Interphase tension

Forces of interphase tension oti/t2 act at the contact surface between the two liquid phases of Ba(OH)2 (or Ca(OH)2) and -CV,Hi4. The thermodynamic condition for a liquid to spread over the surface of another liquid is [19]... 

The values of the surface tensions of the individual phases were calculated, as well as the value of the interphase tension. The results confirmed that the inorganic phase was dispersed in the organic phase. 

Similar dependences for describing the viscosity of emulsions on the basis of the volume averaging method were derived by Mellema and Willemse [60]. They took into account the contribution to the effective viscosity of the interphase tension and obtained a general expression, different from (52) ... 

The adsorption of impurities in the composition of an adhesive onto the substrate surface can be judged by the change of the system interphase tension. To study the interphase tension on the boimdary... 

With purified resin, alcohol decreases the interphase tension, with the exception of OP-10 (allyl phenol oxyethylated ester) for which it produces some increase (by ImN/m) [12]. These findings can be explained by the fact that the alcohol facilitates desorption of the low-molecular weight resin fractions with surface-active properties from the boundary between the resin and the mercmy by increasing their compatibility with the bulk resin. The free energy advantage for alcohol adsorption on the mercury surface is less than that for low-molecular weight fractions, which is why it results in increase of the interphase tension. 

Figure 1.1 Change over time of interphase tension at the interface of mercury and ED-20 epoxy resin with different additives (1) undistilled ED-20 without additives distilled ED-20 with (2) 1% ethanol, (3) 1% butanol, (4) 1% hexane, (5) 1% octane, (6) 1% decane (7) distilled ED-20 without additives.

Earlier we considered the regularities of the effect of simfactants on the oligomer surface tension. The properties of adhesive-bonded joints are also determined by the character and the magnitude of the interaction at the adhesive-substrate interface, which to a greater extent is characterized by the magnitude of the interphase tension [82]. Study of the interphase tension is of great theoretical and practical interest, but at present there are no direct methods for its determination. 

For the study of the interphase tension at the boundary between the oligomer and a metal, a model system was used in which liquid mercury was used for the metal. The mercury was thoroughly purified and distilled prior to use. The interphase tension was determined hy the sessile drop method [230]. The error was estimated as 3mN/m. The oligomers and adhesives based on them were ED-20 epoxy resin-hased adhesive with PEP A curing agent (12%) and PN 609-2IM polyester resin-based adhesive with MEKP(O). 

The equilibrium interphase tension for the studied oligomers is achieved in 10-100 min. The low rate of reaching equilibrium even for the noncuring oligomers is evidently explained by both the increased viscosity of the oligomer in the hoimdary layer and the diffusion processes on the boundary. 

Now let us consider the effect on the adhesion strength of surface-active substances at the houndaiy between the adhesive and the metal. Figure 2.22 presents the dependence of the adhesive-to-steel adhesion strength on the difference between the interphase tension of the solid adhesive-mercury stem and the adhesive sinface tension. Because it is impossible to measure the adhesion strength at the boimdaiy with mercury, it was determined at the bormdary between the adhesive and steel. The interphase tension will surely differ here from that at the bormdary with mercmy, although this is only a quantitative difference and the quahtative relationship will be maintained. 

As Fig. 2.22 shows, the adhesion strength increases as o-adj,Hg fTadhair decreases. RS substances were used to control the interphase tension. The thermodynamic work of adhesion is related to the surface tension at the bormdaries between adhesive and air, between adhesive and sohd, and between sohd and air by the relationship... 

Given that the metal surface tension was the same in all csises, the thermodynamic work of adhesion depends on the interphase tension and on the adhesive surface tension. In this case (see Fig. 2.22) a correlation is seen between the thermodynamic work of adhesion and the adhesion strength. This correlation was determined earlier by Lipatov and Myshko by applying the modified equation of Dupre-Young [102]. Epoxy, polyester, pol30irethane, and polyaciylate adhesives were used the type of the adhesive did not have a significant effect on the correlation dependence. 

As can be seen, the interphase tension of the polyester and the polyurethane is much higher than that of their mixture. One can assume that the formation of the loose transition region in the IPN type mixtures facilitates the adsorption of the polymer s polar groups on the substrate surface and their closer packing. 

The water resistance of these systems of ofigoiu-ethanes imder conditions of increased humidity has been studied in coimection with their possible use as adhesives. The low water resistance of prepol5rmers obtained through TDI reaction and of individual polyols, especially of oligourethanes based on ODA-800 and P-2200, may be explained by the high interphase tension, which facilitates the conditions of selective water sorption at the interphase boundary of adhesive and substrate [145]. 

It is apparent from Table 3.23 that for both orders of mixing the process of system formation is accompanied by reduction of the interphase tension, though when macro-triisocyanate (MTI) is formed this reduction is greater with diisocyanate in excess. 

For the interphase tension change at the boundary with air during prepolymers formation based on individual polyesters and mixtures, the surface tension displayed no change and was 28mN/m for the prepolymer based on L-3003 polyether, 40 mN/m for that based on P-2200 polyester, and 32 mN/m for that based on a 40 1 mixture of polyether and polyester (Table 3.23). For the interphase tension at the boundary with mercury, it was shown that formation of prepolymers based on individual oligoethers and oligoesters and on mixtures is accompanied by significant reduction of interphase tension, this... 

Time (min) Interphase tension (mN/m) ( 2%) of MTI formation process based on Surface tension (mN/m) ( 1%) of MTI formation process based on ... 

Resolution of the effect of the medium into separate components is of great interest, hut the problem presents substantial difficulties in that the various factors act simultaneously and the result is not simply the sum of separate components but the resultant of their interaction. In some cases it is advisable to restrict the approach to treatment of the predominant mechanism, which in our opinion is adsorption. In this case the principal means of increasing the water resistance of the joints is the creation of conditions that decrease or eliminate the selective sorption of water onto the substrate surface under the adhesive layer. This condition is decrease of the interphase tension of solid adhesive-substrate. 

Figure 5.4 shows the correlation between the water resistance of steel adhesive-bonded joints and the adhesive—mercury interphase tension. Given the different states of aggregation of the steel and the mercury, such a correlation can only be qualitative, but it nevertheless clearly indicates ways of controlling the water resistance of adhesive-bonded joints. 

The epoxy, acrylate, polyurethane, and polyester adhesives described earlier in Chapter 2 were used for investigations, and RS agents were used to control the interphase tension. Water resistance was evaluated by the retention of strength of the joints after hold-up in water for 30 days no noticeable hydrolysis was observed in this period. 

For interphase tension below 220-230 mN/m and use of hydrolytically stable adhesives, the joints do not fail along the adhesive—metal interface even after hold-up in water for periods of years. Presumably with this interphase tension the process of substitution of adhesive... 

Figure 5.4 Dependence of the water resistance B of adhesive-bonded joints on the adhesive-mercury interphase tension.

Let us consider a particular example of the effect of RS substances on the water resistance of adhesive-bonded joints. Adhesives based on imsaturated polyester resins, such as PN-1, are distinguished by low water resistance. The influence of water on a steel joint cemented by such an adhesive actually results in some initial increase of the specific electrical resistance along the adhesive-steel interface and then in an abrupt drop (Fig. 5.5). The increase is explained by more complete consumption of the monomer in the system. When ATG is added to the adhesive (which decreases the interphase tension) the specific electrical resistance stabilizes after a drop. The decrease seems to be related to the processes of relaxation of the internal stresses in the adhesive interlayer. The stresses facilitate the diffusion of liquids in polymeric materials, in particular the stress concentration at the polymer-metal interface. 

Thus, minimizing the adhesive-substrate interphase tension is a necessary condition for obtaining both strong and liquid-resistant adhesive-bonded joints. But it is not necessarily sufficient, because the strength and the water resistance can be determined by various factors, such as internal stresses, low hydrolytic stability, etc. 

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