Structure and properties of irradiated HDPE high-density polyethylene/calcium carbonate composites

Abstract

First, some groups of C=O and C–O were introduced onto high-density polyethylene (HDPE) chains by ultraviolet irradiation for short time in ozone atmosphere. The content of the groups was increased with increasing the irradiation time. The irradiated HDPE was blended with calcium carbonate (CaCO₃) to prepare the composites. The melting temperature and crystallinity of HDPE in irradiated HDPE/CaCO₃ composites were lower than the HDPE in HDPE/CaCO₃ composites. Compared with the HDPE/CaCO₃ composites, the dispersion of CaCO₃ and the interfacial interaction between CaCO₃ and HDPE in the irradiated HDPE/CaCO₃ composites increased respectively. With increasing the irradiation time, the mechanical properties (especially impact strength) of the irradiated HDPE/CaCO₃ composites were markedly enhanced, while their thermal stability decreased slightly. For example, the tensile strength and impact strength of the irradiated (20 min) HDPE/CaCO₃ composites increased from 25.7 MPa and 72 J m⁻¹ to 30.3 MPa and 416 J m⁻¹, respectively, compared with those of the HDPE/CaCO₃ composites, and stiffened and toughened HDPE composites were obtained.

Introduction

Polyolefin is a common polymer material, which is low in cost and simple to process. Through blending and filling, it is possible to prepare polyolefin composites with high performance. However, polyolefin is hydrophobic and inert; so the compatibility between polyolefin and inorganic fillers or engineering plastics is usually poor. Consequently, the polyolefin blends often have worse mechanical properties than themselves. Functionalizing polyolefin to improve its compatibility with inorganic fillers or engineering plastics has been studied to prepare high-performance polyolefin composites. The most common way is grafting polar monomer-containing active functional group onto the polyolefin chains through copolymerization. However, the grafting approach is complicated, polluting, and may cause damage to the apparatus. Furthermore, residual graft monomers and other auxiliaries may negatively affect the thermal, electrical, and mechanical properties of the composites. Through ultraviolet irradiation in air or oxygen atmosphere without the addition of any monomers and auxiliaries, the authors Lei, Guan, and others ^{1 –18} functionalized polyolefin and significantly enhanced its compatibility with inorganic fillers or engineering plastics. Thus, stiffened and toughened polyolefin composites were obtained. The technique of functionalizing polyolefin by ultraviolet irradiation is a green chemical method, which is environment friendly and without any chemical pollution. However, the disadvantages of functionalizing polyolefin by ultraviolet irradiation in air or oxygen atmosphere were the process was time consuming (24–48 h) with low efficiency.

In this article, the groups of C=O and C–O were introduced onto the high-density polyethylene (HDPE) chains by ultraviolet irradiation for 10–20 min in ozone atmosphere. The irradiated HDPE/calcium carbonate (CaCO₃) composites were obtained by melting the blend. Compared with those in HDPE/CaCO₃ composites, the dispersion of CaCO₃ and the interfacial interaction between the matrix and CaCO₃ were heightened. Therefore, the mechanical properties (especially impact strength) of irradiated HDPE/CaCO₃ composites were higher than those of HDPE/CaCO₃ composites.

Experiment

Materials

HDPE with a melt index of 1.0 g/10 min and a density of 0.954 g cm⁻³ was manufactured by Yangzi Petrochemical Co. Ltd (China). CaCO₃ treated by silane coupling agent with an average particle size of 1.2 µm was provided by Nanjing University of Technology (China).

Ultraviolet irradiation of HDPE

Ultraviolet irradiation of HDPE was performed at a temperature of 60°C and light intensity of 78 W m⁻² in ozone atmosphere; the wavelength of the lamp was in the range of 365–450 nm.

Preparation of HDPE/CaCO₃ composites

HDPE and CaCO₃ were blended with a twin roller at 155°C for 10 min to prepare HDPE/CaCO₃ (60/40) composites. The HDPE/CaCO₃ composites were prepared into sheets of 1 or 4 mm thickness.

Measurement and characterization

For FTIR analysis, the HDPE powder was made into films with the thickness about 30–50 μm, and the analysis was carried out on a VECTOR22 spectrometer (Bruker, Germany) scanned from 4000 cm^–1 to 400 cm^–1 with a resolution of 4 cm^–1.

For X-ray diffraction (XRD), differential scanning calorimetric (DSC), and X-ray photoelectron spectroscopic (XPS) analyses, HDPE powder was directly used for the characterization of XRD, DSC, and XPS as follows.

XRD analysis was performed with a D/Max II diffractometer (Rigaku, Japan). The sample was scanned from 10° to 50° at a scan rate of 4°/min⁻¹.

DSC analysis was performed on a Pyres 1 thermal analyzer (Perkin-Elmer, Massachusetts, USA). The sample was scanned from 50°C to 180°C at a scan rate of 10°C min⁻¹. Crystallinity (C) was determined using the relation $\mathrm{\Delta }{H}_{\mathrm{m}}/\phantom{\mathrm{\Delta }{H}_{\mathrm{m}}\mathrm{\Delta }{H}_{\mathrm{m}}^{{}^{\circ }}}\mathrm{\Delta }{H}_{\mathrm{m}}^{{}^{\circ }}\times 100\mathrm{\%}$ , where ΔH_m is the melting enthalpy of HDPE and $\mathrm{\Delta }{H}_{\mathrm{m}}^{{}^{\circ }}$ is 286 J g⁻¹.

XPS analysis was carried out on an ESCALB MK II spectrometer (VG Scientific, UK) with aluminum K_α source (1486.6 eV). The irradiated HDPE/CaCO₃ composites were extracted by refluxing xylene for 72 h in a Soxhlet extractor. The remaining CaCO₃ in the extractor was dried completely and then analyzed by XPS.

Molau test: 1 g of HDPE/CaCO₃ composites was dissolved in 19 g of hot xylene. The suspensions were laid for 72 h at 90°C, and then the dispersion of the suspension was observed.

Scanning electron microscopey analysis was performed with a LEO-15300VP scanning electron microscope (Zeiss, Germany).

Tensile test was carried out on an Instron 4466 all-purpose tester (Norwood, Massachusetts, USA) according to ASTM D268 standard. Notched Izod impact strength was measured using an XJ-40A apparatus (Wuzhong Material Testing Machine Factory, Jiangsu, China) according to GB/T 1843 standard.

Thermogravimetric analysis (TGA) was performed using a TA2100-SDT2926 (TA Instruments, New Castle, Delaware, USA). The initial weight loss temperature (T_i) and the corresponding maximum weight loss temperature (T_p) were noted.

Results and discussion

FTIR analysis of irradiated HDPE

FTIR spectra of HDPE and the irradiated HDPE are shown in Figure 1. The intensity of absorption peaks of the irradiated HDPE around 1735 and 1180 cm^–1 (corresponding to C=O and C–O) increased compared with those of HDPE, indicating that the groups of C=O and C–O were introduced onto the HDPE chains by ultraviolet irradiation for short time in the ozone atmosphere. The content of the groups augmented with increasing the irradiation time. Compared with the irradiated polyolefin by ultraviolet irradiation in air or oxygen atmosphere ^{5 –7,10} , the introduction rate of the groups of C=O and C–O by ultraviolet irradiation in ozone atmosphere enhanced remarkably. We thought that the compatibility between the irradiated HDPE (matrix) and CaCO₃ can be improved by introducing the polar groups of C=O and C–O, and the mechanical properties of the irradiated HDPE/CaCO₃ composites can be increased.

Figure 1.

FTIR spectra of the irradiated HDPE for 0 min (a), 15 min, (a) and 20 min (c). FTIR: Fourier transform infrared; HDPE: high-density polyethylene.

XRD and DSC analysis of irradiated HDPE/CaCO₃ composites

Figure 2 shows the XRD spectra of irradiated HDPE/CaCO₃ composites. HDPE in the irradiated HDPE/CaCO₃ composites was an orthorhombic structure as that of HDPE.

Figure 2.

XRD spectra of irradiated HDPE/CaCO₃ composites for 0 min (a), 15 min (b), and 20 min (c). XRD: X-ray diffraction; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Figure 3 shows DSC curves of irradiated HDPE/CaCO₃ composites. Compared with those in the HDPE/CaCO3 composites, the melting temperature and crystallinity of HDPE in the irradiated HDPE/CaCO₃ composites decreased with increasing the irradiation time. The decrease of the crystallinity suggested that the interfacial interaction between the irradiated HDPE and CaCO₃ in the irradiated HDPE/CaCO₃ composites was heightened (Table 1).

Figure 3.

DSC curves of irradiated HDPE/CaCO₃ composites for 0 min (a), 10 min (b), and 20 min (c). DSC: differential scanning calorimetry; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Table 1.

DSC data for irradiated HDPE/CaCO₃ composites.

Irradiation time (min)	Tm (°C)	ΔHm (J g−1)	C (%)
0	132.1	169	59.1
15	130.9	168	58.7
20	130.4	166	58.0

DSC: differential scanning calorimetry; T_m: melting temperature; ΔH_m: melting enthalpy; HDPE: high-density polyethylene; CaCO₃: calcium carbonate; C: crystallinity.

Molau test of irradiated HDPE/CaCO₃ composites

The xylene suspensions of HDPE/CaCO₃ composites separated into two parts after placement, namely, transparent xylene solution contained HDPE component in the top part and CaCO₃ deposited on the bottom part. This indicated that the interfacial interaction between the CaCO₃ and the HDPE matrix was poor, and CaCO₃ was completely separated from the HDPE matrix. Concerning the xylene suspensions of irradiated (10 min) HDPE/CaCO₃ composites, the top part of the test tube was a turbidity suspension (containing CaCO₃), while a little amount of CaCO₃ deposited on the bottom. For the xylene suspension of irradiated (20 min) HDPE/CaCO₃ composites, little amount of CaCO₃ deposited on the bottom. Compared with those in the HDPE/CaCO₃ composites, the dispersion degree of CaCO₃ and the interfacial interaction between CaCO₃ and HDPE in the irradiated HDPE/CaCO₃ composites enhanced.

XPS analysis of CaCO₃ separated from irradiated HDPE/CaCO₃ composites

The XPS analysis data for CaCO₃ obtained from irradiated HDPE/CaCO₃ composites by solvent extraction are shown in Table 2. Compared with that obtained from HDPE/CaCO₃ composites, the HDPE content on the surface of the CaCO₃ particles obtained from the irradiated HDPE/CaCO₃ composites increased. Furthermore, the content further increased with increasing the irradiation time. This also suggested that the interfacial interaction between CaCO₃ and HDPE in the irradiated HDPE/CaCO₃ composites was stronger than that in the HDPE/CaCO₃ composites.

Table 2.

XPS data for CaCO₃ separated from irradiated HDPE/CaCO₃ composites.

Irradiation time (min)	C (%)	O (%)	Ca (%)	C/Ca
0	48.42	43.15	8.43	5.74
15	61.66	31.24	7.10	8.71
20	67.70	26.83	5.47	12.38

XPS: X-ray photoelectron spectroscopy; HDPE: high-density polyethylene; CaCO₃: calcium carbonate; C: carbon; O: oxygen; Ca: calcium.

SEM analysis of irradiated HDPE/CaCO₃ composites

The SEM photographs of liquid nitrogen frozen-fractured surface of irradiated HDPE/CaCO₃ composites are shown in Figure 4. Some agglomerated CaCO₃ and cavity emerged on the fractured surface of the HDPE/CaCO₃ composites, indicating poor dispersion of CaCO₃ in the HDPE matrix and weak interfacial interaction between the CaCO₃ and the HDPE matrix. The size of the agglomerated CaCO₃ and cavity on the fractured surface of irradiated (15 min) HDPE/CaCO₃ composites decreased, while the amount of the bare CaCO₃ reduced markedly. There was no bare CaCO₃ on the fractured surface of irradiated (20 min) HDPE/CaCO₃ composites, and the CaCO₃ was coated by the HDPE. Compared with those in the HDPE/CaCO₃ composites, the dispersion of the CaCO₃ and the interfacial interaction between CaCO₃ and HDPE in irradiated HDPE/CaCO₃ composites improved. The SEM photographs of the impact-fractured surface of irradiated HDPE/CaCO₃ composites are shown in Figure 5. The impact-fractured surface of the HDPE/CaCO₃ composites was smooth, which indicated that the HDPE matrix had no plastic deformation during the impact process and a brittle fracture. Some fibrils were observed on the impact-fractured surface of the irradiated HDPE/CaCO₃ composites, and the fibrils were further thinned with increasing the irradiation time. The irradiated HDPE/CaCO₃ composites showed a tough fracture, and its toughness enhanced with increasing the irradiation time.

Figure 4.

SEM photographs of liquid nitrogen frozen-fractured surface of irradiated HDPE/CaCO₃ composites for 0 min (a), 15 min (b), and 20 min (c). SEM: scanning electron microscopic; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Figure 5.

SEM photographs of impact-fractured surface of irradiated HDPE/CaCO₃ composites for 0 min (a), 15 min (b) and 20 min (c). SEM: scanning electron microscopic; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Mechanical properties of irradiated HDPE/CaCO₃ composites

Mechanical properties of irradiated HDPE/CaCO₃ composites are shown in Table 3. The HDPE/CaCO₃ composites showed low mechanical properties because of poor dispersion of CaCO₃ and the interfacial interaction between CaCO₃ and HDPE. Compared with the HDPE/CaCO₃ composites, the mechanical properties (especially impact strength) of irradiated HDPE/CaCO₃ composites were enhanced with increasing the irradiation time. This was attributed to the fact that the dispersion and the interfacial interaction of the irradiated HDPE/CaCO₃ system further improved with increasing the irradiation time.

Table 3.

Mechanical properties of irradiated HDPE/CaCO₃ composites.

Irradiation time (min)	Tensile strength (MPa)	Impact strength (J m−1)
0	25.7	72
5	28.6	138
10	29.3	275
15	29.6	340
20	30.3	416

HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

TGA of irradiated HDPE/CaCO₃ composites

The TG-derivative TG curves of irradiated HDPE/CaCO₃ composites are shown in Figures 6 and 7, and their analysis data are listed in Tables 4 and 5, respectively. The thermal decomposition temperature of irradiated HDPE/CaCO₃ composites was slightly lower than that of the HDPE/CaCO₃ composites. The temperature of the melting process for irradiated HDPE/CaCO₃ composites was around 200°C; therefore, the melting process was not influenced.

Figure 6.

TG-DTG curves (under N₂ atmosphere) of irradiated HDPE/CaCO₃ composites for 0 min (a) 15 min (b), and 20 min (c). TG-DTG: thermogravimetric–derivative thermogravimetric; N₂: nitrogen; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Figure 7.

TG-DTG curves (in air atmosphere) of irradiated HDPE/CaCO₃ composites for 0 min (a), 15 min (b), and 20 min (c). TG-DTG: thermogravimetric–derivative thermogravimetric; N₂: nitrogen; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Table 4.

TGA (under N₂ atmosphere) data for irradiated HDPE/CaCO₃ composites.

Irradiation time (min)	Ti (°C)	Tp (°C)
0	447.6	477.5
15	441.0	476.4
20	440.9	475.7

TGA: thermogravimetric analysis; N₂: nitrogen; T_i: initial weight loss temperature; T_p: maximum weight loss temperature; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Table 5.

TGA (air atmosphere) data for irradiated HDPE/CaCO₃ composites.

Irradiation time (min)	Ti (°C)	Tp (°C)
0	365.1	434.2
15	363.4	418.4
20	362.2	412.2

TGA: thermogravimetric analysis; T_i: initial weight loss temperature; T_p: maximum weight loss temperature; HDPE: high-density polyethylene; CaCO₃: calcium carbonate.

Conclusions

The groups of C=O and C–O were quickly introduced onto HDPE chains by ultraviolet irradiation in ozone atmosphere. The content of the groups increased with increasing the irradiation time. The irradiated HDPE was used to prepare composites by melting blend with CaCO₃. Compared with the HDPE/CaCO₃ composites, the crystal structure of the HDPE in the irradiated HDPE/CaCO₃ composites did not change in an orthorhombic structure, while its melting temperature and crystallinity decreased. With increasing the irradiation time, the degree of dispersion of CaCO₃ and the interfacial interaction between CaCO₃ and HDPE in the irradiated HDPE/CaCO₃ composites heightened. The mechanical properties (especially impact strength) of the composites increased, while the thermal stability of the composites decreased.