PP and PE: Why Are They So Hard to Bond?

Whether you’re manufacturing automotive parts, packaging bottles, consumer goods, or carbon fiber components, you likely face the same persistent challenge: coatings, inks, and adhesives simply refuse to stick to these materials.

In short, the goal is to get coatings, inks, and adhesives to firmly grip materials that are inherently “ungrippable.” Today, we’ll take a deep dive into why PP (polypropylene) and PE (polyethylene) – the most notorious “troublemakers” in this category – are so difficult to bond.

To start with the conclusion:

It’s not that PP and PE are “hard to bond”, it’s that they are “naturally antisocial.” Their surface energy is too low, and their molecules are too “reclusive” – meaning that when inks or coatings are applied, they can neither spread out properly nor gain a firm hold. To tame them, you first have to understand them.

First, let’s look at a “material profile” table: Surface Energy – the master key to adhesion.

When it comes to any adhesion issue, the first physical property to examine is “surface energy” (often referred to in the industry as the “dyne value,” measured in mN/m, or dynes per centimeter):

• PTFE (Teflon) ~18 Extremely low: Doesn’t even “interact” with water.
• PP 29–31 Low: A top contender for the title of “hardest-to-bond” material.
• PE 31–33 Low: Another top contender for “hardest-to-bond.”
• Nylon ~46 Medium: Bondable, but can be temperamental.
• PET ~43 Medium: A standard, conventional material.
• Glass / Metal Hundreds High: Naturally “clingy” (easy to bond to).

Surface energy can be thought of, in simple terms, as a material’s willingness to “get close” to something else. A higher value means the material is more receptive to being wetted by liquids or gripped by coatings; a lower value means it is more “aloof” or resistant.

So, what counts as a passing grade? Most inks, coatings, and adhesives have a surface tension between 25 and 40 mN/m. Industry experience suggests that for successful adhesion, a material’s surface energy needs to be at least 38 mN/m (a threshold easily checked with a “dyne pen”). This is why plastic manufacturers keep a “dyne pen” on hand – if you draw a line across the surface and the ink remains continuous without beading up or shrinking, it means the surface energy meets the requirement.

PP (polypropylene) and PE (polyethylene) have values ​​of 29–31 and 31–33, respectively – falling significantly short of the threshold for acceptable adhesion. In the first article of this series, I used the lotus leaf as an example: water droplets roll around on its surface because the waxy layer there has an extremely low surface energy, leaving the water with “nowhere to take hold.” The “aloof” nature of PP and PE follows the same principle – all three belong to the same family of low-surface-energy materials.

The First Hurdle for Adhesion: The Liquid Must “Spread Out”

The prerequisite for adhesion is wetting – the liquid must first spread across the solid surface before it can “stick.”

When a drop of water hits a metal surface, it spreads out; when it hits PP, it beads up into a round droplet. The criterion for whether a liquid spreads or not is the “contact angle“:

• Contact angle < 90°: The liquid tends to spread (lyophilic/wetting surface).
• Contact angle > 90°: The liquid beads up (lyophobic/non-wetting surface) – this is exactly what happens with water droplets on PP.

However, there is a common misconception in the industry: Spreading out does not equal sticking.

Some solvent-based inks have a low surface tension (25–35) and can indeed spread across PP – yet once they dry, they can be easily scraped off. Why? Because adhesion involves two hurdles: the first is “spreading out” (wetting), and the second is “holding on” (anchoring). PP presents a challenge for both: while many liquids can clear the first hurdle, they all fail at the second.

The Second Hurdle for Adhesion: Molecules Must “Hold On”

The second hurdle must be examined at the molecular level.

The molecular chains of PP and PE are essentially long, hydrocarbon “strands” – carbon atoms bonded to hydrogen atoms, clean and simple, lacking any “charged” polar groups like hydroxyl (-OH) or carboxyl (-COOH) groups. You can think of “polarity” in terms of magnets: polar molecules carry positive and negative charges. When they encounter a similarly charged surface, they are drawn to it just like a magnet—sometimes even forming chemical bonds, a process known as “chemical anchoring,” which creates an exceptionally strong bond.

In contrast, materials like PP (polypropylene) and PE (polyethylene) are like “non-magnetic” wood. When paint or ink molecules approach them, there is no magnetic-like attraction; they can only rely on weak van der Waals forces – a sort of tenuous, “accidental” connection between molecules. How long can such a flimsy bond last? Once exposed to water, heat, or mechanical bending, it simply fails.

Metal surfaces have high surface energy – they are naturally “clingy” – whereas PP and PE are “reclusive.” It’s not that the liquids aren’t trying; it’s that the materials offer no “grip” for them to hold onto. This is precisely why these materials are notorious as the “hardest-to-bond” substances.

The Three Standard Solutions: Corona Treatment, Flame Treatment, and Surface Primers – or, a “Molecular Double-Sided Tape.”

Once the root cause is identified – low surface energy combined with a lack of polar “anchors” – engineers can apply the right remedy: “modifying” the surfaces of PP and PE to introduce polarity. The industry relies on three primary methods:

First: Corona treatment. A high-voltage electrical discharge is applied to the material’s surface, causing oxidation and the formation of polar groups – such as hydroxyl or carboxyl groups. This causes the surface energy to spike, often exceeding 38 mN/m. However, note the phrase “short-term”: these polar groups are unstable on the surface. Over a few days, they tend to “flip” or rotate back into the interior of the molecular chains, leaving the surface once again “unreceptive” to anything else. Consequently, the effective window for corona treatment lasts only from a few hours to a few days; factories must apply coatings or inks immediately after treatment.

Second: Flame treatment. The principle is the same – using intense, momentary heat to oxidize the surface. Like corona treatment, this has a short effective window and requires a high level of operator skill.

Third: Surface primers (surface treatment agents). This represents a “molecular-level” solution. Take a PP (polypropylene) treatment agent as an example. Its key component is chlorinated polypropylene (CPP) – the name itself gives it away: it’s a “relative” of polypropylene.
By taking the molecular chains of PP and attaching chlorine atoms to them, you create a substance that acts like a “double agent”:

The backbone remains that of PP, allowing it to bond intimately with the PP surface – the chains swell and entangle, much like siblings gripping each other’s hands in a firm, unyielding hold. Meanwhile, the side with the attached chlorine atoms is polar, making it highly compatible with paints and inks – polar attracts polar, providing a solid anchor. It functions like a molecular-level double-sided tape: one side grips the PP, while the other welcomes the coating.

Once the treatment agent dries into a film, it retains its polarity far longer than corona treatment – remaining effective for days or even weeks. It can even be used in conjunction with corona treatment, providing a “double safety” measure. This is why applying a layer of treatment agent before painting has become the standard procedure for coating plastic parts.

There is a general rule: every material has its own “temperament.” Metals are “enthusiastic” – they bond easily; PP and PE are “reclusive” – they resist bonding unless you go to great lengths to persuade them; nylon is sensitive to moisture – it “changes its tune” the moment things get damp; and carbon fiber is “haughty” – its surface is too smooth and often contaminated with residual mold-release agents.

Back to the question in the title: why are PP and PE considered the “kings of hard-to-bond materials”?

Their surface energy is too low (29–33 mN/m, well below the 38 mN/m threshold) – liquids cannot spread across them, causing a failure at the very first hurdle. Furthermore, their molecules are “reclusive” (non-polar, lacking any “handles” for adhesion) – even if a liquid manages to spread, it cannot “take hold,” leading to failure at the second hurdle. The solution lies in modifying the surface.

While methods like corona or flame treatment offer only short-term effects, a treatment agent acts as a “molecular double-sided tape” – bonding with the PP on one side and the coating on the other – providing a much longer-lasting solution. If water is nature’s most restless chemist, then PP and PE are the “aloof” residents in humanity’s material library. The romance of chemistry lies in the fact that even this “aloofness” can be calculated, modified and tamed.