The benefits of the bamboo knitted fabrics are myriad and generally, the water vapour permeability and air permeability tend to increase with the increase of the loop length of the bamboo knitted fabrics, affirms Alaa Arafa Badr.
In today's world, naturally renewable resources are increasingly being required as a result of humans dedication to protect environment. Chemical methods are being developed to generate new environmental materials. Such materials are able to effectively replace or develop the accessible non-natural or natural materials. Bamboo fibre is a new regenerated cellulose fibre, created from the bamboo tree. Bamboo plant is an essential forest biomass resource .
It developed at the beginning of the 21st century and does not require any pesticides or chemicals and is also biodegradable. Bamboo textile goods have a multitude of unbelievable, distinctive properties and advantages. The mainly well-known characteristic of bamboo material is its antibacterial, incredible capability to breathe and its intrinsic coolness, pleasant luster, better drape and tremendously comfortable soft feel[1-3]. The cross-section of bamboo fibre is chiefly filled with numerous voids and micro-holes.
Bamboo fibre has the lowest degree of crystallinity. It is as well extraordinarily hygroscopic and has more excellent moisture and temperature management properties than other traditional fibres such as cotton and polyester[4-7]. Currently, regenerated bamboo fibres have high demands in market and are being used in apparels including underwear, T-shirts, sports textiles, socks, towels, bathing suits, sanitary napkins, absorbing pads, bandages and surgical gowns[5, 8].
Comfort, which defined as states where there are no motivating impulses to correct the environment by the behaviour. Clothing comfort should guarantee proper heat transfer between the human body and its environment to sustain the physiological thermal balance of the wearer. The garments should let the perspiration to pass through; if not it will result in uneasiness[11-12]. Microclimate relative humidity measurements acquired from the human back is more than the chest particularly for high activity time.
Currently, protection properties of fabrics for example hot, cold, wind, rain, antimicrobial and also comfort properties are getting more valued. Thus, there are many challenges to manufacture the clothes having functional properties and also providing high life quality. Due to the development of science and technologies and the progress of social standard, comfort of textile materials becomes increasingly significant in making decisions on purchasing. The comfort provided by clothing relies on some factors. One of them is thermal comfort; other factors implicate durability, softness, drapability, appearance, etc [11-12]. The best thermophysiological comfort of a knitted structure can be accomplished if all parametres of the manufacturing and finishing methods are chosen studiously with the requirements set by the purpose of the product .
Thermal comfort properties of fabrics are really influenced by major factors like: the fibre type, spinning technology, yarn twist, yarn hairiness, fabric thickness and fabric tightness[7,9,15-17]. The hairiness of bamboo yarns is much lower than that of equivalent cotton yarns.
Fabrics knitted with higher bamboo content have considerably less thickness, mass per square metre, thermal resistance and more water permeability[1,18]. As the yarn gets finer the thermal resistance and thermal conductivity of the bamboo knitted fabrics decrease, while the water vapour permeability and air permeability increase. Ucar and Yilmaz investigated the thermal comfort properties of 1x1, 2x2 and 3x3 rib structures and found that the heat loss reduced with the decrease in rib number. According to Tzanov et al  material finishing does not affect on the thermal resistance, but it affects water vapour resistance.
Consequently, it becomes vital to gain from the unusual properties of new fibres for high levels of body comfort by using bamboo fibres. Therefore, the most important goal of this research is to study the thermal comfort behaviour of bamboo knitted fabrics produced with three levels of loop length and six different structures which are single jersey, single pique, double pique, 2-thread fleece, rib 1x1 and interlock structures. The study will be of immense help to the researchers who are investigating the thermal comfort characteristics of fabrics for different seasonal dress wears.
Material and methods
In this study, 100% bamboo yarn having linear density 30.2 Ne, 12.5% elongation and 14.4 CN/Tex – tenacity, was applied to knit all the knitted fabric structures. The bamboo fibre properties used in this study are given in Table 1.
The specification of the single jersey, rib and interlock machines is shown in Table 2. All the single jersey, single pique, double pique and 2-thread fleece (with alternating 3-needle floats for the inlay yarn) knitted samples were produced on the same single jersey machine through changing the cam segment type inside the machine cam doors.
The loop length was changed for every fabric structure to three levels for producing a slack, medium and tight weight knitted fabrics. All fabric samples were dyed in industry with a medium colour shade inside the overflow dyeing machine and then finished through the same standard finishing line applied for the close-form knitted fabrics. Table 3 shows the properties of the knitted fabric samples used, in which the following symbols are used: SJ = single jersey, SP = single pique, DP = double pique and Fleece = 2-yarn fleece knitted fabric.
The effect of the experimental factors: fabric structure and fabric loop length on the bamboo fabric thermal conductivity, thermal absorption, thermal resistance, air permeability, water vapour permeability and water absorption was estimated for significance through applying the ANOVA analysis using R statistical program.
After leaving the finished samples 72 hours in standard conditions, the thermal comfort properties were measured. Thermal properties (thermal conductivity, absorptivity, resistance and also fabric thickness) were determined using Alambeta instrument. In this device the fabric is laid among a hot and cold plate. The hot plate comes in contact with the tested fabric sample at a pressure equal to 200 Pa. Once the hot plate feels the fabric surface, the amount of heat flow from the hot surface to the cold surface through the fabric is detected by heat flux sensors. Additionally, the air permeability was measured according to the ASTM D 737.
Water vapour permeability was measured by means of ASTM E96 method - "Standard Test method for Water vapour Transmission of materials"- Cup method. Furthermore, water absorption % was evaluated according to BS 3449: [Method for resistance of fabrics to water absorption-static immersion test]. The percentage gain of the tested samples were evaluated by preparing a 20*20 cm sample and sinking it into water for five minutes and after that the sample was suspend vertically to permit extra water to drip for another five minutes. At last the water absorption % was calculated in terms of the following equation:
Water absorption % = Absorped water mass g*100Dry mass (g) ---------- (1)
Results and discussion
All the thermal properties; thermal conductivity, thermal absorptivity, thermal resistance, water vapour permeability, water absorption and air permeability are significantly affected at 5% significance level by fabric structure and loop length. The influence of studied parametres on the bamboo thermal comfort properties was investigated better as shown in the coming figures.
Thermal conductivity is essential to determine the heat transfer through knitted fabrics. Still air inside the fabric geometry has the least thermal conductivity rate as compared to all textile fibres (Λair = 0.025). So, air conveys a low quantity of energy via conduction and thermal conductivity decreases likewise.
Comparing the values of thermal conductivity of the bamboo knitted samples in relation to the fabric structure as shown in Figure 1, the fleece and interlock structures have more thermal conductivity than other structures. This case can be explained by the high bulk density; high weight per square metre and less still air in the unit area of 3:1 fleece and interlock structures. Moreover, from the same figure, as the fabric becomes slacker, the thermal conductivity decreases. This could be referred to higher porosity value and the less fabric weight of the samples knitted with longer loop length.
Thermal absorptivity is the aim measurement of the warm-cool feeling of textile fabrics and is a surface related characteristic. If the thermal absorptivity is high, it provides a cooler feeling at initial touch with the skin. The 2-yarn fleece and interlock structures have more thermal absorptivity values while the single jersey structure has the least value (Figure 2). The configuration of these structures generates more contacts points between the bamboo fabric and human skin.
Furthermore, fabrics produced from short loop length have more thermal absorptivity than those produced from medium and long loop length (Figure 5). The more fibre material contained with high tightness level is the possible reason of this trend.
Thermal resistance expresses the thermal insulation of fabrics and is inversely proportional to thermal conductivity , and is proportional to the fabric structure. There is an inverse relationship between thermal conductivity and thermal resistance for the: fleece with rib structure- fleece with double pique structure- DP with SP structure (Figure 3), where the increase in fabric thickness is approximately less than the amount of increase in thermal conductivity (R=h/Λ). So, thermal resistance will be decreased.
On the other hand, there is a direct relationship between thermal conductivity and thermal resistance for the: fleece with interlock structure- SP with SJ structure. This unusual trend might be referred to the extra amount on increase in fabric thickness than the increase in thermal conductivity value . Also, fabrics knitted from small loop length have high thermal resistance, where there is more fabric thickness and weight associated with this level of loop length.
Water vapour permeability
Water vapour permeability of the tested fabrics increases from SJ to SP, DP, rib 1x1, fleece until reaching to the interlock structure (Figure 4). The fleece structure consists of ground and inlay yarns. The miss and tuck loops constituting the inlay yarn cover the back of the fabric and as a result the water vapour permeability of this common structure will be low.
Additionally, there is significant difference between the relative water vapour permeability values of bamboo fabrics produced with different tightness levels. The higher values of the water vapour permeability of the SP and DP structures are mostly reliant on their lower mass per square metre and thickness, which support the easy passage of water vapour throughout the fabric.
Water absorption percentage of the fabrics were obtained according to the equation 1 and shown in Figure 5. From this figure, the wickability rate of interlock structure makes it to have the highest water absorption amount while the SJ has the least amount in comparison with the other structures. Interlock can take up the water with higher amount and allow less air passage. These findings indicate that the interlock clothing for the consumer made of bamboo generated fibres can absorb a lot of sweat, perspiration moves fast from skin to outside and as a result user can feel dry and more comfort.
Furthermore, interlock and rib and 3:1 fleece structures demonstrated the highest water up-take capacity and very high initial wicking rate. This performance is most likely due to the structures' capability to act like a capillary system, rapidly removing and transporting water through the structure.
As the fabric becomes tighter, it will have high water absorption capability due to the more fibre material contained and the high fabric thickness accompanied with this fabric specification.
The results indicate that fleece and interlock structures have the least air permeability value than others bamboo fabric structures as noticed in Figure 6. The existence of this decline is most probably a consequence of the thicker structure of fleece and interlock fabrics, where the transportation of air through a thick fabric will be difficult.
Also from Figure 5, the open structure of the fabric knitted with longer loop length gives the ability to the air to pass through fabric without any obstacles.
In the current study, thermal comfort properties of bamboo knitted fabrics knitted with different structure and tightness level were examined.
On the basis of the results obtained it could be concluded that all the transmission properties: thermal conductivity, thermal resistance, thermal absorptivity, water vapour permeability and water absorption values are strongly correlated to fabric structure and loop length.
SJ, SP and DP structures have less thermal conductivity, thermal absorptivity, water absorption and more water vapour permeability, air permeability than other structures. This case can be clarified by the high amount of entrapped air inside the fabric and the lower mass per square metre and thickness respectively.
There is a direct relationship between thermal conductivity and thermal resistance for the: fleece with interlock structure – SP with SJ structure. This unusual trend might be referred to the extra amount of increase in fabric thickness than in thermal conductivity value.
The miss and tuck loops constituting the inlay yarn for the 2-yarn fleece structure covers the pores inside the fabric and as a result the water vapour permeability of this common structure will be low.
Lighter fabrics that contain high still air (as SJ), have lower thermal conductivity, thermal absorptivity, thermal resistance, water absorption, more air permeability and water vapour permeability. This could be referred to higher porosity value and the less fabric weight of these fabrics.
The author would like to express my sincere thanks to Dr Ramsis Farag, Polymer and Fibre Engineering, Auburn University, USA, for the support in performing all the tests. Also, the author appreciates the co-operation of Mr Osman AY, Country Manager of CSA Textile for his help in providing the regenerated bamboo yarn.
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Alaa Arafa Badr
Textile Engineering Dept
Faculty of Engineering
Alexandria University, Egypt.