Positive Model Temperature and Its Effect on Stiffness and Percent Crystallinity of Polypropylene Samples

The field of prosthetics and orthotics relies on the properties of thermoplastics to fabricate finished devices. Many of these thermoplastics are semi-crystalline materials, meaning that they are composed of both crystalline and amorphous regions. The amorphous regions consist of randomly oriented polymer chains, which enhance material flexibility, while the chains in the crystalline regions have a much higher degree of organization lending the properties of rigidity and brittleness to a material. The ratio of amorphous to crystalline structure determines the final properties of the thermoplastic as a whole. This ratio of crystalline to amorphous material, and thus the microstructure of the thermoplastic, is determined by the thermal history of the specimen. In general, the longer the molten sample of polypropylene has to reorganize during the cooling process, the higher the percent crystallinity and the greater the stiffness of the sample (1).

In this study, researchers examined both the percent crystallinity and stiffness of 1/4” polypropylene [*Polypropylene .25 inch from Southern Prosthetic Supply, Alpharetta, GA 30005.] samples to assess whether these properties were altered during the thermoforming process as a result of changes in positive model temperature. Six conditions of polypropylene were evaluated, an un-thermoformed stock square and five squares drape-formed over positive models of varying temperatures (one hot, one cold and three ambient). Percent crystallinity was evaluated by nuclear magnetic resonance while stiffness was assessed by means of a three-point flexural test (2).

Mean percent crystallinity was found to be greatest among the hot samples (77.50%), lowest among the cold samples (74.32%) and intermediate among the ambient samples (74.70%, 74.80% and 75.10%). This resulted in a linear relationship between initial surface temperature of the positive model and percent crystallinity. However, the differences between the conditions were not found to be statistically significant. This trend agrees with the literature which states that decelerated cooling favors an increase in percent crystallinity (1) because the polymer chains have an extended period of time to reform into an optimal state involving increased secondary bonding between chains. However, a significant difference in percent crystallinity between the conditions was anticipated. This lack of significant difference may be attributed to the temperature at which the samples were stored prior to testing. Once the polypropylene sheets were thermoformed and the samples from each condition were harvested, they were stored at room temperature (~75°F) for three weeks. It is possible that since the samples were stored above the glass transition temperature, vibration and movement of the polymer chains in theamorphous regions of the samples may have resulted in their reorganization into crystalline regions creating an increase in percent crystallinity in these samples.

The second variable assessed was stiffness of the polypropylene samples. The greatest flexural stiffness was found in the samples of the hot and cold conditions, 1178.20 MPa and 1197.80 MPa respectively. The lowest average stiffness was found among the stock samples, 838.29 MPa, while the three ambient conditions produced samples with intermediate stiffness 988.62, 960.64 and 942.88 MPa. The stiffness of the stock samples were significantly lower than those of any other condition. The stiffness of the hot and cold samples was significantly greater than all other conditions but were not significantly different from each other, while the stiffness of the ambient samples was significantly greater than the stock samples but lower than the hot and cold conditions. There was no significant difference in the stiffness of any of the ambient conditions.

Although a linear trend was observed between initial positive model surface temperature and percent crystallinity, the anticipated parallel trend between percent crystallinity and stiffness that is described in the literature was not seen. Unexpectedly, the cold condition had the highest flexural stiffness which seems to contradict its lower percent crystallinity. The literature clearly states that accelerating cooling should decrease the percent crystallinity and subsequently lower the stiffness of the thermoplastic (1). The fact that the average stiffness of the cold condition was significantly higher than that of the ambient conditions yet did not have a significantly different percent crystallinity, leads the authors to believe that the added stiffness of the cold samples was not influenced by the percent crystallinity, but rather by the increased rigidity of the amorphous region due to surface temperatures of the positive model being below the glass transition temperature, Tg. Immediately prior to forming the heated polypropylene, the surface temperature of the positive model was 10 °F. This temperature is below the glass transition temperature (Tg) of polypropylene (3) and as such this may have altered the molecular movement and structure in the amorphous region. Below Tg the amorphous regions of a thermoplastic are in what is considered a glassy state. In this state the molecules are frozen in place. They may be able to vibrate slightly, but do not have any segmental motion in which portions of the molecule to reorganize. When the amorphous regions of a polymer are in the glassy state, they will generally be hard, rigid, and brittle forgoing the flexibility that they generally bestow (3). All three of the ambient conditions demonstrated statistically similar values in percent crystallinity and stiffness. These results lend credibility to the theory that there is no significant difference between thermoforming events by the same individual in the same lab provided lab conditions are preserved.

In summary, the properties of polypropylene samples were influenced by changes in positive model surface temperature. Thus final orthotic function may be influenced not only by material selection, thickness, trimline placement and corrugation but also by changes to the thermal history of the thermoplastic. This is important to keep in mind not only for quality control, but also in engineering specific properties and functions in the finished device.

The average % crystallinity of each thermoforming condition based on positive model temperature at the time of forming. The average value presented represents an average of five samples per condition which were each assessed three times.

The six tested conditions and their respective thermoforming conditions.

The average stiffness of the uniform polypropylene samples from each thermoforming The average value presented represents an average of 5 samples per condition.

References

1. Maier C, Calafut T. Polypropylene: the definitive user's guide and databook. Norwich (NY): Plastics Design Library, 1998.

2. ASTM International. Annual book of ASTM standards. Philadelphia: ASTM; 2003.

3. Callister WD. Materials science and engineering: An introduction. 5th ed. New York: John Wiley & Sons, Inc, 2000.