Polypropylene PP
Chemical composition
Section titled “Chemical composition”Polypropylene consists exclusively of propylene monomers with the molecular formula (C₃H₆)ₙ, where each repeating unit contains 3 carbon atoms and 6 hydrogen atoms.
| Component | Value |
|---|---|
| Monomer | Propylene (propene) |
| Molecular formula | (C₃H₆)ₙ |
| IUPAC name | Poly(1-methylethylene) |
| CAS number | 9003-07-0 |
| Polymer type | Semicrystalline, non-polar thermoplastic |
| Predominant tacticity | Isotactic (chains with methyl groups on the same side) |
Physical properties
Section titled “Physical properties”The density of polypropylene varies between 0.855 g/cm³ in the amorphous state and 0.946 g/cm³ in the crystalline state, with most commercial grades around 0.905 g/cm³.
| Property | Typical value (SI) | Typical value (Imperial) |
|---|---|---|
| Density (amorphous) | 0.855 g/cm³ | 53.4 lb/ft³ |
| Density (crystalline) | 0.946 g/cm³ | 59.1 lb/ft³ |
| Water absorption (24 h) | ≤0.01 % | ≤0.01 % |
| Appearance | Translucent white to opaque | – |
| Isotactic crystallinity | 50 – 70 % | – |
| Refractive index | 1.49 | – |
| Shore D hardness | 70 – 80 | 70 – 80 |
Thermal properties
Section titled “Thermal properties”The melting point of isotactic homopolymer ranges from 130 °C to 171 °C (266 °F to 340 °F), while the glass transition temperature is around -10 °C (14 °F).
| Property | Typical value (SI) | Typical value (Imperial) |
|---|---|---|
| Melting temperature | 130 – 171 °C | 266 – 340 °F |
| Glass transition temperature (Tg) | -10 °C | 14 °F |
| Heat deflection temperature (HDT, 0.455 MPa) | 90 – 105 °C | 194 – 221 °F |
| Continuous service temperature without load | 90 – 100 °C | 194 – 212 °F |
| Coefficient of linear thermal expansion | 100 – 150 ×10⁻⁶ /°C | 56 – 83 ×10⁻⁶ /°F |
| Thermal conductivity | 0.17 – 0.22 W/(m·K) | 1.18 – 1.53 BTU·in/(h·ft²·°F) |
| Specific heat | 1.9 – 2.0 kJ/(kg·K) | 0.45 – 0.48 BTU/(lb·°F) |
Mechanical properties
Section titled “Mechanical properties”Injection-molded polypropylene homopolymer achieves a tensile yield strength of 30–40 MPa (4350–5800 psi) and an elongation at break often exceeding 200 %.
| Property | Isotactic homopolymer | Copolymer (impact) | Units |
|---|---|---|---|
| Tensile strength (yield) | 30 – 40 | 20 – 30 | MPa |
| Tensile modulus | 1.2 – 1.6 | 0.8 – 1.2 | GPa |
| Elongation at break | 150 – 600 | 200 – 500 | % |
| Flexural modulus | 1.2 – 1.8 | 0.8 – 1.4 | GPa |
| Izod impact strength (notched, 23 °C) | 2 – 5 | 10 – 25 | kJ/m² |
| Izod impact strength (notched, -20 °C) | 1 – 3 | 4 – 10 | kJ/m² |
| Compressive strength (yield) | 40 – 55 | – | MPa |
| Fatigue strength (10⁷ cycles) | 10 – 15 | – | MPa |
Chemical resistance
Section titled “Chemical resistance”At 20 °C (68 °F) polypropylene resists most acids, bases, and salts excellently (rating 1 out of 5), maintaining good resistance (rating 2) at 60 °C (140 °F) against organic acids and many alkalis, but it is attacked by chlorinated solvents and aromatic hydrocarbons.
| Chemical | Rating 20 °C | Rating 60 °C |
|---|---|---|
| Hydrochloric acid 10 % | 1 (Excellent) | 1 (Excellent) |
| Sulfuric acid 10 % | 1 | 1 |
| Nitric acid 5 % | 2 (Good) | 3 (Acceptable) |
| Acetic acid 100 % | 2 | 3 |
| Sodium hydroxide (caustic soda) | 1 | 1 |
| Sodium hypochlorite | 2 | 3 |
| Acetone | 2 | 2 |
| Benzene | 3 | 4 (Unacceptable) |
| Gasoline | 4 | 4 |
| Carbon tetrachloride | 4 | 4 |
| Chloroform | 4 | 4 |
| Chlorobenzene | 3 | 4 |
Rating: 1 = excellent (no attack), 2 = good (insignificant attack), 3 = acceptable (slight attack, limited use), 4 = unacceptable (significant attack), 5 = poor (possible cracking or dissolution).
Processing
Section titled “Processing”The polymerization of propylene is carried out at pressures from 10 to 40 bar and temperatures between 70 and 100 °C in the presence of Ziegler‑Natta or metallocene catalysts, obtaining the polymer in powder or granule form with a controllable melt flow rate (MFR) between 5 and 60 g/10 min.
| Process parameter | Typical value |
|---|---|
| Polymerization method | Slurry, gas phase, or bulk melt |
| Catalysts | Ziegler‑Natta, metallocenes |
| Extrusion temperature | 200 – 240 °C |
| Injection molding temperature | 220 – 250 °C |
| Blow molding temperature | 190 – 230 °C |
| Mold shrinkage | 1.0 – 2.5 % |
| Pre-drying (granules) | Usually not required; if stored in humid environment: 2‑4 h at 80 °C |
Available grades:
- Homopolymer: high stiffness and thermal resistance.
- Random copolymer: improved transparency, lower melting point.
- Block copolymer (impact): excellent low-temperature impact resistance.
Applications
Section titled “Applications”With a global production exceeding 70 million tons per year, polypropylene is massively used in sectors such as packaging, automotive, and textiles.
| Industry | Representative applications |
|---|---|
| Packaging | Bottles, yogurt containers, caps, cups, straws, food films, woven bags (big‑bags), labels |
| Automotive | Bumpers, dashboards, interior trims, battery cases, fluid reservoirs |
| Textile | Thermal underwear, ropes, carpets, nonwovens (geotextiles, filters), carpeting |
| Construction | Pipes and fittings for drainage (grey water), waterproofing sheets, landscape edging |
| Laboratory and medical equipment | Beakers, disposable syringes, sterilization containers, surgical trays |
| Consumer goods | Garden furniture, reusable containers, stationery, speakers, brushes, rakes |
| Banknotes and security documents | Polymer banknotes (high‑durability copies) |
Comparison with similar materials
Section titled “Comparison with similar materials”Compared to high-density polyethylene (HDPE), polypropylene offers approximately 30 % higher tensile strength and a heat deflection temperature about 20 °C higher, although its impact resistance at subzero temperatures is lower.
| Property | PP homopolymer | HDPE | Rigid PVC | PS (polystyrene) |
|---|---|---|---|---|
| Density (g/cm³) | 0.90 – 0.91 | 0.94 – 0.97 | 1.35 – 1.45 | 1.04 – 1.06 |
| Tensile strength (MPa) | 30 – 40 | 20 – 30 | 40 – 55 | 35 – 55 |
| Elongation at break (%) | 150 – 600 | 300 – 800 | 2 – 80 | 1 – 3 |
| Melting temperature (°C) | 130 – 171 | 125 – 135 | – (decomposes) | – (amorphous) |
| HDT at 0.455 MPa (°C) | 90 – 105 | 70 – 80 | 65 – 75 | 75 – 95 |
| Resistance to acids/bases | Excellent | Excellent | Good | Good (limited to some) |
| Resistance to organic solvents | Low (chlorinated, aromatic) | Low (same tendency) | Variable | Acceptable (some) |
| Recyclability | Very good | Very good | Good (limited by additives) | Good |
| Relative cost per kg | 1.0 (reference) | ~0.9 | ~1.1 | ~1.2 |
Recycling and sustainability
Section titled “Recycling and sustainability”Polypropylene takes between 20 and 30 years to decompose in landfills, therefore a secondary recycling process has been developed that starts with collection, sorting, cleaning, and remelting at 240 °C (464 °F) to obtain recycled granulate ready for processing.
Standard recycling process (SPI identification code #5):
- Collection – Accumulation of mixed plastic waste from the domestic stream.
- Sorting – Separation of PP products using their recycling code.
- Cleaning – Removal of labels, content residues, and contaminants; subsequent grinding and washing to obtain “clean flake”.
- Melt reprocessing – The flake is pelletized and extruded at 240 °C, cut into small homogeneous granules.
- Production of new products – The melt flow rate (MFR) and other properties are measured to direct the recycled material toward suitable applications.
Products made from recycled PP:
- Battery cables
- Brushes and brooms
- Ice scrapers
- Pallets and trays
- Landscape edging
- Signal lights
- Automobile battery cases
- Bicycle racks
- Rakes and trash bins
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”Below are 6 common questions about polypropylene, with quantitative data to aid in engineering selection.
What is the maximum continuous service temperature of PP?
Section titled “What is the maximum continuous service temperature of PP?”The maximum continuous service temperature without mechanical load is approximately 100 °C (212 °F); under light loads it can remain at 90 °C for thousands of hours without significant degradation.
Is polypropylene suitable for food contact?
Section titled “Is polypropylene suitable for food contact?”Yes, it complies with FDA and EU regulations, withstanding repeated heating up to 100 °C (212 °F) in microwave ovens, although contact with very hot oils (>120 °C) may soften it.
What mechanical differences exist between PP homopolymer and copolymer?
Section titled “What mechanical differences exist between PP homopolymer and copolymer?”The isotactic homopolymer has a flexural modulus above 1.2 GPa and low impact resistance at -20 °C (<3 kJ/m²), while the block copolymer maintains an Izod >10 kJ/m² at -20 °C, sacrificing up to 20 % stiffness.
How does UV radiation affect polypropylene?
Section titled “How does UV radiation affect polypropylene?”Without stabilization, 12 months of outdoor exposure causes a loss of more than 50 % in tensile strength due to photo‑oxidation; UV stabilizers and carbon black extend service life to more than 5 years outdoors.
Can polypropylene be welded?
Section titled “Can polypropylene be welded?”Yes, hot gas or hot plate welding at 220–240 °C (428–464 °F) produces joints with up to 80–90 % of the base material strength, but it does not accept common adhesives without pretreatment.
How long does PP take to biodegrade in landfill conditions?
Section titled “How long does PP take to biodegrade in landfill conditions?”In an anaerobic landfill, complete decomposition of PP takes 20 to 30 years; in industrial composting systems above 60 °C it can fragment faster, but without achieving full biodegradation.
Technical notes
Section titled “Technical notes”The melt flow rate (MFR) of commercial polypropylene ranges from 5 to 60 g/10 min (230 °C / 2.16 kg), a critical parameter for grade selection according to the processing method (injection, extrusion, blow molding).
- Fire behavior: Pure PP is flammable, with a limiting oxygen index (LOI) around 17–18 %. With flame retardant additives, classification V‑2 or even V‑0 in UL 94 testing can be achieved.
- Sterilization: Withstands steam sterilization at 121 °C for short cycles (autoclave), but without nucleating agents it may deform. Gamma radiation and ethylene oxide do not significantly affect its properties.
- Bonding and painting: Low surface energy (~29 mN/m) makes gluing and painting difficult; corona, plasma treatments, or primer with adhesion promoters are recommended.
- Machining: Can be cut, milled, and turned with high-speed steel tools, but the heat generated can cause local softening if not properly cooled.
- Electrical insulation: Excellent dielectric, with surface resistivity >10¹⁶ Ω and dielectric strength of 20–30 kV/mm, ideal for battery casings and electronic components.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=7899
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/polypropylene-pp-chemical-resistance-d_435.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=2557