engineering

Low-density polyethylene

Low-density polyethylene is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Low-density polyethylene rather than just read about it. In short: Low-density polyethylene (LDPE) is one of several varieties of polyethylene (PE). PE's, the dominant synthetic polymer, are produced in many forms that differ in terms of molecular weight, branching, and the incorporation of comonomers.

Key takeaways

Low-density polyethylene belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Low-density polyethylene to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Low-density polyethylene from memory before moving on to harder problems.

Reference excerpt

Low-density polyethylene (LDPE) is one of several varieties of polyethylene (PE). PE's, the dominant synthetic polymer, are produced in many forms that differ in terms of molecular weight, branching, and the incorporation of comonomers. Often these features are captured in terms of the density of the material. All forms of PE are colorless, odorless, rather chemically inert solids produced by the polymerization of ethylene, the monomer. LLDPE is a substantially linear polymer (polyethylene), with significant numbers of short branches. In 2013 the worldwide LDPE market reached a volume of about US$33 billion. A variety of LDPE's are known, which mainly differ by the degree of linearity vs branching. LDPE can be further classified: LDPE and linear low-density PE. The former is produced by free-radical polymerization and the latter by coordination polymerization (e.g. Ziegler-Natta catalysis.)

Properties LDPE is a thermoplastic. PE's are often classified by their densities.

LDPE has more branching (on about 2% of the carbon atoms) than HDPE, so its intermolecular forces (instantaneous-dipole induced-dipole attraction) are weaker, its tensile strength is lower, and its resilience is higher. The side branches mean that its molecules are less tightly packed and less crystalline, and therefore its density is lower.

Chemical resistance At room temperature it is not reactive, except to strong oxidizers; some solvents cause it to swell. It can withstand temperatures of 65 °C (149 °F) continuously and 90 °C (194 °F) for a short time. Made in translucent and opaque variations, it is quite flexible and tough.

Excellent resistance (no attack/no chemical reaction) to dilute and concentrated acids, alcohols, bases, and esters Good resistance (minor attack/very low chemical reactivity) to aldehydes, ketones, and vegetable oils Limited resistance (moderate attack/significant chemical reaction, suitable for short-term use only) to aliphatic and aromatic hydrocarbons, mineral oils, and oxidizing agents Poor resistance, and not recommended for use with halogenated hydrocarbons.

Applications

Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic. LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and various molded laboratory equipment. Its most common use is in plastic bags. Other products made from it include:

Trays and general purpose containers Corrosion-resistant work surfaces Parts that need to be weldable and machinable Parts that require flexibility, for which it serves very well Very soft and pliable parts such as snap-on lids Six-pack rings Juice and milk cartons are made of liquid packaging board, a laminate of paperboard and LDPE (as the waterproof inner and outer layer), and often with of a layer of aluminum foil (thus becoming aseptic packaging). Packaging for computer hardware, such as hard disk drives, screen cards, and optical disc drives Playground slides Plastic wraps Plastic bags Plastic containers Pipes Housewares Battery cases Automotive parts Electrical components

History LDPE was the first grade of polyethylene. It was discovered by John C Swallow and M.W Perrin at Imperial Chemical Industries (ICI) using a high pressure process and free radical polymerization. Major innovations were made in the 1950 by commercial, governmental, and university labs, focusing especially on catalysts, which allowed the production of PE without free radicals. Some of this work led to the Nobel Prize in Chemistry.

Recycling

The EPA estimates 5.7% of LDPE (resin identification code 4) is recycled in the United States. Despite its designation with the recycling symbol, it cannot be as commonly recycled as No. 1 (polyethylene terephthalate) or 2 plastics (high-density polyethylene).

See also Film blowing machine High-density polyethylene (HDPE) Linear low-density polyethylene (LLDPE) Medium-density polyethylene (MDPE) Polyethylene terephthalate (PET/PETE) Stretch wrap Ultra-high-molecular-weight polyethylene (UHMWPE)

References

External links Media related to Low-density polyethylene at Wikimedia Commons 2010_MSW_Tables_and_Figures_508.pdf. epa.gov

Illustrations

Worked examples

Example 1 — a first encounter with Low-density polyethylene

Start with the simplest possible case. Write down what Low-density polyethylene claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Low-density polyethylene before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Low-density polyethylene ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Low-density polyethylene

In research: Low-density polyethylene appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Low-density polyethylene in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Low-density polyethylene is common in secondary-school and first-year university syllabi. It links to neighbouring topics British inventions, Food packaging, Packaging materials, so understanding it makes those chapters shorter.
In everyday life: Look for Low-density polyethylene outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Low-density polyethylene in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Low-density polyethylene means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Low-density polyethylene out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Low-density polyethylene in simple terms?

Why does Low-density polyethylene matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Low-density polyethylene?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Low-density polyethylene.