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The Concept of Surface Energy

الكلية كلية هندسة المواد     القسم قسم البوليمرات والصناعات البتروكيمياوية     المرحلة 3
أستاذ المادة ذو الفقار كريم مزعل أل عبيد       6/15/2011 6:59:00 AM

the concept of surface energy

 

 

the ease with which an adhesive or sealant wets (makes intimate contact with) a substrate surface and the work necessary to separate the adhesive from the substrate can be related to the surface energies of the adhesive, substrate, and subsequent interface. surface energy, 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">  (gamma), is used interchangeably with the terms "surface free energy" and "surface tension."

 

the proof that liquids have a surface energy is easily demonstrated by the fact that a finely divided liquid, when suspended in another medium, assumes a spherical shape. in the absence of gravitational distortion of shape (i.e., the energy associated with having a surface), the liquid tends to go to its lowest energy state-that of a sphere

 

surface tension and surface energy are numerically identical for liquids. surface energy is generally given in units of millijoules per meter squared (mj/m2), while surface tension is given in units of dynes per centimeter (dynes/cm) or newtons per meter (n/m). the sur­face tensions of organic liquids and of most inorganic liquids rarely exceed the value for water (72 dynes/cm).

 

the surface energies of liquids are readily determined by measuring the surface tension with a dunouy ring or a wilhelmy plate2 as shown in fig. 2.1. with the dunouy tensiometer

 

 

                wilhelmy platedu                                                                                  nouyring

 

 

 

 

 

 

 

figure 2.1 wilhelmy plate and dunouy ring methods of measuring surface tension of a liquid. (courtesv: kruss usa)

 

 

 

a clean platinum ring is placed under the surface of the test liquid, and the liquid is slowly moved downward until the ring breaks through the liquid surface. the force is recorded, and by means of appropriate conversion factors, the surface tension of the liquid is calcu­lated. the wilhelmy plate is a similar method, which measures the force of a liquid on an immersed plate passing through the surface as it is removed.

 

another method of measuring surface tension is the "droping weight/droping volume" method.3 with this technique, the average volume of test liquid to cause a droping to fall from a carefully calibrated syringe is used to calculate the surface tension of the liquid. the surface tension of a liquid is a real surface stress however, the same cannot be said of a solid surface. with a solid, work is done in stretching a surface and not in forming the sur­face. for a solid surface, surface energy and surface tension are not the same. still, it is often convenient to refer to y indiscriminately as either surface energy or surface tension, but it is inaccurate because the "tension" in the surface of the solid is greater than the surface energy. it is an easy matter to measure the surface tension of a liquid, but it is not so straight­forward to measure the surface energy of a solid. direct surface tension measurements on solids are mostly made near the melting point however, it is the lower temperature prop­erties that mainly concern adhesive studies. therefore, surface energies of solids have been indirectly estimated through contact angle measurement methods as explained below. in a contact angle measurement, a droping of liquid is placed upon the surface of a solid. it is assumed that the liquid does not react with the solid and that the solid surface is perfectly smooth and rigid. the droping is allowed to flow and equilibrate with the surface. the mea­surement of the contact angle,   ?   (theta), is usually made with a goniometer .that is simply a protractor mounted inside a telescope. the angle that the droping makes wlth" t1ie surface is measured carefully. a diagram of the contact angle measurement is shown in fig. 2.2.

 

 

vapor

 

 

 

 

 

 

 

figure 2.2 schematic diagram of the contact angle and. its surface free energy (tension) components

 

 

the contact angle that the liquid makes with the substrate surface can be quantified by the various tensions that act at the point where the three phases (liquid, solid, and vapor) meet. the subscripts l, s, and v in fig. 2.2 stand for liquid, solid, and vapor.

 

a force balance between the liquid and the solid can be written as:

 

12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_lv  cos   12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75">   = 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sv - 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sl

 

 

where   12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75">   = contact angle,

 

              12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_lv  = liquid-vapor interfacial tension

 

              12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_sv  = solid-vapor interfacial tension

 

          12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_sl  = solid-liquid interfacial tension

 

 

 

 

 

 

this is known as the young equation after the scientist who originated the analysis.

 

the ysv is the solid/vapor interfacial energy and not the true surface free energy of the . solid. the surface energy is related to 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">sv through the following relationship

 

12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_sv = 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> - 12د€" style="width: 11.25pt height: 25.5pt" type="#_x0000_t75"> _e

 

 

where  12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">  is the true surface free energy of the solid, and 12د€" style="width: 11.25pt height: 25.5pt" type="#_x0000_t75">_e is a quantity known as the equi­librium spreading pressure. it is a measure of the energy released through adsorption of the vapor onto the surface of the solid, thus lowering the surface free energy.   however, in most cases, 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_sv is quoted as the surface energy.

 

  it can be calculated from young s equation by measuring the contact angle that a liquid makes on the surface of the material, and from knowing the surface free energies ( 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75"> _lv and 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_sl) of the liquid, which can be found in various chemical reference books.

 

a rather simple method of estimating the surface energy of solids was developed by zisman.   zisman proposed that a critical surface tension, 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_c can be estimated by measur­ing the contact angle of a series of liquids with known surface tensions on the surface of interest. these contact angles are plotted as a function of the 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_lv of the test liquid. the crit­ical surface tension is defined as the intercept of the horizontal line, cos 12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75">  = 1, with the extrapolated straight-line plot of cos 12خ¸" style="width: 10.5pt height: 25.5pt" type="#_x0000_t75">  against 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_lv as shown in fig. 2.3. this intersection is

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

figure 2.3 zisman plots for various low energy polymeric surfaces. (a) polytetrafluoroethylene (ptfe) with n-alkanes as the liquid series. (b) ptfe with a wide range of different liquids. (c) polyethylene with a liquid series: commonly used by zisman.

 

 

. the point where the coi1tactangle is 0 degrees. a hypothetical test liquid having this 12خ³" style="width: 9.75pt height: 25.5pt" type="#_x0000_t75">_lv would just spread over the substrate.                .

 

the surface tension value for most inorganic solids is on the order of hundreds or thou­sands of millijoules per square meter, and for polymers it is at least an order of magnitude lower: solid substrates are often considered to be either "high energy surfaces" (metals, glass, ceramics) or "low energy surfaces" (polymers). organic liquids have surface tensions that are in a similar range as solid polymers. in fact, an epoxy adhesive composition will have approximately the same surface tension in the cured state as it does in the uncured or liquid state. values of critical surface tensions for common solids and surface tensions  of common liquids are shown in table 2.2.

 

 

epoxy resin (dgeba type)

 

fluorinated epoxy resin

 

  polydimethyl siloxane (mw 162)

 

  petroleum lubricating oil

 

  silicone oil

 

formamide

 

dmso

 

glycerol

 

hexane

 

acetone

 

n-propanol

 

toluene

 

trichloroethane

 

methyl ethyl ketone

 

tricresylphosphate

 

octane

 

ethylene glycol

 

propylene glycol

 

mineral spirits.

 

vm& p naphtha

 

water

 

 

aluminum

 

  copper

 

nickel

 

iron oxide

 

  beryllium oxide

 

lead

 

graphite

 

  platinum

 

silver

 

table 2.2 room temperature surface tension of several liquids (top) and critical surface tension of various high energy (middle) and low energy substrates (bottom) (continued)