Differences Between HDPE
Plastic and LDPE Plastic
By John Brennan, eHow Contributor
Plastics are ubiquitous in the modern world.
Polyethylene is the world's most common plastic. It finds innumerable applications in
everything from bottles and jugs to shopping bags and children's toys. It owes its
surprising versatility to its properties and molecular structure. There are different kinds
of polyethylene available; two of the most common are high-density polyethylene or HDPE
and low-density polyethylene or LDPE. While they have many similarities, they are
distinguished by several important differences.
Structure
Ethylene is a simple hydrocarbon with a pair of double-bonded carbon atoms and four
hydrogen atoms. All polyethylenes are long polymers of ethylene subunits. They have a
central chain of carbon atoms, each of which is bonded to the carbon ahead of it and the
carbon behind it and two hydrogen atoms. Some polyetheylenes have secondary chains that
branch out from the first chain; HDPE, however, is unbranched. This structural difference
between HDPE and other polyethylenes is important for determining its function. Both
high-density polyethylene and low-density polyethylene are two common plastics made
from petroleum.
Density
Since HDPE is unbranched, the polymers or chains can be packed together much more
tightly, making HDPE more dense than LDPE or other similar polyethylenes. Some
polyethylenes formed from exceptionally long chains are even more dense than HDPE;
these are called ultra high molecular weight polyethylenes (UHMWPEs).
Properties
Differences in structure and density make HDPE tougher, stronger and more opaque than
LDPE. Its ability to withstand heat and its chemical resistance are also somewhat superior
to LDPE.
Uses
Since it's stronger and more opaque than most other polyethylenes, HDPE is often used
for bottles, jugs, trays and containers. LDPE, medium density polyethylene (MDPE) and
very low density polyethylene (VLDPE) are more often used for shrink wrap, plastic bags,
films, hoses and tubing and other applications. UHMWPE is so strong it's sometimes used
for machine parts, edge protection on ice rinks and even artificial joints.
Characteristics of LDPE
LDPE was the first PE to be developed. It has low-density levels and only a small amount of
branching. It is very flexible and easy to clean. It is often used to make plastic film wrap
and plastic bags. Additionally, it is used to make plastic items that need to be molded, such
as plastic bottles used in labs and some prostheses.
Characteristics of HDPE
HDPE has higher density levels; it is also characterized by a linear structure consisting of
no branching. That makes HDPE stronger and more resistant to chemicals. It is most
commonly used for items requiring blow molding techniques, such as toys, automobile parts
and bottles. It is also used to create cutting boards since it meets FDA food service
standards.
Environmental Concerns
Although most of these plastics can be recycled, much of it ends up in our trash cans.
There are concerns in the environmental industry that, since these plastics do not break
down easily in a landfill, there will be many future negative effects due to our overuse of
these products. According to the American Chemistry Council's Resin Production and
Sales Stats report, released in November 2009, HDPE is the most common plastic and
LDPE is the fifth most common plastic produced in the U.S. The slow rate at which plastic
degrades vs. the amount produced makes the disposal of it an environmental issue.