Insights on the Mechanical Properties of UHMWPE Material

Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a thermoplastic polyethylene with a high molecular weight. It’s also known as high-modulus polyethylene (HMPE), and it contains exceptionally long chains ranging from 3.5 to 7.5 million amu. By enhancing intermolecular contacts, the longer chain helps to transfer load more effectively to the polymer backbone. As a result, the material is extremely robust, with the highest impact strength of any thermoplastic available.

UHMWPE has no odor, no taste, and is harmless. It has all of the properties of high-density polyethylene (HDPE), the ability to withstand intense acids and alkalis, and a variety of organic solvents. It is highly resistant to corrosive chemicals, except oxidizing acids; it has very low moisture absorption and a very low coefficient of friction; it is self-lubricating (see boundary lubrication); and it is highly resistant to abrasion, being 15 times more resistant to abrasion than carbon steel in some forms. It has a lower coefficient of friction than nylon and acetal and is equivalent to polytetrafluoroethylene (PTFE, Teflon). However, UHMWPE has superior abrasion resistance than PTFE.

UHMWPE

UHMWPE is a kind of polyethylene (PE) with an extremely high molecular mass.

UHMWPE is defined by the International Standards Organization (ISO 11542) (ISO, 2001) as having a molecular weight of at least 1 million g/mol, while the American Society defines UHMWPE for Testing and Materials (ASTM) as having a molecular weight higher than 3.1 million g/mol. UHMWPE is a semi-crystalline polymer made up of at least two interpenetrating phases: a crystalline phase in which the macromolecules fold into ordered, crystalline lamellae, and an amorphous, disordered phase, perhaps intercalated by a partially ordered, so-called all-trans, interphase. The microstructure of the polymer, in addition to its molecular mass, plays a vital role in determining its physical, chemical, and mechanical properties.

The molecular weight of UHMWPE utilized in orthopedic applications is normally between 3.5–6 million, and semi-finished bars and rods have a crystallinity of 50–55 percent. The careful balance of superior mechanical qualities and wear resistance that has made UHMWPE the material of choice in arthroplasty is due to this specific combination of chemical structure, molecular mass, and microstructure. The ultra-high molecular weight is linked to a high entanglement density; entanglements act as physical crosslinks, affecting the crystalline morphology when the polymer is melt-crystallized. Unlike lower molecular weight cousins, UHMW crystallizes into a non-spherulitic structure with convoluted, faulty lamellae. Compared to medium and low molecular weight, linear, high-density polyethylene (HDPE), which may melt crystallized to 70–80 percent, the poor crystallinity is due to the high entanglement density.

UHMW IS WIDELY USED FOR:

• Chute, hopper, and truck bed liners are just a few of UHMW applications.
• Guide rails and wear strips
• Idler sprockets and star wheels
• Conveyors that move at a high rate
• Parts for packaging machines
• Parts for food processing machinery
• Dock fenders, bumpers, and pile guards

Material Options for UHMW

• UHMW Liners: To protect metal surfaces and keep solid materials like sand, wood chips, or coal moving smoothly, UHMW sheet is frequently used to line chutes and hoppers.
• UHMW Parts: Easy to construct, UHMW is also utilized for star wheels, idler sprockets, and under-chain wear strips in packaging, conveyor, and food processing gear.
• UHMW Wood Replacement: UHMW is a popular choice for marine construction applications such as dock fender pads, piling guards, and anti-skid walkways since it has a low moisture absorption rate. Like wood, UHMW does not rust, rot, or splinter.
• UHMW Grades: UHMW comes in various grades, including reprocessed, enhanced bearing and wear, and FDA-approved.

TYPICAL PROPERTIES OF UHMW MATERIALS

Performance characteristics:

• Extremely robust and durable properties include:
• low friction, 
• excellent abrasion resistance, 
• good chemical resistance, 
• low moisture absorption, 
• and ease of fabrication.

UHMWPE’s replacement property:

It can replace many materials by its property for several applications. The below table will help to understand.

UHMWPE’s benefits include, but are not limited to:

• Excellent stress resistance and strong crack resistance
• Abrasion resistance – 15 times stronger than carbon steel in terms of abrasion resistance.
• Its percentage is 40%. Aramid yarns are stronger.
• Its chemical resistance – it can withstand most alkalis and acids, organic solvents, degreasing chemicals, and electrolytic attacks.
• It’s completely safe to use.
• Dielectric characteristics are excellent.
• Low coefficient of friction – self-lubricating (comparable to PTFE)
• Non-staining
• The FDA has approved food and medicinal applications.
• Floats in water due to low specific gravity.

Over the last few decades, the necessity to extend the life of orthopedic implants in order to fulfill the demands of younger and more active patients has resulted in a significant evolution in the fabrication of UHMWPE orthopedic devices. The necessity to extend the life of orthopedic implants in order to fulfill the demands of younger and more active patients has resulted in significant advancements in UHMWPE orthopedic device manufacturing during the last several decades.

The wear resistance, mechanical characteristics, and overall performance of UHMWPE biomaterials have substantially improved, as has been extensively reported in the literature, from gamma-air sterilized “historical” polyethylenes to the first and second generation of highly cross-linked materials. UHMWPE is still the most often utilized bearing material in total joint arthroplasty for its lengthy and reasonably successful history. However, because UHMWPE is a complex material, its mechanical and tribological behavior is heavily influenced by the morphological and chemical changes caused by processing conditions such as ionizing radiation sterilization or cross-linking.