Physicochemical Wool Functionalization to Achieve Anti-Felting Characteristics

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The 19th Romanian Textiles and Leather Conference
November 7– 9, 2024, Iași, Romania
DOI: 10.2478/9788367405829-027
© 2025 Melinda Pruneanu et al. This is an open-access article licensed under the Creative Commons Attribution-
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176
PHYSICOCHEMICAL WOOL FUNCTIONALIZATION TO ACHIEVE
ANTI-FELTING CHARACTERISTICS
Melinda PRUNEANU, Ingrid Ioana BUCIȘCANU, Romeo PRUNEANU
Gheorghe Asachi Technical University of Iași, Faculty of Industrial Design and Business Management, Iasi, Romania
melinda.pruneanu@academic.tuiasi.ro
Abstract. The present study aims to synthesize a water-soluble oligomeric reactive product, which can act as
an anti-felting agent of wool fibers. The potential anti-felting agent is an isocyanate-bisulfite adduct with
blocked isocyanate groups (HDMIS), obtained by solubilization of 1,6 hexamethylene diisocyanate (HMDI) in
aqueous medium with a nonionic surfactant, followed by sulfitation with sodium bisulfite. Raising pH to 8.0-
8.5 unblocks the isocyanate group, which reacts with the keratin functional groups and establishes cross-links
between HDMIS and wool surface. The FT-IR spectrum of the hexamethylene diisocyanate-bisulfite adduct
confirmed the complete blockage of the -NCO groups by sulfitation, necessary to increase the water solubility
and to regulate the tendency of the isocyanate group to hydrolyze due to its increased reactivity. The HDMIS
product was applied to the wool fibers by the exhaustion method. The surface of the treated wool fiber and the
treatment performance were studied by scanning electron microscopy (SEM). The SEM images of the wool
fibers treated with HMDIS compared to the untreated ones, showed that HDMIS was uniformly deposited on
the hydrophobic wool surface and covered the epicuticular scales, making the wool fiber surface smoother and
the scales less available for entangling. A certain swelling effect of the keratin matrix that confirms the
physicochemical functionalization of the wool fiber was also noticed. An isocyanate derivative synthesized and
applied by straightforward procedures imparted properties to wool fiber necessary for obtaining an anti-
felting effect.
Keywords: keratin, wool shrinking, oligomeric reagents, hexamethylene diisocyanate, isocyanate-bisulfite
adduct.
1. INTRODUCTION
Wool is a natural protein fiber with a complex morphological structure, which has been used since ancient
times for the manufacture of clothing, household articles, carpets, building materials, etc. due to its excellent
thermal insulation, hygroscopic and fireproofing properties, and for obtaining textiles with special,
multifunctional properties such as anti-soiling, antistatic, or oil absorption. However, wool fibers are
susceptible to felting, which produces shrinkage of wool fabrics and is detrimental to the aesthetic
appearance and dimensional stability of woolen fabrics and properties related to specific density and
mechanical strength, due to loss of keratin substance [1, 2].
Wool felting is an irreversible process consisting in the compressive contraction of fibers; it is strongly
influenced by temperature, humidity, liquor ratio, the intensity of mechanical stress during wet processing,
the type of chemical auxiliaries employed, and centrifugation and drying parameters. The felting effect is
due to the unique scaly structure of the cuticular layer of the wool fiber. It occurs because of the friction
between the tips of the scales relative to the direction of movement, which is opposite to the orientation of
the scales. This friction causes the fibers to entangle with adjacent fibers, increasing the friction coefficient
and ultimately leading to a macroscopic contraction of up to 50-55% of woolen fabrics and apparel, as
illustrated in Figure 1 [3, 4].
Shrinkage resistance (anti-felting capacity) of wool fabrics can only be achieved by significantly reducing
the friction between scales so that they do not shift during washing or using the manufactured articles.

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Therefore, applying an effective and permanent anti-felting treatment to increase the shrinkage resistance
of wool fibers is of paramount importance. This is a critical issue that has led to many scientific studies
worldwide.
Figure 1. The felting effect and the morphological appearance of wool fibers without and with an anti-felting
treatment.
The specialty literature describes three main types of anti-felting processes applicable to wool fabrics: (1)
based on the use of chlorine-based compounds, (2) based on chlorine-free chemical products, and (3) non-
conventional, sustainable processes.
The methods in the first category, such as chlorinated-Hercosett [5], Kroy Hercosett [5, 6], and DCCA-
Hercosett [7], are the industrial standard for producing washable wool because they provide a contraction
of only 0.5 - 1.2% to woolen items. However, they have the disadvantage of significant losses of keratin
substance and the contamination of effluents with chlorinated compounds containing AOX, which are
harmful to the environment. As a result, these methods have been banned in the EU countries. Mori and
Matsudaira [8] reported the use of calcium hypochlorite (Ca(ClO)
2
) in combination with hydrogen peroxide
(H
2
O
2
), but the achieved shrinkage resistance was lower compared with the acidic chlorination process [8].
The most popular chlorine-free antifelting processes are Sirolan ZAOX, based on the use of poly(p-
methylstyrene) (PMS) and silicone resins, and those using peroxy-mono sulfuric acid (PMS) [4]. These
methods provide wool fabrics with a contraction of less than 10%, which is less performant than that
achieved by the chlorinated Hercosett method [9].
Amongst the sustainable anti-felting methods for woven and knitted woolen fabrics, the Securlana process
[10, 11] is the most popular. This process uses a strong reducing agent (e.g., sodium sulfite) and a polyether
(Securlana K). It achieves an acceptable level of contraction resistance; however, this treatment affects the
colour fastness and resistance to soiling of the treated wool fabrics. The use of chitosan alone or mixed with
PMS [12], and proteolytic enzymes [13] reduces the contraction of wool fibers and enhances their
hydrophilicity but is detrimental to the dyeing performance.
Erra et al. [14] combined chitosan with plasma treatment of wool fabric and achieved a shrinkage resistance
of 8% after five washing cycles. Plasma treatment has remarkable effects on wool, resulting in shrinking
decreasing to 4.5% - 8.0% [15, 16] multi-functionalizing by oxidation, and an alteration of the fiber surface
topography [17]. Both low-pressure and atmospheric-pressure plasma technologies have been investigated
to study their effect on anti-felting capacity [18, 19].
Treatment with high-concentration ozone in an acidic environment is an eco-friendly method suitable for
textile materials containing wool fibers. It results in good contraction resistance, does not affect the fiber's
strength properties, and preserves its dye affinity [20, 21].
Enzyme anti-felting treatments have been developed as a sustainable and eco-friendly alternative to
chemical processes involving chlorine-based agents. Amongst proteolytic enzymes, protease [22 - 24],
papain [25], and lipase [26, 27] are the most studied for reducing the contraction of wool fibers. The results
are promising but come with a reduction in dye absorption capacity on the surface of the wool fiber. Enzyme
treatment alone cannot provide the required shrinkage resistance without a polymeric coating.

M. PRUNEANU, I. BUCIȘCANU, R. PRUNEANU
178
Using reactive polymers based on polyisocyanates and/or polyurethanes produced by Bayer [28] for special
finishing of wool fibers provides treatment durability and achieves excellent results related to wool
contraction resistance. However, these processes have the disadvantage of using organic solvents to
dissolve the oligo- and polyisocyanate compounds, which is undesirable in current ecological processes.
The main disadvantages of poly- and diisocyanate compounds are their insolubility in aqueous
environments and the tendency of the isocyanate group to hydrolyze due to its high reactivity, making them
unsuitable for use in their raw form [29].
To function as an anti-felting agent, several criteria related to both the intrinsic structure of the isocyanate
compound and the treatment parameters must be met, namely:
• Polyfunctionality: It is recommended to use bifunctional, aliphatic, or cycloaliphatic isocyanates
with a molecular weight between 140 and 400 g/mol to ensure good diffusion in the keratinous
fiber matrix and increased ability for forming cross-links between keratin fibers [30].
• Solubilization in aqueous environments: The compound must be soluble in water because all wet
processing operations of textiles are conducted in aqueous media.
The isocyanate group reactivity can be controlled by its temporary blocking. The general reactions for
temporary blocking with adduct formation (1) and subsequent unblocking through interaction with a co-
reactant (2) are given below:
𝑅−𝑁=𝐶=𝑂
(

)
+𝐵𝐻
(

)
∘
⇄𝑅−𝑁𝐻−𝐶
∥
−𝐵


 (1)
𝑅−𝑁𝐻−𝐶
∥
−𝐵+𝑅󰆒𝐻, 
󰇒
⎯
󰇏
𝑅−𝑁𝐻−𝐶
∥
−𝑅󰆒+𝐵𝐻 (2)
Reactions (1) and (2), where compound A is an oligo- or polyisocyanate, are fundamental to polyurethanes
production. Common blocking agents (B) include alcohols, phenols, pyrazoles, oximes, sodium bisulfite,
etc., while the co-reactant R'H is an alcohol [29 - 31].
This work aims to synthesize a water-soluble hexamethylene diisocyanate-bisulfite adduct with temporarily
blocked isocyanate groups and to investigate its anti-felting effect on wool fibers by chemical
functionalization.
2. EXPERIMENTAL
2.1. Materials and chemical reagents
For the synthesis of the hexamethylene diisocyanate-bisulfite adduct (HMDIS), the following reagents are
required: 1,6-hexamethylene diisocyanate (HMDI), Tergitol 15-S-5 (non-ionic secondary alcohol
ethoxylate surfactant), and sodium bisulfite (NaHSO3). To test the anti-felting capacity of the synthesized
compound, indigenous merino wool fibers and common reagents (sodium bicarbonate - NaHCO3, sulfuric
acid 96%- H2SO4) were used. All chemical reagents were of analytical grade and used as received from
Sigma-Aldrich and Merck.
2.2. Synthesis of the hexamethylene diisocyanate-sodium bisulfite adduct (HMDIS)
The synthesis procedure was conducted according to the methodology described by Maier et al. [32]. ] In
a reaction vessel, 500 mL of 30 - 32 % (w/w) sodium bisulfite solution was heated to 25° - 30° C. In parallel,
100 grams of hexamethylene diisocyanate was emulsified in 20 grams of Tergitol 15-S-5 under vigorous
mechanical stirring at 25° - 30° C. The emulsion was added in a thin stream over the sodium bisulfite
solution under continuous stirring. After 30 min the temperature of the reaction mixture was raised to 45 -
50 °C and kept at this value for 90 min under constant control of the pH of the reaction medium, which was
corrected to 5,0 - 5,5 with 10 % H2SO4 solution. The reaction mass was cooled to room temperature under
continuous stirring for 60 min and left overnight to allow the maturation of the reaction mass and
completion of the sulfitation process. The synthesized diisocyanate-bisulfite adduct, in the form of a white
suspension, was separated by centrifugation. The sediment was resuspended in ethyl alcohol, dried in a
vacuum dryer, and ground in a ball mill. The procedure is depicted in Figure 2.

The 19th Romanian Textiles and Leather Conference CORTEP 2024, Iasi, Romania
179
Figure 2. Preparation of the hexamethylene diisocyanate-sodium bisulfite adduct (HMDIS)
The HMDIS synthesis is based on the chemical reaction below (3):
𝑂=𝐶=𝑁−(𝐶𝐻)−𝑁=𝐶=𝑂+ 𝑁𝑎𝐻𝑆𝑂
 𝒕,℃
󰇒
󰇏
𝑁𝑎𝑆𝑂−𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆𝑂
𝑁𝑎
HMDI
S
(3)
Temporary blocking of isocyanate-reactive groups in adducts by using sodium bisulfite (NaHSO3) is more
effective than blocking with organic blocking agents [29, 30], because sodium sulfite has low molecular
weight, is inexpensive, and has much lower toxic potential than the organic counterparts. The deblocking
temperature of bisulfite adducts is lower than that of similar adducts obtained with organic blockers.
The hydrolysis of isocyanate-sulfite adducts is pH-dependent, in that the reaction rate increases with
increasing pH; the optimal reaction pH is 8.0 - 8.5, which is convenient to facilitate the cross-linking
reaction with the functional groups of keratin amino acids in wool fiber.
2.3. FTIR analysis of the HMDIS product
The FTIR spectrum of the obtained HMDIS adduct was recorded on a Digilab Excalibur FTS 2000
spectrophotometer, by the anhydrous KBr pellet technique, in the range 400 - 4000 cm-1, number of scans
24 and resolution 4 cm-1.
2.4. Anti-felting procedure/treatment for/of wool fibers
The wool anti-felting treatment with HMDIS involves the following steps, as outlined in Table 1: (1)
Preparation of the wool fibers; (2) Creating conditions for the diffusion of the HMDIS adduct into the
keratin fiber microstructure, by pH and temperature adjusting; and (3) ensuring the combination of free
isocyanate groups with the functional groups of keratin-specific amino acids in a slightly alkaline
environment.
Table 1
Anti-felting treatment of wool fibers with HMDIS
No. Operation Procedure
1 Moistenin
g
of scoured wool fibers Wool fibers are s
p
ra
y
ed with 35% ethanol solution.
2 Air drying of wool fibers Wool fibers are ai
r
-dried at room temperature for 1-2 hours.
3 Conditioning of wool fibers Conditioning takes place in an oven at 40°C.
4. Anti-felting treatment (diffusion
stage)
Treatment is carried out by exhaustion, in HMDIS solution, at
liquor ratio =1:20, at 40°C for 30-40 min. HMDIS offer: 15%
relative to the weight of the wool fibers (w.o.f.).
5 Anti-felting treatment
(combination stage)
Alkalinization of the treatment solution with 0.1 - 0.3% NaHCO3
w.o.f. Wool is left in the solution overnight, with the pH kept at
pH = 8.0-8.5.
6
Acidification of wool fibers
In the same treatment bath, 0.5-1.0% diluted acetic acid (dilution
rate 1:10) is dosed under stirring for 10 min, to attain a final pH
of 5.0 - 5.5.
7 Washin
g
of treated wool Treated wool fibers are washed in cold runnin
g
water.

M. PRUNEANU, I. BUCIȘCANU, R. PRUNEANU
180
2.5. SEM morphological analysis of wool fibers treated with HMDIS
The morphology of the pristine and treated wool fibers was examined using a Vega-II Tescan scanning
electron microscope (SEM, Czech Republic), and Atlas Tescan software for image analysis. Before the
SEM investigation, all samples were sputtered with gold.
3. RESULTS AND DISCUSSION
3.1. FTIR analysis of the hexamethylene diisocyanate-bisulfite adduct (HMDIS)
The FTIR spectrum of the synthesized diisocyanate-bisulfite adduct, presented in Figure 3, confirms the
chemical functionalization of HMDI. The characteristic signal at 2264 - 2270 cm⁻¹, corresponding to the
isocyanate group (–NCO), is absent [31], indicating the complete blocking of isocyanate groups by
sulfitation.
Figure 3. The FTIR Spectrum of the hexamethylene diisocyanate-bisulfite adduct (HMDIS)
The convoluted peak at 3336 cm⁻¹ corresponds to the absorption band of the polyurethane group (-NH-CO-
) and indicates the stretching of the N-H groups. The peak at 2928.6 cm⁻¹ is assigned to the asymmetric
stretching of the -CH₂ group, while the symmetric stretching of the -CH₂ group is observed at 2860.0 cm⁻¹.
The peak at 1524 cm⁻¹ indicates the bending of H-NC bonds, and the peak at 1250 cm⁻¹ corresponds to the
stretching of characteristic bonds in the hexamethylene groups [32, 33]. The peak at 1120 cm
-1
is assigned
to the vibration of the hydrated S = O group present in the sulfitizing agent, NaHSO
3
(1230-1120 cm
-1
)
[34]. The peak at 653 cm
-1
indicates the vibration of the C-H groups.
3.2. SEM microscopic analysis of wool fibers treated with HMDIS
The surface and cross-section morphology of untreated and HMDIS-treated keratin fibers, investigated by
Scanning Electron Microscopy (SEM), is illustrated in Figures 4 and 5.
a
b
Figure 4. SEM images of wool fibers: (a) Pristine wool fiber; (b) Wool fiber treated with HMDIS.

The 19th Romanian Textiles and Leather Conference CORTEP 2024, Iasi, Romania
181
The SEM image from Figure 4a shows an appearance and arrangement of scales characteristic of the
morphology of native wool fibers. In the case of functionalized wool fibers (Figure 4b), the epicuticular
layer is covered by a protective HMDIS coat and suffers a certain degree of swelling, which leads to scales
flattening. These alterations reduce friction between scales and produce an anti-felting effect on the wool
fibers.
A comparative examination of the SEM cross-section images of untreated (figure 5a) and functionalized
(figure 5b) wool fiber shows the filling of interfibrillar spaces in the case of the latter, which proves the
ability of the HMDIS adduct to penetrate the fiber and participate in cross-linking reactions between the
isocyanate groups and the amino acid functional groups specific to keratin. The possible cross-linking
reactions are: with non-ionized amino groups (reaction 4); with hydroxyl groups, (reaction 5); with
sulfhydryl groups of cysteine, the keratin-specific amino acid, (reaction 6); and with the carboxyl groups
from the side chains of keratin (reaction 7) [35]. The filling of the interfibrillar spaces with the chemical
compounds resulting from the cross-linking reactions has a "cementing" effect on the keratin fibrous matrix,
which considerably restricts the movement of the scales and demonstrates the anti-felting effect of the
HMDIS product.
𝑁𝑎𝑆𝑂−𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆𝑂
𝑁𝑎+𝟐𝑯𝟐𝑵−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+𝐻 𝑂
↓𝑡,°𝐶,𝑝𝐻
𝐾𝑒𝑟𝑎𝑡𝑖𝑛 − 𝑁𝐻 − 𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑁𝐻−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+2𝑁𝑎𝐻𝑆𝑂

U
rea type compoun
d
(4)
𝑁𝑎𝑆𝑂−𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆𝑂
𝑁𝑎+𝟐𝑶𝑯−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+𝐻 𝑂
↓𝑡,°𝐶,𝑝𝐻
𝐾𝑒𝑟𝑎𝑡𝑖𝑛 − 𝑂 − 𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑂−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+2𝑁𝑎𝐻𝑆𝑂

U
rethane type compoun
d
(5)
𝑁𝑎𝑆𝑂−𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆𝑂
𝑁𝑎 +𝟐𝑺𝑯−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+𝐻 𝑂
↓𝑡,°𝐶,𝑝𝐻
𝐾𝑒𝑟𝑎𝑡𝑖𝑛 − 𝑆 − 𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+2𝑁𝑎𝐻𝑆𝑂
Thiourethane type compound
(6)
a b
Figure 5. Cross-section SEM images of wool fiber: (a) untreated wool fiber; (b) wool fiber treated with HMDIS.

M. PRUNEANU, I. BUCIȘCANU, R. PRUNEANU
182
𝑁𝑎𝑆𝑂−𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑆𝑂
𝑁𝑎+𝟐𝑯𝑶𝑶𝑪−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+𝐻 𝑂
↓𝑡,°𝐶,𝑝𝐻
𝐾𝑒𝑟𝑎𝑡𝑖𝑛 − 𝐶
∥
− 𝑂− 𝐶
∥
−𝑁𝐻−(𝐶𝐻)−𝑁𝐻−𝐶
∥
−𝑂−𝐶
∥
−𝐾𝑒𝑟𝑎𝑡𝑖𝑛+2𝑁𝑎𝐻𝑆𝑂
C
arbamic anhydrid
e
type compoun
d
(7)
4. CONCLUSIONS
A water-soluble diisocyanate-bisulfite adduct with temporarily blocked isocyanate groups was obtained by
an affordable procedure, involving solubilization of 1,6 hexamethylene diisocyanate (HMDI) in aqueous
medium with a nonionic surfactant, followed by sulfitation with sodium bisulfite.
The absence of the characteristic signal in the 2264-2270 cm-1 band from the FTIR spectrum of the HMDIS
adduct confirms the complete blockage of the -NCO groups by sulfitation, necessary to increase the water
solubility and to regulate the tendency of the highly reactive isocyanate group to hydrolyze.
Due to its linear structure, water solubility, and controlled availability of the reactive isocyanate group, the
HMDIS adduct can be considered as an efficient anti-felting agent for wool.
The anti-felting treatment consists in the physical deposition and chemical reaction of HMDIS with the
keratin functional groups, which cross-links the adjacent wool fibers and functions as a "cementing agent".
Morphological analysis by SEM of HMDIS-treated versus untreated wool fibers highlights the ability of
the product to cover the scales on the wool fiber surface and to confer a degree of swelling to the keratin
matrix.
The mechanism of the anti-felting process is related to HMDIS ability to uniformly distribute itself on the
hydrophobic surface of wool fibers and participate in cross-linking chemical reactions with the specific
functional groups of the keratin amino acids, which considerably restricts the friction between the epicuticle
and exocuticle scales, which confirm the anti-felting capacity of the studied diisocyanate compound.
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