Porous substrate adhesive penetration

Fillers are added to emulsion adhesives to build the total soflds content, to reduce penetration into a porous substrate, and to lower costs. 

Solvents are used, however, for special applications. For example, solvents may be added to reduce viscosity and assist penetration on porous substrates. On certain polymeric substrates, solvents may be added to improve adhesion by assisting the diffusion of the adhesive molecules into the substrate. On nonporous substrates, volatile solvents must be evaporated before cure because the solvent could interfere with the degree of crosslinking, and under certain curing conditions, gaseous bubbles could form in the bond line and degrade joint strength. 

Control of Viscosity. To ensure that adhesives and sealants function well during their application and end use, the formulator must be able to control the flow properties of the product. The challenge that faces the formulator is that the adhesive or sealant may need different flow characteristics at different times. For example, adhesives must flow readily so that they can be evenly applied to a substrate and wet out the surface. Yet, there should not be an excess of penetration into porous substrates, nor should the adhesive run or bleed to create a starved joint. Certain adhesives and sealants must be capable of convenient flow application by trowel or extrusion, but they must also exhibit sag and slump resistance, once applied. Therefore, the flow properties, or rheology, of the material must fit the desired method of application. 

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. 

Sometimes primers can take the place of surface treatments. Two examples are with porous substrates and with certain plastic substrates. With weak porous substrates, such as wood, cement, or porous stone, the primer can be formulated to penetrate and bind weakly adhering material to provide a new, tightly anchored surface for the adhesive. Chlorinated polyolefin primers will increase the adhesion of coatings and adhesives to polypropylene and to thermoplastic olefins. The chlorine atoms in the outer surface of the primer increase surface energy and enhance adhesion of adhesives, sealants, and paints. 

As a family of curing agents for epoxy resins, the amidoamines are lower in viscosity than the polyamides. They exhibit very good adhesive properties due to their chemical structure and easy penetration. Amidoamine cured epoxy adhesives have shown very good properties on concrete and other porous substrates. They cure extremely well under humid conditions. In fact amidoamine cured epoxy formulations have been used to cure underwater in certain applications. A typical general-purpose room temperature curing epoxy-amidoamine system is described in Table 11.7. This adhesive is used as a general-purpose metal-to-metal adhesive and body solder in the automotive industry. 

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. 

However, good penetration into the wood is a very important aspect of wood bonding. Standards such as ASTM D 2559 require bond formation within the minimum and maximum of the recommended open and closed assembly times [9], Sufficient penetration into the wood is considered important for good bond formation, but overpenetration produces a starved bondline that is the weak link. Overpenetration does not occur with non-porous substrates thus, different factors need to be considered in formulating and using wood adhesives. A lower viscosity adhesive is normally better for the wetting and adhesion, but for wood the adhesive can be so thin as to overpenetrate into the wood. 

Although a major advantage of epoxy adhesives is their ability to be formulated without solvents, under certain circumstances solvents may be included. On porous substrates solvents may be added to reduce viscosity and assist penetration. On certain nonporous substrates, particularly some plastics, addition of a small percentage (1 to 3%) of a suitable solvent will improve adhesion. Common solvents are low-boiling aromatic solvents, ketones, or esters. 

Water-based epoxy primers are ideal for damp porous substrates, as such primers will penetrate to an adequate depth to ensure good adhesion and produce a sound surface for bonding. Emulsifiable resins and hardeners are available, and the better systems deactivate the emulsifier system during cure to ensure that the cured system is not unduly water sensitive (Section VI.A). Solventless epoxy primers are used for bonding new concrete to existing concrete. Polyamide hardeners are preferred because of their ability to cure satisfactorily in the presence of water. Accelerators and diluents may be added, but fillers are generally omitted. The primer is applied to the existing concrete, and the fresh concrete cast before the resin has set (Section VI.B). 

Adhesive penetration into wood can be categorized (i) on micrometer level as a result of the hydrodynamic flow and capillary action of the liquid resin from the outer surface into the porous and capillary structure of wood, mostly filling cell lumens, as well as fractures and surface debris caused by processing [5], and (ii) on sub-micrometer level as diffusion penetration into cell walls and micro- fissures. Hydrodynamic flow is initiated by the external compression force as a result of pressure applied to the wood surface to be bonded. The flow then continues into the interconnected network of lumens and pits, with flow moving primarily in the direction of lowest resistance [6]. The extent of utilization of an adhesive may be limited due to excessive penetration into the substrate, since this portion of the applied adhesive is lost within porous substrate structures for the adhesion effect. 

Adhesives frequently penetrate into a porous substrate and act as con-solidants (Figure 5.9) so can be very difficult to remove (Bonet et al., 2007). These materials require higher standards of stability than most adhesives. It is often wise to use separate processes for consolidation and adhesion. Similarly, it may be wise to separate the functions of adhesive and gap filler. 

UP resins have a long and continuing use as stone adhesive for the reasons stated early in its use (Hanpel, 1968) - the commercial products can be readily modified to suit the needs of stone workers, and can be further adjusted in the field. UP resins are similarly used as strong adhesives for coarse ceramics where the viscosity prevents penetration into the porous substrate (Buys and Oakley, 1996). 

Thickeners added to an adhesive will raise viscosity and permit dilution with water. This reduces the total solids of the adhesive and thus lowers its (wet) cost. Thickeners release water slowly, and when combined with lower solids, they also slow the setting (prolong open time) of the adhesive. Thickeners such as polyvinyl alcohol, starch, and hydroxy ethyl cellulose will improve adhesion to porous substrates by reducing penetration of an emulsion into the substrate and decreasing the likelihood of a starved joint. ... 

Fillers are added to emulsion adhesives to reduce cost by replacing resin solids without decreasing total solids, to reduce penetration into porous substrate, and to change the rheology of the compound. Depending on their individual properties, fillers can also add stiffness and strength or decrease tack and blocking. Un-... 

Clays and other fillers impart stiffness to adhesive films. Clays reduce the penetration of adhesives into porous substrates. Large-particle clays are better able to control penetration and also impart more rapid setting speed. A highly plastic clay, bentonite, makes adhesives thixotropic—fluid under high shear, thicker upon application. A study has been made of the effect of various fillers on the adhesive properties of polyvinyl acetate. 

The controlled gelation of PVOH solutions is important where penetration into a porous substrate is undesirable, e.g., paper coatings and adhesives. Boric acid and borax react strongly with PVOH and are widely used industrially as gelling agents. Poly (vinyl alcohol) is extremely sensitive to borax, which causes gelation by forming a bisdiol complex as shown below ... 

While solvent cements may provide greater ultimate strength than latex systems, the latter types find wide use and are often necessary for certain bonding requirements. Solvent systems penetrate into a porous substrate, with the result that multiple applications are often necessary to put enough adhesive on the surface to form a satisfactory bond. Since latex systems exhibite better hold-out on porous surfaces, one coat of such an adhesive is often sufficient for good bonding. 

B. A. Morris, Understanding why adhesion in extrusion coating decreases with diminishing coating thickness. Part I Penetration of porous substrates, Annual Technical Conference — Society of Plastics Engineers, 63, 2964-2968 (2005). 

Highly reactive (unstable) and nonselective species tend to react with any surface on which the species strike and form a polymer deposition with a high level of bonding or adhesion to the surface. Because of this aspect, plasma polymerization tends to form a thin film with a good adhesion with various kinds of substrate materials. Because of nonselective reactivity, the reactive species of plasma polymerization have poor penetration into small cavities such as those of porous structures. Reactive species tend to react with wall material at the entrance of a cavity rather than penetrating into the cavity (which requires that the species not react with the wall at the entrance). 

---