Introduction

Because of excellent physical and chemical properties, ultra-high-molecular-weight polyethylene (UHMWPE) is known as an outstanding engineering plastics [1–4], and has extensively been applied in construction, transport, electronic engineering, and cable industry especially, which has extremely high requirements for flame retarding property [5–9]. To achieving satisfied flame retarding ability, halogen-included flame retardants are selected as common additives for UHMWPE [10]. However, the halogen-included flame retardants are notorious and severely restricted for applications due to the environment protection regulations [11, 12]. Therefore, researchers turn their attempts into the halogen-free flame retardants (HFFR), for example, the commonly applied magnesium hydroxide (MH) [13–16].

MH is one of the most important HFFR due to its practical properties, such as low cost, odorless, toxicity-free, easy to handle, and corrosive gas free [17, 18]. For the flame retarding process, on one hand, MH gives a swollen multicellular char to protect the underlying material from firing [19]. On the other hand, the MH retardants can act as a physical barrier against the heat transmission and oxygen diffusion to prevent the pyrolysis of the material to volatile and combustible products [20]. Besides, the decomposition of MH is an endothermal process and can produce H2O which also contributes to flame retarding property [21]. In our previous study [22], it was also found that MH performed as a superiority and practical flame retardant for low-density polyethylene. However, the ratio of MH in the UHMWPE-MH composites should be no less than 50% to obtain satisfied flame retarding property [23]. With such extremely high concentration of MH, the physical and mechanical properties of the UHMWPE are interviewing serious challenges. The incompatibility between inorganic MH and organic UHMWPE chains is unignorable and needs to be fully concerned. It is highly expected to conduct a coupling reagent to overcome the incompatibility problem without sacrificing the flame regarding ability. Phenoxycyclophosphazene (HPCTP), another novel flame retardant (The molecular form is show in Fig. 1) [24–26], possesses both organic (phenyl groups) and inorganic parts (N–P–O groups). It is principally possible to use HPCTP as an effective coupling reagent to promoting the compatibility without sacrificing the flame regarding ability of UHMWPE-MH composites. Based on the discussion above, we tried to introduce HPCTP into UHMWPE-MH composites to prepare the UHMWPE-MH-HPCTP composites, which was then observed in details by flame retarding testing, and physical and mechanical quality measurements.

Fig. 1
figure 1

Molecular formula of the HPCTP

Full size image

Experiment

Materials

UHMWPE is purchased from Sinopec Beijing Yanshan Company, Beijing, China. The detailed physical and chemical parameters of UHMWPE are listed in Table 1.

Table 1 Physical and chemical parameters of UHMWPE
Full size table

The MH (Magnifin H10), with a median particle size of 0.80–1.10 μm, was obtained from Albemarle Corporation. The HPCTP (Fig. 1) with a purity of 99% was supplied by Huazhong University of Science and Technology.

Preparation of UHMWPE-MH-HPCTP composites

The certain amounts of UHMWPE, MH, and HPCTP were first abundantly mixed by a double roller mill SK-160B at 170 °C. Then, the mixed samples were pressed by a pressure of 10 MPa to sheets at 180 °C. The samples sheets will be tested. The compositions for each sample are present in Table 2 (phr = parts per hundred ratio based on 100 parts of UHMWPE).

Table 2 Compositions of UHMWPE-MH-HPCTP composites
Full size table

Characterization and measurement of UHMWPE-MH-HPCTP composites

The melt flow rate (MFR) tests were conducted by an SRZ-400E Melt Flow Tester (Changchun, China). The measurements were repeated four times at 190 °C.

FTIR spectroscopy of Attenuated Total Reflectance (ATR) mode was employed to study the chemical properties of the samples. The measurements were performed on a Perkin Elmer Spectrum 100 FTIR spectrometer with a 4 cm−1 resolution and 32 scans per spectrum from 4000 to 500 cm−1.

X-ray diffraction (XRD) was performed on Bruker AD8 (Cu Kα, λ = 0.154 nm) at 40 kV and 40 mA. The XRD data were collected at 2θ from 5° to 90°.

The morphologies of samples were studied by a JEOL JSM-6500F scanning electron microscope (SEM), with an acceleration voltage set at 10 kV. Prior to SEM examination, the samples were quenched in liquid nitrogen, and then coated with an Au layer.

The tensile strength and elongation at break points were measured through an electronic tensile strength meter (Wance, Shenzhen, China) at room temperature with a cross-head speed of 250 mm/min. The measurements were repeated five times to get the average values.

Differential scanning calorimeter (DSC) tests were performed on METTLER DSC822E with a procedure: Heating rate 10 °C min−1 in nitrogen atmosphere, from 30 to 180 °C and held at 180 °C for 3 min to erase thermal history, then cooled to 30 °C, and then heated to 180 °C again. The crystallinity (X c) was calculated according to the following equation:

$$X_{\text{c}} = \frac{{\Delta H_{\text{m}} }}{\Delta H} \times 100\%$$
(1)

where ∆H m is the melting enthalpy and ∆H 100 is the theoretical enthalpy (291 J/g) of completely crystalline for UHMWPE.

Thermogravimetric (TG) was conducted by TG (NETZSCH TG 209 F3 Tarsus) in air with a heating rate of 10 °C min−1 from 50 to 650 °C.

Limiting-oxygen index (LOI) measurements were carried out on an HC-2C oxygen index meter (Jiangning, China) according to ASTM D2863-77. The sample size was in a dimension of 100 mm × 6.5 mm × 3 mm. The LOI test was repeated five times for each sample. The experimental results were reproducible within ±0.5%.

The UL-94 measurements were performed, according to, by Fire Testing Technology (Cangzhou Ke Shuo Construction Equipment Ltd., China). The sample size is 130 mm × 13 mm × 4 mm for all measurements.

The cone calorimeter tests were carried out following the procedures indicated in the ISO 5660 standard with a FTT cone calorimeter. Square specimens (100 mm × 100 mm × 3 mm) were irradiated with a heat flux of 35 kW/m2.

Results and discussion

Characterization of UHMWPE-MH-HPCTP composites

FTIR analysis of UHMWPE-MH-HPCTP composites

In Fig. 2, the FTIR spectroscopies were conducted to observe the UHMWPE-MH-HPCTP composites. For the spectra of PE-0-0, four characteristic peaks were observed: the peak at 2918 cm−1 attributed to the CH2 asymmetrical stretching, the peak at 2849 cm−1 attributed to the CH2 symmetrical stretching, the peak at 1465 cm−1 attributed to the bending deformation of CH2, and the peak at 720 cm−1 attributed to the rocking deformation of CH2, respectively [27, 28]. In the FTIR spectra of PE-60-0 composites, a new peak appeared at 3694 cm−1 attributed to the –OH stretching vibration of the MH crystal [29]. In the FTIR spectra of PE-60-5, PE-60-15, and PE-60-25 composites, peaks at 1590 and 1486 cm−1 attributed to the phenyl groups, which were proved to be mono-substitution of benzene by the peaks at 767 and 685 cm−1. The peaks at 1263 and 1176 cm−1 belonged to asymmetrical and symmetrical stretching of P–N, respectively [30]. The peak at 949 cm−1 was believed to be the stretching vibration of P–O–C [31]. The appearing of characteristic peaks of P–N and P–O–C confirmed the existing of HPCTP in UHMWPE-MH composites. In addition, the increasing intensities of these characteristic peaks (P–N and P–O–C) are consistent with the continuously increasing contents of HPCTP in the composites.

Fig. 2
figure 2

FTIR spectra of UHMWPE-MH-HPCTP composites

Full size image

XRD analysis of UHMWPE-MH-HPCTP composites

The crystalline structure is one of the important basic properties for UHMWPE. In Fig. 3, XRD analysis was used to investigate the effect of HPCTP on the crystalline structure of UHMWPE-MH composites. In the curve of PE-0-0, the reflections at 2θ of 21.6°, 23.9°, 30.1°, and 36.3° refer to (1 1 0), (2 0 0), (2 1 0), and (3 0 0) lattice planes of the spherical UHMWPE crystal [32, 33]. In the curves of PE-60-0, PE-60-5, PE-60-15, and PE-60-25 composites, the reflections at 2θ of 18.5°, 38.1°, and 50.8° corresponded to the (0 0 1), (1 0 1), and (1 1 0) lattice planes of the MH crystal [34, 35]. In addition, the unchanged intensities of diffraction peaks for MH were coincident with the settled content of MH in the experiment. However, the diffraction peak at 2θ of 9.4° increased continuously with the increasing HPCTP content, which indicated that the peak at 2θ of 9.4° attributed to HPCTP. The positions of all the diffraction peaks of UHMWPE were motionless, which meant that the incorporation of MH and HPCTP did not break the crystalline structures of UHMWPE. This means that the HPCTP merely played a role of coupling the PE and MH from the interface linking and the adding of HPCTP did not cause to obvious flaws in the crystalline structures of the composites. Therefore, the HPCTP is a suitable coupling reagent which will not reduce the composite properties which are related with the crystalline structures of the composites.

Fig. 3
figure 3

XRD spectra of UHMWPE-MH-HPCTP composites

Full size image

Morphology of UHMWPE-MH-HPCTP composites

By the SEM measurements, the section morphologies of UHMWPE-MH-HPCTP composites were shown in Fig. 4. All the images revealed that MH particles with the size of around 1 μm were uniformly dispersed in the UHMWPE matrix. Due to the incompatibility of organic UHMWPE and inorganic MH [36], many tiny gaps between MH and UHMWPE were observed in PE-60-0 composites. For the PE-60-5 and PE-60-15 composites, the gaps between MH and UHMWPE disappeared completely (Fig. 4b, c). It indicated that HPCTP played a role of coupling reagent to enhance the compatibility between MH and UHMWPE. For PE-60-25 composites, lots of cavities were observed in the UHMWPE-MH-HPCTP composites due to aggregations of superfluous HPCTP (Fig. 4d). After washing in ethanol, the HPCTP was effectively removed and then more MH particles were exposed in the UHMWPE-MH-HPCTP composites (Fig. 4e, f). This phenomenon further confirmed the coupling effect of HPCTP in the UHMWPE-MH-HPCTP composites.

Fig. 4
figure 4

SEM images of HPCTP-MH-UHMWPE composites (a PE-60-0; b PE-60-5; c PE-60-15; d PE-60-25; e PE-60-15 with the amplification of 5 k times; f PE-60-15 with the amplification of 5 k times after washing in ethanol. For a–d, the amplification is 10 k)

Full size image

Processability of UHMWPE-MH-HPCTP composites

Processability of UHMWPE-MH-HPCTP composites was characterized by melt flow rate (MFR). In Fig. 5, it can be seen that the MFR increased proportionally with the increasing of HPCTP contents in UHMWPE-MH-HPCTP composites. It indicated that HPCTP effectively enhanced the MFR of UHMWPE and made the melting process much easier. The MFR for PE-60-0 is 0.28 g/10 min, which is much lower than that of pure PE-0-0 (0.39 g/10 min). The great frictional resistance between the MH and UHMWPE seriously hindered the flow of UHMWPE molecule. While in the UHMWPE-HPCTP-MH composites, the HPCTP played as a lubricant, which effectively whittled the frictional resistance. More HPCTP in the UHMWPE-MH-HPCTP composites caused lower frictional resistance and then higher MFR. The MFR was even 100% promoted for the composite of PE-60-25. The increasing MFR meant lower energy consumption and improved production efficiency in the industrial extrusion and molding process [37, 38]. This means that UHMWPE-MH-HPCTP composites show superiority in energy consuming and cost by the addition of HPCTP.

Fig. 5
figure 5

MFR of the UHMWPE-MH-HPCTP composites

Full size image

Mechanical property of UHMWPE-MH-HPCTP composites

The elongation at break of the composites increased first as the increasing contents of HPCTP, and then decreased, as shown in Fig. 6a. After adding moderate content of HPCTP in UHMWPE-MH composites, HPCTP, on one hand, effectively reduced the frictional resistance during the stretching process. Therefore, the elongation at break increased nearly 600%. However, surplus of HPCTP in the UHMWPE-MH composites, on the other hand, could cause serious aggregations (Fig. 4d), which then leaded to the decreasing of elongation at break. The similar phenomenon was also reported by other researchers [39]. The reduced frictional resistance between the MH and UHMWPE affected the tensile strength of UHMWPE-MH-HPCTP composites slightly (Fig. 6b).

Fig. 6
figure 6

Elongation and tensile strength of the UHMWPE-MH-HPCTP composites (a elongation; b tensile strength)

Full size image

Crystallization and melting behavior of UHMWPE-MH-HPCTP composites

In Fig. 7, DSC measurements were carried out to investigate the crystallization and melting behavior of UHMWPE-MH-HPCTP composites and the values were listed in Table 3. Comparing with PE-0-0, the onset temperature of UHMWPE-MH-HPCTP composites increased first, and then decreased slightly with the increasing HPCTP contents. The reason could be given that HPCTP played a role to couple MH and UHMWPE together then hindered the movement of UHMWPE chains. However, the superfluous HPCTP, possessing comparative low melting point, contributed to the movement ability of UHMWPE-MH-HPCTP composites. The uncrystallization HPCTP contributed to non-crystallization property of UHMWPE, and hence, the crystallinity of UHMWPE-MH-HPCTP composites decreased linearly with the increasing of HPCTP contents, from 62.85 to 35.25%.

Fig. 7
figure 7

DSC of UHMWPE-MH-HPCTP composites (a heating process, b cooling process)

Full size image
Table 3 DSC data of UHMWPE-MH-HPCTP composites
Full size table

Thermal property of UHMWPE-MH-HPCTP composites

In Fig. 8, TG measurements were used to investigate the thermal stability of the UHMWPE-MH-HPCTP composites and the values were listed in Table 4. The decomposition temperatures were defined as T 5%, at which 5% weight of loss occurred. The maximum temperatures (T max) were used to observe the steeply degradation of UHMWPE-MH-HPCTP composites. The decomposition temperatures, T 5%, decreased continuously with increasing of HPCTP contents in the UHMWPE-MH-HPCTP composites (Table 4). In the TG plots of PE-60-0, two degradation steps were observed. The first step was associated with the degradation process of MH; the second step was associated with the degradation process of UHMWPE [13]. In the TG plots of PE-60-15 and PE-60-25, one more decomposition step was observed. This new step between 340–344 °C was associated with the degradation of HPCTP. These results of TG indicated that HPCTP could affect the thermal stability of composites due to lower decomposition temperatures of HPCTP.

Fig. 8
figure 8

TG and DTG of UHMWPE-MH-HPCTP composites (a TG; b DTG)

Full size image
Table 4 TG data of the UHMWPE-MH-HPCTP composites
Full size table

Flame retardance of UHMWPE-MH-HPCTP composites

The flammability of UHMWPE-MH-HPCTP composites was investigated by conducting the LOI and cone calorimeter tests. According to Fig. 9, LOI of the composites performed a continuously increasing trend with the increasing HPCTP contents. The heat released rate (HRR) and total heat released rate (THR) plots of the samples were depicted in Fig. 10a, b respectively. The HRR and THR values of PE-60-0 were lower than that of PE-0-0. This could be explained by the decomposition and dehydration reaction of MH. Through these reactions, the MH particles absorbed amounts of heat from the surface of the polymer during the combustion process [40]. Comparing to the ignition time of PE-0-0, ignition times of PE-60-0 and PE-60-5(or PE-60-25) were delayed about 59 and 75 s, respectively. It indicated that HPCTP could improve flame retarding ability of the composites.

Fig. 9
figure 9

LOI % of UHMWPE-MH-HPCTP composites

Full size image
Fig. 10
figure 10

HRR and THR of UHMWPE-MH-HPCTP composites (a HRR; b THR)

Full size image

The ash photos of the composites in Fig. 11 displayed that the ash shape of PE-60-25 sample was more expansive than the ash shape of PE-60-0. This indicated that PE-60-25 sample could produce more non-combustible gases during the combustion process. In Fig. 12, it was found that huge amounts of organic fragments covered the around the HM particles. However, this did not happen to the PE-60-5 and PE-60-25. It indicated that PE-60-5 sample was more completely carbonization under the influence of HPCTP.

Fig. 11
figure 11

Ash photos of the UHMWPE-MH-HPCTP composites (a PE-60-0; b PE-60-25)

Full size image
Fig. 12
figure 12

SEM images of ashes of UHMWPE-MH-HPCTP composites (a PE-60-0; b PE-60-5; c PE-60-25)

Full size image

From EDS curves (Fig. 13), it can be seen, the presence of phosphorus in ashes of the composites, and the strength of phosphorus signal increased with the increasing HPCTP content. In Fig. 14, the FTIR spectra for the ashes of UHMWPE-MH-HPCTP composites were shown with the region from 4000 to 500 cm−1. There were three characteristic peaks: 3447 and 1440 cm−1 attributed to the O–H (from dehydration of MH) stretching vibration and rocking vibration, and the wide peak from 500 to 650 cm−1 attributed to the Mg–O stretching vibration. In the FTIR spectra of PE-60-5 and PE-60-25 composites, the new bands were found at 1076 cm−1 which attributed to the P–O–P stretching vibration in polymeric phosphoric acid produced by combustion of HPCTP, and the intensity of the new peak increased with increasing HPCTP content.

Fig. 13
figure 13

EDS of ashes of UHMWPE-MH-HPCTP composites

Full size image
Fig. 14
figure 14

FTIR spectra of ashes of UHMWPE-MH-HPCTP composites

Full size image

HPCTP acted as effectively flame retardant in both solid phase and gas phase simultaneously during the combustion process. For the solid phase flame retarding, HPCTP, phosphorus-containing compounds, produced the phosphoric acid by thermal decomposition process. The phosphoric acid played as a stable polymeric phosphoric cover, which prevented the oxygen and combustibles from the UHMWPE-MH-HPCTP composites. For the gas phase flame retarding, the nitrogen element in HPCTP produced amounts of non-combustible gases in decomposition process, such as NH3, N2, NO, NO2, etc. These gases not only largely diluted the concentrations of oxygen and combustibles in the circumstance, but also effectively reduced the temperature by absorbing amounts of heat from the surface of UHMWPE-MH-HPCTP composites. In addition, the heated phosphorus and nitrogen compounds could generate PO· radicals and nitrogen oxides, which were able to capture the OH·, H·, etc., in the combustion reaction [41–43]. The UL-94 measurements were conducted to evaluate the flame regard ability for industrial application and the results were listed in Table 5. By comparing, it was suggested that the addition of HPCTP enhanced the flame regarding ability of UHMWPE-MH-HPCTP composites.

Table 5 UL-94 testing of UHMWPE-MH-HPCTP composites
Full size table

Conclusion

The HPCTP was introduced into UHMWPE-MH composites as a coupling reagent to prepare the UHMWPE-MH-HPCTP composites. SEM and FTIR results confirmed that HPCTP successfully enhanced the compatibility of UHMWPE and MH. The prepared UHMWPE-MH-HPCTP composites, with over 100% increasing of MFR, demonstrated great advantages in processability. The elongation was promoted nearly 600% by adding certain amounts of HPCTP. Although the thermal property was slightly reduced, the UHMWPE-MH-HPCTP composites presented significantly enhanced flame retarding ability, which was sufficiently confirmed by 100% promotion of limiting-oxygen index (LOI), 75 s delaying of ignition time, and advantageous ash results. The previous studies of UHMWPE-MH composites other have high flame retarding ability with sacrificing the processability, or show excellent processability with reduced high flame retarding ability [23, 36, 44–46]. Comparing to the previous studies of UHMWPE-MH composites, the present UHMWPE-MH-HPCTP composites performed priorities in processability, mechanical property, especially flame retarding ability, and supplied a practical candidate material for the industrial applications.