Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low- Density Polyethylene Modified with Microspheres by Image Analysis

Transcription

1 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low- Density Polyethylene Modified with Microspheres by Image Analysis Aneta Tor-Świątek*, Tomasz Garbacz, and Tomasz Jachowicz Faculty of Mechanical Engineering, Lublin University of Technology, 36 Nadbystrzycka str., Lublin, Poland Received: 22 May 2015, Accepted: 7 September 2015 SUMMARY The paper investigates the macroscopic structure of low-density polyethylene modified with polymer microspheres. The analysis is performed on specimens produced by extrusion and injection molding with the following contents of the modifying agent: 0.25%, 0.5% and 1%. The structure of the specimens is examined in six measuring regions by image analysis. In addition, the number and diameter of produced micropores and their area fraction are determined. Keywords: Structure, Low-density polyethylene, Extrusion, Injection molding, Microspheres INTRODUCTION The structure modification of polymer plastics has become indispensable due to a constant development of applications for these materials, the necessity of changing or enhancing their functional and operational properties as well as higher product requirements. In extrusion and injection molding, product structure modification can be achieved by changing their processing parameters, design features of the plasticizing unit and processing tools or by the addition of different auxiliaries which can affect the plastic being processed in a variety of ways [1, 2, 3, 4, 5]. *Corresponding author: a.tor@pollub.pl Smithers Information Ltd Cellular Polymers, Vol. 35, No. 2,

2 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz A higher interest in two-phase products has led to development of new processing methods such as cellular extrusion and cellular injection molding. Depending on decomposition characteristics of the blowing agent applied, these processes can be run using conventional technological lines if suitable processing conditions are provided [5, 6, 7]. A key factor here is the temperature of successive zones in the plasticizing unit. This parameter should be set such that the blowing agent undergoes decomposition in a suitable zone of the machine s plasticizing unit. Another important factor is mixing efficiency which is determined by screw and barrel design. Due to variable processing conditions (high temperature and pressure as well as considerably high shear stresses) and their effect on material in the plasticizing unit, blowing processes are difficult to run [8]. One of the reasons for using cellular plastics is the fact that their use ensures lower product weight. This pertains to both complex systems such as vehicles or cars and simple systems such as packaging. In most cases their manufacturing and operational costs are lower, too. Given their insulating properties, cellular materials are used in building and refrigerating engineering. These materials are good for soundproofing purposes; they can also be used for dampening machinery vibration [9, 10]. A cellular plastic in a plastic state is affected by surface tension on the plasticgas interface and diffusion. However, this system is not stable. Consequently, pores grow larger, while their number in the plastic decreases, which is undesired. The desired form is a structure with small pores which can be produced by the fastest possible cooling and solidification of the extrudate. The range and rate of cooling have a significant effect on extruded structures. For instance, it is possible to produce physical structures with varying degrees of porosity by changing the temperature of cooling. Although cellular plastics have lower thermal capacity, they require longer time for solidification than solid extrudates. This stems from the fact that their thermal conduction is lower [1, 11]. Cellular injection molding is used for producing cellular products which are free from cavities on the outside surface and exhibit minimal mold shrinkage. If processing conditions, input plastic and blowing method are selected properly, it is possible to manufacture products with new modified properties such as lower weight, higher rigidity, higher/better damping and insulating properties, uniform mold shrinkage in all directions as well as good machinability. In addition, molded products are free from internal stresses due to lack of oriented structure and have very good acoustic properties [3, 4, 12]. The size of a solid surface layer (skin) and plastic density distribution in a molded piece depend on the type of injected plastic, the structure and 68 Cellular Polymers, Vol. 35, No. 2, 2016

3 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis producibility of a molded piece, the applied blowing agent and injection conditions. The degree of product core porosity increases with the distance from the surface layer. From a practical point of view, it is favorable to produce molded pieces with a density ranging from 550 kg/m 3 to 850 kg/m 3. Products with a lower plastic density do not have satisfactory mechanical properties [13]. The research on characteristics of cellular structures, including the size and distribution of pores in polymers and other plastics, was conducted by different research centers. The authors of the study [14] investigated the structure and size of pores produced in supercritical fluid processing by adding CO 2 to PLGA. As a result, they obtained porosity ranging from 34% to 78% and pores with diameters ranging from 38.8 µm to 580.1µ, respectively. Moreover, the results revealed a significant effect of processing conditions as well as indicated that the blowing process can be controlled via suitable manipulation of processing parameters. Moreover, it was found that decreasing the venting rate to 120 minutes permits the nucleation sites to grow into larger pores. The problems of pore distribution and size were also investigated by a team of Yvonne I. Heit et al. [15]. Their research involved measuring pore sizes in five representative polyurethane foams used in medicine for wound healing. The pore sizes were classified under three groups as small, medium and large. It was found that small pores of 0.72 mm lead to higher resistance to material distortion and damage, while large pores (of approx. 3.1 mm) cause a higher tissue increment. The study [16] reports the results of research on the size and distribution of open pores in geotextiles by image analysis. The research involved examining the cross section of tested specimens. The results demonstrate that the number of open pores depends on the type of tested geotextiles. The results were significantly affected by the specimen preparation procedure and employed research method. The pore sizes determined by image analysis are smaller that those obtained by the analytical method. The aim of this study is to identify changes in the macroscopic structure of a polymer plastic modified with polymer microspheres by two processes: extrusion and injection molding, using variable contents of the modifying agent and constant processing temperatures. Cellular Polymers, Vol. 35, No. 2,

4 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz EXPERIMENTAL Materials The input material for extrusion and injection molding was low-density polyethylene (LDPE) in the form of pellets, sold under trade name Malen E FABS 23 D-022. Manufactured by Płock-based company Basell Orlen Polyolefins, this material additionally contains anti-block and anti-slip agents to ensure that the friction factor is maintained low. This plastic is mainly used for producing foils. Owing to its good processing properties, however, the material can also be used for injection molding. This polyethylene has a density of 921 kg/m 3, tensile strength of 18 MPa, ultimate elongation of 450% and Shore hardness (D) of 48. The polymer was modified with a microcellular agent in the form of microspheres. The modifying agent is manufactured by Swedish company AkzoNobel under trade name Expancel 951 MB 120. Expancel microspheres have the form of spherical thermoplastic capsules containing a hydrocarbon gas. When heated, the capsule softens and the expanding gas causes its volume to increase, without damaging the thin capsule shell. Expancel 951 MB 120 is an endothermic agent. Spheres cannot bond with one another because the capsules retain their blocking properties, which is also an effective way of preventing closed gas release. Expancel 951 MB 120 is a mixture containing 65% microspheres in ethylene-vinyl acetate (EVA) [39, 41]. The size of microspheres ranges from 28 µm to 38 µm, their density is < 12 kg/ m 3, expansion start temperature T start is between 138 C and 148 C, and their processing temperature ranges from 140 C to 200 C. In compliance with the experimental program, the above blowing agent in the form of microspheres was applied as 0%, 0.25 %, 0.5% and 1%, respectively. Process To investigate the blowing agent s effect on the properties and macroscopic structure of low-density polyethylene, the specimens were prepared by two leading polymer processing methods: extrusion and injection molding. The cellular extrusion process was performed using a profile section extrusion line. This technological line consists of a single-screw extruder type T for extruding pelleted thermoplastics, an extrusion head and two standard auxiliary devices: a water cooling bath and a collecting device (Figure 1). It is fitted with a single-screw plasticizing unit with four heating zones. Cellular 70 Cellular Polymers, Vol. 35, No. 2, 2016

5 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis extrusion is performed by a slotted flat extrusion head. The extruder head has one heating zone equipped with an electric heater band as well as polymer pressure and temperature sensors. The extrudate produced thereby has the shape of a strip with a width of 20 mm and a thickness of 4 mm. Figure 1. View of a fragment of the extrusion line: 1 plasticizing unit of the extruder, 2 extrusion head, 3 extrudate strip, 4 cooling device, 5 cooling agent The cellular injection molding process was performed using a laboratory screw injection molding machine type CS 88/63. This screw injection molding machine fitted with an injection mold is used for processing thermoplastics. The machine s plasticizing unit comprises a barrel mounted in the body of the plasticizing unit and a screw with a diameter of 36 mm. On the barrel, there are heater bands which provide heat for four zones of the plasticizing unit. The tooling of the injection molding machine consists of an injection mold, a stationary platen, a movable platen as well as a closing/opening unit. The movable subassembly of the injection mold (Figure 2) is fixed to the movable platen. This part of the injection mold contains mold cavities, the shape and dimensions of which correspond to tensile specimens. The injection mold is equipped with ejector pins. The processing conditions of both processes are given in Table 1. Cellular Polymers, Vol. 35, No. 2,

6 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz Figure 2. View of the movable subassembly of the injection mold: 1 movable part of the injection mold, 2 mold gating system, 3 molded part, 4 temperature sensor, 5 mounting bolts, 6 cooling agent input and outlet Table 1. Processing conditions for extrusion and injection molding of microporous LD-PE Process Parameter Value Injection time, s 10 Injection molding Extrusion Cooling time, s 7 Mould temperature, ºC 25 Temperature in successive zones of the 160, 180, 190, 190 plasticizing unit, ºC Temperature of the extrusion head, c 170 Temperature of the cooling liquid, C 19 Rotational speed of the screw, rpm 45 Methodology Specimens for physical structure tests were cut out by a microme from produced extrudates and cellular molded pieces and measured in the cross section of the product. The specimens had a length of 20 mm, while their thickness was 1.2 mm. In the experiment we established six measuring points, where regions 1, 2, 3 denote the physical structure of the specimen surface layer, while regions 72 Cellular Polymers, Vol. 35, No. 2, 2016

7 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis 4, 5, 6 are located in the core of the test specimen. Figure 3 shows the arrangement of the measuring ranges in a sample microporous molded piece. Figure 3. Schematic arrangement of the measuring ranges in the cross section of the examined specimens and their division into surface layer and core The structure examination by image analysis was performed on a test stand for microscopic examination consisting of an optical microscope, Nikon Eclipse LV100ND, a digital camera, a digital controller type DS. U3, and a PC equipped with the NIS Elements software (Figure 4). Figure 4. Test stand for microscopic examination of physical structure of cellular products EXPERIMENTAL RESULTS Microscopic Assessment of the Structure Based on the image analysis results of the extruded and injection molded specimens, we performed a quantitative assessment of micropore distribution Cellular Polymers, Vol. 35, No. 2,

8 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz and sizes. The application of the image analysis technique enabled determination of the number of micropores, N, mean diameter of the micropores, Dp, mean unit surface of the micropores, Ap, mean area fraction of the micropores, Up, in particular regions of the test specimens. The microscopic observations of the products revealed significant differences between the surface layer (regions 1, 2, 3) and core (regions 4, 5, 6) of the extruded and molded specimens. The morphology of the products is compared in Figure 5. (a) (b) Figure 5. Morphology of the produced structures in selected measuring regions: a) molded piece - region 2, b) extrudate - region 2 74 Cellular Polymers, Vol. 35, No. 2, 2016

9 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis (c) (d) Figure 5. Cont'd... c) molded piece - region 5, d) extrudate - region 5, with the following microspheres contents: A - 0%, B 0.25%, C 0.5%, D 1% The image analysis results enabled determining the distribution of micropores in the cross section of the specimens. Both extruded and molded products have closed micropores. The extruded products have large round micropores; in the molded pieces, however, the micropores are smaller and their shape is not uniform. The extruded products containing 0.25% and 0.5% microspheres Cellular Polymers, Vol. 35, No. 2,

10 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz have relatively uniformly developed micropores. As for the molded parts, the growth of micropores is significantly lower when the microspheres are added at a quantity of 0.25% and 0.5%, respectively. Increasing the microspheres content in the extrudate to 1% resulted in a smaller growth of micropores and their bonding in the entire cross section of the specimens; in the molded pieces, however, the micropores increased, mainly in the core of the test specimens. Quantitative Assessment of the Microscopic Structure by Image Analysis Image analysis enabled a quantitative assessment of the tested specimens depending on the applied content of microspheres and measuring range. The results of quantitative assessment of the tested microporous structures are given in Figures Figure 6. Micropore area fraction in the extrudate versus microspheres content in particular measuring regions The area fraction of micropores (Figure 6) in the extrudate does not significantly change with increasing the microspheres content from 0.25% to 0.5%. It is only when the microspheres are added in the quantity of 1% that the area fraction considerably increases to 35.71% in the surface layer and to 34.12% in the core of the specimen, respectively. The results demonstrate that the measuring range does not affect the area fraction of micropores, while the values reported in the surface layer and core of the examined extrudate do not significantly differ. 76 Cellular Polymers, Vol. 35, No. 2, 2016

11 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis It was found that increasing the content of microspheres led to a higher area fraction of micropores in the molded piece (Figure 7). The highest increase in area fraction was reported when the microspheres content was set to 2%, amounting to 16.85% in the specimen core and to 7.55% in the surface layer. It was observed that the micropore area fraction in the molded part with the 2% microspheres content is much lower compared to that in the extrudate with the 1% content. This can result from a constrained surface of the injection mould, which prevents micropores from further expansion and growth. Figure 7. Micropore area fraction in the molded part versus microspheres content in particular measuring regions With increasing the microspheres content, the diameters of micropores in the extrudate become smaller (Figure 8), while those of micropores in the molded pieces increase (Figure 9). The largest diameter is observed for the extruded products containing 0.25% microspheres and it amounts to µm, while the smallest diameter is reported for the molded pieces produced with the 1% microspheres content it is µm. The decrease in the diameter can result from a higher number of micropores in the extrudate, which probably prevented their further growth. It can also be claimed that the values of micropore diameters in the extrusion of flat parts do not depend on the measuring point. The measuring range is of vital importance when examining the diameters of micropores in the molded pieces. In regions 1, 2, 3 of the specimen s surface layer, the diameters of the micropores range from 52 µm to 64 µm. In regions 4, 5, 6 which are located in the specimen s core, one can observe a clear increase in the micropore diameter by approx %. Cellular Polymers, Vol. 35, No. 2,

12 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz Figure 8. Micropore diameter in the extruded product versus microspheres content in the measuring regions Figure 9. Micropore diameter in the molded piece versus microspheres content in the measuring regions The highest increase in the number of micropores in the extrudate was observed when the microspheres content was increased from 0.5% to 1% (Figure 10). As a result, the number of micropores increased by 20%. When the measuring range was changed, the number and diameter of micropores changes, too. 78 Cellular Polymers, Vol. 35, No. 2, 2016

13 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis An analysis of the number of micropores in the produced molded pieces (Figure 11) demonstrates that the tested parameter increases with increasing the content of microspheres. However, this increase is not proportionate. The largest number of micropores can be observed in the core of the molded piece (regions 4, 5, 6) per each content applied. Figure 10. Number of micropores in the extrudate versus microspheres content in the measuring regions Figure 11. Number of micropores in the molded piece versus microspheres content in the measuring regions Cellular Polymers, Vol. 35, No. 2,

14 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz The results of structure examination of the molded pieces reveal that the measuring point has a significant effect on the investigated variables. All variables have higher values in the core of the test specimens (Figures 12 and 13). Compared to the extruded products, the molded parts have a surface layer with a considerably smaller number of micropores with smaller diameters. As a result, the area fraction of micropores is almost over two times smaller than that in the specimen core. The surface layer of the molded part has direct contact with the walls of the mold cavity, which prevents the production and growth of microspheres in this region. Figure 12. Number of micropores versus microspheres content in the surface layer and core: a) extrudate, b) molded piece 80 Cellular Polymers, Vol. 35, No. 2, 2016

15 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis The number of produced micropores increased with increasing the blowing agent content both in the core and surface layer of the microporous extrudate. It can be observed that there are far more micropores in the core of the specimen. The production of a smaller number of micropores in the surface layer can result from more intense cooling of the extrudate. The surface layer of the extrudate comes into contact with the cooling water faster, which prevents production of microspheres in this region. Figure 13. Area fraction of micropores versus microspheres content in the surface layer and core a) extrudate, b) molded piece The results also demonstrate that the size of micropores has a significant effect on their area fraction in the specimens. At the same time, however, this Cellular Polymers, Vol. 35, No. 2,

16 Aneta Tor-Świątek, Tomasz Garbacz, and Tomasz Jachowicz relationship for extruded products differs from that for molded pieces. It can be observed that the extruded products reveal the relationship between area fraction of micropores and micropore diameters (Figure 14). The application of the highest microspheres content (1.0%) yields the smallest micropore diameters (83 µm -90 µm) but the highest area fraction of micropores (33%- 39%). When the microspheres content is 0.5%, the diameter of micropores is between 87 µm and 99 µm, while their area fraction ranges from 15% to 24%. At the lowest microspheres content of 0.25%, micropores have the largest diameters ranging from 100 µm to 114 µm, while their area fraction is small, between 17% and 24%. Figure 14. Area fraction of micropores versus diameter of micropores on increasing the microspheres content in the extrudate As for the molded pieces, this relationship is not so direct (Figure 15). It can be observed that the range of micropore area fraction is similar to the range of micropore diameters (3-22% and µm, respectively) when the microspheres content is 0.5% and 1%. When the microspheres content is 0.25%, the above ranges are narrower and amount to 5-15% and µm, respectively. 82 Cellular Polymers, Vol. 35, No. 2, 2016

17 Quantitative Assessment of the Microscopic Structure of Extruded and Injected Low-Density Polyethylene Modified with Microspheres by Image Analysis Figure 15. Area fraction of micropores versus diameter of micropores on increasing the microspheres content in the molded part CONCLUSIONS A dynamic growth of the polymer processing sector, new directions in polymer applications and higher product requirements have lead to the development of innovative technologies and enhancement of auxiliaries used in polymer processing. Cellular extrusion and cellular injection molding processes are fast developing methods for processing thermoplastics. The experimental results demonstrate that the products manufactured by cellular extrusion and cellular injection molding significantly differ with respect to their macroscopic structure. The extruded products are porous in their entire cross section, while the molded pieces are much less porous in the surface layer but highly porous in their core. In addition to this, the sizes and numbers of micropores in the extruded products and molded pieces are different. Apart from processing temperature and microsphere contents, the produced structures were also significantly affected by the method of product cooling and cooling agent type. The extrudates produced by cellular extrusion do not have a solid surface layer due to direct contact with the cooling water. The intense of cooling process also influences of surface structure of obtained samples and sizes and amount of micropores on each extrudates and molds samples. Consequently, it is necessary to use an additional cooling device, e.g. a calibrator. Cellular Polymers, Vol. 35, No. 2,

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