microcapsules composites

Polymer Degradation and Stability 96 (2011) 84e90

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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

The thermal and dielectric properties of high performance cyanate ester resins/microcapsules composites Li Yuan*, Guozheng Liang, Aijuan Gu Department of Materials Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 July 2010 Received in revised form 15 October 2010 Accepted 23 October 2010 Available online 2 November 2010

Novel high performance bisphenol A dicyanate ester (BADCy) resins/poly(urea-formaldehyde) microcapsules filled with epoxy resins (MCEs) composites have been prepared. The effects of different contents of MCEs on the thermal and dielectric properties of cured BADCy were investigated using dynamic mechanical analyzer (DMA), thermalgravimetric analyzer (TGA) and broadband dielectric analyzer. The dielectric properties of BADCy/MCEs treated in hot water and hot air were also discussed. The morphologies of BADCy/MCEs composites were characterized by scanning electron microscopy (SEM). Results indicate that the appropriate content of MCEs can improve or maintain the thermal stability, the low dielectric constant and dielectric loss of cured BADCy mainly owing to higher conversion of cyanate ester (eOCN) groups. After aged in hot water and hot air, respectively, BADCy/MCEs composites with small content of MCEs can retain the low dielectric constant and dielectric loss. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: High performance polymer Thermal properties Dielectric properties

1. Introduction Cyanate esters (CE) resins are currently important thermosetting materials for the encapsulation of electronic devices, hightemperature adhesives, and structural aerospace materials since they have low dielectric constant and dielectric loss, excellent mechanical properties, thermal stability, etc. However, like most highly crosslinked thermosetting resins, CE are brittleness. Many attempts have been made to improve the toughness and other properties of CE. Inclusion of 2.5% by weight of layered silicates (OLS) can lead to a 30% increase in toughness and water resistance of CE [1]. The addition of polysulfone and an organic montmorillonite in CE can improve the fracture toughness, impact strength and water resistance of cured pure CE [2]. A 15 wt% inclusion of random soluble polyimides can lead to a 65% increase in the fracture toughness of CE with a slight loss of flexural strength [3]. The addition of epoxidized polysiloxane can enhance the processing property of CE such as the curing temperature, toughness and water resistance of cured CE [4]. The incorporation of hydroxyl terminated butadiene-acrylonitrile rubber into the matrices can improve the mode I interlaminar fracture toughness [5]. 10 phr of carboxyl terminated butadiene-acrylonitrile rubber leads to a 200% increase in the impact strength of CE with a loss of storage modulus [6]. Polyetherimide can enhance the mode I interlaminar fracture

* Corresponding author. Tel.: þ86 512 65880967; fax: þ86 512 65880089. E-mail address: [email protected] (L. Yuan). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.10.013

toughness of CE [5]. Polysulfone [7e9], amorphous poly(arylene ether sulfone) [10,11], hydroxyl- or cyanateterminated poly(ether sulfone) [12], amorphous poly(arylene ether ketone)s and poly (arylene ether phosphine oxide)s [11] can modify CE to obtain thermoplastic-blended polycyanurate with higher fracture toughness. N-Phenylmaleimide-N-(p-hydroxy)phenylmaleimide-styrene terpolymer, carrying reactive p-hydroxyphenyl groups has been used to improve the toughness of CE resins [13]. Because the rapid evolution of electronic circuit board and microwave transparent structures such as radomes requires the production of smaller, more powerful, and faster multilayer units, materials with high toughness, lower dielectric constant and dielectric loss, high thermal stability, low water absorption and high toughness are urgently needed. It is necessary to toughen CE without the sacrifice of low dielectric constant and dielectric loss, low water absorption, etc. Although many methods of toughening CE resins have been developed successfully, only little attention has been paid to the dielectric properties of the modified CE systems [14,15]. Our previous research has been reported that the appropriate content of poly(urea-formaldehyde) microcapsules filled with epoxy resins (MCEs) can improve the mechanical properties and the hot water resistance of cured CE [16], which may attract substantial commercial and scientific interests. So, in this study, the effects of MCEs on the thermal properties, and dielectric properties of CE were investigated using thermalgravimetric analyzer (TGA), dynamic mechanical analyzer (DMA) and broadband dielectric analyzer.

L. Yuan et al. / Polymer Degradation and Stability 96 (2011) 84e90

85

Fig. 1. Morphologies of MCEs.

2.2. Preparation of BADCy/MCEs systems

110

BADCy/MCEs system was prepared by the casting method. Firstly, BADCy was heated to 130e140  C and kept at the temperature for about 40 min with stirring. Secondly, MCEs were added into the above-mentioned BADCy, mixing quickly, and then the mixture was poured into a mould. After out-degassed, BADCy/MCEs system was cured via the following process: 120  C/1 h þ 150  C/1 h þ 180  C/2 h þ 200  C/2 h.

100

Weight(%)

90 80

0 wt% 2 wt% 5 wt% 8 wt% 10 wt%

70 60

2.3. Characterization

50 40 30 50

100

150

200

250

300

350

400

450

500

550

600

650

o

Temperature( C) Fig. 2. TGA curves of BADCy composites with different content of MCEs.

The thermal stability of sample was performed using a thermogravimetric instrument (TGA, Q50, TA) at a heating rate of 10  C/ min in a nitrogen atmosphere. The morphology of sample was observed using a scanning electron microscope (SEM, QUANTA200, FEI) and optical microscope (OM, XSP-XSZ, Beijing Tech Instrument Co., Ltd, China). Dynamic mechanical analysis (DMA) of sample was performed with a TA Instrument (DMA 2980). The sample dimension is 35 mm  10 mm  4 mm. The tan delta were determined as the

2. Experimental 2.1. Materials Bisphenol A dicyanate ester (2,20 -bis(4-cyanatophenyl)isopropylidene, BADCy, molecular weight: 278) was bought from Zhejiang Shangyu Shengda Biochemical Co. Ltd, China. Poly(ureaformaldehyde) (PUF) microcapsules filled with epoxy resins (MCEs) were synthesized in our laboratory according to the reference [17]. The mean diameter of MCEs used in this work is 86 mm, the mean thickness of wall shell and the core content of MCEs are 8 mm and 81%, respectively. Fig. 1 shows the morphologies of MCEs.

Table 1 The temperature corresponding to weight loss of BADCy composites with different content of MCEs. Content of MCEs(wt%)

Td5 ( C)

Td10 ( C)

Td15 ( C)

Td20 ( C)

Tmax ( C)

0 2 5 8 10

400 396 384 373 371

410 414 401 395 391

417 421 410 401 400

423 427 415 413 407

429 433 425 423 418 Fig. 3. SEM morphology of BADCy/MCEs composite.

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sample was subjected to temperature scan mode at a programmed heating rate of 3  C/min from 50 to about 320  C at a frequency of 1 Hz. Dielectric measurements were performed with Novocontrol Concept 80 broadband dielectric analyzer (Germany) at a room temperature by the two parallel plate modes in the frequency range between 100 and 106 Hz. Samples are about 2 mm in thickness and about 10 mm in diameter. Before testing, samples were dried at

100  C for 1 h. In order to investigate the effects of the hot water and hot air aging on the dielectric properties of composites, samples were also boiled in water at 100  C for 100 h and aged in an oven apparatus at 100  C for 100 h, respectively. Fourier-transform infrared (FTIR) spectra were obtained using a FTIR spectrometer (NICOLET 5700) to identify the chemical structure of the specimen, which was prepared by grinding the sample with potassium bromide (KBr).

O 3R O

C

N

Trimerization

R N

N R O

O

N

R

Ar yl Cyanurat e

O

R O

+ 3 R' O

N

Alk

O

R N

N

a

O

CH 2

R

Insertion

CH CH 2 O

N

N

Alk O

O

N

Alk

Alkyl Cyanurate

CH2 CH CH 2 O

Alk=

O

O

Alk Rearrangement

O Alk O

N

O

b

O

Alk N

N

R'

R

Alk

Alk

N

N N

O

Alk Alkyl Isocyanurate

c

O Alk

N

N

O

N

Alk +

3 R'

O

CH2

O

CH CH2 O

Ring Cleaveage Reformation

3 R'

O

CH2

CH

CH2 N Alk

O

Alk Alkyl Isocyanurate

O

d

Oxazolidinone R O

C

N

R'

O

CH CH2 Ring Formation O

CH2

R'

O

CH2

CH

CH2 N R

O O

e Alkyl

CH2 CH CH 2 O R'

Elimination

CH2 CH CH2 O R'

O R

R OH

R'

R

OH

Phenol

Alkene

f O

CH2

CH CH2 O

Addition

Phenoxy Resi n

R'

O

CH2

Fig. 4. The polymerization mechanism of CE and epoxy resins.

CH OH

CH2

R

g

L. Yuan et al. / Polymer Degradation and Stability 96 (2011) 84e90

3. Results and discussion 3.1. The effects of MCEs on the thermal properties of BADCy The thermal properties are important characterization of polymer composites, and they must be considered to ensure the practicable properties. Fig. 2 shows TGA curves of BADCy composites with different content of MCEs. Table 1 shows the temperature at different weight loss of BADCy/MCEs composites. Tdi represents the i% weight loss temperature, and Tmax represents the temperature at maximum mass loss rate. For the cured pure BADCy, a slight weight loss below 400  C can be attributed to the random scission and cross-linking of the hydrocarbon backbone; the weight loss in the temperature range of 400e500  C is mainly due to the breakdown of the triazine ring; and the weight loss above 500  C is attributed to the decomposition of the residue [18]. From Fig. 2, it can be seen that when the temperature is below about 400  C, the thermal stabilities of all BADCy/MCEs composites are lower than that of cured pure BADCy, and they decrease with the increase of MCEs content, the phenomenon is mainly attributed to the lower thermal stability of MCEs [19]. In addition, a trace of released core materials of MCEs may react with eOCN/triazine ring, forming oxazolidinone structure with lower cross-linking density and thermal stability [20], which is indicated by the transition areas as shown in Fig. 3, and the thermal stability of BADCy decreases. Fig. 4 shows the reaction mechanism of CE and epoxy resins. Here, FTIR spectral evidence of the transition area materials (Fig. 5) was provided to clarify the reaction mechanism. The specimen for FTIR was prepared using microtome equipped with diamond knife sharpening the transition areas. Compared with the cured pure BADCy, the transition area materials show an obvious strong absorption peak of oxazolidinone cycle at 1750 cm1, which can indicate the reaction between epoxy groups and eOCN/triazine ring [20]. When the temperature is above below about 400  C, BADCy composite with 2 wt% MCEs shows slight higher thermal stability than that of cured pure BADCy. The main reason is the fact that during the heating process, PUF wall shell and epoxy resin core materials can catalyze the reaction of BADCy, and the conversion of cyanate ester (eOCN) groups increases [16], resulting in higher thermal stability tendency of composite. As the content of MCEs increases, MCEs and oxazolidinone with low thermal stability become factors dominating the thermal stability of cured BADCy,

87

and the thermal stability of cured BADCy reduces as shown in Table 1. Although Tmax values of most BADCy/MCEs composites are lower than that of cured BADCy, they were only decreased by 4e11  C. Obviously, the additions of MCEs have no significant influence on Tmax of cured BADCy. Fig. 6 shows Tan Delta curves of cured BADCy composites with different contents of MCEs. The peak temperature of Tan Delta curve corresponds to the glass transition temperature (Tg) of the composites. It can be seen from Fig. 6 that BADCy composite with 2 wt% MCEs shows a slight higher Tg, whereas BADCy composites with 5 wt%, 8 wt% and 10 wt% MCEs show a slight lower Tg, compared with cured pure BADCy. The phenomena can be explained by the following reason: MCEs can catalyze the reaction of BADCy and reduce the unreacted eOCN [16], which may facilitate the thermal stability of cured BADCy. When the content of MCEs is lower, the decreased amount of unreacted eOCN may be the main factor affecting the thermal property of BADCy, so BADCy with 2 wt % MCEs shows a slightly higher Tg value, compared with cured pure BADCy. But on the other hand, MCEs have lower thermal stability, and the addition of MCEs can decrease the cross-linking density of BADCy owing to the formation of oxazolidinone structure, and as the content of MCEs increases, the above-mentioned effects of MCEs on BADCy become dominant, thus BADCy/MCEs composites with higher content of MCEs show a decreasing tendency in Tg. It must be mentioned here, the curing temperatures for BADCy and BADCy/MCEs composites are relatively lower, BADCy can not be completely cured, so there is an excess of unreacted eOCN, directly leading to lower cross-linking density and reaction inhomogeneity of BADCy [21], and an inflection apparently occurs at about 250  C. 3.2. The effects of MCEs on the dielectric properties of BADCy BADCy resins are good dielectric functional materials, the dielectric constant (3) and dielectric loss (tand) should be as low as possible. The studies of the dielectric properties of BADCy/MCEs composites are necessary for both fundamental and practical interests. Fig. 7 shows the dielectric properties of BADCy/MCEs composites in the range of frequency between 100 and 106 Hz at room temperature. The dielectric constant of cured pure BADCy is about 4.4e4.5. As the content of MCEs increases, the dielectric constants of BADCy composites decrease slightly firstly, reaching their minimum values (4.2e4.3) when the content of MCEs is 2 wt%,

1.0

1750

0.8

Tan Delta

Absorbance

Transition region materials

0.6

0.4

0 wt% 2 wt% 5 wt% 8 wt% 10 wt%

0.2

BADCy 0.0

4000

3500

3000

2500

2000

1500

1000

-1

Wave number (cm ) Fig. 5. FTIR spectra of the cured pure BADCy and the transition area materials indicated by Fig. 3.

50

100

150

200

250

300

o Temperature( C) Fig. 6. Tan Delta curve curves of cured BADCy composites with different contents of MCEs.

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conversion, which leads to low dielectric loss of composites. In addition, MCEs can prevent the movement of the polymer matrix, leading to the decrease of the dielectric loss of composite. Obviously, the introduction of MCEs into BADCy cannot significantly sacrifice the low dielectric characteristic of BADCy matrix.

6

Dielectric consta

a

4

3.3. The effects of hot water on the dielectric properties of BADCy/MCEs

0wt% 2wt% 5wt% 8wt% 10wt%

During boiling period, moisture absorption will increase the dielectric constant of materials. Thus, low moisture absorption is necessary for BADCy/MCEs. In our previous study, results show that the appropriate content of MCEs can decrease the water absorption mainly owing to the larger steric hindrance effect of MCEs on water molecule [16]. Here, the dielectric properties of boiled BADCy/MCEs composites are investigated. Fig. 8 shows the dielectric properties of BADCy composites with different content MCEs boiled in water for 100 h. The dielectric constant of the treated BADCy is 4.4e4.5, and the dielectric constants of treated BADCy/MCEs composites are 4.2e4.8. Compared with the corresponding untreated composites, the treated composites show only slight change in dielectric constants. The reason is the fact that during the drying process

2

100

Frequency(Hz)

Dielectric constant 0.040 0.035

0wt% 2wt% 5wt% 8wt% 10wt%

0.030

Tan Delta

0.025 0.020

a

0.010 0.005 0.000 100

6

5

0.015

1000

10000

100000

1000000

Frequency(Hz)

Dielectric loss

Dielectric constant

b

4

0wt% 2wt% 5wt% 8wt% 10wt%

3

2

1

Fig. 7. Dielectric properties of BADCy/MCEs composites as a function of MCEs content. 0 100

1000

10000

100000

1000000

Frequency(Hz)

Dielectric constant

b

0.040 0.035

0wt% 2wt% 5wt% 8wt% 10wt%

0.030 0.025

Tan Delta

and then increase slightly to 4.8 as shown in Fig. 7(a). The low dielectric constant of BADCy with 2 wt% MCEs can be mainly attributed to the fact that MCEs can catalyze the reaction of BADCy, the dipolar unreacted eOCN decreases, resulting in a minimal decrease in dielectric constant of BADCy [15]. The increased dielectric constant of BADCy with higher MCEs can be explained by the following reason: Firstly, MCEs contain polar eOH and eNHe groups, higher content of MCEs can result in higher dielectric constant of BADCy. Secondly, a trace of diffused core materials can react with eOCN/sym-trizaine to form oxazolidinone as shown in Fig. 4, increasing the polar networks and decreasing the crosslinking density. Thirdly, the dielectric constant of PUF wall shell materials is higher than that of cured BADCy, the increase of the content of MCEs enhances the proportion of the interface between MCEs and polymer matrix, which enhances the dipole of the interface [22]. The dielectric loss of cured pure BADCy is 0.0017e0.0052. Although the compositions of MCEs have higher dielectric loss, compared with cured BADCy, the addition of MCEs causes a slight change in the dielectric loss of BADCy/MCEs composites (0.0018e0.0075) as shown in Fig. 7(b). It can be explained by the main fact that MCEs can catalyze the reaction of BADCy, the conversion of eOCN increase, thus the movement of the matrix becomes more and more difficult with the improvement of its

0.020 0.015 0.010 0.005 0.000 100

1000

10000

100000

1000000

Frequency(Hz)

Dielectric loss Fig. 8. Dielectric properties of BADCy/MCEs composites boiled in water for 100 h.

L. Yuan et al. / Polymer Degradation and Stability 96 (2011) 84e90

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before dielectric testing, the water molecules unbonded to matrix desorb rapidly through capillary paths generated during the sorption process, and dry voids maybe exist in matrix [23], the dielectric constant of treated composites may drop or return to their original untreated values with the loss of water [24]. The dielectric loss of the treated BADCy is 0.0024e0.0054, and the dielectric loss of the treated BADCy/MCEs composites are 0.0018e0.0075. Owing to some moistures bounded to the matrix existing in composites [25], the treated composites may show a slight higher dielectric loss, compared with the corresponding untreated composites. Because higher content of MCEs may lead to higher moisture content [16], there is apparent increase in dielectric loss for composites with 8 wt% and 10 wt% MCEs at low frequency. Similar features in dielectric loss also have been observed in the drying of epoxy resins [24]. Although the dielectric constant and dielectric loss of all boiled composites increase slightly, the boiled BADCy composite with 2 wt% MCEs can retain lower dielectric characteristic, compared with the boiled BADCy, indicating that the appropriate content of MCEs can facilitate the low dielectric properties of BADCy when boiled in water. Fig. 10. SEM morphologies of aged CE/MCEs aged in hot oxygen at 200  C for 100 h.

a

3.4. The effects of hot air aging on the dielectric properties of BADCy/MCEs composites

6

Dielectic constant

5

4

0wt% 2wt% 5wt% 8wt% 10wt%

3

2

1

0 100

1000

10000

100000

1000000

Frequency(Hz)

Dielectric constant

b

0.040 0.035

0wt% 2wt% 5wt% 8wt% 10wt%

0.030

Tan Delta

0.025 0.020

Fig. 9 shows the dielectric properties of BADCy/MCEs composites aged in hot air at 200  C for 100 h. The dielectric constant and dielectric loss of treated BADCy are 4.3e4.4 and 0.0019e0.0067, respectively, and the dielectric constant and dielectric loss of treated BADCy/MCEs are 4.3e4.7 and 0.0013e0.0079, respectively. Although the long term heat process causes the breaking of chain bonds, reducing the cross-linking density in the matrix or forming new polar groups, the aged BADCy/MCEs composites show a slight change or decrease trends in dielectric constant and dielectric loss, compared with the corresponding unaged composites. The main reason is the fact that the composites are prepared at lower temperature cycle, the heating process initially facilitates the polymerization of unreacted eOCN groups, decreasing the dielectric constant and dielectric loss. As the heating time increases, owing to the lower thermal stability of MCEs, MCEs may decompose, which can be implied by the local degraded areas as shown in Fig. 10, and the decomposition products containing eNHe and eOH can react with the unreacted eOCN groups, thus improving the crosslinked densities of matrix, and the dielectric constant decreases. But on the other hand, gaseous degradation products evolving from MCEs might not easily diffuse out to the surface through the matrix due to its tight molecular structure, therefore, the dielectric constant and dielectric loss of BADCy/MCEs composites increase. Generally, the aged BADCy/MCEs composites can remain the low dielectric constant and dielectric loss of BADCy, especially for the composites with lower MCEs content.

0.015

4. Conclusions 0.010 0.005 0.000 100

1000

10000

100000

1000000

Frequency (Hz)

Dielectric loss Fig. 9. Dielectric properties of BADCy/MCEs composites aged in hot air at 200  C for 100 h.

The high performance BADCy/MCEs composites were prepared. Because MCEs can improve the conversion of eOCN and reduce the unreacted eOCN, the addition of 2 wt% MCEs can slightly improve Tmax and Tg values of BADCy. But on the other hand, MCEs have lower thermal stability, and the addition of MCEs can decrease the cross-linking density of BADCy, the thermal stability properties of BADCy composites with higher content of MCEs show lower thermal stability, compared with cured pure BADCy. The dielectric constants and the dielectric loss of all BADCy/MCEs composites are 4.2e4.8 and 0.0018e0.0075, respectively. Although MCEs contain

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L. Yuan et al. / Polymer Degradation and Stability 96 (2011) 84e90

polar eOH and eNHe groups, when the content of MCEs is smaller, the reduced unreacted eOCN is the main factor influencing the dielectric properties of BADCy, and BADCy with 2 wt% MCEs composite may show lower dielectric constant and dielectric loss, compared with cured pure BADCy. The addition of 2 wt% MCEs also can well maintain the low dielectric constant and dielectric loss of BADCy when treated in hot water or hot air. Acknowledgements The authors thank China Postdoctoral Science Foundation (No. 20080440165), the Third Batch of Special Sustentation of China Postdoctoral Science Foundation (No.599), Jiangsu Provincial Postdoctoral Science Research Sustentation Fund (No. 0802019B), National Natural Science Foundation of China (No. 50903058) and Provincial Natural Science Foundation of Jiangsu (No. BK2009124). The authors also thank Analysis and Testing Centre of Soochow University. References [1] Ganguli S, Dean D, Jordan K, Price G, Vaia R. Mechanical properties of intercalated cyanate esterelayered silicate nanocomposites. Polymer 2003;44 (4):1315e9. [2] Mondragón I, Solar L, Nohales A, Vallo CI, Gómez CM. Properties and structure of cyanate ester/polysulfone/organoclay nanocomposites. Polymer 2006;47 (10):3401e9. [3] Iijima T, Kaise T, Tomoi M. Modification of cyanate ester resin by soluble polyimides. J Appl Polym Sci 2003;88(1):1e11. [4] Yang CZ, Gu AJ, Song HW, Xu ZB, Fang ZP, Tong LF. Novel modification of cyanate ester by epoxidized polysiloxane. J Appl Polym Sci 2007;105 (4):2020e6. [5] Hillermeier RW, Seferis JC. Environmental effects on thermoplastic and elastomer toughened cyanate ester composite systems. J Appl Polym Sci 2000;77 (3):556e67. [6] Feng Y, Fang ZP, Gu AJ. Toughening of cyanate ester resin by carboxyl terminated nitrile rubber. Polym Adv Technol 2004;15(10):628e31. [7] Marieta C, del Rio M, Harismendy I, Mondragon I. Effect of the cure temperature on the morphology of a cyanate ester resin modified with a thermoplastic: characterization by atomic force microscopy. Eur Polym J 2000;36(7):1445e54. [8] Suman JN, Kathi J, Tammishetti S. Thermoplastic modification of monomeric and partially polymerized bisphenol A dicyanate ester. Eur Polym J 2005;41 (12):2963e72.

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