Melt Blowing & Spunbonding Technologies I

We have demonstrated the strong dependence of TPU and PEBA elastomeric melt-blown web strength on polymer crystallization kinetics in previous proceedings. Melt-blowing air temperature and velocity profiles were measured and correlated to polymer crystallization kinetics of the TPU and PEBA elastomeric polymers to predict the distances below the die where polymer solidification starts. However, air temperature and velocity profiles were measured without the polymer fiber, and the effect of the presence of the polymer fiber on the air flow field was ignored. Moreover, it was assumed that the polymer melt temperature at any position below the die is the same as the air temperature. But, fiber temperatures will be hotter than observed air temperatures and thus polymer crystallization will be delayed somewhat. Expensive techniques are required to make measurements with the inclusion of the polymer fiber and present difficulties at positions very close to the melt-blowing die. Thus, simulation tools were used to provide the most cost effective and simplest technique to predict the polymer fiber temperatures below the die and answer how much polymer crystallization will be delayed.

For air flow field simulations without the polymer, the agreement between simulation and experimental results was very good except at distances very close to the die surface. As expected due to recessed die configuration, air jets from the two individual air slots completely merged into a single centerline jet just below the die face within the first 1-2mm, where the maximum peak in the air velocity profile is observed. Since polymer melt having low viscosity meets directly with this maximum air velocity when exiting the die, air drag force acting upon the polymer melt will be the greatest, thereby providing the most significant fiber attenuation rate i.e. fiber diameter reduction.  This was confirmed by fiber diameter profiles i.e. most of the fiber deformation was limited to the region of significant air velocity drop, 0< z<5 cm. Beyond this region, the fiber diameter profile reaches a plateau of almost final fiber diameter values with further increasing distances. We observed that fiber temperature profiles do not reach lower temperatures as quickly as that of the air profile. And, even the fastest crystallizing polymer grades do not reach their solidification temperatures within the distances used to collect melt-blown webs. This was attributed to inadequacy of heat-transfer correlations in modeling fiber temperature profiles. Even though the fiber simulations did not provide quantitative agreement with the web tensile properties explained in terms of crystallization kinetics, it was concluded that simulation tools can be used to qualitatively predict fiber solidification behavior in melt-blowing.  This allows prediction of desired melt-blown web properties by simply (1) analyzing polymer crystallization kinetics through a simple method: differential scanning calorimetry (DSC), (2) modeling air flow field of melt-blowing equipment and fiber temperature profiles with simulation tools, and (3) correlating polymer crystallization behavior to melt-blowing air flow field characteristics and distances between the die and collector.

Recently, there has been significant advancement in the commercialization of metallocene catalyzed polypropylene (mPP).  After more than 20 years of intense academic and industrial activity in the development of mPP significant progress has been achieved, overcoming the challenging fundamental technical issues related to catalyst development as well as the implementation of these new catalyst systems to a production environment.  These catalysts are now being successfully used to manufacture mPP in full scale commercial plants. The expected benefits promised by this technology are finally being delivered.

These mPP resins contribute to enhanced resin attributes which include, among other, an unsurpassed level of product cleanliness and purity, broad regulatory clearance, enhanced product consistency and improved fabric properties.  In addition to improvements in fabric properties these resins also contribute to manufacturing advantages for the resin-to-fabric converter. 

This paper will review the general properties and characteristics of Basell’s metallocene catalyzed polypropylene resins (sold under the Metocene trade name) as well as the current state of commercialization of these products with a primary focus on new metallocene catalyzed melt blown resins for nonwoven applications.

Specialty Polyolefin Elastomers (SPE), a new generation of metallocene catalyst-based polymers endowed with isotactic propylene crystallinity, can be processed in conventional spunmelt equipment to produce elastic nonwoven fabrics. For meltblown applications, SPE resins are produced at a nominal melt flow rate (MFR) of either 200 or 300 g/10 min.

Meltblown elastic nonwovens were produced on a 0.5 m Reicofil® pilot line using SPE resins varying in MFR from 80 to 300.  For each SPE resin, the process air temperature and flow rate were suitably optimized to produce nonwovens relatively free of shots and other defects. The parameters varied during meltblown processing of the SPE resins were die to collector distance (DCD), set back, and air gap and fabric basis weight. Additional studies combining SPE resins with a meltblown polypropylene grade (1600 MFR) were completed over an extensive range of SPE to polypropylene (PP) blend ratio. Both the SPE and PP resins were dry blended and introduced in-line in the extruder hopper. The addition of PP modifies the surface features of the SPE nonwoven and provides a "dry touch" (low COF) at increasingly higher levels of PP in the blend composition.

The tensile, elastic and barrier properties (Hydrohead, Air Permeability) of each nonwoven composition was characterized. The elastic properties were measured using cyclic testing to an extension of 100 % strain and return to zero load. The permanent set, load loss, mechanical hysteresis and retractive force were obtained after both first and second cycle testing. The morphology of the nonwoven fiber was characterized using both Scanning Electron Microscopy (SEM) for fiber diameter and distribution and Atomic Force Microscopy (AFM) particularly in SPE/PP nonwovens, where phase contrast between the SPE and PP phases is obtained because of modulus differences between the component resins.

Meltblown elastic nonwovens comprising SPE resins offer a novel elastomeric product that can be used in applications such as hygiene, personal care, medical and industrial fabrics. The combination of SPE resins with other polyolefins provides further latitude in fine tuning the performance to suit customer end use application.

Nonwoven fabrics, such as spunbond web, are basically a net work of individual fibers.  The physical properties of the individual fibers affect lay down of the fiber, bonding performance of the web, and stress-strain behavior of the fabric. 

The fiber properties are dictated by the base polymer and the molecular orientation during fiber formation.  Polypropylene homopolymer and Specialty Polyolefin Elastomer (SPE) are selected in this study.  SPE's are a new generation of propylene, alpha olefin polymers synthesized with metallocene catalyst.   Key processing parameters such as output rate, draw down ratio, spinning speed are examined using a fiber spinning line.  These parameters are correlated with degree of molecular orientation and tensile properties.

For homopolymer fiber, the tensile strength increases and elongation decreases with increasing molecular orientation.  For elastic fibers produced from Specialty Polyolefin Elastomer, the elastic properties are favored by the lower molecular orientation. 

Hence, it is possible to optimize the nonwoven web properties by carefully selecting the appropriate fiber spinning condition during the nonwoven production.