Reducing Hydrophilic Characteristics of Kraft Paper Insulation by Reinforcing With Surface Modified Rutile-TiO2 Nanoparticles

The hydrophilic nature of cellulose compromises the dielectric properties of kraft paper insulation when exposed to water and moisture. The effort to address this setback is an ongoing research enquiry. In the present work, surface-modified rutile-titanium dioxide nanoparticles (rutile-TiO2 NPs) are reinforced into cellulose pulp to fabricate a nanocomposite kraft paper insulation. Surface modification of the nanofiller particles is intended to reduce the hydrophilicity of the resultant nanocomposite kraft paper insulation. The nanoparticles (rutile-TiO2 NPs) improve other dielectric properties of the insulation. The chemicals used for the surface modification of the rutile-TiO2 NPs were the alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). The nanofiller retention, dispersion and hydrophobic properties of the resultant reinforced kraft paper were experimentally analyzed. The results show that more than 50% of the nanofillers were retained within the reinforced kraft paper and translated to 1.5% by weight loading of the nanoparticles in the kraft paper. Scanning electron microscopy (SEM) images showed evenly distributed nanofillers, with some traces of agglomeration. The moisture absorption property of the kraft paper specimen modified with rutile-TiO2 NPs surface conditioned with 5 vol/vol% ASA improved by 74% compared to the control (reference) specimen. Water vapor transmission rate of the surface-modified nanoparticles reinforced kraft paper insulation decreased by 30%. Compared to the unfilled paper, the contact angle of water droplets on the surface of the reinforced kraft paper improved by 12%. Water absorption rate improved by being 4 times slower in paper specimens containing rutile-TiO2 NPs surface conditioned with 5 vol/vol% ASA. Dielectric dissipation factor measurement results showed that the specimen modified with rutile-TiO2 NPs surface conditioned with 5 vol/vol% ASA had 40% lower dielectric losses than the reference samples. This study, therefore, has successfully improved the hydrophobic properties of kraft paper by filling it with surface-modified rutile TiO2 using 5% ASA as the nanoparticle surfactant.

INDEX TERMS Cellulose, dissipation factor, insulation, kraft paper, nanoparticles, nanofiller, moisture absorption, power transformer, rutile, titanium dioxide. 24 The reliability of power transformers largely depends on the 25 insulation [1], [2], [3], [4], [5]. In power transformers, the 26 agents of insulation degradation include moisture, oxidation, 27 The associate editor coordinating the review of this manuscript and approving it for publication was Ajit Khosla . and pyrolysis (heat) under the influence of electrical and 28 mechanical stresses [1], [3], [6], [7], [8]. 29 Existing thermally upgraded kraft paper technologies for 30 transformer applications use compounds such as amine and 31 cyanoethylation. However, these techniques have the risk 32 of producing toxic byproducts. Consequently, the literature 33 often highlights the need to explore alternative means to 34 improve the thermal stability of kraft paper insulation [4]. been found to be superior among the various alternatives. 68 TiO 2 has a low cast, and its rutile phase is reported to be 69 chemically stable, has a high dielectric constant, withstands 70 higher temperatures and does not readily decompose as com-71 pared with other phases of the TiO 2 [4]. However, metal 72 oxide NPs, including TiO 2 are hydrophilic. The hydrophilic 73 nanoparticles therefore compromise the effort to improve 74 the dielectric properties of the cellulose nanocomposite kraft 75 paper insulation [4], [17], [18]. 76 The present work explores the possibility of reduc-77 ing kraft paper's water absorption characteristics using 78 surface-modified rutile-TiO 2 nanoparticles (NPs). The 79 authors' previous work [4], [5] presents details on 80 how the rutile-TiO 2 nanoparticles were fabricated and 81 surface-modified to reduce moisture absorption and improve 82 thermal stability properties. In the rest of this article, the 83 process of the kraft paper nanocomposite fabrication is 84 presented, followed by the analysis of the nanofiller reten-85 tion rate within the kraft paper, morphology analysis of 86 the paper, and the hydrophobicity tests in comparison with 87 unfilled paper.

II. FABRICATION OF THE RUTILE-TiO 2 NANOCOMPOSITE
The raw materials for the kraft paper fabrication process 92 were; electrical grade unbleached kraft pulp (unbleached sul-93 phur) as supplied by Sappi TM Technology Centre (Pretoria, 94 South Africa), distilled water, filler retention aid and the 95 surface-modified rutile-TiO 2 nanoparticles. The nanoparti-96 cles were surface modified with alkyl ketene dimer (AKD) 97 in one case and in the other, with alkenyl succinic anhydride 98 FIGURE 1. The process of kraft paper fabrication using a Handsheet maker. The rutile-TiO 2 kraft paper composite was fabricated follow-106 ing the procedure illustrated in Figure 1. The same procedure 107 was followed for the control (reference) sample but without 108 adding the nanofillers and retention aid. Both samples (con-

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According to the authors' previous work [5] and 122 El Omari et al. [20], the rate of nanoparticles dispersion, 123 particle agglomeration and amount of nanoparticles retained 124 within the kraft paper nanocomposite influence the resultant 125 properties of the fabricated paper. Therefore, reducing the 126 water absorption characteristics of kraft paper with reinforced 127 surface-modified nanoparticles is proportional to the amount 128 of retained nanofillers, filler distribution and agglomeration. 129 This section examines the percentage of nanofiller (rutile-130 TiO 2 NPs) retained in the composite kraft paper using the 131 ash analysis method and the nanofiller distribution uniformity 132 examination using scanning electron microscopy (SEM). Nanofiller retention is the amount of filler content in the com-136 posite paper [21]. The nanofiller retained in the sheets was 137 determined using the ash content method in accordance with 138 the International Standard ISO 2144:1997(E) [22]. A muffle 139 furnace set at 900 • C was used to determine the ash content. 140 The samples were weighed before and after heating using 141 the analytical weigh balance. Figure 2 depicts the quantity of 142 nanofillers that were retained. At least half of the nanofillers 143 are retained in all the samples within the kraft paper rein-144 forced with surface-modified TiO 2 NPs and the unmodified 145 rutile-TiO 2 NPs. The kraft paper reinforced with unmodified 146 nanoparticles has the highest retention of 57%. The retention 147 of the nanofillers indicates the fibre-filler-fibre bond formed 148 between the cellulose fibre and the nanofillers; the retention 149 aid enhances the bonding.

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Cellulose wood fibres are often inert to numerous chemi-151 cals. However, cellulose wood fibre contains water molecules 152 at room temperature, which can react to hydrolyze the 153 silane [23]. According to Zhang et al. [24], the dioxygen 154 complex nature of TiO 2 NP attracts H 2 O to complete its 155 FIGURE 3. Interaction mechanism of cellulose kraft paper (a) with surface-modified rutile-TiO 2 NPs using ASA, (b) with surface-modified rutile-TiO 2 NP using AKD.   The handsheets were exposed to humid atmospheric air for 186 72 hours to study the sizing efficiency. The atmospheric con-187 ditions were temperature and relative humidity of 23 ±3 • C 188 and 53 ± 5%, respectively. At least three specimens of each 189 FIGURE 5. Surface morphology of the kraft paper test specimens (a) without nanofiller (b) with nanofiller particles. specimen type were used. The samples were cured (dried) in a 190 laboratory oven at 105 • C for 20 minutes and weighed. They 191 were then exposed to the atmospheric air for 72 hours after 192 which they were weighed again. This approach to measuring 193 moisture absorption characteristics of kraft paper has also 194 been used by other researchers such as; Gasser et al. [28], 195 Cigre Working Group A2.30 [29] and Koch [30]. 196 The results as presented in Figure 6, show significant 197 increase in mass in all samples due to the absorbed moisture. 198 However, the control (unfilled) and the unmodified rutile-199 TiO 2 NPs kraft paper specimens show a significantly higher 200 percentage increase in mass than the rest of the samples. 201 Specimens reinforced with AKD surface-modified nanofiller 202 show more hydrophilic properties than those with nanopar-203 ticles modified with ASA. After 72 hours, the kraft paper 204 specimens reinforced with surface-modified rutile-TiO 2 NPs 205 (KP/T-ASA5%) has absorbed the least moisture; about 74% 206 less than the unfilled paper.

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The water vapour transmission rate (WVTR) of the kraft 209 paper specimens was measured following the ISO 2528 stan-210 dard procedure [31] and the conditions were as specified in 211 ISO 187 [32]. The experimental process is as illustrated in on the paper's surface. The Young's model [37], [38]   The present work examined the contact angle and rate at 241 which the contact angle changes with time as the water is 242 absorbed into the kraft paper surface.

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The measurements were carried out using a Kruss Easy 244 Drop (Model FM40Mk2). A droplet of distilled water was 245 placed on the sample and captured using a high-resolution 246 VOLUME 10, 2022 camera. Five spots were tested on each of three replicated accordance with the TAPPI T 558 standard [39].

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As per the contact angle measurement (Table 1) Figure 10. Con-279 sequently, the kraft paper that is reinforced with the modi-280 fied rutile-TiO 2 NPs reacts more hydrophobically with the 281 surrounding moisture. However, it is essential to understand 282 that hydrolysis of the ester bond (of AKD and ASA) between 283 the sized molecules and polymer can result in sizing reversion 284 over time. This is explained in the literature as a phenomenon 285 of loss of size response [33], [35], [36], [42], [43], [44]. 286 Since moisture and water is detrimental to the insulation 287 integrity of kraft paper, the presence of moisture in kraft paper 288 insulated equipment needs to be diagnosed. In that regard, 289 dielectric loss measurement is a common tool for moisture 290 condition diagnosis of kraft paper-based insulation. The next 291 section presents the dielectric loss measurement results of the 292 unfilled and the kraft paper specimens reinforced with rutile-293 TiO 2 nanoparticles.

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The overall result of the work presented in this paper shows 334 that when a cellulose paper (with or without rutile-TiO 2 335 NPs) is directly or indirectly exposed to air-containing water 336 molecules, the air humidity is absorbed. However, reinforc-337 ing the kraft paper with surface-modified rutile-TiO 2 NPs 338 significantly increases the resultant hydrophobicity of the 339 nanocomposite kraft paper. Compared with the control sam-340 ple, it was observed in this work that the specimen modified 341 with rutile-TiO 2 NPs (containing 5 vol/vol% ASA) absorbed 342 less moisture and dropped in water vapor transmission rate 343 by about 74% and 30%, respectively. The contact angle of 344 water droplet on the surface of the composite kraft paper 345 compared with the control specimen increased by 12%. The 346 dielectric loss constant of the kraft paper reinforced with 347 surface-modified rutile-TiO 2 NPs (containing 5 vol/vol% 348 ASA) compared with the control sample decreased by 40%. 349 The results also showed that the efficiency of the sizing AKD 350 and ASA surfactants depends on temperature, and AKD is 351 more effective at a higher temperature.

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In this research, rutile-TiO 2 NPs were surface modified 353 (sized); however, the internal sizing of the kraft pulp itself 354 reinforced with nanoparticles is an alternative technique that 355 needs to be explored.