Silicone Coating: Release Liners, Fabric and the Cure Window

Three ways to put silicone on a web

Silicone coating is a thin film, usually 0.5 to 3 grams per square metre, applied to a moving web of paper, film or fabric and then cured. The three methods differ in how the film thickness is metered, and that decides the coat weight range and the tolerance each one can hold.

A knife-over-roll or knife-over-blanket coater drags a blade across the web with a gap set above it. It is simple and forgiving, it handles viscous compounds and high coat weights, but the coat weight depends on web tension and speed as well as on the gap, so the tolerance is the loosest of the three.

A comma bar or a multi-roll coater meters the compound on a roll surface first and then transfers it, which decouples the film from the web. It holds a tighter tolerance and is the standard for release liners. A slot die pushes the compound through a machined slot under pressure, which gives the tightest control and the highest speed, but the die is specific to one coat weight and one viscosity.

Table 1 — Coating methods, their working range and the tolerance each one holds.

Coat weight sets the line speed

The line speed is limited by how fast the coating can be metered and how fast the oven can cure it, and both scale with coat weight. At 5 g/m2 a line runs at about 120 metres a minute; at 10 g/m2 it drops to 80, at 20 to 50, at 35 to 30 and at 50 to 18.

The relationship is steeper than a simple inverse. Doubling the coat weight from 5 to 10 costs a third of the speed rather than half, because the oven has spare capacity at low coat weights and becomes the limit at high ones. Quadrupling it from 5 to 20 costs more than half the speed, and by 50 g/m2 the line is running at a sixth of where it started.

This is why release liners are coated at 0.8 to 1.5 g/m2 rather than at a comfortable 5. The coat only has to be continuous and defect-free; anything more is silicone that the customer pays for and the oven has to cure. The lower limit is set by dewetting and pinholes, not by performance.

The same arithmetic explains why a converter with one line will not quote both a 1 g/m2 liner and a 40 g/m2 fabric coating at the same price per square metre. They are different machines running at different speeds, and the oven residence time is what differs.

Adhesion starts at the surface

Silicone is a low-surface-energy material, and that is the point: it releases. The problem is that it also does not want to stick to the substrate. A release coating has to adhere to the liner strongly enough to survive winding, shipping and repeated peel, while releasing the adhesive that is pressed onto it. Those two requirements pull against each other, and the surface treatment is what balances them.

Corona treatment oxidises the surface of a PET or polypropylene film, raising its surface energy and giving the silicone something to bond to. The effect on peel strength is steep at first and then flat: 0.5 N/25 mm untreated, 1.8 at 10 W min/m2, 3.2 at 20, 3.6 at 40 and 3.7 at 80.

Past about 20 W min/m2 the treatment is doing nothing useful, and it is doing harm. Over-treatment creates a weak, chalky boundary layer on the film, and the coating then fails cohesively in that layer rather than at the interface, which shows up as a transfer of silicone onto the adhesive and a liner that will not release cleanly. The right dose is the lowest one that reaches the plateau.

Table 2 — Corona dose against measured peel, and what happens past the plateau.

Corona is not the only treatment. Flame treatment works on thicker films and on some papers, and a chemical primer is used where corona cannot reach, such as on a fabric with a deep weave. All three do the same thing: they raise the surface energy enough for the silicone to wet and bond.

Release force is not adhesion

These two numbers are measured differently and are often confused. Adhesion is how hard it is to remove the cured silicone from the substrate, tested by rubbing or by a tape pull. Release force is how hard it is to peel an adhesive away from the cured silicone surface, tested at a fixed angle and speed against a standard tape.

A liner can have excellent adhesion and the wrong release force, and that is the more common failure. Release force is set by the formulation, the coat weight and the cure state, not by the treatment. A heavier coat gives a lower release force because the adhesive sees more silicone and less substrate; an undercured coat gives a higher and less stable release force because the network is incomplete and the low-molecular-weight fraction migrates into the adhesive.

The migration is the mechanism behind the classic complaint that a liner released well in the lab and grabbed in the field. An undercured or over-coated liner transfers silicone oil into the adhesive, and the adhesive loses tack. It shows up weeks later as labels lifting off a product, and the liner tested perfectly when it was made.

Curing without a runaway

Almost all release coatings are platinum-catalysed addition cure, applied solventless and cured in an oven at 120 to 160 C for a few seconds. The reaction is inhibited at room temperature, which gives the coater a working pot life, and it is triggered by heat in the oven.

The cure has to be complete before the web is wound. An undercured coating is tacky, it blocks on the roll, and it migrates. An overcured coating is brittle and can craze, and on a thin film the oven heat also distorts the substrate. The window between the two is narrow at high line speed, because the residence time is short and the oven has to deliver more energy per second.

Two things make the window wider. The first is a catalyst level matched to the coat weight and the line speed, rather than a single formulation used everywhere. The second is a controlled oven profile, with the first zone slightly cooler so that the coating gels before the solvent or the water in the substrate flashes, which prevents pinholes and blisters.

Cure is verified by a rub test, a smear test or a solvent extraction, and the rub test is the one that catches the marginal cases. A coating that passes a visual inspection and fails a rub is a coating that will block on the roll.

What to specify

A silicone coating specification has to pin down the things that a supplier can quietly change. Six lines do it.

• Coat weight in grams per square metre, with a tolerance and a method.
• Substrate, including the film grade and the surface treatment.
• Release force against a named test tape, at a named speed and angle.
• Cure state, with the test method, usually a rub or a solvent extraction.
• Adhesion, tested by a rub or a tape pull after conditioning.
• For fabric, the coat weight per side and the penetration requirement.

The release force and the cure state are the two that matter in service, and they are the two that are most often left as 'silicone coated, release grade'. That phrase covers a factor of three in release force, which is the difference between a liner that works on a high-speed labelling line and one that does not.