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Preparation and properties of continuous glass fiber reinforced anionic polyamide-6 thermoplastic composites
Materials and Design 46 (2013) 688–695
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Preparation and properties of continuous glass fiber reinforced anionic polyamide-6 thermoplastic composites Chun Yan, Hongzhou Li, Xiaoqing Zhang, Yingdan Zhu, Xinyu Fan ⇑, Liping Yu Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo 315201, PR China
a r t i c l e
i n f o
Article history: Received 3 September 2012 Accepted 19 November 2012 Available online 29 November 2012 Keywords: Polymer–matrix composites Mechanical properties Microstructures Thermoplastics
a b s t r a c t Continuous glass fiber (GF) reinforced anionic polyamide-6 (APA6) composites were prepared via in situ ring-opening polymerization of caprolactam monomers. The effects of catalyst content, polymerization temperature and time on the viscosity average molar mass (Mv) and degree of crystallinity (Xc) were investigated in detail. The final mechanical properties of GF/APA6 composites were also studied. The results indicated that both high molecular weight and the high degree of crystallinity of resin matrix lead to the high mechanical properties of composites. Furthermore, the mechanical test results showed that the composites of plain woven fabric had tensile strength of 434 MPa and flexural strength of 407 MPa. The morphologies of tensile fracture surfaces of the composites specimens were observed through Scanning Electron Microscope (SEM). The SEM analysis showed that many disorganized nanofiber crystals appear in the tensile fracture surfaces, which improve the mechanical properties of the matrix resin. The mechanical properties of the composites with different post-heat treatments were further investigated. The mechanical properties of the composites are significantly reduced after quenching treatment, but hardly improved after annealing. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced thermoplastic composites were developed since 1970s. The reinforcements vary from short fiber, long fiber to continuous fibers in the composites. According to the previous reports, mechanical properties of thermoplastic composites improve as the fiber length increases [1–4]. Therefore, people normally prefer fiber length as long as possible in the composites, especially for those composites which are used as structural applications. Thus, continuous fiber reinforced thermoplastic composites have much higher mechanical performance than those composites with short or long fiber reinforcements. Compare to thermoset composites, thermoplastic composites have better impact resistance, unlimited shelf-life, rapid fabrication cycle, and recyclability. They could have wide applications in aerospace, automotive, sporting goods and transportation industries. Thermoset resins such as epoxies and unsaturated polyesters have low viscosities, which can easily impregnate fiber reinforcements. Continuous glass fiber reinforced composites with low cost and good mechanical properties have attracted large-volume markets such as the automotive industries. Moreover, continuous glass fiber reinforced composites can be easily prepared using resin transfer molding (RTM) process [5,6]. RTM is now a proven lowcost manufacturing technique for thermoset resin composites [7]. ⇑ Corresponding author. Tel.: +86 574 86685802; fax: +86 574 86382329. E-mail address: [email protected] (X. Fan). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.11.034
Whereas, thermoplastic resin is naturally in a solid state and must be heated to the melting point before impregnation. It usually has a melt viscosity of 100–10,000 Pa s, by which the resin cannot directly impregnate fiber reinforcements during manufacturing because the required viscosity is normally lower than 1 Pa s [8]. Hence, a condition of pressure is required to impregnate fiber reinforcements and the composite must be cooled under this pressure. In this processing, special tooling, technique, and equipment must be used, many of which are expensive. Therefore, the cost of continuous fiber reinforced thermoplastic composites can be significantly reduced using RTM process. Recently, some research has reported that some monomers or cyclic oligomers have low molten viscosity and can form linear polymer via in situ ring-opening polymerization in the presence of catalyst. And continuous fiber reinforced thermoplastic composites can be prepared with these monomer (e.g. lauryllactam [9] and caprolactam [10–14]) or cyclic oligomer (e.g. cyclic butylene terephthalate [15,16]) through vacuum assisted resin infusion (VARI) and RTM methods. Caprolactam is usually used for cast nylon-6 as raw material in the industry. Cast nylon-6 can also be called as anionic polyamide-6 (APA6) and it has better properties than nylon-6 produced by the conventional hydrolytic polymerization process [17]. The molten caprolactam monomer has a water-like viscosity to be able to impregnate fiber reinforcements completely and can form a linear polymer with a sufficiently high molecular weight through in situ anionic ring-opening polymerization of monomer. Continuous fiber reinforced anionic polyamide-6 (APA6)
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(1)
(2)
(3)
Fig. 1. Structures of materials used in this study: (1) caprolactam, (2) sodium caprolactamate, and (3) hexamethylene-1,6-dicarbamoylcaprolactam.
composites processed through VARI and RTM methods have been extensively investigated by many researchers [10–14]. van Rijswijk et al. [18,19] studied the vacuum infusion process, the choice of catalyst and the effect of polymerization conditions on APA6 matrix properties in detail. The results showed that different catalyst system had different polymerization time. The polymerization time was longer using caprolactam magnesium bromide and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Therefore, large-scale thermoplastic composites can be manufactured using this catalyst system [10]. However, the polymerization time was shorter using sodium caprolactam and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Thus, small-scale continuous fiber reinforced APA6 thermoplastic composites can be produced efficiently using this catalyst system. Pillay et al. [13] prepared carbon fiber reinforced APA6 matrix composites using 3% of sodium caprolactam and 1.5% of difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Researches about the mechanical properties and microstructure of continuous fiber reinforced APA6 thermoplastic composites were not reported in detail. In this study, GF/ APA6 composites were prepared using sodium caprolactam and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. The effects of the preparation condition, such as catalyst content, polymerization temperature and time, on the viscosity average molecular weight and degree of crystallinity of the matrix resin, and on the mechanical properties and microstructure of GF/APA6 composites were discussed. The mechanical properties of the composites with different post-heat treatments were studied.
(1 mol/kg concentration in caprolactam, ‘C10’) were obtained from Brüggemann Chemical (Heilbronn, Germany). The chemical structures of these materials are presented in Fig. 1. Both the activator (C10) and the initiator (C20) were dried in a vacuum oven at 50 °C for at least 24 h before using. Plain woven fabric of glass fiber (S-glass SW100A-90a, 400 g/m2) was supplied by Jiansu Jiuding new material Co., Ltd. (Jiansu, China). The fabric was dried in a vacuum oven at 120 °C for 2 h before using. 2.2. Processing The process used in this paper is based on the VARI process. According to a certain proportion, caprolactam and C10 were melted under a nitrogen flowing atmosphere at 100 °C in an oil bath and mixed by a mechanical stirrer in a 500 mL three-neck round bottom flask. Then a certain amount of C20 was added into the mixture and mixed uniformly. The stirrer was stopped for about 1 min, and then the three phase mixture was infused into the glass fiber fabric by negative pressure of vacuum. The diagram of VARI process setup for GF/PCBT composites is shown in Fig. 2. Polyimide film is used as the vacuum bag and polyimide or polyfluortetraethylene film with pores of 2 mm diameter is used as the peel ply. The distance between two pore centers is about 10 mm. 2.3. Characterizations 2.3.1. Viscosity average molar mass The viscosity average molar mass (Mv) of APA6 resin of composites was determined using micro-Ubbelohde Capillary II (SCHOTT AVS 370, Germany). The GF/APA6 composites were dissolving in methanoic acid and the glass fiber was separated by filtering. Then de-ionized water precipitated the APA6 polymer from its methanoic acid solution. The polymer was dried at least 24 h at 50 °C in a vacuum oven. The dried polymer was dissolved in aqueous H2SO4 (40%) and the viscosity average molar mass was obtained by a single-point measurement at a concentration of 0.5 g/dl. The inherent viscosity (ginh) was calculated according to the following equation.
ginh ¼
2. Experimental details 2.1. Materials Caprolactam was supplied by BASF. The monomer was dried in a vacuum oven at 50 °C for at least 24 h before using. Both difunctional hexamethylene-1,6-dicarbamoylcaprolactam (2 mol/kg concentration in caprolactam, ‘C20’) and sodium caprolactamate
In
t t0
ð1Þ
c
In which t is the flow time of the polymer solution, t0 is the flow time of the pure solvent and c is the concentration of the polymer solution. The viscosity average molar mass can be calculated from ginh by the Mark–Houwink equation [20], see the following equation.
ginh ¼ K 0 Mav
ð2Þ
PTFE tube N2
Heating tape
Oil-bath heating plate
Vacuum pump Resin trap
Polyimide film Flow mashes Peel ply Glass fiber fabric Polyimide film
Fig. 2. Diagram of VARI process for GF/APA6 composites.
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1.5x10
4
3. Results and discussion
50
3.1. Effects of catalyst contents Catalyst contents have large effects on the rate of polymerization and molecular weight of polymer. The rate of polymerization is fast when catalyst content is high, which results in too short
45
40
5.0x10
Xc, %
Mv
1.0x10
4
(a)
3
Mv
1.0
1.2
1.4
30 1.6
C20 content,% Fig. 3. Effects of catalyst contents on the viscosity average molar mass and degree of crystallinity of the matrix resin of GF/APA6 composites.
Tensile strength, MPa
0.8
In which K0 and a are the Mark–Houwink constants, K0 = 5.92 104 and a = 0.69 [21].
30
300
20
ð4Þ
in which w and t are the width and thickness of the test specimen, Fm is the maximum force. All specimens were dried in a vacuum oven at 50 °C for 12 h before testing. 2.3.4. Morphologies The morphologies of the fracture surfaces were observed by a Scanning Electron Microscope (FE-SEM S-4800, Hitachi, Japan) after gold sputtered.
0.8
1.2
10 1.6
C20 content, mol%
(b)
50
450
400
Flexural Strength, MPa
ð3Þ
40
350 30 300 20 250
200 0.4
0.8
1.2
10 1.6
C20 content, mol%
(c)
50
45
ILSS, MPa
2.3.3. Mechanical properties The mechanical properties of composites were tested by using Instron 5985 universal testing machine. Tensile properties were carried out according to ASTM D-3039 norm, flexural properties were determined according to ASTM D-7264 norm and the shortbeam strength of composites was conducted according to ASTM D-2344 norm. A three point bending jig equipped with 3 mm diameter supports and a 6 mm diameter loading nose was adjusted to a span of 10 mm to perform tests on rectangular shape speciments (16 5.0 2.5 mm). The inter-laminar shear strength (ILSS) was calculated according to the following equation.
Fm wt
350
250 0.4
where DH100 is the melting enthalpy of fully crystalline PA6: DH100 = 190 J/g [22].
ILSS ¼ 0:75
40
Flexural modulus, GPa
2.3.2. Degree of crystallinity Degree of crystallinity (Xc) of APA6 resin of composites was measured by Differential Scanning Calorimetry (Mettler Toledo DSC1, Switzerland) and Thermogravimetric Analyses (Mettler Toledo TGA/DSC1, Switzerland). The melting enthalpy (Hm) of APA6 resin of composites was tested by DSC at a heating rate of 10 °C/min over a temperature range of 25–250 °C under a nitrogen flowing atmosphere. The resin quantity was measured by TGA with the same sample at a heating rate of 20 °C/min over a temperature range of 150–800 °C under an air flowing atmosphere. Then the melting enthalpy per unit of mass (DHm = Hm/mresin) was obtained. Degree of crystallinity was calculated according to the following equation
DH m Xc ¼ 100% DH100
400
Tensile modulus, GPa
0.6
50
35
Xc
0.0 0.4
450
40
35
30
25 0.4
0.8
1.2
1.6
C20 content, mol% Fig. 4. Effects of catalyst contents on the mechanical properties of GF/APA6 composites.
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Fig. 5. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different catalyst contents (tensile fracture surfaces: (a) C20 = 0.5 mol%, (b) C20 = 0.75 mol%, (c) C20 = 1.0 mol%, (d) C20 = 1.25 mol%, (e) C20 = 1.5 mol%, and brittle fracture: (f) C20 = 1.0 mol%).
1.5x104
55
50
45
Xc, %
Mv
1.0x104
5.0x103
40
Mv Xc
0.0 140
160
180
35 200
Polymerization temperature, °C Fig. 6. Effects of polymerization temperature on the viscosity average molecular weight and degree of crystallinity of the matrix resin of GF/APA6 composites.
infusion time to impregnate fiber reinforcements completely. In this study, the infusion time is enough to obtain the composites
(300 mm 300 mm nominal thickness 2.5 mm) in the range of catalyst content. C20 content (C10:C20 is 2:1) increases from 0.5 to 1.5 mol%, and the polymerization reaction is carried out at 180 °C for 60 min. The effects of catalyst contents on the viscosity average molar mass and degree of crystallinity of APA6 resin of GF/APA6 composites are presented in Fig. 3. The viscosity average molar mass increases from 9.98 103 to 1.22 104 with the increase of C20 contents until it reaches a maximum when C20 content is 1.0 mol%, then slightly decreases when C20 content further increases. With the increase of the catalyst contents, the reaction rate is improved, leading to higher molecular weight of APA6 resin. However, the viscosity average molar mass drops when catalyst content increases beyond a certain extent, which is consistent with the reported results [23–26]. The reason is that higher catalyst content will increase the amount of initiation points for chain growth and free isocyanate groups are produced by de-blocking reactions of C20 when reaction temperature exceeds 160 °C [27]. Therefore, higher catalyst content will easily lead to more branching and crosslinking reactions when reaction temperature is 180 °C. The crosslinking polymers are usually filtered and removed before measuring the inherent viscosity of APA6. Therefore, the obtained inherent viscosity will decrease, resulting in the reduction of the final viscosity average molar mass of APA6. In addition, the degree of crystallinity increases from 37.1% to 43% with the increase of C20
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(a)
450
50
400
30
350
Tensile Modulus, GPa
Tensile Strength, MPa
40
20
300 140
150
160
170
180
190
10 200
Polymerization temperature, °C
(b)
50
450
40 350 30 300 20
Flexural Modulus, GPa
Flexural Strength, MPa
400
Fig. 8. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different polymerization temperature ((a) 150 °C and (b) 190 °C).
1.5x104
50
250 45 200 140
150
160
170
180
190
10 200
1.0x10
4
Mv
(c)
40
50 5.0x103
Mv
ILSS, MPa
45
35
Xc
0.0
40
Xc, %
Polymerization temperature, °C
0
20
40
60
80
100
120
30
Polymerization time, min Fig. 9. Effects of polymerization time on the viscosity average molecular weight and degree of crystallinity of the matrix resin of GF/APA6 composites.
35
30 140
150
160
170
180
190
200
Polymerization temperature, °C Fig. 7. Effects of polymerization temperature on the mechanical properties of GF/ APA6 composites.
contents from 0.5 to 1.5 mol%. The reason may be that the reaction rate accelerates with increasing catalyst contents. Thus, the polymer has relatively full crystallization time to obtain the higher degree of crystallinity. The effects of catalyst contents on the mechanical properties of GF/APA6 composites are shown in Fig. 4a–c. The tensile properties in Fig. 4a show that tensile strength increases from 328 MPa to 434 MPa with the increase of C20 contents until it reaches a max-
imum when C20 content is 1.0 mol%, then decreases slightly when C20 content further increases. Tensile modulus changes very little with the increase of C20 contents. van Rijswijk et al. [12] had computed the mechanical properties of PA-6/GF composites with the classic lamination theory and obtained 303.6 MPa of tensile strength. Obviously, the tensile strength in this study is higher than the calculated value. The flexural properties of composites in Fig. 4b show that flexural strength increases from 320 MPa to 407 MPa and flexural modulus remains fundamentally unchanged with the increase of C20 contents. The ILSS properties of composites in Fig. 4c show that ILSS increases from 33 MPa to 43 MPa until it reaches a maximum when C20 content is 1.0 mol%, then remains constant when C20 content further increases. The reason is that the molecular weight and degree of crystallinity of APA6 resin have large effects on the mechanical properties of composites. Both high
C. Yan et al. / Materials and Design 46 (2013) 688–695
(a)
50
450
420
390 30 360
Tensile Modulus, GPa
Tensile Strength, MPa
40
20 330
300
10 0
30
60
90
120
Polymerization time, min
400
40
Flexural Strength, MPa
50
350
30
300
20
250
0
30
60
90
120
Flexural Modulus, GPa
(b) 450
10
Polymerization time, min
(c)
693
Many fiber crystals appear in the tensile fracture surfaces of the composites, especially in resin-rich area among fibers. The fiber crystals have a diameter in the nanoscale and look disorganized. However, there is no any fiber crystal in the brittle fracture of the composites. There are two possible reasons to explain this phenomenon. The first one is that fiber crystals might be formed under tensile stress at ambient temperature. So fiber crystals appear in the tensile fracture surfaces of the composites, but there is no fiber crystal in the brittle fracture of the composites. The second one is that fiber crystals might be formed in the process of preparation of composites. The mechanical properties of fibrous crystalline region are usually higher than those of non-fibrous crystalline region. Therefore, tensile fracture will first happen in non-fibrous crystalline region in the tensile process. Thus, fiber crystals of the fibrous crystalline region emerge in the tensile fracture surfaces of the composites. After freezing treatment by liquid nitrogen, the composites will become brittle. Therefore, fracture failure of the composites might happen simultaneously in both non-fibrous crystalline region and fibrous crystalline region, which causes that fiber crystal cannot be found in Fig. 5f. The real forming reason and mechanism of fiber crystals in the GF/APA6 composites will be further studied. 3.2. Effects of polymerization temperature The polymerization temperature changes from 150 °C to 190 °C, and C20 content (C10:C20 is 2:1) and the polymerization time keep 1.0 mol% and 60 min, respectively. Polymerization temperature also has large effects on the molecular weight and degree of crystallinity of APA6 resin of GF/APA6 composites. The effects of polymerization temperature on the viscosity average molar mass and degree of crystallinity are presented in Fig. 6. The viscosity average molar mass was increased from 1.08 104 to 1.22 104 with the polymerization temperature from 150 °C to 190 °C. The reason is that high polymerization temperature can accelerate the rate of polymerization, obtaining high molecular weight. The change trend of viscosity average molar mass with polymerization tem-
50
ILSS, MPa
45
40
35
30
0
30
60
90
120
Polymerization time, min Fig. 10. Effects of polymerization time on the mechanical properties of GF/APA6 composites.
molecular weight and the high degree of crystallinity of matrix resin lead to the high mechanical properties of GF/APA6 composites. Fig. 5a–e is the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different catalyst contents and Fig. 5f is the micrograph of the brittle fracture surface of composites cooled in liquid nitrogen. The fracture shape of composites manifests that most fibers are pulled out and some resins adhere to the surface of fibers, and it is typically an accumulating failure.
Fig. 11. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different polymerization time ((a) 5 min and (b) 120 min).
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Table 1 Mechanical properties of the GF/APA6 composites with different post-heat treatments.
Untreated Quenched Annealed
Tensile strength (MPa)
Tensile modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
ILSS (MPa)
433.5 ± 10.0 330.3 ± 13.5 442.8 ± 24.0
22.0 ± 1.2 17.0 ± 1.3 22.1 ± 1.6
396.4 ± 29.4 330.7 ± 31.8 402.9 ± 28.5
23.5 ± 1.2 20.0 ± 0.5 21.5 ± 0.8
43.3 ± 1.9 38.3 ± 2.2 51.5 ± 2.0
perature is the same as that of pure APA6 resin reported [19]. However, the viscosity average molar mass of APA6 resin of composites is lower than that of the pure APA6 resin. This phenomenon is caused by thermal and chemical interactions. Glass fibers will absorb the exothermic heat of the polymerization reaction, which results in the decrease of reaction temperature. Then the rate of crystallization of APA6 resin increase with decreasing reaction temperature, which inhibits the polymerization reaction [11,13]. In addition, the active groups (the acidic groups and hydroxyl groups) on the surface of glass fiber will easily terminate the polymerization reaction [11]. Therefore, the viscosity average molar mass of APA6 resin obtained from GF/APA6 composites is lower than the pure APA6 resin. The degree of crystallinity decreased from 45.2% to 39.8% with the increase of polymerization temperature. The possible reason is that increasing polymerization temperature can cause much more branch point, which will hinder the process of crystallization [28,29]. Therefore, the degree of crystallinity of the matrix resin reduces with the increase of polymerization temperature. The effects of polymerization temperature on the mechanical properties of GF/APA6 composites are shown in Fig. 7a–c. The tensile properties in Fig. 7a show that tensile strength increases from 363 MPa to 434 MPa with the increase of polymerization temperature until it reaches a maximum when polymerization temperature is 180 °C, then remains unchanged when polymerization temperature further increases. Tensile modulus changes very little with the polymerization temperature. The flexural properties of composites in Fig. 7b show that flexural strength increases from 333 MPa to 396 MPa and flexural modulus remains fundamentally unchanged with the increase of polymerization temperature. The inter-laminar shear properties of composites in Fig. 7c show that ILSS increases from 38 MPa to 44 MPa with the increase of polymerization temperature. The reason is that the molecular weight of APA6 resin increases with increasing polymerization temperature, leading to improve the mechanical properties of composites. Fig. 8 show the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different polymerization temperature. It is typically an accumulating failure according to the fracture shape of the composites which most fibers are pulled out and some resins adhere to the surface of fibers. It is also found that many disorganized nano-fiber crystals appear in the fracture surfaces of composites. 3.3. Effects of polymerization time Polymerization time increases from 5 min to 120 min, and C20 content (C10:C20 is 2:1) and the polymerization temperature keep 1.0 mol% and 180 °C, respectively. The effects of polymerization time on the viscosity average molar mass and degree of crystallinity of APA6 resin of GF/APA6 composites are shown in Fig. 9. The viscosity average molar mass changes little with polymerization time. Degree of crystallinity increases slightly from 41% to 44% with the polymerization time. The polymerization reaction has been basically completed within 5 min. Therefore, the molecular weights of APA6 resin keep constant after 5 min. The effects of polymerization time on the mechanical properties of GF/APA6 composites are shown in Fig. 10a–c. The tensile properties in Fig. 10a show that tensile strength increases from
382 MPa to 437 MPa with the increase of polymerization time. Tensile modulus changes very little with the polymerization time. The flexural properties of GF/APA6 composites in Fig. 10b show that flexural strength increases from 364 MPa to 395 MPa and flexural modulus remains fundamentally unchanged with the increase of polymerization time. The inter-laminar shear properties of composites in Fig. 10c show that ILSS remains constant when polymerization time increases. From the results above, the mechanical properties of composites change little with the polymerization time. The tensile strength and the flexural strength increases by 14% and 8.5%, respectively. Fig. 11 show the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different polymerization time. It is also found that many disorganized nano-fiber crystals appear in the fracture surfaces of the composites. 3.4. Mechanical properties and microstructure of the GF/APA6 composites with different post-heat treatments Post-heat treatments have a larger effect on the mechanical properties of GF/APA6 composites. The mechanical properties of the composites with different post-heat treatments are listed in Table 1. The mechanical properties of composites are significantly reduced after quenching treatment. It shows that, tensile strength decreases by 23.8%, tensile modulus decreases by 22.7%, flexural strength decreases by 16.6%, flexural modulus decreases by 14.9%, ILSS decreases by 11.5%. However, other mechanical properties of composites are hardly improved after annealing in addition to ILSS increasing by 18.9%. 4. Conclusion Continuous glass fiber (GF) reinforced anionic polyamide-6 (APA6) composites were prepared via in situ ring-opening polymerization of caprolactam monomer. Catalyst content and polymerization temperature have significant effects on both the molecular weight and degree of crystallinity of APA6 resin of GF/ APA6 composites. The viscosity average molar mass was up to 1.22 104 with 1.0 mol% of C20 (C10 and C20 is a 2:1 ratio) at 180 °C for 60 min and degree of crystallinity decreased by 11.9% as polymerization temperature risen from 150 °C to 190 °C. Both high molecular weight and the high degree of crystallinity of matrix resin lead to the high mechanical properties of composites. The composites have tensile strength of 434 MPa and flexural strength of 407 MPa. Many disorganized nano-fiber crystals are found in the tensile fracture surfaces of composites. The mechanical properties of composites are significantly reduced after quenching treatment, but hardly improved after annealing. Acknowledgments The authors acknowledge financial support from Innovation Program of the Chinese Academy of Sciences (KGCX2-EW-210), National 863 plan (2012AA03A206), National Key Basic Research and Development Program (973 Program) (2010CB631104), National Natural Science Foundation of China (51103173), Zhejiang Provincial Natural Science Foundation of China (Y4110609), Ningbo Science and Technology Project (2011B1015) and Ningbo Natu-
C. Yan et al. / Materials and Design 46 (2013) 688–695
ral Science 2012A610090).
Foundation
(2011A610113,
2011A610115,
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Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Preparation and properties of continuous glass fiber reinforced anionic polyamide-6 thermoplastic composites Chun Yan, Hongzhou Li, Xiaoqing Zhang, Yingdan Zhu, Xinyu Fan ⇑, Liping Yu Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo 315201, PR China
a r t i c l e
i n f o
Article history: Received 3 September 2012 Accepted 19 November 2012 Available online 29 November 2012 Keywords: Polymer–matrix composites Mechanical properties Microstructures Thermoplastics
a b s t r a c t Continuous glass fiber (GF) reinforced anionic polyamide-6 (APA6) composites were prepared via in situ ring-opening polymerization of caprolactam monomers. The effects of catalyst content, polymerization temperature and time on the viscosity average molar mass (Mv) and degree of crystallinity (Xc) were investigated in detail. The final mechanical properties of GF/APA6 composites were also studied. The results indicated that both high molecular weight and the high degree of crystallinity of resin matrix lead to the high mechanical properties of composites. Furthermore, the mechanical test results showed that the composites of plain woven fabric had tensile strength of 434 MPa and flexural strength of 407 MPa. The morphologies of tensile fracture surfaces of the composites specimens were observed through Scanning Electron Microscope (SEM). The SEM analysis showed that many disorganized nanofiber crystals appear in the tensile fracture surfaces, which improve the mechanical properties of the matrix resin. The mechanical properties of the composites with different post-heat treatments were further investigated. The mechanical properties of the composites are significantly reduced after quenching treatment, but hardly improved after annealing. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced thermoplastic composites were developed since 1970s. The reinforcements vary from short fiber, long fiber to continuous fibers in the composites. According to the previous reports, mechanical properties of thermoplastic composites improve as the fiber length increases [1–4]. Therefore, people normally prefer fiber length as long as possible in the composites, especially for those composites which are used as structural applications. Thus, continuous fiber reinforced thermoplastic composites have much higher mechanical performance than those composites with short or long fiber reinforcements. Compare to thermoset composites, thermoplastic composites have better impact resistance, unlimited shelf-life, rapid fabrication cycle, and recyclability. They could have wide applications in aerospace, automotive, sporting goods and transportation industries. Thermoset resins such as epoxies and unsaturated polyesters have low viscosities, which can easily impregnate fiber reinforcements. Continuous glass fiber reinforced composites with low cost and good mechanical properties have attracted large-volume markets such as the automotive industries. Moreover, continuous glass fiber reinforced composites can be easily prepared using resin transfer molding (RTM) process [5,6]. RTM is now a proven lowcost manufacturing technique for thermoset resin composites [7]. ⇑ Corresponding author. Tel.: +86 574 86685802; fax: +86 574 86382329. E-mail address: [email protected] (X. Fan). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.11.034
Whereas, thermoplastic resin is naturally in a solid state and must be heated to the melting point before impregnation. It usually has a melt viscosity of 100–10,000 Pa s, by which the resin cannot directly impregnate fiber reinforcements during manufacturing because the required viscosity is normally lower than 1 Pa s [8]. Hence, a condition of pressure is required to impregnate fiber reinforcements and the composite must be cooled under this pressure. In this processing, special tooling, technique, and equipment must be used, many of which are expensive. Therefore, the cost of continuous fiber reinforced thermoplastic composites can be significantly reduced using RTM process. Recently, some research has reported that some monomers or cyclic oligomers have low molten viscosity and can form linear polymer via in situ ring-opening polymerization in the presence of catalyst. And continuous fiber reinforced thermoplastic composites can be prepared with these monomer (e.g. lauryllactam [9] and caprolactam [10–14]) or cyclic oligomer (e.g. cyclic butylene terephthalate [15,16]) through vacuum assisted resin infusion (VARI) and RTM methods. Caprolactam is usually used for cast nylon-6 as raw material in the industry. Cast nylon-6 can also be called as anionic polyamide-6 (APA6) and it has better properties than nylon-6 produced by the conventional hydrolytic polymerization process [17]. The molten caprolactam monomer has a water-like viscosity to be able to impregnate fiber reinforcements completely and can form a linear polymer with a sufficiently high molecular weight through in situ anionic ring-opening polymerization of monomer. Continuous fiber reinforced anionic polyamide-6 (APA6)
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(1)
(2)
(3)
Fig. 1. Structures of materials used in this study: (1) caprolactam, (2) sodium caprolactamate, and (3) hexamethylene-1,6-dicarbamoylcaprolactam.
composites processed through VARI and RTM methods have been extensively investigated by many researchers [10–14]. van Rijswijk et al. [18,19] studied the vacuum infusion process, the choice of catalyst and the effect of polymerization conditions on APA6 matrix properties in detail. The results showed that different catalyst system had different polymerization time. The polymerization time was longer using caprolactam magnesium bromide and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Therefore, large-scale thermoplastic composites can be manufactured using this catalyst system [10]. However, the polymerization time was shorter using sodium caprolactam and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Thus, small-scale continuous fiber reinforced APA6 thermoplastic composites can be produced efficiently using this catalyst system. Pillay et al. [13] prepared carbon fiber reinforced APA6 matrix composites using 3% of sodium caprolactam and 1.5% of difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. Researches about the mechanical properties and microstructure of continuous fiber reinforced APA6 thermoplastic composites were not reported in detail. In this study, GF/ APA6 composites were prepared using sodium caprolactam and difunctional hexamethylene-1,6-dicarbamoyl-caprolactam as catalyst system. The effects of the preparation condition, such as catalyst content, polymerization temperature and time, on the viscosity average molecular weight and degree of crystallinity of the matrix resin, and on the mechanical properties and microstructure of GF/APA6 composites were discussed. The mechanical properties of the composites with different post-heat treatments were studied.
(1 mol/kg concentration in caprolactam, ‘C10’) were obtained from Brüggemann Chemical (Heilbronn, Germany). The chemical structures of these materials are presented in Fig. 1. Both the activator (C10) and the initiator (C20) were dried in a vacuum oven at 50 °C for at least 24 h before using. Plain woven fabric of glass fiber (S-glass SW100A-90a, 400 g/m2) was supplied by Jiansu Jiuding new material Co., Ltd. (Jiansu, China). The fabric was dried in a vacuum oven at 120 °C for 2 h before using. 2.2. Processing The process used in this paper is based on the VARI process. According to a certain proportion, caprolactam and C10 were melted under a nitrogen flowing atmosphere at 100 °C in an oil bath and mixed by a mechanical stirrer in a 500 mL three-neck round bottom flask. Then a certain amount of C20 was added into the mixture and mixed uniformly. The stirrer was stopped for about 1 min, and then the three phase mixture was infused into the glass fiber fabric by negative pressure of vacuum. The diagram of VARI process setup for GF/PCBT composites is shown in Fig. 2. Polyimide film is used as the vacuum bag and polyimide or polyfluortetraethylene film with pores of 2 mm diameter is used as the peel ply. The distance between two pore centers is about 10 mm. 2.3. Characterizations 2.3.1. Viscosity average molar mass The viscosity average molar mass (Mv) of APA6 resin of composites was determined using micro-Ubbelohde Capillary II (SCHOTT AVS 370, Germany). The GF/APA6 composites were dissolving in methanoic acid and the glass fiber was separated by filtering. Then de-ionized water precipitated the APA6 polymer from its methanoic acid solution. The polymer was dried at least 24 h at 50 °C in a vacuum oven. The dried polymer was dissolved in aqueous H2SO4 (40%) and the viscosity average molar mass was obtained by a single-point measurement at a concentration of 0.5 g/dl. The inherent viscosity (ginh) was calculated according to the following equation.
ginh ¼
2. Experimental details 2.1. Materials Caprolactam was supplied by BASF. The monomer was dried in a vacuum oven at 50 °C for at least 24 h before using. Both difunctional hexamethylene-1,6-dicarbamoylcaprolactam (2 mol/kg concentration in caprolactam, ‘C20’) and sodium caprolactamate
In
t t0
ð1Þ
c
In which t is the flow time of the polymer solution, t0 is the flow time of the pure solvent and c is the concentration of the polymer solution. The viscosity average molar mass can be calculated from ginh by the Mark–Houwink equation [20], see the following equation.
ginh ¼ K 0 Mav
ð2Þ
PTFE tube N2
Heating tape
Oil-bath heating plate
Vacuum pump Resin trap
Polyimide film Flow mashes Peel ply Glass fiber fabric Polyimide film
Fig. 2. Diagram of VARI process for GF/APA6 composites.
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1.5x10
4
3. Results and discussion
50
3.1. Effects of catalyst contents Catalyst contents have large effects on the rate of polymerization and molecular weight of polymer. The rate of polymerization is fast when catalyst content is high, which results in too short
45
40
5.0x10
Xc, %
Mv
1.0x10
4
(a)
3
Mv
1.0
1.2
1.4
30 1.6
C20 content,% Fig. 3. Effects of catalyst contents on the viscosity average molar mass and degree of crystallinity of the matrix resin of GF/APA6 composites.
Tensile strength, MPa
0.8
In which K0 and a are the Mark–Houwink constants, K0 = 5.92 104 and a = 0.69 [21].
30
300
20
ð4Þ
in which w and t are the width and thickness of the test specimen, Fm is the maximum force. All specimens were dried in a vacuum oven at 50 °C for 12 h before testing. 2.3.4. Morphologies The morphologies of the fracture surfaces were observed by a Scanning Electron Microscope (FE-SEM S-4800, Hitachi, Japan) after gold sputtered.
0.8
1.2
10 1.6
C20 content, mol%
(b)
50
450
400
Flexural Strength, MPa
ð3Þ
40
350 30 300 20 250
200 0.4
0.8
1.2
10 1.6
C20 content, mol%
(c)
50
45
ILSS, MPa
2.3.3. Mechanical properties The mechanical properties of composites were tested by using Instron 5985 universal testing machine. Tensile properties were carried out according to ASTM D-3039 norm, flexural properties were determined according to ASTM D-7264 norm and the shortbeam strength of composites was conducted according to ASTM D-2344 norm. A three point bending jig equipped with 3 mm diameter supports and a 6 mm diameter loading nose was adjusted to a span of 10 mm to perform tests on rectangular shape speciments (16 5.0 2.5 mm). The inter-laminar shear strength (ILSS) was calculated according to the following equation.
Fm wt
350
250 0.4
where DH100 is the melting enthalpy of fully crystalline PA6: DH100 = 190 J/g [22].
ILSS ¼ 0:75
40
Flexural modulus, GPa
2.3.2. Degree of crystallinity Degree of crystallinity (Xc) of APA6 resin of composites was measured by Differential Scanning Calorimetry (Mettler Toledo DSC1, Switzerland) and Thermogravimetric Analyses (Mettler Toledo TGA/DSC1, Switzerland). The melting enthalpy (Hm) of APA6 resin of composites was tested by DSC at a heating rate of 10 °C/min over a temperature range of 25–250 °C under a nitrogen flowing atmosphere. The resin quantity was measured by TGA with the same sample at a heating rate of 20 °C/min over a temperature range of 150–800 °C under an air flowing atmosphere. Then the melting enthalpy per unit of mass (DHm = Hm/mresin) was obtained. Degree of crystallinity was calculated according to the following equation
DH m Xc ¼ 100% DH100
400
Tensile modulus, GPa
0.6
50
35
Xc
0.0 0.4
450
40
35
30
25 0.4
0.8
1.2
1.6
C20 content, mol% Fig. 4. Effects of catalyst contents on the mechanical properties of GF/APA6 composites.
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Fig. 5. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different catalyst contents (tensile fracture surfaces: (a) C20 = 0.5 mol%, (b) C20 = 0.75 mol%, (c) C20 = 1.0 mol%, (d) C20 = 1.25 mol%, (e) C20 = 1.5 mol%, and brittle fracture: (f) C20 = 1.0 mol%).
1.5x104
55
50
45
Xc, %
Mv
1.0x104
5.0x103
40
Mv Xc
0.0 140
160
180
35 200
Polymerization temperature, °C Fig. 6. Effects of polymerization temperature on the viscosity average molecular weight and degree of crystallinity of the matrix resin of GF/APA6 composites.
infusion time to impregnate fiber reinforcements completely. In this study, the infusion time is enough to obtain the composites
(300 mm 300 mm nominal thickness 2.5 mm) in the range of catalyst content. C20 content (C10:C20 is 2:1) increases from 0.5 to 1.5 mol%, and the polymerization reaction is carried out at 180 °C for 60 min. The effects of catalyst contents on the viscosity average molar mass and degree of crystallinity of APA6 resin of GF/APA6 composites are presented in Fig. 3. The viscosity average molar mass increases from 9.98 103 to 1.22 104 with the increase of C20 contents until it reaches a maximum when C20 content is 1.0 mol%, then slightly decreases when C20 content further increases. With the increase of the catalyst contents, the reaction rate is improved, leading to higher molecular weight of APA6 resin. However, the viscosity average molar mass drops when catalyst content increases beyond a certain extent, which is consistent with the reported results [23–26]. The reason is that higher catalyst content will increase the amount of initiation points for chain growth and free isocyanate groups are produced by de-blocking reactions of C20 when reaction temperature exceeds 160 °C [27]. Therefore, higher catalyst content will easily lead to more branching and crosslinking reactions when reaction temperature is 180 °C. The crosslinking polymers are usually filtered and removed before measuring the inherent viscosity of APA6. Therefore, the obtained inherent viscosity will decrease, resulting in the reduction of the final viscosity average molar mass of APA6. In addition, the degree of crystallinity increases from 37.1% to 43% with the increase of C20
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(a)
450
50
400
30
350
Tensile Modulus, GPa
Tensile Strength, MPa
40
20
300 140
150
160
170
180
190
10 200
Polymerization temperature, °C
(b)
50
450
40 350 30 300 20
Flexural Modulus, GPa
Flexural Strength, MPa
400
Fig. 8. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different polymerization temperature ((a) 150 °C and (b) 190 °C).
1.5x104
50
250 45 200 140
150
160
170
180
190
10 200
1.0x10
4
Mv
(c)
40
50 5.0x103
Mv
ILSS, MPa
45
35
Xc
0.0
40
Xc, %
Polymerization temperature, °C
0
20
40
60
80
100
120
30
Polymerization time, min Fig. 9. Effects of polymerization time on the viscosity average molecular weight and degree of crystallinity of the matrix resin of GF/APA6 composites.
35
30 140
150
160
170
180
190
200
Polymerization temperature, °C Fig. 7. Effects of polymerization temperature on the mechanical properties of GF/ APA6 composites.
contents from 0.5 to 1.5 mol%. The reason may be that the reaction rate accelerates with increasing catalyst contents. Thus, the polymer has relatively full crystallization time to obtain the higher degree of crystallinity. The effects of catalyst contents on the mechanical properties of GF/APA6 composites are shown in Fig. 4a–c. The tensile properties in Fig. 4a show that tensile strength increases from 328 MPa to 434 MPa with the increase of C20 contents until it reaches a max-
imum when C20 content is 1.0 mol%, then decreases slightly when C20 content further increases. Tensile modulus changes very little with the increase of C20 contents. van Rijswijk et al. [12] had computed the mechanical properties of PA-6/GF composites with the classic lamination theory and obtained 303.6 MPa of tensile strength. Obviously, the tensile strength in this study is higher than the calculated value. The flexural properties of composites in Fig. 4b show that flexural strength increases from 320 MPa to 407 MPa and flexural modulus remains fundamentally unchanged with the increase of C20 contents. The ILSS properties of composites in Fig. 4c show that ILSS increases from 33 MPa to 43 MPa until it reaches a maximum when C20 content is 1.0 mol%, then remains constant when C20 content further increases. The reason is that the molecular weight and degree of crystallinity of APA6 resin have large effects on the mechanical properties of composites. Both high
C. Yan et al. / Materials and Design 46 (2013) 688–695
(a)
50
450
420
390 30 360
Tensile Modulus, GPa
Tensile Strength, MPa
40
20 330
300
10 0
30
60
90
120
Polymerization time, min
400
40
Flexural Strength, MPa
50
350
30
300
20
250
0
30
60
90
120
Flexural Modulus, GPa
(b) 450
10
Polymerization time, min
(c)
693
Many fiber crystals appear in the tensile fracture surfaces of the composites, especially in resin-rich area among fibers. The fiber crystals have a diameter in the nanoscale and look disorganized. However, there is no any fiber crystal in the brittle fracture of the composites. There are two possible reasons to explain this phenomenon. The first one is that fiber crystals might be formed under tensile stress at ambient temperature. So fiber crystals appear in the tensile fracture surfaces of the composites, but there is no fiber crystal in the brittle fracture of the composites. The second one is that fiber crystals might be formed in the process of preparation of composites. The mechanical properties of fibrous crystalline region are usually higher than those of non-fibrous crystalline region. Therefore, tensile fracture will first happen in non-fibrous crystalline region in the tensile process. Thus, fiber crystals of the fibrous crystalline region emerge in the tensile fracture surfaces of the composites. After freezing treatment by liquid nitrogen, the composites will become brittle. Therefore, fracture failure of the composites might happen simultaneously in both non-fibrous crystalline region and fibrous crystalline region, which causes that fiber crystal cannot be found in Fig. 5f. The real forming reason and mechanism of fiber crystals in the GF/APA6 composites will be further studied. 3.2. Effects of polymerization temperature The polymerization temperature changes from 150 °C to 190 °C, and C20 content (C10:C20 is 2:1) and the polymerization time keep 1.0 mol% and 60 min, respectively. Polymerization temperature also has large effects on the molecular weight and degree of crystallinity of APA6 resin of GF/APA6 composites. The effects of polymerization temperature on the viscosity average molar mass and degree of crystallinity are presented in Fig. 6. The viscosity average molar mass was increased from 1.08 104 to 1.22 104 with the polymerization temperature from 150 °C to 190 °C. The reason is that high polymerization temperature can accelerate the rate of polymerization, obtaining high molecular weight. The change trend of viscosity average molar mass with polymerization tem-
50
ILSS, MPa
45
40
35
30
0
30
60
90
120
Polymerization time, min Fig. 10. Effects of polymerization time on the mechanical properties of GF/APA6 composites.
molecular weight and the high degree of crystallinity of matrix resin lead to the high mechanical properties of GF/APA6 composites. Fig. 5a–e is the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different catalyst contents and Fig. 5f is the micrograph of the brittle fracture surface of composites cooled in liquid nitrogen. The fracture shape of composites manifests that most fibers are pulled out and some resins adhere to the surface of fibers, and it is typically an accumulating failure.
Fig. 11. SEM micrographs of the fracture surfaces of the GF/APA6 composites with different polymerization time ((a) 5 min and (b) 120 min).
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Table 1 Mechanical properties of the GF/APA6 composites with different post-heat treatments.
Untreated Quenched Annealed
Tensile strength (MPa)
Tensile modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
ILSS (MPa)
433.5 ± 10.0 330.3 ± 13.5 442.8 ± 24.0
22.0 ± 1.2 17.0 ± 1.3 22.1 ± 1.6
396.4 ± 29.4 330.7 ± 31.8 402.9 ± 28.5
23.5 ± 1.2 20.0 ± 0.5 21.5 ± 0.8
43.3 ± 1.9 38.3 ± 2.2 51.5 ± 2.0
perature is the same as that of pure APA6 resin reported [19]. However, the viscosity average molar mass of APA6 resin of composites is lower than that of the pure APA6 resin. This phenomenon is caused by thermal and chemical interactions. Glass fibers will absorb the exothermic heat of the polymerization reaction, which results in the decrease of reaction temperature. Then the rate of crystallization of APA6 resin increase with decreasing reaction temperature, which inhibits the polymerization reaction [11,13]. In addition, the active groups (the acidic groups and hydroxyl groups) on the surface of glass fiber will easily terminate the polymerization reaction [11]. Therefore, the viscosity average molar mass of APA6 resin obtained from GF/APA6 composites is lower than the pure APA6 resin. The degree of crystallinity decreased from 45.2% to 39.8% with the increase of polymerization temperature. The possible reason is that increasing polymerization temperature can cause much more branch point, which will hinder the process of crystallization [28,29]. Therefore, the degree of crystallinity of the matrix resin reduces with the increase of polymerization temperature. The effects of polymerization temperature on the mechanical properties of GF/APA6 composites are shown in Fig. 7a–c. The tensile properties in Fig. 7a show that tensile strength increases from 363 MPa to 434 MPa with the increase of polymerization temperature until it reaches a maximum when polymerization temperature is 180 °C, then remains unchanged when polymerization temperature further increases. Tensile modulus changes very little with the polymerization temperature. The flexural properties of composites in Fig. 7b show that flexural strength increases from 333 MPa to 396 MPa and flexural modulus remains fundamentally unchanged with the increase of polymerization temperature. The inter-laminar shear properties of composites in Fig. 7c show that ILSS increases from 38 MPa to 44 MPa with the increase of polymerization temperature. The reason is that the molecular weight of APA6 resin increases with increasing polymerization temperature, leading to improve the mechanical properties of composites. Fig. 8 show the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different polymerization temperature. It is typically an accumulating failure according to the fracture shape of the composites which most fibers are pulled out and some resins adhere to the surface of fibers. It is also found that many disorganized nano-fiber crystals appear in the fracture surfaces of composites. 3.3. Effects of polymerization time Polymerization time increases from 5 min to 120 min, and C20 content (C10:C20 is 2:1) and the polymerization temperature keep 1.0 mol% and 180 °C, respectively. The effects of polymerization time on the viscosity average molar mass and degree of crystallinity of APA6 resin of GF/APA6 composites are shown in Fig. 9. The viscosity average molar mass changes little with polymerization time. Degree of crystallinity increases slightly from 41% to 44% with the polymerization time. The polymerization reaction has been basically completed within 5 min. Therefore, the molecular weights of APA6 resin keep constant after 5 min. The effects of polymerization time on the mechanical properties of GF/APA6 composites are shown in Fig. 10a–c. The tensile properties in Fig. 10a show that tensile strength increases from
382 MPa to 437 MPa with the increase of polymerization time. Tensile modulus changes very little with the polymerization time. The flexural properties of GF/APA6 composites in Fig. 10b show that flexural strength increases from 364 MPa to 395 MPa and flexural modulus remains fundamentally unchanged with the increase of polymerization time. The inter-laminar shear properties of composites in Fig. 10c show that ILSS remains constant when polymerization time increases. From the results above, the mechanical properties of composites change little with the polymerization time. The tensile strength and the flexural strength increases by 14% and 8.5%, respectively. Fig. 11 show the SEM micrographs of the tensile fracture surfaces of the GF/APA6 composites with different polymerization time. It is also found that many disorganized nano-fiber crystals appear in the fracture surfaces of the composites. 3.4. Mechanical properties and microstructure of the GF/APA6 composites with different post-heat treatments Post-heat treatments have a larger effect on the mechanical properties of GF/APA6 composites. The mechanical properties of the composites with different post-heat treatments are listed in Table 1. The mechanical properties of composites are significantly reduced after quenching treatment. It shows that, tensile strength decreases by 23.8%, tensile modulus decreases by 22.7%, flexural strength decreases by 16.6%, flexural modulus decreases by 14.9%, ILSS decreases by 11.5%. However, other mechanical properties of composites are hardly improved after annealing in addition to ILSS increasing by 18.9%. 4. Conclusion Continuous glass fiber (GF) reinforced anionic polyamide-6 (APA6) composites were prepared via in situ ring-opening polymerization of caprolactam monomer. Catalyst content and polymerization temperature have significant effects on both the molecular weight and degree of crystallinity of APA6 resin of GF/ APA6 composites. The viscosity average molar mass was up to 1.22 104 with 1.0 mol% of C20 (C10 and C20 is a 2:1 ratio) at 180 °C for 60 min and degree of crystallinity decreased by 11.9% as polymerization temperature risen from 150 °C to 190 °C. Both high molecular weight and the high degree of crystallinity of matrix resin lead to the high mechanical properties of composites. The composites have tensile strength of 434 MPa and flexural strength of 407 MPa. Many disorganized nano-fiber crystals are found in the tensile fracture surfaces of composites. The mechanical properties of composites are significantly reduced after quenching treatment, but hardly improved after annealing. Acknowledgments The authors acknowledge financial support from Innovation Program of the Chinese Academy of Sciences (KGCX2-EW-210), National 863 plan (2012AA03A206), National Key Basic Research and Development Program (973 Program) (2010CB631104), National Natural Science Foundation of China (51103173), Zhejiang Provincial Natural Science Foundation of China (Y4110609), Ningbo Science and Technology Project (2011B1015) and Ningbo Natu-
C. Yan et al. / Materials and Design 46 (2013) 688–695
ral Science 2012A610090).
Foundation
(2011A610113,
2011A610115,
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