The release liner is such a basic component to the pressure sensitive label that it is often overlooked. By definition, a release liner is the component of the pressure sensitive label stock which functions as a carrier for the pressure sensitive label. Prior to application it protects the adhesive, and readily separates from the label immediately before the label is applied to the product. In short, the release liner is the backbone of the pressure sensitive label. Without the liner and its ability to consistently release the label for dispensing, whether by hand or by machine, there would be no label as we know it today.
Ninety percent of all release liners for labels are paper based. Super calendered kraft (SCK) and glassine are the dominate base substrates. The other 10 percent of the release liner production consists of film substrates. In this category, synthetic liners have some advantages over paper liners, offering cost savings over film and more strength over paper.
SCK papers are the most popular choice for liners in North America. An SCK liner is one that has been super-crushed or calendered for the purpose of making it very smooth and allowing for precise thickness, which is critical throughout the production process. It also creates a barrier for the silicone so that it will not seep into the paper. Matthew Shields, director of marketing at Loparex, Willowbrook, IL says, “The SCK’s density affords the liner good silicone holdout capabilities, good caliper control, high basis weight per caliper, good die-cutting ability, and high temperature resistance which enhances processability.”
SCK liners also allow for a low coating weight of silicone. Lower coat weights save money and make drying easier to accomplish. Another advantage of SCK liners is that they achieve a stable release of the labels, allowing them to come off predictably and uniformly time after time.
Film liners, however, are required when a film is used for the label stock. The most dominant type of film liner is polyester. The stretching factor of film requires a liner that performs in the same manner as the face stock itself. Shields says, “Polyester film has excellent dimensional stability and layflat characteristics. Obvious benefits include the elimination of paper fibers, high tear strength which minimizes web-breaks and increases productivity.” But, he adds, “polyester is more costly as well.”
One of the disadvantages of film is that it has heat limitations. With increased temperatures, film’s tensile properties soften. “You can also get gauge band issues, which are basically ridges in the machine direction. Another drawback of films is bagginess which can affect processability.” How much of an issue these obstacles present depends on the requirements of a job.
Paper also has its disadvantages. One of the drawbacks can be moisture content, which can vary depending on the ambient humidity. With paper comes the potential for poor dimensional stability. A paper gets wet and dries out again. It can have curl issues, which can lead to poor registration farther along in the converting process. It can tear, which can cause problems again in diecutting operations. If you nick a liner when you’re diecutting against it, you have a web break. Heavy dust in the atmosphere can cause contamination. It’s opaque. And paper, after all, is not as smooth as film.
As an alternative to film and paper, Loparex pioneered poly-coated papers. Shields explains: “High density, low density or polypropylene resins are extruded on to one or both sides of the base paper. This produces a substrate with improved smoothness, curl resistance,and good layflat properties. Poly-coating provides excellent dimensional stability for the paper as well.”
Poly-coated paper picks up a lot of the benefits of films but also leans toward the price of papers. Shields acknowledges that paper release liners are used in most processes, but “there are some high-end clear label applications where you want that clear-on-clear look. Clear label, clear adhesive, and clear liner.”
Doug Goldstein, new business development manager of CPFilms Inc., Fieldale, VA, says that polyester film is preferred over paper because it diecuts better and has better dimensional stability. “Dimensional stability is important for subsequent coating and adhesive coating on the machine,” he says. Those qualities represent film’s main advantages, he adds, but probably its biggest disadvantage is that is still not as inexpensive as a paper liner.
The temperature issue can’t be overlooked. “Polyester is good only up to around 325° F, whereas paper can withstand higher temperatures,” Goldstein says. “This presents a problem in an application where it needs to be cured at a higher temperature. In that case the film would melt or shrivel up.”
Robert Benson, product manager for release coatings and pressure sensitive adhesives at GE Silicones, Waterford, NY, identifies five characteristics to consider when deciding which substrate should be used with the liner best suited to a converter’s need: compatibility with the pressure sensitive adhesive; release force; release profile; diecuttability; and cost.
Benson acknowledges that paper liners dominate the market, but he observes that a film release liner “gives you a very smooth surface. This smooth release surface translates to a smooth adhesive surface, critical to many optically clear applications. The other advantage of films are that they tend to be stronger than papers, which is important if you’re running at very high line speeds.” Because film use is growing faster than paper, GE Silicones has been developing film products with lower and faster cure profiles.
Diecutting is key
Diecutting performance contributes to the reason different types of liners are used. Shields of Loparex says certain surfaces are more conducive to diecutting than others. However, he says, “It depends on how you are going to diecut. Are you kiss cutting against or diecutting through the web? For example, polypropylene extruded on paper provides a very hard surface to diecut against. Films such as polyester and biaxially-oriented polypropylene are also suitable surfaces to diecut against. SCK is not as hard a surface.” He adds that although film is more expensive, it may be worth paying more to get a better product, because constant web breaks cost more money in downtime and labor and materials.
Price, of course, is always a factor, but it’s not independent of performance. Shields says, “No matter how inexpensive a product may be, if it’s not performing its intended use, then it’s not going to be qualified.”
There are two main ways to cure silicones: thermally and radiation. As Shields explains, thermally cured silicones, the largest group, includes condensation/tin chemistry, which is delivered in emulsions and solvent. Addition chemistry, such as platinum, can be delivered in solvent, solventless or in emulsions. Radiation cured silicone systems include electron beam (EB) and UV light.
Shields believes that “a customer chooses a specific silicone system based on the desired performance requirements of the end-use product. Those influence what kind of chemistry you’re going to use.”
Proper silicone application ensures that the labels dispense correctly. Correctly applied silicone will not soak into the paper, will not rub off, and will not scrape off. If it does any of these, then the label will not dispense properly.
Liners that are cured either with EB or UV are growing in popularity. EB is the curing process that utilizes electron particles. One benefit these systems is that the paper does not have to bake at 300° for several seconds, which causes it to lose moisture, often to below 1 percent. With EB or UV, curing the moisture content is not affected at all, leaving it at 3 or 4 percent.
EB and UV curing work well with film liners, which is where they gained their early foothold. Film liners stretch and distort during the baking process, which caused manufacturers to explore alternative curing methods.
Goldstein says, “Our UV cured silicone release coatings have low extractable silicone levels,” and they target them mainly to the electronics market where silicone contamination is an issue. “A big industry issue with silicone liners is the amount of silicone transfer,” he says. “In terms of good, better, best, you go from tin, good; platinum, better — both tin and platinum are thermally cured systems — then you have UV as best. UV cured is best as far as low silicone transfer.”
CPFilms offers other non-silicone releases. One is a wax based, the other chrome based. All of these are used when silicone contamination is an issue, i.e., electronics, computers, hard drives and medical markets.”
According to Shields, “tin catalyzed — condensation chemistry — is the most mature silicone chemistry out there. It’s very reliable. The liners have a very low coefficient of friction and a medium gloss level.”
|Film liners, polycoated liners and direct coated liners from Loparex are used in a wide variety of pressure sensitive adhesive applications.|
The strength of the release liner is an important consideration. It must be strong enough to stand up against repeated diecutting without breaking or tearing during the printing process.
Breaks can be costly and time consuming. The liner also must be strong enough to carry through to the label applicator. Breaks during the application process are also wasteful, and so it is critical that the liner be properly matched to the end use.
Uniform caliper results in good diecutting capability. If the caliper is too thick in certain areas of the web, the die could cut through the label stock and the adhesive, quite possibly nicking the liner. If it is too thin in places, the die might not cut through the stock or the adhesive. Problems with matrix winding could then develop.
The liner also must have a good release value. If the label has too tight a release, the diecutting will not work because the matrix won’t remove easily during the weeding process, nor will the label dispense properly, if at all. When the release value is too loose, the facestock might have trouble sticking to the paper, the paper might have bubbles or ridges in it, or it might move.
“The required strength of a liner depends on its end use and also on the processing equipment being used,” says Shields. “When we sit down with a customer, it’s important that we have the ability to openly discuss what the requirements are for the liner. By understanding the adhesive type and process conditions the liner will be exposed to, we can plan accordingly. That way we can deliver a product with the optimal performance in the shortest cycle time.”
After examining sheet samples, “we like the customer to evaluate the production samples to further validate the product’s performance. The final step is to specify the attributes that are critical to the end use.”
Free silicones are basically un-reacted or un-crosslinked silicone. Every liner produced has some level of silicone extractables. Generally, the free silicone level should be lower than 5 percent extractables, but some manufacturers can go as low as 1 percent.
Free silicones are a function of two parts of the production process: one is the amount of heat used during the curing process, and the other is the speed of the converting equipment. By turning up the heat or slowing the equipment, the supplier can lower the amount of free silicones.
Free silicone is a negative because it changes the release value of the liner, making the release unpredictable. Shields says, “Free silicone can transfer to the adhesive and reduce the tack level of the adhesive.”
When the silicone migrates to the surface, the adhesive will not adhere, thus causing another stability problem. However, he adds, free silicones also have a positive side-effect. “They can give you an advantage by reducing the initial spike or peel force.”
Loparex has developed a product called LO-EX. “It’s taken silicone extractables to a level where they’re almost undetectable,” Shields says. “We’ve corroborated the results in tests we’ve done with an independent laboratory.”
Demands of the customer
Benson of GE Silicones says customers are demanding productivity, and one way to achieve that is with higher speed. Benson says, “Faster line speeds are being seen across the entire industry. Sometimes this means changes in raw materials, other times it means setting up your process to run more efficiently.
“When we look at the faster line speeds and how that carries through to supply of release coatings, we’re seeing requests for lower and flatter stripping profiles. The lower and flatter profiles keep the release forces down as converters apply labels at increasingly faster rates.”
Goldstein of CPFilms says there is a new area of growth, and that is fluoro-silicone release liners. “Those are used with silicone pressure sensitives,” he says.
Shields says customers are asking for new products. “They’re asking for traction coatings for sheet feedability, and they’re asking for anti-static properties to address static build up.”
There is no clear cut formula for deciding which liner is best. Shields says, “Liners are very end-application specific. It’s important that release liner manufacturers and converters work together to address what is needed.”