High temperatures and humidity can wreak havoc on assemblies bonded with pressure-sensitive adhesives.
Technical Services Manager
Siliconized release liners play a key role in pressure-sensitive adhesive (PSA) systems. They protect adhesives during transport and storage as well as while they pass through various converting and assembly operations. To protect the adhesive in varying atmospheres, liners must not see excessive heat and long-term exposure to humidity.
That's because unprotected release liners can wrinkle and buckle, especially during the summer months.
This so-called cockling of siliconecoated paper release liners can destroy the adhesive system and cause major assembly problems. Technicians are often forced to unwind and discard a couple of wraps of PSA tape to reveal noncockled product. However, this wasteful practice may only solve some, but not all, of the problems excessive heat and humidity cause.
Release liners are throwaway components, yet play a critical role during assembly. Designers must carefully spec them so they perform well in the particular production processes they will see. A properly designed release liner will function reliably in high-speed and automated processes so manufacturing can be more productive.
Eighty-five percent of PSA release liners are made from wood pulp-derived paper that inherently absorbs a lot of moisture. The paper swells in all three dimensions in high humidity. In contrast, pressure-sensitive tape is more dimensionally stable. It expands only in two dimensions along the horizontal plane. Excess expansion of the release liner makes the liner move in the third dimension, so it separates from the tape and the adhesive web.
Temperature fluctuations can also cause cockling. Elevated temperatures dry out and damage wood fibers, making release liners more vulnerable to subsequent higher humidity. There can be problems as well from condensation that accompanies cooling cycles after high-temperature processes such as heat lamination.
THE COST OF COCKLING
Cockling causes waste. The wrinkles form air pockets that can dry-out the adhesive before assembly.Cockling also brings creases in the first and second laminating processes. This not only wastes the pressure-sensitive adhesive, but also destroys the substrate material. In addition, wrinkles and creases in the release liner, or premature release caused by cockling, can clog converting, die-cutting, and printing machinery.
Uneven adhesive performance caused by cockling can be particularly problematic for small diecut parts. Parts positioned at an area where the adhesive is compromised won't bond properly. Another problem is that small kiss-cut parts are apt to be pulled off the release liner during the cutting operations, causing waste and potentially jamming the equipment.
Many printing operations demand that release liners have good "layflat" properties so they won't curl. Even minute amounts of cockling are unacceptable for many graphics applications.
It pays to prevent cockling not only to keep costs down, but also for aesthetics. Some applications remove liners during converting and assembly. In others, a release liner remains intact until the end user removes it. In this case, the liner must look good at the point of sale, which often means a release liner free of cockling. For example, cockled release liners typically won't do for bandages and other medical products because they give the appearance of poor quality.
The first step in heading off cockling is the selection of material for the silicone-coated release liners. Unfortunately, the types of liners that resist cockling best are also the most costly. The least-expensive material, densified kraft paper (with a 60 to 80-lb basis weight, i.e., weight of one 500 sheet ream of paper), is also the most common. But it is highly susceptible to cockling. It is fine for generalpurpose use and rotary die cutting, but not for kiss cutting.
Kraft papers are sealed with a polycoating for better dimensional stability. Polycoated liners have a polyolefin coating on both sides of the base paper. The plastic layers help encapsulate the paper and keep it from drawing in moisture. This configuration resists tearing and wrinkling and can be used for all diecutting operations.
Another option is to use paperboard liners with a higher basis weight and caliper (width) of 0.012 to 0.014 in. A 12-pt board with a heavy, 140-lb basis weight, can be coupled with polycoating (on both sides) to virtually eliminate cockling. The liner's large caliper also maximizes kiss-cutting performance and eases removal of small parts and waste pieces.
Plastic film liners made of polyester, polyethyleneterephthalate, and other polymers are also available. They absorb little moisture compared to paper and are thus not susceptible to cockling. They resist tears and will work with high-speed rotary die-cutting and hot-wire-cutting equipment. Plastic film liners are becoming more widely used, but represent only 15% of liners. Paradoxically, film introduces static electricity challenges during drier seasons though it addresses humidity concerns in summer months.
STORAGE AND HANDLING
"To control cockling it's important to maintain moisture stability during the entire life cycle of the release liner, especially with kraftpaper release liners," says Eric Bjork, technical team leader at Loparex Inc., Willowbrook, Ill., a supplier of siliconized release papers and films.
Bjork emphasizes that "the quality and composition of the paper itself is critical to maintaining dimensional stability. Designers should spec good-quality, uniform paper material with an appropriate mixture of softwood and hardwood pulp. The better the layflat, the more square and resilient the paper will be to humidity and temperature fluctuation."
To ensure PSAs perform as anticipated no matter the weather, designers should include a few simple storage and handling specs on assembly drawings before throwing them "over the wall" to manufacturing. Doing so will help prevent cockling problems and minimize waste down the road.
- Rolls of tape with paper liners should be kept cool and dry and protected from dirt, extreme temperatures, and damage.
- Plastic wrap around each roll, including both ends, will help keep moisture from getting to the paper.
- When a laminating job is complete, the remaining roll should be wrapped in plastic, even if it is still on the machine.
- Prelaminated parts including foam sheets and precoated graphic arts should be protected from humidity by storage in a controlled environment or wrapping in plastic.
- Any product already threaded through the laminator will be unprotected and may need to be discarded if it develops severe cockling before the next job.