Pretreatment of bamboo (Regenerated Cellulose - Bamboo Rayon)

Bamboo fiber is a regenerated cellulose fiber produces from bamboo pulp. Bamboo pulp is refined though a process of hydrolysis-alkalization and multi-phase bleaching before converting in to the fiber. Physical and chemical properties of bamboo fiber are nearly close to viscose. It has good durability, stability, moderate tencity and thus good sping ability. Bamboo products are further characterized by its good hydrophilic nature, excellent permeability, soft feel, excellent dyeing behavior and color vibrancy and over all its antimicrobial property. This paper will provide an overview of special properties of bamboo fibers along with its behavior during processing and dyeing.

Characteristics of Bamboo fiber : Bamboo fiber is characterized by its chemical functional properties such as :
Natural antibacterial : It has been demonstrated by various research that bamboo owns a unique anti-bacteria agent named “bamboo kun�?. This substance has particular and natural functions of anti-bacteria and deodorization. The content of bamboo fiber in blended yarns decides the anti-bacterial behavior of the final product. The higher the bamboo content the better the anti-bacterial properties. It is suggested to keep 70% bamboo fiber in blended yarn in order to obtain satisfactory anti-bacterial effect. Natural anti-bacteria function of bamboo fiber differs largely from that of chemical additive or finishes applied on the fabric. The later often tend to cause skin allergy when applied to apparel. Antibacterial function of bamboo fabric continues even after fifty times of washing.

Green & biodegradable : Bamboo fiber is produced out of bamboo stem which is cultivated in northern part of china. Bamboo is fast growing renewable resource, its growth is approx 3 feet over a night thus considered environment friendly. Bamboo products are 100% biodegradable in soil by microorganism and sunshine and the decomposition process doesn’t cause pollution in environment.

Breathable and cool : The cross-section of the bamboo fiber is filled with various micro-gaps and micro-holes thus is has much better moisture absorption and ventilation. With this unparalleled micro-structure, bamboo fiber apparel can absorb and evaporate humans sweat in seconds. According to authoritative testing figures, apparels made from bamboo fibers keep body temperature 1-2 degrees lower than normal apparels in hot summer.

Soft in touch : Being generated cellulose from bamboo pulp softness is the inherent property of bamboo fiber. It gives hygienic and delicate touch to skin.

Luxurious and shiny appearance : Because the cross-section of the bamboo fiber is filled with various micro-gaps and micro-holes, it absorbs more dye take up as compared to other cellulose fibers and appears luxurious and shiny.

Apart from above positive attributes, bamboo fiber is considered as a low tenacity fiber. Its dry tensile strength is merely 2.33 CN/Tex and wet tensile strength is 60% lower of dry tensile strength i.e. 1.37 CN/Tex. Keeping this fact in view it requires special care at different stages of production right from spinning to final finish stage of the garment.

Apart from above positive attributes, bamboo fiber is considered as a low tenacity fiber. Its dry tensile strength is merely 2.33 CN/Tex and wet tensile strength is 60% lower of dry tensile strength i.e. 1.37 CN/Tex. Keeping this fact in view it requires special care at different stages of production right from spinning to final finish stage of the garment.

Pretreatment and Dyeing : Bamboo fiber being regenerated cellulose is sensitive to both acid and alkali in pretreatment. It has low tensile strength thus low or negative tension is applied during the process. Reactive dye can react better with bamboo fiber molecules under weak alkali condition.

Part I : Bambro Tex Technical Guidance Documents : 

1. Characteristics of bamboo fiber
Strength :
Natural anti-bacteria
Green & Biodegradable
Breathable and Cool
Soft and feeling
Luxurious shiny appearance

Low tensile strength (wet tensile strength is lower : 60% of dry tensile strength) weak cohesion is spinning

Anti-bacteria function :
The content of bamboo fiber in blended yarns influence the anti-bacterial behaviour of the final product. The higher the bamboo content the better the anti-bacterial properties. Commonly we suggest maintaining 70% bamboo fiber in blended yarn to reach a satisfying anti-bacteria effect.

Tensile strength :
As bamboo fiber tensile strength is low, we recommend using yarn count ranging from Ne8 – Ne60. When 50s-60s count pure bamboo yarn, we suggest doubling from the yarn.

Tips in bamboo yarn producing process 

Bamboo yarn producing is very similar to traditional viscose producing. You only need to make minor adjustments when producing.
1. As bamboo fiber tends producing flying in roving and drawing, we recommend adopting a high humidity (65%-70%) and a low temperature (25OC) in workshop. If bamboo fiber is too dry before feeding, we recommend adopting a vapour pre-treatment to improve its humidity.
2. As bamboo fiber shows weak cohesion, we recommend adopting high coefficient of twist. Low card web tension and low roving tension is recommended.
3. In order to control hairiness in spinning, we recommend using high-quality steel ring and ring traveler.

3. Bamboo fabric weaving

1. To meet weaving requirement on tensile strength and to have excellent efficiency we recommend using bamboo yarn with a twist coefficient in the rang of 350 to 410 turns per meter. This ensures to reduce floss on yarn surface and it helps to also improve yarn tensile strength.
2. As bamboo yarn’s moisture regain rate and elongation is relatively high, you should maintain even and relatively low tension during the process of warping and sizing of the yarn. When weaving high-count, high-density woven fabrics with fine yarns, we recommend to choose arc process flow in warping, control each sections elongation in sizing and maintain low tension in both warping and sizing. In this way, it helps avoiding broken ends caused by increased elongation in weaving.
3. For single yarn weaving, you should choose a suitable sizing agent in sizing. Bamboo fibers are hydrophilic fibers. They are easy to be sized. So it is better to choose denatured starch as main component of sizing agent. In order to decrease hairiness, we recommend to choose some acrylic acid as part of sizing agent rather than PVA. Acrylic acid can improve yarn softness and makes it easy dividing the yarn. Careful : do not add too much acrylic acid as excess of acrylic acid will causes humidity absorbance and viscosity regain.
4. Bamboo fiber is sensitive to moisture regain. We recommend that you keep a medium moisture regain rate. Bamboo fibers is breakable when moisture regain is too low. However bamboo fibers tensile strength also decreases dramatically when moisture regain rate is too high. The suitable moisture regain rate is 8-9%. Additionally please make sure to maintain a medium speed when sizing.
5. You should maintain lower tensions when weaving. We recommend weaving at relative humidity : 65-75%. You can adopt lower relative humidity when yarn’s moisture regain rate is high, Vice versa.

Note : Bamboo yarn tends to elongate, it may cause weft stripiness.

4. Pretreatment and Dyeing

1. Because bamboo fiber is regenerated cellulose fiber, it is sensitive to both acid and alkali. So you must notice the volume of alkali in pretreatment.
2. Because bamboo fiber fabric has lower wet tensile strength, you should adopt lower tensions or negative tensions during dyeing & pretreatment.
3. Recommending to choose reactive dye-stuffs in dyeing. Reactive dye can react with bamboo fiber molecular under weak alkali condition.
4. General principles in dyeing & pretreatment :
a. Light – singeing: You should choose a mild condition in singeing in order to avoiding damaging bamboo fiber.
b. Enzyme – desizing : We request a high desizing rate aove 80%.
c. Light – Scouring : For 100% bamboo fiber fabric / yarn you may skip process of scouring because pure bamboo fiber contains title trash and pigment. If you insist on scouring you need to make sure the volume of calcined soda does not exceed 10g/L. (Note : Both excess calcined soda oxydol can damage bamboo fiber.)
d. Light – mercerizing : Because bamboo fiber owns an excellent natural shine it does not need mercerizing. However you can use light-mercerizing so as to improve absorbing rate of dyes. (please note that acid and alkaline resistance ability of bamboo fiber is weak.)
e. Dyeing : Bamboo fiber’s wet tensile strength is low. It swells acutely in water. Bamboo textiles are suitable to be dyed on jigger or winch loose rope dyeing machine. (Note : In dyeing, the volume of calcined soda can not exceed 25g/L. The temperature can not exceed 100oC. ) In drying you should keep low temperatures and maintain low tensions.

Light-singeing : The surface of pure bamboo fabric or bamboo/cotton blended fabric may have a lot of floss, cotton seed shell, or neps. It gassing does not process completely, cloth cover will not get a satisfying result. In subsequent dyeing, the remaining floss may cause dye unevenness, color fading, and even defects on cloth cover.

Gassing techniques :
Gassing Speed : 80-100 m/min
Burner Temperature : 1100OC
Burner fire should be even.
Gassing result should reach to Grade 3-4.

Enzyme- desizing : Use of high-activation, high-stability and selective enzyme which can remove starch sizing agent. To make sure that enzymes applied are not damaging the fiber properties. Enzyme need to be kept active in the range pH 5-pH 9. Recommended enzyme Tinozym L40.

Press Enzyme – desizing Techniques and Formulations (mL/kg)
Tinozym L40 2-5
Ultravon GP (humectation & wash agent) 3-4
Mangle expression 100%
Soakage temperature room temperature – 90O C
Stacking time : 6-18 hr.
Wash in hot water

Non-continuous Enzyme – desizing Techniques and formulations (ml/kg)
Tinozym L40 1-23
Ultravon EL (humectation & floatation agent) 1-2
Temperature 60oC – 90OC
Time 30-60 min
Wash in hot water

Scouring & bleaching : Natural appearance of bamboo fiber is similar to viscose thus bleaching its whiteness. For bamboo cotton blend, bleaching is essential in order to have better homogeneity in dye take-up.

Techniques and formulations (g/l)

For fabric :
Tinoclarite CSW 2-4
Caustic soda 1-3
Hydroperoxidation (35%). Ml/L 8-10
Temperature, oC 90-98
Time, min 40-60

For Yarn :
Tinoclarite CSW 1-2
Caustic Soda 1
Hydroperoxidation (35%), ml/L 5-8
Temperature, oC 90-98
Time, min 40-60

Mercerization :

For bamboo fabric and bamboo / cotton blended fabrics, mercerization not only improves fibers absorbance of dyestuffs, but also it can increase the brightness of the dyed fabrics due to plain surface and even luster Further more it is also able to a substantial degree

Techniques and formulation (g/L) :

Caustic Soda : 210-240
Temperature room temperature
Time : 40 – 60s