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Figure 5- 29: UV transmittance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different fibre diameter.

Figure 5- 30: UV transmittance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different yarn linear density.

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Figure 5- 31: UV transmittance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different yarn twist.

Figure 5- 32: UV transmittance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different cover factor setting.

Figure 5- 29 to Figure 5- 32 show the comparison between calculated (predicted) and measured (actual) results, for wool single jersey fabrics with different mean fibre diameter, yarn linear density, yarn twist and cover factor setting. The predicted and experimental results had the same trend: when mean fibre diameter, yarn linear density, yarn twist and fabric cover factor settings were kept constant, the fabric with finer fibres provided lower UV transmittance. This

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stitch density was similar. However, with increasing fibre diameter, the weight per area was decreased, whereas the thickness and porosity were increased (Table 5- 1). Based on statistical analysis, the effect of porosity was greater than the thickness on the UPF values of fabrics. The porosity was increased from the calculation using ‘total porosity of the knits equation’ [211], which is associated with bulk density of the knitted fabric, the fibre density, yarn bulk density and yarn packing factor. These resulted in a decrease of the UV protection with increasing fibre diameter, although the thickness of fabrics was increased slightly. For yarns with the same yarn linear density (constant mass of fibres), there are fewer coarse fibres in the cross section of a single yarn, therefore, fewer coarse fibres in a yarn can block fewer UV rays than yarns with smaller diameter fibres.

Both predicted and experimental results showed (Figure 5- 30): when the fibre diameter, yarn twist and cover factor setting were kept constant, the fabrics with a larger yarn linear density yarns had a lower UV transmittance, resulting in higher UV protection. This was because the yarns with a larger linear density provided less open area in the fabric surface. In addition, their thickness, weight per area, and stitch density increased with this increase of yarn linear density.

Similarly, as shown in Figure 5- 31, the higher the cover factor, the greater the UV protection, as fabric with a larger density can shield more UV rays. Figure 5- 31 shows that the UV transmittance was slightly increased with increased yarn twist. This was because the larger yarn twist could decrease the yarn diameter, so that it relatively increased the spaces between the yarns for UV light penetrating through the fabrics.

Figure 5- 29 to Figure 5- 32 illustrated that the models had a good agreement with the test results. ܽ݅ݎΨ and area modulus of stitch contributed to this agreement between calculated and actual results. The structure influences the pore size, pore distribution, pore density, pore connectivity and the percentage of total pore volume, and these properties of the macro-pore are important in determining UVR transmission of a fabric [315]. Total porosity of the knits is defined as the portion of all air spaces in knitted fabric both between yarns and inside them [211], and this was the same as ܽ݅ݎΨ, which was involved in the optical model in this work.

Area modulus of stitch also influences the tightness of the fabric. When the linear density of yarns was different, the tightness of the knitted fabric was different, even though the weight per area was the same [287].

Figure 5- 33 to Figure 5- 36 show the comparisons of UV reflectance results between calculated

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finer fibre diameter, larger yarn linear density and greater cover factor had a higher UV reflectance. In the twist group, there was no significant change in UV reflectance results with increasing yarn twist. In these four groups, all trends of UV reflectance results were in accordance with the trends of fabric areal density. This was because, the fabric with a higher areal density had more fibres on the surface to reflect UV light. For both trends and results, the calculated results agreed with the measured results for UV reflectance of fabrics.

Figure 5- 33: UV reflectance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different mean fibre diameter.

Figure 5- 34: UV reflectance (at 350 nm wavelength) of calculated and measured results for wool fabrics

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Figure 5- 35: UV reflectance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different yarn twist.

Figure 5- 36: UV reflectance (at 350 nm wavelength) of calculated and measured results for wool fabrics with different cover factor setting.

There were some deviations between calculated and actual results in the acceptable range. The variability of the fibre diameters, the actual arrangement of the fibre arrays in the cross section of the yarns could not be controlled and this resulted in deviations between actual test results and theoretical idealized assumptions. The test errors of the measurements and calculations of

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Figure 5- 37: Air permeability results for wool fabrics with different mean fibre diameters (Mean Fibre Diameter group).

Figure 5- 38: Air permeability results for wool fabrics with different yarn linear densities (Yarn Linear Density group).

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Figure 5- 39: Air permeability results for wool fabrics with different yarn twists (Yarn Twist group).

Figure 5- 40: Air permeability results for wool fabrics with different cover factor settings (Cover Factor group).

The trends of air permeability of fabrics in Figure 5- 37 to Figure 5- 40 agreed with the trends of UV transmittance of fabrics in Figure 5- 33 to Figure 5- 36, and were opposite to the trends of UV protection of fabrics (UPF values) in Figure 5- 25. This was because air permeability can reflect the openness of the fabrics, which is directly related to the air passing through the fabrics, and thus the UV light penetrating through the fabrics.

In the previous studies, there was a simple relationship between UV protection and fabric cover

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However, it is hard to understand the way in which the UV light penetrates through the fabrics from this equation. In addition, knitted fabrics have more complex constructions than woven fabrics. Besides fabric cover factor, there are other factors affecting UV protection, such as the fabric weight, bulk density, areal density, stitch density, yarn linear density. Therefore, compared with Eq.5- 4, this work provides a more conclusive way to predict UV transmittance for knitted fabrics.

The statistical model and optical model presented in this work have taken all these factors into account for predicting the UV protective properties. The predicted results from the optical model fitted well with the experimental results. Therefore, the model can be used to predict the UV properties of single jersey fabrics.

Combining the predictive results from both statistical and optical models, the optimised parameter range can be obtained for the fabric to confer a high UV protection. The initial optimised parameters were inferred as: smaller fibre diameter, larger yarn linear density, lower yarn twist and greater cover factor setting. The calculated results from optical model are shown in Figure 5- 41. The fabric with the parameters: mean fibre diameter 13.7 ȝm, yarn count 40 tex , yarn twist 586 T/m and cover factor setting 1.54 tex^1/2mm^-1 had the lowest UV transmittance, giving it the highest UV protection (Figure 5- 41). Thus, fabrics close to this parameter range were included in the experiments to verify optimised results.

Figure 5- 41: The predictive results of UV transmittance calculated using optical model for the initial

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