Understanding the Surface Energy of Plastic Containers & How It Affects Your Labeling System
Labeling plastic containers isn’t always as simple as rolling on a label. Adhesives need to be spread across and cling to the surface. However, not all plastics are the same. Low surface energy plastics won’t stick to adhesives; they actually repel them.
So, how do you determine if your containers are causing label adhesion problems, and how can you change the surface energy of plastic containers?
What Is Material Surface Energy?
Material surface energy is a measurement of the amount of disruption of intermolecular bonds. The higher this energy is, the more the molecules in the plastic are attracted to each other. This attraction helps these molecules bond to other substances, including adhesives.
Energy measurements (wetting) are based on the spread of water, called contact angle measurements. The higher the energy, the more a drop of water spreads out over the surface. Adhesives perform similarly to water on these surfaces. This energy is measured in Dynes per centimeter. Materials are divided into low (LSE) and high (HSE) surface energy levels. The dividing line between LSE and HSE is around 40 Dyne/cm.
Glass ranges between 200 and 300 Dyne/cm, while aluminum has an energy of 840 Dyne/cm. Polytetrafluoroethylene (PTFE), the coating used on non-stick pans, has an energy rating of just 18 Dyne/cm. (Teflon is a common brand of PTFE.) Plastics used in packaging range from 29 to 50 Dyne/cm, which makes labeling plastic containers more difficult than other types of containers.
Another way to think of energy is by contact degrees. A perfect wetting surface has a zero-degree contact label. When the adhesive is applied, it spreads out, creating an even layer across the entire surface. A non-wetting surface has a 180-degree contact angle. The adhesive is repelled, forming a sphere that minimizes surface contact. High surface energy (HSE) materials have over 90 degrees of contact. The adhesive lays flat across the surface but still has some surface tension, much like a drop of water on a plate.
The energy on the surface of the plastic can be raised using several surface treatment methods. Applying heat, oxidation, chemicals, corona treatments, or plasma treatments to the plastic substrates alters the chemical structure of the surface.
Abrasion increases contact area, and while the adhesive’s grip can still be weak, there’s more area for it to attach to on the surface. Primer uses molecules that have one end that bonds to the plastic and another end that bonds to the adhesive. This makes the surface wettable without affecting the chemical structure of the container. Adhesion promoters serve the same function. These chemicals can be blended into the label’s adhesive.
Which Plastics Cause the Most Problems with Label Adhesion?
While plastics have lower surface energy than other common packaging materials, the range in wettability drastically affects the adhesive bond.
Low Surface Energy Plastics
In general, low surface energy (LSE) plastics tend to be soft. They have a low molecular density and a low melting point.
Polypropylene (PP) is a more rigid plastic with high resistance to chemicals, and it has no problem with hot-fill products. PP bottles and jars are common, but this plastic is mostly used in bottle caps. Polypropylene is also food safe and works well for medical devices and containers, including test tubes. Unfortunately, at just 29 Dyme/cm, it also has the lowest surface energy of any commonly-used container plastic.
Polyethylene and Low-Density Polyethylene
Polyethylene(PE) and Low-Density Polyethylene (LDPE) plastics are flexible and impact resistant but not translucent. High chemical resistance makes this plastic type a favorite choice for household chemicals. They have slightly higher surface energy than PP.
Although Polystyrene (PS) is mostly known for its foam form (Styrofoam), it’s also used in packaging. This plastic is clear, rigid, and has low impact resistance. It’s mostly used for cosmetics and personal care products. At 36 Dyme/cm, it’s easier to get adhesives to work with this plastic than PE or PP.
Polyethylene and Polyethylene Terephthalate Glycol
While mostly used to make transparent bottles, polyethylene (PET) and polyethylene terephthalate glycol (PETG) can be opaque. Polyethylene and polyethylene terephthalate glycol are often used for water, juice, peanut butter, and salad dressing bottles. PETG has an added glycol molecule and is more flexible than PET, and both of these plastics have the same energy as PS.
High-density polyethylene (HDPE) is used for milk jugs. This plastic is chemical and impact resistant, so it’s also used for household chemicals and personal care products. It has the same energy as PS and PET.
Environmental concerns over polyvinyl chloride’s (PVC) production have prompted a move toward PET containers. However, the high chemical resistance and availability in translucent forms make it popular for industrial chemicals and automotive products. At 39 Dyme/cm, it’s right between LSE and HSE categories.
High Surface Energy Plastics
Higher surface energy (HSE) plastics are sturdy and easy to mold into complicated shapes.
Although rarely used for disposable containers, this rugged plastic is common for reusable containers and lab equipment, including test tubes. Polycarbonate has an energy rating of 42 Dyme/cm.
Thermoplastic Polyolefin (TPO)
Thermoplastic polyolefin (TPO) is mostly used for industrial storage containers. It’s also a popular choice for label stock. Since it’s waterproof and temperature-resistant, this plastic is ideal for frozen and refrigerated products. Surface energy varies depending on how it is formed.
Treated HDPE, PE, and LDPE
Treated HDPE, PE, and LDPE plastics usually have a flame treatment or plasma treatment. Just five minutes of plasma exposure nearly doubles the energy of LDPE, while even shorter flame treatments greatly increase the surface energy of these plastics. Wettability varies depending on the treatment process and the amount of time used to treat the plastic.
How Do I Address Issues With Adhesion?
Packaging and labels need to be looked at holistically. By changing both the treatment or texture of the container together with the formulation of the label adhesive, you can usually find a solution that lets you use the plastic that best fits the needs of your products.
To get the best bond on difficult surfaces, you need pressure. Labeling machines like our 360a Series Wrap System (WR) use a wipe-on applicator, which presses down on the label as it wraps around the container. For containers with uneven or recessed surfaces, an air blow system applies the label using force from the air. This application is best for products that have unusual surfaces that physical applicators cannot match.
Energy isn’t the only thing that affects adhesion on plastic containers. Static electricity can keep labels from contacting surfaces, while off-gassing from plastic containers and labels can lead to bubbling and peeling. If you want to know more about addressing these problems, read our blog article “Labeling Glass vs. Plastic Containers.”
Labeling Solutions That Work With Your Plastic Packaging
Is the surface energy of plastic containers causing issues with your label application? When you need help labeling plastic containers, contact CTM Labeling Systems.
We manufacture a wide array of equipment to work with almost any type of packaging, and our distributors across the country will work with you to find the right labeling solution for your business.