Film Test Methods Thomas I. Butler Film Testing Testing of fabricated film is preferred to compression molded samples because they give results indicative of what a consumer might experience when using the product. The following sections will give a brief description of some of the more common film tests used in the polymer industry to predict performance in a specific application. If more information regarding a specific test is desired, an ASTM reference number is provided with each test. It is important to remember that orientation and crystallization, which will change with fabrication conditions, will also impact film properties. Film properties listed on data sheets from polymer suppliers are generally run under specific conditions, and are usually published with the film properties. TENSILE PROPERTIES ASTM D-882 A film specimen of certain specified dimensions is placed in the grips of a universal tester capable of constant crosshead speed and initial grip separation. The crosshead speed is determined based on the polymer being tested. Typically for LDPE and LLDPE films the crosshead speed is 20 in/min (500 mm/min) with a grip separation of 2 in (50 mm) for LDPE and LLDPE see Figure –1. HDPE films are typically run at 2 in/min (50 mm/min). The force as a function of time is measured using a load cell. The elongation is determined from the crosshead speed as a function of time. At least five samples are averaged to determine the tensile values for a film.
Figure –1
Tensile test
The sample is tested to the point of rupture. The force measured on the specimen to yield and rupture and the length to which it was stretched at these two points are recorded by computer (or by strip chart on older equipment). Using the width and thickness of the film sample the stress and strain are calculated (see Figure –2). Typically film tensile properties are measured in both the MD and CD. Tensile properties include yield tensile, ultimate tensile, elongation, and modulus are used to determine the relative strength and flexibility of films. Figure –2
Stress-Strain curve obtained from a tensile test.
Stress, psi Fracture
Ultimate Tensile Modulus
Strain hardening Necking
E
Yield C D B Strain softening Elastic A
Strain, in/in
Ultimate Elongation
The stress-strain contains a lot of information about the film. The initial slope of the curve is the region that recovers any deformation when the stress is removed and is represented by region A in Figure –2. In this region as shown in Figure –3 only the amorphous regions of the film morphology exhibit any deformation, but they are able to recover to their original dimension when the stress is removed. The crystalline regions of the film experience no deformation in this region. Figure –3
Elastic region of the stress-strain curve
Figure –4 shows where some strain softening begins to occur. Here the amorphous regions have begun to be extended enough to prevent full recovery when the stress is removed producing a change in dimension. The crystalline regions have not experienced any deformation. The maximum stress measured in this region is referred to as the tensile yield strength.
Figure – 4
Strain softening region of the stress-strain curve.
After the maximum stress is reached more amorphous deformation might occur as shown in Figure –5. Figure –5
Yield region of a stress-strain curve.
The necking region of the stress-strain curve is shown in Figure –6. Here the crystalline regions are beginning to experience deformation by slipping or sliding between folded chain structures. A long necking region usually indicated that there is significant crystalline orientation in this direction. It also implies that the film could be a good candidate for secondary orientation processes. Figure – 6
Necking region of a stress-strain curve.
The final region of the stress-strain curve is the strain hardening region as shown in Figure – 7. Deformation causes the crystalline regions that are connected to tie molecules begin to unfold rapidly, increasing the stress levels until the film ruptures. Figure –7
Strain hardening region of the stress-strain curve.
Tensile Yield Strength Yield strength measures the highest stress where a film, when deformed, will resume its original dimensions when the force is removed. The yield stress is expressed as force per original area (engineering stress) and is calculated per Equation (1) and the elongation at yield per equation (2). The tensile yield of a film is strongly correlated with the polymer density, but is also a function of the crystallization rates the film encountered during fabrication. Faster cooling will lower the yield stress. (1)
σy
=
F y / ( width * thickness),
psi (MPa)
(2)
εy
=
L y / (L o ) * 100 ,
%
Ultimate Tensile Strength Ultimate tensile is measurement of the force per original area where the film ruptured as shown in Equation (3). The ultimate tensile strength is used to determine the relative strength of the film. Film thickness is included in the calculation of ultimate tensile strength, however it is strongly influenced by orientation and
therefore the values can vary significantly even at the same film thickness. Increasing orientation in a film will increase the ultimate tensile strength of a film. Higher molecular weight and narrow MWD produce higher tensile strength. (3)
σT
=
F T / (width * thickness),
psi (MPa)
Ultimate Elongation Ultimate elongation is measurement of deformation per original length where the film ruptured as shown in Equation (4). Elongation is strongly influenced by orientation and therefore the values can vary significantly even at the same film thickness. Increasing orientation in a film will increase the ultimate tensile strength of a film. (4)
εT
=
L T / (L o ) * 100
%
Secant Modulus The elastic region also relates to the stiffness/flexibility of the film and is referred to as the secant modulus. The secant modulus is determined by the ratio of force to strain at either 1% or 2% elongation and reported as psi (MPa). Toughness The tensile toughness (tensile energy to break) is defined as the area under the stress-strain curve. The results are reported as ft-lb f /in3 (J/cm3). IMPACT STRENGTH PROPERTIES Dart Impact ASTM D-1709 The drop dart impact test is the most common method utilized for determining impact strength. A weighted round-headed dart is dropped onto a tightly clamped sheet of film, and the sample is examined for failures (tears or holes in the film) see Figure - 8. Enough drops of varying weights are made to determine the weight in grams for a 50 percent failure point. Figure – 8
Drop dart impact test equipment
Dart
Drop height -
Method A = 26 in Method B = 70 in
Report weight in grams of 50% failure Film clamp
Dart impact value is very dependent on film thickness since the test is sensitive to orientation effects of the film. The tackiness of the film surface (slip additives) and the condition of the dart head have also been shown to influence the values determined by this test. Since dart impact is not tested in any film direction,
it is sensitive to orientation effects in both directions. Best results are usually obtained when the orientation is balanced in the MD and CD. There are two methods used in drop dart impact testing: Method A drops the dart from a height of 26 inches (660 mm) above the film sample. The values will typically range between 40 grams and 1400 grams. This method works well for thin films (< 3 mils or < 75 microns). Method B drops the dart from a height of 60 inches (1524 mm). This method is used for thick films or films with very low crystallinity. The values from the two methods (A or B) can not be converted to the other method. Spencer Impact ASTM D-3420 The Spencer impact measures energy required to burst and penetrate the center of a clamped sample struck by a ball on a pendulum (an Elmendorf tear test unit). The loss in mechanical energy required to rupture the film is calculated using Equation (5). (5)
ES
=
(scale reading)/(10.2)
(J)
Total Energy Drop Dart ASTM-4272 Total energy drop dart (TEDD) determines the resistance of a film by measuring the residual energy of a free falling dart passing through the film. The apparatus is similar to the drop dart impact test with the addition of a timing mechanism. A sensor determines the time (t 1 ) for the dart to drop in free fall (no sample) for a specified distance (d), and the time (t 2 ) the dart takes to pass through the film sample. The key difference between Drop dart impact and TEDD is that in TEDD the mass (m) of the dart must be high enough to rupture the film and pass completely through the sample. The acceleration of the falling dart is determined using the gravitational constant (g). The energy required to rupture the sample is calculated using Equation (6). (6)
Er
=
(m/2)*((d2*((1/t 1 2))-((1/t 2 2)))+((g2/4)*((t 1 2-t 2 2)))
ft-lb f (J)
Puncture Resistance This procedure is not an ASTM test. The puncture resistance is a measure of the toughness of a film to a relative low velocity probe. A tensile tester unit is modified using a circular clamp to hold the film sample and a probe mounted on the load cell. The probe design is not standardized throughout the film industry so the results can not be compared. The probe is used to puncture the film at 10 in/min (39.4 mm/min) and the energy (E P ) (area under the stress-strain curve) is used to calculate the puncture resistance (see Equation (7). (7)
Pr
=
(E P / (thickness * sample area)
ft-lb f / in3 (J/cm3)
TEAR PROPERTIES Elmendorf Tear ASTM D-1922 Elmendorf tear strength is the force required for propagating a tear through a specified distance/direction (1.7 inch, 43 mm) with an initial slit across a semicircular specimen (see Figure –9). The
test is typically reported in both MD and CD. This test originally developed for the paper industry is used extensively in the plastic film industry. Elmendorf tear is dependent upon the orientation in the film, with higher tear values obtained when orientation is increased in the opposite direction. Elmendorf tear is reported in grams, will change with film fabrication conditions at the same film thickness. It has been noted that the average of MD and CD tear for a given polymer may be a constant. Therefore as fabrication conditions are changed to improve MD tear, the CD will decrease in tear strength. A special shaped film specimen is clamped in the grips of an apparatus, which has as its moving part a free-swinging pendulum of a certain weight. As the pendulum is released, the force of its weight is transferred to the specimen to propagate from the point of the initial slit. The force in grams required to fully tear the specimen is measured either manually or electronically. A minimum of ten (10) samples should be used to determine the average tear in each direction. Figure –9
Elmendorf Tear sample
PPT ASTM D-2582 Puncture-Propagation-Tear (PPT) test is used to measure the resistance of a film to tearing when snagged. Failures due to snagging hazards occur in many end use applications such as industrial liners and trash bags. The PPT test determines the dynamic tear resistance of plastic film. A falling weighted carriage (W) falling from a given height (H) 20 inches (508 mm) to puncture the film. The film tear is propagated and the distance (L) is measured. There are 5 tests run, and the results are averaged for both MD and CD. The tear resistance is calculated using Equation (8). (8)
F PPT
=
((W * H)/L) + W
lb f (Kg)
Tensile Tear ASTM D-1938 This test method covers the determination of the force required to propagate a tear in a plastic film using a tensile tester. A film specimen (1-inch x 3-inch) dimensions with a 2-inch slit is placed in the grips of a universal tester capable of constant crosshead speed (10 inch/min). The maximum force required to tear the sample is measured. At lest five samples are averaged to determine the tensile tear strength lb f (N) values for a film. OPTICAL PROPERTIES Gloss ASTM D-2457 The method is used measure gloss use a light from a standardized source lamp projected onto a film sample at a preset angle. The amount of light reflected by the film is measured and reported in gloss scale units relative to a specified mirror. The gloss can be measured at various preset angles of reflectance (20, 45, or 60) degrees. As the gloss of a product improves, more light is reflected and the
reported value increases. The lower angle measurements are best for high gloss film and the higher angles are use for lower gloss films. Spectral gloss correlates to the shine or sparkle of the film surface. This trait can impact desirability of consumers to purchase the film or product packaged within it. Polymers with a higher melt index, lower density, or a narrow MWD will generally produce films with better gloss. Gloss can also be optimized by adjustment of extrusion parameters. Generally higher melt temperatures produce films with higher gloss values. Surface melt fracture will lower gloss. Haze ASTM D-1003 This method utilizes an instrument called a hazemeter. The hazemeter is set up to transmit a beam of light (T t ) through a sample and measure the light that is diffused (T d ), or forward scattered more than 2.5o, from its original path. The results are reported in terms of percent haze (see Equation – 8). The lower the haze value, the better are the optical properties of the film. (8)
Haze
=
(T d /T t ) * 100
%
In many packaging applications good optical properties are desired. Haze of a film has been found to be a combination of (1) surface roughness and (2) polymer crystalline structure. Each of these will diffuse the light as it passes through the film increasing the haze. The internal haze can be measured by coating the film surfaces with oil of similar reflective index of the film. The surface haze is then determine using Equation – 9. (9)
Haze s
=
Haze t – Haze i
%
Extrusion parameters can also be optimized to improve haze. Higher melt temperature and larger BUR will usually improve haze values. Polymers with a higher MI, lower density, and narrow MWD will generally have lower haze values. Shrinkage ASTM D-2732 This test determines the degree of unrestrained linear thermal shrinkage of a film at a given temperature. A standard-size specimen (100 mm x 100 mm) is immersed in a temperature-controlled liquid for a specified length of time (10 sec) allowing the sample to shrink unrestrained. The sample is then removed and measured for changes in linear dimensions. Results are reported in percentage of shrinkage in both MD and CD directions (see equation –10). (10)
Shrinkage
=
((L o – L f ) / L o ) * 100
%
Extrusion parameters can change the internal stresses locked into the film. These stresses can be released when heated allowing the film to recover to a smaller dimension. The magnitude of the shrinkage will vary with the temperature of the film. When the film is heated to a high enough temperature and for a long enough time, all the residual stress will be released and the maximum shrinkage value obtained. Polymer properties that increase shrinkage include lower MI, broad MWD, and long chain branching. Lower density polymers tend to shrink at lower temperatures. Blocking ASTM D-3354 Blocking is the term given to describe the degree of adhesion existing between layers of film. The load required to separate a film is measured by a balance beam or load cell system. One sheet of a blocked specimen is secured to an aluminum block (100 cm2 area) suspended from the end of the load
cell. The other sheet is secured to an aluminum block fastened to a base. The force required to separate the two films is reported in grams. Polymer films having a smooth glossy surface will readily block or stick together. High levels of block make opening the package or bag very difficult. High winder tension, insufficient cooling, and high nip roll pressure are a few of the possible causes of blocking in film processes. Additives such as antiblock (diatomaceous earth, talc, or calcium carbonate) can be incorporated into the film to roughen up the surface, which reduces block. Coefficient of Friction ASTM D-1894 A metal block weighing (B) equipped with a sponge rubber pad is covered with the film specimen. The block is pulled across a plane covered with the same film at a constant speed. A load cell measures the frictional force (A) is used to determine the static (load at start) and kinetic (load during steady pulling) friction (see Equation – 11). (11)
µ
=
A/B
The COF is important in determining how that film will perform on conversion equipment and in final form such as in openability or stacking. This test is used to determine the level of additives required allowing efficient downstream processing of the film. Wetting Tension ASTM D-2570 The film surface is exposed to drops of mixtures of formamide and ethylene glycol monoethyl ether in the presence of air. Gradually increasing surface tension mixtures are applied to the film surface until a mixture is found that just wets the film surface. The surface tension of that mixture is equal to the wetting tension. The results are reported in dynes/cm. The ability of polyolefin films to retain inks, coatings, adhesives, etc. is dependent upon the surface tension of the surface and can be improved by surface-treating techniques such as corona discharge or flame treatment. Wetting tension is utilized to determine the degree or level of treatment applied by establishing a correlation between surface tension (wetting tension) and treatment level.
