The short answer
Glass-filled PP is lighter, cheaper, absorbs almost no moisture and resists water, acids, bases and detergents. Glass-filled nylon is much stronger, stiffer when dry, and handles far more heat. After it absorbs moisture, its stiffness can fall close to coupled PP's. Moving from nylon to coupled glass-filled PP is a common cost-down for parts that do not run hot or carry high loads.
Buyers look at this swap when nylon prices rise or supply tightens. It can save real money, but only on the right parts.
Side by side: 30% glass-filled grades
| Property | PP, 30% glass | Nylon 6, 30% glass |
|---|---|---|
| Density | ~1.12 g/cm³ | ~1.35–1.37 g/cm³ |
| Tensile strength | ~75–100 MPa (coupled) | ~150–185 MPa (dry) |
| Flexural modulus | ~5.5–7 GPa | ~8–9.5 GPa (dry); drops when conditioned |
| Heat deflection, 1.8 MPa | ~140–155°C | ~200–210°C |
| Water absorption | Almost none | Significant; properties change in use |
| Drying | Usually not needed | Required |
| Chemical resistance | Excellent with acids, bases, water | Good with fuels, oils, greases |
| Relative cost | Lower | Higher |
Typical values for general-purpose grades; individual grades vary. Glass-filled PP values assume a chemically coupled grade.
Where glass-filled PP works
- Housings, covers and brackets at moderate temperature
- Fan shrouds, battery trays and cooler under-hood parts
- Appliance parts in contact with water or detergents
- Parts where moisture swelling has been a problem with nylon
- Weight-sensitive parts, since the same part weighs about 15 to 20% less
Where to stay with nylon
- Parts above roughly 120°C in service, or carrying load when hot
- Gears, structural parts and snap fits that need toughness and fatigue resistance
- Parts in hot fuels or oils
- Approvals that specify nylon
Switching from nylon to PP
- Check heat and load at the real service temperature.
- Check stiffness. PP is less stiff than dry nylon; thin walls may need ribs or a higher glass level.
- Check the tool. Shrinkage differs, so measure first parts.
- Check assembly. PP is harder to bond and paint than nylon.
- Get approvals before the change if the part carries any.
See also glass-filled nylon vs. PBT, PA66 shortage alternatives and reducing part weight.
Common questions
Can glass-filled PP replace glass-filled nylon?
In many parts that do not run hot or need high stiffness, yes, and it usually costs less and weighs less. It is not a good swap where the part sees sustained heat, high loads, fuels at temperature, or needs nylon's stiffness.
Is glass-filled PP lighter than glass-filled nylon?
Yes. At 30% glass, PP is around 1.12 g/cm³ against about 1.35 to 1.37 for nylon, so the same part weighs roughly 15 to 20% less.
Does glass-filled PP absorb moisture?
Almost none, so its dimensions and stiffness stay steady and it usually does not need drying. Nylon absorbs moisture and changes properties as it does.
What is chemically coupled glass-filled PP?
PP compounded with a coupling agent that bonds the glass to the polymer. Coupled grades are much stronger and stiffer than uncoupled ones, so check that the grade you compare is coupled.
Which is better for under-hood parts?
Glass-filled PA66 for hot parts near the engine. Coupled glass-filled PP is common for cooler under-hood parts such as fan shrouds, battery trays and housings.
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