Polypropylene

Polypropylenes are the most frequently used sheet resins in today's Orthotic and Prosthetic appliances.  Their popularity is due to their exceptional versatility in both application and manufacturing.  There are two primary classifications of polypropylenes used in Orthotics and Prosthetics; Standard Grade and Orthopedic Grade.  The two grades are made from the same resins, but are significantly different in their characteristics.

Standard Grade Polypropylene has the highest stiffness and rigidity factors in the polypropylene family of resins.  It is excellent for use in orthotic designs that experience high torsional and directional forces.  The high stiffness value of this material contributes to its increased brittleness compared to Orthopedic Grade Polypropylene and therefore is not recommended for high impact situations.

Orthopedic Grade Polypropylene has characteristics which include; high impact resistance, good cosmetic capabilities, and excellent formability.  Its reduced stiffness compared to regular grade polypropylene contributes to increased patient comfort in a variety of orthotic designs.  It can be used in direct contact with skin or combined with interface padding.  It is not recommended for applications where higher stiffness and rigidity values are required unless it is supported by reinforcements.  
Manufacturing
Polypropylene is one of the most versatile materials in its manufacturing capabilities and may be saw cut, routered, die cut, milled, or punched.  It can be molded by using; drape, frame, injection, vacuum, and pressure techniques.  The sheet must be clamped securely and minimal cuts should be taken to avoid chipping and cracking, especially when using Standard Grade Polypropylene.

The normal forming temperature range for Standard Grade and Natural Orthopedic Grade is 375-400 F.  Fleshtone and Black Orthopedic Grade have an upper limit of 395 F.  Once molded, they may be modified using localized heat, although specific temperatures are required.  They are self adhering at forming temperature and may be laminated.  Predrying is not required for thermoforming.  Moisture contained in the model should be sealed with a moisture barrier to resist bubbling and rippling.  Medium polypropylenes may not be solvent welded or glued.  Medium strength welds can be obtained using hot welds employing polypropylene welding rods and hot air welding irons.  Welding rods are available in sizes and colors to match sheet materials.

Sterilization
Polypropylenes can only be sterilized using ethylene oxide Used in a 12%/88%; ethylene oxide/freon gas mix).

Precautions
Polypropylenes are combustible thermoplastics and therefore should be stored in a cool dry area.  Because they are petroleum derivative plastics, they give off a heavy black toxic smoke that is dangerous to inhale and difficult to see through.  This material has ultraviolet stabilizers and is tolerable to many outdoor applications.  Due to their excellent mold replication properties, vacuum interface is almost always required to prevent marking of plastic.  If air holes are required to draw vacuum they should be small. Using vacuum holes of no larger than 1/32" diameter is required to prevent marking of the plastic.

Applications
Applications of these materials are almost infinite.  They can be used in all applications where non-optical characteristics are required.  The advantage of two different brittleness and impact strengths leads to it being the principal plastic of the industry. It is the standard sheet material used for most shell and cuff designs in upper and lower extremity orthotics.  Its use in spinal orthotics includes everything from girdles and support pads for superstructures to back panels and full body jackets.