Originally Published PMPN September 2001
Donald Barcan, president of Donbar Industries Inc., and Tom Adams, consultant, Sonoscan Inc.
Recently there has been a growing interest in acoustic micro imaging (AMI) for characterizing the bonds that preserve the integrity of pharmaceutical and medical packages. In AMI, an image is produced by pulsing high-frequency ultrasound into a package seal and collecting the echoes from the various depths within the seal. The resulting image shows both defect and non-defect internal features.
The transparency of a given material to light is unrelated to its transparency to ultrasound. Polymers, foils, and other commonly used packaging materials are all good transmitters of ultrasound. What matters in making an acoustic image are the specific properties of each material in a package. (Each material has its own acoustic velocity—the speed at which sound travels through it—and its own density.) The product of these two values is the acoustic impedance of the material.
When high-frequency ultrasound is pulsed into a material, echoes are returned only from interfaces between materials. The degree of reflection can be calculated from the acoustic impedance change between materials. The extreme case is a gap-type defect such as a void, delamination, or disbond. These defects are filled with a gas (air) rather than a solid. Their density is therefore far lower than the density of any solid. This results in an abnormally low acoustic impedance value, which in turn means that gap-type internal defects reflect virtually all of the ultrasound. Well-bonded material interfaces reflect only a portion of the ultrasound; the rest travels deeper into the package seal to be reflected from deeper material interfaces.
As a result of this interaction with materials, AMI visually displays voids, delaminations, disbonds, and other gas-filled defects; defects where a solid material is in the wrong location; and variations in bond or seal strength. These internal features are critical for package integrity.
In order to determine the utility of AMI in the inspection of medical and pharmaceutical packaging seals, a series of test packages with induced defects was created. The materials used were PET/PE sealed to uncoated Tyvek, PET/foil/PE sealed to PET/foil/PE, and PET/PE film sealed to PET/PE film.
Each sample was heat-sealed along an area about 4 in. long and 0.25 in. wide. To create an induced defect, a strip of Teflon glass tape 0.375 in. wide was inserted across the seal before sealing. The sample was then heat-sealed, and the glass tape was withdrawn to create a void or disbond that would be filled with air and would therefore reflect ultrasound strongly.
After sealing, each of the samples was imaged by AMI using ultrasonic frequencies ranging from 10 to 30 MHz. As a general rule, ultrasonic frequencies above 200 MHz produce higher image resolution and are capable of detecting defects having diameters of 5–10 µm. Typically, defects that cause concern in medical packages have diameters around 0.001 in., which requires ultrasonic frequencies in the 10 to 50 MHz range.
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| Figure 1: Acoustic image of PET/PE film heat-sealed to uncoated Tyvek shows an induced defect (an air gap) at the center. |
The acoustic image of the sample, where the film was heat-sealed to the Tyvek, is shown in Figure 1. The induced defect and the large nonbonded regions above and below it appear bright because they reflect virtually all of the ultrasound pulsed into the sample. At either end of the bottom seal, the bonded area, which appears dark gray, joins and lies on top of the package's side seals.
The bonded area, however, does not reflect ultrasound evenly—the edges are dark and the center is light. The variation in gray scale represents a significant variation in reflection intensity of the ultrasound and therefore of bond quality. One of the values of AMI is that it can nondestructively image such variations before normal process drift causes them to become outright defects during production.
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| Figure 2: The dark areas in this acoustic image of PET/foil/PE sealed to PET/foil/PE indicate bad bond quality. |
Figure 2 is the acoustic image of the defect induced in the foil-on-foil material. The following observations can be made:
- The gray area of the seal is rounded where it meets the induced defect. This may suggest a difference in heat transfer properties in the foil material.
- As in the previous sample, there is some variation in the gray level of the seal itself. That is, there is some variation in bond quality.
- A small defect (white) is present near the left end of the seal.
The white area on the outside of the defect at left is a void that reflects all of the ultrasound and therefore appears bright. The dark feature at the center of the defect may be a foreign particle that prevented local bonding and created the void.
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| Figure 3. Magnified view of the induced defect in a PET/PE sealed to a PET/PE sample. |
Figure 3 shows the results of bonding PET/PE film to PET/PE film. The dark bonded areas in this sample show little variation, indicating good bond quality. The two dark vertical lines crossing the induced defect are surface features at the top of the sample where the thickness of the glass tape caused a ridge to form. The small defect (white) to the left of the induced defect is a void.
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| Figure 4. Another portion of the PET/PE sealed to a PET/PE sample shows weak bond areas (light gray and white). |
Figure 4 is the acoustic image of the PET/PE to PET/PE sample in an area away from the induced defect. The package seal is referred to as a "triple-track seal." The parallel curved lines are heat-sealed bonds, and the edge of the package is at the top of the picture. There is considerable variation in the quality of the bond. Some areas of the bonds are nearly white, indicating poor bond quality. There are also small dark bonded areas outside of the intended bonds.
The acoustic images for each sample were produced by a standard Sonoscan C-SAM AMI instrument, in which the ultrasonic transducer scans over the package while pulsing and receiving ultrasound several thousand times per second. These are all planar x–y images of the bond interface. Other AMI modes produce nondestructive cross-sections and Thru-Scan images. Produced by ultrasound traveling completely through the package, Thru-Scan images show defects at all depths within the package.
The bonded areas in pharmaceutical and medical packages may contain several layers involving different materials. One member of a foil-foil package, for example, may consist of an external PET layer, an adhesive bonding the PET to the foil, the foil, and an inner polyethylene adhesive and/or a peelable layer. The assembled package thus consists of 8 or 9 layers, although the 2 polyethylene layers typically fuse during bonding. If the package contains a separate peelable layer, that layer would be visible acoustically as well.
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| Figure 5: This acoustic image of the edge seal on a foil/foil pouch shows numerous voids between the foil layers. |
Because return echoes arrive at the transducer at slightly different times, AMI can use electronic gating to limit imaging to a narrowly defined depth within a package. The resulting image displays only features at the gated depth. For critical work, gating could be on the bond of the PET adhesive to the foil—an important depth to image in a case where the foil has ruptured and created a leak.
AMI is useful during package development to establish those process parameters that will avoid defects during production, as batch sampling during production, and for failure analysis of field returns. Most of the packages that have come into Sonoscan's applications laboratory during the past year are R&D samples, or in some instances, samples from late-stage R&D, where a package has been adopted but is experiencing failures during production.
Copyright ©2001 Pharmaceutical & Medical Packaging News
