INFLUENCE OF PARTICLE SIZE IN MULTI-LAYER ROTATIONAL MOLDING WITH A MULTIPHASE INTERLAYER TO GENERATE MECHANICAL ADHESION

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1 INFLUENCE OF PARTICLE SIZE IN MULTI-LAYER ROTATIONAL MOLDING WITH A MULTIPHASE INTERLAYER TO GENERATE MECHANICAL ADHESION Martin Löhner and Dietmar Drummer, Institute of Polymer Technology (LKT), Friedrich-Alexander- Universität Erlangen-Nürnberg, Germany Abstract Rotational molding shows the potential to build up multi-layer parts by sequential adding of different materials into a rotating cavity. The limited compatibility of several materials to each other reduces the potential material combinations significantly. Former investigations showed the general applicability of a multi-phase interlayer to bond incompatible materials during the rotational molding process. Within this interlayer interlocking occurs between the two materials. This work investigates the influence of particle size on the material distribution and peel strength for the material combination Polyethylene and Polyamide 12. It is shown, that the material distribution is depending on the particle size added to generate the interlayer whereas the peel strength is mostly unaffected if the interlayer thickness exceeds the particle size. For thinner interlayers smaller particles show higher peel strengths and a varying interphase region. Introduction Rotational molding shows distinct advantages for the production of hollow parts. Especially the design freedom, the potential to produce seamless hollow parts as well as the very low resulting residual stresses show potentials compared to competitive production technologies [1-4]. Besides the possibility to generate single-layer parts, additionally multi-layer build-ups can be generated by sequential adding of materials into the hollow mold [5, 6], e.g. by a so called dropbox which was patented by Duffy [7]. The different Polyethylene types are the by far most used material in rotational molding [3]. As characterized in various publications, Polyethylene shows only very limited specific adhesion to most other thermoplastic materials [8, 9], limiting the potential material combinations significantly. Besides specific adhesion additionally mechanical adhesion, which is based on interlocking, is a potential way to bond materials to each other [10, 11]. Sakaki et al. showed the general applicability of mechanical adhesion for multi-layer rotational molding [12]. In previous publications, the influence of relevant processing and geometric factors on the resulting bonding strength was characterized [13, 14]. The aim of this work is to reveal the influence of the particle size of the utilized polymer powders on the resulting layer build-up and bonding strength. The materials used in this investigation are Polyethylene (PE) and Polyamide 12 (PA 12). Fundamentals Multi-layer Rotational Molding The forming in rotational molding of thermoplastic materials takes place by polymer particles in powdery form adhering to the mold wall which is heated above the melting temperature of the polymer. By sequential adhering of additional powder particles, a loosely sintered polymer layer is generated. The resulting air inclusions within the polymer melt are removed while it is held in the melt state. This effect is driven by surface tension, which also leads to a smoothening of the inner melt surface [15-17]. Multi-layer parts can be generated by adding a second, inner material after the first outer material completely adhered to the mold surface. The above described process of adhering and homogenization of a material layer is then repeated on the inside of an already existing melt layer. As shown in previous publications, no relevant melt flow appears in rotational molding [15]. Only very limited flow on a micro-scale occurs due to the above mentioned surface tension effects. Besides mono-material layers also multi-phase layers can be generated. To achieve this, a blended or dry blended material mixture can be added [13]. Multiphase Interlayers to Generate Mechanical Adhesion in Rotational Molding Several works focused the possibility to generate mechanical adhesion between two base layers in rotational molding by adding a multiphase interlayer. Sakaki et al. [12] investigated the general applicability of the concept with the material combination PE and PP characterizing the resulting bonding strength as well as the fracture behavior. Further investigations were conducted with alternative material combinations as for example PE and SPE ANTEC Anaheim 2017 / 2235

2 PA 12 [13, 18] as well as with PE and thermoplastic Polyurethane (TPE-U) [14]. The influences of several processing parameters as well as layer build-ups, for example regarding interlayer thickness were characterized in these investigations. Adhesion of Polymers The adhesion of polymeric material to each other can be divided into serval mechanisms. Ehrenstein [10] divided them into three main groups which are described below. Other classifications of these mechanisms were for example defined by Kinloch [11] and Habenicht [19]. Mechanical adhesion Mechanical adhesion describes the interlocking of two materials due to a phase boundary incorporating undercuts. This adhesion mechanism is not significantly influenced by the material combination and the compatibility of the materials to each other. Mainly the structure of the phase boundary as well as the mechanical properties of the base materials determine the resulting bonding strength. A schematically bonding of two materials by mechanical adhesion is shown in Figure 1. [10, 19] secondary valence bonds are van der Waals bonds (which are divided into London, Debeye and Keesom forces). [10, 19] Materials Two thermoplastic rotational molding resins in powdery form, which show no relevant specific adhesion to each other, were used in this investigation. The used materials were Polyethylene Lupolen 4021 K RM Black Powder, provided by LyondellBasell Industries AF S.C.A. (PE) and Polyamide 12 Icorene 9005 provided by ICO Polymers (A. Schulman) (PA 12). Both materials were fractionated into three fractions utilizing sieving. The used screen sizes of the sieves were 250 µm and 315 µm. The resulting fractions as well as the respective envisaged particle size range is shown in the following table: Table 1: Generated fractions and their aimed particle size range Fraction Aimed particle size (by sieving) [µm] Fraction 1 < 250 Fraction 2 > 250; < 315 Fraction 3 > 315 Experimental Material Characterization Figure 1: Mechanical adhesion, according to [10, 19] Diffusion of polymers Between compatible polymers, segments of macromolecules can diffuse in the area of the phase boundary into the other polymer system. In this region entanglements of polymer chains develop and bonding between the polymer emerges. [11, 20] Valence and secondary-valence bonds of molecules Various bonds can build up between the molecules of different polymers. These can be divided into valence bonds and secondary valence bonds. Examples for The materials were characterized utilizing different procedures within this work. The particle shape of all fractions used was analyzed by scanning electron microscopy (SEM). The secondary electrons were detected to get information about the surface topology. Additionally, the powder was characterized in terms of flowability and bulk density. The respective measurements were conducted according to DIN EN ISO 6186: (opening: 25 mm) (flowability) and DIN EN ISO 60: (bulk density). Both characterizations were repeated seven times and the mean values were calculated. The average particle size was analyzed optically using a Morphologi G3 (Malvern Instruments). The thermal properties of the materials were characterized by a Differential Scanning Calorimetry (DSC). Here the first heating was analyzed with a heating rate of 10 K min -1. Processing Trials Processing trials were conducted using a uniaxial rotational molding machine (FILL Ges.m.b.H, Gurten, Austria) and a cylindrical mold with a length of 210 mm SPE ANTEC Anaheim 2017 / 2236

3 and a diameter of 211 mm. The rotation speed was set to 15 rpm for all experiments. The heating took place by contour following infrared heaters with adjustable intensity to ensure a homogeneous temperature over the mold length. Compressed air which was applied to the outside of the mold was used for cooling. Adding of material was possible during the whole process by an axial opening of the mold in combination with an adding device to spread polymeric material homogeneously over the mold length. The test setup is schematically shown in Figure 2. Figure 3: Temperature progress and adding times [13] In this work the influence of the particle size is investigated. Therefore, three different fractions were utilized for the interlayer. Doing so, the respective corresponding fractions of PE and PA 12 were utilized. For the PE and PA 12 base layers, non-fractioned material was used. A variation of the interlayer thickness was conducted between 0.5 and 1.0 µm. Specimen Characterization Figure 2: Schematic drawing of the rotational molding setup used [13] The different layers were generated by sequentially adding materials into the mold. In advance to the process start the mold was heated to 50 C and the first material (PE) was added into the mold. The multiphase material, a PE-PA 12 dry blend with equal material shares, was added during heating at 190 C, after the first component (PE) completely adhered to the mold wall. The third, inner material (PA 12) was added at 210 C, following to the spreading of the multiphase layer. After heating, the temperature was kept constant for a defined time t H = 600 s at processing temperature T P = 210 C. In the further progress the mold was cooled down to below 70 C before demolding the part. The progress of temperature and the respective adding times are schematically shown in Figure 3. The produced specimens were characterized using mechanical and optical methods. The peel strength of the resulting parts was characterized according to DIN EN ISO as schematically shown in Figure 4 and described below. Figure 4: Test setup of peel test according to DIN EN ISO Specimens with a length of 200 mm and a width of 25 mm were cut out of the produced parts following the SPE ANTEC Anaheim 2017 / 2237

4 circumference of the cylinder. The two layers were separated over a length of 50 mm at one side of the sample. The separation took place by cutting in the middle of the interlayer. These separated regions were clamped in the tensile testing machine. According to the mentioned standard, the testing took place with a testing speed of 100 mm min -1. For the characterization of the peel force a test distance of 200 mm was analyzed. Within this distance the maximum and minimum force was detected and the peel forces were calculated by averaging these values. The cross sections of the resulting specimens were characterized by transmitted light microscopy. Therefore, samples were cut out of the middle area of the cylindrical parts. Additionally, the resulting fracture surfaces of both layers were characterized by scanning electron microscopy (SEM). Results Material Characterization Table 2: Average particle size of the used materials and fractions Material Average particle size (volumetric) [µm] PE; non fractioned 456 PE; Fraction PE; Fraction PE; Fraction PA 12; non fractioned 328 PA 12; Fraction PA 12; Fraction PA 12; Fraction This difference between the fractions can also be seen in Figure 6. The fractions of both materials show a clear difference regarding particle size to each other. The shape of the particles is not influenced by sieving. All fractions show comparable particle forms and surfaces. The PA 12 particles have smoother surfaces, whereas the PE exhibits a micro-structured surface. The results of the DSC analysis are shown in Figure 5. The Polyethylene shows a melting peak at C and an enthalpy of fusion of J g -1. The Polyamide 12 whereas shows a melting peak at C and an enthalpy of fusion of 50.0 J g -1. The materials therefore exhibit a difference in melting temperature of about 51.3 K. The PE material shows a more distinctive endothermal transition while melting the crystalline structures. Figure 5: DSC analysis, 1 st heating [13] In Table 2 the average volumetric particle size is shown for both base materials as well as for the three fractions of each material. The PA 12 reveals slightly smaller average particle sizes for all fractions. This difference is most pronounced at fraction 1 with a difference of 82 µm. The generated fractions show distinctive differences between each other. Figure 6: SEM analysis of different powder fractions, left: PE; right: PA 12 The resulting flow characteristics are given in Table 3. The bulk density shows a clear difference between PE and PA 12, which can be traced back to the difference in the material density. The bulk porosity, which is not affected by the material density, shows no differences SPE ANTEC Anaheim 2017 / 2238

5 between both materials. For the PA 12 the bulk porosity increases for smaller fractions. Additionally, the pourability decreases for smaller PA 12 fractions. The PE shows no influence of the average particle size on the pourability or bulk porosity. Table 3: Powder characteristics Material Bulk density [g cm-3] PE; non fract. PE; Fract. 1 PE; Fract. 2 PE; Fract. 3 PA 12; non fract. PA 12; Fract. 1 PA 12; Fract. 2 PA 12; Fract. 3 Bulk porosity [%] Outflow time [s] Interlayer By adding a multiphase interlayer in between two base layers in rotational molding, a specific phase boundary results. This is shown for fraction 1 and fraction 3 in Figure 7. Over the aimed interlayer thickness of 1.0 mm a random phase boundary builds up. Within this region, interlocking occurs due to undercuts and entanglements on a mesoscopic scale. For the used fractions no clear difference in the resulting interlayer appeared for interlayer thicknesses much higher than the average particle size used for the interlayer. This is shown in Figure 7 for an interlayer thickness of 1.0 mm. Figure 7: Cross section of typical interlayers with a thickness of 1.0 mm In contrast to interlayer thicknesses much higher than the average particle size, the size of the particles has a clear influence on the resulting phase boundary for thin interlayers. Fractions with an average diameter in the range of the aimed interlayer thickness do not result in undercuts between both materials. Here, nearly no structures can be observed, which act for mechanical adhesion between both materials. This is shown on the right side of Figure 8 where non fractioned materials (average particle sizes: 328 µm PA 12; 456 µm PE) were utilized to form an interlayer with an aimed thickness of 0.5 mm. In contrast, by using fraction 1 (average particle sizes: 168 µm PA 12; 250 µm PE) undercuts occur between both materials over an interlay thickness of 0.5 mm, which is shown on the left side of Figure 8. Figure 8: Cross section of exemplary specimens with a interlayer thickness of 0.5 mm Peel Strength The resulting peel strength for the investigated interlayer thicknesses of 0.5, 0.75 and 1.0 mm are shown in Figure 9. Figure 9: Peel strength for different particle sizes and interlayer thicknesses For non-fractioned materials as well as for fractions 2 and 3 a comparable peel behavior was detected. Whereas for 0.5 mm interlayer thickness peel strengths of below 2 N mm-1 were characterized, the bonding increases up to around 5 N mm-1 (0.75 mm interlayer thickness) and to 9 11 N mm-1 for an interlayer of 1.0 mm. In contrast, interlayers generated out of small particles (fraction 1) show higher bonding strength for 0.5 mm and 0.75 mm aimed interlayer thickness. This effect can t be observed for 1.0 mm interlayer thickness. SPE ANTEC Anaheim 2017 / 2239

6 Fracture Behavior The fracture surfaces characterized by SEM are shown in Figure 10. The general facture behavior is comparable for all used powder fractions. Both fracture surfaces exhibit a coarse structure. Approximately spherical areas indicating ductile deformation and cohesive failure can be observed. In these areas undercuts or entanglements led to a mechanical adhesion of the two materials to each other. Beside these spherical areas, no cohesive failure can be detected in the surrounding material. No deformation of the surface at the phase boundary due to peeling was observed in these surrounding regions. The above described cohesive failure of the materials also led to PE remains on the PA 12 layer. Partially also PA 12 remains were detected on the PE layer. The fracture surfaces of the three used powder fractions show a distinctive difference in the size of the above discussed structures. With increasing average particle size used for the interlayer the cohesively failed areas grow, whereas the amount of these structures at a defined area tends to decrease with increasing particle size. smaller average particle sizes within all fractioned materials. Segregation effects during the time the interlayer material is not molten yet or during adhering could result from these differences. The outflow time and the bulk porosity as indicators for the flowability are comparable for both materials, whereas for the PA 12 both characteristic values are influenced by the particle size. Taken into account that the interlayer material was added in a preheated mold, segregation effects can be assumed to be negligible due to the adhering and melting of the interlayer material within a short period of time. For all particle sizes an increasing bonding strength was observed at thicker interlayers up to 1.0 mm. The reason therefore is presumably the more distinctive buildup of interlocking regions between both materials. Thicker interlayers lead to a broader region in which mechanical interaction of both materials can occur. This behavior, which was already discussed in previous publications [13, 14], is schematically shown in Figure 11. Thin interlayers lead to single interaction points which contribute to bonding, whereas a thicker interlayer results in various undercuts. Figure 11: Build-up of mechanical connection points for different interlayer thicknesses Figure 10: SEM analysis of fracture zones for the three used powder fractions, left: PA 12, right: PE, interlayer thickness 1.0 mm As shown in Figure 9, depending from the interlayer thickness, differences in the peel strength can occur utilizing different particle sizes. The ratio between the resulting peel strength using fraction 1 and fraction 3 are shown in Figure 12. For an interlayer thickness of 0.5 mm the resulting peel strength utilizing fraction 1 is about 2.7 times higher compared to the peel strength achieved with fraction 3. This ratio decreases with increasing interlayer thickness. Whereas for 0.75 mm, fraction 1 reveals about 1.7 times higher bonding, the ratio is balanced at 1.0 mm interlayer thickness. Discussion The material characterization showed significant differences in the melting behavior of both used polymers, exhibiting a difference in the peak melting temperature of about 50 K. Additionally, the PA 12 material showed smaller particle sizes what resulted in SPE ANTEC Anaheim 2017 / 2240

7 Figure 10. The size and the amount of the mechanical connection points differs depending on the used particle size. The resulting bonding strength whereas is comparable for different particle sizes, see Figure 12. The possible reason is that the area fraction acting for the mechanical adhesion is comparable using different particle sizes within a thick interlayer. Only the scaling of the structures differs regarding the utilized particle size. Conclusions In former publications the general potential to bond incompatible material in rotational molding was shown [13, 14]. In this work, the influence of the particle size on this approach was investigated. Figure 12: Ratio of peel strength using fraction 1 and fraction 3 for different interlayer thicknesses The possible reason for the above discussed dependence is the build-up of the interphase. Smaller particles lead to a small-scaled interphase region allowing undercuts and entanglements already within a thin interlayer. Larger particles with an average particle size in the range of the interlayer thickness would not lead to undercuts, as shown in Figure 8. Therefore, for thin interlayers a dependency of the particles size can be detected. These mechanisms are schematically shown in Figure 13. For thicker interlayers, significantly larger than the utilized average particle sizes, this effect is not applicable. Here, the thickness of the interlayer is sufficient to generate a network of undercuts and entanglements independent from the particle size, as shown in Figure 7. Multi-layer parts with a multiphase interlayer were produced by sequential adding of Polyethylene and Polyamide 12. For the interlayer different powder fractions were utilized with defined average particle sizes. For interlayer thicknesses clear above the average particle sizes utilized to generate the interlayer, no influence of particle size on the peel strength was detected. For interlayer thicknesses in the range of the average particle size used the build-up of interlocking in between the materials is hindered. Therefore, smaller particles lead to higher bonding strengths compared to larger fractions used for thin interlayers. For 0.5 mm interlayer thickness differences of about 170 % in peel strength were detected by using different particle sizes. The fracture behavior shows comparable structures for different particle sizes. Whereas the size and amount of mechanical connection points is affected by the particle size. This work therefore contributes to gain knowledge for the material selection as well as on the part design, especially regarding interlayer thickness, for multi-layer rotational molded parts bonded by mechanical adhesion. Acknowledgements This project has received funding from the European Union s Horizon 2020 research and innovation program under grant agreement No References Figure 13: Schematic drawing of the resulting material distribution within a thin interlayer utilizing different particle sizes The fracture behavior for thick interlayers in dependency of the used particle size is shown in 1. R.J. Crawford and J.L. Thorne, Rotational Molding Technology, William Andrew Publishing, Norwich (2002). 2. G.L. Beall, Rotational Molding, Carl Hanser Verlag, Munich (1998). 3. R.J. Crawford and M.P. Kearns, Practical Guide to Rotational Moulding, Rapra Technology Limited, Shawbury (2003). SPE ANTEC Anaheim 2017 / 2241

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