Influence of Commercial Additives and y-Irradiation on Structural and Mechanical Properties of rHDPE/rGFRP

The mentioned bone-shaped specimens were used for the uniaxial tensile test. Tensile strength was determined as the engineers' stress. The test was conducted on a Zwick/Roell Kappa 50 DS machine (ZwickRoell, Ulm, Germany) with a speed of 5 mm/min in speed and 20 Hz of sampling under 20 °C and 1010 hPa. The test was conducted in accordance with ASTM D638-14 [29].

Tensile modulus was determined based on the inclination of 24 consecutive measurements of the stress-strain test with the beginning at ε = 0.002. Re_0.2 was considered as offset yield strength, when elongation at maximum strength. The results were averaged according to normal distribution. In every series 8 samples were tested with additives and 4 specimens of irradiated ones.

In every series, 8 samples with additives were tested and 4 irradiated specimens.

Microstructure images were captured using the Keyence VHX-1000 microscope (Keyence, Osaka, Japan). Images were taken on cross-sections of bone-shaped specimens [30]. The cut was done 10 mm from the outer edge of the holder location SEM analysis was done with Hitachi Su-70 (Hitachi, Tokyo, Japan), 15 kV.

Irradiation was carried out at the Institute of Nuclear Chemistry and Technology in Warsaw. Bone-shaped specimens of rHDPE and rHDPE/rGFRP were irradiated using ⁶⁰Co Gamma Chanber 5000 (BRIT, India) source, Figure 2 a. The average dose rate was ~1.4 kGy/h. Dose was determined by standard Fricke dosimeter [31,32] using radiation yield of Fe³⁺ equal to 1.61 μmol J⁻¹, molar absorption coefficient of Fe³⁺ at 303 nm 2174 M⁻¹ cm⁻¹ and mass density of dosimetric solution 1.023 g cm⁻³, The test was conducted at 30 °C. During the process, specimens were fixed in a holder that was rotating to ensure homogeneous distribution of radiation dose in specimen volume. Dose absorbed by the samples were in range: 0 – 100 kGy. This range was taken into consideration of previous results published by Cota [33]. Tensile modulus, strain at tensile strength, and tensile strength were determined.

Fig. 2.

a)Gamma Chamber 5000 equipment; b) schematic of radioactive decay of ⁶⁰Co

Irradiated specimen names correspond to doses, as listed in Table 2.

Differential scanning calorimetry (DSC) was carried out using DSC Q1000 (TA Instruments, New Castle, PA, USA). Samples of 8 g were placed in the aluminum hermetic pan. Then it was heated from -80 °C to 200 °C (1^st heating cycle), next cooled from 200 °C to -80 °C (cooling cycle), and then heated again to 200 °C was repeated (2^nd heating cycle). Processes were conducted at a rate of 10 °C/min in a nitrogen atmosphere. Based on the experiment following parameters were determined: melting temperature (T_m), melting enthalpy (ΔH_m, 2^nd heating curve), crystallization temperature (T_c) and crystallinity degree (X_c) using formula (1): where: 1Xc[%]=ΔHmΔHm0ωHDPE*100{X_c}[\% ] = {{\Delta {H_m}} \over {\Delta H_m^0{\omega _{HDPE}}}}*100 where: ΔH_m - melting enthalpy (from 2^nd heating cycle), ΔHm0=288 J/g[34]−\Delta H_m^0 = 288{\rm{J}}/{\rm{g}}[34] - , ω_HDPE - weight fraction of HDPE in composites.

The sample containing 1 wt% S (AX2292) shows the highest tensile modulus, 1.91 GPa, which is 3% higher than the reference material (rHDPE/rGFRP) 1.82 GPa. The relative standard deviation does not exceed 8.5%, Figure 3 a. There are no significant differences in parameters when studying additives content, Figure 3 b,c.

Fig. 3.

Mechanical parameters of rHDPE/rGFRP material with additives described below the chart: a) tensile modulus; b) tensile strength; c) offset yield strength

The highest strength is exhibited by material with maleic anhydrate additive which equals 40.08±0.14 MPa. 1 wt% of of this additive translates to a 10% strength increase, respectively 2 wt% to 12% and 3 wt% to 14%. Such a relation is not observed considering silane-based additives. The maximum relative standard deviation was 9.5%.

Offset yield strength is also highest for samples with maleic anhydrate additive, which is 16% higher than the reference sample. The maximum relative standard deviation reaches 7.6%.

S. da Silva and JRM d/Almeida [35] investigated the influence of 0, 2, and 3 wt% HDPE-alt-MAH on HDPE/PA12 blends. They reported that the addition of 2 wt% of the additive significantly improved tensile modulus by over 40%, while the 3 wt% addition led to a smaller increase. Similarly, tensile strength increased for 2 wt% content of compatibilizer and decreased for 3 wt%.

Figure 4 presents the microstructure of the composites. The non-modified composite does not exhibit higher porosity than the modified.

Fig. 4.

Surface of the materials rHDPE/rGFRP 40%: a) without additives; b) 3S; c) 3MAH

The brighter and more colorful particles are fragments of the filler.

In some areas, there are significantly more chaotic, non-linear regions, as shown in Figure 4b.

Figures 5 a-i shows the SEM images of a cross-sections of the composites. Pores are formed in the matrix around the resin and fiber elements, which can be observed as brighter areas. The cross-section surfaces do not exhibit any significant differences compared to the reference material (rHDPE/rGFRP), Figure 5 b. However, when studying a close-up view of fibers, Figure 6, shows that pores form around the fibers, suggesting that the matrix and filler create a homogeneous microstructure. The fibers in Figure 6 h-I, the sample with MAH additive, are covered with a thin polymer layer that was not picked up during grinding.

Fig. 5.

SEM images of the composite material with additives: a) rHDPE; b) rHDPE/rGFRP; c) 0.2P; d) 0.5P; e) 1S; f) 2S; g) 1MAH; h) 2MAH; i) 3MAH

Fig. 6.

SEM images of the composite material with additives close-up view on fibre: a) rHDPE; b) rHDPE/rGFRP; c) 0.2P; d) 0.5P; e) 1S; f) 2S; g) 1MAH; h) 2MAH; i) 3MAH

Analyzing failure zones, it can be concluded that composite failure was brittle. Fibers protrude from the matrix in every sample and in the vast majority are oriented in line with the stretching axis. It can be seen that MAH samples present a much more perpendicular character of structure, which is convergent with the direction of tensile test force direction. This pattern is also seen in the 2S sample, which exhibits almost comparable durability to MAH samples.

Yu S. et al. [36] examined a BF/PLA/PLA-g-MAH composition and it’s tensile strength and observed that system does not show interfacial void channels among inter-filament threads; thus, the microstructure formed a single matrix composite system when some other tested polymeric compositions do. Poor wettability can be observed, as it was reported in several previous studies on thermoplastics filled with rigid bodies [37].

Fig. 7.

Optical microscope images of the composite material: a) rHDPE/rGFRP; b) 0.2P; c)0.5P; d) 0.3S; e) 1S; f) 2S; g) 1MAH; h) 2MAH; i) 3MAH

Tensile modulus increases with increasing absorbed dose, no matter which material (with filler or without) it was, Figure 8a. A similar relation can be observed for both materials – the modulus is higher for the material with filler, which could be predicted and was tested before. Offset yield strength values do not change in such a clear way, as shown in Figure 8 b. The highest strain at maximum stress (ε_Rm) presents specimens with the lowest absorbed dose. Further, the proportional relation can be observed between εRm and absorbed dose, Figure 8 c.

Fig. 8.

Mechanical parameters of irradiated specimens: a) tensile modulus of irradiated materials; b) offset yield strength; c) strain at tensile strength

When analyzing stress-strain curves, Figure 9, significant differences can be observed. γ-irradiation does not influence the tensile modulus (similar results in this dose range are presented by Cota [33]). However, it changes the character of σ(∊) relation and tensile strength. For A material (rHDPE/rGFRP – black curve on Figure 9, it can be seen that the offset yield strength is lower. A Linear decreasing character of strain at tensile strength can be observed, in Figure 9, when the strain at break parameter rises with higher irradiation dose. Further, there is a plateau fragment for ε=0.02-0,08 (A40). For a raw material, rHDPE, the relation changes even more. The slope of the function descends softly. Considering all B series, single curves behave differently. R_{m_rHDPE}=36.5±0.3 MPa, when the average maximum strength for irradiated samples equals R_{m_B40-B100}=33.5±2 MPa. The decrease is not significant. Absorbed doses 40-100 kGy do not impact tensile strength. However, ε_Rm transfers to lower values, when elongation at break is increasing for B40 and under the influence of increasing γ-irradiation dose, decreasing from 0.6 to 0.35. Partially the same results were reported by M. Zayat et al. [38] who studied HDPE/modified sugarcane bagasse. The tensile modulus increased by over 25% for neat HDPE, and tensile strength improved at 100 kGy by circa 15%. However, there is a discrepancy when analyzing elongation at break (for doses 0-100 kGy), as it increased in this study, but decreased in the mentioned paper (by 15%).

Fig. 9.

Engineering stress-strain relationship for non-irradiated (rHDPE/rGFRP and rHDPE) and irradiated samples (A40-A100 and B40-B100).

Shershneva et. all. [39] were investigating the influence of γ-irradiation on LDPE/GF and concluded that at irradiation doses up to 60 kGy, the process of gel formation proceeded with a greater speed than destruction. They also tested the simultaneous effects of nitrile butadiene filler rubber and radiation on strength, consequently reaching similar conclusions that the material with filler has higher stiffness [39]. The presence of filler changes the rHDPE degradation mechanism, which was observed by Valandez Gonzalez [40] when studying UV irradiation influence on HDPE and HDPE/CaCO3. In our study, irradiation of the material decreases tensile strength but increases yield strength. In the mentioned studies, UV irradiation increased gel content and crystallization. Normally, the oxidation starts from the outer layers since the process takes place in the air atmosphere. There is a directed structure of the matrix that is coaxial with the direction of tensile test axial force. The cross-linking process occurred in the whole sample, hence the entire sample stretched much more evenly.

The microstructure images of the failure location show that external layers of specimens are stiffened when the core is more flexible, Figure 10, which is a consequence of receiving higher dose of radiation. Material failure starts near the resin and fibre fragments where pores are created, Figure 10 b-c. However, the comparing results to samples modified with additives, the fracture zone is more homogenous.

Fig. 10.

The surface of irradiated specimens after the tensile test: a) macro view on the degraded surface; b) close-up view on fibres; c) close-up view on resin fragment on the surface of the rHDPE/rGFRP sample after the tensile test

Failure sections of the rHDPE/rGFRP irrigated composite. The inner layers are more ductile as shown in Figure 11 b-d. Fracture of the A series samples is much more brittle than B series. A significant difference can be observed when comparing B80 and B100 samples. The surface of the B80 sample does not detach as it does for the B100 sample.

Fig. 11.

Failure locations of specimens after the tensile test: a) rHDPE; b) B40; c) B60; d) B80; e) B100; f) A40; g) A60; h) A80; i) A100

The results of the DSC analysis of the composites modified with commercial additives before the irradiation test are shown in Figure 12 and collected in Table 3. The 2^nd heating curves (Figure 12a) for all materials have single melting peak corresponds to melting of the crystal phase. It occured at 136 °C for the reference composite rHDPE/rGFRP, and it does not change after modification with additives. Similarly, the crystallization curves presented in Figure 12b have the same shape with narrow peak occur between 113-115°C for all composites. In the case of crystallinity content, a slight increase is observed for composites containing silica-based compatibilizer; however the highest increase is reached for composites modified by 3 wt% of MAH, from 61.1% up to 68.3%. It suggests that MAH increase the nucleation capacity of rHDPE or enhance the crystallization rate.

Fig. 12.

a)2^nd heating curves and b) cooling curve for analyzing composites before irradiation test

Similar conclusions were reached by A. Hassan et al. [41], who used PP-g-MA for compatibilization of the glass fibres with polypropylene. These results are consistent with the mechanical properties, which are the highest for the composite modified with 3 wt% MAH. More crystal phase in the material results in better mechanical performance. The shift of T_c towards higher temperatures usually indicates faster crystallization associated with increased mobility of the polymer matrix chains [42]. Such an increase would be expected for the 3MAH sample. However, the observed rise is insignificant (only 1 °C higher compared to the reference sample).

The result of the DSC analysis of irradiated composite samples is is presented in Figure 13 and in Table 4. There are no effect or changes about maximum up to 4 °C on the melting and crystallization temperature of the irradiation doses used. However, some changes are observed in the crystallinity content due to the applied irradiation. After exposure to 60 kGy dose the crystallinity content decreases from 61.1% to 56.7%. Similarly, H. Ahmad et al., investigated HDPE cross-linking with dicumyl peroxide (2.5 wt%), thus reducing the crystallinity content by almost half. They explained this phenomenon by creating a three-dimensional crystalline lattice and lower mobility of chains, which was previously mentioned, making it more difficult for crystallization and increasing the time required to form crystallites and the crystal network [43].

Fig. 13.

a) 2^nd heating curves and b) cooling curve for analyzing composites after irradiation test