Internal strength, adhesives

Cohesive strength is the internal strength of an adhesive or the ability of the adhesive to resist splitting. Unlike tack and adhesion strength, cohesive strength is not influenced by the substrate. 

Scherf, J., Cohen, Y. and Wagner, H.D. (1992). Interfacial strength measurements in poly (p-phenylene benzobisthiazole)/epoxy composites. Intern. J. Adhesion Adhesive, 12, 251-256. 

COD. See chemical oxygen demand, cohesive failure. Occurs when internal strength of the adhesive is not as great as the forces applied to it. 

The internal strength of the cake. If the cohesion of the latter is less than the adhesion to the cloth, the cake will il internally and leave solids on the cloth. The moisture content will vary across the depth of the cake. 

Another important consideration is the mode of failure experienced during the 180 peel test. Pressure-sensitive adhesives with internal strength relatively low in comparison to their adhesion to a test substrate will fail cohesively while the same type of adhesive with improved internal strength will often fail adhesively. This adhesive failure can be important for temporary applications but is of less importance for high strength, permanent uses. Some systems change from the cohesive to the adhesive failure mode as the amount of cure is increased, a point which will be illustrated later. 

The first case includes the assumption that there is a distinguishable layer between the adherend and the on-laminated joint. However, there is no specific layer that can be said to be the adhesive layer thus the thickness of the adhesive layer is not accurately definable. This leads to the second assumption, that the joint strength is dependent on the internal strength of the laminate. In reality, the performance of laminated joints indicates that the first assumption is closer to the truth. 

Composition. An adhesive is composed of basic raw materials, which are called binders [1] and which determine its adhesiveness (adhesion) and its internal strength (cohesion), and of frequently necessary auxiharies, which establish particular end-use and processing characteristics. The adhesiveness of an adhesive, its internal strength after setting, and its processing characteristics are the fundamental properties that determine its suitability for use in forming adhesive joints. Adhesive joints are the joints formed between substrates and adherents using adhesives. 

The internal strength of the adhesive mass or its resistance to flow or creep under an applied load is referred to as the cohesive strength. PSTC defines cohesive strength as the ability of the adhesive to resist splitting " upon deformation, the extended fibrils of a PSA" with low cohesive strength will rupture (split), leading to a residue on the adherend surface. 

Minimum tensiley shear or peel strength at specified temperatures. For the most part, only one of these types of strength evaluation need be considered. There is no need for adhesives to develop strengths far in excess of the internal strength of the weaker of the materials being bonded. 

If the failure is cohesive, the breaking load gives the internal strength of the cured adhesive. 

Adhesives are high-molecular-weight substances which bond materials to one another without significantly changing their structure. The action of adhesives is based on two key properties they must firstly wet solid surfaces and adhere to them, and secondly, they must be cohesive, i.e. have internal strength. 

The cohesive polyester-polyol crystals of commercially available PU adhesives melt at about 50 °C - the lamination adhesive is thermally activated and becomes soft and capable of heat-sealing (Fig. 8-26). During coohng, the polyester-polyol segments re-crystaUize, resulting in a rapid increase in the internal strength of the adhesive film. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Rowell, R.M., Youngquist, J.A. and Sachs, LB. (1987a). Adhesive bonding of acetylated aspen flakes, part 1. Surface changes, hydrophobicity, adhesive penetration and strength. International Journal of Adhesion and Adhesives, 7(4), 183-188. 

Estimating the Adhesive Quality of Lignins for Internal Bond Strength... 

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