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

Sawers, Andrew MSPO, CO; Parsons, Sarah MSPO, CO; Geil, Mark PhD; Hovorka, Christopher MS, CPO

Author Information
JPO Journal of Prosthetics and Orthotics: July 2007 - Volume 19 - Issue 3 - p 75-83
doi: 10.1097/JPO.0b013e318074ee98
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As a semi-crystalline material, polypropylene is composed of both crystalline and amorphous regions. The amorphous regions consist of randomly oriented polymer chains, whereas the polymer chains in the crystalline regions have a much higher degree of organization. Long polymer chains are created by propylene monomers that form primary bonds with each other. During cooling, secondary bonds are formed between the polymer chains, and the chains then fold upon themselves to create flat plate-like structures referred to as lamellar plates. Stacked lamellar plates constitute the crystalline regions. Between each of these lamellar plates are amorphous regions referred to as tie-points. Amorphous regions can consist of segments of polymer chains extending from the lamellar plates (tie-points) or entirely separate polymer chains.1 During recrystallization, the crystalline and amorphous regions grow radially from focal points to create spherulites, which form the microscopic structure of polypropylene. Semi-crystallinity is a desirable property for most plastics because it combines the rigidity and brittleness of the crystalline region with the flexibility of the amorphous region, allowing semi-crystalline polymers (e.g., polypropylene and polyethylene) to bend prior to failure.2 The thermal history of the specimen determines the microstructure of polypropylene and thus the plastic’s macroscopic material properties. In general, the longer duration 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

It wasn’t until the 1960s, when polypropylene reached forms of mass distribution and process viability, that practical thermoforming could be realized in the orthotics and prosthetics industry.3 Thermoplastics such as polypropylene quickly became the preferred medium over traditional materials such as leather and metal because of their clarity, flexibility, rigidity, fast processing time, adjustability, light weight,4 ease of flow during the forming process,5 ease of cleaning, and hypoallergenic nature.

Despite the implementation of thermoforming technology for the better part of 40 years, little of the thermoforming process in the field of orthotics and prosthetics (O&P) has been quantified or controlled. As a result, the O&P profession has not kept pace with other industries in establishing standardized fabrication procedures to ensure quality control and reliable device function. Most thermoformed devices in O&P are fabricated on a trial-and-error basis with no standardization supported by quantifiable research. Some professionals have offered advice on producing consistent products,5–7 but little experimental evidence has been collected to quantify optimal thermoforming conditions.

Recently, several studies have addressed quality control in the thermoforming process of device fabrication as it pertains to the O&P profession. In an unpublished report, Bedard8 investigated four polypropylene samples cooled in different environments. Analysis of these samples indicated that percent crystallinity increased as cooling was decelerated. It can be inferred from the literature that with increased percent crystallinity comes greater stiffness, greater flexural strength, increased yield stress,1 and greater resistance to creep and stress cracking.5 A limitation of Bedard’s study was that bending stiffness was not assessed. Convery et al.9 investigated whether variations in the fabrication process created differences in thermoformed ankle-foot orthoses (AFOs). The investigators found that there was no significant difference in the thickness or resistance to plantarflexion or dorsiflexion of the thermoformed AFOs fabricated by two different technicians at the same facility following identical thermoforming protocols. This suggests that individual variation within a given facility in the thermoforming process does not contribute to or cause variability in the physical properties or function of thermoformed AFOs. However, the study did not focus on the evaluation of optimum thermoforming protocols.

Although many variables may influence the thermoforming process, the current study investigates positive model temperature for several reasons. It has been suggested that the ideal condition for vacuum forming polypropylene involves a positive mold at or near 190°F (the set temperature of polypropylene).5,10 This is advocated to prolong cooling and provide the polymer chains with an extended period of time to reform into an optimal state.2 This has been shown to favor an increase in percent crystallinity, whereas very rapid cooling can inhibit crystallization.10 Accelerating the cooling rate of polypropylene could also potentially weaken its strength by a factor of 16%, whereas its strength is increased by extending the cooling time.7 Extending the cooling times by using warm positive models has also been recommended to reduce the thermal shock to the hot plastic as it is formed over the positive model, thus preventing an increase in internal stress and subsequent failure.4

O&P professionals currently influence the properties and functions of the orthosis/prosthesis by careful selection of materials, material thickness, trim line placement, and corrugation design. To date there is no documentation regarding the curing or thermoforming process and how manipulating the variables within this process may influence the properties and function of the orthotic or prosthetic device.

RATIONALE FOR METHODOLOGY

There is no consensus on thermoforming procedures within the O&P profession. As a result, documented standards by manufacturers and other industries were tailored to the specific needs of this investigation. The variables requiring control were: 1) positive model moisture content, temperature, and dimensions; 2) oven temperature; 3) heating duration; 4) forming temperature of the polypropylene; 5) duration of vacuum; 6) cooling time before removal of the polypropylene square from the positive model; and 7) sample orientation.

  • 1) Positive model moisture content, temperature, and dimensions: A major concern using plaster of Paris to create the positive models was the variation in moisture content among models. Moisture content was controlled by creating all positive models with the same plaster-to-water ratio and forming all models on the same day. The temperatures of the positive models were selected out of convenience.
  • Several key processing temperature variables were selected as guidelines for heating, forming, vacuum draw, and cooling (Figure 1 and Table 1).
  • F1-5
    Figure 1.:
    Key temperatures affecting the characteristics of polypropylene. These temperatures served as guidelines to help determine the appropriate heating/forming temperatures, duration of vacuum, and cooling time.3
    T1-5
    Table 1:
    Key temperatures for polypropylene
  • Dimensions were selected to be smaller than the 12″ diameter of the blister forming stand. All models were made to have consistent dimensions.
  • 2) Oven temperature: Ranges of oven temperature have been proposed for the heating of polypropylene, including 356° to 392°F11,12 and 350° to 375°F.13 In this investigation, the oven set point was established at 400°F, and an internal oven temperature range was measured between 355° and 375°F throughout the experiment with a standard oven thermometer.
  • 3) Heating duration: Observing changes in thermoplastic sheet clarity or color is a common technique to estimate when the sheet is ready for removal from the heat source and for thermoforming over the positive model. Sheet clarity increases as the secondary bonds between the polymer chains are broken. This eases flow and chain mobility, creating a more uniform thickness distribution.1 A more objective and measurable guideline is to heat the polypropylene sheet for 3 minutes per millimeter of thickness.11,12 Using this guideline, ¼″ polypropylene should be heated for a duration of approximately 19 minutes. These two methods, observation of sheet clarity and heating duration based upon sheet thickness, were applied to determine the required heating time to ensure an appropriate forming temperature. To standardize this procedure, each sheet was heated for the same duration.
  • 4) Forming temperature: Several guidelines have been proposed for polypropylene forming temperatures, including 310° to 325°F9 and 331°F.5 Sheet temperature should closely approach the upper forming limit without exceeding it and reaching melt temperature (Tm).10 This enhances chain mobility while avoiding permanent damage to the primary bonds within the polymer chains. The Tm for polypropylene varies depending on its structural configuration, with isotactic polypropylene having a Tm of approximately 340° to 347°F.2 Accordingly, we aimed to establish a forming temperature near the upper limits without reaching melt phase temperatures. A noncontact infrared (IR) thermometer was used as an economical means to measure temperature of the heated polypropylene sheet because it does not disturb the molten plastic surface. Although this method does not indicate the core temperature of the sheet, IR thermometry is the most efficient and practical means to measure temperature of the thermoplastic sheet.
  • 5) Duration of vacuum: The lower processing limit of polypropylene at which recrystallization begins is 290°F. Establishing a vacuum seal with the positive model and drawing vacuum before the plastic cools below this temperature prevents internal stress, springback, and shrinkage.4 Based on this principle, the temperature of all thermoformed squares had to be at or preferably above 290°F at the time of vacuum application. The set temperature of polypropylene is 190°F5 and signifies the point at which the majority of crystallization is complete. This is the temperature at which the vacuum source can be safely switched off.5 These process guidelines were adhered to in this study: all the thermoformed squares were at or below the set temperature before the vacuum was switched off, and each square was maintained under vacuum for the same duration. One assumption was that the polypropylene that is vacuum-formed over the hot positive model should take the longest duration to reach the set temperature, followed by the ambient and cold conditions. The duration required for the hot condition to reach the set temperature was used as the time that all subsequent conditions were to be maintained under vacuum.
  • 6) Cooling time: Because of the low heat conductivity of plastics, there is longer cooling time for parts having a wall thickness in excess of SYMBOL″.10 Cooling a thermoformed part to ambient temperature is recommended because it ensures complete recrystallization. Slight shrinkage can occur in thermoformed parts for as long as 24 hours after thermoforming if the part is removed while still warm.11 In our investigation, the thermoformed squares were allowed to cool to ambient temperature during a period of 24 hours before they were removed from the positive model and prepared for subsequent testing. In contrast to the Tm, which corresponds to changes in the crystalline portion of the semi-crystalline polymer, the glass transition temperature (Tg) corresponds to changes in the amorphous region. Tg is approximately 15°F for isotactic polypropylene and 0°F for atactic polypropylene.2 Above Tg, the polymer chains in the amorphous regions move much more freely than at or below Tg.1 Although the Tg may not influence the heating of the polypropylene sheet, it can influence the forming, cooling, and storage properties of the thermoformed polypropylene parts.
  • FSM1-5
    Symbol
  • 7) Sample orientation: Sheet orientation was recorded prior to heating the polypropylene squares to ensure consistency in alignment with sheet extrusion direction. This was done in an effort to control shrinkage, which is greater along the extrusion direction than in the transverse alignment,9 and to standardize subsequent testing of material properties.

AIMS AND RESEARCH HYPOTHESIS

In this investigation the authors sought to determine if differences in positive model temperature resulted in changes to the molecular structure (percent crystallinity) and material properties (stiffness) of polypropylene samples. Because decelerated cooling provides the polymer chains with greater time to reorganize, the authors hypothesized that as positive model temperature increases, there will be a corresponding increase in the percent crystallinity and stiffness of the polypropylene samples harvested from the vacuum-formed squares.

METHODS

POSITIVE MODEL FABRICATION

The polypropylene samples were drape-formed over the positive models using a blister-forming vacuum stand. Accordingly, the model diameter was smaller than the 12″ diameter of the platform to allow a seal around the outer rim. Templates for creation of the positive models were constructed of SYMBOL″ polyethylene rings. The polyethylene template rings were secured to a ¼″ thick square of polypropylene, which served as a flat base during initial pouring of the liquid plaster to create the positive models. The plaster of Paris slurry consisted of 22 parts plaster (United States Gypsum Plaster, Chicago, IL) and 15 parts water, which was mixed and then poured into the template rings. The templates were tapped to dislodge air bubbles from the setting plaster and to ensure a flat surface at the base of the model. All plaster models were allowed to cure for an 18-hour period. Once cured, the surfaces of the positive models were smoothed with sand screen to remove rough edges and imperfections. This was intended to reduce the likelihood of stretching or rupturing the plastic during thermoforming and to provide a uniform forming surface (Figure 2). Dimensions of each positive model were compared to ensure consistency, and then each model was allowed to equilibrate in one of three environments: room temperature (77°F, for 24 hours); cold environment (−4°F freezer for 24 hours); hot environment (400°F oven for 330 minutes). A nylon was also applied to act as a wick in the thermoforming process.

F2-5
Figure 2.:
Positive model resting on the vertical vacuum forming station.

POLYPROPYLENE PREPARATION

Polypropylene was selected for this investigation because of its availability and frequent use in the fabrication of orthotic and some prosthetic devices. Two sheets of ¼″ thick polypropylene were obtained from a plastic distributor (Southern Prosthetic Supply, Alpharetta, GA). These sheets were cut into six 16" × 16" squares to satisfy each of the following conditions: stock (unthermoformed square for baseline values), hot (thermoformed over a positive model above the set temperature to allow for decelerated cooling), cold (thermoformed over a positive model below the set temperature to accelerate cooling), and ambient (three squares thermoformed over positive models at room temperature, the industry standard). The machine extrusion direction was marked and recorded on each plastic sheet prior to cutting each square. This allowed the investigators to maintain a consistent orientation of the samples during harvesting of the test samples.

Of the three ambient squares, ambient 1 was from the same sheet of plastic as the stock, hot and cold conditions. The other two ambient squares were from a second sheet of polypropylene. Comparisons of ambient samples allowed the researchers to evaluate the consistency of the heating and vacuum cycles and to determine whether the particular plastic sheet influences the final material properties.

THERMOFORMING PROTOCOL

A PDQ infrared (IR) oven (OTS Corporation, Barnardsville, NC) was preheated to a set-point of 400°F. Once the oven achieved this set temperature, there was a 30-minute equilibration period to minimize hot spots and ensure uniformity of the heating environment.

For each of the five positive models, the following steps were executed:

  1. The set-point of the oven was compared to the actual internal temperature of the oven using an analog oven thermometer (Acu-rite Incorporated, Jamestown, NY).
  2. Plastic was placed in the infrared oven for 18 minutes.
  3. The pre-forming percent humidity, ambient temperature, and vacuum pressure were measured and recorded. (A thermocouple was used to measure ambient humidity and temperature [Model TMH-360, EAI Education, Oakland, NJ]).
  4. The surface temperature of the positive model prior to thermoforming with the noncontact laser thermometer was measured and recorded.
  5. The surface temperature of plastic in the oven prior to removal from the oven with the noncontact laser thermometer was measured and recorded.
  6. The surface temperature of the plastic on the positive model as vacuum was applied was measured and recorded.
  7. The surface temperature of the plastic on the model every 30 seconds for 30 minutes after the vacuum was applied with the noncontact laser thermometer was measured and recorded.
  8. Vacuum was maintained for 30 minutes to ensure core plastic temperature cooled below the set temperature (190°F) before the vacuum is switched off.
  9. Thermoformed plastic remained on the positive model for 24 hours to continue cooling and to reduce the likelihood of distortion.

All surface temperatures were measured with a noncontact infrared MiniTemp thermometer (Raytek Corporation, Santa Cruz, CA). The center of the positive model was marked to ensure consistency of location of temperature measurements. The accuracy and repeatability of the infrared thermometer is ±2% of the reading for targets at 30° to 500°F, with a range of 0° to 500°F. Thermoforming conditions for all sample groups are presented in Table 2.

T2-5
Table 2:
The six tested conditions and their respective thermoforming conditions

SAMPLE HARVESTING

Samples from each of the thermoforming conditions were harvested from the formed square to complete stiffness and percent crystallinity testing. For flexural stiffness evaluation, sample dimensions were selected following ASTM standard D 790-03.14 This standard requires a support span-to-depth ratio of at least 16:1 and a specimen length at least 20% longer than the support span. Samples of dimension ¼″ × ½″ × 5″ (thickness × width × length) were cut for percent stiffness testing while small associated samples were retained for crystallinity evaluation.

TESTING OF SAMPLES (STIFFNESS AND PERCENT CRYSTALLINITY)

Five samples from each of the thermoformed squares and from the unthermoformed square of stock plastic were tested for flexural stiffness via ASTM standard D 790-03.14 If a sample displayed visible surface imperfections, such as dimpling, bubbling, or discoloration, it was not used in the three-point bending test as per ATSM 790-03 standards. A testing apparatus (Figure 3) was developed to ensure proper and consistent orientation of the plastic samples with respect to the Series 8521 Material Testing Instrument (Instron Industrial Products Group, Norwood, MA).

F3-5
Figure 3.:
Polypropylene sample loaded on testing apparatus undergoing flexural testing via Instron.

The testing apparatus was nonelastic, so the load transfer from the crosshead to the load cell of the Instron was not dampened. The apparatus maintained a consistent span of 4" (102 mm) for material testing. Four eyebolts were mounted upright to a ½″ section of Vivac (Durr-plex) (Southern Prosthetic Supply). Three ¼″ × 3″ bolts served as the loading and support cylinders. The apparatus was aligned on the Instron load cell, and samples were centered across the support span (Figure 3).

Following Procedure B of ASTM D 790-03, the rate of crosshead motion was determined as 2.16 mm/s, and excursion of the crosshead was set to 17.34 mm (Appendix A).

The Instron was programmed to deflect the sample based on these parameters. Stiffness was then calculated by the accompanying Series IX software package (Instron Industrial Products Group).

The degree of crystallinity was assessed using a Resonance Maran Ultra Nuclear Magnetic Resonance (NMR) machine (Austin, TX). Each test consisted of 32 scans of the sample at a preset frequency. Each sample was measured three times, from which an average value was calculated. The degree of crystallinity was determined by quantifying the percentage of protons present in the amorphous region.

STATISTICAL ANALYSIS

All percent crystallinity and stiffness values were analyzed with SPSS software (Chicago, IL) using a Bonferroni post-hoc ANOVA test. This test was selected as a conservative tool in assessing any significant differences in pair-wise comparisons between conditions. A value of p < 0.05 was used to indicate significant difference between conditions.

RESULTS

Surface cooling profiles for each condition are presented in Figure 4. As expected, the surface of the hot condition produced the longest duration to cool below the set temperature (24 minutes), followed by the three ambient conditions (4 minutes) and the cold condition (3 minutes). In all five conditions, a plateau was noticed at approximately 240°F.

F4-5
Figure 4.:
Surface cooling profiles of the five ¼″ polypropylene thermoformed squares under various conditions. Note the brief plateaus around 240°F.

The greatest average 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, whereas the three ambient conditions produced samples with intermediate stiffness or 988.62, 960.64, and 942.88 MPa (Figure 5, Table 3). The stiffnesses of the stock samples were significantly lower (p = 0.00) than those of any other condition. The stiffnesses of the hot and cold samples were significantly greater than all other conditions (p = 0.00) but were not significantly different from each other (p = 1.00), whereas the stiffnesses of the ambient samples were significantly greater than those of the stock samples (p = 0.00) but lower than the hot and cold conditions. There was no significant difference in the stiffness between any of the ambient conditions (p = 1.00, 0.744, and 1.00, Tables 4 and 5).

F5-5
Figure 5.:
Average stiffness of five samples from each of the six conditions. Error bars display one standard deviation above and below the average.
T3-5
Table 3:
Mean values and standard deviations for percent crystallinity and stiffness of all thermoformed conditions
T4-5
Table 4:
P values for average stiffness between each of the six conditions
T5-5
Table 5:
P values for percent crystallinity between each of the six conditions

Mean percent crystallinity was 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 correlation between initial surface temperature of the positive model and percent crystallinity (Figure 6, Table 3). However, the differences between the conditions were not found to be statistically significant. All p values for stiffness and percent crystallinity are reported in Tables 4 and 5.

F6-5
Figure 6.:
Percent crystallinity versus initial positive model surface temperature of the five thermoformed squares.

DISCUSSION

A brief plateau was noted in the cooling profiles of each of the five thermoforming conditions at approximately 240°F (Figure 4). This is precisely midway between the lower processing limit (290°F) and the set temperature (190°F) of polypropylene. The authors speculate that it may correspond to an increase in secondary bonding between the polymer chains during the recrystallization process, with the energy released from this interaction acting to sustain the surface temperature for an extended period of time. Upon examining the cooling profiles of the three ambient conditions, it becomes apparent that they experienced nearly identical cooling cycles (Figure 4). This implies that the cooling of the plastic does not vary during vacuum forming trials and is not influenced by uncontrolled variations in the thermoforming process or the specific plastic sheet used. As depicted in Figure 4, the samples exposed to the cold condition reached the set temperature first, followed by the ambient conditions, and finally the hot condition. This implies that the polymer chains in the plastic draped over the hot positive model had more time to rearrange and recrystallize and as such should have the highest percent crystallinity.

As expected, a linear relationship was noted between initial positive model surface temperature and percent crystallinity (Figure 6). This trend is consistent with the literature in that decelerated cooling favors an increase in percent crystallinity1 because as cooling is delayed, the polymer chains have an extended period of time to reorganize. However, a significant difference in percent crystallinity between the conditions was anticipated. This absence of significant difference may be attributed to the temperature at which the samples were stored prior to testing. Once the thermoplastic squares were thermoformed and the samples from each condition were harvested, they were stored for 3 weeks at room temperature (75°F, which is above Tg). It is possible that since the samples were stored above Tg for a 3-week period, vibration and movement of the polymer chains in the amorphous regions of the cold and ambient samples may have resulted in their conversion to the crystalline regions, creating an increase in percent crystallinity in the samples of those 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 observed. Unexpectedly, the cold condition produced the highest stiffness, which seems to contradict its lower percent crystallinity. The literature clearly states that accelerated 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 Tg. Immediately prior to forming the heated ¼″ polypropylene sample, the surface temperature of the positive model was 10°F. This temperature is below the glass transition temperature (Tg) of polypropylene,2 and as such this may have altered the characteristics of the amorphous region. Below Tg, the amorphous regions of a thermoplastic are in what is considered a glassy state, where the molecules are frozen in place. Slight molecular vibration may occur, but there is no segmental motion in which portions of the molecule move. 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 in other conditions.2

All three of the ambient conditions demonstrated statistically similar values in percent crystallinity and stiffness. These results lend further credibility to the theory that there is no significant difference between vacuum forming trials performed by the same individual laboratory, provided that lab conditions are preserved.

In interpreting these results, several questions have emerged with regard to the thermoforming of polypropylene:

  • 1) The literature suggests using a positive model at or near the set temperature.5 If the goal is to provide an extended period of time for the polymer chains of the plastic to optimally align and increase secondary bonding, then why only heat the positive model to the set temperature? Why not heat it to a higher temperature? Is this a practical approach? Should the temperature of the positive model not be as close as possible to that of the heated thermoplastic?
  • 2) Are the changes that occur in percent crystallinity and material properties as a result of manipulating the thermoforming process permanent or transient? If the majority of thermoplastic orthoses are used at room temperature and are in contact with the human body, which are both above Tg, perhaps percent crystallinity and subsequent material properties change with time because of movements by polymer chains in the amorphous regions. This presents the question of whether it is worthwhile to manipulate the variables of the thermoforming process to achieve different material properties if these properties may vary with time.
  • 3) What is the threshold for change in percent crystallinity that causes a change in the material properties of polypropylene and subsequently the performance of the orthosis or prosthesis? This is an important question to address because if there is a change in percent crystallinity but no resulting change in performance of the orthosis or prosthesis, influencing crystallinity may not be of concern.

CONCLUSION

Profound effects on thermoplastic characteristics can be observed by altering parameters of the thermoforming process. Specifically, the temperature of the positive model and its effects on the stiffness of polypropylene can be influenced. Clinically, the results derived from this research may provide orthotists/prosthetists with the ability to exert greater control on the function of orthotic/prosthetic devices outside of material selection, material thickness, trim line placement and corrugation, thus allowing them to fine-tune the final function of the device. To do this accurately would first require expanding and broadening this technique from the simple uniform geometric shapes assessed in this investigation to thermoformed orthoses or prostheses. Although not statistically significant, a literature-supported trend of increasing crystallinity with decreasing cooling time was observed.

If this investigation is to be reproduced, the authors suggest that thermoforming of all conditions occur on the same day because the stiffness and percent crystallinity of polypropylene may be time and temperature dependent. Completion of the material testing should be conducted at consistent times from the thermoforming stage, and the samples should be stored in the same environment.15,16 Another recommendation is to examine the morphology of the spherulite structures in each condition. Recorded surface temperatures may be more representative of the core temperature of the plastic if a thinner sheet is used. Alternatively, using indwelling thermocouples to record core temperature would be recommended if this investigation were to be repeated, thus eliminating the need to infer data from the surface temperature.

Future directions of this research may include:

  • 1) Using positive models with temperature differences over smaller intervals;
  • 2) Determining positive model threshold temperatures at which significant changes to the physical properties of thermoplastics occur;
  • 3) Testing percent crystallinity and physical properties over an extended period of time to check for time-dependent changes;
  • 4) Examining the morphology of the spherulite structures in each condition;
  • 5) Testing of other material properties (fracture, fatigue, etc.);
  • 6) Extending the testing beyond polypropylene to include other thermoplastics, additives, resins and fiber reinforced plastics (FRP);
  • 7) Manipulating other variables in the thermoforming process and quantifying the influence of the anatomical shape of an orthotic/prosthetic device and how that interacts with changes to material properties in determining the performance of the orthosis/prosthesis.

Our goal is that eventually clinicians will be able to use this information to help optimize device performance through fabrication to achieve the desired function of the device.

ACKNOWLEDGMENTS

The authors thank the following people for their assistance in the design of the experiment, carrying out the investigation, and analyzing the data: Rob MacDonald, RTPO (c), and Gary Bedard, CO, FAAOP.

APPENDIX A

Equation for crosshead motion:

where:

R = rate of crosshead motion, mm/min

z = rate of strain in outer fibers, 0.10 mm/mm/min

L = support span, 102 mm

d = approximate depth of samples; 5 mm

Equation for crosshead excursion:

where:

D = displacement of sample at midspan, mm

r = maximum strain of sample; 0.05 mm/mm

L = support span; 102 mm

d = approximate depth of samples; 5 mm

REFERENCES

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3. Kogler G. Materials and technology. In: Lusardi MM, Nielsen CC, eds. Orthotics and Prosthetics in Rehabilitation. Woburn, MA: Butterworth-Heinemann; 2000:11–32.
4. Clover W. Lower extremity thermoplastics: an overview. J Prosthet Orthot 1991;1:9–13.
5. Lunsford TR. Strength of materials. In: Goldberg, B, Hsu JD, eds. Atlas of Orthoses and Assistive Devices. 3rd ed. St. Louis, MO: Mosby; 1997:15–66.
6. Pritham CH. Thermoplastics in lower extremity prosthetics: equipment, components and techniques. J Prosthet Orthot 1991;3(1):14–21.
7. Fairley M. Thermoforming technology: controlling the process. O&P Edge March 2004;20–26.
8. Bedard G. Preliminary report on measuring the rate of polypropylene recrystallization after clinical thermoforming through the use of differential scanning calorimetry. Paper presented at: AAOP Annual Meeting & Scientific Symposium, March 8–10, 2004; Orlando, Florida.
9. Convery R, Greig RJ, Ross RS, Sockalingam A. A three center study of the variability of ankle foot orthoses due to fabrication and grade of polypropylene. Prosthet Orthot Int 2004;28:175–182.
10. Gruewald G. Thermoforming: A Plastics Processing Guide. 2nd ed. Lancaster, PA: Technomic Publishing; 1998.
11. North Sea Plastics: Materials Information. Available at: http://www.northseaplastics.com. Accessed December 5, 2004.
12. Orthotic Materials Heating Guidelines. Available at: http://www.orthotictechnicians.com/html/archive/plastemp/pdf. Accessed December 5, 2004.
13. American Plastics: Homopolymer Polypropylene. Available at: http://www.americanoandp.com. Accessed December 5, 2004.
14. American Society for Testing and Materials International. Annual Book of ASTM Standards. Philadelphia: ASTM; 2003.
15. Min B, Pae KD. Physical ageing of polypropylene in glassy state. J Mater Sci 1989;24:3613–3615.
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Keywords:

crystallinity; orthosis; polypropylene; prosthesis; stiffness; thermoforming

© 2007 American Academy of Orthotists & Prosthetists