Surface-Modified Polyimide Fiber-Filled Ethylenepropylenediene

Dec 19, 2012 - Polyimide (PI) short fiber-filled ethylenepropylenediene monomer (EPDM) insulations for a solid rocket motor were fabricated by surface...
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Surface-Modified Polyimide Fiber-Filled Ethylenepropylenediene Monomer Insulations for a Solid Rocket Motor: Processing, Morphology, and Properties Zhongqiang Han, Shengli Qi, Wei Liu, Enlin Han, Zhanpeng Wu,* and Dezhen Wu Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: Polyimide (PI) short fiber-filled ethylenepropylenediene monomer (EPDM) insulations for a solid rocket motor were fabricated by surface modification of PI short fibers in an alkaline aqueous solution and subsequently by mixing formulation of EPDM insulations on a two-roll mill. The effects of PI short-fiber surface modification and short-fiber content on the mechanical and ablative properties were investigated. The excellent mechanical and ablative properties of PI fiber-filled EPDM insulations were based on their unique fiber/polymer adhesive property because of the rough surface character of modified PI fibers. The microstructures of char layers of the insulations were also characterized by scanning electron microscopy and energydispersive spectrometry. fiber and inorganic material, such as silver layers, can be improved via surface modification of the PI fibers.16,17 Surface modification of the PI fibers mainly relies on immersion of the fibers into a potassium hydroxide (KOH) aqueous solution to form carboxylic acid groups via an imide-ring cleavage reaction. Herein, surface-modified PI fibers were first used to prepare EPDM-based insulations for a SRM. We expect that this approach could demonstrate many advantages, such as good mechanical properties, excellent thermal resistance, and ideal char layers during working of the SRM. This technique is expected to extend to fabrication of elastomeric insulations or other functional organic composite materials filled with PI fibers to protect the case of a SRM or other potential applications.

1. INTRODUCTION It is common practice to utilize silica- and/or fiber-filled elastomeric composites to protect solid rocket motor (SRM) combustion chambers from the severe temperatures generated when it was firing. For example, during firing of a SRM, internal insulations positioned between the case and solid propellant are subjected to temperatures in the range of 2000−4000 °C with pressures of 6.89 MPa or greater.1,2 The most common heatshield elastomeric insulation of choice is ethylenepropylenediene monomer (EPDM) rubber because of its low specific gravity, aging resistance, low-temperature flexibility, chemical stability, and low erosion rate.3−6 Not only can fibers confer insulations’ high strength and outstanding thermal stability, but they can also reinforce the formed char layers during working of the SRM. Short fiber-reinforced EPDM composites have been widely applied as the thermal insulation of a SRM.3−11 However, not all fibers are satisfying fillers for the elastomeric thermal insulation of a SRM. For example, asbestos has been banned because of its carcinogenicity, and it has been replaced by other man-made short fibers or pulps. Carbon and glass fibers are susceptible to deterioration into smaller particles or shreds because of their fragile characteristics during processing of the insulation materials under high shear forces. Thus, flexible organic fibers, such as short aramid fiber, are often used to reinforce EPDM insulations because of their flexible properties to withstand shear stress during mixing of the insulation materials. Compared with aramid fibers, polyimide (PI) fibers have better thermal stability and higher char residues because of their stiff aromatic backbones and aromatic imide groups of the molecular main chains (Figures 1 and 2).12−15 However, the adhesion properties between the PI fiber and EPDM matrix are unacceptable because of the smooth surface of the PI fiber, which hardly adheres to the EPDM matrix. As a consequence, it is quite difficult to achieve ideal ablation and mechanical properties of EPDM/PI fiber insulations. More recently, our group reported that the adhesion properties between the PI © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. PI (PMDA-ODA) fibers were prepared by a two-step processing method in our group,17 and their properties are shown in Table S1 in the Supporting Information. EPDM [type J4045; ethylene content, 49−55 wt %; 5-ethylidene-2norbornene content, 6.7−8.7 wt %; Mooney viscosity ML(1 + 4) @100 °C, 40−50; tensile strength, >15 MPa (160 °C, 30 min)] was provided by Jilin Chemical Co., Ltd., China. Kevlar fiber (3−5 mm) was supplied by Jiangmen Six Part Fiber Co. Ltd., China, and dried in a vacuum-drying oven for 5 h at 120 °C before use. Fumed silica (size, 12 nm) was purchased from Shenyang Chemical Co., Ltd., China. A boron-containing phenol/formaldehyde resin was purchased from Shanxi Taihang Impede-fire Polymer Co., Ltd., China. All other additives were commercially available materials and were used as received. Received: Revised: Accepted: Published: 1284

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Figure 1. Chemical structures of (a) PMDA-ODA-type PI and (b) Kevlar fibers.

2.3. Preparation of Short Fiber-Filled EPDM Thermal Insulations. In the formulations of EPDM/PI short fiber insulations (Table S2 in the Supporting Information), the variable contents of surface-modified PI fibers were 0 g per 100, 2.5, 5, 10, 13, and 15 parts of rubber (phr), respectively, while the contents of other ingredients were constant. For comparison, we also fabricated EPDM insulations filled with 10 phr of parent PI fibers and 10 phr of Kevlar short fibers. Procedures of the thermal insulations were described as follows: First, EPDM and additives were mixed on a two-roll open mill [X(S)K-160, Jiangdu Xinzhenwei Testing Machinery Co., Ltd., China] at a friction ratio of 1:1.1 using conventional processing technologies of elastomeric products, and then fumed silica and all other additives were added. Second, the organic short fibers were slowly added with distribution of the fibers in the master batch. Then dicumyl peroxide and sulfur were added. After that, the materials were extruded 10 times to ensure better dispersion after the nip gaps of the rolls were set to less than 1 mm, except for only one sample (EPDM/Kevlar insulation) extrusion for 20 times in Table 1. Finally, the resulting materials were stripped off the mill and vulcanized within an iron box on a flat vulcanizing machine (QLB-25T, Jiangdu Xinzhenwei Testing Machinery Co., Ltd., China) for 40 min at 160 °C. 2.4. Characterization. The ablation properties were conducted on an oxyacetylene ablation tester (YS-22, Xi’an Huian Chemical Co., China). The flow rate and pressure of oxygen were set to 1512 L/h and 0.4 MPa, respectively, while the flow rate and pressure of acetylene were set to 1116 L/h and 0.095 MPa, respectively. The total ablation time was 20 s. The distance between the specimen face and torch tip was 10 mm, and the angle between the torch and specimen was 90°. The test specimen was a cylinder with a thickness of 10 mm and a diameter of 30 mm. The linear ablation rate is to evaluate how fast the material is damaged or burned by the flame. The ablative thickness is tested as a product of the thickness change before and after ablation tests of each specimen, and the linear ablation rate was calculated by dividing the ablative thickness by the total ablative time (20 s). That is, a lower linear ablation rate represents a higher ablation-resistance property.

Figure 2. Thermogravimetric and differential thermal analysis curves of (a) PI and (b) Kevlar fibers.

2.2. Surface Modification of PI Fibers. PI fibers were cut into 3−5 mm and then immersed in a 2 M KOH aqueous solution for 0.5 h at ambient temperature. After being rinsed by a 1 M hydrochloric acid aqueous solution, the PI fibers were rinsed by a copious amount of deionized water. Finally, the modified PI fibers were dried in a vacuum-drying oven for 5 h at 120 °C.

Table 1. Mechanical and Ablative Properties of EPDM Thermal Insulations

a

fibers

contents of fibers (phr)

yield stress (MPa)

modified PI modified PI modified PI modified PI modified PI modified PI parent PI parent Kevlar

0 2.5 5 10 13 15 10 10

no 4.4 ± 0.2 6.1 ± 0.3 8.1 ± 0.2 9.7 ± 0.5 13.9 ± 0.1 7.1 ± 0.2 5.6 ± 0.5 (6.8 ± 0.2)a

tensile strength (MPa) 15.4 12.8 13.2 13.8 14.3 13.9 11.9 6.3

± ± ± ± ± ± ± ±

1.1 0.7 0.2 0.3 0.2 0.1 0.2 0.9 (10.1 ± 0.3)a

elongation at break (%) 807.5 778.1 666.3 727.7 710.6 379.0 661.0 517.3

± ± ± ± ± ± ± ±

ablation rate (mm/s)

3 33 18 18 12 10 13 89 (739.3 ± 18)a

0.20 0.18 0.06 0.04

0.06 0.09

Data in brackets were obtained for the sample that was extruded 20 times after the nips of the rolls were set to less than 1 mm. 1285

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Scheme 1. Ideal Changing Process for Surface Modification of PI Fibers via a Chemical Hydrolysis Reaction in an Alkaline Solution

The mechanical properties of EPDM thermal insulations were tested by an electronic universal testing machine (SANS, MTS systems (China) Co., Ltd.). The dumbbell-shaped test sample with a working length of 25.0 mm, a width of 6.0 mm, and a thickness (measured before the test) of 2−3 mm was cut from a thermal molded sheet. The dumbbell specimen was placed in the grips of the testing machine, and the test was carried out with a separation of the grips at a rate of 500 mm/ min. Both the tensile strength and elongation at break of insulation were calculated by the average value of five replicates for each formulation. Thermogravimetric analysis of the PI and Kevlar fibers was carried out on a TA Instruments (Q 50, USA) analyzer. The sample (6.0 ± 0.3 mg) was uniformly sprawled on the sample pan and heated from ambient temperature to 800 °C with a temperature procedure of 10 °C/min under a nitrogen atmosphere. Micromorphologies of the surface and cross sections of PI fibers, fracture surfaces of tensile samples, and char layers of insulations were characterized by a scanning electron microscope (S-4700, Hitachi, Japan) at an accelerating voltage of 20 kV after samples were coated with ca. 5 nm of a palladium/ gold alloy. Samples for testing of the cross sections of PI fibers were fabricated by burying PI fibers within an epoxy matrix and then fractured in liquid nitrogen. The chemical composition of the exposed fiber was examined by an energy-dispersive spectrometer (EDAX GENSIS 2000). The ablated surface of the cooling ablated specimen was also recorded using a Canon Power Shot G12 camera. Dispersion of organic fibers in a EPDM phase was characterized using a digital optical microscope [Motic DM-BA200-C, Motic (Xiamen) Instrument Co., Ltd.] on unvulcanized rubber sheets of ca. 0.5 mm thickness. The sample was fabricated via dispersion of 10 phr fibers in an EPDM matrix without any other filler. The total specific surface area values for PI fibers before and after treatment were estimated using nitrogen adsorption equilibrium data gathered at −196 °C in a Micromeritics ASAP 2020 instrument. The unit was equipped with turbo molecular drag pumps that permitted evaluation of the adsorption data at ultralow pressure.

Figure 3. SEM images of both (a) parent and (b) surface-modified PI fibers.

treatment,19 resorcinol formaldehyde latex,20 and mechanical5 and chemical modification.21 It is well-known that suitable modification methods depend on the chemical structures of both the fiber and matrix. PI is vulnerable to hydrolysis in an alkaline solution under certain conditions to form carboxyl and amino groups via an imide-ring cleavage reaction.22,23 In view of this characteristic of PI, it facilitates to enhance the roughness and surface area of PI short fibers, and hence the interactions between fibers and EPDM matrixes would be enhanced accordingly. The ideal changing process of PI short fibers during surface modification is listed in Scheme 1. Scanning electron microscopy (SEM) morphologies in Figure 3

3. RESULTS AND DISCUSSION 3.1. Surface Modification of PI Fibers. Conventional modification methods of fibers include coupling agents,18 plasma 1286

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modification of PI fibers would enhance the interface-adhesive property between the fibers and EPDM matrixes, compared with the original PI and/or parent Kevlar fibers. 3.2. Effects of the Content of PI Fibers on the Ablative Properties and Microscopy of Char Layers. The linear ablation rates of EPDM insulations with various contents of modified PI short fibers are shown in Table 1. The ablative resistance of EPDM insulation was markedly enhanced when certain contents of PI short fibers were added. The linear ablation rate of EPDM insulation containing no PI short fibers was only 0.20 mm/s, while upon incorporation of 10 phr PI

Figure 4. SEM images of char layers after ablation testing for EPDM insulations filled with various contents of modified PI fibers from (a) 2.5, (b) 5, and (c) 10 phr and (d) a cross section of the char layer with 10 phr modified PI fibers.

clearly show that the surface morphologies of modified PI short fibers became much rougher than those of parent PI short fibers. Brunauer−Emmett−Teller (BET) measurement of the surface areas for parent PI short fibers also shows that it was too smooth to absorb nitrogen and its BET specific surface area was 0.0051 m2/g. In comparison, it was 0.1568 m2/g for the modified PI short fibers. In this case, we can deduce that surface

Figure 5. (a) One bare fiber in the char layer selected from Figure 4c. (b) Magnification of the fiber surface of part a. (c) EDS results of part b. 1287

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short fibers into EPDM insulations, it decreased dramatically to 0.04 mm/s. Furthermore, char layers were formed when the insulations were exposed for certain times to combustion gases in SRMs. Also, char layers can protect the remaining virgin material from the thermal and mechanical effects of the combustion gases.1,24 A looser char layer was found for the insulation without any PI fibers after the ablation test, which can be peeled off from

the remaining virgin material (Figure S1 in the Supporting Information), while an intact char layer with integrated and rigid characteristics remained on the surface of the remaining virgin insulation with 10 phr PI fibers after the ablation test. We suggest that the char layers with integrated and rigid characteristics show better erosion characteristics than those without fiber fillers under combustion gases. To further inspect the surface morphology of char layers of PI short fiber-filled EPDM insulations, SEM was applied for selected samples in Table 1. As shown in Figure 4, many holes were formed in the char layers because of the emission of pyrolysis gases through decomposition of the insulations under combustion gas measurement. With the addition of modified PI short fibers, it was found that insulations have integrated char layers with fewer holes and/or pores after the ablation test. Furthermore, we can also found that many fibers remained in the char layers of the insulations (Figure 4c). These PI short fibers still maintained their fibrous structure during formation of cthe har layer. The fibers shrank during the ablation measurement, which left gaps between the fiber and matrix. The fibers were coated with quantities of continuous nanospheres with ca. 50−150 nm diameter (Figure 5). We suggest that these nanospheres probably mainly consist of carbonaceous char residues because of pyrolysis of both the PI fibers and insulations. The results were supported by energydispersive spectrometry (EDS) surface composition data for the insulation in Figure 5b. The data showed 91.64% C, 7.65% O, and 0.71% Si. Several literatures have also reported carbonization behavior of the PI fibers at temperatures lower than 3000 °C.25,26 Thus, PI fiber-filled insulations enhanced the ablation performance because the char layers with integrated and fewer-hole structures are able to protect the remaining virgin material from further ablation. 3.3. Mechanical Properties of EPDM with Various Fiber-Filled Insulations and Microscopy. The mechanical properties of EPDM insulations filled with two kinds of organic fibers were also displayed in Table 1. Because of improvement of the surface roughness of PI fibers, the modified PI fiber-filled EPDM insulations show better mechanical properties than those filled with the same content of parent PI and Kevlar fibers. For instance, tensile stress and strain of EPDM insulations filled with 10 phr modified PI fibers were ca. 13.8 MPa and 727.7%, while those filled with 10 phr parent PI were 11.9 MPa and 661.0% respectively. Also, the mechanical properties of the insulation filled with 10 phr of Kevlar fibers were only 6.3 MPa and 517.3% because of the inhomogeneous

Figure 6. Optical microscopic photographs of the dispersion of fibers in EPDM rubber as a function of the extrusion times: (a) parent Kevlar fibers (10 times); (b) parent Kevlar fibers (20 times); (c) parent PI fibers (10 times); (d) modified PI fibers (10 times).

Figure 7. Stress−strain curves of EPDM insulations filled with different contents of modified PI fibers. 1288

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the stress−strain curves of these EPDM materials were varied with different contents of the PI short fibers. Figure 7 shows the stress− strain curves of modified PI fiber-filled EPDM composites. The yield stress of EPDM insulations increased with an increase of the modified PI fiber content, which indicates the reinforcing effect of PI fibers on the insulations.8 Figure 8 shows SEM micrographs of the tensile fracture surfaces of EPDM insulation samples. The fracture surface of EPDM insulation filled with parent PI fibers exhibited fiber pull-out effects, leaving some holes in the matrix, which indicates a weak interface action between the fiber and matrix because of the smooth surfaces of the parent PI fibers (Figures 3a and 8b).28 On the other hand, no obvious fiber pullout phenomena were found on the fracture surface of surfacemodified PI fiber-filled EPDM insulation (Figure 8c). The result was similar to what Saikrasun et al. found on the fractured surface of a treated Kevlar/Santoprene thermoplastic elastomer.29 Because of the strong interface action between the modified PI fibers and the EPDM matrix, the PI fibers were broken near the fracture surfaces of tested EPDM materials, and some matrixes closely adhered to the surfaces of the modified PI fibers (marked with an arrow in Figure 8c). Therefore, we can conclude that the adhesion properties between PI fibers and elastomeric substrates can be improved by surface modification of these fibers. Ablation performances of these materials can be enhanced by adjusting of the microstructures of the char layers accordingly.

4. CONCLUSIONS In conclusion, PI short fiber-filled EPDM insulations with excellent mechanical and ablative properties were fabricated by pretreated PI short fibers in an alkaline aqueous solution and subsequently by mixing formulation of EPDM insulations on a two-roll open mill. These ablation measurements clearly show that the PI fiber-filled EPDM insulations have a much higher heat erosion resistance than common Kevlar fiber-filled EPDM insulations. To enhance the fiber−polymer adhesive property, modification of the PI fibers is needed via improvement of its surface roughness. It is expected that the current approach based on surface modification of the PI fiber as fillers can extend to fabrication of composites for solid rocket and some other potential applications.

Figure 8. SEM images of the tensile fractured surfaces of EPDM insulations filled with 10 phr different organic fibers: (a) filled with Kevlar fibers, indicating large amounts of fiber pull-outs and a poor distribution; (b) filled with parent PI short fibers, showing the occurrence of PI fiber pull-out, leaving some holes in the matrix, and clean surfaces of fibers; (c) filled with modified PI short fibers, indicating a reduction of fiber pull-out and the bonding of some matrixes on the surface-modified PI fiber (marked with an arrow).

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ASSOCIATED CONTENT

S Supporting Information *

Physical and thermal properties of PI and Kevlar fibers (Table S1), formulation of EPDM thermal insulations (Table S2), photographs of surfaces of postablated EPDM/PI samples in Table 1 (Figure S1), and SEM images of cross sections of both (a) parent PI fibers and (b) surface-modified PI fibers buried in epoxy matrixes (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

dispersion of Kevlar in the EPDM matrix.27 The inhomogeneous dispersion can be improved through our severe control over the fabrication processes. For instance, the tensile stress and strain of the insulation filled with 10 phr of Kevlar fibers could reach to ca. 10.1 MPa and 739.3%, respectively, via its extrusion 20 times after the nip gap of the rolls was narrowed less than 1 mm. These results suggested that the characteristics of the fibers have effects on their dispersion in insulations. Figure 6 shows optical microscopic photographs of the dispersion effects of different fibers in EPDM rubber. We can see that surface-modified PI fibers were uniformly dispersed in the EPDM matrix, while Kevlar fibers showed agglomeration and entanglement in the EPDM matrix. As mentioned above, the maximum stress and strain of all PI fiber-filled insulations present similar good properties. However,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-10-6442-1693. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Project 51273018) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0706). 1289

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