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Effect of injection speed on phase morphology, crystallization behavior, and mechanical properties of polyoxymethylene/poly(ethylene oxide) crystalline/crystalline blend in injection processing.

INTRODUCTION

Polyoxymethylene (POM) is one of the important engineering plastics and has been widely applied to many fields such as cars and electronics. However, the toughness and wearability of POM are poor. Many efforts have been made but failed due to its bad miscibility with the modifiers. Up to now, only TPU can effectively improve the toughness of POM (1-3). No literature has been reported that single modifier can simultaneously improve the toughness and wearability of POM. Therefore, how to prepare POM with excellent performance is of great significance. Our previous researches showed that poly(ethylene oxide) (PEO) was a novel modifier of POM, which could not only largely increase the notched impact strength, but also greatly decrease the frictional coefficient and wear width of POM (4). More importantly, we found that POM/PEO blend was a rare and interesting crystalline/crystalline system.

Crystalline/crystalline polymer blending is an effective approach to prepare materials with high performance, because the blend can form multiscale and multilevel structures during the liquid-to-solid process, quite different from crystalline/amorphous and amorphous/amorphous blends (5). However, the difference of components' crystalline temperatures and that of melting points of the conventional miscible crystalline/crystalline blends are usually both less than 50[degrees]C, making it difficult to study and control the crystalline structures of the components through change of temperature and stress during processing (6-31). However, for the systems with a big gap between the components' melting points or crystallization temperatures, the components are commonly immiscible, and their crystallizations are independent (32), (33).

Different from above two kinds of polymer crystalline/ crystalline blends, POM and PEO both have good compatibility at amorphous and melting states as revealed by DMA results and viscosity measurements in our previous studies (34), and there are as big as 100[degrees]C differences in both crystallization temperatures and melting points, which make POM/PEO blend a rare and excellent example to study the crystallization behaviors of crystalline/ crystalline blends. The reason is that the small difference in the chemical unite of POM [[--([CH.sub.2]--0),--].sub.n] and PEO [--([[CH.sub.2]--[CH.sub.2]--0).sub.n]--] results in a big difference in chain conformation of POM ([H9.sub.5]) and PEO ([H7.sub.2]), which induces the stereo intermolecular complexation of POM and PEO due to the match in size and shape of their conformations, a structure base for the compatibility of the system; on the other hand, a big difference in their crystalline structures and properties causes the gap between their crystallization temperatures and melting points to exceed 100[degrees]C. Therefore, it is easy to study the crystallization behavior of the blend and to control the multiscale crystalline structures by altering the content, temperature, and stress during processing, so as to understand the relationships among molecular structures, chain conformation, crystalline structure, and material properties of polymer crystalline/crystalline blends.

As part of a series of study of POM/EPO system, this work investigated the effect of injection speed on the phase morphology, the crystalline behavior, and mechanical properties of the POM/PEO blend.

EXPERIMENTAL

Materials

POM (M90) was produced by Yuntianhua Company of China, [M.sub.w] was about 8 X [10.sup.4]. PEO was provided by Shang-hai Jicheng Company of China, [M.sub.w] was about 5 x [10.sup.5].

Blending

Before the blending, POM was dried for 4 h at 90[degrees]C, while PEO was for 6 h at 50[degrees]C. Then, POM and PEO were mixed in a mixer FW-400A (Zhongxing Company of China) and extruded at about 180[degrees]C and granulated by twin-screw extruder TSSJ-25-33 (Chengguang Chemical Engineering Research Institute of China). The screw diameter was 25 mm, and ratio of the length to diameter was 33:1.

Injection

After dried for 6 h in an oven at 60[degrees]C, the granulated POM/PEO blend was injected into samples through K-TEC40 injector. The injection temperatures from charging regions to nuzzle were 70, 190, 195, and 193[degrees]C, respectively. Lock pressure was 220 kN, and mold temperature was 30[degrees]C. The injection pressures and the injection speeds were listed in Table 1.

TABLE 1. Injection conditions for POM/PEO blends.

Code  Injection speed (rnm/s)  Injection pressure (MPa)

1                           5                        13
2                          10                        20
3                          20                        25
4                          50                        80
5                          80                       100
6                         120                       160


Differential Scanning Calorirnetry

Differential scanning calorimetry (DSC) was carried out by NETZSCH DSC 204 with a TAC 7/7 Controller. Flux of N2 was 50 ml/min in order to balance the temperature change, and isothermal operation was done 5 min before dynamic operations. The heating rate was 10[degrees]C/min.

Scanning Electron Microscopy

Electron microscopy experiments were performed at 10 kV through a high-resolution scanning electron microscope (Inspect FSEM, FEI Company). Before investigation, the specimens were eroded by water to remove PEO and then get coated with Au-Pd.

Mechanical Properties Test

Tensile tests were performed by Instron 4302 (Instron Company of American) at room temperature and 50 mm/ min constant crosshead rate according to ASTM D-638. Impact tests were performed by ZBC-4B (MST Company of American) according to ASTM D-1709.

RESULTS AND DISCUSSION

Effect of Injection Speed on the Morphology of POMI PEO Blend

In our previous research, it was found that POM/PEO blends were sensitive to shear rate. With the increasing shear rate, the shear viscosity of POM/PEO blends melt decreased (4). Accordingly, the morphology and crystalline structure of POM/PEO blends can be controlled by adjusting the shear rate during processing. However, it is difficult to obtain the accurate shear rate value in injection processing due to complex module sprue and high-injection speed. However, the relationship between the injection parameters and the shear rate could be analyzed by simplifying the injection module sprue into the capillary.

In the capillary, the shear rate can be expressed as follows (35):

[gamma] = 4Q/[pi][R.sup.3] = 4 [[[integral].sub.0].sup.R] V(r)2[pi]rdr/[pi][R.sup.3] (1)

In Eq. 1, Q is the volume flow rate, R is the radius of the capillary, and V(r) is the flow rate of the melt.

Based on Eq. 1, it can be seen that the shear rate has direct relationship with the volume flow rate Q, and the flow rate of the melt V(r), which increases with the increase in the injection speed. Therefore, the shear rate and the shear stress of the injection process could be controlled by adjusting the injection speed.

Commonly, under shear stress during the processing, the dispersing phase in the polymer blend melt, especially for ystalline/amorphous blends and incompatible crystalline/crystalline blends, will get deformed, elongated, and then break into smaller droplets. The different morphologies of the dispersing phase could be obtained by controlling the viscosity ratio of the components or shear rate in the processing, which has been confirmed in many polymer blends such as POM/TPU, PP/PC, PP/PE, and PP/ EPDM (36), (37). However, the effect of shear stress on compatible crystalline/crystalline blends may be different. Figure 1 shows the SEM photos of POM/PEO blend (90/ 10) prepared at different injection speeds. Before investigation, the blends were eroded by water to remove PEO, so that the holes or interstices represented PEO. Clearly, when the injection speed was 5 mm/s, most of PEO dispersed into big and irregular droplets in POM. With the injection speed increasing to 50 mm/s, the phase morphology of PEO changed from big droplets to the big lamellar, and PEO was inserted into the crystal lamellar fibers of POM. When the injection speed further increased to 120 mm/s, almost all PEO phase became smaller and dispersed into the crystal lamellas of POM.

According to the above SEM results, it could be concluded that in POM/PEO crystalline/crystalline blend, with shear rate increasing, the location of PEO changed from the amorphous region among the spherulites to the crystal lamellar fibers and finally to the crystalline lamellar of POM. It was ascribed to the good compatibility between POM and PEO at the amorphous and melting states and big differences between their crystallization temperatures and melting points. The crystallization temperature of POM was about 145[degrees]C while that of PEO was about 45[degrees]C. When POM began to get crystallized, PEO was still in melting state. Because PEO could not enter the crystal lattice of POM and form cocrystal with POM, which had been testified by our previous works (34), the PEO melt was only pushed out of the crystallization region of POM and dispersed in the amorphous region between the crystallization regions of POM crystal. If the injection speed was low, the shear rate in the sprue of the injector would be low, shear stress would be weak, and the viscosity of PEO melt would be higher. PEO would not disperse evenly, and most of PEO only existed in the amorphous region between the spherulites of POM. If the injection speed increased, the shear rate in the sprue would increase, the shear stress would become strong, and the viscosity of PEO melt would decrease, resulting in that PEO melt could disperse in a smaller scale. After POM crystallization, PEO could exist in the amorphous region between crystalline lamellar fibers and even between crystalline lamellas of POM spherulites. Therefore, with increasing the injection speed, the formed phase morphology of the POM/ PEO (90/10) blend was shown in Fig. 1.

Effect of Injection Speed on the Crystalline Behavior of POM/PEO Blend

Figure 2 was DSC curves of the POM/PEO(90/10) blend prepared at different injection speeds, and the relative data were listed in Table 2. Clearly, when the injection speed was lower than 20 mm/s, the crystalline peak of PEO appeared at ~43[degrees]C and that of POM appeared at ~143[degrees]C. When the injection speed increased to 50 mm/s, there appear two faint crystalline peaks of PEO at fNi38 and ~7[degrees]C, respectively, while the crystalline peak of POM did not change and appeared at about ~143[degrees]C. When the injection speed was beyond 80 mm/s, the crystallization temperature of PEO decreased to ~5[degrees]C and that of POM decreased to ~141 [degrees]C. Similarly, the melting points of PEO and POM in the blend decreased from ~63 to ~57[degrees]C and ~168 to ~l62[degrees]C, respectively. In addition, the crystallization peak areas of POM and PEO had no clearly change with increasing the injection speed, which indicated that the injection speed had minor effect on the crystallinity of the components in the bleed.

TABLE 2. Data of DSC curves in Fig.2.

Injection     Crystallization   Crystallization   Melting point
speed          temperature of    temperature of       of PEO
([s.sub.-1])  PEO ([degrees]C)  POM ([degrees]C)  ([degrees]C)

5                         42.8             143.4           63.7
20                        41.7             143.2           63.8
50                   38.1, 7.0             143.5           62.5
80                         5.6             140.9           57.2
120                        5.1             141.2           57.6

Injection     Melting point  Crystallization   Crystallization
speed             of POM       peak area of PEO  peak area of POM
([s.sub.-1])  ([degrees]C)         (J/g)               (J/g)

5                     167.9              27.4             117.3
20                    168.2              28.3             115.8
50                    167.6              26.5             116.4
80                    166.2              25.5             115.2
120                   166.2              25.8             114.6


Because the crystallization temperature of POM was higher than that of PEO, after POM crystallization, PEO still was at the melting state. Therefore, when PEO began to get crystallized, the whole process only took place within the limited space formed by the crystallized POM. On one hand, because PEO had good compatibility with POM at the amorphous and melting states, there were strong interactions between PEO and POM preventing POM movement from changing from irregular state to regular state or vice versa during POM crystallization or melting processing. As a result, the crystallization temperatures and melting points of POM decreased. On the other hand, the size of the limited space formed by crystallized POM would exert a great influence on the crystallization of PEO. If the limited space was large enough, PEO would be crystallized freely. But if it is too small, the movement and the crystallization of the PEO would be restrained. The smaller the limited space became, the more strongly the crystallization of PEO was restrained. According to the effect of the injection speed on the morphology of PEO, it could be seen that the increasing injection speed determined the morphology size of PEO. When the injection speed was lower than 20 mm/s, the phase morphology of PEO was so large that PEO could get crystallized relatively freely, and the crystallization temperature and melting point of PEO were high. With the increasing injection speed, the phase of PEO became smaller and the restraint of PEO crystallization increased, causing the crystallization temperature and melting point of PEO to decrease.

Based on this result, it could also be deduced that if the morphology of PEO was small enough to fully limit the movement of PEO, PEO would only exist in POM matrix at amorphous state, which was testified in the POM/PEO blend-containing 5% PEO and will be stated in other works.

Figure 3 showed the crystalline morphology of POM, PEO, and POM/PEO (90/10) blends prepared at different injection speeds. It was clear that neat POM and PEO both produced the spherulites, and the spherulites of POM were equirotal and far smaller than that of PEO. In POM/ PEO blend, when the injection speed was 5 mm/s, the spherulites were also formed in the blend, and they were asymmetrical. When the injection speed increased to 50 or 120 mm/s, the spherulites became symmetrical and obviously were larger than that of neat POM.

From SEM and DSC results, it could be known that when the injection speed was low, PEO was pushed out of the spherulites of POM to form asymmetric and larger phase morphology and then could get crystallized more freely, that is, not only POM could get crystallized independently, but also PEO could form crystals and grow independently. Consequently, the spherulites of the blend demonstrated different sizes. With the increasing injection speed, PEO would gradually get inserted into the amorphous region between the crystal lamellar fibers or between the crystal lamellas of POM spherulite to enlarge the amorphous regions in POM spherulites. As a result, the size of the spherulites in the blend increased with the increasing injection speed, larger than that of neat POM.

Effect of Injection Speed on the Mechanical Properties of POM/PEO Blend

PEO was the material with high toughness at the room temperature and has good compatibility with POM at the amorphous state. Therefore, POM/PEO blends had higher notched impact strength than those of neat POM, which was proved in our previous research (4).

TABLE 3. Effect of injection speed on the mechanical properties of
POM/PEO(90/10) blend.

Injection        Tensile     Elongation at    Notched impact
speed (mm/s)    strength       break (%)        strength
                  (MPa)                       (kJ/[m.sub.2])

Neat POM      58.2 [+ or -]  28.5 [+ or -]   6.7 [+ or -] 0.5
                        0.5              5
5             46.2 [+ or -]  82,3 [+ or -]   7.9 [+ or -] 0.5
                        0.5              5
10            46.5 [+ or -]  89.7 [+ or -]   7.8 [+ or -] 0.5
                        0.5              5
20            46.0 [+ or -]  88.3 [+ or -]   8.8 [+ or -] 0.5
                        0.5              5
50            46.2 [+ or -]  89,5 [+ or -]  10.8 [+ or -] 0.5
                        0.5              5
80            46.8 [+ or -]  85.0 [+ or -]   113 [+ or -] 0.5
                        0.5              5
120           45.7 [+ or -]  86.5 [+ or -]  10.5 [+ or -] 0.5
                        0.5              5


The mechanical properties of neat POM and POM/ PEO (90/10) blends prepared at different injection speeds were listed in Table 3. It showed that the tensile strength and elongation at the breaking point of the blends did not change obviously with the increasing injection speed. However, the notched impact strength of the blend varied greatly with the increasing injection speed. When the injection speed was beyond 50 mm/s, the notched impact strength of the blend reached 11.3 kJ/[m.sup.2], increasing by about 40% compared to that of the blend, 7.8 kJ/[m.sup.2], prepared at 5 mm/s injection speed.

Because the crystallinity of the components in the blend had minor change with the increasing of the injection speed, we considered that the variety of notched impact strength of the blend with the injection speed increasing was mainly attributed to the change of PEO phase morphology. When the injection speed was low, most of PEO dispersed between the spherulites of POM, and its phase morphology was larger and irregular, which could not effectively prevent the growth of the craze when the blend was subjected to impact. Therefore, the blend had lower notched impact strength. However, when the injection speed increased, the PEO could get inserted into the amorphous region between the crystal lamellar fibers or between the crystal lamellas of POM, so as to form the symmetrical soft-hard interlayer phase morphology in a very small scale, which could not only prevent the craze growing to the crack but also effectively absorb the impact energy when the sample was impacted. As a result, the notched impact strength increased.

CONCLUSION

The injection speed in the processing greatly affected the phase morphology of PEO in POM/PEO (90/10) blend and finally determined its crystalline behaviors and the mechanical properties. The results showed that, because the shear rate in the sprue of the injector increased and the viscosity of PEO decreased, PEO could disperse in the amorphous region of POM more easy, and the size of PEO phase decreased and its location changed from the amorphous region between POM spherulites to that between crystal lamella fibers or crystal lamellas in POM with the injection speed increasing. Consequently, the crystallization of PEO was restrained, and the crystallization temperatures and melting points of PEO decreased from ~43 to ~5[degrees]C and ~63 to ~57[degrees]C, respectively. As a result, the notched impact strength of the blend increased from 7.8 to 11.3 kJ/[m.sup.2].

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Correspondence to: Shibing Bai; e-mail: baishibing@scu.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 50773041, 51010004.

Shibing Bai, Qi Wang

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China

DOI 10.1002/pen.23131
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Author:Bai, Shibing; Wang, Qi
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Date:Sep 1, 2012
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