LDPE Vs HDPE: What are the Differences and Similarities

Q: Why is HDPE called the best food grade plastic?

HDPE is called the best food safe plastic. It is given the FDA symbol "2". HDPE is one of the most stable and most inert forms of plastic with features like corrosion resistance and less moisture absorbent, making it the best choice for any food-storing application.

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LDPE Vs HDPE

Both HDPE and LDPE belong to the Polyethylene family of thermoplastic materials extracted from petroleum through the process of polymerization. As we all know, polyethylene is one of the most common and utilized plastic materials globally. It has a simple molecular structure, and its applications can be found in everything from children’s toys to grocery bags to water bottles.

Distributively, under the polyethylene class of materials, HDPE (High-density Polyethylene) and LDPE (Low-Density Polyethylene) stand out as they’ve made their way into the packaging and manufacturing industry and transformed them forever into a more affordable and reliable asset.

Here are the basic attributes of the two materials:

Low-density Polyethylene – The clear or transparent plastic shows incredible chemical resistance, flexibility, and waterproofing capacity. It is utilized to make a wide array of products – the most common applications being flexible packaging material, grocery bags, plastic wrap & films, and injection molded components.
High-density Polyethylene – The highly popular plastic material is known for exhibiting high impact strength and durability. It is commercially available in many variations, such as transparent and opaque, and like LDPE, it also demonstrates excellent chemical resistance. The most popular HDPE applications include automotive parts, toys, kitchen equipment, plumbing pipes, outdoor furniture, and structures.

Although LDPE and HDPE are thermoplastic ethylene polymers, they have several differences in their properties, structure, and uses. One stark distinction between them is that the LDPE is more branched out in its structure than in perfect rows compared to HDPE. Branching occurs during polymerization, where polymer chains are attached with secondary polymer chains joined to them through replacing an atom in the primary chain with a monomer group.

LDPE Vs HDPE: Properties –

Property	LDPE	HDPE
Chemical Structure	More branching	More linear, less branching
Flexibility	Low density – 0.91-0.94 g/cm3	High density – 0.95-0.97 g/cm3
Heat Resistance	A substantial decrease in density when kept through temperatures beyond 20°C	Capable of resisting heat with temperatures more than 100°C
Flexibility	Low crystallinity (50-60%), leading to more flexibility	High crystallinity (>90%), therefore, makes it tougher and more rigid
Strength	Increased impact strength in cold conditions	High tensile strength and specific strength
Transparency	High, because of amorphous condition	Low, due to high crystallinity
Chemical Resistance	Resistant to most alcohols, alkalis, and acids; low resistance to oxidizing agents and selected hydrocarbons	Outstanding resistance to acids, alcohols, solvents, and alkalis; low resistance to hydrocarbons
Maximum allowable stress at 20°C	6–17 MPa	14–32 MPa

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Similarities Between LDPE and HDPE –

Since they are fundamentally made up of the same polymerized ethylene molecules, LDPE and HDPE have many similar attributes. Below are similar properties.

Low material weight
High impact strength
High recyclability
Cost-effectiveness
Tensile strength ranging from 0.20 to 0.40 N/mm2
Low cost of manufacturing
Resistance to chemicals and weathering

The following are the similar characteristics seen in both materials when processed in injection molding:

Melting Point – 180 ̊ to 280 ̊ C (355 ̊ to 535 ̊ F)
Rapid injection speeds
Drying of finished component not necessary

The materials have several similar applications. Here are some industries using LDPE and HDPE extensively.

Packaging
Pipes & Fittings
Electrical
Automotive
Hydraulics & pneumatics

Difference Between LDPE and HDPE –

As we discussed the similarities between LDPE and HDPE, now time to have a look at the differences. Owing to different compositions and branching, there are many attributes within both materials that are drastically different.

Differences in Physical Attributes:

LDPE is more flexible and softer than HDPE. It is also translucent and has a lower melting point of 115° C. However, when compared to HDPE, it is more likely to rupture under stress.

On the other hand, HDPE is more rigid and durable and offers better chemical resistance. It has a higher melting point of 135°C, allowing it to resist higher temperatures than LDPE. In addition, its crystalline structure provides it greater strength and opacity.

Differences in Production of LDPE and HDPE:

How is LDPE produced?

LDPE is produced by either a stirred autoclave or tubular reactor. Its general manufacturing process involves the compression of ethylene gas, polymerization using an initiator, and gas separation.

How is HDPE produced?

HDPE is produced by heating the petroleum at very high temperatures using slurry polymerization or gas phase polymerization. The process begins with the polymerization of a solution of ethylene monomers after operation and drying.

Differences in Processing of LDPE and HDPE:

The thermoplastic can be very easily processed in the following methods:

Injection Molding:

The injection molding process transforms LDPE or HDPE granules into custom shapes and sizes determined by the mold. First, the granules are sent to a hot barrel, from which the materials are liquified or melted through a screw barrel and heater bands. The molten plastic is then injected into a pre-designed mold cavity, which also cools down the material. After the material is solidified, it is ejected out of the injection molding machine.

Blow Molding:

This processing method is utilized to manufacture hollow-shaped plastic components. Rather than injecting the molten plastic, blow molding uses compressed air to blow the material into the mold.

Extrusion:

Like injection molding, this method also uses heat to melt the plastic granules. However, the difference is seen in the final part of the machine, where instead of going through the mold, the melted plastic goes through a pre-designed opening and is then cooled down to solidify.

Differences in Recyclability:

Both LDPE and HDPE are recyclable, but recycling for both should be done separately. LDPE is categorized as under recycling number 4, and HDPE under recycling number 2. Although it heavily depends on the product, recycling LDPE can be challenging due to softness; it is very likely to get stuck in the machinery. On the side, HDPE is easier to recycle and transport thanks to its hardness.

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Advantages & Disadvantages of HDPE and LDPE –

	Advantages	Disadvantages
LDPE	Good electrical insulation Waterproofing ability Low cost A wide array of uses Easy to process and mold	Meager UV resistance Not useful in extremely high and low temperatures High permeability to Co2 and other gases More susceptible to stress crack
HDPE	Low cost High tensile strength Waterproofing Good electrical insulation resistance to low temperatures	Meager UV resistance More risk of more shrinking May crack under stress.

HDPE vs LDPE: Applications –

LDPE

Wash bottles
Plastic bottles
Computer hardware covers and packaging
Light packaging materials
Corrosion protection layer for work surfaces

HDPE

Automotive parts like fuel tanks
Pipes and fittings
Boat parts
Plastic chairs and tables
Consumer products: ice cube containers, trash, and recycling bins, ice chest, toys
Fibers for ropes, industrial fabrics, and nets
Packaging materials: bottle caps, plastic milk bottles, drums, bulk containers for industrial use.

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FAQs –

1. Is LDPE cheaper than HDPE?

Ans. Yes. LDPE is cheaper than HDPE owing to the lower impact strength and chemical resistance than HDPE. However, the price difference between both is not very substantial.

2. Can HDPE and LDPE be mixed?

Ans. Unlike other polymers, HDPE and LDPE can be mixed. However, to be recycled together, it’s better to separate them to maintain their specific properties and get the best results.

3. What dissolves HDPE?

Ans. The most useful solvents for dissolving HDPE are trichlorobenzene, trichloroethylene, tetralin, Benzene, toluene. But if you want to go for less hazardous options than toluene, xylene, and trichloroethylene are good choices.

4. Why is HDPE called the best food grade plastic?

Ans. Ans. HDPE is called the best food-safe plastic. It is given the FDA symbol “2”. HDPE is one of the most stable and most inert forms of plastic with features like corrosion resistance and less moisture absorbent, making it the best choice for any food-storing application.

5. What to Choose LDPE or PET bottles?

Ans. Hands down, PET is the best material for manufacturing all types of bottles, but sometimes using LDPE bottles makes more sense. LDPE is a more squeezable material making dispensing sauces, honey, creams, condiments, and syrups. It is also better at withstanding colder temperatures than PET.

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Final Thoughts –

That was all I had to say about HDPE vs. LDPE. Choose carefully between the two for smoother operations and the best results. Also, go through the detailed guides of each material for understanding the materials in detail.

Kindly share our thoughts and question in the comment box.

Have a Phenomenal day.