MazeEngineers uses autoclavable polypropylene for a wide variety of mazes. If you need specialized autoclavable mazes, please contact us for more information. We are able to create certain mazes with this specification.

The early 1950’s witnessed the discovery of polypropylene (Sailors & Hogan, 1981), a polyolefin resulting from the chain polymerization of the propylene monomer. By 1957, polypropylene was commercially produced in the US and Europe in the following year (Sailors & Hogan, 1981). Since its discovery, the demand for polypropylene has increased over the years with applications ranging from household appliances to medical devices.

The petrochemical product is similar in chemical structure to polyethylene, another popular commodity thermoplastic in use around the world. Like polyethylene, polypropylene is also a non-toxic, inert and hydrophobic material that does not degrade in vivo. However, unlike polyethylene, that can be produced as a clear material, polypropylene has a milky, translucent appearance. Polypropylene is often colored to make it opaque. In clinical set-ups, polypropylene’s high melting point and resistance to chemicals and organic solvents make it ideal for construction of apparatus and equipment that can be autoclaved and reused. Further, it is light-weight, moldable and resistant to cracking.

In comparison to polyethylene that tends to be expensive due to its purity, polypropylene provides a cost-effective alternative. Given its many beneficial properties, polypropylene is seen slowly replacing polyethylene in many applications. Not only is polypropylene a versatile and compatible material, but it also recyclable up to four times. Polypropylene can be recycled as shredded/granulated flakes or extruded into dense plastic pellets which can be mixed with virgin plastic to create new products.

The production of polypropylene starts with obtaining propylene which is produced as a by-product of the downstream distillation of crude oil. Propylene can also be obtained as the co-product of ethylene production. With the help of a catalyst, usually Ziegler-Natta or metallocene catalyst, the propylene undergoes polymerization to create the polymer polypropylene. Advancements in catalysts have led to improved stereospecificity and productivity of polypropylene.

Although different industrial production processes (bulk polymerization and slurry polymerization) of polypropylene exist, the gas-phase polymerization is a popular method since it provides an economical and flexible process that can accommodate a range of catalysts. Early manufacturing units made use of the slurry process technology that was designed to utilize the first- and second-generation catalysts. This process, however, had its disadvantages. Not only was the cost of operation and capital required high, but the procedure also required treatment of the catalyst with alcohol to deactivate and extract it. Further, extraction and removal of the unwanted atactic polymer were also required, making this method labor intensive and energy inefficient. The slurry technologies eventually evolved into the bulk processes. In comparison to these processes, the gas-phase technology eliminated the need for the separation and recovery of large quantities of solvents and liquid propylene. This method also produced polypropylene that was dry and required minimal deactivation of residual catalysts. The technologies for production evolved with the catalysts advancements to allow better and efficient procedures (Karian, 2003). The polypropylene is obtained as a powder at the end of these processes that is mixed with additives and converted into pellets.

The processing conditions and the catalysts used to enable the production of polypropylene with desirable properties suitable for specific applications. Property variations can be achieved by introducing additives or varying the crystallinity in the polymer.

In general, two categories of polypropylene, homopolymers, and copolymers, are available commercially. Polypropylene homopolymer consists of only the monomer propylene in the polymer chain while the copolymer version typically uses the comonomer ethylene. The copolymer category is further divided based on the arrangement of the comonomer. The block copolymer polypropylene, typically, contains 5 to 15 % ethylene comonomer units which are arranged in regular patterns. On the other hand, the random copolymer grade has an irregular, random arrangement of the ethylene units (copolymer level is generally 1 to 7%). In comparison to the general-purpose grade homopolymer polypropylene, the copolymer category polypropylene offers many advantages which can include impact resistance, more flexibility, and enhanced clarity. Another grade of polypropylene, the impact copolymer, are usually formed by the addition of ethylene-propylene rubber to homopolymers or random copolymers (copolymer level range from 5 to 25 %) to produce a product with increased low-temperature impact strength. Impact copolymers can also be formed using ethylene-propylene-diene, polyethylene, or plastomers.

Based on tacticity, polypropylene can exist as isotactic, syndiotactic and atactic. Isotactic polypropylene is what is generally available commercially. Isotactic polypropylene has a high crystallinity that results in desirable physical, mechanical and thermal properties in its solid state. On the other hand, atactic polypropylene has low crystallinity that results in an amorphous polypropylene generally used as sealants, caulks and for modifying rubber, asphalt, polyethylene, and bitumen. Lastly, the syndiotactic polypropylene, in comparison to its counterparts exhibits excellent electrical, thermal and mechanical properties due to its low crystallinity, smaller spherulites, different crystal lattice and less residual catalysts (Yoshino et al., n.d.).

The growth of polypropylene in medical application is mostly due to its cost-effectiveness. The plastic has a variety of use in clinical and sterile environments. Most commonly it can be seen used as a sterile, non-absorbing surgical suture for skin closure and general soft tissue approximation and ligation. Polypropylene’s good tensile strength and long flex life make it ideal for use as a suture. Further, the product is also considered safe and effective, though care must be taken to avoid instrumentation trauma and kinking stresses at knots (Calhoun & Kitten, 1986; Macfarlane et al., 2014). Polypropylene meshes are also frequently used as implants notably as a transvaginal mesh and in hernia surgery. However, concerns regarding the safety of these implants are still under debate since polypropylene can erode surrounding tissues over an uncertain period (Gold et al., 2012; Moalli, Brown, Reitman, & Nager, 2014).

Polypropylene nonwoven fabrics are used in fabrication products such as surgical masks and gowns. These fabrics usually produced using the melt-blown process are soft, light-weight and allow passage of water vapor while preventing the penetration of aqueous solutions. Given its autoclavability and good impact resistance among other features, polypropylene is often used to produce equipment such as disposable hypodermic syringes, gallipots, instrument trays, test tubes, drug delivery systems, infant feeding tubes, and preclinical research mazes.

Strengths

• Cost-effective alternative to plastics like polyethylene and other materials such as stainless steel used in sterile environments.
• Has good chemical resistance to a range of acid solutions, alcohols, bases and mineral oils.
• Is autoclavable, heat passive and does not retain heat.
• Is very resistant to absorbing moisture.
• Has high flexural strength, good impact resistance, and good fatigue resistance.
• Is a good electrical insulator.

Limitations

• Has a high thermal expansion coefficient.
• Susceptible to UV degradation.
• Poor resistance to concentrated acids, bases, chlorinated solvents and aromatics.
• Is highly flammable and susceptible to oxidation.
• Poor low-temperature impact strength.