XIIIth International İzmir Textile and Apparel Symposium April 2-5, 2014

KNITTING TECHNOLOGY AND TECHNICAL TEXTILES Ahmet Ünal Reutlingen University / School of Textile and Design / Department of Textile Technology and Textile Management, Alteburgstr. 150 Reutlingen, Germany [email protected]

ABSTRACT Textile preforms produced from glass, carbon and aramid yarns, are widely used for the production of composite materials in aerospace renewable energies industries to name but a few. Weaving, braiding, stitch-bonding and warp knitting machines are suitable for the production of 2D and 3D textile structures. However their possibilities are limited when it comes to producing complex geometries. Flat knitting machines with their single needle selection, stitch transfer allow for the manufacture of such fabrics, which are generally used in the clothing industry and also increasingly for technical textiles. The article investigates the present and future possibilities of knitting technology for the technical textiles particularly flat knitting machines for composite materials. Key Words: Knitting, Technical Textiles, 2D and 3D knitted fabrics

1. TECHNICAL TEXTILES AND FABRIC PROPERTIES Textile products are widely used from classical clothing textiles to technical textiles fulfilling a huge number of completely different requirements. Textile structure for compression stocking must be highly elastic but geotextiles have to be very stiff. Whereas medical textiles used as artificial arteries in the body have to serve for a whole human life, geotextiles used to prevent erosion have to dissolve as soon as roots of threes and plants have developed. We need a fully fashioned 2D or 3D fabric structure for a complex shape composite reinforcement. As will be seen from the examples, each product has an optimum combination of materials, fabric production, dyeing-finishing and clothing technologies. 2. KNITTING TECHNOLOGY AND KNITTED FABRICS Textile surfaces can be produced mainly with weaving, knitting and nonwoven technologies and the choice of the technology depends on the structure and properties of the textile to be produced. Knitting technology can be defined as a conversion system in which yarn loops are intermeshed to form a textile surface. The fabric structure is based on knit loops and depending on the application area, additional structural elements can be integrated into the fabric such as weft yarns, warp yarns, nonwovens (eg. maliwatt technique) etc. to increase fabric properties.

2.1 Warp knitting machines Warp knitted fabrics can be produced on raschel, tricot, crochet and malimo stitch bonding machines. The main feature of these techniques is the high production speed and possibility to 11

XIIIth International İzmir Textile and Apparel Symposium April 2-5, 2014

produce wide spectrum of fabric structures. Raschel and tricot machines produce generally clothing and home textiles (curtains, laces, sport textile, etc.), while crochet machines yield narrow textiles. Malimo stitch bonding machines are particularly suitable to the production of technical textiles as they allow for processing chopped glass fibers, multiaxial reinforcing yarns and nonwoven fabrics. 2.2. Circular knitting machines Circular knitting machines can run several knitting systems at a time (eg 120 system) and thus are highly productive. Due to the construction of these machines they are more often used for production of clothing and home textiles. 2.3 Flat knitting machines Computer controlled modern flat knitting machines offer the individual needle selection and needle bed racking. These two features enable fully fashioning and endless possibilities for the production of 3D knitted fabric with complex geometry. For this purpose, two independent fabrics are produced on the front and rear needle bed and subsequently joined with different methods. Three out of these are described below in different fields of application. 3. APPLICATION EXAMPLES 3.1 Reinforcing of a timber construction with 3D fully fashioned flat-knitted fabric Wood can be densified under pressure, vapor and heat to increase strength and stiffness. With the same methods it can be 3D shaped for timber constructions. A new timber construction [1], made from densified and 3D shaped wood, is shown in Fig 1-d. In this construction, the connection area between wood layers causes problem under load and needs support [2]. 3D fully fashioned flat-knitted fabric from glas fiber is a very suitable reinforcement for this type of junctions [3]. The 3D fabric geometry necessary for this reinforcement is depicted in Fig. 1a, the flat knitted fabric in Fig. 1b, application on the timber construction is shown in Fig. 1c.

a) Fabric geometry

b) 3D fully fashioned flat-knitted fabric

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XIIIth International İzmir Textile and Apparel Symposium April 2-5, 2014

c) Application of fabric on timber construction

d) Reinforced timber construction

Fig 1. Reinforcing a timber construction with flat-knitted 3D-fabric

3.2 3D flat-knitted fabric for composite materials Another example is a 3D flat-knitted fabric for lightweight composite materials. In this application, we need two textile surfaces connected by a U-shaped or V-shaped third surface (Fig. 2).

Fig 2. U-shaped and V-shaped fabric structures for composite materials

These fabrics are produced from hybrid yarns, which consist of glas and polypropylene components [4, 5]. The mold pieces are inserted into the fabric to support its 3D-shape during processing (Fig. 3).

Fig 3. Processing of flat-knitted composites

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XIIIth International İzmir Textile and Apparel Symposium April 2-5, 2014

The knitted fabric and mold pieces are placed in the annealing oven where polypropene yarns melt and keep the glass yarn in shape (Fig. 4).

Fig. 4. 3D composite material from flat knitted fabric

3.3 3D flat-knitted fabric with monofilament yarns Two fabrics, knitted on front and rear needle bed can also be connected with monofilament yarns. This technique can be applied on circular, flat and warp knitting machines and these fabrics are widely used as a spacer fabric in sport shoes, in car seat, in mattress fabrics etc.. Spacer height between two outer fabric surfaces is very important in determining the fabric properties and depends on different parameters particularly the distance between two needle beds. A new type of warp knitting machine (Karl Mayer HDR 6-EL HighDistance ®) allows adjusting this distance between 20 to 65 mm. With this possibilty, the warp knitting technology offers the widest range of spacer heights and thus warp knitted spacer fabrics are used in a lot of different applications. However the production possibilities of shaped or fully fashioned fabric on warp kniting machines are very limited. On the contrary flat knitting machines with their single needle movement have endless possibilty to produce fully fashioned fabrics. Yet due to the machine construction the distance between two needle beds is not adjustable. Therefore spacer height can only be adjusted indirectly changing parameters like monofilament diameter, connection angle of monofilament with outer surfaces, integration of elastic yarns. Initial investigations have shown that with these parameters up to 30mm spacer height can be achieved (Fig. 5).

Fig. 5. 3D flat-knitted fabric with monofilament yarns

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XIIIth International İzmir Textile and Apparel Symposium April 2-5, 2014

4. CONCLUSION Flat knitting machines are very important for the production of clothing textiles but require further investigation for producing technical textiles. 2D and 3D fully fashioned fabric structures from cotton or wool yarn can be knitted on these machines successfully. This knowledge must be transfered to processing carbon and glass yarns and the construction of machine must be adapted to these yarns. This will on one hand open the new production area for flat knitting machines and on the other hand enable the production of fully fashioned technical textiles from carbon and glass yarns.

5. REFERENCES [1] HALLER, P. Concepts for textile reinforcements for timber structures. Materials and Structures, (2007) 40, pp 107–118. [2] HALLER, P. BIRK, T., OFFERMANN, P., CEBULLA H. Fully fashioned biaxial weft knitted and stitch bonded textile reinforcements for wood connections. Composites: Part B 37 (2006), pp 278–285. [3] ÜNAL, A., OFFERMANN P. Effects of the knit structure on the deformation behaviour of weft knitted reinforced fabrics. Melliand Textilberichte, Frankfurt, 86 (2005) 4, pp 50-51. [4] ÜNAL, A.; HOFFMANN G., CHERIF Ch. Development of weft knitted spacer fabrics for composite materials. Melliand Textilberichte, Frankfurt, 87(2006) 4, pp 49-50. [5] TORUN, A. R., HOFFMANN, G., ÜNAL, A., CHERIF Ch. Spacer fabrics from hybrid yarn with fabric structures as spacer. 16. International conference on composite materials. 2007, Kyoto-Japan.

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