Standard Test Method for
Corona-Treated Polymer Films Using Water Contact Angle Measurements¹

1. Scope

1.1 This test method covers measurement of the contact angle of water droplets on corona-treated polymer film surfaces, with subsequent estimation of the film's wetting tension.

Note 1-There is currently no ISO standard that duplicates this test method.

1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents

2.1 ASTM Standards:
D618 Practice for Conditioning Plastics and Electrical Insulating Materials for Testing²
D724 Test Method for Surface Wettability of Paper (Angle-of-Contact Method)³
D2578 Test Method for Wetting Tension of Polyethylene and Polypropylene Films⁴
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of Test Method⁵

3. Terminology

3.1 Descriptions of Terms Specific to This Standard:

3.1.1 polarity, n-in surface chemistry, a value that quantifies the concentration of polar functional groups on the polymer surface and is measured as a polar component of surface energy over the total surface energy value.

3.1.2 Static contact angle, 0 (degrees), n-the angle between the substrate surface and the tangent line drawn to the droplet surface at the three-phase point, when a liquid drop is resting on a plane solid surface.

3.1.2.1 Discussion- Contact angle values can be determined as follows: (1) by measuring the contact angle directly with a protractor by using tangential alignment of a cursor line; (2) by measuring the angle between the substrate surface and the line from the three-phase point to the apex of the droplet, and then multiplying the obtained number by 2 to calculate the contact angle;⁶ or (3) from the dimensions of the droplet's image, as follows (Test Method D 724):

Where:

H = height of a droplet's image, and
R = half its width.
Method (1) is biased due to the subjective nature of finding a tangent to the droplet image at the three-phase point; if Method (1) is to be used, the bias of each operator's measurements must be determined; Methods (2) and (3) are generally free of bias.

3.1.3 Surface energy (mN/m), n-for a given solid, the molecular forces of its interaction with other interfaces; it is measured as free energy per unit area.

3.1.4 Surface tension of a liquid, n-definition to be determined later.

3.1.5 Wetting tension, (mN/m, n-for a given solid, this value is accepted to be equal to the surface tension of the liquid that just exhibits a zero contact angle on the solid; it is used as an estimate of the solid's surface energy.

3.1.5.1 Discussion- A different unit of measurement is used in the industry for surface energy and wetting tension: dyne/cm; 1 mN/m = 1 dyne/cm.

4. Summary of Test Method

4.1 In this test method, drops of water are placed on the surface of a film sample, and the contact angle values are measured and then averaged. The estimate of the film's wetting tension is then found in a conversion chart. Practically any polymer film can be tested.

5. Significance and Use

5.1 The ability of polymer films to retain inks, coatings, adhesives, etc. is primarily dependent on the character of their surfaces and can be improved by one of several surface-treating techniques. The electrical discharge treatment, such as corona treatment, has been found to increase the wetting tension of a polymer film surface by increasing the surface polarity. The stronger the treatment, the higher the polarity. The higher the polar component, the more actively the surface reacts with different polar interfaces. It is therefore possible to relate the wetting tension of a polymer film surface to its ability to accept and retain inks, coatings, adhesives, etc., if the ink, coating, or adhesive contains the necessary polar functionalities. Wetting tension in itself is not a completely acceptable measure of ink, coating, or adhesive adhesion.

5.2 The wetting tension of a polymer film belongs to a group of physical parameters for which no standard of accuracy exists. The wetting tension of a polymer cannot be measured directly because solids do not change shape measurably in reaction to surface energy. Many indirect methods have been proposed.⁷ Different test methods tend to produce different results on identical samples. Practical determination of a solid's surface energy uses this interaction of the solid with test liquids.

5.3 Although the level of surface treatment of polymer films has been traditionally defined in the industry in terms of dynes/cm (mN/m), these values are derived from a subjective interpretation of the observed test liquid behavior.

5.4 The surface energy of a solid material consists of two components, dispersive and polar. Water will interact with different polymers, treated or untreated, according to the polar and dispersion forces acting on it, but it will not chemically react with the polymer. A water droplet will assume a shape in response to the surface energy and polarity of the film's surface. As a result, different polymers that are treated to different levels can be considered a homologous series in terms of their interaction with water. Thus, each time a film is subjected to corona treatment, a new member of the homologous series of polymers is produce. For such a series, the water droplet will assume a shape in response to the particular surface energy and polarity of the film's surface.

5.4.1 For such a series, an empirical equation exists that describes a relationship between the wetting tension and cosine of contact angles of a test liquid, such as water. That equation can be used to estimate the wetting tension of the substrate from the water contact angle values. See Appendix A1.

5.5 The conversion chart for estimating the range of wetting tension of the substrate from the contact angle values is shown in Appendix X2. This conversion chart is shown for the purpose of providing a bridge between previous test data obtained using Test Method D 2578. Otherwise, contact angle values should be used for defining the film's surface. Because this test method and Test Method D 2578 use different test liquids, it is impossible to equate a water contact value to a single wetting tension value. For the purpose of this test method, each water contact angle value is equated to a range of wetting tension values.

5.6 The following ranges of water contact angle values can be used as guide for defining the level of surface treatment of polyolefins and many other polymer films with initial low surface energies:

Marginal or no treatment >90° (under approximately 34 dynes/cm)
Low treatment 85 to 90° (approximately 36 to 34 dynes/cm)
Medium treatment 78 to 84° (approximately 39 to 36 dynes/cm)
High treatment 71 to 77° (approximately 43 to 40 dynes/cm)
Very high treatment <71° (above approximately 43 dynes/cm)

5.6.1 The suitability of the test for specification acceptance, manufacturing control, and end use of polymer films will have to be established through capability studies.

5.7 Almost all materials have variations in surface energy as one moves from point to point. Non-uniform treatment of film with corona treaters may also add variability to the results. Therefore, the surface energy of a specimen must be described by the average wetting tension and its variability over the sample surface.

6. Interferences

6.1 The wetting tension of a polymer film in contact with a drop of liquid in the presence of air is a function of the surface energies of both the air-film and film-liquid interfaces; any trace of surface-active impurities in the test liquid or on the film may affect the results. It is therefore important that the portion of the film surface to be tested not be touched or rubbed, that all equipment be scrupulously clean, and that water purity be guarded carefully. Glass apparatus, in particular, is likely to be contaminated with detergents having very low surface tension, unless specific precautions are taken to ensure their absence, such as cleaning with chromic sulfuric acid and rinsing with distilled water.

6.2 The presence of slip, antistatic, or antiblocking and other additives can change the surface of the film since these additives leach out to the surface with time.

6.3 Finely embossed films add more variability to the measurements.

6.4 Polymer films are known to generate static charges when they are processed. Corona treatment usually exacerbates the problems. The film sample for surface measurement may carry the static charge generated in processing or may acquire a charge by friction when sliding over the specimen holder surface.

6.4.1 The static charges may cause errors in contact angle measurements. These errors occur as the electrostatic field generated by the charged sample pulls the water droplet to the film surface stronger than it would be in the absence of charges.

6.4.2 To prevent measurement errors, the film sample must be kept free of static buildup before and during measurements by placing an air ionizer, also known as a static eliminator, in the vicinity of the specimen holder. These devices produce air ions of positive and negative polarity. The charged object in the vicinity of the ionizer will attract ions of the opposite polarity and will be neutralized.

6.5 The transfer of a droplet onto the measured surface must be accomplished following the procedure described in Section 11. An improper transfer technique increases the variability of measurements.

6.6 The contact angle measuring method using tangential alignment of the cursor line of a protractor, as described in 3.1.2.1 (1), is biased due to the subjective nature of finding a tangent to the droplet image at the three-phase point. The smaller the measured contact angle, the larger the error. The bias of each operator's measurements must be determined if this method is to be used.

7. Apparatus

7.1 Contact Angle Meter, or Goniometer-The apparatus for measuring contact angles must contain the following: (1) a liquid dispenser capable of suspending a precise droplet of the specified volume from the tip of the dispenser, (2) a sample holder that can stretch a film sample flat without wrinkles and distortions, (3) provisions for bringing the sample toward the suspended droplet in a controlled manner to accomplish droplet transfer onto the measured surface, and (4) means for projecting a "silhouette" image of the drop with minimal distortions. The apparatus must have means for direct angle measurements, such as a protractor, or direct measurements of the droplet's dimensions, such as a grid or scale, or both, or a video camera with a video interface card to digitize the image of the droplet for subsequent calculations.

8. Reagents and Materials

8.1 Purity of Water-Deionized ultra filtered water should be used for testing purposes. The test liquid must be kept in scrupulously clean containers.

9. Sampling

9.1 The minimum amount of film required for this test is an approximately 25-mm wide and 300-mm long sample strip. If a sample is taken from a roll, the direction of the sample relative to the machine direction of the web must be noted.

9.2 Extreme care must be taken to prevent the surface of the film sample from being touched or handled in the areas upon which the test is to be made.

9.3 The number of measurements per film sample can be determined using published tables for sampling plans. Complete tables can be found in most books on quality control.⁸ The recommended number of readings per sample is ten.

9.4 For the purpose of determining the wetting tension profile across the width of the roll, one contact angle measurement can be taken for each inch of the sample.

10. Conditioning

10.1 Conditioning is not generally required for routine quality assurance (QA) or process control measurements since conditioning may affect the measured value and misrepresent the actual conditions.

10.2 Conditioning is required for interlaboratory measurements intended to compare the results. Condition the test specimens at 23 + 2°C (73.4 + 3.6°F) and 50 + 5 % relative humidity for not less than 40 h prior to testing in accordance with Procedure A of Practice D 618, for those tests in which conditioning is required. In cases of disagreement, the tolerances shall be +1°C (+1.8°F) and +2% relative humidity.

10.3 Test Conditions-Special test conditions are not generally required for routine QA or process control measurements.

10.4 Test Conditions for Interlaboratory Studies-Whenever possible, conduct tests in the standard laboratory atmosphere of 23 + 2°C (73.4 + 3.6°F) and 50 + 5% relative humidity, unless otherwise specified in the test methods or in this test method. In cases of disagreement, the tolerances shall be +1°C (+1.8F°) and +2% relative humidity.

11. Procedure

11.1 Place a sample strip onto the specimen holder of the instrument. Make sure the film sample is lying flat without wrinkles and distortions.

11.2 Suspend a 5 to 8-uL droplet at the end of a syringe needle. Bring the surface with the mounted sample upward until it touches the pendant drop. Than lower the surface with the sample to complete the droplet transfer. Do not drop or squirt droplets on the surface. See Fig. 1.

11.3 Follow the instrument manufacturer's instructions for a maximum time between dropping of the water droplet and measurement. Use the measurement techniques listed in 3.1.2.1.

11.4 Advance the sample to place the next droplet onto a previously untouched area.

11.5 Take ten contact angle measurements on the sample.

12. Calculation and Interpretation of Results

12.1 Calculation:
12.1.1 Calculate the average of the ten measurements.
12.1.2 Calculate the standard deviation of the measurements.
12.1.3 Example:

Average contact angle: 70°. This result corresponds to the lower end of the very high treatment level. Standard deviation: 2.4°.

12.2 Interpretation of Results-Look up the conversion chart to find the estimate of wetting tension range that can be attributed to the measured contact angle values.
12.3 Example-In the conversion chart in Appendix X2, the 70° contact angle corresponds to a wetting tension range of 40 to 46 mN/m.

13. Report

13.1 Report the instrument used, number of readings, average value of the contact angle measurements and standard deviation, wetting tension value found in the conversion chart, temperature, humidity, and any comments and observations.

14. Precision and Bias

14.1 Precision:

14.1.1 Instrument error depends on the instrument used. The instrument error can be assessed by measuring a uniform surface such as solidified paraffin wax and calculating the standard deviation of the measurements.

14.1.2 Precision of the Test-Table 1 is based on an interlaboratory study conducted in 1995 in accordance with Practice E 691 involving four materials and eight laboratories. Each test result was the average of ten individual determinations. Each laboratory obtained three test results for each material on two days. The results demonstrate that the precision of the results is generally independent of the treatment level (wetting tension) of the film.

Note 2: Caution-the following explanations of r and R are only intended to present a meaningful way of considering the approximate precision of this test method. The data in Table 1 should not be applied rigorously to the acceptance or rejection of material, as those data are specific to the interlaboratory study and may not be representative of other lots, conditions, materials, or laboratories. Users of this test method should apply the principles outlined in Practice E 691 to generate data specific to their laboratory and materials, or between specific laboratories. The principles of 14.1.3 through 14.2 would then be valid for such data.

14.1.3 Concept of r and R- If S and S have been calculated from a large enough body of data, and for test results that were averages of ten test determinations:

14.1.3.1 Repeatability, r (Single Operator)-The two test results should be judged not equivalent if they differ by more than the r contact angle degrees for that treatment level.

14.1.3.2 Reproducibility, R (Multilaboratory)-The two test results should be judged not equivalent if they differ by more than the R contact angle degrees for that treatment level.

14.1.3.3 Any judgment per 14.1.3.1 and 14.1.3.2 would have an approximate 95 % (0.95) probability of being correct.

14.2 Bias-No statement can be made regarding the bias of this test method because the contact angle is defined in terms of the test method.

15. Keywords

15.1 contact angle; contact angle meter; corona treatment; goniometer; polymer films; surface treatment; water; wetting tension

APPENDIX

(Nonmandatory Information)

X1. ADDITIONAL INFORMATION

X1.1 When a drop of liquid rests on the surface of a solid, and a gas is in contact with both, the forces acting at the interfaces must balance. These forces can be represented by various surface tensions acting in the direction of the surfaces, and it follows that

Y_GL ^cos0 = Y_GS - Y_SL

Where:
0 = angle of contact of the edge of the drop with the solid surface,
Y_GL = surface tension of the gas-liquid interface,
Y_GS = surface tension of the gas-solid interface, and
Y_SL = surface tension of the solid-liquid interface.
When the air is saturated with vapors of the liquid, Y_GL will be the surface tension of the liquid.
X1.2 For the polymer series described in 5.4, a unique condition exists in which films treated to different levels can be considered a homologous polymer series. For such a series, the relationship between the cos0 of water droplets and wetting tension, Yc was found to be linear:⁹

Y_c = k₁ Y_GL - ( 1 - cos0) / k₂

Where the parameters k₁ and k₂ are constants.

X1.2.1 In this form, the relationship can be used to estimate the wetting tension of corona-treated surfaces from the water contact angle data. In this case, the value YGL is faxed at 72 mN/m for water, while the contact angle and its cosine will be changing depending on the surface energy of the substrate.

X1.2.2 Since the wetting tension of a polymer film belongs to a group of physical parameters for which no standard to accuracy exists, a range of values for wetting tension corresponding to a given water contact angle value is used to correlate the contact angle data to the wetting tension. The values of k1 and k2 range from 0.75 to 0.9 for k1 and from 0.35 to 0.46 for k2. A conversion chart is given in Appendix X2.

X2. WATER CONTACT ANGLE TEST FOR WETTING TENSION (WCAT)

1. This test method is under the jurisdiction of ASTM Committee D-20 on Plastics and is the direct responsibility of Subcommittee D20.19 on Film and Sheeting. Current edition approved April 10, 1996. Published June 1996.
2. Annual Book of ASTM Standards, Vol 08.01.
3. Annual Book of ASTM Standards, Vol 15.09.
4. Annual Book of ASTM Standards, Vol 08.02.
5. Annual Book of ASTM Standards, Vol 14.02.
6. U.S. Patent No. 5 268 733.
7. Wu, S., Polymer Interace and Adhesion, Marcel Dekker, Inc., New York, NY, 1982.
8. Duncan, J.A., quality Control and Industrial Statistics, 3d Ed., Irwin, Homewood, IL, 1965.
9. Blitshteyn, M., "Wetting Tension Measurements on Corona-Treated Polymer Films," TAPPI Journal, Vol 78, No. 3, March 1995, pp. 138-143.

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