Substrate failure adhesion

Adhesives need to produce high peel forces when the adherends are pulled apart — preferably high enough to induce substrate failure. To accomplish high peel, the... 

Adhesion depends on a number of factors. Good adhesion is defined by most customers as substrate failure. The major adhesive manufacturers possess equipment that allows them to make bonds with customer substrates under conditions that closely simulate actual packaging lines. These bonds are peeled either automatically or by hand to gauge adhesion. The most important factors influencing adhesion are the wet-out of the substrate, partieularly by the polymer component of the adhesive system, and the specific adhesion with the substrate. Choice of resin is critical for both. Rosin, rosin esters and terpene phenolics are eommonly added for these purposes in EVA and EnBA-based systems. Adhesion at low temperatures is also influenced by the overall toughness of the system at the test temperature. 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

These adhesives have been found to adhere strongly to metals, glass, wood, ceramics, masonry, asphalt, leather, and plastics like polystyrene, phenolics, polycarbonates, ABS, cellulose acetate, polyesters, rubbers, and some polyolefins. In general, the most favorable results are noted in the bonding of steel and aluminum, perhaps because the bond strengths are more easily observed before substrate failure. 

Another method for measuring either die- or substrate-attach adhesion strength involves attaching a contact tool to the top of the die or substrate with an adhesive that is stronger than that of the die-attachment adhesive. The tool applies a vertical force until the die detaches from the substrate or until a minimum specified strength is achieved. The test method and failure criteria are specified in Method 2027 of MIL-STD-883F. i... 

Figure 7 Durability of cyanoacrylate adhesives. Asterisk denotes substrate failure.

Figure 4 shows failure types of the adhesive bonded joint under a pull strength test. The cohesive failures in Fig. 4(a) and (b) occur when fracture is developed either within the adhesive and substrate, while the adhesive failure in Fig. 4(c) separates the substrate and adhesive at the interface. 

Thus, the adhesive contacts the substrate via a layer of substances that frequently differ from the adhesive in composition. If the cohesion strength of these substances is less than that of the adhesive, this will determine the failure stress of the adhesive-bonded joint. The adhesive, which has the same composition as that of the adhesive in bulk, can form a weak zone in the substrate surface. Adhesives are polymers and the particular nature of a polymer must have effects at all stages of formation and operation of an adhesive-bonded joint. 

FAILURE THROUGH ORDINARY CEMENT MORTAR (SUBSTRATE) FAILURE through LATEX-MODIFIED MORTAR FAILURE THROUGH THE INTERFACE (ADHESIVE FAILURE)... 

In most cases, failure cracks arise in the weak bormdary layers. Bikerman distinguished many groups of weak boundary layers according to their origin. Similar to the formation of the adhesion joint, three phases usually participate in failure adhesive, substrate and air (or other medium). Various admixtru-es form a weak boundary layer, being concentrated at the phase border. The defects in the structure of the substrate and adhesive and admixtures determine... 

Further credence was lent to this interpenetration theory when a similar effect was observed using a free radical-cured acrylic adhesive system [61]. In this system, the primer consisted of a solution of a copper (II) salt and the adhesive was based on methyl methacrylate monomer with a small amount of trifunctional monomer and a hydroperoxide/amine-based cure system. It was found that bonds formed on low-density PE result in substrate failure and that an interphase of mixed adhesive and PE is formed that is <1.5 mm-thick. 

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