Bubble, Bubble, Outgassing and Trouble

We all love that ‘new car’ smell. It sure is difficult to describe, though. This is because the famously intoxicating odor is really a byproduct of manmade materials. Fresh paint, carpets, foams, adhesives, and a whole host of other artificial substances emit much stronger scents when they’re new. But why?

Outgassing (or off-gassing) is the origin of these odors. When plastics go through extrusion or injection molding processes, they can trap tiny pockets of gas as they solidify. Heat, sunlight, and time can cause these trapped gas bubbles to expand and blow microscopic holes in the surface of the plastic, releasing the gas—outgassing.

New glues, finishes, and laminating polymers can also contain gases trapped microscopically within the material. Whatever the form of the material, those gases will slowly escape into the air.  Some of them we can smell. Over time, the gases dissipate completely, and the odor fades away.

This is an especially common headache for pressure-sensitive adhesive (PSA) labels, as any manufacturer of appliances employing plastic will tell you. And odor alone is not the only issue. The gradual escape of captive gases from injection-molded thermoplastics can cause bubbles to build up under adhered labels. This would be a non-issue in a porous label, but stickers, tapes, and protective laminates are generally airtight.

You’re Full of Hot Air!

Intense UV exposure and high temperatures are known to accelerate outgassing, which can make blistering and delamination issues even more pronounced. The automotive industry knows this well, as engine heat and harsh sunlight regularly stress labels to the max.

Modern cars are increasingly filled with low-surface energy thermoplastics that outgas. Injection-molded polycarbonate, for example, is the primary material for headlamp lenses and optical reflectors. Acrylic, polypropylene, and polyamide are also major culprits. The numerous thermoplastic polyolefin compounds and polyurethanes used in car bumpers are also known to outgas heavily.

At the same time, a car also features a full complement of required labels, such as:

  • Vehicle Emission Control Information (VECI) labels
  • VIN stickers
  • Tire pressure labels
  • Caution labels
  • Brand emblems
  • Instrument labels
Conventional labels lift and blister due to outgassing from polymers (above), whereas a label stock boasting high permeability (below) does not suffer this fate. Images courtesy of Lintec Automotive.

Automotive labeling is also nuanced work. Many of these critical labels must already compete with challenging curvatures, low surface energy, exposure to chemicals, contaminants, weather, abrasions, and intense heat. Add outgassing to the mix, and what’s an automaker to do?

To prevent unsightly bubbles—or worse, labels and laminates that peel away—manufacturers have put R&D to work.

Cellular Plastics

Virtually any plastic (thermoplastic or thermoset) can be given a “cellular” structure by infusion with air during formulation. The term essentially refers to plastic foams. However, cellular plastics can be rigid or flexible, lightweight or dense, a large block, or a thin film. It all depends on the material and the process used to make them cellular.

Some suppliers of auto data labels have painstakingly designed adhesive carriers from incredibly fine-celled polymers. These “blister-free” films allow air to pass through without creating any visible holes in the surface. To the naked eye, the best examples look like any other smooth, airtight sticker. However, the difference is clear when you adhere the sticker to a surface with a pocket of air trapped in the middle, press on it, and the bubble just disappears.

How do they do it?  Well, there are actually several technologies at work. These composite pressure-sensitive films are formed by laminating a surface base with cellular film—often polyester. Any outgasses generated by the substrate (or air bubbles trapped during application), can then pass through the cellular structure film. Blisters and bubbles disappear quickly with a little pressure.

In addition, a special cavitated underlayer is designed with a clever lateral air-release mechanism: micro-channels formed on the adhesive surface allow air to escape out the sides of the label, keeping it flush with the surface as outgasses flow towards the edges.

Industries of Interest

These cellular films and laminates make ideal label stock for bubble-free application and long-term adhesion on high-outgassing plastics. They have swiftly been adopted for use over a variety of surface bases for blisterless stripes, stickers, and labels. Appliance manufacturers often use cellular PSAs to create large nameplates or applied graphics on polycarbonate. The automotive industry relies on outgas-friendly films for engine covers, polyurethane bumper stripes, and more.

The potential uses in other industries—construction, eyewear, retail, aerospace, toys—are nearly limitless. These polyester films are not only conformable to curved surfaces, but offer excellent mechanical strength, heat resistance, and smoothness.  Depending on their purpose, the films can be manufactured transparent, opaque, gloss, matte, textured, or even with a metalized finish that won’t compromise air-permeability. The surface base and adhesive layer should also be carefully matched to the needs of the application.

In conclusion it looks like outgassing is no longer the trouble for PSA labels that it used to be.

The author is Account Manager at Lintec Automotive.