Fractionation of textile fibres

from denim jeans

GUSTAF CHROONA

KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF CHEMICAL SCIENCE AND ENGINEERING

Fractionation of textile fibres

from denim jeans

GUSTAF CHROONA

Master Thesis, 2016 KTH Royal Institute of Technology Chemical Science and Engineering Department of Fibre and Polymer Technology SE -100 44 Stockholm, Sweden

Cover image: microscopy image of textile fibres found in denim jeans (Innventia AB)

Summary

The structure and composition of denim jeans is complex. In addition to cotton, which is the

dominating type of textile fibre, there may be up to about 20 % synthetic fibres. The synthetic

fibres are found in the sewing thread and in the elastic yarns that are used to make stretch den-

im jeans. In total it was found that up to six different types of textile fibres may be present in

the material.

To be able to recycle cotton in jeans by producing regenerated cellulose fibres a very high

purity with respect to cotton is required. The purpose with this project was to investigate the possibility to fractionate the textile material found in denim jeans to obtain a pure cotton frac-

tion that can be used in the viscose process to produce regenerated cellulose fibres, which

then can be used to manufacture new clothes. In this project traditional wet mechanical sepa-

ration equipment found in the pulp and paper industry, in the form of a laboratory screen

(used as a model for a pressure screen) and hydrocyclone, was used to fractionate the textile

material from cut and shredded denim jeans. The degree of separation of synthetic fibres from

cotton fibres was quantitatively evaluated by measuring the glucose content after acid hydrol-

ysis.

The results from the experimental work showed that there were runnability problems both

regarding disintegration and fractionation. Regarding the fractionation, plugging was found to

be a problem and no significant separation of synthetic fibres from cotton fibres was obtained

with the conditions of the experiment.

Sammanfattning

Strukturen och sammansättningen i denimjeans är komplex. Utöver bomull, vilken är den

dominerande typen av textilfiber, kan de innehålla upp till 20 % syntetiska fibrer i tyget. De

syntetiska fibrerna finns i sytråden och i elastiska garner som används för att tillverka stretch-

denimjeans. Den här studien visade att upp till sex olika typer av textilfibrer kan förekomma i

materialet.

För att kunna återvinna bomull i denimjeans genom a producera regenererade cellulosafibrer

krävs en mycket hög renhet med avseende på bomull. Syftet med projektet var att undersöka

möjligheten att fraktionera textilmaterialet i denimjeans för att erhålla en ren bomullsfraktion

som kan användas i viskosprocessen för att tillverka regenererade textilfibrer, vilka sedan kan

användas för att tillverka nya kläder. I projektet användes, inom massa och pappersindustrin

traditionell våtmekanisk separationsutrustning i form av en laboratoriesil (här använd som en

modell av en trycksil) och en hydrocyklon, för att fraktionera textilmaterialet från klippta och

sönderslitna denimjeans. Separationsgraden av syntetiska fibrer från bomullsfibrer bestämdes

kvantitativt genom att mäta glukoshalten efter sur hydrolys.

Resultatet från det experimentella arbetet visade att det finns körbarhetsproblem både gäl-

lande uppslagning och fraktionering. För fraktioneringen visade sig pluggning vara ett pro-

blem och ingen signifikant separation av syntetiska fibrer från bomullsfibrer erhölls med för-

hållandena i experimentet.

Foreword

First, I would like to thank my supervisor and examiner at KTH, Prof. Mikael Lindström and

my supervisor at Innventia, Dr. Hannes Vomhoff for valuable guidance during the project.

Then, I would also like to give special thanks to the following people at Innventia AB,

Fredrik Aldaeus and Per Törngren for the help with the chemical analyzes, which was essen-

tial for completing the project, Lennart Hermansson for the help with the hydrocyclone trials,

Mikael Bouveng and Elisabeth Björk for sharing their experience and contributing with valu-

able advices and Lars-Åke Hammar for helping me with the practical work in the laboratory.

Finally, I would like to thank all the other people that have helped me during the project and

also the friendly fellow students at the office!

This thesis was performed at Innventia AB in Stockholm, Sweden, during the spring 2016. It

has been a very interesting project and a valuable experience for my future career.

Gustaf Chroona

Stockholm, June 2016

Table of contents 1. Introduction ............................................................................................................................ 1

1.1 Background ...................................................................................................................... 1

1.2 Problem description .......................................................................................................... 1

1.3 Purpose ............................................................................................................................. 1

1.4 Objectives ......................................................................................................................... 2

1.5 Delimitations .................................................................................................................... 2

2. Theoretical background .......................................................................................................... 3

2.1 Jeans fabrics ..................................................................................................................... 3

2.2 Disintegration ................................................................................................................... 3

2.3 Mechanical separation ...................................................................................................... 4

3. Experimental .......................................................................................................................... 8

3.1 Raw material .................................................................................................................... 8

3.2 Microscopic observation of some fibre properties ........................................................... 9

3.3 Wet disintegration experiment ......................................................................................... 9

3.4 Separation with a laboratory screen and hydrocyclone .................................................... 9

4. Results and discussion .......................................................................................................... 17

4.1 Fibre properties .............................................................................................................. 17

4.2 Wet disintegration experiment ....................................................................................... 18

4.3 Separation with a laboratory screen ............................................................................... 19

4.4 Separation with a hydrocyclone ..................................................................................... 21

4.5 Separation efficiency ...................................................................................................... 22

5. Conclusions .......................................................................................................................... 29

References ................................................................................................................................ 30

Appendix I - Classification of textile material ......................................................................... 33

Appendix II - Structure and composition of denim jeans ........................................................ 34

Appendix III - Detailed fibre properties in tabular form .......................................................... 37

1

1. Introduction

1.1 Background

The world demand for cotton is high in relation to the production. [1] The production has re-

mained at a fairly constant level during the last 30 years, which is an indication that it has

reached a peak were it does not increase any more. This is a problem as the material standard

in the world increases which will make it even harder to meet the demand in the future. [2]

Cotton is the main textile fibre in denim jeans. Today discarded jeans are dumped or com-

busted [3] and it is clear that a large value then is lost. Cotton production has a large negative

environmental impact. To produce one kilogram cotton 20 000 liter of water is needed. Pesti-

cides are also frequently used and cotton production stands for about 15 % of the world total

pesticide usage. Pesticides are also harmful to the workers in the cotton fields. [3]

With respect to the large demand in relation to the production, the high value of the cotton

and the negative impact on the environment, it is highly desirable to recycle the cotton. With

an effective process for recycling of the cotton, both environmental and economic benefits

could be obtained at the same time as the problem with meeting the demand in the future

would be solved.

1.2 Problem description

It is possible to recycle the material in cotton fibres by using its cellulose to make new regen-

erated textile fibres. This can be done in the viscose process. The problem with this process is

that it is demanding with respect to the purity of the dissolving pulp. The demand is that < 0.1

% of the material should be insoluble in the regeneration process. For textile material from

denim jeans this can be assumed to be equivalent to a purity requirement of > 99.9 % with

respect to cotton before the dissolving pulp can be used in this process.

In almost all modern denim jeans synthetic fibres are used together with cotton to improve the

properties of the product, usually to give higher stretch and strength. This means that the

shredding and cutting stage in the recycling process of denim jeans will give a material that is

a mixture of cotton and synthetic fibres. Material recycling of cotton fibres from denim jeans

is therefore only interesting if an effective separation method is available that can separate

synthetic fibres from cotton fibres to give a cotton fraction with enough purity for the regen-

eration process.

1.3 Purpose

The purpose was to find a mechanical separation method for separating synthetic fibres from

cotton fibres in denim jeans. The cotton can then be used in a regeneration process to produce

regenerated cellulose fibres, which then can be used to manufacture new clothes.

2

1.4 Objectives

To give a detailed description of the structure and composition of modern denim jeans.

To disintegrate the textile fibres in a water suspension to an acceptable level sufficient

for the mechanical separation equipment.

To experimentally investigate the usability of traditional mechanical separation

equipment in the pulp and paper industry in the form of a pressure screen and a hydro

cyclone for the fractionation of a fibre blend from cut and shredded denim jeans. The

objective was to obtain a pure cotton fraction that fulfilled the demand for the viscose

process.

To find a quantitative analysis method that could be used to determine the degree of

separation of synthetic fibres from cotton fibres in denim jeans.

1.5 Delimitations

The starting material was cut and shredded denim jeans from a textile recycling com-

pany. The material was mainly from denim jeans but it also contained a small amount

of textile fibres from other garments with similar properties.

The investigation only considered separation of textile materials found in denim jeans.

Non textile material, such as metal parts, was not considered and was assumed to be

able to be completely separated with other equipment.

3

2. Theoretical background

2.1 Jeans fabrics

Jeans is a type of trousers that are made of denim or other cotton fabric [4], but denim is today

without doubt the dominating type of fabric. Denim is a cotton fabric that is woven in a 3/1

twill structure. [3] The pieces of denim fabric are held together with stitches of a sewing

thread. There are therefore two main components in denim jeans that are based on textile fi-

bres, the denim fabric and the sewing thread for making the stitches. The sewing thread used

for making the stitches is commonly a core spun yarn with a core of polyester and a wrapping

of cotton. [3] A classification of textile material is found in Appendix I and a detailed descrip-

tion of the material in denim jeans is found in Appendix II.

An increasing trend is to wear stretch denim jeans. When stretch denim jeans are made, elastic

yarn is used together with cotton yarn in the weaving process, where the elastic yarn replaces

some of the cotton yarn in the fabric. [3] When elastane is used to produce a high elastic yarn

the elastane is put as a core inside the cotton weft yarn, to form a core spun elastic yarn. [5] A

detailed description can be found in Appendix II and III.

2.2 Disintegration

Before the fibres can be fractionated they must be disintegrated into a pulp suspension of wa-

ter and fibres. [29]. Producing a pulp suspension with the fibres found in a pair of jeans re-

quires both dry and wet processes.

The main focus in the dry disintegration is to cut and then shred the raw material. This is done

with equipment where different fractions then can be collected. Overall the dry cutting and

tearing process has three outputs. One stream with reclaimed fibres, consisting mainly of long

fibres, one stream with dust fibres, consisting mainly of short fibres and one fraction mainly

consisting of non-textile material and unopened pieces. See figure 1 for an illustrative over-

view of the principle.

Figure 1 Cutting and tearing process, modified after [6]

The main focus in the wet disintegration is to suspend the fibres in water which will provide

to the ability of free motion. [7] The basic method to obtain the disintegration is to simply add

4

water and fibres, followed by stirring the fibres in a disintegrator, a process known as disinte-

gration. [8] A common type of disintegrator is the hydra pulper.

2.3 Mechanical separation

The traditional types of solid-solid separation equipment used in the pulp and paper industry

are the screen and the hydrocyclone. A screen uses a physical barrier in the form a screen

plate (perforated barrier) while a hydrocyclone uses the hydrodynamics of a suspension as a

separation mechanism. [9] [10] The separation principles are different for the two types of

equipment. The separation with a screen will be by length (primary) and flexibility (second-

ary) [10] while the separation with a hydrocyclone will be by density and specific surface

area. [9] The separation equipment split the feed into two streams. In this report the streams

will be named coarse and fine fraction according to what is shown in figure 2 and 4.

2.3.1 General demand on the feed

In order for the separation mechanisms to operate properly the solid particles must have the

ability to move freely in the pulp suspension. Pressure screens have a rotor that helps to pre-

vent flocculation and can handle relatively high concentrations. Hydrocyclones are more sen-

sitive as they have no rotor and also require a laminar flow in the feed. They must therefore

operate at lower concentrations. [9]

2.3.2 Screen

Today the most common type of screening equipment used in the pulp and paper industry is

the pressure screen. Other types of screening equipment that traditionally have been used are

vibration screens and gravity centrifugal screens, but these are not as efficient as the modern

pressure screen. [11]

There are two important parts that are needed in any screening equipment, a perforated barrier

and a cleaning mechanism to prevent the screen from plugging. [11] There are several possi-

bilities for the design related to the operation and the shape of the internal parts. However the

basic working principle is still the same. The most common construction of a pressure screen

is to have the screen plate fixed in a screen housing combined with a rotor that rotates inside

the screen. [9] The most common design is to have a foiled shaped rotor. [11] This setup is

illustrated in figure 2.

5

Figure 2 Pressure screen modified after [12]

The feed enters from the top into the center of the cylindrical screen plate where it is acceler-

ated by the rotor. It will then move in three directions at the inside of the screen plate. Down-

wards to the reject valve, tangential around the inner wall of the screen plate and radial from

the feed side to the accept side. [9] A fraction of the fibres in the feed will pass through the

screen plate as fine fraction while the remaining fibres will leave the screen in the bottom as

the coarse fraction. [13]

The rotor is an important component as it creates pressure pulses [9] that act as a cleaning

mechanism for the screen apertures. It also generates turbulence at the screen surface which is

an effective mechanism for breaking flocks and it orientates the long dimension of the fibres

in the direction of rotation which improves the length based fractionation. [14]

The screens plate is cylindrical and usually made of stainless steel. There are two main types

of apertures, hole and slots. Modern holed screens are drilled with a conical profile to reduce

the pressure drop. In older variants of screen plates the holes were punched out. [11] Modern

slotted screens are made with a wedge-wire construction. Commonly wedge-shaped wires are

placed in the vertical direction and are held together by metal bars. The slots become wedge-

shaped which reduces the pressure drop. In older variants of slotted screens the slots were cut

out from solid material. [11] The different types of aperture shapes are shown in figure 3.

Figure 3 Aperture shapes, a) punched holes, b) conical shaped holes, c) machined slots, d) slots with

wedge-wire design [15]

6

The surface facing the feed side of the screen plate may be made smooth or with a contour. A

contoured surface has elevations that create micro turbulence. The main purpose with the con-

tour is to increase the capacity of the screen plate. But it is important to be aware of that the

contour negatively affects the length based fractionation as the rejection of longer fibres based

on the mechanism via alignment becomes less efficient. [12]

2.3.3 Hydrocyclone

A hydrocyclone consists of a stationary pipe, see figure 4, and has no moving parts. [16] The

feed is pumped in tangentially at the top (base) and obtains a rotational motion. An up going

vortex is formed at the center. The suspension will simultaneously flow downwards towards

the bottom (apex) and towards the center in the radial direction. The suspension that flows

towards the center will, if it gets close to the central vortex, change the flow direction from

vertical to upward towards the top (base). [14] The separation occurs in the radial direction

where the particles are affected by two opposing forces, an outward centrifugal force that acts

to drag the particles towards the wall and an inward drag force that acts to drag the particles

towards the center. [17] A fraction of the particles in the feed will be dragged to the central

vortex and leave at the base in the fine fraction while the remaining particles will move down

at the wall and end up in the coarse fraction. A hydrocyclone rig can consist of a large number

of pipes.

Figure 4 Hydrocyclone modified after [9]

7

It is more difficult to separate particles with similar density and specific surface area. As an

example separating fibres from fibres is more difficult than separating sand from fibres. To

obtain an acceptable degree of fibre-from fibre separation, it is necessary to use a fractiona-

tion hydrocyclone pipe. Compared to a standard hydrocyclone it has a narrower feed inlet and

a narrower radius at the bottom section of the pipe. This results in larger forces inside the hy-

drocyclone which improves the separation. The negative effect is higher energy consumption.

[18]

2.3.4 Important process parameters

The mass reject rate is an important process parameter for the control of the fractionation.

[19] [10] In practice the mass reject rate is controlled indirectly by changing the volumetric

reject rate.

The volumetric reject rate is the fraction of the volume in the feed that goes to the coarse frac-

tion and is given by equation 1.

RRv =V̇R

V̇F∙ 100 (1)

volumetric reject rate 𝑅𝑅𝑉 [%], volumetric flow rate of feed V̇F [m3/𝑠], volumetric flow rate of coarse fraction V̇R [m

3/𝑠]

The mass reject rate is the fraction of the mass in the feed that goes to the coarse fraction and

is given by equation 2.

RRm =CRV̇R

CFV̇F∙ 100 =

CR

CFRRv ∙ 100 (2)

Mass reject rate 𝑅𝑅𝑚 [%], mass flow rate of feed ṁF [kg/s], mass flow rate of coarse fraction ṁR [kg/s]

Separation efficiency:

The separation efficiency is a measurement of how well the equipment has performed the

desired separation. The efficiency can be described in several ways. The best choice is to

some extent depending on the separation task. A simple alternative is to directly compare the

feed with the fine and coarse fraction. [15] It is then important to both take the degree of sepa-

ration and the mass in each fraction into account.

8

3. Experimental

The experimental investigation was divided into three parts. In the first part the shape and

maximum width of polyester and elastane fibres were determined in a sample of cut and

shredded denim jeans. In the second part, an experiment with a laboratory wet disintegrator

was run with the aim of determining if the material could be disintegrated. This was important

since the material must be disintegrated before it can be separated in the separation processes.

In the third part, an experiment with a laboratory screen and a hydrocyclone was run with the

aim of investigating the potential for using a pressure screen and a hydrocyclone for the frac-

tionation of the textile material. The laboratory screen was here used as a model for a pressure

screen while the hydrocyclone was a pilot scale system. This part also involved a chemical

analysis to determine the degree of separation.

3.1 Raw material

The raw material came from a large textile recycling company. Two types of material were

used in the experiment, see figure 5.

Figure 5 The raw material a)”Yarn like” (art: 42087), b) “Dust” (art: 42148”Dust”)

Both types of materials mainly consisted of worn-out denim jeans that had been cut and

shredded. The main difference between the materials was their physical state. The material

“Yarn like” consisted of a large amount of unopened pieces and long yarns in addition to the

individual fibres, while the material “Dust” mainly consisted of individual fibres but it also

contained smaller pieces of yarn. The materials were highly inhomogeneous. As they came

from worn-out denim garments they contained dirt and other material accumulated during

usage. A strong blue coloration of the water and some foam formation probably from traces of

washing agents was observed in the wet state. The material “Dust” was delivered as blocks of

compressed dry pulp, see figure 6, while the material “Yarn like” was delivered packaged in a

plastic wrapping.

9

Figure 6 Compressed blocks of the material ”Dust”

3.2 Microscopic observation of some fibre properties

A microscope was used to estimate the shape and maximum width of polyester and elastane

fibres from the material. In that observation the material “Dust” was used. The polyester fi-

bres were identified by shape and the width was reported. The elastane fibres were identified

from what was known about their physical behavior (high elasticity) and shape. The maxi-

mum width and the number of filaments were reported.

3.3 Wet disintegration experiment

In the wet disintegration experiment a standard laboratory wet disintegrator was used and the

disintegrating effect was obtained by a propeller attached to a rotating shaft. Both types of

raw materials were investigated in the experiment. The disintegration was performed accord-

ing to the standard ISO 5263 - Pulps - Laboratory wet disintegration with a concentration of

15 g/l and a volume of 2 l. The degree of disintegration was determined based on the number

of remaining fibre knots that could be observed after that a small piece of the pulp had been

highly diluted in a measurement glass.

3.4 Separation with a laboratory screen and hydrocyclone

The separation experiment was only carried out with the material “Dust”. A laboratory screen

and a hydrocyclone were used in the experiment.

3.4.1 Preparation of the feed

To keep the effect on the outcome related to the state of the feed at a relatively constant and

equal level in all trials the preparation of the feed had to be done carefully. This was handled

by preparing one large batch that then was used as the feed in all trials.

The material was torn apart into smaller pieces before the wet disintegration, see figure 7.The

disintegration was done with a hydra pulper model Grubben, see figure 8, which could hold a

maximum volume of 0.5 m3.

10

Figure 7 Raw material torn apart Figure 8 Hydra pulper Grubben

The disintegration was carried out at a concentration of approximately 24 g/l and with a total

time of 15 min. A control of that the pulp was fully disintegrated was done by a visual obser-

vation. It was then pumped to a tank that could hold a maximum volume of 2.5 m3 were it was

diluted to an appropriate concentration. One part was taken out for the trials in the screen

while the remaining suspension in the tank was used for the hydrocyclone trials.

3.4.2 Separation with a laboratory screen

For large scale screening a pressure screen is the type of equipment that should be used as it is

the most effective type of screening equipment. In this experiment a laboratory screen was

used as a model for a pressure screen because it is easy to operate and does not require an

experienced operator or any extensive piping, nor any large amounts of pulp.

The laboratory screen was a centrifugal screen in small scale of model Valmet laboratory

screen FS-03 (TAP03). It becomes weakly pressurized when the feed funnel is filled with the

feed suspension, but the pressure is well below that in a real pressure screen. The manufactur-

er states that the results obtained with the screen are representable for a real pressure screen,

but as a special kind of material was used in this trial, that statement should be handled with

great care. The results should therefore only be seen as an indication of the results that would

be obtained with a real pressure screen. The overall experimental setup is shown in figure 9.

11

Figure 9 Laboratory screen

Both the fine and coarse fraction outlet had ball valves to individually regulate the openings

between 0 - 100% and thereby the volumetric rates of the fractions. The ball valve on the

coarse fraction outlet was kept fully open during all trials and the opening was instead con-

trolled by a hose clamp. This was a necessary modification as plugging in the ball valve made

it impossible to carry out the experiment. The reject and accept was collected in buckets.

The type of screen plate could easily be switched between the different experiments. The two

types of screen plates that were used in the experiment are shown in figure 10 and data for the

screen plates are presented in table 1.

12

Figure 10 Screen plates used in the experiment

Table 1 Data for the screen plates used in the experiment

Screen aperture

Aperture size

Hole (diameter)

Slot (width)

[mm]

Open area

[%] Contour Aperture design

Hole 2 18 yes Punched holes

Slot 0.2 8 yes Wedge-wire

Before the experiment was started the volumetric reject rate was determined for several con-

figurations of the coarse and fine fraction valves. A pre-experiment had also been run that

gave an indication of the volumetric reject rates that corresponded to the target mass reject

rates. The experiment was then carried out by running the equipment at some volumetric re-

ject rates around the point where each target mass reject rate was expected to be located. In

each trial, a feed volume of 10 l at a concentration of 3 g/l was used. In some trials several

fractionations were done to collect enough material in both streams for the evaluation. The

feed was poured into the funnel. This was done quickly and always from the same direction.

To avoid sedimentation in the feed funnel, that had a tendency to block the feed, the suspen-

sion was stirred with a gentle motion. The feed was then screened and thereby separated into a

fine and coarse fraction that were separately collected. The target trial points that were deter-

mined after the pre-experiment are shown in table 2.

Table 2 Parameter values for experiment with laboratory screen

Target trial point 1 2 3 4

Screen plate Hole 2 mm Hole 2 mm Slot 0.2 mm Slot 0.2 mm

Mass reject rate [%] 65 85 85 90

13

3.4.3 Separation with a hydrocyclone

In this experiment a full scale hydrocyclone located at FEX pilot plant was used. The hydro-

cyclone rig was of model Noss Radiclone 4937. It was fitted with single pipe mounted in a

horizontal direction, see figure 11.

Figure 11 Hydrocyclone rig with a fractionation pipe

Different pipes could be used but in this trial a fractionation pipe was used in all trials. The

prepared feed was under constant stirring in a large tank and was then directly pumped to the

hydrocyclone feed inlet during the trials. The feed and the coarse and fine fraction outlets

were all equipped with individual pressure gauges. Both the fine and coarse fraction outlet

were equipped with a ball valves and flow meters for control of the volume rates.

The experiment was carried out with a continuous flow of 100 l/min with a feed concentration

of 1.9 g/l. With a volume of prepared feed of about 2.5 m3 the experiment had to be carried

14

out in 25 min. It was planned to collect samples at 9 volumetric reject rates in the following

order, 90 %, 80 % ⋯ 10 %. The tendency for plugging increases with decreased volumetric

reject rate, which was the reason why the experiment was started at the highest volumetric

reject rate and then successively continued towards lower values. For each volumetric reject

rate the flows was given time to stabilize. The pressure at the feed and the coarse and fine

fraction outlet were then recorded followed by sampling both at the fine and coarse fraction

outlet by collecting the material in buckets. The target trial points are shown in table 3.

Table 3 Parameter values for experiment with hydrocyclone

Target trial point 1 2 3

Mass reject rate [%] 25 50 85

3.4.4 Evaluation of the trials

The trials were evaluated according to the following:

Measuring of flows, volumes and concentrations

Glucose after acid hydrolysis

Acid insoluble substance

Laboratory sheets

A measurement of the flows, volumes and concentrations were done in all trials. The meas-

urement of the degree of separation with glucose after acid hydrolysis and acid insoluble sub-

stance was only done on the samples closest to the target trial points. Laboratory sheets were

also only made for these samples.

In the hydrocyclone trial the volumetric flow rates were measured by flow meters on both the

fine and coarse fraction outlet. The laboratory screen was run in a non-continuous mode and

the volumes in each trial were then instead measured. This was done by weighting the amount

of water in the coarse and fine fraction, followed by a calculation of the volume by using the

density. The volumetric reject rate was then calculated according to equation 1.

The concentration was determined in both the fine and coarse fraction according to ISO 4119.

It was found to be very difficult to determine the concentration in the coarse fraction as it had

a high concentration and the long textile material had a high tendency to form flocks. The

error from determining the concentration in the feed and fine fraction was assumed to be

smaller than the error from determining the concentration in the coarse fraction. Therefore,

the concentration in the feed and fine fraction was used to calculate the concentration in the

coarse fraction by a mass balance and this value for the concentration in the coarse fraction

was then used in equation 2 to calculate the mass reject rate.

15

The quantitative determination of the degree of separation of cotton and synthetic fibres was

done by performing a quantitative chemical analysis. First, one sample from the batch of feed

was analyzed and assumed to be representative for all trials. Then both the fine and coarse

fraction from each trial were analyzed in the same way. The degree of separation could then

be determined by directly comparing the fractions of each trial, with the feed sample as a ref-

erence. The determination was primary done by measuring the glucose content in the samples

after acid hydrolysis. A gravimetric determination of the acid insoluble substance was also

done, but the results from that analysis will only be discussed and no conclusions about the

degree of separation were based on these results. The measurement of acid insoluble sub-

stance was found to be simpler and less time consuming, and therefore also less expensive

compared to the measurement of glucose after acid hydrolysis, but on the other hand it was

found to be give more uncertain results.

In the method with measuring glucose after acid hydrolysis, the amount of cellulose fibres in

the sample is directly proportional to the amount of glucose after the hydrolysis stage. This is

a chromatographic method that selectively measures glucose after acid hydrolysis without

being affected by the other material that is present in the sample. The mass of glucose will be

equal to the mass of cellulose in the sample if the hydrolysis is complete. A pure cotton fabric

was analyzed with the method to evaluate the completeness of the hydrolysis stage as a refer-

ence and the result indicated that the hydrolysis was complete. To give the content of cellu-

lose fibres in the sample a correction factor must be applied to correct for the fraction of non-

cellulose material found in the fibres. This factor will vary depending on the treatment of the

material.

To determine if an observed change in the glucose content relative to the feed was statistically

significant, a pairwise calculation was done were the glucose content in each of the streams

was compared with the glucose content in the feed. In this calculation a standard deviation of

repeatability of 1.2 % for the analytical method was used and a confidence of approximately

95 %. This calculation did not take the error related to the experimental procedure (not analy-

sis) into account. A judgment of the significance of the observed results, both from what logi-

cally could be expected and what was known about the error in the experimental procedure,

therefore had to be combined with this calculation. The error in the experimental procedure

was expected to be highly related to the homogeneity of the material. An inhomogeneous ma-

terial gives problems with sampling and it is also difficult to keep a constant composition of

the feed into the equipment.

The method with measuring acid insoluble substance gives the proportion of cellulose fibres

in the sample. This method is developed for determining the proportion of cellulose fibres in a

binary mixture of cellulose fibres and polyester fibres. It is a gravimetric method where the

principle is that the cellulose fibres are acid soluble, while the polyester fibres are acid insolu-

ble. After the material has been treated with the acid it is filtered and the acid insoluble sub-

stance will be collected on a filter paper followed by a determination of its weight and the

proportions of cellulose and polyester can be calculated. There is a standard, ISO 1833‑

11:2006, for this procedure but this was not followed. Instead the determination was done as a

part of the determination of the glucose after acid hydrolysis as this method includes a proce-

16

dure where the material is treated with sulfuric acid followed by a filtration stage, which is

very similar to the ISO standard. A problem with applying this method for determining the

proportion of cellulose fibres in mixtures that have a different composition than only cellulose

fibres and polyester is that everything that dissolves in the acid will be treated as cellulose

fibres. If there are synthetic fibres and other material (dirt) present in the sample that dis-

solves in the acid it will result in an error. The results with this method will therefore be un-

certain when the components in the sample not have been identified. The use of the method to

evaluate the separation efficiency was found to problematic as one type of elastane fibres in

the material dissolved in the acid. Moreover there is a high probability that the material con-

tain other material (dirt) that that dissolves in the acid.

The standard methods used in the evaluation are shown in table 4.

Table 4 Standard methods used in the evaluation

Analysis Method

Concentration ISO 4119 Pulps Determination of stock concentration

Glucose SCAN 71:09 Carbohydrate composition.

Laboratory sheets were also made to study the appearance of the fractions. From this an un-

derstanding of how the material had been split in the separation equipment could be gained.

17

4. Results and discussion

4.1 Fibre properties

The first goal in the project was to give a detailed description of the structure and composition

of modern denim jeans. Most of the results related to this goal are given in Appendix II and

III. It was found that denim jeans may contain up to six types of fibres when considering both

the sewing thread and the fabric. These are cotton, polyester (in the form of PET, PTT and

PBT), elastane and T400. Taking both the trends and the composition of both the sewing

thread and fabric into account, a mixture of cut and shredded denim jeans is expected to main-

ly consist of cotton and the remaining fraction of synthetic fibres will mainly consist of poly-

ester in the form of PET, PTT and PBT with smaller amount of elastane and T400. To obtain

an indication on the relative amount of the fibres used in the fabric in stretch denim jeans a

small survey over the composition of some common types of stretch denim jeans, from a large

clothes retailer, was carried out. This showed that fabric in stretch jeans mainly consists of

cotton but that it also may contain up to about 20 % synthetic fibres.

Data for each type of fibre is presented in table 5.

Table 5 properties of fibres found in denim jeans

Fibre

Apparent

density

(wet state)

[g/cm3

]

Maximum

width

(wet state)

[µm]

Average

length

(wet state)

[mm]

Water uptake

(g water/g dry fibre)

[%]

Swelling

[%]

Cotton 1.36 [1]

14 -23 [3]

22 [7]

22 Diameter: 14

Length: 1.2

PET 1.38 17.5 [4]

Unknown [8]

0.4 negligible

PTT 1.34 17.5 [4]

Unknown [8]

0.4 negligible

PBT 1.31 17.5 [4]

Unknown [8]

0.4 negligible

T400 1.36 [2]

37.5 [5]

Unknown [8]

0.4 negligible

Elastane 1.0 - 1.2 86 [6]

Unknown [8]

0.3 – 1.2 negligible

[1] Including water filled lumen, assumed to be 5 % of the total volume

[2] Calculated for PTT/PET (40/60) which is the average of the available compositions

[3] Valid for virgin fibres (calculated by assuming a swelling of 14 % of dry fibres)

[4] Estimated from a microscopy image of cut and shredded denim jeans

[5] Estimated from a microscopy image of a single fibre

[6] Estimated from a microscopy image of cut and shredded denim jeans (valid for fibres with 3 filaments)

[7] Valid for virgin fibres (calculated by assuming a swelling of 1.2 % of dry fibres)

[8] Made as filaments

18

4.2 Wet disintegration experiment

The laboratory wet disintegrator with the material “yarn like” is shown in figure 12.

Figure 12 Problem with wet disintegration of “Yarn like”

As can be seen the disintegration was problematic as the material spun around the rotating

shaft and the propeller. The degree of separation for a different number of revolutions is

shown in figure 13.

Figure 13 Disintegration of material ”Yarn like”

This shows that after 60 000 revolutions (20 min) the disintegration was not complete since a

lot of fibre knots remained.

19

The material “Dust” was easier to disintegrate as it did not spin around the rotating shaft and

propeller. The degree of disintegration for different number of revolutions is shown in figure

14.

Figure 14 Disintegration of material "Dust"

After 30 000 revolutions (10 min) the material was well disintegrated but some diffuse fibre

knots still remained. After 60 000 revolutions (20 min) the disintegration was considered

complete and no fibre knots remained.

The reason behind that it was possible to disintegrate the material “Dust” but not the material

“Yarn like” was probably because of the state of the material. The material “Yarn like” con-

tained a lot of yarns that were very long compared to the individual fibres and this was proba-

bly the reason to that it did spin around the rotating shaft which resulted in that no disintegra-

tion was obtained. As the pulp must be fully disintegrated before fractionation a decision was

made to only continue with the material “Dust”.

4.3 Separation with a laboratory screen

During the fractionation with the laboratory screen, plugging was found to occur at three posi-

tions, in the fine fraction outlet, in the coarse fraction outlet and at the rotor. In figure 15 the

plugging at the rotor and in the coarse fraction outlet is shown.

20

Figure 15 Plugging in the screen

The plugging in the coarse and fine fraction outlet made the screen inoperable while plugging

at the rotor resulted in that the screen vibrated. The plugging at the coarse fraction outlet was

found to be the most problematic and at first the screen could only be operated at a very lim-

ited interval. Sedimentation of the feed in the feed funnel was also a problem as it did block

the feed inlet to the screening compartment.

The plugging was partly solved by regulating the opening at the coarse fraction outlet with a

hose clamp instead of the ball valve that initially was used. With a hose clamp a smoother

throttling was obtained. The problem with sedimentation in the feed funnel was solved by a

gentle stirring of the feed in the funnel

The mass reject rate is plotted versus the volumetric reject rate in figure 16.

Figure 16 Mass reject rate vs volumetric reject rate for laboratory screen. (Encircled points were

analyzed for the separation efficiency)

21

The lowest points for each screen plate represent the lower limit for stable operation. As can

be seen it was possible to get more material through the holed screen plate than the slotted

one. Still, plugging at the rotor occurred at some of the lowest points, but this was not regard-

ed as making the operation instable as the equipment could be operated. The separation effi-

ciency was evaluated for the encircled points.

It is important to be aware of that the formation of a plug is a time dependent phenomena. As

the equipment only was run for a short time in the experiments, nothing is known about the

runnability of the equipment in a continuous process. It is also important to emphasize that the

results only should be taken as an indication of the results that would be obtained with a real

pressure screen as a laboratory screen was used as a model for a pressure screen.

4.4 Separation with a hydrocyclone

During the fractionation with the hydrocyclone plugging was occurring at two positions, in

the feed inlet and in the coarse fraction outlet. In figure 17 the plugging in the feed inlet is

shown together with the plugs that were taken out from the feed inlet and coarse fraction out-

let.

Figure 17 Plugging in the hydrocyclone

Also here the equipment only was run for a short time during the experiments and nothing is

known about the runnability of the equipment in a continuous process. The experiment was

started with a feed concentration of 3.8 g/l which resulted in that it only was possible to reach

a volumetric reject rate of about 70 %. This trial was not evaluated for the separation efficien-

cy. Instead the concentration was lowered to 1.9 g/l, which made it possible to operate the

hydrocyclone down to volumetric reject rate of about 50 % and this trial was then evaluated

for the separation efficiency. A drawback of the high dilution is that the energy consumption

will be high.

The mass reject rate is plotted versus the volumetric reject rate in figure 18.

22

Figure 18 Mass reject rate vs volumetric reject rate for the hydrocyclone. (Encircled points were

analyzed for the separation efficiency)

The point at the lowest mass reject rate was the lowest mass reject rate that was reached. After

this point the system plugged and became inoperable. As can be seen, the lowest mass reject

rate that was reached with the hydrocyclone was almost equal to that for the holed screen

plate. The separation efficiency was evaluated for the encircled points.

4.5 Separation efficiency

For the evaluation of the separation efficiency, the samples for the points closest to the target

mass reject rates were taken out. The points taken out for the screen are encircled in figure 16.

For the hydrocyclone two of the target mass reject rates were far below what was possible to

reach before it plugged. Therefore, the points were instead chosen by taking the highest and

lowest point and then one point in the middle of this interval, these points are encircled in

figure 18.

4.5.1 Laboratory sheets

The laboratory sheets for all fractions that were taken out for evaluation are shown in figures

19 - 21.

The results for the slotted screen plate are shown in figure 19.

23

Figure 19 Laboratory sheets for the slotted screen plate. (The percentage at each image shows the

mass split in the screen)

With the slotted screen plate the separation of yarn was complete and the fine fractions did

not contain any yarn.

The results for the holed screen plate are shown in figure 20.

Figure 20 Laboratory sheets for the holed screen plate. (The percentage at each image shows the

mass split in the screen)

24

With the holed screen plate the fine fractions contained smaller amounts of yarn but most of

the yarn ended up in the coarse fractions. There was no significant difference in the separation

between the two mass reject rates.

The results for the hydrocyclone are shown in figure 21.

Figure 21 Laboratory sheets for the hydrocyclone. (The percentage at each image shows the mass

split in the hydrocyclone)

With the hydrocyclone the fine fractions contained smaller amounts of yarn but most of the

yarn ended up in the coarse fractions. The separation of yarn improved with an increase in the

mass reject rate. At a mass reject rate of 63 % the fine fraction contained almost as much yarn

as the coarse fraction while it at a mass reject rate of 96 % contained almost no yarn.

The interesting observation is that coarser material was separated from individual fibres.

Comparing the ability to separate yarns for all the experimental configurations at the about

same mass reject rate ~ 85 %, the holed screen and the hydrocyclone seems to have given

almost the same separation, with a minor number of yarns in the fine fraction. The slotted

screen gave a much better separation, with no yarns in the fine fraction. Comparing the holed

screen and the hydrocyclone at about the same mass reject rate ~ 65 % the separation with the

holed screen was better than with the hydrocyclone. From this a conclusion is that the separa-

tion of the yarn was better with a screen compared to the hydrocyclone. Moreover, the slotted

screen gave the best separation, with no pieces of yarn in the fine fraction.

That the hydrocyclone proved to be less effective than the screen for separating yarn, can

probably be explained by a high specific surface area of the yarn which increased the tenden-

cy for the yarn to go to the fine fraction. The reason that the slotted screen plate gave the best

25

separation was likely because the slot width was smaller than the diameter of the yarn and all

the yarn were therefore blocked by the screen plate.

4.5.2 Degree of separation

The results from the quantitative chemical analysis that was used to determine the degree of

separation of synthetic fibres from cotton fibres are shown in figures 22 - 24. Both the results

from the method with measuring glucose after acid hydrolysis and acid insoluble substance

are shown together in each figure. A change in the glucose content in one stream relative to

the feed represents an equal change in the content of cellulose fibres. A change in the content

of acid soluble substance in one stream relative to the feed represents an equal change in the

content of cellulose fibres providing that the non-cellulose based material not has dissolved in

the acid.

The results for the slotted screen plate are shown in figure 22.

Figure 22 Glucose after acid hydrolysis and acid insoluble substance for the slotted screen plate (The

percentage at each bar shows the mass split in the screen.)

The results show that both the glucose content and the acid soluble content was almost the

same in the feed and in all the fractions. None of the small changes in the glucose content

relative to the feed were proved be statistically significant with a confidence of 95 %.

For the screen the separation is with respect to length (primary) and flexibility (secondary).

One explanation that supports the observed results is that, as the material was treated in the

same way in the dry disintegration it may have resulted in an equal length distribution for

each fibre type. As the separation is primary with respect to length, it is according to this ex-

planation difficult to obtain a separation with a screen.

26

An interesting observation from the result is that the amount of glucose in the glucose method

distributes around 80 % while the amount of acid soluble substance distributes at around 90

%. This can be explained by the occurrence of non-cellulose material in the cotton fibres. To

give the content of cellulose fibres in the sample with the glucose method a correction factor

must be applied to correct for the fraction of non-cellulose material found in the fibres. This

factor will vary depending on the treatment of the material. Furthermore, any non-cellulose

based materials that have dissolved in the acid have erroneously been counted as cellulose

fibres in the method with acid insoluble substance.

The results for the holed screen plate are shown in figure 23.

Figure 23 Glucose after acid hydrolysis and acid insoluble substance for the holed screen plate. (The

percentage at each bar shows the mass split in the screen.)

The results obtained with the holed screen plate are similar to those obtained with the slotted

screen plate, as both the glucose content and the acid soluble content was almost the same in

the feed and in all the fractions. None of the small changes in the glucose content relative to

the feed were proved be statistically significant with a confidence of 95 %.

The results for the hydrocyclone are shown in figure 24.

27

Figure 24 Glucose after acid hydrolysis and acid insoluble substance for the hydrocyclone (The per-

centage at each bar shows the mass split in the hydrocyclone.)

Also for the hydrocyclone the acid soluble content was almost the same in the feed and in all

the fractions, but as can be seen there was a slightly larger change in the glucose content rela-

tive to the feed for three of the streams. These were the fine fraction (mass reject rate of 96

%), the coarse fraction (mass reject rate of 14 %) and the coarse fraction (mass reject rate of

63 %).

In a calculation, that only considered the error related to the analytical procedure, all the three

streams with a slightly larger change in the glucose content relative to the feed were found to

be statistically significant with a confidence of 95 %. But after comparing the results with

what logically could be expected and taking the error in the experimental procedure into ac-

count, an final assessment were that none of the observed changes in the glucose content were

statistically significant. The split of the material between the streams in each trial could not be

logically explained by what was expected from a mass balance. When the feed is split into

two streams and the glucose content decreases in one of the streams relative to the feed, it

must also increase in the other stream. If the glucose content decreases in the stream that ob-

tained the largest mass fraction of the feed, the decrease in that stream must also be lower

than the simultaneous increase in the other stream. This is not shown in the results which in-

dicate that the observed changes in the glucose content are ordinary fluctuations related to the

error in the procedure. The error related to the experimental procedure was also expected to

be large as the material was highly inhomogeneous.

For the hydrocyclone the separation is with respect to density and specific surface area. The

density for cotton compared to the three polyester types PTT, PET, PBT and also the T400

fibre is very similar in the wet state while the elastane fibre has a much lower density. Re-

28

garding the specific surface it is assumed to be similar for cotton and the polyester fibres and

some higher for the T400 and elastane fibres as they consist of partly fused multi filaments.

As most of the fibres in the mixture are expected to be cotton and polyester, see Appendix II,

the results may be largely dependent on how these are separated. As their density and specific

surface area are similar the separation was difficult.

The result of the measurements of acid insoluble substance showed smaller differences in

separation than what were seen in the results from the method with glucose after acid hydrol-

ysis. Therefore the same conclusion about the degree of separation would probably have been

drawn from these results. This indicates that, in this specific case, the error related to the pro-

cedure of measuring acid insoluble substance was lower than for the method with glucose. An

explanation to this may be that the amount of acid soluble non-cellulose based material may

have been low in the material. The analytical procedure for measuring acid insoluble sub-

stance was also simpler than that for determining glucose, which probably gave a smaller ran-

dom error.

It is important to emphasize that the results only should be taken as an indication as, a labora-

tory screen was used and that only a very limited number of configurations were tested in the

experiment. This is not enough to give a general conclusion for the usability of a pressure

screen or hydrocyclone for the separation problem.

29

5. Conclusions

This work has focused on the separation of synthetic fibres from cotton fibres in denim jeans

with wet mechanical separation equipment in the form of a laboratory screen and hydrocy-

clone.

Denim jeans mainly consist of cotton but may contain up to 20 % synthetic fibres. Synthetic

fibres are used both in the sewing thread and in elastic yarns used to make stretch jeans. The

structure of denim jeans is complex. Synthetic fibres are tightly combined with cotton fibres.

It is also common to use core spun yarns where the synthetic fibres are completely encapsu-

lated in cotton.

It was possible to disintegrate the material “Dust” (mainly consisting of individual fibres) to

an acceptable level for the mechanical separation. On the other hand the material “Yarn like”

(mainly consisting of longer yarn) could not be disintegrated. This sets a limitation for what

types of materials that can be used as the raw material.

With respect to the demand for the viscose process of > 99.9 % cotton, it seems to be difficult

to use a screen or hydrocyclone to reach this high level of purity. Within certain limits it was

possible to operate a laboratory screen and hydrocyclone with a textile material as the feed,

but nothing is known about the ability for the equipment to operate in a continuous mode.

There were problems both regarding disintegration and fractionation. Regarding the fractiona-

tion, plugging was found to be a problem both in the laboratory screen and hydrocyclone and

no significant separation of synthetic fibres from cotton fibres was obtained with the condi-

tions of the experiment. It is important to emphasize that these results are valid for the raw

material used in the experiment and with the configuration and operation of the equipment.

An observation of the visual appearance of the fractions showed that coarse yarn could be

separated from finer material and the best separation of yarn was obtained with the laboratory

screen.

Finally, it was found that the best method for the evaluation of the degree of separation of

synthetic fibres from cotton fibres was to measure the glucose content after acid hydrolysis. It

selectively measured cellulose fibres and no control of the other material in the sample was

required, but to get a quantitative measure of the cellulose fibres content the method needs to

be calibrated. The measurement of acid insoluble substance was found to be simpler and less

time consuming and less expensive compared to the method there the glucose after acid hy-

drolysis was measured, but on the other hand it was found to give more uncertain results.

30

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33

Appendix I - Classification of textile material

The definition of fibre in the context of textiles is according to BISFA, “A morphological

term for substances characterized by their flexibility, fineness and high ratio of length to cross

sectional area”. [20] Fibres are the raw material for all textiles and they will therefore affect

both the processing and the final properties of the textile material.

The common way to classify fibres is with respect to their origin. They are first divided into

the two main groups, natural and man-made fibres. Natural fibres are fibres that are complete-

ly formed in nature into the fibrous form. Man-made fibres are fibres that are formed by hu-

mans into a fibrous form. After the division into natural and man-made fibres the man-made

fibres can further be classified into two main subclasses, natural polymer fibres and synthetic

fibres. Natural polymer fibres are man-made fibres based on fibre forming parts derived from

nature. Synthetic fibres are man-made fibres where the fiber has been completely formed by

humans. [21] The complete scheme is given in figure 25.

Figure 25 Classification of textile fibres [21]

According to the general definition a yarn is a “textile product of substantial length and rela-

tively small cross section, composed of fibres with or without twist” [20] and a fabric is "a

structure made of yarns or fibres which has a much greater surface area in relation to its

thickness." [20] The fabric is the final product in a chain of processes starting from the indi-

vidual fibres that are the building blocks of the structure. Both the properties of the fibres and

the way that they are put together will affect the final properties of the fabric. [22]

34

Appendix II - Structure and composition of denim jeans

Denim fabric

The denim fabric is made by weaving. Weaving uses yarns to produce an interlocking struc-

ture. Two assemblies of yarns are used in the weaving machine. The vertical parallel assem-

bly of yarns is called the warp. Another yarn called the weft is then inserted horizontally to

the warp. [22] The 3/1 twill wave is made by alternately letting the warp thread go over three

and under one weft yarn. The repeating pattern is also moved systematically by one position

at each warp thread to create an offset in the structure [23], see figure 26. The 3/1 twill weav-

ing structure gives rise to the characteristic diagonal lines in the fabric. [24]

Figure 26 Twill weave, blue is the warp (horizontal lines in the figure) and grey is the weft (vertical

lines in the figure) [24]

Sewing thread

The sewing thread used for making the stitches is commonly a core spun yarn with a core of

polyester and a wrapping of cotton. [3] In this type of yarn the polyester core is in the form of

a filament yarn and cotton staple fibres are then spun around the core, see figure 27. [22] The

polyester core makes the sewing thread much stronger than if only cotton had been used and

the risk of breakage during manufacturing and wear is reduced. [3] In the literature all the

three common types of polyesters, PET, [25] PBT, [26] and PTT [27] have been found to be

in use as the core filament. Which type of polyester that is most common is not clear.

Figure 27 Core spun yarn [3]

Elastic yarns

The elastic yarn used for stretch denim fabrics can broadly be put into two groups depending

on the elasticity. There are high elastic yarns and low elastic yarns. The high elastic yarns can

often be extended 4 - 6 times their own length [5] while the low elastic yarns only can be ex-

tended about 1.5 times their own length. [20] The high elastic yarns are made of elastane fi-

bres and the low elastic yarns are made of PBT fibres (a type of polyester fibres) or T400 fi-

bres (a type of elastomultiester fibres). [5] Therefore, according to the fibre classification

35

chart the elastic yarns are based on man-made synthetic fibres. The elastic yarn is commonly

only used in the weft but there is an increasing trend in using it both in the weft and warp.

[24] From observation of stretch denim it is clear that the synthetic fibres in these yarns are

made as filaments.

When elastane is used to produce a high elastic yarn the elastane is put as a core inside the

cotton weft yarn, to form a core spun elastic yarn. [5] The construction of the yarn is illustrat-

ed in figure 28. The cotton staple fibres, in the form of slivers, are winded around the elastane

core. The elastane core can either be a monofilament yarn or a partly fused multifilament

yarn. [28]

Figure 28 Construction of a core spun elastic yarn [20]

Elastic yarn based on PBT fibres is classified as a textured multifilament yarn. It becomes

elastic first after texturing, [29] which is a process where a filament yarn is provided with

durable distortions, commonly crimps, coils or loops. [30]

Textured PBT yarn is made by first forcing the PBT filaments into a folded state, then heat

and cool them down and finally relax them, which will “set” the fibres in the folded state. [31]

The overall shape of the yarn is a crimped structure, see figure 29 and figure 30.

Figure 29 Elastic textured yarn in stretch

denim jeans chemically identified

as polyester (probably PBT)

Figure 30 Elastic textured yarn chemically

identified as polyester (probably

PBT)

Elastic yarn based on T400 fibres is classified as a bi-component multifilament yarn. The

elastic effect is obtained after heat treatment. T400 is a fiber that is composed of the two pol-

yester types PTT/PET. They are aligned as filaments side by side. Each fibre thereby gets two

36

sides, with PTT on one side and PET on the other. After heat treatment the shrinkage of the

two polyester types will be different which results in a helical structure of the fibres. [32], see

figure 31 for an illustrative overview.

Figure 31 Illustration of yarn based on T400 fibres

Relative amounts of the different fibres

A conclusion from this literature study is that denim jeans may contain up to six fibre types

when considering both the sewing thread and the fabric. These are cotton, polyester (in the

form of PET, PTT and PBT), elastane and T400. A discussion now follows about the relative

amounts of the fibres that can be expected to be found in a mixture of fibres from cut and

shredded denim jeans.

First the relative amounts will be affected by the trend of wearing stretch jeans. Ordinary non-

stretch denim jeans only contain synthetic fibres in the sewing thread in the form of polyester

which can be any of PET, PTT or PBT, while stretch denim jeans contain synthetic fibres in

the fabric in addition to those in the sewing thread. As stretch denim jeans are popular today it

can be expected that both non-stretch and stretch denim jeans will be present in significant

amounts.

To obtain an indication on the relative amount of the fibres used in the fabric in stretch denim

jeans a small survey over the composition of some common types of stretch denim jeans,

from a large clothes retailer, was carried out. The labeling found on denim jeans only refers to

the fabric and the sewing thread is not taken into account. [33] This showed that fabric in

stretch jeans mainly consists of cotton but that it also may contain up to about 20 % synthetic

fibres. It was also clear that low elastic yarn (yarn based on PBT or T400) often is used to-

gether with high elastic yarn (yarn based on elastane) in stretch jeans. The second point to

notice was that a much greater amount of the low elastic yarn is put into the fabric when it is

used compared to the amount of high elastic yarn. The third point was that PBT seems to oc-

cur more often than T400 for obtaining the low elastic effect. This is also supported by the

fact that the elastic yarns based on PBT and elastane are old types of fibres that already were

used in 1950, [34] [29] while the elastomultiester in the form of T400 is a relatively new type

of fibre introduced to the market in 2002. [34] In this small survey polyester was assumed to

be textured PBT yarn and the elastomultiester was considered as being a yarn based on T400

fibres, to be consistent with what is given in source [5].

Therefore taking both the trends and the composition of both the sewing thread and fabric into

account, a mixture of cut and shredded denim jeans is expected to mainly consist of cotton

and the remaining fraction of synthetic fibres will mainly consist of polyester in the form of

PET, PTT and PBT with smaller amount of elastane and T400.

37

Appendix III - Detailed fibre properties in tabular form

Cotton

Morphology

(dry)

Circular (hollow) Twisted

In the dry state a cotton fibre is a flattened and twisted tube. [35] The cross

section is shown in figure 32 and the overall shape in figure 33.

Morphology

(wet)

The morphology in the wet state is different for a never dried fibre and a once

dried fibre. When the fibre dries, the structure undergoes an irreversible

change. A wet never dried fibre has a completely cylindrical shape. [36] A

rewetted once dried fibre will straighten out and the number of convolutions

that were present in the dry state will decrease [37]. Some of the internal parts

have also collapsed. The cross section of a once dried wet fibre will be round

and hollow [38].

Surface

structure Rough [36]

Chemical

structure

A cotton fibre is to 95% made of cellulose. [39] Cellulose consists of glucose

molecules that are bonded together with covalent bonds into long chains, see

figure 34. Important properties are that the chain has a direction as it is

asymmetric. It also has hydroxyl groups that will be involved in hydrogen

bonding with other cellulose molecules or water. The hydrogen bonding is

only in one plane and will be both intra and inter molecular, see figure 35.

This forms sheets of cellulose chains that are bonded together by hydrogen

bonds. Between the sheets there are only weaker van der Waals forces. [35]

Figure 33 – Overall shape [20] Figure 32 – Cross section [36]

Figure 2 – Cellulose monomer [35]

Figure 1 – Cellulose structure [36]

38

The fine structure of a cotton fibre is illustrated in figure 37. The first layer is

the cuticle which consists of fats, pectin and waxes. The second layer is the

primary wall which is a thin “basket-weave” structure of cellulose fibrils. [36]

The secondary wall consists of three main layers, S1, S2, S3 that all are made

of cellulose fibrils, see figure 36, that are spiraling around the fibre to form a

helical structure. The difference between the layers is the spiral angle of the

fibrils and the thickness of the layers. The winding also alternately reverses in

direction in these layers. The center of the fibre is the lumen, which is a liquid

filled hollow part of the fibre that will collapse when the fibre dries. [40]

The molecular structure of the secondary fiber wall has been investigated

under many years and many theories about the structure have been proposed,

but still it is not fully understood. According to X-ray analysis of cotton the

structure is 2/3 crystalline and 1/3 non crystalline, but today the generally

accepted view is that cotton is almost 100% crystalline. The x-ray diffraction

result can, according to this view, then be explained by an imperfect packing

of the crystalline micro fibril units and hence the amorphous phase will be

very low. [35]

Dimensions

Wet

Maximum width: 14 - 23 µm (Valid for virgin fibres, calculated by assuming

a swelling of 14 % of dry fibres)

length: 22.13 mm (Valid for virgin fibres, calculated by assuming a swelling

of 1.2 % of dry fibres)

Dry

Maximum width: 12 - 20 µm (valid for virgin fibres) [21]

length: 21.87 mm (valid for virgin fibres)

Calculation of average fibre length for cotton

The fibre length for cotton is commonly reported as staple length which is the

same as the upper half mean length (UHML). This value is calculated by tak-

Figure 3 – Assembly into micro fibrils a)

cellulose molecule, (b) cellulose fibril [35]

Figure 4 – Fine structure

of a cotton fibre [36]

39

ing the mean value of the length by number of fibres in the longest half with

respect to weight. Another related parameter is the uniformity index (UNI)

which is the ratio between mean length (ML) and upper half mean length

(UHML) expressed as percentage, [35]

UNI = (ML/UHML)*100

The mean value can then directly be obtained if the staple length and the UNI

value is known.

The dominating cotton variety is the Upland cotton (G. hirsutum) which

stands for 90 % of the world production. The typical staple length of this va-

riety is in the range between 25.4 mm to 28.6 mm. Of the remaining share,

8% is represented by the Pima-type cotton (G. barbadense) which has a typi-

cal staple length between 32 to 50.8 mm. The rest, 2%, is represented by very

short fibres, with staple length less than 25.4 mm. [36]

Considering an average value for the uniformity index (UNI) of 81 [35] and

an average value for the typical staple length of Upland cotton of 27 mm, the

mean length of Upland cotton was calculated to 21.87 mm.

Swelling

Diameter: 14 % (going from dry to wet state) [36]

Length: 1.2 % (going from dry to wet state) [36]

Regain

(Water

uptake)

Fully wetted cotton: 22 % g water/g dry cotton [41]

Average humidity conditions: 8.5% g water/g dry cotton [41]

Density

Wet

Only fibre wall: 1.38 g/cm3

[36]

Apearent density: 1.36 g/cm3

(including water filled lumen, assumed to be 5

% of total volume)

Dry

Only fibre wall: 1.55 g/cm3

(only fibre wall) ) [36]

Apearent density: 1.40 g/cm3

(including air filled lumen, assumed 10 % of

total volume)

40

Polyester (PET/PTT/PBT)

Morphology

Circular (solid) Cylindrical

Surface

structure

Smooth [42]

Chemical

structure

Figure 40 – PET (polyethylene tereftalat) [43]

Figure 41 – PTT (polytrimethylene terephthalate) [43]

Figure 42 – PBT (Polybutylene Terephthalate) [43]

Dimensions

Maximum width: 17.5 µm (Estimated from a microscopy image of cut and

shredded denim jeans)

length: unknown (Made as filaments)

Swelling

(Assumed to be negligible)

Regain

(Water

uptake)

0.4% [44]

Density

PET: 1.38 g/cm3

[44]

PTT: 1.34 g/cm3 [44]

PBT: 1.31 g/cm3 [44]

Figure 5 – Cross section Figure 6 - Overall shape

41

Elastane

Morphology

Multichannel (solid) Partly fused multifilament

The number of filaments that are used to make the fibre varies. Both single

and multifilament fibres are in use. [28] By microscopic observation of a

sample of cut and shredded denim jeans, fibres with three filaments were

found to be the most common. But, this conclusion is uncertain as only a lim-

ited number of fibres were observed.

Surface

structure

Smooth [45]

Chemical

structure

Figure 45 – Elastane [46]

An elastane fibre is according to the definition, “Fibre composed of at least

85% by mass of a segmented polyurethane and which, if stretched to three

times its unstretched length, rapidly reverts substantially to the unstretched

length when the tension is removed.” [20] An elastane fibre is a polymer of

hard and soft segments. The soft segments are either polyethers or polyesters

while the hard segments are structures that contain aromatic rings which pro-

vide rigidity. [44] The hard segments will provide physical crosslinks between

the chains. [47] The structure is shown in figure 46.

Figure 44 - Overall shape Figure 43 - Cross section

42

Dimensions

Maximum width: 86 µm (estimated from a microscopy image of cut and

shredded denim jeans, valid for fibres with 3 filaments)

length: unknown (made as filaments)

Swelling

(Assumed to be negligible)

Regain

(Water

uptake)

0.3 – 1.2 % [48]

Density 1.0 – 1.2 g/cm

3 [49]

Figure 7 – Elastane fibre under stretched and relaxed

mode [44]

43

T400

Morphology

Multichannel (solid) Partly fused multifilament

Surface

structure

Smooth [50]

Chemical

structure

T400 is a type of elastomultiester that is composed of the two polyester types

PTT/PET. The polymeric weight proportions can vary between 30/70, 40/60

or 50/50 for these types of PTT/PET fibres. [32] The concept is clarified in

figure 49.

Figure 49 - T400 fibre

According to BISFA a elas