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Study on the creep behavior of polypropylene.

INTRODUCTION

Polypropylene (PP) is one of the common commodity polymers with very wide applications. Nevertheless, because of its very poor weatherability, PP cannot be used in certain applications requiring heat-resistance and lightfastness. Over the past decades, many researchers have carried out a great deal of research on the aging and antiaging of PP, obtaining some good results in these fields (1-5). However, PP products are often subjected to stress in actual use, even throughout their whole service, and hence the main reason for PP failure may be the effect of stress (i.e., creep).

Studies on the creep behavior of plastics have gotten some important results. Read and Tomlins investigated the creep behavior and physical aging of PP under different environmental conditions (temperature, stress) and established the corresponding creep model (6-9). Dean et al. created a model for nonlinear creep and physical aging in PVC under high stress (10), (11). Lai and Bakker carried out short- and long-term tensile creep tests on HDPE under different levels of stress at an ambient temperature of 20[degrees]C (12).

In these previous studies, the individual effects of temperature, UV light, and mechanical stress, or at most a combination of two of them, were considered. Recently, we focused our research on the combined effects of temperature, UV light, and mechanical stress on creep behavior and obtained some unexpected results. In this work, we have studied the tensile creep behavior of PP to understand the long-term stability of its size and load capacity, as well as to delineate its creep failure law and to predict its service lifetime. This information proved to be useful for the rational selection of PP for deployment under different environmental conditions. In this work, we used four kinds of PP to study the influence of molecular weight and molecular structure on its critical failure.

EXPERIMENTAL

Materials

PP1 (polypropylene, F401, homopolymer) and PP2 (polypropylene, J802, homopolymer) were supplied as pellets by Langang Petrochemical Company, China. PP3 (propylene-ethylene copolymer, 7726) and PP4 (propylene-ethylene copolymer, EPS30R) were supplied as pellets by Yanshan Petrochemical Company, China. PP copolymer mainly consists of PP phase and ethylene-propylene rubber (EPR) phase. The melt index (MI), viscosity-average molecular weight ([M.sub.v]), weight-average molecular weight ([M.sub.w]), number-average molecular weight ([M.sub.n]), and glass transition temperature ([T.sub.g]) of four kinds of PP are listed in Table 1. The mechanical properties are listed in Table 2.
TABLE 1. Material data of four kinds of PP used.

Sample MI (g/10 min) [M.sub.v] [M.sub.w] [M.sub.n]
 (x[10.sup.5]) (x[10.sup.5]) (x[10.sup.5])

PP1 2.1 2.50 3.20 0.60
PP2 14.0 1.89 2.29 0.62
PP3 29.0 2.21 2.65 0.54
PP4 1.8 3.70 4.46 1.32

 [T.sub.g] ([degrees]C)

Sample PP phase EPR phase

PP1 12.81 -
PP2 13.62 -
PP3 9.02 -35.22
PP4 13.49 -44.58

TABLE 2. Mechanical properties of four kinds of PP used.

Sample Yield strength (MPa) Tensile strength Elongation at break
 (MPa) (%)

 PP1 34.7 34.7 316.3
 PP2 36.3 36.3 26.6
 PP3 26.9 26.9 90.3
 PP4 26.6 26.6 188.3


Specimen Preparation

PP pellets were molded into dumbbell-shaped tensile bars (GB 1040 type II specimens, 150 mm X 10 mm X 4 mm) by means of a Nissei PS40E5ASE machine.

Gel Permeation Chromatography Analysis

The molecular weights of PP samples were measured with a PL-GPC220 apparatus (Polymer Laboratories, USA) at I50[degrees]C. The solvent was trichlorobenzene, and polystyrene was used as a standard. The molecular weight of aged samples was assessed by means of measurements of PP surfaces that had been irradiated for 12 h with UV light; the surface thickness was about 1 [micro]m.

X-Ray Photoelectron Spectrometer Analysis

The surface chemical structures of PP were observed by means of an X-ray photoelectron spectrometer (XSAM800, Kratos, UK); the oxygen and carbon contents of the PP surfaces were measured.

Creep Properties

The creep properties of PP samples were assessed using a multifunction stress aging testing machine that was capable of performing creep tests under different environmental conditions (temperature, UV, and stress). The tensile stress applied in the creep experiments was less than the yield stress of PP. UV light was provided by an iodine-gallium lamp with a wavelength range of 350-450 nm and a power output of 500 W. In this work, the UV intensity of an iodine-gallium lamp was 300 [micro]W/ [cm.sup.2] which was denoted as 1UV. Thus, 2UV represented a UV intensity of 600 [micro]W/[cm.sup.2].

To understand the process of the creep failure, the true stress-true strain dependence must be determined. Thus, logarithmic (true) strain and true stress were used in our study. According to the procedure developed by G'Sell et al. (13), true stress-strain curves were measured using an INSTRON 4301 machine equipped with a CCD camera, which regulated the crosshead speed so that a constant local strain rate in the narrowest part of the specimen was maintained. Because the oriented PP samples can deform homogeneously, rather than with the aid of video-controlled measurements in the common cases, true stress-strain dependencies could be obtained without video control and therefore with a higher accuracy. The true stress ([sigma])--true strain ([epsilon]) curve could be directly obtained assuming a constant volume relationship by

[sigma] = F/A = (F/[A.sub.0]) * [lambda] (1)

where F is the load, A the varying sample cross section, [A.sub.0] the initial cross section, and [lambda] the extension ratio and

[epsilon] = In [lambda]. (2)

Scanning Electron Microscopy

Surface morphologies of aged samples were investigated using a Hitachi S-450 scanning electron microscope (SEM). The specimen surfaces were coated with a thin layer of gold, about 50 [Angstrom] thick.

RESULTS AND DISCUSSION

Influence of Temperature, Stress, and UV Light on the Creep Behavior of PP1

Figure 1 shows the influence of temperature, stress, and UV light on the creep behavior of PP1. As shown in Fig. 1a, the creep rate of PP1 increased rapidly with increasing temperature. At applied force of 500 N. the creep rate was very slow at 50 C. When the temperature was increased to 60[degrees]C. creep failure occurred at about 18,000 s, and when it was further increased to 70[degrees]C, the creep failure time was only about 1800 s.

[FIGURE 1 OMITTED]

The free volume of PP1 increased with increasing temperature as the intermolecular forces decreased, which allowed the molecular chain segments to slip more easily within PP1. thereby accelerating the creep failure of PP1 under the applied stress. The effect of degradation also needs to be considered. As shown in Table 3, sample tested at a higher temperature had a lower molecular weight, which would weaken the tensile strength of PP1 and accelerate creep failure of PP1. This phenomenon acted synergistically with the previous one that the free volume of PP1 increased rapidly with increasing temperature in increasing the creep rate of PP1.
TABLB 3. The GPC result of PP1 samples before and after exposure to UV
light for 12 h.

Temperature Load (N) [M.sub.w] [M.sub.n] [M.sub.v]
([degrees]C) (x [10.sup.5]) (x [10.sup.4]) (x [10.sup.5])

 23 0 (a) 3.20 5.96 2.50
 0 1.58 5.19 1.36
 300 0.90 2.90 0.78

 60 0 1.38 2.10 1.13
 300 0.31 0.59 0.25

(a) Without UV light.


As shown in Fig. 1b. at room temperature (23[degrees]C), the creep rate was very slow under applied force of 800 N. When the applied force was increased to 900 N. creep failure occurred at about 13.000 s. Further increased the applied force to 1000 N, the creep failure time was only about 2000 s. This result shows that the creep rate of PP1 increases quickly with increasing tensile stress. Because stress decreases the activation barrier for bond dissociation, thus allowing the molecular chains to move more easily, with increasing tensile stress, the movement of the molecular chains increases rapidly and creep is accelerated.

Figure 1c shows the effect of UV light on the creep failure of PP1. Under irradiation with UV light, the creep failure time was only about 14,400 s at applied force of 800 N at 23[degrees]C. It may be that UV light provokes an increase in the degradation rate of PP1. As shown in Tables 3 and 4, the molecular weight ([M.sub.w], [M.sub.n], and [M.sub.v]) decreased significantly and the oxygen content increased rapidly under irradiation with UV light for 12 h at 23[degrees]C. When the temperature was increased to 60[degrees]C, [M.sub.n] decreased by about 60% and the oxygen content increased by about 116%. In the case of stress, the changes were more obvious. These results show that stress acted synergistically with UV light in accelerating the photooxidative degradation of PP1. The effect was similar to that studied by Chen and Tyler (14).
TABLE 4. The relative content of carbon and oxygen of PP1 sample
surfaces before and after exposure after exposure to UV light for 12 h.

Temperature ([degrees]C) Load (N) C (At%) O (At%)

 23 0 (a) 96.5 3.5
 0 96.2 3.8
 300 92.4 7.6

 60 0 91.8 8.2
 300 90.4 9.6

(a) Without UV light.


In addition, the increase in oxygen content showed that the molecular chain segments of PP1 had higher activity and greater freedom of movement under UV light irradiation. It was useful for stretching of the molecular chain segments. The decrease in molecular weight indicated that the molecular chain segments had ruptured, and the anticreep ability of PP1 was weakened under irradiation with UV light.

SEM Analysis

To investigate the microstructure variation of PP1 surfaces under different aging environmental conditions, the surface morphologies of the aged PP1 specimens were examined by SEM. Figure 2 shows the micrographs of PP1 sample surfaces under different aging environmental conditions. The destruction of UV light and stress on surface morphology was not obvious at 23[degrees]C, as shown in Fig. 2b and c. When the temperature was increased to 60[degrees]C, the micro-cracks of PP1 surface increased obviously (Fig. 2d). In the case of stress, the changes were more obvious in Fig. 2e.

[FIGURE 2 OMITTED]

This phenomenon further proved the aforementioned conclusion that stress acted synergistically with UV light in accelerating the photooxidative degradation of PP1.

The aforementioned results indicate that temperature, stress, and UV light weaken the anticreep ability of PP1 and accelerate the creep failure.

Critical Failure Strain of PP1

Figure 3 shows critical failure strain ([[epsilon].sub.crit]) of PP1 sample under different aging conditions. It can be seen clearly that the [[epsilon].sub.crit] of PP1 remained unchanged at the same temperature, which demonstrated that the [[epsilon].sub.crit] was independent of the tensile stress or irradiation with UV light. While the [[epsilon].sub.crit] of PP1 increased gradually with the increase of temperature, as shown in Fig. 4. Increasing tensile stress and irradiating with UV light only accelerated the creep failure: the creep failure law of PP1 did not change.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

In addition, it can also be seen in Fig. 3 that the values of [[epsilon].sub.crit] in a and b or in c and d are very close, whereas there are large differences in [[epsilon].sub.crit] between a and c and b and d. The corresponding temperatures were either above or below the [T.sub.g].

The aforementioned results could be related to the [T.sub.g] of PP1. The differences above and below [T.sub.g] were too great to be attributable to the free volume of PP1. Below [T.sub.g], the amorphous phase was frozen and the distances between molecules were shortened. Thus, the free volume was greatly decreased and the force between molecules was strongly enhanced. As a consequence, slippage of the molecular chain segments within PP1 was more difficult under the applied stress, and so the molecular chain segments ruptured more easily. As time elapsed, the damage gradually spread, leading to rapid creep failure of PP1 under a relatively low strain. Above [T.sub.g], the opposite was true.

The [[epsilon].sub.crit] of PP1 was almost the same when the temperature was above 23[degrees]C, which was useful for predicting the service lifetime of PP above room temperature.

From the results mentioned earlier, it could be concluded that the value of [[epsilon].sub.crit] is independent of the tensile stress and UV irradiation, whereas it depends only on the temperature.

Influence of Molecular Weight and Molecular Structure on the Creep Behavior of PP

Figure 5 shows the [[epsilon].sub.crit] of another homopolymer PP2 sample with lower molecular weight under different aging conditions. It can be seen clearly that there is also a [[epsilon].sub.crit], for PP2 during the creep course, and the creep failure law of PP2 was similar with PP1. The [[epsilon].sub.crit] of PP2 was smaller than that of PP1 at almost all experiment temperatures. This result shows that, in the case of the same structure, the [[epsilon].sub.crit] of low-molecular weight PP was less than that of high-molecular weight PP, which was further proved by the creep failure law of PP3 and PP 4 (Figs. 6 and 7). The [[epsilon].sub.crit] of low-molecular weight PP3 was less than that of high-molecular weight PP4 at nearly all experiment temperatures (above - 10[degrees]C).

[FIGURE 5 OMITTED]

This result may be interpreted by assuming that, in the case of the same structure, the longer molecular chain of high-molecular weight PP allows greater deformation under the applied stress. Thus, greater damageable deformation and a higher value of [[epsilon].sub.crit] were obtained.

The [[epsilon].sub.crit] could not be observed for PP3 and PP4 (propylene-ethylene copolymer) during the creep course when temperature was below -10[degrees]C, as shown in Figs. 6 and 7. The creep curves were linear and did not show apparent critical change point, which was similar with that of EPR.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The reasonable explanation about the result was that propylene-ethylene copolymer mainly consists of PP phase and EPR phase. EPR phase has lower [T.sub.g] and greater deformation ability under the same conditions. When temperature was below -10[degrees]C. EPR phase was still in rubber state and its creep strain was much greater than that of PP phase in glass state. The creep behavior of PP phase was screened by EPR phase. Therefore, creep curves of PP copolymer were linear and similar to that of EPR. So, the [[epsilon].sub.crit] could not be observed for PP copolymer during the creep course when temperature was below -10[degrees]C.

The [[epsilon].sub.crit] of four kinds of PP was listed in Table 5 at a few typical temperatures. The [[epsilon].sub.crit] of PP1 (homopolymer) was higher than that of PP3 (propylene-ethylene copolymer) at nearly all temperatures (above -10[degrees]C). This result shows that, in the case of similar molecular weights, the [[epsilon].sub.crit] of PP homopolymer was greater than that of propylene-ethylene copolymer.
TABLE 5. The critical failure strain ([[epsilon].sub.crit]) of
different PP samples at various temperatures.

 [[epsilon].sub.crit] (%)

Temperature ([degrees]C) PP1 PP2 PP3 PP4

 -30 4.45 2.50 - (a) -
 -20 5.96 3.76 - -
 -10 7.45 4.74 6.50 14.46
 0 9.83 6.02 8.70 15.20
 10 10.30 8.70 9.21 15.50
 23 16.48 13.83 11.50 16.23
 60 17.16 16.47 11.70 17.10

(a) Without [[epsilon].sub.crit].


It was maybe that the incorporation of ethylene molecules into the main propylene chain decreased the tacticity of molecular chain of propylene-ethylene copolymer, which made the stretch and slippage of molecular chains more difficult along the direction of the applied stress. This resulted in the breakup of molecular chains in a relatively small strain and smaller irreversible deformation. As a consequence, the smaller [[epsilon].sub.crit] was observed for PP copolymer with respect to PP homopolymer with similar molecular weight.

CONCLUSIONS

The creep behavior and creep failure law of PP have been studied using a multifunctional stress aging testing machine. The four kinds of PP displayed a [[epsilon].sub.crit], the value of which was independent of the tensile stress and UV irradiation, depending only on the temperature and the nature of the PP. In the case of the same structure, the [[epsilon].sub.crit] of low-molecular weight PP was less than that of high-molecular weight PP. In the case of similar molecular weights, the [[epsilon].sub.crit] of PP homopolymer was greater than that of propylene-ethylene copolymer. The [[epsilon].sub.crit] could not be observed for PP copolymer at low temperature.

Increasing the tensile stress and irradiation with UV light accelerated the creep failure of PP, but did not change the creep failure law of PP. These results have led to a new way to predict the service lifetime of PP.

ACKNOWLEDGMENTS

The authors are grateful to Xinyuan Zhang and Hong Cheng (Analysis and Testing Center, Sichuan University) for their technical support during the research.

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Xiaolin Liu, (1) Yajiang Huang, (1) Cong Deng, (2) Xiaojun Wang, (2) Wei Tong, (1) Yuxin Liu, (1) Jianqian Huang, (1) Qi Yang, (1) Xia Liao, (1) Guangxian Li (1)

(1) The State Key Laboratory for Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, People's Republic of China

(2) Analysis and Testing Center, Sichuan University, Chengdu 610065, People's Republic of China

Correspondence to: Guangxian Li; e-mail: guangxianli@scu.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50533080; contract grant sponsor: National Basic Research Program of China; contract grant number: 2005CB623800. DOI 10.1002/pen.21330

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Liu, Xiaolin; Huang, Yajiang; Deng, Cong; Wang, Xiaojun; Tong, Wei; Liu, Yuxin; Huang, Jianqian; Yan
Publication:Polymer Engineering and Science
Article Type:Technical report
Date:Jul 1, 2009
Words:3242
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