Mechanical performance of nano-silica modified insulating paper with mechanical-thermal synergy

Abstract

In this paper, the mechanical properties of nano-silica modified insulating paper under the combined action of mechanical vibration and temperature conditions are studied. Unmodified and nano-silica modified cellulose insulating paper with 2 wt% and 4 wt% were prepared, respectively, and a series of mechanical-thermal synergy experiments were carried out. With the same mechanical stress and temperature, and with the same aging duration of 144 h (6d), the tensile strength of modified insulating paper with 4 wt% nano-silica, increased 0.99 kN/m and 0.55 kN/m, respectively, compared with those of the unmodified and the 2 wt% nano-silica modified insulating paper. The experiments indicate that the nano-silica modification can effectively improve the mechanical properties of insulating paper. In this work, the modified mechanism of nano-silica is analyzed from the interface effect of modified polymer and the quantum effect of the modified polymer interface two aspects. It is shown that the interface formed in the modified insulating paper can transfer the mechanical stress acted on the insulating paper and prevent the cracks formed in the aging process of the test sample from further expansion, while the quantum effect discretizes the electron energy level, which can restrict the motion of the molecular chain segment to some extent. The conclusion can be used for reference to improve the performance of insulating paper.

Keywords

Degree of polymerization (DP)mechanical-thermal synergy nano-silica particle tensile strength (TS)

1. Introduction

AT present, cellulose insulating paper is still the main solid insulating medium in large oil-immersed power transformers for peculiar characteristics such as easy accessibility of raw materials, completely degradation and excellent insulating performance of chain polymerized hydrocarbons. As far as insulating oil is concerned, its insulating performance can be recovered or improved through filtering or replacing oil. But the insulating paper, which is wrapped around the windings, is irreplaceable. Once either the electrical or the mechanical strength of the insulating paper degrades below its security threshold, the transformer must exit from service to avoid blackout caused by insulation fault [1]. So the health condition of the insulating paper relates to the life expectancy of the transformer directly, it is critical to evaluate the aging status effectively to ensure the safety and stability of large power transformers.

During long-term transformer operation, the insulating paper always endures the co-effect of mechanical, thermal, electrical stresses, and environmental factors, which may gradually deteriorate its insulating performance, and power system fault may be induced. Related works of literature have shown that thermal aging [2] is the main cause of insulation degradation, and the moisture generated during aging process can accelerate the deterioration rate [3,4]. And other factors including copper sulfide deposition [5], electric fields [6], may play an active role in the degradation. But little attention was focused on the effect of vibration induced by DC bias in converter transformers [7,8] on the insulation degradation. On the other hand, to meet the ever-growing energy demand, more and more ultra HVDC (UHVDC) projects have been constructed and put into use, and converter transformer numbers showed a proliferation trend [9]. In [10], the negative effect of mechanical vibration within converter transformers on the deterioration of insulating paper mechanical properties was studied. Hence, it is necessary to improve the anti-aging ability of insulating paper and extend its residual life expectancy of in-service transformers to ensure a sustained and stable power supply.

It is well known that the insulation properties of the cellulose insulating paper are mainly embodied by cellulose whose weight ratio accounts for about 90% within insulating paper, and cellulose is a linear polymerized hydrocarbon composed of D-anhydroglucose units (AGU) linked by 𝛽 −1, 4 glycoside bonds, with C₁-chair conformation [11]. Therefore, it can be achieved that the anti-aging ability of insulating paper can be modified by increasing the deterioration resistance of cellulose. Now, there are two kinds of methods to enhance the anti-thermal-aging ability of insulating paper, namely chemical modification and physical modification of cellulose. The former uses a more stable chemical group to replace the hydroxyl (−OH) in cellulose, while the latter uses heat stabilizer or inorganic nano-particles to improve the cellulose properties [12]. As far as the chemical modification method is concerned, during the process of the hydroxyl replacement, the linkage bonds between AGU may be damaged even broken, so the degree of polymerization will reduce and its mechanical strength may decline. On the other hand, the heat stabilizer such as amine compounds and inorganic nano-particles including montmorillonite, aluminium oxide (Al₂O₃), titanium dioxide (TiO₂) and so on, used inphysical modification method, have no effects on the linkage of celluloses. Meanwhile, quantum effect and large specific surface area at nano-scale and some other peculiar merits of nano-particles can be used to promote the electrical properties, such as permittivity, space charge characteristics and so on. So, most scholars focus attention on the cellulose physical modification method [13,14]. However, few researchers pay attention to the experimental or theoretical aspects of the degradation properties of nano-particles modified insulating paper (MIP) with the mechanical-thermal synergy effect.

Based on the previous research, nano-silica modified cellulose insulating paper was prepared with different weight ratios of 0 (unmodified), 2% and 4%, respectively. Then the mechanical-thermal aging (MTA) experiments were carried out in a high-temperature-vibration aging oven. And the mechanical properties including the tensile strength (TS) and the degree of polymerization (DP) of the specimens were measured. Finally, the anti-mechanical-degradation mechanism of the nano-silica modified insulating paper was explored.

2. Preparation and aging experiments

2.1. Determination of nano-particle type and weight content

It is well known that so many kinds of nano-particles can be used, such as aluminium oxide, titanium dioxide and montmorillonite as previously described, to improve the properties of cellulose, while neither determined selection principles nor weight content guidance can be found. Considering the fact that nano-silica particles have a large number of unsaturated bonds and hydroxyl groups in different bond states on their surfaces besides economical, chemical and electrical characteristics, and degree of difficulty to connect with cellulose base, nano-silica particle was adopted here. And the parameters of selected nano-silica particles are shown in Table 1.

Table 1
Parameters of selected nano-silica particles

Index Description

Shape Spherical particle

Diameter 40 ± 5 nm

Purity quotient 99.8%

Specific area 170 m²/g

Hydroxyl ≥45%

Index	Description
Shape	Spherical particle
Diameter	40 ± 5 nm
Purity quotient	99.8%
Specific area	170 m²/g
Hydroxyl	≥45%

The weight content of nano-particle is critical to determine the mechanical and electrical properties of the modified insulating paper. It proved that the nano-particle aggregation appeared when its weight content exceeded one given value for all nano-particles, and then the anticipated effects could be affected seriously. Though different physical and chemical methods are adopted to overcome aggregation, the optimal weight content of nano-particle is hard to determine and the trial and error method is used in practice. Here, for simplicity, two groups of modified paper with nano-silica 2 wt% and 4 wt% are received.

2.2. Preparation of the proposed insulating paper

This paper selects the following raw materials including the unbleached coniferous kraft pulp produced in Russia and Hydrophobic-170 hydrophobic nano-silica particles produced by Shanghai Tuosheng Biotechnology Co, Ltd, PRC. According to literature [16], virginal and nano-silica modified cellulose insulating paper with 2 wt% and 4 wt% were prepared, through soaking, beating, dissociating and paper-making steps in turns.

During the paper-making process of MIP, the surface treatment of nano-particle is critical. Polyamic acid (PAA) prepolymer solution was firstly prepared with N,N-dimethylacetamide (DMAc, made by Sinopharm Chemical Reagent Co., Ltd., PRC), 4,4^′-diaminodiphenyl ether (ODA, made by Shanghai Jingchun Biochemical Technology Co., Ltd., PRC) and pyromellitic anhydride (PMDA, made by Shanghai Jingchun Biochemical Technology Co., Ltd.) solvent. Secondly, KH-550 silane coupling agents (SCA) were obtained through magnetic stirring the mixed solution of DMAc, distilled water and a small amount of aminopropyltrie-thoxysilane (APTES, made by Nanjing Youpu Chemical Co., Ltd., PRC). Thirdly, the nano-silica particles were surface treated with deionized water with melamine dissolved and the KH-550 SCA mentioned above to make the nano-particles more compatible with cellulose. Fourthly, nano-silica particles were put into PAA resolution, and immersion liquid was achieved by churning the silica/PAA mixture. Finally, the dissociated wood pulp was mixed with the prepared immersion liquid, and the sheet can be manufactured after churning and ultrasonic vibration of the pulp.

Here, the prepared nano-silica MIP has a diameter of 200 mm, an average thickness of 0.13 mm, a tightness of 0.96 g/cm³, tensile strength of 8.8 kN/m, which is in accordance to the literature [13,15]. For convenience, the group of unmodified insulating paper labeled with A, and the other two groups with the mass ratio of nano-silica 2% and 4% labeled with B and C, respectively.

2.3. MTA experiments

According to the method described in the document [10], the aforementioned three groups of insulating paper were pre-processed to obtain particular rectangular samples with 200 mm length and 10 mm width. Then, the prepared specimen was fixed in the high temperature-vibration aging oven to carry out aging experiments with the same method as literature. The MTA experiments include three kinds of conditions named E1, E2 and E3. The aging conditions of E1 were as follows: temperature 130 °C, no vibration, aging duration was 2d, 4d, 6d, 8d, 10d. For kind E2, the vibration frequency was 10 Hz, the amplitude was 1 mm, the temperature and aging time were the same as those of E1. And for kind E3, the amplitude was 2 mm, the other conditions were the same as those of kind E2.

2.4. Mechanical properties measurements

∙ Tensile Strength (TS)

With the variation of TS, the deterioration rate of insulating paper under given aging conditions can be characterized quantitatively, and the residual life assessment criterion of insulating paper can also be obtained [10]. Here, ZL-100A/300A Shauber-type tension tester for paper and board (produced by Changchun Paper Testing Machine Factory, PRC) was adopted to determine TS values of the aged samples according to the national standard GB/T 12914-2008 [16]. It should be noted that the constant speed tension method was used to measure TS with the speed of 20 mm/min to reduce the viscoelasticity effect of the paper. The average of TS values of each group was adopted as the measurement result.

∙ Degree of Polymerization (DP)

DP is the most objective characteristic for evaluating the residual life of insulating paper, and the method of viscosity for measuring DP proposed by Oomen and Arnold [17] was widely used in practice. Based on DP, first-order and second-order dynamic models were developed [18,19]. This work adopts the viscosity method to measure the DP values of each group of specimens according to technical guideline [20], and the used pulp viscosimeter was produced by Beijing Hengcheng Technology Co., Ltd. The results are the average of the three measurements.

3. Results and analysis

3.1. Change rule of unmodified insulating paper

According to the aforementioned method, the TS values of each group of the prepared insulating paper are measured as shown in Figs 1, 2 and 3, respectively. The initial TS value of the unmodified insulating paper sheet is 7.96 kN/m without aging.

Fig. 1.

TS values of unmodified insulating paper.

Fig. 2.

TS values of MIP with 2 wt% nano-silica.

Fig. 3.

TS values of MIP with 4 wt% nano-silica.

It is of interest to compare the obtained three figures, where three typical featurescan be found. Firstly, all TS values of the three groups decrease monotonically with the increase of aging duration under the same mechanical and thermal aging conditions (E1, E2 or E3). This tendency is consistent with the experimental results of Li [10] and Hill [18], which can be illustrated with TS values of group A with condition E1, TS decreases from the original 7.96 kN/m to 7.51 kN/m with 2 days’ duration, and it decreases a step further to 7.37 kN/m after 4 days’ degradation. Secondly, the TS values also decrease with the augmenting of the amplitude, and the larger the amplitude is, the decrease of TS is more obvious under the condition of the same aging duration. This feature can be discussed by the case of TS values of group B with the condition of the same aging duration of 6 days, the TS value is 8.42 kN/m without vibration, but it reduces 1.06 kN/m and 0.28 kN/m to be 7.36 kN/m and 7.08 kN/m, respectively, when the amplitude increased to 1 mm and 2 mm. This trend is also as same as that showed in literature [10]. Meanwhile, TS values of nano-particles modified specimens in each group are greater than those of the original specimen under the same aging conditions, which is the most important feature of the three figures. Here, TS values with aging condition E3 are taken as a precedent, after 8 days’ aging, TS values of group B and C are 6.42 kN/m and 6.32 kN/m, respectively, both greater than that of group A, 6.08 kN/m. And the changing tendency of TS values after 2, 4, 6 and 10 days are in line with the above ones. Considering the above facts, it can be concluded that the nano-silica particle can make salient progress in the mechanical property of insulating paper.

From [21], it is known that the degradation rate of cellulose can be depicted with the first-order reaction kinetics equation. That is, the relationship between the TS loss rate ϵ_TS (defined as ϵ_TS = 1 − TS∕ TS₀, TS₀ is the initial TS) and the aging duration t can be shown as Eq. (1). $\begin{eqnarray}\displaystyle {\displaystyle \frac{d{\varepsilon}_{\text{TS}}}{dt}}=k_{\text{TS}}({\varepsilon}_{\text{TS}}^{\ast }-{\varepsilon}_{\text{TS}}),\quad {\varepsilon}_{\text{TS}}|_{t=0}=0 & & \displaystyle\end{eqnarray}$ (1) where k _TS is the TS degradation rate constant. It is assumed that cellulose has a constant reaction rate during the aging duration, that is k _TS = constant, and ${\varepsilon}_{\text{TS}}^{\ast }$ is the capacity of the TS degradation reservoir. The exact solution of Eq. (1) can be inferred as shown with Eq. (2). $\begin{eqnarray}\displaystyle {\varepsilon}_{\text{TS}}={\varepsilon}_{\text{TS}}^{\ast }(1-e^{-k_{\text{TS}}t}). & & \displaystyle\end{eqnarray}$ (2) With the measured TS values and Eq. (2), Fig. 4 can be obtained, all of which have the same shape as those in literature [21]. The constants k _TS and ${\varepsilon}_{\text{TS}}^{\ast }$ of TS in the three groups are shown in Table 2. In these figures, curves A, B, and C are the fitting curves satisfying Eq. (2), respectively, and discrete remarks such as ○, □ and Δ are the measured values of each group under the given aging conditions.

Fig. 4.

Relationship of ϵ_TS vs. aging duration for three groups of the unmodified specimen with MTA experiments.

From Fig. 4 and Table 2, the following conclusions can be drawn that with constant aging temperature, the degradation rate and the degradation reservoir of the unmodified insulating paper will increase with the mechanical vibration amplitude strengthens, which confirms the conclusions of the literature [10]. In other words, mechanical vibration accelerates the loss of the insulating paper mechanical life, and the greater the amplitude is, the faster the insulating paper mechanical life losses. Further, it can be seen from Fig. 4 that if the mechanical vibration amplitude keeps constant, the longer the aging time is, the larger the ϵ_TS gets, and the aging duration that ϵ_TS required reaching its steady state decreases with the amplitude increases.

3.2. TS change rule of modified insulating paper

When the weight ratio of nano-silica is 2%, the TS value of the unaged MIP is 10.28 kN/m. Similar to Section 3.1, Fig. 5 can be obtained according to Eq. (1) and the constants k _TS and ${\varepsilon}_{\text{TS}}^{\ast }$ are shown in Table 2. Furtherly, it can be found that the TS of MIP increases obviously when the weight ratio of nano-silica is 2%. Taken the data aged 8 days as an example, the TS increases from the original 6.94 kNm to 7.50 kN/m under the 130 °C thermal aging condition. When the vibration amplitude increases from 0 to 1 mm and keeps the temperature and aging duration constants as 8 days, the TS increases from the original 6.76 kN/m to 6.86 kN/m. While the amplitude turns to be 2 mm and other aging conditions keep as the same as the above, the measured TS is 6.42 kN/m, increases 0.34 kN/m compared to the original 6.08 kN/m. Correspondingly, the increment of ${\varepsilon}_{\text{TS}}^{\ast }$ is 0.1699, 0.2111, and 0.18, respectively, and the increment of k _TS is 0.0804, 0.09, and 0.0196, respectively. The results are firm evidence that nano-silica can improve the TS of cellulose insulating paper to a limited extent, and the increment of k _TS indicates that the ability of the MIP has an enhanced ability to withstand mechanical degradation.

Fig. 5.

Relationship of ϵ_TS vs. aging duration for three groups of MIP with MTA (with 2 wt% nano-silica).

Table 2

Calculated parameters for fitting Eq. (2) to the data in Figs 4, 5 and 6

Aging condition	Fig. 4			Fig. 5			Fig. 6
	${\varepsilon}_{\text{TS}}^{\ast }$	k _TS∕d ⁻¹	R ²	${\varepsilon}_{\text{TS}}^{\ast }$	k _TS∕d ⁻¹	R ²	${\varepsilon}_{\text{TS}}^{\ast }$	k _TS∕d ⁻¹	R ²
E1	0.1352	0.2348	0.8933	0.3051	0.3152	0.8971	0.3542	0.1556	0.8327
E2	0.1785	0.2466	0.8876	0.3896	0.3366	0.8741	0.4868	0.1664	0.8648
E3	0.2524	0.3362	0.9188	0.4324	0.3558	0.9122	0.5124	0.1712	0.9023

When the weight ratio of nano-silica is up to 4%, the TS of MIP is 10.36 kN/m before aging. Correspondingly, the relationship between the TS and aging duration is shown in Fig. 6. The constants of k _TS and ${\varepsilon}_{\text{TS}}^{\ast }$ are also listed in Table 2.

Fig. 6.

Relationship of ϵ_TS vs. aging duration for three groups of MIP with MTA (with 4 wt% nano-silica).

Compared Fig. 5 with 6, it can be seen that when the weight ratio of nano-silica is up to 4%, the increments of ${\varepsilon}_{\text{TS}}^{\ast }$ of the three MIP groups are 0.0491, 0.0972 and 0.08 compared to those of 2 wt%, which gives a powerful illustration that the TS of MIP can be enhanced by increasing the weight ratio. However, from Table 2, it can be learned that k _TS deceases as the weight ratio increases from 2 wt% to 4 wt%, and the decline of the three groups is 0.1556, 0.1702 and 0.1846, respectively, which indicates that the time increases for the TS to reach the steady state value with the mechanical-thermal synergy. That is, the rate constant of TS degradation per unit time of the sample becomes slower.

3.3. DP change rule of modified insulating paper

During the mechanical-thermal aging process of insulating paper, the insulating performance degrades as the breaking of 1–4 glycoside bonds between glucose monomer units [22]. Among the thermal and mechanical effects, the former activates the reactive groups in the cellulose fibers such as aliphatic hydroxyl groups, making further degradation of the fibers, while the fibers embrittle gradually. While the latter plays a mechanical shear act, which will consolidate the fragmentation of fibers. As the aging conditions and aging duration changing, the number of the broken bond of 1–4 glycosidic between glucose monomer units varies, which results in the DP changing accordingly. The measured DP data of the samples are shown in Table 3.

Table 3
Measured DP values of groups A, B, and C

Aging time/d Group A Group B Group C

E1 E2 E3 E1 E2 E3 E1 E2 E3

2 1164 1133 1129 1262 1235 1209 1141 1130 1100

4 1045 987 960 1214 1084 1104 1129 1075 1062

6 832 793 737 1053 967 964 1062 916 910

8 819 697 601 962 869 728 951 855 828

10 599 593 540 805 633 584 792 709 641

Aging time/d	Group A	Group B	Group C
2	1164	1133	1129	1262	1235	1209	1141	1130	1100
4	1045	987	960	1214	1084	1104	1129	1075	1062
6	832	793	737	1053	967	964	1062	916	910
8	819	697	601	962	869	728	951	855	828
10	599	593	540	805	633	584	792	709	641

From literature [21], it can be seen that the relationship between the accumulated DP loss of cellulose, ϵ_DP (defined as ϵ_DP = 1 − DP∕DP₀, DP₀ is the initial DP), and the aging duration t is similar to that of ϵ_TS and t, which is described as Eq. (3). Hence, the curves shape of ϵ_DP vs. t is as same as those of ϵ_TS vs. t. Here only curves of groups A and B are given with Figs 7 and 8. The data of k _DP (DP degradation reaction rate) and ${\varepsilon}_{\text{DP}}^{\ast }$ (the degradation reservoir of DP) are presented, which are listed in Table 4, respectively. $\begin{eqnarray}\displaystyle {\varepsilon}_{\text{DP}}={\varepsilon}_{\text{DP}}^{\ast }(1-e^{-k_{\text{DP}}t}). & & \displaystyle\end{eqnarray}$ (3)

Fig. 7.

Relationship of ϵ_DP vs. aging duration for three groups of the unmodified specimens with MTA experiments.

Fig. 8.

Relationship of ϵ_DP vs. aging duration for three MIP groups of with MTA experiments (4 wt% nano-silica).

Table 4

Calculated k _DP and ${\varepsilon}_{\text{DP}}^{\ast }$ for fitting similar to Eq. (2) to the data in Figs 7 and 8

Group	k _DP			${\varepsilon}_{\text{DP}}^{\ast }$
	E1	E2	E3	E1	E2	E3
A	0.1753	0.1895	0.1986	0.493	0.577	0.608
C	0.1456	0.1529	0.1543	0.382	0.407	0.446

Compared Fig. 7 with Fig. 8, it can be seen that both the parameters of ${\varepsilon}_{\text{DP}}^{\ast }$ and k _DP of the MIP decrease drastically than those of the unmodified paper. Here, parameters of the group A and C with aging condition E2 are taken as examples. Connected with Table 4, ${\varepsilon}_{\text{DP}}^{\ast }$ was 57.7% 1.610 and 40.7% before and after modification, respectively, with a decrease of 29.5%, meanwhile the k _DP reduced from 0.1895 before modification to 0.1529, with a decrease of 19.3%. Suppose DP = 300 is adopted as the criterion of the paper degraded to its life end. In that case, it can be inferred that under the experimental conditions of E2, the aging duration required to deteriorate to its life end will be 43 days and 91 days, respectively, which showed that the mechanical life of the insulating paper increased 112%. The results state clearly that the MIP can work longer under the same MTA conditions compared to the unmodified one.

Here, the change rate of k _DP when the aging condition is changed is defined with Eq. (4). $\begin{eqnarray}\displaystyle {\alpha}_{\text{DP}}={\displaystyle \frac{k_{\text{DP}}^{i}-k_{\text{DP}}^{1}}{k_{\text{DP}}^{1}}}\times 100\%\quad (i=2,3) & & \displaystyle\end{eqnarray}$ (4) where $k_{\text{DP}}^{1}$ and $k_{\text{DP}}^{i}$ are the k _DP values of group E1 and those of E _i (i = 2, 3), respectively. If 𝛼_DP is positive, it means that k _DP increases with amplitude increasing, and vice versa. With the measurement data in Table 4, it is known that with the aging condition deteriorating, that is, from E1 → E2 → E3, the value of a _DP of the unmodified insulating paper increased from 8.1% to 13.3% with an increment of 64.2%, while that of the modified one, increased from 5% to 5.9%, and its increment is 18% only. It is powerful evidence that, though all k _DP values have a positive relationship with amplitude, the a _DP values of the modified sample are significantly lower than those of the unmodified sample, which indicates that nano-silica MIP can significantly enhance the mechanical strength of the insulating paper.

4. Discussions

4.1. Cellulose modification mechanism with nano-silica

Though cellulose molecules are polar polymer, when the appropriate amount of nano-silica particles is added, the quantum effect of the nano-particles can significantly increase the mechanical strength of the MIP. The modification mechanism can be analyzed from two standpoints: one is the interface effect of the nano-modified polymer [23] and the other is the quantum effect of nano-particles at the interface of the modified polymer [24].

∙ Interface effect of nano-particle modified polymer

As we know, the cellulose in insulating paper is linear polymer in which a plurality of 𝛽-D-glucopyranosyl groups are bonded to each other by a (1-4)-𝛽-glycosidic bond. When nano-silica particles are appended, cellulose acts as matrix, nano-silica particles act as reinforcement, and silane coupling agent acts as a bridging agent between the matrix and the reinforcement, so that the reactive functional group- hydroxyl group on the surface of the cellulose is bonded to H₂N in the form of a chemical bond, which the interface connection structure can be formed.

The formation of the interface plays a key role in improving the mechanical strength of the insulation owing to both the transfer effect and the blocking effect of the interface. The transfer effect means that the interface can transmit the mechanical force acting on the insulating paper to the reinforcing phase. The blocking effect means that the interface can prevent the cracks induced by the machine-thermal synergy from further expanding and it also slows down the stress concentration partly [23]. Therefore, the interface structure in the nano-silica MIP can improve the force-receiving manner, and the paper can withstand higher-intensity machine-thermal synergy, which can explain the experimental results in this paper.

∙ Quantum size effect of nano-particles

When the size of nano-silica particles is reduced to specific nanometer size, the electron level near the Fermi level will be changed from quasi-continuous level to discrete level, and the energy gap will be widened with the particle size decreases, which is called the quantum size effect [24]. The interface multi-core model theory believes that the interfacial region of nano-silica doped in insulating paper can be divided into three layers, named from the outside to the inside as loose layer, tie layer and bonding layer, which are shown in Fig. 9.

Fig. 9.

Multi-core model for nano-particle MIP interfaces.

In the multi-core model, the bonding layer plays a key role in the interface effect for its strong bonding role though it has an nm-level thickness only. Meanwhile, the bonding layer is the interaction zone between the coupling agent and the cellulose chain, and the thickness is slightly larger than the bonding layer. In the loose and tie layers, the interface region with nano-scale and independent dielectric properties can be formed between the dipolar cellulose molecule and the nano-silica particles. This region can limit the movement of the molecular segments, so that the MIP has a higher tensile strength. Microscopically, the number of broken cellulose chains in the modified samples is less than that in the unmodified samples within the same aging duration, hence the measured DP is higher than that of the unmodified samples.

4.2. The weight content of nano-particles in MIP

It can be seen from part 2.1 that the nano-silica weight contents are 0, 2 wt%, and 4 wt%, respectively, and when the aging duration of the E3 group is 6 days, the measured DP of the samples is 737, 964, and 910, respectively. It indicates that continuing increase of the weight content of nano-silica particles in MIP cannot continue to improve the performance of the insulating paper.

Meanwhile, the above phenomena also exist in the TS values of the specimens in the above tables. Therefore, the mechanical properties of insulating paper cannot be improved by simply increasing the weight content of nano-silica particles. The agglomeration characteristics of nano-silica particles also make it true that the weight content of nano-silica particles has a maximum. Hence, it is necessary to determine a reasonable weight content of nano-silica particles based on the economics of the MIP and more systematic experimental data.

4.3. Insulation life prediction of insulating paper

With the synergy of MTA experiments, cellulose degrades accompanying the DP reducing, interfiber bonding destroying, mechanical strength losing, all of which contributed to tearing and defibrillation [25]. Among the chemical, electrical and physical indicators of degradation of cellulosic insulating paper, TS and DP can be regarded as the most reliable ones to evaluate the residual life of insulating paper. Considering the positive acceleration effect of vibration on the deterioration of insulating paper [10], TS may be more effective. With a given value TS_end at temperature T, the terminal percentage TS loss ϵ_TSend can be determined, connected with Eq. (2), the residual life of insulating paper can be obtained with Eq. (5). $\begin{eqnarray}\displaystyle t=-\ln \left(1-{\displaystyle \frac{{\varepsilon}_{\text{TS}_{\text{end}}}}{{\varepsilon}_{\text{TS}}^{\ast }}}\right)/k_{\text{TS}}. & & \displaystyle\end{eqnarray}$ (5) According to the data in Table 4, and the terminal TS value of each group are set to half of the original of the unmodified paper, that is 0.5 ∗ 6.94 = 3.47 kN/m, then the calculated lifetime values are shown in Table 5.

Table 5

Lifetime of three groups of insulating paper (days)

Group	E1	E2	E3
A	51.1	47.8	45.2
B	68.5	65.3	47.8
C	103.4	96.7	93.4

It can be seen that under aging conditions of E1, the residual life of unmodified insulating paper is about 51 days, and it prolongs 17 and 52 days with 2 wt% and 4 wt% nano-silica weight content, respectively. Under conditions of E2, the increment of lifetime is 17 and 40 days, respectively. And under conditions of E3, it shows the same tendency as those of E1 and E2. Therefore, it proves the validity of the proposed method.

Further, it should be noted that the calculated lifetime values shown in Table 5 are based on the TS value of the unmodified paper. On the other hand, the real lifetime of the MIP can be obtained with Eq. ((5)) connected with the their initial and terminal TS values. Interested readers can work out the corresponding values, which will not be given in this paper.

5. Conclusions

Based on the study of mechanical properties of cellulose insulation modified by nano-silica particles under accelerated MTA experiments, the following conclusions can be arrived at.

(1) Both TS and DP change rules of correlation coefficients in the first-order reaction kinetics equations including TS and DP are obtained. It shows that the anti-vibrational-degradation characteristics of nano-silica MIP are obviously enhanced. Therefore, the modified method put forward in the paper has more practical significance.

(2) The mechanism of improving the mechanical properties of nano-silica MIP is the interface effect of nano-silica modified polymer and the quantum effect of nano-silica particles at the interface of modified polymer. The interface formed by the former can transfer the mechanical action of the MIP. It can also prevent further crack growth during aging to a certain extent, and play a role in reducing stress concentration. The quantum effect can discretize the electronic energy levels and limit the movement of the molecular segments.

(3) Life prediction method is received and the predictions of the MIP can characterize the effectivity quantitatively. But the weight content of nano-particles has interesting potential for further study.

Footnotes

Acknowledgements

The authors are grateful for the financial support from the Key R&D Program of Shandong Province, China (No. 2019GGX102049).

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Mechanical performance of nano-silica modified insulating paper with mechanical-thermal synergy

Abstract

Keywords

1. Introduction

2. Preparation and aging experiments

2.1. Determination of nano-particle type and weight content

Table 1 Parameters of selected nano-silica particles Index Description Shape Spherical particle Diameter 40 ± 5 nm Purity quotient 99.8% Specific area 170 m2/g Hydroxyl ≥45%

2.3. MTA experiments

2.4. Mechanical properties measurements

3. Results and analysis

3.1. Change rule of unmodified insulating paper

4.1. Cellulose modification mechanism with nano-silica

4.3. Insulation life prediction of insulating paper

Footnotes

Acknowledgements

References

Table 1
Parameters of selected nano-silica particles

Index Description

Shape Spherical particle

Diameter 40 ± 5 nm

Purity quotient 99.8%

Specific area 170 m²/g

Hydroxyl ≥45%