The Five Classes of Seaming Solvent

You are choosing a seaming solvent. You have data sheets from two or three suppliers and they all say roughly the same thing: fast drying, strong bond, broad compatibility. None of them tell you why one would work on your line and another would not.

The problem is that “seaming solvent” covers products that do fundamentally different things. A solvent formulated for PVC at 150 m/min has almost nothing in common with a product designed to bond polyolefin film. Treating them as interchangeable — or selecting on price — is how you end up scrapping a reel while trying to figure out what changed.

This page lays out a five-class framework that sorts every seaming solvent by two axes: solvency strength (how aggressively it attacks the polymer surface) and evaporation rate (how quickly it leaves the joint). Those two properties, together with substrate identity, determine whether a seam will hold.

How a Seam Forms

A seam is a solvent weld. There is no adhesive and no third material in the joint. The solvent dissolves and swells a few microns of polymer on both mating faces of the overlap. Polymer chains from each face interdiffuse across the interface — a process called autohesion. The solvent then evaporates, and the chains re-entangle into a continuous polymer network. If the joint is done right, the seam is as strong as the parent film.

Correct dosing is thin. A common starting point is 1.5 mL/min per 100 m/min of line speed. At 200 m/min, that is 3 mL/min of solvent distributed over 200 metres of seam per minute — roughly 15 microlitres per metre. Across a 3 mm overlap (assumed throughout this page), that liquid film is approximately 5 microns thick, against sleeve film that is typically 40–50 microns. You are laying down about a tenth of the film’s own thickness.

This is why “just add more solvent” is rarely the answer. Overdosing does not improve interdiffusion — it floods the interface, extends drying time, and risks the solvent still being active when the flattened tube winds onto itself.

The Two Axes

Solvency strength is the product’s ability to dissolve and swell a given polymer. This is governed by thermodynamic compatibility between the solvent blend and the substrate — predictable, to a first approximation, by Hansen solubility parameters. A solvent whose Hansen parameters sit close to the polymer’s will dissolve it readily. One that sits far away will wet the surface but not penetrate.

Evaporation rate determines how long the solvent stays wet on the overlap. At 200 m/min the web moves at 3.3 metres per second. The solvent must remain active long enough for chain interdiffusion to occur before the nip closes, then be substantially gone before the tube is wound. Too slow and you get blocking on the roll. Too fast and the film skins over before the chains have time to entangle.

Every commercial seaming solvent is a blend positioned somewhere on these two axes. The five classes below are not arbitrary marketing bins — they reflect real clustering in how products are formulated and what substrates they serve.

Class 1 — General Purpose

Moderate solvency, standard evaporation rate. This is the default starting point for PVC and PETG shrink sleeves at line speeds up to roughly 150–200 m/min.

Class 1 solvents work because PVC and amorphous PETG are both relatively easy to dissolve. Their polymer chains are accessible — PVC because of its polar chlorine substituents, PETG because its comonomer (cyclohexanedimethanol) disrupts chain packing and prevents crystallisation. A moderate-strength solvent blend penetrates the surface, the chains interdiffuse, and the solvent leaves at a rate matched to mid-range line speeds.

Use when: You are running PVC or PETG at moderate speeds and your seam quality is acceptable. This is the class to start with before reaching for anything more aggressive or faster-drying.

Do not use when: Your line speed exceeds the drying window (seams are still wet at the winder — move to Class 2), your substrate is PET (move to Class 4), or your substrate is polyolefin (move to Class 5).

Class 2 — High Speed

Moderate solvency, fast evaporation. Formulated for the same substrates as Class 1 — PVC and PETG — but tuned for higher line speeds where a standard-drying solvent would still be active at the winder.

The solvency is comparable to Class 1. What changes is the evaporation profile: the blend includes a higher proportion of faster-evaporating components, so the solvent flashes off sooner. This keeps the drying window inside the available distance between the applicator and the winder, even at speeds above 200 m/min.

The tradeoff is that faster evaporation compresses the time available for chain interdiffusion. If the solvent evaporates before enough entanglement has occurred, the seam will be weak even though the substrate is compatible. This is why Class 2 is not unconditionally better than Class 1 — at lower speeds, a fast-drying solvent may skin over too quickly.

Use when: You are running PVC or PETG and your seams are blocking or tacky at the winder, indicating the solvent is not fully gone by wind-up.

Do not use when: You are running at moderate speeds (Class 1 will give better interdiffusion time), your substrate is PET, or your substrate is polyolefin.

Class 3 — High Solvency

Aggressive solvency, fast evaporation. Class 3 solvents hit the polymer harder than Classes 1 or 2. They dissolve more material, deeper, and they do it quickly.

This class exists for situations where a moderate-solvency product is not wetting or penetrating the substrate adequately — thicker films, certain PETG grades with higher orientation, or PVC compounds with heavy plasticiser loading that dilutes the polar sites. The more aggressive chemistry compensates for reduced surface accessibility.

The risk is proportional. A Class 3 solvent on a thin, easily dissolved film will overdose the interface — dissolving too much material, distorting the overlap zone, or leaving visible witness marks. It is not a substitute for correct flow rate adjustment.

Use when: Classes 1 and 2 are giving weak seams on PVC or PETG despite correct dosing and drying, and you have confirmed that the substrate itself is the variable (thicker gauge, different compound, higher orientation).

Do not use when: A Class 1 or 2 product is working. More solvency is not better — it is more risk. Class 3 will not solve a PET seaming problem; the failure mode on PET is crystallinity, not solvency intensity. Move to Class 4.

Class 4 — PET Dedicated

Substrate-specific formulation for PET and rPET. This is where the framework crosses a boundary that Classes 1–3 cannot bridge.

Standard PET (polyethylene terephthalate) retains the ability to crystallise. Its chains pack into ordered crystalline domains that resist solvent penetration. A solvent that beautifully seams PETG — which is amorphous by design — will wet the surface of PET but fail to dissolve and swell the crystalline regions. The seam peels apart cleanly because no interdiffusion occurred.

Class 4 solvents are formulated specifically to interact with PET’s morphology. The exact mechanisms are proprietary to each supplier, but the functional requirement is the same: the product must penetrate or disrupt enough of the crystalline structure to allow chain interdiffusion across the overlap.

This is a different class, not a stronger version of Class 3. Increasing the aggressiveness of a general-purpose solvent does not turn it into a PET solvent. The problem is not intensity — it is specificity.

For a detailed treatment of why PET resists standard solvents and what Class 4 products do about it, see Why Your Solvent Won’t Seam PET.

For a side-by-side comparison of PET and PETG at the molecular level, see PETG vs PET: The Seaming Boundary.

Use when: Your substrate is PET or rPET. There is no other option in the solvent-welding space.

Do not use when: Your substrate is PETG (Class 1–3 will work and are simpler to run), PVC (same), or polyolefin (Class 5 territory — different mechanism entirely).

Class 5 — Polyolefin

Different bond mechanism entirely. Class 5 is here for completeness and because converters running polyolefin sleeves will arrive at this page looking for a solvent. The answer is that conventional solvent welding is categorically impossible on polypropylene and polyethylene.

PP and PE are non-polar and semicrystalline. Their Hansen solubility parameters sit far from any practical solvent system — the dispersive component is comparable to other polymers, but the polar and hydrogen-bonding components are near zero. No solvent blend will dissolve and swell the surface the way it does for PVC or PETG. This is not a gradient. It is a boundary. No amount of dosing or solvency adjustment crosses it.

Class 5 products use a fundamentally different bonding mechanism — purpose-formulated chemistries that achieve adhesion without relying on solvent welding. Alternatives include UV-cured adhesive seaming, thermal sealing, and laser sealing, each with different equipment requirements and line-speed implications.

For the full treatment, see Seaming Polyolefin Shrink Sleeves.

Use when: Your substrate is PP or PE. No other class applies.

Do not use when: Your substrate is PVC, PETG, or PET — you have better, simpler options in Classes 1–4.

The Diagnostic Order

When a seam is failing, the instinct is to change the solvent. That is usually step four, not step one. Before switching classes, work through this sequence:

1. Substrate. What polymer are you actually running? Confirm it. If you have switched from PETG to PET — even on the same supplier’s data sheet calling both “polyester” — you may have crossed a class boundary. If you are on polyolefin, you are in Class 5 territory regardless of anything else.

2. Flow rate. Is the solvent volume correct for your line speed? The starting point is 1.5 mL/min per 100 m/min. Measure it. A clogged applicator nozzle, a pump drift, or a changed overlap width can move you out of the dosing window without any change to the solvent itself.

3. Drying speed matched to line speed. Is the solvent still wet at the winder, or is it flashing off before the nip? If you increased line speed without changing solvents, you may need to move from Class 1 to Class 2 — not because Class 1 is wrong, but because its evaporation rate no longer fits your timing window.

4. Solvency class. Only after confirming substrate identity, flow rate, and drying match should you consider whether you need a different solvency level. Moving to Class 3 when the real problem is a clogged nozzle wastes time and introduces new variables.

Using This Framework

Start with your substrate. That immediately tells you which classes are even candidates:

• PVC or PETG: Classes 1, 2, or 3. Start with Class 1. Move to Class 2 if you need faster drying. Move to Class 3 only if solvency is genuinely insufficient.
• PET or rPET: Class 4. No alternative within solvent welding.
• PP or PE: Class 5, or a non-solvent seaming method.

Then match drying to line speed. Within your candidate classes, choose the evaporation profile that fits the distance and time between your applicator and your winder.

Then — and only then — adjust solvency. If the substrate is right and the drying is right and the seam is still weak, you may need more aggressive chemistry. But that is the last lever, not the first.