Innovative Products from Stitch-Bonded-Hydroentangled Nonwoven Composites for Technical Applications

Elke Schmalz, Hilmar Fuchs, MargotBrodtka, Jochen Schreiber

Sächsisches Textilforschungsinstitut e. V., Chemnitz, Germany

1. Introduction

Apart from conventional applications, new types of using stitch-bonded nonwovens have been opened up in recent years by fibre material and process developments. Applications, among others, in high-tech-technology, vehicle manufacturing, in the field of filtration and particle separation, in the building industry, in medicine, in the field of hygienics belong to them.
New requirements are constantly raised as to properties, performance and the economical and the eco-friendly production of textile fabrics. As alternative technologies of fabric production, the technological variants of the processes of stitch-bonding appear to be ideal ways of manufacturing nonwovens that meet the purpose. From the group of stitch-bonded materials, stitch-bonded nonwovens, more than any other materials, find increasing interest due to their properties, such as textile touch, voluminosity and softness.

2. Manufacturing process and structural characteristics

The manufacturing of stitch-bonded-hydroentangled nonwoven composites is carried out in two steps. In the first step, a pile stitch-bonded nonwoven Kunit with a one loop surface is manufactured. The following second step represents the compression of the one loop surface of pile stitch-bonded nonwoven by putting a hydroentangled nonwoven on it.
With the Kunit process, a card web is formed and pleated by means of an oscillating element and then, loops are formed on one surface by means of compound needles. The resulting stitch-bonded nonwoven is highly voluminous, shows loops of fibres on one surface and high standing pile pleats of the very fibres on the other surface. The pile surface is soft bushels of fibre fixed in the loops only and, generally, oriented uprightly towards the loop surface. The share in pile ranges from 98 to 99%. The number and size of cavities between the fibres depend on the fineness of the fibres used and the fabric thickness. The height of the pile surface depends on how you set the oscillating element at the stitch-bonding position. The larger the stroke of oscillation the higher the pile pleats and, consequently, the fabric thickness. Practicable are heights of pile between 2 and 10 mm. The surface with the loops of the stitch-bonded nonwoven is comparable with a knitted fabric from fibres. The percentage of pores of the loop surface is between 93 to 95%.

If even surfaces are wanted on both sides, it is possible to loop the pile surface of the stitch-bonded nonwoven Kunit by means of a stitch-bonding machine MALIMO, type Multiknit, which involves a continuous or discontinuous process. The pile stitch-bonded nonwoven Multiknit is achievable both from one and two Kunit layers. Further fabrics may be involved.

The large-pore loops of the pile stitch-bonding nonwovens as well as the heavy tendency to rolling up of the unprocessed stitch-bonded nonwoven Kunit appear to be a disadvantage for application in the above fields.

One process of fibre compression without any chemical agents is putting on a web by hydroentangling. This process maintains the structure and volume of the pile web layer. A stitch-bonded nonwoven Kunit is fed into an equipment for hydroentanglement. In front of the bonding unit, a tangled web is bonded to a hydroentangled nonwoven and joined with the stitch-bonded nonwoven Kunit. This is carried out in one step by high energy water jets hitting the web. A number of injectors with increasing water jet pressure are used for manufacturing the bonded material. The water jet pressures and the diameter of the aperture depend on the mass per unit area of the bonding layers.

Water jet treatment is followed by drying and fixing. The temperature and exposure time in the drying machine depend on the fibre type used and the mass per unit area of bonded fabric.
With our experiments the bonded fabrics were treated in a stentering, drying and setting machine.

The stitch-bonded-hydroentangled nonwoven composites (Fig. 1) manufactured in this process show high voluminosity, a fine-pore layer of web on one or two surfaces and preferable processing performance. The linking between the stitch-bonded nonwoven and fibre web and the hydroentangled nonwoven respectively is solid high quality. The tendency to rolling up is overcome.

Figure 1: Stitch-bonded-hydroentangled nonwoven composite

3. Properties and fields of application

The stitch-bonded-hydroentangled nonwoven composites are achievable from a variety of fibre materials and their combinations, respectively so they can be used in many branches of industry. Their cross-section structure and their textile-technological values depend on the application wanted and the machine-setting chosen. The spectrum of properties of the stitch-bonded-hydroentangled nonwoven composites makes them interesting alternative products for the following applications (among others):

3.1. Deep filter media for wet and dry filtration

For deep filtration, even, voluminous fabrics with a progressive gradient of density are required. The stitch-bonded layer consists of bushels of fibres incorporated in loops. The gradient of density raises in the direction of the loop surface, thus contributing to the progressive cross-section structure. Air permeability depends on the number of fibres in the cross-section and ranges from 1000 to 4000 l/m2s. Due to its high porosity, a needle bonded layer is well suitable for particle deposition. Contrary to standard deep filter media, fibres are oriented a different way. With conventional deep filter media fibre orientation is mainly perpendicular to the flow-in direction. With stitch-bonded nonwovens Kunit, the fibres of the upper pile surface are random, oriented in flow-in direction. The bottom surface of the stitch-bonded nonwoven is made up of a layer of loops of fibres with relatively wide pores in the loop heads and between the loops.
During filtration a large proportion of small particles may enter through these openings into the clean gas. Putting on a nonwoven of fine fibres by means of water jets of high energy, we succeeded in compressing the loop surface and thus, we achieved a combination of the good filter properties of the upper pile surface with a fine-pore layer serving to reclaim small particles (up to 0.3 µm). The voluminosity of pile stitch-bonded nonwovens is largely maintained with this process. Air permeability at the loop surface may be reduced to 200 l/m2s, depending on the density of the hydroentangled nonwoven. Thickness of filter layers between 3 and 15 mm are practicable.

The stitch-bonded-hydroentangled nonwoven composites are cut-edge proof, well processable (e. g. into bags) and may be used for fine filtration (e. g. with gas filtration up to filter classification F9). Products from special fibres are particularly well suitable for vehicle filtration.
Due to the possibility to integrate antibacterial materials, they are also suitable e. g. for water filtration.

It is possible to incorporate conductive materials or to specially finish the filter media (electrostatic treatments, incorporation of adsorbents).

3.2. Insulation material

Stitch-bonded-hydroentangled nonwoven composites for insulation show the following advantages: stable shape, large pore surface and, opposite to it an even surface of fine pores. The hydroentangled nonwoven put on the stitch-bonded nonwoven compresses the pores of the loop surface in such a way that the air permeability of the basis stitch-bonds is reduced to 1/3 or even 1/4. The volume, the active air permeability and insulation performance of the basis stitch-bonding nonwovens are maintained. Due to the compression layer put on, the stitch-bonded-hydroentangled nonwoven composite provides further special functions. As the user requires, thin layers of heat-proof or conductive fibres may be put on or, prior to drying, may be subjected to additional treatment with textile auxiliaries such as flame-proof, stiffening agents.

In acoustics, mass systems unable to rebound are used for reducing sound. The stitch-bonding process Multiknit is very well suitable to achieve such-like structures as it builds nonwoven composites with density areas varying across the cross-section. The putting on or, respectively, integration of thin, hydroentangled layers of nonwoven for the manufacture of stitch-bonded-hydroentangled nonwoven composites may be used to reduce selected frequencies analogous to thin membranes able to swing as often used in acoustics. When using heat resistant fibre materials for these acoustically working thin layers of nonwovens, these may, at the same time, function as fire-blockers. Due to there little mass per unit area, the high cost incurred in heat resistant fibres is compensated for since it is possible to use inexpensive fibre materials across all the nonwoven cross-section. Another advantage of the composites is their high shapeability.

The stitch-bonded-hydroentangled nonwoven composites are especially well suitable for sound and heat insulation as well as in the building, vehicle manufacturing and machine building industries.

3.3. Upholstery material

For upholstery material, the use of textiles for substituting the standard materials is of current interest. This mainly focuses on the substitution of foams. This is based on trends to reduce hazardous emissions in the manufacturing process, to better recycleability and a better sitting climate. You will achieve this creating systems of composites of equal materials and, in addition, using eco-friendly processes. In order to substitute upholstery materials of foam, voluminous, stabile nonwovens are especially suitable as for instance stitch-bonded-hydroentangled nonwovens composites. The latter provide the pre-requisites to meet the economical and ecological requirements. This is due to the fact that a wide range of fibre materials is processable, bonding is achieved without any chemical agents and the fabric structure being easy to set and tardily resistant to pressure.

Potential fields of application are carpets, mattresses, the vehicle interior as well as prophylaxis is against decubitus. The main requirements with these applications are stability as to measurements and shape in frequently changing climatic conditions, high rebound, good moisture exchange as well as insulation against heat and sound. In order to achieve really good rebound after compression, it is advisable to use fibre blends of different fibre fineness between 3.3 and 12 dtex, incorporating a certain share in bi-component fibres. You will reach the necessary stability in both length-wise and horizontal direction and in shaping the material by reinforcing or compressing the base stitch-bonded nonwovens using a nonwoven of the same fibre raw material but of fibre fineness of 1.0 to 3.6 dtex and 50-100 g/m2 mass per unit area. Applying hydrophilic fibres in the compressed layer, a capillary effect is caused which will allow for the transportation of low amounts of moisture into the interior of the stitch-bonded nonwoven. The positive properties of the bi-component fibres will become fully effective in the process of the composites drying out, bendability and rebound behaviour of the stitch-bonded nonwovens being thus adapted to the application in question and causing the necessary stability against pressure. Stability properties may be set as follows:

- maximum perpendicular tensile strength: 200-700 N at an elongation of 15-70%;
- maximum cross tensile strength: 100-350 N at an elongation of 30-85%;
- bursting vaulting height: 32-40 mm at a bursting pressure of 1.1-2.7 daN/cm2.

The greatest advantage when making the upholstery composite is its softness and much better gliding properties of the novel stitch-bonded-hydroentangled nonwoven composites as compared to stitch-bonded nonwovens Kunit. For regeneration, these composites are washable.

3.4. Absorbent fabrics in technical, medical and hygienic applications

Stitch-bonded-hydroentangled nonwoven composites are an interesting alternative to conventional products, also for applications in which fabrics with high absorbency are needed. The effect is achieved by means of a structure of high interior storage capacity together with the use of fibre blends from/with hydrophilic fibres in a voluminous stitch-bonded nonwoven.

Dependent on the planned application, the hydroentangled nonwoven layer may consists of hydrophilic or hydrophobic split fibres as well as finest fibres.

One important aspect when developing textile absorbent materials is high moisture absorbency at the utmost smallest area. The moisture absorbency capacity of the stitch-bonded-hydroentangled nonwoven composites ranges, depending on the fibre material used and the mass per unit area, from 500 to 1400%.

Even higher values are achievable with hydrophilic split fibres. These are split up into finer fibres when being bonded which is due to the impact of high energy water jets and, consequently, larger surfaces being created. Draining the moisture from the surface into the interior of the fabric structure is available in case the hydroentangled layer is manufactured from synthetic fibre materials such as PP, PES. The latter then functions as conveying layer.

Additional advantages, except for high absorbency, are softness and flexibility as well as the fine-pore, fluffy hydroentangled layer of nonwoven and its cut-edge stability. Depending on the application chosen and the requirements the material can easily regenerated, which is due to its washability. Combining the stitch-bonded nonwoven Kunit with a layer of nonwoven, shrinking as a consequence of washing was reduced by about 50%.

4. Summary

The stitch-bonded-hydroentangled nonwoven composites described are compounds consisting of at least one stitch-bonded nonwoven and a surface compressed by fibres. The compression or reinforcement of stitch-bonded nonwovens are effected by putting on a web with high energy water jets. The manufacturing process facilates the use of all kinds of fibres with textile characteristics, the combination of different fabric properties, variable layer structures and fibre combination without blending. The pores of stitch-bonded nonwoven surface may be reduced by compression in such a way that the applied web layer meets protective functions but active air penetration is maintained.

The stitch-bonded-hydroentangled nonwoven composites show the following essential advantages: voluminosity, easy processing due to much improved slideability, cut-edge proofness, combinations between special functions, e. g. insulation effect of stitch-bonded nonwovens with flame and heat resistance of web layer, high storageability with separation of smallest particles in application as a filter medium, connected with conductivity or anti-static effects and high absorbency.

This article is published on NT New Textiles, see the contents.