Understanding the failure mode mechanisms of adhesive bonds – how to design robust, durable bonded joints that perform over products lifetime.
Adhesive failure is an interfacial bond failure between the adhesive (or coatings, printing inks, sealants) and the adherend. Cohesive failure occurs when a fracture allows a layer of adhesive to remain on both surfaces. Examination of the failure mode provides valuable insights about process variables that affect the strength and durability of the bonded joint. The bonding process involves both adhesive and cohesive forces and failures frequently involve more than one failure mode. Four failure modes are Adhesive failure (interfacial), Cohesive Failure (within adhesive), Substrate Failure (material failure), and Mixed-mode Failure (a combination of adhesive and cohesive failures). Delamination can occur during the manufacture process or at any point during the product's storage or service life. Robust adhesive bonds must possess both bond strength and bond durability. Too often, critical failure mechanisms are not properly identified, and manufacturers end up purchasing unnecessary and costly equipment while chasing a solution.
Adhesion between two materials involves surface science, joint designs, and adhesive (or ink/coating) chemistry. Adhesive joints may fail adhesively or cohesively. The failure mode refers to the mechanism by which an adhesive bond breaks or fails as a result of applied stress or force. When bonds fail, it is critical to identify the underlying causes.
Adhesive failures occur at the interface between the adhesive and the adherend (substrate). The adhesive separates cleanly from the substrate. Visually, there will be residual adhesive remaining at any location on only one surface. Adhesive failures almost always originate in the manufacturing process such as foreign contamination, migration of chemical additives into the adhesive bond line, viscosity and thickness attributes of the bonded interface, improper curing, and fluctuations in temperature can generate excessive stress on adhesive bonds due to different thermal expansion properties. The adhesive must properly wet the surface of the substrate or the bond strength will be less than maximum.
Cohesive failure results in fracture of the adhesive and is characterized by the visual presence of the adhesive material on the matching faces of both adherends (substrates). The bond between the adhesive and the substrates is stronger than the adhesive itself. Cohesive failures that occur in-the-field are typically caused by poor joint design.
Substrate Failure (or adherend failure) is the optimal bond strength in which the actual material has broken rather than the bond.
Mixed-mode failures exhibit characteristics of both adhesion failure and cohesion failure because the interface is partially degraded. Essentially, this failure mode is a transitional phase in which the strength of the adhesive bond is lower than the cohesion failure strength.
Adhesive bonds fail for a variety of reasons, including part design, molding, materials and storage, pretreatment, assembly, and operating field conditions. In today's world of global manufacturing, components are frequently produced in one country and shipped to another for assembly. This protocol alone introduces factors that make bonding more difficult.
Adhesive bond failures can occur for a variety of causes, including part design, molding, materials and storage, pretreatment, assembly, and service conditions. In today's global manufacturing environment, components are routinely made in one country and delivered to another for assembly. This protocol alone introduces factors that make bonding more difficult.
In-depth publications and references on adhesion science, surface preparation, and bonding methods.
In this recorded webinar, you will gain a better understanding of the science behind surface pretreatment of different plastics and learn how to determine the best solutions for your adhesion challenges.
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Polymer morphology and processing will ultimately affect the ease or difficulty of bonding operations. Semi-crystalline plastics are generally more difficult to bond than amorphous plastics. Acetals, nylons, polyolefins, PPS, polycarbonates, PTFE, and rubber/elastomers present the greatest challenges. Migratory external lubricant sprays, such as silicones, should never be used in molding facilities because they are carried in the air to all other machines and deposited on molded parts.
Surface preparation is critical to achieving high adhesive bond strength. Surfaces must be clean and contamination-free from dirt, grease and oils. Low molecular weight materials (LMWM) such as silicones, mold release products and anti-slip agents inhibit bonding. When cleaning plastic surfaces, excess solvent will create weak boundary layers leaving a haze build-up that will inhibit bonding. When using plasma surface pretreatment there are optimal wetting levels — overtreatment and undertreatment must be avoided.
Contact angles and Surface Energy. Understanding the contact angle and its physical properties of interaction between solids and liquids provides valuable information in determining optimal adhesion bonding surface wettability conditions. The higher the surface energy of the solid substrate relative to the surface tension of a liquid (water, printing inks, adhesives, coatings, etc.), the better will be its "wettability," and a smaller contact angle. Contact angles are affected by both changes in surface chemistry and changes in surface topography.
The advancing contact angle is most sensitive to the low energy (unmodified) components of the substrate surface, while the receding angle is more sensitive to the high energy, oxidized groups introduced by surface pretreatments. Thus, the receding angle is actually the measurement most characteristic of the modified component of the surface following pretreatments, as measured using dyne solutions. Therefore it is important to measure both the advancing and receding contact angles on all surface-modified materials.
The SABREEN Group is widely recognized internationally as The Plastics Process Manufacturing Experts. We develop and implement robust process solutions for all types of applications, including extreme environmental reliability. From mission critical zero-defect jet fighter canopy ejection systems to drones, electronics, and underwater structures. SABREEN, with over 40 years of hands-on experience and the author of more than 150 technical publications, offers custom engineered adhesion bonding process solutions. When we work at your site, we bring plasma pretreatment and surface analysis equipment to demonstrate adhesion solutions on your production line.
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