How to improve chemical resistance of adhesives and sealants?

Chemical resistance refers to the ability of adhesives and sealants to withstand exposure to various chemicals without significantly degrading their properties. This includes maintaining their bond strength, flexibility, and overall integrity when in contact with substances (like acids, alkalis, solvents, oils, and fuels).

The influence of chemical resistance on adhesive and sealant performance is crucial for ensuring product longevity and reliability. Insufficient resistance to chemicals may hamper the following: 
 

• Bond integrity: Chemical exposure can degrade the adhesive's polymer structure. This leads to weakened bonds, loss of adhesion, and eventual failure.
• Material degradation: Chemicals can cause swelling, softening, cracking, or dissolution of the adhesive. This compromises its structural integrity.
• Longevity: Poor chemical resistance shortens the lifespan of bonded components. Thus, requiring premature replacements and increasing maintenance costs.
  
   

Impact of chemicals on adhesives and sealants
 

Here is a list of some chemicals that can damage your adhesives and sealants. 
 

Thus, the property of chemical resistance is crucial for ensuring long-term performance, particularly in applications involving chemical exposure.

 

Ways to protect adhesives and sealants from chemical exposure

There are many ways of protecting adhesives and sealants from exposure to chemical environments including:
 

• Polymer type and chemical structure.
• Degree of crosslinking in the polymer network.
• Presence of fillers and additives.
• Environmental conditions like temperature, humidity, and duration of exposure.
• Higher concentration of a chemical.
• Minimizing the area of exposure.

When selecting chemistry for chemical resistance, your choice should be guided by the specific chemicals your application will encounter.

 

Adhesive & sealant chemistries
 

Here are the common adhesive and sealant chemistries and their resistance profiles:

 

Acid-resistant adhesives

They are formulated to withstand exposure to strong acids. Epoxy-based adhesives and some specialized polyurethane adhesives are often used.

As formulations vary, specific resistance depends on the type of the epoxy. Compared to other structural adhesives, those based on epoxy chemistry have outstanding resistance to solvents, weak acids, and alkalis. This accounts for their use as adhesives, sealants, and coatings in many industrial applications. As a result, epoxy compounds are used to assemble water softener tanks, chemical containers, brewery tanks, and other industrial components in the chemical industry.

Epoxy adhesives are more resistant to a variety of liquid environments than other structural adhesives. However, the resistance to a specific environment is greatly dependent on the type of epoxy curing agent used and the formulation employed. Generally, the factors that tend to promote thermal stability also improve chemical resistance. Thus, the adhesives having the greatest thermal stability will also provide the greatest chemical resistance.

 

Solvent-resistant adhesives
 

They maintain their bond strength even when exposed to various solvents. Examples include certain epoxy, polyurethane, and fluoropolymer adhesives.

 

Oil-resistant adhesives
 

They are designed to resist degradation from exposure to oils, fuels, and other hydrocarbons. Nitrile rubber-based and some epoxy adhesives are commonly used. Polyurethane adhesives offer good resistance to oils and fuels. However, they may be more vulnerable to strong solvents and acids. Selection depends on the specific chemical exposure.

 

Acrylic adhesives
 

Chemical resistance varies significantly. Some acrylics, particularly methyl methacrylate (MMA) types, offer good resistance to various chemicals, including certain acids and oils. However, they may be more susceptible to strong solvents.

A chemical resistance chart is a valuable resource when choosing the right adhesive for a particular application. MMA adhesives are known for their compatibility with a wide range of plastics, including chemical-resistant plastics. This compatibility ensures that the adhesive will maintain its adhesion strength even in the presence of chemicals. For example, the chemical resistance of Plexus® MA8105 (a two-part methacrylate adhesive) in different chemicals is shown in Figure 1.
 

Figure 1: Strength retention of MMA adhesives (Plexus® MA8105 is a two-part methacrylate adhesive) after being exposed to different chemicals for 500 hours and 1000 hours³

In applications where adhesives are subjected to wear and tear, MMA adhesives still hold their ground. Their exceptional long-term adhesion characteristics mean that they maintain their bond strength even under dynamic mechanical stress and environmental factors. This durability ensures that the bonded components remain intact, or structural for an extended period.

 

Silicone and fluoro silicone sealants

Silicone sealants are known for excellent resistance to a wide range of chemicals, including many acids, bases, and solvents. They are generally more versatile in harsh chemical environments. They provide broader chemical resistance, especially to temperature extremes and a wider range of chemicals.
 

Pros vs. cons of chemically resistant adhesives and sealants
 

Parameters to consider while selecting the right chemically resistant adhesive

When selecting a chemically resistant adhesive or sealant, it is crucial to consider the specific chemicals and environmental conditions that the material will encounter. Some of the parameters to consider during the selection are given below:
 

1. Identification of chemicals: The specific chemicals must be determined to which the adhesive will be exposed.
2. Concentrations and temperatures: Higher concentrations and temperatures accelerate chemical reactions, affecting adhesive performance.
3. Reviewing compatibility charts: Manufacturer charts should be checked. This helps to assess adhesive resistance to the identified chemicals.
4. Evaluation of application: Factors like bond strength, flexibility, and application methods must be considered.
5. Testing: Small-scale tests must be conducted under simulated conditions to validate adhesive performance.

Once the right adhesive has been selected using the above parameters, the surface for bonding must be prepared to ensure maximum adhesion. The surface preparation of the aluminium alloy involves using the standard procedure.
 

• Cleaning: Dirt, grease, oil, and other contaminants are removed from the surfaces to be bonded.
• Abrasive blasting or sanding: These methods can create a rougher surface, improving adhesion.
• Chemical etching: Certain chemicals can etch the surface, further enhancing adhesion.
• Priming: Applying a primer can improve the bond between the adhesive and the substrate.
• Dryness: Surfaces must be ensured to be completely dry before applying the adhesive. This is because the moisture can interfere with the bonding process.

Let's have a look at some of the suppliers offering chemically resistant adhesives:
 

• 3M: They produce specialized adhesives offering excellent resistance to oil, fuel, and aliphatic hydrocarbons. Examples include 3M™ Chemical-Resistant Adhesive Transfer Tape 96100CR series and 3M™ universal adhesive 847.

• Henkel: Their LOCTITE® brand produces many chemical-resistant products, like thread lockers, and retaining compounds. These solutions are ideal for industries like automotive, aerospace, and electronics.

• Sika: They produce polyurethane and silicone sealants with varying degrees of chemical resistance. Their products are used in demanding applications like construction, infrastructure, and industrial settings.

Essential safety measures
 

There are several safety precautions to be kept in mind during the selection and use of chemically resistant adhesives. These are given below:
 

• Ventilation: Many chemical adhesives release fumes, so adequate ventilation is crucial.
• Personal Protective Equipment (PPE): Gloves, eye protection, and respirators to be used as needed. This helps to prevent skin and respiratory irritation.
• Handling: The manufacturer's instructions must be followed for mixing and applying the adhesive.
• Storage: Adhesives must be stored in designated areas, away from heat and incompatible materials.
• Safety Data Sheet (SDS): The SDS should always be read and understood for the specific adhesive being used. This document contains critical information about hazards, handling, and first aid.

Chemical resistance in adhesives and sealants is measured by assessing a variety of techniques. This includes analyzing how materials perform under specific chemical exposures. Here is a comprehensive overview:
 

• Physical analysis: Changes in tensile strength, elongation, adhesion strength, hardness, and weight after exposure are measured. Visual inspections are done for cracking, blistering, swelling, or discoloration.
• Chemical analysis: Chemical changes in the materials are analyzed. Techniques like Fourier Transform Infrared Spectroscopy (FTIR) or Gas Chromatography-Mass Spectrometry (GC-MS) are used.

Methods for testing chemical resistance

Some of the best methods for testing sealant chemical resistance are as follows:
 

• Immersion testing: Exposing sealant samples to the chemical environment for a specified time and temperature.
• Spray testing: Simulating exposure to chemical sprays or mists.
• Environmental chambers: Using controlled environments to simulate real-world exposure conditions.

Commonly used chemicals in resistance testing are listed below:
 

• Acids: Sulfuric acid, hydrochloric acid
• Bases: Sodium hydroxide, potassium hydroxide
• Solvents: Toluene, xylene, acetone
• Oils and fuels: Motor oil, gasoline, diesel
• Salt solutions: Sodium chloride

ASTM standards for chemical resistance
 

The consequences of exposure to chemical environments are so severe that it is usually necessary to test preproduction joints. This should be done both in the laboratory and the field, under conditions as close to the actual service environment as possible. The parameters that will likely affect the durability of a given joint are:
 

• maximum stress level
• average constant stress level
• nature and type of environment
• cyclic effects of stress and environment (rate and period)
• time of exposure
   

Most standard tests to determine the chemical resistance of adhesive joints last only 30 days or so. Exposure tests lasting less than 30 days are not applicable to many real-life service requirements. Practically all adhesives are resistant to these fluids over short periods and at room temperatures. Some epoxy adhesives even show an increase in strength during aging in fuel or oil.¹

ASTM provides numerous standards for chemical resistance testing.
 

1. ASTM D896: Standard test method for resistance of adhesive bonds to chemical reagents, specifies the testing of adhesive joints for resistance to solvents and chemicals.
2. ASTM D471: Standard test method for rubber property - Effect of liquids.
3. ASTM D543: Standard test method for resistance of plastics to chemical reagents.
4. ASTM D1141: Standard practice for the preparation of substitute ocean water.
5. ASTM C661: Standard test method for indentation hardness of elastomeric - Type sealants by means of a durometer.
6. ASTM D391: Standard test method for chemical resistance of reinforced thermosetting resins used in equipment.
7. Military Specification (MMM-A-132): Standard test fluids and immersion conditions used by many adhesive suppliers are specified here.

Role of accelerated aging in chemical resistance testing

Accelerated aging plays a significant role in chemical resistance testing. These are explained below:
 

• Predicting long-term performance: Accelerated aging simulates long-term exposure in a shorter timeframe.
• Evaluating material durability: Helps to predict how materials will perform over their service life.
• Temperature: Increasing the temperature of the test, increases the rate of the chemical reaction. Therefore, accelerating the aging process.

Figure 2 shows the long-term effect of a heat-cured epoxy joint in various chemical environments. As can be seen, the temperature of the immersion medium is a significant factor in the aging properties of the adhesive. As the temperature increases, more fluid is absorbed by the adhesive and the degradation rate increases.
 

Figure 2: Effect of immersion in various chemical environments on a one-part heat curing epoxy adhesive (EA929, CytecHysol Division) in (a) Gasoline at 75°F, (b) Gear oil at 2SQoF, (c) Distilled water at 75°F, (d) Tap water at 212°F, and (e) Shell zone at 2SQoF¹

The effect of simultaneous exposure to both mechanical stress and a chemical environment is often more severe than the sum of each factor taken separately. Mechanical stress, elevated temperatures, and exposure to chemicals or solvents can be a fatal combination for certain adhesives and sealants if all occur at the same time. The stress can act to accelerate the degradation caused by the environment, and vice versa.

Joints that will be exposed to both chemical environments and load at the same time are especially vulnerable. The prototype specimens need to be appropriately tested.

Figure 3 shows the failed surfaces produced by the fatigued boron epoxy sample immersed in the reference fuel for 283 days before testing. The fracture has propagated from the corner and close to the FM73 and metal interface. This locus of failure was also observed for similar specimens that were unconditioned. However, the unconditioned samples also failed at a lower load, around 26kN.²
 

Figure 3: Failure surfaces of the fatigued boron-epoxy SDS immersed in the reference fuel for 283 days before testing²

Chemically resistant adhesives are crucial in industries facing harsh chemical exposures. This includes chemical processing, oil and gas, and water treatment. Here, adhesives must withstand acids, alkalis, solvents, hydrocarbons, and treatment chemicals for tanks, pipes, and equipment.
 

• Automotive and aerospace industries rely on these adhesives for resistance to fuels and fluids, ensuring vehicle and aircraft longevity.
• Medical devices require adhesives compatible with sterilization and bodily fluids.
• The electronics and food and beverage sectors require adhesives resistant to manufacturing/cleaning chemicals and food products, respectively.
• For underwater applications, certain chemically resistant adhesives are formulated. For example, epoxy-based adhesives are designed to cure in wet or submerged environments. It is important to verify that the specific product is rated for underwater use.
• In paints, the adhesive and the paint type are crucial, as adhesives may not provide a suitable surface for paint adhesion. For example, silicone sealants are difficult to paint. It is recommended to test paint adhesion on a small, inconspicuous area before painting the entire surface.
• The development of eco-friendly chemically resistant adhesives is an ongoing area of research. Some manufacturers are exploring bio-based epoxies and polyurethanes derived from renewable resources. It is important to look for products having low VOC.

Because of the large variety of chemical resistance needs, it is very hard to give a top-rated or most affordable list. Because each application will have very different needs. It is very important to consult the manufacturer data sheets, and chemical resistance charts, to verify that any product will meet the needs of the application.
 

Figure 4: Applications of chemical-resistant adhesives and sealants