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What’s the difference between LDPE and HDPE?

clock 6 minutes | 04 Jan 2019

In the beginning

To truly understand Low-Density Polyethylene and High-Density Polyethylene, we have to understand their origins. Both materials derive from polyethylene (PE), which is the most popular plastic in the world, used in everything from supermarket bags to bullet-proof vests. 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 the structure of their molecules.

In a nutshell: the chemistry of LDPE and HDPE

LDPE’s polymer chains have side branches. Picture a single, straight line – a chain – of molecules. Within this line another chain branches off in one direction. From that chain another chain branches off. These side branches stop the polymer molecules from lining up in an orderly fashion.

  • LDPE’s structure is not crystalline.
  • This irregularity is what gives LDPE its lower density.
  • The forces of attraction between polymer molecules are weakened.

HDPE’s polymer chains line up regularly. Picture an army of soldiers in formation, side by side, row by row, column by column, and you’ve got the idea of HDPE.

  • HDPE’s structure is crystalline.
  • HDPE’s formidable structure gives it a higher density than LDPE.
  • The forces of attraction between polymer molecules are strong.

LDPE’s long- and short-chain branches keep the material from packing tightly in its crystalline form. This gives it less tensile strength than HDPE, but greater ductility.

On the flip side, HDPE doesn’t have much branching going on. The molecules are tightly packed together during crystallisation, making HDPE dense and possessing higher resilience than LDPE.


At a glance: Characteristics

LDPE

HDPE

Flexible

Semi-rigid; tough

Good compression set

Good compression set

Good abrasion resistance

Excellent abrasion resistance

Lightweight

Lightweight

Good chemical resistance (variable(

Good chemical resistance (variable)

Weatherproof

Weatherproof

Low cost

Low cost


Industrial applications

Here’s a look at what you can expect when it comes to the performance of LDPE and HDPE in different applications.

Electrical insulation

Chemically inert, polyethylene has excellent electrical and mechanical properties, which make it a preferred material for cable insulation in high voltage. The application determines whether you should use LDPE or HDPE.

HDPE has higher abrasion and tear resistance than LDPE, along with higher tensile and shear strength. If you’re going to bury your cables underground, then opt for HDPE. Otherwise, LDPE is an excellent choice. LDPE cable conduits give you outstanding flexibility, a high fatigue life and high-impact strength. LDPE’s high strength also make it a good choice for cable cleats.

At a glance: Electrical properties

Dielectric constant@ 1MHz

Dielectric strength kV mm-1

Disspipation 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



Hydraulics and Pneumatics

Pneumatics is another application where you’ll find polyethylene, and not just because of economics. As HDPE is semi-rigid, it excels as tubing with push-to-connect fittings without clamps. It’s suitable for low-pressure pneumatics or fluid-handling applications. Its rigidity also gives it excellent properties as a banjo union cap, allowing it to be applied quickly and without fuss while protecting the application from dirt and moisture ingress.

different plugs

HDPE is excellent for protecting hoses. Spiral wrap, which is made from the material, has exceptional anti-crushing and abrasion resistance.

LDPE is also used for tubing, working particularly well for:
  • Air and pneumatic lines
  • Fluid feeds
  • Wire jacketing
  • Food and beverage processing
  • Potable water systems
  • Drains

LDPE is an excellent material for specialist hydraulic caps and plugs. As a translucent SAE Butt Weld Cover, the material allows the O-ring to remain visible, while an internal locking bead provides secure protection. LDPE’s flexibility shines as side release and compression fit plugs, with a continuous sealing lip. The material can also be formulated to be a bit more rigid, making it outstanding as a hexagonal cap.

Automotive

A special grade of HDPE is used in fuel tank manufacturing. In fact, it accounts for around 44% of the plastic used in fuel systems, according to Automotive IQ. HDPE is ideal for the extrusion blow-moulding process, giving the tanks excellent performance

  • Ability to manufacture complex shapes, allowing for the optimum use of fuel in tight spaces
  • Outstanding behaviour during impact
  • Lightweight

HDPE is also used in the car’s body, and because it’s lightweight, it helps to reduce fuel consumption.

LDPE can be found on the car’s exterior as lightweight parts. It’s particularly popular as a component used to protect car parts during manufacture and shipment. Take, for example, a car’s powertrain. As electrical connector caps, LDPE’s flexibility makes them quick and easy to apply and remove, either by automation or by hand. Stretch fit cover caps made of LDPE are the perfect solution for masking awkward shapes. Again, LDPE’s flexibility makes this possible.

Packaging

Low cost and easy to process, HDPE is used widely in the production of plastic packaging. It’s an especially effective barrier with chemical stability, which makes it perfect for containers and bottles, particularly household and industrial chemicals.

Note, unpigmented bottles made from HDPE are translucent and stiff, so they’re well suited for packaging products with short shelf life cycle. On the other hand, pigmented bottles have better stress crack resistance than unpigmented HDPE.

LDPE’s toughness, flexibility and transparency means it’s usually used in packaging in film applications where heat sealing is needed. It’s also used to make some flexible lids and bottles.

Pipe and flange protection

The caps and plugs used to protect pipes and flanges are mostly LPDE. Again, it’s all about the material’s flexibility and high-impact strength. The flexibility means fasteners are usually not needed, which adds to the ease of applying caps and plugs and removing them. The high-impact strength is critical to protect the pipes and flanges during transit and storage.It’s also corrosion resistant, another attribute that makes it so popular with the construction and oil and gas industries. The variety of protection on offer is vast. Just some examples include:

Manufacturing

Now let’s look at how these materials perform in another context.

Injection moulding: LDPE and HDPE

You can use either material, of course. Your choice will depend on what you’re producing. Both are extremely popular resins used in injection moulding, and both are cost effective.

LDPE is an easy flow material because of 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

Because HDPE is a crystalline material, 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 – and that, of course, depends on the processing conditions and even the design of the part that you’re manufacturing.

At a glance: injection moulding properties

LDPE

HDPE

Drying

Not usually necessary

Not usually Necessary

Melt Tempreature

180˚ - 280˚C (355˚ - 535˚F)

180˚ - 280˚C (355˚ - 535˚F). For higher molecular weigh gains, 200˚ - 250˚C (392˚ - 482˚F)

Mould Tempreature

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


A closer look at LDPE and HDPE

Finally, the British Plastics Federation offers these values so that you compare them for yourself.


Property

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


Resistance to chemicals

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

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How does the injection moulding process work?