Module 4: Polymer-extrusion based Technologies

The two important polymer-extrusion based technologies that are mainly used to convert the molten polymer into nonwoven fabrics are spunbond technology and meltblown technology.

1. 
   Spunbond technology
   

Spunbond technology uses preferably thermoplastic¹ polymers with high molecular weight and broad molecular weight distribution such as polypropylene (PP) and polyester (PET) [1]. To a small extent, other polyolefins such as polyethylene of high density (HDPE) and linear polyethylene of low density (LLDPE) as well as a variety of polyamides (PA), mainly PA 6 and PA 6.6 are found. Out of these polymers, polypropylene is mostly used primarily due to its low price and advantageous properties such as low density, chemical resistance, hydrophobicity, sufficient or even better strength. The fibre grade polypropylene (mainly isotactic) is the principal type of polypropylene which is used in spunbond technology. The important raw material parameters for polypropylene to be a suitable candidate for spunbond technology are melt flow index (MFI) of about 20–40 g/10 min and polydispersity ratio (M_w/M_n) of around 3.5–7. The molecular weight can be around 180000. On the other hand, the important raw material parameters for polyester are intrinsic viscosity of about 0.64, low share in COOH-groups, high crystallinity, and low water content (as low as 0.004%). Spunbond nonwovens are exclusively made from crystalline polyester. Crystallinity influences pre-drying and extrudability as well as filament drawing orientation, which is basic to make products that meet the requirements and that are of proper strength. Pre-drying is inevitable as PET at thermal strain is subject to hydrolytic degradation when extruded. In addition, low water content avoids air pockets in the melt that might cause filament breakage. Frequently, requirements can only be met by means of polymer modification. Except for the mechanical properties, UV-resistance and flame-retardancy are important with technical applications. Nowadays, the bicomponents are found in spunbond fabrics. The cross-section of these bicomponent filaments has at least two different polymer components. Figure 2.1 shows different geometry of cross-sections of the bicomponent filaments. Sometimes the bicomponent filaments are splitted or fibrillated into microfibres by means of hydroentangling energy. The resulting fabrics are extremely soft, particularly after finishing, and have therefore been considered for use in clothing, hygiene, and medical dressing components. In addition, bicomponent fibres with eccentric sheath core arrangement are used to develop crimp in spunlaid fabrics by differential thermal shrinkage of the two polymer components.

Figure 4.1 displays a schematic diagram of spunbond machine. The spunbond technology, in its simplest form, consists of four processes namely, spinning, drawing, web formation, and web bonding. The spinning process largely corresponds to the manufacture of synthetic fibre materials by melt-spinning process. In the drawing process, the filaments are drawn in a tensionally locked way. The web formation process forms a nonwoven web. Web bonding is generally possible by means of the web bonding processes discussed earlier. The bonding process includes mainly thermal calender bonding. Mechanical bonding and chemical bonding of spunlaid webs are also reported. The sequence of processes is as f

Figure 4.1

A positive displacement volume metering device is used for uniform melt delivery to the die assembly. It ensures the consistent flow of clean polymer mix under process variations in viscosity, pressure, and temperature. The metering pump also provides polymer metering and the required process pressure. The metering pump typically has two intermeshing, counter-rotating, toothed gears. The positive displacement is accomplished by filling each gear tooth with polymer on the suction side of the pump and carrying the polymer around to the pump discharge. The molten polymer from the gear pump goes to the feed distribution system to provide uniform flow to the die nosepiece in the die assembly.

The die assembly is one of the most important elements of the spunbond technology. The die assembly has two distinct components: the polymer feed distribution section and the spinneret.

The feed distribution in a spunbonding die is more critical than in a film or sheeting die for two reasons. First, the spunbonding die usually has no mechanical adjustments to compensate for variations in polymer flow across the die width. Second, the process is often operated at a temperature range where thermal breakdown of polymers proceeds rapidly. The feed distribution is usually designed in such a way that the polymer distribution is less dependent on the shear sensitivity of the polymer. This feature allows the processing of widely different polymeric materials using just one distribution system. The feed distribution balances both the flow and the residence time across the width of the die. There are basically two types of feed distribution that are employed in the spunbonding die, the T-type (tapered and untapered) and the coat-hanger type. An in-depth mathematical and design description of each type of feed distribution is given by Mastubara [2-5]. The T-type feed distribution is widely used because it gives both even polymer flow and even residence time across the full width of the die.

From the feed distribution channel the polymer melt goes directly to the spinneret. The spinneret is one of the components of the die assembly. The web uniformity partially hinges on the design and fabrication of the spinneret, therefore the spinneret in the spunbonding process requires very close tolerances, which has continued to make their fabrication very costly. A spinneret is made from a single block of metal having several thousand drilled orifices or holes. The orifices or holes are bored by mechanical drilling or electric discharge machining (EDM) in a certain pattern. The spinnerets are usually circular or rectangular in shape. In commercial spunbonding processes, the objective is usually to produce a wide web (of up to about 5 m), and therefore many spinnerets are placed side by side to generate sufficient fibers across the width.21 The grouping of spinnerets is often called a block or bank. In commercial production lines, two or more blocks are used in tandem in order to increase the coverage of the filaments.

The proper integration of filament spinning, drawing, and deposition is critical in the spunbonding process. The main collective function is to solidify, draw, and entangle the extruded filaments from the spinneret and deposit them onto an air-permeable conveyor belt or collector.

Filament drawing follows spinning. In conventional extrusion spinning, drawing is achieved using one or more set of draw rollers. While roller drawing can certainly be used in spunbonding, a specially designed aerodynamic device such as a Venturi tube is commonly adopted.

Filament deposition follows the drawing step. Filament deposition is also frequently achieved with the aid of a specially designed aerodynamic device referred to as a fanning or entangler unit. The fanning unit is intended to cross or translate adjacent filaments to increase cross-directional web

The concept of spunbond technology was developed sometime in late 1950s simultaneously in Europe and USA. Since then numerous innovations are disclosed on spunbond production system. This technology is derived from filament spinning technology. Many patents were granted on filament spinning technology. The basic principles involved in it, as proposed by Hartman [6], are explained with the help of Figure 4.2.

Figure 4.2a illustrates a system of filament formation. Here air as hot as melting temperature emerges from closely to the nozzle holes, takes the filaments and draws them. The emerging air, at the same time, intermingles with the ambient air. It uses longitudinal spinnerets, with air slots on both sides for the expulsion of the drawing gas ‘1’ (primary air). The room air (secondary air) ‘2’ is carried along and after lay down of the filaments, the air is removed by suction ‘3’. This process is well suited for tacky polymers, such as linear polyurethene. The continuous filaments after web collection bond themselves (self-bond) at their crossover points due to their inherent tackiness. Crystallization, which then sets in, subsequently eliminates the stickiness of the filaments after bonding.

F

Figure 4.2

Figure 4.2c depicts one more system. Here the cooling and drawing air are separated. It operates with regular cooling duct ‘1’ and drawing jet ‘3’. The drawing and cooling arrangements can be operated to give very high spinning speeds with the result that highly oriented filaments are produced. The room air ‘2’, of controlled temperature and moisture content, can be entrained to control the development of filament properties. The air is removed by suction ‘4’ after web formation.

Figure 4.2d illustrates another system that has a mechanical drawing step ‘2’ between the spinneret and lay down zones. This route is similar to conventional spinning and is especially useful for polymers, which in regular air drawing do not give optimum filament ‘4’. Webs with high strength and low elongation are generally made using this particular system.

Some of the commercial spunbond production systems are Docan system, Lutravil system, Ason/Neumag system, Reicofil system and Rieter system. We will discuss one system, the interested readers can learn about other systems from [1]. Figure 4.3 displays the schematic diagram of Reicofil spunbond system. The polymer pellets or granules are vacuum fed to dosing station on top of the extruder. Inside the extruder the polymer pellets are melted and homogenised. The molten polymer is then passed through a filter system and a spin punp, the melt is distributed by a coathanger die, feeding the spinneret which forms a curtain of filaments. The filaments are cooled by means of a stream of air in a blowing area, drawn by aerodynamic forces and then transported to the downstream discharge channel. Here, the primary blow ducts, located below the spinneret block, continuously cool the filaments with conditioned air. The secondary blow ducts, located below the primary blow ducts, continuously supply the auxiliary air at room temperature. A ventilator operating across the width of the machine, generates under-pressure and sucks the filaments together with the mixed air down from the spinnerets and cooling chambers. The filaments are swirled around and then deposited on the wire mesh belt as a random nonwoven material. This is transferred to the heat bonding calender which by heat and pressure sets the physical properties as tensile and elongation of the final product. After calendering the material is cooled by a water-cooled pair of rolls and then wound up. The continuous filaments are sucked through a Venturi (high velocity low pressure zone) to a distributing chamber, where fanning and entangling of the drawn filaments takes place. Finally, the entangled filaments are deposited on a moving suctioned mesh belt to form a web. Filament orientation in the web is influenced by turbulence in the air stream, which generally serves to increase randomization.

Figure 4.3