What PETG Is

PETG is polyethylene terephthalate glycol-modified — a copolyester where a portion of the ethylene glycol in standard PET is replaced with cyclohexanedimethanol (CHDM). That substitution disrupts the regular chain packing that produces crystallinity in standard PET. The result is an amorphous polymer: no crystalline domains, a lower glass transition temperature, and a structure that is accessible to solvents.

This is the single most important fact about PETG for seaming purposes. Amorphous polymers let solvent molecules penetrate between polymer chains without having to break through ordered crystalline regions first. That is why PETG seams easily and why PET does not — see PETG vs. PET for the boundary in detail.

Why PETG Seams Easily

A solvent-welded seam requires the solvent to dissolve or swell the polymer surface on both faces of the overlap, allow the polymer chains to interdiffuse across the interface, and then evaporate. In PETG, the amorphous structure offers minimal resistance to the first step. The solvent reaches mobile chains quickly, interdiffusion begins almost immediately, and the bond develops fast.

This responsiveness is what makes PETG the default substrate for shrink sleeve seaming. It works with Class 1 (general purpose), Class 2 (high speed), and Class 3 (high solvency) solvents. The choice between classes depends on line speed and how aggressive you want the solvent attack to be — not on whether the solvent can bond the film at all.

Shrink Properties

PETG shrink sleeve films typically offer 40–78% shrinkage in the transverse direction (TD), with low shrinkage in the machine direction (MD). The exact ratio depends on the film grade, stretching conditions during manufacture, and the shrink tunnel temperature profile. These are typical industry ranges; specific values vary by supplier and grade.

The high TD shrink combined with low MD shrink is what makes PETG suitable for full-body sleeves on contoured containers. The seam must survive the shrink process — a well-formed solvent weld on PETG generally does, because the bond strength exceeds the film’s yield strength in the overlap zone.

Compatible Solvent Classes

  • Class 1 (general purpose): Standard solvency, standard drying. Suitable for moderate line speeds on PETG. The most forgiving option.
  • Class 2 (high speed): Same solvency level but faster evaporation. Use when line speed outpaces the drying window of Class 1.
  • Class 3 (high solvency): More aggressive solvent attack, fast drying. Use when bond strength on Class 1 or 2 is marginal — but on PETG, this is rarely needed and increases the risk of over-solvation.

For the full classification framework, see solvent classes.

Dosing Sensitivity

PETG is the substrate where over-dosing is more common than under-dosing. Because the polymer is amorphous and solvent-receptive, even a modest excess of solvent will penetrate deeper than intended. At standard dosing — approximately 5 µm of liquid on a 40–50 µm film — the solvent should dissolve only the surface layer needed for chain interdiffusion.

Apply too much and the solvent reaches the ink layer beneath the overlap, softens or dissolves it, and creates print distortion or ink transfer. In severe cases, the solvent penetrates through the film entirely and causes blocking — adjacent layers of the wound sleeve stick together on the mandrel or in the roll.

The practical consequence: when troubleshooting PETG seaming problems, check dosing first. The instinct is often to increase solvent when a seam looks weak, but on PETG, many apparent bond issues are actually caused by too much solvent disrupting the overlap zone rather than too little failing to activate it.

Common PETG Seaming Defects

Over-solvation. The most frequent PETG-specific problem. Symptoms: wrinkled or distorted print in the overlap area, ink smearing visible through the outer film layer, or a seam that looks wet or glossy longer than expected. Reduce dosing or switch from Class 3 to Class 1. See troubleshooting over-solvation for step-by-step resolution.

Blocking. Wound sleeves stick together — layers bond where they should not. Almost always a dosing excess or insufficient drying time before winding. On PETG, blocking can occur at surprisingly low solvent levels because the polymer surface is so receptive.

Print distortion at the overlap. The solvent softens the ink layer beneath the overlap, and the impression from the outer layer transfers or shifts. This is distinct from general print quality issues — it appears only in the seam zone. Reducing dosing usually resolves it; if the seam weakens unacceptably at lower dosing, switching to a faster-drying class can help by limiting how deep the solvent penetrates before it evaporates.

Weak seams at high speed. If line speed exceeds the drying-and-bonding window, the solvent evaporates before sufficient interdiffusion occurs. The fix is usually a move from Class 1 to Class 2, which provides faster solvent action at the same solvency level.

PETG in the Broader Substrate Landscape

PETG replaced PVC as the dominant sleeve material because it avoids PVC’s recycling contamination issues. It is itself now under some pressure from virgin PET and rPET, which avoid the CHDM comonomer that can affect recycled PET stream quality. For converters, the transition from PETG to PET is more significant than PVC to PETG was — it requires a move from Classes 1–3 to Class 4, with a narrower seaming window and less tolerance for dosing variation.