To measure the properties of Tyvek®, DuPont uses test methods that are based on recognized industry methodologies created by organizations such as ASTM, ISO, CEN, TAPPI and AATCC. Many of the methods developed to measure properties of porous packaging materials were developed for paper. As a result, DuPont has modified some of these methods to allow them to work with Tyvek®. Additionally, the use of internal methods based on those of organizations such as ASTM and ISO allows for the assessment of the impact on test results for any standardized test method modification.
Basis weight is the measure of the weight per unit area of the sheet, typically expressed in oz/yd2, g/m2, or lb/3,000 ft2 ream. For example, Tyvek® 1073B has a basis weight of 2.20 oz/yd2, 74.6 g/m2, or 45.8 lb/ream. The reference standards are ASTM D3776 and EN ISO 536, both with modified sample size.
Delamination is the measure of the internal bonding level of a given substrate. It is the weakest point of any substrate, which is almost exactly in the middle for Tyvek®. This property is very important for process control of medical packaging. To measure it, a split is initiated in a 1-in. (2.5-cm) wide sample. The split is the starting point to peel the layers apart. The average force to continue the peel is measured using a tensile tester. Results are given in lbf/in. or N/2.54 cm. The reference standard is ASTM D2724, modified for speed and gauge length.
Elongation is the measure of the extent a substrate will stretch before it breaks. The units are a percentage (%) of sample length. For example, a 10-in. (25-cm) sample of a substrate that has 20% elongation will stretch 2 in. (5 cm) before breaking. Total elongation, or how much energy the substrate can absorb (i.e., resiliency), relates to the protective nature of the material. The initial slope of the stress-strain curve, known as the initial modulus of the material, relates to how rapidly the substrate elongates when initial forces are applied. In web-fed equipment, the initial modulus also relates to how much the material resists loss of registration.
Elongation is measured by taking a 1-in. x 8-in. (2.5-cm x 20-cm) strip of product, clamping it so that 5 in. to 6 in. (13 cm to 15 cm) are between the jaws of a tensile tester, then applying force to the ends until the sample breaks. The reference standards are ASTM D5035 and EN ISO 1924-2, both modified for speed and gauge length.
Hydrostatic head is the measure of the pressure required to force three drops of water through a substrate. It is converted to the height of a column of water, which corresponds to the pressure. The units are typically inches (in.) or centimeters (cm). This property is impacted by the substrate’s affinity for water. For Tyvek® medical and pharmaceutical packaging styles, the surface energy is 32 dynes/cm to 25 dynes/cm. The reference standards are AATCC TM 127 and EN 20811, with rate of use set at 60 cm H2O/min.
Microbial barrier is the measure of the ability of a porous substrate to prevent bacterial spore penetration. One standard test method, ASTM F1608, measures the “filtration” efficiency of a substrate to remove spores from an aerosol being forced through the substrate in an air stream.
A completely impermeable control sample (microbial penetration is zero) is challenged with one million or 106 colony forming units (cfu). The number of cfu 106 has a log10 value of 6. If a sample challenged in the same way as the control allows 10 cfu (log10 10 = 1) to penetrate, then its log reduction value (LRV) is 5 (6 - 1 = 5). Therefore, the higher the LRV, the more resistant the packaging is to bacteria and microorganisms. Tyvek® 1073B has an LRV of 5.2 and is the best porous substrate available for medical packaging.
There are two disadvantages associated with ASTM F1608. The first is that the test method’s air flow rate significantly exceeds the air flow rate seen during typical distribution of a medical device. The second disadvantage is that it takes a long time to incubate the spores to get a count of how many spores penetrated the test material.
ASTM F2638 eliminates both of these problems. It’s a real-time test, eliminating the need to incubate spores. It functions by counting inert particles as they penetrate the barrier material. More importantly, the air flow rates are close to those experienced during transportation, eliminating the other disadvantage. This test method also varies the flow rate and thereby generates a penetration curve. On this penetration curve, most substrates tested have a maximum. Therefore, it is possible to report a pMax, which is the maximum penetration for the given substrate. The flow rate at which the maximum occurs depends on the mass, fiber diameter and density of the substrate.
Moisture Vapor Transmission Rate (MVTR)
MVTR is the measure at which moisture vapor is transmitted through a sample. There are several different manufacturers of MVTR equipment. It is important to note that MVTR results are highly dependent on the test method used and the material type. Important variables between test methods include: pressure gradient; volume of air space between liquid and sheet sample; temperature; air flow speed over the sample; and test procedure. Therefore, the results are not comparable from one company to another, nor between different pieces of equipment. The reference standard is TAPPI T523. The test conditions used by DuPont are 73°F (23°C), 85% relative humidity.
Mullen burst is a measure of the ability of a substrate to resist forces applied uniformly throughout the substrate. This property indicates how a package may perform in environments where pressure changes take place and the package balloons or where a force is applied over a relatively large area, such as when a heavy object is placed on top of a lidded tray. Mullen burst is measured by clamping a sample in a ring stand and expanding a diaphragm under the sample until the sample ruptures in the weakest spot. The pressure in the diaphragm (psi or kPa) is then recorded.
Because Tyvek® is isotropic (exhibits the same value when measured along axes in all directions), it has a very high Mullen burst for a low weight material. Mullen burst is proportional to the basis weight of the material, the bonding level, and to some extent, the elongation. The Mullen burst value increases as these three property values increase. The reference standards are ASTM D774 and ISO 2758.
Opacity measures how much light passes through a substrate. It is a ratio of the reflected light through a sample with a white and a black background. If the reflected light is the same from both backgrounds, then the opacity is 100%. A white background without any sample reflects 100% of light, while a black background has zero reflectance. The opacity of Tyvek® depends on the basis weight and bonding level. Because Tyvek® medical and pharmaceutical packaging styles are highly bonded, the opacity is relatively low. Opacifiers, such as TiO2 used in Tyvek® 2FS™, improve visual appearance and bar code readability. The reference standards are TAPPI T425 and ISO 2471, modified for different backing standards, area and illumination.
Porosity measures the ability of a substrate to permit flow of air at a given pressure differential. In the United States, the Gurley Hill method is used. In Europe and most of the rest of the world, the Bendtsen air permeability method is used. The Gurley Hill method measures the time to pass 100 cc of air through 1 in.2 (6.45 cm2) of sample at a pressure of approximately 5 in. (13 cm) of water. The Bendtsen method measures the actual flow rate of air in mL/min through a 10 cm2 sample at a pressure differential of 1.5 kPa (6 in. of water).
Porosity is important for gas sterilization processes to ensure that a sufficient amount of sterilant saturates the package in a short time and that the subsequent flushing and aeration of any residuals of the sterilant are efficient. Porosity also allows the packages to equilibrate rapidly from the pressure changes that occur in sterilization, shipping and storage environments. If any material in the device develops an odor after gamma radiation, the porous material allows the odor to vent so that none is evident when the package is opened. The reference standards for Gurley Hill porosity are TAPPI T460 (modified sample size) and ISO 5636-5 (modified for sealing fluid characteristics). The reference standard for Bendtsen air permeability is ISO 5636-3.
Spencer puncture is the method for determining the impact resistance of plastic films and packaging materials under conditions that closely approximate the strain rate that these materials are subject to in the healthcare industry. This property indicates how a package will perform if an object falls on the package or if an object in the package strikes the lid. DuPont uses procedure B of ASTM D3420, modified with a 9/16-in. (14.28-mm) diameter hemispheric-shaped probe tip with a 6,400-gram pendulum, which is necessary to puncture tough materials like Tyvek®. Results using different test apparatus are not comparable.
Tear is the measure of the ability of a substrate to resist tearing when a highly localized force is applied. Elmendorf tear measures the energy required to propagate an initiated tear for a unit distance. The units are lbf or Newtons. This property is important because nicks and cuts may occur at the edge of a lid and could affect its clean peel. The tear strength of Tyvek® is significantly higher than that of medical-grade paper. The reference standards are ASTM D1424 and EN 21974.
Tensile strength is the measure of the ability of a substrate to resist loads in the plane of the sheet. The units are lbf/in. or N/2.54 cm. Along with elongation, tensile determines the ability of a material to absorb energy before failure. Tensile is measured by taking a 1-in. x 8-in. (2.5-cm x 20-cm) strip of product, clamping it so that 6 in. (15 cm) are between the jaws of a tensile tester, and then applying force to the ends until the sample breaks. The reference standards are ASTM D5035 and EN ISO 1924-2, both modified for speed and gauge length.
Thickness is measured by placing the material on a hard, flat surface and determining the distance from the base using a presser foot that is parallel to the base and applied to the top surface of the material. The pressure applied to the presser foot depends on the material being measured. The measurement corresponds to the highest spot covered by the presser foot. The larger the cross-sectional area of the presser foot, the greater the chance to pick the highest spots on the sheet. For this reason, the average thickness value measured for a sheet is lower as the area of the presser foot decreases. The standard deviation for thickness is approximately 1 mil (25 µm). The reference standards are ASTM D1777 (modified to use 7.15 psi pressure and 0.625-in. diameter presser foot), EN 20534 (modified to use 14.5 psi [100 kPa] pressure and 2 cm2 surface), and EN ISO 534.