Surface Tension epoxy

In a recent case study (see Svendsen et al, 2007 and also Problem 6.1), in collaboration with a paint company, the adhesion of six different epoxies-silicon systems has been studied. These paints are used in marine coating systems. Some epoxies showed adhesion problems in practice while others did not. The purpose of the study was to understand the origin of these problems and whether adhesion could be described/ correlated to surface characteristics, e.g. surface tensions. An extensive experimental study has been carried out including both surface analysis (contact angle measurements on the six epoxies, surface tension of silicon at various temperatures, atomic force microscopy (AFM) studies of the epoxies), as well as measurements of bulk properties (pull-off adhesion tests and modulus of elasticity). Theoretical analysis included both estimation of Zisman s critical surface tensions and surface characterization using the van Oss-Good theory. 

Estimate the LW, acid/base and the total surface tension of all three epoxies. Surface tension components for the van Oss-Good method for water and MEG are given in Table 3.4, while for benzaldehyde it can be assumed that only LW contribution exists and the surface tension is 38.5 mN m. Comment on the values obtained for the individual components of the surface tensions for the three epoxies. 

TPX shows an excellent peelability from a wide variety of materials. Therefore, TPX is used in applications in that separating properties are important. For this reason, it can be used as a release material in the process of curing thermosetting resins. In Table 4.4, the separating force expressed as surface tension of various materials against an epoxy resin are shown. 

The three EME coupling agents in Table 1 were analyzed using contact angle measurements to determine their polar and dispersion components of surface tension. From the surface tension data, wettability envelopes were constructed and compared with the surface tension properties of the epoxy coating [4], These data predicted that EME 47 would be wet by the epoxy, but not EME 23. This is believed to be the reason for the very low peel strength when EME 23 was employed [4],... 

TABLE 3.3 Surface Tension of Several Liquids Including Epoxy Adhesive Formulations (Top) and Critical Surface Tension of Various Substrate Materials (Bottom)... 

Most common adhesive liquids readily wet clean metal surfaces, ceramic surfaces, and many high-energy polymeric surfaces. However, epoxy adhesives do not wet low-energy surfaces such as polyethylene and fluorocarbons. The fact that good wetting requires the adhesive to have a lower surface tension than the substrate explains why organic adhesives, such as epoxies, have excellent adhesion to metals, but offer weak adhesion on many untreated polymeric substrates, such as polyethylene, polypropylene, and the fluorocarbons. 

The surface energetics that control wetting are largely related to the general chemical composition of the epoxy polymer molecule. However, the surface tension of an epoxy... 

FIGURE 3.4 Contact angle of an uncured epoxy adhesive on four substrates of varying, critical surface tension. Note that as the critical surface tension of the substrates decreases, the contact angle increases, indicating less wetting of the surface by the epoxy adhesive.4... 

TABLE 3.4 Surface Tension of Liquid Epoxies and an Epoxy-Amine Mixture6... 

DEAPA was used in several early commercial epoxy adhesive formulations. Schonhom and Sharpe9 have shown that this amine is surprisingly effective in reducing the surface tension of epoxy resins (Table 5.4). It is speculated that the utility of DEAPA adhesives is in part due to better wetting than other epoxy formulations. 

TABLE 5.4 Surface Tension of Several Epoxy Resin Formulations... 

Fluoroepoxies have gained interest because of the unique adhesion properties that can be provided by the fluorine groups. There have been several attempts to marry the properties of epoxy resins with those of fluorocarbon resins. In general these have focused on adhesive systems that (1) have a lower surface tension than unmodified epoxy or (2) have significant hydrophobicity to resist exposures in moist environments. 

Surfactants act as wetting agents by lowering the surface tension of the waterborne epoxy. Silanes can be used to increase adhesion to certain substrates and fillers, as shown in Table 14.4, formulation C. Water-compatible thickeners and protective colloids such as polyvinyl alcohol, substituted cellulosics and sugars, and some acrylics improve application properties and offset viscosity decrease seen with water dilution. 

To obtain a usable adhesive bond with polyolefins, the surface must be treated. A number of surface preparation methods, including flame, chemical, plasma, and primer treatments, are in use. Figure 16.4 illustrates the epoxy adhesive strength improvements that can be made by using various prebond surface treatments to change the critical surface tension of polyethylene. 

As observable from the SFM topography measurements, the surface of the epoxy inside the holes of the copper film exhibits a slight convex curvature, probably due to the surface tension of the epoxy formulation. Hence, the epoxy surface nearby the Cu/epoxy interface was not in contact with the mica substrate when curing the mixture of epoxy resin and curing agent, and any interference on the curing reaction from the presence of the mica can be ruled out. On the other hand, the local slope of the curved epoxy surface is... 

In a somewhat similar fashion, Ishii et alP- have demonstrated inkjet fabrication of polymeric microlenses for optical chip packaging. UV curable epoxy resin is deposited onto optical devices by inkjet printing. When the droplets hit the surface, they form into partial spheres due to their surface tension, and are UV-cured to form the microlens with diameters from 20 to 40 tm with /-numbers of 1.0 to 11.0. Their uniformity in a microlens array was measured to be within 1% in diameter and 3 tm in pitch (total count of 36 lenses). They have also demonstrated hybrid integration of inkjetted microlenses with a wire-bonded vertical-cavity-surface-emitting laser (VCSEL) with coupling efficiencies of 4 dB higher than without the microlens. 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

Several very interesting classes of materials may also result from proper fluorination of conventional monomeric or polymeric diepoxides. The conventional diepoxides or epoxies" are extremely useful adhesive materials. The epoxy resin which we have examined in this study has a surface tension of 47.2 dynes per cm. Appropriate fluorination of this... 

Completion of the reaction and the degree of conversion of the epoxy groups were determined by IR spectroscopy. To determine the equUi-brium value of the surface tension, a quantity of PEPA was added such that the extent of epoxy group conversion would not exceed 30—35%. In this case the resin remained in the liquid state. 

At low degrees of epoxy conversion, CCMF of OP-10 does not change (see Fig. 2.7, curves 1-3). Only at 30% conversion does CCMF begin to decrease (curve 4). With increase of the degree of conversion, one observes the decrease of the system surface tension at the surfactant concentration that corresponds to CCMF. The kink on the sirnface tension isotherm at low surfactant concentrations can be related to the formation of the surfactant pre-associations in the system. 

Figure 2.7 Isotherms of the surface tension of OP-10 solutions in DEG-1 at various conversion levels of the epoxy groups (1) 5%, (2) 10%, (3) 20%, and (4) 30%.

Consider the change in the system siuface tension with the extent of epoxy group conversion at different siufactant contents (Fig. 2.8). The transition of the surfactant molecules into the associated state with increase of the degree of epoxy groups conversion (with increase of the oligomer molecular weight) results in depletion of the siuface layer of surfactant molecules by diffusion to lower-concentration regions, which causes the increase of the system surface tension (see Fig. 2.8, curves 2 and 3). Consequently, the formation of the... 

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