Polymer Basics for Acetals
Acetals are easy to machine, general-purpose engineering plastics of outstanding dimensional stability. They are high molecular and high crystalline plastics produced by the polymerization of formaldehyde or its derivatives. Acetals are tough and boast of high stiffness and a low coefficient of friction. They are commonly used in medical applications, including inhaler components, Luer caps, and blood filtration materials.
Characteristics and Properties of Acetals
Acetals are rigid, strong, and have a low coefficient of friction against plastics, metals, and other acetals. They also boast of excellent creep resistance, making them a popular choice in applications that require dimensional stability. Additionally, acetals have low water absorption, high wear characteristics, and good electrical properties. They are FDA approved for use with food because they are easy to machine and fabricate. Acetals also boast of superior moisture and fatigue resistance.
Acetals are geminal-diether derivatives of ketones or aldehydes created by reacting two equivalents of alcohol and eliminating water. Acetals are made when an aldehyde or ketone reacts with alcohol in the presence of an acid catalyst. During the formation of acetals, a hemiacetal is formed as an intermediate. The mechanism of acetal formation follows the seven steps outlined below:
- Protonation of carbonyl - the transfer of a proton (H) to a molecule or atom to form a bond
- Nucleophilic attack by alcohol - alcohol is used to perform a nucleophilic attack on the product formed in step one
- Deprotonation to form a hemiacetal - a hemiacetal is created when alcohol is added to acetone or aldehyde
- Protonation of alcohol - this guarantees alcohol leaves the molecule in step five due to its unstable charge.
- Removal of water
- Nucleophilic attack by alcohol
- Deprotonation by water
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Common Uses in Medical Devices and Life Sciences
Acetal is an excellent material for medical device applications that require low friction. Acetals are slippery and are therefore good for sliding mechanisms. They are also easy to machine, chemically resistant to hydrocarbons, neutral chemicals, and solvents. They make an excellent choice for medical device applications that require complex tight tolerances. Acetal is commonly used in the following medical applications:
- Medical instruments - equipment that must be sterilized regularly, such as handles, trays, prosthesis parts, scraper blades
- MRI machines, surgical instruments, dental instruments, handheld diagnostic wands, and sterilization trays
- Endoscopic probes, diagnostic, anesthetic, and imaging equipment
- Biomedical applications such as drug conjugation and regenerative medicine
The Best Method of Sterilization
The FDA regulates four types of sterilization of medical devices made from natural polymers. These are autoclave, dry heat, ethylene oxide, and irradiation sterilization methods. Each method's effectiveness and suitability is typically dependent on the polymer material's specific properties.
Acetal medical devices can be best sterilized using ethylene oxide gas, a process that is highly reliable and effective. The devices sterilized using this method requires several hours of aeration to eliminate residual gas.
It is worth noting that ethylene oxide is highly toxic, and devices should be dried entirely before subjecting them to the gas.
Acetal devices can also be sterilized using gamma irradiation. This method of sterilization involves exposing devices to gamma rays. Cobalt-60 is one of the standard gamma radiation sources used for ionizing radiation sterilization. Additionally, acetals can also be sterilized using the E-beam sterilization technique. This technique involves exposing devices to high energy electron beams to rid them of contaminants. E-beam sterilization method reduces the exposure time needed for sterilization, which results in less chemical degradation of the devices.