Siloxanes as Additives for Plastics

Transcription

1 Degussa, Goldschmidt Industrial Specialties, Essen, Germany SUMMARY Organomodified siloxanes combine the high efficiency of silicone oils with good compatibility with polymeric resins. The synthesis of organic siloxanes reveals a high degree of freedom, giving access to a variety of different representatives. Depending on the nature of the organic moiety and the overall molecular weight, organomodified siloxanes can be adapted to special applications within polymers. Thus their performance covers the range from performance additives, as lubricant or dispersing agent, as in filled compounds, to surface modifying additives, helping to improve surface characteristics, such as scratch resistance. Various polypropylenic compounds containing representatives of such organomodified siloxanes were evaluated. Process parameters such as energy consumption, pressure build up and out-put rate on a twin screw extrusion line could be increased. A positive influence on surface properties could be obtained by selecting favourable additives from this class of products. 1 INTRODUCTION The use of plastics has grown continuously in recent years. Today, they replace metals or natural rubber in many technical fields such as the automotive industry. The surfaces of polymeric materials often face special requirements in daily use. In external applications, influences such as weather, including moisture or UV-radiation must be considered. Special exposure of the surface to the effects of mechanical forces through friction, shock or impact means increased wear. Attempts are under way to counter these effects with additives, either during the manufacture of the basic polymers or during the compounding operations. Owing to their favourable surface characteristics, applications for silicones range from silicone rubbers, used as sealants for joints, to silicone surfactants for cosmetic products 1-4. In these different applications, worldwide consumption amounts to approximately 700,000 tonnes/annum. Of this, approximately 80% originates from the major producers Dow Corning, Bayer-GE/Plastics, Wacker, Rhodia and Shin-Etsu. Silicones are increasingly used in the plastics sector, in particular as process additives, and for the modification of polymers. Organomodified siloxanes combine the high efficiency of silicone oils with good compatibility in polymeric matrices. The general principles of this class of materials as well as the role of silicones in the modification of polymers are presented. This paper describes how organomodified siloxanes can provide several advantages during the processing of plastics and filled compounds during extrusion, injection- and blow moulding. 2 SILICONES AND ORGANOMODIFIED SILOXANES (OMS) 2.1 Properties and Structure of Silicones and Organomodified Siloxanes The success of silicone compounds is based on their extraordinary material characteristics (Figure 1), which clearly distinguish them from organic compounds. Silicones, often called silicone oils, carry organic groups, in most cases methyl groups, which are linked to the silicon atom. The units within the silicone are linked by way of Si-O-Si bonds. Silicone oils are usually liquid products, since intermolecular interaction is low even at molecular weights greater than,000. Silicone oils possess a low glass transition temperature (146 K). The glass transition temperature is higher in polyethylene, which contains only hydrogen on the carbon (148 K). Polymers & Polymer Composites, Vol. 10, No. 1,

2 Siloxanes are characterised by their high thermal and chemical stability and excellent surface characteristics. For instance, they have low surface tension of approximately 22 mn/m. Compared to this, the surface tension of organic oils is in the range of mn/m. Owing to their hydrophobic and lyophobic character, silicone oils are highly incompatible in most media, which often limits their use. This disadvantage can be overcome by the introduction of organic groups on the siloxane backbone. They are then called organomodified siloxanes. Organomodified siloxanes (oms) possess the favourable surface characteristics of silicone oils but have improved compatibility with organic materials, such as thermoplastics, as shown (Figure 2). Figure 1 Characteristics of silicone oils physiologically inert high chemical stability low surface tension liquid over a broad range of temperatures good gliding properties good spreadability hydrophobic and oleophobic high temperature stability There are many different ways to modify siloxanes. Substituents attached to the siloxane backbone via Si C or Si O C linkages give access to a variety of copolymers. They may be distributed statistically or as a block. The chain length may be varied. The equilibration therefore enables the design of polysiloxanes with reliable and reproducible properties. Comb-type siloxanes (Figure 3) with substituents as lateral chains and linear siloxanes, which are modified at the terminal silicon of the siloxane chain, are distinguished. These structures also differ from a polymeric point of view. Comb-like structures show a classical Gaussian curve concerning both chain length and substitution pattern. Therefore, a minimum chain length is required for comb-like siloxanes to ensure that every molecule has at least one modification, otherwise a certain fraction of silicone oil is also obtained. In α,ω-siloxanes, a uniform structure is accessible which resembles an ABA block copolymer. Here, there is no difference in the amount of functional groups per molecule. Reactive groups, for instance hydroxy, amino, epoxy or acryloxy on the siloxane backbone may get involved in subsequent reactions, forming genuine chemical bonds with the environment. Figure 3 Organomodified siloxanes (OMS) Figure 2 Characteristic profile of organo-modified siloxanes 3 SILICONES AND OMS IN POLYMER APPLICATIONS Apart from silicone oils, silicone rubbers for sealants, elastomers (modified siloxanes), resins and adhesive agents (alkoxysilanes) have the widest distribution in terms of quantity. 50 Polymers & Polymer Composites, Vol. 10, No. 1, 2002

3 Organomodified siloxanes constitute approximately 15% of the silicone market, which has a total volume of approximately 5 billion Euro. Main applications are in the fields of cosmetics, stabilisers for polyurethane foams, release coatings and additives for varnishes and paints. OMS are frequently employed in applications where different phases meet, for instance liquid-liquid (oil in water), liquidsolid (liquids on solid surfaces). They serve to bridge these interfaces and phase boundaries. The versatility of silicone products can be utilised for applications in the field of plastics. The addition of OMS to formulations serves to improve process parameters and/or surface and material properties. Today, Dow Corning silicone masterbatches 5-7 are arguably the best-known representatives of this class of compound, with the widest distribution in the field of plastics. These highly filled silicone masterbatches are mainly ultra-high molecular silicone oils in a polymer substrate (Figure 4). In recommended application concentrations of up to 2%, these additives improve the process characteristics of the polymer as a lubricant during processing. Since these additives migrate to the surface during the processing step because of incompatibilities with the basic polymer, they additionally promote the surface durability. In these polymer blends, the siloxane is not chemically linked with the basic polymer. The retention of the siloxane in the polymer is due to its high molecular weight and the resultant impaired mobility. Spherical silicone particles 8 of a few µm particle diameter were developed by Toshiba as anti-blocking additives and are now marketed in a joint venture with Bayer-GE (Figure 5). As recommended additives, especially for rubber, thermosets and polyolefin films they serve to improve surface slip characteristics and reduce surface friction resistance without visual turbidity. Under product names such as UVHC 3000, General Electric - Bayer markets formulations, which form a transparent protective film on polycarbonate to impart scratchproof properties. Following the application by spraying, the film is hardened thermally or by UV curing. This application is used in the automobile sector for headlights. The weight savings are advantageous as an alternative to glass. In addition to this there are many patents and applications dealing with the special modification of plastics with silicone compounds. 3.1 OMS additives by Goldschmidt Industrial Specialties for Plastic Applications Organically modified siloxanes (OMS) can be employed as additives during the processing of polymers as process aids for extrusion, injection Figure 4 Dow-Corning silicone master batches Polymers & Polymer Composites, Vol. 10, No. 1,

4 Figure 5 Toshiba Tospearls moulding or blow moulding In addition, they can also be used to enhance the material characteristics, in particular the surfaces. The advantage of this class of compound lies in the favourable property profile of silicone oils combined with their good compatibility with the matrix. This is made possible by controlling the number and nature of organic substituents. If, for instance, functional groups such as hydroxy, amino or ester functions are located on the siloxane backbone, they can be chemically reacted e.g. during a reactive extrusion with a polymer also containing functional groups, and permanently incorporated. The organic modification of the additive has an effect on the preferred orientation (Figure 7) in the plastics, be that in the bulk phase (A) or on the polymer surface (B). Consequently this orientation determines the effect of the OMS. When incorporating fillers in polymers, additional internal boundary surfaces exist between filler bodies and polymer. An OMS may accumulate on the boundary surfaces and act as a dispersing agent. As indicated, the examples of organic modifications can be manifold. In the following test series the siloxane wax Tegopren 6846, a siloxane co-polyester Tegomer Figure 6 Silicone resin films as scratchproof coating 52 Polymers & Polymer Composites, Vol. 10, No. 1, 2002

5 Figure 7 Possibilities of orientation of OMS in the polymer phase 64P 12 and a siloxane co-polymer Tegomer PP- Si1 were applied and compared with the unmodified material as standard in each case (Table 1). The incorporation of OMS takes place during a premixing step or by direct addition during compounding, for instance on an extrusion line. The effect of the various OMS additives on the processing parameters and material properties was tested using unfilled polypropylene (PP), formulation 1 (F1) and PP compounds filled with chalk (F2) and talc (F3). The plain polymer or compound without additives was used as a control. The basic values were established as follows. A PP with a melt flow index, MFI = 2.9 [g/10 min], Stamylan 14E10, was employed as a standard for comparison. This material was extruded at 220 C at a pressure of 39 bar and a power consumption (amperage draw) of 53% (twinscrew extrusion line 27/ L/D). The granules produced were injection moulded into standard bars (DIN T2) and subjected to impact testing according to the notched Izod method (DINISO180). The filled systems containing % by weight of chalk (Millicarb OG, OMYA; Talc Luzenac 10MO), showed Table 1 Representatives of OMS applied in the test series OMS T egopren 6846 T egomer 64P T egomer PP-Si1 Structure Comb-type structure Linear; ABA type Grafted PP with reactive siloxane Modification Melting p oint [ C] Alkyl 65 Polyester 55 Polyolefin 1 a lower power consumption and lower pressure build-up when processed in the extruder than the pure PP. The melt flow index for the compound filled with chalk is highest with 3.5 g/10min (230 C), while the pure PP has the highest Izod notched impact strength with 3.6 KJ/m 2. Based on these values, the influence of the addition of various OMS in increasing dosage (0.5% and 2%) was examined (Figure 9). The tests were performed on the extrusion line such that the throughput was maintained constant at 10 kg/h. The addition of Tegomer 64P and Tegomer PP- Si1 to PP causes a reduction in the power consumption and a decrease of pressure in the extruder during processing. While the changes [%] are small with an additive level of 0.5%, they show clear differences with an addition of 2%. The power consumption with the addition of 2% Tegomer 64P is reduced by up to 25%. Because of the lubricant effect of the OMS, the polymer can be processed more easily using less energy. The sliding effect consists of a reduction in the internal and external friction and occurs immediately after addition. In addition to this, the melt flow index and the impact strength are increased. The differences as a function of the concentration increase for the MFI in absolute values from 3.1 (0.5%) to 5.6 g/10min (2.0%) with Tegomer 64P, and are very pronounced. Even minor concentrations of both additives result in an improvement in the notched-bar impact strength. The elastic characteristics of the material remain unchanged under these process conditions. The modulus of elasticity (DIN 53455) is 15 MPa with % PP and 1500 MPa with 2% Tegomer 64P. Polymers & Polymer Composites, Vol. 10, No. 1,

6 Figure 8 Comparison of PP with and without filler (F 1-3) Figure 9 Processing parameters with unfilled PP and OMS additives F 4-7 (0.5/2.0%). The graph shows the percentage differences relative to F1 3.2 Effect of OMS in Compounds Dispersant and lubrication effects can be interlinked or interacting occurrences. Additives guaranteeing such effects include the polyolefin waxes, fatty acids and their derivatives. Special compounds such as fluoropolymers, silicones or special types such as boron nitrides are used. In principle it is possible to distinguish between optimisation with regard to the rheological and mechanical characteristics of the polymer. The rheological characteristics for instance aim at the improvement of the melt flow behaviour, by reducing the viscosity, or the compatibility of the various components in a compound. This can be achieved by influencing the polymer phase internally or by covering interfaces between the polymer phase and external surfaces, for instance the metal surfaces of processing equipment. The structural and chemical characteristics of the additive determine the preferred effect. The results for a PP compound filled with % chalk are shown in Figure 10. Again the percentage deviation 54 Polymers & Polymer Composites, Vol. 10, No. 1, 2002

7 Figure 10 Processing parameters with PP filled with chalk (%) and OMS additive (0.5/2.0%); F 8 11 from the standard is presented. The addition of OMS results in a lower power consumption and pressure build-up, while 0.5% Tegomer 64P (F8) initially increases the pressure slightly from 37 to 38 bar. For the MFI, 0.5% Tegomer 64P produces an increase of more than 2% while the situation is reversed with regard to the notched-bar impact strength. This may be due to the competitive behaviour of lubrication and dispersing effects. Tegomer 64P obviously possesses good compatibility with the matrix due to its organophilic character, achieving excellent dispersion of the filler material. The silicone wax Tegopren 6846 (F10, F11) also shows effects on the process parameters while no positive effects are noticeable on MFI and notchedbar impact strength. A further example (Figure 11) shows a talc-filled (%) compound (F12 15). The use of Tegomer Figure 11 Processing parameters with PP filled with talc (%) and OMS additive (0.5/2.0%) F12 15 Polymers & Polymer Composites, Vol. 10, No. 1,

8 64P in these formulations also results in advantages. An addition of 0.5% additive initially increases the MFI, obviously at the expense of the notched-bar impact strength, which can be increased in turn by increasing the dosage. For a % chalk PP compound (F25 33), too, all the tested additives resulted in an increase in the throughput during extrusion, while pressure (bar) and power consumption (%) were again regulated to standard level (Figure 13). 3.3 Comparison of OMS with Organic Process Aids Polyolefin waxes or fatty acid derivatives are frequently used as process additives for plastic formulations. Investigations were conducted to assess the reaction profile of OMS compared with these substance classes. The following additives were examined during this comparison: Tegomer 64P as representative of the OMS, erucamide (fatty acid amide), N,N ethylenebisstearamide (fatty acid amide) and an unmodified PP wax. These additives were added at 0.5% and 1% to PP and PP-% chalk compound and processed on the extruder. The unmodified material was first passed through the extrusion line and the power consumption (33%) and pressure build-up ( bar) were measured (F16). The individual formulations were subsequently processed so that the power consumption and the pressure build-up had the same values as the standard (F17 24). In return, throughput [kg/h] increased in all cases (Figure 12), indicating good slip and antiblock properties. Tegomer 64P achieved the highest throughputs with 10.5 kg/h (0.5%) and 11.5 kg/h (1%), corresponding to a percentage increase of up to % compared with the standard (8.0 kg/h). The addition of Tegomer 64P again resulted in the highest increases (+15/27%). With the purely organic additives, the increases were at a comparable level, i.e., up to 17% on the addition of 1% N, N ethylenebisstearamide (F30). The notched-bar impact strength values were established for the compounds by means of standard bars (Figure 14). The best results were achieved with erucamide, while Tegomer 64P also resulted in a clear increase in the notched-bar impact strength of 48% (0.5%) and 62% (1%). The comparison clearly showed that Tegomer 64P is able to increase the throughput compared with oleochemicals and polymer additives, while ensuring good dispersion of the filler material. 3.4 Influence of OMS on Surface Characteristics For surface examinations PP containing % talc was again compounded with 2% Tegomer 64P (F34 35). The process parameters were changed during these tests relative to F F13. Extrusion was performed at lower power consumption and pressure build-up than the initial test series (Table 2). Figure 12 Throughput kg/h of PP formulations with 0.5% of various additives (F16 24) 56 Polymers & Polymer Composites, Vol. 10, No. 1, 2002

9 Figure 13 Throughput kg/h of PP chalk compounds with 1% of various additives (F25-33) Figure 14 Izod notched [KJ/m 2 ] of PP formulations with various additives (F25-33) Table 2 Test parameters of F34 and F35 F ormulation Power [%] PP talc : reference sample Pressure [bar] KSZI [ KJ/m 2 ] E-mod. [MPa] Tens. Strength [MPa] Elongation at break [%] , ± 0. 5 PP talc : 2% T egomer 64P ± 1. 0 Polymers & Polymer Composites, Vol. 10, No. 1,

10 The OMS has the same effects on the talc compound even under milder processing conditions, i.e. there is an improvement in the processing parameters and the notched-bar impact strength. It can also be seen from the Table that the stiffness, expressed by the modulus of elasticity, is reduced from 35 MPa to 2810 MPa. On the other hand, the tensile strength is reduced, while the elongation at break is increased. In addition to tensile bars, plates ( x 80 x 0.5 mm) were injection moulded from the compounds and subjected to a scratching test according to GM E 280. This test, also known as the Erichson lattice cut, makes use of a semi-spherical metal tip of 1.0 mm diameter moved over the plate surface at a constant speed with a contact force of 5N to produce a lattice cut. The individual cut lines are mm long at 2 mm intervals. This process creates scratches on the surface. The resulting scratches and furrows were examined with a confocal laser-scanning microscope (Figure 15). This method provides 3D images of surfaces via optical sectioning and computer image analysis. The following results were obtained from the evaluation. The scratches on the compound (% talc without additive) were highly furrowed. The profile view makes this obvious, showing scratch depths of µm (Figure 15A). The scratch depth is very uneven as if caused by a slip-stick-effect. The scratches are visually prominent as a result. Figure 15B shows the results of the compound with 2% Tegomer 64P. The scratch in this case is not furrowed, but proceeds at a constant depth of 6 µm, which is clearly less pronounced. This effect could be interpreted as the sum of several characteristics. On the one hand the addition of Tegomer 64P increases the mechanical properties (notched-bar impact strength) and consequently the resistance to mechanical impact. Elasticity, as indicated by Young s modulus, is also increased. Figure 15 Surface structure of a scratch on PP filled with talc (%) and OMS additive (2.0%) 58 Polymers & Polymer Composites, Vol. 10, No. 1, 2002

11 Figure 16 Friction coefficients of PP films This allows the material to resist the effect of an external force and subsequently rebound to its original position, while the anti block properties of the OMS ensures improved sliding of the needle tip. The optical effect makes the scratches less perceptible. These characteristics are of interest, especially for parts that are subjected to high mechanical loads, and where appearance is important. 3.5 OMS as Process Agent During Film Production Polymer process additives are state-of-the-art and widely used for reducing the friction in the production of films and foils. As an example, OMS was tested in this field of application. The sliding friction according to DIN53375 was measured on a polypropylene film and compared with a film containing an additive. By adding OMS it was possible to reduce the friction resistances from 0.58 to 0.27 and 0.14 (0.5%/2% Tegomer 64P) and 0.38 and 0.23 (0.5%/2% Tegomer PP-Si1), improving the sliding characteristics of the film. improved even at an application concentration of 0.1%. The advantage of the OMS is that it is effective immediately, rendering conditioning superfluous. CONCLUSIONS The continual growth of the plastics industry and the need for continuous improvements in material characteristics are equal motivations for the entire plastics industry to keep on working for innovative solutions. Figure 17 Melt fracture behaviour of LLDPE film During the manufacture of LLDPE blown film (Figure 17), modifications were also conducted with OMS and the melt fracture behaviour (formation of shark skin) examined. Melt fracture behaviour was Polymers & Polymer Composites, Vol. 10, No. 1,

12 Table 3 Overview on tested formulations Formulation PP % Filler % Additive Energy Consum. % Pressure bar MFI g/10 min Izod notched 2 KJ/m F F F3 Talc F4-0,5% Tegomer 64P , 0 F5-0% Tegomer 64P 36 5, 6 4, 1 F6-0,5%Tegomer PP-Si , 3 R7-0% Tegomer PP-Si , 4 F8 0,5% Tegomer 64P , 1 5 F9 0% Tegomer 64P , 0 9 F10 0,5%TP F11 0% TP F12 Talc , 0 7 F13 Talc 0%Tegomer 64P , 5 5 F14 Talc 0,5%TP F15 Talc 0%TP , 5 2 F , 5 F17-0,5% Tegomer 64P F18-1,0% Tegomer 64P , 3 F19-0,5% Erucamide F20-1,0% Erucamide F21-0,5% N,N Ethylenebisstearamide , 2 F22-1,0% N,N Ethylenebisstearamide F23-0,5% PP-wax , 2 F24-1,0% PP-wax , 2 F F17 0,5% Tegomer 64P 7 4, 3 F18 1,0% Tegomer 64P 1 4, 7 F19 0,5% Erucamide , 4 F20 1,0% Erucamide , 8 F21 0,5% N,N Ethylenebisstearamide 3 9 F22 1,0% N,N Ethylenebisstearamide 1 4, 3 F23 0,5% PP-wax F24 1,0% PP-wax , 2 F 34 * Talc , 9 F 35 * Talc 0,5%Tegomer 64P PP: Stamylan 14 E 10; * RCW 284 Eltex (Solvay) : Millicarb OG (Omya) Talc: 10 MO (Luzenac) Polymers & Polymer Composites, Vol. 10, No. 1, 2002

13 Since their industrial-scale production, siloxanes have been used in a wide range of commercial applications. Structurally modified derivatives have been increasingly tested for the plastics sector and used commercially for some time. The first representatives of organomodified siloxanes yielded advantages in the polymers and compounds employed. The advantages are due to the unique characteristics of this class of materials. Selection of suitable OMS, adaptation of structures to certain polymer types, optimisation of the formulations and co-ordination of processing conditions create the potential for further improvement in the property profiles and performance of plastics materials and for matching them to the set requirements. ACKNOWLEDGEMENTS The author gratefully acknowledge the assistance of Dr. J. Venzmer and H. Dumm for the investigations with the Confocal Laser Scanning Microscope; A. Schulman (Kerpen) for cooperation concerning the studies on scratch resistance; Grafe Color Batch GmbH (Blankenheim) for measuring of coefficient of friction of films; M. Scheiba, M. Nols for conducting the extrusion trials and mechanical tests. REFERENCES 1. W. Noll, Chemie und Technologie der Silicone, Verlag Chemie, Weinheim (1968) 2. I. Schlachter, G. Feldmann-Krane, Marcel Dekker, Inc. New York (1998), G.E. Hahn, K.-D. Klein, I. Yilgör, and C. Gould, Silicon Containing Polymers. 4. B. Arkles, Chemtec 12 (1999) 7 5. V.B. John, H. Rubröder. Gummi, Fasern, Kunststoffe 11 (2000) M. Jones, J. Mat. Chem. Royal Society of Chemistry, Cambridge (1995) K. Ryan et al., J. Vinyl & Additive Technol., 3 (2000) 6 8. R. J. Perry, M. E. Adams Chemtec 29 (1999) 2 9., Der Verarbeiter als Compoundeur, Würzburg, (1999). 10. R.G. Jones, Silicone Containing Polymers, Royal Soc. Chem., Spec. Publ. (1995) H.H. Chen J. Appl. Polym. Science, 37, (1989) I.Yilgör, J.E. McGrath, Adv. Polym. Sci. 86 (1988)1 Polymers & Polymer Composites, Vol. 10, No. 1,

14 62 Polymers & Polymer Composites, Vol. 10, No. 1, 2002