Hot melt adhesives are a cornerstone of industrial manufacturing, valued for their quick set times and versatility. As thermoplastic, polymer-based adhesives applied in molten form, they adhere through mechanical anchorage, ensuring durable bonds. Yet, even the best adhesives face challenges, resulting in adhesive failure. Understanding the nuances of why adhesives fail to properly bond can prevent production setbacks and ensure optimal performance.

Adhesion vs. Cohesion: Understanding the Differences

The choice between cohesive and adhesive strength depends on application needs. Cohesive strength ensures the adhesive material holds together under stress, preventing internal splitting — this is ideal for high-stress environments, flexible bonds, and thin film applications.

Adhesive strength focuses on the bond between the adhesive and substrate, which is the right solution for bonding dissimilar or low-energy materials and preventing delamination or peeling. Strong adhesion is key in challenging surfaces like treated plastics or for sealing against corrosion. Many applications, such as automotive or woodworking, require a balance of both to ensure long-term performance. Proper surface preparation and adhesive selection optimize both cohesive and adhesive bonds.

Cohesive and adhesive failures in manufacturing can result in bond failure, leading to costly downtime, product recalls, and reduced efficiency. Cohesive failure occurs when the adhesive material splits within the bulk layer or along a thin film, often due to improper surface preparation or poor stress distribution. Adhesive failure happens when the adhesive separates from the substrate, typically caused by low surface energy or contamination. Surface analysis methods, such as contact angle measurements, help diagnose issues in the bond line. Environmental factors, corrosion failure, and bonding of dissimilar materials further impact failure mode. Proper surface preparation is critical for strong adhesive and cohesive bonds.

Adhesion: The Bond Between Adhesive and Substrate

Adhesion refers to the forces that hold an adhesive to the surface of a substrate. It is the strength of the bond at the interface where two different materials—such as glue and cardboard, plastic, or metal—meet. Essentially, adhesion represents the “stickiness” of the adhesive to the surface, making it essential to ensure that the two materials remain joined under stress or environmental exposure.

Adhesion occurs due to the attraction of unlike molecules, or those of the adhesive and the substrate. Multiple factors, including the surface characteristics of the substrate and the chemical properties of the adhesive, influence the bond’s effectiveness.

There are several different types of mechanisms through which adhesion can occur:

Mechanical Adhesion: This occurs when the adhesive penetrates the microscopic ridges and valleys of the substrate, anchoring itself in place. Rougher, more porous surfaces allow better “grip,” improving adhesion strength.
Adsorption (Intermolecular Bonding): In this process, adhesive molecules form chemical or physical bonds with the substrate’s surface molecules. These bonds are often weak individually, such as van der Waals forces, but together, they create significant adhesive strength.
Diffusion: When the adhesive’s polymers interlace with the molecules of the substrate (common in plastic-based materials), the two materials effectively “mingle” at a molecular level, forming an intertwined structure.

Key Factors Affecting Adhesion

Surface Energy: Materials with high surface energy, such as metals and glass, promote strong adhesive bonds, as their surfaces attract adhesive molecules. Low surface energy materials, like polyethylene and Teflon, are naturally more “slippery” and resist adhesion unless treated (e.g., with primers or surface roughening).
Surface Cleanliness: Contaminants such as oils, dust, or moisture on the substrate can form a barrier between the adhesive and the surface, weakening the bond. Even small amounts of residue can drastically reduce adhesion strength.
Viscosity and Flow of the Adhesive: Adhesives with the correct viscosity are better able to fill surface imperfections and form a uniform bond. If the adhesive is too thick, it may sit on top of the surface without creating enough contact; if too runny, it may drip or fail to build up sufficient thickness.

Signs of Adhesive Bond Failure

The adhesive peels away cleanly from one or both substrates, leaving no material behind.
The failure point occurs entirely at the boundary layer between the adhesive and the substrate.
There is no internal splitting of the adhesive layer (which would indicate a cohesive failure instead).

Common Adhesive Issues and Their Solutions

Bond Failure: When bond failure occurs, you may see a clean separation of the adhesive from the substrate or observe weak coverage.

Causes:

Insufficient adhesive coverage
Premature cooling of the adhesive
Unsuitable adhesive for temperature extremes
Substrate with low surface energy
Low application temperature, long open time, or poor compression

Solutions:

Increase bead size, pump pressure, or temperature
Choose an adhesive with the appropriate open or set time

Cold Cracking: When adhesive loses flexibility in cold temperatures, it may crack or fail under stress.

Causes:

Adhesive surpassing its pot life
Exposure to extreme cold or improper storage

Solutions:

Use adhesives designed for flexibility in cold environments
Store bonded items in controlled environments

Additional Tip: To mitigate potential failures, keep bonded assemblies in climate-controlled conditions during curing and storage.

Preventing Adhesive Failure

Prepare Substrates Properly: Ensure surfaces are clean, dry, and free from oils or contaminants before applying adhesive. Consider using surface treatments such as plasma cleaning or primers for low-energy surfaces.
Optimize Surface Roughness: When working with smoother materials, lightly abrade the surface to create microscopic anchoring points for the adhesive.
Choose the Right Adhesive for the Job: Not all adhesives bond well to all substrates. When working with difficult materials like polypropylene or treated metals, use adhesives formulated for those surfaces.
Maintain Proper Application Temperature: Adhesives applied too cold may not flow properly and fail to penetrate the substrate surface, leading to weak adhesion.

Cohesion: The Internal Strength of Adhesives

Cohesion refers to the internal forces that hold the adhesive together as a unified material. It is the measure of how well the adhesive can resist internal stress, such as stretching, pulling, or shear forces, without splitting apart. This concept is often described as the “glue within the glue”—it defines the structural integrity of the adhesive layer itself.

How Cohesion Works

Cohesion is the attraction between like molecules. When these internal molecular bonds are strong, the adhesive holds together even under challenging conditions like fluctuating temperatures, mechanical impact, or vibrations. Conversely, weak cohesion can cause the adhesive to tear or fracture when force is applied, even if the bond to the substrate remains intact.

Key factors influencing cohesion include:

Polymer Composition: The chemical structure of the adhesive determines its cohesive strength. Long-chain polymers, for instance, provide flexibility and toughness, while shorter chains may lead to brittle bonds.
Temperature Sensitivity: Adhesives are formulated with service temperature ranges in mind. If an adhesive is exposed to temperatures outside this range, it can become brittle (in extreme cold) or too soft and weak (in excessive heat), leading to cohesive failure.
Aging and Degradation: Over time, exposure to oxygen, UV light, or repeated heating and cooling can degrade the adhesive’s internal bonds, reducing its cohesive strength. This degradation often manifests as cracks, splitting, or powdery residue.

Signs of Cohesion Failure

The adhesive layer remains adhered to both substrates but shows a visible split or crack in the middle.
There is no fiber tear or substrate damage—just a clean break within the adhesive layer.
The adhesive may appear brittle, hardened, or discolored due to thermal stress or chemical breakdown.

Thin Film Cohesive Failure Occurrences

Cohesive failure occurs when the adhesive itself splits, often leaving material on both substrates without pulling fibers.

Causes:

Inconsistent compression during bonding
Weak internal adhesive strength
Environmental temperature outside the adhesive’s service range
Thermal degradation due to poor heat stability

Solutions:

Check for uniform compression across all bonded surfaces
Ensure the adhesive selected meets your application’s heat and durability requirements

If adhesive failures persist despite adjustments, an adhesive specialist can evaluate your environment and materials to recommend a more appropriate solution.

Preventing Cohesive Failure

Select the Right Adhesive: Use adhesives with formulations designed for your specific environment (e.g., high-temperature stability or low-temperature flexibility).
Monitor Temperature Ranges: Ensure that the adhesive operates within its recommended service temperature to maintain optimal cohesion.
Apply Consistent Compression: Inconsistent pressure during the bonding process can weaken the adhesive’s internal structure, leading to cracks or incomplete bonding.

Why Troubleshooting Matters

Identifying the root cause of adhesive failures can drastically reduce downtime, improve product integrity, and boost overall efficiency. By diagnosing adhesion and cohesion failures, manufacturers can implement targeted solutions to restore optimal bond performance.

Equipment Challenges

Reduced Adhesive Flow

Reduced adhesive flow can severely impact production by causing uneven adhesive application or even complete line stoppages. This issue often stems from blocked filters or nozzles, which prevent the adhesive from passing through the system smoothly. Low adhesive temperatures or faulty temperature controls can also thicken the adhesive, making it difficult to dispense properly.

To resolve this, it is crucial to clean or replace clogged filters and nozzles regularly to ensure consistent flow. Additionally, calibrating or repairing faulty temperature controllers helps maintain optimal adhesive viscosity. Regular preventive maintenance can significantly reduce these disruptions and keep the adhesive system running smoothly.

Adhesive Charring

Adhesive charring occurs when the adhesive overheats and breaks down, forming burnt residue that can lead to clogs, poor flow, and contamination. This buildup increases downtime and can cause significant damage to dispensing equipment.

Common causes of adhesive charring include excessively high tank temperatures and low adhesive levels that expose the adhesive to oxygen, promoting oxidation. To prevent charring, it’s essential to monitor and maintain proper tank temperatures and keep adhesive levels consistent. Adding a nitrogen blanket to the tank can further minimize oxidation by reducing the adhesive’s exposure to air. These steps help extend the life of your adhesive and prevent costly equipment failures.

Specific Troubleshooting Scenarios

Adhesive Stringing

Adhesive stringing occurs when thin strands of adhesive stretch and break as the applicator moves away from the substrate, leading to messes, weak bonds, and inefficient adhesive use. This issue often arises when the applicator is positioned too far from the substrate, causing the adhesive to stretch before it can properly adhere. Stringing can also result from adhesive that is too viscous or applied at a temperature below its optimal range.

To prevent stringing, adjust the nozzle position so it is closer to the substrate during application. If the adhesive is too thick or cold, consider switching to a lower-viscosity adhesive or increasing the application temperature to improve flow and bonding.

Disrupted Bonds

Disrupted bonds occur when the adhesive fails to form a complete or durable bond, often leading to premature separation under stress. This failure can be caused by selecting an adhesive that is unsuitable for the specific application or environmental conditions, such as extreme temperatures or humidity. Additionally, improper compression during bonding or exposure to environmental extremes can weaken the adhesive’s hold.

To address disrupted bonds, optimize the compression timing and ensure even pressure is applied across the bond line during assembly. Selecting adhesives with better hot tack or resistance to environmental stress can also improve bond performance. Additionally, substrates should be free of oils, moisture, or contaminants that may interfere with adhesion, as surface preparation plays a crucial role in achieving strong, lasting bonds.

Preventing Adhesive Failures

Storage and Maintenance Tips

Keep adhesive tanks covered to prevent oxidation
Use nitrogen blankets to minimize air exposure
Maintain adhesive at recommended storage conditions to avoid premature degradation

Application Best Practices

Ensure proper nozzle alignment and distance
Avoid mixing incompatible adhesives to prevent gelling
Avoid over-pressurizing lines, as this can lead to air pockets and inconsistent flow

Environmental Considerations

Cold and Heat Resistance

Choose adhesives designed for extreme environments
Implement solutions to prevent cold cracking or thermal failure

Moisture Management

Identify and dry substrates prone to moisture absorption
Reduce humidity levels during storage and bonding processes

For example, drying equipment such as dehumidifiers can make a significant difference in maintaining optimal conditions.

Equipment Maintenance

Routine Checks

Monitor pump pressure, air supply, and temperature regularly
Clean nozzles, filters, and key components to avoid clogs

Replacing Worn Components

Recognize signs of wear such as leaks or irregular pressure
Implement a proactive replacement schedule to avoid failures

A small leak in one surface of a dispensing line could indicate more significant internal wear that, if ignored, could lead to costly shutdowns.

Professional Adhesive Solutions for Long-term Success

Proactive maintenance and proper adhesive selection can drastically improve manufacturing outcomes. You can avoid costly disruptions by distinguishing between cohesive and adhesive failures and following best practices.

When you need expert guidance, APPLIED Adhesives provides tailored solutions to optimize your processes and support your long-term success. Whether you’re facing bonding challenges or optimizing for environmental factors, our expert field technicians are here to help you achieve reliability and efficiency in your operations.
Whether you need preventative maintenance, custom dispensing solutions, or an urgent part replacement, we’re ready to tackle any challenge. Let APPLIED be the partner you can count on for all your adhesive needs—contact us today to experience the APPLIED difference firsthand.