Mechanical Properties,Morphology,Thermal Performance,Crystallization Behavior,and Kinetics of PP/Microcrystal Cellulose Composites Compatibilized by Two Different Compatibilizers

Abstract

Polypropylene (PP)/microcrystalline cellulose (MCC) composites and PP/MCC composites modified by maleic anhydride grafted PP (PP-g-MA) and methyl acrylic acid glycidyl ester grafted PP (PP-g-GMA) respectively were prepared in a twin-screw extruder. The mechanical properties, morphology, and thermal performance were investigated. The nonisothermal crystallization, melting behavior, and nonisothermal crystallization kinetics were investigated by DSC. The results indicated that the addition of MCC had led to the increase of the tensile strength, impact strength, and flexural strength of PP. PP-g-GMA modification was more conducive to the improvement in tensile strength, impact strength, and flexural strength. The three types of PP/MCC composites have higher thermal decomposition temperatures, Vicat softening temperatures, and dimensional stability. Nonisothermal crystallizations of PP/MCC composites were in accordance with tridimensional growth with heterogeneous nucleation. Meanwhile, MCC was acted as the nucleating agent in PP matrix, which increased the crystallization temperature. PP-g-GMA further increased the crystallization temperature while PP-g-MA weakened the heterogeneous nucleation effect of MCC. Avrami equation and Mo method give a satisfactory description of the crystallization kinetics process. The activation energy of crystallization, nucleation constant, and fold surface free energy of PP were markedly reduced in PP/MCC composites and its compatibilized composites. The value of F(T) systematically increased with increasing relative degree of crystallinity. The addition of microcrystalline cellulose has greatly reduced the spherulitic size of PP.

Keywords

polypropylene microcrystalline cellulose mechanical properties nonisothermal crystallization compatibilizer.

INTRODUCTION

Polypropylene (PP) has been widely used due to its attractive combination of good processability, mechanical properties, and chemical resistance. However, inadequate stiffness and high brittleness of PP have limited its versatile application to some extent. Thus, a variety of methods such as filling, blending, enhancement, and so on were adopted to improve the performance of PP [1–6]. Microcrystalline cellulose (MCC), whose amorphous regions are removed by acid hydrolysis, can be a very promising cellulosic reinforcement of polymers. MCC is presented as short rod-like or powdered particles with the size of 20–80 µm. Compared with other natural polymers, MCC possesses strong rigidity, high crystallinity, and excellent mechanical properties [7,8]. Therefore, MCC has received a great deal of attention in the preparation and modification of materials. Especially nowadays, when oil fields are becoming depleted, MCC can be used to alleviate the problems of resources and the environment as a renewable resource. Since MCC containing polyhydroxy has inherent hydrophilic property, incompatibility problems would exist when it is introduced into nonpolar PP matrix. Thus, the key issue is to improve the interface compatibility between MCC and thermoplastic resin matrix. In recent years, compatibilizers prepared by grafting polyolefins have been reported [9]. Currently, a good compatibilizer of maleic anhydride grafted polypropylene (PP-g-MA) is being widely used [10]. Due to the fact that glycidyl methacrylate (GMA) with high reactivity of epoxy groups has the advantages of owning a single high boiling point, low toxicity and corrosion for equipment, and so on, GMA grafted polyolefins has also received a lot of attention [11].

The influence of PP and various additives on the crystalline structure and crystallization behavior of PP have been reported by many researchers [12–14], however, the effects of different compatibilizers on the crystallization kinetics of PP/MCC composites have not been reported yet. Taking the actual processing of polymers into account, techniques such as injection molding and other processes are carried out under nonisothermal conditions. Therefore, the nonisothermal crystallization kinetics study is more practically significant. In this article, crystallization behaviors of PP/MCC composites at different cooling rates were investigated. Four dynamic models were employed to deal with the nonisothermal crystallization process of PP/MCC. The kinetic parameters, like the Avrami exponent, was evaluated from the data based on differential scanning calorimeter (DSC) crystallization exotherms for pure PP, PP/MCC composites, PP grafted with maleic anhydride (PP-g-MA), and methyl acrylic acid glycidyl ester grafted PP (PP-g-GMA) modified PP/MCC composites. Furthermore, polarized optical microscopy technique was used to display the effect of MCC and compatibilizer on the nucleation density of PP.

EXPERIMENT

Materials and Equipments

A commercial grade isotactic PP (H030SG, MFR = 3 g/10 min at 200°C) used in this study was supplied by Reliance Industries Limited. MCC was obtained from Guangzhou Gaoli Additives Factory. PP-g-MA and PP-g-GMA were supplied by Guangzhou Lushan Chemical Materials Co. of China with MA and GMA grafting ratio of 1.0% and MFI of 15 g/10 min.

The following equipments were used:

The twin-screw extruder: SHJ-20, Nanjin Jieya Extrusion Equipment Co., Ltd. of China. The injection molding machine: HM7DENKEY, Nissei Plastic Industrial Co., Ltd. The universal testing machine: Zwick/Roell Z005, Zwick Roell Testing Machines Pvt. Ltd. The impact testing machine: ZBC-50, China Shenzhen SANS Testing Machine Co., Ltd. The scanning electron microscope: XL30S-FEG, Netherlands Philips Ltd. The thermogravimetric Analysis: Q5000, TA Instruments USA. The vicat softening machine: GT2HV2000, Taiwan Gotech Testing Machines Co., Ltd.

Sample Preparation

PP, PP-g-MA, PP-g-GMA, and MCC were dried at 70°C for 24 h and mixed according to the composition in Table 1. The mixtures were blended in a twin-screw extruder at the temperature of 160–190°C. The screw speed was fixed at 150 rpm while the feed speed was fixed at 15 rpm. After being fully dried out, the products were injected into the standard sample with the injection mechanism, and the injection temperatures were set to 160–190°C.

Table 1

Composition of PP/MCC composites

Component	PP	PP-g-MA	PP-g-GMA	MCC
(wt %) Sample
PP	100	–	–	–
MCC2	98			2
MCC5	95			5
MCC10	90			10
MCC15	85			15
GMA5MCC2	93	–	5	2
GMA5MCC5	90	–	5	5
GMA5MCC10	85	–	5	10
GMA5MCC15	80	–	5	15
MA5MCC2	93	5	–	2
MA5MCC5	90	5	–	5
MA5MCC10	85	5	–	10
MA5MCC15	80	5	–	15

Mechanical Testing

Tensile, flexural, and Izod impact tests were carried out according to ASTM Standard. For each test as well as each type of the composites, five specimens were tested and the average values were reported. Tensile tests were conducted according to ASTM D 638 with a Universal Testing Machine at a crosshead speed of 50 mm/min. Static flexural tests were carried out according to ASTM D 790 with the same Testing Machine mentioned above at a crosshead speed of 2 mm/min. Izod Notch impact tests were conducted according to ISO 179 with a Universal Impact Testing Machine.

Microstructure Analysis

The impact specimens were frozen in liquid nitrogen for 3 h, and then quickly smashed. The fracture surfaces of the specimens were sputter-coated with gold before scanning electron microcopy (SEM) analysis. The fracture surface morphology of the composites was observed with a Philips XL-30 ESEM scanning electron microscope with an acceleration voltage of 15 kV.

DSC Characterization

A TA Instruments Q200 DSC was used to study the thermal behavior of the PP/MCC composites, and was calibrated using the melting temperature and enthalpy of a pure indium standard. About 8–9 mg of the sample was accurately weighted for DSC testing, and all measurements were performed in nitrogen atmosphere.

In nonisothermal crystallization and melting behavior characterization, a sample of the composite was rapidly heated to 220°C and held for 5 min. Subsequently, it was cooled to 60°C at the cooling rate of 10°C/min for crystallization behavior study. And then, it was reheated to 220°C at 10°C/min for melting behavior study.

In nonisothermal crystallization kinetics study, a sample of the composite was rapidly heated to 220°C and held for 5 min. Subsequently, it was cooled to 60°C at selected cooling rates ranging from 5 to 40°C/min. Each sample was used only once.

Polarized Optical Microscopy

The spherulite morphology and growth of PP/MMC composite were studied using a polarized optical microscope (POM, Zeiss) equipped with a hot stage (Linklam LTM350). The samples were hot pressed into films with the thickness of 0.02 mm. The crystallization process was recorded in the second heating scan at the rate of 0.5°C/min.

TGA Analysis

The thermal decomposition behavior of the composites was studied by a thermogravimetry (TA Instruments) in nitrogen atmosphere with the heating rate of 10°C/min.

3.1 The Effect of Compatibilizers on the Mechanical Properties of PP/MCC Composites

Figures 1–3 show the mechanical properties of unmodified PP/MCC, PP-g-MA modified PP/MCC, and PP-g-GMA modified PP/MCC composites. It can be seen that tensile strength decreases slightly with the increase of MCC content; however, it is still higher than that of pure PP on the whole. Particularly, PP/MCC composites modified by PP-g-GMA have an obvious increase in tensile strength. This indicates that compatibilizer improves the interfacial adhesion of PP and MCC, which is favorable to the mechanical properties of composites. The impact strength of unmodified composites is higher than that of PP/MCC composites modified by compatibilizer while the impact strength of composites modified by PP-g-MA and PP-g-GMA is basically equaled or slightly higher than that of pure PP. The impact strength of PP/MCC composites modified by PP-g-GMA increases obviously. The possible reason may be that the reaction occurred between the epoxy group of PP-g-GMA and hydroxyl groups on MCC particles, which is also effective in improving the compatibility of fibers and plastics. The flexural strength of composites filled with MCC and compatibilizer is better than that of pure PP, and shows a rising trend with the increasing MCC content. The flexural strength of MCC15, GMA5MCC15, and MA5MCC15 has improved by 32.6%, 43.6%, and 35.2%, respectively than those of pure PP at 15% MCC content.

Figure 1

Tensile strength of unmodified PP/MCC composites, PP-g-MA and PP-g-GMA modified PP/MCC composites.

Figure 2

Impact strength of unmodified PP/MCC composites, PP-g-MA and PP-g-GMA modified PP/MCC composites.

Figure 3

Flexural strength of unmodified PP/MCC composites, PP-g-MA and PP-g-GMA modified PP/MCC composites.

SEM Images of PP/MCC Composites

Figure 4 shows the fracture surface of unmodified PP/MCC, PP-g-MAH modified PP/MCC, and PP-g-GMA modified PP/MCC composites with 2–15 wt% of MCC. It can be seen that unmodified PP/MCC has a rough fracture surface. With increasing MCC content, the MCC agglomeration becomes more severe, and the interface is clearer between MCC particle and PP matrix, which indicates that the decrease of tensile strength is due to the bad interface between MCC and PP. Compared to unmodified PP/MCC composites, the fracture surface of modified composites is relatively smooth. On the whole, massive MCC aggregates cannot be seen, which means only a few MCC particles inlayed in PP matrix can be observed. There is a tight interface between MCC particles and PP matrix, which shows the two compatibilizers can improve the interface bond and increase the tensile strength effectively. The size of MCC particles is observed smaller in the fracture surface of PP-g-GMA modified PP/MCC composites than that of PP-g-MA modified PP/MCC composites, which indicates PP-g-GMA has a more obvious effect on MCC dispersive action and interface modification.

Figure 4

SEM photos of unmodified PP/MCC composites, PP-g-MA and PP-g-GMA modified PP/MCC composites: (a) MCC2; (b) MCC5; (c) MCC10; (d) MCC15; (e) GMA5MCC2; (f) GMA5MCC5; (g) GMA5MCC10; (h) MA5MCC15; (i) MA5MCC2; (j) MA5MCC5; (k) MA5MCC10; (i) MA5MCC15.

Thermal Stability for PP/MCC Composites

Thermal gravimetric analysis (TGA) and heat distortion temperatures of the polymer materials reveal the thermal stability at high temperatures and under conditions of use. Figure 5 shows the vicat softening temperature of unmodified PP/MCC composites, PP-g-MA modified PP/MCC composites, and PP-g-GMA modified PP/MCC composites. Figure 6 shows TG curves of unmodified PP/MCC composites, PP-g-MA modified PP/MCC composites, and PP-g-GMA modified PP/MCC composites. The related thermal stability parameters are provided in Table 2. Only one decomposition stage occurred on the TG curve of pure PP. However, there are two obvious decomposition stages on the TG curves of unmodified PP/MCC composites, PP-g-MA modified PP/MCC composites, and PP-g-GMA modified PP/MCC composites on the boundary of decomposition temperature 336°C and mass loss 20 wt%, and all the TG curves are quite similar, which shows the two compatibilizers have no effect on the heat decomposition behavior of PP/MCC composites. As is reported, there are two main stages: the first decomposition stage occurs between 270°C and 320°C due to the noncombustible gas transformation, such as carbon dioxide and oxides of carbon and silicon. The second decomposition stage occurs between 330°C and 480°C due to the splitting decomposition and combustible gas transformation [15]. The decomposition stages of PP/MCC composites are in accordance with MCC decomposition and PP decomposition, respectively. MCC decomposition slightly advanced to PP below 336°C, but above 336°C, MCC decomposition in composites delays 50°C more than that of pure PP. The possible reason is that decomposed MCC forms coke. The coke gets evenly dispersed to the surface and inside of the composites, which effectively hinders the release of PP decomposer. Thus, the temperature of fastest weight loss composites is increased to above 462°C compared to that of pure PP (430.57°C). Vicat softening temperature analysis shows that vicat softening temperature gradually increases with the increasing MCC content, which reveals that semi-rigid natural polymer MCC in composites has a protective effect on PP and improves the resistance to thermal degradation of soft and long PP leading to the improvement of thermal stability. However, the addition of the two compatibilizers as well as lowering the softening temperature increases the vicat softening temperature, which demonstrates that macromolecule compatibilizer exists on the interface between PP and MCC.

Table 2

Thermal properties of pure PP, PP/MCC, PP-g-MA, and PP-g-GMA modified PP/MCC composites with the content of 10 wt% of MCC.

Sample	Onset decomposition temperature (°C)		Decomposition peak temperature (°C)		Residual weight (wt%)	Vicat softening temperature (°C)
PP	276.05	430.57	0.69	153.0
MCC10	300.5	431.1	339.2	462.9	2.83	155.5
MA5MCC10	302.3	434.8	336.3	462.9	2.36	156.2
GMA5MCC10	308.0	436.0	334.7	462.9	2.55	157.1

Figure 5

Vicat softening temperatures of unmodified PP/MCC composites, PP-g-MA and PP-g-GMA modified PP/MCC composites.

Figure 6

TG curves of pure PP, PP/MCC, PP-g-MA, and PP-g-GMA modified PP/MCC composites with the content of 10 wt% of MCC.

Nonisothermal Crystallization and Melting Behavior

The nonisothermal crystallization and melting data of pure PP, PP/MCC composites, PP/MCC composites modified by PP-g-GMA, and PP-g-MA can be seen in Table 3, in which the crystallization enthalpy (ΔH_c) and melting enthalpy (ΔH_m) were normalized by the weight percent of PP. It can be seen that the crystallization temperatures (

T_{c}^{p}

) of PP were improved when being blended with MCC. Moreover,

T_{c}^{p}

of the composites increased with the increasing MCC content. This indicates that MCC had heterogeneous nucleation effect on PP crystallization and enabled PP crystallization at higher temperatures. Both

Δ \overset{\cdot}{}

H_c and

Δ \overset{\cdot}{}

H_m of the PP/MCC composites were smaller than those of pure PP, which might be due to the fact that the disturbance of MCC on the crystal growth of PP led to the decrease in crystallization degree of PP in the PP/MCC composites. It is also observed that with the introduction of MCC, PP/MCC composites exhibited higher crystallization peak temperature

T_{c}^{p}

and initial crystallization temperature

T_{c}^{onset}

than those of pure PP, from 111.69°C and 116.88°C of pure PP to above 116°C and 121°C. However, with the increase of the content of MCC, there was no change of

T_{c}^{p}

and

T_{c}^{onset}

. This indicates that MCC had obvious heterogeneous nucleation effect on PP crystallization. There was little effect of MCC on the melting peak temperature

T_{m}^{p}

. If we fix the content of PP-g-MA and PP-g-GMA in PP/MCC as 5 wt%,

T_{c}^{p}

and

T_{c}^{onset}

would be further increased, which indicates that PP-g-MA and PP-g-GMA also had heterogeneous nucleation effect on PP crystallization. With the increasing content of MCC,

T_{c}^{p}

of PP in modified PP/MCC composites was gradually increased.

Table 3

Nonisothermal crystallization and melting parameters of pure PP, PP/MCC, PP-g-GMA, and PP-g-MA modified PP/MCC composites.

Sample	$T_{c}^{p}$ (°C)	$T_{c}^{onset}$ (°C)	ΔH_c (J/g)	$T_{m}^{p}$ (°C)	ΔH_m (J/g)
PP	111.69	116.88	98.13	165.63	95.64
MCC2	116.93	121.26	88.39	165.21	79.72
MCC5	118.13	123.11	93.76	163.84	88.61
MCC10	120.78	125.98	95.69	163.99	83.68
MCC15	122.36	127.63	96.15	164.73	90.47
GMA5MCC2	118.36	124.19	88.20	164.02	80.40
GMA5MCC5	117.50	122.01	87.65	164.85	77.75
GMA5MCC10	122.96	127.91	81.46	163.71	76.51
GMA5MCC15	122.29	127.29	87.84	163.75	72.04
MA5MCC2	116.82	121.43	91.59	164.28	74.77
MA5MCC5	118.54	123.33	94.44	163.45	82.38
MA5MCC10	120.85	125.74	75.86	163.31	68.90
MA5MCC15	121.17	126.11	97.15	163.19	88.19

The crystallization and melting behavior of PP-g-GMA modified PP/MCC composites differed from that of PP-g-MA modified PP/MCC composites. $T_{c}^{p}$ and $T_{c}^{onset}$ of the former are both higher than those of the latter and unmodified PP/MCC composites, while the $T_{m}^{p}$ of PP/MCC composites and modified PP/MCC composites was slightly less than that of pure PP. This demonstrates that MCC and the compatibilizers are not beneficial to forming crystals with high melting point.

Nonisothermal Crystallization Kinetics

Four methods have been widely adopted to describe the nonisothermal crystallization process, that is, Avrami equation, Ozawa equation, Jeziorny equation, and Mo’s method. The double logarithmic form of Avrami equation is: [16]

(1)

where X(T) is the relative crystallinity after the crystallization time t, Z_t and n are the crystallization kinetic constant and Avrami exponent, respectively, and both are related to the rate and mechanism of crystallization.

Jeziorny [17] calibrated the crystallization kinetic constant of Avrami equation using cooling rate Φ:

(2)

where Z_c is the calibrated crystallization kinetic constant.

Ozawa derived an equation whose double logarithmic form is: [18]

(3)

where R is the cooling rate, K(T) is a function related to the overall crystallization rate that indicates how fast crystallization proceeds, and m is the Ozawa index, which is somewhat similar to the Avrami exponent and depends on the type of nucleation and growth dimensions.

Liu and co-workers [19] proposed a different equation by combining the Avrami and Ozawa equations, giving rise to the relationship between cooling rate R and crystallization time t at a given relative crystallinity:

(4)

(5)

where the kinetic parameter, F(T) = [K(T)/Z_t] ^1/m , refers to the value of the cooling rate, which has to be chosen at unit crystallization time when the measured system amounts to a certain degree of crystallinity; the Mo exponent α is the ratio of the Avrami exponent n to the Ozawa exponent m, that is, α = n/m. F(T) has a definite physical and practical meaning – the smaller the value of F(T), higher the crystallization rate becomes.

Moreover, Kissinger [20] suggested a method to determine the activation energy for the transport of the macromolecular segments to the growing surface, ΔE, by calculating the variation of T_p with the cooling rate Φ:

(6)

where R is the gas constant.

According to Equations (1)–(6), nonisothermal crystallization kinetic plots of MCC10 composite (Figure 7) were obtained and they showed a nice linearity in Avrami method, Mo method, and Kissinger method. This suggested that the above three methods may provide a satisfactory description to the composites in this study. All nonisothermal crystallization kinetic parameters of the composites with 10 wt% MCC content were listed in Table 4 according to the same processing procedures as those of MCC10 composite. It can be observed that pure PP gave a value of about 2.8 < n < 3.4, suggesting a spherulitic growth from nuclei initiated at time zero, which was in accordance with what is reported in the literature [21]. The n value of MCC10, GMA5MCC10, and MA5MCC10 composites ranged from 3.1 to 4.3, which means that MCC, PP-g-GMA, and PP-g-MA would not influence the nucleation and crystalline growth of PP significantly. With the increase of the cooling rate, the t_1/2 and T_p of pure PP, MCC10, GMA5MCC10, and MA5MCC10 composites decreased. At the same cooling rate, the t_1/2 and T_p of MCC10, GMA5MCC10, and MA5MCC10 composites were higher than those of pure PP. The F(T) and α of pure PP, MCC10, GMA5MCC10, and MA5MCC10 composites increased with the increase of X(T), suggesting that it needs higher cooling rate to reach certain X(T) at the same crystallization time. At the same X(T), the values of F(T) of pure PP were higher than that of PP/MCC composites, and there was no significant difference in the values of F(T) among PP/MCC composites, PP-g-GMA, and PP-g-MA modified PP/MCC composites. In other words, to reach the same X(T), the crystallization time needed by PP is the longest and the crystallization time of MA5MCC10 composite is the shortest, which shows the heterogeneous nucleation effect of MCC, PP-g-GMA, and PP-g-MA. Moreover, the values of ΔE for these specimens are ranked as: PP > MA5MCC10 > MCC10 > GMA5MCC10, indicating that the addition of PP-g-GMA obviously depressed the ΔE of PP in the composites and made the crystallization of PP in the composites easier on nonisothermal crystallization condition.

Table 4

Nonisothermal crystallization kinetic parameters of pure PP, PP/MCC, PP-g-GMA, and PP-g-MA modified PP/MCC composites with 10 wt% MCC.

Sample	Φ (°C/min)	n	Z _c	t_1/2 (min)	T_p (°C)	ΔE (kJ/mol)	X(T) (%)	F(T) (K/min)	α
PP	5	3.4	0.62	1.71	115.45	221.16	20	7.1	1.4
	10	3.1	0.95	0.96	111.72		40	9.3	1.4
	20	3.3	1.06	0.59	107.14		60	11.5	1.5
	30	2.8	1.07	0.48	105.44		80	14.4	1.6
	40	3.0	1.06	0.45	103.82
MCC10	5	3.2	0.58	1.87	125.62	183.69	20	2.2	1.5
	10	3.4	0.89	1.20	121.68		40	2.5	1.5
	20	3.5	1.02	0.80	114.85		60	2.7	1.5
	30	3.4	1.05	0.56	113.90		80	3.0	1.6
	40	3.6	1.05	0.49	111.33
GMA5MCC10	5	4.3	0.47	2.19	124.71	155.95	20	2.3	1.3
	10	3.8	0.93	1.05	123.29		40	2.5	1.4
	20	3.4	1.05	0.77	119.18		60	2.7	1.5
	30	3.1	1.06	0.58	115.43		80	3.0	1.5
	40	3.9	1.06	0.50	108.92
MA5MCC10	5	3.8	0.53	2.06	122.90	213.08	20	2.2	1.4
	10	3.3	0.93	1.09	121.24		40	2.4	1.4
	20	3.2	1.04	0.73	115.29		60	2.7	1.5
	30	3.3	1.05	0.60	112.69		80	2.9	1.5
	40	3.4	1.06	0.46	112.35

Figure 7

Nonisothermal crystallization kinetic plots of MCC10 composite. (a) the Plot of relative Crystallinity Versus time (b) th Plot of $\ln [- \ln (1 - X (T))]$ Versus In t (c) the Plot of In Φ Versus In t (d) the Plot of In (Φ/T_p)² Versus 1/T_p.

POM Observation

The final performance of crystalline polymer-matrix composites depends on the crystallization behavior and crystal morphology of polymer-matrix to a large degree. In this article, the crystal morphology of pure PP, unmodified PP/MCC composite, PP-g-GMA, and PP-g-MA modified PP/MCC composite at 10 wt% of MCC were observed by polarized optical microscopy (POM) (Figure 8). The spherulites with black cross extinction phenomena and the obvious crystal interface were observed in pure PP (Figure 8(a)).

Figure 8

POM micrographs of PP/MCC composite, PP-g-GMA and PP-g-MA modified PP/MCC composite with 10 wt% of MCC: (a) PP; (b) MCC10; (c) GMA5MCC10; (d) MA5MCC10.

In Figure 8, it is clear that the nucleation rates of PP/MCC composites are larger than that of PP due to the heterogeneous nucleation of MCC and compatibilizers particles. Crystal growth is ultimately limited by facing spherulite boundary, and it will lead to an irregular final structure and smaller crystalline dimension. It is well known that crystalline morphology affects the mechanical properties of a material significantly since a smaller crystal is prone to develop into a crack to absorb impact energy while a big spherulite tends to form local stress which makes the material brittle. Therefore, the crystalline morphology of PP/MCC is favorable to its mechanical properties.

Water Absorption for PP/MCC Composites

The samples (10 mm × 10 mm × 4 mm) were impregnated in the breaker filled with distilled water for 48 h at room temperature after being dried in an oven at 50°C for 24 h, and then, all the water on the sample surface was quickly wiped off with a filter. Subsequently, the sample was quickly weighed to get the weight after water absorption. The percent of weight gain after being soaked is the percentage of water absorption of the sample, as is shown in Table 3. The water absorption of composite remained unchanged with the addition of MCC containing hydroxyl groups, which indicates that MCC has good dimensional stability as the filler in wood-plastic composites. Thus, MCC is of great potential in the application of wood-plastic composites.

CONCLUSIONS

In this study, the PP/MCC composites were prepared by extrusion blending and injection molding. Also, two different types of compatibilizers, that is, PP-g-GMA and PP-g-MA, were used to modify the interface of PP/MCC composites. The results indicated that the addition of MCC led to the increase of tensile strength and impact strength of PP. PP-g-GMA modification is more conducive to the improvement in tensile strength and impact strength while PP-g-GMA modification is more favorable to improve the tensile properties and toughness. PP-g-MAH and PP-g-GMA modification improve the dispersion and interfacial adhesion between MCC and PP, which can explain why mechanical properties of composites are improved. The three types of PP/MCC composites have higher thermal decomposition temperature and Vicat softening temperature and dimensional stability. To sum up, compared to the mineral fiber reinforcements (such as glass fiber), MCC has the advantages of being lightweight, inexpensive, and high-quality as a plastic reinforcement. Four methods were used to deal with the nonisothermal crystallization process of PP/MCC composites and PP-g-GMA and PP-g-MA modified PP/MCC composites. The Avrami equation, Ozawa equation, Jeziorny’s, and Mo’s methods can give a satisfactory description of the nonisothermal crystallization behavior of PP/MCC composites. According to the results obtained by Avrami equation, the primary crystallization stage for nonisothermal melt crystallization might correspond to a three-dimensional spherical growth with heterogeneous nucleation. The values of the kinetics parameter F(T) reveal that adding MCC and compatibilizers increases the crystallization rate of PP from molten state obviously. The results of DSC and POM techniques indicate that MCC can increase the crystallization temperature and the nucleation density of PP. Thus, MCC is an effective nucleating agent for PP. When the compatibilizers of PP-g-GMA and PP-g-MA are added to PP, the crystallization rate is increased.

Footnotes

ACKNOWLEDGMENTS

This work was supported by China Guangdong scientific and technological project (No2010B080701060) and Guangdong natural science fund (No8451063201000041).

References

Yang

Zhang

X.Q.

Zhang

L.J.

Zhang

(2007). Largely Improved Toughness of PP/EPDM Blends by Adding Nano-SiO₂ Particles, Polymer , 48(3): 860–869.

Chow

W.S.

Ishak

Z.A.M.

Karger-Kocsis

(2005). Morphological and Rheological Properties of Polyamide 6/Poly(propylene)/Organoclay Nanocomposites, Macromol. Mater. Eng., 290(2): 122–127.

Rachtanapun

Selke

S.E.M.

Matuana

L.M.

(2004). Relationship Between Cell Morphology and Impact Strength of Microcellular Foamed High-Density Polythylene/Polypropylene Blends, Polym. Eng. Sci., 44(8): 1551–1560.

Zheng

Y.H.

Shen

Z.G.

Cai

C.J.

S.L.

Xing

Y.S.

(2009). Influence of Nonmetals Recycled from Waste Printed Circuit Boards on Flexural Properties and Fracture Behavior of Polypropylene Composites, Mater. Des., 30(4): 958–963.

Nourbakhsh

Ashori

Tabari

H.Z.

Rezaei

(2010). Mechanical and Thermo-Chemical Properties of Wood-Flour/Polypropylene Blends, Polym. Bull., 65(7): 691–700.

Bao

S.P.

Tjong

S.C.

(2009). Temperature and Strain Rate Dependences of Yield Stress of Polypropylene Composites Reinforced with Carbon Nanofibers, Polym. Compos., 30(12): 1749–1760.

X.F.

Chang

P.R.

J.G.

(2008). Properties of Biodegradable Thermoplastic Pea Starch/Carboxymethyl Cellulose and Pea Starch/Microcrystalline Cellulose Composites, Carbohyd. Polym., 72(3): 369–375.

Ashori

Nourbakhsh

(2010). Performance Properties of Microcrystalline Cellulose as a Reinforcing Agent in Wood Plastic Composites, Compos. Part B-Eng., 41(7): 578–581.

Abacha

Fellahi

(2005). Synthesis of Polypropylene-Graft-Maleic Anhydride Compatibilizer and Evaluation of Nylon 6/Polypropylene Blend Properties, Polym. Int , 54(6): 909–916.

10.

Shashidhara

G.M.

Biswas

Pai

B.S.

Kadiyala

A.K.

Feroze

G.S.W.

Ganesh

(2009). Effect of PP-g-MAH Compatibilizer Content in Polypropylene/Nylon-6 Blends, Polym. Bull., 63(1): 147–157.

11.

Pascu

Vasile

Gheorghiu

(2003). Modification of Polymer Blend Properties by Argon Plasma/Electron Beam Treatment: Surface Properties, Mater. Chem. Phys., 80(2): 548–554.

12.

Sobkowicz

M.J.

Dorgan

J.R.

Gneshin.

K.W.

Herring

A.M.

McKinnon

J.T.

(2008). Renewable cellulose derived carbon nanospheres as nucleating agents for polylactide and polypropylene, J. Polym. Environ., 16(2): 131–140.

13.

Tomlal

J.E.

Joseph

Skrifvars

Thomas

Joseph

(2010). Thermal and Crystallization Behavior of Cotton-Polypropylene Commingled Composite Systems, Polym. Compos., 31(8): 1487–1494.

14.

Zhu

X.L.

Wang

C.S.

Wang

H.P.

(2008). Non-Isothermal Crystallization Kinetics and Nucleation Activity of Filler in Polypropylene/Microcrystalline Cellulose Composites, Iran. Polym. J., 17(4): 297–309.

15.

Levan

S.L.

(1989). Concise Encyclopedia of Wood and Wood-based Materials , 1st edn, pp. 271–274, Pergamon PressElmsford, NY.

16.

Avrami

(1940). Kinetics of Phase Change. II Transformation–Time Relations for Random Distribution of Nuclei, J. Chem. Phys., 8: 212–224.

17.

Jeziorny

(1978). Parameters Characterizing the Kinetics of the Non-Isothermal Crystallization of Poly(ethylene terphthalate) Determined by DSC, Polymer , 19(10): 1142–1149.

18.

Ozawa

(1971). Kinetics of Non-Isothermal Crystallization, Polymer , 12(3): 150–158.

19.

Liu

T.X.

Z.S.

Wang

S.E.

Zhang

H.F.

(1997). Nonisothermal Melt and Cold Crystallization Kinetics of Poly(aryl ether ether ketone ketone), Polym. Eng. Sci., 37(3): 568–575.

20.

Kissinger

H.E.

(1956). Variation of Peak Temperature with Heating Rate in Differential Thermal Analysis, J. Res. Natl. Bur. Stand., 57(4): 63–66.

21.

D.F.

Wu.

L F.

Zhang

Y.S.

Zhang

(2007). Nonisothermal Cold Crystallization Behavior and Kinetics of Polylactide/Clay Nanocomposites, J. Polym. Sci. Pol. Phys., 45: 1100–1113.