1. Introduction

Usage of colorimetry in the textile industry has improved the quality and precision of dyers’ and printers’ work. More objective estimation has minimised misunderstandings between the manufacturers of textiles and their clients. Unfortunately colorimetry is much less used when fabrics are made of multicolour threads than when yarns and fabrics are dyed or printed. When multicolour threads are used for patterning, the particular colour effects of a woven fabric depend on combinations of thread colours and must be measured. Moreover, the constructional parameters of woven structure, namely thread density, fineness and weave, can strengthen or reduce the colour effect on the surface [1].

When two or more colour threads are used in warp and weft pattern, the colour effect of the entire fabrics is caused by optical mixing of light reflected from the surfaces of warp and weft threads and by the influence of foundation. In this case, optical mixing can be just partly explained as additive colour mixing, because the colour surfaces are not light emitting objects, but only its converters [2].

Moreover, the relation between the colour values of different colours and their size must be carefully considered. When two colours are placed close to each other, their influence can be explained by the terms of contrast and harmony. The differences between colours are explained by the contrast. Quantitative contrast means that the size of the two colour surfaces are different, and as a consequence the contribution of reflected light from different areas differ from each other. We can find this type of contrast on the woven structures, where, the surface of warp threads differ from weft threads surface due to different density and fineness. Taking into account colour values of warp and weft, the most interesting are complementary colour contrasts and colour harmony. Complementary colours lie on the opposite side of the colour circle, and their sum of reflected light give an unsaturated colour, which can be observed as a greyish hue on the fabric. On the other hand, the close position of

parameters give uniform colour values to the entire fabric surface, the usage of colorimetry may contribute considerably to improving the quality of products, which in turn would result in a reduced number of claims [5].

The aim of this research is to enable corrections to colour values of woven fabrics by changing the constructional parameters. It was assumed that when a yarn of a certain fineness was not available, it was substituted with a yarn of lower fineness. The different thread fineness caused colour deviations between two fabrics, and to this end our experimental work aimed to find out whether the changes of other constructional parameters (density) could correct differences in colour effects on fabric surfaces.

2. Theoretical Part

2.1. Theoretical Calculation of Colour Values of a Fabric Made of Differently Coloured Threads

The warp and weft threads’ interlacing point can be divided into three surfaces which differ in colour and construction: the warp thread surface, the weft thread surface and the spacing between the threads, as shown in Figure 1 [1].

incident light

do 1/go 1 2 3 w arp	wef	t	dw	1/gw 3 12

reflection from warp and weft interlacing

points = fabric entire

colour

Figure 1. Schematic view of a fabric with individual colour surfaces of warp – 1, weft – 2, inter-thread spacing – 3, and final colour effect of the woven fabric - 4

On the basis of the thread diameter , the thread density and the weave, the fractions of individual colour threads in the pattern are calculated by using the following equation [2]:

u ⋅ n + u ⋅ nu ⋅ n + u ⋅ n

on,ot ot,oi on,wt wt ,oi wn,ot ot,wi wn,wt wt ,wi

Ui =+ (1)

nn

oi wi

where n is the number of all points in a colour repeat; uon,ot is the content of the warp thread colour in the warp interlacing point; not,oi is the number of the warp points on i colour warp threads; uon,wt is the content of the warp thread colour in the weft interlacing point; nwt,oi is the number of the weft points on i colour warp threads; noi is the sum of the warp and the weft points on i colour warp threads = not,oi+nwt,oi; uwn,ot is the content of the weft thread colour in the warp interlacing point; not,wi is the number of the warp points on i colour; uwn,wt is the content of the weft thread colour in the weft interlacing point; nwt,wi is the number of the weft points on i colour wefts; nwi is the number of the warp and the weft points on i colour wefts = not,wi+nwt,wi.

The colour differences between the standard woven fabric and the fabric made of multicolour threads are calculated by using the following equation [5,6]:

where a^*v, b^*v and L^*v are colour values of the pattern; a*s, b*s and L*s are the colour values of the standard fabric; a*i, b*i and L*i are the colour values of the i component; Ui is the fraction of the i component in a colour repeat.

2.2. The Influence of Constructional Parameters on the Size of Colour Components

The effect of the increase of threads fineness and its density is presented in Figure 2. The dimensions of colour components change as a consequence of changes in relation between constructional parameters.

On the left side of Figure 2 the original warp and weft points are presented. With the increase of warp or weft fineness the fraction of these threads increases which causes a smaller surface of other thread system and of the foundation. The size of colour repeat remains constant. In the second case, the increase of density causes the change of repeat dimensions. The fraction is higher for the thread system with increased density, and reduced for the other. Finally, due to different relations between colour components in the colour repeat, four different colour effects can be observed on the right side of the figure [7].

warp colour effect

4a

4b

4c

4d

Figure 2. Change of the thread density and fineness in the warp and weft interlacing points: 1-warp, 2-weft, 3-foundation, 4.a-d- different colour effects

3. Experimental part

In the experimental part, computer simulations of woven fabrics were made with the Arahne CAD system [8]. The simulations were printed out on paper with the Epson Stylus Pro XL 720 dpi printer. The colour values of the simulations and fabrics were measured with DataColor SpectraFlash SF 600Plus-CT (D65). The warp threads were red, the weft threads were blue by the first group of fabric simulations and yellow by the second group. The colour of foundation (space between threads) was white. In Table 1, the colour values L*, a*, b*, hab and C*ab of the warp, weft and foundation are presented.

Table 1. Colour values L*, a*, b*, hab and C*ab of warp, weft and foundation Table 2. Thread diameter, density and fractions of warp (o), weft (w) and foundation (f) by different constructional parameters of threads

Table 3. Theoretically calculated and spectrophotometrically determined colour values and ∆E*ab values of red/blue and red/yellow patterns

Five simulations of woven structure with different constructional parameter were prepared. The constructional parameters of standard woven fabric-pattern 1 were as follows: fineness of warp and weft threads Tto = Ttw = 27tex and thread density go =gw =27 threads/cm. In pattern 2, the weft fineness was changed to 24 threads/cm, and the thread density and colour remained the same. With pattern 3, the colour difference was corrected by increasing the weft threads’ density; in pattern 4, the correction was made by changing the warp yarn fineness, and in pattern 5, by decreasing the warp threads’ density. The constructional parameters and fractions are presented in Table 2. The symbols do and dw are the diameters of warp and weft thread (mm), go and gw are the densities of warp and weft threads and Uo, Uw and Uf are the fractions of warp, weft and foundation in one colour repeat of woven fabric. The colour values of the simulations and fabrics were determined:

- theoretically, with calculations on the basis of the theoretically defined fractions and the L*, a* and b* values of the yarns used (equation (1)) and

- spectrophotometrically, on the printouts of simulations of woven fabrics.

Finally, colour deviations ∆E*ab between colour values of planned woven fabric-pattern 1 and woven fabrics with changed constructional parameters (2-5) were calculated and the correction of colour values using changes of constructional parameters was objectively investigate.

4. Results of measurements

The colour values of red and blue patterns with ∆E*ab values are presented in Table 3, with theoretically calculated and spectrophotometrically determined colour values of simulations.

5. Discussion

In the experimental work, two different colours of weft threads were used, blue and yellow. These colours were chosen because they are located in different parts of the CIE L*a*b* colour system. Blue has a very high negative value of colour parameter b* (-44,77), whereas yellow is located at the opposite end of the colour system, where there are high positive values of b* colour parameter (70,41). Woven fabrics with equal constructional parameters in the warp and weft system (go=gw=27 threads/cm, Tto=Ttw=27 tex) should be made. The desired colour effect was supposed to be achieved on the surface of the simulation of woven fabrics with such constructional parameters and colour values of threads. However it wasassumed that weft yarn of fineness 27 tex was not available, and thus it was replaced with threads of lower fineness 24 tex. Different fineness of weft threads caused certain colour deviation in comparison with the desired effect as a decrease of chroma, because of the bigger fraction of space between threads and the change in hue angle in the direction of the red axis. This was because in the woven construction with weft fineness of 24 tex, the red warp threads have a higher fraction in colour repeat (Table 2) and a greater influence on the colour effect of the surface.

In the table, it can be observed that the fraction of single-colour component (warp, weft, and foundation) changes with the change of the thread fineness and density. The fractions Uo, Uw and Uf of pattern 2 differ considerably from those of the fractions of pattern 1, which was taken as the standard. This caused high colour differences between patterns 1 and 2, as can be observed in table 3. For the blue weft colour ∆E*ab, the value between patterns 1 and 2 is 1.94 by theoretical calculation and 1.64 by spectrophotometrically measured colour values, while for the yellow weft these values were 1.99 and 2.26.

The aim of the research is to explore the possibility of correcting colour by changing constructional parameters. Patterns 3-5 were designed with this aim in mind. By these patterns, changes to constructional parameters were carried out in order to correct the fractions of colour components, so that they would be closer to the fractions of the standard pattern. These can be seen in Table 3, where patterns 3-5 have fractions Uo, Uw and Uf more similar to fabric 1 in comparison with pattern 2. So, the colours of warp and weft threads have a similar influence on the colour of the simulation. The consequence is that by theoretical calculations, the ∆E*ab values between patterns 3-5 and 1 are lower in comparison with those between fabrics 1 and 2. In the case of blue weft, the ∆E*ab values of the theoretically determined colour values between pattern 1 and the patterns with changed fineness or density 3-5 range from 0.21 to 1.24, and in the case of the yellow weft from 0.21 to 1.50. Moreover, the success of colour corrections achieved by changing constructional parameters is proved with

∆E*ab colour deviations between spectrophotometrically determined colour values. In this case, the ∆E*ab values range from 0.13 to 0.69 by the blue weft, and from 0.64 to 1.79 by the yellow weft.

In Table 3, it is evident that the colour corrections using changes to constructional parameters are effective in both colours of weft threads, blue and yellow. The lowest ∆E*ab values are present between patterns 3 and 1 by spectrophotometrically determined colour values, so when lower fineness of weft thread (24 tex) is corrected with higher density weft threads (28.5 threads/cm). The same results give us the theoretical predictions of colour values, which by pattern 3 in fact predict the closest fraction values to pattern 1 (Uo1= Uw1 = 0.476, Uo3= 0.478, Uw3 = 0.473) and give us the lowest colour deviations.

On the other hand, the patterns with blue and yellow weft show differences between theoretical calculations and spectrophotometrical measurements. The theoretical calculations of colour values predict higher colour differences between the colour values of pattern 1 and the corrected colour values of fabrics 3-5 by patterns with blue weft (theoretical: 0.21-1.94, measured: 0.13-1.64). Meanwhile, in patterns with yellow weft spectrophotometrical measurements are less successful, and the ∆E*ab values are 0.64 to 2.26 in comparison with the theoretical calculations, which give ∆E*ab values from 0.21 to 1.99.

6. Conclusions

The colour of a woven structure made of different coloured threads in the warp and weft system depends on the colour values of the threads and on the constructional parameters. On the basis of the experimental work the following conclusions can be drawn: -changes to constructional parameters in one thread system can be substituted with changes to

density or fineness in another system to achieve a similar colour effect on the woven surface; -colour corrections can be made by changing constructional parameters; -correction by the increase in weft threads’ density is most successful with minimal ∆E*ab values in

the case when colour differences are caused by lower weft thread fineness;

- theoretically predicted ∆E*ab colour differences can be compared with those which have been spectrophotometrically determined, and depend on the threads’ colour.

Colour corrections using modifications to constructional parameters indicate new possibilities of usage of colorimetry in weaving technology. This enables objective estimation of colour effects due to changes in fineness, thread density and weave, as well as achievement of the desired colour values on the surface of a woven structure with the choice of proper constructional parameters.