Chemical Recycling of Textile Polymers(2)

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Chemical Recycling of Textile Polymers

INTRODUCTION

Use of polymer in textile field is abundant. Very first is textile fibre itself is a polymer. But every

polymer is not a textile fibre. There are certain properties that a polymer should possess then

only we can use certain polymer as textile fibre. Such properties are length to diameter ratio,

flexibility, abrasion resistance, etc.

Polymers are also use in processing of textile goods e.g. sizing. For sizing of cotton, we use

starch, which is a polymer. For cotton-PET blend we use poly vinyl alcohol for sizing, which is

again a polymer.1 In printing, polymer is used in the preparation of the screen for the printing

Also, thickening agent, which is main constituent of printing, is polymer based e.g. thickening

agent based on polyacrylate.2 While in finishing, we use poly acrilamide, poly vinyl alcohol, poly

vinyl acetate and poly acrylate based formulations are used at times with other functiona

chemicals e.g. silicones, reactive softeners, resins. Also anti-static finish as poly glycol ester is

also applied and many more.1

But if these are synthetic polymers like nylon, PET, poly acrylic, etc, which are not degradable,

affect environment badly. The landfill effect of all textile polymers is studied. 3 It says, all natural

polymers were degraded after one year without any significant effect on land. On the other handsynthetic polymers were hardly degraded but had ill effect on land. So here arises the need to do

something with these polymers. So we can recycle them.

There are three different ways of recycling such polymers as shown below4: -

1. Mechanical recycling: Mechanical recycling just changes the form of polymer e.g

polyester shirts can be torn and use this for pillow. It actually doesn’t solve the problem,

since polymer doesn’t changes its chemical form.

2. Thermal recycling: We melt the polymer and again extrude it into fibres e.g. PET bottles

can be converted into fibre. Though better from previous one, we cannot go for repeated

thermal recycling since this degrades the polymer properties such as crystallinity

strength, etc.




3. Chemical recycling: We subject the polymer to different processes so as to get the

original raw material back i.e. total depolymerization of polymer, or partial

depolymerization to the oligomers, and again reacting them back, will give a polymer of

virgin quality.

Hence chemical recycling is the best option to follow today though expensive.

Chemical recycling5:

The term chemical recycling is most often applied to the depolymerization of certain

condensation or addition polymers back to monomers (the basic building blocks from which

plastics are made). Examples of these types of plastics are polyesters (e.g., the PETE used in

soda bottles), polyamides (e.g., the nylon used in carpeting), and polyurethanes (for e.g. foam)

Long chain polymers can be treated chemically and/or thermally to break the chains into short

segments. When the treatment is done to re-create the chemicals from which the polymers were

initially made, we call that feedstock or monomer creation. If the treatment breaks the polymers

into an assortment of chemical species, the processor can decide whether to recover specific

chemicals for feedstock use or to use the assortment of chemical species for fuel or to use some

combination of both end products.

a. Recycling of Polyester:Lets have a closer look at the basic raw materials and how they are transformed to

produce PET. The basic raw material is crude oil, which by fractional distillation will

produce ‘naptha’; separation of the aromatic and aliphatic fractions leads to terephthalic

acid (TPA) and ethylene glycol (EG). The first step in the production of PET polymers

out of TPA is an esterification reaction and out of DMT is an ester exchange reaction. In

this reaction, one molecule of TPA/DMT and two molecules of EG produce one molecule

of Bis-hydroxy ethylene terephthalate (BHET) and two molecules of methanol. This is

reversible reaction6.

The next step of this reaction is polycondensation, in which polycondensation of BHET

takes place to give PET and glycol. This is also a reversible reaction. The chemical




recycling of PET uses this possibility of reversing the reaction to the BHET monomer or

even to the PTA/DMT and EG stage6.

There are mainly four methods to depolymerize PET, to convert the polymer back into its

raw materials.

1. Glycolysis (using ethylene glycol or diethylene glycol),

2. Methanolysis (using methanol),

3. Hydrolysis (using water),

4. Diolysis (using butanediol)7.

After depolymerization, the monomers and/or oligomers are recovered, sometimes

purified via vacuum distillation, and repolymerized to give virgin PET. The main

depolymerization processes that have reached commercial maturity are glycolysis and

methanolysis. However, other methods such as hydrolysis have been researched

extensively. Collectively these chemical recycling processes are termed chemolysis

Selection of the most appropriate recycling technique is dependant upon the quality of the

available feedstock and type of the end product required7. The different chemica

recycling routes for PET essentially differ in the purity and consistency of the

intermediates they produce.

b. Recycling of Polyamide9:

Nylons are also polymers that can be depolymerized efficiently to recover monomer feedstocks. Two nylons, nylon 6 and nylon 66, are used for carpet fiber and high performance

molded applications. Nylon 6 is made by reacting caprolactam to form polymers

Caprolactam is a ringed chemical of seven atoms (six carbons and one nitrogen). During

the polymerization the ringed molecule is opened up to form a monomer chain. The

monomer chains are then link together to form a polymer. Nylon 66 is a condensation

polymer formed by combining a hexamethylene diamine (HMD, the first "6") with adipic

acid (the second "6"). Unlike polyesters, nylons do not simply depolymerize by operating

the synthesis reactions in reverse.

Approach has been developed to recover monomers. Carpets are physically taken apart

and the mixture of nylons isolated. The mixed nylons are then subjected to ammonolysis

at above 300 C by combining nylons and ammonia in the presence of catalysts. The




ammonolysis products are distilled to recover crude HMD, crude caprolactam, and other

monomers. The crude caprolactam is further refined to make monomers for nylon 6

manufacture. The HMD and other monomers are hydrogenated and purified to make pure

HMD monomer for manufacture of more nylon 66.

The recovery of monomers from mixtures of nylon 6 and nylon 66 requires substantial

capital equipment and engineering expertise. The economics of such recovery are assisted

by the inherent value of the nylon monomers and subsequent nylon polymer. Post-

consumer nylon from carpets and automotive parts are targeted for a process that is slated

for commercialization by 2002. Now, we will study the depolymerization of nylon-6 and

nylon-6,6 separately.

• Nylon-6:

Recovery of 6-aminocaproic acid 10: Nylon-6 can be depolymerized by hydrolysis, either

in presence of alkali or acid, to get 6-aminocaproic acid (6-ACA). If the reaction

conditions are severe then we get the acid or alkali salt of 6-ACA. And caprolactam being

amphoteric in nature, it is very difficult to prepare in pure form, by the process of

neutralization.

Recovery of caprolactam10, 11, 12: Depolymerization of nylon-6 waste can be accomplished

by the reaction of water, acid or basic. Caprolactam is separated from main chain of

polymer as a result of an intermolecular exchange reaction. Nylon-6 waste is converted into caprolactam by heating with superheated steam to 200-

400 0C, if necessary under pressure, in the presence of non-volatile organic or inorganic

acids and alkalis, preferably in 3-15:1 weight ratio of waste with acid/alkali hydroxide

Fresh waste is added along with the acid/alkali hydroxide as the caprolactam distills off

with the steam. The waste can be in any form. The rate of depolymerization is decided by

the available surface area per unit weight of the waste and fine structure of the fibrous

material. When the thermal splitting process is started in the presence of caprolactam-

H3PO4 (a compound formed by heating equimolar quantities of caprolactam and

phosphoric acid), significant improvement in the purity of caprolactam is achieved

Separation of caprolactam molecule is possible from any part of the polymer chin as a

result of an intermediate exchange reaction. The caprolactam formed must be removed




efficiently from the reaction zone to displace the polymer-monomer equilibrium towards

monomer formation.

The alkali catalyzed depolymerization reaction proceeds rapidly at 250 0C. Besides

caprolactam, great amounts of impurities are produced, the removal of which is very

troublesome. At lower temperatures the velocity of distillation of caprolactum depends on

the extent of vacuum. Such processes therefore are rarely used commercially. Acid

catalysts are found to give purer caprolactam than alkali catalyst.

Caprolactam is recovered from nylon-6 waste in good yield by passing superheated steam

through molten mass at 225-350 0C. A mixed phosphoric acid-boric acid catalyst at 0.1-

5.5.0wt% based on the polymer, is used. The steam containing caprolactam is condensed

and the solution is worked up to get pure caprolactam. A concentration of catalyst in the

range of 0.3-1.0 wt%, based upon the polymer, gives 93-95% caprolactam recovery.

• Nylon-6,613:

The monomers for Nylon-6,6 are hexamethylenediamine (HMD) and adipic acid and the

depolymerization processes such as, hydrolysis, aminolysis leads to these monomers and

or derivatives of adipic acid. The direct hydrolysis of nylon-6,6 can be done. Isopropanol

is used to extract the HMD and the adipic acidis recovered through electrodialysis from

sodium adipate. The aminolysis process is performed under high pressures of ammonia

and high temperature and leads to HMD, adiponitrile, caprolactam and aminocapronitrileHydrogenation of these components leads to pure HMD. The key issues are the yield of

the process and the harsh condition required to keep reaction times reasonable. The direct

aminolysis of pure nylon-6,6 has poor yields and generates tar. The inclusion of nylon-6

improves the yield significantly.

A two stage process is developed to depolymerizenylon-6,6 separately. First, n-

butylamine is used as reagent to break the amide linkage and generate HMD and N,N’-

dibutyladipamide. Aminolysis of this is then carried out to recover the n-butylamine and

adiponitrile, which can be hydrogenated to HMD. The overall yield of adiponitrile from

this process is about 48% due to condensation side reactions.




Overall the low yields and harsh process conditions have not favored the scale-up of

these technologies to depolymerize nylon-6,6. Hence, the depolymerization of nylon-6,6

is done by adding nylon-6 to it, which improves the yield.

C. Recycling of Polyvinyl chloride14:

Scrap PVC can be oxidized in oxygenetaed NaOH solution at temperature in the range

150-260 0C and at elevated pressure to give oxalic acid, a mixture of benzenecaboxylic

acid and CO2. it was found that the yield of oxalic acid increases with increasing NaOH

concentration until 15M. one tonne of scrap PVC can yield 600Kg of oxalic acid and 300

Kg of benezecarboxylic acid under optimum conditions. Oxalic acid is a valuable

intermediate in the chemical industry.

In chemical reaction involved in the solution oxidation of PVC, the first step involves the

dehydrochlorination of PVC to give polyene sequences. Then in an oxygen atmosphere

aromatica rings are formed by bimolecular addition reactions and coiling of the polyenes

In third step, liquid phase oxidation of the dehydrochlorinated PVC containing these

aromatic rings gives various benezecarboxylic acid. Oxalic acid and CO 2 are produced at

every stage, by base catalyzed oxygen oxidation of PVC in the liquid phase.

A PVC pallet decomposes completely by the reagent (15M NaOH) at 250 0C and an

oxygen partial pressure of 5 MPa after 12 hours. This process also handles highly

plasticized or filled PVC. The process to oxalic acid oxidizes organic plasticizers such as

phthalates.

d. Recycling of Polyurathane15:

Polyurethanes (PU) can also be depolymerized to form useful monomers. Polyurethanes

are typically formed by reacting diisocyanate with a glycol, such as EG. Polyurethanes

can be theoretically depolymerized by hydrolysis or glycolysis like polyesters or by

ammonolysis, like nylons. The principle source of polyurethanes to be depolymerized

would be foams, either pre-consumer or post-consumer such as auto seat cushions8.

In hydrolysis, PU are broken down into their original precursors; the base polyol and an

amine, by the application of superheated steam. This process causes hydrolysis of the

urethane linkages and is described in number of patents. In principle it is possible to

separate the amine obtained from the hydrolysis process and, after purification, use it




again as a raw material for the isocyanate process. However, the unfavorable economics

of this process limits its commercial exploitation21.

In aminolysis, the PU scrap is chemically cleaved amines such as dibutylamine

ethanolamine is added to the PU scrap. The decomposition of PU with amines leads to

quite a different product mix than with glycolysis or hydrolysis. Aminolysis converts the

urethane linkage to polyols and disubstituted urea, which in turn breaks down to yield

oligomeric ureas and amines. Glycolysis of PU is by far the most promising chemical

recycling route for PU12.

Commercial polyurethane depolymerization has been by glycolysis. The polyurethane is

mechanically cleaned of other polymers and other material. The mechanically separated

and cleaned polyurethane is reacted with glycols, such as diethylene glycol, in the

presence of catalysts at about 200 C to form polyols. The polyol product is then purified

for use in new polyurethane manufacture. Glycolysis of polyurethanes can be

economically acceptable, but still requires more development in order to tolerate more

contamination in the post-consumer material8.

CONCLUSION

Advanced recycling of plastics represents a significant technological advancement that in the

case of some polymers is already supplementing existing mechanical recycling processes. These

processes signal a significant technical advancement in plastics recycling because the productsafter purification, are identical to current feedstocks and monomers used to produce new plastics.

Many of the plastic feedstock recycling processes appear to be technically feasible and robus

enough to warrant further development in the future. At this stage the technology is stil

developmental. Much more needs to be learned about infrastructure requirements, feedstock

quality, processing, and economics. This recent development in plastics recycling shows promise

toward achieving the industry's goal of increasing the environmentally and economically sound

recovery of polymers, and may someday provide viable recycling options beyond conventiona

mechanical recycling for many more types of post-consumer plastics. Present economics

however, suggest that major new investments in the commercial development of chemical and

feedstock recycling will be limited in the short term.




REFERENCE

1. Shenai, V.A., ‘Bleaching And Mercerizing’, Pub, Sevak Publications, 1995, pp

2. Shenai, V.A., ‘Technology Of Printing’, Pub, Sevak Publications, 1995, pp

3. Lindsay, A. and Woodings, C., Managing Non-woven Product Waste, Courtaulds

Research, UK, 1991, pp 78-87.

4. ‘Recycling Non-woven’, Tappi Journal , 79(3), 1996, pp 215-219.

5. http://www.plasticsresource.com/recycling/recycling_rate_study/index.html

6. Dr. H. Meierkord, ‘Recycling Opportunities For Polyester’, Chemiefasern/

Textilindustrie, Vol. 43/95, June 1993, pp E 80.

7. Scheirs, J., ‘Polymer Recycling’, Wiley series in polymer science, pp. 158-159.

8. Dr. Sharma, N.D., and Shubhada, ‘Management of Polyethylene Terephthalate

Waste’, Asian Textile Journal , 3(11), 1995, pp 92-99.

9. http://www.plasticsresource.com/recycling/recycling_backgrounder/bk_advanced.ht

ml

10. Sharma, N. D., ‘Utilisation of Nylon-6 Waste’, Textile Asia, 22(6), 1991, pp. 66-73.

11. U.S. Patent Number: - 5,852,115 (from site www.uspto.gov)

12. U.S. Patent Number: -5,169,870 (from site www.uspto.gov)

13. Realff, M.J., Newton, D. and Ammons, J.C.,’ Modelling and Decision Making For

Reverse Production system Design For Carpet Recycling’, Journal Of Textile Institute, 91, 2000, part 3/-, pp. 176.




http://www.plasticsresource.com/recycling/recycling_rate_study/index.html

http://www.plasticsresource.com/recycling/recycling_backgrounder/bk_advanced.html


http://www.uspto.gov/

http://var/www/apps/conversion/tmp/scratch_6/www.uspto.gov

http://www.plasticsresource.com/recycling/recycling_rate_study/index.html



http://www.uspto.gov/

http://var/www/apps/conversion/tmp/scratch_6/www.uspto.gov

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Chemical Recycling of Textile Polymers(2)