“House-of-cards” structures in silicone rubber composites for superb anti-collapsing performance at medium high temperature

Abstract

Ceramifiable polymer composites, especially silicone rubber composites, are widely used at elevated temperatures. These composites could convert into freestanding ceramified residues at temperatures above ∼1100 °C through eutectic reactions between the pyrolyzates of polymer matrices and ceramifying agents. However, it remains challenging for polymer composites to be used at the medium high temperatures (500–900 °C), because of the gap between the temperatures of polymer decomposition and the eutectic reaction. Here, we demonstrate that when the Laponite platelets in the preformed “house-of-cards” structure (Laponite-armored hollow composite particles, referred to as LHCPs) are used as fillers for silicone rubber, the resultant composites can well maintain their shape and mechanical strength in this critical temperature range as mentioned above. As revealed by microscopic and crystallographic studies, the superb anti-collapsing performance at medium high temperatures was indicated to stem from the “house-of-cards” structure of the LHCPs. These composites may find potential in some high temperature applications, for example, fire resistance cables, where both shape-maintenance and mechanical support at medium high temperatures are required in order to ensure electric supply in fire.

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Introduction

Polymeric materials are flexible, light-weight and easy to process, thus are widely used nowadays; however, it remains a challenge for them to be used at elevated temperatures (for example over 200 °C). When heteroatoms (e.g. silicon^1–3) or aromatic rings are introduced to the backbone, polymers may be used at slightly higher temperatures (up to 500 °C at most), such as silicone rubber, polyimide and polyamide. For even higher temperature applications, ceramifiable polymers have been developed, which can burn into ceramic residues at higher temperatures (i.e. >1100 °C), thus maintaining a certain shape and mechanical strength.^4–8 For example, silicone rubber may form tough fuel and heat barrier layers composed of inorganic silicon compounds through the decomposition of the polymer matrix and the fusion between the pyrolyzates and the ceramifying agents. Nevertheless, there is a temperature gap between the degradation of polymer and ceramic formation, which is roughly in the range of 500–900 °C. This is a critical temperature range, where most of the polymeric materials fail to maintain their shape and mechanical strength while the other mechanism supported by ceramified residual has not been in function yet. If this critical temperature range is not passed quickly, even the ceramifiable polymers will collapse into powders. Therefore, developing polymer-based materials which can be used in the above temperature range is one of the greatest challenges in materials community and new strategies have to be explored.

Inorganic fillers including mica^9–11 and wollastonite^12–14 are frequently used as ceramifying agents, which could form percolation structures thus may provide a supporting framework after polymer has decomposed. They may also facilitate the densification of the silica residues through a eutectic reaction,¹⁵ achieving a much enhanced strength at severely high temperature (i.e. above 1100 °C). However, in the critical temperature range mentioned above (500–900 °C), when polymer decomposes, these filler particles often form aggregates in order to achieve the minimal interfacial energy, thus losing their original network arrangement as well as mechanical support. Laponite platelets are good examples (discoidal platelet with a lateral diameter of ca. 25–35 nm and a thickness of ca.1 nm (ref. 16)). When dispersed in a medium, they are easy to form interconnected networks, due to their discoid structure.¹⁷ However, they tend to aggregate in a face-to-face style in order to maximize their contact with each other and minimize the interfacial area,^18–22 thus their composites with silicone rubber would still collapse after polymer decomposition.

If the Laponite platelets are pre-organized in an edge-to-edge style (Scheme 1a) in the silicone rubber matrix, they might still maintain this structure after polymer decomposition. Consequently, the ceramifiable polymer composites, containing such pre-organized filler particles, would preserve the initial shape integrity and sufficient mechanical strength in this critical temperature range (Scheme 1b) and further be ceramified at the eutectic reaction temperature.

Scheme 1 Schematic illustrations of (a) the pre-organized nano-architecture; (b) a hypothetical pyrolysis pathway of filler-silicone rubber composite heated at 500–900 °C.

Herein, we are aiming to address the above challenge by using such novel filler particles, i.e., Laponite-armored hollow composite particles (LHCPs) with a pre-organized “house-of-cards” structure. The LHCPs could be fabricated via a Pickering emulsion template method^23–27 and be readily dispersed in the silicone rubber matrix, yielding ceramifiable polymer composite materials. These composite materials exhibit superior anti-collapsing performance at the medium high temperature (i.e. in the critical temperature range as mentioned above) after the decomposition of silicone rubber matrix.

Experimental

Materials

3-(Trimethoxysilyl)propyl methacrylate (TPM) and potassium persulfate (KPS) were purchased from Alfa Aesar. Laponite RD was purchased from Rockwood Additives Ltd. Isoamyl acetate (PEA) was purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethylorthosilicate (TEOS) was purchased from Xilong Chemicals Corp., Guangdong, China. Dibutyltindilaurate was supplied by Beijing Yili Fine Chemicals Co., Ltd., Beijing, China. Ammonia water (25–28 wt%) was purchased from Beijing Chemical Works, Beijing, China. Hydroxyl-terminated room temperature curing silicone rubber with a brand name of 107 Rubber (η = 3900 mPa s, M_w = 45 000 by GPC) was purchased from China National Bluestar (Group) Co., Ltd., Beijing, China. All materials were used as received. Water used in all experiments was purified by an ELGA Purelab® system, with a resistance greater than 18.2 MΩ cm.

Characterizations

Scanning electron microscopy (SEM) observations were performed on a JEOL JSM 6700 F instrument operated at a voltage of 5 kV. Samples were sputter-coated with Pt before SEM observations. Transmission electron microscopy (TEM) images were obtained at 200 kV on a JEOL JEM 2200 FS electron microscope. The average particle size and size distribution were measured by dynamic light scattering (DLS) using a Malvern Zetasizer Nano. Thermogravimetric measurements were conducted with Pyris 1 TGA Thermogravimetric analyzer to investigate the thermal stability of the samples. The samples were pyrolyzed from room temperature to the target temperatures at a ramping speed of 10 °C min⁻¹ in flowing air. N₂ adsorption isotherms were obtained at 77 K on a Quantachrome AUTOSORB-1 instrument, and samples were degassed at 100 °C for 8 h prior to testing. Pore size distributions were evaluated from the adsorption isotherms using the classical Barrett–Joyner–Halenda (BJH) model.

The compressive strengths were tested by compression measurements with a 5 kN load cell on rectangular specimens of approximately 6 × 6 × 3 mm. The specimens were subjected to uniaxial compression under a cross-head speed of 1 mm min⁻¹, employing an INSTRON 3365 mechanical testing machine. Three to five pieces were tested to obtain the average property. The chemical structures of samples were analyzed by FT-IR (Bruker EQUINOX55) employing the potassium bromide pellet technique,²⁸ in the range of 4000–400 cm⁻¹ at a 4 cm⁻¹ resolution. X-ray diffraction spectra were collected on a RigaKu D/MAX 2500 spectrometer. Each scan was conducted from 2θ angle of 5° to 70° at a scanning rate of 2° min⁻¹.

Synthetic procedures

Preparation of the Laponite-armored hollow composite particles (LHCPs). The Laponite-armored hollow particles were prepared via a modified method from that established in our previous work.²³ The recipes used were given in Table 1. Typically, 0.2 g of Laponite RD was dispersed in 40 mL deionized water using sonication to obtain a visually clear colloidal dispersion. The pH of the resulting colloid system was adjusted to around 10 with ammonia water. TPM (varied amounts with respect to PEA) and/or PEA was then poured into the above dispersion. A stable emulsion system was obtained after stirred at room temperature for ∼8 h. The emulsion was then heated to 70 °C after being degassed by purging with nitrogen for 0.5 h. The emulsion polymerization was started upon the addition of ∼10 mg KPS and subsequently carried out at 70 °C for 12 h. The resultant particles were collected by centrifugation at 8000 rpm for 10 min. After washing with ethanol twice to remove unreacted reagents, the products were dispersed in water or dried for further characterization.

Table 1 Formulations for preparing the LHCPs

Ingredients	Water (g)	Laponite (g)	TPM (mL)	PEA (mL)	KPS (mg)
LHCP-1	40	0.2	0.1	1.0	10
LHCP-2	40	0.2	0.2	1.0	10
LHCP-3	40	0.2	0.4	1.0	10
LHCP-4	40	0.2	0.4	0	10

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Preparation of the specimen for pyrolysis. The silicone rubber was compounded with 30 wt% (weight percentage in the final silicone rubber composite) Laponite or LHCPs in two roll mixing mill until a homogeneous batch was obtained. 1 wt% (weight percentage in the final silicone rubber composite) dibutyltindilaurate as catalyst and 2 wt% TEOS as crosslinking agent were then added and processed until a visually good dispersion was achieved. The mixtures were poured into molds, vulcanized at room temperature for 24 h, and then postcured at 60 °C for 6 days in an oven. The hydroxyl-terminated silicone can be crosslinked with TEOS through the following reaction:²⁹

Sample pyrolysis. Pyrolysis of the specimens was performed using a tube furnace. Samples were heated from room temperature to various temperatures in air at a heating rate of 10 °C min⁻¹.

Results and discussion

Our preparation strategy involved a modified Pickering emulsion system consisted of water, Laponite platelets, PEA and TPM. After mixing together and stirring for 8 to 12 h, a visually milky emulsion was obtained, which was then further polymerized overnight initiated by potassium persulfate. The morphology of resultant LHCPs could be controlled by the ratios of TPM to PEA (Table 1 and Fig. S1†). The product obtained from the recipe of LHCP-3 was of the optimal structures so that it was used as the filler for silicone rubber for further research. As shown in Fig. 1, the LHCPs were of hollow structure and with Laponite platelets armoring on the surface of the polymeric shell. Close-up inspection of the LHCPs revealed that dark “filaments” were on the surface of the LHCP (inset in Fig. 1b), corresponding to the Laponite platelets viewed parallel to the observation direction, with a strong scattering contrast. Laponite platelets with different orientations did not generate such a contrast thus were hardly detected in the images. The LHCPs were with an average diameter of 100 ± 35 nm and a shell thickness of ∼20 nm, by measuring 400 particles. Dynamic light scattering (DLS) study (Fig. 1c) revealed the size distribution by number for the LHCPs with a peak value of ∼116 nm. The dimension of the LHCPs acquired from DLS was reasonably comparable with the microscope images. More importantly, the LHCPs could maintain their “house-of-cards” structure very well at the temperature of interest (500–900 °C) (Fig. S2†), which was essential for them to be used at these elevated temperatures.

Fig. 1 Characterizations of the LHCPs: (a) SEM images, (b) TEM images, (c) particle size distribution given by DLS study, (d) IR spectrum, (e) TGA curve of the LHCPs. Inset in (b) shows a high magnification TEM image of one LHCP.

In addition, the LHCPs had an inorganic–organic hybrid structure with an organic moiety content of ∼50 wt% judged by TGA measurement (Fig. 1d and e). The characteristic adsorption peaks in Fig. 1d at 2950, 2847, 1395, 1467 and 1726 cm⁻¹ were assigned to the alkane groups and carbonyl group of the polymerized TPM (PTPM); those at 3446 and 1638 cm⁻¹ were indexed to the stretching vibration and bending vibration of –OH groups in the PTPM resulted from the hydrolysis of TPM. The absorption bands in the region of 1000–1100 cm⁻¹ were typical of the stretching vibration of Si–O–Si and that at ∼800 cm⁻¹ was assigned to Si–O deformation. The partial hydrolysis of TPM will give –OH groups for the silicone moiety in LHCPs, which can chemically bond with the hydroxyl-terminated silicone rubber and the crosslinking agent TEOS. Besides, the compatibility between PTPM and silicone rubber is rather good; the solubility parameters of PTPM and silicone were calculated to be 6.78 and 6.12 respectively,³⁰ which were very close to each other, indicating these two polymers were well compatible.

The composites with LHCPs in silicone rubber matrices (abbreviated as S-LHCP hereafter) were prepared. Those with the same amount of original Laponite platelets (abbreviated as S-L hereafter), surface modified Laponite platelets by TPM (referred as S-ML), as well as the blank silicone rubber were also studied for comparison. It was immediately evident that the LHCPs and modified Laponite platelets were dispersed better in silicone rubber matrix than the original Laponite platelets, without apparent sedimentation (Fig. S3a†) and aggregation or local detachment at filler–matrix interface (Fig. S3c and d†). While in the case of S-L composite, serious sedimentation was observed (Fig. S3a†), as well as microscopic aggregation (Fig. S3b†). This is rather expected as Laponite platelets are hydrophilic thus surface modification is needed to enhance their compatibility with silicone rubber matrix.

The thermal behaviors of silicone rubber and its composites were presented in Fig. 2 and corresponding characteristics were summarized in Table 2. Two characteristic temperatures were used to evaluate the thermal stability: T₅ (temperature when weight loss reaches to 5 wt%) and T_d (temperature at maximum rate of weight loss). It can be seen that silicone rubber had two weight loss processes, 275 °C–385 °C and 400 °C–500 °C, with T_d at 364.7 °C and 469.4 °C, respectively. The first weight loss was due to the thermal oxidation of their methyl side groups, while the other one to the decomposition of the siloxane backbones producing cyclic oligomers.^31–33 When original Laponite platelets were incorporated (S-L composite), the thermal stability was not improved significantly or somehow worsened a bit (i.e. the second T_d). For the case of S-ML, it can be seen that both T₅ and T_d improved to some extent due to the better dispersity of modified Laponite platelets in silicone matrix. Similarly, in the case of S-LHCP composite, the thermal stability has been much improved, with two T_ds at 468.4 °C and 572.2 °C, respectively. The measured residue yields of silicone and S-LHCP composite were found to be 17.9% and 24.8%, respectively, which were in agreement with the theoretical value for each recipe. Interestingly, no obvious thermal decomposition event of the LHCPs was observed in silicone rubber composites, but only decomposition at higher T_d, which seemed to suggest that silicone rubber also improved the thermal stability of the LHCPs, presumably because the encapsulation of the LHCPs by silicone rubber matrix hindered them from contacting with oxygen. Inorganic fillers are well recognized in enhancing the thermal degradation temperature of polymers.^34,35 Because only the polymer segments restricted by filler particle surface are more affected, a better dispersion of filler particles in polymer matrix is thus desired in order to increase the direct contact of polymer with particle, which is the case in this study. Furthermore, due to the chemical bonding and hydrogen bonding between fillers and rubber matrix, the restriction of polymer segments by filler particles is even stronger, thus the enhancement in polymer degradation temperature is more pronounced.

Fig. 2 Thermal behaviors of different samples: (a) TGA, (b) DTG traces of silicone rubber (black, i), LHCP (magenta, ii), S-L (red, iii), S-ML (blue, iv), S-LHCP (green, v).

Table 2 Thermal characteristics of different samples

Samples	T5 (°C)	Td (°C)	Residue (%)
LHCP	331.2	423.4, 595.8	50.6
Silicone	337.8	364.7, 469.4	17.9
S-L	332.3	373.5, 442.7	37.7
S-ML	375.5	457.3	29.0
S-LHCP	340.0	468.4, 572.2	24.8

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As mentioned earlier, the anti-collapsing performance of polymer composites at medium high temperatures (500–900 °C) remains a big challenge. It can be seen that even with these filler particles, the degrading temperature of silicone rubber matrix cannot be much higher than 500 °C, thus the anti-collapsing performance cannot count on the support from silicone rubber. The residual mass fraction at a given temperature is regarded as the most useful index to estimate their performance at that temperature. Usually, the residual mass fraction of polymer composites could be very high at high inorganic filler content, which will cause a significant increase in their density unfortunately, hampering their application as low weight materials. Therefore, it is not a wise option to enhance polymer composites' anti-collapsing performance by using extremely high filler content. It can be seen that all the composites samples in Fig. 2 had some residual mass left in this critical temperature range, and the residual mass fraction increased with fillers addition. Their anti-collapsing performances were evaluated by visual check after heated at different temperatures from 500 °C to 1100 °C, mainly on their ability to maintain their shape integrity. As shown in Fig. 3, the silicone rubber composite filled with LHCPs (S-LHCP) can maintain its original shape and size up to a temperature of 900 °C (Fig. 3b₄, c₄ and d₄). After that, the specimen showed some shrinkage, which was owing to the commencement of fusion at that high temperature, but still successfully maintained its shape integrity (Fig. 3e₄). While the other three samples, the pure silicone rubber (Fig. 3b₁, c₁, d₁ and e₁), the composite with original (S-L) (Fig. 3b₂, c₂, d₂ and e₂) and modified Laponite platelets (S-ML) (Fig. 3b₃, c₃, d₃ and e₃), all collapsed into powders.

Fig. 3 Digital photos of the samples: (a₁–a₄) samples before pyrolysis (a₁) silicone, (a₂) S-L, (a₃) S-ML and (a₄) S-LHCP; (b₁–e₁) silicone after pyrolysis at (b₁) 500 °C, (c₁) 700 °C, (d₁) 900 °C and (e₁) 1100 °C; (b₂–e₂) S-L after pyrolysis at (b₂) 500 °C, (c₂) 700 °C, (d₂) 900 °C and (e₂) 1100 °C; (b₃–e₃) S-ML after pyrolysis at (b₃) 500 °C, (c₃) 700 °C, (d₃) 900 °C and (e₃) 1100 °C; (b₄–e₄) S-LHCP after pyrolysis at (b₄) 500 °C, (c₄) 700 °C, (d₄) 900 °C and (e₄) 1100 °C.

It was worth noting that the residual mass fraction of S-LHCP was actually lower than S-L and S-ML, thus the better anti-collapsing performance of S-LHCP in this critical temperature range should not be simply attributed to the residual mass fraction. Therefore, we need to find out what were the possible reasons responsible for the exceptional anti-collapsing performance of the S-LHCP composites.

A microscopic investigation on their structure at different temperatures provided some evidences. In S-LHCP composites, after pyrolysis at 500 °C and 700 °C, a porous network was formed (Fig. 4a and b). Interestingly, LHCPs remaining their “house-of-cards” structure and they were in close proximity to each other (Fig. 4a and b), serving as backbones of the network as well as the main support for shape integrity. This was verified by the porosity measurements (Fig. S4†), showing an obvious hysteresis as a result of the presence of mesoporosity, with a specific surface area of 172 m² g⁻¹ (Fig. S4a†). In addition, the mesopores showed a broad distribution centered at around approximately 22 nm, exhibiting a pore volume of 0.716 cm³ g⁻¹. The porosity measurements further confirmed the shape retention property of the LHCPs and the polymer composites after heat treatment. The special structural nature of the LHCPs might be one of the reasons why the S-LHCP composites preserved their shape integrity in this critical temperature range. Some irregularly shaped residues resulted from the pyrolysis of the silicone matrix were found to adhere to the LHCPs, further enhancing the strength of LHCPs networks. The above network could further fuse together through chemical bonding upon pyrolysis at higher temperatures, thus improving the connection between network building blocks. As seen in Fig. 4c, the fracture surface of the pyrolyzate at 900 °C showed the coexistence of localized dense regions (marked with red dash line) and porous regions, suggesting the occurrence of fusing between LHCPs. By 1100 °C, the fusion continued; the dense regions expanded and became dominating (Fig. 4d), which corresponded to a shrinkage of the specimen (Fig. 3d₄). As comparisons, pure silicone rubber, S-L and S-ML composites all formed loose aggregates of residual ashes after pyrolysis, thus falling into powders macroscopically (Fig. S5 in the ESI†).

Fig. 4 Microstructures of the S-LHCP after pyrolysis: SEM images of the fracture surfaces of the S-LHCP after pyrolysis at (a₁) 500 °C, (b₁) 700 °C, (c₁) 900 °C, (d₁) 1100 °C; the corresponding close-ups are shown in a₂–d₂.

On the other hand, X-ray diffraction studies also suggested a similar process (Fig. 5). It can be seen that pyrolyzates of S-LHCP and S-L composites exhibited almost the same diffraction lines except for some subtle differences of several diffraction peaks. Strong distinct humps around 2θ values of 20–30° were arisen from the amorphous Si_xO_yC_z derived from the pyrolyzates of the composites. Before pyrolyzed at 900 °C, only diffraction peaks of Laponite platelets were observed. While starting from 900 °C, new crystalline phases of cristobalite (majority phase with main peaks at 22°, 28°, 31°, 35–36°, 52° etc.) and enstatite (minority phase with main peaks at 20°, 26–27°, 33–34°, 52°, 56–57° etc.) started to form and became dominant when pyrolysis at 1100 °C. The almost identical diffraction patterns for these two pyrolyzates indicated that there was no production of new additional phases responsible for the superb anti-collapsing performances when LHCPs was used as the fillers. Consequently, their difference in anti-collapsing performance should result only from the “house-of-cards” structure of the filler particles.

Fig. 5 XRD patterns of (a and c) S-LHCP, (b and d) S-L after pyrolysis at 500 °C (black, i), 700 °C (red, ii), 900 °C (blue, iii), 1100 °C (green, iv); (c) and (d) are the corresponding partially enlarged details.

As discussed above, the exceptional anti-collapsing behaviors of the S-LHCP composites may be due to the following reasons. When the filler content was sufficiently high, fillers can form a network after polymer decomposition anyway. However, the “house-of-cards” structure can make most use of Laponite platelets in forming a network at relatively low concentration, thus can provide good anti-collapsing property. Furthermore, the strong interactions between the LHCPs and the silicone matrix were also responsible for the effective coherent residue production during thermal degradation of silicone rubber.³⁶

The excellent anti-collapsing performance of the S-LHCP composites in the medium high temperature range makes it a potential candidate for various applications including fire resisting cables. Besides the good shape integrity, sufficient mechanical strength is another prerequisite for the application as fire resisting cables so that the residues can ensure the normal operation of electric power and communication on fire. Compressive stress–strain curves of S-LHCP composites after pyrolysis at various temperatures were presented in Fig. 6 and corresponding characteristic values were summarized in Table 3. It can be seen that the S-LHCP after pyrolysis showed rather high compressive strength and modulus, which increased with pyrolysis temperature due to the formation of chemical linkages between building blocks as revealed earlier in this study. Even pyrolyzed at 500 °C, the compressive strength could reach to a relatively high value, ∼2.1 MPa. Dramatic increases in both compressive strength and modulus were observed after pyrolyzed at 1100 °C (37.1 MPa and 310.5 MPa, respectively) as a result of eutectic reactions (Fig. 4d₁ and d₂). On the contrary, no measurements could be done on the pyrolyzed pure silicone rubber, S-L or S-ML composites as they were all powders.

Fig. 6 Mechanical performance of S-LHCP after pyrolysis: stress–strain curves of S-LHCP after heating at 500 °C (black, i), 700 °C (red, ii), 900 °C (blue, iii), 1100 °C (green, iv) under uniaxial compression.

Table 3 Compressive strength of S-LHCP composites after pyrolysis at various temperatures

Pyrolysis temperature (°C)	Compressive strength (MPa)	Compressive modulus (MPa)
500	2.1 ± 0.1	10.7 ± 2.5
700	5.7 ± 1.8	21.8 ± 6.6
900	11.7 ± 1.0	78.4 ± 8.5
1100	37.1 ± 7.7	310.5 ± 7.7

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Conclusions

In summary, we have introduced an innovative view that incorporating pre-organized “house-of-cards” structures of Laponite platelets into the silicone rubber matrices could bring about excellent anti-collapsing performance at medium high temperature (500–900 °C), after the decomposition of polymer. The anti-collapsing properties of the composites system may be ascribed to the following reasons: (i) the “house-of-cards” structure of the LHCPs and (ii) the strong interactions between LHCPs and the silicone matrix. These uniquely structured fillers are readily prepared and well compatible with silicone rubber matrix. Furthermore, apart from outstanding anti-collapsing performance at medium high temperatures of the resultant composites, the LHCPs also impart the residuals of composites with sufficient mechanical strengths. The strategy of introducing such a structure to enhance the anti-collapsing behaviour is versatile and might be extended to a wide range of polymer matrices. This presents new opportunity for applications where maintaining the original shape and good mechanical strength is prevalent at elevated temperature especially after the decomposition of polymer, for example, fire resistant cables.

Acknowledgements

This work was supported by State Key Development Program of Basic Research of China (2012CB933200), National Natural Science Foundation of China (51173193, 21474122) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020300).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26937g

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