Critical surface tension additives

The critical surface tension concept has provided a useful means of summarizing wetting behavior and allowing predictions of an interpolative nature. A schematic summary of 7 values is given in Fig. X-10 [123]. In addition, actual contact angles for various systems can be estimated since )3 in Eq. X-38 usually has a value of about 0.03-0.04. 

For self-releasing systems the presence of an IMR agent on the surface of the part, which makes It self-releasing, also lowers the value of the solid s critical surface tension, making It less wettable by the paint, than If It were free of the IMR surface coating. This problem, however, can be solved by either one of two ways (1) by the addition of solvent to the paint formulation that... 

In addition to overcoming experimental difficulties, the Naval Research Laboratory group has contributed many important generalizations and new concepts-for example, the concepts of low energy solid [33,69], critical surface tension [33,61], and autophobic liquid [32,41]. These developments have also stimulated the search for an interpretative scheme capable of yielding values of solid-vapor and solid-liquid interfacial tensions. 

The present investigation describes the successful modification of the surface properties of polymeric solids by the adsorption of appropriate partially fluorinated compounds at polymer-air interfaces during the formation of the polymer surfaces. The extent of additive adsorption was foxmd to be dependent upon the molecular structure, fluorine content, and solubility of the additives in the solute—i.e., their organophilic-organophobic balance with respect to the solute. Certain effective additives were able to decrease the critical surface tension, of such polymers as poly(methyl methacrylate) and polyacrylamide to 20 and 11 dynes per cm., respectively. These low values correspond to surfaces containing closely packed CF2 and CF3 groups. 

Four polymers with different surface compositions were used in this study—polystyrene (PS), poly(methyl methacrylate) (PMMA), polyacrylamide (PAM), and a poly(vinylidene chloride) (PVeC) copolymer (containing 20% polyacrylonitrile). Polystyrene has essentially a hydrocarbon surface, whereas the surfaces of poly (methyl methacrylate) and polyacrylamide contain ester and amide groups, respectively. The surface of the poly(vinylidene chloride) copolymer on the other hand will contain a relatively large number of chlorine atoms. The presence of acrylonitrile in the poly(vinylidene chloride) copolymer improved the solubility characteristics of the polymer for the purposes of this study, but did not appreciably alter, its critical surface tension of wetting. Values of y of these polymers ranged from 30 to 33 dynes per cm. for polystyrene to approximately 40 dynes per cm. for the poly(vinylidene chloride) copolymer. No attempt was made to determine e crystallinity of the polymer samples, or to correlate crystallinity with adsorption of the fluorocarbon additives. 

In many studies of wettability Zisman and coworkers have used the contact angles of a series of n-alkanes as a convenient means for determining for low energy solid surfaces [5,6,13,20]. In Figure 2 are plotted the cos 9 vs. 7lv° curves for the n-aUtanes on PMMA surfaces containing 0.5% additive I and 1.0% additive II. The critical surface tensions with additives I and n were 19 and 20 dynes per cm., respectively, representing a decrease of about 20 dynes per cm. from the value of obtained with the additive-free surface. Since the y values of 19 and 20 dynes per cm. are very close to that of 18 dynes per cm. reported by Fox and Zisman [l3] for the n-alkanes on poly-tetrafluoroethylene surfaces, it is apparent that a number of perfluoro-alkane groups are present in the outermost part of the surface phase with the principal axis of each carbon-carbon chain parallel to the surface. 

PVeC copolymer surface are in good agreement, generally within the limits of experimental error. These values of 6 are very similar to those reported previously by Ellison and Zisman[S] on a poly(vinylidene chloride) surface, which for comparison are plotted in Figure 3 along with the present results. The plot of cos 6 vs. in Figure 3 shows the critical surface tension of the additive-free PVeC copolymer surface to lie between 38 and 44 dynes per cm. 

Modification of a spreading oil through additive agents. Selected solutes can be added which act either as additives of low surface tension capable of adsorbing as monolayers whose critical surface tension is lower than the surface tension of the liquid or as additives of higher volatility which create a surface tension gradient. 

Antifogging additives function by increasing the critical surface tension of the polymer surface, allowing the molecules of water to wet the surface, forming a continuous layer of water that does not scatter fight and therefore does not interfere with transparency. 

Chemical or physical surface treatments are especially required for structural bonding of low-surface-energy plastics. Low-surface-energy plastics include polyethylene, polypropylene, TPO, and fluorinated polymers. These surface treatments are designed to increase the critical surface tension and improve wetting and adhesion. In addition to increasing the critical surface tension, surface treatments are designed to remove contaminants or weak boundary layers, such as a mold release. 

Many novel high-performance soft contact lenses contain siloxane backbones because of the high oxygen transmissibility DJL) provided by such a chemistry [12]. The ocular environment remains healthier with siloxane-based lens materials. However, siloxane molecules are not inherently water wettable because of their low critical surface tensions [llj. In addition, siloxane surfaces attract lipid-soluble material, which can penetrate the lens [16]. Siloxane-based lenses can also attract and attach proteins and mucus on the surface of the lens [16]. The physical and chemical changes in the lens surface and bulk associated with deposit formation include tear film disruption, decreased vision, discomfort, decreased lens life, conjunctival hyperemia, and bacterial adhesion [17,18]. 

Conditions may require the use of surface preparation on plastics. Previously mentioned were cases in which the surface is too smooth for mechanical interlock or has too low a critical surface tension for wetting. In addition, other conditions may be critical. This includes the presence of mold releases, plasticizers, or other contaminants on the surface of the part. Methods of surface preparation are discussed below. 

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