Adhesion Explained for Industrial Contexts (and beyond)
Adhesion is the fundamental force that makes materials stick together—from the paint on a car to the glue in a smartphone assembly. Technically, it describes the attractive interactions between different materials at their interface, whether those forces are mechanical, chemical, or physical in nature. In simpler terms, adhesion determines how well one material “holds on” to another—an essential property for nearly every manufacturing and engineering process.
Because of its importance, adhesion is not only a practical concern but also a key area of research and technological innovation. Scientists and engineers continually work to understand and control the mechanisms that govern adhesion, developing new materials, surface treatments, and verification methods to meet ever-higher industrial standards. In this context, modern surface pretreatment technologies, such as plasma activation, have become crucial tools for improving adhesion on challenging materials—bridging the gap between theory and industrial application.
How Adhesion Works: The Science Behind Strong Bonds
Adhesion describes the set of physical and chemical interactions that cause two different materials to hold together at their interface. At the microscopic level, several mechanisms act simultaneously. Van der Waals forces create weak but widespread attractions between molecules, forming the foundation of most adhesive bonds. In many industrial processes, chemical bonding—such as covalent, ionic, or hydrogen bonds—adds stronger and more specific connections between surfaces. Mechanical interlocking occurs when a liquid adhesive or coating penetrates surface irregularities and hardens, physically anchoring to the substrate. In certain systems, electrostatic effects also contribute, as oppositely charged surfaces attract each other across the interface.
For effective bonding, these mechanisms depend heavily on the surface energy of the substrate and its ability to be wetted by an adhesive, coating, or ink. High-energy surfaces (like metals or glass) allow liquids to spread easily, promoting close molecular contact and stronger adhesion. Low-energy materials (such as polyethylene or polypropylene) tend to repel liquids, requiring surface treatment to improve wettability. Temperature, surface roughness, and material compatibility further influence how well adhesion develops. For example, elevated temperature can enhance molecular mobility, enabling better diffusion and chemical bonding, while excessive roughness may reduce real contact area if valleys remain unfilled.
A clear distinction must also be made between adhesion and cohesion: adhesion refers to the attraction between different materials, whereas cohesion describes the internal strength within one material or adhesive layer. Both are crucial—strong adhesion without sufficient cohesion leads to internal failure, and vice versa.
In practical terms, adhesion governs the success of countless industrial applications—from painting and gluing to printing, coating, and sealing. Understanding the balance of forces and surface properties behind adhesion allows engineers to optimize processes, choose suitable materials, and ensure long-term bond reliability.
Why Adhesion Matters in Manufacturing
Adhesion plays a critical role across numerous industrial sectors where reliable bonding, coating, sealing or printing processes are foundational to product performance and durability.
In the automotive industry, for instance, adhesive bonds must withstand vibration, temperature cycles and material-mix combinations (e.g., plastics bonded to metals) while maintaining integrity throughout a vehicle’s lifetime. In electronics manufacturing, high‐precision adhesion is essential for reliable printed circuit boards, display coatings and micro-assemblies. In medical technology, secure bonding and sealing of components (such as implants, catheters or instrumentation) is vital for safety and biocompatibility. The packaging industry demands consistent adhesive and print performance even on low-energy materials like polypropylene or recycled films, while textile fabrication and broader transportation manufacture (rail, aerospace, shipbuilding) increasingly depend on material combinations and surface treatments to maintain bond strength, reduce weight and improve surface functionality.
At Plasmatreat, our industry solutions are applicable to plastics, metals, glass, cardboard, textiles and composites in automotive, electronics, packaging, life sciences and transportation applications.
Across all these contexts, the underlying challenge is the same: achieving and maintaining sufficient adhesive interaction between surfaces that may be very different, under demanding environmental or operational conditions. Ensuring that adhesion is robust not only at the moment of bonding, but across the product lifecycle, is therefore a top priority for manufacturing decision-makers.
Typical Causes of Poor Adhesion
In industrial manufacturing, adhesion failures are among the most frequent and costly quality issues. Problems such as delamination, peeling, or detachment can occur when coatings, adhesives, or sealants fail to form a stable bond with the substrate. A typical example is when paint flakes off a plastic surface or a glued joint separates under mechanical or thermal stress. The root cause often lies in the surface characteristics of the material – especially when dealing with low-surface-energy plastics such as polypropylene (PP), polyethylene (PE), or polytetrafluoroethylene (PTFE). These polymers are chemically inert and nonpolar, meaning that adhesives or coatings cannot easily wet or anchor to their surfaces. Other difficult materials include certain fluoropolymers, silicones, and composite laminates with incompatible surface layers.
However, material properties are not the only issue. Insufficient cleaning, residual release agents, or oxidized films can create weak boundary layers that prevent intimate contact between adhesive and substrate. Likewise, an incorrect material pairing or the use of an adhesive incompatible with the substrate’s chemistry can lead to premature bond failure. Even when the right adhesive is used, missing or inadequate surface activation—for example, skipping pretreatment before bonding or coating—can drastically reduce adhesion strength.
To address these issues, engineers rely on a combination of surface pretreatment, optimized material selection, and precise adhesive matching. Pretreatment processes such as plasma activation, corona discharge, or flame treatment can increase surface energy and enable strong, durable bonds on previously non-wettable materials. Choosing adhesives formulated for specific substrate chemistries and maintaining rigorous cleaning protocols further minimizes the risk of adhesion failure. These methods form the basis for the next step: how surface modification and pretreatment improve adhesion performance.
Surface Cleaning & Activation: How Pretreatment Boosts Adhesion
A strong, durable bond begins long before any adhesive, coating, or sealant is applied—it starts with a clean and properly activated surface. Even microscopic traces of oil, dust, release agents, or oxidation layers can prevent the intimate contact required for reliable adhesion. Contaminants act as weak boundary layers that interrupt molecular interaction between materials, leading to adhesion failure such as peeling, blistering, or delamination. Therefore, thorough cleaning is the essential first step in any bonding, coating, or sealing process. Once the surface is free of impurities, activation further enhances its chemical and physical readiness for adhesion.
The key to successful activation lies in increasing surface energy and improving wetting behavior. A high-energy surface allows liquids—like paints, adhesives, or sealants—to spread evenly, maximizing the area of molecular contact and promoting stronger interfacial forces. In contrast, untreated surfaces, especially low-energy plastics, tend to repel liquids, resulting in poor wetting and unreliable bonding.
Several technologies are available to modify surface energy and cleanliness. Corona treatment, for example, is widely used for plastics and films. It works by generating a high-voltage electrical discharge that oxidizes the surface and introduces polar functional groups. However, its effectiveness is generally limited to non-conductive materials. Plasma treatment, by contrast, offers far greater flexibility—it can be applied to plastics, metals, glass, composites, and even textiles. According to the Fraunhofer Institute for Manufacturing Engineering and Automation, plasma processes not only remove organic residues but also chemically activate the topmost molecular layers of a surface, achieving both microfine cleaning and functionalization in one step.
The special advantages of plasma include its dry, environmentally friendly operation (no solvents or chemicals), precise control, and suitability for inline integration in automated production systems. It can selectively treat small, complex, or sensitive components without affecting the bulk material. By simultaneously cleaning and activating, plasma significantly raises surface energy, improves wettability, and creates the optimal conditions for long-term adhesion reliability—making it one of the most versatile and effective pretreatment technologies available in modern manufacturing.
Improving Adhesion with Plasmatreat
Plasmatreat’s Openair-Plasma® technology is a globally proven solution for improving adhesion in manufacturing processes where conventional pretreatments reach their limits. The principle is based on the generation of an atmospheric-pressure plasma jet using electricity and compressed air. When directed onto a surface, this plasma removes organic contaminants, breaks molecular bonds in weak boundary layers, and introduces new polar functional groups that significantly increase surface energy. The result is a surface that is microscopically clean, highly wettable, and chemically active—ideal conditions for strong and durable bonding, painting, coating, or sealing.
The advantages of Openair-Plasma® go far beyond traditional methods like solvent cleaning or corona treatment. It is a dry, contact-free, and selective process, meaning it can be integrated directly into production lines without altering the base material or requiring post-cleaning steps. The technology works on a wide range of substrates—plastics, metals, glass, composites, and textiles—and can precisely treat complex geometries or sensitive components. This makes it ideal for automotive manufacturing (e.g., reliable paint adhesion on bumpers or bonding of headlamp housings), medical technology (e.g., activation of device housings or tubing for adhesive sealing), and electronics production (e.g., improved potting, conformal coating, or encapsulation). Across these industries, Openair-Plasma® ensures consistent, repeatable adhesion results, even on challenging low-energy materials like polypropylene or PTFE.
Choosing Plasmatreat means partnering with a market leader that combines deep process expertise with decades of practical experience. The company supports customers worldwide in designing, testing, and integrating plasma pretreatment systems inline, ensuring high efficiency and minimal downtime. Beyond performance, plasma also offers clear sustainability advantages: it eliminates the need for chemical primers or solvents, reduces waste, and lowers energy consumption compared to thermal or chemical processes. This not only helps manufacturers meet stricter environmental regulations but also cuts costs and simplifies logistics.
Ensuring Adhesion Quality
In industrial manufacturing, measuring and documenting adhesion quality is essential to ensure process reliability and long-term product performance. Since adhesion is highly dependent on surface condition, objective testing methods and consistent monitoring form the backbone of quality assurance. The most widely used diagnostic tool is the contact angle measurement, which evaluates how a liquid droplet interacts with a surface. A small contact angle indicates strong wettability and high surface energy—conditions that favor robust adhesion. More sophisticated approaches calculate the surface free energy (SFE), providing numerical criteria that can be used to validate pretreatment efficiency and maintain consistent production quality.
Beyond surface characterization, mechanical testing quantifies the actual strength of the bond. The peel test measures resistance to peeling forces, especially relevant for films, foils, or flexible substrates. The pull test determines the tensile force needed to separate bonded components, while the shear test evaluates how well a joint resists forces acting parallel to the bond line—closely simulating many real-world stress conditions. Together, these tests create a complete picture of adhesion performance across materials and joining methods.
To maintain high process consistency, all measurements must be systematically documented and integrated into quality control workflows. Parameters such as pretreatment intensity, adhesive type, curing conditions, and test results should be linked for full traceability. Modern manufacturing increasingly employs inline measuring systems, which enable automated process monitoring and immediate feedback during production. By capturing data directly from the line—such as changes in surface energy or bond strength—companies can prevent deviations before they lead to failures.
At Plasmatreat, such analytical and validation capabilities are supported through advanced facilities like the Plasmatreat Academy, where materials, process parameters, and adhesion tests are evaluated under real production conditions. This integration of measurement, documentation, and automation ensures that adhesion moves from a variable factor to a measurable, certifiable element of modern industrial quality assurance.
Common Questions on Adhesion (FAQ)
What are the most common causes of adhesive bond failure in industrial processes?
Adhesion failures usually result from poor surface preparation, contamination, or incompatible materials. Residual oils, release agents, oxidation layers, or moisture can block molecular contact between adhesive and substrate. Inconsistent curing conditions, incorrect adhesive selection, or mechanical stress beyond design limits can also lead to premature failure. Effective cleaning, activation, and process control are therefore critical for durable adhesion.
What role does surface roughness play in adhesion performance, and is more roughness always better?
Surface roughness can improve adhesion by promoting mechanical interlocking—but only when the adhesive fully wets the surface. Excessive or irregular roughness may trap air or contaminants and reduce true contact area. The optimal roughness depends on the material and adhesive viscosity; in many cases, controlled micro-roughness combined with high surface energy yields the strongest and most consistent bonds.
How does surface energy of a substrate affect the choice of adhesive or coating process?
Surface energy determines how easily a liquid adhesive or coating spreads on a surface. High-energy materials (like metals or glass) support excellent wetting and bonding, while low-energy polymers (such as PP, PE, or PTFE) resist adhesion. For these materials, the process choice must include surface treatment—for example, plasma, corona, or flame activation—to raise surface energy and enable reliable wetting and bonding.
How can engineers integrate adhesion measurement and monitoring into high-volume production lines?
In automated environments, adhesion quality can be monitored using inline systems that measure surface energy or contact angle directly on the production line. These sensors provide real-time feedback on surface readiness before bonding or coating. Combined with routine mechanical tests (e.g., peel or pull samples) and digital process documentation, such systems make adhesion a measurable and controllable variable in large-scale manufacturing.
Can surface treatments (like plasma, flame, corona) fully replace primers, or are there cases where primers are still essential?
Modern plasma and corona treatments often eliminate the need for chemical primers by cleaning and activating the surface more efficiently and sustainably. However, primers may still be essential when specific chemical coupling, corrosion protection, or aesthetic functions are required. The optimal approach depends on the substrate, adhesive chemistry, and application environment—so process validation remains necessary.
When bonding dissimilar materials (e.g., plastic to metal), what specific adhesion challenges arise?
Bonding dissimilar materials introduces challenges from differences in surface energy, thermal expansion, and elasticity. Plastics may expand or contract more than metals, creating stress at the interface during temperature changes. Additionally, nonpolar polymers often require surface activation to achieve chemical compatibility with metal substrates. Adhesive selection must account for flexibility, curing behavior, and long-term environmental stability.
How important is adhesive joint design (gap, thickness, geometry) to achieving strong adhesion?
Joint design is critical. Proper gap size ensures full wetting without creating voids; adhesive thickness must balance stress distribution and load absorption; and geometry influences how forces act on the bond line. Designs that avoid peel or cleavage stresses and favor shear or compression loads typically provide higher durability. Good joint design is as essential as good surface preparation.
What are the limitations of adhesion as a joining method compared to mechanical fastening or welding?
While adhesive bonding enables lightweight, uniform stress distribution and multi-material design, it has limitations. Adhesive joints require surface preparation, curing time, and are generally less tolerant of high continuous temperatures than welded or bolted joints. They can also be harder to disassemble or inspect non-destructively. Nonetheless, when properly engineered, adhesive bonding often delivers superior aesthetics, corrosion resistance, and fatigue performance.
