Key takeaways
- PP homopolymer is made from propylene only: stiffest, highest melting point (about 160–165 °C), the standard for woven tape, fiber and BOPP film.
- Random copolymer adds about 1–7% ethylene at random: clearer, more flexible, melts lower (about 135–150 °C).
- Impact copolymer adds an ethylene-propylene rubber phase: much tougher in the cold, slightly less stiff.
- Woven sacks use raffia-grade homopolymer (melt flow index about 2–4 g/10 min) because it draws into strong oriented tapes.
The difference between polypropylene homopolymer and copolymer is what goes into the chain. Homopolymer PP is made from propylene alone and is the stiffest, most heat-resistant type; copolymer PP adds a small amount of ethylene, either randomly (random copolymer, for clarity and flexibility) or as a rubbery second phase (impact copolymer, for toughness in the cold).
Quotable definition: a polypropylene homopolymer is PP whose chains contain only propylene units, while a polypropylene copolymer also contains ethylene (or another comonomer), which lowers stiffness and melting point but improves clarity or impact strength.
For general PP properties, see what is polypropylene. This guide goes one level deeper into the resin types and explains why woven bag makers use homopolymer.
What is the structure and formula of polypropylene?
- Polypropylene abbreviation: PP.
- Monomer for polypropylene: propylene (propene), CH2=CH–CH3.
- Polypropylene formula: (C3H6)n, a chain of propylene units, each with a methyl (CH3) side group.
The arrangement of those methyl groups (the tacticity) controls the properties:
| Tacticity | Methyl group arrangement | Result |
|---|---|---|
| Isotactic | All on the same side of the chain | Crystallizes well; stiff, strong, melts ~160–165 °C. Almost all commercial PP |
| Syndiotactic | Alternating sides | Crystalline but softer and clearer; niche use |
| Atactic | Random | Amorphous and sticky; used in adhesives and as a by-product |
How PP polymerization works
Commercial PP is made by coordination polymerization using Ziegler-Natta or metallocene catalysts, which place each propylene unit in the same orientation to give isotactic chains. Production uses bulk (liquid propylene in loop reactors), gas-phase or slurry processes. Adding ethylene to the reactor, or in a second reactor, produces the copolymer grades.
Types of polypropylene compared
| Property (typical) | Homopolymer | Random copolymer | Impact (block) copolymer |
|---|---|---|---|
| Ethylene content | 0% | about 1–7%, random | about 5–15%+, in a rubber phase |
| Density (g/cm³) | 0.900–0.910 | 0.890–0.905 | 0.895–0.905 |
| Melting point | 160–165 °C (320–330 °F) | 135–150 °C (275–300 °F) | 160–165 °C (matrix) |
| Glass transition (Tg) | about −10 to 0 °C | about −15 to −5 °C | PP phase ~0 °C, rubber phase ~−50 °C |
| Tensile strength at yield | 31–37 MPa | 25–30 MPa | 20–28 MPa |
| Flexural modulus | 1.3–1.8 GPa | 0.8–1.2 GPa | 1.0–1.4 GPa |
| Cold impact | Poor below 0 °C | Moderate | Good to about −20 °C or lower |
| Clarity | Translucent | Best (with clarifier) | Opaque |
| Typical uses | Woven tape, raffia, fiber, BOPP film, thermoformed parts | Clear containers, bottles, hot-water pipes, film | Crates, buckets, car bumpers, luggage, cold-store packaging |
Values are typical published ranges for unfilled grades; data sheets for specific resins vary.
PP homopolymer
Homopolymer is the workhorse grade. Its high crystallinity gives the best stiffness, the highest heat deflection and excellent response to orientation (stretching), which aligns the chains and multiplies tensile strength. The trade-off is brittleness on impact below about 0 °C.
Random copolymer
Scattering a few percent of ethylene through the chain disrupts crystal formation. Random copolymer PP is clearer, more flexible and seals at lower temperatures, with a lower melting point. It is used for transparent housewares, medical items, blow-molded bottles and sealing layers in film.
Impact copolymer
Impact (also called heterophasic or block) copolymer contains rubbery ethylene-propylene particles dispersed in a homopolymer matrix. Those particles absorb impact energy, so the material stays tough well below freezing while keeping a high melting point. It is the choice for crates, pails and automotive parts.
What is melt flow index and why does it matter?
Melt flow index (MFI or MFR) is the mass of molten polymer, in grams, that flows through a standard die in 10 minutes at 230 °C under a 2.16 kg load (ISO 1133 / ASTM D1238). Low MFI means long chains and a viscous melt; high MFI means shorter chains and an easy-flowing melt.
| Process | Typical MFI (g/10 min) |
|---|---|
| Raffia / woven tape | about 2–4 |
| BOPP film | about 2–4 |
| Pipe, sheet | below 1 |
| Injection molding (containers, caps) | about 10–50+ |
| Fibers, spunbond non-woven | about 25–40 |
| Melt-blown non-woven | several hundred to over 1,000 |
Choosing the right MFI is as important as choosing homopolymer or copolymer.
Why do woven sacks use raffia-grade homopolymer?
Raffia grade PP is a homopolymer with a low MFI (about 2–4) and a narrow molecular weight distribution, designed to be extruded as a flat film, slit into tapes and stretched. Stretching at a draw ratio of roughly 5–7:1 aligns the chains, raising tape tenacity to several times that of unoriented PP; typical sack tapes reach about 4–6 grams per denier.
Homopolymer is chosen because:
- It orients best, giving the highest tape strength for the least resin.
- Its higher melting point lets bags take hot-filled products such as cement and some chemicals.
- Its stiffness gives sacks shape on filling and stacking.
Common additives in raffia compounds:
- UV stabilizer (usually hindered amine light stabilizers, HALS) for any outdoor use. Unstabilized PP can lose much of its strength after a few months of strong sun.
- Calcium carbonate filler masterbatch, typically a few percent, to improve tape extrusion and opacity. Heavily filled tapes are cheaper but weaker, so specify a minimum tensile strength rather than accept unknown filler levels.
- Color masterbatch and sometimes anti-slip treatments.
Examples: Industrial PP Woven Bag Tear-Resistant Fabric High for heavy industrial goods, Heavy Duty PP Valve Bag 45kg Virgin PP Anti-Slip made from virgin PP, and UV Stabilized PP Woven Bag with Top Drawstring for Sand with UV stabilizer. More options are in PP Woven Sacks. The fabric side is explained in what is woven fabric, and the bulk-bag process in how FIBC bulk bags are made.
What is polypropylene density and tensile strength in practice?
Polypropylene density is about 0.90–0.91 g/cm³, the lowest of the major commodity plastics, so PP floats in water. Copolymers are similar or slightly lower; mineral-filled grades are heavier.
Polypropylene tensile strength for molded homopolymer is about 31–37 MPa at yield. Oriented tape and fiber are much stronger along their length, which is why a 1,000 kg bulk bag can be made from only a few kilograms of PP.
Polypropylene glass transition temperature (Tg) is around −10 to 0 °C. Below it, the amorphous regions stiffen, which explains why homopolymer becomes brittle in deep cold.
What is polypropylene fiber?
Polypropylene fiber is PP extruded through spinnerets as multifilament, staple fiber or monofilament, mainly from homopolymer grades with higher MFI than raffia. It is used in carpets, ropes, geotextiles, non-woven fabrics (spunbond and melt-blown), concrete reinforcement fibers and technical textiles. Woven sacks use flat tapes rather than round fibers, but the chemistry is the same.
Polypropylene vs ABS
| Property | PP (homopolymer) | ABS |
|---|---|---|
| Structure | Semi-crystalline polyolefin | Amorphous terpolymer (acrylonitrile-butadiene-styrene) |
| Density | 0.90–0.91 g/cm³ | 1.03–1.07 g/cm³ |
| Glass transition / softening | Tg ~0 °C; melts 160–165 °C | Tg ~105 °C; no sharp melting point |
| Chemical resistance | Excellent | Fair; attacked by some solvents |
| Fatigue / living hinge | Excellent | Poor |
| Surface finish, paintability | Waxy, hard to bond | Glossy, easy to paint and glue |
| Recycling code | 5 | 7 |
ABS suits rigid housings and parts needing a good surface; PP suits chemical containers, hinges, packaging and anything that must be light. For PP vs PE, see polypropylene vs polyethylene bags.
Sourcing woven PP products
EGP converts polypropylene into woven sacks, valve bags, bulk bags and fabrics in Yantai, China. If you need specific tape strength, UV stabilization or virgin resin, contact us with your requirements.
Frequently asked questions
Which is stronger, PP homopolymer or copolymer?
Homopolymer is stiffer and has higher tensile strength at room temperature, and it orients into the strongest tapes and fibers. Impact copolymer is tougher, absorbing much more energy in a sudden impact, especially below freezing.
What are the types of polypropylene?
The three main commercial types are homopolymer, random copolymer and impact (block) copolymer. Within each type, grades are further defined by melt flow index, additives and the intended process, such as raffia, film, fiber or injection molding.
What is the density of polypropylene?
Polypropylene has a density of about 0.90–0.91 g/cm³ for homopolymer, with copolymers similar or slightly lower. It is lighter than water and lighter than polyethylene, PET, PVC and ABS.
What is the glass transition temperature of polypropylene?
The glass transition temperature of polypropylene is roughly −10 to 0 °C. Below this temperature the amorphous part of the polymer stiffens, making homopolymer PP more brittle on impact.
What is raffia grade PP?
Raffia grade PP is a homopolymer resin with a low melt flow index, typically about 2–4 g/10 min, designed to be extruded into film, slit into tapes and stretched. Its tapes are woven into sacks, bulk bags, ground covers and tarpaulins.
What is the monomer for polypropylene?
The monomer is propylene (also called propene), CH2=CH–CH3. It is produced mainly from oil refining and steam cracking, and from dedicated propane dehydrogenation plants.
