TPEs Meet 'Real World' Requirements for Replacing PVC in Tubing

PVC Tubing
Medical tubing shown with connectors. Teknor Apex has developed adhesive systems for bonding its Medalist medical elastomer-based tubing to traditional connectors.

While PVC has long been the material of choice for medical tubing, issues such as those focusing on the phthalate plasticizers used to make PVC flexible are causing some manufacturers to look for alternative materials. Until now the proposed replacements have failed to fully duplicate the advantages of PVC. Teknor Apex Company set out to develop Medalist MD-500 TPEs as the first fully practical alternative to PVC for tubing, providing equivalent or better performance at every stage of production and end use.

Basics: Flexibility, Clarity, Sterilization Resistance

Medalist TPEs exhibit values comparable to PVC for mechanical properties critical in tubing, such as tensile strength and tensile stress. Their specific gravity is 30% lower, indicating that a pound of TPE yields more linear feet of tubing than a pound of PVC. Here is how they compare in terms of basic functional properties:

  • Flexibility.  In PVC compounds this property is the result of high levels of plasticizer--as much as one-third of the formulation by weight. TPEs are inherently elastic.
     
  • Clarity. Medalist MD-500 compounds provide the same crystal clarity as PVC.
     
  • Resistance to sterilization. In tests for color shift following heat aging of gamma-irradiated samples, the Medalist compounds proved superior to PVC. Gamma exposure causes degradation of the PVC polymer, resulting in discoloration, embrittlement, and a falloff in mechanical properties. The Medalist compounds even surpassed special gamma-resistant grades of PVC, with heat-aged color shift values less than a third as great.

Everyday Clinical Considerations

Assembled tubing (made from Medalist medical elastomer) shows the results of post-extrusion processes, including hole punching, tipping, printing, and insert molding.

Among the greatest failings of previously proposed alternatives to PVC are those involving properties important in daily use by healthcare workers. Medalist MD-500 elastomers have exhibited performance comparable to PVC in:

  • Kink resistance. In many applications, tubing is used for infusion of fluids into the body. If the tube should be bent through a radius that is too tight, it collapses, with consequences that are potentially life-threatening.
     
  • Clamp resilience. To temporarily halt the flow of liquid, medical tubing is often clamped using either roller or hemostat clamps. Both compress the tube to block flow, and after release of pressure, the tube must recover and allow flow to resume in a short space of time.
     
  • Necking resistance. Necking is a phenomenon in which the diameter of a tube is permanently reduced when exposed to a longitudinal force.
     
  • Haptics or 'feel.' Most tubing sets are used manually, with the familiar feel of PVC being the accepted norm in the healthcare industry. In blind tests, experienced individuals from the device industry failed to distinguish between PVC and TPE tubing.

Extrusion and Fabrication

Teknor Apex has worked collaboratively with companies having expertise in medical tubing manufacture. In a demonstration run on a fully equipped, commercial-scale tubing extrusion line, equipment manufacturer American Kuhne Inc. achieved high-speed, tight-tolerance production of tubing from MD-585, an 85 Shore A Medalist compound. The company ran tubing with a 0.105 in. (2.67 mm) OD and 0.020 in. (0.508 mm) wall thickness at speeds up to 830 ft/min (253 m/min.). At a 600 ft/min (183 m/min) line speed, tolerances of +/- 0.0004 in. on the OD and +/- 0.0001 in. on the wall thickness were held.

To demonstrate the capabilities of Medalist MD-500-based tubing in post-extrusion processes, Teknor Apex worked with prominent medical device manufacturers Dunn Industries, Inc., which specializes in medical tubing, and Pelham Plastics Inc., with expertise in assembly techniques. The companies achieved good results in these techniques:

  • Cutting to length.This requires a wide processing window, close tolerances, and ease of cutting in-line with extrusion.
     
  • Hole punching posed no problems, even allowing for tube rotation producing a serpentine pattern; though this process is typically difficult for elastic materials because of stretching or tearing.
     
  • Tipping--the process of forming a tapered closure at the end of a tube--was carried out without need for pretreatment or release agents, which would require additional regulatory approvals. Cycles were fast, there was no sticking, tapers were consistent, and the aesthetic quality of the finished product was excellent.
     
  • Printing was carried out successfully using standard automated corona surface treatment and traditional pad printing with conventional inks. Teknor Apex suggested inks that could be used without the need for pre-treatment.
     
  • Insert molding, in which luers were applied to the ends of tubing, involved short cycle times, no tube distortion, and excellent bonding of the tubing to pre-colored luers made of either polypropylene or Medalist elastomers.
     
  • Attachment to standard connectors. Teknor Apex has developed patent-pending bonding systems involving both room temperature-cured and light-cured adhesives, enabling common-size TPE infusion tubing to achieve bonds exhibiting a retention force significantly greater than the minimum required by device manufacturers. In addition, Clean Room Devices LLC has developed an automatic fitting inserter that carries out high-speed assembly of tapered-barb connectors and tubing produced from Medalist MD-500 TPEs.

Elliott Pritikin is senior medical market manager for the Thermoplastic Elastomer Division of Teknor Apex Company.

 

Table 1.

Sterilization Stability: Gamma Irradiation

Property and Unit

Test Method

Tubing Compound

Medalist MD-575

Elastomer

General Purpose Flexible PVC

Gamma-Stable Flexible PVC

Specific gravity

ASTM D792

0.88

1.2

1.2

Hardness, Shore A

   (15 sec. delay)

ASTM D2240

72

75

75

Tensile strength, MPa

ASTM D412

(D638)

13.5

(14.8)

(14.8)

Tensile stress at 100%, MPa

ASTM D412

(D638)

4.3

(6.2)

(6.2)

Elongation at break, %

ASTM D412

(D638)

720

(420)

(420)

Post-gamma color shift, Delta E

   (dosage of 30 kGy)

CIELab

1.91

2.55

1.42

Post-gamme color shift, Delta E

   (heat-aged at 50ºC. for 48 hrs.)

CIELab

2.02

10.52

7.01

Table 2.

Kink Resistance in Outer Diameter

Tubing Sample

(4 mm OD; 2.5 mm ID)

Unit

Minimum Bend Diameter for Tubing Made from Three Medalist Compounds

MD-565

MD-575

MD-585

Before gamma irradiation

cm

1.43-1.59

1.43-1.59

1.43-1.59

After gamma

irradiation

cm

1.43-1.59

1.43-1.59

1.43-1.69

Table 3.

Functional Properties of Tubing Compounds

Property

Tubing Compounds

Medalist MD-500 Series

Traditional Alternatives to PVC

General-Purpose Flexible PVC

Gamma-Stable Flexible PVC

PVC-like haptics ('feel')

++

0

++

++

Clarity

++

0

++

++

Kink-resistance

+

0

+

+

Clamp resilience

++

0 / -

++

++

Tube necking

++

0 / -

++

++

Flexibility

++

+

+ / 0

+ / 0

Gamma stability after aging

+

0

0 / -

+ / 0

Key: 0 = Equivalent; + = Better; - = Worse

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