Nano-silica reinforced epoxy resin/nano-rubber composite material with a balance of stiffness and toughness

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Nano-silica reinforced epoxy

resin/nano-rubber composite material with a balance of stiffness and toughness

Shuo Wang

^1,2

, Meng Cao

¹

, Hongqian Xue

²

, Fanglin Cong

³

, Xiaodong Li

¹

, Changbao Zhao

¹

and Weiguo Su

⁴

Abstract

In the electronics and aerospace industries, epoxy resins are generally regarded as economical and efficient adhesives and have a high status. However, epoxy resins are highly crosslinked polymers and are very brittle adhesives where they are prone to fast crack propagation under dynamic loads. Therefore, it is very necessary to enhance the toughness of epoxy resin adhesives. Nano-rubber has been proved to be an important toughening agent for epoxy resin, which can significantly improve the fracture toughness of epoxy resin. However, increasing the toughness of epoxy resin by adding nanomaterials is often accompanied by decreasing the strength and stiffness of resin. Therefore, in this work, rigid nano- silica particles were added to improve the rigidity and tensile strength reduction caused by the addition of rubber particles.

And further increase the toughness of the epoxy resin to obtain an epoxy adhesive with balanced stiffness-toughness. As a result, it can be found that the addition of silica particles can significantly improve the decrease in stiffness caused by the addition of rubber particles. For example, Young’s modulus and tensile strength are increased by 28%, and 23%, respectively, with 4% silica is added based on rubber particles. Through the single lap shear experiment, it is found that the shear strength of the epoxy/RnP/silica composite adhesive has increased, which further proves that the addition of nano- silica particles can increase the stiffness of the epoxy composite. The dynamic mechanical analysis experiment found that after adding nano-silica particles, the storage modulus of epoxy composites increased, which also shows that adding nano- silica particles can improve the stiffness of epoxy composites. Scanning electron microscopy analysis was performed to study the reinforcement mechanism of epoxy/RnP/silica composite materials. The thermal stability of epoxy composites was characterized by Dynamic mechanical analysis and thermogravimetric analysis.

Keywords

Nano-silica, rubber nanoparticles, balanced stiffness-toughness, nanocomposite, adhesive

Introduction

Bonding technology has been considered as an acceptable alternative to other joining methods because of many advantages such as uniform stress distribution, anti-corrosion, low weight and cost-effectiveness. Epoxy resin is a widely used structural adhesive^1–5 owing to its excellent mechanical strength, simple preparation process, excellent chemical and corrosion resistance. Despite these advantages, epoxy resin has low fracture toughness and hence possesses poor crack resistance due to the tight, three-dimensional crosslinked molecular structures. During the past decades, numerous attempts have been made to improve the brittleness and fracture energy of epoxy resins by mixing with soft rubber nanoparticles and rigid nanofillers, such as silica,^6,7 alu- mina^8,9 and carbon-based fillers.^10,11 Of these nanofillers,

elastomeric nanoparticles of *55 nm in diameter are regarded as the most effective, as shown by an improvement

1College of Aerospace Engineering, Shenyang Aerospace University, Shenyang, China

2School of Mechanical Engineering, Northwestern Polytechnical University, Xian, China

3College of Civil Aviation, Shenyang Aerospace University, Shenyang, China

4National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering, Wuhan, China

Corresponding author:

Weiguo Su, National Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering, Wuhan 430033, China.

Email: [email protected]

High Performance Polymers 1–10

ªThe Author(s) 2021 Article reuse guidelines:

sagepub.com/journals-permissions DOI: 10.1177/0954008320988752 journals.sagepub.com/home/hip

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of 877%in fracture energy observed at 5.0 vol%nanoparticle fractions.¹²It is well established that rubber particles can produce the desired effect on toughening brittle epoxy,^12–16 but they usually cause deterioration to other important properties, such as mechanical strength and Young’s modulus.

To strike a balance in mechanical performance, epoxy composite adhesives reinforced and/or toughened by hybrid fillers have gained a plethora of studies. By combining different proportions and/or types of particles, specific key mechanical properties and other essential properties can be enhanced at the same time. Hybrid composites have lately attracted the attention of researchers using different types of carbon fillers, for example, multiwalled carbon nanotube (MWCNT) with carbon black,¹⁷MWCNTs with GnPs,^18–20 nano silicone rubber core-shell particles,²¹ and GnPs with single-wall CNT and nano-diamonds.²²

As a promising alternative to improve the fracture toughness of polymers and adhesives, the use of nanosized particles, especially silica nanoparticles, has attracted widespread attention. This new method has the potential to increase the fracture toughness of polymers without reducing the detrimental effects of the lower glass transition temperature associated with rubber and thermoplastic polymer particles. Nanosized silica particles can produce considerable or even greater improvement in fracture toughness, but will not lose stiffness and strength at high temperatures like rubber particles. It is investigated that 4.4 vol%of silica particles can increase the fracture energy release rateG1cof epoxy resin by 40%.²³By contrast, the same volume fraction of silica nanoparticles increases the G1cvalue of a tough and brittle epoxy system by 274%and 76%, respectively.²⁴Based on epoxy toughened by liquid rubber, Kinloch observed that by using 4.1 wt% silica nanoparticles, the adhesive toughness was maximum increased by 92%.²⁵

As for rubber/epoxy composites, the continuous rubber network is considered a critical factor in toughening. How- ever, due to the characteristics of the soft nano-rubber particles, the stiffness of the composite material itself decreases at the same time of toughening. The rigid nano-silica particles can greatly increase the stiffness of the polymer due to their own characteristics. In addition, some suitable nanoparticles can contribute to the compat- ibility of the polymer blend, which is also an important factor in improving its performance.³ Therefore, in this article, rubber nanoparticles (RnPs) and silica nanoparticles are added to the epoxy resin. The relationship between structure and performance was studied, including lap shear strength and toughness of epoxy composite adhesive. The silica, which is widely used in rubber reinforcement, is selected to achieve the purpose of enhancing the tensile strength, fracture toughness and other properties of the adhesive. Preparation of adhesives with high toughness and mechanical properties has great application potential in the industrial field, which can be made into a high-performance

resin matrix for advanced aerospace composites with excellent specific stiffness and specific strength. Furthermore, the fracture toughness of the resin matrix increased by RnPs can also improve the composite structure’s fatigue performance. Besides, a comprehensive study of the hybri- dization between silica and RnPs has provided readers with a clear method to determine the efficiency of the hybridiza- tion of nanofillers into polymer composites.

Experiment details Materials

55 nm-diameter spherical rubber nanoparticles (Kane ACE MX-120) were provided in a colloidal solution at 25 wt%in epoxy by Kaneka, Japan. Silica nanoparticles (Nano-pox F400) were supplied as a colloidal sol (40 wt%) in epoxy by Hanse Chemie AG, Germany.²⁶ Epoxy resin (DGEBA) was purchased from Nantong Xingchen Synthetic Material Co., China and Jeff-amine 230 (J230) hardener was from Huntsman, China. All materials were used as received.

Preparation

In this study, three epoxy composite systems were prepared: (i) epoxy/rubber nanoparticle (RnP) composite adhesives (ii) epoxy/silica composite adhesives; and (iii) epoxy/1 wt% RnP/SIO2composite adhesives. Fabrication of epoxy/RnP composite is detailed as follows. Epoxy resin was mechanically stirred at 100C for 1 hr with pre- weighed amounts of rubber nanoparticles (RnPs) batch to produce epoxy/RnP composite adhesives with fraction range 0.5–2.0 wt%. A quantity of J230—at weight ratio epoxy: J230¼3.3:1—was added to the mixture and mag- netically stirred for 10 mins at 40–50C. The mixture was degassed in a vacuum oven for 10 mins to remove any trapped air bubbles. The composite mixture was poured into silicone rubber moulds and left in a fan over to cure over two stages: 80C for 2 hrs and then 120C for 10 hrs.

The cured samples were sandpaper-polished to eliminate visible flaws or marks, and finally, the samples were treated at 120C for 2 hrs to lessen the defects made by sandpaper polishing. Epoxy/nano-silica composites were prepared by following the same procedures used for the epoxy/RnP composites.

In epoxy/RnP/SIO2 composites, RnP was kept at 1.0 wt% and composites were produced as follows. The epoxy/silica and epoxy/RnP viscous mixtures were added and homogenously mixed after 10 mins magnetic stirring.

The mixture was further degassed for 10 mins to remove trapped air bubbles. The hardener–J230 was slowly added to the three-phase mixture and manually stirred using a glass rod for 5 mins, at room temperature. Finally, the epoxy/(1 wt% RnP)/SIO2 viscous mixtures were then poured into the rubber moulds, cured and prepared for

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testing as discussed in epoxy/RnP system. In this composite system, silica wt%was ranged from 1–4 wt%.

Characterizations

Morphology.Scanning electron microscopy (SEM) was car- ried out to observe the fracture surfaces (crack tip and propagation zone) of compact tension (CT) specimens using (SEM, ZEISS Sigma 300). A thin layer of platinum was used to coat the fractured surface and then examined at 10 kV accelerating voltage.

Compact tension and tensile testing.Using compact tension (CT) experiment to test the fracture toughness (KIC) of pure epoxy and its bulk nanocomposites, and calculate according to ISO 13586 standard. The size of the CT sample is 30 30(5–6) mm.

Using dumbbell samples conduct tensile testing at a speed of 0.5 mm/min on XIANGMIN machine. Capture elastic strain with an extensometer; The Young’s moduli was determined at strain range 0.05%–0.15%.

Mechanical testing of adhesive joints.The single lap shear test of adhesive joints is usually used to check the bond strength. The surface treatment of bonded joints is significant to bond strength. This work adopts the following methods to treat the surface of the bonded joint. First, the aluminum plate used for the lap shear test is sanded with sandpaper to remove surface impurities and roughen the surface. Wash the polished aluminum plate with deionized water. Then, the aluminum plate is placed in a 10%sodium hydroxide solution for 3 mins to corrode the surface further, so that the aluminum surface has sufficient adhesion. After taking it out, clean the surface with acetone for subsequent experiments.

Single lap shear tests were conducted on the adhesive joints using a universal tensile testing machine (MTS 100 kN, USA). According to ASTM D1002 standard, the bonded joint is subjected to a tensile load with a strain rate of 1.3 mm/min. In order to ensure the accuracy of measur- ing the shear strength of adhesive joints, at least five samples of each material were tested.

Thermal characterization. Dynamic mechanical analyzer (DMA2980, TA Instruments, Inc, USA) was used to determine glass transition temperatures (Tg) of the prepared samples. The DMA test was performed at 1 Hz with double cantilever clamp of a 20 mm span within temperature range 25–150C, and data were captured every 2 sec.

The thermal stability of the epoxy and nanocomposites was studied using a thermogravimetric analyzer (NETZSCH STA 449C Jupiter, Germany) by heating cured samples to 600C with a ramp rate of 10 K/min under argon atmosphere.

Result and discussion

Mechanical properties of epoxy/RnP composites

Mechanical properties of epoxy resin after incorporating rubber nanoparticles are plotted in Figure 1 as a function of RnP content; namely fracture toughness (K1c), critical strain energy release rate (G1c), Young’s modulus and tensile strength. The details of the properties of epoxy composites are given in Table 1. In Figure 1(a) and (b), bothK1c

andG_1cof epoxy composites were exceptionally increased by adding RnPs to 1.0 wt%. At 1.0 wt%RnPs,K1candG1c

of the epoxy composites augmented by 188% and 711%, respectively. Beyond 1.0 wt%RnPs, the increment of these properties was plateaued. Rubber nanoparticles (RnPs) are viscoelastic phase which in turn reduces the brittleness of a high crosslinked matrix, epoxy, provided that they were uniformly dispersed in the matrix. At low fraction (<1 wt%), RnPs were able to disperse evenly while at a high fraction (2 wt%), RnPs start to aggregate and form clusters creating microsize rubber particles; this retreats the toughening effect of viscoelastic fillers. Meng et al.¹³ reported that size of rubber nanoparticles has a significate effect on the epoxy toughness; for example, at 2.5 vol%, critical strain energy release rate (G1c) of the epoxy composite containing microsize rubber particles increased by 56%; on the other hand, when nanosize rubber particles were added, the G1c of epoxy composite exceptionally increased by 604%.

Figure 1(c) and (d) shows that Young’s modulus and tensile strength of epoxy/RnP composites moderately decrease with RnPs. Since rubber nanoparticles are a viscoelastic phase in the high brittle matrix (epoxy), it is expected that the stiffness and strength of the epoxy composite will be less than neat epoxy. However, the drop is minor; for example, 1.0 wt%RnPs reduces Young’s modulus and strength of epoxy by 7% and 9%, respectively.

Considering the substantial enhancement in toughening the epoxy by adding 1.0 wt%RnPs, this reduction in stiffness is acceptable. It is noteworthy to mention that our results agree with previous studies.¹³This section concludes that 1.0 wt% RnPs is an ideal content to achieve maximum toughness for the epoxy resin. The 1.0 wt% RnP fraction is stored in a three-phase epoxy composite system (epoxy/

1.0 wt%RnP/Silica) to develop epoxy composite adhesives with high toughness and balanced stiffness.

Mechanical properties of epoxy/RnP/silica composites

Considering the advantage of 1.0 wt% RnP in enhancing the toughness of epoxy resin, but the effect of its strength and rigidity decreased slightly. 1.0 wt%RnP were selected with various fractions of rigid nanoparticle-silica to improve further the performance and balance loss of strength and stiffness of epoxy resin due to the addition of nano-rubber. Figure 2 contains fracture toughness

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(K1c), critical strain energy release rate (G1c), Young’s modulus and strength of neat epoxy and epoxy/1.0 wt% RnP/

Silica composites. Furthermore, the details of the properties of epoxy composites are given in Table 2. The toughness of epoxy resin has been further enhanced with adding nano- silica (Figure 2(a) and (b)). TheK1candG1cof the epoxy/

rubber/silica composites adhesive are slightly improved when adding a low fraction of nano-silica until 3.0 wt%. For example, when 3%silica is added, the K1cand G1cof the composite material reach 1.93 MPa m^1/2 and 1747 J/m², improved of 24% and 8% respectively compared to 1%

epoxy/rubber composite material. Obviously, this is related to the fact that silica is a rigid particle. In the fracture process

of epoxy composites, it will hinder the propagation of cracks and cause cracks to turn since silica is a rigid particle, which will consume much energy during the crack propagation process, thereby improving the toughness of epoxy composites. However, the fracture toughness of epoxy/RnP/silica composites begins to decrease when the content of silica is higher than 3%. For example,K1candG1cdecreased by 9%

and 10%respectively, adding 4%silica. The reason is that the content of silica particles in the composite material is higher causes the particles to aggregate and lead to a decrease in toughness.

Figure 2(c) and (d) shows that silica reinforces the stiffness of epoxy/1.0 wt%RnP composite. For example,

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.5 1.0 1.5 2.0

,ssenhguoterutcarFK1Cm.aPM(1/2)

RnP fraction (wt%)

epoxy/RnP

(a)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0 300 600 900 1200 1500

etaresaelerygrenEG1C, (J/m2 )

RnP fraction (wt%)

epoxy/RnP

(b)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

40 45 50 55 60

)aPM(htgnertselisneT

RnP fraction (wt%)

epoxy/RnP

(c)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75

)aPG(suludoms'gnuoY

RnP fraction (wt%)

epoxy/RnP

(d)

Figure 1.Toughness and mechanical properties of epoxy/RnP composite (a) fracture toughness (K1c), (b) energy release rate (G1c), (c) tensile sterngth, and (d) Young’s moudulus.

Table 1.Toughness and mechanical properties of epoxy/RnP composites.

Fraction of RnP nanoparticle (wt%)

Tensile strength (MPa)

Young’s modulus (GPa)

Fracture toughness,K_Ic (MPa m^0:5)

Energy release rate,G_Ic (J=m²)

0 (neat epoxy) 55.1+0.6 1.69+0.03 0.60+0.03 173.44+38.67

0.5 53.2+0.5 1.63+0.02 0.91+0.05 411.88+39.59

1 50.6+0.5 1.57+0.02 1.72+0.09 1406.36+70.32

2 47.3+0.5 1.54+0.03 1.63+0.08 1292.93+64.65

3 44.9+0.4 1.50+0.04 1.55+0.08 1169.13+58.46

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at 4.0 wt% silica increased Young’s modulus of epoxy/

1.0 wt% RnP composite from 1.57 GPa to 2.02 GPa, increased by 28%. However, tensile strength reached its maximum value at 2.0 wt%silica then declines. The tensile strength reaches 64.5 MPa at 2% silica is added, which is improve 23% than 1% epoxy/RnP composite.

The Young’s modulus and tensile strength of the epoxy/

rubber/silica composite material have been greatly improved due to the excellent stiffness and strength of the silica particles. It complements the soft nano-rubber particles, balances the toughness and stiffness of the epoxy composite material, and enables the epoxy resin adhesive to obtain the best mechanical properties and toughness.

Fracture analysis

Fracture analysis is imperative to understand the failure mechanism in composites which is due to debonding between fillers and matrix or both phases share loads and resist deformation. A scanning electron microscope (SEM) was used to study the fracture surface of the epoxy/RnP and epoxy/1.0 wt%RnP/silica composite CT samples cured by J230. SEM micrographs of pure epoxy were discussed in previous studies,^6,12so they are not shown here. Figure 3 contains representative images of 1.0 wt%epoxy/RnP composites. In Figure 3(a), a rougher fracture surface and many thin river lines can be observed compared with pure epoxy

0 1 2 3 4

1.70 1.75 1.80 1.85 1.90 1.95 2.00

,ssenhguoterutcarFK1Cm.aPM(1/2)

Silica fraction (wt%)

epoxy/1wt% RnP/Silica

(a)

0 1 2 3 4

1500 1600 1700 1800

etaresaelerygrenEG1C, (J/m2)

epoxy/1 wt% RnP/Silica

(b)

0 1 2 3 4

52 56 60 64 68

)aPM(htgnertselisneT

(c)

0 1 2 3 4

1.6 1.8 2.0 2.2

)aPG(suludoms'gnuoY

(d)

Figure 2.Toughness and mechanical properties of epoxy/1 wt% RnP/silica composite (a) fracture toughness (K_1c), (b) energy release rate (G1c), (c) tensile sterngth, and (d) Young’s moudulus.

Table 2.Toughness and mechanical properties of epoxy/RnP/silica composites.

Fraction of RnP-Silica nanoparticle (wt%)

Tensile strength (MPa)

Young’s modulus (GPa)

Fracture toughness,K_Ic (MPa m^0:5)

Energy release rate,G_Ic (J=m²)

0 (1% RnP) 52.7+0.8 1.57+0.02 1.72+0.02 1614.74+16.15

1 62.8+0.9 1.77+0.01 1.73+0.02 1647.87+16.48

2 64.4+0.6 1.89+0.03 1.85+0.03 1720.70+17.21

3 63.0+0.9 1.94+0.03 1.93+0.02 1747.25+17.47

4 61.6+0.7 2.02+0.05 1.77+0.04 1575.33+15.75

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resin. Figure 3(b) is an enlarged image of the tiny square in Figure 3(a). From Figure 3(b), a significantly rougher scaly surface with a large number of hackles and ridges can be observed. A large number of stress whitening areas indicate that large-scale plastic deformation has occurred, which is not observed on the fracture surface of pure epoxy resin.¹² It is consistent with the increase of the fracture toughness value of the toughened epoxy resin with the size of the stress whitening zone. The rough surface can be observed in the high-magnification image enlarged in Figure 3(c).

The highly improved surface roughness means that matrix deformation (such as shear bands) is the main fracture surface phenomenon that absorbs fracture energy.

Figure 4 contains the microfracture surface of 1.0 wt%

epoxy/1.0 wt%RnP/Silica and 3.0 wt% epoxy/1wt%RnP/

Silica composite adhesives. In Figure 4(a, panel 1) and (b, panel 1), rough fracture surfaces with dense scaly fracture zones on the surface can be observed, which is the main reason for epoxy toughening. Figure 4(a, panel 2) and (b, panel 2) are enlarged areas of the small squares in Figure 4(a, panel 1) and (b, panel 1). A large number of tortuous ridges can be clearly observed in Figure 4(a, panel

2) and (b, panel 2). The reason is that the addition of silica particles will cause the front end of the crack to be deflected when it encounters the silica particles when it grows, which makes the crack consume much energy during the growth. It is one of the most important reasons for the improvement of the toughness of epoxy adhesives.

Moreover, in Figure 4(b, panel 2), a denser crack zone can be observed, and the crack is more tortuous than in Figure 4(a, panel 2). It is mutually verified by the higher toughness of 3.0 wt% silica adhesive in Figure 3.

Figure 4(a, panel 3) and (b, panel 3) are high- magnification microscopic images. It can be seen that the fracture surface in the Figure 4(b, panel 3) is rougher than the fracture surface in Figure 4(a, panel 3) and has higher ridges, which means that more nanoparticles will promote greater matrix deformation.

Lap shear strength of the adhesive joints

Adhesive-joint strength is crucial for the safe function of a complete system. Lap shear strength (LSS) test measures the joint strength by dividing the failure load by adhesive Figure 3.(a to c) SEM fracture surface of 1 wt% epoxy/RnP.

Figure 4.SEM micrograph of fracture surface of the 1 wt% epoxy/RnP-silica (a1 to a3) and 3 wt% epoxy/RnP-silica (b1 to b3) composites.

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bond area. Figure 5 shows the influence of different weight fractions of additives into the lap shear strength of epoxy- bonded joints. The effect of mixing different mass fractions of rubber on the lap shear strength of epoxy-bonded joints in Figure 5(a). As the proportion of nano-rubber in epoxy resin increases, the strength of bonded joints continues to increase, reaching a maximum value of 13.91 MPa at 1.0 wt%, then began to decrease slowly. The increase in lap shear strength of adhesive joints is attributed to the increase in toughness brought about by the addition of nano-rubber.

Nonetheless, due to the decrease of the stiffness of epoxy adhesive with the addition of nano-rubber, the shear strength of the bonded joint has not been greatly improved.

It only increased from 11.52 MPa (neat epoxy) to 13.91 MPa (epoxy/1 wt%RnP), by an increase of 21%.

Figure 5(b) shows the change of the lap shear strength value of adding different content of silica-based on adding 1.0 wt%nano-rubber. It can be seen from Figure 5(b) that the lap shear strength has been dramatically improved with the addition of nano-silica. Furthermore, when the silica content at 3.0 wt%, the lap shear strength reaches the maximum value of 15.97 MPa, which is improved by 39%and 15% compared with the pure epoxy resin adhesive and adding 1.0% nano-rubber epoxy adhesive, respectively.

The reason is since the addition of rigid nano-silica particles to the epoxy resin complements the existing rubber particles, which not only slightly improves the toughness, dramatically improves the stiffness of the epoxy adhesive.

Based on the above reasons, the strength of the lap shear joint of epoxy/RnP/Silica composite adhesive has significantly been improved. The strength is about 15%greater than the improvement achieved by using the nano-rubber additive alone, indicating that the two fillers interact well, thereby enhancing the shear strength of the joint.

Failure modes of the adhesive joints

By observing from the bonded joints fracture surfaces of pure epoxy and composite adhesives, all joints suffer

cohesive failure. The optical image of the complementary fracture surface of the pure epoxy resin, 1.0 wt% epoxy/

RnP and epoxy/1.0 wt%RnP/3.0 wt%silica adhesive head is shown in Figure 6. In the image, it can be found that the pure epoxy adhesive joint is 100%cohesive failure in the adhesive layer, and the 1.0 wt%epoxy/RnP and epoxy/1.0 wt%RnP/3.0 wt%silica joint shows a mixture of the cohesive failure within the adhesive and cohesive failure in closer proximity to the adhesive and aluminum adherend interface, and epoxy/1.0 wt%RnP/3.0 wt%silica lap joints have a larger range of close cohesive failure than 1.0 wt%

epoxy/RnP lap joints. The damaged surface confirms that the surface treatment provides excellent adhesion, but the addition of nanophase brings the fracture surface closer to the adhesive-aluminum interphase area, which is common in high-strength structural adhesives.

Dynamic thermomechanical analysis

Dynamic mechanical analysis (DMA) is another tool to examine the mechanical performance of polymer composites as a function of temperature, namely properties include glass transition temperature (Tg), storage modulus and mechanical loss factor (tan ). The glass transition temperature (Tg) is the temperature that marks the viscoelastic state of a polymer from the glassy phase; it is com- monly defined as the peak oftancurve. Figure 7 contains the relationship between the glass transition temperature (measured at the peak of tan) and the storage modulus of the epoxy/1.0 wt%RnP composite with the silica fraction and temperature. The glass transition temperature is shown in Table 3. Figure 7(a) is the curve of the tan value of epoxy and its composites with temperature. It can be seen that the glass transition temperature of epoxy resin is increased when 1.0%nano-rubber is added. And the glass transition temperature gradually increases with the further addition of nano-silica particles. For example, when 1.0%

nano-rubber is added into neat epoxy, the glass transition temperature (Tg) increases from 87C to 91C. On this

0 2 4 6 8 10 12 14 16 18 20

3.0 RnP/EP

2.0 RnP/EP

1.0 RnP/EP

0.5 RnP/EP

)aPM(htgnertsraehspaL neat epoxy

(a)

0 2 4 6 8 10 12 14 16 18 20

4.0 SIO2/EP 3.0 SIO2/EP 2.0 SIO2/EP 1.0 SIO2/EP

)aPM(htgnertsraehspaL yxopetaen 1.0 RnP/EP

(b)

Figure 5.Lap shear strength of (a) neat epoxy and epoxy/RnP composites adhesives, and (b) neat epoxy and epoxy/1wt% RnP/SiO2

composites adhesives.

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basis,Tgis further improved with silica particles are added.

Tg of the epoxy/1.0 wt% RnP/4 wt% Siliica composite reaches 99C on 4.0 wt%silica particles are added, which is 12C (increase 13.4%) higher than pure epoxy resin. The tan value continues to decrease as the proportion of filler nano-silica continues to increase. It can be explained by the limited chain mobility of silica nanoparticles in epoxy systems.

Figure 7(b) shows the storage modulus of epoxy composites as a function of temperature. It can be seen that with the continuous increase of nano-silica fillers, the storage modulus continues to increase, and the stiffness of the epoxy composite material is improved. When 4.0 wt%particles are added, the storage modulus reaches 1879 MPa, which is a 40%improvement over pure epoxy resin. This Figure 6.Fracture surfaces of neat epoxy, 1 wt% epoxy/RnP and epoxy/1 wt% RnP/3 wt% silica lap shear joints.

20 40 60 80 100 120 140

0.0 0.4 0.8 1.2 1.6

Tan delta

Temperature(°C) neat epoxy epoxy/1 wt% RnP

epoxy/1 wt% RnP/1wt% Silica epoxy/1 wt% RnP/2wt% Silica epoxy/1 wt% RnP/3wt% Silica epoxy/1 wt% RnP/4wt% Silica

(a)

40 60 80 100 120 140

0 500 1000 1500 2000 2500 3000

neat epoxy epoxy/1 wt% RnP

epoxy/1 wt% RnP/1wt% Silica epoxy/1 wt% RnP/2wt% Silica epoxy/1 wt% RnP/3wt% Silica epoxy/1 wt% RnP/4wt% Silica

)aPM(suludomegarotS

Temperature (°C)

(b)

Figure 7.DMA curves of epoxy and its nanocomposites (a) tan delta and (b) storage modulus.

Table 3.The glass transition temperature and initial degradation temperature of pure epoxy and epoxy composite adhesives.

Epoxy composite

Glass transition temperature,Tg

(C)

Onset degradation temperature (C)

Neat epoxy 87 358

1 wt% rubber 91 361

1 wt% rubber/1 wt%

silica

93 363

1 wt% rubber/2 wt%

silica

95 364

1 wt% rubber/3 wt%

silica

97 362

1 wt% rubber/4 wt%

silica

99 360

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enhancement indicates that silica nanoparticles have a strong influence on the stiffness of epoxy resin. The stiffness of the epoxy composite material is greatly improved by adding nano-silica particles.

Thermal stability analysis

The thermal behavior of pure epoxy resin and nanocomposites of 1.0 wt% rubber nanoparticles and contain of 1.0 wt%, 2.0 wt%, 3.0 wt%, 4.0 wt%silica nanoparticles are analyzed by TGA experiment. The result is shown in Fig- ure 8. Table 3 shows the initial degradation temperature (the temperature at which the decomposition begins) of pure epoxy and its composite materials. The TGA results showed that the thermal stability of the epoxy adhesive modified with nano-rubber and nano-silica did not decrease. Besides, the initial thermal decomposition temperature of epoxy resin is further increased due to the addition of rubber and silica nanoparticles. The reason may be due to the large specific surface area of nano-silica, high surface activity, significant adsorption of antioxidants and strong control release. This leads to a decrease in the activity of the antioxidant and an increase in the thermal stability of the matrix. Moreover, the rigid silica particles affect the heat conduction of the matrix, causing thermal degradation to lag. The residual (carbon) amount of pure epoxy is 9.1% at 600C. After adding nano-rubber particles, the residual amount is lower than pure epoxy, but the residual amount is higher than pure epoxy after adding nano-silica particles. It indicates that the heat resistance of rubber particles is lower than that of pure epoxy, and the heat resistance of silica particles is higher than that of pure epoxy.

Moreover, the epoxy composite material has the highest thermal degradation temperature (364C) at adding 1 wt%

rubber/3 wt% nano-silica particles. In short, the thermal

stability of epoxy can be effectively improved after adding nano-rubber and nano-silica particles.

Conclusions

In this work, an epoxy/rubber/silica composite adhesive with balanced stiffness-toughness is successfully prepared. The addition of soft rubber particles dramatically improves the toughness of the epoxy resin. Based on adding the best proportion of rubber particles, rigid silica particles are added to successfully improve the stiffness and strength reduction caused by the addition of rubber particles. Also, the toughness of epoxy resin is improved.

When the optimal ratio of rubber is added, theK1candG1c

of the epoxy resin increase by 188%and 711%, respectively. However, the stiffness of epoxy has decreased. On this basis, silica particles are added, and Young’s modulus and tensile strength of epoxy/rubber composite materials are increased by 28% and 23% respectively, which are much higher than pure epoxy resin. Toughness is also improved compared with epoxy/rubber composite materials. Furthermore, due to the increase in toughness and rigidity, the lap shear strength of epoxy composites is increased by 39%compared with pure epoxy. After adding silica particles, the glass transition temperature and storage modulus of the composite material are both improved. Through thermogravimetric analysis experiments, it is found that the addition of nano-rubber and nano-silica particles leads to an increase in the initial degradation temperature of epoxy composites, and the highest initial degradation temperature is 364C (6C higher than pure epoxy) when adding 1 wt%rubber/2 wt%

silica particles. It shows that the composite material has good thermal stability. It is hoped that this work will provide insight into the design and preparation of high- performance epoxy adhesives.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Scientific Research Funds from Liaoning Education Department (JYT2020006), the Scien- tific Research Funds from Liaoning Education Department (JYT2020007), Shanxi Province Key Research and Development Program (2019KW-063) and the National Natural Science Foun- dation of China (Grant No. 51903249).

ORCID iDs

Meng Cao https://orcid.org/0000-0002-8877-008X Fanglin Cong https://orcid.org/0000-0002-9079-3723 Weiguo Su https://orcid.org/0000-0003-0827-7269

0 100 200 300 400 500 600

0 20 40 60 80 100

300 320 340 360 380

80 100

)%(thgieW

Temperature (°C) neat epoxy

epoxy/1wt% RnP

epoxy/1wt% RnP/1wt% Silica epoxy/1wt% RnP/2wt% Silica epoxy/1wt% RnP/3wt% Silica epoxy/1wt% RnP/4wt% Silica

Figure 8.TGA curves of cured pure epoxy and its composite adhesives.

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