Performance Properties of Metallocene Polyethylene, EVA, and Flexible PVC Films

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published September 1998

TECHNICAL PAPER SERIES

As manufacturers of medical devices and supplies continue efforts to conserve resources and reduce waste, they are seeking new materials that can meet application needs while providing opportunities for volume or weight reduction. Fabricators are looking for ways to reduce the thickness, weight, or volume of device components without sacrificing the structural integrity or functionality of the device. For disposable devices and supplies, a reduction in raw materials will result in a direct reduction of waste.

Flexible polyvinyl chloride (PVC) is one of the largest-volume medical film materials, and as such provides a significant opportunity for both raw material and waste reduction. Because PVC film provides a wide array of functional performance characteristics at a low cost, any potential replacement material will need to provide similar performance at a comparable total system cost. Among the materials that have been introduced as alternatives to flexible PVC are films made with metallocene polyethylene (mPE) or ethylene-vinyl acetate (EVA) resins.

It has been demonstrated in a previous study that mPE films offer a number of attractive performance attributes, including superior tensile strength, elongation, and toughness, as well as excellent resistance to puncture, impact, and bursting.¹ EVA films have also been introduced as alternatives to flexible PVC for medical device applications, primarily because they can be fabricated with radio-frequency (RF) sealing equipment. Films made with EVA have been promoted as combining toughness and low-temperature sealability with clarity, flexibility, and impact and puncture resistance.^2,3

This study was designed to evaluate the performance properties of mPE and EVA films and compare them with those of PVC film. The properties were studied over a range of film thicknesses to determine the effect that thinning the films has on performance. The objective was to determine if either of the alternative materials could allow for the manufacture of thinner films that would still meet the requirements of medical device applications.

EXPERIMENTAL PROCEDURE

Three types of film were evaluated in this study, as described in Table I. They included an mPE film, an EVA film, and a PVC film. The mPE film (MDF 7200; The Dow Chemical Co., Midland, MI) was a cast, embossed film with a density of 0.905 g/cm³, and was evaluated in a range of thicknesses from 0.07 to 0.35 mm (3 to 14 mil). The medical-grade PVC and EVA films (E30-194 and EVA 1800; Ellay, Inc., City of Commerce, CA), were evaluated in thicknesses ranging from 0.15 to 0.33 mm (6 to 13 mil). Because all films were produced with a fine embossed finish on one side, thickness was reported as a nominal thickness per ASTM E 252.⁴

Standard physical properties including tensile strength, elongation, toughness, modulus, tear resistance, and barrier characteristics were evaluated for all films. The films were conditioned according to ASTM D 882 and then tested following the ASTM methods indicated in Tables II–IV.

Table II. Physical properties of mPE films.
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Table III. Physical properties of EVA films.
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Table IV. Physical properties of PVC films.
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Additional tests were conducted to examine other attributes that are important to film performance in many medical device applications. When devices are sterilized, they are exposed to moderately elevated temperatures; EtO sterilization, for example, can involve cycles of 12 to 24 hours at temperatures ranging from 54° to 60°C (130° to 140°F).⁵ Transportation and storage conditions can also involve elevated temperatures. If multiple film plies of a medical device are in contact with one another when the device is exposed to elevated temperatures, the layers can have a tendency to stick or "block." This can result in a device not functioning as designed—for example, a collection bag may not fill with liquid because the bag can't open easily.

In order to test this tendency of the films to block, and to demonstrate differences among film types and surfaces, a blocking load by parallel plate test (ASTM D 3354) was conducted. Two 10 x 15-cm plies of film were positioned between two 10-cm² glass plates, and a load of 1816 g was placed on the top plate to create a force of 18.2 g/cm² (0.25 psi). The films were tested first with the embossed surface as the inner contact layer of the pair, and a second time with the smooth side of the film as the inner contact surface, since both configurations can occur during device fabrication. Following oven-aging at 57°C under load for 24 hours, the films were cooled for 2 hours. The film plies were subsequently attached to the 10-cm² fixture on a Kayeness blocking tester and pulled apart in a direction perpendicular to the plane of the film-ply interface with a force that increased at a rate of 90 g/min. The force required to completely separate the two plies of film was then recorded.

Another important performance property for many medical devices is the extent to which a material acts as a barrier to the transmission of oxygen, water vapor, and other gases. Oxygen- and moisture-transmission rates are often erroneously used to predict the transmission rates of other gases or odors. The transmission rate of a particular substance through a polymer film is related to the solubility and diffusivity of the permeant in the specific film; therefore, the transmission of another gas or of odors may not be related to the transmission rate of oxygen.⁶

The ability of a film to serve as a barrier to odors can be important in applications such as ostomy bags or urological collection bags. To evaluate the barrier performance of the films to an odor-causing substance, a test for the transmission rate of ammonia—a compound that often accumulates in medical collection bags—was developed. Since it has been shown that the permeability of NH₃ gas and of aqueous ammonia NH₄OH solutions are equivalent, this test method evaluates the permeation rate of aqueous ammonia to represent the barrier to ammonia gas or odor.⁷ A 10-cm² pouch was made from each film type, filled with a 5% solution of aqueous ammonia, and sealed and placed in 1 L of water. The water was periodically titrated with HCl, and the transmission rate of ammonia calculated from the bag area and the amount of HCl used to neutralize the water after a given diffusion time.

RESULTS

The results of the physical property tests are shown in Tables II, III, and IV for the mPE, EVA, and PVC films, respectively. Plots of selected properties are shown in Figures 1–4. The results of the secant modulus test were similar for the three film types, indicating that the mPE and EVA films have flexibility similar to that of the PVC films and would be suitable for flexible device applications.

Figure 1. Ultimate tensile strength versus film thickness. (Data shown for machine direction test only.)

Tensile tests showed that although the yield strengths of the tested film types were similar, there were significant differences among the ultimate tensile strengths of the films. As Figure 1 illustrates, the ultimate tensile strength of the mPE film is somewhat constant across the range of thicknesses tested, and is about 20% greater than that of the EVA film and 25% greater than the PVC film. Both the EVA or PVC films decreased slightly in ultimate tensile strength as film thickness increased. This indicates that a device fabricated with an mPE film can have greater strength than a similar device made with EVA or PVC film, and that a much thinner mPE film could be used without sacrificing strength.

Figure 2. Tear-propagation resistance versus film thickness. (Elmendorf tear test; data shown for machine direction only.)

Significant differences among the films were also demonstrated by the results of the tear resistance test, and are illustrated in Figure 2. Resistance to tear propagation for all tested films increased as film thickness increased, although the mPE films increased at a greater rate than the EVA and PVC films. The tear resistance of the mPE film is about twice that of the EVA and PVC films at the thinner gauges tested, and is about three times stronger in the thicker films tested. For an application requiring strong tear resistance, one could, without reducing performance, employ a much thinner mPE film as compared with either an EVA or a PVC film.

Figure 3. Film blocking resistance versus thickness (24 hours at 57°C under 0.25-psi load).

Figure 4. Film permeability (normalized for thickness).

The tendency of a film to block is related to its composition and surface characteristics. The potential for blocking can be reduced by embossing the surface of the film to minimize the surface area that is in contact with an adjacent film layer, or by adding small particulate solids (e.g., silicon dioxide) to the film, which modifies the surface roughness. The results of the resistance-to-blocking test demonstrated both differences among the film types and differences resulting from the type of surface finish on the film. For the temperature and pressure conditions used in this test, neither the mPE or EVA film was susceptible to blocking when the film plies were tested with the embossed surfaces in contact with one another. When the PVC film was tested with the embossed layers in contact, the plies stuck together and, depending on film thickness, required between 40 and 164 g to pull apart the 100-cm² test area. For the PVC films, the blocking force increased as the film thickness decreased.

As a means of evaluating the effect of the embossed surface on resistance to blocking, the film samples were tested under the same conditions as above, but with the smooth surfaces in contact with one another. All of the film types tested exhibited a tendency to block when the smooth surfaces were in contact; however, the mPE film was much more resistant to blocking than the EVA or PVC film. The mPE film required a negligible amount of force to separate the smooth film plies (10–60 g), as compared with the PVC film (100–210 g) and the significantly adhered EVA film (175–225 g). The force required to separate the PVC film increased rapidly as film thickness decreased, while that of the mPE and EVA films decreased slightly with decreasing film thickness. The data demonstrate that the tendency for a medical device film to block during sterilization, transportation, or other exposure to elevated temperatures could be substantially reduced by using an mPE film and by embossing the surfaces that are in mutual contact.

For a specific film type, the permeation rates to oxygen, water vapor, and ammonia are inversely related to film thickness: as the film thickness decreases, the transmission rates increase. This relationship is approximately linear throughout the thickness range tested, so that a normalized permeation rate can be calculated for each film type. The mPE film tested had a water-vapor transmission rate that was approximately 4.5 times lower than the PVC film and 3.5 times lower than the EVA film (a lower transmission rate indicates a better barrier to the permeant). Normalized for thickness, the water-vapor transmission rate of the mPE film was 0.96 g•mm/m²/day at 37°C and 90% RH, as compared with 4.2 g for PVC and 3.5 g for EVA films. The PVC film had the lowest oxygen transmission rate, about 2 times lower than the EVA film and about 2.7 times lower than the mPE film. The oxygen transmission rate, expressed as cm³•mm/m²/day at 23°C and 1 atm, was 130 for the PVC film as compared with 280 for the EVA film and 390 for the mPE film.

The data on ammonia permeability show that oxygen transmission rates cannot be used as an indicator of the permeability of other gases or odors. Although the PVC film was a better barrier to oxygen compared with the mPE film (by approximately 3 times), the mPE film was the best barrier to ammonia (about 1.5 times better than PVC and 2 times better than EVA film). These results illustrate the need to test barrier properties with a permeant related to the needs of a specific application, and indicate that mPE film would be a better choice than PVC in an application in which a barrier to ammonia odor was important.

CONCLUSION

Results of this study demonstrate that mPE and EVA films can be suitable alternatives to PVC film for some applications, and that the mPE film can provide a number of properties superior to those of the other film types, including tensile strength, tear resistance, resistance to blocking, and barriers to water vapor and ammonia. It was also shown that the use of mPE can allow for significantly thinner films than with EVA or PVC, without sacrificing performance.

In addition to the properties noted, mPE film provides many other attributes that can make it suitable for certain medical device applications, such as excellent flexibility over a wide temperature range, good cold-crack and pinhole resistance, low-temperature toughness and impact resistance, and the ability to be sterilized by EtO or by gamma with no discoloration.

The fabrication of devices with mPE film can be accomplished through many types of operations, including heat and impulse sealing, adhesive bonding, and thermoforming. An mPE film can also be welded using RF welding equipment that has been modified with a catalyst film. Strong welds and tear seals can be produced by this method.

EVA film can be fabricated with conventional RF welding equipment, provided that the total vinyl acetate content in the film is above 14% (and preferably above 19%).⁸ Although higher vinyl acetate content contributes to more-effective RF welding, it can also reduce some performance properties, such as strength, barrier to some permeants, and resistance to blocking. EVA and mPE films are not suitable for autoclave sterilization, since they have melting points lower than 121°C.

Because the density of mPE film is about 30% lower than PVC film and 4% lower than EVA film, a lighter weight product that provides more film area per pound can be produced. In addition, the superior physical properties of mPE film allow for a reduction in film thickness of 25% or more as compared with PVC film, with no sacrifice in performance. This thinner, lighter film can meet performance specifications while reducing the volume of material required, making the end product lighter to ship and creating less waste material for disposable devices.

REFERENCES

1. Lipsitt B, "Metallocene Polyethylene Films as Alternatives to Flexible PVC Film for Medical Device Fabrication," in Proceedings of the Society of Plastics Engineers, Inc., Annual Technical Conference (ANTEC 97), Brookfield, CT, SPE, pp 2854–2858, 1997.

2. Technical Data Sheet for Elvax 3165 EVA Resin, Wilmington, DE, DuPont Co., 1996.

3. Technical Literature for EVA 1800 Film, City of Commerce, CA, Ellay, 1997.

4. Storer R (ed), Annual Book of ASTM Standards, West Conshohockem, PA, ASTM, 1993.

5. Guideline for Industrial Ethylene Oxide Sterilization of Medical Devices, ANSI/AAMI ST27, Arlington, VA, AAMI, 1988.

6. Vieth WR, Diffusion in and through Polymers: Principles and Applications, Munich, Hanser, 1991.

7. Shterenzon A, et al., Transfer of Electrolytes through Polymer Films, Mater, USSR, Lakokrasoch, 1969.

8. Desmarais R, Method of welding thermoplastic film, U.S. Pat. No. 5,449,428, 1995.

Bruce Lipsitt is the development leader for the Specialty Olefin Films Technical Service and Development (TS&D) of The Dow Chemical Co. (Midland, MI), with responsibilities that include the identification and development of new film applications.

Copyright ©1998 Medical Plastics and Biomaterials