Understanding the Basics: What Are PP and FRP Ducting Systems?

Ducting made out of polypropylene (PP) is a thermoplastic polymer. Due to its lightweight properties and excellent chemical resistance, PP ducting has become very popular in the construction of sustainable buildings. PP ducting can be extruded or welded. It is often used for fume extraction, exhaust ventilation systems, and corrosive gas emissions in industries with moderate to severe chemical exposure.

Fibre Reinforced Plastic ducts (FRP), aka (GRP), are made out of a polymer with glass fibres as reinforcement. Duct systems made from fibre reinforced plastic possess excellent strength characteristics, considerable stiffness, and high chemical resistance, and are manufactured using hand-lay up, filament winding or pultrusion processes, which expand the possibilities for creating unique shapes and sizes of duct systems.

Performance in High-Humidity Environments

Industrial ducting faces a major challenge of high humidity, where constant exposure to moisture can cause deterioration of the materials, development of micro-organisms, failure of joints and reduce the ability for airflow through the ducts. Below you will find how FRP compares to PP in regard to performance criteria for use in high moisture conditions.

1. Moisture and Corrosion Resistance.

Material-wise, PP and FRP share the property of being non-metallic, meaning they don’t corrode the same way as galvanised steel or aluminium ducting does. Neither material will corrode through traditional methods.

• PP is nearly impervious to all water or water-based chemicals, making it an excellent choice for wet scrubber systems, acid fume extraction systems and areas that are prone to condensation.
• FRP will also perform very well in high moisture environments, especially if the outside of the FRP product has been finished with a gel coat or surface veil. The outer layer of the FRP product (having a high resin content) is a barrier to moisture entering the glass fibre core.

Summary: Both materials are equal in their ability to resist moisture; however, FRP has a slight advantage in areas where there is direct water spray or immersion on the surface of the FRP product.

2. Compatibility with chemicals

Due to high humidity levels in industrial processes, acid vapour, chlorine-type chemicals, solvent vapour, and alkaline mist are present in humid air. Each of these chemicals possesses an aggressive nature that can easily damage improper materials.

• Polypropylene (PP) performs well against all strong acids like sulphuric acid, hydrochloric acid, and nitric acid (in moderate concentrations). Additionally, it has resistance to most alkaline chemicals (lots of alkali chemicals) and all organic solvents. The nonpolar structure of PP allows many corrosive agents to bond only with the surface.
• The chemical resistance of FRP (aspects of it) is determined by the resin that has been selected for use. For example, vinyl ester type (vinyl ester) resins provide better resistance (more resistant) to chemicals than a traditional polyester resin would. Due to the large number of different chemicals that FRP can be used with, PP would be the better choice for corrosive vapour when the vapour being ducted contains a large percentage of aggressive vapour.

Summary: When ducting concentrated acidic vapour, PP would be the preferred choice. When there isn’t an abundance of aggressive vapour in the duct, FRP is usually a good choice for chemical vapour, as long as you select the appropriate resin for the chemical vapour being ducted.

3. Duct Structural Integrity and Rigidity

Ducting systems in larger-scale industrial plants must frequently run considerable distances with very little support, be exposed to negative pressures due to suction operation, and carry heavy items (fittings) such as dampers and flanges.

• PP has medium structural capacities. In applications using larger PP, this requires the use of more supporting brackets or ribbing for stiffness to avoid sagging and/or becoming deformed under vacuum conditions.
• FRP’s composite manufacturing develops 2x as much stiffness and tensile strength as PP. Consequently, FRP ducting can support longer unsupported periods, handle larger amounts of positive/negative pressure differentials from the inside/outside, and carry heavier loads without deformation.

Summary: For structural capabilities, FRP wins clearly over PP’s capabilities. Therefore, for large duct installations, overhead installations, and high-pressure installations, FRP is the more dependable medium for supporting ductwork.

4. Heat Tolerance

Industrial environments that are both humid and hot often consist of hot steam, exhaust gases from industrial boilers, and moisture combined with heat generated by the process. 

• Polypropylene has a maximum working temperature of approximately 100°C. When this limit has been exceeded, the material will start to soften and deform during continuous exposure to high temperature.
• FRP composites, depending on the resin used to manufacture them, can withstand a temperature range from 120°C to 180°C+; thus allowing the use of high-performance materials in their construction.

Summary: For applications where humidity and heat exist together (e.g., steam exhausts and kiln ventilation), FRP would be the choice of material.

5. Weight & Installation Ease 

Ducting in industrial plants that are complicated often needs to be installed at high locations or in tight spaces. Since the weight of duct material affects the time taken to install, the cost of labour, and the load on the structure that is supporting it, the ducting materials must be chosen carefully.

• Polypropylene (PP) is lighter in weight than Fibreglass Reinforced Plastic (FRP) for equivalent duct dimensions. This makes transporting and handling PP much easier than FRP and installs more quickly as less labour force is needed to get the ducting into place.
• The density of the FRP is greater than that of the PP. Therefore, if you are installing custom FRP ducting pieces, there is a need for heavier lifting devices and skilled labour to install the ducting and make the joints.

Summary: If speed of installation, ease of handling, and labour force cost are important, choose PP.

6. Outdoor Durability and UV Resistance

Humid areas can also be outdoors, such as coastal vegetation, outdoor scrubbers, and exhaust systems on rooftops, which receive UV radiation as well as moisture.

• When unmodified PP is exposed to UV light, it can become brittle and eventually crack due to sunlight. The cost of UV-stabilising PP materials typically increases the overall material cost.
• FRP products will maintain their mechanical properties when exposed to UV light, making them superior to other materials for outdoor use.

Summary: Using FRP is the best option when considering longevity and low maintenance in outdoor applications with high humidity.