What’s the difference between LDPE and HDPE?
What is LDPE made of? What are HDPE properties and uses? The difference between high-density polyethylene and low-density polyethylene lies in their chemical structure. This guide will cover what that means in terms of characteristics, and common examples and applications of the two materials. We’ll also cover:
- LDPE vs. HDPE: a quick introduction
- What is LDPE?
- What is HDPE?
- Differences between LDPE and HDPE
- Differences between LDPE and HDPE in physical characteristics
- Similarities between LDPE and HDPE
- Differences between LDPE and HDPE in recyclability
- Differences between LDPE and HDPE in production methods
- LDPE vs. HDPE examples
- Applications of LDPE and HDPE
LDPE vs. HDPE: a quick introduction
To truly understand high-density polyethylene and low-density polyethylene, we have to understand their origins. Both plastic materials are made from polyethylene (PE), which is the most popular plastic in the world, used in everything from plastic milk bottles and flexible packaging material to outdoor furniture.
PE is a thermoplastic created by polymerising ethylene. This can be done in different ways that produce LDPE and HDPE. In other words, these polymers have different properties because of their molecular structure.
What is LDPE?
First, let’s consider the structure of low-density polyethylene. Its polymer chains have side branches. The illustration here represents a chain of LDPE molecules. Within this line another chain branches off in one direction. From that chain another chain branches off.
The occurrence of branching stops the polymer molecules from lining up in an orderly fashion.
LDPE is a semi-crystalline structure. This irregularity is what gives LDPE chemistry its lower density. The forces of attraction between polymer molecules are weakened compared to HDPE.
What is HDPE?
You can see the structure of HDPE illustrated here. With a crystalline structure, its polymer chains line up regularly.
The lack of branching gives HDPE’s formidable structure a higher density compared to the density of LDPE. HDPE strength is the result, with the forces of attraction between polymer molecules strong.
Differences between LDPE and HDPE
It comes down to HDPE vs LDPE chemical structure. LDPE’s long- and short-chain branching keep this plastic material from packing tightly in its crystalline form. LDPE crystallinity gives it less tensile strength than HDPE. LDPE structure, however, has greater ductility.
Why is HDPE stronger than LDPE? As already mentioned, HDPE doesn’t have much branching going on. The molecules are tightly packed together due to HDPE crystallisation. This makes HDPE dense, possessing higher resilience than LDPE.
Differences between LDPE and HDPE in physical characteristics
Everything comes back to LDPE vs HDPE branching. However, additives can enhance all plastic materials – including HDPE and LDPE – to achieve the characteristics you’re after. The characteristics below are typical of HDPE and LDPE plastic, but keep in mind you can enhance your desired qualities.
HDPE vs LDPE properties
| Characteristic | LDPE | HDPE |
|---|---|---|
| Flexibility | Low crystallinity (50-60%) = more flexibility | High crystallinity (>90%) = more rigidity & toughness |
| Heat resistance | Significant decrease in density when subjected to temperatures over 20°C | Withstands heat of temperatures over 100°C |
| Melting point (varies with density) | Lower melting point, typically 106°C to 115°C | Typically 125°C to 135°C |
| Transparency | High, due to amorphous condition | Low, due to increased level of crystallinity |
| Abrasion resistance | Good | Excellent |
Similarities between LDPE and HDPE
The materials share the same polymerized ethylene molecules, so you’ll find many similarities between LDPE and HDPE material properties.
- Low material weight
- High impact strength
- High recyclability
- Cost-effectiveness
- Low cost to manufacture
- Weather resistance
Sometimes their properties are almost identical, as these values from the British Plastics Federation show. Note the difference between high density and low density.
| Properties | LDPE | HDPE |
|---|---|---|
| Tensile Strength | 0.20 - 0.40 N/mm² | 0.20 - 0.40 N/mm² |
| Thermal Coefficient of expansion | 100 - 220 x 10-6 | 100 - 220 x 10-6 |
| Max. Cont. Use Temp. | 65˚C / 149˚F | 65˚C / 149˚F |
| Density | 0.917 - 0.930 g/cm3 | 0.944 - 0.965 g/cm3 |
| Chemical resistance | LDPE | HDPE |
|---|---|---|
| Dilute Acid | Excellent | Excellent |
| Dilute Alkalis | Excellent | Excellent |
| Oil and Greases | Moderate (Variable) | Moderate (Variable) |
| Aliphatic Hydrocarbons | Poor | Poor |
| Aromatic Hydrocarbons | Poor | Poor |
| Halogenated Hydrocarbons | Poor | Poor |
Differences between LDPE and HDPE in recyclability
Not all types of plastics are easy to process when it comes to recycling. Generally, the lower the classification number, the easier the plastic is to recycle.
Thermoset plastics can not be recycled. HDPE and LDPE are thermoplastics, so they can be. Any plastic that can be recycled are separated according to their resin type. This means that LDPE – classed under recycling number 4 – and HDPE – classed as recycling number 2 – are recycled separately. The process for recycling both plastics is the same, however. Grinders shed the plastics into flakes, which are cleaned, dried, and melted into pellets, or granules. Recycled plastic granules are often used in manufacturing.
Typically, LDPE can be trickier to recycle than HDPE, as it’s a softer material. It can get caught in the recycling machinery, while denser HDPE is easier for machines to process.
Differences between LDPE and HDPE in production methods
The LDPE manufacturing process is done by compressing monomer ethylene gas in an autoclave or tubular reactor. This facilitates the linking of monomers into polymer chains, called polymerization.
The manufacturing process of HDPE involves heating petroleum to high temperatures. This releases the ethylene gas monomers, which combine to form polymer chains.
Both materials are extremely popular resins used in injection moulding, and both are cost effective. It’s helpful to look at how they perform in the manufacturing process.
LDPE and HDPE in injection moulding
The crystallinity of LDPE makes it an asy-flow material with its long-chain branching. It’s also well suited to hot runner moulds, so fast injection speed is recommended. Likewise, HDPE flows easily.
And again, like LDPE, use a fast injection speed. If frequent colour changes are necessary with HDPE, use an insulated hot tip runner.
LDPE Shrinkage
Actual shrinkage values depend on the moulding conditions. As a general rule, you’re looking at shrinkage of 0.02 – 0.05mm/mm, or 2% - 5%, when density is between 0.91 – 0.925 g/cm˄3.
When density is between 0.926 - .04 g/cm˄3, expect shrinkage of 1.5 – 4%.
HDPE Shrinkage
The crystallinity of HDPE means the shrinkage is high: about 0.015 – 0.04mm/mm or 1.5 – 4%. This will depend on the degree of orientation and the level of crystallinity in the part. That, of course, depends on the processing conditions and even the design of the part that you plan to manufacture. Also note, HDPE processing temperature is higher than that of LDPE.
LDPE vs HDPE: properties for injection moulding
|
LDPE |
HDPE |
|
|
Drying |
Not usually necessary |
Not usually Necessary |
|
Melt Temperature |
180˚ - 280˚C (355˚ - 535˚F) |
180˚ - 280˚C (355˚ - 535˚F). For higher molecular weigh gains, 200˚ - 250˚C (392˚ - 482˚F) |
|
Mould Temperature |
20˚ - 70˚C (68˚ - 158˚F) For uniform and economic heat removal, cooling channel diameters should be at least 8 mm. The distance from the surface of the mould to the edge of the cooling channel not exceed 1.5 times the diameter of the cooling channel |
20˚ - 95˚C (68˚ - 194˚F). Higher temperatures for wall thickness of up to 6mm. Lower temperature for wall thickness greater than 6mm. Cooling rate should be uniform to minimise shrinkage variations. Cooling channel diameters should be at least 8mm and must be within a distance of 1.3 d from mould surface (where ‘d’ is the diameter of the cooling channel) |
|
Material Injection Pressure |
Up to 150 MPa |
70-105Mpa |
|
Pack Pressure |
Up to 75Mpa |
|
|
Injection Speed |
Fast. Profiled speeds can limit warpage problems of large surface area parts |
Fast. Profiled speeds can limit warpage problems of large surface area parts |
You can learn more in our guide What is plastic injection moulding and how does it work?
LDPE vs. HDPE examples
Uses of LDPE and HDPE are everywhere in the home. But what about more demanding applications? Instead of considering HDPE vs LDPE bottles, it might help you more if we look at examples of industrial uses of HDPE and also industrial uses for LDPE.
Applications of LDPE and HDPE
HDPE vs LDPE crystallinity. How does that affect their applications? They each play critical roles across different uses and industries.
Applications that need electrical insulation
HDPE or LDPE? Chemically inert, polyethylene has excellent electrical and mechanical properties, which make it a preferred material for cable insulation in high voltage. Both LDPE insulation and HDPE insulation are excellent.
HDPE vs LDPE structure: electrical properties
| Dielectric constant@ 1MHz | Dielectric strength kV mm-1 | Dissipation factor @ 1MHz | Surface resistivity Ohm/sg | Volume resistivity Ohm/cm | |
|---|---|---|---|---|---|
| HDPE | 2.3-2.4 | 22 | 1-10 x 10-4 | 1013 | 1015-1018 |
| LDPE | 2.2-2.35 | 27 | 1-10 x 10-4 | 1013 | 1015-1018 |
Cable management
Both are used in cable management, with HDPE taking on more heavy-duty roles. HDPE and LDPE examples include:
HDPE has higher abrasion and tear resistance than LDPE, along with higher shear and tensile strength. This particular HDPE cable clip is a blind loop fastener that not only secures cables to walls or ceilings, but can also handle pipes, or tubes. It’s simple to install: feed the cables or tubes through the U-shaped clip and push into the hold in the wall.
Universal cable grip is ideal for bundling multiple cables while keeping them separated at the same time. The base is rigid HDPE for a stable, strong hold, which is secured with screws. Due to LDPE tensile strength, HDPE is more appropriate here. The inserts need extreme flexibility, which is why they’re made of rubber-like TPE.
When you need to combine flexibility and strength, then LDPE cable protection is a good choice.
Ideal as cable conduit if you need outstanding flexibility, high-fatigue life and high-impact strength. Its high-impact strength aids its recovery capability when crushed. Typical applications include general factory wiring and connections to machines.
LDPE’s strength also makes it a good choice for cable cleats, which are used to secure, fix and route electrical cables in positions along the route of an electrical installation. LDPE material protects cables from damage, vibration, and chafing due to excessive movement.
You can learn more in our Ultimate guide to cable management.
Hydraulics and pneumatics
Pneumatics is another application where you’ll find polyethylene, and not just because of economics. Is HDPE rigid? It’s semi-rigid, so it excels as tubing with push-to-connect fittings without clamps. It’s suitable for low-pressure pneumatics or fluid-handling applications. The chemical properties of LDPE and HDPE make both of these materials ideal for these applications.
You can also check out our complete range of hose protection to see more options – not only in LDPE or HDPE, but other materials.
HDPE enables these caps to be applied quickly and without fuss while protecting the application from dirt and moisture ingress.
HDPE spiral wrap hose protectors
HDPE is excellent for protecting hoses with exceptional anti-crushing and abrasion resistance. It’s also available in a more heavy-duty HDPE for high-material memory and maximum coverage of the hydraulic hose. The thicker wall thickness provides a more robust protection.
HDPE plastics are also used for threaded o-ring plugs, while LDPE properties make it an excellent material for specialist hydraulic caps and plugs.
Available in two colours. LDPE red option is ideal for SAE flange nipples. The Natural option protects split-flange coupling surfaces. The translucent material also allows the O-ring to remain visible, while providing secure protection by an internal locking bead.
LDPE flexibility enables easy fitting and removal when protecting hexagonal connections and assemblies. They can also be used for the protection of round components, making them a versatile choice for a range of projects.
Pipe and flange protection
HDPE vs LDPE pipe and flange protection isn’t much of a contest. HDPE properties mean the material is too rigid to be as effective as LDPE caps and plugs, which are used to protect pipes and flanges. LDPE’s flexibility means fasteners are usually not needed, which adds to the ease of applying caps and plugs and removing them. It’s also corrosion resistant, another attribute that makes it so popular with the construction and oil and gas industries.
You can see our entire range of pipe and flange protection now.
Available in two styles with flexible ribs of different diameters to fit multiple schedules of NPT and drill pipe. The straight wall on the plug’s outside diameter provides a secure friction fit with the mating pipe’s inside diameter providing excellent protection against contaminants entering the pipe.
LDPE push-in full face flange protector
The flexible fins ensure a snug fit to protect the full face of a flange.
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