INTRODUCTION
The folded yarn is produced by folding two or more singles yarns together. Folded yarns are used extensively in the textile industry. Many woven and knitted fabrics use folded yarns.
The consumption of knitted fabric is constantly growing in the clothing industry because the knitted structure offers several advantages, such as extensibility, drape, wrinkle recovery and cheap production cost.
The study of the properties of the knitted fabric was published by Manich et al. (2001) who studied the abrasion kinetics of woven fabrics. He showed that the knitted fabrics properties depend on the yarns characteristics.
Cannon and Onal (2002) presented the abrasion properties of knits made from open end and ring spun yarns. He showed that the abrasion resistance of the fabrics depends on the fiber properties, yarn and fabric structures.
Cheikhrouhou et al. (2001) indicated the influence of stitch length on the adiathermic power of the knit for two yarns of different linear density, also that the adiathermic power increase with the stitch length because the number of air inside the knit increases, which reduces the thermal exchange with the outside and thus improves the calorific retention capacity.
Ceken and Eylûl (2001) investigated the pilling properties of plain knitted fabrics, he concluded that not only yarn properties that have some effect on pilling, which occurs on fabric surfaces during washing and wearing of wool and wool blend flat knitted fabrics, but also fabric tightness.
Slah et al. (2006) used the neural network to foresee the global quality of the knitted fabric from the fiber, yarn and knitting parameters. He estimated the quality of the knit by the properties of comfort, permanency, usefulness and the mechanical resistance properties.
In the following study, we try to estimate the effect of the spinning and folding parameters and the linear count of folded yarns on the quality of knitted fabrics using the experimental design method and linear regression. And to compare the comfort properties such as the adiathermic power, the air permeability and abrasion resistance of plain knitted fabrics made of hybrid and ring folded yarns.
MATERIALS AND METHODS
Production of the cotton folded yarns: In this study, we used three ring spun yarns (R) and three rotor yarns (OE) of different linear density Yd (tex) with respectively three spinning twist factor (α_{SNm}).
The value of twist factor α_{SNm} obtained when the twist is multiplied by the square root of the linear density of the yarn. It presented by the following formula:
Where:
α_{SNm} 
= 
The spinning twist factor. 
T 
= 
Twist in turns per meter. 
Yd (tex) 
= 
Linear density in tex. 
Two types of folded yarns are discussed: the first, the ring folded yarn (FR)
is composed of two ring spun yarns and the second, the hybrid folded yarn (FOE/R)
is formed with one ring spun yarn and one Open End yarn.
Three folding twist factor (α_{FNm}) were tested. α_{FNm} is given by the following formula:
Where:
α_{SNm} 
= 
The folding twist factor. 
T 
= 
Twist in turns per meter. 
Yd (tex) 
= 
Linear density in tex. 
Practical studies showed that it is a rapport between the spinning and the
folding twist factors for the folded yarns. In that, (Dréan and Renner,
1994) presented the relationship between the value of spinning and folding twist
for the cotton folded yarns. He concluded that in knitting, the folding twist
value of folded yarns is the half of the spinning twist value in order to have
more voluminous folded yarns favoured in knitting.
This result verifies the choice of the value of the spinning and the folding
twist factors. The combination of the linear count, the spinning and folding
parameters are arranged according to a fractional set (9 experiments) (Table
1).
Ring spun yarns (R) were produced on spinning machine type Zinser 321 in Sitex Industry/Sousse/Tunisia. Autocorner Schlafhorst 338 was used for the winding operation. Rotor yarns (OE) were produced by using Autocoro Schlafhorst of type ACO 240U/288. Assembly and folding operation were carried out, respectively on "SSM" machine and the Twoforone yarn folding machine Volkmann of the VTS07 type.
The mechanical properties of single and folded yarn were tested by the Uster
Tensorapid. Unevenness and hairiness were controlled by using the Uster Tester
3 (Table 2).
Production of Jersey knitted fabrics: The ring and hybrid folded yarns were knitted using a Protti PV 92SX flat knitting machine with the English gauge E7 in Textile Research Unit of ISET Ksar Hellal/Tunisia.
We estimated the abrasion resistance of the knitted fabrics by measuring the loss of weight (LW(g m^{–2})) after 2500 rounds, this operation is made on Martindale abrasiometer according to the standard NFG 07 121.
The adiathermic power (PA) is the property of a fabric to conserve the body heat that it surrounds, which permits to measure the protective calorific power of the knit. The French Norm NFG 07108 makes use of the following formula for calculating this parameter.
Table 1: 
Levels and factors used in fractional set 

Table 2: 
Ring (FR) and hybrid (FOR/R) folded yarns properties 

FOE/R: Hybrid folded yarn; FR: Ring folded yarn 
PA = 17.4 Log ((E^{2}/Ms)H100),
with E: The thickness of the knitted fabric (mm); Ms: The specific mass of the
knitted fabric in (g m^{–2}).
Air Permeability of a knitted fabric is given by the following formula:
The average debit of air in m^{3} sec^{–1} measured by the air permeabler SDL with reference to standard ISO 9237 (1995); A: The surface of the test tube (m^{2}).
RESULTS AND DISCUSSION
The quality of the knitted fabric is determined by its comfort properties,
for example the adiathermic power, the air permeability and the abrasion resistance
(Table 3).
We used the statistic Delta which is the difference between the high and the low effect of each factor in order to classify the most influential factors of folded yarns on the properties of knitted fabrics.
The dominant parameters that influence the air permeability, the adiathermic
power and the abrasion resistance of knitted fabric of ring and hybrid yarns,
are the yarn linear density Yd (tex) and the folding twist factor (α_{FNm})
(Table 1, 4a and b).
By the value of statistic delta, we can present the significance of the parameters which is revealed as: level of importance: Yd (tex) , α_{FNm},α_{SNm }
By means of this ranking, we can compare the properties of the two kinds of
plain knitted fabrics by Fig. 16 to give
the mean data of properties of plain knitted fabrics and to evaluate the effect
of the most significant factors, the linear density and the folding twist factor
on the comfort properties.
According to the Table 3, we notice that the plain knitted
fabrics properties made of hybrid and ring cotton folded yarns are very similar.

Fig. 1: 
Relationship between adiathermic power PA and Yd (tex) 

Fig. 2: 
Relationship between adiathermic power PA and α_{FNm} 
Table 3: 
Table for optimization of the plain knitted fabric quality
of cotton 

From Fig. 1 and 2, we noted that the adiathermic
power is not affected by the linear density Yd (tex) and the folding twist factor
α_{FNm}. And the plain knitted fabrics made of hybrid folded
yarns have an adiathermic power approaching to those made of ring folded yarns.

Fig. 3: 
Relationship between air permeability P and Yd (tex) 

Fig. 4: 
Relationship between air permeability P and α_{FNm} 
Table 4a: 
Effect of different factors on knitted fabric properties made
of ring folded yarns 

The Yd (tex) has significant influence on the air permeability and abrasion
resistance of the two plain knitted fabrics. As a matter of fact, we notice
that when Yd (tex) increases for a constant plain knitted fabric LFA (the yarn
length absorbed by stitch), the volume occupied by the folded yarn in the knitted
fabric increases and consequently the air permeability decreases (Fig.
3).

Fig. 5: 
Relationship between abrasion resistance LW and Yd (tex) 

Fig. 6: 
Relationship between abrasion resistance LW and α_{FNm} 
Table 4b: 
Effect of different factors on knitted fabric properties made
of hybrid folded yarns 

In deed, for a Yd (tex) varying from 80 tex to 200 tex, the air permeability
knitted fabric decreases of 48.07%.
The hairiness increases with the 2fold yarn count as shown in Table
2, because the surface area of the yarn will increase with the yarn count
and it is likely that the hairiness will increase as well and the abrasion resistance
of plain knitted fabrics will decrease (Fig. 5). Consequently,
the folded yarns of linear density = 80 tex has a low hairiness. These yarns
gives a plain knitted fabrics showing a increase of the abrasion resistance
of 32.03% that the plain knitted fabrics by the folded yarns of Yd = 200 tex.
The fact that confirms the clarification of Cannon et al. (2003).
The folding twist factor α_{FNm} affects the air
permeability and the abrasion resistance of plain knitted fabrics. Actually;
when α_{FNm} increases the volume occupied by the
folded yarn in the stitch decrease and the air permeability increases (Fig.
4). And also by the increase of α_{FNm}, the
hairiness decreases (Table 2) and the abrasion resistance
increases (Fig. 6).
As a result, working with α_{FNm} of 60 instead of 50 improves the plain knitted fabric air permeability because high folding decreases the diameter of folded yarns and increases the porosity of the realized knitting. This improvement is about 31%.
Figure 6 shows that the abrasion resistance increases as
the folding twist increases of 11%.
We noticed also that the comfort properties of knitted fabrics depend with the properties of folded yarns. In fact, we concluded that the spinning twist factor α_{SNm} has not a significant effect on knit properties because this factor influences essentially the properties of the single yarns and not folded yarns ones.
According to results of Table 3, it was difficult to discuss
the relationship between each parameter of spinning and folding process and
the linear density of folded yarns with the comfort properties of knitted fabrics.
Equations describing these relationships were then determined.
By via the data of the Table 3 and the linear regression,
six statistical models can be used to describe the effect of the selected parameters
on the properties of the plain knitted fabrics.
For the plain knitted fabric realised by the ring folded yarns:
For the plain knitted fabric realised by the hybrid folded yarns:
(R^{2}):The coefficient of the determination of the statistical model
By means of the data of Table 3 and the linear regression,
we can obtain the optimum level of the three factors for comfort properties
of the plain knitted fabrics by the relationship between each property and each
factor.
The optimum spinning and folding condition for the selected folded yarns obtained
from the experimental design for the properties of plain knitted fabric which
can be defined in Table 5.
By means of regression equations of 1 to 6, we estimate the calculated value
of the adiathermic power, the air permeability and abrasion resistance of the
two types of the knitted fabrics.
Table 5: 
Optimum level of plain knitted fabrics properties 

For testing our models, we used the optimum spinning and folding conditions
(Table 5). A comparison of the estimated values from the model
with those from the experiment shows very good agreement (Fig.
79).
As a result, we can note that the best optimum spinning and folding conditions
are α_{FNm} = 60, α_{SNm }=
120_{ }and Yd (tex) = 80 which presented improvements of the plain knitted
comfort properties made of hybrid and ring folded yarns especially for the air
permeability and for the abrasion resistance (Table 5).

Fig. 7: 
Comparison between the experimental and calculated PA 

Fig. 8: 
Comparison between the experimental and calculated P 

Fig. 9: 
Comparison between the experimental and calculated LW 
CONCLUSIONS
The interest of this study was to compare the two plain knitted fabrics made of hybrid and ring folded yarns properties. It aims also at studying the influences of the linear density, the spinning and the folding twist factors on the characteristics of plain knitted fabrics.
Among the results, we have found that the plain knitted fabrics properties made of hybrid and Ring cotton folded yarns were very close.
In addition to that, we noticed that the linear density and the folding twist factor have a considerable influence, especially on the air permeability and on the abrasion resistance of the plain knitted fabrics. On one hand, these factors have several effects on the Jersey knitted fabrics air permeability and abrasion resistance. On the other hand, they can be influential on other plain knitted fabrics criteria.
Consequently, the prediction equation obtained can be used by the spinner as a guide to evaluate the influence of folding and spinning parameters on the plain knitted fabrics properties.