spopsisFurther studies of a new and highly effective method for the surface treatment of low surface energy polymers for adhesive bonding are reported. Mechanisms are suggested for the increase in the cohesive strength in the surface region of polyethylene when it is exposed to activated species of inert gases. The technique is unique because, in contrast with results obtained with other methods, bulk properties of the polymer such as color or tensile strength and elongation are unaffected and surface properties such as wettability and dielectric properties such as surface conductivity are essentially unchanged.
The kinetics of wetting of both high-energy (aluminum, mica) and low-energy (FEP Teflon) surfaces by thermostated polyethylene and ethylene-vinyl acetate copolymer melts in air were followed by the rate of approach of the apparent contact angle θ to its final value θ∞ and the change of the radius of the base of the polymer melt drop, r, with time. The volume of the drops studied varied from 0.006 cm3 to 0.028 cm3 and θ could be studied in the interval from about 100°>θ≥θ∞. The melted polymer drops maintain the shape of a spherical segment. The reduced dimension r/r0, where r0 is the value of r when the contact angle is 90°, is a universal function of the reduced time aTt, a dimensionless quantity. The reduced contact angle cosθ/cosθ∞, where cosθ∞ is the value of cosθ at infinite time, can be represented by a function of the same variable aTt. The shift factor aT is given by γ/Lwη, where γ is the surface tension of the liquid, η is the viscosity, and Lw is a length characteristic of the polymer-substrate system.
An extended equation is proposed, y = yNe-B/("-'") relating absolute viscosity ( 7 ) to surface tension y. This relationship is shown to be valid for a variety of liquids, over the entire liquid range extending from the critical temperature (T,) to the temperature at which the viscosity becomes infinite ( J N ) . R E C E N T L Y , Pelofsky (11) proposed the following relationship between surface tension and the absolute viscosity which is, apparently, valid for a large variety of liquids note is to extend the above empirical treatment by introducing the concept of the vapor viscosity (qL) in equilibrium with the liquid. The author proposes that Equation l be written asIn the above form, Equation 1 can apply only to a restricted temperature range and will necessarily fail a t the critical temperature as viscosity is finite while y = 0.Although there is no apparent theoretical justification a t the present time for Equation 1, the purpose of this -7 E 70.0 0 \ m a 0 E 60.0 0 I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -E -e VL-T" Figure 1. Surface tension for several liquids plotted as a function of over entire liquid range Water A Benzene 0 Argon . where, A = yN. At T, Equation 2 satisfies _the boundary condition that as ~L V -0, exp. ' l -B i i q -q I ' -0. When ql >> vu, which is a t T << T,, Equation 1 is obtained. I n the vicinity of the critical temperature, when q L makes a sizable contribution, considerable deviations from linearity are observed when Equation 1 is employed.The author selected several liquids where data are available over an appreciable temperature interval to demonstrate the validity of Equation 2. These data are plotted in Figure 1. Apparently, argon is the only liquid for which there are extensive and reliable data near the critical region. Here, the agreement is excellent. The discrepancy between the literature values of for water in the critical temperature region makes the extension of the plot to the region doubtful ( 3 ) . For all the liquids quoted by Pelofsky, as well as those shown in Figure 1, the plots pass through the origin.As has been indicated by Pelofsky, the slope may be an indication of the surface tension of the substance at the temperature where q = a. Table I attempts a correlation between the temperature where q --+ m and the temperature of homogeneous nucleation. Homogeneous nucleation as opposed to heterogeneous nucleation results from a spontaneous phase change without the assistance of impurities. The principal component of the energy barrier in homogeneous nucleation is the thermal energy that keeps two particles from clustering together. A special feature of nucleation phenomena is that in forming particles of a new phase, energy is required to form the new surface. Opposing these forces are the intermolecular attractive forces. When the Table I. Correlation between TN and
Heterogeneous nucleation and crystallization of polymer melts against high-energy surfaces (e.g., metals, metal oxides, and alkali halide crystals) have been found to result in marked changes in both the surface region morphology and wettability of these polymers even though the chemical constitution of the polymer is unchanged. The critical surface tensions (7c) of a variety of polymers nucleated against gold are considerably in excess of the commonly accepted values. Employing a modified Fowkes-Young equation can account for these sizable differences if the surface layer of these crystallizable polymers generated against high-energy surfaces is essentially crystalline.
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