There are three types of olefinic groups present in sufficient concentrations to warrant consideration, one or more of that can normally be found in any polyethylene. The three types are trans-vinylene, R-CH=CH-R′, sometimes referred to as trans-internal unsaturation; vinylidene or pendent methylene, RR′C-CH2; and vinyl unsaturation, R-CH=CH2, also referred to as terminal unsaturation.

The type and quantity of these groups can influence the chemical and physical properties of the resin. Information concerning their presence may also be used to characterize or identify unknown resins or blends of resins.

Additives may interfere with unsaturation measurements. The use is cautioned to determine which additives are present and if there any absorbance bands caused by additive presence that overlap or interfere with unsaturation absorbance bands used in this test method in the range of 1050 to 850 cm^-1. If overlapping bands occur, the method is not applicable.

Interference fringes resulting from smooth sample surfaces can cause measurement errors. This test method requires the use of aluminum foil in the compression molding of samples to provide an adequately rough surface to minimize interference fringes.