Acetal and polypropylene are two types of plastic. While polypropylene is less expensive and more resistant to moisture than acetal, acetal has greater strength and heat resistance. Both types of plastic are appropriate for different kinds of fittings.
Acetal is difficult to break and is resistant to heat, chemicals and everyday wear. However, its durability comes with a high price tag, and its restricted processing makes it more difficult to obtain than other plastics.
Polypropylene is a common type of plastic that's best known among consumers for its use in food storage containers. It resists chemicals, heat and cold; absorbs very little moisture; and is relatively inexpensive and easy to purchase.
Acetal absorbs water more rapidly than polypropylene, which may make it less appropriate for plumbing fittings. However, it has more than double the tensile, flexural and compressive strength of polypropylene, making it better for fittings in particularly high-stress environments. Furthermore, while polypropylene is only safe at 180 degrees Fahrenheit or less, acetal's melting point is 347 degrees Fahrenheit. These benefits may make acetal worth the extra cost and may even balance out the water absorption issue for some users.