The Industrial Revolution transformed our world into one driven by consumerism. In pursuit of higher consumption, the focus shifted to increasing productivity and ease of processing, often without considering their environmental impact.

However, in recent years, there has been a significant shift in focus due to the increasing accumulation of non-biodegradable waste in the world. Research and development efforts have turned towards developing eco-friendly, biodegradable materials. The textile processing industry, known for its environmental pollution, has made efforts to adopt textile fibers that are biodegradable.

One such textile fiber with considerable biodegradability is poly-lactic acid fiber. This article aims to provide a systematic documentation of the chemistry and processing details of this fiber, which has been lacking until now.

From nature, we obtain two types of materials:

1. Renewable materials: These materials grow, get biodegraded, and re-grow.
2. Non-renewable finite materials: These materials may regenerate but may exhaust if used intensively and are, therefore, non-sustainable.
In the pursuit of greener technology, research and development have been focused on the growth of biomaterials in various fields of science and technology. Biomaterials are biopolymers with numerous applications. They have the advantage of being derived from natural sources, reducing the need for synthesized chemicals in their manufacturing. This, in turn, reduces the potential for environmental hazards and pollution. While not all biomaterials are usable in their natural form and require re-engineering in most cases, the ultimate goal is to develop biomaterials that can be used in their natural form, minimizing the intervention of synthetic chemicals.

Biomaterials, being made from natural sources, are generally bio-compostable or biodegradable, meaning they can be broken down by microbes once their use is over. This helps mitigate the problem of waste disposal associated with synthetic plastic materials. The need for dedicated spaces to dump waste materials can be reduced, leading to improvements in aquatic environments and ecosystems by avoiding the introduction of non-biodegradable materials into water bodies. However, it's worth noting that most biomaterials may still involve the use of some synthetic chemicals during their manufacturing and processing, making them only partially decomposable or leading to slow decomposition. Research is ongoing to make all biomaterials completely biodegradable.

Biomaterials are sometimes referred to as biopolymers or renewable polymers. They are generally classified into three main categories:

Polynucleotides: These comprise RNA and DNA.
Polypeptides: This category chiefly contains amino acids.
Polysaccharides: These primarily contain carbohydrates.

Among the numerous end uses of biomaterials is their use as polymers, textiles, medicines, composite materials, machine parts, electronics and many others. In case of textiles, it is important for a biomaterial to achieve the length to diameter ratio of at least 500:1, and thus, all biomaterials cannot qualify as textile fibres because of their polymer morphology and matrix structure. Among the few that can are poly-lactic acid (PLA), soybean protein fibre, chitosan, alginate, poly-hydroxybutyrates (PHB), collagen, catgut, zein, starch-based polymers and some biomaterials preferred as super absorbent polymers.


Since the textile industry is notorious for environmental pollution caused by the short fibres and more importantly, the chemicals used in manufacturing and subsequent processing, the need to find more and more biomaterials that are suitable for use as textiles has increased rapidly. Along with that, the need to pick the correct and most optimum processing methods and parameters has also been there. As a result, a lot of research is being devoted into dyeing and chemical processing of the biopolymers of value as textiles.


2. Biomaterials for textiles


To find out the possibilities of biopolymers as textile fibres, we looked at the sources, dyeing properties and end-uses of certain biopolymers that are gaining increasing commercial popularity. Due to the limited scope of volume here, we have restricted our study to three biopolymers that have shown maximum promises in the textile industry, viz., PLA, soybean and chitosan. An important consideration behind choosing the three of them was that they belong to separate classes of fibres. While PLA is a regenerated polyester fibre from a natural source, soybean and chitosan are regenerated protein fibres, both from natural sources, although the sources varied from terrestrial to aquatic origins.


3. Poly-lactic acid (PLA fibre)


Most synthetic polyester fibres have their sources in petroleum products. The fast depletion of petroleum raises the question as to the future of this fibre, although it has many good properties that have made it popular globally. This depleting level of the source has been a continuous and tremendous force towards the search for biodegradable polyester fibre that can be made out of a natural source. Being natural, the source will be eternal. But the challenge has been to identify natural polyester that can be processed into fibre and used as durable end products.


A potential solution towards environmental impact has been tried and achieved to some extent using biodegradable polyesters like poly-lactic acid (PLA). They have been tried in biomedical uses successfully and also as eco-friendly packaging materials and textile fibres. They are made out of natural resources that are annually renewable, such as corn. The probability of engineering at the molecular level while processing at fibre stage adds versatile diversity in properties to these fibres.


PLA fibre is also a polyester fibre in which class Polyethylene terephthalate (PET) fibre is still the most commercially used variety. So, when the chemical properties of PLA are discussed, a comparison with PET fibre becomes inevitable. PLA and PET fibres have some similarities owing to their close chemical nature and structure. Both are prone to hydrolysis during melt spinning, so drying of the polymer chips before melting is of utmost importance. Both polymers readily form fibres through melt extrusion. Both fibres after extrusion can undergo spin-draw processes to achieve tensile properties commercially acceptable.

 

But similarities only end here, and in order to study PLA fibre dyeing, its dissimilarities with PET dyeing become very significant. A major difference between PLA and PET fibre is that PLA is aliphatic polyester while PET is aromatic. While PET is a linear polymer owing to the steric hindrance posed by the aromatic rings, PLA is helical in structure, owing to its aliphatic structure. The helical structure of PLA helps in easy crystallisation, and thus, the extent of maximum crystallinity achievable in PLA is higher than in PET. These dissimilarities lead to huge differences in properties of PLA and PET related to strength, shrinkage and bulk, and chemical processing of the fibres as well.


Avinc and Khoddami claims in a detailed study on wet processing of PLA fibres that these fibres enjoy the ecological advantages of excellent performance as textiles, ease of melt processing, renewable source origin, unique property gamut and ease of composting and recycling. As a result, PLA fibre has found innumerable ways of end uses that include medical, pharmaceutical, packaging, house wares and clothing.


4. Chemistry


Poly-lactic acid fibre, often abbreviated as PLA fibre, was discovered by Carothers in 1932. It can be produced either by direct condensation of lactic acid or ring opening polymerisation of cyclic lactide dimer. In direct condensation process, high molecular weight is not possible. Also, a large reactor is required for the process, and there are needs for additional units for vaporisation and recovery of solvent. So, it's a costly process. In case of ring opening polymerisation, higher molecular weight is possible.


PLA fibre has two isomeric forms in which it exists. They are Poly-L-lactic acid (PLLA) and Poly-D-lactic acid (PDLA). Chemically synthesised PLA fibre is a racemic mixture of PLLA and PDLA in 50:50 ratios. If PLA is made out of fermentation, it generally contains 99.5 per cent PLLA. While PLLA rotates polarised light in clockwise direction, PDLA does so in the counter clockwise direction. The meso-PLA variety, which is a racemic mixture of the two isomers in more or less equal proportions, is optically inactive. Figure 1 below gives the structure of PLA fibre polymer:-


PLA having higher proportion of PLLA is preferred for textile fibre manufacturing, as this isomer is easy to crystallise, and the percentage of crystallinity achieved is higher. In case of PLA with more than 15 per cent of PDLA, crystallisation becomes difficult. For textile fibres, a minimum amount of crystallinity is desired for strength and durability properties. So, PLLA is more preferred in textiles.


PLA fibre can be produced by both melt spinning and solution spinning processes. The melt spinning process is preferred over solution spinning owing to its lower cost of manufacturing and environment friendliness due to avoidance of synthetic chemicals as solvents. In fact, PLA fibre is the only melt spun fibre out of annually renewable raw materials.

 

PLA is one of the rare thermoplastic natural fibres that we have. It is known for its sustainability. Firstly, the raw materials used are non-polluting. Added to that, PLA products are easily compostable. In presence of 98 per cent relative humidity and 60OC temperature, they are easily consumed by microbes. The left-over can be reused to grow corn, beet or rice cultivation, or to give back PLA polymer. Hence, it is recyclable to a larger extent than the other biopolymers, many of which are not recyclable and need fossil fuels as raw materials, which take many years to be produced. So, sustainability of PLA fibre is superior to all non-cellulosic fibres and even better than a few natural fibres. However, it cannot be claimed to be 100 per cent sustainable as some chemicals and energy used in the processes involved are irretrievable.


The glass transition temperature for PLA has been reported to be within 55-65OC, depending on the relative ratio of PLLA to PDLA, while its melting point is in the range of 171-180OC. The melting point of PET is 260OC. PLA fibre is thus thermally unstable as compared to PET. Thus for dyeing with dispersed dyes, dyeing temperature for PLA is supposed to be lower than PET. The refractive index of PLA, which is in the range of 1.35-1.45, is less than that PET (1.54). So, with the same amount of dyestuff, the shade depth of PLA is expected to be darker than that achieved in PET. The moisture regain of PLA is marginally higher than PET, indicating easier dye uptake and better dye absorption and retention in PLA as compared to PET.


5. Dyeing


PLA is very sensitive to acidity or alkalinity of the medium. This remains a big challenge so far as dyeing of PLA is concerned. Dyeing in acidic, alkaline as well as neutral condition has some or the other negative effect on PLA fibre. In case of alkaline condition, PLA fibre suffers surface erosion. The fibre surface no more remains smooth due to degradation of polymer from the surface, which results in a rough fibre with poorer handle, harsher feel and inferior draping qualities. On the other hand, treatment in neutral or acidic media causes bulk erosion in which the main chain scission of polymer chain occurs, causing a sharp decrease in the degree of polymerisation. This has more detrimental effect on the durability properties of the fibre. So, dyeing in alkaline medium is comparatively safer and gives better properties post dyeing. This suits the fibre when it is blended with cotton, since the latter is mostly dyed in alkaline medium for reactive and vat dyes.


With disperse dyes, the optimal dyeing temperature reported for PLA is 110OC, which is 130OC for PET. As reported by Suesat et al, the dyeing time for both fibres was 30 minutes, and while PET is dyed at pH 4, PLA requires pH of 5 for the same. An interesting observation while comparing the application of same dyestuff on PET and PLA was that between PLA and PET, the former takes lesser dyestuff to reproduce the same depth of shade, hence becoming more eco-friendly fibre to dye between the two. However, poorer dye-fibre interaction is indicated in PLA as compared to PET, as inferred by Suesat et al. Also for PLA, not only the shade depth is deeper with the same amount of dye, but also the lmax shifts slightly towards the greener side of the visible spectrum, leaving a more greenish tinge as compared to PET. This is the hypsochromic shift of hue that is observed in dyeing of PLA.


Another study by Yang and Huda found the optimum dyeing conditions for PLA fibre to be 110OC at pH 5 to 6. Although higher pH values increased dye uptake, this range of pH was the best when loss of fibre strength and mechanical properties owing to dyeing was considered. A more acidic (pH 4 or lower) or a more alkaline (pH 7 or higher) dye bath resulted in a substantial loss of elongation and strength.

 

A very interesting study by Bilal indicated quite a number of interesting observations with regard to dyeing behaviour of PLA fibres. Firstly, it confirmed the dyeing temperature of PLA fibre to be 110OC as above this, there was considerable degradation in the fibre polymer structure owing to thermal behaviour and resulting in loss of mechanical properties. Secondly, it observed that it was only a few degrees above Tg of PLA fibre that dye bath exhaustion was initiated. It was also observed that with the rise in dye bath temperature, the pH of the dye bath decreased substantially. This increase in acidity was found to be due to degradation of polymer chains giving rise to free COOH group and thus enhancing the acidity. The DSC was found to show double melting behaviour which was initially assumed to be due to crystal size and morphology perfection. However, further investigation using SAXS-WAXS technique confirmed the reason to be melt-recrystallisation. The double melting behaviour is highly affected by the dyeing temperature.


An interesting study by Bach, Knittel and Schollmeyer was performed to evaluate the option of dyeing PLA fibre in supercritical carbon dioxide. The study yielded noteworthy results. It was observed that with supercritical carbon dioxide, deep shades were not possible in PLA fibre although dyeing temperature above 50OC showed the fibre cross sections to be completely dyed. Besides, maximum dyeing temperature was found to be 90OC above which the polymer was degraded beyond further usage. Carbon dioxide dyeing was found advantageous with respect to shrinkage of fabrics out of non-heat-set PLA fibre as compared to water-finishing processes. Fibre damage and elongation at break were found to be comparable between water-finishing and carbon dioxide methods, while the latter was found to take much higher time of dyeing up to almost 10 times. There was a good potential, though, for PLA fibres to be dyed successfully in supercritical carbon dioxide, as claimed in the study, although further investigation involving disperse dyes suitable for dyeing at low temperatures was required. Also, ways to dye heavier shades needed to be found out.


A noteworthy study by He, Lu, Zhang and Freeman indicated that optimal dyeing conditions for yellow azo-anthraquinone dyes having affinity for PLA fibres could be dyed at optimal conditions of 90OC, pH 5.0 and 60 minutes. The dyed samples exhibited moderate to good wash fastness and very good light fastness properties.


6. Dye-fibre bond


In case of polyesters, the dye-fibre interaction has been found to be highly dependent on the hydrophillicity of the sub-groups in the disperse dye. The dyes with hydrophobic sub-groups are found to dye PET more easily than the ones with hydrophilic sub-groups, since PET itself is a hydrophobic fibre. PLA fibre has been reported to be comparatively more hydrophilic than PET, Nylon or Polypropylene fibres owing to the presence of the carboxylate (-COOH) group.Disperse dyes with N(C2H4OCOCH3)2, -(CO)2NC3H6OCH3, -SO2NHC6H5, -NO2, -CN(NH)C6H4 and CH(CO)2C6H4 as sub-groups were observed by Suesat et al to have better dye-fibre interaction with PLA fibre. On the other hand, disperse dyes with Cl and Br sub-groups have very weak dye-fibre bonding in case of PLA fibre. Azo-based disperse dyes have been found to be best suited for dyeing of PLA fibre.


During disperse dyeing of PLA fibre, the heat involved causes decomposition, resulting in lower molecular weight. Less exposure to heating during solvent dyeing process has been reported to cause less decomposition, making achieving higher molecular weight possible.

 

Karst, Hain and Yang evaluated the loss in tensile properties of PLA fabrics under various conditions of wash care pH and temperature, followed by different techniques of drying. It was revealed in the study that when home laundered at 35OC and pH=8 and line dried at 21O C and 65 per cent relative humidity, the PLA fabrics retained their tensile properties to a larger extent than when laundered at 50OC and pH=10 followed by tumble drying at 50O C or 70O C. Thus, care instructions for PLA fabrics essentially mention the use of mild detergents at pH=8, cold machine wash at 35OC and line dry. The study concluded that the durability and extent of hydrolysis of PLA were affected not only by pH and temperature but also the relative percentage of L-lactide and D-lactide in the PLA polymer. A 50:50 ratio of PLLA: PDLA was found to give maximum resistance to hydrolysis than 100 per cent PLLA or PDLA. Also, for PLA of given percentages of PLLA and PDLA, the poly-blends are more resistant to hydrolysis than the only the copolymers of L-lactide and D-lactide. This was an important observation considering the textile variety of PLA has about 99.5 per cent L-lactide.


7. Fastness Properties


As reported, the dye-fibre bonding in case of PLA fibre is inferior to that of PET. As a consequence, the rubbing fastness in case of disperse dyes with PLA fibre is found to have ratings inferior by 0.5 to 1.0 in the grey scale, as compared to PET dyeing. In case of wash fastness, the difference is 0.5 to 1.0 for colour change and 1.0 to 2.0 for staining of colour, PLA fibre being the inferior of the two. The light fastness is found to be lower by 1.0 rating on the blue wool scale, whereas in case of some dyes, it is found to be equal. The presence of cyanoethyl, acetoxyethyl, cyano, chloro, phenoxy, and anthraquinone groups have a positive effect on the light fastness of disperse dyes on PLA fibre, while methyl group affects it adversely. Also, longer the aliphatic chain in the disperse dyes, lesser is the light fastness reported. The aminoanthraquinone sub-groups, due to the presence of both aromatic and cyclo-fatty amino groups, have a positive effect on light fastness. In order to improve wash fastness, more intense reduction clearing (RC) process after dyeing of the polyester component is the only way to ensure higher wash fastness in the dyeing of PLA, as is with PET dyeing.


Avinc, in another study stated that higher concentration of sodium dithionate during reduction clearing improves the wash fastness to a great extent. The air present during reduction clearing degrades sodium dithionate by oxidation, affecting the effectiveness of reduction clearing. In order to compensate for the loss, increase in amount of sodium dithionate is helpful and effective, but is not economical or environment-friendly. Thus, reducing the amount of air during reduction clearing is a better option. It has been tried and found to give satisfactory improvement in wash-fastness. Higher liquor ratio helped in the process and the ideal condition to give best results was found to be treatment at 60OC for 15 minutes.


Avinc, Phillips and Wilding in a study compared the washing, alkaline perspiration and wet rubbing fastness of PLA fibres with that of PET dyed at the same visual depth. It was found that before heat treatment, the fastness ratings in all cases were found to be the same with virtually no staining on nylon fabric. However, after heat setting under typical industrial finishing conditions, the wet fastness properties of both fibres were found to decrease. This was attributed to the thermal migration of disperse dyes under heat treatment at 110OC and above. The same was thought to be higher in case of PLA fibre as compared to PET, resulting in inferior wet fastness of dyed PLA fibre.

 

8. End uses


PLA fibres are recommended to end uses that are desired to be compostable, like diapers, shoe linings, active wears and apparels. The cost of PLA fibre lies somewhere in between PET and Nylon, and the strength properties, although inferior to many man-made fibres, are commercially acceptable. PLA can give 4DGTMfibre or splittable fibre of desired cross section, amalgamated with Nylon. The resiliency of PLA fibres is good, and the natural flame retardant properties are better than PET. The melting point can be varied from 120OC to 175OC, based on the relative composition of isomers. Due to a relatively lower melting point compared to other man-made fibres, PLA fibre is well suited for bi-component fibres.


The major drawback of PLA fibre is that its abrasion resistance is very poor. Also, it is easier to hydrolyse PLA than PET during chemical processing. This puts up a challenge in processing of the fibre. Also, end-uses involving such chances are better avoided with PLA fibre.


Avinc et al in another study revealed that flame retardant PLA fabrics could be achieved that were as durable as 50 wash cycles. The application was optimised to the conditions of drying at 110OC followed by thermo-fixation at 135OC for 90 seconds. These conditions were found to have minimal effect on the fabric handle due to the flame retardant finish. It was also interestingly observed that when oil and water repellency finish was applied to PLA fabrics in the same bath along with flame retardant finish, and also fabric softener, their combined effect on colour change of PLA fabric was minimal in the range of 0.5 grey scale as compared to if they were applied sequentially as separate chemical baths. In such sequential application, the visible colour change was found to be greater than 1.0 on the grey scale.


References:

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  3. Jacobs, A.N., "Monitoring the molecular weight of Poly (Lactic Acid) during fibre spinning and colouration," A thesis of The School of Human Ecology, Kansas State University, submitted in May, 2012.
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  8. Yang, Y. and Huda, S., "The Balance between Dyeing and Physical Properties of PLA," Connection.ebscohost.com
  9. Bach, E., Knittel, D. and Schollmeyer, E., "Dyeing poly (lactic acid) fibres in supercritical carbon dioxide,"
  10. He, L., Lu, L., Zhang, S. and Freeman, H.S., "Synthesis and application of yellow azo-anthraquinone disperse dyes for polylactide fibres," Colouration Technology, 126, 2010, 92-96.

11   Karst, D., Hain, M. and Yang, Y, "Care of PLA Textiles," Research Journal of Textile and Apparel, Vol. 13 No. 4, 2009, 69-74.

12   Avinc, O., "Maximizing the wash fastness of dyed poly (lactic acid) fabrics by adjusting the amount of air during conventional reduction clearing," Textile Research Journal, 81(11), April, 2011, 1158-1170.

13   Avinc, O., Phillips, D. and Wilding M., "Influence of different finishing conditions on the wet fastness of selected disperse dyes on polylactic acid fabrics," Colouration Technology, 125, 2009, 288-295.

14 Avinc, O., Day, R., Carr, C. and Wilding, M., "Effect of combined flame retardant, liquid repellent and softener finishes on poly (lactic acid) (PLA) fabric performance," Textile Research Journal, 82(10), January, 2012, 975-984.