Key process factors
The key process factors of spunbond nonwoven technology are polymer throughput rate, polymer melting temperature, quench air temperature, quench air velocity, and lay-down velocity. These process factors play important roles in deciding the morphology and diameter of the filaments which are the building block of any spunbond nonwovens. The bonding parameters are also important and their effects are already discussed earlier.
The polymer throughput rate determines the morphology and diameter of the filaments. The morphology of the filaments spun at lower throughput rate is better developed than those at higher throughput rate. Because the rhelogical conditions are more favorable for crystallinity and orientation of the filaments spun at lower throughput rate. The filaments spun at lower throughput rate are thus more stable than those spun at higher throughput rate. The filament diameter increases with increasing throughput rate.
The polymer melting temperature influences on the drawing of the filaments through the spinneret that in turn decides the diameter of the filaments. The lower polymer melting temperature results in increase in melt viscosity of the polymer that leads to difficulty in drawing of the filaments. On the other hand, the higher melting temperature results in decrease in the melt viscosity of the polymer that makes drawing easier. Too high polymer melting temperature can cause polymer degradation leading to filament breakages.
There is a great debate going on the effect of quench air temperature on the diameter and morphology of the filaments. One group of researcher argues that the lower quench air temperature results in increase of viscosity that leads to slower draw-down which finally resulting in higher filament diameter. As the draw-down takes place slowly, an increase in crystallinity and orientation is observed. The other group argues that lower quench air temperature is helpful in generating higher spinline stress that leads to reduction in filament diameter. As the draw-down takes places under higher stress, an increase in crystallinity and orientation is observed.
The quench air pressure has a role to decide filament diameter. Higher quench air pressure increases spinline draw ratio that in turn reduces filament diameter. The pressure drop is known to be proportional to air velocity.
The web is formed by the pneumatic deposition of the filament bundles onto a moving belt. In order to obtain maximum uniformity and cover, the individual filaments must be separated before reaching to the belt. This can be accomplished by inducing an electrostatic charge onto the bundle while under tension and before deposition. This can be achieved by high voltage corona discharge. The belt is usually made of an electrically grounded conductive wire, which discharge the filaments upon deposition. Sometimes mechanical or aerodynamic forces can also separate filaments. If the lay-down conveyor belt is moving and filaments are being rapidly traversed across the direction of motion, the filaments are being deposited in a zig-zag pattern on the surface of the moving belt.