**Protein Fibre Surface Modification**

## Jolon Dyer1,2,3,4 and Anita Grosvenor1

*1AgResearch Lincoln Research Centre, Christchurch, 2Lincoln University, Canterbury, 3Biomolecular Interaction Centre, University of Canterbury, Christchurch, 4Riddet Institute at Massey University, Palmerston North, New Zealand* 

#### **1. Introduction**

Many natural fibres, including wool, cashmere and silk, are protein-based materials; the dry weight of wool is almost entirely derived from proteins (Maclaren & Milligan 1981). As such, they possess an inherent structural and chemical heterogeneity not found in synthetic polymers. Although typically less heterogeneous than biological fibres, the rapidly emerging range of commercially available protein-based biomaterials also contain a wide range of functionality derived from their constituent primary and secondary protein structure.

The response of fibres to processes such as dyeing and finishing treatments correlates directly to their structural and chemical properties, and this is particularly true for surface treatments. Due to its barrier function in the fibre, modification of the surface has a profound impact on processing and performance. Keratinous fibres such as wool and cashmere have an outer lipid layer which results in a hydrophobic surface. Recently a range of innovative and novel fibre surface technologies has been developed, many of which involve altering surface properties by the removal of the lipid layer, which exposes a proteinaceous surface with a variety of reactive chemical moieties. Treatments that can be covalently bound to fibre surface components, rather than simply physically applied to the surface, offer the potential for superior durability.

The surface modification of proteinaceous fibres has a long history. These include plasma applications, which expose and generate functional groups on the protein surface, etching into the surface of the cuticle scales, to improve properties such as surface wettability, dyeability, shrink-resistance and felting-resistance. Chemical approaches utilised include ozone treatments, which cause oxidation of the surface and altered ionic balance, leading to a more plastic and reactive fibre surface and shrinkage control; chlorination, which improves sorption characteristics and reduces shrinkage; hydrogen peroxide treatment; and acid anhydride acetylation of silk and wool, which improves the textile response to dyeing, shrink resist, and setting treatments. Enzymatic treatments have been utilised to decuticulate the surface and improve properties such as shrink resistance. More recent developments include reaction of functional chemical agents or branched molecules to exposed reactive groups on the fibre surface, enabling the attachment of covalently-bound smart treatments, or the amplification of reactive groups for increased functionality.

This chapter outlines developments in the area of targeted surface modification of proteinbased fibres and textiles, including summarising applications and future directions. It is not

Protein Fibre Surface Modification 113

Yuen 2007). As an alternative to ablation, with the use of carbon- and hydrogen-rich plasma

The physical changes (roughening) caused by ablative plasma treatments result in altered yarn strength, frictional and spinning properties and decreased felting behaviours (Höcker 2002). A detrimental effect is a firm or harsh handle (Rakowski 1997). The chemical changes (surface oxidation and the loss of surface-bound fatty acids) result in improved dyeing and resistance to shrinkage (Rakowski 1997). Shrink-resistance is obtained at levels comparable to those obtained from other treatments (like chlorination, reduction and resin application) by the addition of resin, causing surface smoothing. Dyeing depth, speed and bath exhaustion are also improved (Höcker 2002). Atmospheric pressure plasma treatment is also seen to improve the dyeability of mohair (Demir 2010). An unusual application of wool plasma treatment is the deposition of antibacterial polymers for woollen wound dressings

Due to the wide variety of techniques utilised to treat protein-based fibres with plasma, it can be difficult to compare the effects reported by different groups. Relating this problem to dyeing, Naebe *et al*. showed that the effect of plasma on dyeing depended on the dye used – with hydrophobic dyes unaffected by plasma treatment, and hydrophilic dyes greatly affected (Naebe *et al.* 2010). One general feature of plasma-based approaches to modifying the surface of protein materials is the high level of surface oxidation observed after

Hydrophilicity can be a desirable attribute for textiles, allowing improved dyeing, and enhancing comfort and wear properties. The natural hydrophobic surface of wool makes it practically impossible to print wool fabrics without prior treatment. Plasma treatment improves fabric wettability, and also reduces felting (Kan & Yuen 2007). This can be accomplished by the removal of surface lipids (oxygen plasma) or by the deposition of hydrophilic monomers, such as the plasma-assisted deposition of acrylic acid (Kutlu *et al.* 2010). Aside from dye properties and processing, the wool dyeing rate is primarily determined by fibre morphology and by the state of adsorbed water in the fibre (Ristić *et al.* 2010). These properties may be modified by a variety of surface treatments. When the outer lipid layer is even partially removed from wool, the wettability and dyeing rate are reduced. Plasma treatments remove this lipid layer and generate functional groups (such as thiols) that are more reactive to certain dyes. Corona discharge has been reported to incorporate oxygen atoms into the fibre and enhances wettability, which increases the acid-dye intensity of printed fabrics (Ryu *et al.* 1991). Combining plasma treatments with chitosan results in increases in colour intensity and dyeability over those observed with either treatment alone

UV/ozone (UVO) treatments also achieve textile improvement effects able to compete with aqueous treatments. UV radiation at certain frequencies generates atomic oxygen and ozone from molecular oxygen (at 184.9 nm) and atomic oxygen from ozone (at 253.7 nm). Organic hydrocarbons may also be excited at 253.7 nm. Textiles placed close (within 5 mm) of these UVO sources are cleaned by the reaction of UV-excited surface molecules with atomic

This surface treatment results in more wettable wool, which improves dyeing and printing properties, even at low temperature (Xin *et al.* 2002). The wool also yellows, although this can be lessened when the process is combined with peroxide pad bleaching (Shao *et al.*

oxygen. The volatile reaction products desorb from the surface (Shao *et al.* 1997).

gases, polymers may be deposited on the surface (Kan & Yuen 2007).

using fluorinated post-discharge plasmas (Canal *et al.* 2009).

treatment (Meade *et al.* 2008b).

(Ristić *et al.* 2010).

**2.2 UV/Ozone** 

intended to cover every aspect of fibre surface modification, but rather focus on key types of surface modification that exploit the unique properties of proteinaceous fibres.

#### **2. Physical modification**

The increasing pressure for environmentally friendly processing approaches has motivated the textile industry to exchange aqueous treatments involving potential chemical pollutants and effluents with high adsorbable organic halide (AOX) contents for physical 'dry' processes, such as plasma and UV/ozone treatment. With respect to dyeing, the nature of physical modification to protein fibre surfaces and where this occurs in production are important and must be taken into consideration.

#### **2.1 Plasma**

Plasma treatments, which utilise a gaseous electrical discharge, are reported to be surface specific for protein fibres (Höcker 2002) and offer significant potential in terms of being simple, clean, solvent-free and relatively inexpensive. These treatments can be used to modify surfaces by the deposition of polymers or can 'clean' the surface by surface etching. Plasma treatments are increasingly replacing wet (chemical) textile treatments to achieve outcomes such as shrink-resistance and improved dyeability. Plasma and corona treatments oxidise the surface of protein-based textiles, generate chemically active radicals, induce functionalisation, and etch the surface (Höcker 2002; Ceria *et al.* 2010). Those utilising certain gases may result in the deposition of atoms from those gases, such as fluorine (Höcker 2002). The effects of plasma treatments are restricted to the wool surface, and are therefore unlikely to result in changes to the bulk properties resulting from damage to the interior of the fibres.

Within the context of the textiles industry, plasma means the products of the interaction of an electromagnetic field with gas; namely, a partially ionised gas that contains ions, electrons and neutral particles (Kan & Yuen 2007). At gas pressures similar to atmospheric pressure and high voltage, corona discharge is generated. At gas pressures of 0.1-10 MPa at lower voltage, glow discharge is generated (Rakowski 1997; Kan & Yuen 2007). Atmospheric corona discharge plasmas lack uniformity due to their filamentous nature (Prat *et al.* 2000). Glow discharge is most commonly referred to when plasma textile processes are described. Many plasma treatments have employed low pressure (vacuum) systems to keep the plasma stable, called cold plasma or non-equilibrium plasma. Lowered pressure also assists with the penetration of plasma effects through the thickness of a textile (Poll *et al.* 2001). Atmospheric pressure plasma systems, however, have an industrial advantage for large-scale textile applications because of the expense, time and space involved in maintaining a vacuum (Sugiyama *et al.* 1998; Demir 2010). These require a source frequency of 1-20 kHz and a carrier gas of helium (Prat *et al.* 2000). Microwave-induced glow-discharge plasma can be kept stable at atmospheric pressure (Sugiyama *et al.* 1998). Several types of discharge may be used to generate plasma, including direct current discharge, radio-frequency discharge, and microwave.

In wool, plasma treatment oxidises and partially removes (ablates) the hydrophobic surface lipid layer (both the loosely adhering lipids and those covalently bound). The disulfide bonds in the surface protein layer of wool (epicuticle) are also oxidised (Höcker 2002). The surface is the only part of the fibre affected; this becomes rougher (surface area increases), while its protein contents are hardly affected (Höcker 2002). The free radicals that remain on the wool surface following ablative plasma treatment stimulate the formation of functional groups and of bonds between the fibre surface and customised surface coatings (Kan &

intended to cover every aspect of fibre surface modification, but rather focus on key types of

The increasing pressure for environmentally friendly processing approaches has motivated the textile industry to exchange aqueous treatments involving potential chemical pollutants and effluents with high adsorbable organic halide (AOX) contents for physical 'dry' processes, such as plasma and UV/ozone treatment. With respect to dyeing, the nature of physical modification to protein fibre surfaces and where this occurs in production are

Plasma treatments, which utilise a gaseous electrical discharge, are reported to be surface specific for protein fibres (Höcker 2002) and offer significant potential in terms of being simple, clean, solvent-free and relatively inexpensive. These treatments can be used to modify surfaces by the deposition of polymers or can 'clean' the surface by surface etching. Plasma treatments are increasingly replacing wet (chemical) textile treatments to achieve outcomes such as shrink-resistance and improved dyeability. Plasma and corona treatments oxidise the surface of protein-based textiles, generate chemically active radicals, induce functionalisation, and etch the surface (Höcker 2002; Ceria *et al.* 2010). Those utilising certain gases may result in the deposition of atoms from those gases, such as fluorine (Höcker 2002). The effects of plasma treatments are restricted to the wool surface, and are therefore unlikely to result in changes to

Within the context of the textiles industry, plasma means the products of the interaction of an electromagnetic field with gas; namely, a partially ionised gas that contains ions, electrons and neutral particles (Kan & Yuen 2007). At gas pressures similar to atmospheric pressure and high voltage, corona discharge is generated. At gas pressures of 0.1-10 MPa at lower voltage, glow discharge is generated (Rakowski 1997; Kan & Yuen 2007). Atmospheric corona discharge plasmas lack uniformity due to their filamentous nature (Prat *et al.* 2000). Glow discharge is most commonly referred to when plasma textile processes are described. Many plasma treatments have employed low pressure (vacuum) systems to keep the plasma stable, called cold plasma or non-equilibrium plasma. Lowered pressure also assists with the penetration of plasma effects through the thickness of a textile (Poll *et al.* 2001). Atmospheric pressure plasma systems, however, have an industrial advantage for large-scale textile applications because of the expense, time and space involved in maintaining a vacuum (Sugiyama *et al.* 1998; Demir 2010). These require a source frequency of 1-20 kHz and a carrier gas of helium (Prat *et al.* 2000). Microwave-induced glow-discharge plasma can be kept stable at atmospheric pressure (Sugiyama *et al.* 1998). Several types of discharge may be used to generate plasma, including

In wool, plasma treatment oxidises and partially removes (ablates) the hydrophobic surface lipid layer (both the loosely adhering lipids and those covalently bound). The disulfide bonds in the surface protein layer of wool (epicuticle) are also oxidised (Höcker 2002). The surface is the only part of the fibre affected; this becomes rougher (surface area increases), while its protein contents are hardly affected (Höcker 2002). The free radicals that remain on the wool surface following ablative plasma treatment stimulate the formation of functional groups and of bonds between the fibre surface and customised surface coatings (Kan &

surface modification that exploit the unique properties of proteinaceous fibres.

**2. Physical modification** 

**2.1 Plasma** 

important and must be taken into consideration.

the bulk properties resulting from damage to the interior of the fibres.

direct current discharge, radio-frequency discharge, and microwave.

Yuen 2007). As an alternative to ablation, with the use of carbon- and hydrogen-rich plasma gases, polymers may be deposited on the surface (Kan & Yuen 2007).

The physical changes (roughening) caused by ablative plasma treatments result in altered yarn strength, frictional and spinning properties and decreased felting behaviours (Höcker 2002). A detrimental effect is a firm or harsh handle (Rakowski 1997). The chemical changes (surface oxidation and the loss of surface-bound fatty acids) result in improved dyeing and resistance to shrinkage (Rakowski 1997). Shrink-resistance is obtained at levels comparable to those obtained from other treatments (like chlorination, reduction and resin application) by the addition of resin, causing surface smoothing. Dyeing depth, speed and bath exhaustion are also improved (Höcker 2002). Atmospheric pressure plasma treatment is also seen to improve the dyeability of mohair (Demir 2010). An unusual application of wool plasma treatment is the deposition of antibacterial polymers for woollen wound dressings using fluorinated post-discharge plasmas (Canal *et al.* 2009).

Due to the wide variety of techniques utilised to treat protein-based fibres with plasma, it can be difficult to compare the effects reported by different groups. Relating this problem to dyeing, Naebe *et al*. showed that the effect of plasma on dyeing depended on the dye used – with hydrophobic dyes unaffected by plasma treatment, and hydrophilic dyes greatly affected (Naebe *et al.* 2010). One general feature of plasma-based approaches to modifying the surface of protein materials is the high level of surface oxidation observed after treatment (Meade *et al.* 2008b).

Hydrophilicity can be a desirable attribute for textiles, allowing improved dyeing, and enhancing comfort and wear properties. The natural hydrophobic surface of wool makes it practically impossible to print wool fabrics without prior treatment. Plasma treatment improves fabric wettability, and also reduces felting (Kan & Yuen 2007). This can be accomplished by the removal of surface lipids (oxygen plasma) or by the deposition of hydrophilic monomers, such as the plasma-assisted deposition of acrylic acid (Kutlu *et al.* 2010). Aside from dye properties and processing, the wool dyeing rate is primarily determined by fibre morphology and by the state of adsorbed water in the fibre (Ristić *et al.* 2010). These properties may be modified by a variety of surface treatments. When the outer lipid layer is even partially removed from wool, the wettability and dyeing rate are reduced. Plasma treatments remove this lipid layer and generate functional groups (such as thiols) that are more reactive to certain dyes. Corona discharge has been reported to incorporate oxygen atoms into the fibre and enhances wettability, which increases the acid-dye intensity of printed fabrics (Ryu *et al.* 1991). Combining plasma treatments with chitosan results in increases in colour intensity and dyeability over those observed with either treatment alone (Ristić *et al.* 2010).

#### **2.2 UV/Ozone**

UV/ozone (UVO) treatments also achieve textile improvement effects able to compete with aqueous treatments. UV radiation at certain frequencies generates atomic oxygen and ozone from molecular oxygen (at 184.9 nm) and atomic oxygen from ozone (at 253.7 nm). Organic hydrocarbons may also be excited at 253.7 nm. Textiles placed close (within 5 mm) of these UVO sources are cleaned by the reaction of UV-excited surface molecules with atomic oxygen. The volatile reaction products desorb from the surface (Shao *et al.* 1997).

This surface treatment results in more wettable wool, which improves dyeing and printing properties, even at low temperature (Xin *et al.* 2002). The wool also yellows, although this can be lessened when the process is combined with peroxide pad bleaching (Shao *et al.*

Protein Fibre Surface Modification 115

Keratin fibres such as wool, human hair and cashmere are covered in an outer lipid layer which is covalently bound to the surface to form a hydrophobic barrier. The major component of the surface wool lipids is 18-methyleicosanoic acid (18-MEA). 18-MEA is attached to the underlying protein mainly via covalent thioester bonding. A range of treatments have been reported to cleave the thioester bonds to form thiols on the epicuticle surface (Meade *et al.* 2008b). The generation of reactive surface sulfhydryl groups, with the sulfur able to act as a strong nucleophile, make these thiols attractive potential sites for subsequent covalent attachment of novel surface modifications (Meade *et al.* 2008a). Most research and development in this area has been performed with wool, but the principles

Wool fibres are comprised of a core of cortical cells surrounded by an outer sheath of overlapping cuticle cells. Each cuticle cell is enclosed within a resistant membrane termed the epicuticle (Höcker 2000). The epicuticle of wool covers the cuticle, and is comprised of both proteins and lipids (fatty acids); the hydrophobicity of the wool surface is largely attributable to this external lipid layer. The fatty acid component of the epicuticle accounts for approximately quarter of the epicuticle mass, with the surface-bound fatty acids forming a hydrophobic surface layer (Meade *et al.* 2008b). The branched chain fatty acid 18 methyleicosanoic acid (18-MEA) has been identified as the major lipid component of the wool surface, comprising approximately 65-70% of the surface lipid content (Negri *et al.* 1991; Ward *et al.* 1993). This 18-MEA is covalently bound to surface proteins via thioester linkages to cysteine, with the epicuticle estimated to have a content of 35% half-cystine (Negri *et al.* 1993; Evans & Lanczki 1997). The identity of the proteins that the surface lipids are bound to, forming proteolipids, is not yet well understood (Dauvermann-Gotsche *et al.* 1999). Thioesters are a relatively reactive group that can be cleaved relatively readily

If this outer lipid layer is removed in a controlled manner, it is possible to expose the underlying proteinaceous surface so that there are a variety of reactive functional groups (including hydroxyl, carboxyl and amine moieties) available for potential covalent attachment of surface treatments. There are various alkaline reagents that have been used to release covalently bound surface lipids from wool, yielding a hydrophilic and anionic

The use of alcoholic or aqueous alkali conditions cleaves the thioester bonds to form a substituted lipid and a thiol at the epicuticle surface (Negri *et al.* 1991; 1993; Dauvermann-Gotsche *et al.* 2000) The chemical mechanism for nucleophilic substitution of the thioester

Alcoholic alkali treatments use sodium butoxide, potassium tert butoxide, potassium hydroxide and hydroxylamine in water or in an anhydrous solvent such as tert butanol, dehydrated butanol, or ethanol (Leeder & Rippon 1983; Brack *et al.* 1996; Taki 1996; Meade *et al.* 2008b). These treatments are applied to control felting shrinkage, to improve polymer application, dyeing and printing, shrink resistance, and electrical conductivity (Leeder & Rippon 1983; Leeder *et al.* 1985). Alcoholic alkali treatment is less damaging than chlorination, as the effects are limited very much to the surface. The cortex is assumed to be unaffected; only the cuticle is removed, or de-scaled (Taki 1996). Lipid material is removed and a polar surface is generated. Covalently bound lipids are removed from the surface, revealing the polar surface of proteins beneath (Leeder & Rippon 1985; Brack *et al.* 1996; Meade *et al.* 2008b). As potassium t-butoxide is not found to be as good at removing the

surface with an increased frictional coefficient (Dauvermann-Gotsche *et al.* 2000).

bond is shown in Figure 1 [modified from Meade *et al*. 2008b].

have potential for application in other mammalian fibres used in textiles.

through nucleophilic substitution reactions.

**3.5 Delipidation** 

2001). This is due to oxidation of the surface layers: Cysteic acid is detected, as disulfide bonds are broken. Carboxylic acid and carbonyl groups are detected at higher levels than after chlorination (Shao *et al*. 1997). The lipids at the surface are modified or volatised. UVO treatment results in a colour yield and dyeability comparable to that obtained after chlorination, permitting printing (Shao *et al*. 1997; 2001).
