3.1.1. Basic Morphology and Bending Properties
Micrographs of impact fractured surfaces of the BF/Talc/HDPE composites at two different filler combination levels are shown in
Figure 1. For composites with more talc loading (
Figure 1a), the bulk of failure occurred due to the talc-particle pull-out from the matrix. The interfacial interactions between the talc and matrix were not strong enough to resist the pulling-out force during the fracturing process. A poor interface between talc and HDPE resulted from the existence of unmodified talc particles that did not allow much of the interaction among the two and made the particle slippage easy during the pulling-out. In general, BFs were well distributed in the matrix (
Figure 1b), with a good interfacial bonding between the fiber and HDPE matrix. Most BFs were well in-bedded in the HDPE matrix as observed from sample fracture surface. Static mechanical properties of the BF/Talc/HDPE composites are summarized in
Table 1.
The data showed that the incorporation of BFs into Talc/HDPE composites improved the flexural properties, tensile modulus, and impact strength of the hybrid composites. Flexural properties of BF/HDPE composites exhibited an increasing trend as the BF content increased. The use of well dispersed and high strength BFs accounted for the improved flexural properties of the hybrid composites. In addition, the good interface interaction between BF and HDPE matrix, as observed from the SEM image, effectively helped transfer stresses from plastic matrix to the reinforcing fiber. Tensile modulus of BF/Talc/HDPE hybrid composites also increased as the BF content increased. The tensile strength of hybrid composites did not show additional increase the BF content increased. As the BF content increased, the impact strength of composites also showed an increasing trend, indicating a better interface interaction between BFs and HDPE compared to that between talc and HDPE.
Figure 1.
Morphology of basalt fiber (BF) and talc filled high density polyethylene (HDPE) composites. (a) BF/Talc = 4:36 wt % and (b) BF/Talc = 20:20 wt %.
Figure 1.
Morphology of basalt fiber (BF) and talc filled high density polyethylene (HDPE) composites. (a) BF/Talc = 4:36 wt % and (b) BF/Talc = 20:20 wt %.
Table 1.
Mechanical properties of the basalt fiber (BF)/Talc/ high density polyethylene (HDPE) composites.
Table 1.
Mechanical properties of the basalt fiber (BF)/Talc/ high density polyethylene (HDPE) composites.
BF/Talc (wt %) | Flexural | Tensile | Impact Strength (KJ/m2) a,b |
---|
Strength (MPa) a,b | Modulus (GPa) a,b | Strength (MPa) a,b | Modulus (GPa) a,b |
---|
0/40 | 29.00 (0.80)A | 2.35 (0.08)A | 22.22 (0.56)B | 3.24 (0.17)A | 4.60 (0.39)A |
4/36 | 30.38 (0.51)B | 2.39 (0.08)AB | 21.37 (0.55)A | 3.21 (0.11)A | 4.87 (0.17)AB |
8/32 | 31.27 (0.34)C | 2.48 (0.04)BC | 21.33 (0.62)A | 3.46 (0.07)B | 5.01 (0.16)B |
12/28 | 32.16 (0.50)D | 2.49 (0.04)C | 21.53 (0.51)AB | 3.60 (0.18)B | 5.13 (0.12)B |
20/20 | 32.44 (0.55)EF | 2.58 (0.08)D | 21.79 (0.75)AB | 3.88 (0.11)C | 5.49 (0.25)C |
40/0 | 33.11 (0.37)F | 2.75 (0.05)E | 21.31 (0.27)A | 4.11 (0.09)D | 6.70 (0.25)D |
3.1.2. Surface energy Estimation
The surface energy of material is a crucial property in determining polymer wetting on the surface, which governs the interface characteristics of the two adhering surfaces [
26]. The strength of the interface between the reinforcing agent and the matrix plays an important role in determining the efficiency of stress transferred from matrix to the reinforcing agent. The strength of the interface in turn governs the overall strength of the composite. A complete wetting of the matrix to the fiber or particulate surface provides an opportunity to have an intimate contact and thus adhesion between the two. Wetting is, thus, an interfacial phenomenon that is governed by the surface energy or tension of the two interacting phases, in this case matrix (HDPE) and fiber (BF) or inorganic particulate (Talc).
Surface energy of the material is defined as sum of the two main components that are based on the molecular interactions. These are based on polar (γ
p) and nonpolar/disperse (γ
d) interaction energy components. They are referred as Lewis acid-base (γ
AB) and Liftshitz-van der Waals (γ
LW) interactions refined by Van Oss
et al. [
26]. γ
AB is further refined into the acid (
) and base (γ
Θ) components and calculated from their geometric mean [
27,
28,
29]:
The total or net surface energy (γ) is the sum of the γ
LW and γ
AB:
The surface energy values of HDPE, BF and Talc are listed in
Table 2 [
28,
30,
31,
32].
Table 2.
Surface energy parameters of HDPE, BF and Talc from published literature [
28,
30,
31,
32].
Table 2.
Surface energy parameters of HDPE, BF and Talc from published literature [28,30,31,32].
Phase | γ (mN/m) | γLW (mN/m) | (mN/m) | γΘ (mN/m) | γAB (mN/m) |
---|
HDPE | 40.9 | 32 | - | - | 8.9 |
BF | 140–240 | 61 | 159 | 11 | 83.64 |
Talc | 47.7–53.3 | 45.5 | 0.02 | 57.01 | 2.14 |
Interfacial strength of the reinforcement and matrix is well correlated with the interfacial energy and interfacial tension [
33]. A generalized expression for an estimation of the interfacial tension between the two condensed interfaces is given by Giese and Van Oss [
28,
34,
35]:
where
LW are the Liftshitz-van der Waals interactions,
and Θ are the polar interactions and
X,
Y are the interacting phases.
The interfacial strength can also be quantified from the thermodynamic work of adhesion (
WA) between the two interacting phases.
WA was related to surface tension components of the two interacting phases [
36,
37,
38].
WA is taken as the geometric mean of the polar and the nonpolar interactions given by the following Equation:
Table 3 provides the interfacial tension and thermodynamic work of adhesion between the HDPE-BF and HDPE-Talc interfaces calculated using the above equations. The theoretical values of the interfacial surface energy and
WA of the HDPE-BF interface are higher than these of the HDPE-Talc interface. This suggests positive and better interfacial interactions of HDPE with BF than with talc. A higher interfacial energy or work of adhesion led to a stronger interface between the two phases. This was reflected in the improved impact and flexural strength of the HDPE with the BF addition and is verifiable from the SEM micrographs of the fractured specimens of the hybrid composite. High γ value of the BF provides an opportunity for the low γ HDPE matrix to completely wet its surface, which led to more intimate interactions of the two surfaces. At the higher temperature (melt temperature), γ of the polymeric materials further reduces [
34], thus enhancing the wettability of HDPE. The observed stronger interfacial interaction between the HDPE and BF surface are possibly due to the contributions from high acidic surface energy (
) component of BFs. For the purpose of theoretical evaluation, the surface energy values of all the materials were adapted from the different sources in literature. The marked variation in the γ of BFs is also due to its dependence on the type of processing (grinding/milling), geographical location of resource, size of particle, ratio of exposed faces (basal/lateral, lateral faces contribute to γ
AB and basal to γ
LW) and methods of evaluation (experimental as well as theoretical). Another approximation taken for the
and
WA calculations was the assumption of equal contribution of
and (
=
) to the γ of HDPE and BFs.
It should be pointed out that the above analysis is only based on theoretical calculation using published surface energy data for the studied raw materials (i.e., HPDE, Talc, and BFs), and no actual experimental measurements were carried out in this study to verify the calculated data.
Table 3.
Calculated interfacial energy and thermodynamic work of adhesion for HDPE/Talc and HDPE-BF composites.
Table 3.
Calculated interfacial energy and thermodynamic work of adhesion for HDPE/Talc and HDPE-BF composites.
Interface Type | (mN/m) | WA (mN/m) |
---|
HDPE-Talc | 20.2 | 85.0 |
HDPE-BF | 29.9 | 142.9 |
3.1.3. DMA Properties
Figure 2 shows measured storage modulus E’, loss modulus E’’ and loss tangent, tanδ, of BF/Talc/HDPE composites with varying BF/Talc contents as a function of temperature. For the BF/Talc hybrid composites with a fixed total content of BF and Talc, a slight increase trend of storage modulus with the increased BF content in the composite was observed. The storage modulus of all BF/Talc/HDPE composite decreased with an increase in temperature, converging to a narrow range at high temperatures. The incorporation of BFs imposed more mechanical limitations than Talc, thereby reduced the mobility and the deformation of the matrix with increased temperatures. For the loss modulus of composites, E’’ increased as the BF content increased and had a peak in the transition region around 50–60 °C, known as α-relaxation of HDPE [
39]. The α-relaxation is considered as a complex multi-relaxation process due to the molecular motion of PE crystalline region [
40]. The loss modulus at this relaxation region varied with changes of BF/Talc ratio in the composite due to varying constraints on the segmental mobility of polymer molecules introduced by different fillers. For the loss tangent of composites, the BF/Talc/HDPE composites showed a slightly decreased value of tanδ as the BF content increased, indicating that the use of BFs gave the composite more prominent elastic nature compared with talc filled composites.
Figure 2.
Temperature dependence of storage modulus E' and loss modulus E" (a) and loss tangent tanδ (b) with varying composition levels of BF in BF/Talc/HDPE hybrid system. a. BF/Talc (0/40), b. BF/Talc (4/32), c. BF/Talc (12/18), and d. BF/Talc (20/20).
Figure 2.
Temperature dependence of storage modulus E' and loss modulus E" (a) and loss tangent tanδ (b) with varying composition levels of BF in BF/Talc/HDPE hybrid system. a. BF/Talc (0/40), b. BF/Talc (4/32), c. BF/Talc (12/18), and d. BF/Talc (20/20).
3.1.4. Thermal Expansion Property
The LCTE of fiber reinforced plastic composites is affected by the mismatch of high LCTE of the plastic matrix and low LCTE of fibers. A low LCTE value is a desirable property for composites in order to achieve dimensional stability. The measured LCTE values of BF/Talc/HDPE composites over two temperature ranges (
i.e., 60 to −30 °C; and −30 to 60 °C) are shown in
Figure 3 as a function of BF content in the BF/Talc mixture. The cooling and heating cycles led to almost identical LCTE values at each given BF content level. Generally, the LCTE values of BF/Talc/HDPE composites showed a decreasing trend as the BF loading level increased. The result was attributed to the lower LCTE of BFs and also to the fact that BFs were well-bonded to the plastic matrix, which posed a mechanical restraint on the opening or closing of the polymer chains during the heating or cooling cycles and thus helped decrease the overall LCTE of composite. A noticeable reduction (almost 20% in LCTE value) was observed when BF content increased from 0 to 4 wt %. The LCTE value of BF/Talc/HDPE composites with 20 wt % BFs were, respectively 86.6 × 10
−6/°C and 87.8 × 10
−6/°C from the cooling and heating cycles. However, these values are still higher than the reported LCTE values of well-made commercial WPCs filled with wood flour and other fillers. It is believed that improved interphase adhesion through surface modification of the filler can help improve the thermal expansion behaviors of filled composites with hybrid fillers [
1].
Figure 3.
Effect of BF content on LCTE value of BF/Talc/HDPE composites.
Figure 3.
Effect of BF content on LCTE value of BF/Talc/HDPE composites.