The scenario is always the same. You have been running PETG sleeves on a general-purpose solvent — Class 1 or Class 2 — and the seams are fine. Your customer switches the sleeve substrate to PET. Same solvent, same applicator, same flow rate, same line speed. The seam peels apart in your hands.
Your first thought is dosing. You increase the flow rate. The seam gets wetter but not stronger. You try a more aggressive solvent — a Class 3. Still fails. The film surface looks wetted, the overlap is visibly saturated, but the bond has no strength. The seam separates cleanly, as if the two faces were never chemically joined.
They were not. That is the problem, and more solvent will not fix it.
What Makes PET Different
PET (polyethylene terephthalate) and PETG (glycol-modified PET) are both polyesters built from the same base chemistry. On a supplier data sheet they may both appear under “polyester shrink film.” But their behaviour under solvent is fundamentally different, and the reason is crystallinity.
PETG contains a comonomer — cyclohexanedimethanol (CHDM) — that replaces a portion of the ethylene glycol in the polymer backbone. The bulky cyclohexane ring disrupts the regularity of the chain. PETG chains cannot pack into ordered crystalline structures. The polymer remains amorphous: a tangle of disordered chains with no long-range order.
Standard PET has no such disruption. Its chains are regular enough to fold and pack into crystalline domains — regions where chains are aligned in tight, ordered arrays. These crystalline domains are dense, thermodynamically stable, and resistant to solvent penetration.
When a general-purpose seaming solvent contacts amorphous PETG, it penetrates the disordered chain network, swells the surface, and allows interdiffusion across the overlap. That is a normal solvent weld.
When the same solvent contacts PET, it encounters crystalline domains that act as physical barriers. The solvent may wet the surface and interact with whatever amorphous material exists between the crystallites, but it cannot dissolve or swell the crystalline regions themselves. Without adequate penetration, there is no meaningful chain interdiffusion. The “seam” is two wetted surfaces pressed together, not a welded joint.
Why Solvency Intensity Does Not Help
This is the critical misunderstanding. The failure on PET is not that the solvent is too weak. It is that the solvent is the wrong type.
Crystalline domains in PET are held together by dense chain packing and intermolecular forces along regular, repeating segments. A more aggressive version of the same solvent chemistry — moving from Class 1 to Class 3 — increases the rate and depth of attack on amorphous regions, but it does not grant access to the crystalline domains. The crystallites are a different structural environment, and general-purpose solvency does not address them.
Increasing flow rate compounds the problem. You are now flooding the surface with a solvent that cannot do the job, extending drying time, risking blocking, and potentially distorting the film — all without improving the seam.
What Class 4 Solvents Do
Class 4 solvents are formulated specifically for PET. They are not stronger versions of Class 1 — they are a different class because the substrate demands a different chemical approach.
The exact formulation strategies are proprietary to each supplier, and we will not speculate on specific chemistries. Functionally, a Class 4 product must interact with PET’s semicrystalline morphology in a way that enables chain interdiffusion across the overlap. Whether that involves disrupting crystalline order at the surface, selectively attacking the amorphous intercrystalline regions with enough depth to anchor the bond, or some combination — the result is a seam with structural integrity on a substrate that resists general-purpose solvents.
The dosing principles still apply. A Class 4 solvent is not a product you pour on — the target is still a thin, controlled film across the overlap. The starting point of 1.5 mL/min per 100 m/min of line speed (assuming a 3 mm overlap) remains the baseline, adjusted for the specific product’s instructions.
The rPET Variable
Recycled PET introduces variability that does not exist with virgin PET or PETG. The degree of crystallinity in rPET film depends on the feedstock blend, the reprocessing conditions, and the film manufacturer’s extrusion and orientation parameters. Two nominally identical rPET films from different suppliers — or different lots from the same supplier — can present meaningfully different crystallinity profiles to the seaming solvent.
This means a Class 4 solvent that works on one rPET film may require flow rate adjustment on another. The class is correct, but the operating window may shift between batches.
If you are running rPET and seeing inconsistent seam quality across lots, confirm that the variability is in the film, not in your process. Hold your dosing, line speed, and applicator setup constant, and test the seam on retained samples from both the good lot and the bad lot. If the good lot seams and the bad lot does not with identical parameters, the film has changed. [INFERRED]
The Diagnostic Path
If you are troubleshooting a PET seaming failure, work through the diagnostic order:
1. Confirm the substrate. Is it actually PET, or is it PETG? They are often loosely labelled. If the film supplier’s data sheet says “PET” without the “G,” assume semicrystalline until proven otherwise.
2. Check your solvent class. If you are using a Class 1, 2, or 3 product, that is the problem. No adjustment to flow rate or line speed will make a general-purpose solvent weld PET. You need a Class 4 product.
3. Check flow rate. Once you are on a Class 4 solvent, confirm that dosing is in the correct range for your line speed. Underdosing a Class 4 product will still give a weak seam — the substrate-specific chemistry needs adequate contact time and coverage to work.
4. Check drying. Class 4 products have their own evaporation profile, which may differ from the Class 1 or 2 product you were running before. Verify that the solvent is substantially gone before the tube reaches the winder.
The key point: if your seam fails on PET, the first question is not “how much solvent” or “how aggressive.” The first question is “which class.” On PET, only Class 4 is a candidate.