Adhesive and Cohesive Bond Strength

To obtain maximum adhesion, the adhesive bond strength between the adhesive and adherend should be greater than the cohesive bond strength of the adhesive, as indicated in Fig. 6.2. (Of course, the overall strength is also limited by the cohesive strength of the substrates.) 

Concave meniscus adhesive cohesive force good bond 

Convex meniscus cohesive adhesive force bad bond 

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Fillers. Fillers are not commonly added to CR adhesives. Calcium carbonate or clay can be primarily added to reduce cost in high-solids CR mastics. Maximum bond strength is obtained using fillers with low particle size (lower than 5 [jim) and intermediate oil absorption (30 g/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently flame retardant, aluminium trihydrate, zinc borate, antimony trioxide or... 

The second frontier is a nondestructive technique to monitor the development of an adhesive bond in place. Neither the theoretical approach of surface free energy nor the empirical approach of bond destruction are as desirable as being able to measure adhesive and cohesive strength as they are developing in the glueline. By constantly observing the science of other materials, we may obtain ideas and techniques that will let us move these and other frontiers in wood adhesion. 

Once it is solidified, the performance of the adhesive depends on its adhesive bond strength, as discussed, and on its cohesive bond strength. In many applications, the adhesive is selected so that the adhesive bond strength exceeds the cohesive bond strength. In that case, the overall strength of the adhesive joint will be the cohesive bond strength of the adhesive itself, or of the substrates, whichever is less. 

Cohesive bond strength depends on both the chemical nature and the physical state of a material. Temperature and the molecular weight of the adhesive are two important factors. Increasing the molecular weight of an adhesive increases its cohesive strength, but also increases its viscosity and decreases wettability. 

Estimate the cohesive bond strength of a PE-based adhesive with a surface tension of 31 dynes/cm and d = 5 x 10" cm. 

What are adhesive strength and adhesive bonds What are cohesive strength and cohesive bonds ... 

As wood adhesives, isocyanates have found applications as binders for composition board (100). The advantages of isocyanates are many high adhesive and cohesive strengths, flexibility in formulation, versatility of various cure temperatures and curing rates, excellent structural properties, ability to bond with material having moisture content, and lack of formaldehyde emission. The most important advantage is their ability to form waterborne adhesives. 

Product technologies that have high adhesion include elastomeric adhesives and sealants (see Silicone adhesion and Rubber-based adhesives). Their adhesive strength reflects both adhesion and cohesion properties (see Peel tests and Rheological theory). In each of these product technologies, the unique physico-chemical properties of silicones, in general, and in adhesion, in particular, originate from the atomic and chemical bond structures of the PDMS backbone. 

An adhesive is a material which is applied to form a thin interlayer between solid substrates, with sufficient adhesive and cohesive strength to form a strong and durable bond. When applied, adhesives are fluid, because their flow units are independent or only weakly connected. By means of cooling, solvent evaporation or chemical crosslinking, they set to form viscoelastic solids. In both liquid and solid forms, adhesives are rheologically interesting materials. To understand the rheology of adhesives, therefore, it is necessary to study colloidal and polymeric systems in both fluid and solid states. This chapter will deal fully with fluid properties fortunately, much of the theory is equally applicable to the solid state. 

In the first paper, Petke reviews the current literature for developments in structure-property relationships of adhesives. He illustrates how chemical and physical structure affect both the adhesive and cohesive properties of polymers. In his review, he shows how the balance of adhesive and cohesive properties determine the ultimate bond strength of an adhesive. Relationships are cited for several types of adhesives. 

Cohesive bond strength. Cohesive bond strength is the force within the adhesive itself when bonding two substrates. The cohesive bond strength depends mainly on the intermolecular forces of the adhesive, molecular mass, and temperature. 

The carboxylic polyacrylates are effective as laminating adhesives for both flexible and rigid surfaces. The particular carboxylic acrylate used for a given laminating application is determined by the characteristics of the surfaces to be adhered, whether adhesive or cohesive bond failure is desired, the bond strength required of the adhesive, and many other factors. These may include stiffness, friability, extensibility, clarity, cold flow, moisture resistance, solvent or chemical resistance, heat and light insensitivity, radiation resistance, compatibility with resins and other polymeric materials or pigments, and vulcanizability. The carboxylic polyacrylates may be varied to meet the adhesive requirements of a specific application by ... 

In van Ooij s model, brass develops a high bond strength to rubber because the sulfide not only has good adhesive and cohesive properties, but is also very porous so that the rubber molecules become entangled in this film during vulcanization. This phenomena is depicted in Fig. 20. 

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