1. Panimula
Polyethylene (PE) is a polymer produced by polymerisation of the monomer ethylene (CH₂=CH₂).
First commercialised in the 1930s, PE is now available in multiple engineered forms whose properties are defined by molecular weight, chain architecture (branching), and processing (including crosslinking).
PE’s combination of chemical inertness, kakayahang maproseso, low cost and a spectrum of mechanical behaviors—from flexible films to ultra-tough solids—explains its ubiquity across packaging, konstruksiyon, transport, consumer, medical and industrial sectors.
2. What Is Polyethylene (PE)?
Polyethylene (PE) is a family of semi-crystalline thermoplastics produced by polymerizing ethylene (CH₂=CH₂).
It is the world’s most widely used plastic because of its combination of low cost, kemikal na kawalang-kilos, wide processing window and a tunable range of mechanical behaviours — from soft, flexible films to very tough, wear-resistant solids.
Key properties
- Paglaban sa kemikal: excellent to most acids, alkalis, solvents and fuels.
- Mekanikal: wide range — LDPE is soft and extensible; HDPE is stiff and strong; UHMWPE combines high strength with exceptional impact toughness.
- Thermal: melting points typically ~105–135 °C depending on grade; service temperatures are generally limited compared with engineering plastics.
- Moisture: essentially non-hygroscopic (negligible water uptake).
- Wear & friction: UHMWPE has outstanding low friction and abrasion resistance.
3. Commercial PE grades and what makes them different
PE is typically categorised as follows:
- LDPE (Low-Density Polyethylene): density ~0.910–0.925 g/cm³; may kakayahang umangkop, good clarity (Mga pelikula), low tensile strength. Common for squeeze bottles, Mga pelikula, cable jackets.
- LLDPE (Linear Low-Density Polyethylene): density similar to LDPE; superior tensile strength and puncture resistance in films due to short-chain branching. Widely used for stretch film and co-extruded structures.
- MDPE (Medium-Density PE): density ~0.926–0.940 g/cm³; used for gas pipes and some blow moulding.
- HDPE (High-Density Polyethylene): density ~0.940–0.970 g/cm³; stiff, good chemical resistance, used for pipe, Mga lalagyan, rotomoulding parts.
- UHMWPE (Ultra-High-Molecular-Weight PE): Mw typically >3×10⁶ g/mol; outstanding abrasion resistance, very low friction; used for liners, mga bearing, sliding applications and some medical implants.
- XLPE (Cross-linked PE): PE chemically or radiation crosslinked to improve temperature, creep and chemical resistance; used for high-temperature piping and cable insulation.
- Metallocene-catalysed PE (mPE / mLLDPE): tighter molecular weight distribution and improved mechanical property control — enables high clarity films and tailored mechanical behaviour.
Each grade is optimised for processability and application performance by adjusting Mw, comonomer content and catalysts.
4. Typical physical and mechanical properties
The table below gives representative, typical ranges for common PE grades. Use manufacturer datasheets for design-critical values.
| Pag-aari | LDPE | LLDPE | MDPE | HDPE | UHMWPE |
| Densidad ng katawan (g·cm⁻³) | 0.910–0.925 | 0.915–0.930 | 0.926–0.940 | 0.940–0.970 | 0.930–0.940 |
| Lakas ng paghatak (MPa) | 8–15 | 12–20 | 14–25 | 20–37 | 30–45 |
| Elongation at break (%) | 200–800 | 200–600 | 200–400 | 100–600 | 100–400 |
| Young’s modulus (GPa) | 0.2–0.4 | 0.3–0.6 | 0.6–0.9 | 0.8–1.5 | 0.8–1.5 |
| Melting point (°C) | 105–115 | 105–120 | 120–130 | 125–135 | 130–138 |
| Notched Izod (kJ·m⁻²) | 30–100 (matigas ang ulo) | 30–100 | 20–60 | 10–40 | 50–200 (very tough) |
| Email Address * | Mababa ang | Katamtaman | Katamtaman | Mabuti na lang | Napakahusay |
| Continuous service temp (°C) | ~65–80 | ~65–80 | ~80–90 | ~80–110 | ~80–120 |
| Paglaban sa kemikal | Napakahusay | Napakahusay | Napakahusay | Napakahusay | Napakahusay |
| Water absorption | Hindi gaanong mahalaga | Hindi gaanong mahalaga | Hindi gaanong mahalaga | Hindi gaanong mahalaga | Hindi gaanong mahalaga |
5. Processing methods and manufacturing considerations
PE is processed by almost every thermoplastic technique:
- Paglabas — pipes, mga sheet, pelikula, mga profile. HDPE and UHMW in pipes and liners are extruded or ram-extruded.
- Blow-molding — bottles and containers (HDPE, LDPE).
- Pag iiniksyon paghubog — fittings, housings and components (HDPE, LDPE variants).
- Rotational (rotomoulding) — large hollow parts (mga tangke, mga kayak).
- Film casting / blown film — packaging films (LDPE, LLDPE, mLLDPE).
- Compression sintering / ram extrusion / compression pagmomolde — UHMWPE often processed this way because of extremely high Mw (no conventional melt flow).
- Crosslinking methods — chemical (peroxides), silane grafting or electron-beam / gamma radiation to produce XLPE for higher temperature or improved creep resistance.
6. Key applications by grade
- LDPE / LLDPE: flexible film, shopping bags, Mga liner, film packaging, cable sheathing, agricultural films.
- HDPE: water and gas distribution piping, blow-moulded containers (milk bottles), geomembranes, rotomoulded tanks, mga bahagi ng istruktura.
- MDPE: gas distribution pipe, geomembranes.
- UHMWPE: wear strips, chutes and liners, sliding bearings, chain guides, orthopaedic implants (hip and knee components), ballistic fibres (UHMWPE fibres like Dyneema® / Spectra®).
- XLPE: high-temperature pipe applications (hot water/industrial), cable insulation.
7. Performance challenges and failure modes
Although chemically robust, PE has several known failure mechanisms to design against:
Environmental stress cracking (ESC)
- Kahulugan: crack formation and propagation under stress in the presence of specific chemicals or surfactants.
PE’s most critical failure mode — stress levels below yield can cause cracking over time in contact with detergents, glycol, or some hydrocarbons. - Pagbawas: choose ESC-resistant formulations, reduce residual/trapping stress (improve processing and anneal), avoid sharp notches and reduce sustained tensile stresses.
Creep and long-term deformation
- PE exhibits significant creep under sustained load, especially at elevated temperature.
Design for creep with safety factors; use HDPE, XLPE or select UHMW for reduced creep where needed.
UV / oxidative degradation
- Unstabilised PE degrades under UV and oxygen: surface chalking, embrittlement and loss of mechanical properties.
Stabilisation with UV absorbers, carbon black pigmentation and antioxidants is routine for outdoor applications.
Low stiffness at high temperature and dimensional limits
- PE’s modulus falls with temperature; for structural applications approaching service temperature limits select materials with higher stiffness or crosslinking to raise heat deflection.
Fusion / welding considerations (for piping)
- HDPE piping is typically joined by butt fusion or electrofusion; poor welding leads to weak joints and premature failure — welding procedures and operator qualification are critical.
8. Environmental, recycling and sustainability aspects
- Recyclability: PE is highly recyclable (mechanical recycling); HDPE and LDPE are commonly reprocessed into packaging and non-critical products. PE is assigned recycling codes: #2 (HDPE) at #4 (LDPE).
- Mga Limitasyon: kontaminasyon, mixed polymers and additives complicate recycling streams. UHMWPE and filled grades are more difficult to reprocess into high-value products.
- Biobased options: ethylene can be produced from bioethanol (bio-PE) with identical properties to fossil-based PE.
- End-of-life: incineration with energy recovery and chemical recycling (depolymerisation) are technical options; lifecycle analysis depends on application and recovery rates.
- Environmental concerns: microplastics generation from films and wear particles (hal., from conveyor liners) requires consideration.
9. Comparative Analysis — Polyethylene (PE) mga bes. other common materials
The table below compares PE with several materials engineers commonly consider as alternatives for parts, Mga pelikula, pipes or wear components.
| Pag-aari / Pamantayan | PE (LDPE / HDPE) | PP (Polypropylene) | PVC (Matigas) | POM / Acetal | Naylon (PA6 / PA66) |
| Densidad ng katawan (g·cm⁻³) | 0.91–0.97 | 0.90–0.91 | 1.34–1.45 | ≈ 1.41 | 1.12–1.15 |
| Lakas ng paghatak (MPa) | 8–37 (LD→HD) | 30–40 | 35–60 | 50–75 | 50–90 |
| Young’s modulus (GPa) | 0.2–1.5 | 1.0–1.8 | 2.7–3.5 | 2.8–3.5 | 2.5–3.5 |
| Natutunaw na / usable temp (°C) | Tm ~105–135 / use ≈ 65–110 | Tm ~160–170 / use ≈ 90–120 | Tg/softening ~75–80 / use ≈ 40–60 | Tm ~165–175 / use ≈ 80–100 | Tm ~215–265 / use ≈ 80–120 |
| Paglaban sa kemikal | Napakahusay (mga asido, mga base, many solvents) | Napakahusay (similar to PE) | Mabuti na lang (mga asido, salts, many chemicals) | Mabuti na lang (mga gasolina, mga langis) | Mabuti na lang (hydrocarbons, mga langis) |
| Pagsipsip ng kahalumigmigan | Hindi gaanong mahalaga | Hindi gaanong mahalaga | Hindi gaanong mahalaga | ~0.2–0.3% | 1–3% (hygroscopic) |
Wear / friction behavior |
Mabuti na lang (HDPE better than LDPE) | Katamtaman | Katamtaman | Napakahusay (mababang alitan, Mababang Wear) | Mabuti na lang |
| Dimensional na katatagan | Katamtaman (creep under load) | Katamtaman | Mabuti na lang | Napakahusay | Katamtaman (affected by moisture) |
| Paglaban ng UV (unstabilized) | Mga Maralita (needs stabilizers) | Mga Maralita | Mas mahusay na (formulation dependent) | Mga Maralita | Mga Maralita |
| Processability | Napakahusay (paglabas ng mga, blow, injection, rotomolding) | Napakahusay | Mabuti na lang (but narrow processing window) | Mabuti na lang (injection, machining) | Mabuti na lang (requires drying before molding) |
| Recyclability | Napakahusay (HDPE/LDPE widely recycled) | Napakahusay | Limitado (chlorine content) | Limitado | Katamtaman |
| Mga tipikal na aplikasyon | Films, bottles, mga tubo, mga tangke, Mga liner | Automotive trim, mga bisagra, Mga lalagyan | Mga tubo, window profiles, mga angkop na bagay | Mga gears ng katumpakan, mga bushing, Mga balbula | Mga Gear, mga bearing, mga pabahay, tubing |
10. Konklusyon
Polyethylene is a versatile thermoplastic family whose different grades cover a very wide span of mechanical and processing behaviours.
PE’s strengths are chemical resistance, kakayahang maproseso, low cost and a capability range from flexible films to ultra-tough sliding parts.
The most common engineering pitfalls are environmental stress cracking, creep and UV degradation — each addressable through grade selection, stabilisation and design.
For most industrial designers, PE remains an economical and robust choice when its limitations are understood and managed through specification and testing.
Mga FAQ
What is the difference between LDPE and HDPE?
LDPE has more chain branching, lower crystallinity and lower density (≈0.91–0.925 g/cm³) → softer, more flexible films.
HDPE has little branching, higher crystallinity (≈0.94–0.97 g/cm³) → stiffer, stronger parts and pipe.
Why does PE crack under mild chemicals sometimes?
That is environmental stress cracking (ESC): certain surfactants and detergents promote slow crack growth under tensile stress. Selecting ESC-resistant grades and reducing stress concentrations mitigates the risk.
Can PE be used for pressure piping?
Yes — HDPE and MDPE are widely used for potable water and gas distribution. Proper fusion welding and qualified materials/processes are essential.
When should I choose UHMWPE?
Choose UHMWPE when very high abrasion resistance, low friction and impact toughness are required (conveyor liners, wear pads, sliding bearings, certain medical implants).
Is polyethylene recyclable?
Oo nga: HDPE and LDPE are among the most recycled plastics, but contamination and mixed polymers influence recycling quality.
Mechanical recycling and emerging chemical recycling routes are both used.
