Adhesives play a critical role in modern manufacturing and engineering, enabling the creation of durable joints without the need for mechanical fasteners. To ensure adhesives meet performance standards, rigorous adhesive testing methods are applied to evaluate their strength, durability, and reliability under various conditions. Accurate test results are critical for selecting the right adhesive and evaluating its suitability for specific applications.
However, adhesive testing is not without challenges—many tests are performed incorrectly, or their results are misinterpreted, leading to flawed decision-making. This article explores some of the most common adhesive testing methods and highlights areas where misunderstandings or incorrect interpretation often arise.
1. Peel Strength Test
The peel test is a fundamental mechanical testing method used to measure the adhesive properties of bonded joints. It evaluates how well an adhesive performs when subjected to a tensile force that separates a flexible substrate from another substrate, which may be either flexible (e.g., tape or film) or rigid (e.g., metal or plastic).
These results provide essential data on the adhesive’s ability to maintain a secure bond under tension and environmental conditions. Peel tests measure several key parameters, including:
- Initial Peak Force: The maximum force observed at the beginning of the peel.
- Average Force: The force required to peel the adhesive over the entire bond length.
- Peel Strength: The average force per unit width of the adhesive bond (usually recorded in units such as lbs/in or kN/m).
Why Peel Testing Matters:
Peel testing is critical for evaluating the seal strength of adhesive bonds. For instance, in consumer packaging, adhesives may be designed to break at low forces to allow easy opening. In contrast, automotive applications demand adhesives that can withstand extreme conditions and stresses. Understanding how adhesive bonds behave in their end-use environment is essential for manufacturers to ensure product reliability and functionality.
Factors Affecting Peel Strength:
- Peel Angle: The angle at which the peel force is applied is crucial to the test outcome. A peel angle of 90° or 180° is most commonly used. Variations in the peel angle can cause significant fluctuations in peel strength.
- Rate of Peel Loading: The viscoelastic nature of the adhesive causes different behaviors at varying rates of strain. For example, a faster rate of peel might result in different adhesive performance than a slower rate, especially for elastomeric adhesives.
- Adhesive Thickness: The thickness of the adhesive bondline can have a significant impact on peel strength. Thicker adhesive layers in elastomeric adhesives generally lead to higher peel strengths, while thicker layers of more rigid adhesives may reduce peel strength.
- Substrate Properties: A flexible adhesive on a rigid substrate may behave differently from a rigid adhesive on a flexible substrate. Rougher or smoother surfaces can also affect adhesion and test consistency.
Types of Peel Tests:
Several peel testing methods are used based on the type of adhesive bond and the desired result:
- T-Peel Test: used for two flexible substrates bonded together, often in packaging for consumer or medical products. In this test, the two bonded substrates are pulled apart at the ends using tensile grips, forming a “T” shape.
- 90° Peel Test: commonly used to assess the bond strength between a flexible substrate and a rigid substrate. The flexible substrate is pulled in a tensile direction, while the rigid substrate is held in place and moved laterally, maintaining a constant 90° angle.
- 180° Peel Test: In this test, a specimen is bonded to a rigid substrate (usually a plate) and then subjected to tensile forces applied at a 180° angle.
Peel Testing Systems and Equipment:
Peel tests are typically performed on universal testing machines, which consist of a test frame, load cell, software for data analysis, and various grips and fixtures designed for specific tests. For tests that require a constant peel angle, such as the 180° or T-peel tests, specialized test fixtures are used to maintain the angle throughout the test.
2. Tensile/Shear Strength Test
A shear test evaluates the strength of an adhesive bond under forces applied parallel to the bonded surfaces, while tensile tests assess the adhesive’s resistance to forces applied perpendicular to the bonded surfaces. Both tests are critical for understanding how adhesives perform under different stress types in structural joints and load-bearing applications.
The test involves applying a tensile load to two bonded substrates until the adhesive layer fails. The results indicate the adhesive’s ability to resist shear forces and provide valuable data on joint integrity and durability.
Why Tensile/Shear Testing Matters:
Industries such as aerospace, automotive, and construction rely on adhesives to create joints capable of enduring significant tensile and shear forces during operation. These tests ensure bonded assemblies meet safety and durability standards while playing a vital role in quality control by identifying inconsistencies or flaws across production batches. Understanding tensile and shear strength also aids in selecting the right adhesive for specific applications, ensuring reliability and long-term performance.
Factors Affecting Tensile/Shear Test Results :
- Substrate Material: The type, surface finish, and stiffness of the bonded materials impact stress distribution. Smooth, rigid, or uniform substrates may yield different results compared to rough or flexible ones.
- Adhesive Thickness: Thinner adhesive layers typically exhibit higher strength because they reduce flexibility and distribute loads more uniformly.
- Loading Rate: Faster loading rates can lead to different failure modes compared to slower rates, especially for viscoelastic adhesives
- Bond Area: Larger bond areas tend to spread the stress, potentially leading to higher measured shear and tensile strengths.
- Edge Fillets: Fillets formed by excess adhesive increase lap length and reduce stress discontinuities, enhancing strength in shear applications.
Types of Shear Strength Test:
- Single Shear Test: In this test, one substrate overlaps the other with adhesive applied between them to form a lap joint. The size of the overlap can be adjusted based on the test requirements. This method is commonly used for testing lap joints, sheet materials, and fasteners.
- Double Shear Test: This test involves placing the adhesive between two substrates, with both sides overlapping to create a double lap joint. The setup distributes stress more evenly and allows for higher load capacity compared to a single lap test. This setup minimizes bending loads and is ideal for testing bolts, rods, and materials susceptible to bending in a single shear setup.
In addition to the single lap and double lap tests, other shear testing methods include the double butt lap test, bevelled lap test, and joggle lap test. These methods, particularly suited for complex joint configurations, offer valuable insights into adhesive performance in specialized applications.
Shear/Tensile Strength Testing Systems and Equipment:
Peel testing systems typically use universal adhesive testing machines equipped with specialized fixtures, grips, and load cells to ensure accurate and consistent measurements. Adhesive testing software captures force-displacement data, providing precise calculations of tensile and shear strength. For support in selecting the right test method or equipment, feel free to contact us!
3. Tack and Green Strength Test
Tack and green strength tests evaluate an adhesive’s ability to create an immediate bond (tack) and its strength during the early stages of curing (green strength). Tack measures the adhesive’s initial stickiness, often important for applications requiring quick adhesion, while green strength determines the adhesive’s ability to hold materials together under load before it fully cures.
You can find detailed information about green strength at: What is Green Strength? High Green Strength Adhesive
Why Tack and Green Strength Testing Matters:
Testing tack and green strength is essential for ensuring adhesives perform as expected in applications where quick initial bonding or early load-bearing capacity is critical. Industries like packaging, construction, and automotive often rely on adhesives to hold components in place during assembly or curing. These tests help determine whether an adhesive can meet these demands, reducing assembly errors, improving production efficiency, and enhancing product reliability.
Factors Affecting Tack and Green Strength Test Results:
Green strength is influenced by more than just the adhesive’s formulation. It depends on several interrelated factors, such as:
- Adhesive Composition: Green strength is influenced mostly by the adhesive ’s formulation. Different formulations exhibit varying levels of tack and green strength depending on their chemical makeup.
- Surface Properties: The texture, porosity, and cleanliness of the bonded surface significantly affect the adhesive’s performance.
- Application Dynamics: Contact pressure, duration of contact, and separation rates all impact green strength results.
- Mechanical Stiffness: The rigidity or flexibility of the bonded materials also affects green strength outcomes.
- Curing Conditions: The curing time and conditions directly influence green strength, as faster-curing adhesives typically achieve higher early strength.
Types of Tack and Green Strength Tests:
- Rolling Ball Tack Test: A rolling ball is allowed to roll across an adhesive surface to measure how far it travels, indicating tackiness.
- Loop Tack Test: A loop of adhesive tape is pressed onto a test surface and peeled away to measure the force required. This test is commonly used in the packaging and labeling industries.
- Probe Tack Test: A probe is pressed onto the adhesive and then pulled away, measuring the force required to break contact. This test is suitable for adhesives used in electronics and medical devices.
- Shear Test for Green Strength: This test measures the adhesive’s ability to hold two substrates together under a shear load during the early stages of curing. This test is often used in structural bonding applications.
4. Impact Test
The impact test evaluates an adhesive’s ability to withstand sudden and dynamic forces, simulating real-world conditions where joints experience shocks or rapid stress. This test measures the adhesive’s toughness, durability, and resistance to fracture under high-speed loading conditions.
Why Impact Testing Matters:
Impact testing is crucial for applications where adhesives are exposed to unexpected loads or vibrations. Industries like automotive, aerospace, and consumer electronics rely on adhesives to maintain joint integrity during impacts, crashes, or drops. This test ensures that adhesive bonds can absorb and dissipate energy effectively, reducing the risk of failure and improving product safety and performance.
Factors Affecting Impact Test Results:
Several factors influence the performance of adhesives during impact testing:
- Adhesive Composition: The adhesive’s chemical formulation, including its viscoelastic properties, significantly affects impact resistance.
- Substrate Material: The stiffness, flexibility, and surface energy of the bonded materials play a crucial role in how the joint responds to impact.
- Bondline Thickness: Thicker bondlines may absorb more energy but can also introduce weaknesses depending on the adhesive type.
- Environmental Conditions: Temperature and humidity can alter the adhesive’s properties, impacting its performance under impact forces.
- Rate of Loading: The speed of the applied impact affects the adhesive’s ability to absorb energy and resist fracture.
Types of Impact Tests:
- Drop Weight Test: A weight is dropped from a specified height onto the bonded joint to evaluate the adhesive’s ability to absorb and dissipate energy.
- Pendulum Impact Test: A pendulum strikes the bonded joint at a controlled speed to measure energy absorption. It is suitable for assessing adhesives in crash-sensitive applications.
- Charpy Impact Test: Measures the energy required to break a bonded joint using a swinging hammer that strikes the specimen horizontally.
- Izod Impact Test: Similar to the Charpy test but measures the energy to fracture a vertical specimen clamped at one end.
Systems and Equipment:
Core components in the impact testing systems include impact testers or drop towers for applying controlled impacts, sensors and load cells for measuring force and energy absorption, and specimen clamping fixtures to ensure stability during tests. High-speed cameras are often employed to observe failure modes, while environmental chambers simulate real-world conditions like temperature and humidity.
5. Environmental Adhesive Testing
Perhaps more than any other adhesive testing method, environmental testing reveals the complex interaction of multiple test parameters. Short-term strength values from laboratory tests often fail to fully predict adhesive performance during continuous exposure to real-world environments.
Environmental testing evaluates how adhesives perform under varying conditions such as temperature, humidity, UV exposure, and chemical contact. For example, the combination of heat and moisture can degrade adhesives significantly faster than either factor alone, as one may accelerate the degrading effects of the other.
Conclusion
Adhesive testing can be a complex process due to the numerous factors that influence the results. Because of these challenges, instead of navigating the intricacies of adhesive testing on your own, you can rely on professional services to ensure your adhesive bonds meet performance standards. Prostech provides access to fully equipped laboratories, experienced engineers, and tailored testing solutions to meet specific application needs. Feel free to contact us for expert support and guidance!
