impact modifier boosts toughness of films

Plastics Additives & Compounding January/February 2008

30ISSN1464-391X/08 © 2008 Elsevier Ltd. All rights reserved.

The trend towards ever-thinner and yet stronger films is currently fuelling interesting market developments. At the present time, about 75% of all films in Western Europe are made of polyethylene. Worldwide, about 50% of the total polyethylene production goes into the manufacture of films. However, market growth can be mainly put down to the linear copolymers that are produced by means of the low-pressure method using ethylene and higher alpha-olefins (LLDPE) that offer better material properties. Indeed in striving to obtain higher-quality products, manufacturers are even willing to accept the somewhat worse optical properties of LLDPE in comparison to LDPE. The same applies to the newer mLLDPE grades that are produced using metallocene catalysts, where a narrower processing window and even higher material prices are tolerated with an eye towards meeting the growing demand for products of better quality.

Improving properties of PE films

Films made of mLLDPE have a number of advantageous properties. They exhibit exceptional toughness and strength as well as good visual properties (in blends with LDPE). This means that they lend themselves for use not only in food packaging (for example, freezer films), but also for film applications such as high-quality stretch films and heavy-

duty bags. And yet, the property level of the mLLDPEs has still not reached what is technically feasible. As is the case with other film materials, the mechanical characteristic values of films made of this material can still be markedly improved through the targeted use of special additives – for example, by admixing small quantities of Styroflex® 2G 66, one of BASF’s thermoplastic elastomers. The idea of improving film materials by adding an impact modifier is not new, but it runs up against certain limitations in the case of mLLDPE. For a long time, the toughness and tackiness of PE-based cling wraps have been improved by adding VLDPE; polypropylene films are modified through the addition of PP-based thermoplastic elastomers. For this purpose, copolymers such as ethylene vinyl acetate (EVA) or ethylene butyl acrylate (EBA) are offered for LDPE films. However, as a rule these additives have the drawback that they have to be used in fairly large amounts, which has a detrimental effect on the stiffness of the blends. Another aspect regarding the modification of mLLDPE is that this polymer is already a polyethylene variant with a very high toughness – after all, it is used itself as an impact modifier for LDPE. So the challenge is to find an even tougher material that raises the properties of mLLDPE to the next level without impairing its modulus of elasticity. Styroflex® 2G 66 can improve the toughness of mLLDPE without impairing the stiffness of the blend in the concentration ranges recommended.

Polyolefin compatibility

Styroflex® 2G 66 is a block copolymer made of styrene and butadiene (SBS) having the properties of a thermoplastic elastomer (see Figure 1). It stands out for its extreme toughness and enormous resilience: after being stretched by 500%, Styroflex®

films recover by 400%. In conventional PE-based stretch films, this resilience is limited to about 100%. Drop impact tests have shown that 10-litre stand-up pouches made of films containing a Styroflex® layer can withstand drop heights of up to 2.5 metres – similarly structured mLLDPE/

Impact modifier boosts toughness of films

Films are among the most important products to be made of polyolefins. Since they are primarily mass-produced, even slight reductions in the film thickness have the potential to lead to considerable savings. Robert Huber and Jörg Auffermann of BASF explain how Styroflex® 2G 66 SBS block copolymer can be used to increase the puncture resistance and the tear propagation resistance of mLLDPE and PP films. However, this is only the case if the additive is optimally distributed in the polyolefin phase and is present in dispersed form within the nano range.

The secure packaging of sharp-edged products is a task that only a few high-quality film materials can master. In fact, Styroflex® 2G 66 made by BASF AG helps to attain considerable savings. (Photo: BASF AG)

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F i l m p r o d u c t i o n


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LDPE films reach only one-third of this value. Due to the puncture resistance and tear propagation resistance of the material, Styroflex® films are also used wherever pointed or sharp-edged products have to be securely packaged (see Table 1).In addition, Styroflex® 2G 66 elastomer can be combined with a number of other polymers. Due to the fact that Styroflex®

2G 66 contains slightly less butadiene than commercially available SBS grades, its polarity lies between that of PE, PP and PS,

making it compatible with styrenic plastics as well as with polyolefins. Studies of injection-moulded test specimens as well as industrial injection moulding applications have shown that Styroflex® 2G 66 not only increases the tensile strength of HIPS and gives PS and ABS a higher (impact) strength, but that it can also function as a compatibilizer between PS or ABS and PE or PP. Moreover, it can be used in blends as well as in coextrusion with polyethylene and polypropylene. The toughness can also

be further enhanced further, particularly for the high-performance film material mLLDPE.

PolyethyleneBASF has recently undertaken experiments in cooperation with the University of Aalen in Germany to demonstrate that by incorporating the additive puncture resistance can be increased without altering the modulus of elasticity. These experiments demonstrated that the puncture resistance of mLLDPE blown films (made of Exceed® 1018A, an mLLDPE from ExxonMobil; thickness of 50µm; production parameters: screw speed of 80 1/min, blow-up ratio of 3:1, melt temperature of approximately 180°C [356°F]) in a dart drop test (according to ASTM D 1709) was already markedly improved by adding a mere 2.5% Styroflex® 2G 66 to the polyolefin. High-speed puncture tests (according to DIN 53373) and measurements of the tear propagation resistance (ASTM D 1922) especially in the lengthwise direction also yielded much higher values.Moreover, measurements of films containing 0-5% Styroflex® suggest that the property improvement at low additions follow an approximately linear course. The tear propagation resistance values according to Elmendorf (ASTM D 1922) in the lengthwise

Figure 1: Styroflex® 2G 66 is an extremely tough block copolymer made of styrene and butadiene that was specially developed for film applications. When used as an additive, even at low concentrations, it can already increase the puncture resistance of polyolefins without impairing their stiffness. (Photo: BASF AG)

Table 1: Styroflex® 2G 66: Selected material properties.

Property Test Method Unit ValueDensity ISO 1183 kg/m3 1.000

Melt-flow index (190 ºC/2.16 kg) ISO 1133 cm3/10 min 3

Melt-flow index (230 ºC/2.16 kg) ISO 1133 cm3/10 min 14

Melting temperature during film extrusion ºC 190-220

Tensile modulus ISO 527-1/-2 MPa 120

Yield stress, 50 mm/min ISO 527-1/-2 MPa 4

Yield stress, 50 mm/min ISO 527-1/-2 % 5

Nominal strain at break, 50 mm/min ISO 527-1/-2 % > 650

Flexural modulus ISO 178 MPa 140

Flexural strength ISO 178 MPa 4

Sheet tear propagation resistance, d=50 µm, longitudinal ASTM D 1922 cN 650

Sheet tear propagation resistance, transversal ASTM D 1922 cN 800

Free falling dart drop ASTM D 1709 B g > 1.000



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direction are 9.6% higher at a Styroflex®

content of 2.5% and 26.8% higher at a Styroflex® content of 5% than is the case with pure mLLDPE. A similar picture is obtained with the results of the dart drop tests according to ASTM D 1709: the addition of 2.5% Styroflex® to the mLLDPE increases the values by 15.5% - in fact, films with 5% Styroflex®

have a value that is 24% better (see Figure 2). In contrast, divergences from the linear pattern were observed in the puncture test according to DIN 53373: 2.5% Styroflex® yielded a value that was 153% better than the starting polymer - 5% Styroflex® yielded a plus of ‘only’ 82%. In the Elmendorf test as well, it seems that a divergence from the linear pattern can be seen in the transversal direction already at an additive content of 5%. Nevertheless, the values measured in the transversal direction are still considerably better than those of the unblended mLLDPEs. It can be assumed that the mechanical properties can be improved even further using larger amounts of Styroflex®,although the improvements will no longer be linear. Best results were achieved by using a thin Styroflex® layer in a multilayer film (for

example, in stand-up pouches). Changes in the visual film properties such as, for example, the haze, are mainly due to differences in the phase morphology and in the achieved particle size and dispersion of Styroflex® in the PE matrix. However, it is crucial for the moduli of elasticity of the blends to remain unaffected by the admixture: in the concentration range from 0-5%, these moduli were always on the same level as pure mLLDPE, both longitudinally and transversally. In a similar manner, the elongation at break and tensile strength were not influenced by the addition of Styroflex®.Relatively small Styroflex® additions are already sufficient to improve important properties of metallocene PE considerably – and without any losses in terms of the stiffness. Figure 3 shows a compilation of a number of findings.

PolypropyleneAs previously mentioned, the impact modification of mLLDPE is a special case because this plastic is already a very tough polymer. In order to study the influence of Styroflex® in blends with other

important film materials, BASF conducted another series of tests with heterophase polypropylene copolymers. It is generally known that PP films excel over those made of PE because of their high stiffness and heat resistance, but that they have the drawback of lower tear propagation resistance and less impact strength. However, these two properties can also be significantly improved through the addition of Styroflex®. In the test of the tear propagation resistance according to Elmendorf (ASTM D 1922; details of the film production as in the example with mLLDPE, but with melt temperatures of about 220°C [428°F]), a 50 µm-thick film made of Basell’s PP Moplen® EP310D exhibited values that, depending on the Styroflex® content, were higher by up to 37.5% in the machine direction. Similar increases were also found in the puncture test according to DIN 53373 and in the dart drop test (ASTM D 1709): when 10% Styroflex® was added, an increase in the penetration energies by up to 34% and in the puncture resistances by 128.2% was obtained in each case

Figure 2: Puncture resistance via dart drop test of a mLLDPE-Styroflex® blend according to ASTM D 1709. (Graph: BASF AG)

Figure 3: Selected properties of a blend of mLLDPE and 5% Styroflex® 2G 66, measured on a 50 µm-film. The changes in comparison to mLLDPE without the addition of Styroflex® are shown. (Graph: BASF AG)

Figure 5: Selected properties of a blend of PP and 10% Styroflex® 2G 66, measured on a 50 µm-film. The changes in comparison to PP without the addition of Styroflex® are shown. (Graph: BASF AG)

Figure 4: Puncture resistance via dart drop test of a PP-Styroflex® blend according to ASTM D 1709. (Graph: BASF AG)




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(see Figure 4). However, with polypropylene not all additions of Styroflex® necessarily lead to better mechanical properties. The tear propagation resistance in transversal direction according to Elmendorf and the dart drop test results only increased significantly, for example, after the addition of 5% SBS. This could possibly be explained by a more complex phase behaviour; even though Moplen® EP310-Styroflex® blends exhibited hardly any differences in the haze values. In summary, however, it can be stated that films made of Moplen® EP310D can also benefit from the addition of Styroflex®

2G66.It should be pointed out that as well as the moduli of elasticity, the elongation at break and the tensile strength of the specimens all remain at about the same level, irrespective of the Styroflex® content and the film orientation. Figure 5 again shows a compilation of a number of selected findings.However, it must be emphasized that this cannot be assumed for every PP type. In a series of tests with a heterophase polypropylene copolymer from another manufacturer (BHC 5012 from Borealis), considerable improvements could be achieved in the tear propagation resistance in the machine direction (up to 100% with 10% Styroflex®). In the puncture tests as well as in the dart drop tests, the results of the specimens containing Styroflex®

were worse than of blends with Moplen®

EP310D, but markedly better than pure polypropylene.

Small particles and good distributionImprovements in the properties due to Styroflex® do not automatically occur to the full extent: for optimal results, certain prerequisites have to be met. Although both PP types are heterophase copolymers that were developed for film applications, they differ, for example, in terms of their melt-flow index or their molecular weight distribution, which can lead to a different distribution of the Styroflex®

particles in the PP matrix during the extrusion procedure. So, in the large polyethylene material family, not every product variant is equally able to benefit from the Styroflex®

additions. While this SBS develops its greatest effect as an additive in mLLDPE, the effects in LLDPE and especially in LDPE fall far short of the optimum.The reason for this phenomenon is probably to be found in the size of the Styroflex® particles that are dispersed in the polyolefin phase. The rule of thumb is that the smaller the particles, the more positive the influence on the puncture resistance and on the crack propagation (see Figure 6). This can be explained by means of a simple model: the more finely the Styroflex®

particles are distributed in the matrix, the higher the probability that a propagating crack will run into one of the particles at some point in time: then it can no longer propagate. The sooner a crack strikes such a particle, the better. On the other side: the larger the SBS particles, the more strongly the macroscopic properties of the film are affected by the more or less

pronounced Styroflex® domains. This results in a drop of the modulus of elasticity. In the tested mLLDPE specimens, the Styroflex® particle size was about 120 nm, that is to say, within the nano range.

ConclusionThe greater the shear forces during the distribution of the additive in the polyolefin phase, the greater the positive effects of the addition of Styroflex®. BASF is currently examining the extent to which this can also be influenced by processing parameters. However, currently the best results can be anticipated in tough film materials such as mLLDPE and a few polypropylene variants.However, the advantages are considerable. Using Styroflex®, it is not only the strength of mLLDPE and PP films that can be increased substantially: if no extreme puncture resistances and toughness values are required, then it is also possible to use Styroflex® 2G 66 in order to reduce the film thicknesses while retaining the same characteristic mechanical values. Thus the system costs of the film production can be lowered. Model calculations performed by BASF have shown that mLLDPE films containing 2.5% by weight of Styroflex® 2G 66 displaying the same or even a slightly improved property level can be made about 5% thinner. In view of current raw material prices, this translates into cost savings of 3%. In PP films, the potential savings lie in a comparable order of magnitude, in spite of the requisite higher Styroflex® contents.The use of the additive is not associated with any drawbacks for processors and customers. Styroflex® 2G 66 was developed specifically for film applications, for instance it stands out for high thermal stability during processing as well as for having a very low level of gel particles; cracking of the polymer is not to be expected, even under PP processing conditions. Styroflex® 2G 66 is supplied in the form of granules and is simply admixed into the polyolefin granules in the extruder. Due to the similar haze values, films with additives barely differ visually from standard grades.Contact:BASF AG, Ludwigshafen, GermanyTel: +49 621 60 41446Email: [email protected] Website: www.basf.de/Styroflex

Figure 6: Important for an optimal property profile of mLLDPE-Styroflex® blends are the size and distribution of the additive particles. In mLLDPE (left), because of the high shear energy during the extrusion of the metallocene PE type, the particles were all dispersed down to nanometre-size. As a result, crack propagation is effectively suppressed without impairing the modulus of elasticity of the polyolefin. For comparison see LDPE on the right, LLDPE in the middle (Photograph: BASF AG).


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impact modifier boosts toughness of films

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