Crystal distribution and molecule orientation of micro injection molded polypropylene microstructured parts.
The use of microstructured parts in the field of MEMS has been strongly increasing over the past decade (1-3). Microstructured parts have outer dimensions of several millimeters up to a few centimeters with three-dimensional microstructures on their surface (4-7). Micro injection molding (MIM) is a promising technology for the replication of microstructured parts because of its low production cost, mass production capability, applicability for many materials, good tolerance (8), (9). At present, the investigation on MIM contains mold technology, special machine, production process, filling performance analysis, numerical simulation, etc. (10), (11). However, seldom research on the crystal characters and molecule orientation of microstructured parts produced by MIM is reported. Large numbers of microstructured parts are produced from polymers especially polypropylene (PP) (12). Isotactic polypropylene (i-PP) may have three different crystal structures ([alpha] monoclinic phase, [beta] hexagonal phase [gamma] orthorhombic phase), a mesomorphic and an amorphous phase (13). The presence of one or more polymorphic forms in i-PP strongly depends on crystallization conditions (temperature, cooling rate, pressure, etc.) and product size. Furthermore, the crystal phase and molecule orientation have a significant impact on the mechanical and optical properties of the finished polymer article (14), (15). MIM has many features which are different from conventional injection molding such as higher injection pressure and speed, higher mold temperature and mold vacuum (16). (17). On the other hand, the size of microstructures decreases to micron or sub-micron. So, investigating the crystal distribution and molecule orientation is helpful to understand the special properties of microstructured parts. In this article, the crystal style and molecule orientation of PP microstructured parts were investigated by X-ray diffraction. The hardness of shear zone of micro columns was evaluated by Nano Indenter.
The experimental material i-PP with isotactic index of 98.3%, density of 0.91g/[cm.sup.3] and melting point of 164-170[degrees]C is a commercialized product from Harbin Huaao plastic Ltd., China. These microcavities on the silicon mold insert were produced by inductive couple plasmasion etching. The specimen designed in this article has outer dimensions of 12 mm x 7 mm x 1.5 mm (L x W x H) with 10 x 10 micro columns ([PHI]100 [micro]m x 250 [micro]m) array locating on one surface, as shown in Fig. 1. Test specimens were fabricated by the Babyplast6/10 MIM machine (Cronoplast S.L., Espana). The process parameters contain mold temperature of 90[degrees]C, injection pressure of 100 MPa and holding pressure of 3 s. To understand the crystal style and molecule orientation of the microstructured part, X-ray diffraction tests were introduced. The base plate was test by R1GAKU X-ray diffractometer (Rigaku, Japan). D8 D1SCOVER-GADDS X-ray micro beam ([PHI]100 [micro]m) diffractometer (Bruker AXS, USA) was used to detect micro column and different areas of the base plate. Firstly, a row of micro columns connecting with a part of base plate were cut off from the whole mcirostructured part by a thin blade. Then micro beam diffractometer tests were conducted on the slide surface of micro column along the axial direction. The hardness of shear zone in micro columns was evaluated by Nano Indenter XP (MTS, USA) at room temperature with an indentation depth of 1000 nm. Before nanoindentation test, the whole microstructured parts were embedded in a supporting material to prevent the distortion of micro columns during grinding and polishing. Then, micro columns were ground along axial direction and radial direction, respectively. Afterward, micro columns were polished to form smooth cross sections and longitudinal sections through the principal axis of micro columns. Finally, nanoindentation tests were performed on the shear zone of cross section and longitudinal section, respectively.
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSION
The morphology distribution of [PHI]100 [micro]m micro column and a part of the base plate are shown in Fig. 2. As displayed, both micro column and base plate present "skin-core" morphology which comprises a noncrystalline skin layer, a shear zone, and a spherulites core, It is well known that most conventional injection molded crystal polymer macroscopical parts represent ''skin-core" morphology. Microstructures made by MIM still represent "skin-core" morphology, which has been proven in our previous investigation (18). However, the morphology distribution in micro columns is quite different from that of the base plate. The relative proportion of different structures is defined as the quotient between the thickness of different structures and of overall sample. It can be concluded by comparing Fig. 2a and b that the thickness of skin layer and shear zone do not decrease with the reduction of the microstructures size. However, the thickness of the base plate and micro column are 1.5 mm and 100 [Micro]m, respectively. Thus, the relative proportion of shear zone in micro columns is markedly higher than that of the base plate.
[FIGURE 2 OMITTED]
Figure 3 shows the diffraction profiles of different areas of i-PP microstructured part by X-ray diffractometer and micro beam diffractometer, respectively. The diffraction profile of the whole base plate is displayed in Fig. 3a. As shown, the 20 corresponding to the main peaks contain 14[degrees], 17[degrees], 18.5[degrees] 21[degrees], and 22 of [alpha] phase and 16[degrees] and 21[degrees] of [beta] phase which indicates that the whole base plate contains both [alpha] and [beta] phase. Micro column also comprises [alpha] phase and [beta] phase, which can be concluded from Fig. 3b. [alpha] phase is the most common and stable crystal form of i-PP. [beta] phase was found to increase by the influence of high shear rates. The fraction of [beta] phase ([K.sub.[beta]]) was calculated using the following relation proposed by Turner-Jones et al. (19).
[FIGURE 3 OMITTED]
K = [I.sup.[beta].sub.300]/([I.sup.[alpha].sub.110] + [I.sup.[alpha].sub.040] + [I.sup.[alpha].sub.130] + [I.sup.[beta].sub.300]) (1)
where [I.sup.[beta]] 300, [I.sup.x] 110, [I.sup.[alpha]] 130 correspond to the diffraction intensities of the [beta] phase (at 2[theta]= 16[degrees]) and [alpha] phase (at 2[theta] = 14[degrees], 17[degrees], 18.5[degrees]), respectively. The [k.sub.[beta]] of the whole base plate and micro column are 0.276 and 0.413, respectively, calculated from the diffraction intensities shown in Fig. 3a and b. It means that the fraction of [beta] phase in micro columns is markedly higher than that of the base plate. To understand the distribution of [beta] phase in the base plate. X-ray micro beam diffraction was introduced. Figure 3c shows the diffraction result of the core zone from the base plate which reveals that [alpha] phase is the only crystal phase of the core zone. Test result of the area including skin layer, shear zone, and core zone from the base plate indicates that this area contains both y.[alpha] and [beta] phase, as shown in Fig. 3d. On the other hand, the skin layer is a noncrystalline layer. So, [beta] phase must distribute in the shear zone of the base plate. There is no special nucleator in i-PP used in the experiments. The appearance of [beta] phase is induced by the effect of shearing action. During the filling course, molecule in shear zone gets high shearing stress, and crystal could appear in this zone. The relative proportion of shear zone in micro column is markedly more than that of the base plate which induces the increase of [K.sub.[beta]] in micro column. The [beta] i-PP has several different characteristics in comparison with the traditional [alpha] phase. Studies have proved that [beta] i-PP phase shows an enhanced toughness and impact strength at room temperature. On the other hand, the hardness will decrease with the increase of [beta] phase fraction (20). So, the mechanical properties of these micro columns must differ from that of the base plate.
Figure 4 shows the Debye diffraction patterns of the base plate's core zone and micro column by X-ray micro beam diffractometer. As shown in Fig. 4a, the shape of the arcs is changeless and the brightness of the arcs is homogeneous which indicates that the crystals in the base plate's core zone are not oriented. The reason is that molecule of core zone gets low shearing action during the filling stage and experiences a relative long time for release during the cooling stage. Furthermore, the shearing rate of molecular increases markedly when melt PP flows into microcavities. Molecular of micro column must experience a higher shear stress during the filling stage compared with the base plate. However these micro columns only hold slight orientation, as shown in Fig. 4b, the brightness of these arcs is slightly nonhomogencous. The slight orientation is induced by the special process conditions of MIM. The cooling method used in MIM process is water cooling. The average cooling rate is small and the mold temperature is high up to 90[degrees]C. These conditions give the molecule of micro columns a long time to get relaxation after the injection stage. Despite the skin layer holding marked orientation, it is too thin to affect the whole micro column. Thus, only slight orientation is determined by micro beam X-ray diffraction for micro columns.
[FIGURE 4 OMITTED]
Anisotropy is a special mechanical property of many microstructures with the decrease of their size. Molecule orientation resulted in the injection cycle is one of the typical factors which induce the anisotropy. So, nanoin-dentation tests were used to analyze the effect of slight orientation of micro columns on their mechanical property. Nanoindentation was conducted on the shear zone of micro column along axial direction and radial direction, respectively. Figure 5 shows the seriate values of hardness from the nanoindentation tests on the cross section and longitudinal section of micro column, respectively. As shown, these values become constant gradually with the increase of indentation depth. It is clear that the hardness of shear zone along the axial direction of micro column is approximate with that along the radial direction. Therefore, the mechanical anisotropy of micro columns induced by orientation could be ignored, such as the hardness of shear zone.
[FIGURE 5 OMITTED]
I-PP micro columns ([empty set]100 [micro]m x depth 250 [micro]m) array locating on a macroscopical base plate was produced by MIM. Micro columns have the same types of crystal as the macroscopical base plate, which are [alpha] and [beta] crystal. [beta] phase crystal is detected only in the shear zone of the microstructured part. The relative proportion of shear zone in micro columns is markedly more than that of the base plate, which induces the increase of [K.sub.[beta]] in micro columns (0.413) compared with the base plate (0.276). Molecule of micro columns retains slight orientation after the solidification stage due to the special process parameters of MIM. The hardness of shear zone along the axial direction of micro column is approximate with that along the radial direction.
The authors thank the 49th research institute of China Electron Science and Technology Combine Company for making the silicon insert.
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Zhen Lu, K.F. Zhang
School of materials science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Correspondence to: Zhen Lu; e-mail: luzhen-hit@I63.com
Published online in Wiley InterScience (www.interscience.wiley.com).
[C] 2009 Society of Plastics Engineers
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|Author:||Lu, Zhen; Zhang, K.F.|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2009|
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