Vis To Assess Surface

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Combined Use of Streaming Potential and UV/Vis To Assess Surface Modification of Fabrics via Soil Release Polymers Alessandra Valentini,*,† Serafim Bakalis,† Kostantinos Gkatzionis,† Gerardo Palazzo,‡ Nicola Cioffi,‡ Cinzia Di Franco,§ Eric Robles,∥ Anju Brooker,*,∥ and Melanie M. Britton⊥

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†

School of Chemical Engineering and ⊥Department of Chemistry, University of Birmingham, Birmingham B15 2TT, United Kingdom ‡ Dipartimento di Chimica, Università degli studi di Bari “Aldo Moro”, Bari 70100, Italy § CNR−Istituto di Fotonica e Nanotecnologie, Bari 70100, Italy ∥ The Procter & Gamble Company, Newcastle Innovation Centre, Newcastle NE12 9TS, United Kingdom ABSTRACT: Polymers have become a widespread part of laundry detergent formulations because of their benefits which are usually delivered via surface modification of fibers. Therefore, there is a growing interest in understanding their deposition on fabrics. In this work, we have used streaming potential to assess changes in surface charge of polyester and knitted cotton after modification via soil release polymers (SRPs). Results identify a relationship between the measured zeta potential for the modified fabrics and the charge of the polymer. The effects of parameters, such as agitation speed and bulk concentration during deposition, have been investigated. Streaming potential data were then correlated to adsorption isotherms from UV absorbance data, and a Langmuir−Freundlich model was proposed to describe the isotherms for polyester. The stain removal index for some common hydrophobic stains was determined via image analysis. A link between SRP deposition efficiency and their effectiveness on greasy soil removal was observed.

1. INTRODUCTION

Among all the chemical methods, polymer deposition on fabrics has been proposed as a way to modify the hydrophilicity of the surface. This type of surface modification is often performed by soil release polymers (SRPs), a class of polymer that is gaining attention in the laundry industry. An SRP is a polymer that deposits on, and modifies, surfaces to ease removal of soil.5,6 When SRPs deposit, they hydrophilize hydrophobic surfaces, such as polyester fabrics or plastic substrates. In the case of fabrics coated with a SRP, the deposited film lowers the adhesion of greasy soil. Consequently, an enhancement of soil removal is possible from the substrate. A variety of SRPs have been investigated,3 which are able to deliver soil removal and resistance to redeposition.6−8 Previous results have shown that cleaning is strongly improved for fabrics with lower porosity and that a prewash with the SRP is able to strongly improve soil removal. However, there is still a lack of understanding about the way in which this is achieved. A possible mechanism involves a delamination process (Figure 1) in which the soil adheres on top of the SRP film and then a delamination-failure in the polymer film occurs.7 The soil

Soiling on a substrate is a complex, multistage process which is controlled by interactions between the substrate and soil. The two main stages of the soiling process are the initial wetting of the surface by the soil and the penetration of the soil in the structure of the fabric, with subsequent entrapment of the soil in the interyarn (micro-occlusion), in the pores (macroocclusion), or between fibers.1,2 Hence, by varying the surface energy or porosity of a fabric (via deposition of polymers, plasma treatment), soil adhesion can be controlled. It is known that a soil, with lower surface energy (e.g., oil), spontaneously wets a substrate that has a lower surface energy (e.g., hydrophobic fabrics); therefore, its adhesion is stronger. It is then easy to understand why modifying the surface energy of fabrics has become quite common in the textile industry, particularly as a result of the increased use of synthetic fabrics.3 This is because synthetic fabrics typically have a higher hydrophobicity compared to cotton, meaning the adhesion of greasy soil is strongly enhanced on this type of garments, which are more difficult to clean as a consequence.4 One way to improve the cleaning of new synthetic fabrics is to make the textile more hydrophilic, via chemical or physical modification, which will improve the wettability and cleanability of the material. © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

May 11, 2019 July 12, 2019 July 26, 2019 July 26, 2019 DOI: 10.1021/acs.iecr.9b02604 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram showing the mechanism of action of SRPs: (a) untreated polyester fabric; (b) fabric modified via SRP deposition; (c) soil adhesion on fabric modified via SRP; (d) soil removal from fabric surface via surfactant; (e) emulsified soil in solution.

Figure 2. General molecular structure of the PEG−POET soil release polymers covered in this paper (a) and molecular structure of SRA300 (b) and SRN240 (c).

attaches on top of the SRP film, and this will promote its emulsification and suspension by surfactants in the solution.7 Redeposition is inhibited because of electrostatic repulsion between hydrophilized surface and the soil suspended in the bulk solution. Block copolymers of poly(ethylene glycol)−poly(ethylene terephthalate) (PEG−POET) (Figure 2) are widely used for surface modification of fabrics.8−12 They are amphiphilic block copolymers, which can have complementary chemical properties, which makes them suitable to hydrophilize hydrophobic textiles such as polyester. This is because a simple PEG homopolymer is unlikely to deposit on polyester. Hence the hydrophobic POET block, which interacts with the fabric via its aromatic ring, enables deposition. The hydrophilic PEG block is exposed to the washing liquor,13 leading to hydrophilization of the textile and providing either electrostatic or steric repulsion of the soil. However, characterizing the deposition of polymers on a fabric surface remains a significant challenge. Most added SRPs are complex mixtures, containing polymers of different chain length and degree of polymerization, and therefore chemical analysis is extremely difficult in solution. Moreover, analysis is even harder on surfaces because the polymers can have multiple anchoring points and therefore solvent pre-extraction is typically not possible; hence, they need to be analyzed directly either via their elemental composition (e.g., X-ray

photoelectron spectroscopy, XPS)14,15 or via mass spectrometry techniques such as TOF-SIMS16 or MALDI.17 The use of XPS is not effective as SRPs are generally composed of light elements (C, H, O), just like the fabric they are deposited on, and therefore it is very hard to differentiate the signal from the SRPs on substrate. As SRP modifies the fabric surface energy, an alternative approach to assess SRPs deposition could be by measuring the contact angle or Wilhelmy method.18−20 However, measuring and analyzing the contact angle on porous substrate, such as fabric, is not straightforward.21 In fact, an extremely fast camera is required to be able to measure the contact angle on fabrics before the liquid is wicked into the porous structure of the fabric via capillary action, and sometimes it requires a different data analysis versus nonwoven rigid textile. As a consequence, the understanding of the effect of SRP structure and charge on deposition has not been determined. In this paper, a novel approach has been employed to indirectly collect information about fabric hydrophilization and about the surface charge of fabric after SRP deposition. This approach is based on a streaming potential technique, a powerful tool used to assess zeta potential of fabrics before and after surface modification.22,23 In this work, this technique enabled indirect verification of the deposition of SRPs, which was then confirmed by measuring the residual SRP in the bulk solution via UV/vis spectroscopy and imaging the SRP deposit B

DOI: 10.1021/acs.iecr.9b02604 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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of adsorbent) qE occupied sites to the concentration of adsorbate (say a polymer P) at equilibrium, [P]e:

on the substrate via SEM and image analysis. Results are presented for two different SRPs on polyester, as well as knitted cotton as negative control. The effect of deposition parameters, i.e., as initial polymer concentration and agitation, on deposition efficiency and soil removal will be discussed.

qE = Q sat

dIstr η L × × dΔP ε*ε0 A

qE = Q sat

dUstr η × *K ε*ε0 dΔP

(1)

(2)

where K (S m ) is the electrical conductivity:

1 L × R A

(5)

3. MATERIALS AND METHODS 3.1. Sample Preparation for Streaming Potential. Polyester and cotton fabrics were acquired from WFK Testgewebe GmbH, and they were boiled in deionized (DI) water for 30 min to remove any finishes in the fabric production process. Texcare polymers SRN240 SRA300F were acquired from Clariant Gmbh, Muttenz, Switzerland as a wax/granular raw material with an activity equal to 40% for SRN240 and 100% for SRA300F. Both the polymers are block copolymers with PEG−POET structure (Figure 2a), and the backbone for SRA300F is functionalized with sulfonate groups (Figure 2b) that are not present in SRN240 (Figure 2c). The two SRPs have similar molecular weight and the same ratio between PEG and POET blocks and were used at the same molar concentration. Polymer stock solutions at 5000 ppm (around 1 mM for both the polymers) in Milli-Q water were prepared. Fabrics were cut in 5 × 5 cm2 pieces and conditioned with SRP solution in a tergotometer (Copley Scientific, Nottingham, U.K.), keeping the weight ratio between solutions and fabrics equal to 24, which is in line with the normal load for Europe and North America.36 The stock SRP solutions were diluted to 50 ppm, and the liqueur was mixed at 200 rpm for 10 min. Fabrics were added at last, and treatment was performed for 1

−1

K=

K ([P]e )m 1 + K ([P]e )m

The variable K is related to the median binding affinity (K0) via K0 = K1/m. In this model m is the heterogeneity index, which varies from 0 to 1. For a homogeneous material, m = 1, and we retrieve the Langmuir isotherm. A heterogeneity in the affinity of the surface for noninteracting adsorption sites corresponds to m < 1. However, when multiple adsorption sites occur, another phenomenon connected with inhomogeneity can emerge. The polymer affinity for an adsorption site can depend on the occupancy of the neighboring sites. In such a case, the adsorption is said to be cooperative, as happens for the binding of a ligand to certain proteins (indeed the reader with a life science background will easily recognize that the form of the above Langmuir−Freundlich isotherm coincides with Hill’s equation ruling the cooperative ligand binding).35 In the case of positive cooperativity, the presence of occupied binding sites makes the further ligand binding favorable and m > 1. In the case of negative cooperativity, the binding to a site reduces the affinity of the ligands for the other sites and m < 1.

where ΔP is the hydraulic pressure (Pa) applied at the two ends of the streaming channel, η is the viscosity (Pa s) of the electrolyte solution, ε is the dielectric coefficient of the electrolyte solution, ε0 (C2 N1− m−2) is the vacuum permittivity, and L (m−1) is the cell constant. A For flow through porous substrates having a poorly defined geometry, a different arrangement is preferred in which the ends of the streaming cell are connected by a high impedance electrometer that avoids any external current, and the accumulation of charge sets up a measurable electric potential difference called the streaming potential Ustr(V) that can be translated to zeta potential through a modified Helmholtz− Smoluchowsky equation that does not require knowledge of geometrical parameters of the substrate: ξ=

(4)

where Qsat denotes the saturation level of the solid and K (Langmuir’s constant) is the equilibrium constant for the adsorption process reflecting the adsorption energy. A real solid surface is generally heterogeneous, characterized by adsorption sites with different adsorption energies and therefore different Langmuir’s constant. For such system the overall adsorption isotherm is obtained integrating the Langmuir’s isotherms over all the KL-values29 obtaining the following isotherm often called Langmuir−Freundlich or Sips isotherm.30−34

2. THEORETICAL BACKGROUND 2.1. Streaming Potential. Streaming potential is based on the Stern electrochemical double layer (EDL) theory.24−26 According to this, a substrate in contact with an electrolyte solution induces a counterion distribution with an inner plane, very dense in counterions, and a diffuse layer that progressively reaches the ion content typical of the bulk. Substrates experience surface charge due to ion adsorption or dissociation of acid/base groups present on the surface. Hydrophobic surfaces (e.g., polyester, polyacrylonitrile), although they contain nonionizable monomer units, have highly negative surface charge due to adsorption of OH− ions.27 The accumulation of charge sets up an electric field E that in turn induces a current flowing in the opposite direction through the bulk of the liquid. This back current is called the conduction (or leak) current. Streaming potential is measured by flowing an electrolyte solution tangentially to a surface; the flow causes movement of the counterions generating a net ionic streaming current Istr. Such a current can be directly measured by short-circuiting the two ends of the cell through a low impedance path of a slit cell of surface A and length L. In such an arrangement the dependence of the streaming current can be related to surface zeta potential ζ(V) via the Helmholtz−Smoluchowsky equation:22,28 ζ=

K [P]e 1 + K [P]e

(3)

2.2. Adsorption Isotherms: Langmuir and Langmuir− Freundlich. In the simplest case, the adsorption isotherm follows the Langmuir model describing the surface as an ensemble of fixed and independent adsorption sites that can be either empty or occupied by the adsorbed molecule. The adsorption isotherm relates the amount of adsorbate (per gram C


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Industrial & Engineering Chemistry Research h at 200 rpm and (35 ± 1) °C, which reproduces the conditions used in cold and fast washing cycles. Fabrics were then dried overnight under humidity and temperature control (50% RH, 20 ± 2 °C). The effect of agitation speed was evaluated for polyester by comparing the previous results to the streaming potential of polyester sheets pretreated at the same conditions listed before but using an agitation speed of 100 rpm instead of 200 rpm. The effect of two SRPs’ concentration was tested in a range between 25 and 200 ppm of SRP, keeping the same solution/ fabric weight ratio of 24 at an agitation speed of 200 rpm. 3.2. Streaming Potential. The streaming potential of fabric was measured on a SurPASS 3.0 (Anton Paar GmbH), using a cylindrical cell. Fabrics were cut into 5 × 5 cm2 pieces to achieve required permeability for the measurement. The streaming solution used contains 1 mM KCl, and it was kept at room temperature during the analysis. A pressure drop between 600 and 200 mbar was generated during the measurement. Measurements were performed from pH 6 to 9 (normal pH-window experienced during laundry process), and three data points were collected. Three zeta cycles were measured for each piece of fabric at each pH (three internal replicates) and then averaged to get a unique value at each pH for each sample. Data shown are presented as an average zeta potential value measured on three different pieces of fabrics pretreated in the same cycle (three external replicates). 3.3. UV/Vis Analyses. The amount of undeposited polymer in solution, in contact with the fabric, was measured via UV/vis spectroscopy (Cary UV/vis 60 spectrophotometer by Agilent, using a dip probe with path length of 1 cm). The amount of SRP adsorbed on polyester was measured indirectly from the concentration left in the bulk solution, after incubation of the polyester fabric in 50 ppm of SRP solution (section 3.1). The equivalent concentration of SRP in solution was extrapolated from a calibration curve, previously built using polymer concentrations between 10 and 60 ppm. Both polymers studied, Texcare SRA300F and Texcare SRN240, exhibit a maximum UV/vis absorbance around 240 nm. The depletion of SRPs’ concentration in solution, as a function of time, was monitored, and the resulting adsorption isotherms were fitted to Langmuir and Langmuir−Freundlich using Origin 9.0 software (OriginLab, Northampton, MA). The percentage of polymers adsorbed was compared with the streaming potential values, and the results are exhibited in stain removal test. 3.4. Stain Removal Test: Sample Preparation. The effect of fabric surface modification by SRP on grease removal was determined using a stain removal test. For this, SRPmodified fabrics (see Sample Preparation above) were sent to Lubrizol Advanced Materials Inc. for application of circular stain on fabrics. Commercially available round stains of artificial sebum,37 collar and cuf f soil and lard38 (Lubrizol, Netherlands), were mixed with a purple/red dye and applied on 5 × 5 cm2 squares of polyester and cotton. Collar and cuff soil is an artificial body greasy soil that has artificial sebum and particulates (clay, soot, etc.) as its main component, and it is intended to be representative of soils created from human and external sources, known to be a relevant and hard to remove soil normally accumulating in the collar and cuff. Stain removal tests were run in automatic tergotometer with addition of a normal laundry detergent formulation at 25 for 20 min at 1800 rpm, followed by a 7 min rinse cycle at 7200 rpm. Measurement repetitions were run for each polymer type,

using four artificial sebum stains, four lard stains, and four collar and cuff stains and adding cotton ballast to keep the ratio wash solution/fabrics equal to 16. The soil removal test was run on polyester and cotton. Image analyses of the stains on a white background, before and after washing, were taken by DigiEye (Verivide Ltd., Leicester, U.K.) and analyzed with DigiEye software version 6.2. The analysis is based on determining the coordinates Li*, ai*, and bi*,39 defined by the Commission Internationale de l’Eclairage (CIE), for each stain before (i = 1) and after the wash (i = 2). These coordinates were used to calculate differences in lightness (ΔLi*), redness (Δai*), and blueness (Δbi*) for each stain in contrast to the background. The total difference was then calculated as * ,i = ΔEab

2 2 2 ΔLi* + Δai* + Δbi*

(6)

and stain removal index (SRI) was determined as follows: ij ΔE − ΔE2 yz zz × 100 SRI (%) = jjj 1 zz j ΔE1 k {

(7)

Indication of different efficiency of soil removal is given by comparing SRI for untreated and SRP-treated fabrics. 3.5. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was used to assess the effect of on fabric morphology by the SRP surface modification. Squares of 0.5 × 0.5 cm2 were cut and coated by electron-beam deposition with palladium. SEM was performed with a Field Emission Zeiss ΣIGMA instrument (Jena, Germany, tension 3 kV; emission 120 μA) directly on metallized samples.

4. RESULTS 4.1. SRP Deposition on Polyester. A study of streaming potential for untreated fabrics and SRP-treated swatches was performed. The zeta potential results for untreated polyester are shown in Figure 3. A typical value of zeta potential for polyester was found to be between −60 and −65 mV, which is in agreement with literature.40 The negative charge has been previously associated with adsorption of OH− ions on the

Figure 3. Electrokinetic curves of polyester fabrics in the presence and absence of SRPs. The curve for untreated polyester (■) is compared with same material after deposition of SRN240 (▲) and SRA300F (●). Zeta potential values at each pH are the result of the average between three replicates for three different samples. D


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Figure 4. Adsorption isotherms fittings for SRA300F (left) and SRN240 (right). Estimated parameters associated with Langmuir (solid line) and Langmuir−Freundlich (dashed line) are listed in Table 1.

surface of the polyester fiber.41 Increasing pH is found to cause a progressive decrease in zeta potential because of the increasing amount of available OH− ions present in the bulk electrolyte solution, which consequently increases their absorption on polyester. As expected, there is a change in zeta potential when the polyester has been pretreated with an SRP. Polyester pretreated with SRA300F results in a lower value of zeta potential, i.e., shifting the surface zeta-potential to a more negative value (around −90 mV). This shift is expected, as SRA300F is an anionic polymer and, therefore, its deposition on top of the fabric renders the charge on the polyester surface to be negative due to the sulfonate groups in SRA300F at pH from 6 to 9. Further evidence of this behavior is indicated in the small variation of zeta potential between pH 6 and 9, given the pKa of the polymer expected to be below pH 6. Conversely, SRN240 causes a shift of the zeta potential for polyester, to less negative values, because of the nonionic character of this polymer. Hence, the SRN240 polymer chains are expected to displace the OH− ions adsorbed onto the polyester surface or at least decrease their density on the surface with uncharged moieties. Moreover, a progressive decrease of zeta potential, with increasing pH, confirms that, for this sample, OH− adsorption on the surface of polyester is still the main driver for surface charging. As shown in Figure 3, the two SRPs have an opposite effect on the streaming potential, as a result of their different charge nature. There is a shift in the zeta potential of the fabrics after SRP deposition compared to the untreated polyester, and the difference in zeta potential is almost the same for both polymers. This might suggest that the two SRPs have the same efficiency of deposition. To test this hypothesis, the amount of adsorbed polymer was measured, indirectly, by UV/vis, which showed that about 26 ppm (51.2% w/w) of SRN240 deposited on polyester while only 7.8 ppm (15.6% w/w) for SRA300F, disproving the initial hypothesis. Based on this, it can be concluded that the zeta potential shift could not be used to assess efficiency of deposition, as it strongly depends on the charge density of the SRP. In fact, lower amount of SRA300F is deposited, but the difference in zeta potential vs untreated polyester is about the same (but with opposite sign) observed for higher amount of SRN240. Hence, the negatively charged SRA300F, even at a lower degree of deposition, is sufficient to significantly reduce the zeta potential. Conversely, the neutrally

charged SRN240 is only able to change the zeta potential by a similar amount because of the significantly higher degree of deposition. Further confirmation of the greater degree of deposition for SRN240 compared to SRA300F was observed in the analysis of adsorption isotherms (Figure 4). The unadsorbed amount of each SRP, in bulk, was measured via UV/vis, and the equivalent amount of polymer deposited per mass of fabric (qE) was calculated. The relationship between the amount of adsorbed SRP and the corresponding equilibrium bulk concentration of the SRP can be quantified by the adsorption isotherm. In the adsorption isotherms shown in Figure 4, the Langmuir−Freundlich model fits the data best. By comparing the parameters associated with these fittings (Table 1), it is Table 1. Fitting Parameters Obtained by Fitting the Adsorption Isotherms for SRA300F and SRN240 on Polyester with Langmuir and Langmuir−Freundlich Models SRA300F Qsat (mg/g) K (ppm) Qsat (mg/g) K (ppm)−m m

SRN240

Langmuir 1.0 ± 0.3 (5.0 ± 0.2) × 10−3 Langmuir−Freundlich (5.3 ± 0.4) ×10−1 (1.6 ± 0.2) × 10−4 2.1 ± 0.4

(7.3 ± 1.1) × 10−1 (6 ± 4) × 10−2 (6.0 ± 0.2) × 10−1 (9.0 ± 0.2) × 10−5 3.6 ± 0.9

evident that SRN240 requires a lower amount to saturate the polyester, having a lower Qsat. For both SRPs, the adsorption is cooperative. Moreover, we could calculate the binding affinity of the two SRPs to the polyester as K0 = K1/m, where K is the Langmuir−Freundlich constant obtained in the fitting. This comparison identifies SRN240 as the SRP with the highest affinity to the polyester, which could be a consequence of its neutral charge. In fact, the electrostatic repulsion between the anionic SRA300F and the negatively charged polyester surface could be the reason for the lower affinity of this SRP to the polyester fabric. The zeta potentials for modified fabrics, as a function of SRP concentration, were measured and found to behave similarly. The data for SRA300F are presented in Figure 5a. It is evident that a plateau in the zeta potential is reached at around 75 E


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Figure 5. (a) Zeta potential for polyester fabric at pH 6 (■) and pH 8 (●) after pretreatment with different concentrations of SRA300F. (b) Zeta potential of polyester modified by SRA 300F at pH 6 and 8 vs ppm of SRA300F adsorbed on fabrics. Pretreatment was run in tergotometer at 200 rpm at pH 8 for 1 h, and fabrics were analyzed after drying.

ppm, and zeta potential remains fairly constant above this concentration. This behavior is in line with multilayer formation, which explains why, as shown by the UV/vis, when a higher amount of SRP is deposited, there is not a corresponding change in zeta potential. If the concentration of deposited SRP is plotted against zeta potential, a linear trend is observed in the first part of the concentration range. This behavior means that the zeta potential parameter is directly proportional to the amount of SRP adsorbed. Conversely, a plateau is reached for a concentration >75 ppm which means that streaming potential is not sensitive enough to detect the formation of multilayers. The streaming potential is then a useful technique to assess deposition of actives on fabrics. However, it shows limits in the detection of SRP multilayers, as the detected charge remains constant even when a new layer is deposited. 4.2. Impact of Agitation Shear Forces. The zeta potential of polyester and pretreated polyesters as a function of pH is presented in Figure 6. The plot indicates that agitation

speed does not play a role in SRPs deposition, as the zeta potential curves of fabric pretreated at 200 and 100 rpm are close. This was expected, as agitation might play a role in the kinetic of deposition, but it does not affect the thermodynamic equilibrium between SRP deposited and SRP left in bulk. 4.3. Soil Release Properties of the Polymers: Polyester. A stain removal test was performed on polyester samples with and without preconditioning with the SRPs. Stain removal indexes (SRIs) were calculated via image analysis of stains before and after the washing and are shown in Figure 7.

Figure 7. Chart of stain removal index for polyester fabrics that have been washed with and without SRPS and then stained with artificial sebum (in red), lard (in blue), and collar and cuff (in black).

Comparison of the SRIs, between untreated and treated polyester, suggests that SRPs deliver soil-release benefits at the low concentrations (50 ppm) typically used in washing formulations. The SRI achieved for the stains is comparable for the two SRPs, except on collar and cuff where SRN240 seems to deliver better results. This may be due to the higher amount of SRN240 deposited, compared to SRA300F, that can result in lower adhesion of the soil on the surface and easier removal of complex soil mixture including particulates from polyester in the first case.

Figure 6. Zeta potential of polyester fabric pretreated per 1 h at 35 °C with SRA300F (■, ●) and SRN240 (▼, ▶) using an agitation speed of 200 rpm (■, ▼) and at 100 rpm (●, ▶) vs untreated polyester (▲). F


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Industrial & Engineering Chemistry Research 4.4. SEM Analysis. The comparison between SEM images of unmodified polyester and polyester post-SRP deposition (Figure 8) identified differences in fabric morphology after

Figure 9. Electrokinetic curves of cotton fabrics with and without conditioning stage with soil release polymers (1 h, 35 °C). The curve for untreated cotton (■) is compared with same material after deposition of SRN240 (▲) and SRA300F (●).

Figure 8. SEM images of untreated and SRP-modified polyester at 10 000× (a, c, e) and 50 000× (b, d, f) magnification. Figures represent untreated polyester (a, b) and polyester pretreated with SRA300 F (c, d) and SRN240 (e, f). In all cases fabrics have been metalized by electron-beam deposition with palladium.

surface modification via SRP. Unmodified polyester shows higher roughness even at lower magnification, and the porosity appears evident when reaching a magnification of 50 000×. A smoothing effect can be observed for the fabric modified via SRA300 (Figure 8c), with a change of granulometry of the fiber surface versus unmodified polyester. However, in the same picture some nonhomogeneous areas were observed that could be unmodified areas of the fabric. Deposition of SRN240 results in a smoother surface (Figure 8e), and deposition seems more homogeneous on the fibers. This suggests the formation of a homogeneous polymer film on the fabric that will block the pores and will then result in a lower penetration of soil inside the fabrics pores. This observation justifies the higher SRI observed on fabrics modified via the nonionic soil release polymers and is in line with the higher degree of deposition for this SRP compared to SRA300F. 4.5. SRP Deposition on Cotton. The zeta potential curves for treated vs untreated cotton, as a function of pH, were measured (Figure 9). No significant difference can be observed in the zeta potential between SRP-treated samples, and no differences in SRP concentration in bulk were confirmed via UV/vis. This might be explained by the lower affinity of cotton for POET block adsorption which confirms that this SRP structure is not compatible to deposition on natural fibers. To verify the absence of polymer deposition on knitted cotton, stain removal tests were performed, and the results are shown in Figure 10. In general, the SRIs observed are higher than polyester, as cotton is more hydrophilic and

Figure 10. Chart of stain removal index for cotton fabrics that have been washed with and without SRPS and then stained with artificial sebum (in red), lard (in blue), and collar and cuff (in black).

has weaker interaction with hydrophobic soil and hence is easier to remove and emulsify the greasy soil. However, no significant improvement in grease removal from cotton was observed after polymer deposition at the tested level suggesting weaker soil adhesion on cotton. This is a further confirmation of the lack of deposition of SRPs on cotton, as the SRI is the same for untreated and SRP treated knitted cotton.

5. CONCLUSIONS This work has identified that polymer charge is the most relevant parameter in affecting streaming potential values for polyester after the deposition of soil release polymers. This observation revealed that the direction of the shift is determined where the SRP is anionic or nonionic in nature. The use of UV/vis to measure SRP concentration left in the bulk solution after deposition onto the fabric surface has helped to estimate the amount of polymer deposited on polyester. The nonionic SRN240 was identified to have the higher degree of deposition. Comparison of amount of SPRs deposited, with shift in streaming potential curves, implies that the net-charge of the anionic SRP affects the zeta potential G


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more than the nonionic SRP, even at lower deposition concentrations. However, determination of stain removal indexes, of oily and particulate stains, reveals that the percentage of adsorption has a greater impact on enhancing cleanability of the fabrics. Improved deposition and surface modifications could be achieved via an increase in concentration of the polymer. This study has shown that streaming potential is not sufficient to detect further changes of zeta potential after that SRP saturates the outer layer of the fabric. Analysis of cotton, after SRP’s deposition, revealed that these polymers are not efficient in conditioning cotton fabrics, and this was confirmed by UV/vis data and stain removal tests. Further investigation is necessary to understand the effect of SRPs on a fabric’s morphology and on the effect of surfactant on the deposition.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected]. *E-mail address: [email protected]. ORCID

Alessandra Valentini: 0000-0002-9304-7443 Gerardo Palazzo: 0000-0001-5504-2177 Nicola Cioffi: 0000-0002-6765-440X Eric Robles: 0000-0003-2021-4850 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the European Union’s Horizon 2020 research innovation programme under grant agreement No. 722871 in the scope of the Marie Skłodowska-Curie Action ITN BioClean.

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REFERENCES

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