SIGNIFICANCE OF POLYMER

7 downloads 0 Views 259KB Size Report
Polytetrafluoroethylene (PTFE) can provide a coefficient of friction as low as 0.05 in .... Inclusion of expanded PTFE filaments aligned perpendicular to the sliding ...
SIGNIFICANCE OF POLYMER NANOCOMPOSITES IN TRIBOENGINEERING SYSTEMS Ing. Olga KONOVALOVA, Prof. Ing. Jan SUCHANEK CSc. Czech Technical University, Faculty of Mechanical Engineering, Department of Manufacturing Technology, Technicka 4, Prague 6, Czech Republic

Abstract It is over a century passed since polymers, due to their inimitable specific properties, started to supplant such reliable and time-proved materials like wood, metals and ceramics in engineering systems. This process is not going to the end, but continuing to search new ways and methods of improvement and correction of polymers properties for the greater resort to its application in mechanical high-reliability parts. Development of composite materials with macro and micro additives became a first stage at the way of improvement or supplement of required properties of polymers. But fillers of such scale can also dramatically affect other properties, changes of which are undesirable or even prohibitive. By this reasons appeared a next stage of development which deals with nanoscale level of additives. It was found, that in right proportions nanofillers can enhance required properties even better and by smaller amount than macro- or micro-fillers. And what is more important, improvement of required properties can be achieved with keeping other properties at the same level. This article contains a review of reasons, advantages and disadvantages of tribological applications of different kinds of plastics and polymer composites in triboengineering systems, and the latest ways of enhancement of their properties by adding different types of fillers. Key words: tribology, polymer, polymer nanocomposite, friction, filler.

INTRODUCTION Polymers in the natural world have been around since the beginning of time. Because of the extraordinary range of properties of polymeric materials, they play an essential and ubiquitous role in everyday life. This role ranges from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life. Natural polymeric materials such as shellac, amber, and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper. The list of synthetic polymers includes synthetic rubber, bakelite, neoprene, nylon, polyvinyl chloride, polystyrene, polyethylene, polypropylene, polyacrylonitrile, polyvinylbutyral, silicone, and many more. Tribology is a science which focuses on friction, wear and lubrication of interacting surfaces in relative motion [1].The development of new technologies, often motivated by global issues such as environmental pollution, creates new requirements for bearings and wear resistant materials that can not be satisfied by traditional metallic materials. Polytetrafluoroethylene (PTFE) can provide a coefficient of friction as low as 0.05 in the complete absence of any lubricant. An absence of corrosion of parts which can be affected by solted water or humid climate. These and many others are of considerable interest to engineers. Recently the application of polymers has rapidly increased generally in technology and also as materials for dubbing components in machines and devices. This is particularly connected with low cost of materials and manufacturing in large amount of components. When the polymeric materials are rubbing in tribological contacts it is very useful and often the lubrication is not necessary. This kind of contact is often called as oilless [2]. The fiction coefficient can be similar to the lubricated metallic or ceramic contacts. The wear and friction of non-metallic solids have some fundamental similarities to that of metals, there are also significant differences in the wear mechanisms involved and the level of friction or wear which occurs. These differences can be exploited to produce valuable new bearing materials which can change commonly accepted expectations of tribological performance. The diference of application of polymers in frictional contacts in comparison to metals and ceramic materials relates mainly to the chemical and physical

structures as well as to the surface and bulk properties. The polymers show very low surface free energy and also have the viscoelastic properties. It effects in drastic tribological differences when we consider adhesive and mechanical components of fiction force. A few polymers do have valuable tribological properties and most research is directed towards this relatively limited number of polymers [3]. Common polymers, with actual or potential tribological function together with their basic tribological characteristics, are listed in table 1. Table 1 Tribological characteristics of typical polymers. Polymer

Tribological characteristics

Polytetrafluoroethylene (PTFE) Nylons

Low friction but high wear rate; usually blended with other polymers or reinforced as a composite material. High operating temperature limit. Moderate coefficient of friction and low wear rate. Medium performance bearing material. Wear accelerated by water. Relatively low temperature limit. Performance similar to nylon. Durable in rolling contacts.

Polyacetals Polyetheretherketone (PEEK) Ultra high molecular weight polyethylene (UHMWPE) Polyurethanes Polyimides Epoxies and phenolics

High operating temperature limit. Resistant to most chemical reagents. Suitable for high contact stress. High coefficient of friction in pure form. Very high wear resistance even when water is present. Moderate coefficient of friction. Good abrasive wear resistance. Relatively low temperature limit. Good resistance to abrasive wear and to wear under rolling conditions. Relatively high coefficient of friction in sliding. High performance polymers, suitable for high contact stresses and high operating temperatures. Used as binders in composite materials.

The structural features of polymers and the possibility of changing their properties within a wide range provide a variety of tribological applications of polymers and polymer composites.The application of the different fillers gives an opportunity of improving the tribological behavior of polymers. In spite of their outstanding properties the polymer composite materials are not finally investigated and there is still staying some of old but very important aspects which need our further researches and improvements of materials. The wide range of ways of use polymer composite materials in the vital part lies from the aircraft and car industries to the medical equipment and prosthetic appliances. It’s increasing importance of tribological aspect of material properties. Improvements of tribological properties lead to life prolongation of critical details and as the result increase reliability of structures. The general idea behind the addition of the nanoscale filler (usually a few percent by weight, wt%) is to create a synergy between the various contituents, such that novel properties capable of meeting or exceeding design expectations can be achieved [4]. The properties of polymer nanocomposites rely on a range of variable, particularly the matrix material, which can exhibit nanoscale dimensions, loading, degree of dispersion, size, shape and orientation of the nanoscale second phase and interaction between the matrix and the additive. The nanoscale reinforcing phase can be grouped into three categories, namely nanoparticles, nanotubes and nanoplatellets. In the field of mechanical properties, the changes in modulus and strength depend strongly on the degree of interaction between the particle and the polymer [5]. For example, in Polymethylmethacrylate (PMMA) polymer nanocomposite reinforced with alumina, the modulus decreased, whereas in polystyrene nanocomposite reinforced with silica nanoparticles, the modulus increased, due to weak and strong interaction between the matrix and the nanoparticles respectively. Another advantage of using nanoparticles as reinforcement is that their size is smaller than the critical crack length that typically initiates failures in composites [6]. As a result nanoparticles can act as voids and provide

improved toughness and strength to the composites. However, agglomeration of the nanoparticles should be prevented at all costs. Nanoparticles can also significantly affect glass transition temperature (Tg). Typically this occurs because nanoparticles influence the mobility of the polymer chains due to bonding between the nanoparticles and the polymer and bridging of the polymer chains between the nanoparticles [7]. Higher the interaction between matrix and nanoparticles, more it affects Tg, as it will increased. Another benefit of using the nanoparticles in polymer matrix nanocomposite is on enhancement in wear resistance. For example, when nylon is reinforced with silica nanopaticles, the wear resistance of the nanocomposite increased. In tribological field of applicationas improvement example of some polymeric composite coatings of machine parts by nano-particles can be a covers of calendar rollers [8]. In many technical systems, especially related to the energy technology, there are machine parts that are mainly loaded by their own weight due to rotational movements, multiple accelerations and decelerations, and they need to be transported while being used for energy storage purposes. An enhancement of the efficiency of such components in terms of higher rotational speeds is only possible, if a material with a high specific strength is used. The cover has to be hard and stiff, has to show a certain dynamic deformation behavior, the impact behavior must be excellent and predictable, and the surface has to be smooth and wear resistant.Polymer based composites have an enormous advantage compared to conventional metals. 1. Advantages and disadvantages of tribological applications of plastics and polymer composites over metallic and ceramics materials The most representative example of use plastics in tribology is plastic bearing. Materials mainly include PA, POM, PTFE, PEEK kinds, and mainly have the below features [9]. Advantages of polymer composites:  Low coefficient of friction (Surface energy of polymers is much lower than that of Ceramics and Metals. The effect of the surface energy on the coefficient of friction is also explained by the formation and disruption of the adhesive bonds between the rubbing surfaces. Lower surface energy results in lower coefficient of friction.);  High adhesive resistance;  Good chemical and corrosion resistance (Many bearing failures are caused by corrosion. Plastic ball bearings can be utilized in environments destructive to conventional steel bearings. They can operate in hostile environments such as sea water, film processing solutions and swimming pools. In many cases the working medium can be used as a lubricant);  Lubrication free;  Non magnetic;  Light weight;  Viscoelasticity ( The rubbing scratches on the polymer surface may heal due to the viscoelastic “flow” of the material.);  Formation of the transfer film;  At stage of running-in (for example in bearings) the final working shape of polymer composite detail caused by a plastic deformation instead of high wear rate of materials both of details in joint;  Less sensitive to shaft misalignment;  Better manufacturability and processability;  When plastic detail destroys (because of high temperature etc.) it doesn’t injure metal part of joint, so repair isn’t very expensive and complicated;  Economicaly to produce;  It can be very complex shape with good functional integration. Disadvantages : 

Low thermal resistance;

    

Low thermal conductivity; High Coefficient of Thermal Expansion; Low stiffness (modulus of elasticity); Low strength; Polymer can swell in contact with lubricants, water and other liquids.

2. Antifriction additives for plastics and basic comparison of their properties and efficiency (including nanoparticles). Antifriction additives: 

Graphite; MoS2; Al2O3; CrO2; ZrO2; TiO2; ZnO; CuO; SiO2; Si3N4; SiC; Liquid crystals; Polytetrafluoroethylene (PTFE); Nanotubes; Liquid synthetic lubricants.

Coefficient of friction at high temperature in friction zone changes very dramatically, therefore improvement of thermal conductivity results better antifriction properties of composite detail. Thermal conductivity improves: 

Copper ; Graphite; Carbon fibers. Ceramic powders (every polymer require a special selection of type of particles, because of the huge varying of their synergy mechanisms).

Some polymers are disposed to absorb different liquids. The swell of polymer detail in friction joint leads to decrease permissible clearance, thus it increases the normal load at a contact zone, so that cause a high friction and wear of joint. Dimension stabilizing additives: 

Glass fibers; Carbon fibers; Kevlar fibers; Ceramic powders; Nanotubes

2.1 PTFE PTFE is widely used as an additive in lubricating oils and greases [10]. The inclusion of PTFE in dramatically reduces both the friction coefficient and the wear rate of the virgin materials. Due to the low surface energy of PTFE, stable unflocculated dispersions of PTFE in oil or water can be produced. Contrary to the other solid lubricants, PTFE does not have a layered structure. The macro molecules of PTFE slip easily along each other, similar to lamellar structures. The low abrasion resistance due to the soft nature of the PTFE induce a pronounced wear of the disperse domains which leads to the formation of a uniform and continuous transfer film on the counterpart. PTFE shows one of the smallest coefficients of static and dynamic friction, down to 0.05(0.04, in different sources). Operating temperatures are limited to about 260ºC. Inclusion of expanded PTFE filaments aligned perpendicular to the sliding surface enabled reduction of wear rate by an order of magnitude as compared to particle filled composites. 2.2 Graphite The original graphite flakes with a thickness of 0.4–60 mm may expand up to 2–20,000 mm in length. These sheets/layers get separated down to 1 nm thickness, forming a high aspect ratio (200–1500) and high modulus (~1TPa) graphite nanosheets. When dispersed in the matrix, the nanosheet exposes an enormous interface surface area. Under sliding conditions, stacks of graphene layers are easily sheared off particles exposed at the surface. This debris can lead to the formation of a stiffer transfer film, as compare to the PTFE, effectively providing a stable reduction of the coefficient of friction over long period of time. 2.3 Carbon nanotubes Carbon nanotubes (CNT) reducing the penetration depth of microscratches [11]. When the CNTs diameter is smaller, bigger stress transference from the polymer to the filler can be obtained in the nanocomposite due to the higher surface of contact between the polymer and nanotubes, and a bigger reduction is achieved. The acidic functionalization of CNTs did not considerably increase the scratch resistance possibly for the inadequate dispersion of the reinforcement agent because of the strong interaction among nanotubes, which could promote its agglomeration and reduce the dispersion degree of CNTs in the polymer. The carbon nanotubes constitute an alternative promissory in order to develop textile materials with high performance due to the increase in the tribological properties as use wear of these polymeric matrices.

2.4 MoS2 MoS2 is a predominant materials used as solid lubricant having a lamellar or plate-like crystal structure usually used to reduce wear rates and increase pressure–velocity limits [12]. In the form of dry powder these materials are effective lubricant additives due to their lamellar structure. The lamellas orient parallel to the surface in the direction of motion. Even between highly loaded stationary surfaces the lamellar structure is able to prevent contact. In the direction of motion the lamellas easily shear over each other resulting in a low friction. Large particles best perform on relative rough surfaces at low speed, finer particle on relative smooth surface and higher speeds. 2.5 Liquid crystals Polymers with mixed liquid crystals show filler-rich skin layers, giving rise to surface modified polymers. It results an achievement of the lowest friction coefficient (even with increased temperature) and shows even superior lubricating ability than MoS2 [12]. This preferential distribution of liquid crystal at the surface does not affect base-polymer bulk properties such as crystallinity percentage. 2.6 Fibers Carbon fibers are generally observed to be less abrasive reinforcement due to the essentially graphic structure of the wear debris. Glass fibers are commonly used in systems where the chemical inertness of the reinforcement is important. 2.7 Ceramic powders (Al2O3, CrO2, ZrO2,TiO2,Si3N4,SiC,SiO2,ZnO,CuO) Depending on the material system used, an improved tribological behavior was mainly attributed to the positive influence of such particles on the mechanical properties such as strength and hardness, or to tribochemical reactions leading to an improved adhesion between the transfer film and the counterpart material. 2.8 Advantages of nanoadditives compare to macro- and micro-scale additives A significant reduction of the particle size down to the nanoscale level leads to a completely distinct wear behavior and better properties under dry sliding wear conditions. Advantages of nanoadditives:    

generally lower abrasiveness due to reduced angularity; enhanced strength, modulus and toughness due to defect-free structure; higher specific surface areas and, thus, improved adhesion; high effectiveness at very low contents.

3. Typical examples of tribological applications of plastics and composites in mechanical engineering. Examples for the use of polymers and polymer composites: In areas where high wear resistance is require at low friction coefficients and ever higher surrounding temperatures 

  

slide bearings (for the automotive industry) ; Plastic slide bearings are subdivided into polymer-coated bearings with metallic supports and solid plastic bearings. bearings for shock absorbers ; grooved belt wheels in components such as ignitions, alternators or diesel injection pumps; elastomer O-rings;



compressor plate valves(aerospace, automotive, electronic and chemical process industries);

In areas where tribological loading is required but the goal is to obtain abrasive wear resistance –  roller coatings in paper machines or calenders;

 lubricated pump bearings that must continue to work under extremely abrasive conditions;  machine parts that are mainly loaded by their own weight due to rotational movements, multiple accelerations and decelerations (technical systems related to the energy technology);  carbon brushes;  gears, wheels, buses;  stirring bars;  piston parts. CONCLUSION Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined. Polymers already have a range of applications that far exceeds that of any other class of material available to man. Wide range of material properties variations gives to engineers a great opportunities to choose from large amount of polymers and nanofillers the best composite material for each application. And further with the advent of the nanotechnology and the precision to observe the nanomaterials properties at nanoscale using hi-tech machines like TEM, SEM, AFM , PSA etc. [4] provides an edge towards getting better and better nanocomposites for the benefit of the human society. ACKNOWLEDGEMENTS This study was supported by Grant No. 161-802590B of the SGS of CTU-Faculty of Mechanical Engineering. REFERENCES [1] STACHOWIAK, G. W., BATCHELOR, A.W. Engineering Tribology, Ed.3. Elsevier. Boston: Butterworth-Heinemann, 2005, 801p. [2] Rymuza, Z. Tribology of Polymers. Archives of Civil and Mechanical Engineering. 2007, Vol.7,No. 4, pp.177-184. [3] STACHOWIAK, G. W., BATCHELOR, A.W. Engineering Tribology. Ed.2. Elsevier. Boston: Butterworth-Heinemann, 2001, 740p. [4] MITCHELL, B. S. An Introduction to Materials Engineering and Science for Chemical and Materials Engineers. Departament of chemical Engineering. Tulane University. Wiley-Interscience, 2004, 976 p. [5] ZHOU, R., BURKHART, T. Mechanical and Optical Properties of Nanosilica-filled Polycarbonate Composites. Journal of Thermoplastic Composite Materials. Elsevier, July 2010, Vol. 23, No. 4, pp. 487-500. [6] ZHOU, R., BURKHART, T. Influence of Nanoparticle Loading on Craze Formation and Crack Propagation of Polycarbonate in Different Environments. Journal of Thermoplastic Composite Materials. Elsevier, September, 2010. Vol. 23, No.5, pp. 607-621. [7] ASHBY, M. F., FERREIRA, P.J., SCHODEK, D.L. Chapter 1 - Nanomaterials and Nanotechnologies: An Overview. Nanomaterials, Nanotechnologies and Design - An Introduction for Engineers and Architects. Boston: ButterworthHeinemann, 2009, pp. 1-16 [8] FRIEDRICH, K. Recent Trends in Polymer Composite Materials for Tribology Applications – from Macro- to Nanoscale. Institut für Verbundwerkstoffe GmbH (IVW). University of Kaiserslautern. Chem. Listy. Symposia, 96, 2002, pp.S3–S16. [9] Information is available at: http://cysbearing2009.en.made-in-china.com/product/nMvEchTbnykB/China-PlasticCeramic-Bearings.html [10] VAIL, J.R., KRICK, B.A., MARCHMAN, K.R., SAWYER W.G. Polytetrafluoroethylene (PTFE) fiber reinforced polyetheretherketone (PEEK) composites. Wear. Vol. 270, Issues 11–12, May, 2011, pp. 737-741 [11] GIRALDO, L.F., LOPEZ, B.L., BROSTOW, W. Effect of the type of carbon nanotubes on tribological properties of Polyamide 6. Polymer Engineering and Science. Wiley InterScience, May, 2009. [12] BERMUDEZ, M. D., CARRION-VILCHES, F. J., MARTINEZ-MATEO, I., MARTINEZ-NICOLAS, G. Comparative Study of the Tribological Properties of Polyamide 6 Filled with Molybdenum Disulfide and Liquid Crystalline Additives. Journal of Applied Polymer Science. Vol. 81, 2001, pp. 2426-2432.