier). But if one succeeds and the other fails, the substrate has changed, and your solvent class needs to change with it.

Why You Cannot Bridge the Gap

The intuitive response is to escalate within the existing class. If Class 1 does not work on PET, try Class 2 for faster drying. If that does not work, try Class 3 for higher solvency. Push the chemistry harder until it breaks through.

This does not work, and it is important to understand why — not just as a practical matter, but as a structural one.

Classes 1, 2, and 3 differ in their evaporation rate and the intensity of their solvency. But they all operate on the same principle: dissolve amorphous polymer, allow interdiffusion, evaporate. A more aggressive Class 3 product attacks amorphous material faster and deeper. It does not gain the ability to dissolve crystalline domains. The crystallites are not “harder to dissolve amorphous material” — they are a different structural state of the polymer, with different thermodynamic stability.

Increasing flow rate has the same limitation. More solvent means more liquid on the overlap, longer drying times, and greater risk of blocking — but the additional volume contacts the same crystalline barriers. You are flooding the surface with a solvent that cannot do the job you need it to do.

This is why the five-class framework places PET in its own class. The boundary between Classes 1–3 and Class 4 is not a solvency gradient. It is a substrate boundary.

The Correct Response: Class 4

Class 4 solvents are formulated specifically for PET’s semicrystalline morphology. They are not “stronger” versions of Class 1 — they are chemically different products designed to interact with a substrate that general-purpose solvents cannot weld.

If you have confirmed that your film is PET (not PETG) and your seam is failing despite correct dosing and applicator setup, the answer is a class change, not a parameter adjustment. Move to Class 4.

The rPET Complication

Recycled PET adds a variable that neither virgin PET nor PETG presents: batch-to-batch inconsistency in crystallinity.

The degree of crystallinity in an rPET film depends on the recycled feedstock — the mix of PET grades, the reprocessing history, and the conditions under which the film was extruded and oriented. A film made from a feedstock with higher recycled content, or a different blend of source materials, may have a different crystallinity profile than the previous lot.

This can manifest as a Class 4 solvent working well on one delivery and giving marginal seams on the next, with no change in your process. The solvent class is correct — the substrate has shifted within the class.

If you are running rPET, tighter incoming quality control on the film is worth the effort. Retain samples from each lot. When seam quality drifts, test against the retained sample from the last good lot using identical parameters. If the retained sample still seams well, the new lot has changed. That information goes back to the film supplier — it is not something you can solve by adjusting the seaming process.

Summary

PETG and PET are both polyesters, but PETG is amorphous and PET is semicrystalline. That structural difference determines whether a general-purpose seaming solvent can form a weld. On PETG, Classes 1–3 work. On PET, they do not — and no amount of dosing, solvency escalation, or flow rate adjustment changes that.

The correct path is to identify the substrate, recognise the boundary, and move to Class 4. The diagnostic order starts with substrate identification for exactly this reason.