Adhesive penetration

With typical values of = 6 x 10 N s/m and b = 0.01 s, penetration will be about ten diameters under a driving pressure of 1 N/mm- (ca. 10 atmospheres). With a higher initial viscosity of 6 x 10 N s/m — perhaps as a result of delay in applying the same adhesive — penetration would only be 3.2 diameters. 

During the production of wood-based panels, part of the adhesive penetrates into the wood surface. An overpenetration causes starved glue lines, whereas a low penetration limits the contact surface between wood and the adhesive low penetration often is the consequence of bad wetting behavior. 

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. 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

Adhesives. A description of the honeymoon system of wood gluing was first published in 1974 (1). It is, in principle, a system in which two different adhesive compositions are applied to the two surfaces to be mated. Since wood adhesives penetrate the wood surface (and have to do so in order to provide a good bond), it is important that both individual components of the system ultimately cure to form a solid durable polymer. This can be achieved either by designing the components such that each will fully cure by itself or by providing ingredients in the two components that react after mutual diffusion takes place ultimately, all layers of the glueline must reach the fully cured state. 

The surface energetics of wetting can be broken down into three different processes [12], adhesion, penetration, and spreading. These steps are schematically shown in Table 9.2. For a smooth sohd surface with complete liquid solid contact, the work of adhesion, penetration, and spreading are given by the following equations [13] ... 

In general, for liquids, which are normally smooth, the value of i is equal to 1. For solids, i is always greater than 1. The effect of surface roughness on adhesion, penetration and spreading for a rough cube is shown in Table 9.3. The total work for the sum of all of these steps is referred to as the work of dispersion. When the work of dispersion is negative, dispersion is spontaneous. 

Petroleum Plant Toxicity Water Threat Viscosity Adhesion Penetration Degradation... 

Mechanical adhesion, as the title implies, is achieved by the open or porous nature of the substrate(s), whereby the adhesive penetrates into the cracks, crevices or gaps in the surface(s) involved. Specific or chemical adhesion involves the bonding of nonabsorbent or non-porous surfaces such as found with glass, metals, including foil, and plastics. The bond achieved between these materials and the adhesive depends on molecular or electrical forces based on van der Waals forces. Polar and non-polar materials need to bond with like adhesives. 

Flow of the adhesive to fill the surface micro-roughness is important for all bonding, but adhesive penetration into the substrate is not a significant issue in most bonding applications. 

FIG. 2—Models to illustrate the difference between an interfacial bond and those involving adhesive penetration into the wood cell wall, including interdigitation, adlayer, and a fidly interpenetrating polymer network. 

Many classes of adhesives are used in wood bonding because of different production and end-use conditions. Most adhesives can give acceptable wood bonds if the use conditions are not too strenuous or at high moisture levels. The interaction of a hot-melt adhesive with wood should be quite different from that of a water-borne adhesive, not only because of viscosity differences but also because of the lack of cell wall swelling by hot-melt adhesive. In addition, some adhesives penetrate and change the mechanical properties of cell walls [10,18], but it not known if all adhesives that penetrate cell walls change their mechanical properties. 

Johnson, S. E. and Kamke, F. A., Quantitative Analysis of Gross Adhesive Penetration in Wood Using Fluorescence Microscopy, J. Adhesion, Vol. 40, 1992, pp. 47-61. 

Mechanical interlocking occurs when the adhesive penetrates pores and openings in the substrate, producing a "lock and key" effect. This requires the adhesive to "wet" the substrate. Its effect can be enhanced by roughening the surface, thus creating pores and increasing the surface area. 

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