Abstract
Mineral fillers (MFs) have been widely employed for polymer hybrid composites to enhance mechanical properties for the past few decades. In this study, we explored structural impacts of kaolin and talc on the mechanical, heat-resistant, rheological and flame retardant (FR) properties of the polycarbonate (PC) composites. The incorporation of these fillers into the polymer matrix enhanced mechanical properties like flexural and tensile modulus while elongation at break, impact strength and melt flow index (MFI) were reduced as a function of filler concentration. The heat deflection temperature (HDT) was slightly increased with increasing talc content. Infiltration of even 5 phr fillers dramatically enhanced the flame retardancy. Talc showed somewhat higher mechanics and FR characteristic, compared to kaolin. These PC/filler hybrid composites with enhanced mechanics and flame retardancy would be useful for a specific application by tailoring their ratios.
1 Introduction
During the past few decades, various mineral fillers (MFs) have been widely used for particulate reinforced thermoplastic composites such as talc, kaolin, wollastonite and mica (1), (2), (3), (4), (5), (6). These fillers influence mechanical properties, heat deflection temperature (HDT), melt flow index (MFI), flame retardancy, dimensional stability and crystallinity for a thermoplastic matrix with low cost (1), (2), (3), (4). For example, the incorporation of talc in thermoplastic polymers improved stiffness, strength, dimensional stability, and crystallinity, decreasing impact strength (3). In contrast, kaolin-based polymer composites such as epoxy and polypropylene exhibited an increase in toughness, maintaining other mechanical properties (4). Moreover, to enhance various properties such as mechanics, dimensional stability and coefficient of thermal expansion (CTE) with low cost, talc and kaolin have been widely used in industry (3), (4).
Polycarbonate (PC) has been a promising material for various applications such as electronic devices, lighting materials, home appliances and in the automotive industry due to its excellent balance of mechanics, transparency, dimensional stability, and electrical properties (7), (8). For the past decades, flame retardancy for these applications has been necessary to commercialize a polymeric product due to safety issues (7), (8), (9). For PCs and PC blends, bromine-, sulfonate salt-, silicone- and phosphate-based flame retardants (FRs) have been widely used with anti-dripping additives such as polytetrafluoroethylene (PTFE) when transparency is not required (8), (10), (11), (12), (13), (14). Phosphate-based FRs are less harmful to the environment and are effective as a plasticizer as well as a FR in PC. Commonly used phosphate-based FRs are triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP), and bisphenol A bis(diphenyl phosphate) (BDP) (10), (11). TPP is contributable to a gas-phase action (flame inhibition) while RDP and BDP are assigned to a condensed phase action (char) with some gas-phase action, producing a crosslinking network and acid precursors to hinder heat transfer (11). BDP shows the highest thermal stability and facilitates mechanical feeding and mixing due to its liquid phase (8), (10). Inorganic additives such as talc have also been investigated for flame retardancy in PC and PC blends (15), (16), (17), (18). In this study, we investigated the effect of talc and kaolin combined with BDP in the PC matrix on the required properties for industry such as mechanics and flame retardancy.
2 Experimental
2.1 Materials
Two PC grades (SC-1620 and SC-1080) with 62 and 8 g/10 min of MFI were supplied by Samsung SDI Chemistry Co. (South Korea). The average particle size of talc and kaolin (Lotte chem, South Korea) was 8 and 4 μm, respectively. Various additives such as antioxidants and lubricants were used for stable extrusion processing. Bisphenol A bis(diphenyl phosphate) (BDP, CAS:5945-33-5, C39H34O8P2, boiling point: 679°C) was used as a FR. All materials were used as received for extrusion without further purification, similar to what is used in industry.
2.2 Sample preparation
The two grades of PC resins, talc, kaolin, antioxidants (0.5 phr) and lubricants (0.5 phr) were mixed by a tumbler mixer at 20 rpm for 10 min and extruded via an intermeshing co-rotating twin-screw extruder with barrel temperature of 250°C at 250 rpm. The screw diameter and L/D ratio were 45 mm and 36 mm, respectively. The composites for flame retardancy tests consist of 15 phr of BDP. All components were fed all at once into the extruder vertically through a specially designed mixer at a rate of 80 kg/h. To avoid phase separation of components, a powder type and a pellet type of PCs were utilized together. The extrudates from the die were cooled down in a water bath at room temperature, and subsequently pelletized. Test specimens for tensile, flexural, UL94, Izod impact, and HDT measurements were prepared by injection molding at a range of 210–250°C of injection molding zones and at 50°C of a mold. Prior to the injection molding, the pellet-type composite samples were dried at 80°C for 4 h.
2.3 Sample characterization
MFI is more widely used for processability than spiral flow due to its convenience. MFI was determined at 220°C with a load of 5 kg, according to ASTM D1238. The tensile measurements were performed using an Instron model 5580 according to ASTM D638. Due to the embedded MFs, it was elongated at a drawing rate of 5 mm/min at room temperature. Average results were obtained from five specimens. Flexural modulus and strength were measured using a three-point flexural test in accordance with ASTM D790 at a crosshead rate of 2.8 mm/min at room temperature. The average values were taken among five specimens for each sample. Izod impact strength was performed according to ASTM D256 at a thickness of 3.2 mm at room temperature. The average value was determined among ten specimens for each sample. The morphology of the composites was observed using scanning electron microscopy (SEM) at 10.0 kV. The samples were coated with 5 nm gold. Transmission electron microscopy (TEM, JEM-1400, JEOL) and microtome (RMC, Leica) were also used at 120 kV. To determine the embedded filler content in the composites, a polymer burn-off test, called ashing was performed at 750°C for an hour. HDT was measured according to ASTM D648 at a heating rate of 2°C/min and a load of 1.82 MPa. The residue was weighed and then the weight fraction was calculated. The specific gravity was measured based on ASTM 792 water displacement method. The flame retardancy test was performed using the UL94V method where a vertically clamped composite sample was ignited twice for 10 s to the bottom of the sample with the thickness of 1.0 mm.
3 Results and discussion
Tensile and flexural results reflect the mechanics of composites for applications. The tensile modulus of talc and kaolin with and without the FR at various ratios was measured in Figure 1. The tensile modulus of each sample was increased with increasing filler loading. For example, the tensile modulus of talc+FR was increased from 2.64 to 4.69 GPa, which is contributable to the Halpin-Tsai equations (19), (20).
Tensile modulus of MF/PC composites (talc and kaolin) with and without the flame retardant (FR) at various ratios (A).
The left and right insets indicate talc- and kaolin-embedded tensile samples, respectively. From left to right tensile samples for each inset, the filler concentration increased; 0, 5, 10, 20, 30, 40 phr. Stress-strain curves for talc-(B) and kaolin-(C) embedded PC composites.
where
where E, Em, and Ep are the tensile moduli of the composite, the matrix, and the filler, respectively. ρ and ϕ are the aspect ratio and volume fraction of the filler, respectively.
The incorporation of talc into the PC composite resulted in the higher tensile modulus, compared to kaolin. The tensile modulus of talc and kaolin-embedded composites was increased from 2.08 GPa to 4.40 and 3.21 GPa, respectively, due to the difference in aspect ratio according to Einstein-Guth-Gold (EGG) equation (21) despite difficulty in determining the aspect ratio. The MF/PC composites containing BDP at most concentration ratios exhibited somewhat higher tensile modulus compared to non-FR composites due probably to plasticization effect of BDP. The stress-strain curves for both fillers indicate the ductile and the brittle behavior before and after 20 phr, respectively (Figure 1B and C).
Similarly to tensile modulus, the flexural modulus has been widely employed to evaluate mechanics of a material. The flexural modulus of all MF-filled PC composites increased almost linearly as a function of filler content (Figure 2). The increment in modulus for talc and kaolin-filled polymer composites are usually contributable to higher aspect ratio and particle orientation of these two fillers, compared to other fillers such as calcium carbonoate (3). Talc-embedded composites showed a higher flexural modulus, compared to kaolin. As kaolin tends to agglomerate, caused by its fine particles, the mobility of polymer matrix is increased, decreasing the stiffness of the composite (3). The correlation between the flexural and tensile modulus of a polymeric composite is complicated, although the ideal ratio between the two of them is 1.0 with a circular or square sample shape. The rectangular samples often show a higher flexural modulus than the tensile modulus. The rectangular test samples of MF/PC composites investigated in this study showed a higher flexural modulus in comparison with the tensile modulus.
Impact of different fillers (talc and kaolin) with and without the flame retardant (FR) at various ratios on the flexural modulus of PC composites.
HDT is another important thermal property for practical applications and often determines the surface quality of products during injection molding. The injection molding conditions are also dependent on HDT such as a mold temperature and pressures. The HDT of talc-embedded composites (circle) was slightly increased from 128.5 and 86.8 to 130.9 and 90.1°C for talc and talc+FR composites, respectively. However, the HDT for kaolin-filled composites (triangle) was not changed regardless of the FR (Figure 3).
Heat deflection temperature of MF/PC composites (talc and kaolin) with and without the flame retardant at various ratios.
For mechanical properties of electronic devices, the impact strength of the material is of importance. The Izod impact strength of talc-embedded composites was gradually decreased as a function of filler content due to the nature of the talc filler (4). The particle size of the talc filler examined in this study is also larger than that of the kaolin filler. Large particles usually act as flaws, initiating cracks (22). Talc particles tend to agglomerate, leading to a week point and thus fracture by weak stress, which is ascribed to Griffith’s theory (23). The morphology of these fillers is discussed in Figure 4. The incorporation of kaolin below 10 phr increased the impact strength from 9.9 up to 13.1 kgf cm/cm. After 10 phr, the impact strength decreased with increasing filler concentration. Most mechanical properties of MF-infiltrated PC composites increased with increasing filler content, while the impact strength was reduced, which indicates a proper amount of fillers should be determined, depending on the required mechanics of an application.
SEM morphology of MF/PC composites (talc and kaolin).
(A, C): 10 phr talc composites. (B, D): 10 phr kaolin composites. The insets of (C) and (D) indicate talc and kaolin without a polymer matrix. The scale bar in the insets is 10 μm, respectively.
MFI offers the flow behavior of molten materials. The filler-embedded composites showed reduction in MFI as the incorporation of MFs suppresses plastic flow, increasing the viscosity of a molten polymer. Talc and kaolin particles tend to slide in the composites under shear force due to their platy shape, and thus, MFI for the four samples decreased slightly at high loading (40 phr) from 52.1 and 5.3 to 39.1 and 4.1 for kaolin and kaolin+FR composites, respectively (Figure 5). Most fillers except calcium carbonate (may cause chain scission of PC) induce flow limit at a low shear rate that helps enhance flame retardancy whereas the viscosity is less deteriorated at a high shear rate like the processing shear rate (17), (24).
Melt flow index (MFI) of MF/PC composites (talc and kaolin) with and without the flame retardant at various ratios.
The tensile and flexural strength, elongation at break, HDT, specific gravity, ash, flame retandancy test are displayed in Table 1. The tensile and flexural strength slightly changed without a trend. These properties are dependent on extrusion and injection process conditions and these values tend to lightly vibrate before and after a sudden concentration point, requiring every single concentration point (3). The variance in the values is negligible. The elongation at break dropped significantly by incorporation of MFs up to 20 phr, representing that the infiltration of fillers changed the failure mode of PC from ductile to brittle behavior. The fillers hindered the ability of the PC polymer matrix to undergo a plastic deformation process. The incorporation of talc resulted in lower elongation at break, compared to kaolin. Specific gravity and char residue increased with increasing filler content, due to heavy weight of fillers and residual fillers after ashing, respectively. The infiltration of MFs significantly enhanced flame retardancy of the composites, while the PC matrix composed of FR without fillers burned and failed. Even a small amount (5 phr) of each MF generated V-0 whereas higher loading (30 and 40 phr) of kaolin led to failure due largely to agglomeration of fine kaolin particles (Figure 6). However, talc-embedded composites showed V-0 at all the concentrations regardless of agglomeration due to its several potential retardancy mechanisms. Talc typically hinders gas diffusion, protects the fire residue, and generates flow limit at low shear rate, which induces the anti-dripping effect (18). MgO from talc may react with two carboxylic acids on PC generated via Kolbe-Schmitt rearrangement at high temperature, producing Mg2+ and two carboxylates (17). The ionic interaction between Mg2+ and carboxylates may lead to a crosslinking structure. The hydroxyl group on the phosphorus of BDP formed via hydrolysis reacts with the hydroxyl group on PC caused by Fries rearrangement at high temperature (11). This condensation reaction results in a crosslinking network between BDP and PC, which is contributable to char formation. Talc may accelerate the hydrolysis and the condensation reaction, enhancing flame retardancy.
Tensile and flexural strength, elongation at break, HDT, specific gravity, ash, and flame retardancy of MF/PC composites [talc (T) and kaolin (K)] without the flame retardant, and talc+FR (TF) and kaolin+FR (KF) at various ratios.
| phr | 0 | 5 | 10 | 20 | 30 | 40 | |
|---|---|---|---|---|---|---|---|
| Tensile strength (MPa) | T | 66 | 67 | 63 | 62 | 62 | 62 |
| TF | 70 | 68 | 67 | 66 | 65 | 64 | |
| K | 66 | 63 | 62 | 63 | 62 | 63 | |
| KF | 70 | 70 | 68 | 65 | 65 | 64 | |
| Flexural strength (MPa) | T | 97 | 97 | 98 | 100 | 101 | 102 |
| TF | 111 | 111 | 113 | 109 | 108 | 106 | |
| K | 97 | 96 | 94 | 99 | 103 | 99 | |
| KF | 111 | 112 | 113 | 109 | 110 | 104 | |
| Elongation at break (%) | T | 97 | 88 | 59 | 11 | 5 | 4 |
| TF | 57 | 51 | 15 | 7 | 4 | 3 | |
| K | 97 | 92 | 71 | 31 | 5 | 4 | |
| KF | 57 | 52 | 35 | 19 | 6 | 4 | |
| Heat deflection temperature (°C) | T | 128.5 | 129 | 129 | 130.6 | 130.6 | 130.9 |
| TF | 86.8 | 86.8 | 87.3 | 89 | 89.3 | 90.1 | |
| K | 128.5 | 127.6 | 128.3 | 127.9 | 128.8 | 128.6 | |
| KF | 86.8 | 86.9 | 87.1 | 87 | 87.1 | 87 | |
| Specific gravity | T | 1.217 | 1.241 | 1.295 | 1.358 | 1.407 | 1.461 |
| TF | 1.205 | 1.233 | 1.258 | 1.290 | 1.339 | 1.392 | |
| K | 1.217 | 1.256 | 1.288 | 1.359 | 1.419 | 1.471 | |
| KF | 1.205 | 1.229 | 1.263 | 1.303 | 1.349 | 1.389 | |
| Char residue (%) | T | 0 | 4.5 | 8.9 | 16.3 | 22.8 | 28.7 |
| TF | 0 | 4.1 | 7.9 | 14.9 | 20.2 | 25.5 | |
| K | 0 | 4.4 | 8.8 | 16.2 | 23.0 | 28.1 | |
| KF | 0 | 4.0 | 8.0 | 15.0 | 20.6 | 24.9 | |
| UL94 (s) | TF | Fail | V-0 (24) | V-0 (19) | V-0 (18) | V-0 (20) | V-0 (25) |
| KF | Fail | V-0 (27) | V-0 (29) | V-0 (33) | Fail | Fail |
TEM morphology of MF/PC composites (talc and kaolin).
(A, B and C): 5, 20 and 40 phr talc composites, respectively. (D, E and F): 10, 20 and 40 phr kaolin composites, respectively. The scale bar is 5 μm.
Figure 4 shows the fracture surface morphology of talc and kaolin particles in the polymer matrix. Although agglomeration of some talc and kaolin particles was observed, the particles were well-dispersed in the PC polymer matrix. Fine particles were caused by the shear force during the intermeshing twin-screw extrusion. It should be noted that there is interplay between breaking particles and agglomeration of finer particles.
Due to the limited capability of SEM to observe agglomeration of fillers, TEM was utilized to correlate the impact strength result (Figure 7) with the composite morphology (Figure 6). The talc-embedded PC composites showed large talc agglomeration at all the composites whereas 10 phr kaolin-infiltrated PC composite indicates a good dispersion with tiny particles <1 μm and above 10 phr, agglomeration was observed. This may lead to the enhanced impact strength of kaolin-embedded composites with <10 phr. When a polymeric material is deformed or damaged, shear yielding and crazing compete. Ductile-brittle transition occurs (large shift) at a critical value of surface-to-surface interparticle distance (ligament thickness) (25), (26), (27), (28), (29), which is typically a submicron distance. In addition, below ca. 5 μm of a particle size, debonding (small shift) at the interface between fillers and polymeric matrix may occur, inducing shear yielding behavior. The cavity at the interfaces helps reduce sensitivity towards crazing (30). In contrast, large particles deteriorate shear yielding, facilitating crazing. The 10 phr kaolin-infiltrated PC composite (Figure 6D) shows well-dispersed submicron particles, which may result in shear yielding, enhancing the impact strength. It should be noted that each filler particle sizes were finer than the pristine filler particles and the broken fillers were caused by the intermeshing co-rotating twin-screw.
Izod impact strength of MF/PC composites (talc and kaolin) with and without the flame retardant at various ratios.
4 Conclusion
Incorporation of kaolin and talc into the PC matrix enhanced mechanical, heat-resistant, rheological and FR properties of the PC composites. The tensile and flexural modulus of 40 phr talc-filled PC composites was increased from 2.06 and 2.33 to 4.40 and 5.08 GPa, respectively, while that of kaolin-filled composites was enhanced from 2.06 and 2.33 to 3.21 and 3.97 GPa, respectively. However, the Izod impact strength was reduced from 9.9 to 4.1 and 2.5 kgf cm/cm for 40 phr talc and kaolin in the composites, respectively, whereas 5 and 10 phr kaolin showed higher impact strength: 12.9 and 13.1 kgf cm/cm, respectively, compared to pure PCs. The HDT was slightly increased with increasing talc content. V-0 was achieved with addition of talc and kaolin by UL 94 FR tests. Talc showed somewhat higher mechanics and FR characteristic, compared to kaolin. These hybrid composites can be useful for applications accepting somewhat lower impact strength, but requiring higher mechanics and excellent flame retardancy.
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