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Rubber-to-substrate bonding is a critical process across industries such as aerospace, transportation, industrial applications and more. Adhesives play a critical role in in achieving strong and reliable bonds when bonding rubber to substrates. When the adhesive fails, the entire bonding process can collapse, leading to costly complications in the final application.

Failures in rubber-to-substrate bonding can arise from various factors, including:

• Incompatible materials
• Improper adhesive storage
• Processing issues such as inadequate mixing, insufficient drying time, poor surface preparation and handling, and incorrect coating film thickness

H.M. Royal, a leading distributor of chemical products, has extensively analyzed the potential causes of bonding failures when bonding rubber to substrates. Their expertise sheds light on how proper adhesive selection and application can prevent these issues and ensure successful outcomes.

Common Causes of Bond Failures

Because bonded systems can fail for a number of reasons, it is essential to understand the many causes and possible types of bond failures. By thoroughly understanding these factors, you can select an adhesive system that balances strength and flexibility, reducing the risk of failures and ensuring the longevity of the bond. The failures we see most often are:

• Rubber failure – The ideal scenarios for a bond failure is a rubber tearing or rubber retaining bond failure, where the adhesive system adheres to both polymer and substrate.
• Cement-to-metal/substrate failure – This occurs when the bonding system fails to achieve adhesion at the substrate surface, usually due to improper surface preparation. (e.g., contamination, oxidation). This failure can be avoided with proper cleaning/degreasing, surface roughening and other surface treatments.
• Rubber-to-cover failure – This happens when the bonding system fails to achieve any adhesion with the polymer. This may result from using an adhesive that is unsuitable for the application, issues with raw materials in the rubber formula, or even inadequate curing conditions. Make sure to use adhesives that are compatible with both the polymer and the substrate and account for temperature, chemical exposure, and physical stressors.

Adhesive Selection and Compatibility

The strength of the rubber to substrate bond relies heavily on the type of adhesives.  Without selecting the appropriate adhesive system, bond failures can occur, compromising the assembly’s overall strength and durability.

Importance of Compatibility

Adhesive compatibility with both the polymer and substrate is crucial for a durable bond. Selecting an adhesive that aligns with your materials’ properties prevents performance issues and ensures long-term stability.

Prioritizing material selection and compatibility can prevent failures when bonding rubber to substrates. With so many adhesives on the market, it can be difficult to decide on the right adhesive for your application. Be sure to contact our material experts if you’re looking for a recommendation.

The Role of Solvent-Based Adhesives

Selecting the appropriate adhesive for the materials involved, as well as for the end-use environment, is vital.  For example, let’s consider two solvent-based adhesives:

1. THIXON 2001-EF: This adhesive is specifically formulated to bond natural rubber, chloroprene, butyl, SBR, EPDM, and nitrile to various substrates. It is designed to provide an adequate bond for these materials as well as resistance to typical environments in which these materials are used.
2. THIXON 405: In contrast, THIXON 405 is formulated for bonding castable, millable, and thermoplastic urethanes to metal substrates, including various grades of steel and aluminum. It offers excellent adhesion properties tailored for these specific materials.

Although both adhesives are solvent-based, they perform optimally in different applications with distinct substrates. By carefully considering the available adhesive options and their recommended applications, you can select the adhesive system that best meets your needs and ensures a durable bond.

Sustainability Considerations

In today’s world, sustainability is a growing concern for many companies. If you want to produce environmentally conscious products, selecting the right adhesive can make a significant impact. Certain adhesives can eliminate problematic raw materials, contain lower volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), and have varying levels of flammability.

Ensure that you follow all environmental, health and safety regulations when selecting a solvent. Our materials experts understand the importance of sustainability and can recommend compatible materials and application-specific adhesives to meet your needs.

Best Practices for Surface Preparation

Bonds can fail if the adhesive system does not effectively bond the polymer to all coated areas. Contaminants such as oil, dust, solvent, rust or mold release agents may create a barrier on the substrate surface, preventing proper adhesion.

When a bond fails on a substrate that is receptive to bonding, it’s usually due to poor surface preparation because contaminants interfere with the bonding process. Potential contaminants include:

• Oil from pressurized air lines
• Residue left behind by grit blasting
• Accumulation of dust in areas where dust is present

Below are some best practices for effective surface preparation.

Degrease Surfaces

Degreasing removes oils, loose dirt and other contaminants from the substrate surface. Use solvents that are appropriate for the lubricants involved. Notes for degreasing the substrate:

1. Use vapor, submersion, or a wipe with an appropriate solvent or degreasing chemical.
2. Perform an additional solvent wipe or rinse if the substrate is subsequently blasted, abraded, or otherwise treated in a way that might leave behind contaminants.
3. Allow degreased parts to dry completely before application.
4. Avoid using a dirty cloth or towel for drying, as this could lead to recontamination. Instead, use a clean cloth to dry the substrate carefully after the degreasing.

Perform Mechanical Abrasion

Mechanical abrasion should take place after degreasing to remove any surface oxidation or heavy contaminants not removed during the degreasing process. This step not only removes surface films, scale and oxides, but also increases the surface area available for bonding.

Effective methods of abrasion include:

• Wire brushing
• Roughening with sandpaper
• Steel grit or other media blasting

The chosen method largely depends on the type of substrate being used. Certain substrates, such as beryllium, can pose risks to workers during abrasion, so always check health and safety requirements before starting the process. For metals, a surface finish with a roughness mean square (RMS) between 150 microinches and 250 microinches is usually recommended.

After abrasion, remove all loose particles from the surface. Neglecting this step could lead to bond failure.

Apply Surface Treatments

Improving the physical and chemical properties of the surface can enhance adhesion, especially when bonding plastic and metal substrates. A variety of acids and alkalis can be used for surface treatment. The type of chemical will depend on the surface being treated.

Common surface treatments include:

• Phosphitization/Phosphate Treatment (Fe, Ca, and other elemental treatments)
• Corona/Plasma Treatment (particularly for difficult-to-wet or bond plastics)
• Acid Etch (Nitric, Chromic, Hydrofluoric, Hydrochloric, etc.)
• Alkaline Etch (Sodium Hydroxide, Potassium Hydroxide, etc.)

Workers should wear appropriate personal protective equipment and need to be well-trained in the use of these treatments. Treatment and rinse baths must be maintained at suitable temperatures and concentrations to ensure consistency across batches.

Application Methods

The appropriate application method for adhesion depends on several factors, including cost, size, complexity of the application area, and desired throughput speed.

Mixing

Proper adhesive mixing is critical to avoid settling of solids and ensure consistent bonding performance. Use a propeller-style mixer and follow the manufacturer’s technical data sheet

Adhesives that are mixed incorrectly may contain settled solids, which often include raw materials critical in the bonding process.  Even slight deviations from the proper mixing procedure can compromise bond integrity.

For the best mixing results, H.M. Royal recommends using a propeller-style mixer at 60 RPM or higher setting, depending on the specific technology. Lower mixing speeds are required for waterborne bonding systems. To ensure safety, operators should wear personal protective equipment and mix the adhesive thoroughly in a well-ventilated area, as specified in the technical data sheet.

Spraying

Although spraying can be more expensive, it is highly efficient in terms of throughput. In some cases, it may even be more cost-effective to spray large parts rather than brush them. To achieve proper atomization, a lower viscosity adhesive is often required for spraying and diluting the adhesive may yield the best results. However, care must be taken to avoid overspraying the parts, as this can lead to wasted adhesive.

Dipping

Dipping typically requires less expensive equipment than spraying. Like spraying, dipping can be automated to increase throughput speeds.  It is important to use an adhesive with the right viscosity to ensure it remains on the substrate after being removed from the dip tank.

Brushing

Though it is often a more time consuming method, brushing may be necessary to effectively coat the substrate of intricate parts.

Both brushing and dipping have nearly 100% transfer efficiency, meaning they minimize waste. This allows a gallon to go three times further compared to spraying, which has a transfer efficiency of only 25-30%, unless highly optimized.

Curing

After applying and drying the adhesive, you are ready to introduce the rubber and cure it. Curing processes are crucial for ensuring a strong, reliable and long-lasting bond. Deviating from specified processing instructions can significantly impact the quality of the bond.

Keep in mind that the adhesive should cure alongside the rubber. They must have overlapping cure windows to prevent curing too quickly or slowly. Since heat and pressure are necessary for curing, it is essential to ensure that coated inserts are subjected to this heat and pressure within these specified parameters.

If coated inserts are exposed to mold cure temperatures for an extended period before the rubber is introduced, bonding failures may occur due to a phenomenon known as prebaking.

Generally, a low prebake time is preferable for bonding, but each adhesive varies in how much prebake it can resist. DuPont’s Thixon™ 550, for example, is extremely resistant to prebake.

Storage and Handling of Adhesives

Proper storage conditions are essential for optimal bonding performance.  For example, DuPont adhesives should be sealed and stored in a dry environment when not in use. Generally, the recommended storage temperature is between 60°F and 80°F.

Improper storage of unopened adhesive containers can negatively impact bond integrity.  Extreme temperatures, whether too high or too low, can cause the adhesive to degrade before application. This degradation reduces the effectiveness of cross-linking agents, making it difficult for the adhesive to bond with rubber.  Additionally, it may render the adhesive unsuitable for application methods such as brushing, dipping or spraying, as the material may become too viscous to use.

Exposing coated inserts to dust, humidity, high/low temperature, and UV light also can have an impact on bond integrity. To maximize the shelf life and effectiveness of adhesives, it is crucial to follow the manufacturer’s storage instructions. Keeping adhesives at the recommended storage temperatures helps ensure they retain their optimal bonding properties.

Environmental Factors and Testing

Environmental factors such as temperature fluctuations and chemical exposure can weaken bonds over time. Conducting destructive testing on bonded components helps identify potential weaknesses and ensures they meet performance requirements.

Understand the Environment

Bonded rubber components experience many stressors and environmental conditions that can cause bond failures in the field. Your part may be exposed to elevated temperatures for an extended period of time. Or it may suffer damage from a caustic agent or oil.  Take environmental factors into consideration when choosing an adhesive best suited for your application.

Testing for Bond Integrity

The only definitive way to verify the integrity and reliability of a bond is through testing. With rubber to metal parts, the only way to evaluate bond strength is to destroy the part. Destructive testing techniques can offer valuable insights into the bond quality and help identify any potential weaknesses.

Adhering to Processing Directions

Viscosity

The optimal viscosity for your adhesive will depend on the application method. Generally, spraying will require a lower viscosity, while dipping and brushing will require a higher viscosity. The static viscosity of your adhesive can be roughly measured with a Zahn cup. At H.M. Royal, we can measure the dynamic viscosity of your adhesive using a Brookfield viscometer.

Dilution ratio

Adjusting the viscosity of the adhesive requires proper dilution. To dilute your adhesive, check the technical data sheet for the recommended dilution ratio and appropriate diluents.

Be cautious not to overdilute the material, as this can weaken the reactivity of the adhesive system. By strictly following the processing instructions, you can ensure that the bonding process occurs optimally, maximizing the strength and reliability of the rubber-to-substrate bond.

Dry film thickness

Determining a dry film thickness is important but requires special instrumentation. The dry film thickness is directly related to the degree of the dilution of the adhesive.

By carefully selecting adhesives, preparing surfaces, and following application best practices, you can significantly reduce the risk of bond failures when bonding rubber to substrates. If you’re facing challenges, contact the experts at H.M. Royal for personalized recommendations and technical support.

We can also recommend the right adhesive for your application, whether it involves rubber, polyurethane or other materials. Our lab evaluates the performance of each adhesive with your specific elastomer and provides data to support our recommendations. Contact us for a technical consultation today.

You can also email us at info@hmroyal.com. Find out more about the products we represent at www.hmroyal.com.