Factors Affecting the Strength of Spunbond Nonwovens

Factors Affecting the Strength of Spunbond Nonwovens

 

The flow of spunbond nonwovens production process is as follows: polymer chip--> screw extruder--> melt filter--> metering pump--> spinneret--> lateral quenching--> filament drafting--> fabric web laying--> nonwoven fabric forming via thermal calendering--> winding. The factors that affects nonwovens’ strength most are: the performance of fiber filaments, which is related to the quality of resin chips, setting conditions for spinning and stretching; web deposition modes; temperature and pressure of, contact time with thermal roller; fiber web thickness and the height and area of the embossing point of engraved roller.

1. Main factors affecting the performance of fiber filaments

1) Chip quality

The impurity content in the raw polymer should be limited to less than 0.025% to ensure smooth spinning. In addition, the moisture content should be within 0.05%.

 

2) Process parameters

① Spinning box: the temperature should be slightly higher to increase the flow performance of the melt and ensure smooth spinning; but too high temperature will cause a large number of broken and raw-edged filaments, which are not conducive to the smooth production. Generally, to obtain quality spinning, the actual temperature of the melt should be 20℃~25℃ higher than the melting point of the resin.

② Spinning speed: prudent design of the spinning pump speed is crucial to a stable production and improved product quality.

③ Selection of the drawing point: the drafting point is the distance at which the melt stream begins to be drawn by external air forces after leaving the spinneret surface, so that the macromolecular chain is stretched. The oriented area is the key to measuring the quality of spinning. For polypropylene, the drafting point is generally between 15cm~20cm.

 

3) Stretching conditions.

① Drafting device structure: mainly the design of the nozzle width and the drafting air duct. To increase the drafting speed, the nozzle and the drafting air duct should be small.

② Quenching conditions: have a great influence on drafting. Both insufficient or excessive cooling have an negative impact on nonwoven fabric strength.

 

2. Web deposition method

The key consideration for web deposition is to ensure the uniformity of fabric web, without wavering or fluttering of the filament due to external factors. There are mainly three filament laying methods, each with their own characteristics:

① Brush-type deposition: MD strength difference of the fabric is small, but with more filament strands on the nonwovens’ surface;

② Scattering deposition: if frequency of the diffuser is not high enough, "cloud spots" will be formed on the fabric surface;

③ Jet deposition: nonwovens laid with this method are with few strands, no cloud spots, good softness, and high elongation; but the MD & CD strength difference is big.

 

3.Temperature and pressure of, contact time with thermal roller

1) Temperature: the tensile strength of nonwoven fabrics maintains an approximately linear relationship with the calender roller temperature. However, the temperature reaches a certain value, a deorientation effect will take place to the filament, which means the fiber structure is destroyed, resulting in a decrease in the nonwoven strength. Thus, the thermal roller temperature should be set between the melting point and softening point of the fiber.

2) Pressure: when the line pressure increases, the CD and MD strength of the nonwoven fabric increases; but after a certain value, due to excessive heating and pressing, the physical properties of the thermally bonded fiber are destroyed, resulting in a decrease in strength. Generally, the line pressure increases with the increase of fabric basis weight.

3) Contact time: determined by the roller line speed, generally does not need to be set separately; it changes synchronously with the production line speed, within the range of 5 N/mm and 300 N/mm.

 

To sum up, the above three factors need to be taken into account as a whole to achieve the best nonwoven quality.

 

4. Fiber web thickness

Within a certain range, the strength increases with the rising of fiber web thickness; but over a certain GSM, the excessive thickness of the fiber web will weaken the bonding effect, bringing even no strength improvement.

 

5. Height and area of the embossing point of engraved roller

The pattern and nip shape of the embossing roller: the larger and more in quantity of the nip, the stronger the nonwoven fabric, but its softness and breath ability will deteriorate. Increasing the height of the nip will reduce the strength of the fabric, but increase its air permeability and softness. However, due to the limitation of the nip strength, the height cannot be too large, generally below 1 mm.

Factors Affecting the Strength of Spunbond Nonwovens

 

The flow of spunbond nonwovens production process is as follows: polymer chip--> screw extruder--> melt filter--> metering pump--> spinneret--> lateral quenching--> filament drafting--> fabric web laying--> nonwoven fabric forming via thermal calendering--> winding. The factors that affects nonwovens’ strength most are: the performance of fiber filaments, which is related to the quality of resin chips, setting conditions for spinning and stretching; web deposition modes; temperature and pressure of, contact time with thermal roller; fiber web thickness and the height and area of the embossing point of engraved roller.

1. Main factors affecting the performance of fiber filaments

1) Chip quality

The impurity content in the raw polymer should be limited to less than 0.025% to ensure smooth spinning. In addition, the moisture content should be within 0.05%.

 

2) Process parameters

① Spinning box: the temperature should be slightly higher to increase the flow performance of the melt and ensure smooth spinning; but too high temperature will cause a large number of broken and raw-edged filaments, which are not conducive to the smooth production. Generally, to obtain quality spinning, the actual temperature of the melt should be 20℃~25℃ higher than the melting point of the resin.

② Spinning speed: prudent design of the spinning pump speed is crucial to a stable production and improved product quality.

③ Selection of the drawing point: the drafting point is the distance at which the melt stream begins to be drawn by external air forces after leaving the spinneret surface, so that the macromolecular chain is stretched. The oriented area is the key to measuring the quality of spinning. For polypropylene, the drafting point is generally between 15cm~20cm.

 

3) Stretching conditions.

① Drafting device structure: mainly the design of the nozzle width and the drafting air duct. To increase the drafting speed, the nozzle and the drafting air duct should be small.

② Quenching conditions: have a great influence on drafting. Both insufficient or excessive cooling have an negative impact on nonwoven fabric strength.

 

2. Web deposition method

The key consideration for web deposition is to ensure the uniformity of fabric web, without wavering or fluttering of the filament due to external factors. There are mainly three filament laying methods, each with their own characteristics:

① Brush-type deposition: MD strength difference of the fabric is small, but with more filament strands on the nonwovens’ surface;

② Scattering deposition: if frequency of the diffuser is not high enough, "cloud spots" will be formed on the fabric surface;

③ Jet deposition: nonwovens laid with this method are with few strands, no cloud spots, good softness, and high elongation; but the MD & CD strength difference is big.

 

3.Temperature and pressure of, contact time with thermal roller

1) Temperature: the tensile strength of nonwoven fabrics maintains an approximately linear relationship with the calender roller temperature. However, the temperature reaches a certain value, a deorientation effect will take place to the filament, which means the fiber structure is destroyed, resulting in a decrease in the nonwoven strength. Thus, the thermal roller temperature should be set between the melting point and softening point of the fiber.

2) Pressure: when the line pressure increases, the CD and MD strength of the nonwoven fabric increases; but after a certain value, due to excessive heating and pressing, the physical properties of the thermally bonded fiber are destroyed, resulting in a decrease in strength. Generally, the line pressure increases with the increase of fabric basis weight.

3) Contact time: determined by the roller line speed, generally does not need to be set separately; it changes synchronously with the production line speed, within the range of 5 N/mm and 300 N/mm.

 

To sum up, the above three factors need to be taken into account as a whole to achieve the best nonwoven quality.

 

4. Fiber web thickness

Within a certain range, the strength increases with the rising of fiber web thickness; but over a certain GSM, the excessive thickness of the fiber web will weaken the bonding effect, bringing even no strength improvement.

 

5. Height and area of the embossing point of engraved roller

The pattern and nip shape of the embossing roller: the larger and more in quantity of the nip, the stronger the nonwoven fabric, but its softness and breath ability will deteriorate. Increasing the height of the nip will reduce the strength of the fabric, but increase its air permeability and softness. However, due to the limitation of the nip strength, the height cannot be too large, generally below 1 mm.