Determining Polyethylene Specific Gravity Thresholds for Tariff Entry
Polyethylene tariff entry requires density testing per ISO 1183-1 Method A on annealed plaques, where crossing 0.940 g/cm³ shifts classification from 3901.10 to 3901.20.
Comonomer Incorporation
Meaning
Polymerization reaction steps integrate secondary olefin monomers into a polymer backbone during resin synthesis. This process, known as comonomer incorporation, introduces short chain branches into the linear structure of polyethylene. These branches prevent the polymer chains from packing closely together into crystalline regions.
Density Reduction
Chemical branch density determines the physical classification of the polymer resin. High levels of comonomer incorporation produce lower density resins such as linear low density polyethylene. The addition of comonomers like butene or hexene lowers the density from 0.960 to 0.918 grams per cubic centimeter.
This density shift is controlled by adjusting the ratio of comonomer to ethylene in the reactor.
Polymer Properties
Mechanical strength and flexibility are altered by the branch distribution of the polymer. Higher comonomer incorporation improves the tear resistance and environmental stress crack resistance of the material. This makes the polymer suitable for flexible film applications and blow molded containers.
It also increases the melt strength of the resin during processing.
Process Economics
Production costs are influenced by the choice and amount of comonomer used in the reaction. Hexene and octene comonomers are more expensive than butane but provide superior mechanical properties. High levels of comonomer incorporation require longer reaction times and higher catalyst costs.
Moulders choose the resin grade based on the required performance and the cost of the raw material. This selection balances the part cost against the performance requirements of the final product.