Comparison of bending stiffness of six different colours of copolymer polypropylene

Abstract

This paper compares the bending stiffness of 5 different colours of copolymer polypropylene (CCP) with that of natural copolymer polypropylene (NCP). Flesh coloured and natural sheets are supplied thicker than other pigmented sheet. The bending stiffness of a specimen may be defined as EI, i.e. the product of E, Young's modulus of elasticity and I, the 2nd moment of area. Strips of “as supplied” (AS) and “post-draped” (PD) specimen were clamped and subjected to bending to assess the effect of pigmentation on bending characteristics. The gradient of the graph of bending deflection δ versus bending moment enables EI to be estimated. The process of thermoforming polypropylene reduces EI, the bending stiffness. However, the manual draping and vacuum procedure introduces so many variables that it is difficult to quantify the effect of pigmentation. The E of a bent specimen may be estimated from the gradient of the graph of δI versus bending moment. In the case of AS sheet, the effect of pigmentation on E is inconclusive. PD specimens indicate a significant reduction in E due to thermoforming. This was verified by an electron-microscope study of AS and PD specimens. Draping an ankle-foot orthosis (AFO) results in a non-uniform wall thickness. The results of this study with respect to the effects of pigmentation on the bending stiffness of AFOs are inconclusive. More detailed studies require to be completed in order to confirm which factors are responsible for this non-uniformity in wall thickness and consequent variation in bending stiffness.

Physical sciences

“…68 It was near this time that Prosthetics and Orthotics International published a related analysis of the bending stiffness of six different colors of copolymer polypropylene. 69 The authors described that pigmentation of copolymer altered its innate bending stiffness, but subsequently found that the variability in wall thickness inherent to drape forming appeared to negate the impact of such variability in the ultimate biomechanical performance of the device. 69 As the behavior of plastic was increasingly understood, the effect of both supplementary and replacement materials began to be explored.…”

“…69 The authors described that pigmentation of copolymer altered its innate bending stiffness, but subsequently found that the variability in wall thickness inherent to drape forming appeared to negate the impact of such variability in the ultimate biomechanical performance of the device. 69 As the behavior of plastic was increasingly understood, the effect of both supplementary and replacement materials began to be explored. For example, Major et al 70 reported upon the resistance to dorsiflexion observed with four variations of the polypropylene AFO.…”

Design of an Automated Device to Measure Sagittal Plane Stiffness of an Articulated Ankle-Foot Orthosis

“…Bench analyses have been conducted to investigate the torque-angle or torque-deflection relationships using a tension meter, [13][14][15][16][17] a strain gauge, 18 a load cell, 19,20 a dial gauge, [21][22][23] a forceplate, 24 or a muscle training machine. 25 Methods used to apply force to an AFO in previous bench studies have been generally grouped into two types, either direct application of force to a specific area of an AFO 13,16,22 or indirect application of force via a surrogate shank. 14,15,17,18,20,21,23 The advantage of conducting bench analyses is it is easier to control the experimental parameters in comparison to functional investigations.…”

“…However manual operation would induce a human error. The use of a load cell 19,20 and a dial gauge [21][22][23] would enable objective measurement of the stiffness of an AAFO. However, previous devices reported in the literature were primarily designed to test a non-articulated AFO (NAFO) [19][20][21][22][23] or require drill holes to bolt an AFO to the device.…”

“…The use of a load cell 19,20 and a dial gauge [21][22][23] would enable objective measurement of the stiffness of an AAFO. However, previous devices reported in the literature were primarily designed to test a non-articulated AFO (NAFO) [19][20][21][22][23] or require drill holes to bolt an AFO to the device. 21,22 Moreover, we considered that the devices reported in the precedent studies using a tension meter, [13][14][15][16][17] a strain gauge 18 or a dial gauge [21][22][23] were not able to perform a controlled dynamic measurement of the stiffness.…”