Plastic bottles on a production line.
By CTM Admin Labeling • labeling systems 0 Comments

Best Practices for Labeling Plastic Bottles

While containers like glass, aluminum, steel, and cardboard are mostly uniform, plastics can be made up of a wide range of materials (in addition to variations in casting techniques, surface textures, and chemical additives).

Even if a label tests well on a sample, other issues can crop up in your production environment. Here’s how you can address these issues when labeling plastic bottles.

Table of Contents

  • Considerations for Custom Labels on Plastic Containers
  • Surface Energy, Texture, and Wetting
    • Effects of Matte vs. Shiny Surfaces
  • Off-Gassing, Contamination, and Adhesive Degradation
    • Solutions for Off-Gassing
  • Surface Contamination
    • Solutions for Condensation
  • Static Electricity
  • Flexibility
  • Choosing the Right Label Shape
    • Label Application Considerations
  • Take the Guesswork Out of Plastic Bottle Labels

Considerations for Custom Labels on Plastic Containers

Let’s take beverage bottles as an example. Most commonly, manufacturers elect to use plastic or glass bottles (ex. beer bottles are always made of glass, while water bottles are almost always plastic).

No matter the material, bottle label printing can be difficult. Custom label designs have to take into account the curvature of the bottle and ensure all required information is included and visible where necessary (whether on the name label or the information panel). Your label printer and applicator should adapt to custom shapes and custom sizes.

In addition to shape and size, each company must also consider potential concerns like surface energy, adhesion, off-gassing and more.

Surface Energy, Texture, and Wetting

Have you ever noticed the water beads that sit on your car after a wax finish? The wax coating gives the car low surface energy. When that wax layer wears off, water spreads out across the surface. The unwaxed paint has high surface energy. Plastic bottles and adhesives work in much the same way. Adhesives wet out easily on high-surface energy plastics, spreading out and forming a strong bond to the container. On low-surface energy containers, the adhesive can’t spread and attach, weakening the bond.

Surface energy is measured in dynes per centimeter. Teflon has a surface energy of 18 Dynes/CM. This low surface energy makes it great for non-stick coatings. Polystyrene, polyethylene, TPO, and polypropylene have surface energy between 29 and 31 Dynes/cm, keeping adhesives from wetting out. However, wetting can be increased by heat treating these plastics or by applying a primer. Nylon, polyester, ABS, polycarbonate, PVC, and acrylic have a surface energy of over 40 Dynes/cm, so they readily accept adhesives without added processing.

Effects of Matte vs. Shiny Surfaces

Choosing between matte and shiny surfaces affects more than aesthetics: it also affects adhesion. Rough surfaces give adhesives more to bite on by letting them flow into depressions on the container’s surface. However, this only works up to a point. On high-energy plastics, this reduces the effective surface area, because the adhesive can’t flow into these crevices. You may find that you get better adhesion on low-energy plastics by choosing containers with flat, shiny surfaces.

Pressure is also important for wetting. A wipe-on system, like that used by our 360a wrap system, uses physical pressure to push the label onto the container. This added force increases wetting, providing a stronger bond than other label applicators.

Off-Gassing, Contamination, and Adhesive Degradation

Casting plastic leaves small voids. These voids can be filled with chemical additives to create an impermeable container, but those chemicals can leach into the product. Most plastic containers, including nearly all food-safe containers, skip these additives which allows gasses to pass from the inside of the bottle to the outside. That off-gassing can then in turn break the adhesive bond and form bubbles underneath the label.

Plasticizers make plastics more flexible, but they can leach out onto the surface of the container. This reduces contact with the adhesive and can break down its chemical bonds. Release agents, fillers, hardeners, and binders used to improve container manufacturing also affect the chemical composition and wetting of the adhesive.

Solutions for Off-Gassing

Choose a permeable product label with a compatible adhesive to resolve these issues. Fine adhesive carriers maintain performance while allowing gasses to pass through gaps between molecules. This permeability requires the use of waterproof label stock for products exposed to moisture, like cold drinks. Bi-oriented polypropylene (BOPP) is the most popular choice for water-resistant face stock, but it’s possible to get similar performance from paper stock with the right coating.

Surface Contamination

While you can do sample tests for your labels, this doesn’t take into account the contamination that your bottles may pick up on the factory floor (which can be anything from spillage at filling stations to dust and lubricants on factory equipment). That contamination can also stem from condensation. While condensation isn’t typically an issue in a clean environment, it can begin to pose problems (bacteria growth) when left unaddressed.

Solutions for Condensation

Condensation is a common problem for cold products. Small amounts of moisture can be dealt with by choosing an adhesive that works with wet surfaces. However, for most applications, it’s easier to add an air knife before the labeling station to blow off moisture before labeling.

Static Electricity

Static electricity is caused by the transfer of electrical charges between objects. In production, this happens between containers and the conveyors and rollers they rub against. If the container and the label have the same charge, they repel each other. If they have opposite charges, they attract each other. Either charge disrupts the application, causing flagging, wrinkles, and bubbling.

Moisture in the air can discharge this electricity, making it less of an issue on warm and humid days. In dry environments, eliminators and ionizers can be added to send an electric current through containers and neutralize the charge before labeling.

Flexibility

If you’re using flexible plastic containers, like PET bottles, you need label materials that can bend with the container. Naturally, laminate and paper labels have different levels of flexibility. Paper labels wrinkle as they flex, but most plastic face stocks will retain their shape. Take water bottle labels as an example. Some brands glue the two ends of the label and only connect it to the bottle in one singular location. This custom water bottle label technique allows the thin plastic container to bend and move without affecting the integrity of the label.

As temperatures drop, most adhesives become brittle and won’t wet out. Low-temperature adhesives stay pliable, helping with the application while keeping the label on the bottle. Likewise, some label stock retains its flexibility better than others at low temperatures. When choosing the label for any product, consider both the minimum application temperature (MAT) and the service temperature range (STR.) For example, flash freezing can make the container far colder than storage, requiring a low MAT. If the container may be microwaved, it could experience extremely high temperatures requiring a wide STR.

Choosing the Right Label Shape

Plastic bottles have slanted surfaces to help them release from molds. Although the sides of the bottle may have an angle of just one or two degrees, this is enough to cause issues with label skewing. Each change in the container’s surface requires a different strategy for coverage. Decreasing the size of the label makes it less prone to application problems. Labeling steeply angled surfaces with front and back labels reduces the chance of skewing that you get with wrap-around labels.

Label Application Considerations

Your bottle material, bottle shape, label material, and label shape all dictate how to apply labels. It also largely depends on your capacity.

While small businesses may have the luxury of DIY application from paper sticker sheet labels, larger manufacturers need to add it as part of their production line, where label applicators connect to roll labels and automatically place them on the product accurately and quickly. Some label applicators also double as label printers, making high-quality label applications a breeze.

Take the Guesswork Out of Plastic Bottle Labels

So, how do you begin labeling plastic bottles correctly? Start by contacting CTM Labeling Systems. We make flexible, reliable equipment that can be customized for any product – from wine bottle labels to cosmetic labels (like lotion or chapstick). We’ll help you find the right labeling solution for your needs.