What Is PE (Polyethylene)?
Polyethylene (PE) is a linear macromolecular polymer obtained by addition polymerization of ethylene monomer (CH₂=CH₂). It is one of the most widely used plastics worldwide.
At its core, the molecular structure of polyethylene is very simple: the –CH₂–CH₂– unit repeats continuously to form long chains.
This simple and stable structure gives rise to several key properties:
1. Good chemical stability: It shows good resistance to most acids, bases, and salt solutions and is not easily corroded. (Note: In strongly oxidizing media—such as concentrated nitric acid, fuming sulfuric acid—and in certain organic solvent systems, degradation or swelling may still occur. Specific media and temperature conditions must be evaluated.)
2. Thermoplastic behavior: It softens and melts upon heating and resolidifies upon cooling, allowing repeated processing and molding.
3. Appearance and feel: Typically ranges from translucent to milky white, is lightweight, and offers a certain degree of toughness and softness.
4. Electrical insulation: It has high volume resistivity and low dielectric loss, providing excellent electrical insulation performance, and is therefore widely used in cable sheathing and related applications.
Thanks to its low cost, wide processing window, and tunable properties, PE is extensively used in:
1. Packaging materials (plastic bags, cling film, stretch film)
2. Various bottles, containers, drums, and pipes
3. Disposable medical and laboratory consumables and packaging materials
4. Packaging for household cleaning products, etc.
Main Types of PE: HDPE, LDPE, LLDPE
The properties of polyethylene vary significantly with changes in density and the degree of branching in the molecular chains. In engineering and everyday applications, the three most common types are:
HDPE (High-Density Polyethylene)
LDPE (Low-Density Polyethylene)
LLDPE (Linear Low-Density Polyethylene)
Comparison of Three Common PE Types
Type | Full Name | Typical Density (g/cm³) | Molecular Structural Features | Key Performance Keywords | Typical Melting Point Range (°C) | Typical Application Examples | Recycling Code |
HDPE | High-Density PE | ~0.94–0.97 | Molecular chains are essentially linear with very few branches; high crystallinity | High stiffness, good strength, impact resistant, resistant to many chemicals | ~120–130 | Detergent bottles, bleach bottles, milk bottles, oil drums, pipes, garbage bins, etc. | ♻️2 (HDPE) |
LDPE | Low-Density PE | ~0.91–0.93 | Molecular chains are more highly branched; lower crystallinity and density | Soft, highly extensible, tear resistant, suitable for films | ~105–115 | Plastic bags, shopping bags, garbage bags, cling film, food packaging films, etc. | ♻️4 (LDPE) |
LLDPE | Linear LLDPE | ~0.91–0.94 (similar to LDPE) | Linear backbone with short side branches; properties between HDPE and LDPE | Good toughness, puncture resistant, suitable for stretch film and packaging films | ~115–125 (formulation dependent) | Shopping bags, stretch wrap film, cling film, agricultural films; often blended with LDPE | Generally also classified as ♻️4 |
Note: Density and melting point ranges may vary slightly depending on specific grades and comonomers. The values in the table represent typical ranges.
A brief explanation of recycling codes:
The “♻️ + number” symbol on plastic products is formally called the Resin Identification Code (RIC). Its primary function is to indicate to recycling systems “what type of plastic this is.” It does not guarantee that the product will actually be recycled or that it is inherently environmentally friendly. Nor does it imply that appropriate recycling channels necessarily exist locally. Whether and how a product can be recycled depends on regional recycling policies and facilities.
Brief Overview of Key Types
(1) HDPE: High-Density Polyethylene
The molecular chains of HDPE have almost no long branches, and both crystallinity and density are relatively high. As a result, the material is stiffer, more impact resistant, and more resistant to most chemicals. This makes it particularly suitable for:
(1) Detergent bottles, bleach bottles, and other cleaning product packaging
(2) Milk bottles, oil drums, and storage containers
(3) Garbage bins, pipes, fittings, and other structural products
In food-contact applications, HDPE that meets regulatory requirements is also widely used for food containers and packaging materials (such as dairy bottles and certain food pails).
(2) LDPE: Low-Density Polyethylene
LDPE has a higher degree of chain branching and lower density and crystallinity. The material is soft and highly extensible, making it particularly suitable for various types of films:
(1) Plastic bags, shopping bags, garbage bags
(2) Cling film, food packaging films
(3) Flexible packaging pouches and certain flexible medical packaging
Many of the “soft plastic bags” people encounter in daily life are made of LDPE or LDPE/LLDPE blends.
(3) LLDPE: Linear Low-Density Polyethylene
Structurally, LLDPE lies between HDPE and LDPE: its backbone is relatively linear with short side branches. This structure retains a certain level of strength and puncture resistance while maintaining softness and stretchability.
Its most typical applications include:
(1) Shopping bags, cling film, stretch wrap film
(2) Various packaging films and agricultural mulch films
(3) Blends with LDPE to balance “soft hand feel” and “mechanical strength”
In simple terms, LLDPE is an excellent material for producing strong, tough films and is very common in the packaging industry.
Advantages and Limitations of PE
1. Key Advantages
These can be summarized in three points:
1. Resistant to most chemicals: Generally stable in the presence of acids, bases, and salt solutions, making it very suitable for containers and piping.
2. Lightweight, tough, and tunable: By adjusting density and molecular structure, PE can cover a wide spectrum from “soft films” to “rigid bottles and pipes.”
3. Easy to process and cost-effective: Compatible with multiple processing methods such as blow molding, extrusion, and injection molding, enabling large-scale manufacturing.
These are the fundamental reasons why PE is “everywhere” in packaging, home and personal care, medical applications, and laboratory consumables.
Limitations and Points of Attention in Use
(1) Limited heat resistance
(a) LDPE is generally suitable for short-term use at temperatures up to around 80°C.
(b) HDPE can withstand conditions close to the temperature of boiling water (about 100°C) for short periods. Certain grades can tolerate higher temperatures briefly, but for long-term use, temperatures are typically recommended to be controlled within the 80–100°C range, and actual limits should follow the material datasheet or product specification.
The key point is:
The melting point is not the same as the safe long-term service temperature. For long-term use, repeated heating, or contact with high-temperature media, the product manual or relevant standards must be followed.
Therefore:
(a) It is not recommended to expose ordinary PE films or plastic bags directly to hot oil or boiling water.
(b) PE containers that are not explicitly labeled as suitable for high-temperature use are generally not appropriate for ovens or prolonged high-temperature environments.
(2) Moderate weatherability (outdoor use)
(a) Long-term exposure to sunlight will cause polyethylene to age, become brittle, and discolor due to UV radiation.
(b) For long-term outdoor applications, UV stabilizers are typically added to the formulation, or alternative weather-resistant materials are selected.
(3) Mechanical properties depend on product geometry
(a) HDPE bulk materials or thick-walled parts exhibit good impact resistance and stiffness.
(b) Very thin PE films (such as very thin plastic bags) are easily punctured by sharp objects—this is primarily a matter of product design and thickness, rather than PE itself being “inherently brittle.”
(4) Safety at elevated temperatures depends on “designed operating conditions”
When used in compliance with food-contact regulations and under specified conditions, PE is widely regarded as a safe food-contact material:
(a) United States:
In FDA Title 21 of the Code of Federal Regulations (21 CFR), polyethylene is listed among the polymers permitted for use in indirect food contact applications.
(b) European Union:
Regulation (EU) No 10/2011 on plastic materials intended to come into contact with food sets specific requirements for composition and migration limits for plastics including PE.
Risks often arise when:
(1) Use conditions exceed the design range (e.g., unsuitable high-temperature microwave heating, direct flame contact).
(2) Non-food-grade materials are mistakenly used for food-contact applications.
(3) Manufacturing processes and quality control do not meet relevant specifications.
A more rigorous statement is therefore:
Within the designed service temperature range and the limits permitted by regulations, food-grade PE is generally safe. If the temperature range is exceeded or the material is used contrary to instructions, the risk of thermal degradation and increased additive migration may rise.
Comparison of PE and PP: How to Make Practical Material Choices?
In everyday life and in the laboratory, the materials most commonly compared are PE (polyethylene) and PP (polypropylene).
1. Fundamental Differences Arising from Structure
PE:
(a) The backbone consists of –CH₂–CH₂– units, with a simple structure.
(b) Density and properties are strongly influenced by crystallinity (i.e., the degree of chain branching).
PP:
(c) Formed by polymerization of propylene monomers; the main chain carries methyl (–CH₃) side groups.
(d) More readily achieves higher crystallinity; its stiffness and heat resistance are generally higher than those of PE.
2. Key Performance Comparison
Aspect | PE (Polyethylene) | PP (Polypropylene) |
Heat resistance | LDPE: around 80°C; HDPE: up to about 100–120°C (depending on formulation) | Typically has higher heat deflection temperature and heat resistance than PE. Certain specialized grades can be used at about 100–130°C and are among the common materials for “microwaveable” food containers (actual use must follow explicit “microwave-safe” labeling or the specified heat resistance rating). |
Mechanical properties | Good toughness and extensibility; suitable for soft films and flexible products | Higher stiffness and tensile strength, but impact toughness drops markedly near 0°C or at lower temperatures, making it more prone to brittleness. |
Softness | Generally softer (especially LDPE and LLDPE films) | More rigid overall; commonly used for boxes, cup lids, and reusable containers. |
Transparency | Mostly semi-transparent or opaque (depending on thickness and formulation) | Can be made semi-transparent or even relatively transparent. |
Typical applications | Plastic bags, cling film, cleaning product bottles, flexible packaging, film-type laboratory consumables | Microwaveable meal boxes, cup lids, storage boxes, reusable tableware, and certain laboratory vessels. |
Recycling code | HDPE: 2; LDPE / LLDPE: 4 | PP: 5 |
3. Practical Material Selection Guidelines
1. When heat resistance is required (e.g., microwave heating, hot-water sterilization)
→ Give priority to PP (#5), and always follow the product’s explicit “microwave-safe” label or specified heat resistance rating.
2. When high flexibility and film-type packaging are needed
→ Prefer LDPE, LLDPE, or their blends (commonly used in plastic bags, cling film, etc.).
3. When higher stiffness and structural strength are required for bottles, drums, or pipes
→ Common choices are HDPE (#2) or PP (#5), selected according to the specific heat resistance and chemical resistance requirements.
PE and the Environment: Recyclable ≠ Necessarily Environmentally Friendly
PE is often labeled as “recyclable,” but this does not mean it will necessarily be recycled in practice, nor that it is automatically environmentally friendly.
1. Recycling Codes and Material Identification
On the triangular recycling symbol found on plastic products:
1. #2: HDPE (High-Density Polyethylene)
2. #4: LDPE / LLDPE (Low-Density or Linear Low-Density Polyethylene)
These numbers are essentially resin identification codes, used to indicate the type of plastic. They are not equivalent to “definitely recyclable” or “definitely recycled.”
2. Why Are Actual Recycling Rates Low?
The main reasons include:
1. Films are too light and thin:
Items such as cling film and shopping bags are very light, often contaminated, and costly to collect and wash on a per-piece basis.
2. Contamination:
Packaging contaminated with oil and food residues is more difficult and expensive to process, making recycling less feasible.
3. Limited value of recyclates:
Many recycled PEs are used for garbage bags, non-critical building materials, and other low value-added applications.
4. Lack of efficient systems for film recycling in some regions:
In many areas, there is no highly efficient recycling infrastructure specifically for thin plastic films.
By contrast, thick-walled HDPE bottles, containers, and drums are easier to collect and sort, and in some countries their actual recycling rates are relatively higher.
3. Typical End Uses of Recycled PE
After washing, melting, and pelletizing, recycled PE is commonly used in:
1. Garbage bags
2. Plastic flooring and road protection boards
3. Construction fill and related materials
4. Secondary plastic containers, etc.
At present, recycled PE in many regions is still mainly used in relatively low value-added applications such as garbage bags and construction liners.
The use of high-quality recycled PE in high-demand fields such as food-contact materials and pressure pipes remains restricted by regulatory requirements and by the consistency of material quality.
4. What Can Users Do?
1. Reuse whenever possible:
Reduce the frequency of using single-use products.
2. Prioritize durable, reusable materials:
For example, repeatedly washable PP containers or glassware, where appropriate for the experiment and safety requirements.
3. Support proper sorting and recycling:
In particular, separately sort clean, relatively large-volume HDPE containers, which increases the likelihood that they will actually be recycled.
Frequently Asked Questions (FAQ)
Q1: Can PE be used for food contact? Is it safe?
When used in compliance with relevant regulations and according to the instructions, PE is one of the most widely recognized food-contact materials globally.
1. United States:
In FDA 21 CFR “Indirect Food Additives: Polymers,” polyethylene is included in the list of polymers permitted for food-contact applications, with detailed requirements on use conditions and additives.
2. European Union:
Regulation (EU) No 10/2011 establishes harmonized rules for plastic materials and articles intended to come into contact with food, including requirements for overall migration, specific migration limits, and a Union List of permitted substances.
Usage recommendations:
(a) Choose PE products labeled “food grade” or “for food contact”.
(b) Follow the temperature limits and use conditions specified in the product manual; avoid “pushing beyond” the recommended range.
Q2: Can PE containers be used in a microwave oven?
In general, it is not recommended to heat ordinary PE containers in a microwave oven.
1. Most PEs are only suitable for use in the range of about 80–120°C, whereas local hotspots in a microwave can exceed this range significantly.
2. Excessive temperatures may cause the material to soften and deform, and may also accelerate thermal degradation and additive migration.
If microwave heating is required, it is recommended to:
(a) Give priority to containers made of PP (#5) that are explicitly labeled as “microwave-safe.”
(b) Avoid microwaving films, bags, or packaging that are not clearly labeled as microwave-safe.
Q3: Can PE containers hold hot water?
1. For short-term contact with warm water around 70–80°C, some HDPE / LDPE containers are designed to withstand such conditions. However, it is necessary to consider:
(a) The material type (HDPE vs. LDPE vs. copolymers)
(b) Wall thickness and overall product design
(c) Use conditions specified by the manufacturer
2. Prolonged contact with near-boiling water or repeated high-temperature rinsing will accelerate material aging and deformation, shortening the service life.
If an application involves long-term contact with hot water or repeated high-temperature treatment (such as laboratory cleaning and disinfection, hot water flushing, etc.), a more robust approach is to:
(a) Use heat-resistant PP containers or glassware specifically designed for such conditions, or
(b) Refer to relevant standards and the manufacturer’s instructions and select products explicitly rated for high-temperature use.
Overview of Aladdin Polyethylene (PE) and Polypropylene (PP) Products and Melt Flow Rate Reference Materials
Category | Aladdin Cat. No. | Name | CAS No. | Specification / Purity | Product Features / Application Notes |
PE melt flow rate reference material | Polyethylene melt flow rate reference material | 9002-88-4 | Melt flow rate: 1.94 g/10 min | Low melt flow rate (MFR) polyethylene reference material, suitable for melt flow rate instrument calibration, method validation, and routine quality control. | |
PE melt flow rate reference material | Polyethylene melt flow rate reference material | 9002-88-4 | Analytical reference material, melt flow rate: 2.01 g/10 min | Analytical-grade PE melt flow rate reference material, used to establish or verify the accuracy and repeatability of PE melt flow rate test methods. | |
PE melt flow rate reference material | Polyethylene melt flow rate reference material | 9002-88-4 | Analytical reference material, melt flow rate: 3.98 g/10 min | Medium-to-high melt flow rate PE reference material; can be combined with other MFR levels to construct multi-point calibration or to assess the linear range of test methods. | |
PE resin (HDPE) | Polyethylene (PE) | 9002-88-4 | High density, melt flow index 12 g/10 min (190°C/2.16 kg) | High-density polyethylene (HDPE) with a medium-to-high melt flow index, suitable for process and formulation studies on HDPE containers, pipes, and injection/blow-molded products. Can also serve as a high-MFR reference material for comparison with low-MFR pipe-grade HDPE. | |
PE resin (low molecular weight characterization) | Polyethylene (PE) | 9002-88-4 | Average Mw ~4000 (GPC), average Mn ~1700 (GPC) | Low-molecular-weight polyethylene with specified Mw/Mn values, suitable for GPC method evaluation, studies on molecular weight–property relationships, or as a model material for wax-like polyethylene. | |
PE resin (LLDPE film grade) | Polyethylene (PE) | 9002-88-4 | Linear low density, melt flow index 1.0 g/10 min (190°C/2.16 kg) | Linear low-density polyethylene (LLDPE) with a typical film-grade melt index, suitable for packaging films, stretch films, agricultural films, and related formulation and process studies. | |
PE resin (UHMWPE powder) | P434346 | Polyethylene (PE) | 9002-88-4 | Ultra high molecular weight, surface-modified, powder, average particle size 125 μm | Ultra-high-molecular-weight polyethylene (UHMWPE) powder with surface modification, designed to improve dispersion and reinforcement in coatings, composites, and wear-resistant filler systems. |
PE resin (medium density) | Polyethylene (PE) | 9002-88-4 | Medium density | Medium-density polyethylene (MDPE), with properties between HDPE and LDPE; suitable for studying the influence of density/crystallinity on mechanical and barrier properties. | |
PE resin (LDPE, high melt index) | Polyethylene (PE) | 9002-88-4 | Low density, melt flow index 21 g/10 min (190°C/2.16 kg) | Low-density polyethylene (LDPE) with high melt flow index and good flowability, suitable for film extrusion, coating applications, and studies on process windows and flow behavior. | |
PP melt flow rate reference material | Melt flow rate reference material (polypropylene) | 9003-07-0 | Melt flow rate: 20.71 g/10 min | High melt flow rate polypropylene reference material, suitable for high-range calibration of melt flow rate instruments and for evaluating test methods for high-flow-grade PP. | |
PP resin (low MFR) | Polypropylene (PP) | 9003-07-0 | Melt flow index 0.5 g/10 min | Low melt flow index PP with high viscosity and high strength potential, suitable as a model for high-molecular-weight PP systems and for studies on high-strength structural components. | |
PP resin (medium MFR) | Polypropylene (PP) | 9003-07-0 | Melt flow index 2.2 g/10 min | PP with a general-purpose melt flow index, suitable as a base resin for PP in teaching, fundamental formulation research, and comparison among PP grades with different MFR levels. | |
PP resin (medium MFR) | P110849 | Polypropylene (PP) | 9003-07-0 | Melt flow index 4 g/10 min | PP resin with relatively high flowability, suitable for injection molding studies of thin-walled parts and complex molds. |
PP resin (very high MFR) | Polypropylene (PP) | 9003-07-0 | Melt flow index 35 g/10 min | Very high melt flow index PP, suitable for rheological studies of extremely high-flow systems, process window exploration for ultra-thin parts, or use as a flow-modifying component. | |
PP resin (high MFR) | Polypropylene (PP) | 9003-07-0 | Melt flow index 12 g/10 min | High melt flow index PP for applications such as thin-walled parts and fibers where high flowability is required, and useful for performance comparison with low-MFR PP. | |
PP resin (ultra-low MFR) | P110853 | Polypropylene (PP) | 9003-07-0 | Melt flow index 0.3 g/10 min | Ultra-low melt flow index PP, suitable as a representative of high-viscosity/high-molecular-weight systems for rheology, crystallization, and mechanical property studies. |
PP resin (high MFR) | P301642 | Polypropylene (PP) | 9003-07-0 | Melt flow index 28 g/10 min | High melt flow index PP with flowability between 12 and 35 g/10 min; can be combined with other MFR levels to construct a PP melt index gradient system. |
PP melt flow rate reference material | Polypropylene melt flow rate reference material | 9003-07-0 | Melt flow rate: 1.65 g/10 min | PP reference material with medium melt flow rate, suitable for calibration of melt flow rate instruments and routine quality control of general-purpose PP test methods. | |
PP melt flow rate reference material | P119573 | Polypropylene melt flow rate reference material | 9003-07-0 | Melt flow rate: 1.65 g/10 min | Same MFR level as P308012; can be used as a reference material for working curves or for verifying method consistency across different batches. |
PP resin (molecular weight characterization) | Polypropylene (PP) | 9003-07-0 | Average Mw ~12000, average Mn ~5000 | Polypropylene resin with specified Mw/Mn values, suitable for GPC method evaluation, studies on the relationship between molecular weight and flow/mechanical properties, and teaching demonstrations. |
Aladdin: https://www.aladdinsci.com/