ProComp® Overview

Advanced Vacuum Thermoforming with Carbon Infused Polypropylene

Overview

ProComp® is the outcome of material science engineering resulting in the development of a patented proprietary method of producing a thermoplastic prepreg composite that is compatible with standard orthotic and prosthetic vacuum thermoforming. Utilizing the predicated materials of homopolymer polypropylene and carbon fiber in a prepreg composite not only improves the performance of lower extremity clinical devices, but it also improves the process of drape encapsulation and bubble vacuum forming. Therefore, the definition of advanced vacuum thermoforming is the combination of material science and process engineering which raises the standard of practice when providing thermoplastic based exoskeletal lower extremity components or devices. 

Composite Design

The composite is consolidated with a proprietary process whereby discontinuous carbon fiber strands are infused into the core of the polypropylene laminate. The base resin is an FDA food grade homopolymer polypropylene. The FDA classification stipulates a minimum of resin additives which translates into a decrease in the likelihood of patient dermal irritation. The carbon fibers are infused as discrete fiber plies in the core of the laminate. The fibers on average are 0.25 – 0.5 inches in length. The fiber distribution is omnidirectional which provides reinforcement and strength in any direction. No fibers are infused into the central core of the laminate offering an “I” bean mechanical effect to improve the stiffness of the material. No fibers are infused into the outer ply of the composite thus eliminating any change of dermal contact. 

The fiber volume ratio of the composite is by design below 40 percent. The theoretical fiber maximum ratio for any composite is over 90 percent. A typical thermoset prepreg composite will have a fiber volume over 70 percent. The common wet thermoset resin vacuum bag assisted lamination as utilized in any O&P lab will have a fiber ration with a range of 50 to 65 percent dependent on individual technique and equipment performance. In the commercial composite arena, the process is called vacuum bag resin transfer molding (VBRTM).   In ProComp® the fiber ratio is purposely below 40 percent which allows the physical characteristics of the resin to be paramount versus the carbon fiber. The result is a thermoplastic prepreg composite sheet that improves the good physical qualities of polypropylene while minimizing the poor characteristics.  

When the composite was tested within ASTM D-790 flexural modulus guidelines the results have demonstrated a stiffness improvement as high as 25 percent over the base polypropylene resin statistics. Since thermoplastic exoskeletal ankle foot orthoses (AFO) provide contact support to the lower limb increasing the structural stiffness of the device will have a direct impact on a patient’s gait. Tensile Strength and Izod Impact testing under ASTM D638 and D256A also demonstrate an increase the base polypropylene resin. 

The outcome is the ability to fabricate a thermoplastic based exoskeletal lower extremity device as the discontinuous carbon fiber segments flow with the melt temperature polypropylene resin allowing the fabrication of exoskeletal lower extremity devices in a composite rather than a plastic while utilizing standard vacuum forming process technology. 

Clinical Benefits

Improvement in the clinical device is the outcome of material science as well. The infused carbon segments in discrete plies inhibit clinical creep or device shape change. The polyolefin family of thermoplastic resins which include polypropylene as well as the polyethylene variants all have a semicrystalline molecular structure. Since no cross linking occurs between the long macromolecular polymer chains in the resin, the material is susceptible to creep or a change in shape over time when subjected to stress. Clinical creep is the change in shape of a clinical device due to gait forces and body temperature impacting the molecular structure. In a ProComp® based clinical device the shape as derived from fabrication on a rectified positive model is thus retained through the life cycle of the product. 

Since the composite is thermoplastic based the ability to implement a spot shape change is retained. A heat gun can be utilized to heat a small area that as an example may be creating undue dermal pressure can be thermally adjusted as has been the practice standard with plastic devices. The fiber segments will move with thermally softened area. 

Process Engineering Benefits

The design specifications for ProComp® were defined to allow the physical characteristics of the thermoplastic prepreg composite to be completely compatible with current vacuum forming process equipment universally found in O&P labs as well as individual levels of technical competence. The transition from plastic sheeting to the prepreg composite is a simple exchange of materials. 

The composite due to the infused carbon fibers does offers several processing benefits. The fibers improve the sag strength of the melt temperature fabrication sheet allowing improved control during the molding process. The increase in sag strength allows for control of wall thickness of the sheet during molding and thus the stiffness of the final product. 

Since the carbon fibers are infused into the polypropylene resin as discrete plies the fibers are completely encapsulated by the resin. When the trimlines of the clinical device are finished using an ablative technique via sanding or finishing cones on a power router, the resulting particulate matter is rather large and does not become airborne nor do the particles create dermal irritation. 

The need to orient the machine direction of the sheet with the long axis of the positive model especially in open cylindrical orthotic fabrication as is typical with extruded thermoplastic sheet fabrication to overcome shrinkage is not required. The proprietary process utilized for the consolidation of ProComp® is not unlike an annealing process of heating, stabilization, pressure, and cooling, thus machine direction is eliminated. 

The shape change typical from the shrinkage of extruded plastic sheeting is virtually eliminated with use of the composite. Individual fabrication technique as associated with drape encapsulation molding can induce thinning and thus induced forces into the composite. The result is usually only present in extreme manipulation of the melt temperature composite during molding, otherwise the molded shape is locked into place by the presence of the carbon fibers in the base resin. 

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Effects of Materials, Reinforcement, and Heat Treatment on Thermoplastic Solid Ankle-Foot Orthosis Mechanical Properties: A Preliminary Study
Authors: Gao, Fan PhD; Bedard, Gary G. CO, FAAOP
Journal of Prosthetics and Orthotics: July 2013 - Volume 25 - Issue 3 - p 140-53

    In this article practitioners are provided test results on several methods of increasing the stiffness of solid ankle foot orthoses (AFO). The range of materials tested included homopolymer polypropylene, copolymer polypropylene and ProComp® carbon infused polypropylene. Two reinforcement methods for homopolymer polypropylene solid ankle AFOs were tested which included the use of corrugations molded into the AFO and the insert bonding of CompCore™ reinforcement coupons of carbon/glass/polypropylene. 

    In addition, the heat treatment method of annealing was conducted on homopolymer polypropylene. The test results validate the use of carbon infused polypropylene in the form of ProComp® and the reinforcement method of bonding CompCore™ into a polypropylene AFO.

https://journals.lww.com/jpojournal/fulltext/2013/07000/effects_of_materials,_reinforcement,_and_heat.8.aspx

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Bench Test Validation of a Dynamic Posterior Leaf Spring Ankle-Foot Orthosis
Authors: Bedard, Gary G. BSc, CO, FAAOP; Motylinski, Jennifer MPO; Call, Benjamin MPO; Gao, Fan PhD; Gray, Leslie MEd, CPO. Journal of Prosthetics and Orthotics: January 2016 – 

Volume 28 - Issue 1 - p 30-37

 

    In this article practitioners are provided with test results on the mechanical properties of a flat blade posterior leaf spring ankle foot orthosis (PLS AFO) fabricated from both ProComp® and homopolymer polypropylene. The provision of kinetic energy return posterior blade style of AFOs has become very common. In this paper the properties of PLS AFO fabricated from plastic and of a thermoplastic composite are reported. The use of standard polypropylene and ProComp® provide the practitioner the option of fabricating a PLS AFO utilizing standard commercial materials. The paper also reports on the use of non-commercial carbon infused polypropylene that has a higher carbon fiber volume than the current commercial ProComp® produce line to explore future composite development. 

https://journals.lww.com/jpojournal/FullText/2016/01000/Bench_Test_Validation_of_a_Dynamic_Posterior_Leaf.6.aspx