Taking the measure of surface treatment is a learning process

If you surface-treat your materials (and it's very likely you do), you should be up to date on methods for measuring the level of treatment.

As a growing number of end users switch from paper to plastic substrates to package or label their products, many converters are being forced for the first time to learn the intricacies of printing on these "foreign" materials.

Fortunately for these newcomers, long-time suppliers and converters of polypropylene, polyethylene, and other poly-based materials have already developed (though not necessarily perfected) methods of measuring the surface energy of treated films.

Coming to 'Terms'

The term "surface energy," or wetting, is normally used to describe the reactivity of the surface of a solid substrate, while "surface tension" is used in reference to a liquid. Frequently, the two terms are used interchangeably, since both refer to the same force at which molecules at the surface of the substrate ultimately cling to one another.

The phenomenon of surface energy is based on the relative energies of the solid substrate and the liquid in contact with it. For converters of plastic films, knowing the surface energy of a polymer surface is critical in assuring good coating and print quality, as well as the adhesion of laminated films - particularly with the growing popularity of water-based inks, coatings, and adhesives.

The surface energy of a solid polymer cannot be measured directly because solids typically show no reaction to the exertion of surface energy. Consequently, practical measurements of surface energy involve the interaction of the solid with a test liquid to determine wetting tension as a measure of surface energy.

Surface tension is expressed in units of force per unit of width, similar to web tension. However, since surface tension forces are so much smaller, it is more convenient to express them in dynes per centimeter, rather than pounds per inch. Hence, the act of measuring surface energy, or tension, is typically known as a "dyne test."

Measuring Surface Energy

Until recently, converters have primarily used dyne liquids and pens to test the level of surface treatment on flexible films. In a "dyne test," wetting tension liquids (typically mixtures of ethylene glycol monoethyl ether and formamide) are spread over a film surface to determine printability, coating laydown, and heat sealability of treated films. Mixing ethylene glycol monoethyl ether and formamide in different concentrations yields a solution with different surface tensions; solutions of increasing wetting tensions are applied to the polymer film until a solution is found that just wets the polymer surface.

From a practical standpoint, water-based ink systems normally require a substrate surface tension of 42 to 45 dynes/cm in order to provide acceptable adhesion and wetout. Therefore, all plastic films, including nylon and polyester, will require some amount of surface treatment in order to accept a water-based ink. The surface tension of the film should be between 3 dynes/cm and 10 dynes/cm greater than the surface tension of the ink in order to ensure acceptable performance. Thus, even pretreated films should be checked before use to make sure proper dyne energy is present.

To facilitate testing, cotton-tipped applicators, full-etch coating rods, and solution-tipped "dyne pens" are typically used to smoothly apply wetting solutions across polymer surfaces. When a cotton applicator is used, the solution is spread lightly over approximately a 1-sq-in. area. If the continuous film of liquid remains intact - or fails to wet-out - for two or more seconds, the next higher surface tension solution is applied to a similar area. If the liquid dissipates in less than two seconds, the next lower numbered solutions are tried until an exact measurement is attained.

Advantages and Problems

The greatest advantage of this method, which is based on ASTM standard test method D2578-84, is its speed and simplicity. For example, it can be used on the converting room floor to easily identify problems with corona treaters. Though it provides an established procedure for determining surface tension, this method presents several problems in reliability and consistency of results:

* Care must be taken to limit evaporation of solutions, since it changes concentrations and, consequently, changes dyne level values.

* Cotton swabs have been found to have widely different effects on dyne indications, because different swab manufacturers use various binders to hold the cotton to the end of the stick.

* Although the mixtures of ethylene glycol monoethyl ether and formamide used in this method are relatively stable, exposures to extremes in temperature or humidity should be avoided.

Also, while this method is generally effective for shop floor measurements of dyne levels, they can be subject to contamination and operator influence and therefore may not be 100% accurate. For example, in recent round-robin testing by the Flexible Packaging Assn., results of measurements of identical materials tested by different labs using dyne liquids varied by as much as 11 to 15 dynes/cm. This can be significant, since a minimum of 42 dynes/cm of adhesive energy is needed to assure proper ink adhesion on a printed plastic sheet.

One problem may be that lab personnel using cotton swabs have a tendency to vary the amount that is picked up by the cotton swab, thus varying the results of the test. Also, the method of rubbing the liquid onto the polyolefin surface with a cotton-tipped applicator varies from person to person. This rubbing also tends to give erratic dyne level results.

In addition, contaminants could affect the test. Possible contaminants could include impurities on the polymer surface itself, or, in the case of dyne pens, on the surface of the pen tip from a previous test. Surfactants incorporated with certain polymers to change their characteristics can also change the results. That's why it is critical that polymer films being tested not be touched or rubbed prior to the test to prevent contamination. Also, a polymer surface that has been unevenly treated will deliver different readings in different test areas, rendering the test useless.

Because of these potential problems, some converters use a drawdown rod rather than a cotton tip applicator to establish a uniform thickness of wetting tension solution on the polyolefin sample.

An Alternative Emerges

The emergence of a new ASTM-certified method of measuring surface energy, called the Water Contact Angle Test (WCAT) method (ASTM #D5946), offers converters another alternative. The new standard, finalized by ASTM earlier this year, is an analytical technique that uses an instrument to measure the contact angle of a water droplet placed on the surface of a film. Unlike dyne tests, the WCAT method also allows for materials outside of the PE and PP "families," such as PET, ethylene vinyl acetate, and colored and holographic materials, to have their surface energies determined.

Against a droplet of liquid placed on the surface of a polyolefin substrate, the gas pressure in the surrounding atmosphere applies a certain force, causing it to have a particular shape as it rests on the surface of the material. Its surface tension and its interaction with the polyolefin surface determines the eventual bond.

The WCAT test method is based on a simple concept: the higher the surface energy of the treated film, the smaller the contact angle of water placed on the surface. Contact angle is determined through direct measurements.

To conduct the contact angle test, a series of colored test liquids is applied to the treated surface. When a droplet is placed on a solid substrate, its contact angle has a single value for smooth surfaces. In general, the droplet of any liquid will have an angle of contact determined by the surface energy of the substrate. The higher the surface energy of the solid substrate in relation to the surface tension of the liquid, the smaller the contact angle.

For example, a water droplet on PE, which exhibits a low surface energy, will "stand up" on the material at an angle of greater than 90 deg. The same droplet on a high-energy surface will lay down or cling more closely to the surface, resulting in a contact angle of less than 60 deg.

Factors to Consider

There are two factors to consider regarding the WCAT method of wetting tension: the accuracy of the contact angle measurements and the correlation between the wetting tension plot data and the data generated with dyne solutions.

Generally, the accuracy of contact angle measurements is not limited by the experiment technique but rather by the reproducibility of the surfaces measured. Accurate measurements of a contact angle are simple enough: Accuracy and precision depend on several factors, such as droplet deposition technique, droplet size and consistency, and the presence of electrostatic charges on the sample.

There is the question of how closely wetting tension values obtained using WCAT correlate with the dyne liquid test. In fact, results of both tests on the same substrate rarely offer the same results. Why? One reason is that different test methods give different surface values. Plus, accuracy and precision of the two test methods depend on film type and the presence of additives.

Proponents of the WCAT method point to a number of "advantages" over the more traditional dyne tests: It can test practically any material, can be used for time studies of corona treatment aging, can be used to quantify uniformity of corona treatment, can detect overtreatment, can measure backside treatment, can accurately measure embossed films, does not use toxic materials, and can use a variety of test liquids.

A "goniometer," the instrument used to measure the surface energy in the WCAT method, is typically used to directly measure the contact angle of a liquid. Common applications include the analysis of the degree of surface hydrophilicity (i.e. wettability) and hydrophobicity (nonwettability). The unit offers the user the visual appearance of a microscope, but the easy-to-read scales require no adjustments due to magnification problems. Precalibrated protractor and micrometer scales are built into the ocular lens. Because the instrument is a reflective type, it provides greater accuracy and precision than other measurement instruments, such as the "Optical Comparator."

What's Available

A variety of industry suppliers offer surface testing equipment to the converting industry. (For a full list of testing equipment suppliers, see p. 239 of the June 1996 Paper, Film & Foil CONVERTER Buyers Guide.) Here are just a few examples of the types of surface testing equipment available (by no means a comprehensive list):

Pillar Technologies offers a range of DyneTest pens and solutions to verify the level of corona treatment on any substrate. Pillar Poli-Test and DyneTest pens are magic marker-type pens that are able to quickly verify whether a substrate is treated to a 36-dyne level or higher. If the material is properly treated, a test mark drawn on the material remains uniform; if not, the line of testing liquid retracts in a form of small globules until it nearly disappears. Poli-Test liquids can test levels from 30 to 70 dynes in a similar fashion.

Corona Designs offers a similar line of surface tension test markers, called PowerPens, that are available in 14 standard levels from 30 through 56 dynes/cm.

Tantec's CAM-FILM-T Contact Angle Meter uses pure water to measure wetting tension on all types of films using a US-patented Half-Angle measuring method. The meter has a substantiated repeatability of [+ or -]1 dyne/cm for single-operator use, and the same lab measurements and reproducibility of [+ or -]2 dyne/cm for multilaboratories. The company also offers improved contact angle meter Model CAM-FILM-T for measuring wetting tension on films and coated paper, and the CAM-MICRO, for use with pure water, that measures wetting tension on substrates greater than 10 mils in thickness, such as boardstock.

Kernco Instruments produces a video/optical multiple retrofit contact angle meter system that allows material surface characteristics to be visually monitored. The VCA2000 unit encompasses a high-resolution video camera, a sample stage with 4-deg motion, and a finely adjustable dual lighting system.

Companies contributing to this report include:

Corona Designs Inc., Garland, TX; ph: 214/272-0471; fax: 214/272-0379.

Enercon Industries Corp., Menomonee Falls, WI; ph: 508/694-9000. fax: 508/694-9046.

Kernco Instruments Co. Inc., El Paso, TX; ph: 915/852-3375; fax: 915/852-4084.

Pillar Technologies Ltd., Hartland, WI; ph: 414/367-3060; fax: 414/367-3206.

Tantec Inc., Schaumburg, IL; ph: 847/529-5506; fax: 847/529-6956.

Sherman Treaters, Ont., Canada; ph: 905/670-9117; fax: 905/670-9799.

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