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The Chemmunicator

Welcome to this issue of The Chemmunicator, a quarterly publication of Chemsultants and ChemInstruments.

Volume 2, Issue 3

October, 2010

Surface Energy and Surface Tension: Measurements key to ink, adhesive and coating wet out

by Joe Mausar

SURFACE TENSION (SURFACE ENERGY)

Surface tension is a property of the surface of a liquid caused by the cohesion of molecules within the liquid, which impacts the behavior of virtually all liquids including inks, adhesives and liquid coatings. The surface of any liquid is made up of an interface between that liquid (i.e. ink or adhesive), some other solid medium (i.e. paper substrate) and the gas (i.e. air) surrounding both. Since the molecules on the surface of the liquid are not surrounded by similar molecules on all sides (they are in contact with the solid and the gas at some points) they are more attracted by adhesion to their neighboring molecules on the surface. For example, the top surface of a pool is an interface between the pool water and the air. This phenomenon, and the concept of contact angle measurement as a means to describe it, is best described by Young's Equation. Simply stated, the phenomenon is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not a liquid will spread over a surface or "wet it out."

FIGURE 1: Young's Equation

SG = SL + LGcos

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Surface tension is not simply a property of the liquid alone, but also a property of the liquid interface between that liquid and other surrounding mediums (the gas and the solid). Based on the materials being studied, this relationship is what causes a portion of liquid to be attracted to another solid surface, such as a film or paper, or a release coated liner. Surface tension has the dimension of force per unit length or energy per unit area. Surface tension and surface energy are related - but when referring to energy per unit of area, we typically use the term surface energy - which is a more specific term in the sense that it applies to solids. This is all very interesting, but why is it important?

SURFACE ENERGY AND WETTING

Surface energy and wetting are important material considerations in a variety of industries including coatings, packaging, printing and converting and pressure-sensitive adhesive materials manufacturing. In manufacturing, printing and converting operations, we need to know if the surface we are attempting to coat or print will readily accept the applied coating. If we are using a water based formulation, we need to know if the substrate is hydrophilic (likes water or some other liquid) or hydrophobic (repels water or some other liquid). This will give us an indication of how well the adhesive or ink liquid will spread out (wet) over the surface.

FIGURE 2: Hydrophobic and Hydrophilic Surfaces

Surface tension, surface energy and contact angle are key material surface properties which are very closely related to the phenomenon of "wetting." In order to gain optimum adhesion an adhesive or coating must wet out the surface to which it is coated. In the same respect an ink must wet out the surface to be printed. For both of these to occur the surface tension of the adhesive or ink must be lower than the surface energy of the surface to which they are applied. Most adhesive and ink formulations wet out very well to high surface energy (HSE) surfaces. Low surface energy surfaces (LSE) including polyolephins such as polyethylene or polypropylene are more problematic.

Surface Energy Classifications of Selected Solids and Liquids

· · ·

VERY HIGH SURFACE ENERGY SOLIDS (greater than 50 dynes / cm2) Stainless Steel, Aluminum, Copper, Tin, Zinc, Lead HIGH SURFACE ENERGY SOLIDS (35 to 45 dynes / cm2) Alkyd Enamel, Polycarbonate, Polyester, Acrylic, Melamine, ABS, Vinyl, Nylon, Kapton LOW SURFACE ENERGY SOLIDS (30 to 35 dynes / cm2) Polypropylene, Polyethylene, Polystyrene, Tedlar, PVA, EVA, Acetal

LIQUID SURFACE TENSION · 50 -70 Water, Glycerin, Blood · 25 -35 Benzene, Xylene, Toluene, Acetic acid, Butyl acetate · 20 -25 Acetone, Methanol, Ethanol, Isopropyl acetate, Heptane

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When the surface energy of the ink or adhesive is higher than the surface energy of the substrate to which it is to be applied, the surface energy of the substrate must be raised to ensure sufficient wet out of the liquid. There are a number of methods to accomplish this including flame treatment, corona discharge and plasma treatment. An example of the increase in surface energy that can be achieved by treating various films is shown below:

Untreated vs. Treated Films ­ Surface Energy in Dynes

LDPE Film OPP Film PET Film UNTREATED 31 31 42 TREATED 38 ­ 42 38 ­ 42 50+

SURFACE ENERGY MEASUREMENT

There are two common methods for measuring surface energy: dyne measurement and contact angle measurement. There are several differences between the two methods.

DYNE MEASUREMENT

Of the two methods the dyne measurement method using a simple solution applied to the solid substrate is the easier, faster and less costly of the two methods. This method is based on ASTM standard Test Method D 2578. The most common solution used is comprised of ethyl cellosolve, formamide and a dye to make it easier to detect with the naked eye. The variation in concentration of the ethyl cellosolve % vs. the formamide % results in different dyne level solutions. In this process dyne solutions of various dyne level concentrations are applied to the substrate until one is found to completely wet out the surface. The dyne level of the substrate then corresponds to the dyne level of that solution. Three solution application processes are available: cotton swab application, dyne pen application and a draw-down coating application using a lab draw down coater. Variation in the pressure applied by the operator when applying the dyne solution and speed of solution application can affect the dyne result. While the dyne solution method of surface energy measurement is faster, easier and less costly it also suffers from the inherent problems of being subjective and inconsistent. The dyne method suffers from the following: · · · · · Open to potential contamination of the pens or applicators which can affect results requires the mechanical spreading of the dyne solution over the surface of the substrate in order to allow the solution to reach equilibrium which takes an imprecise time period, does not allow for an easy true average measurement of the surface energy of the substrate, requires the user to "read" and interpret the behavior of the dyne solution on the substrate and typically results in variations of 3 to 10 dynes/cm in measurement of the same surface.

It should also be noted that dyne solutions have a limited shelf life of only about six months. Use of outdated dyne solutions can result in false dyne level information. Use of dyne testing is best suited to shop floor / production testing that will be used as a quick check of previously established information such as that recorded after substrate treatment. There are also a broad number of suppliers of dyne solutions and dyne pens including: Diversified Enterprises (ACCU DYNE TESTTM solutions and pens), ENERCON (EnerDyneTM pens), Jemmco LLC (DYNE TEST 3

SOLUTIONS and Accu-Flo Felt Tip Dyne Pens) and Flexart Inks, Paints & Coatings, Inc (Dyne Test Solutions and Poly/Dyne Test Pens). These are just a few of the suppliers of liquid dyne solutions and dyne pens.

CONTACT ANGLE MEASUREMENT

The surface energy of a solid substrate such as a release coated liner or a label facestock can not be measured directly, because solids do not change shape in reaction to their surface energy. Because of this the use of contact angle measurement is the most reliable method to measure the surface energy of such solids. Practical measurements of surface energy use the interaction of a liquid droplet of a test solution resting on the solid surface. The term "contact angle" actually describes the shape of a liquid droplet resting on the solid surface. The contact angle is comprised of a tangent line from the liquid droplet to the solid surface (see Figure 1). The higher the surface energy of the solid substrate in relation to the surface tension of the liquid, the smaller is the resulting contact angle. Most instruments used for measuring contact angles are based on the observation and measurement of the tangent line of the liquid drop placed on the solid substrate surface. There are three general categories of contact angle measurement instruments: 1. Video-based, with a computerized automated contact angle calculation system 2. Microscope-based, with an ocular viewing tube with internal protractor 3. Projection screen-based, with an amplified image and protractor screen. The video-based system is quite accurate and because it includes a CCD camera it offers the advantage of being able to maintain visual records of completed contact angle measurements. Video-based systems are, however, quite costly. Microscope-based systems are low power and less accurate because they rely to a greater extent on operator interface and decision making. Projection screen-based systems are relatively simple to use and offer the added benefits of ease of droplet formation and location, and ease of operator viewing of the test in progress. In most types of projection screen-based systems it can be difficult for the operator to properly determine the tangent line (see FIGURES 1 & 2). In fact, it can be tricky to identify both the tangent line and the contact point of the liquid / solid interface. There are any number of contact angle meters (also called goniometers) available from various manufacturers including the Ramé ­ Hart Model 190 Contact Angle Goniometer, KRUSS DSA 100 Contact Angle Goniometer, Firsttenangstroms FTA 32 meter and the FIBRO DAT 500 Dynamic Contact Angle meter to name a few. These contact angle meters all function in essentially the same manner to determine the contact angle and require some level of operator experience in order to determine the exact interface point of the liquid drop with the solid surface. This is an important point to properly determining the correct contact angle. However, there is a much easier and more reliable method that employs a concept called the "Half-Angle Measurement Technique" (U.S. Patent No. 5,268,733). The Half-Angle Measurement Technique is available on the CAM-PLUS series of contact angle meters. This technique is much easier for the operator to use allowing for easy identification of the liquid / solid interface and the apex of the liquid drop which is used in calculating a Half-Angle.

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FIGURE 3: CAM-PLUS Contact Angle meter

Operation of the CAM-PLUS to determine the Half Angle is a quite simple and straightforward series of steps: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Charge the droplet syringe with water. Place the solid substrate sample onto the specimen holder. Locate the sample to be tested precisely under the syringe tip. Release one drop (10 screen divisions wide) of the water solution from the syringe. Move the syringe and raise the sample surface to contact the drop. Adjust the height of the sample surface to align it with the display horizontal cross-line. Align the left side of the drop with the display vertical cross-line. Locate the apex (centerline) of the drop. Align the protractor hairline indicator with the apex of the drop. Read the Half Angle on the protractor scale.

A diagram of the Half-Angle technique follows in Figure 4.

FIGURE 4: Half-Angle diagram

The contact angle is then calculated using the formula:

contact angle = 2X arc tan (height of drop / radius of drop)

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Calculation of the contact angle using the Half-Angle technique is not only quite simple, but it is also quite fast compared to other contact angle measurement techniques due to the ease in aligning the base of the solution drop to a horizontal line and the edge of the drop to a vertical post rather than attempting to locate the exact interface point of the liquid / solid and the correct tangent line typical in other contact angle measurement techniques.. The Half-Angle technique has substantiated measurement-to-measurement repeatability of ± 1 dyne / centimeter for tests completed by the same operator. The technique also offers excellent repeatability of ± 2 dynes / centimeter for tests completed between different operators or different locations. If we remember, one of the drawbacks of using dyne solutions or dyne pens is the significant variation one can expect between measurements of the same substrate surface by the same operator of 3 to 10 dynes. The HalfAngle technique for determining contact angle is therefore 1.5 to 5 times more accurate than using dyne solutions or dyne pens. Thus the Half-Angle technique offers repeatability far superior to the use of dyne solutions or even other contact angle measurement techniques.

Examples of high and low contact angles using the Half-Angle technique are shown in Figure 5 below.

FIGURE 5: Half Angle Examples 140 120 100 80 60 40 20 0 140 120 100 80 60 40 20 0

Low Contact Angle = Good Surface Wet out

Half Angle Contact Angle

High Contact Angle = Poor Surface Wet out

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SUMMARY

The acceptable wet out of adhesives on solid surfaces or inks on printing surfaces is key to the production of suitably performing products. The characterization of ink, coating or adhesive wettability of solid materials has presented challenges to both scientists and printing and coating technicians alike. For converters and printers of plastic films it is important to know the surface energy of the solid surface and the surface tension of the liquid that will be applied to it. The proper balance of liquid component surface tension and solid component surface energy is critical in assuring good coating adhesion and acceptable print quality. Dyne testing of the solid surface is a good basic test methodology, contact angle measurement is a better, more consistent process, but the Half-Angle Measurement Technique may be the most accurate and most repeatable measurement option of all.

Definitions

Contact Angle: the equilibrium angle of contact of a liquid on a solid surface, measured at the contact line where the three phases (liquid, solid, gas) meet. Cohesion: a physical property of a substance, caused by the intermolecular attraction between like-molecules within a body or substance that acts to unite them. Dyne: the force required to accelerate a mass of one gram at a rate of one centimeter / second squared. Surface energy: The energy equal to the surface tension at an interface. Surface tension: a phenomenon at the surface of a liquid caused by intermolecular forces. Wetting: the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces.

References

1. Renate Förch, Holger Schönherr, A. Tobias A. Jenkins, Surface design: applications in bioscience and nanotechnology. Wiley-VCH. p. 471. ISBN 3527407898, 2009. 2. P.G. de Gennes, "Wetting: statics and dynamics", Reviews of Modern Physics 57 pgs. 827-863, 1985 3. Tadmor, Rafael (2004). "Line Energy and the Relation between Advancing, Receding, and Young Contact Angles". Langmuir 20: 7659. 4. Jacob Israelvilli, Intermolecular and Surface Forces, Academic Press (1985­2004) 5. D.W. Van Krevelen, Properties of Polymers, 2nd revised edition, Elsevier Scientific Publishing Company, Amsterdam-Oxford-New York, 1976. 6. Shieh, Sarah, An Analysis of Contact Angle Measurement, AST Products, March 2001 7. Blitshteyn, Mark and Wetterman, Bob, Testing for Surface Energy, reprint, Converting Magazine, Delta Publications, 1993. 8. Wetterman, Robert P., Surface Tension Measurement and Coatings Development, Paint & Coatings Industry, pgs. 202 ­ 206, October 1998. 9. ASTM D2578 (Test Method for Wetting Tension of polyethylene and polypropylene), and ASTM D5946 (Test Method for Corona Treated Polymer Films using water Contact Angle measurements), ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA, 2010. 10. TAPPI T-458 (Surface Wettability of paper ­ angle of contact method), TAPPI, 15 Technology Parkway S., Norcross, GA, 30092, 2010. 11. Utschig, Steven, Measuring treatment of non-porous materials, 2 pages, copyright by Enercon Industries Corporation, 12/22/2006.

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