Ningjing Wu* and
Zhaoxia Xiu
Key Laboratory of Rubber-Plastics, Ministry of Education, Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao City 266042, People's Republic of China. E-mail: ningjing_wu@qust.edu.cn; ningjing_20132013@163.com; Tel: +86-0532-84022420
First published on 28th May 2015
Surface microencapsulated aluminum hypophosphite (SiAHP) was successfully prepared via the condensation polymerization of N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane. The notched impact strength of the ABS/SiAHP composites was significantly enhanced compared to the corresponding ABS/AHP composites because the microencapsulated SiAHP improved the compatibility of SiAHP and the ABS matrix, and the vertical burning rate of the ABS composite with only 22.0 wt% SiAHP achieved V-0. The cone calorimeter tests demonstrated that the peak heat release rate (PHRR) and peak smoke production rate (PSPR) values of the ABS/22 wt% SiAHP composite were decreased by 81.1% and 49.5%, respectively, compared to those of the ABS/22 wt% AHP composite. Moreover, the total heat release (THR) and the total smoke production (TSR) values of the ABS/SiAHP composites were all lower than those of the ABS/AHP composites. These results clearly indicated that the silicone microencapsulation modification of SiAHP not only enhanced the flame retardancy efficiency of the FR ABS/SiAHP composite but also effectively restrained the smoke production rate of the ABS. A comparison of digital photographs and SEM images of the residues of the ABS/AHP and ABS/SiAHP composites after the cone calorimeter tests revealed that the residue of the ABS/SiAHP composites exhibited a denser and more compact surface char layer structure than that of the ABS/AHP composite. Energy-dispersive X-ray spectroscopy (EDS) measurement indicated that SiAHP more effectively promoted the carbon formation in the FR ABS composite at the surface compared to AHP. The three-dimensional compact char layer network containing C and Si effectively improved the flame retardancy of the ABS/SiAHP composite. Therefore, the flame retardancy of the ABS/SiAHP composite was attributed more to condensed-phase mechanisms than the flame retardancy of the ABS/AHP composite.
Hypophosphite salts and salts of alkylphosphinic acid have been used as new effective flame retardants because they exhibit good flame retardant properties, thermal stability and water resistance.8–22 The salts of alkylphosphinate and hypophosphite are promising flame retardants for applications in engineered plastics and other heterochain polymers, such as poly(1,4-butylene terephthalate) (PBT),8–10 polyamide-6 (PA6),11,12 polyamide 66 (PA66),13 polylactic acid (PLA),14–16 polyvinyl alcohol (PVA),17 polyurethane (PU).18 The combination of different flame retardant systems, including aluminum hypophosphinic acid and the alkyl phosphinic acid salt,19 aluminum phenyl hypophosphinic acid and melamine polyphosphate,20,21 melamine phosphate and ammonium phosphate and zinc borate composite,13 effectively improved the flame retardancy of glass-fiber reinforced polyamide 6 (GFPA6) and polyamide 66 (GPPA66). The flame retarded poly(butylene succinate) (PBS) in combination with silica as a synergistic agent exhibited good flame retardancy and antidripping properties.22 The combinations of aluminum hypophosphate (AHP) and melamine cyanurate,23 aluminum hypophosphate and metal oxide,24 aluminum hypophosphate and POSS25 have also enhanced the flame retardant efficiency of FR PBT or FR PET composites.
Because of the low flame retardancy efficiency of the polyolefin, there are few literatures about salts of alkylphosphinic acid and hypophosphite salts employed as flame retardants on polyolefin. Aluminum hypophosphate (AHP) was ever used in the flame retardant polystyrene (PS)26 and ethylene-propylene-diene monomer rubber (EPDM).27 When the content of AHP reached 25 wt%, the LOI of PS/AHP composite was 25.6% and it passed UL-94 V-0 rating. The incorporation of nanosilica improved the flame-retardancy and the mechanical properties of the EPDM/AHP composite. Three metal hypophosphites, including aluminum hypophosphite (AP), magnesium hypophosphite (MP), and calcium hypophosphite (CP) were applied to flame retarded ABS. The results indicated that AHP could endow the best flame retardancy for ABS with a UL-94 V-0 rating and LOI value of 25.1%. The flame-retardant mechanism was attributed to the formation of a two-layer protective barrier consisting of an organic P–O–C char layer and inorganic layer to insulate material from fire and oxygen in the condensed phase, and the generation of PO˙ and P˙ to capture the reactive radicals in the vapor phase.28 In our previous paper, ABS composites with aluminum hypophosphite (AHP) and different synergistic agents presented good flame retardancy and different synergistic flame retardancy mechanisms.29 Because AHP is one kind of inorganic phosphorous-containing flame retardant, its surface property is different from ABS matrix, thus the incompatibility of ABS matrix and AHP and uneven dispersion of AHP particles in ABS matrix leads to the great decrease of toughening property of FR ABS/AHP composites. In addition, toxic gases, such as PH3, are released during the decomposition of AHP. These issues have thus hindered the development of hypophosphite salts in flame retardant polyolefins. Therefore, modifying the surface of aluminum hypophosphite and improving the compatibility between the flame retardant and the polymer are important for the large-scale application and development of the hypophosphite salt in the high-performance flame retardant polyolefins.
The surface microencapsulated modification of flame retardants has been demonstrated to be an effective method for improving the interfacial adhesion between the polymer matrix and the flame retardant.30–32 Microencapsulated red phosphorus using melamine cyanurate not only exhibited high flame retardant efficiency but also improved the compatibility between the PA66 or GFPA66 matrix and the flame retardant.33 Moreover, the microencapsulation of red phosphorus using melamine-formaldehyde also reduced the solubility of the phosphorus in water and reduced the emission of phosphine (PH3) during the burning process.34 Microencapsulated magnesium hydroxide modified with urea formaldehyde and melamine formaldehyde resins greatly improved the flame retardancy of EVA.35 Microencapsulated ammonium polyphosphate modified with melamine formaldehyde resin altered the flame retardancy mechanisms and improved the mechanical properties of PP.36
The study of microencapsulated salts of alkylphosphinic acid and hypophosphite has rarely been reported. In this study, aluminum hypophosphite (AHP) was microencapsulated by N-(β-aminoethyl)-γ-aminopropylmethyl siloxane with the aim of enhancing both the flame retardancy efficiency of AHP and the compatibility between the ABS matrix and AHP. The structure of silicone microencapsulated-aluminum hypophosphite (SiAHP) was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) measurements. The mechanical properties and morphologies of the FR ABS/AHP composites and FR ABS/SiAHP composites were compared. The flame retardancy of the FR ABS composites was systematically investigated using a limiting oxygen index (LOI) test, a UL-94 test and a cone calorimeter test. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were further used to investigate the flame retardancy mechanisms of ABS/SiAHP composites.
Thermogravimetric Fourier-transform infrared spectroscopy (TG-FTIR) measurements of the samples were performed using a TGA-7 type thermo-analysis instrument (Perkin-Elmer Co., USA) linked to the Vector 33 FTIR spectrophotometer (Bruker Company, Germany). The samples were measured in an alumina crucible with a mass of approximately 5.5 ± 0.3 mg; the measurements were performed from room temperature to 800 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere.
Flexural tests were conducted according to GB/T 9341-2000. Tensile tests were conducted at a crosshead speed of 20 mm min−1 according to GB/T 1040.1-2006. The Izod impact properties were tested according to GB/T 1843-2008 and the depth of the nick was 2 mm.
The limited oxygen index (LOI) was measured using a HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China) according to the standard oxygen index test ASTM D2863. The specimen dimensions were 130 × 6.5 × 3.2 mm3.
UL94 vertical burning tests were performed using a vertical burning instrument (CFZ-1 type, Jiangning Analysis Instrument Co., China) according to UL94 test ASTM D3801-2010. The dimensions of the samples were 130 × 13 × 3.2 mm3.
The cone-calorimeter tests were performed on a cone calorimeter (Fire Testing Technology, U.K.) according to ASTM E1354/ISO 5660. The dimensions of the samples were 100 × 100 × 3.2 mm3. Each specimen was wrapped in aluminum foil and exposed horizontally to a 50 kW m−2 external heat flux.
Scanning electron microscopy (SEM) was used to observe the surface and inner morphologies of the residue samples obtained from the cone-calorimeter tests. The observations were performed on a JSM-6700F (Japan Electronics Corp.) scanning electron microscope. The specimens were coated with a conductive layer of gold prior to imaging.
Energy-dispersive X-ray spectroscopy (EDS) was performed to obtain the relative elemental compositions of the residue samples at the surface and inner obtained from the cone-calorimeter test using an EDS apparatus (INCA type, British Oxford Instrument Co.).
Transmission electron microscopy (TEM) was conducted to investigate the microstructures of the microencapsulated SiAHP using a JEM-2100 (Japan Electronics Corp.) transmission electron microscope. The specimens were prepared in 0.1% (w/w) water dispersion and dropped onto copper grids.
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Fig. 3 (a) FT-IR spectra of AHP and SiAHP particles. (b) Enlarged FT-IR spectra in the range of 1750–1250 cm−1, (c) 1200–1000 cm−1. |
Thermal degradation analysis curves of AHP and SiAHP under a nitrogen atmosphere are shown in Fig. 4 and the related data are listed in Table 1. The onset decomposition temperatures (T5%) of AHP and SiAHP were 327.2 °C, and 319.0 °C, respectively. AHP exhibited three temperatures of the maximum weight-loss rate (Tmax): 339.9, 422.3 and 454.9 °C, whereas SiAHP exhibited two Tmax at approximately 333.9 and 427.1 °C in Fig. 4(B). The residue of SiAHP at 700 °C was 76.4%, which was higher than that of AHP. These results indicated that the incorporation of the silicone microencapsulation minimally influenced on the thermal stability of SiAHP.
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Fig. 4 TGA (A) and DTG (B) curves of AHP and SiAHP in a nitrogen atmosphere (heating rate: 10 °C min−1). |
Samples | T5% (°C) | Tmax1 (°C) | Tmax2 (°C) | Tmax3 (°C) | Total residue yield at 700 °C (wt%) |
---|---|---|---|---|---|
AHP | 327.2 | 339.9 | 422.3 | 454.9 | 75.2 |
SiAHP | 319.0 | 333.9 | 427.1 | — | 76.4 |
ABS/25 wt% AHP | 360.3 | 355.2 | 443.3 | — | 19.9 |
ABS/25 wt% SiAHP | 344.4 | 338.3 | 440.3 | — | 20.3 |
TG-FTIR measurements that provide direct identification of the evolved gaseous products contribute to the understanding of the influence of the silicone microencapsulation modification on the thermal degradation process of AHP. The FT-IR spectra of the thermal decomposition gaseous products for SiAHP are shown in Fig. 5(a). The characteristic peak of P–H at 2400 cm−1 appeared at 332.6 °C, and the peak intensity increased with increasing temperature. To clearly observe the thermal decomposition process of SiAHP, the spectra of SiAHP were enlarged in Fig. 5(b) in the range of 2600–2200 cm−1 and eliminated the influence of CO and CO2 during the TGA measurements. When the temperature was increased to 348.0 °C, the characteristic peak intensity at 2400 cm−1 reached approximately maximum, which was attributed to the further release of PH3 at the thermal decomposition stage of AHP. When the temperature was increased to 414.3 °C, the characteristic peak intensity decreased because of the decrease in the released amount of PH3. Compared to the reported literatures,28,29 the silicone microencapsulation modification of SiAHP has little influence on the thermal degradation process of AHP.
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Fig. 5 TG-FTIR spectra of gaseous product of SiAHP in the thermal degradation process (a) 3500–800 cm−1, (b) 2500–2200 cm−1. |
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Fig. 6 TGA (A) and DTG (B) curves of ABS/AHP and ABS/SiAHP composites in a nitrogen atmosphere (heating rate: 10 °C min−1). |
The mechanical properties of the ABS and FR ABS composites are listed in Table 2. Compared to the tensile strength and flexural strength of neat ABS, those of the FR ABS composites with 22 wt% and 25 wt% AHP exhibited no obvious changes, whereas the tensile strength and flexural strength of the FR ABS composites with 22.0 wt% and 25.0 wt% SiAHP exhibited a slight increase. However, the notched impact strength and the elongation yield at break of ABS/AHP composite dramatically decreased. In addition, the notched impact strength of the ABS/22 wt% SiAHP composite dramatically increased from 3.3 kJ m−2 of ABS/22 wt% AHP composite to 7.5 kJ m−2. Similarly, the notched strength for the FR ABS composite with 25.0 wt% SiAHP improved by approximately twofold compared to that of the ABS/25.0 wt% AHP composites. These results indicated that the silicone microencapsulation of SiAHP promoted the enhancement of the notched impact strength of the ABS/SiAHP composites.
Sample | Flexural strength (MPa) | Tensile strength (MPa) | Elongation yield at break (%) | Notched impact strength (kJ m−2) |
---|---|---|---|---|
ABS | 62.6 ± 0.5 | 38.0 ± 0.6 | 10.0 ± 1.5 | 17.0 ± 0.3 |
ABS/22 wt% AHP | 63.1 ± 0.4 | 37.6 ± 0.4 | 2.0 ± 0.2 | 3.3 ± 0.2 |
ABS/25 wt% AHP | 61.1 ± 0.5 | 36.1 ± 0.3 | 1.4 ± 0.1 | 2.1 ± 0.3 |
ABS/22 wt% SiAHP | 64.5 ± 0.3 | 39.5 ± 0.5 | 2.1 ± 0.1 | 7.5 ± 0.3 |
ABS/25 wt% SiAHP | 64.7 ± 0.4 | 38.2 ± 0.4 | 2.0 ± 0.2 | 6.9 ± 0.2 |
To investigate the effect of microencapsulated SiAHP on the mechanical properties of the FR ABS composites, we observed the surface morphologies of the ABS/25 wt% AHP and ABS/25 wt% SiAHP composites at the fracture using SEM analysis. As shown in Fig. 7, the bright and irregular AHP particles were dispersed in the ABS/AHP composites and most of AHP particles were aggregated in the FR ABS composites in Fig. 7(A). The dimensions of the AHP white particles were determined to be from 500 nm to 1um and the interface between AHP particles and ABS matrix was clear, as shown in Fig. 7(B). In the case of the ABS/25 wt% SiAHP composites, most of SiAHP particles were relatively uniformly dispersed in the ABS matrix in Fig. 7(C). The dimensions of the white AHP particles were about 500 nm and the interface between SiAHP particles and ABS matrix was vague, as shown in Fig. 7(D). These results indicated that the silicone microencapsulation modification of SiAHP improved the dispersion in ABS matrix and enhanced the interfacial adhesion between the flame retardant and the ABS matrix. It is considered that the amino group of silicone and the cyano group of the ABS matrix have similar polarities, which promoted the improvement of the compatibility between the ABS matrix and the SiAHP particles.
Samples | ABS (wt%) | AHP (wt%) | SiAHP (wt%) | LOI (%) | UL-94 classes | Smoke |
---|---|---|---|---|---|---|
ABS/22 wt% AHP | 78 | 22 | 24.5 | V-1 | Light | |
ABS/25 wt% AHP | 75 | 25 | 25.0 | V-0 | Light | |
ABS/22 wt% SiAHP | 78 | 22 | 25.0 | V-0 | Lighter | |
ABS/25 wt% SiAHP | 75 | 25 | 25.5 | V-0 | Lighter |
The cone-calorimeter test data for the flame retardant ABS composites are listed in Table 4. The peak heat release rate (PHRR) value of the ABS/22 wt% SiAHP composite was 213.7 kW m−2. With the increasing of SiAHP content, the PHRR value of the ABS/25 wt% SiAHP composite was decreased to 195.0 kW m−2, which was reduced by 68.6% compared to that of the ABS/25 wt% AHP composite. The heat release rate (HRR) and total heat release (THR) curves of the flame retardant ABS are shown in Fig. 8. It clearly indicated that the HRR and THR values of the ABS/SiAHP composites were decreased with the increasing of SiAHP content in Fig. 8(a) and (b). Moreover, with the incorporation of SiAHP, the TPHRR values of the ABS/22 wt% SiAHP and ABS/25 wt% SiAHP composites were delayed to 265.0 s and 307.5 s, respectively. These cone-calorimeter test data indicated that surface microencapsulated SiAHP could effectively enhance the flame retardancy of ABS composites.
Sample | ABS/22 wt% AHP | AHP/25 wt% AHP | ABS/22 wt% SiAHP | ABS/25 wt% SiAHP |
---|---|---|---|---|
TTI (s) | 12.5 ± 0.5 | 12.0 ± 1.0 | 11.5 ± 0.5 | 13.0 ± 1.0 |
PHRR (kW m−2) | 387.0 ± 27.7 | 279.5 ± 1.0 | 213.7 ± 7.8 | 195.0 ± 2.5 |
TPHRR (s) | 245 ± 15.0 | 257.5 ± 12.5 | 265.0 ± 5.0 | 307.5 ± 7.5 |
THR (MJ m−2) | 125.8 ± 14.2 | 99.0 ± 9.5 | 101.6 ± 5.6 | 96.5 ± 5.2 |
PMLR (g s−1) | 0.172 ± 0.005 | 0.171 ± 0.005 | 0.129 ± 0.002 | 0.139 ± 0.001 |
CR (%) | 12.3 ± 1.6 | 13.0 ± 0.3 | 20.8 ± 0.1 | 23.0 ± 0.1 |
PSPR (m2 s−1) | 0.139 ± 0.002 | 0.130 ± 0.005 | 0.093 ± 0.002 | 0.086 ± 0.002 |
FGR (kW m−2 s) | 1.58 | 1.09 | 0.80 | 0.54 |
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Fig. 8 HRR (a) and THR (b) curves of ABS/AHP and ABS/SiAHP composites during the cone calorimeter test. |
The peak mass loss rate (PMLR) value of ABS composite with 22 wt% SiAHP was 0.129 g s−1, as shown in Table 4, which was approximately reduced by 25% compared to that of ABS/22 wt% AHP composite. The char residue yield (CR) curves of the FR ABS composites are shown in Fig. 9. The CR values of the ABS/22 wt% SiAHP composite after the cone calorimeter test was 20.8%. With the increasing of SiAHP content, the CR value of the ABS/SiAHP composites was increased to 23.0%, which was increased by 43.5% compared to that of ABS/25 wt% AHP composites. These results indicated that the silicone microencapsulated SiAHP effectively promoted the formation of the residues of the FR ABS composites.
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Fig. 9 Char reside yield curves of ABS/AHP and ABS/SiAHP composites during the cone calorimeter test. |
The smoke production rate (SPR) and total smoke production (TSP) curves of flame retardant ABS are shown in Fig. 10. For FR ABS/SiAHP composites, the TSR values of the FR ABS composites were decreased with the increasing of SiAHP content, and the TSR values of ABS/22 wt% SiAHP and ABS/25 wt% SiAHP composites were lower than those of the ABS/AHP composites with the same flame retardant content. As shown in Table 4, the peak smoke production rate (PSPR) values of the ABS/22 wt% SiAHP and ABS/25 wt% SiAHP composites was 0.093 m2 s−1 and 0.086 m2 s−1, which represented reductions of approximately 49.5% and 51.2% compared to those of ABS/22 wt% AHP and ABS/25 wt% AHP composites, respectively. These results demonstrated microencapsulated SiAHP more efficiently suppressed the smoke production of ABS composites compared to AHP in the FR ABS composites.
Generally, a lower fire growth rate (FGR) value indicates that the time to flashover is delayed. For the ABS/22 wt% SiAHP composite, the FGR value was 0.80 kW m−2 s. With the increasing of AHP content, the FGR value of ABS/25 wt% SiAHP composites was decreased to 0.54 kW m−2 s, which was reduced by 101.9% compared to that of the FR ABS composites with the same amount of AHP, respectively. Therefore, the microencapsulation modification of SiAHP greatly reduced the FGR of the ABS/SiAHP composite. These results demonstrate that SiAHP effectively improved the fire safety of FR ABS composite.
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Fig. 11 Digital photographs of the residues of FR ABS composites after cone calorimeter test. (A) ABS/22 wt% AHP (B) ABS/25 wt% AHP (C) ABS/22 wt% SiAHP (D) ABS/25 wt% SiAHP. |
The surface and inner microstructure of the residues of the ABS/25 wt% SiAHP composites were further compared by SEM analysis. Fig. 12(A) shows the surface morphology of the residue of ABS/25 wt% SiAHP composite at 1 × 104 magnification after the cone calorimeter test. Compact and dense char layers with many relatively small cluster-particles were observed. There were some microvoids with the dimension of approximately 0.5–2 μm. In the 3 × 104 magnification micrograph in Fig. 12(A), numerous small particles were closely accumulated and connected into semi-continuous network structure, and the dimensions of these particles were approximately 200-300 nm in Fig. 12(B). From the literatures,38 these small particles were speculated to be generated from the carbonized product of ABS, indicating that the addition of SiAHP promoted the formation of small carbon particles at the surface of the ABS resin. The compact surface microstructure of the residue was more effective in inhibiting the propagation of oxygen and heat into the interior polymer. Fig. 12(C) shows the inner morphology of the ABS/SiAHP composite residue at 5 × 103 magnification. Two different types of particles were distributed in the interior of the char layers. The wrinkled particles were relatively irregular with dimensions from 1 to 5 μm. In the 1 × 104 magnification micrograph in Fig. 12(D), many smooth, round particles with dimensions ranging from 100 nm to 2 μm were distributed in the inner residue of the ABS/SiAHP composite. These different particles were speculated to be generated from the pyrolysis product of AHP and a small amount of carbonized product of ABS resin. On the basis of the aforementioned analysis, the residue morphologies at the surface and interior for the ABS/25 wt% SiAHP composites were differed substantially. The results showed that the incorporation of silicone-microencapsulated SiAHP strongly influenced on the formation of the surface and inner morphologies of the residue.
The chemical components of the surface and inner residual char layers for the ABS/25 wt% AHP and ABS/25 wt% SiAHP composites after the cone calorimeter test were investigated by EDS analysis, as shown in Table 5. For the ABS/25 wt% SiAHP composite, the average relative weight content of C in the residue at the surface was 19.89 wt%, which was higher than that for the ABS/25 wt% AHP composite, and the average relative weight percents of O, P and Al in the residue were 39.10 wt%, 32.19 wt% and 7.69 wt%, respectively, which were lower than the corresponding compositions of residue for the ABS/25 wt% AHP composite, indicating that the incorporation of the same amount of SiAHP dramatically promoted the carbon element formation of the residue at the surface. At the inner char layer of the residue, the relative composition of C was slightly higher than the corresponding composition of residue for the ABS/25 wt% AHP composite. Specifically, the relative weight content of Si of the residue was 1.13 wt%, which was higher than the corresponding composition at the inner layer of the ABS/25 wt% SiAHP composite. This result indicated that the silicon-containing component in the residue easily migrated and gathered at the surface during the cone calorimeter test. The three-dimensional compact char residue network containing more C and Si exhibited a more stable structure and effectively improved the flame retardancy of the ABS/SiAHP composite. The flame retardancy mechanism of the ABS/SiAHP composite can therefore be reasonably attributed to the occurrence of more condensed-phase mechanism in the ABS/SiAHP composite than in the ABS/AHP composite.
Sample | Average elemental composition | |||||
---|---|---|---|---|---|---|
C | O | P | Al | Si | ||
ABS/25 wt% AHP (surface) | wt% | 9.31 | 49.29 | 31.53 | 9.83 | 0 |
ABS/25 wt% AHP (inner) | wt% | 9.11 | 47.00 | 32.16 | 11.73 | 0 |
ABS/25 wt% SiAHP (surface) | wt% | 19.89 | 39.10 | 32.19 | 7.69 | 1.13 |
ABS/25 wt% SiAHP (inner) | wt% | 10.09 | 58.66 | 21.17 | 9.62 | 0.47 |
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