Both LDPE and HDPE have the exact same monomer of ethylene. They are chemically the exact same substance - a super long-ass chain of ethylene units:
They are both relatively low melting thermoplastics. LDPE has a slightly lower melting point of about 110 °C as compared to HDPE which melts at around 131 °C (see footnote 1). These low melting points allow for fast injection molding and there is no need for drying the molded part. They are both relatively low weight materials (light weight compared to other materials for objects). They have high impact resistance and resistance to chemicals, water vapor, and weathering. They are both great for recycling and their ease of use makes them a low cost choice for manufacturing. Common industries that incorporate polyethylenes are automotive, packaging, piping, electrical, and hydraulics.
Well the first obvious difference is their densities. The LD is LOW-density and the HD is HIGH-density. What is that in numbers? It is sutle, but significant. LD ranges around 0.91-0.93 g/mL while HD is in the range of 0.94-0.97 g/mL. Believe it or not, that is enough of a difference that these plastics can be physically separated based on their densities in float tanks.
LDPE is a softer, more flexible plastic than HDPE. It melts at a lower temperature (110 °C vs 131 °C). Softer unfortunately also means not as strong. LDPE is more likely to crack and fracture when under stress. HDPE, due to its much more cystalline structure (see below) is simply stronger, harder, and more resilient towards both stress and chemicals.
And... even though they are both very recyclable, they must be processed differently which is why they have different recycle symbols of ♶ (LDPE) and ♴ (HDPE). Most LDPE plastics are films and light weight materials that can easily jam up recycling equipment. HDPE will more easily run through a recycling plant with no issues.
LDPE and HDPE are great examples of how the three-dimensional structure of a compound will influence and govern the overall physical properties of a substance. In general, this is true for all similar compounds and substances:
The more ordered/layered the molecules are, the more crystalline, and therefore the harder the substance and higher the melting point. The more disordered, the less crystalline (more amorphous) and lower melting point.
Branching! The amount of branching in a polymer chain influences the ability to stack closely. Branching always tends to produce a more disordered and open substance. Just think about packing 100 lbs of 3 foot sections of tree limbs with little branches sticking out of each one. It is tough to get a good tight packing. But do the same with relatively straight sticks (think wooden dowels) and you can easily pack them neatly into a much much smaller space. Realize that my example is a bit extreme, but the fact is that HDPE is way less branched (almost no branching) than LDPE (about 2% of the carbons have branching). This is why the density is higher for HDPE and why the melting point is higher. The intermolecular forces (IMFs) for HDPE are greater than for LDPE because the chains can make closer approaches to each other and therefore have a slightly higher forces of attraction. This more crystalline structure is also much stronger in tensile strength. So HDPE is a much better choice for objects that need some extra strength.
Below is a schematic of the long carbon chains in both LDPE and HDPE. Note how the LDPE chains are more branched and more loosely packed, while the HDPE shown has no branching and is packed a bit tighter.
1. Note that both LDPE and HDPE melting points are not really exact - which is true for any polymer. Polymers will usually melt over a range of temperatures and it depends on the method of manufacturing and ultimately the length and branching of the carbon chains. LDPE generally melts over the range of 105-115 °C, while HDPE melts over a range of 120-140 °C. The numbers given in the text are in the middle of these ranges and are therefore a good approximation.
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