Forming Barrier Materials for Blister Packages

How blister materials react to forming should be considered during material selection and equipment design.

by Daphne Allen, Editor

When selecting blister packaging materials for today's sensitive new drug formulations, packaging engineers seek materials that will provide the best moisture and oxygen barrier at an attractive cost. They have a lot to choose from, given the myriad combinations of PVC, PVdC, and Aclar (manufactured by Allied Signal Specialty Films) as well as foil and other materials.

In addition to barrier properties and cost, however, engineers must consider the forming characteristics and requirements of the materials. To successfully form and seal certain blister materials, engineers must understand how the materials react to certain processes. Such an understanding is key to producing blister packaging that doesn't crack or become unsealed during shipping or storage. For instance, if the film thickness is not consistent throughout the blister cavities or if the film separates from its lidding, then moisture, oxygen, or other elements will compromise drug integrity—no matter how good a barrier the unformed material itself provides.

To run certain materials, companies may need special tooling and equipment. Some materials require special mold coatings, and others need specific machinery modifications.

Fortunately, most new blister-forming machines are already equipped to handle a variety of materials with minimal tooling or equipment accessory changes. However, to understand how to utilize the features of the new machines, to effectively change over from one material to another, or to retrofit older machines to run new materials, engineers must work closely with material and equipment and tooling providers.

POLYVINYL CHLORIDE

Because of its favorable cost-to-barrier ratio and its compatibility with a number of other blister materials, polyvinyl chloride (PVC) is used widely for drug packaging. It is also the easiest blister material to form and to seal, say suppliers and users. "Calendered materials, such as PVC, are better suited for ease of use on pharmaceutical-style packaging machinery because of the control offered in the mechanical properties, such as shrinkage and gauge control," says Kent Sides, business unit manager of pharmaceutical films at Klöckner Pentaplast of America (Gordonsville, VA). He adds that PVC-based materials generally accept a wide range of heat-seal coatings for lidding materials and are easily perforated. According to Ulrich Frauchiger of Perlen Converting AG (Princeton, NJ, and Perlen, Switzerland), materials other than PVC are less tolerant to temperature and forming conditions, and they are also more sensitive to inadequate maintenance.

If engineers change from one type of PVC to another, John Blum, healthcare technical specialist for AlliedSignal Specialty Films (Morristown, NJ), recommends "adjusting thermoforming temperatures and perhaps the timing of the thermoformer to ensure that blister thickness is optimized."

POLYVINYL DICHLORIDE

Blister materials coated with polyvinyl dichloride (PVdC) are also easy to form, but because the PVdC tends to release a gas during forming that corrodes equipment and because it is sticky, all equipment intended to touch the PVdC needs to be coated. Bob Hartwig, director of sales and marketing for Uhlmann Packaging Systems Inc. (Towaco, NJ), says that European packagers have found a way to avoid problems related to the stickiness of PVdC. "They form the material with the PVdC layer on the inside of the blister rather than on the outside, where it directly touches molds and sealing plates."

U.S. packagers, however, have chosen to form PVdC-coated materials with the PVdC layer on the outside because of forming issues. "It is difficult to fill blisters with the PVdC layer on the inside because the capsules don't slide in easily—they stick to the PVdC layer," says Hartwig. If companies do decide to form PVdC as the Europeans do, Blum advises that packagers keep this difficulty in mind when determining the materials and shape of plug assists, so that the PVdC doesn't stick to the assists.

The U.S. industry has dealt with PVdC's stickiness by using release coatings made of Teflon or Teflon derivatives on preheat plates, mold dies, and sealing plates. John Hopkinson, general manager of Service Industries (Rolling Meadows, IL) explains that the industry standard has been to apply a hard coating of Teflon to the aluminum tooling. However, chemicals in the PVdC can wear down the coating, he says, causing the forming dies to deteriorate in less than a year. The use of a Teflon-impregnated ceramic coating, however, may be the answer. "We've been supplying the dies for PVdC for about a year now, and we have found no degradation," he says.

The coating may also prove to be cost-effective. "When Teflon hard coatings deteriorate, you can't recoat the tooling. You cannot remove the old coating because it actually penetrates the aluminum. Instead, you need to replace the tooling," says Hopkinson. Teflon-impregnated ceramic-coated tooling does cost more than the hard-coated dies, but depending on the design and layout of the forming die, most tooling can be recoated because the coating doesn't penetrate the aluminum. Also, using tooling that doesn't deteriorate often reduces machine downtime and keeps lines running at optimum speeds.

In addition to using the proper coating, packagers should use tooling made of materials that are suitable for aggressive acids, explains Perlen's Frauchiger. "V4A stainless steel will do the job," he says.

ACLAR

According to Blum, Aclar laminates using the new homopolymer materials Rx160, SupRx900, UltRx2000, UltRx3000, and the coextruded homopolymer Cx130 can be formed similarly to the carrier web to which they are laminated. Mold surfaces can be run with or without coatings. However, if the draft angles are very shallow or the part is complicated, using a textured or coated mold surface will help distribute material thickness across the die cavity and facilitate material removal from the mold.

Rexam's blister packaging materials satisfy a broad range of needs.
 

"Aclar does not melt or stick to the mold, but it has a high coefficient of friction, which may complicate mold release," says Blum. "Eject pins are recommended for the mold if the parts being formed have a severe geometry." In addition, preheat plates should be textured and should also have a slip coating on the surface to reduce drag through the heater section.

Blum says that molds and preheat plates typically used for PVdC should also work for Aclar. Cx130 will form under the same conditions as PVC. However, Aclar is a tough polymer and does not exhibit the notch sensitivity of PVC. So, when using Aclar, perforating dies must be set at a depth where the Aclar is being scored.

POLYPROPYLENE

Because of its high shrinkage ratio, polypropylene is one of the most difficult materials to form. "It has an unstable shrinkage factor, and, as a soft material, it tends to want to return to its original shape," says Steve Hamaday, director of engineering at Sharp (Conshohocken, PA). Uhlmann's Hartwig explains that polypropylene shrinks continuously on both sides, so there can be tremendous registration problems. To control shrinkage, Blum explains that polypropylene applications require preheat lengths three times the index length so that the temperature can be raised slowly and then taken to forming temperature at the last index. Molds must be redesigned to allow for the unusually high postforming shrinkage, and dedicated rails are required. Post-cooling is required after the molding and sealing. Other machinery additions include grippers and gripper combs necessary for holding the polypropylene in place.

The TLT 1400 thermoformer from Bosch/TL Systems features push-button changeover.
 

A few newer machines, such as those from Bosch/TL Systems (Minneapolis), Klöckner Medipak (Clearwater, FL), Romaco/Noack (Morris Plains, NJ), and Uhlmann, are supplied ready to form polypropylene. Sharp recently purchased two UPS 1070 thermoformers from Uhlmann, which already incorporated cooling plates into the machines. Older machines can be converted, however. According to Klöckner's Sides, the cost of such a conversion depends upon the size of the machine and tooling, ranging from as little as $15,000 to as much as $35,000.

Using polypropylene also means using more polymer. "Because of polypropylene's pliability and softness, film gauge must be increased compared with that of other polymers to retain blister rigidity," explains Sides.

CYCLIC OLEFIN COPOLYMER

According to Peter Schmitt, president of Montesino Associates (Wilmington, DE), cyclic olefin copolymer (COC) has excellent thermoformability, but it tends to be brittle on its own, so it is usually laminated to polypropylene to withstand forming. Blum says that it can even be laminated to Aclar. COC can be formed on most of today's high-end thermoformers, says Carsten Gruber of Bosch/TL Systems.

According to Nic Hunt, global product director of films at Rexam Medical Packaging (Mundelein, IL, and Bristol, United Kingdom), COC is a candidate for a lamination to cold-form foil as a sealant side because of its ease of forming.

FOIL

Because cold-formable foil (CFF) has an elongation percentage of only 15 to 18%, monolayer CFF cannot be formed without cracking. Consequently, the foil is laminated in a sandwich between two polymers. One commonly used structure is OPA/adhesive/aluminum/adhesive/PVC, explains David Sciubba, healthcare sales manager for Hueck Foils (Wall, NJ). The structure can be customized to meet specific forming or product contact layer requirements. OPA is used primarily for its forming capabilities.

According to Sciubba, the OPA "enhances the forming process due to its elasticity. As it stretches, it brings the aluminum with it to create the cold-formed cavity." He adds that special grades of aluminum have been developed for cold forming. These are designed to stretch and form more easily than conventional grades.

Sciubba explains that PVC is used for two reasons. First, it has been commonly used as the contact layer and seal-to layer on pharmaceutical blisters. The second reason is that PVC is very stiff, and does not tend to shrink or spring back during cold forming. Once the cold-forming process is complete, the PVC helps the cavity hold its shape.

Despite the addition of OPA, CFF still has forming limitations. "The CFF structure doesn't form easily, so blister cavity designs can't be too sharp or great because the foil will break," explains James Guida, pharmaceutical and healthcare products technology coordinator for Reynolds Metals (Richmond, VA). "The goal is to make sure that the material is stretched uniformly and that you don't have localized stretching built into the design of the cavity."

Because curves must be gradual, there is a significant increase in package size when comparing CFF packages with thermoformed packages, says Sides. The typical draft for the side walls is 50% of the depth at 30° and 50% at 45°. (Thermoformed materials can form well with as little as 3° to 4° drafts on the side walls of the cavities.) "As a rule of thumb, the width of the cavities for CFF is three times the depth for the formed part. These requirements may increase material usage by more than 60% in some cases," he explains.

However, to contend with this drawback, says Steve DiAngelis, Romaco's division manager of packaging, printing, and secuirty, Romaco/Noack implemented algroup wheaton pharmacenter shelbyville's (Shelbyville, KY) patented advanced forming technology (AFT) system on Noack 920 and 623-series blister packaging machines. AFT consists of a two-step process in which a flat punch first draws the aluminum laminate and then a Teflon step-plug forms the drawn cavities. The AFT process enables reduction of the CFF cavity size, and thereby the pack size, by up to 20% while using the same aluminum laminate as in conventional aluminum cold forming. In addition to material savings, the AFT process results in increased output in many applications when, for example, it enables five packs instead of four to fit across the machine's web.

If a company is using an older thermoformer that was not originally designed to cold form, a cold-forming station will need to be added. Most new blister formers, however, are already equipped to run CFF, and only a few modifications need to be made to begin forming. For instance, "when changing to aluminum, different sized parts and forming plugs will need to be added," says Gruber.

To prevent CFF from slipping around and to control stretching, Hopkinson says that precision-ground gripper bars are commonly used to hold the material in place. These have all but replaced rubber O-rings, the more economical but unreliable-over-time mechanisms of steadying the material. "Rubber tends to lose its tackiness, so it can lose its ability to hold the foil in place," he says. The gripper bars have a knurled surface that enables them to lock the material in place. "You want all the stretching to take place in the cavity, not anywhere else," says Guida.

In addition to the forming station and mechanisms for steadying CFF, Blum recommends adding inspection systems to ensure that no pinholes are produced during forming. Additional equipment may be needed to ensure that each cavity is filled with product, because inspection systems suitable for transparent films do not work unmodified with all-aluminum packages, says Montesino's Schmitt.

SEALING

According to Bosch's Gruber, both rotary and platen sealing systems handle all blister packaging materials very well. However, each method may offer some specific benefits when it comes to certain material combinations or package sizes.

For high-speed running, Blum feels that rotary sealing with preheats has a few advantages over platen sealing. "With multilayer child-resistant lid stocks, it is difficult to conduct heat through to the sealant layer. Platen sealing does this by increasing dwell time. On the other hand, rotary sealers can preheat the sealant surface with radiant heat or hot air. With child-resistant lid stocks, it is also difficult to release air through platen sealing. If the upper sealing tool is patterned to release air, the heating is localized to the contact points only. Flat or matted sealing tools should be used for these lid materials."

Klöckner's Pentapharm vinyl is designed for pharmaceutical blister packaging.
 

But Klöckner's Sides says that rotary equipment may have difficulty with child-resistant packaging that uses supported lidding materials composed of paper/adhesive/polyester/adhesive/aluminum/heat-seal coating. "Due to the complex makeup of the materials and the multiple layers involved, rotary equipment has a more difficult time sealing these products. The heat-seal coatings are commonly the same as those used in push-through applications, but are heavily insulated from the heat source in the sealing operation. These structures may require a combination of lidding preheaters and higher sealing temperatures or lower operating speeds."

Gruber explains that most of the currently available rotary sealers can be supplied with optional lid-stock preheaters for use with paper-laminated lidding as well as with polypropylene lidding.

Gruber points to other advantages of rotary sealers. "Rotary systems offer twice the sealing density and tightness when compared with platen-type machines because of their higher sealing pressure, i.e., linear contact compared with area contact. Also, because of their continuous motion and higher speed, the product is not exposed to heat as long as it is exposed in platen systems. Finally, platen-type systems sometimes create damaged cavities because of incorrect registration. Materials cannot get out of register in a rotary system because the male blister cavities are always in the female sealing roller cavities, ensuring positive control."

However, rotary sealers do not provide adequate dwell time for some adhesive systems to bond to the blister, Blum adds. "Rotary sealers do not provide uniform sealing pressure across the card as the loading is fixed but the surface area of contact is continually changing."

Hartwig explains that platen systems are better suited to seal large blister cards and physician samples. "Blisters don't come out as flat on rotary sealers as they do on platen sealers. This is a problem for blister styles commonly used in the United States, which tend to be larger than those used in Europe," he explains. Also, blisters with reinforcing ribs, such as physician samples, "don't go around the curves of a rotary drum too easily." Blisters that do not lie flat after sealing can often present problems during cartoning.

According to Sciubba, lidding foils are designed to seal on both platen and rotary sealers. "The key issue is to ensure that the materials receive the proper amount of heat—via the combination of temperature, pressure, and sealing dwell time—to activate the heat-seal coating on the lidding material," he says. To help packagers determine the strength and temperature needed to seal foil lidding structures to blister films, Hueck Foils provides customers with a heat seal curve, which shows strength versus temperature at a fixed pressure and dwell time (see Figure 1). "This curve, while never an exact representation of the performance of each individual machine, can act as a guideline or troubleshooting guide for proper sealing," Sciubba explains. The firm can create one that more closely represents actual conditions by using the exact blister film, dwell time, and pressure.


Figure 1. Seal strength by temperature for a child-resistant foil consisting of paper/adhesive/PET/adhesive/foil sealed to PVC blister film. Dwell time is 1 second, sealing pressure is 340.5 psi, and sealing plane is 0.465 in2. Source: Hueck Foils


To achieve a better seal, packagers who employ platen sealing will often use seal plates with knurled patterns. "Knurling was developed to maximize sealing surface areas. The knurling pattern etched into tools seals and traps air in the seal, reducing the likelihood of the occurrence of snakes or bubbles, which can cause leaks or work their way back into the cavity," Hopkinson says.

 

Lid/Web
Materials
Platen Environment
Seal Load Upper Die Knurl Lower Die Knurl
Foil/Foil 2–4 tn x  
4–8 tn x  
Paper/Foil 2–4 tn   x
4–8 tn   x
Foil/Films 2–4 tn x  
4–8 tn x  
Paper/Films 2–4 tn x x
4–8 tn   x
CR/Film 2–4 tn x x
4–8 tn   x


Table I. Guideline for determining when to use knurled dies for upper and lower sealing platens. Source: Service Industries, originally presented at HCPC's Seventh Annual National Symposium on Patient Compliance.

However, packagers must choose a pattern suitable for their combination of lidding and blister. For instance, if a foil-to-foil or a foil-to-film combination is used, engineers may not want to have both the top and bottom plates knurled, because the etched patterns that press on the combination from both sides could puncture the package, explains Hopkinson. For advice on when to use knurled upper and lower sealing plates, see Table I.

COMMUNICATING WITH SUPPLIERS

Many suppliers offer special services designed especially to help packagers investigate new material combinations as well as improve their current operations. For instance, Klöckner Pentaplast has set up a Packaging Technology Center at its Gordonsville, VA, headquarters. "We can conduct tests to help customers evaluate the performance of alternate materials and tooling designs as well as perform troubleshooting to save their production downtime. The center also allows us to test newly developed films internally and bring them to market faster," says Klöckner's president Tom Goeke.

A number of engineers have already attended seminars at the center, including Jim Greene, QC supervisor at NCS Healthcare/Vanguard Labs (Glasgow, KY). "We were considering using a 10-mil PVC with an amber tint and looking at forming and sealing temperatures," explains Greene. "We needed to find out how to raise the core temperature of the PVC to forming temperature and how to validate the temperature ranges." During the seminar, Greene and his team also sought to determine the proper forming parameters for Aclar Cx130 with a two-ply PVC. "We discovered that we needed to run different temperatures for each platen. One side needs to run 10°C cooler because of Aclar's high shrinkage ratio."

On the tooling side, many equipment providers, such as Uhlmann, offer design assistance and tool production. Third-party tooling manufacturers, such as Hopkinson's firm, also work closely with packaging engineers. Regardless of where an engineer obtains tooling, Hopkinson recommends getting the tooling provider involved early on in material selection. "It is important to share technical information with all suppliers. The last thing you want to do is to purchase material and tooling and then find out that the tooling is too aggressive for the film type," says Hopkinson.

Engineers may also benefit from knowing how a material is formed. While the manner in which a material is produced may not affect its forming characteristics, Rexam's Hunt explains that the fewer processes materials are subjected to during converting, the fewer the chances there are to induce material variability. If materials are consistent, engineers will have to make fewer machinery adjustments. To minimize such processes, Perlen recently invested in a four-head coating system for the production of its PVdC-coated films. "There is less wear on the material when it goes through the machine only once," explains David Faghani, Perlen's sales and marketing director.

Sophisticated packaging engineers may already be well acquainted with the forming requirements of today's commonly used blister packaging materials. After all, PVC has been used for blister packaging for decades. However, new combinations of materials, such as new ways of applying PVdC to PVC, or new formulations like the new Aclar Cx130, may form differently than older combinations, so it is important to work closely with material suppliers and converters as well as with equipment and tooling providers.

Opening photo by Roni Ramos

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