2.1. Materials

Two types of the E-glass multi-filament yarns were used in this work. Their characteristics are shown in Table I.

Table I. Characteristics of the glass filament yarns used

2.2. Knitting process

The knitting was carried out on a hand-manipulated V-bed flat knitting machine SGE9007 made in China. The gauge was selected according to the above yarn linear densities used, and was 11 needles per inch. In order to improve feeding conditions for the glass filament yarns, the yarn feeding system was replaced by a yarn tension device used on a Stoll CMS TC-320 flat knitting machine. Also, all contact points on the machine were polished in order to minimise any damage to the yarn by abrasion. During knitting, the yarn input tension and take-down tension were kept constant.

Three kinds of knitted structures were selected. They were single jersey, and two ribs knitted with different needle arrangements as shown in Fig. 1.

2.3. Assessing method and tensile test

The method used for assessing the degree of glass filament breakage is based on the yarn strength loss due to the weft knitting process. It consists in measuring the filament yarn strengths before and after knitting, and calculating their strength loss as a percentage according to the following formula:

DFB = (T0 - T1) / T0 *100, %

where DFB (%) is defined as the degree of glass filament breakage as a percentage, and T0 and T1 are the yarn breaking forces before and after knitting respectively.

It is evident that the measurement of the breaking force of a yarn after knitting can only be made when it has been unravelled from the fabric which was knitted. In order to reduce possible further filament breakage during unravelling handing, special care must be taken. According to a preliminary microscopic observation, the filament breakage due to unravelling handing was very small and could be counted as negligible in comparison to the breakage of filaments due to the knitting process. Therefore, it could be considered that the unravelling handing had only a little influence on the experimental results.

The glass filament yarns were tested on the HD021 automatic tensile-testing installation according to Chinese National Standard GB/T 7690.3—2000, in which the tested length is 50 cm, and the crosshead speed is 20 cm/min. 10 tests were preformed for each testing case, and the mean value of the breaking forces was calculated. Because the glass filament yarns are brittle, it is very easy to damage them under the action of the clamps. To solve this problem, all yarns tested were coated with the liquid epoxy resin Bisphenol at both ends, in order to avoid yarn breakage at the clamp positions.

3.1. Variation trend of the degree of filament breakage with cam setting

It can be seen from Figures 2-4 that all the curves have the same variation trend, in which the degree of filament breakage firstly decreases and then increases with the increase in the loop lengths for all the knitted structures and glass yarns. This means that a minimum value of the degree of filament breakage exists if a suitable stitch cam position is set. This minimum value can be considered as an optimum cam setting position for knitting glass filament yarns. In addition, it was found that the optimum position is closer to the lower limit of the loop length. In order to explain this phenomenon, it is necessary to analyse the degree of filament breakage caused at different knitting steps in the knitting cycle.

In a knitting cycle, from the old loop clearing to the new loop forming, three kinds of force actions (i.e. tension, bending and friction) can result in the breakage of glass filaments. For a glass filament yarn, a single tensile action along with its axe does not readily cause high filament breakage, because a glass yarn normally has high tensile strength. However, during a knitting cycle, as loops are being formed, the yarns are subjected to tension, bending and friction simultaneously. These simultaneous actions can easily cause glass filament yarns to break due to their brittleness in bending and on friction. The degree of filament breakage for a glass yarn can be considered as the sum of the filament breakages caused at different steps in a knitting cycle. However, the degree of the filament breakage caused at each knitting step varies with the cam setting (i.e. loop length). When the loop lengths are proximate to the lower limit, the degree of filament breakage during the loop clearing and knock-over predominates.

Because the loop lengths decrease, higher frictional actions are produced between the yarn/yarn, yarn/needle and high yarn tension in the old loops in the course just knitted. There is no doubt that the higher frictional action will cause considerable filament breakages on the yarn’s surface. The lesser the loop length is, the higher the degree of filament breakage is on the left side of the optimum cam setting position. However, as the loop lengths are increased, the degree of the filament breakage at the loop formation step increases. The reason is that when the loop lengths increase, the fabric structures are loosened, and the degree of the filament breakage at the clearing and knock-over steps decreases.

100

Yarn A95 Yarn B

90 85 80 75 70 65

DFB( %)

60 55 50 45 40 35 30 25 20

0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95

Loop Length (cm) Single Jerssy Stitch

Figure 2. Variation of DFB in term of the loop length for single jersey 60

55 50 45 40 35 30 25 20 15

DFB（%）

0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95

Loop Length (cm) 1x1 Rib Stitch (half needles in action)

Figure 3. Variation of DFB in term of the loop length for 1 × 1 Rib I (half needles in action)

DFB( %)

80 75 70 65 60 55 50 45 40 35 30

Loop Length (cm) 1x1 Rib Stitch (all needles in action)

Figure 4. Variation of DFB in term of the loop length for 1 × 1 Rib II (all needles in action)

However, the maximum yarn tension at new loop formation step is increased because the wrapping angles of the yarn with the knitting elements are increased. At the same time, the bending and friction effects of the yarn also increase. When the loop lengths start to increase from the lower limit, the reduction of the degree of filament breakage during the old loop clearing and knock-over is faster than the increase of the degree of filament breakage during the formation of new loops. Consequently, the degree of filament breakage decreases in the zone near the lower limit. However, after a short increase in the loop lengths, the filament breakages accelerate because the increase of the yarn tension during loop formation continues to accelerate. Thus, the degree of filament breakage begins to increase once more. The greater the loop length is, the higher the degree of filament breakage is on the right side of the optimum cam setting position.

3.2. Effects of the knitted structures on the degree of filament breakage

In Figures 2-4, all the curves have the same variation trend. However, the degree of the filament breakage for each knitted structure is different. Knitting single jersey results in higher filament breakage than in knitting 1×1 ribs. The maximum value of the filament breakage degree in knitting a jersey structure can reach over 90%. There are two reasons for this. The first one is that the jersey is only knitted on a needle bed. When a jersey fabric is taken down from the knitting zone, a severe friction effect is produced due to the fabric’s contact with the needle bed. As a result, a higher degree of filament breakage is obtained. The second reason is that the yarn in the part of the sinker loops is subjected to more bending action during loop formation. This severe bending can also result in a high filament breakage.

If a comparison is performed between two types of 1×1 ribs, it can be found that the rib knitted with all needles in action has a higher degree of filament breakage than the rib knitted only with half needles in action. This is normal because the tightness of the rib knitted with all needles in action increases. Higher tightness will cause higher friction between yarn/yarn, yarn/knitting elements and high yarn tension during loop formation. Consequently, higher filament breakage is produced.

3.3. Effects of the filament diameter and yarn torsion on the degree of filament breakage

It can be seen from Figures 2-4 that the effects of two glass filament yarns on the degree of filament breakage for 3 knitted structures are very similar. When the loop lengths are at lower values, the degree of filament breakage of yarn B (with bigger filament diameters and lower torsion) is lower than that of yarn A (with smaller filament diameters and higher torsion). However, as the loop lengths increase, the situation changes inversely. This phenomenon can be easily explained as follows; when the loop lengths are at the lower values, the frictional effects during knitting increase due to the higher tightness of the kitted structures. In this case, the yarn with the smaller filament diameter can be more easily damaged by friction. At the same time, higher yarn torsion makes the yarn surface more uneven. The more contact points between the yarn surfaces exist, the more contact points between knitting elements are produced. Consequently, the degree of filament breakage increases. Contrarily, when the loop lengths are at higher values, the simultaneous tension and bending actions to which the yarns are subjected during loop formation is increased. As yarn with a bigger filament diameter can be easily broken in bending, the filament breakage degree is expected to be higher than that of the yarn with a smaller filament diameter. In addition, the lower yarn torsion can also reduce the yarn tensile strength because there is a decrease in the ability of all the filaments to be extended simultaneously .

3.4. Comparison between commonly used knitwear yarns and glass filament yarns

In order to confirm the particular properties of the glass filament yarn in weft knitting, two types of commonly used knitwear yarns, a cotton yarn and a wool/acrylic-fibre blend yarn (wool 30%, acrylic-fibre 70%), were knitted and their strengths before and after knitting were tested under the same conditions as glass yarns. Figure 5 shows a comparison of the degree of fibre breakage between the commonly used yarns and glass yarns for single jersey structure.

Blend Yarn

100	Cotton Yarn
90	Glassfiber Yarn A Glassfiber Yarn B
80
70
60 DFB (%)
50
40
30
20
10
0 0.50 0.55	0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95
	Loop Length (cm)

Figure 5. Comparison of DFB between commonly used yarns and glass yarns

It was found that the fibre damage during knitting to the commonly used yarns is very small compared to that to the glass yarns. In addition, no obviously regular variation trends of the degree of fibre

breakage in term of loop length are observed. This confirms that knitting glass yarn is more difficult than knitting normal yarn, and choosing a suitable cam setting is very important when a glass yarn is being knitted.