Using Corona Beam Technology for Testing
By Gregory J. Gormley, President
ConverTec Corp. (Newtown, PA)
A novel technique locates anomalies in thin, flexible barrier materials.
|Figure 1. Corona beams flowing as a result of Griebel-Gormley Aperture Effect.
(click image to enlarge)
Electron beam technology can be used to determine the integrity, porosity, and permeability of conductive and nonconductive medical flexible barrier packaging materials. Materials include Tyvek, paper, plastic film, laminated composites, and adhesive-backed materials coated on metal foil.
The technology facilitates 100% real-time, in-line testing. It is nondestructive, noncontact, nonabrasive, and completely dry. Because the corona beam is drawn and not projected, it will follow a tortuous path through materials and containers, and it can curve to follow its anodic high-voltage potential in milliamps or microamps, which makes it nondestructive.
When used with electronic instrumentation and a nitrogen cover gas in an open atmosphere, the electron beam maintains its prescribed frequency and ionizes the gas to create a corona beam. This corona beam discharge, maintained at a high positive voltage, forms from the holes or anomalies in the flexible barrier material. If detected, the anomalies can be analyzed to determine the presence of viral- and sub-viral-sized voids or holes. Blisters, contaminants, stress fractures, overlapped materials, and formulation defects can also be found. These holes or voids may occur during barrier-product manufacturing, forming, or sealing.
The process can also be used to determine a material’s permeability. Flexible-barrier film manufacturers can perform this process to certify a specific quality level. Material fabricators can perform it to ensure quality standards for preformed materials. Product manufacturers can confirm the integrity of the final sealed package. There are many other packaging applications that can utilize this technology for film characterization and packaging validation and integrity testing.
Figure 2. Four-micron laser-drilled hole detected in FDA study. (Courtesy FDA)
Figure 3. SEM picture of a 2-micron formulation defect in blister pack material.
Figure 4. SEM picture of a 40-micron piece of dirt contamination in PTFE film.
Figure 5. SEM picture of a 30-micron formation defect in blister pack material.
Once the corona beam system’s foundation resources are in place, such as the power supply and digital controller, the system has modular add-on capability for additional test locations at a fraction of the initial component cost. There can be multiple test stations for many different aspects of the package process performing unrelated tasks. Determinations can range from the integrity of a glass vial to the presence of bubbles in liquid fills to packaging film validation. Gross over- or underfills and product presence can also be detected.
The test can be performed as a secondary operation if the material has to be cured in a secondary process. However, if the material is fully cured at the end of the process line, the product can be tested with a series of sensors that would account for the entire area of the product’s surface (100%) by a moving or stationary gantry with an array of beams each averaging a focal area of an eighth of an inch to a full half-inch diameter.
The method of reject removal from the process is critical, and many different options for handling are available. The idea is to eliminate the labor factors of rehandling the product in off-line statistical testing and keep the cost as low as possible by testing in-line.
The technology also has other unique applications. Since we can test for blisters and bubbles in flat material, a similar type of test, for validation, has evolved that provides for testing sealed material. After the product has been sealed in the pouch or blister and there has been an atmosphere change because of the addition of a head gas or a drawn vacuum, the atmosphere in the pouch, vial, syringe, etc., has been altered. The corona beam passing through the sealed container will provide a signal that validates that the gas has not escaped and that at that point in time there is a solid sealed package. The validation of the atmosphere inside a package provides a sure indication the thermal seals are correct.
In general terms, the integrity or presence of anomalies can be determined by using an electronic sensor in an open atmosphere under a fluid cover gas or a flow of cover gas. The cover gas is directed at the material and, if there is a small aperture, hole, or anomaly in the material, a change in the electric discharge or corona (also known as an electron beam, an electrostatic corona or a corona discharge) occurs. This change is measured by an AM radio sensor. The corona beam gun comprises an electrode and a sensing mechanism that records electrons that are drawn through the hole or anomaly in the barrier material. The sensor also contains a series of focusing resistors for attenuating the beam. The occurrence of this change in discharge is due to the below-described Griebel-Gormley Aperture Effect (referred to herein as the Aperture Effect; see Figure 1).
During the characterization of materials, a prescribed range of acceptability can be calibrated. This means that a window of desirably sized permeable material can be defined as a calibration standard. Items measured include porosity, permeability, and the consistency of the material signature. The corona beam carries an applied frequency that creates a consistent “noise” or “signature” when it passes through normal material that has a solid or permeable designed structure. The electronic corona beam with its imposed frequency will always flow through the structure of the test material. It is the destructive interference of the frequency moving through the material with the electronic corona beam that is digitally compared to the calibration standard for valuation.
The overall design of the system is based on the type of material being tested and the perceived or desired outcome required. Factors that relate to the quality and length of the electronic corona discharge (i.e., the corona beam) include the diameter of the anode needle’s tip in the corona beam sensor gun, the quality of the needle material (e.g., barium, platinum, gold, silver), and the heating of the anode and cathode needle tips. Other important factors are the dielectric quality of the material being tested, the type of defect that is being tested, and the operating parameters of the testing equipment for characterization, such as the frequency, the amplitude, the wave shape, and the voltage. The proper combination of these factors leads to the ability to detect and monitor sub-micron-sized permeability, porosity, apertures, holes, or anomalies in the test material.
The figures at left illustrate a samples of the sizes and types of anomalies that this technology pinpointed on a sheet of material so that SEM photos could further identify these types of anomalies, which are normally not detected (see Figures 2, 3, 4, and 5).
The Aperture Effect is based on the point-to-point effect, a well-known effect in physics. The point-to-point effect in practical terms is the passage of a static electrical charge from a cathode electrode to an anode electrode in an open atmosphere. (An example is how static electricity from a carpet in a dry room can be collected by your body and then discharged when you near a grounded item.) The Aperture Effect is shown by the use of a smooth, grounded cylindrical cathode electrode (i.e., approximately cylindrical) in proximity to a needle-tip anode electrode (a needle point). Very few electrons (or corona discharges) are discharged if the voltage is too low. But when the cathode is masked with a thin film material containing a very small void of material (a hole or anomaly), an electrical cathode electrode point is masked out on the grounded cathode. A point-to-point effect is created on a microscopic level, and electrons flow from the cathodic roller through the hole or anomaly in the thin film material to the anodic tip of the corona beam gun without increasing the applied voltage. This flow of electrons (the corona beam) carries a prescribed frequency that is measured to determine the amount of constructive and destructive changes occurring in the frequency.
It is important to understand that the corona electrons are always flowing through the material. The electrons of the corona beam do not crash or arc like static electric discharges. The corona electrons that flow through the material at the location that is predefined as “good material” create the material’s baseline signature. It is from this baseline signature that the range of acceptability is established. The amount of change can then be compared to a library of calibration readings contained in the digital controller system for comparative identification and analysis. The system can also be set to a range of acceptability for a pass-fail response. A tremendous amount of data can be produced and easily digitally filtered and processed in a presentable format.
A cover gas is also important in achieving the Aperture Effect. Typical cover gases include nitrogen, noncombustible gases, noble gases, and dehydrated air. The results vary with the particular cover gases used. The use of nitrogen, as opposed to dry air, neon, other noble gases, or other attractive sources, makes a dramatic difference. The flow rate and gas pressure are also important factors. The higher the gas pressure is, the more of a gas flow, and the beam lengthens. As the pressure increases, the gas becomes denser, and the electrons flowing from the cathode to the anode move more slowly. It should be noted that the beam may move or wander in the cover gas environment. The beam is self-seeking within the material’s focal area. The focal area is the circular area of the test material being hit by the fluid cover gas. Thus, the beam moves in the area of the material bounded by the fluid cover gas in order to locate the properly sized aperture or anomaly, and the pressure of the gas creates a focal area on the surface of the test material that can vary based on the required parameters.
Other important factors in the creation of the Aperture Effect are the power supply’s voltage, the frequency of the pulsed dc signal, and the distance from the cathode to the anode electrodes. Moreover, the distance between the cathode and the material being tested is an important factor in obtaining the Aperture Effect. If the material being tested is too far from the cathode (i.e., the cylindrical roller), the Aperture Effect will be lost. However, this can be alleviated when a conductive noble gas or other energy source is grounded and used to supplement the difference in the conformal space required between the material and the cylindrical mandrel.
|Figure 6. Laser beam versus corona beam technology.
(click image to enlarge)
The following figure is a simplified comparison of a corona beam and a laser beam. The laser beam uses a series of glass focusing lenses to draw the photon light energy to a point so that it can be used effectively to perform different operations. The beam is projected to the work piece so that, in a mechanical sense, it can cut, weld, etc. In the case of the corona beam, the electrons are drawn from the cylindrical cathode electrode side of the material to the anode electrode side with test material between the two electrodes. To create the corona beam’s Aperture Effect through the material, the relative dielectric weakness of the test point in the material in the focal area to the relatively higher dielectric strength of good solid material induces the flow of electrons of the corona beam. The raw energy from the power supply is drawn through a series of resistors to develop a very-high-density attenuated positive field at the end of the anodic needle tip inside the corona beam gun (see Figure 6).
Corona beam technology is available to validate the integrity of barrier and porous packaging films and test flexible barrier material 100% to an application standard that will be precise, certified, and confirmed to an exact calibration level, while simultaneously saving money and time.
This corona beam technology can be validated by the current standard mechanical tests that are used on a destructive or nondestructive statistical basis. So don’t throw out the tried and true methods of validation. They will still be needed to calibrate the corona beam technology’s validation systems. The fact of the matter is that you will need to use them much less.
The National Institutes of Health funded the original proof-of-concept model that was presented to a group of NIH and FDA scientists for comments on practical applications. FDA purchased the first prototype that was used in a study that conclusively (100%) discovered holes and anomalies in condoms from 1 µm and above in an approximate time of 1 second per item. NASA Tech Briefs NANO2005 Conference presented ConverTec with the Nanotech Briefs Nano 50 Award for its “realized or anticipated impact and accomplishments in the nanotechnology field.”
References Available Upon Request