Polypropylene (PP): Properties, Grades & Chemical Resistance

Bobby Brown

Post 2016-02-19

Polypropylene (PP): Properties, Grades & Chemical Resistance

What is Polypropylene (PP)?
PP Grades
PP Material Properties
PP Applications
PP Chemical resistance chart
Frequently Asked Questions

What is Polypropylene (PP)?

Polypropylene — abbreviated PP, sometimes called polypropene — is a lightweight, semi-crystalline thermoplastic polymer made from propylene, a gas derived from petroleum refining. It belongs to the polyolefin family and is currently the second most produced plastic in the world, with global output exceeding 90 million metric tons per year.¹

In plain terms: PP is a hard, slightly translucent plastic that resists a wide range of acids, alkalis, and organic solvents, absorbs virtually no moisture, and is easy to weld, machine, and fabricate. It's also one of the lightest commercial plastics available — which matters when you're building large tanks, piping systems, or equipment that needs to be installed and handled.

You've already encountered PP more than you realize. The yogurt cup in your fridge, the pill bottle in your medicine cabinet, the white or green pipes in a building's hot water system, hospital syringes, food storage containers — all PP. In industrial settings, it's the standard material for chemical tanks, pipe fittings, pump bodies, and fluid-handling equipment that needs to resist acids and alkalis without corroding. LORRIC's PP-wetted flow meters and spray nozzles work on the same principle: reliable chemical resistance at a cost that makes sense across a wide range of process fluid applications.

PP Grades

Not all PP is the same. The three main types — homopolymer, random copolymer, and impact copolymer — are each engineered for a different job. Understanding which is which saves a lot of confusion when you're specifying a material or reading a product datasheet.²

PP Homopolymer (PP-H)

Stiffest · Most chemical resistant · Highest heat tolerance

Made from pure propylene monomers with no additives to the chain. The result is the stiffest and hardest of the three types, with the best resistance to heat and chemicals. If you're specifying PP for a chemical tank, a pump body, a valve component, or a flow meter wetted part, this is almost always the grade you want.

The trade-off: it becomes brittle in cold temperatures (below 0°C) and has relatively low impact resistance compared to copolymer grades.

You'll find it in: chemical storage tanks, pipe fittings, industrial equipment linings, plating barrels, laboratory fixtures, and fluid-handling components including LORRIC's PP-wetted flow meter parts.

PP Random Copolymer (PP-R)

Clearer · More flexible · Better for piping

A small amount of ethylene is added into the polymer chain randomly during production. This disrupts the crystal structure just enough to make the material more flexible and visually clearer — but the heat and chemical resistance drop slightly as a result.²

You'll find it in: PP-R water supply pipes (the green or white pipes used in buildings for hot and cold water), food packaging films and containers where some clarity matters, and medical containers. The "PP-R pipe" you see in plumbing is this grade.

PP Impact Copolymer (PP-ICP)

Toughest · Best for cold · Absorbs impact

A rubber-like phase is blended into the PP matrix during production, creating a material that can absorb hard impacts without cracking — even at temperatures down to –22°C. The stiffness and chemical resistance are reduced compared to homopolymer, but the toughness is significantly better.³

You'll find it in: car bumpers, appliance housings, industrial containers, and any application where the material will take knocks or be used in cold environments.

PP Material Properties

Mechanical Properties

PP homopolymer is a stiff, hard material with a density of about 0.905 g/cm³ — making it one of the lightest commercial plastics available. That low density means a PP component weighs roughly 10% less than the same part made from HDPE, and dramatically less than metal. Yet it holds meaningful tensile strength (around 33 MPa at yield) and compressive strength. It doesn't absorb water — less than 0.01% water absorption in 24 hours — which means its dimensional stability and mechanical properties are consistent in wet or humid environments.⁴

One mechanical characteristic worth calling out: PP has a natural "hinge memory." It can be bent back and forth repeatedly along a thin section without fracturing — a property called the living hinge effect. This is why PP is used for the flip-top caps on shampoo bottles and condiment containers. That same resilience makes it durable in applications with repeated mechanical stress.

The main mechanical limitation is cold-temperature brittleness. Standard PP homopolymer begins losing impact toughness below 0°C, and becomes noticeably brittle at sub-zero temperatures. If your application involves cold climates, cold process fluids, or refrigerated environments, PP impact copolymer or an alternative material like PVDF should be evaluated.

Thermal Properties

There are two thermal numbers for PP that often get confused, and the difference matters.

The melting point is the temperature at which PP's crystalline structure fully breaks down. For commercial isotactic PP, this is typically 160–166°C. The "perfectly isotactic" theoretical maximum approaches 171°C, but no commercial grade reaches it.⁵ These numbers come up in processing discussions — injection molding, extrusion — but they're not the number you use for application design.

The number you actually design around is the maximum continuous working temperature — the highest temperature at which PP can hold its shape and strength reliably under sustained load. For standard PP-H sheet and rod, this is approximately 82°C (180°F). Above that, PP begins to soften and creep under mechanical stress, well before it anywhere near melts. Heat-stabilized grades extend this to around 100–115°C, but that requires specifying the right grade.⁶

PP also expands significantly with temperature changes — its coefficient of thermal expansion is relatively high for a structural material. This matters in piping design and for components that go through large temperature swings: the dimensional change needs to be accounted for in joints, supports, and connections.

Chemical Resistance

At room temperature, PP resists the vast majority of acids, alkalis, alcohols, and salt solutions. Non-oxidizing acids and alkalis — hydrochloric acid, sulfuric acid at moderate concentrations, sodium hydroxide, potassium hydroxide — can be stored and handled in PP containers and piping without issue. This is one of the main reasons PP is so widely used in chemical processing and water treatment.⁶

Where PP struggles is with strong oxidizing agents and aromatic or halogenated hydrocarbons. Toluene, xylene, chlorinated solvents like chloroform and trichloroethylene, concentrated nitric acid, and concentrated chlorine — these will attack or dissolve PP. Temperature makes things worse: chemicals that PP handles fine at room temperature may cause swelling or degradation at 60–70°C. The chemical resistance chart further down this page covers the specifics.

UV and Weathering Resistance

Standard PP grades are not UV-stabilized, and this is a common specification trap. Outdoor exposure causes photooxidative degradation — the material becomes chalky on the surface, then brittle, then cracks. For equipment installed outdoors or in environments with strong UV exposure, you must specify a UV-stabilized grade (typically formulated with carbon black, which is also why outdoor PP tanks and fittings are often black).

Electrical Properties

PP is an excellent electrical insulator. Its dielectric constant is low (around 2.25), its volume resistivity exceeds 10¹⁴ ohm·cm, and it maintains these properties well even in humid conditions because it absorbs virtually no moisture. These characteristics make it a standard choice for electrical cable insulation, capacitor films, and electrical component housings.⁴

PP Applications

PP's combination of light weight, broad chemical resistance, and reasonable cost has made it one of the most versatile materials across industries. Here's where it shows up and why.

Chemical processing and water treatment — Chemical tanks, pipe fittings, pump bodies, valve components, fume hoods, and equipment linings in acid pickling lines and electroplating plants are commonly made from PP-H. The same properties extend naturally to water treatment: dosing manifolds, filter housings, and piping where chlorinated water or treatment chemicals flow.⁶
Semiconductor and PCB fabrication — Wet bench components, rinse tanks, chemical delivery lines, and wafer handling fixtures are often PP or PVDF — the two dominant materials in these environments. PP handles the dilute HF, HCl, H₂SO₄, and H₂O₂ solutions used in semiconductor wet processing at controlled concentrations and temperatures.
Food and beverage — PP is FDA-approved (21 CFR 177.1520) and USDA-accepted for direct food contact. You find it in food-grade containers, packaging films, dairy packaging, caps and closures, and PP-R hot and cold water distribution pipe in food production facilities.
Medical and laboratory — PP can withstand short-duration steam sterilization (autoclaving), is chemically inert, and is BPA-free. Disposable syringes, vials, lab containers, and some surgical devices are made from PP.
Everyday consumer goods — From yogurt cups to storage boxes, rugs to packaging films. The "#5" resin code on the bottom of plastic containers is PP.

PP in LORRIC products

PP wetted parts appear in several LORRIC paddle wheel flow meters — including the FP-AS510 AxleSense™ and FP-P Series — for general industrial fluid monitoring, water treatment, and chemical dosing lines within PP's temperature and chemical compatibility range. The spray nozzle line includes the MWP-PP hollow cone nozzle, a PP-body nozzle designed for chemical wash applications. For fluids that exceed PP's chemical or temperature limits, LORRIC's PVDF-wetted flow meters are the appropriate step up — see the PVDF material guide for a direct comparison.

PP Chemical resistance chart

How to read this chart:

OK Recommended for continuous service at stated conditions

△ Conditional — test under your specific conditions before use

✕ Not recommended

N/A Data not confirmed from primary sources

ⓘ Note: This chart shows how this material performs against individual chemicals under lab conditions. Real-world results vary — concentration, temperature, and mixed chemicals all affect performance. Always test under your actual operating conditions before finalizing your material selection.

Category	Chemical	PP Rating
Organic Acids	Acetic acid (20%)	OK
	Acetic acid, glacial	N/A
	Acetic anhydride	N/A
	Citric acid	N/A
Organic Compounds	Acetaldehyde (20°C)	OK
	Acetone (20°C)	△
	Aniline	N/A
	Benzaldehyde	OK (70°C) ✕ (100°C)
	Benzene (20°C)	OK (20°C) ✕ (70°C)
	Benzyl alcohol	OK
	Benzyl chloride	N/A
	Corn oil	N/A
	Ethanol (96% @ 70°C)	OK
	Ethylene glycol	OK
	Fatty acid	N/A
	Formaldehyde (10%, 40%)	OK
	Formic acid	N/A
	Hexane (20°C)	✕
	Lactic acid	N/A
	Methanol (20°C)	OK
	Methyl alcohol	N/A
	Paraffin oil	N/A
	Petroleum	N/A
	Phenol	N/A
	Propane, liquid	N/A
	Propanol	N/A
	Stearic acid	N/A
	Tannic acid	N/A
	Tartaric acid	N/A
	Toluene	✕
Urea (aqueous)	N/A
Inorganic Compounds	Ammonia (aqueous)	N/A
	Ammonium chloride (aqueous)	N/A
	Ammonium hydroxide	N/A
	Ammonium nitrate	N/A
	Ammonium sulfate	N/A
	Aqua regia	N/A
	Barium chloride	N/A
	Barium hydroxide	N/A
	Brine	N/A
	Calcium chloride	N/A
	Calcium hydroxide	N/A
	Carbonic acid	N/A
	Chloric acid	N/A
	Chlorine (10%)	OK (10%) △ (30%)
	Detergent	N/A
	Hydrobromic acid	N/A
	Hydrochloric acid	N/A
	Hydrofluoric acid (20°C)	OK
	Hydrogen peroxide	N/A
	Nitric acid (20°C)	OK
	Phosphoric acid (70°C)	OK
	Potassium hydroxide	N/A
	Potassium nitrate	N/A
	Potassium sulfate	N/A
	Sodium carbonate (aqueous)	N/A
	Sodium hydroxide (50%)	OK
	Sodium nitrate	N/A
	Sulfuric acid	OK (30%@70°C) △ (30%@100°C) OK (100%@20°C) △ (100%@70°C)
	Sulfur dioxide	N/A

Frequently Asked Questions

What is the maximum temperature polypropylene can handle?

For standard PP homopolymer in continuous structural or fluid-contact service, the safe upper limit is approximately 82°C (180°F). This is the temperature at which PP starts to soften and lose its shape under load — not to be confused with its melting point (~160–166°C), which is the temperature at which it fully liquefies. Exceeding the working temperature doesn't necessarily cause instant failure, but it will cause gradual deformation and dimensional change over time, which is a problem for piping, tank walls, and flow meter wetted components. Heat-stabilized grades can push this ceiling to around 100–115°C. Above that, PVDF is typically the material to consider.⁶

Is polypropylene resistant to all acids and chemicals?

Not all — PP has clear limits that matter in practice. It handles most non-oxidizing acids, alkalis, alcohols, and salt solutions well at room temperature. Where it fails: strong oxidizing agents (concentrated H₂O₂, concentrated HNO₃, chlorine above 30%), aromatic solvents (toluene, xylene), halogenated solvents (chloroform, trichloroethylene), and elevated temperatures in general push PP's boundaries significantly. A common field mistake is assuming PP handles a chemical because it handles a dilute version — concentrated or heated versions of the same chemical may attack the material. The resistance chart on this page and the original grade datasheet are the right references before finalizing any material selection.⁶

What is the difference between PP and PE (or HDPE)?

PP and HDPE are close relatives — both polyolefins, similar chemical resistance at room temperature, similar look and feel. The differences come down to a few key performance areas. PP runs hotter: its maximum working temperature (~82°C) is higher than HDPE (~60–82°C depending on grade), and its melting point (~163°C) is well above HDPE's (~130–137°C). PP is also stiffer — higher flexural modulus — and harder. HDPE wins on cold-temperature impact resistance: it stays tough below 0°C, whereas standard PP-H becomes brittle. In chemical terms, HDPE is often slightly better with oils and hydrocarbon fuels. For industrial fluid handling and chemical process equipment in moderate-temperature service, PP-H is the more commonly specified of the two.

What does the number 5 on plastic containers mean? Is it the same as PP?

Yes — the triangle with a "5" stamped on the bottom of plastic containers is the resin identification code for polypropylene. It's a global recycling classification system, and PP5 is polypropylene that has been approved for food contact (typically homopolymer or random copolymer grade). It's the same material described throughout this page — just in consumer product form. One thing to note: the #5 code tells you the material, but it doesn't tell you whether the specific container is safe for microwave heating. That requires a separate microwave-safe label (the icon with wavy lines). Not all PP containers are designed or tested for repeated microwave use — single-use takeout containers in particular are PP but not rated for repeated heating cycles. If you see both the #5 code and the microwave-safe symbol, the container has been specifically tested and is cleared for reheating food.

Is polypropylene safe? Is it toxic, food-safe, and microwave-safe?

PP is one of the safer plastics for everyday and food-contact use. It is FDA-approved (21 CFR 177.1520) and USDA-accepted for direct food contact, and carries recycling code #5 on food containers. It does not contain BPA — polypropylene's chemistry has nothing to do with bisphenol A, which is associated with polycarbonate plastics. There are no known cancer-causing effects from PP under normal use conditions.⁷ For microwave use: PP containers labeled "microwave safe" have been specifically tested and cleared for reheating food. The key is to check for both the #5 resin code and the microwave-safe symbol — the resin code tells you the material, but the microwave-safe label confirms the specific product was tested for heating applications. Not all PP containers are rated for repeated microwave use, particularly single-use takeout containers.

What are PP's main disadvantages?

PP is an excellent all-around material, but it has real limitations that affect material selection. The main ones: it degrades under UV and outdoor exposure (standard grades must be UV-stabilized for outdoor use), it becomes brittle below 0°C (homopolymer grades — impact copolymers are better but still limited), and it can't handle strong oxidizing chemicals or halogenated solvents. Its thermal expansion is higher than metals, which requires careful design in piping and connections. And its working temperature ceiling of around 82°C rules it out for hot chemical lines or high-temperature process service — where PVDF typically takes over. For most industrial fluid handling and chemical storage applications below that temperature ceiling, with non-oxidizing process fluids, PP-H is hard to beat on the price-to-performance ratio.

References

^ Grokipedia. Polypropylene. — Used for: global production volume, density range, and melting point range for homopolymers.
^ LyondellBasell. Tech Topic: Material Selection — Polypropylene. — Used for: PP grade types (homopolymer, random copolymer, impact copolymer), grade selection criteria, and characteristic differences.
^ LyondellBasell. Polypropylene, Impact Copolymer. — Used for: impact copolymer structure and applications.
^ Laminated Plastics. Polypropylene (PP) Technical Data Sheet. — Used for: mechanical, physical, thermal, and electrical properties (ASTM test methods) for PP-H and PP-C.
^ Wikipedia. Polypropylene. — Used for: melting point ranges (171°C theoretical max, 160–166°C commercial isotactic), cross-confirmed by multiple primary sources.
^ Röchling Engineering Plastics. Polystone® G (HDPE) and Polystone® P (Polypropylene) — Semi-Finished Thermoplastics Brochure (2017). — Used for: maximum working temperature, chemical resistance table data, and industrial applications.
^ Healthline. Is Polypropylene Safe? — Used for: FDA approval status, BPA-free confirmation, and food safety classification of PP.