Controlled degradation of high molecular weight polypropylene resins to products with high melt flow (> 200) and more narrow molecular weight distribution is an important modification step in the polypropylene industry. This so called controlled rheology or vis-breaking step is usually done by the polymer producer. By using Ciba® IRGATEC® CR 76 this principle can now be extended to the manufacture of meltblown PP grades during the fiber spinning process. The controlled reaction to form the high quality meltblown resin occurs on-line in the extruder of the nonwoven equipment at temperatures higher than 250°C.

This powerful tool enables the nonwoven manufacturer to control the properties of monolithic meltblowns and SMS composite fabrics by the controlled regulation of the melt flow (MF).

The resulting melt flow depends on processing temperature and the concentration of Ciba® IRGATEC® CR 76. It allows melt flows between 25 and 2500 just by varying the processing temperature and/or by changing the concentration of the additive.

Weight average molecular weight (M_w) and molecular weight distribution MWD (M_w/M_n) are the most important molecular characteristics influencing the processing behavior of polypropylene. Resin quality is very important for the formation of a consistent nonwoven fabric.

A resin with a broad molecular weight distribution has the disadvantage of containing high and low molecular weight polymer fractions. These fractions can form gels leading to a defect known as shot or smoke. Therefore, a combination of consistent melt viscosity (MF) and narrow molecular weight distribution is a prerequisite for having improved spinability.

The immense advantage of the polymer modifier Ciba® IRGATEC® CR 76.  is the controlled mode of action resulting in polypropylenes with narrow molecular weight distributions - significantly narrower compared to the ones of commercial meltblown polymers and theoretically equal to the MWD of possible m-PP grades.

Comparison of the weight average molecular weights (M_w) and molecular weight distributions (M_w/M_n) of the starting polymer (MF 25), of meltblown nonwovens manufactured with different concentrations of  Ciba® IRGATEC® CR 76. and commercial meltblown grades.

GPC molecular weight distribution of two commercial meltblown grades compared to a meltblown nonwoven, which has been manufactured with Ciba® IRGATEC® CR 76 from a spunbond PP grade (MF 25). Typically, a bimodal molecular weight distribution (MWD) is observed in the commercial grades with a significant amount of low molecular weights and oligomers.200

These polymer fractions can cause fiber breakages and fumes during spinning resulting in shot and decreased mechanical & barrier properties of the meltblown nonwovens. Ciba® IRGATEC® CR 76 leads to a degradation of the starting polymer without formation of low molecular weight parts. The formation of a monomodal and narrow molecular weight distribution is also an evidence for the controlled degradation mechanism without any side reactions.

The significant improvements in macroscopic properties of meltblown nonwovens manufactured with  Ciba® IRGATEC® CR 76, like improved liquid barrier properties, mechanical strength and thermal stability give rise to analyze the microscopic properties. Electron microscopic (SEM) measurements of meltblown fabrics produced from a commercial resin (MF 1800) shows rather stiff filaments with some film-like areas in the commercial whereas in a spunbond resin (MF 25) modified with 1.5%  Ciba® IRGATEC® CR 76 the filaments are finer and more flexible. The amount of film-like areas is also significant lower. These observation correlates with measurements of the mean diameter and diameter distribution of the filaments.

Product:

IRGATEC® CR 76