The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films

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1 The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films D.J. Whiteman 1, C. Agra-Gutierrez 2, M.J. Bird 1, S.E. Thomas 1, D.R. Skuse 1, and D.M. Ansari 1. 1 Imerys Minerals Limited, Par Moor Road, Par, Cornwall, PL24 2SQ, UK 2 A. Schulman Plastics N. V., Pedro Colomalaan 25, B 2880 Bornem, Belgium Received: 22 July 2007, Accepted: 14 April 2011 Summary The current study considers the use of a selection of engineered ground calcium carbonates in two different polyethylene film systems. The calcium carbonate grades selected cover a range of particle sizes and include uncoated and organically treated examples. Alterations in the processing requirements on mineral addition are reported and their implications for film and masterbatch production considered with respect to energy usage and throughput. Inclusion of the engineered additives allowed production of the same weight of material at lower energy uses or improved productivity at the same energy usage. The effects on the mechanical properties of the composite films with increasing mineral loading have also been measured, demonstrating enhancements particularly in the areas of tear strength and impact resistance. These enhancements allow the possibility of film downgauging relative to the unfilled film without loss of performance and yield a raw material cost saving by reduction of polymer usage. 1. INTRODUCTION The use of polymer films has become increasingly widespread, covering a range of different application areas which include relatively undemanding uses such as stretch film and supermarket bags through to speciality films which might contain multiple layers or exhibit some specific functionality. With an estimated film consumption of 33 million tons in the polyolefins, in particular polyethylene, have come to dominate the film market for many applications especially packaging. However, as with most production polymers these materials will require the addition of functional additives to help fulfil their final function. Such additive packages will potentially include a stabiliser to ensure long-term performance, a pigment for enhanced aesthetic properties or a mineral Smithers Rapra Technology, 2011 which can influence the mechanical performance of the polymer. The current trend in many applications, and particularly packaging, is to downgauge polymer parts or films in order to make material cost savings and weight reductions. However, this can lead to a loss of mechanical performance and in many cases the trade off between downgauging and a fit-for-purpose product has already been optimised. Often the move to a composite material such as fibre reinforced plastics rather than the unfilled polymer can lead to an improvement in these properties compared with the unfilled polymer, and it is well known that the use of reinforcing additives such as glass fibres, talc and mica can influence the mechanical performance of a composite 2,3,4,5. It is not uncommon however, that an enhancement in one property of a filled composite is accompanied by a deleterious effect on another, and so the choice of additive should be made with the properties of the specific application in mind 3. These properties will bear a direct relation to the properties of the additive including the size and aspect ratio. In order to maximise the positive effects of a functional additive care should be taken to make sure that it is well dispersed within the polymer matrix, emphasizing the importance of processing conditions. In many cases the dispersion may be influenced by the use of engineered functional additives with surface treatments which help compatiblise the additive with the polymer matrix 3. With the correct choice of surface treatment the additive can also be reactively coupled to the matrix, allowing further optimisation of properties 3. Historically, conventional ground calcium carbonates (GCC) have Polymers & Polymer Composites, Vol. 19, No. 9,

2 D.J. Whiteman, C. Agra-Gutierrez, M.J. Bird, S.E. Thomas, D.R. Skuse, and D.M. Ansari been used as a cost-reducing fillers in polymer systems, replacing part of the higher cost polymer in a composite with the lower-cost mineral. It is likely that additional benefits in terms of environmental impact can also be made here with the replacement of an oil-based polymer with a mineral potentially reducing the carbon footprint. However, this is likely to be very dependent on the mineral loading as any additional processing steps required to incorporate the mineral must be considered. The use of these low aspect ratio calcium carbonate particles offer little in terms of reinforcement to the composite 3 except at very fine particle sizes 6 of <1 µm. Increasingly, with the development of specialised or engineered calcium carbonate grades additional advantages to the composite can be realised, both in terms of mechanical properties and also in terms of processing improvements and energy savings 7 through the enhanced thermal conductivity of the composite. Such engineered grades will tend to be optimised in terms of the particle size, particle size distribution and surface chemistry for a specific composite property and application. The current work describes the use of a range of ground calcium carbonates, including engineered grades of different particle sizes and particle size distributions, and also the effect of surface treatment on the mechanical properties of an LDPE and an LLDPE/LDPE film blend with respect to their unfilled analogues. The effect of the inclusion of the mineral functional additive on the processing parameters is also considered in terms of the respective thermal properties of the polymer, the mineral and the resulting composite. using a Micromeritics Sedigraph 5100 (Figure 1) which demonstrate the differences in steepness of the particle size distributions that cannot be ascertained from the tabulated data. These grades are commercially available materials commonly used in film applications. The materials chosen include both uncoated and stearic acid coated calcium carbonates. Two film systems have been chosen using the following film grade polyethylenes ExxonMobil LD100BW LDPE (MI 2.0 g 10 min -1 ) 8 and Ineos Innovex LL6208AF LLDPE (MFR 0.9 g 10 min -1 ) 9. Films were produced from either the LDPE or a 90:10 blend of LLDPE/LDPE, the latter blended Table 1. Ground calcium carbonate grades used Calcium carbonate grade Mean particle size (µm) (sedigraph) material being chosen for a balance of its excellent optical properties and mechanical strength 10. In light of the results in the LDPE only the coated grades have been considered in the blend material. Masterbatches of all the calcium carbonates were prepared on an APV Baker Perkins MP2030 twin-screw compounder with a die temperature of 200 C, operating at a constant torque of 50%. Masterbatches of 50 wt.% and 40 wt.% calcium carbonate were prepared in the LDPE and LLDPE/LDPE blend respectively prior to letting down to the final concentrations during film production. 30 µm gauge films were prepared using a Dr. Collin 180/30 d 98 Topcut (µm) (sedigraph) Particle size for which 98% value Coarse particle analysis Particles > 25 µm (ppm) Stearate treated (Y/N) Supercoat Y Engineered grade A Y Engineered grade B Y Engineered grade C Y Carbital N Carbital 110S Y Carbital 115S Y Figure 1. Sedigraph particle size distributions of calcium carbonate grades 2. Materials and Methodology A description of the calcium carbonate grades used in the current study is given in Table 1 along with their particle size distribution curves measured 744 Polymers & Polymer Composites, Vol. 19, No. 9, 2011

3 The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films blown film line with a 60 mm diameter die and a die gap of 0.8 mm. A die temperature of 240 C was used. Films with filler loadings of 3-15 wt.% and 5-30 wt.% were prepared for the LDPE and LLDPE/LDPE blend films respectively. The effect of mineral loading on processing parameters was monitored for each formulation. Film surface temperature was also recorded at a distance of 12 cm above the die using a Minolta Land Cyclops 343 portable infrared (IR) thermometer to monitor the effect of mineral loading on the composite cooling. decreased energy usage indicates the relative ease of dispersing the surface treated mineral. It is also noted that a small decrease in work output is seen for Carbital 110S with loading, when considered on a constant volume basis. This reduction is attributed to the lubricating effect of the surface treatment Processing Benefits Film Production The current demand of the film line extruder is depicted in Figure 3 for Carbital 110S in LDPE at a constant throughput and is an effect noted in the literature 12. With increasing mineral loading the current demand is seen to Figure 2. Compounder work output vs. calcium carbonate loading in LDPE masterbatch 2.1 Film Testing Tensile strength was measured in the machine direction according to ASTM D using a Hounsfield H10K tensometer. Impact strength was measured by free falling drop dart using the staircase method as specified in ASTM D using a Ran Ray FDDI instrument. Single point tear strength in the machine direction was also measured using a Hounsfield H10K tensometer, following ASTM D RESULTS AND DISCUSSION 3.1 Processing Benefits Masterbatching The compounder work output for the 2 µm Carbital 110 and treated Carbital 110S grades in LDPE is shown in Figure 2. The work output relates to the difficulty of compounding 1 kg of material at a constant throughput and can also be used as an indication of the relative ease of dispersion of a particulate material within the polymer melt. For both the coated and uncoated grades the work output is seen to fall with increasing mineral addition corresponding to an energy saving or the potential to increase throughput at a constant energy usage. It can be seen that the use of a hydrophobic surface coating on Carbital 110S produces a lower work output than that of the uncoated Carbital 110. This Figure 3. Current demand in film line extruder vs. calcium carbonate loading in LDPE melt Polymers & Polymer Composites, Vol. 19, No. 9,

4 D.J. Whiteman, C. Agra-Gutierrez, M.J. Bird, S.E. Thomas, D.R. Skuse, and D.M. Ansari fall and can be directly related to the thermal behaviour and properties of the composite melt. On melt processing, the addition of the particulate solid gives rise to a shear in the extruder barrel and an additional shear heating and increased temperature of the melt, thus reducing the melt viscosity 3. This reduced viscosity allows the composite to be conveyed at a constant throughput with less energy use or gives the potential to increase throughput at a constant energy. Further productivity gains can be achieved by considering the thermal conductivity of the composite material compared with the unfilled polymer. The thermal conductivities of some commercial polymers and ground calcium carbonate are shown in Figure 4 13,14. It can be clearly seen that the thermal conductivity of calcium carbonate is ~ 5 times greater than those of the polyolefins with the conductivity of a polymer/calcium carbonate composite having an intermediate value. At low concentrations this intermediate value will follow a simple law of mixtures. However, at higher concentrations where particles will come into contact, this relationship will break down as a percolation threshold is reached 15. A composite with this enhanced thermal conductivity will experience faster melting in an extruder barrel and result in a more homogeneous melt temperature. The produced composite film will cool faster than the unfilled polymer, as demonstrated by the measured bubble temperature at increasing loadings, (Figure 5). This results in a lowering of the freeze line and a more stable bubble. The increased stability and faster cooling can also be utilised for increased productivity. higher performance of the polyethylene blend material is a reflection of the linear low component which intrinsically has a higher performance compared to the LDPE 8, Tensile Strength The tensile strength of the two film sets are shown in Figures 6a and 6b along with the elongation at break in Figures 7a and 7b. On the addition of calcium carbonate to the films a decrease in tensile strength is noted for all grades. In many cases the decrease between the individual loadings is within the experimental error of the technique. However, an overall decrease in strength is noted over the whole range. The tensile strength of these materials is dependent largely on the polymer and polymer chains which may unravel and distort considerably prior to breaking of the individual chains, followed by the sample as a whole. The inclusion of the mineral into the test specimen means that some polymer must be removed. Due Figure 4. Thermal conductivities of calcium carbonate and some commercial polymers Figure 5. Blown film bubble surface temperature vs. calcium carbonate loading for LDPE composite film 3.3 Mechanical Performance of Blown Films The mechanical properties for the LDPE films and the LLDPE / LDPE films are shown in parts (a) and (b) of the following figures respectively. The 746 Polymers & Polymer Composites, Vol. 19, No. 9, 2011

5 The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films to the higher relative density of the mineral the volume of polymer lost, and therefore the number of polymer chains effectively removed, is relatively low and the stress may be distributed over the remaining ones equally without too much loss in tensile strength e.g. 10 wt.% of calcium carbonate is equivalent to 3.5 vol.% in this system. As such the decrease in tensile strength with incremental filler loading increase is small. The presence of coarse particles are known to concentrate stresses leading to crack initiation and also to failure by hampering the movement of the polymer chains. This is demonstrated here with the finer grades such as Supercoat having the higher tensile strength and elongation at break, particularly at higher loadings. Supercoat also benefits from a controlled topcut, as seen by the d 98 results (particle size for which 98% of particles are value) and coarse particle analysis, as shown in Table 1, demonstrating the absence of oversized particles. The complete particle size distributions, as shown in Figure 1, show the correlation between the tensile strength and the particle size with the finer coated grades with improved topcuts having the higher tensile strength. The coated, coarse grades have an intermediate strength and the coarse uncoated materials giving the lowest. This latter group can be explained by the level of dispersion of the mineral within the matrix. Improved dispersion also has an effect on the tensile strength of these materials which can be demonstrated by comparison of Carbital 110 (uncoated) and Carbital 110S (coated) in the LDPE film blends. The surface treatment leads to an improved dispersion and few aggregates which results in a higher relative tensile strength. Figure 6a. Tensile strength of calcium carbonate filled LDPE composite films Figure 6b. Tensile strength for calcium carbonate filled LLDPE/LDPE composite films Figure 7a. Elongation at break for calcium carbonate filled LDPE composite films 3.5 Tear Strength Tear strength in film is related to the propagation of a crack cut into the test sample. Under an applied force the crack concentrates the stress resulting in an area of high stress preceding Polymers & Polymer Composites, Vol. 19, No. 9,

6 D.J. Whiteman, C. Agra-Gutierrez, M.J. Bird, S.E. Thomas, D.R. Skuse, and D.M. Ansari Figure 7b. Elongation at break for calcium carbonate filled LLDPE/LDPE composite films Figure 8. Crack blunting in a polymer composite. The propagating crack induces a second crack at the polymer / filler interface due to a high stress area in front of the crack. The cracks then run together to form a blunt crack the crack tip. As a crack propagates, making two new surfaces, a number of processes occur at the crack tip including plastic deformation, tearing, stretching and flowing of the polymer. The introduction of a mineral, fibre or even a rubber domain can lead to the blunting of a crack by the formation of an induced crack at the additive surface and the running together and arrest of these multiple failures. The induced crack and blunting results in a dissipation of energy and an overall increased tear strength. A schematic of the mechanism of crack blunting is shown in Figure 8. The tear strengths for both film types are shown in Figures 9a and 9b. It can be seen that at lower loadings, up to 10 wt.%, the addition of a fine carbonate results in an increase in tear strength compared with those of the unfilled samples, with only the coarse Carbital 115S showing a reduction in tear strength at these loadings. The finer materials all pass through a maximum value before falling to a value equivalent to that of the unfilled sample at approximately 20 wt.% loading. Tear strength is improved by the presence of the surface coating as demonstrated by the comparison of Carbital 110 and Carbital 110S. This increase can be attributed to improved dispersion resulting in a reduction of coarse aggregates which could lead to weak points in the film. The benefit from the reduction of coarse particles is also seen with the controlled top cut in the Supercoat samples. The presence of the surface coating also affects the debonding stress of the system and the energy required to separate the mineral/ polymer interface. The debonding stress is related to the particle size, the elastic modulus of the polymer and the interfacial energy of the particulate 16. It is anticipated that the coating lowers the interfacial energy resulting in a lower debonding stress and a more efficient blunting of propagating cracks by micro void formation at the polymer/ mineral interface. These improvements in tear strength with the incorporation of a fine coated calcium carbonate can potentially allow for a downgauging of the film without a loss of tear strength and potentially offering a raw material cost saving. 3.6 Drop Dart Impact Resistance For many polymer applications impact resistance and toughness are measured using the Charpy or Izod tests where a notched sample is mounted vertically and impacted with a falling pendulum weight. For film applications these tests are not applicable and so a falling weight or drop dart impact test is used. The drop dart test consists of stretching the film tight over an orifice of a given size and dropping a weighted dart vertically from a standard height. The weight of the dart is incrementally increased and fresh sections of the film tested until a weight is found that consistently causes a failure of the film. Details of this test are given in ASTM D The results for the drop dart impact resistance measurements are given in Figures 10a and 10b. The ability of a film to withstand an impact relies on the energy of the impact to be rapidly dissipated through molecular motions, or other energy dissipation mechanisms, rather than film failure. The ability of the polymer to debond at a polymer additive interface without holing the film is one such energy dissipation mechanism. It can be seen that for all coated grades there is an improvement in impact resistance relative to the unfilled films, up to at least 10 wt.% depending on the grade. The finer coated grades tend to have higher impact resistances than the 748 Polymers & Polymer Composites, Vol. 19, No. 9, 2011

7 The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films coarser materials and the beneficial effect of additive inclusion is seen at higher loadings. The coarse Carbital 115S grade shows a deterioration in impact resistance after 10 wt.%. A likely explanation of this is due to the larger particle size distribution and top cut (d 98 ) resulting in a lower specific surface area and, thus, a lower area available for debonding and energy dissipation. The uncoated Carbital 110 is the only sample to show a reduction in impact resistance with respect to the unfilled film. The lowering of the interfacial energy by surface coating provides a low stress route to debonding and micro voiding which allows for an additional energy dissipation mechanism thus increasing the impact resistance. This improved impact resistance could again allow for downgauging of the film without the loss of mechanical properties and potentially allow for a raw material cost saving. An example of this downgauging is given in Figure 11 for Supercoat in LLDPE film produced under the same conditions as those above. The results show that an equivalent impact energy to the unfilled film can be achieved at 20 wt.% mineral inclusion with a film thickness reduction of 39%. Figure 9a. Tear strength for calcium carbonate filled LDPE composite films Figure 9b. Tear strength for calcium carbonate filled LLDPE/LDPE composite films 4. CONCLUSIONS A selection of calcium carbonate/ LDPE and calcium carbonate/lldpe/ LDPE composite films has been produced by masterbatch production and film blowing. At both stages of production, processing advantages have been demonstrated with the use of calcium carbonate, compared with the unfilled polymer. During masterbatch production a decrease in work output, a measure of the energy required to compound 1 kg of composite at a constant throughput, was noted with the addition of calcium carbonate. A further decrease in work output was noted with the use of a coated calcium carbonate compared with an uncoated grade. This is indicative of the ease of dispersion Figure 10a. Drop dart impact resistance of calcium carbonate filled LDPE composite films Polymers & Polymer Composites, Vol. 19, No. 9,

8 D.J. Whiteman, C. Agra-Gutierrez, M.J. Bird, S.E. Thomas, D.R. Skuse, and D.M. Ansari Figure 10b. Drop dart impact resistance for calcium carbonate filled LLDPE/ LDPE composite films Figure 11. Drop dart impact resistance of calcium carbonate filled LLDPE composite films For tear strength all but the coarsest grades showed an improvement over the unfilled polymer with the presence of a physical barrier providing a mechanism for the blunting of a propagating crack and a relieving of the associated stress. Between coated and uncoated analogous grades a higher tear strength was seen for the coated materials. This is attributed to an improved dispersion leading to the presence of fewer coarse aggregated particles acting as stress concentrators. The organic coating also reduces the interfacial energy of the calcium carbonate, allowing easier debonding and more efficient blunting of propagating cracks. A similar effect is noted in the drop dart impact resistance results with the coated grades offering an additional energy dissipation mechanism via debonding. The coarser grades offered a smaller improvement due to the lower specific surface area and thus, a lower-energy dissipation mechanism. of the hydrophobic coated grade in the polymer melt. This reduction in required energy for compounding can lead to a cost saving at constant production rates or an increase in productivity at a constant energy usage. Further energy savings can be realised in the film blowing stage by the ease of feeding of the composite materials due to shear heating and reduced viscosity in the melt. The addition of the calcium carbonate also results in faster cooling of the film and a more stable bubble which offers the possibility of faster line speeds. The addition of the correct choice of calcium carbonate grade has also been shown to influence greatly the mechanical properties of the studied polyethylene films, particularly in terms of tear strength and impact resistance. In both cases fine surfacetreated calcium carbonate grades have shown the greatest improvement in properties. The noted improvement in both tear strength and impact resistance with the addition of calcium carbonate, in particular the fine stearic acid-coated grades with good topcuts, offers the possibility of down-gauging of the composite film with respect to the unfilled polymer. This could result in raw material cost savings without a loss of mechanical performance. Based on the balance of properties the Supercoat grade fine, treated calcium carbonate is recommended for use in polyethylene films, giving processing improvements and superior mechanical performance particularly at higher loadings. REFERENCES 1. Pardos Marketing. Flexible Packaging 2005, Pira 5 th International Conference, Brussels, Belgium(2005). 2. Katz H.S. and Milewski J.V., Handbook of Fillers and Reinforcements for Plastics, Academic Press, New York, USA (1978). 750 Polymers & Polymer Composites, Vol. 19, No. 9, 2011

9 The Influence of Engineered Calcium Carbonate Functional Additives on the Mechanical Properties and Value Proposition of Polyethylene Films 3. Rothon R.N., Particulate-filled polymer composites, 2 nd edition, Rapra Technology Shrewsbury UK (2003). 4. Wypych G., Handbook of Fillers, 2 nd edition, Toronto, Canada (2000). 5. DeArmitt C., Plastics Additives and Compounding, September (2001). 6. Pukansky B., Composites, 21, (1990) Ansari D.M. and Higgs R.P, Polymers, Laminates and Coatings Conference, 24 th -28 th Aug TAPPI (1997). 8. ExxonMobil data sheet LD100BW. ExxonMobil Corporation. 9. Ineos Datasheet Innovex LL6208AF, Ineos Polyolefins. 10. Yilmazer U., Bakar M., and Sahed Z., Antec 87. (1987) Los Angeles, USA. 11. Hawworth B. and Jumpa S., Plastics, Rubbers and Composites, 28 (1999) Batiste J.L.H., Shaw L.G., and Ballard R.R., Laminations and Coatings Conference. TAPPI (2000), Toronto, Canada. 13. Mark J.E., Physical Properties of Polymers, 2 nd Edition, Dekker, New York, USA (2007). 14. Xanthos M., Functional Fillers for Plastics, 2 nd Edition, Wiley,Weinheim, Germany (2010). 15. Berlin A.A., Volfson S.A., Enikolopian N.S., and Negamatov S.S., Principles of Polymer Composites, Elsevier (1991). 16. Zhuk A.V., Knunyants N.N., Oshmyan V.G., Topolkaraev V.A., and Berlin A.A., Debonding microprocesses and interfacial strength in particle filled polymer materials., J. Mater. Sci., 28, (1993) Polymers & Polymer Composites, Vol. 19, No. 9,

10 D.J. Whiteman, C. Agra-Gutierrez, M.J. Bird, S.E. Thomas, D.R. Skuse, and D.M. Ansari 752 Polymers & Polymer Composites, Vol. 19, No. 9, 2011