Features

Flexible polyvinyl chloride (PVC) is the most common material used to manufacture clear flexible tubing for medical applications. Flexible PVC medical tubing contains plasticizer, the component added to PVC to impart flexibility to the inherently rigid PVC polymer. If this plasticized PVC compound is in direct contact with another polymer surface, the plasticizer can interact with the surface of that polymer and can often result in marring and or cracking of that surface. This interaction between the plasticizer in the plasticized PVC compound and the surface of a second plastic part can lead to reduced physical properties and, in some cases, catastrophic failure of the second plastic part in contact with the flexible PVC.

Table 1 – Summary of plasticizer-induced stress cracking observations at 23°C.
Polycarbonate is a clear polymer commonly used in medical applications. Its rigidity and clarity in conjunction with its history of safe use in medical applications are valued characteristics. However, the lower molecular weight grades of polycarbonate typically used for injection molding applications are known to be susceptible to plasticizer-induced stress cracking when in contact with flexible PVC.

Rigid PVC is another polymer used in medical applications, though not to the level of polycarbonate. When properly formulated, its rigidity and clarity make it a candidate for use in many of the applications where polycarbonates are presently used. Our experience indicated that rigid PVC would be more resistant to plasticizer-induced stress cracking than polycarbonate and we decided to validate this observation.

Adding Stress Increases Cracking

Research shows that when stress is introduced into polymer part regions in contact with plasticized PVC, plasticizer-induced stress cracking will occur much more rapidly than if the region of the part in contact with the flexible PVC is not under stress. This stress can either be in the form of a mechanical force placed on the part while in service, residual stress embedded into the plastic part during its fabrication, or both.

In cases where the stress is introduced into the part during fabrication, the stress can often be released by annealing the part in question. It was also observed that annealing polymer parts prior to placing them in contact with flexible PVC reduces their tendency to incur stress cracking after contact is initiated. However, post fabrication annealing is an added manufacturing step, difficult to do in some cases, and sometimes introduces undesired tradeoffs such as brittleness and surface deformation. Therefore, post-fabrication annealing is not commonly performed on these parts.

Table 1 describes the physical properties of the rigid PVC and the polycarbonate on which we performed plasticizer-induced stress crack testing.

The hardness, tensile modulus, and flexural modulus of the rigid PVC grade and polycarbonate grade are quite similar. The Izod impact—the ASTM standard method of determining the impact resistance of materials—of the rigid PVC grade we tested is lower than that of the polycarbonate grade tested. However, injection molding grades of rigid PVC similar to that of the rigid clear PVC tested are available with Izod impact strength equal to that of the polycarbonate grade tested.

Experiment

Six 70 Shore A (15 sec) durometer flexible PVC formulations simulating flexible PVC medical tubing formulations were prepared. These formulations were milled for 5 minutes on a two-roll mill at 325°F then pressed into 6" x 6" x 0.075" plaques for 5 minutes at 325°F. After the plaques were pressed, they were cut into 6" x 0.5" x 0.075" strips for later use as the flexible PVC compound aggressor compound for the subsequent plasticizer stress crack testing of the rigid PVC and polycarbonate test specimens.

ASTM D-638 Type 1 tensile bar specimens of rigid PVC and polycarbonate of 0.125" thickness were injection molded on a Shinwa DL 110-IQ injection molding machine manufactured by Shinwa Seiki Co. Ltd. The process melt temperature for the rigid PVC compound was 195°C and the mold temperature was 49°C. The process melt temperature of the polycarbonate was 304°C and the mold temperature was 82°C. These resulting molded specimens are depicted in Figure 1.

Strips of the various flexible PVC formulations were placed in direct contact with the surface of the rigid PVC and polycarbonate test specimens. These rigid test specimens were then stressed by affixing them with metal binder clips to a metal fixture that imparted a 3% strain upon the specimens. This resulted in creating a test where a layer of 70 Shore A (15 sec dwell) flexible PVC compound containing a specific plasticizer was in direct contact with the stressed outer arc of the rigid PVC and polycarbonate test specimens.

Once these samples were affixed to the mandrel, visual observations were made at periodic intervals to determine each material’s susceptibility to plasticizer-induced stress cracking. Room-temperature testing was carried out in a constant temperature room at 23°C and 50% relative humidity (RH). Observations of plasticizer-induced stress cracking were made at intervals ranging from 1 hour to 28 days and recorded.

Results

Table 2 – Physical property comparison of polycarbonate and rigid PVC.
The data in Table 2 shows that the rigid polycarbonate samples are far more prone to stress cracking than the rigid PVC sample in our test protocol. The dioctyl adipate (DOA) plasticized formulation began showing visible evidence of inducing stress cracking in the rigid polycarbonate test specimen within 0.2 hours and demonstrated significant stress cracking within 0.5 hours of initiating the test. The DOA plasticized formulation began showing visible evidence of stress cracking the rigid PVC formulation within six hours of initiating the test and significant stress cracking was observed when the sample was viewed after 24 hours.

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