The Crystallinity Problem

PET — polyethylene terephthalate without the glycol modification — is a semicrystalline polymer. Where PETG has its chain regularity disrupted by the CHDM comonomer, standard PET retains ordered crystalline domains interspersed with amorphous regions.

Those crystalline domains are the problem. In a solvent-welded seam, the solvent must penetrate the polymer surface, mobilize chains, and allow interdiffusion. Crystalline regions are tightly packed and thermodynamically stable — solvent molecules cannot easily penetrate them. The amorphous regions between crystallites are accessible, but the crystallites themselves act as physical crosslinks that restrict chain mobility even when the surrounding amorphous phase is swollen.

The result: solvents that work well on PETG — Classes 1, 2, and 3 — will wet a PET surface and may partially swell the amorphous fraction, but they cannot develop a structural bond. The seam looks formed but fails under peel testing. This is not a marginal effect that more solvent can overcome. It is a fundamental incompatibility between general-purpose solvents and semicrystalline PET.

For a detailed discussion of where the boundary falls, see PETG vs. PET seaming.

Class 4 Solvents

Class 4 solvents are formulated specifically for PET. They are not simply “stronger” versions of Classes 1–3 — they work through a different interaction with the substrate. Where general-purpose solvents rely on dissolving an amorphous surface layer, Class 4 formulations are designed to penetrate or disrupt the semicrystalline structure enough to enable chain interdiffusion across the overlap.

The specific mechanisms vary by formulation, but the principle is consistent: the solvent must interact with PET’s crystalline domains in a way that Classes 1–3 do not. This typically involves solvent chemistries with different polarity profiles and hydrogen-bonding characteristics, matched to PET’s Hansen solubility parameters (indicative values: δD 18.7, δP 6.3, δH 6.7 MPa^0.5).

Class 4 is substrate-dedicated. It exists because PET requires it, and using it on PETG or PVC would likely cause severe over-solvation. See the solvent class framework for how the classes relate to each other.

A Narrower Seaming Window

PET is less forgiving than PETG or PVC in every dimension of the seaming process:

Dosing tolerance. The acceptable dosing range is narrower. Too little solvent and the crystalline barrier is not sufficiently disrupted — the seam appears formed but has no structural integrity. Too much and the solvent penetrates beyond the overlap zone, reaching the ink layer or causing film distortion. On PETG, you have perhaps a 2:1 ratio between maximum and minimum acceptable dosing. On PET, that ratio is smaller. [INFERRED]

Dwell time sensitivity. The solvent needs sufficient contact time to interact with the crystalline structure, but excessive dwell time before the overlap closes allows the solvent to penetrate too deep. The window between “enough contact” and “too much penetration” is tighter than on amorphous substrates.

Temperature dependence. PET’s crystallinity is temperature-dependent. Film that has been stored in warm conditions, or that arrives at the seaming station warm from a prior process step, may have a different crystallinity level than the same film at standard conditions. This can shift the seaming window enough to matter.

Shrink Properties

PET shrink sleeve films typically offer 40–70% transverse direction (TD) shrinkage, with controlled low MD shrink. The range overlaps with PETG but tends to be somewhat narrower at the high end. Specific values depend on grade and manufacturer.

PET’s shrink behaviour in the tunnel is somewhat different from PETG — it tends to have a higher onset temperature and a narrower temperature window for controlled shrinkage. This is relevant to seaming because the seam must survive the mechanical stresses of shrink. A Class 4 solvent bond on PET, when properly formed, is strong enough to withstand shrink forces. But a marginal bond — one at the edge of the acceptable window — may fail during shrink even though it passes a room-temperature peel test.

rPET Content

Some PET sleeve films contain recycled PET content. The presence of rPET in the film can shift seaming behaviour because recycled material may have different crystallinity levels, trace contaminants, or degraded molecular weight compared to virgin PET. A solvent and dosing setup validated on virgin PET may need adjustment when the film contains recycled content. See rPET film for a detailed discussion.

Growing Market Share

PET’s share of the shrink sleeve market is increasing, driven by recyclability demands. PET sleeves on PET bottles create a mono-polymer package — provided the sleeve can be separated or is compatible with the recycling stream. Some recycling processes can tolerate PET sleeves; others require sleeve removal via density separation (PET sleeves sink, unlike PETG which has been designed by some suppliers to float).

For converters, the transition from PETG to PET means moving from a substrate that seams easily with general-purpose solvents to one that requires dedicated Class 4 products, tighter process control, and more rigorous seam validation. The economics and sustainability case are clear, but the seaming process demands more attention.

Seaming Best Practices for PET

  • Validate the specific film grade. PET crystallinity varies by supplier and grade. Run seam peel tests on each new film lot, not just each new supplier.
  • Hold dosing tighter. Reduce the acceptable variation range compared to PETG. If your applicator drifts, PET will show it before PETG would.
  • Monitor film temperature. Know the temperature of the film at the seaming station. If ambient or process conditions change significantly, re-validate.
  • Test seam strength after shrink, not just before. A marginal bond may pass at room temperature and fail in the tunnel.

For detailed seaming guidance on PET, see PET seaming.