Choosing Rigid Tray Materials—Not an Open-and-Shut Case

Medical device makers must pay close attention to every detail when choosing a tray material.

by Erik Swain, Senior Editor

There are only a few materials commonly used for rigid medical trays. However, the available options are quite different from one another, so careful attention to many details is needed when choosing the right material for a specific use.

Before picking a material for a tray, a medical device manufacturer must consider, at a minimum, the anticipated characteristics of the formed tray: sterilization method compatibility, clarity, barrier, temperature distortion after sealing, compatibility with device materials, sealability, recyclability, shock and vibration resistance, environmental temperature and humidity resistance, and, of course, cost.

The decision-making process varies, depending on the device maker. A large, established company with a staff of packaging engineers probably already has in place a formula for determining what materials should be used for a specific product's trays. Smaller companies and start-ups are more likely to seek advice from thermoformers, material suppliers, and consultants.

Suppliers are more than happy to help. Many have a standard set of questions they pose to their customers, even if the customers have predetermined specifications. "It is still incumbent upon us to question their choice," says Dick Simmons, vice president—medical marketing of Plastofilm Industries Inc. (Wheaton, IL), an Ivex company. "We get to see everything, and we might know something that can save them money or processing time."

The three materials most commonly used in U.S. medical trays are Eastar PETG copolyester, high-impact polystyrene, and polyvinyl chloride (PVC). Other materials include XT acrylic multipolymer from Cyro Industries (Rockaway, NJ), Barex (acrylonitrile) from BP Chemicals (Warrensville Heights, OH), K-Resin (butadiene styrene) from Phillips Chemical Co. (Houston), high-density polyethylene (HDPE), polycarbonate, and polypropylene, each of which is more likely to be used when specific needs must be met.

In general, polystyrene is preferred when reducing costs is a significant issue, PVC when clarity is required but costs need to be contained, and PETG when clarity, recylability, and use with multiple kinds of sterilization are the primary factors. But generalities should never be relied upon—there are many specific factors that must be considered for each project.

MAJOR FACTORS

The first issue considered is generally sterilization compatibility. Most companies are already committed to one or several types, so if a material is not compatible with a type of sterilization, then it will not be chosen.

Part of PETG's popularity can be explained by its ability to be used with gamma, ethylene oxide (EtO), gas plasma, electron-beam, or hydrogen peroxide sterilization, says Jennifer Lauderback, senior technical services representative at Eastman Chemical Co. (Kingsport, TN), which is the exclusive supplier of raw PETG resin.

Another consideration is whether the package must be clear or opaque. In some applications, clarity is important, not only so that operating room personnel can verify what's in the package before opening it, but so that seal integrity can be verified visually.

PETG, PVC, and Barex are among the right materials if clarity is needed. If the tray needs to be opaque or colored, then polystyrene is the proper choice.

Compatibility with device materials must also be considered. If the device is unusable because it sticks to the side of the tray, it won't matter how much is saved on packaging costs.

"Some materials interact differently with different films," says Patrick Welch, sales manager for Barger Packaging Corp., a division of Welch Packaging Group (Elkhart, IN). "For example, tubing used nowadays in suction devices for surgical instruments can have a tendency to seal themselves to a PETG-type material during EtO sterilization when the cycles have elevated temperatures and humidity levels. In that case, we might suggest a more inert substance like Barex or XT polymer. But if it's just a rigid, injection-formed plastic part, in most cases we can use a PETG material."

Bob Brown, corporate director of marketing for Primex Plastics Corp. (Richmond, IN), adds that devices made of PVC cannot go in polystyrene trays because they stick to the trays. They stick because of plasticizer migration.

BARRIER

Another determining factor in tray material selection can be moisture vapor transfer rate (MVTR), an indication of moisture barrier, says David Rosten, product manager of Perfecseal Mankato's thermoforming division (Mankato, MN). Certain grades of PVC and PETG have lower rates than polystyrene, he says. But, he says, using Aclar, made by AlliedSignal (Morristown, NJ), in conjunction with PETG or polystyrene works for products such as artificial skin, which must lie in an alcohol bath and cannot withstand any outside moisture. The same result can be achieved by using PVC with polyvinylidene chloride (PVdC) or Aclar.

"Oxygen barrier can also be an issue," says John Campbell, business unit manager for Klöckner Pentaplast of America (Gordonsville, VA), which offers PVC, PETG, and Barex. "For this requirement, PVC/PVdC, Barex, and other multiple-structure films are commonly used. Barex is a monofilm with excellent oxygen barrier, chemical resistance, and formability," he says.

THE ENVIRONMENT

Some device manufacturers find that they can get a competitive advantage if they provide recyclable packages, so they choose PETG or HDPE, which are the only rigid medical plastics classified among the most recyclable plastics in the United States. PVC is also recyclable in some areas.

But trays can only be recycled if they do not come into contact with medical waste. Some healthcare practitioners are accustomed to using trays as medical waste bins, so device makers and hospital staff need to work together to actually institute a recycling program for trays.

 

 

 

  PSa PVCb PETGc XTd PCe
Yield
(sq in./lb/mil)
26,800 20,700 21,800 25,200 23,100
Clarity Milky Excellent Excellent Excellent Excellent
Oxygen
Barrier
Poor Good Good Good Poor
Water
Barrier
Poor Good to
Excellent*
Good to
Excellent*
Poor Poor
CO2
Barrier
Poor Good Fair Good Poor
Nitrogen
Barrier
Poor Good Good Good Poor
Compatible
Sterilization
Methods
Gamma,
EtO
EtO,
E-beam
Gamma, EtO,
gas plasma,
E-beam
hydrogen
peroxide
Gamma,
EtO, gas
plasma,
E-beam
Gamma,
heat
EtO
Transition
Temperature
175° F 170° F 185° F
Favorable
Properties
Formability,
stiffness
low cost
Clarity,
formability,
stiffness,
weather and
abrasion
resistance,
recyclability
low cost
Clarity
flexibility of
sterilization
recyclability,
formability,
widespread
current usage
Heat
resistance,
clarity
stiffness,
high
impact,
process-
ability,
formability
Clarity,
stiffness,
high
impact,
ignition
resistance,
strength
Unfavorable
Properties
Low barrier,
incompatible
with devices
made of
PVC
Inability to
use
gamma
sterilization
Price Unsuitable
for
application
requiring
UV stability
Low
Barrier
 
  Barexf K-Resing PPh HDPEi
Yield
(sq in./lb/mil)
24,000 26,000 30,800 29,000
Clarity Excellent Excellent Fair to
Excellent*
Poor
Oxygen
Barrier
Excellent Poor Poor Poor
Water
Barrier
Good Poor Excellent Excellent
CO2
Barrier
Excellent Poor Poor Poor
Nitrogen
Barrier
Excellent Good Poor Poor
Compatible
Sterilization
Methods
EtO,
E-beam,
lower-level
gamma
Gamma,
EtO,
E-Beam
Heat,
EtO
Gamma,
EtO
heat
Transition
Temperature
170°F
Favorable
Properties
Gas
barrier,
stiffnes,
formability,
clarity,
chemical
resistance,
steriliz-
ability,
denest-
ability
High
impact,
clarity,
chemical
resistance,
surface
hardness
Water
barrier,
high
impact,
low cost
chemical,
moisture,
and heat
resistance
High
impact,
moisture
resistance,
crack
resistance,
recyclability
Unfavorable
Properties
Price Incompatible
with devices
made of
PVC
Inability
to use
gamma,
low gas
barrier
Low gas
barrier
* Depending on grade.

a: polystyrene, b: polyvinyl chloride, c: Eastar copolyester, d: acrylic multipolymer, e: polycarbonate, f: acrylonitrile, g: butadiene styrene, h: polypropylene, i: high-density polyethylene.

Sources: Barger Packaging, Eastman Chemical, Klöckner Pentaplast, Perfecseal Mankato, Cyro Industries, Phillips Chemical, Bayer Corp.



 

Manufacturers who sell in Europe tend to shy away from using PVC because of opposition to it by some European government officials and activists. Waste disposal in Europe is done through incineration, and PVC opponents claim some forms of the material give off toxins when burned. PETG, by contrast, burns cleanly, which helps its popularity in Europe, Lauderback says.

PVC is recyclable, and it may burn more efficiently than opponents claim, Campbell says, citing a study conducted by the American Society of Mechanical Engineers a few years ago. Donald Barcan, president of DBI Inc. (Donbar Industries Inc.), a medical device package engineering consulting firm in Long Valley, NJ, agrees, stating that the charges against PVC may have been true for the original versions of PVC, but it has since been reformulated.

AVOIDING STRESS

Other issues that have been the subject of much discussion recently are the flange sealability and distortion of trays, says Barcan. "The sealability issues are directly related to process control and sealing validation, as defined in ISO 11607," he says. "Flange distortion is related to a combination of the heat energy required to seal the lid to the tray, the thickness of the flange, and the stresses in the sheet. If the sheet material and resultant flange thickness are too thin, distortion will probably occur. That can cause potential seal-integrity problems. If the formed tray is highly stressed and subjected to sterilization or distribution temperatures and humidity that approach the glass transition temperature of the polymer, then additional tray distortion will occur. The bottom line is having a thorough knowledge of the materials and forming conditions. These details must be included in the part specifications provided to the thermoformer."

Eastman has been working with Strainoptic Technologies Inc. (North Wales, PA) to help thermoformers accurately measure PETG tray stress, thereby optimizing the process while reducing cycle times and product failures. (For a full description of their work, see PMP News's October 1998 feature on trays.)

"Trays with more-intricate designs tend to contain more residual stress," Lauderback says. "Excessive amounts of stress can be the result of improper package design or forming the package using too cold a sheet temperature. The recommended sheet temperature needs to be at least 300°F. Generous radii or draft angles in the trays should also be used."

One problem that arises during denesting with some materials, especially PETG, is that the trays often stick to one another when stacked. Applying a light coating of silicone to the outer surface solves the problem for PETG, but the procedure should be done carefully. Barex is one material that does not require such a coating.

"In the infrequent case that the silicone is not uniformly applied, it could affect seal quality," says Carl Marotta, president of Tolas Health Care Packaging (Feasterville, PA). "This is a relatively minor occurrence, and advances in adhesives have made it a relatively minor problem." Improvements in the extrusion process have also helped solve the problem.

Device manufacturers should also determine each material's transfer temperature—the temperature at which its molecular structure begins to change. This is a particular concern during shipping, especially now that hospitals will often send products back to the manufacturer rather than stockpile inventory. The most temperature-resistant materials are XT polymer, polycarbonate, and polypropylene.

COST

While most device manufacturers make quality the first consideration in material selection, cost is never too far removed from the equation, especially in recent years as containing healthcare expenses has become a national priority.

"Polystyrene has the advantage there," says Bill Johnson, sales director of Alga Plastics Co. (Cranston, RI). "The supply is readily available, and it is available from more than one extruder."

As such, polystyrene seems to be the favorite material for commodity products, which sell in high volume and need a low price to gain a competitive edge. "If it's a low-end commodity suture or gauze tray, it does not go to PETG because that's too expensive," says Brian Meltzner, sales manager of Merrill's Packaging Inc. (Burlingame, CA).

PETG is aimed elsewhere instead—at the specialty market, the more-intricate, lower-volume devices, Lauderback of Eastman says. The increased cost is because of significant manufacturing modifications to the polymer to prevent crystallization and to make it easier to process, she says. "It is a specialty polymer for a specialty application," she says. "Packaging is a small part of the overall cost of a device, but if there is a packaging failure, large costs are incurred. If you look at price-to-performance ratio, PETG is a very good investment."

PVC, on the other hand, is marketed as a lower-cost alternative to PETG, offering the same clarity and formability, compatibility with EtO and E-beam sterilization, and nearly the same recyclability, according to Klöckner Pentaplast. The company also considers PETG, along with Barex, a specialty material.

Selecting the correct material is crucial. Once the material is chosen for a certain product, it is very costly to change it because new validation and stability tests would have to be performed. "Unless there's a major problem, you cannot dive in and change it very easily," says David Kurisko, staff engineer–R&D packaging at Ethicon Endo-Surgery Inc. (Cincinnati), which usually uses PETG for its trays, but has also used styrene and XT polymer. "The cost to change over is just too big."

For the same reasons, Smith & Nephew Inc. (Memphis) is locked in to using PETG for its trays, says Andrew Fronc, manager of packaging development–orthopedic division. "We have stability data, seal-strength test data, and real-time and 10-year accelerated aging data," he says. "We do a bit of experimenting with flexible materials, but not much as far as rigid goes."

CONCLUSION

The regulatory climate and the maturity of the U.S. market mean that there has not been much activity in developing new materials for rigid medical trays, those in industry say.

Rather, any innovations to be seen in the near future will be incremental ones—attempts to improve on the undesirable qualities of existing materials. "The biggest challenge is finding a more exotic material that withstands a variety of environmental factors but is less expensive," says Simmons of Plastofilm. "We have a lot already, but they are expensive compared to polystyrene, and price drives a lot of everything."

Eastman plans to work on incremental improvements to PETG, such as increasing its transfer temperature and coming up with improved denesting additives without silicone for the applications that cannot handle it, Lauderback says.

Regardless of what new developments occur, the quality of the materials is likely to keep improving. "The methods have improved and have brought up the quality of the product," Welch of Barger says. "We do not reject nearly as much raw material as we used to, and it's still basic PETG, Barex, and XT polymer."

No votes yet