Meltblown & Spunbond Technology II

Manufacturers are currently supplying poly(phenylene sulfide)  (PPS) for use in continuous melt production of nonwoven fabrics by spunbond and meltblowing (MB) processes. Aside from a preliminary study made by these authors in a paper presented on MB PPS at the INDA Filtration Conference in November 2006, little evaluation of the products which might be produced has been reported. The significant thermal shrinkage problem with MB PPS was noted and it was thought that shrinkage might be minimized by slowing the PPS fiber cooling rate hopefully to allow polymer crystallization to occur during the melt blowing process.  In this second paper, this approach was successfully implemented and a better understanding of the causes and control of thermal shrinkage was achieved. Japanese authors have also reported that extensive annealing is necessary to produce a nonwoven PPS fabric with low shrinkage.  The authors have altered the extrusion, quench and collection conditions in an effort to reduce the thermal shrinkage, and report on the effectiveness of this approach.

Outside the difficulties encountered in processing  Halar or ETCFE as a meltblown nonwoven, its properties offer new opportunities and technical solutions to long standing problems.

Bicomponent nonwovens hold strong positions in durable nonwovens markets for years, with emphasis on commercial carpeting, automotive and roofing applications. A well known brand (Colback®) is based on PET core and PA6 sheath. In the 2-step Colback process (spinning and web formation-bonding) properties are tuned by varying sheath percentage and draw ratio, among others.  We compared bicomponent webs with cores of PET, PA6, PBT, PP all with the same sheath polymer. The stress-strain curves are completely different, reflecting properties of core polymers and draw ratio.  We also compared different sheath polymers (PA, PBT, others) with two draw ratios of the same core (PET). Again we see little influence of sheath on stress-strain properties.  Nevertheless, the contribution of the bond forming sheath is unique and manifold: without bonds we would not have any web properties

Considering sheath perspectives:

• The sheath consolidates the lay down pattern and enables transfer of stresses via numerous bonds between filaments. Although bonds are much weaker than filaments, their numbers are so huge that they can transfer high loads when yielding simultaneously.
• The sheath enables some tuning of the E-modulus: high density and strength of bonds restrict macroscopic fabric deformation and shift emphasis fully on straining of filaments, leading to a high E-modulus. The opposite -low density and strength of bonds- provokes more macroscopic fabric deformation, being the lowest strain energy solution to an imposed strain.
• The sheath polymer controls the process of break. Bicomponent nonwovens mostly collapse at strains well below the breaking strain of filaments. Application dependent, the sheath is designed such  that delamination of the consolidated structure initiates break. Keeping a fixed core-sheath ratio and bonding temperature, Load at Break appears to be proportional to the specific mass of the nonwoven.

For melt blowing with a conventional slot (“Exxon”) die or a multi-row Schwarz die, air is injected parallel to or at an angle to the molten fiber.  However, the air is never injected off-center with respect to the fiber orifice. This is not the case with a swirl die. In a swirl die, the molten polymer stream is attenuated and directed (swirled) by a concentric ring of air jets.  This type of die is commonly used as a glue applicator. However, it is a type of melt blowing die, and as such it could potentially be used to produce melt blown fibers.

With polypropylene, experiments were run wherein the effects of air flowrate, polymer flowrate, air temperature, and polymer temperature were examined. The fiber diameter, swirl pattern diameter, and swirl frequency were measured. The experimental results favorably compared to results predicted by a mathematical model. Experiments were also run wherein hollow fibers were produced with the swirl die. The swirl frequency increased with increasing hollowness, a result that could be commercially exploited for increasing line speed or improving fiber laydown pattern.