The grafting reaction of
- grafting process
- response surface methodology
Manufacturers are paying a growing interest to the functionalisation of textiles for innovative applications. Among the most emerging technologies are those based on microencapsulation . This high-tech approach for the global market allows the improving of intrinsic properties or providing entirely new functionalities, which would not be possible or would be too expensive by using other processes . Encapsulation creates a physical barrier that protects the active ingredient from the external environment, enabling the possibility of its controlled release . Such finish allows a broader usability and higher market value of garments in a vast range of applications .
In the beginning, functional textiles focused only on individual value-adding properties. Nevertheless, recent research has targeted combinations of multiple properties and their effects, allowing the development of new multifunctional smart textiles with various functionalities in one product, such as simultaneous aromatic , antibacterial , superhydrophobic  and UV-protective effects . With regards to encapsulated materials, microparticles have been in use for a long time in many fields, involving applications such as carbonless copying paper , adhesives, insecticides, acaricides , pharmaceutics, medicine , coatings , liquid crystals and cosmetics, and others . For the latter newly emerging sector, encapsulated products are designed to transfer an active material upon contact with the human body for cosmetic purposes like perfuming, moisturising, slimming, refreshing, relaxing, revitalising or/and UV-protecting .
As a current trend, researchers nowadays tend to go further by producing cosmetic-loaded microparticles from renewable resources such as cyclodextrins (CDs) , which leads to the usage of green and bio-sourced materials .
CDs are a simple and relatively affordable material  that result from the degradation of starch by
In the textile field, CDs frequently used to be chemically grafted to cotton with polycarboxylic acid in the presence of a catalyst. According to Sharkawy et al.  and Yang et al. , citric acid (CA) was used as a safe crosslinker to covalently bind the microcapsule wall material to hydroxyl groups of cotton via ester bonds. During curing, the binder forms a thin, elastic and transparent layer on the textile surface in which the microcapsules are bound. Hence, the adhesion between the bound layer and the textile substrate plays a crucial role .
In a previous study, the functionalisation of cotton fabric by fragrant
The paper reports on the application of Response Surface Methodology (RSM) on
Pure cotton jersey knitted fabric, bleached, mercerised of specific area 138 g ⋅ m−2 was supplied by Esprit Maille (Bouhjar, Tunisia). CA (Sigma-Aldrich, 99.5%) was used as a crosslinking agent and DHP (Sigma-Aldrich, 99%) was used as a catalyst. Microcapsules of spherical shape and a mean particle size of 29 µm with good thermal stability were synthesised according to our previous published procedure . PU neroline loaded microcapsules have been achieved by the classical process of interfacial polycondensation between
Cotton was impregnated into an aqueous solution containing fragrant microcapsules mixed with CA and DHP for 5 min at 195°C. Fabric samples were then squeezed and dried at 90°C for 15 min. Functionalised textiles were finally rinsed and dried again to remove non-grafted microcapsules.
The functionalisation rate of the fabrics was assessed by the mass gain of the samples upon treatment with
RSM was used to optimise the impregnation process as reported by Box et al. , D’Agostino and Stephens  and Khuri and Cornell . The studied experimental factors were the concentrations of the microcapsules (
Experimental range and levels of independent process variables
|(g ⋅ L−1)||90||100||110|
|(g ⋅ L−1)||80||100||120|
|(g ⋅ L−1)||50||60||70|
CAT, catalyst concentration; CA, citric acid; DHP, disodium hydrogen phosphate
Box-Behnken design and experimental results
The shape and surface features of textile fabrics before and after functionalisation were observed by scanning electron microscopy (SEM) after drying using a Jeol JCM 5000 (Jeol, Japan) microscope. Specimens were examined at room temperature without surface metallization at 5 kV and 10 kV acceleration under moderate vacuum.
Tensile test measurements of untreated and treated fabrics were carried out based on the ISO 13934-1 standard by using a Lloyd dynamometer LS series (Ametek STC, France) under standard conditions (20 ± 2°C and 65 ± 2% of relative humidity) after preconditioning all samples in a standardised conditioning room for 24 h. Measurements in column and row directions were performed using a specimen with dimensions of 200 mm length and 50 mm width. The distance between clamps and the movement speed were 50 mm and 10 mm ⋅ s−1, respectively. Three samples were tested for each type of fabric.
Air permeability of untreated and treated fabrics was measured based on the EN ISO 9237 standard by using a FX3300-III instrument (TEX-TEST AG) under standard conditions (20 ± 2°C and 65 ± 2% of relative humidity). Preconditioning of all samples was done in a standardised conditioning room for 24 h. Measurements were performed under an air pressure of 100 Pa per 100 cm2 fabric surface. Tests were done in triplicate and the measurements were carried out by scanning the entire textile surface on both sides in order to ascertain the reproducibility of the results. Results were expressed as rates of air flow per unit area and time expressed in “mm ⋅ s−1”.
Wash fastness was studied in accordance with ISO Standard 105-C10 of 2010. Functionalised fabrics were washed for 30 min at 40°C in an Autowash device. Then, specimens were rinsed with water for 5 min and dried under ambient conditions.
The fate of bound microcapsules and the residual amount of fragrance were estimated by SEM observations and gas chromatography (GC) analysis using a 7,890 A Gas Chromatograph from Agilent Technologies (USA) equipped with a flame ionisation detector (FID). Residual neroline after washing was extracted using chloroform. Analytes from the 1 µL samples were injected at a split ratio of 1:20 and separated on a 19091J HP-5 column (length, 30 m; internal diameter, 0.32 mm; film thickness, 250 mm). The oven temperature was initially set at 200°C for 10 min and the detector temperature was 280°C. Nitrogen was used as the carrier gas with a constant flow rate of 20 mL ⋅ min−1 and an inlet pressure of 1.5 bar.
Functionalisation of mercerised cotton jersey fabric with neroline-loaded microcapsules based on
The studied variables were the main operating conditions influencing the grafting process, the microcapsules concentration, the CA crosslinking agent concentration and the DHP CAT. The studied response was the mass gain of the functionalised textile (
The microcapsules concentration varied between 90 g ⋅ L−1 and 120 g ⋅ L−1 for a CA crosslinking agent concentration of 80 g ⋅ L−1 and a DHP CAT of 50 g ⋅ L−1. The variation of
For a microcapsules concentration of 110 g ⋅ L−1 and DHP CAT of 50 g ⋅ L−1, the effect of the CA crosslinking agent concentration on the mass gain (
As shown in Figure 3, the mass gain increased with respect to the concentration of CA at levels above [CA] = 120 g ⋅ L−1. However, this ‘optimum’ concentration depends on both the quantity of microcapsules introduced and the textile surface area to be treated. As a polycarboxylic acid, CA acts as a crosslinking agent by reacting with the hydroxyl groups of the
For a microcapsules concentration of 110 g ⋅ L−1 and a CA crosslinking agent concentration of 120 g ⋅ L−1, the mass gain percentage (
It appears that the optimum CAT is about 60 g ⋅ L−1. Indeed, the mass gain decreases for higher concentrations. This may be due to the fact that, at higher concentrations, the reaction takes place mainly between the microcapsules instead of between the microcapsules and the cellulose.
Since the preliminary experiments showed that the variations of mass gain with respect to variables were not linear, and that there was a maximum, a second-order quadratic model has therefore been developed to explain the mass gain. To reach this goal, the three experimental factors were simultaneously varied according to a Box–Behnken design. Results were investigated using the software Minitab 19 which led to Eq. (2), where
ANOVA was first used to evaluate the fitting quality of the model . The assessment is based on an
ANOVA, analysis of variance
To finally evaluate the validity of the model, the residuals were also determined by calculating the difference between the experimental
Once validated, the model was used to visualise the response surfaces as bi-dimensional plots of two factors (
The contour plots in Figure 5 confirm the results drawn by varying parameters one by one: the concentrations of microcapsules and CA should be maximum and the concentration of catalyst should be minimum within the studied window. There are synergistic effects, however. The most relevant ones are those between the concentration of catalyst on the one hand and either the concentration of microcapsules or CA on the other hand. The optimum concentration of catalyst drawn from the one parameter at a time study was 60 g ⋅ L−1 for a microcapsules concentration of 110 g ⋅ L−1 and a CA crosslinking agent concentration of 120 g ⋅ L−1. The quasi-symmetrical feature of the contour plots with respect to the diagonal in Figure 5 shows that the optimum of CAT shifts from 60 g ⋅ L−1 to 50 g ⋅ L−1 as the concentration of microcapsules increases. The same trend is observed for an increase of CA concentration.
The optimal conditions of the cotton grafting process using fragrant microcapsules were predicted by the response optimiser tool of Minitab 19 software for maximum response. Response optimisation described in Figure 6 shows that optimal experimental conditions are a microcapsules concentration of 107.6 g ⋅ L−1, a CA crosslinking agent concentration of 111.9 g ⋅ L−1 and a DHP CAT of 50 g ⋅ L−1, which yields a mass gain of 5.32%.
To verify the validity of the model, the corresponding experiment was performed to compare the experimental results with the response predicted by the model. The theoretical optimal value of mass gain under these optimised conditions is 5.32% while the experimental one is 5.58%. The difference between these two values is well within the 95% confidence interval of the experimental error, thus proving the good predictive performance of the developed model.
After functionalisation of cotton knitted fabric by the grafting process under optimised conditions, several textile properties were measured namely morphology, tensile strength, air permeability and wash fastness.
Grafted cotton fabric was analysed by SEM after being treated in a bath containing fragrant
Though SEM does not provide a quantitative analysis, SEM pictures show that a sufficiently high density of grafting has been reached. Pictures reveal a large collection of grafted microparticles distributed evenly onto the cotton textile fabric: between the textile yarns, the fibers and in the cotton cavity. The density of grafted microcapsules was similar to that of previous work obtained by the impregnation process using PU or epoxy resin  shell microparticles based on
Fragrant microcapsules are not damaged following the heat treatment undergone during the process (grafting and drying temperatures of 195°C and 90°C respectively). The microparticles are also tightly fixed by means of covalent bonds after the reaction and a simple rinse.
The effect of the optimised grafting treatment on the tensile properties was evaluated by a comparison between the tensile curves of the treated and the non-treated samples in both knitting directions (row and column directions), and in particular, the maximum force, the elongation at maximum force as well as the stiffness (Figure 8).
The general tensile behaviours of the different knitted fabrics were similar in the two stress directions and whether they were measured before or after functionalisation. Tensile strength at low loads mainly originates from the friction resistance of yarns in the wales . Meshes progressively line up leading to fabric elongation. The yarns get deformed at higher loads and eventually neck and break.
A comparison between the maximum force, the elongation at maximum force as well as the stiffness of the untreated and the treated fabrics is shown in Figure 9.
The treatment of grafting the microcapsules to the cotton knitwear increased the maximum force in both directions. The establishment of new covalent bonds contributes, in some way, to the improvement of the tensile strength. The elongation at maximum force is slightly higher for the grafted textile than that of the untreated textile. The cause of such changes could be the treatment at high temperatures and without applied tension during grafting that increases the amorphous areas of already mercerised cotton. Hence, the rate of crystallinity decreases and the strain at maximum force increases slightly in both directions of stress.
Examination of Figure 9 shows that the grafting treatment increases the stiffness of the textile stressed in both directions. This increase is more accentuated in the column direction rather than in the row direction, which is probably due to the establishment of new bonds, inter-fibers and inter-yarns, of covalent types by the grafting process. According to these results, it appears that the optimised grafting treatment slightly modifies the mechanical properties of the textile without altering them.
The assessment of air permeability was investigated by measuring the rate of air flow through the fabric under differential pressure.
The air permeability properties of knitted cotton fabrics before and after treatment are displayed in Figure 10. The treatment of grafting microcapsules decreased the air permeability of the functionalised textile by 50% for a mass gain of 5.58%. The textile support is not yet saturated and occluded with bound microcapsules, since porosity between the fibres was clearly seen in SEM pictures. There is no excess of the grafted microcapsules forming an occlusive layer right side up and on the other side of the textile substrate. Microcapsules are well distributed inside the textile sample in the sites between the thread and in statements between the fibers.
Misra et al.  in 2020 reported that fabrics become less permeable to air upon increasing the mass per unit area of the textile. Although the air permeability decreased from 992 mm ⋅ s−1 to 492 mm ⋅ s−1, the wearing comfort is not compromised. Given the low mass per unit area of the jersey structure, the very high level of openness and air permeability of the textile support before treatment allows for a large mass gain without loss of sensory properties.
The uniformity of grafted microcapsules was confirmed by air permeability measurements performed at different points of the fabric. All tests presented similar air permeability behaviour. The present treatment is efficient in, not only promoting a high level of material attachment but also ensuring the coating uniformity .
The effect of washing cycles on grafted textile was investigated according to the ISO 105-C10 standard of 2010 to evaluate the efficiency and the durability of the microcapsules application to the knitted fabric. SEM observations and GC analysis were used to assess the lifetime of scent textiles both as qualitative and quantitative evaluations.
SEM micrographs of the functionalised knitted fabric after 40 washing cycles (Figure 11) show the presence of microcapsules remaining bound to the textile. The amounts of microparticles on the fabric before and after washing are similar; no noticeable fall could be detected. Thus, unlike the impregnation treatment, the grafting treatment provides a good wash fastness. Indeed, grafted microcapsules still remain attached to the textile support by means of covalent bonds. Compared to their original shape in the dispersed state,
The presence and the quantification of residual encapsulated neroline after washing cycles were studied by GC analysis. The area of the fragrance peak decreased as repeated washing cycles have been applied. The amount of residual neroline was determined based on a calibration curve.
Figure 12 shows that the residual neroline concentration decreased slowly, confirming that the microcapsules remained loaded with perfume along subsequent washing cycles.
Residual neroline concentration on the textile treated with
Thereby, this decrease in perfume concentration is essentially due to a diffusion phenomenon of the active material through the pores of the PU microcapsules and the cavity of the
After the successful application of neroline-loaded
The effect of the composition of the grafting bath on the efficiency of the microcapsules deposition shows a clear optimum. The RSM allows optimising the odorous functionalisation of pure knitted cotton. Using the developed model, the maximum possible mass gain reached was 5.32%. The optimal grafting conditions are as follows: 107.6 g ⋅ L−1 for microcapsules concentration, 111.9 g ⋅ L−1 for CA crosslinking agent concentration and 50 g ⋅ L−1 for DHP CAT.
There is justification for optimisation because there is a pronounced maximum mass gain of the cotton knitted fabric according to the variables studied. Specifically, there is an optimum concentration of microcapsules. This disclosure goes against a kind of ‘common sense’ belief that one should deposit more microcapsules for obtaining greater binding to the fabric. It has been shown that RSM is an efficient tool for performing such optimisation.
Some properties of cosmetotextiles produced via the optimised grafting protocol have been measured. SEM micrographs revealed effective adhesion between the microcapsules and the cotton fibers. The tensile strength tests in both directions of stress showed that the grafting treatment slightly modified the mechanical properties of the textile, without dramatically altering them. Such alterations may be viewed as improvements or deterioration of properties depending on the type of end-use of the fabric and its properties before treatment. The air permeability decreased by 50% after functionalisation. Overall, the studied grafting treatment appears adequate for the development of a wide range of other cosmetotextile articles since it makes it possible to add a specific functionality to the textile support without altering its intrinsic properties. The optimised treatment appears to be effective not only in promoting a high level of adhesion of the material but also in ensuring uniformity of the coating. The wash fastness resistance of grafted fabric was quite satisfactory since there was no loss of microcapsules following successive washing cycles. The amount of neroline component decreased slowly with a fragrance loss of 2% after five washes and 77% after 40 washes. After the release of the encapsulated active principle, reloading the microcapsules through
Thus, microencapsulation appeared to be an effective method to control the diffusion of fragrances through the pores of the microparticles shell, protecting the active principle from the environment. Of noteworthy interest, the grafting technique as well as the presence of
Box-Behnken design and experimental results
Experimental range and levels of independent process variables
|(g ⋅ L−1)||90||100||110|
|(g ⋅ L−1)||80||100||120|
|(g ⋅ L−1)||50||60||70|