Polypropylene fibers have two main uses in concrete:
1) Prevent plastic shrinkage cracks in concrete.
During the setting and hardening process of concrete, surface water loss can lead to plastic shrinkage and cracks. Adding polypropylene fibers to concrete can prevent these cracks. Due to the lower elastic modulus of polypropylene fibers compared to hardened concrete, the improvement in crack resistance (cracking caused by temperature stress or mechanical load) of hardened concrete is limited, and the improvement in tensile and flexural strength is not significant, which has a certain degree of improvement in concrete toughness.
2) Improve the fire resistance of hardened concrete. Under the action of high temperature, polypropylene fibers will first soften and burn off, forming many pores in the concrete. The water vaporized by high temperature in the concrete can be discharged along these pores, preventing the formation of internal high pressure due to water vaporization and causing the concrete to burst, thereby greatly improving the fire resistance time and grade of the concrete.
Polypropylene fibers can be divided into long fibers, short fibers, spunbond non-woven fabrics, melt blown non-woven fabrics, etc.
(1) Lightweight
The density of polypropylene fiber is 0.90-0.92 g/cm, which is the lightest among all chemical fibers, 20% lighter than nylon, 30% lighter than polyester, and 40% lighter than viscose fibers. Therefore, it is very suitable for use as a filler for winter clothing or as a fabric for skiing, mountaineering, and other clothing.
(2) High strength, good elasticity, wear resistance, and corrosion resistance
Polypropylene has high strength (the same in dry and wet conditions) and is an ideal material for manufacturing fishing nets and cables; Good wear resistance and resilience, similar in strength to polyester and nylon, with a rebound rate comparable to nylon and wool, much higher than polyester and viscose fibers; Polypropylene has poor dimensional stability, is prone to pilling and deformation, is resistant to microorganisms, and does not cause damage; Chemical resistance is superior to general fibers.
(3) Has electrical insulation and warmth retention properties
Polypropylene fiber has a high electrical resistivity (7 × 10 Ω. cm) and a low thermal conductivity. Compared with other chemical fibers, polypropylene fiber has the best electrical insulation and warmth retention, but is prone to generating static electricity during processing.
(4) Poor heat and aging resistance performance
Polypropylene fibers have a low melting point (165-173 ℃) and poor stability to light and heat. Therefore, polypropylene fibers have poor heat resistance, aging resistance, and are not resistant to ironing. However, the anti-aging property can be improved by adding anti-aging agent during spinning.
(5) Poor moisture absorption and staining properties
The moisture absorption and dyeing properties of polypropylene fibers are the worst among chemical fibers, with almost no moisture absorption and a moisture regain rate of less than 0.03%. Fine denier polypropylene has a strong core suction effect, and water vapor can be eliminated through the capillaries in the fibers. After being made into clothing, the comfort of the clothing is better, especially with ultra-fine polypropylene fibers. Due to the increased surface area, sweat can be transmitted more quickly, keeping the skin comfortable. Due to the non moisture absorption and low shrinkage rate of fibers, polypropylene fabrics have the characteristics of easy washing and quick drying.
Polypropylene has poor dyeing properties, light colors, and poor dyeing fastness. Ordinary fuels cannot dye it, and most colored polypropylene fibers are produced by pre spinning coloring. Raw material coloring and fiber modification can be used, and fuel complexing agents can be mixed before melt spinning.