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.

Within seven days, all of the stressed rigid polycarbonate samples in contact with the flexible PVC formulations had begun to show evidence of stress cracking with the exception of those in contact with the trioctyl trimellitate (TOTM) or epoxidized soybean oil (ESO) plasticized formulations plasticized. After 28 days of testing at room temperature, only the polycarbonate specimen in contact with the ESO plasticized flexible PVC did not exhibit signs of plasticizer-induced stress cracking. Rigid polycarbonate specimens in contact with flexible PVC strips plasticized with DOA and acetyl tributyl citrate (ATBC) had broken in the test fixture within the first week of testing.

After seven days of exposure, only the DOA plasticized PVC compound had induced visible stress cracking in the rigid PVC test specimens. The ATBC plasticized PVC compound was able to induce visible stress cracking in the rigid PVC test specimens within 14 days of exposure.

Flexible PVC compounds plasticized with dioctyl phthalate (DOP) and dioctyl terephthalate (DOTP) were able to induce stress cracking in the rigid PVC test specimens within 28 days of exposure. The flexible PVC compounds plasticized with TOTM and ESO were unable to induce stress cracking in the rigid PVC test specimens after 28 days of exposure. Unlike the polycarbonate samples, none of the rigid PVC test specimens had broken under stress in the test fixture. Due to rigid polycarbonate test specimen breakage, we were unable to obtain 14-day and 28-day plasticizer-induced stress cracking data on most of the polycarbonate samples.

The data in Table 2 demonstrates a surprising correlation between plasticizer viscosity and that plasticizer’s ability to induce stress cracking in both rigid PVC and polycarbonate. The order in which the plasticizers studied induced stress cracking in the polycarbonate test specimens matched the order of plasticizer viscosity from lowest to highest. Although the rigid PVC stress cracking data did not differentiate between DOP and DOTP nor between TOTM and ESO, at no time does a plasticizer with higher viscosity induce stress cracking in the rigid PVC or polycarbonate test specimens more rapidly than a plasticizer with lower viscosity.

Historical Context

In 1984, Emilia Lacatus and James W. Summers of the BFGoodrich Chemical Group proposed a mechanism for plasticizer-induced stress cracking of rigid PVC predicated on the ability of the plasticizer to migrate into microvoids on the surface of the rigid PVC polymer. Their research paper, “Stress Cracking of Rigid Polyvinyl Chloride by Plasticizer Migration,” was published in the Journal of Vinyl Technology, and concluded the following: “The rate of plasticizer caused stress cracking depends on plasticizer molecular size and viscosity with small molecules and lower viscosity causing faster failure.”

The migration of plasticizer into microvoids on the rigid PVC surface is a critical component of Lacatus’ and Summers’ proposed stress cracking mechanism. It would be logical that plasticizer viscosity would be a driving factor in this mechanism, and we definitely observed that lower viscosity plasticizers induce stress cracking in both rigid PVC and polycarbonate more rapidly than higher viscosity plasticizers. Although Lacatus and Summers limited their plasticizer- induced stress cracking mechanism to rigid PVC, their observations also appear applicable to other rigid amorphous polymers such as polycarbonate.

Attempts to correlate Hansen Solubility Parameters with the relative ability of these plasticizers to induce stress cracking in rigid PVC and polycarbonate were unsuccessful. We suspect that plasticizer solubility in the rigid amorphous plastic may be a factor; however, we believe that viscosity is a more significant factor and this interferes with our ability to differentiate plasticizer-induced stress crack performance of these plastics based on solubility parameters.

Stress Cracking Mechanism

Here is a description of what we believe the stress cracking mechanism is for amorphous polymers based on our observations and literature searches:

Step 1: Plasticizer reaches a defect site on the surface of the stressed part.

Step 2: Plasticizer migrates into the defect site on the surface of the stressed part. This is one of the ways that lower plasticizer viscosity can accelerate the stress cracking mechanism of amorphous polymers.

Step 3: The region at the bottom of the defect site is subject to stress concentrations up to 50 times higher than the surrounding regions. This leads to a phenomenon known as stress enhanced fluid absorption, which increases the rate of plasticizer absorption into this region. As the plasticizer absorbs into this region of stress concentration, the polymer chains begin to align in the direction of stress.

Step 4: The polymer chains in the stress concentrated region begin to relieve stress by pulling apart from one another. This widens the defect and creates a new region of stress concentration deeper into the part where stress enhanced fluid absorption can now take place. Steps 3 and 4 can now repeat.

We proceeded under the Lacatus and Summers assumption that plasticizer migrating into microvoids or surface defects lowers the surface energy required for crazing and cracking of the rigid PVC test specimens due to the stress enhanced fluid absorption phenomena. Based on this stress cracking mechanism, we evaluated two different environmental stress crack (ESC) mitigation technologies that we believed would reduce the ability of plasticizers to induce stress cracking in rigid PVC test specimens under stressed conditions.

Fig. 1 – Test specimens exposed to 70 Shore A (15 sec) DOA plasticized flexible PVC compound for 24 hours at 23°C under 3 percent strain.

Both formulation technologies were incorporated into modified versions of the original rigid PVC formulation initially evaluated at various levels. We found that both technologies were successful in slowing the rate at which DOA plasticized flexible PVC compound induced stress cracking in rigid PVC formulations when evaluated at 23°C. As shown in Figure 1, the test specimens were exposed to 70 Shore A (15 sec) DOA plasticized flexible PVC compound for 24 hours. The material on the left is a polycarbonate, while the other three materials are different rigid PVCs.

Conclusions

It was observed that stressed injection molded specimens fabricated from the injection molding grade of polycarbonate designed for medical applications were significantly more prone to plasticizer-induced stress cracking than stressed injection molded specimens fabricated from the clear rigid PVC compound at 23°C based on our test protocol.

There appears to be a direct correlation between a plasticizer’s viscosity and its ability to induce stress cracking in both rigid PVC and polycarbonate. Plasticizers with lower viscosities, such as DOA and ATBC, were found to more readily induce stress cracking in the rigid PVC and polycarbonate test specimens in our test protocol than plasticizers with higher viscosities, such as TOTM and ESO. Attempts to correlate the relative ability of these plasticizers to induce stress cracking of polycarbonate and rigid PVC with Hansen Solubility Parameters were unsuccessful.

The mechanism for plasticizer-induced stress cracking originally proposed by Lacatus and Summers is supported by our observations and appears to be applicable for other rigid amorphous polymers such as polycarbonate.

Two rigid PVC formulation technologies designed to minimize the impact of plasticizer-induced stress cracking were evaluated. Both formulation technologies, when used at sufficient levels in rigid PVC, yielded significant improvements in the ability of these formulations to resist plasticizer-induced stress cracking. These stress crack resistant (SCR) technologies show great promise for use in applications where plasticizer-induced stress cracking of rigid amorphous plastics has been an issue.

This article was written by Paul Kroushl, Technical Service Associate, Teknor Apex Company, Pawtucket, RI. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/61059-164.


Medical Design Briefs Magazine

This article first appeared in the March, 2016 issue of Medical Design Briefs Magazine.

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