Surface modification of meta aramid to enhance dyeing

69 Table 12 Dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with different PAA concentrations and curing durations using a shaking bath.. 73 Table 13 Dyed samples of PAA–grafted N

A Dissertation for the Degree

of Doctor of Philosophy

Surface Modification of Meta–aramid

VU, NGUYEN KHANH Surface Modification of Meta–aramid to Enhance Dyeing (Under the direction of Dr Stephen Michielsen)

Well known for their high thermal and chemical stability, meta–aramid fibers play a very important role in high performance textiles, especially firefighter’s clothing Nonetheless, meta–aramid fibers also have limitations Due to their high crystallinity and inertness, the coloration of these fibers is extremely difficult as confirmed by many references Although many improvements have been made so far, the dyeing of meta–aramid fibers still requires high temperature and long duration to obtain good color strength and shade Owing to this drawback, a surface modification step was implemented using the very popular industrial technique, pad–dry–cure, to modify the surface of meta–aramid fibers via grafting–to technique with poly(acrylic acid) (PAA) As an anionic polyelectrolyte, PAA facilitated the coloration of meta–aramid fibers with cationic dyes A dyed fabric, whose K/S values can be considered industrially acceptable, was produced at room temperature (250C –

270C), and under neutral pH (=7) in 8 or 15 minutes (depending on selected dyeing technique) Dyeings had good crockfastness (both wet and dry), which proved the feasibility

of the proposed treatment

© Copyright 2018 Nguyen Vu All Rights Reserved

by Nguyen Khanh Vu

A dissertation submitted to the Graduate Faculty of

North Carolina State University

in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina

DEDICATION

I would like to dedicate my dissertation to my wife (YÊN), my daughter (VÂN), my parents (MINH and TRÚC), my sister (NGỌC) and my parents–in–law (KHÁNH and TUYẾT) as well as other relatives in my big family Without their mental and physical support, it would have been impossible for me to fulfil this dream

BIOGRAPHY

NGUYÊN was born in the biggest city in Viet Nam, Ho Chi Minh, in 1984 He finished his Bachelor of Science degree in Mechanical Engineering focusing on Textile Technology in 2008 at Bach Khoa University (Formerly Ho Chi Minh City University of Technology HCMUT) After two years of search, he got a chance to start a Master’s degree

in high–tech textiles concentrating on multi–functional textiles at University of Minho, Guimarães, Portugal This program was funded by Erasmus Mundus Action 2 from 2010 to

2012 Subsequently, he returned to his undergraduate university and started working as a lecturer In 2013, he won the 911–Fellowship (a nation–wide application process that takes a year to finalize) sponsored by the Viet Nam Government Thanks to this fellowship, he was finally able to start his doctoral degree at North Carolina State University and earn the Doctor of Philosophy title after a long and arduous journey

ACKNOWLEDGMENTS

It is a truism that the first person whom I have to thank is Dr Stephen Michielsen Thanks to his guidance and directions upon professional knowledge, I could improve and widen my critical view of scientific world, especially in the area of fiber and polymer science Without such support from Dr Michielsen, it is not possible for me to achieve my title

Immediately after, I would like to show my deep gratitude to all of those who have financially supported me so as I can spend my four years here at College of Textiles, North Carolina State University The first person is Dr David Shafer, Assistant Dean at Graduate School Without his aid, I couldn’t have kept going with my doctoral degree Next, Vietnam International Education Cooperation Department, Ministry of Education and Training, Bach Khoa University (Formerly Ho Chi Minh City University of Technology – HCMUT), College of Textiles are organizations I would like to show my thankfulness These institutions have put the first stones forming the pathway on which I could walk step by step

to today’s achievement

Additionally, I would like to send my gratefulness to companies who have sponsored materials (fabrics, chemicals) towards my research Those are TenCate Protective Fabrics (Nomex® IIIA fabric rolls) and M Dohmen USA Inc (Good lightfastness basic dyestuff) Their products play a huge role in my study

Similarly, I would like to thank Ms Birgit Andersen, Research Assistant and Lab Manager, TECS; Mr Jeffrey Krauss Pilot Laboratory Manager, ZTE and Ms Teresa White, Research Specialist, ZTE They have taught me important skills from the very first day I started my research Thanks to those skills I could develop and finish my work

In the end, I am lucky to have many good friends around me in the College I would like to say thank to all of those who ever helped, listened to, discussed and supported me during my student life here at College of Textiles

TABLE OF CONTENTS

LIST OF TABLES ix

LIST OF FIGURES xiii

CHAPTER 1 – INTRODUCTION 1

CHAPTER 2 – PRIOR ART 4

2.1.M ETA– ARAMID 4

2.1.1 History, polymerization, and production 4

2.1.2 Products produced from meta–aramid 8

2.1.3 Applications [21] 10

2.1.4 Coloration, finishing, and functionalization of meta–aramid 12

2.2 P OLY ( ACRYLIC ) ACID (PAA) [83] 35

2.2.1 Definition 35

2.2.2 Chemical structure and synthesis 35

2.2.3 Physical properties 36

2.2.4 Behavior in aqueous solution 37

2.2.5 Applications 38

CHAPTER 3 – MATERIALS 52

3.1 N OMEX ® IIIA FABRIC 52

3.2 P OLY ( ACRYLIC ACID ) – PAA 52

3.3 T OLUIDINE B LUE O (TBO) 52

3.4 G RAFTING OF PAA ONTO N OMEX ® IIIA FABRIC 53

3.4.1 Screening grafting procedure 53

3.4.2 Modified grafting procedure – Design of Experiment 53

3.5 Q UANTIFICATION OF PAA GRAFTED ONTO N OMEX ® IIIA FABRIC 54

3.6 D YEING OF N OMEX ® IIIA FABRIC USING BASIC DYES 55

3.6.1 Preliminary dyeing procedure 55

3.6.2 Modified dyeing procedure 55

3.7 C OLOR STRENGTH MEASUREMENT 56

3.8 F ABRIC HAND 56

3.9 C ROCKING FASTNESS 57

3.10 W ASHING 58

CHAPTER 4 – RESULTS AND DISCUSSION 59

PART I – PRELIMINARY RESULTS 59

4.1 Q UANTITY OF PAA GRAFTED ONTO META– ARAMID FIBERS 59

4.1.1 Standard curve of Toluidine Blue O (TBO) 59

4.1.2 Amount of PAA grafted onto Nomex® IIIA 60

4.1.3 Theoretical calculation of a monolayer of PAA grafted onto Nomex® IIIA fiber 61

4.2 M ECHANISM OF GRAFTING PAA ONTO N OMEX ® IIIA 63

4.2.1 Scheme 1 63

4.2.2 Scheme 2 64

4.3 K/S VALUES 67

4.4 C ROCKING FASTNESS 69

4.4.1 Single thermal treatment 69

4.4.2 With a 2 nd thermal treatment 69

PART II – FINAL RESULTS 70

4.5 D YEING MECHANISM 70

4.6 C OLOR UNIFORMITY 72

4.6.1 Exhaust dyeing with an agitating bath 72

4.6.2 Continuous dyeing with a padding mangle machine 76

4.7 K/S VALUES 80

4.7.1 K/S values of exhaust–dyed Nomex® IIIA samples 80

4.7.2 K/S values of continuous pad–dyed Nomex® IIIA samples 83

4.8 C ROCKFASTNESS 85

4.8.1 Exhaust–dyed Nomex® IIIA samples 85

4.8.2 Improvement for crockfastness of Basic Blue 17 88

4.8.3 Pad–dyed Nomex® IIIA samples 88

4.9 F ABRIC HAND 90

4.10 W ASHING OF EXHAUST – DYED SAMPLES WITH B ASIC B LUE 17 91

CHAPTER 5 – CONCLUSION 92

CHAPTER 6 – FUTURE WORK 94

REFERENCES 96

APPENDICES 104

LIST OF TABLES

Table 1 History of development of Aramid Fibers [16] 5

Table 2 Company, Aramid Type, and Brand Names of Commercial Aramids [15] 6

Table 3 Filament yarns of Nomex® Aramid [24, 25] 9

Table 4 Nomex® paper and pressboard [24] 10

Table 5 Effectiveness of different carriers [31] 14

Table 6 Fastness of dyeing on fabrics of Nomex E–8 [31] 16

Table 7 Dye combinations for certain shades for protective clothing [32] 17

Table 8 Modified design of experiments for grafting of PAA onto Nomex® IIIA 54

Table 9 Statistically calculated of TBO's absorbance after different treating conditions (PAA concentrations and curing duration) 61

Table 10 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples 69

Table 11 Wet crockfastness values after 2nd thermal treatment at different durations 69

Table 12 Dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with different PAA concentrations and curing durations using a shaking bath 73

Table 13 Dyed samples of PAA–grafted Nomex® IIIA cured at 2200C with different PAA concentrations and curing durations using a shaking bath 74

Table 14 Dyed samples of PAA–grafted Nomex® IIIA cured at 2400C with different PAA concentrations and curing durations using a shaking bath 75

Table 15 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at 2000C with different PAA concentrations and curing durations using a padding mangle machine 77

Table 16 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at

2200C with different PAA concentrations and curing durations using a padding mangle machine 78Table 17 Basic Blue 41 dyed samples of PAA–grafted Nomex® IIIA cured at

2400C with different PAA concentrations and curing durations using a padding mangle machine 79Table 18 Color strength of pretreated Nomex® IIIA samples with a fixed PAA

concentration of 0.01 wt% and dyed with four different colors 81Table 19 Numerical values of K/S max of dyed Nomex® IIIA samples with a fixed

PAA concentration of 0.01 wt% and dyed with four different colors 81Table 20 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2000C 86Table 21 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2200C 86Table 22 Crockfastness values (dry and wet) of dyed Nomex® IIIA samples cured

at 2400C 87Table 23 Molecular structures of Basic Blue 17, Basic Red 46, Basic Yellow 2, and

Basic Violet 16 87Table 24 Crockfastness (wet and dry) values of Basic Blue 17 exhaust–dyed

Nomex® IIIA samples after a 2nd thermal treatment at 1800C 88Table 25 Crockfastness (wet and dry) values of Basic Blue 41 (0.1 owf%) pad–dyed

Nomex® IIIA samples of different pretreating conditions 89

Table 26 Crockfastness (wet and dry) values of Basic Blue 41 (0.25 owf%) pad–

dyed Nomex® IIIA samples of different pretreating conditions 89

Table 27 Mean values of bending moduli obtained between two specific curvature points (0.5 cm and 1.5 cm) going one way, and going the opposite way (– 0.5 cm and –1.5 cm) for both warp and filling directions 91

Table A.1 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2000C .105

Table A.2 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2200C .106

Table A.3 K/S–max values of exhaust–dyed Nomex® IIIA samples cured at 2400C .107

Table A.4 K/S–sum values of exhaust–dyed Nomex® IIIA samples as a function of curing duration for PAA concentration of 0.01 (wt%) at different curing temperatures .108

Table A.5 Maximal absorbance values (between 400 nm to 700 nm wavelengths) of pad-dyed (0.1 owf%, Basic Blue 41) Nomex(R) IIIA samples .109

Table B.1 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2000C .110

Table B.2 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2200C .111

Table B.3 Bending rigidity of Nomex® IIIA samples (filling and warp directions) cured at 2400C .112

Table B.4 Bending rigidity of Nomex® IIIA samples (filling and warp directions) padded with 0.01% PAA then cured at different temperature in 3 minutes 113

Table B.5 Bending rigidity of Nomex® IIIA samples (filling and warp directions) padded with 0.05% PAA then cured at different temperature in 3 minutes 114

Table C.1 Hue changing phenomenon in PAA–grafted Nomex® IIIA samples (0.1

wt% PAA, cured at 2400C in 3 minutes) dyed with Basic Blue 17 before and after washing with AATCC 61–2A standard .115Table C.1 (continued) 116

LIST OF FIGURES

Figure 1 Preparation of MPIA at (a) low and (b) high temperatures [15] .6

Figure 2 Wet spinning process of meta–aramid [22] .7

Figure 3 Nomex® honeycomb structure [27] .12

Figure 4 DSC results of m–aramid fiber after different DMSO treatment time at 60°C [57] .23

Figure 5 N–methyformanilide [61] .24

Figure 6 1–phenoxypropan–2–ol (left) and 2–phenoxyethanol (right) [63] 24

Figure 7 Molecular structures of Kevlar® and Nomex® [68] .27

Figure 8 The synthesis route of copolymerization of MeDMA and PEO45 by atom transfer radical polymerization [69] 28

Figure 9 K/S and exhaustion of UV–irradiated meta–aramid films [70] .29

Figure 10 Dye exhaustion (%) and K/S values on m–aramid fiber [73] .30

Figure 11 Comparison between plasma and wet processes [78] .31

Figure 12 SEM images of PMIA fibers treated by plasma under different amplitudes: (a) untreated; (b) treated with 60s; (c) treated with 120s; (d) treated with 180s [76] .31

Figure 13 Schematic Illustration of Procedure for Fabrication of PMIA−PDA/Ag Composite by Poly(dopamine)–Assisted [80] .32

Figure 14 SEM images of (a and b) silver coated PMIA fibers without functionalization of dopamine and (c and d) silver coated PMIA–PDA without exogenous reducing agent [80] .32

Figure 15 Illustration of Procedures for Preparing MPIA–PDA–KH560 Fibers (a)

and MPIA–(PDA+KH560) Fibers (b) [81] .33

Figure 16 ADMH grafting copolymerization and chlorination on the synthetic fibers [82] .34

Figure 17 Poly(Acrylic) acid as a textile sizing agent [84] .35

Figure 18 Polymerization of tertbutyl acrylate and hydrolysis [83] .36

Figure 19 Dehydration of PAA to form cyclic anhydrides [85] .37

Figure 20 Illustration of different methods of preparing graft copolymers [92] .39

Figure 21 Two common approaches to membrane surface modification with macromolecules Shown are strategies that lead to covalently bound polymer modifiers [94] .40

Figure 22 Modification of silica surface by “grafting–to” of poly(acrylic acid) [122] .45

Figure 23 Two different schemes to graft PAA onto silica particle [124] .46

Figure 24 Grafting reaction of cyclodextrin onto cellulose by the intermediate of a polycarboxylic acid bearing more than three carboxylic functions [129] .47

Figure 25 The grafting of poly(acrylic acid) (PAAC) layers on n–heptylamine (HApp) thin films via water–soluble carbodiimide (EDC/NHS) chemistry [130] .48

Figure 26 Amidization of nylon 6,6 using EDC [9] .49

Figure 27 Amidization of a nylon 6,6 using NHS [9] .50

Figure 28 Molecular structure of Toluidine Blue O (TBO) .52

Figure 29 X–rite Color Spectrophotometer .56

Figure 30 Measuring principle of bending rigidity with Kawabata system (KES–F2)

[135] .57

Figure 31 AATCC Crockmeter .57

Figure 32 Standard curve of Toluidine Blue O .59

Figure 33 Absorbance of Toluidine Blue O in terms of initial percentage of PAA solutions used for grafting to Nomex® IIIA .60

Figure 34 Circular cross–section of fiber with a singular coating layer .62

Figure 35 Amidization between carboxyl and primary amine .64

Figure 36 Amidization between anhydride and primary amine .65

Figure 37 Adsorption–grafting process of PAA onto nylon 66 [131] .66

Figure 38 K/S values of dyed Nomex® IIIA samples .67

Figure 39 Color strength of dyed Nomex® IIIA samples .68

Figure 40 Dried PAA–padded Nomex® IIIA .70

Figure 41 Cured PAA–padded Nomex® IIIA 71

Figure 42 Dyeing of PAA–grafted Nomex® IIIA .71

Figure 43 K/S–sum values of exhaust–dyed Nomex® IIIA samples with Basic Blue 17 as a function of curing durations for PAA concentration of 0.01 (wt%) at different curing temperatures .83

Figure 44 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after pre–cured at 2000C under different curing durations and PAA concentrations 84

Figure 45 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after

pre–cured at 2200C under different curing durations and PAA concentrations 84Figure 46 K/S–max values of pad–dyed (0.1 owf%) Nomex® IIIA samples after

pre–cured at 2400C under different curing durations and PAA concentrations 84Figure 47 K/S–sum values of Nomex® IIIA samples pad–dyed (0.1 owf%, Basic

Blue 41) as a function of PAA concentrations cured at different temperatures in three minutes .85Figure 48 Molecular structure of Basic Blue 41 .87

high as well, 272–2750C resulting from hydrogen bonding and chain rigidity [2] This advantage in performance turns out to be a disadvantage from an aesthetic viewpoint The coloration of aramid fibers, in general, is extremely difficult This is due, in part, to the crystallinity of the fiber Nomex® can be produced in staple fiber, filament yarn, industrial paper, and pressboard forms Both staple and filament yarns can be ordered undyed or producer–dyed; however, only limited colors can be obtained

As with many other materials, textile products are experiencing fast developments where, thanks to technological innovations, a wide range of products with novel applications have been developed Many of these value–added functions are closely related to textile surface properties Commonly used techniques for the surface modification of textile materials are described by Wei [3] Many methods have been devised to fulfill the current sustainability trends Inventors have been trying to make textile processing less environmentally damaging According to Sharma [4], the textile industry occupies the top position in the list of most polluting industries based on the volume and composition of discharged water This is the major driver for the application of new benign technologies However, due to the investment cost, commerciallizability, complexity in utilization, etc

traditional aqueous treatment is still the favored treatment in industry During wet processing treatments, chemicals and reagents are deposited onto the surface of textiles (which may either be involved or not in chemical reaction with fibers) to change the product’s surface properties through which new properties are added Better dyeability and improved dyeing yield are two typical objectives in textile wet processing [5-7]

In order to form covalent bonds between compounds with macromolecular weight substances, two possible approaches are applied which are known as “grafting to” and

“grafting from” Three major techniques employed are chemical grafting, radiation grafting and photo–grafting [8] According to Tobiesen and Michielsen [9], although many fibers have only a few graft sites, long polymer chains are able cover the entire surface of the substrate If “grafting from” is applied, monomers will be deposited onto the surface and then polymerized under different activation sources such as plasma [10], UV radiation [11], and γ–ray [12] However, this approach takes a long time to implement and it is difficult or even impossible to determine the molecular weight of the created polymer In the meantime, the

“grafting to” approach is much easier to carry out For example Cai [13] successfully grafted polyacrylic acid onto nylon 66 to study the photostability of surface–bound dyes The polymer used for the addition to the surface of the substrate can be fully characterized and known prior to grafting The most common methods in the textile industry for the application

of chemicals onto textile substrates are ‘pad–dry–cure’, UV irradiation and spraying as they are relatively easy and low cost finishing techniques [14]

In the current work, a chemical treatment process was carried out in order to provide

primary dye sites onto a meta–aramid fabric via the pad–dry–cure technique Specifically,

poly(acrylic acid) (PAA) was padded onto Nomex® IIIA fabric then cured at high

temperature to trigger a surface grafting of PAA chains onto the fabric An experimental design was constructed to implement a series of experiments using three different curing temperatures (2000C, 2200C and 2400C respectively) and three curing times (2, 3 and 4 minutes for each curing temperature) to determine the optimal pretreatment conditions (chemical concentration, curing temperature, and curing duration) to deposit the highest quantity of required chemical onto Nomex® IIIA fabric To evaluate the proposed procedure,

a theoretical quantification of the grafted PAA was carried out and the results were then

compared against the empirical values Thereafter, pretreated meta–aramid fabric was

colored by two different dyeing approaches, namely exhaust– and pad– dyeing processes, under both alkaline conditions at pH 10 and also neutral pH 7 to study their influences on color uniformity of dyed samples Besides color uniformity, the correlation between the amount of grafted PAA and color strength was also investigated using K/S measurements Another important factor in dyeing is crockfastness (wet and dry) which was also assessed for dyed Nomex® IIIA fabric, using AATCC test method 8, to examine how strongly dye molecules were fixed onto the grafted surface Finally, bending rigidity of pre–treated samples was measured with the Kawabata Evaluating System (KES F2) to determine how the proposed treatment technique affected fabric softness

CHAPTER 2 – PRIOR ART

2.1 Meta–aramid

2.1.1 History, polymerization, and production

Nomex®, poly(m–phenylene isophthalamide) (MPIA), belongs to the polyamide family because it has the amide group (–CO–NH–) in its chemical structure but is as an aromatic polyamide Because of the presence of aromatic backbones in the structure, aromatic polyamides possess better mechanical, thermal and chemical properties than the aliphatic polyamides (e.g nylons) The US Federal Trade Commission (FTC) defines wholly aromatic polyamides as synthetic polyamides in which at least 85% of the amide groups are bound directly to two aromatic rings1 [15]

1 Rules and regulations under the Textile Fiber Products Identification Act

(http://www.ftc.gov/os/statutes/textile/rr-textl pdf), Part 303.7 (Generic names and definitions for manufactured

Table 1 History of development of Aramid Fibers [16]

/Producer

Base Polymer

1938 Commercialization of Nylon

1965 Discovery of anisotropic polymer by P.F

Flory

1970 Discovery of air–gap spinning

(ii) PPDT

1976 Introduction of SVM fiber (formerly

Polyhetero arylene

1978 Development of Arenka aramid fiber

1987 Introduction of HMO–50 (Technora)

1996 Introduction of Trevar (discontinued later)

1997 Kevlar 49 HS by new fiber technology

p–aromatic

hydrocyclic copolyamide

MPDI – Poly(m–phenylene isophthalamide);

PBA – Poly(p–benzamide); and

PPDT – Poly(p–phenylene terephthalamide)

Among the aromatic polyamides, Nomex® (registered by DuPont USA) or meta–

aramid is the oldest It was a breakthrough material, which was officially commercialized at the beginning of 1960s Ever since that time the history of thermal and electrical insulation

has turned a new page [17] Besides Nomex®, there are several other commercial meta–

aramids, including Conex® (Teijin company, Japan) and Fenilon (the Soviet Union), but Nomex® and Conex® are the most widely used [18]

Table 2 Company, Aramid Type, and Brand Names of Commercial Aramids [15]

Company Aramid Type Brand name

Para–aramid

Nomex Kevlar Teijin

Meta–aramid Para–aramid

Copolymer ODA/PPTA

Teijinconex Twaron Technora

The synthesis of aromatic polyamides can be done via two routes (Figure 1), (1)

reaction between diacid chlorides and diamines at low temperatures and (2) direct polycondensation of aromatic diacids with diamines in solution at high temperatures (Yamazaki–Higashi method) No matter which route is applied, polar aprotic solvents are

frequently used (for instance HMPA, NMP, N,N–dimethylformamide (DMF), or N,N–

dimethylacetamide (DMA) [15]

Figure 1 Preparation of MPIA at (a) low and (b) high temperatures [15]

The dry spinning process of MPIA has been discussed in detail by Sweeny [19] and Carol [20] During the procedure, the polymer solution is extruded through a die and then comes in contact with a hot air current (at 2250C) to evaporate the solvent To improve the stability of the spinning solution, inorganic salts can be added (e.g CaCl2, LiCl2) [20, 21] In reality, complete evaporation of the solvent is avoided because the fiber should undergo a wet drawing process to improve its properties The speed of the spinning process under this method is rather fast, in the order of one hundred meters per minute [21]

Figure 2 Wet spinning process of meta–aramid [22].

Alternatively, meta–aramids can be produced by a wet spinning process (Figure 2)

where the dry polymer is dissolved at a low temperature in a 100% sulfuric acid to create an aramid dope This dope is heated to 1000C to create a clear solution which will then be pressed through a spinneret submerged completely in a water bath with a high concentration

of an inorganic salt to yield fibers In this bath, the sulfuric acid solvent dissolving the polymer is removed leaving only the fibers, which are then drawn, dried and heat–set to obtain excellent mechanical behavior [2, 22] Tai and his colleague [23] also invented a wet

spinning process specifically for meta–aramids where the spinning solution has a relatively

high concentration of salt In general, due to the high drag of the coagulation solvent, the production speed with this method is in the order of only ten meters per minute [21]

2.1.2 Products produced from meta–aramid

Meta–aramids can be manufactured into a variety of fiber products They can be in

staple or continuous filament yarns, industrial paper and pressboard

2.1.2.1 Staple yarn

Nomex® staple and tow are used in yarn manufacturing The fiber count is usually 1.5

or 2 denier per filament (dpf) In the meanwhile, staple fiber length is frequently from 1.5 to

6 inches and in some cases can even be cut with various lengths Many different materials can be made from Nomex® fibers Owing to their outstanding ability in thermal resistance as well as low flammability, Nomex® fibers are the first choice for protective apparel used in firefighting, military and sports uses Furthermore, Nomex® when transformed into fabrics (both staple and filament yarn) is used for industrial gas filtrations However, when elemental halogens are present, Nomex® fibers degrade and PTFE or Teflon® fibers are used instead [24]

2.1.2.2 Continuous filament yarn

Nomex® continuous filament yarns have low dyeability The yarn’s fineness is usually

in the range from 200 to 2400 denier with 2 dpf Several types of 200 denier yarns are 430,

431, 432, 433 and 434 Those belonging to the 1200–denier group are Type 430, 431 and 432

yarns All these filament yarns are frequently used in applications where the working condition involved high temperatures Lately, several new yarn types, available as dyeable yarns or as specific producer colors, are also produced (as shown in Table 3)

Table 3 Filament yarns of Nomex® Aramid [24, 25]

Type Grade Denier dtex Twist Filament

# Luster Color

430

2.1.2.3 Paper and pressboard

Nomex® paper types comprise 410, 411, 412, 414, and 418 and are provided in different thicknesses and densities depending on the final application Nomex® paper types are most widely used in the electrical industry, for instance armature slot insulation, wire wrap, phase insulator, wedges, lead insulations, end laminates, bushings, coil wrappers and interleaving, and crossover tubing and end caps in motors and generators Some are also used

in transformers, appliances, and for military applications [24]

Nomex® pressboard is another form of meta–aramid fiber There are several types

including 992, 993 and 994, whose densities, thicknesses, physical and electrical properties

are quite diverse (Table 4) These pressboards are popularly exploited as spacers and barriers

in transformers and end lams in motors [24]

Table 4 Nomex® paper and pressboard [24]

Type Thickness Density, g/cc End–use

418 3 – 14 0.08 – 0.36 1.0 – 1.1 High–voltage electrical insulation

419 7 – 13 0.18 – 0.33 0.5 High–voltage electrical insulation

2.1.3 Applications [21]

Meta–oriented aramid fibers can provide a good option to a broad domain of

applications where their core values canbe maximized In practice, protection and industrial

applications are the two principal areas for use of meta–aramid fibers

2.1.3.1 Protection

The most famous and prevalent usage of meta–aramid fibers is in protective garments

particularly developed to prevent workers from hazardous risks of heat and flame Specific areas of application include oil industry, electrical workers, and firefighter suits In the first two cases, direct contact with flame and fire rarely occur but firefighters often have to go into

the fire to do their job Thus, garments with low shrinkage property are required In order to

minimize the shrinkage of the garment, blending with para–aramid fibers improves the performance of the meta–aramid protective clothes that naturally have low shrinkage

In addition to functional performance, aesthetics of the product is another great concern The coloration of the fabric is in fact a major requirement To provide color for the fabrics, some are piece dyed which brings about a good range of colors; while some are dope–dyed or producer dyed fibers which provide better light fastness properties With protective garments, fibers are used in staple form then processed into yarns via conventional spinning technologies for cotton and wool When the material is used for construction of racing driver or pilot suits, filament yarns are utilized

2.1.3.2 Industrial applications

Aircraft seat covers should prevent fire from spreading to their proximity For such

applications meta–aramid fibers are normally blended with flame retardant wool fibers [26]

With good resistance to both chemicals and heat, these fibers are a good solution for hot air filtration Moreover, its exceptional thermal stability makes this fiber a widely used needle felt filter media for high temperature baghouse operation The automotive sector also has certain applications where such fibers may be needed Because of their decent fatigue

properties, meta–aramid fibers, in filament form, are used for mechanical reinforcement of

elastomeric hoses and belts

Figure 3 Nomex® honeycomb structure [27].

Not only in the textile industry but in the paper industry meta–aramids also play an important role During the preparation of paper, short fibers may be combined with meta–

aramid binding particles, namely “fibrids” This type of paper has good electrical insulation even under exposure to a constant temperature of 2200C Furthermore, Pinzelli and Loken [28] designed and developed specific papers (Nomex® 412) into honeycomb structures

(Figure 3) taking advantage of the lightweight, high stiffness and flame retardancy of meta–

aramids to manufacture parts for airplanes [27]

2.1.4 Coloration, finishing, and functionalization of meta–aramid

2.1.4.1 Coloration

It has been 50 years since the introduction of Nomex® to the market in 1967 This

meta–aramid fiber can be considered as the most important milestone in the fibrous thermal

resistant material The foundation of this success is based on the works of Wilfred Sweeney (1926–2011) who was awarded the Lavoisier Medal in 2002 [29] A number of enhancements have been applied to this aromatic polyamide to improve its performance

including the coloration of the fiber, which is still one of the most difficult aspects of processing this fiber

Generally, dye molecules diffuse into non–crystalline or low order regions of a fiber

This implies that fibers whose polymer chains are highly oriented (e.g meta–aramid, para–

aramid, etc.) may be very difficult to dye Solutions devised to overcome such problems include the utilization of plasticizing agents, application of certain dyes which have higher diffusion ability into fibers, or the use of dyeing machinery that contain pressurized chambers [30]

The very first procedure to dye Nomex® was introduced by two researchers working at

Du Pont Co., Wilmington, Delaware, USA [31] The Nomex® used in that study was the E–8 which is normally used for aircraft furnishings Schumm and Cruz [31] tested many carriers

in the dyeing of Nomex® to determine the most appropriate solution Table 5, shows a list of carriers and the strength of the dyed fiber expressed in K/S value K/S increases with an increase in dyeing strength Results indicate that the three best carriers are Chemocarrier KD5W, Latyl Carrier A, and β–Naphthol

Table 5 Effectiveness of different carriers [31]

Carrier K/S of dyed fiber

properties are little affected in the pH range of 2 to 9 In addition, pH lower than 3 is a standard practice for all shades in which Cl Basic Green 6 is used The fabric could be dyed

in piece, stock, and yarn forms Some combinations of dyes to produce certain shades are shown in Table 7

Another attempt to dye meta–aramid fibers was carried out by Richardson and Walck

in 1970 [33] In that work, the authors used a copolymer of poly(m–phenylene isophthalamide) and poly[N,N’–m–phenylene bis(meta amino benzamide)isophthalamide] to produce tows which were then cut into staple fibers 2 inches in length The cut fibers were, in turn, converted into pads and then dyed at a pressure of 20 p.s.i.g The dye bath comprised a basic dye and an organic dye carrier The outcome revealed that spun copolymer fibers without super atmospheric pressure had low dye pickup even with the highest amount of Na–SMPD (sodium salt of 2,4–diaminobenzene sulfonic acid) Thus, besides the pressure, they found that a dye assistant and an adequate amount of Na–SMDP were required Furthermore,

it was recommended that the filament tow must not be heated while taut

Table 6 Fastness of dyeing on fabrics of Nomex E–8 [31]

Xenon Arc (20 Hrs.)

SC

Modified ASTM D620–S7T (192 Hrs.) SC

Washing IIIA SC

Dry Clean

SC

Crocking

Dry rating

Wet rating

Table 7 Dye combinations for certain shades for protective clothing [32]

CI No Brown Olive Navy Black

Pretreatment of polyaramid has also been suggested as a solution to dyeability of the

fiber Hermes [34] suggested a thermal treatment of shaped articles made of polyaramid,

where at first the articles would pass through a hot bath of a high boiling point organic liquid (e.g glycols, glycol ethers, solvents or solvent blends) at atmospheric pressure Thereafter, the pretreated articles would be dyed in another hot bath containing a conventional organic dyestuff (dissolved or dispersed) in a high boiling point organic liquid This process could either be batchwise or continuous and shaped articles could be yarns (staple or filament), tow

or fabric, etc The merit of this invention was the short treatment time, which was favored by industry Another improvement was done by Preston and Hofferbert [35] In their research, the authors pointed out the drawback from prior art was the poor tensile properties of dyed polyaramids In order to overcome all demerits from previous studies, Richardson and Walck

[33] used pyridine for the aqueous dyeing of polyaramids Preston and Hofferbert [35]

confirmed that the usage of pyridine, to a certain degree, had provided good depth of shade and clean shade on bright fibers, had retained good fiber tensile properties and given adequate dyed lightfastness An advantage of pyridine was its water solubility, therefore, after the dyeing process it was completely removed resulting in excellent lightfastness of

dyed fabrics as confirmed by the authors Moore and Weigmann [36] examined the chemical

energy provided from suitable solvents (dimethylformamide (DMF), dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO)) as a means of modifying fiber structure [37] to help improve the dyeability

To improve the affinity of basic dyes for high molecular aromatic polyamides, Nischk and his colleagues [38] invented a method to modify the molecular structure of polyamides with disulfone imide Although there were no specific descriptions confirming the approach

could be applied to meta–aramids, the results showed positive signals To determine the

improvement of dyeability, treated fibres were dissolved in 25 cc of dimethyl acetamide and the extinction values at 475 mμ compared with pure dimethyl acetamide were measured in a photometer The extinction values of modified polyamide were always higher than that of unmodified homopolycondensate product The claimed novel polyamide comprised a conventional high molecular weight aromatic polyamide and 30 to 100 molar percent of the disulfone imide segments

Swelling the polyaramid is another solution to overcome the dyeability difficulties of the fiber In 1984 Kelly realized that preceding endeavors to color aromatic aramid substrates normally required high temperature (212oF or higher), but resulted in poor wash fastness and color fading under exposure to light [39] To overcome these issues, he swelled polyaramid fibers and introduced into those fiber substances that could form strong bonds with anionic dyes into the fiber These substances were amines or substituted amines (both aliphatic and aromatic) For convenience both tasks were done at the same time, thus the swelling solvent had to dissolve the amines as well (e.g dimethylformamide, dimethylsulfoxide and dimethylacetamide) The detailed explanation of this successful patent was provided by Moore and Weigmann [36] Wolf and his co–workers [40] chose a different way to insert

color into polyaramids In this invention, just dry–spun filament yarns went through a dyebath (containing either cationic or anionic dyestuff) prior to or during the stretching stages in the production line The claimed advantage was that the aramid filament yarns had not been modified with any acid or basic groups The proposed temperature in this invention was preferably in the range of 500C – 800C to give the best outcome The solvents used were polar solvents which included dimethylacetamide, N–methyl pyrrolidone, dimethylformamide or hexamethyl phosphoric acid tris amide

In order to have a continuous or semi–continuous dyeing process for meta–aramid

fibers in combination with the enhancement of flame resistance, a series of patents [41-43] have been issued to researchers from Burlington Industry Inc in Greensborough, North

Carolina, USA In all of the suggested procedures, the meta–aramid materials went through

solutions containing swelling agents before being treated with dyes and flame retardants They could also use this process to add flame retardants to improve the substrates flame resistance The LOI for Nomex® T–455 fabric, dyed and flame–retardant treated from this invention, reached 39% compared to 26.6% for the greige Nomex® T–455 fabric Printing of Nomex® fabric by treating N–cyclohexyl–2–pyrrolidone prior to the application of printing

paste was as well executed [44] and shaped articles made of meta–aramid fibers could be

printed to improve flame resistance [45] However, Riggins and Hauser [46] pointed out that

these inventions [41-43] did not use conventionally available machinery and therefore the

process had to be improved with common machinery and thus N–cyclohexyl–2–pyrrolidone

was used to promote diffusion Moreover, this process was specifically suitable for meta– aramids only and few improvements were obtained when applied to para–aramid (e.g

Kevlar®) One year later, another patent [47] was disclosed that improved the previous one

[46] and deployed conventional pressure jet dyeing machines In this upgraded procedure, Riggins and Hauser provided manufacturers with an ability to simultaneously dye meta–

aramid fibers while improving their fire resistance without lessening the fiber’s inherent strength Additionally, the task ensured a high level of fire retardancy with an LOI of 37% – 44% as claimed by Cates [42, 43] Later, Johnson [48] devised another process to impart both color and fire retardancy onto Nomex® substrates The work related to a thermosol/pad process where a “neat” solution of fire retardant contained the required amount of disperse/acid dyestuff This solution was then padded at room temperature onto the substrate and thereafter heated (3500F to 3900F) for 10 seconds to 2 minutes Treated fabrics/fibers were rinsed by a halogenated hydrocarbon (perchloroethylene) The process was also applicable in print form using a paste or via immersion of the whole fabric into a hot bath containing necessary chemicals Thermal treatment and rinsing steps were exactly similar for different routes chosen This work claimed to confer better color fastness and fire resistance for treated Nomex® The fire retardant used was cyclic phosphonate esters, which swell the Nomex® fibers to create openings for the insertion of dyestuff molecules The author also suggested the utilization of common chemicals such as N–methyl pyrrolidone, dimethylsulfoxide (DMSO) and dimethylacetamide to improve the swelling ability of the solution Another work [49] involved coloring MPDI material with acid dyes via printing operation In this method, a solution of diamine salt (hexamethylenediamine dihydrochloride) and a surfactant were imbibed into never–dried MPDI fibers to confer better printing and/or overprinting properties

Ghorashi [50] introduced another method to dye tows of crystalline poly(meta–

phenylene isophthalamide) fibers or filaments with water–insoluble dyes First, an aqueous

dispersion of water–insoluble dye was padded onto the tow Second, the tows were steam heated at a temperature higher than dye–activation temperature but lower than glass transition temperature of the MPIA This temperature can reach up to 1650C so the dye can gradually diffuse into the MPID fibers In short, the task was carried out at low temperature and over a short period (no longer than 30 minutes) More importantly, no swelling agents or carriers were used By modifying dry spinning stages, Headlinger and collaborators [51-53]

successfully obtained patents for the creation of low shrinkage meta–aramid yarns with good

coloration values The extruded filaments after going through the quenching step were conditioned in a solution at 300C or up to 1000C during which the drawing was also implemented The filaments were thereafter washed, dried and heat treated up to 3000C in 0.5

to 5 seconds This process was claimed to be continuous and darkened the color shade

A common nuisance encountered with colored wholly aromatic polyamides (including meta type) is that the light resistance is quite weak, especially when a basic dye is used In order to overcome this problem, prior to the dyeing process, wet or dry spun aromatic polyamide fibers should be treated by dipping in a UV–shielding solution Nonetheless, during the dyeing process utilizing carriers, these “transporters” will shed the UV–shielding substances Another approach was the use of alkylbenzene sulfonic acid onium salt as a dyeing assistant but this was considered costly in practice Finally, pigments resistant to fading under light could be selected but this can result in loss of production time due to aggreation of pigment crystals, difficulty in small–lot production and restricted hues (selected color is dependent on market trend and has to be stored in large quantities If color change is demanded, the extrusion equipment needs a thorough and complete cleaning) [54] Lately, a Japanese research group [55] disclosed a patent which claims to have overcome all the