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INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams
INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams
INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams
INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams
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INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams

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  • 1. Paper - 10 INNOVEX® Polyolefin a novel foam enhancement technology for LDPE foams Sergio Torres-Giner, Carlos Vázquez-Torner, José Luis Feijoo-Gómez FERRO SPAIN, Specialty Plastic Division (PCEM), Almassora, Castellón, 12550, Spain Abstract Nowadays foaming offers several benefits such as weight reduction and efficient thermal insulation to polyolefins processors. One of the most important applications of flexible polymer foams is found in the packaging industry acting as impact energy absorbers in which the mechanical behavior in compression is one of the most desired characteristics. Ferro, as a leader in the technology for nucleation in the extruded polystyrene (XPS) foams market, has recently developed a new foam enhancement masterbatch that improves the mechanism of nucleation and reinforcement in polyolefin foams. This technical paper presents and describes the benefits of the novel Ferro’s masterbatch INNOVEX® LD-0392NU on extruded non-crosslinked low density polyethylene (LDPE) foams. From a structural point of view, the presence of this new nucleating agent made possible to reduce the cell size and the open cell content when compared to a typical talc masterbatch at same process conditions. Furthermore, regarding the mechanical properties, Ferro’s masterbatch produced an overall improvement in the compression performance that was achieved due to a severe reduction of the number of cells that break during compression. The new masterbatch is based on the Ferro’s proprietary technology that is commercialized under the trade name of INNOVEX®. Introduction Today the polyolefin foams represent a fast growing segment within the polyolefin market. Polyolefin foams can be produced in a great variety of densities, ranging from a very few kg/m3 to solid polymer density. Low density polyethylene (LDPE) foams habitually present a closed-cell structure, which makes them suitable for applications where flexibility is important as well as provides certain chemical resistance, thermal and electrical insulation. Such versatile properties make LDPE foams ideal for packaging applications but also open up new possibilities in weight and energy saving for other industries such as construction, transportation, insulation, sports and leisure, and agriculture (1). Additionally this corresponds with the sustainable trend in modern society to create new products with the consumption of fewer raw materials. The industrial process for non-crosslinked foams consists basically on the direct extrusion in a tandem extruder of a multicomponent formulation including a LDPE resin, a physical gas and a chemical blowing agent and/or nucleating agent. Colorants, stabilizers, and other additives are frequently added to obtain the desired properties. After the residence time required for dissolving the gas in the polymer and cooling the melt, the high-pressure drop at the extrusion die exit causes the instantaneous expansion of the gas encapsulated inside the polymer matrix that generates the foam. Ultrathin mineral fillers, i.e. inorganic structures below 1μm, represent a novel generation of functional nucleating agents for producing a large number of heterogeneous nucleation sites and retarding the bubble growth in the early stage of the foaming process. The combination of such low-sized particles with polymer foaming technologies can lead to the achievement of novel foams with improved cellular structure that at the same time present an increase in the macroscopic properties (2). Not only the particle size but also the particle shape and the surface chemistry of the particles play an important role on the bubble formation. Typically non-crosslinked low density LDPE foams have a mean cell size of ca. 1,500 microns by using conventional nucleating agents such as micrometric lamellar talc. However, the uniform dispersion of intercalated organomodified montmorillonite has led to reduce cell sizes and narrower the cell size distribution in LDPE foams (3). A recent study (4) has also shown that the addition of silica nanoparticles produced an improvement of the cellular morphology in LDPE foams, not only in achieving reduced cell sizes and increased cell densities but also in cell size distribution homogeneity. In other polyolefin foams, for instance polypropylene, the incorporation of nanoparticles has not only improved the cellular morphology but has also dramatically increased the melt viscosity and melt strength, which lessened the occurrence of cell coalescence (5). The main objective of this work is to study the effect of Ferro’s novel nucleating agent based on ultrathin natural clays on the morphology, thermal and mechanical properties of non-crosslinked LDPE foams. The relationship between the cell morphology and the foam mechanical properties was also considered.
  • 2. Experimental Materials INNOVEX® LD-0392NU masterbatch was fabricated using a twin-screw technology extruder at FERRO’s facilities. The proprietary composition consists of ca. 50% cocktail of non-modified ultrathin clays pre- dispersed in an olefinic-based carrier. The particles are based on an optimal combination of fibrillar and lamellar morphologies. The resin employed was a LDPE for foam extrusion with a density of 0.92 g/cm3 and a melt flow index of 1.9 g/10 min at 190ºC. A commercial 50% talc masterbatch was also used. Foaming Industrial scale LDPE foamed rectangular sheets of 11 mm of thickness and an approximate density of 20 kg/m3 were obtained using a tandem single/single-screw extruder at 650 Kg/h. Isobutane was used as physical blowing agent. LDPE foams were obtained nucleating with talc masterbatch and with INNOVEX® LD-0392NU masterbatch, in both cases dosing at 1.0% and keeping the same process conditions. LDPE planks of ca. 50 mm were also produced by an industrial thermal welding process of five LDPE sheets. Foam Characterization Density measurements of the LDPE foams were performed by Archimedes principle using the density determination kit for the AT261 Mettler-Toledo balance, following the ASTM standard D1622-08. The open cell content (OP) was measured with an Eijkelkamp 08.06 Lange air picnometer according to ASTM D6226-10. Cellular structures were obtained by optical microscopy. A Leica DM2500M optical microscope was used. Cell size and anisotropy ratio were determined from foams sections in the extrusion and wide direction using an image processing tool based on the software Image J. Thermal properties were analyzed by Differential Scanning Calorimetry. Measurements were performed in a Mettler-Toledo DSC 822e equipment. The thermal program, under nitrogen atmosphere, consisted of: A heating step from 40 to 200°C at a heating rate of 10°C/min, an isothermal step at 200°C for 3min to erase thermal history, a cooling step (crystallization) from 200 to 40°C at 20°C/min and a heating step 40 to 200°C at 10°C/min. Samples mass was ca. 10 mg. Compression tests were performed on LDPE planks of 30 x 30 mm section using a universal testing machine (Instron 5500R6025) at a strain rate of 10 mm/min and maximum static strain of 75%. Collapse stress (σC) and stress at maximum strain (σ-75%) were obtained from the experiments following the standard ISO 604-2002. All foam characterization was conducted at CellMat Technologies SL (Valladolid, Spain). Results Cell Morphology Table 1 presents the density and the open cell content for talc and INNOVEX®, respectively. This shows that density is kept for both talc and INNOVEX® in the LDPE foams and, more notably, foam samples nucleated with INNOVEX® presented a reduction in the percentage of open cells of ca. 47%. Table 1. Cellular characterization of LDPE foams. Sheets Density (Kg/m3 ) OP (%) Talc 19.31 ± 0.02 16.5 ± 0.9 INNOVEX® 19.55 ± 0.09 8.7 ± 2.6 Figure 1 shows the cellular images of the LDPE foams nucleated with talc (left) and INNOVEX® (right), respectively. In the figure it is possible to notice the lower cell sizes obtained with INNOVEX® when compared to talc. Besides it is worthy to mention that the distribution of cell sizes seems to be much more regular. Figure 1. Optical images, in the extrusion direction, of the LDPE foams for: A. Talc; B. INNOVEX®. Table 2 gathers the cell sizes in the three main directions of the foams. This indicates that the influence of the ultrathin clay particles used in INNOVEX® produced a reduction of ca. 12% on the mean cell size for the LDPE foams. Results also show that the cell orientation was not modified by the presence of INNOVEX®. In all cases cells were, to some extent, oriented to the gravity.
  • 3. )1( s f      )1( )1( 0 OP P s f C          Table 2. Cell morphology for LDPE foams. Sheets Dext (m) Dwide (m) Dgrav (m) Dmean (m) CAR* Talc 1339 1379 1714 1477 1.26 INNOVEX® 1175 1226 1520 1307 1.27 * Cell Anisotropy Ratio: Dgrav / (Dext + Dwide) Thermal Properties The effect of INNOVEX® on the thermal properties of the LDPE foams is summarized in Table 3. The similar melting temperature (Tm), crystallization temperature (TC), temperature difference between melting and crystallization (ΔT) and crystallinity (C) of the foams indicate comparable nucleating effect of INNOVEX® and talc on the crystallization process of the LDPE matrix. Table 3. Thermal characterization of LDPE foams. Sheets Tm (ºC) TC (ºC) ΔT (ºC) C (%) Talc 111.33 97.03 14.30 44.00 INNOVEX® 111.35 97.00 14.35 43.31 Mechanical Properties In Figure 2 the stress–strain curves of the LDPE planks for talc and INNOVEX® are depicted. Both foams presented a flexible behavior in the compressive test. The observed response consists on a short period in which the strain occurs immediately on applying the stress that, above the collapse stress, is followed by a substantial deformation as a function of the applied load. Figure 2. Compression test carried out on the LDPE planks for talc and INNOVEX®. The compressive values obtained from the stress- strain curves are reported in Table 4. This shows that the cellular structure of the LDPE foamed samples reinforced with INNOVEX® presented higher compression resistance and lower residual strain. In particular, the collapse stress (σC) and the stress at maximum strain (σ- 75%) were increased ca. 9 and 11%, respectively. The increase in compressive strength is connected to the better cellular structure in terms of cell size and open cell content. Table 4. Mechanical characterization of LDPE foams. Planks σC (KPa) σ-75% (KPa) ԑRESIDUAL (%) Talc 4.2 ± 0.2 155.0 ± 3.9 17.4 ± 0.4 INNOVEX® 4.6 ± 0.3 173.9 ± 2.1 15.1 ± 1.6 The following equation (Eq. 1) can predict the stress (σ) – strain (ԑ) relationship in compression for low density flexible foams, considering isothermal conditions with short timescales (6): (1) Where σC is the collapse stress, P0 is the pressure of gas enclosed in cells and ρR is the relative density of the foam, i.e. the foam density (ρf) divided by the density of the solid polymer (ρs). This considers that, in the post- collapse zone, load is supported partially by the cellular structure and partially by the gas enclosed on the cells. The gas contribution is directly related to the closed cells fraction )1( OP . The lower the percentage of open cells the higher is the stress needed to compress the foam. From Eq. 1 the following term can be detached: (2) This is named ‘gas volumetric strain’ and it is related to the contribution of the gas to the foam compressive response. Curves for the LDPE foams were obtained from the experimental data plotting the compressive stress applied as a function of the ‘gas volumetric strain’ (see Figure 3). According to this isothermal compression
  • 4. model, the slope of the curve represents the effective pressure of the gas contained in the cells and its axis cross-point corresponds to the collapse stress value. Figure 3. Stress vs ‘gas volumetric strain’ for the LDPE planks. Table 5 shows the value of the effective gas pressure inside the LDPE foams obtained from the curves for talc and INNOVEX®. This shows that, during compression, the amount of pressure in the foams reinforced with INNOVEX® was ca. 21% higher. This suggests that the overall improvement in compression performance for the LDPE foamed with INNOVEX® is achieved due to the initial low fraction of open cells and, predominantly, to the reduction of the number of cells that break during compression. The same reasons explain the lower residual strain for the foams containing INNOVEX®. As the amount of gas enclosed in the cells after the compressive cycle is higher for these materials the recovery is quicker and more complete. Table 5. Effective gas pressure enclosed in the cells. Planks )1(0 OPP  (KPa) Talc 42.8 ± 0.9 INNOVEX® 54.0 ± 1.7 Conclusions The current paper describes the main benefits of new Ferro’s masterbatch INNOVEX® LD-0392NU for LDPE foams. The ultrathin particles included in INNOVEX® act as a novel nucleating agent that contributes to the creation of a very regular celled LDPE foam with low cell sizes and reduced open cell content. From a mechanical point of view, the presence of INNOVEX® significantly induces an improvement in the compression and recovery performance which is achieved due to the generation of an enhanced and reinforced cellular structure that reduces the number of cells that break during compression. On account of the above, the INNOVEX® technology for polyolefins can be regarded as novel foam enhancement platform to replace conventional foaming additives such as talc and chemical blowing agents. The technology can be successfully applied to the industrial production of low density polyethylene foams with enhanced mechanical performance or, alternatively, to obtain same foams with lower densities without inducing a mechanical impairment. Results can be extended to polypropylene foams as well as other foams in flexible and semi-flexible packaging applications. References 1. D. Klempner and K.C. Frisch, eds., Handbook of Polymeric Foams and Foam Technology, Oxford University Press, Munich, Vienna, New York (1991). 2. L.J. Lee, C. Zeng, X. Cao, X. Han, J. Shen, G. Xu. Composites Science and Technology, 65, 2344 (2005). 3. S.M. Seraji, M.K.R. Aghjeh, M. Davari, M.S. Hosseini, Sh. Khelgati Sh. Polymer Composites, 32, 1095 (2011). 4. C. Saiz-Arroyo, M.A. Rodriguez-Perez, J.I. Velasco, J.A. de Saja. Composites: Part B, 48, 40 (2013). 5. W. Zhai, T. Kuboki, L. Wang, C.B. Park, E.K. Lee, H.E. Naguib. Industrial & Engineering Chemistry Research, 49, 9834 (2010). 6. E. Laguna-Gutierrez, E. Solorzano, S. Pardo-Alonso, M.A. Rodriguez-Perez. FOAMS® 2012 - 10th International Conference on Foam Materials & Technology, Conference's poster for the meeting, Barcelona, Spain (2012). Keywords: LDPE foams, INNOVEX®, ultrathin particles, flexible packaging. Acknowledgments CellMat Technologies SL (University of Valladolid) is acknowledged for all the foam samples characterization. In particular we would like to thank Dr. Miguel A. Rodríguez-Pérez and Dr. Cristina Saiz- Arroyo for their technical support and helpful discussion about the experimental results.

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