Plasticization of the adhesive

The toughening mechanism of elastomer modified epoxy systems is different from that of flexibilized epoxy systems and can be used in combination with them. Flexibilized epoxy systems reduce mechanical damage by a reduction in modulus or plasticization of the adhesive. This allows stress to be relieved through distortion of the adhesive, but it also generally results in a lowering of the adhesive s glass transition temperature with an accompanying reduction in heat and chemical resistance. 

Of primary importance in moist environments is the plasticization, or softening, of the adhesive, a process that depresses Jg and lowers the modulus and strength of the elastomer [89-91]. Plasticization of the adhesive may also allow disengagement from a microrough adherend surface to reduce physical bonding and thus reduce joint strength and durability [37]. On the other hand, it may allow stress relaxation or crack blunting and improve durability [92]. 

Brewis et al. studied the effects of moisture and temperature on the properties of epoxy-aluminum joints by measuring changes in the mechanical strength properties of the soaked adhesive [90]. The Tg of the wet adhesive and relative strengths of wet and dry joints were evaluated for up to 2500 hours. They concluded that the joint weakening effect of water was due to plasticization of the adhesive that, in turn, was dependent on the rate of water diffusion within the adhesive. 

In a similar study, Brewis et al observed the same type of reductions in the mechanical properties of the adhesive. They demonstrated that the reduction in strength of single lap joints on exposure to moisture was linearly related to the fractional water content of the joints. On saturation with water the joints were weakened by 44%, whereas films of the adhesive were weakened by as much as 74%. It was pointed out that the lack of correlation between the bulk and adhesive joint strength was due to the different stress distributions in the two situations. They concluded that the principal mechanism of loss of adhesive joint strength was by diffusion-controlled water plasticization of the adhesive. There is an apparent discrepency between this work, in which water has a reversible effect on joint strength, and that of Butt and Cotter, in which the effect is irreversible. This will be discussed further in Section III.B. 

Brewis et al have studied the effect of exposure to warm moist air on aluminum bonded with a DGEBA/di(l-aminopropyl-3-ethoxy)ether adhes-ive. They observed a linear relationship between the loss in joint strength and the fractional water content of the joints, calculated under the assumption that the water entered the joint by diffusion through the adhesive. The excellent correlation observed strongly supports the view that the loss of strength is due to plasticization of the adhesive caused by water uptake. 

By working trough the method of the AE diagnostics, and as it was with carbon plastic case, the adhesive joint were tested by the step- and two-multiple loading. 

The test with the step loading shows that acoustic activity of the solid adhesive joints in the tested carbon plastic is quite low. The maximum on the endurance area was fixed at the predestructive moment. The last is evidence to the fact that the prevailing defect of the adhesive joints is starting its development at the loading level, which is close to the destruction point. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Adhesive Transfer Processes. Many polymers, whether dehberately or accidentally, are adhesives, so that much of the adhesive industry can be regarded as a part of the mbber and plastics industry. However, there are several important material-transfer appHcations involving polymer products that are so critically dependent on controlled adhesion that they merit specific mention in that category. They include hot stamping foils, release coatings for pressure-sensitive adhesive products, photocopier materials, transfer coatings, and transfer printing of textiles. 

Fig. 16. The results of Dyckerhoff and Sell for inlerfacial strengths measured hy butt-tensile tests for various lacquers (adhesives) against various plastics as a function of the surface energy, ys of the plastics. Arrows indicate the surface tensions of the adhesive, y, used in the generation of each curve, showing rough agreement with the requirement that a maximum in adhesion is achieved when yt ys (I kp/cm- 0.1 MPa). Redrawn from ref. [71. ...

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

For some applications, such as for repulpable type PSAs, it may be advantageous to incorporate high levels of acrylic acid because this makes the polymer more hydrophilic. At the same time, high levels of acid also improve the water-dispersibility of the adhesive, especially at higher pH where the acid groups are converted to the more water-soluble neutralized salt form. Since the high level of acid increases the of the resulting polymer, a non-tacky material results. To make the adhesive pressure sensitive, the polymer can be softened with water-dispersible or soluble plasticizers, such as polyethers [68]. 

In some cases, plasticization of a PSA may be detrimental to its performance. A well-known example is the deterioration of the performance of an adhesive applied to plasticized PVC. Migration of the plasticizer from the flexible vinyl into the PSA often softens the adhesive to the point where it fails cohesively from the vinyl, leaving sticky residue behind during removal of the adhesive-coated article from the substrate. One way to address this detrimental effect of plasticizer migration is to formulate an already plasticized PSA, perhaps because a better balance exists between the plasticizer in the PVC substrate and the PSA in contact with it [101]. 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

In general, fully compatible resin are desirable. However, there are many applications where borderline compatibility is tolerated, and even in some cases, borderline compatibility or controlled incompatibility may enhance tack in adhesive systems. On the other hand, a resin with a borderline compatibility in combination with an oil or plasticizer in an adhesive formulation, will result in phase separation and therefore the migration of the oil or plasticizer to the adhesive surface is favoured. 

BRs were found to have a rate-sensitive mechanical response with very low tensile and shear strengths [63]. The stress-strain curves of the adhesives were characterized by an initial elastic response followed by a region of large plastic flow. 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

A method of evaluating the adhesive bond to a plastic coating substrate is a tape test. Pressure-sensitive adhesive tape is applied to an area of the adhesive coating, which is... 

The model proposed by Bowden and Tabor has been regarded as the most successful one for presenting a simple and logical theory capable of explaining the Amontons friction law. However, suspicions concerning the two fundamental assumptions in the model were gradually aroused over past years. Friction has been attributed, in Bowden and Tabor s model, to the adhesion between asperities in contact and torn-off of the adhesive junctions when the shear stress exceeds a critical value. This implies that plastic flow and surface destruction may occur at the moment of slip, and that friction is dominated by the shear strength of the adhesive conjunctions, which is material dependent. 

Solvent wiping. Rubbers tend to swell by application of solvents and the mechanical interlocking of the adhesive is favored. Although chlorinated hydrocarbon solvents are the most effective, they are toxic and cannot be used toluene and ketones are currently the most common solvents. The treatment with solvents is effective in the removal of processing oils and plasticizers in vulcanized mbbers, but zinc stearate is not completely removed and antiozonant wax gradually migrates to the mbber/polyurethane adhesive interface. Table 27.1 shows the moderate increase in adhesion produced in SBR by MEK wiping. 

Hot-melt adhesives the molten adhesive wets the surfaces of the plastics to be assembled and interlocks them while solidifying again on cooling. It is necessary for the materials to be assembled to tolerate the temperature of the molten adhesive. Joining is sensitive to the temperature, which involves the melting of the adhesive joint. However, some hot-melt adhesives crosslink after joining and become less sensitive to heat. 

Finally, the solubility parameter of the adhesive and the substrate must be close. Without getting too teehnieal, the solubility parameter is a rough estimate of polarity. The old saying like dissolves like can be extended to like bonds like. More aeeurately, the solubility parameter is the ealeulated potential energy of 1 em of material for eommon solvents. Polymers are assigned solubility parameters of solvents in which they are soluble. Table 19.3 lists solubility parameters for various solvents and polymers. As an example of how to use this table, butadiene-acrylonitrile rubber with 6= 9.5 bonds natural rubber (6= V.9-8.3) to phenolic plastics (6= 11.5). Note that its solubility parameter is between that of the two substrates. 

Energy is needed to break such a contact, more so with some joined items than others. If we consider the adhesion of plastic on glass, the highest adhesion will be obtained if the adhesive fills all the valleys and crevices of each adhered body surface. This will remove any air pockets that do not contribute to adhesion. The role of the adhesive or glue is to provide mechanical interlocking of the adhesive molecules. 

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