Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame retardant

Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame retardant

Journal Pre-proof Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame ret...

13MB Sizes 0 Downloads 24 Views

Journal Pre-proof Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame retardant Junlei Chen, Jihui Wang, Anxin Ding, Aiqing Ni, Hongda Chen PII:

S1359-8368(19)33247-0

DOI:

https://doi.org/10.1016/j.compositesb.2019.107555

Reference:

JCOMB 107555

To appear in:

Composites Part B

Received Date: 9 July 2019 Revised Date:

18 September 2019

Accepted Date: 22 October 2019

Please cite this article as: Chen J, Wang J, Ding A, Ni A, Chen H, Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame retardant, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107555. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Flame retardancy and mechanical properties of glass fibre reinforced polyethylene composites filled with novel intumescent flame retardant Junlei Chena, Jihui Wanga,b, Anxin Dingb,c,*, Aiqing Nib,*, Hongda Chena a

School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road,

Wuhan 430070, Hubei, China b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, China c

Lehrstuhl für Carbon Composites, Technische Universitaet Muenchen (TUM), Germany

ABSTRACT A novel intumescent flame retardant (IFR) system composed of ammonium polyphosphate and poly(1,3-diaminopropane-1,3,5-triazine-o-bicyclic pentaerythritol phosphate) (APP/PDTBP) was mixed with polyethylene to prepare the continuous glass fibre reinforced polyethylene (CGF/PE) unidirectional prepregs by melt impregnation process, and then the CGF/PE/IFR composite laminate were consolidated by hot compression moulding method. The flame retardancy, thermal stability and mechanical properties of CGF/PE/IFR composite laminates were investigated by limiting oxygen index, vertical burning test, cone calorimetric test, thermogravimetric analysis, tensile and flexural strength tests, mode I interlaminar fracture toughness test, and scanning electron microscopy (SEM). The flame retardancy of CGF/PE composite laminates increased with the increase of IFR system loading, which attributes to the formation of intumescent char layer from flame retardants and the weakening of wicking actions of glass fibres. The mechanical properties increased and then decreased with addition of flame retardants, except for the flexural strength with continuous increase. The SEM images showed that IFR system had toughening effect and could improved the strength of fibre-matrix interface. Based on the mechanical properties and the flame retardancy, when the matrix contains 30 wt% IFR system, the CGF/PE/IFR composite laminate had *

Corresponding author E-mail address: [email protected], [email protected]

1

the best comprehensive performance. Keywords: A. Glass fibres; A. Polymer-matrix composites (PMCs); B. Fracture toughness; Intumescent flame retardant; Flame retardancy

1. Introduction Compared with thermosetting composites, fiber reinforced thermoplastic composites have been widely used in recent years due to their high specific strength and stiffness, good impact resistance, short production cycle and recyclability [1, 2]. Thermoplastic resins such as polyphenylene sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) reinforced by carbon fiber or glass fiber have already been used extensively in the field of aerospace sector due to their excellent mechanical properties and high heat stability [3]. However, these matrices are expensive and need high processing temperature and pressure. Polyethylene (PE) is widely used in glass fibre reinforced thermoplastic composites because of its light weight, non-toxic, excellent electrical insulation, chemical corrosion resistance, low cost and easy processing. Unfortunately, glass fibre reinforced polyethylene composites (GF/PE) have poor fire resistance because PE is flammable material and could burns easily, which limits their wider applications [4, 5]. To improve the flame retardancy of GF/PE, the most effective way is to add flame retardant in the composites. Although thermoplastic resins, especially PE, have been widely studied for flame retardant, there are few researches on the flame retardant properties of GF/PE composites [6-8]. Moreover, while glass fibres (GF) can significantly enhance the mechanical properties of the polymer, they deteriorate the flame retardancy of the polymer due to their wicking actions [9-11]. Intumescent flame retardant (IFR) as a halogen-free flame retardant with low smoke and low toxicity have been developed rapidly in the field of flame retardant polymers. The polymer containing IFR has excellent flame retardancy because IFR produces dense and continuous intumescent char layer on the surface to protect the matrix during combustion [12-15]. However, traditional IFR system like ammonium

2

polyphosphate/pentaerythritol/melamine (APP/PER/MEL) has many shortcomings, such as poor water resistance due to the low molecular weight of PER and MEL, and poor compatibility with the matrix, especially for those non-polar polymers such as PE and polypropylene (PP), which deteriorate the flame retardancy and mechanical properties of the material [16, 17]. Hence, novel IFR systems contained macromolecular charring agent have been widely studied in recent years due to the better thermal stability, water resistance and compatibility with the matrix [7, 18, 19]. Among them, triazine-derived macromolecular charring agent [20-23] can effectively solve the shortcomings of traditional IFR and remarkably improve the thermal stability and flame retardancy of the polymers and their composites. In our previous work, a phosphorous-nitrogen based macromolecular charring agent containing

caged

bicyclic

phosphate

and

triazine

structure

named

poly(1,3-diaminopropane-1,3,5-triazine-o-bicyclic pentaerythritol phosphate) (PDTBP) was synthesized and a novel IFR system contained APP and PDTBP could significantly promote the flame retardancy, water resistance properties and thermal stability of high-density polyethylene (HDPE) [24]. To our best knowledge, the effect of IFR system on the flame retardancy and mechanical properties of continuous glass fibre reinforced polyethylene (CGF/PE) composites has not been studied yet. In this work, the flame retardancy, thermal stability and mechanical properties of CGF/PE composites filled with novel IFR system (APP/PDTBP) were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetric test (CCT), thermogravimetric analysis (TGA), tensile and flexural strength tests, mode I interlaminar fracture toughness test, and scanning electron microscopy (SEM), respectively.

2. Experiments 2.1. Materials HDPE (HMA 028, melt flow index of 40 g/10 min (190

, 2.16 Kg), density of

0.954 g/cm3) was obtained from Exxon Mobil Corporation, Avon, Texas, USA. Maleic anhydride-grafted polyethylene (MAPE), CMG5804, was purchased from Fine-blend

3

Compatilizer Jiangsu Co., Ltd., Shanghai, China. APP (polymerization degree ≥1000) was supplied by Jinan Sennuo New Material Technology Co., Ltd., Jinan, China. PDTBP shown in Fig. 1 was synthesized in our laboratory [24]. Glass fibre direct yarn (4305S), pre-treated by sizing agent containing amine group (-NH2), was provided by Chongqing Polycomp International Corporation, Chongqing, China. 2.2. Composite laminate manufacturing Continuous fibre reinforced thermoplastic composites can be prepared by traditional molding compression process which is tedious and inefficient [25]. While the melt impregnation process can complete both resin modification and fibre impregnation by one step using an extrusion die with a twin-screw extruder and an impregnating die, which is a highly efficient method to prepare high-quality continuous fibre reinforced thermoplastic unidirectional prepregs (UD) [26]. Then the CGF/PE/IFR composite laminate was manufactured by UD using a hot compression molding method. The above manufacturing process is illustrated in Fig. 2. 2.2.1. UD production HDPE, MAPE, APP, and PDTBP were firstly dried in an oven (80

for 24 h) and

then mixed using the twin-screw extruder (KTE-36, Nanjing Kerke Extrusion Equipment Co., Ltd., Nanjing, China, length/diameter ratio = 44:1 and screw diameter = 32 mm) attached to an extruding die that extrudes the molten resin onto the surface of the unidirectional glass fiber yarns. The resin-coated fibres then passed through the impregnation die with three wavy contact surfaces, which could make the fiber impregnated more fully. Finally, the UD is obtained by rapid cooling using cold rollers. The thickness of the CGF/PE/IFR UD was 0.32 ± 0.03 mm. Based on our previous work [24], the composition of HDPE, APP, PDTBP and MAPE in the resin matrix of the CGF/PE/IFR UD is listed in Table 1. 2.2.2. Consolidation The hot compression moulding method was used to prepare CGF/PE/IFR composite laminate. Composites with a [0°/90°]3s or [0°/90°]4s layup (orthogonal lay 3

4

or 4 sublaminates and then symmetrical lay) were stacked in a steel mould with dimension of 250×250 mm. The stacks were consolidated in a hot pressing (180

,5

MPa, 10 min), followed by a cold pressing at the same pressure for 5 min. The thickness of the [0°/90°]3s and [0°/90°]4s layup laminates were 3.08 ± 0.13mm and 4.12 ± 0.15 mm, respectively, and the mass fraction of GF in the composite laminate was about 60 wt%. Therefore, the content of IFR in CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR were 4 wt%, 8 wt%, 12 wt% and 16 wt%, respectively. 2.3. Characterization 2.3.1. Flame retardancy The LOI values were measured by an oxygen index instrument (JF-3, Jiangning Analysis Instrument Factory, Nanjing, China) according to ASTM D2863-2008 with sheet dimensions of 130 mm × 6.5 mm × 3.0 mm. The UL-94 vertical burning test was determined with a vertical burn instrument (PX-03-001, Phinix Analysis Instrument Co., Ltd., Suzhou, China) according to ASTM D3801-2010 with sheet dimensions of 125 mm × 13 mm × 3.0 mm. The CCT was conducted on a cone calorimeter (Fire Testing Technology Co., East Grinstead, UK) according to ISO5660. The samples with dimensions of 100 mm × 100 mm × 3.0 mm were wrapped in aluminum foil and exposed under a horizontal heat flux of 50 kW/m2. The residues of the samples after the CCT test were photographed by a digital camera (DSC-RX10 II, SONY Inc., Tokyo, Japan). 2.3.2. Thermal properties The TGA test was carried out with a STA449F3 thermal analyzer (Netzsch Instruments Co., Selb, Germany) from 30

to 700

at a heating rate of 10

/min in

nitrogen atmosphere. 2.3.3. Mechanical properties The tensile and flexural strength of CGF/PE/IFR composite laminates were measured using an universal testing machine (Instron 5967, Instron Corporation, High

5

Wycombe, UK) at a crosshead speed of 4 mm/min according to ISO 527-4 and ISO 14125, respectively. The double cantilever beam (DCB) specimen was employed to determined the mode I interlaminar fracture toughness (GIC) of the CGF/PE/IFR composite laminate according to ASTM D5528. DCB tests were conducted using a universal mechanical testing machine (UTM, Instron 5565), and the DCB specimen size is 150 mm long and 25 mm wide with an initial crack length of a=50 mm (Fig. 3). GIC was calculated by the modified beam theory (MBT) using the following expression:

GIC =

3Pδ 2b(a + ∆ )

(1)

where P is applied load, δ is load line displacement, b is specimen width, a is delamination length, and ∆ is the crack length correction factor which could be determined experimentally from the least squares plot of the cube root of compliance (C1/3) as a function of delamination length (a). The compliance (C) is the ratio of the load line displacement to the applied load (δ/P). 2.3.4. Scanning electron microscopy (SEM) The residual char after CCT and fracture surfaces obtained from fracture toughness test were examined by SEM (JSM-IT300, JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 20.0 kV. All specimens were coated with gold to avoid electrical charging before analysis.

3. Results and discussion 3.1. Flame retardancy 3.1.1. LOI and UL-94 tests The LOI and UL-94 vertical burning tests were used to characterize the flammability of composites. The results of LOI and UL-94 tests for CGF/PE and its IFR composites are listed in Table 2. The LOI values of CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR were 23.2%, 25.3%, 29.5% and 30.6%, respectively, which were 24.1%, 35.3%, 57.8%, and 63.6% higher than

6

CGF/PE/0 wt% IFR with a LOI value of 18.7%, respectively. It indicates that the LOI value and flame retardancy of CGF/PE composite laminates increase with the increase of IFR system loading. Besides, all the tested samples could not be classified according to the UL-94 vertical burning test with no rating because of the wicking actions of GF which could accelerate flame spread during the combustion of the samples [27], and the GF with high mass fraction prevented the HDPE matrix from dripping. However, it was worth noting that different flame spread behavior could be observed among the samples which were related to flame retardancy of IFR system with different content. Fig. 4 presents the combustion progress of all specimens under UL-94 vertical burning test. Apparently, the sample had shorter and shorter burnt length at 60 s after ignition with the increase of IFR system loading. The two samples of CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR had similar burnt length (45.7 and 45.1 mm), which were much smaller than that of other samples. In addition, these two samples were not ignited in the first 10 s and ignited in the second 10 s. It indicates that the addition of IFR system can significantly reduce the flame spread rate of CGF/PE composite laminate. This is largely due to the barrier effect of char layer formed from IFR system, which covers on the GF surface and decreases its wicking action as shown in Fig. 5. When the matrix is burned and the char layer is formed, the original smooth and high-energy GF surface is transformed into rough and carbonized surface, which leads to remarkably weakening of the wetting, spread and flow of the interfacial PE melt, thus markedly reducing the actual flame spread rate [11]. 3.1.2. Cone calorimetric test The CCT was applied to further evaluate the combustion behavior of CGF/PE/IFR composite laminates in a real fire environment [28].The heat release rate (HRR) and total heat release (THR) curves of CGF/PE and its IFR composites are shown in Fig. 6, the correlative characteristic parameters of time to ignition (TTI), peak of HRR (PHRR), time to PHRR (TPHRR), THR, total smoke production (TSP), fire performance index (FPI=TTI/PHRR) and fire growth index (FGI=PHRR/TPHRR) are listed in Table 3. The

7

morphology of residues for CGF/PE/IFR composite laminates after CCT are presented in Fig. 7. It clearly showed that CGF/PE/0 wt% IFR composite laminate had a sharp HRR peak appeared at 82 s with the PHRR as high as 336.4 kW/m2, with a THR value of 128.5 MJ/m2. However, with the addition of IFR system, the PHRR values of CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR decreased to 260.4, 218.7, 195.3, and 179.3 kW/m2, respectively. Similarly, the THR and TSP values of these composite laminates had significantly decreased with the increase of IFR system loading. As shown in Fig. 7, the intumescent char layer covered on the glass fibre could be clearly seen in the CGF/PE/30 wt% IFR (Fig. 7(f)), while only fibres remained for CGF/PE/0 wt% IFR (Fig. 7(a)) after CCT. The above results indicate that the IFR system can form intumescent char layer to cover the GF surface and may inhibit the transfer of heat and combustible gases, thus improving the flame retardant performance of CGF/PE composites. Moreover, FPI and FGI [29] defined as the ratio of TTI value to PHRR value and the ratio of PHRR value to TPHRR value can be used to evaluate the fire hazard of materials, and a higher FPI value and smaller FGI value mean a lower fire hazard [19]. It was obvious that CGF/PE/30 wt% IFR had the highest FPI value and the smallest FGI value, which indicated that its fire hazard was much lower than that of others. 3.1.3. Flame-retardant mechanism Based on the above analysis and our previous research work [24], the possible flame-retardant mechanism of CGF/PE/IFR composites is described below. At low temperature, PDTBP decomposed into triazine oligomers radicals, pentaerythriol and phosphoric acid, and APP decomposed into oligomeric phosphate; meanwhile, the incombustible gases such as NH3 and H2O diluted the concentration of combustible gas and absorbed a lot of heat. As the temperature increases, these decompositon products formed cross-linking structures. Simultaneously, the NH3, H2O and CO2 could make the the system expand and foam, thus forming a compact and intumescent char layer

8

covered on the GF surface, which could decrease the wicking actions of GF and improve the flame retardancy of CGF/PE composites. 3.2. Thermal properties Fig. 8 exhibits the TGA and derivative thermogravimetric analysis (DTG) curves of CGF/PE/IFR composite laminates in nitrogen atmosphere, and the related data are presented in Table 4. Ti meant the initial decomposition temperature when the weight loss was 5 wt%, Tmax meant the temperature at maximum weight loss rate and Rmax meant the maximum weight loss rate. The Ti and Tmax of CGF/PE/0 wt% IFR were 449.5

and 482.6

, respectively. The residue at 800

was 60.8 wt%, which was

consistent with the GF mass fraction of about 60 wt% in the CGF/PE composites. All CGF/PE composites had similar Tmax, but showed a lower Ti when the IFR system (APP/PDTBP) was introduced into the HDPE matrix. This effect was enhanced with the increase of IFR system loading. It mainly because the IFR system occurs thermal decomposition and crosslinking reactions at a low temperature [24], and a higher content of IFR system leads to a lower Ti. However, the addition of IFR system dramatically slowed the decomposition rate which can be indicated by the decreasing Rmax and increased the residue amount at high temperature. Moreover, CGF/PE/30 wt% IFR showed 65.1 wt% residue amount, which increased by 7% compared with CGF/PE/0 wt% IFR at 800

. The residues of CGF/PE/10 wt% IFR, CGF/PE/20 wt%

and CGF/PE/40 wt% IFR were slightly less than that of CGF/PE/30 wt% IFR, indicating that the effect of IFR system on accelerating char formation was not proportional to the content of IFR system. All the above results indicate that IFR system (APP/PDTBP) can form a protective char layer on surface of GF before HDPE degradation, which decreased the wicking action of GF and the transmission of heat and flammable gas, thus inhibiting the decomposition rate and improving thermal stability. 3.3. Mechanical properties 3.3.1. Tensile and flexural strength tests Mechanical properties such as tensile and flexural strength of CGF/PE/IFR

9

composite laminates are shown in Fig. 9. The tensile strength and flexural strength of CGF/PE/0 wt% IFR were 148.3 ± 9.4 MPa and 148.5 ± 5.6 MPa, respectively. The tensile strength was increased to 186.5 ± 27.1 MPa of CGF/PE/10 wt% IFR and 159.7 ± 11.1 MPa of CGF/PE/20 wt% IFR, with a 26% and 8% increment compared with CGF/PE/0 wt% IFR, respectively. Fig. 10 displays the general scheme of the reaction between the functional groups on the GF, IFR, and MAPE. It can be found that both GF and IFR containing amine groups, which can act as reactive sites and react with maleic anhydride that exist in the MAPE [23]. The MAPE and IFR with polar groups could improve the compatibility between GF and HDPE matrix, thus increasing the interfacial strength, which contribute to the increment in the tensile strength [30, 31]. A strong bond between the fibre and matrix of CGF/PE/IFR laminate composites could be further verified by the SEM images of fracture surface of the composite laminates. But the further increase of IFR system loading resulted in a significant decrease in tensile strength, the values of CGF/PE/30 wt% IFR (132.9 ± 16.4 MPa) and CGF/PE/40 wt% IFR (109.7 ± 26.9 MPa) were both lower than that of CGF/PE/0 wt% IFR. The decreased strength attributes to the larger size of IFR agglomerates at higher amount of IFR system, which could act as stress concentration point to induce the fracture of the specimens [32]. However, the flexural strength of CGF/PE/IFR composite laminate increased continuously with the increase of IFR system loading. Compared with CGF/PE/0 wt% IFR, the increments of CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR were 13%, 19%, 25% and 25%, respectively. These improved flexural strength of the composites can be attributed to the high stiffness of the phosphorus layer of APP [8]. 3.3.2. Interlaminar fracture toughness test The load-displacement curves recorded during the interlaminar fracture toughness test for CGF/PE/IFR composite laminates are shown in Fig. 11, which could be divided into three stages: elastic deformation ( ), crack initiation ( ), crack propagation ( ) [33]. In the elastic deformation stage ( ), all specimens had linear behaviour with

10

approximate same slope between load and displacement before the crack initiation. In addition, the linear slop depends on the flexural rigidity of each arm [34], which indicates that the addition of IFR system with different amount has no significant effect on the total flexural rigidity of the delaminated beam. In this stage, there is no visual damage or crack initiation as depicted in Fig. 12( ). In the crack initiation stage ( ), all specimens exhibited a nonlinear increase behavior before the maximum load, and the crack initiation could be visually observed as shown in Fig. 12( ). The specimens of CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR showed a maximum load value of 68.75, 83.29, 72.85, and 63.66 N, respectively, which were higher than that of CGF/PE/0 wt% IFR (56.45 N). It indicates that CGF/PE/IFR composite laminates have higher resistance for crack growth behavior. In the crack propagation stage ( ), all specimens exhibited a slight decrease and reached the stable crack propagation stage. It was observed that slight fibre bridging occurred during the crack propagation as clearly presented in Fig. 12( ). This suggests that the crack propagation causes load reduction and then leads to delamination failure. The resistance curves (R-curves) for DCB specimens drawn between the mode delamination fracture toughness (GIC) and delamination length (a) are presented in Fig. 13. And the average values of the initiation (GIC init.) and propagation toughness (GIC prop.) are shown in Fig. 14. The R-curves showed that the presence of IFR system in the HDPE matrix significantly improved the GIC value of composite laminates. The crack growth rate of neat CGF/PE composite propagated smoothly, however, the crack of composite filled with IFR system was deflected and pinned by the reinforcing obstacles so that more energy was required, resulting in higher fracture toughness [25]. As shown in Fig. 14, both the GIC init. and GIC prop. values increased first and then decreased in general with increasing the IFR system loading. It was clear that the GIC init. increased by about 32.6%, 41.3%, 23.9% and 15.2% from 0.46 kJ/m2 for neat CGF/PE composite to 0.61, 0.65, 0.57 and 0.53 kJ/m2 for CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR, respectively. The GIC prop. increased

11

significantly by about 63.4%, 84.2%, 72.3% and 43.6% from 1.01 kJ/m2 for neat CGF/PE composite to 1.65, 1.86, 1.74 and 1.45 kJ/m2 for CGF/PE/10 wt% IFR, CGF/PE/20 wt% IFR, CGF/PE/30 wt% IFR and CGF/PE/40 wt% IFR, respectively. This improvement may attributes to crack pinning and branching and strong interfacial adhesion between the fibre and matrix of composites filled with IFR system [35]. To further investigate the fracture toughness mechanism, crack propagation macrographs and SEM micrographs of the fracture surface for all specimens were analyzed. Fig. 15 shows the typical crack path and bridging phenomenon of DCB specimens during the crack propagation. The mid-plane surface of neat CGF/PE composite displayed a planar fracture path, while the composites filled with IFR system had a nonplanar fracture path, as shown in Fig. 15(a1-a5). The nonplanar paths (branching of cracks) indicated that the cracks often deflected from the planar path and required a higher driving force to propagate into the adjacent layer with irregular paths, which created large amount of fracture surface area, resulting in higher fracture toughness [36]. Meanwhile, the fibre-bridging phenomenon was observed in Fig. 15(b1-b5), which could support load and thus act as a toughening mechanism. Compared with neat CGF/PE composite, CGF/PE/IFR composites had a longer bridging zone, therefore leading to higher fracture toughness. Furthermore, SEM micrographs of fracture surface are shown in Fig. 16. The micrograph for neat CGF/PE composite (Fig. 16(a)) displayed that the more intensive fibre imprints left after fibre pull-out and clean fibres without matrix could be observed on the fracture surface, which meant the strength of fibre-matrix interfacial bonding was weaker than that of interlaminar HDPE resin, resulting in lower fracture toughness [35]. From the SEM micrographs (Fig. 16(b-e)), CGF/PE/IFR composites presented no fibre pull-out and rougher matrix fracture surface which coats the fibres. The rough areas indicated that IFR system has toughening effect and enhances the interfacial strength between fibre and matrix, which leads to the improvement of fracture toughness. However, fracture toughness did not increase continuously but increased first and then decreased with the increase of IFR

12

system loading, because the IFR system agglomerates would become larger size with increasing the IFR system content. The larger agglomerates could be fractured more easily and thus could not provide an effective barrier for pinning and branching of the cracks [25]. The above results indicate that a low content of IFR system already significantly improves the fracture toughness of composites and higher IFR system content did not help further improvement.

4. Conclusion In this study, the flame retardancy and mechanical properties of CGF/PE/IFR composite laminates were investigated. Adding the novel IFR system (APP/PDTBP) into CGF/PP composites significantly increased the LOI value (up to 30.6%) and reduced the flame spread rate of CGF/PE composite laminate due to the formation of intumescent char layer from flame retardants and the weakening of wicking actions of GF. Meanwhile, the values of PHRR, THR and TSP decreased with the increase of IFR system loading, and CGF/PE/30 wt% IFR had the lowest fire hazard than that of others. A low content of IFR system already significantly improves the tensile strength and mode I interlaminar fracture toughness of composites and higher IFR system content did not help further improvement because of the large IFR system agglomerates. The improved flexural strength can be attributed to the high stiffness of the phosphorus layer of APP. The results reveal that IFR system has toughening effect as a compatibilizer and improves the fibre-matrix interfacial strength, thus results in the tensile strength and fracture toughness improvement. The images of crack path and SEM indicated that crack branching and rough areas were formed verifying a high delamination resistance. In terms of flame retardant properties and mechanical properties, the CGF/PE/30 wt% IFR composite laminate had the best comprehensive performance.

Acknowledgements This research was supported by the Fundamental Research Funds for the Central Universities (2019-zy-036).

Reference 13

1.

Akkapeddi

MK.

Glass

fiber

reinforced

polyamide 6

nanocomposites.

Polym

Compos

2010;21(4):576-585. 2. Kim JW, Lee JS. The effect of the melt viscosity and impregnation of a film on the mechanical properties of thermoplastic composites. Materials 2016;9(6):448. 3. Dı́Az J, Rubio L. Developments to manufacture structural aeronautical parts in carbon fibre reinforced thermoplastic materials. J Mater Process Technol 2003;143(1):342-346. 4. Shen Z, Bateman S, Wu D, McMahon P, Dellolio M, Gotama J. The effects of carbon nanotubes on mechanical and thermal properties of woven glass fibre reinforced polyamide-6 nanocomposites. Compos Sci Technol 2009;69(2):239-244. 5. Lim K-S, Bee S-T, Sin LT, Tee T-T, Ratnam CT, Hui D, Rahmat AR. A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites. Compos Part B 2016;84:155-174. 6. Cao Z, Zhang Y, Song P, Cai Y, Guo Q, Fang Z, Peng M. A novel zinc chelate complex containing both phosphorus and nitrogen for improving the flame retardancy of low density polyethylene. J Anal Appl Pyrolysis 2011;92(2):339-346. 7. Ba M, Liang B, Wang C. Synthesis and characterization of a novel charring agent and its application in intumescent flame retardant polyethylene system. Fibers Polym 2017;18(5):907-914. 8. Khanal S, Zhang W, Ahmed S, Ali M, Xu S. Effects of intumescent flame retardant system consisting of tris (2-hydroxyethyl) isocyanurate and ammonium polyphosphate on the flame retardant properties of high-density polyethylene composites. Compos Part A 2018;112:444-451. 9. Casu A, Camino G, Giorgi MD, Flath D, Laudi A, Morone V, Casu A, Camino G, Flath D, Laudi A. Effect of glass fibres and fire retardant on the combustion behaviour of composites, glass fibres–poly(butylene terephthalate). Fire Mater 1998;22(1):7-14. 10. Monti M, Tsampas SA, Fernberg SP, Blomqvist P, Cuttica F, Fina A, Camino G. Fire reaction of nanoclay-doped PA6 composites reinforced with continuous glass fibers and produced by commingling technique. Polym Degrad Stab 2015;121:1-10. 11. Liu L, Liu Y, Han Y, Liu Y, Wang Q. Interfacial charring method to overcome the wicking action in glass fiber-reinforced polypropylene composite. Compos Sci Technol 2015;121:9-15. 12. Kang T, Li J, Liang X, Zhao X, Xue L, Fan X, Yan Q. A novel phosphazene cyclomatrix network polymer: Design, synthesis and application in flame retardant polylactide. Polym Degrad Stab 2011;96(7):1248-1254. 13. Chen H, Wang J, Ni A, Ding A, Han X, Sun Z. The effects of a macromolecular charring agent with gas phase and condense phase synergistic flame retardant capability on the properties of PP/IFR composites. Materials 2018;11(1):111. 14. Zhan J, Song L, Nie S, Hu Y. Combustion properties and thermal degradation behavior of polylactide with an effective intumescent flame retardant. Polym Degrad Stab 2009;94(3):291-296. 15. Song P, Fang Z, Tong L, Xu Z. Synthesis of a novel oligomeric intumescent flame retardant and its application in polypropylene. Polym Eng Sci 2010;49(7):1326-1331. 16. Chen X, Jiao C, Li S, Hu Y. Preparation and properties of a single molecule intumescent flame retardant. Fire Saf J 2013;58:208-212. 17. Feng C, Liang M, Zhang Y, Jiang J, Huang J, Liu H. Synergistic effect of lanthanum oxide on the flame retardant properties and mechanism of an intumescent flame retardant PLA composites. J Anal Appl Pyrolysis 2016;122:241-248.

14

18. Dai J, Li B. Synthesis, thermal degradation, and flame retardance of novel triazine ring-containing macromolecules

for

intumescent

flame

retardant

polypropylene.

J

Appl

Polym

Sci

2010;116(4):2157-2165. 19. Wen P, Wang X, Wang B, Yuan B, Zhou K, Song L, Hu Y, Yuen RKK. One-pot synthesis of a novel s-triazine-based hyperbranched charring foaming agent and its enhancement on flame retardancy and water resistance of polypropylene. Polym Degrad Stab 2014;110:165-174. 20. Liu Y, Deng CL, Zhao J, Wang JS, Chen L, Wang YZ. An efficiently halogen-free flame-retardant long-glass-fiber-reinforced polypropylene system. Polym Degrad Stab 2011;96(3):363-370. 21. Li B, Xu M. Effect of a novel charring-foaming agent on flame retardancy and thermal degradation of intumescent flame retardant polypropylene. Polym Degrad Stab 2006;91(6):1380-1386. 22. Xie H, Lai X, Li H, Zeng X. Synthesis of a novel macromolecular charring agent with free-radical quenching capability and its synergism in flame retardant polypropylene. Polym Degrad Stab 2016;130:68-77. 23. Chen H, Wang J, Ni A, Ding A, Sun Z, Han X. Effect of novel intumescent flame retardant on mechanical and flame retardant properties of continuous glass fibre reinforced polypropylene composites. Compos Struct 2018;203:894-902. 24. Chen J, Wang J, Ni A, Chen H, Shen P. Synthesis of a novel phosphorous-nitrogen based charring agent and its application in flame-retardant HDPE/IFR composites. Polymers 2019;11(6):1062. 25. Gabr MH, Okumura W, Ueda H, Kuriyama W, Uzawa K, Kimpara I. Mechanical and thermal properties of carbon fiber/polypropylene composite filled with nano-clay. Compos Part B 2015;69:94-100. 26. Xian G, Pu H, Yi X, Pan Y. Parametric Optmisation of Pin-assisted-melt Impregnation of Glass Fiber/Polypropylene by Taguchi Method. J Compos Mater 2006;40(23):2087-2097. 27. Patrick Lim WK, Mariatti M, Chow WS, Mar KT. Effect of intumescent ammonium polyphosphate (APP) and melamine cyanurate (MC) on the properties of epoxy/glass fiber composites. Compos Part B 2012;43(2):124-128. 28. Tang G, Huang X, Ding H, Wang X, Hu Y. Combustion properties and thermal degradation behaviors of biobased polylactide composites filled with calcium hypophosphite. RSC Adv 2014;4(18):8985-8993. 29. Schartel B, Hull TR. Development of fire retarded materials - Interpretation of cone calorimeter data. Fire Mater 2007;31(5):327-354. 30. Lee JH, Jung D, Hong CE, Rhee KY, Advani SG. Properties of polyethylene-layered silicate nanocomposites prepared by melt intercalation with a PP-g-MA compatibilizer. Compos Sci Technol 2005;65(13):1996-2002. 31. Parija S, Nayak SK, Verma SK, Tripathy SS. Studies on physico mechanical properties and thermal characteristics of polypropylene/layered silicate nanocomposites. Polym Compos 2010;25(6):646-652. 32. Pavlidou E, Bikiaris D, Vassiliou A, Chiotelli M, Karayannidis G. Mechanical properties and morphological examination of isotactic Polypropylene/SiO2 nanocomposites containing PP-g-MA as compatibilizer. J Phys Conf Ser 2005;10:190-193. 33. Almansour FA, Dhakal HN, Zhang ZY. Effect of water absorption on Mode I interlaminar fracture toughness of flax/basalt reinforced vinyl ester hybrid composites. Compos Struct 2017;168:813-825. 34. Shokrieh MM, Salamat-talab M, Heidari-Rarani M. Effect of interface fiber angle on the R-curve behavior of E-glass/epoxy DCB specimens. Theor Appl Fract Mech 2016;86:153-160.

15

35. Demirci MT, Tarakçıoğlu N, Avcı A, Akdemir A, Demirci İ. Fracture toughness (Mode I) characterization of SiO 2 nanoparticle filled basalt/epoxy filament wound composite ring with split-disk test method. Compos Part B 2017;119:114-124. 36. Srivastava VK, Gries T, Veit D, Quadflieg T, Mohr B, Kolloch M. Effect of nanomaterial on mode I and mode II interlaminar fracture toughness of woven carbon fabric reinforced polymer composites. Eng Fract Mech 2017;180:73-86.

Table 1. Composition of PE/IFR in the resin matrix of the CGF/PE/IFR unidirectional prepregs. Samples CGF/PE/0 wt% IFR CGF/PE/10 wt% IFR CGF/PE/20 wt% IFR CGF/PE/30 wt% IFR CGF/PE/40 wt% IFR

HDPE (wt%) 95 85 75 65 55

APP (wt%) 0 6.7 13.3 20 26.7

PDTBP (wt%) 0 3.3 6.7 10 13.3

MAPE (wt%) 5 5 5 5 5

Table 2. LOI values and UL-94 results of CGF/PE/IFR composite laminates. Samples

△LOI/%

LOI/%

CGF/PE/0 wt% IFR CGF/PE/10 wt% IFR CGF/PE/20 wt% IFR CGF/PE/30 wt% IFR CGF/PE/40 wt% IFR

18.7 23.2 25.3 29.5 30.6

Rating

— 24.1 35.3 57.8 63.6

UL-94 Burnt length/mm

(60 s after ignition)

No rating No rating No rating No rating No rating

89.2 83.3 74.2 45.7 45.1

Table 3. Characteristic parameters of the CCT for CGF/PE/IFR composite laminates. Samples

TTI (s)

TPHRR (s)

PHRR (kW/m2)

THR (MJ/m2)

CGF/PE/0 wt% IFR CGF/PE/10 wt% IFR CGF/PE/20 wt% IFR CGF/PE/30 wt% IFR CGF/PE/40 wt% IFR

32 28 29 32 28

82 113 100 108 60

336.4 260.4 218.7 195.3 179.3

128.5 108.1 86.3 81.1 69.2

TSP (m2) 9.1 6.3 4.6 3.8 2.4

FPI (s·m2/kW) 0.095 0.108 0.133 0.164 0.156

Table 4. TGA data of CGF/PE/IFR composites in nitrogen atmosphere. Samples

Ti ( )

Tmax ( )

Rmax (wt%/min)

Residue at 800 (wt%)

CGF/PE/0 wt% IFR CGF/PE/10 wt% IFR CGF/PE/20 wt% IFR CGF/PE/30 wt% IFR CGF/PE/40 wt% IFR

449.5 446.9 428.3 416.6 389.7

482.6 483.2 481.9 482.5 481.9

9.7 8.3 7.9 7.3 6.8

60.8 64.6 64.2 65.1 63.7

16

FGI (kW/s·m2) 4.102 2.304 2.187 1.808 2.988

Fig. 1. Chemical structure of PDTBP.

Fig. 2. Schematic illustration for the manufacture process of CGF/PE/IFR composite laminate. Adapted from reference [23].

Fig. 3. DCB specimen dimensions.

17

Fig. 4. Photos of UL-94 test for specimens: (a) CGF/PE/0 wt% IFR, (b) CGF/PE/10 wt% IFR, (c) CGF/PE/20 wt% IFR, (d) CGF/PE/30 wt% IFR and (e) CGF/PE/40 wt% IFR.

18

Fig. 5. Schematic diagram of wicking action (a) before and (b) after adding IFR system.

Fig. 6. Heat release rate (a) and total heat release (b) curves of CGF/PE/IFR composite laminates.

19

Fig. 7. Photographs of residues for (a) CGF/PE/0 wt% IFR, (b) CGF/PE/10 wt% IFR, (c) CGF/PE/20 wt% IFR, (d) CGF/PE/30 wt% IFR and (e) CGF/PE/40 wt% IFR and SEM of residues for (f) CGF/PE/30 wt% IFR after CCT.

Fig. 8. (a) TGA and (b) DTG curves of CGF/PE/IFR composites in nitrogen atmosphere.

20

Fig. 9. Tensile and flexural strength of CGF/PE/IFR composite laminates.

Fig. 10. General scheme of the reaction between the functional groups on the GF, IFR, and MAPE. Adapted from reference [23]

21

Fig. 11. Load-displacement curves of CGF/PE/IFR composite laminates under DCB test.

Fig. 12. Series stages of CGF/PE/0 wt% IFR composite laminate under DCB test.

22

Fig. 13. R-curves of CGF/PE/IFR composite laminates.

Fig. 14. GIC init. and GIC prop. of CGF/PE/IFR composite laminates.

23

Fig. 15. Typical crack path (a1-a5) and bridging phenomenon (b1-b5) of DCB specimens; (a1, b1) CGF/PE/0 wt% IFR, (a2, b2) CGF/PE/10 wt% IFR, (a3, b3) CGF/PE/20 wt% IFR, (a4, b4) CGF/PE/30 wt% IFR and (a5, b5) CGF/PE/40 wt% IFR.

24

Fig. 16. SEM images of fracture surface for (a) CGF/PE/0 wt% IFR, (b) CGF/PE/10 wt% IFR, (c) CGF/PE/20 wt% IFR, (d) CGF/PE/30 wt% IFR and (e) CGF/PE/40 wt% IFR.

25

Conflict of interest statement The authors declared that they have no conflicts of interest to this work.

1