1. Ievads
Polietilēns (PE) ir polimērs, ko iegūst, polimerizējot etilēna monomēru (CH₂=CH₂).
Pirmo reizi komercializēts pagājušā gadsimta trīsdesmitajos gados, 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, Apstrādājamība, low cost and a spectrum of mechanical behaviors—from flexible films to ultra-tough solids—explains its ubiquity across packaging, būvniecība, transports, patērētājs, medical and industrial sectors.
2. Kas ir polietilēns (PE)?
Polietilēns (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, ķīmiskā inertācija, wide processing window and a tunable range of mechanical behaviours — from soft, flexible films to very tough, wear-resistant solids.
Galvenās īpašības
- Ķīmiska izturība: excellent to most acids, sārms, solvents and fuels.
- Mehānisks: wide range — LDPE is soft and extensible; HDPE is stiff and strong; UHMWPE combines high strength with exceptional impact toughness.
- Termisks: 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).
- Valkāt & berze: 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; elastīgs, good clarity (filmas), low tensile strength. Common for squeeze bottles, filmas, 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, konteineri, rotomoulding parts.
- UHMWPE (Ultra-High-Molecular-Weight PE): Mw typically >3×10⁶ g/mol; outstanding abrasion resistance, very low friction; used for liners, gultņi, 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.
| Īpašums | LDPE | LLDPE | MDPE | HDPE | UHMWPE |
| Blīvums (g · cm⁻³) | 0.910–0.925 | 0.915–0.930 | 0.926–0.940 | 0.940–0.970 | 0.930–0.940 |
| Stiepes izturība (MPA) | 8–15 | 12–20 | 14–25 | 20–37 | 30–45 |
| Pagarinājums pārtraukumā (%) | 200–800 | 200–600 | 200–400 | 100–600 | 100–400 |
| Younga modulis (GPA) | 0.2–0,4 | 0.3–0,6 | 0.6–0,9 | 0.8–1.5 | 0.8–1.5 |
| Kušanas temperatūra (° C) | 105–115 | 105–120 | 120–130 | 125–135 | 130–138 |
| Notched Izod (kJ·m⁻²) | 30–100 (grūts) | 30–100 | 20–60 | 10–40 | 50–200 (very tough) |
| Nodilums pretestība | Zems | Mērens | Mērens | Labi | Lielisks |
| Nepārtraukta apkalpošanas temp (° C) | ~65–80 | ~65–80 | ~80–90 | ~80–110 | ~80–120 |
| Ķīmiska izturība | Lielisks | Lielisks | Lielisks | Lielisks | Lielisks |
| Ūdens absorbcija | Nenozīmīgs | Nenozīmīgs | Nenozīmīgs | Nenozīmīgs | Nenozīmīgs |
5. Processing methods and manufacturing considerations
PE is processed by almost every thermoplastic technique:
- Ekstrūzija — pipes, loksnes, plēve, profili. HDPE and UHMW in pipes and liners are extruded or ram-extruded.
- Blow-molding — bottles and containers (HDPE, LDPE).
- Iesmidzināšana — fittings, housings and components (HDPE, LDPE variants).
- Rotational (rotomoulding) — large hollow parts (tvertnes, kajaki).
- Film casting / blown film — packaging films (LDPE, LLDPE, mLLDPE).
- Compression sintering / aunu ekstrūzija / saspiešanas liešana — 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, starplikas, film packaging, cable sheathing, agricultural films.
- HDPE: water and gas distribution piping, blow-moulded containers (milk bottles), geomembranes, rotomoulded tanks, strukturālās sastāvdaļas.
- MDPE: gas distribution pipe, geomembranes.
- UHMWPE: wear strips, chutes and liners, sliding bearings, chain guides, orthopaedic implants (gūžas un ceļgala komponenti), ballistic fibres (UHMWPE fibres like Dyneema® / Spectra®).
- XLPE: high-temperature pipe applications (hot water/industrial), kabeļa izolācija.
7. Performance challenges and failure modes
Although chemically robust, PE has several known failure mechanisms to design against:
Environmental stress cracking (ESC)
- Definīcija: 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. - Mazināšana: 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. Vide, recycling and sustainability aspects
- Pārstrāde: 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) un #4 (LDPE).
- Ierobežojumi: piesārņojums, 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.
- Dzīves beigas: 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 (Piem., from conveyor liners) requires consideration.
9. Comparative Analysis — Polietilēns (PE) vs. other common materials
Zemāk esošajā tabulā ir veikts salīdzinājums PE with several materials engineers commonly consider as alternatives for parts, filmas, pipes or wear components.
| Īpašums / Kritērijs | PE (LDPE / HDPE) | PP (Polipropilēns) | PVC (Stingrs) | POM / Acetāla | Neilons (PA6 / PA66) |
| Blīvums (g · cm⁻³) | 0.91–0.97 | 0.90–0.91 | 1.34–1.45 | ≈ 1.41 | 1.12–1.15 |
| Stiepes izturība (MPA) | 8–37 (LD→HD) | 30–40 | 35–60 | 50–75 | 50–90 |
| Younga modulis (GPA) | 0.2–1.5 | 1.0–1.8 | 2.7–3,5 | 2.8–3,5 | 2.5–3,5 |
| Kūstošs / 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 |
| Ķīmiska izturība | Lielisks (skābes, bāzes, daudzi šķīdinātāji) | Ļoti labs (similar to PE) | Labi (skābes, salts, many chemicals) | Labi (degviela, eļļas) | Labi (ogļūdeņraži, eļļas) |
| Mitruma absorbcija | Nenozīmīgs | Nenozīmīgs | Nenozīmīgs | ~ 0,2–0,3% | 1–3% (hygroscopic) |
Valkāt / friction behavior |
Labi (HDPE better than LDPE) | Mērens | Mērens | Lielisks (Zema berze, zems nodilums) | Labi |
| Izmēra stabilitāte | Mērens (creep under load) | Mērens | Labi | Lielisks | Mērens (affected by moisture) |
| UV pretestība (unstabilized) | Nabadzīgs (needs stabilizers) | Nabadzīgs | Labāks (formulation dependent) | Nabadzīgs | Nabadzīgs |
| Apstrādājamība | Lielisks (ekstrūzija, blow, injection, rotomolding) | Lielisks | Labi (but narrow processing window) | Labi (injection, apstrāde) | Labi (requires drying before molding) |
| Pārstrāde | Ļoti labs (HDPE/LDPE widely recycled) | Ļoti labs | Ierobežots (chlorine content) | Ierobežots | Mērens |
| Tipiskas lietojumprogrammas | Films, bottles, pīpes, tvertnes, starplikas | Automobiļu apdare, eņģes, konteineri | Pīpes, window profiles, armatūra | Precīzijas pārnesumi, bukses, vārsti | Pārnesumi, gultņi, apvalki, caurules |
10. Secinājumi
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, Apstrādājamība, 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.
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, valkāt spilventiņus, sliding bearings, certain medical implants).
Is polyethylene recyclable?
Jā: 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.
