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Influence of polyamide 6 as a charring agent on the flame retardancy, thermal, and mechanical properties of polypropylene composites.

INTRODUCTION

Polypropylene (PP), as an important commodity plastic, has been widely applied in many fields, such as cars, electronic shells, architectural materials, furniture, interior decorations, etc. [1-3] However, PP is inflammable, and may therefore be hazardous when placed close to the open flame. It may also enhance the fire propagation due to its tendency to drip during the combustion. Application of PP in many fields is severely limited due to its flammability and dripping behaviors [4-6]. Therefore, it is very important to enhance the flame-retardant performances and reduce the emissions of smoke and poisonous gases.

One of the most effective methods for improving the flame retardancy is to incorporate intumescent flame retardants (IFRs) into the PP matrix, which has been reported [7-10]. Generally, IFRs mainly contain three components, which are acid source, blowing agent, and charring agent. On heating, fire-retardant materials form foamed cellular charred layers on their surface, which protects the underlying material from the action of the heat flux or the flame. The mechanism is based on slowing up heat and mass transfer between the gas phase and the condensed phase [11-14], In most cases, ammonium polyphosphate (APP) is chosen as acid source and blowing agent in IFRs. Our previous work [15] showed that the addition of elastomer-microencapsulated APP (MTAPP) in the flame-retarded PP composites obtained better mechanical properties and flame retardancy compared with PP/APP composites. However, it is very difficult to reach the flame retardant rating in UL-94 test for the flame-retardant composites filled with MTAPP alone due to the scarcity of charring agents.

As an indispensable component, charring agent plays a very important role that greatly influences the flame retardancy. The currently used charring agents mainly include small molecular compounds, e.g., pentaerythritol (PER), dipentaerythritol (DPER), and macromolecular agents, e.g., polyamide 6 (PA6) [16, 17], thermoplastic polyurethanes (TPU) [18], These studies show that small molecular and macromolecular charring agents possess their respective advantages and disadvantages. Generally, the former shows better charring performance but the thermal stability is relatively poor. However, the situation of macromolecular charring agents is just the opposite. TPU, as a carbonization agent, has already been introduced in the PP matrix by Montaudo et al. [19], This shows that the addition of TPU is able to promote char formation during the combustion of PP. Le Bras et al. [20] studied the synergistic effect of TPU associated with APP in the flame-retardant polyethylene/PP (PE/ PP) blends. The results show that the limiting oxygen index (LOI) values of PE/PP blends increase with a synergetic effect using a 1:3 TPU/APP ratio. PA6, as a charring agent, was also investigated in the flame-retardant ethylene-vinyl acetate copolymers (EVA) composites, and the mechanical and fire properties of EVA composites were improved due to the presence of PA6 [21]. The results show that PA6/APP has a synergetic effect generated a thermal stabilization of a phosphorocarbonaceous structure in the intumescent char which increases the efficiency of the shield. In addition, the formation of a "ceramic" acts as a protective barrier. PA6 plays the role of both a polymeric matrix and a carbonization agent because of its unique mechanical properties and processability, which can improve the mechanical properties of the IFRs/PP composites [22].

This work employs a similar approach with the use of PA6 as a charring agent to improve the flame retardancy of the flame-retardant PP/MTAPP composites. For comparison, the flame-retardant PP composites with only MTAPP as a flame retardant are also studied. The flame retardancy, thermal properties, and mechanical properties of pure PP and the flame retardant PP composites are investigated by LOI, UL-94, scanning electron microscopy (SEM), water resistance, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mechanical properties tests.

EXPERIMENTAL

Materials

Polypropylene (PP: B4808, melt flow rate = 10 g/10 min) was purchased from Yanshan Petrochemical, China. Polyamide 6 (PA6; 1013B) was purchased from Ube industries, Japan. Maleic anhydride grafted PP (PP-g-MAH, Grafting degree: 1.1 MA%) was purchased from Nanjing Deba High Polymer Material Company. Thermoplastic polyurethane (TPU, 481, Polyester grade) was obtained from Bayer Company, Germany. Ammonium polyphosphate (the degree of polymerization, n > 1000) (APP, Z201) was offered by Shifang Taifeng New Flame Retardant, China. (TPU)microencapsulated APP (MTAPP) was prepared according to the method [15]. All materials were dried at 80[degrees]C for 12 h before use.

Preparation of the Flame-Retardant PP Composites

The flame-retardant PP composites were prepared in a twin-screw extruder (TSE-20A1600-4-40, made in Nanjing, China) according to the recipe. The screw speed was 150 rpm, and the temperatures from hopper to die were 165, 190, 215, 230, 240, 250, and 240[degrees]C, respectively. The extruded pellets were dried at 80[degrees] C for 8 h prior to being injection molded into standard text samples of various sizes using an EM80-V (Chen De Plastics Machinery) injection-molding machine. The temperature profiles of the injection-molding machine were 175, 185, and 195[degrees]C from hopper to die. The formulations of the PP composites were presented in Table 1.

Measurements and Characterization

Limiting Oxygen Index. The LOI value was measured using a JF-4 type instrument (Manufactured by Jiangning Analysis Instrument Factory, Nanjing, China) on sheets 120 x 6.5 x 3 [mm.sup.3] according to the standard oxygen index test (ASTM D2863-77).

UL-94 Testing. The UL-94 vertical test was carried out on a CZF-1 type instrument (Nanjing Jiangning Analytical Instrument Factory, China) on sheets 127 x 12.7 x 2.7 [mm.sup.3] according to ASTM D635-77.

Scanning Electron Microscopy. The char formed after UL-94 test was first sputter-coated with a conductive layer, and then its surface morphology was observed by a scanning electron microscopy (Model JSM-7500F, Japan). The accelerated voltage was 20 kV.

Water Resistance. Specimens of the same size as used for the UL-94 test were put in distilled water at 75[degrees]C for 72 h. The treated specimens were subsequently dried at 80[degrees]C for 8 h, and the weight of the specimens was measured before water immersion and after drying. The migration percentage was calculated as the following equation:

Migration Percentage = [W.sub.0] - W/[W.sub.0] x 100%

Where [W.sub.0] is the initial weight of the specimens before water immersion, and W is the remaining weight of the specimens after water immersion and drying.

Thermogravimetric Analysis

TGA of samples was performed on a NETZSCH TG 209FI thermogravimetric analyzer at a heating rate of 10[degrees]C/min under nitrogen with a flow rate of 60 mL/min and the scan range was from 30 to 700[degrees]C.

Differential Scanning Calorimetry

The crystallization and melting behaviors of samples were investigated by using a differential scanning calorimetry (DSC, Netzsch STA 449C Jupiter, Germany). Every sample was heated from room temperature to 250[degrees]C at a heating rate of 10[degrees]C/min, maintained at 250[degrees]C for 5 min to erase the thermal history. The samples after premelting was cooled to room temperature at a cooling rate of 10[degrees]C/min, and then subsequently reheated to 250[degrees]C at a rate of 10[degrees]C/min. All samples were carried out in nitrogen atmosphere and the weight of each sample was about 10 mg.

The melting temperature ([T.sub.m]) of the samples was determined from the maxima of the fusion peaks. The crystallinity ([X.sub.c]) of the composites was calculated using the equation [X.sub.c] = [DELTA][H.sub.m]([w.sub.i] x [DELTA][H.sub.0]), where [DELTA][H.sub.m] is the DSC measured value of fusion enthalpy, and [w.sub.i] is the mass fraction of PP in the composites. [DELTA][H.sub.0], the enthalpy of fusion of 100% crystalline polymer, is 209 J/g of neat PP. (23)

Mechanical Properties

The tensile tests of samples were carried out on a tensile tester AGS-J (Autograph SHIMADZU) with a crosshead speed of 100 mm/min. Notched Izod impact strength was performed according to the ASTM D3420 by using a Pendulum Impact Tester (Model JBS-300B; Shandong Drick Instruments, China). The radius of notch used in the specimens was 2 mm. All the tests were performed at 23 [+ or -] 2[degrees]C. The results were the average values of at least five specimens.

RESULTS AND DISCUSSION

The LOI and UL-94 Tests

The LOI and UL-94 tests are usually used to evaluate the fire resistances of materials, especially for screening the flame retardant formulations of materials [24, 25]. The LOI values and UL-94 testing results of the flame retardant PP composites are presented in Table 1. Obviously, pure PP is easy to be flammable because its LOI value is only 18.2. When 30 wt% MTAPP is added into the PP matrix, the LOI value of the PP/MTAPP composites increases to 22.9 (sample PPMAO); however, PPMAO still failed in the UL-94 test. In order to solve this problem, PA6 is selected as a charring agent with MTAPP to improve the flame retardancy of the PP composites, where total amount of flame retardants including both MTAPP and PA6 is kept at 30 wt%. It can be clearly observed from Table 1 that the addition of PA6 has a great effect on the flammability of PP/MTAPP composites. The flame retardancy of the PP composites is improved due to the presence of PA6. The LOI value increases with increasing the content of PA6. It reaches a maximum as the amount of PA6 is 7.5 wt%, and then decreases with further increasing PA6 content due to the combustion of PA6 itself. At the loading level of only 7.5 wt% PA6, the LOI value is increased to 24.2 and the rating UL-94 V-2 is achieved for the PPMA2 sample (the weight ratio of MTAPP to PA6 is 3:1). Above results illustrate that MTAPP with suitable amount of PA6 as a charring agent exist a favorable synergistic effect on enhancing the flame retardancy of PP/MTAPP composites.

The char morphologies of pure PP, PPMAO, and PPMA2 after the UL-94 tests are shown in Fig. 1. Compared with the flame-retardant PP composites, pure PP sample has no char residue with serious dripping, indicating the flammability of the pure PP is very complete and acute. As shown in Fig. IB and C, the residue of PPMAO is in melted-form without obvious char residue, whereas the char residues of the PPMA2 sample are thick and compact. These indicate the carbonization agent PA6 can improve the char formation generated. However, the UL-94 test of PPMA2 only reaches V-2 rating. The possible reason is that the char residue of the PPMA2 sample has not more integrated and stable. The stable and compact char residues form the surface of underlying polymer materials act as excellent barriers of heat transfer and insulation [26],

The Morphology of Char Residues

To elucidate how the formation of chars affected the combustion of the flame-retardant PP composites, the residue left at 600[degrees] C after 5 min in muffle furnace are examined for change in char appearance with the aid of a camera. Figure 2 exhibits the digital photos of the char residue of PP and the flame retardant PP composites after muffle furnace test. For pure PP, there is essentially no char left. The residue left by PPMAO is very thin and there are many huge crevasses on the surface. The intumescent char layers are formed for PPMA2, and the residue left by PPMA2 is mainly composed of a thicker and more compact char in comparison with PPMAO. These results further confirm the synergistic effect between MTAPP and PA6 on improving the char formation of PP, which is better in protecting the underlying materials from further burning.

The morphology of the residual char formed after UL-94 vertical test is also investigated by SEM, as shown in Fig. 3. It can be observed from Fig. 3A that the chars of PPMAO sample exhibits very porous morphology. In addition, there are many holes and it cannot form a continuous protective char layer. Thus, the flame-retardant effect is poor, which is reflected in lower LOI value and no rating in UL-94 test. In contrast, as shown in Fig. 3B, the intumescent char residues of PPMA2 sample is much more compact and denser than PPMAO. The compact char can slow down heat and mass transfer between gas and condensed phases. In addition, this char structure can offer a good shield to prevent the underlying polymeric substrate from further attack by heat flux in a flame, which is proved by vertical flammability tests.

Water Resistance

Table 2 lists the migration percentage and the flame retardancy of the PP/MTAPP composites with various content of PA6 after water immersion at 75[degrees]C for 72 h. It can be seen from Table 2 that the migration percentage increases with increasing the content of charring agent. The LOI values of the PP/MTAPP/PA6 composites decrease with increasing the PA6 content. In addition, all samples of PPMA0~PPMA3 after water immersion do not pass through UL-94 testing rating. The small migration percentage for PP/MTAPP composites is attributed to the isolation effect of the encapsulated shell effectively protecting the APP powders from being attacked by water. Moreover, the formation of hydrogen bonds between the APP powders and TPU molecular chains improve the compatibility, which decreases the migration of the MTAPP [15]. However, it is well known that PA6 has a high solubility in water due to the strong polar group in the side chain. Therefore, PA6 is easy to migrate from the PP matrix. So the migration percentage of PP/MTAPP/PA6 composites increases with increasing the content of PA6. The PPMA2 sample can reach UL-94 V-2 rating before water treatment, whereas the composites fail in UL-94 test after water treatment. This illustrates that PP/ MTAPP/PA6 composites after water immersion have poor flame retardancy due to the great migration percentage of PA6.

Thermal Stability

To understand well the flame retardancy, the thermal degradation of the PP composites is investigated by TGA. TGA and DTG curves for the above samples under nitrogen at a heating rate of 10[degrees]C/min are shown in Figs. 4 and 5, and the related TGA data are recorded in Table 3. It can be observed that pure PP starts to decompose around 390[degrees]C, and almost decomposes completely at 480[degrees]C. In addition, pure PP leaves only 0.2% charred residues at this temperature of 700[degrees]C. It is seen from Fig. 4 and Table 3 that the initial decomposition temperature (defined as [T.sub.5wt%]) of PP/MTAPP composites is lower than that of pure PP due to the low thermal stability of MTAPP. Compared with PP/MTAPP composites, it is clearly found from Table 3 that the degradation temperatures of 5, 10, and 20 wt% mass loss ([T.sub.5wt%], [T.sub.10w%], and [T.sub.20wt%]) of PP/MTAPP/PA6 composites decrease due to the addition of PA6, and gradually decrease with increasing the content of PA6, which is possibly attributed to be the esterification between acid source and carbonization agent. However, the PP/MTAPP/PA6 composites exhibit an enhanced thermal stability at temperatures ranging from 450 to 700[degrees] C and have higher charred residues when the suitable amount of PA6 as a charring agent is incorporated into the PP/MTAPP composites. Among the weight ratio of MTAPP to PA6 3 : 1 (PPMA2) has obtained a highest charred residue which is 16.2% at 700[degrees]C. The higher charred residue is produced, the higher are the barrier properties of heat and gas transfer between PP matrix and combustion zone. Therefore, the PPMA2 sample has the higher value of LOI and the rating of the UL-94 test than other samples.

Crystallization and Melting Behaviors

Figures 6 and 7 show the DSC crystallization and melting curves of pure PP and the flame-retardant PP composites with heating or cooling rate of 10[degrees]C/min, and the corresponding DSC data are also listed in Table 4. In the case of PP/MTAPP composites, it is observed that [T.sub.m] of PP is lower than that of pure PP, however, the crystallinity increases dramatically. This dramatic increase in the crystallinity ([X.sub.c]) clearly indicates the strong nucleation efficiency of inorganic particles on the crystallization of PP [27]. The presence of a high concentration of dispersed inorganic particles will prevent large crystalline domains from forming due to limited space and restrictions movement on polymer chains [28], Such imperfections in crystalline structure may also explain the lower melting points observed from PP/MTAPP composites. For the samples of PPMA1-3, with increasing the content of PA6 and decreasing the content of MTAPP, the crystallinity of PP in the composites decreases sharply. The decrease of [X.sub.c] of PP in composites is most likely due to the decrease of inorganic particles MTAPP content, which will cause a decrease of its nucleation substrate. In addition, it is also attributed to the enhancement of interaction of PP with PA6 in amorphous domain for the block copolymer formation with increasing the content of PA6, which will limit the movement of molecular chain [29]. So, it is found that the [X.sub.c] values decrease with increasing the PA6 content.

Mechanical Properties

In general, the addition of flame retardants causes a decrease in the mechanical properties due to a poor interfacial interaction between inorganic particles and the polymer matrix, which limits the application of flame retardant materials in many fields.

Table 5 provides the mechanical properties of the pure PP and flame retardant PP composites. As listed in Table 5, the incorporation of MTAPP deteriorates seriously the tensile strength, impact strength and elongation at break of the PP matrix. However, it is found that the introduction of PA6 as a charring agent to PP/MTAPP composites leads to a great improvement in the mechanical properties, especially for the tensile strength (samples PPMA1-3). The increase in the tensile strength and impact strength is attributed to interaction of maleate part with amine and group of PA6 enabling to improve the compatibilization of PP with PA6 [29].

CONCLUSIONS

This work demonstrates the influence of PA6 as a charring agent on the flame retardancy and synergistic effect of intumescent flame-retardant PP composites. The results show that the combination of MTAPP with PA6 to flame-retarded PP composites has a high LOI value than only MTAPP to the PP matrix. Among the weight ratio of MTAPP to PA6, 3:1 is the best one and PPMA2 sample also reaches UL-94 V-2 rating. In addition, PA6, as a charring agent, can make the intumescent char layer thicker and more compact. The water resistance results illustrate that PP/MTAPP/PA6 composites have poor flame retardancy after water immersion in the case of the great migration percentage of the PA6. The TGA results reveal that introduction of PA6 into PP/MTAPP composites is an efficient way to improve the thermal stability of the composites. The DSC analysis shows that the crystallinity of the composites decreases with increasing the content of PA6 and decreasing the MTAPP content compared with that of the PP/MTAPP composites. Moreover, introduction of PA6 to PP/MTAPP composites gains a great improvement in the mechanical properties, especially for the tensile strength.

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Man Chen, (1) Mengqi Tang, (1) Yonghong Ma, (1) Xiaolang Chen, (1) Jun Qin, (2) Weidi He, (1) Zhibin Zhang (3)

(1) Key Laboratory of Advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

(2) Key Laboratory of Karst Drainage, Ministry of Education (Guizhou University), Guiyang 5500003, China

(3) School of Life Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China

Correspondence to: X. L Chen; e-mail: chenxl612@sina.com Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51003088, 21266005; contract grant sponsor: Fundamental Research Funds for the Central Universities; contract grant number: SWJTU12CX009; contract grant sponsor: Sishi Star Foundations of Southwest Jiaotong University (2011).

DOI 10.1002/pen.24075

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. The formulations of the flame-retardant PP composites.

                     MTAPP               PP-y-MAH
Samples   PP (wt%)   (wt%)   PA6 (wt%)    (wt%)     LOI (%)   UL-94

PP          100        0         0          0        18.2       F
PPMA0        70       30         0          0        22.9       F
PPMA1        65       24         6          5        23.8       F
PPMA2        65      22.5       7.5         5        24.2      V-2
PPMA3        65       20        10          5        23.1       F

TABLE 2. Migration percentage and flammability of the flame-retardant
PP composites after treatment with water at 75[degrees]C for 72 h.

Samples   [M.sub.p] (%)   LOI (%)   UL-94

PPMA0          1.4         22.5       F
PPMA1          2.1         22.8       F
PPMA2          2.9         22.0       F
PPMA3          3.5         21.4       F

TABLE 3. TGA data of the pure PP and flame-retardant PP composites.

          [T.sub.5wt%]   [T.sub.10wt%]   [T.sub.20wt%]
Samples   ([degrees]C)   ([degrees]C)    ([degrees]C)

PP            410             422             434
PPMA0         397             424             441
PPMA1         329             380             428
PPMA2         316             370             427
PPMA3         308             355             413

          [T.sub.max]    Charred residues at
Samples   ([degrees]C)   700[degrees]C (wt%)

PP            455                0.2
PPMA0         460               19.9
PPMA1         457                8.0
PPMA2         459               16.2
PPMA3         456               10.5

TABLE 4. DSC data of pure PP and the flame-retardant PP composites.

Samples   [T.sub.m] ([degrees]C)   [T.sub.c] ([degrees]C)

PP                150.9                    118.3
PPMA0             149.0                    110.9
PPMA1             150.1                    112.0
PPMA2             149.9                    112.4
PPMA3             149.7                    112.5

          [DELTA][H.sub.m]   [DELTA][H.sub.c]
Samples        (J/g)              (J/g)         [X.sub.c] (%)

PP              65.8               73.6             31.5
PPMA0           59.2               68.0             40.5
PPMA1           56.1               64.4             38.4
PPMA2           49.2               56.8             33.6
PPMA3           43.8               52.1             29.9

TABLE 5. Mechanical properties of pure PP and the flame-retardant PP
composites.

          Tensile strength       Notched impact        Elongation at
Samples        (MPa)         strength (kj/[m.sup.2])     break (%)

PP              27.1                   7.1                 479.2
PPMA0           19.3                   4.9                 264.1
PPMA1           24.5                   5.4                   8.4
PPMA2           27.5                   5.9                  12.1
PPMA3           26.6                   5.8                  10.8
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Author:Chen, Man; Tang, Mengqi; Ma, Yonghong; Chen, Xiaolang; Qin, Jun; He, Weidi; Zhang, Zhibin
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jun 1, 2015
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