Characterization of Composites Based on Polyoxymethylene and Effect of Silicone Addition on Mechanical and Tribological Behavior.
Unmodified materials have limited scope of application, hence researchers are increasingly looking for modifications which allow to obtain desired properties. The requirements imposed to novel engineering materials provoke that the choice of proper chemical composition or materials, additions, modifiers and processing methods is not trivial. Working conditions, part geometry, and surface quality are some of the broadly understood issues that need to be considered in order to provide the desired effect. Analyzing working conditions, particular care should be paid to strength parameters, wear resistance, dimensional changes and the influence of temperature or moisture and aging factors on strength parameters.
Friction is a set of phenomena occurring in the area of contact between two moving bodies, as a result of which resist of movement appears. Several types of friction can be distinguished according to the accepted criterion. Depending on the type of movement, there is sliding or rolling friction, differentiating between the state of motion--static or dynamic. Analyzing the contact area we can distinguish between internal or external friction. The chosen material may also be a criterion, because friction occurs in both solids and liquids. Another criterion is the contact surface configuration, which may be dry, fluid, mixed or have a boundary layer. The research is constantly being carried out to modify materials in such a way as to achieve the lowest coefficient of friction and to slow down material wear . A typical plastic additive used to reduce friction and wear is polytetrafluoroethylene (PTFE). However, this additive is mainly used to reduce the wear and dynamic friction coefficient at high surface pressure of the friction elements. It is recommended as an additive for materials compatible with fiber-reinforced plastic [2-4,11]. The improvement of wear and durability of plastics is achieved by the use of powder or fibrous fillers, among others in the form of glass fibers, glass beads and mineral powders. Powder fillers have limited impact on strength and heat stability compared to fiber fillers, but usually provide a noticeable increase in material elasticity and limit shrinkage. The use of fiberglass provides significant improvement in the strength and stiffness of the element and reduces wear, but increases surface roughness, friction coefficient and reduces sound attenuation [4,5]. Fibrous filler, which has the advantage of increasing durability and wear resistance, lowers the friction coefficient and reduces noise is aramid fiber [6,7]. Among the many species of thermoplastic polymers in tribotechnical applications, only some of them are used. One of them is POM. Polyoxymethylene (POM) also known as polyacetal and polyformaldehyde is an engineering thermoplastic used in precision parts. It has high stiffness, low friction and excellent dimensional stability. The excellent physical and mechanical properties of POM are mainly due to its high crystallinity [8-10]. For many years, scientists have been working on friction coefficient reduction and the wear of composite materials. Fakhar et al.  investigated tribological properties of POM composites with short aramid fiber (ASF) and PTFE. Tribological measurements showed that both additives reduced the friction and wear of POM composites. However, both additives reduced the breaking energy of the POM in the impact test. On the other hand, the results of the tensile strength showed that the addition of ASF mechanically strengthened the POM while the PTFE deteriorated the mechanical properties of the composition. Luo et al.  examined the effect of morphology of aramid fibers and particles on friction and wear of POM composites. The results showed that the addition of ASFs and particles (AP) affected the friction and wear in two different ways. The mechanical action of ASF in POM/ASF composites was superior to the POM/AP composite due to the better transfer of stress to fibers and better maintenance of the fiber in the matrix. However, this type of morphology was not helpful in improving abrasion resistance. Abrasion rates were strongly associated with the shape of the filler, due to the better coverage on the composite surface; it was shown that aramid powders were much more effective in reducing the high adhesion to the POM substrate than the ASFs. Tribological properties of POM composites with carbon fiber (5-25% by volume) were investigated by Li . The addition of carbon fibers into the POM matrix improved abrasion resistance. As the fiber content increased, the coefficient of friction and wear were reduced. Xiang et al.  tested POM composites with low-density polyethylene (LDPE) and wood fibers. Studies showed that the addition of 5% by weight of LDPE worked effectively as a lubricant. The interpose of wood fibers resulted in an increase in the abrasion rate for POM/LDPE composites, but had little effect on the coefficient of friction in the presence of LDPE, but the coefficients of friction and abrasion were still lower compared with the unmodified POM. Tang et al.  proved that cellulose fiber improved the wear resistance and friction coefficient of POM composites. It is also possible to use thermoplastic elastomeric fillers. Thermoplastic polyurethane can be processed simultaneously with thermoplastic polyacetal to provide a material with lower hardness and sound damping ability . Polyformaldehyde has also been modified by researchers through the addition of LDPE, polyolefin elastomer (POE), and grafted high-density polyethylene glycol. The addition of LDPE had a favorable effect on the coefficient of friction in relation to the unmodified polyacetal. The introduction of a POE caused an increase in impact strength and also provided higher thermal stability [16,17]. The influence of copper particles on thermal conductivity and tribological properties of POM composites was also determined by He et al. [18,19]. The results showed that the addition of Cu to POM had little effect on thermal conductivity and slightly reduced coefficient of friction and abrasion rate. The above analysis of the literature indicates the multitude of forms used as a reinforcement of polyformaldehyde as well as the continuous search for fillers allowing to modify POM. The aim of the study was an investigation on tribological properties of polyformaldehyde modified with various additives, thermoplastic polyurethane and aramid fibers which is a novelty among the previously tested composites based on POM.
The standard dumbbell samples and bars were made at the Plastics Laboratory of Azoty Group SA in Tarnow using the Engel ES 200/40 HSL injection molding machine at temperatures indicated by the manufacturer for Tarnoform T-300. The granules of the composition were prepared by twin-screw extrusion with cold granulation using a line of compounding. Selected, manufactured and tested materials are presented in Table 1.
Method of Testing
Physical, mechanical and tribological tests of modified polyacetal were carried out. Density was estimated using hydrostatic scale RADWAG WAS 22 W. The mechanical properties were tested with a MTS Criterion Model 45 universal testing machine, with a measuring range up to 30 kN using the MTS axial extensometer according to PN-EN ISO 527-1:20100 for static tensile test and PN-EN ISO 178:2011 for the three point flexural tests. The test speed was set to 10 mm/min. Charpy impact test (PN-EN ISO 179-1:2010 was examined on unnotched specimens using a Zwick HIT 5.5P. Tests under variable loading to determine the initial mechanical hysteresis loops for the analyzed compositions were also investigated using the MTS Criterion 43 universal testing machine with the MTS axial extensometer. The force was enforced with a minimum force of 50 N and a maximum force of 1,200 N. The speed of load and unload was 100 mm/min, which is translated into a low frequency of cycles and allows to visualize viscosity phenomena. Microscopic observation was carried out using Scanning Electron Microscope JEOL JSN5510LV. The fracture surface of the sample was sputter coated with a gold layer before examination. Friction coefficient and wear rate were determined by the method "ball on disk" using Tribometer T-01 M. The test was conducted under various conditions of speed sliding, the load was equal 40 N and the wear trace was set to 500 m. The ball is made of steel 100Cr6 according to ISO/EN 683-17 with a diameter of 6 mm. The principle of "ball on disk" method is presented in Fig. 1. For each specimen, the date reported is the average of five specimens.
RESULTS AND DISCUSSION
The first stage was examination of the density and the mechanical properties of the produced composites. A static tensile test and static bending test were carried out at room temperature. Mechanical parameters as a tensile modulus, tensile strength, and strain at break as well as flexural modulus and flexural strength at strain of 3.5% were determined. The results are summarized in Table 2.
It was not noticed a significant influence on density, only addition in form of polyurethane provided reduction density by 20%. The mechanical properties of the tested composites showed that only the addition of aramid fibers caused increase in tensile strength while other compositions reduced. The addition of silicones brought about an increase of the flexural strength as well as a flexural and tensile modulus with a slight decrease in the tensile strength. The addition of 20% of thermoplastic polyurethane lowered tensile and flexural properties but provided increase in strain at break.
The impact strength was also investigated. Impact resistance determines the energy that is needed to break the sample. Each modification influenced on the impact value in relation to neat POM. The smallest effect was observed for the POMSO composition. The introduction of aramid fibers into the polymer matrix caused a 3-fold drop in impact strength. The others compositions with the addition of silicones also lowered impact strength by approximately 50%. The fall in impact strength indicates a change in the nature of fracture from ductile to brittle, which is related to the breakage of macromolecules in the plane of cracking. Such a tendency may be caused by poor adhesion to the polymer matrix, local depolymerization of POM, irregular structure, fractures between crystallites or the formation of micronotchs during, for example, processing processes and associated shrinkage of the material. In the case of POMBK5M and POM2U compositions SEM pictures (Table 1) show a significant irregularity of the structure which may affect on the impact strength. For the compositions with thermoplastic polyurethane (POM2U) the sample was not damaged under the test conditions. This is related to the plasticization effect of polyurethane, which increased the flexibility of the polymer and the mobility of polymer chains.
Mechanical hysteresis loops were conducted in order to understand the visco-elastic properties of the tested materials. It does not belong to typical dynamic or fatigue tests, but it lets compare materials in terms of energy dissipated during load--unload cycles. Figure 2 presents first hysteresis loops. Analyzing the hysteresis loops and the determined parameters (Figs. 2 and 3), it should be noted that the composite with thermoplastic polyurethane had the highest elastic energy from the all tested composites, as well as the highest energy dissipation coefficient which was also confirmed by the impact test. The stiff and durable composition of POM with addition of aramid fiber also exhibited high energy dissipation coefficient. The other compositions were characterized by similar parameters.
In shaping and evaluating the tribological properties of polymer composites, most often in sliding combination with steel, many factors have to be taken into account. These factors are: the change in the structure of the surface layer (SL) of composites and tribological processes occurring during friction between cooperating materials, all of that has a significant influence on the tribological properties of sliding associations. Changes in the structure of the composite SL depend on both the chemical composition and the initial structure of the composite (depending on the technology), external forces of the friction process as well as the interaction of the fillers on each other and on the composite polymer matrix. These factors have a fundamental influence on the phenomena occurring in the friction process and as a consequence on the tribological properties of the sliding association. The properties of composites depend on the size of the filler and their surface, which affect on the characteristics of the friction process.
Polyformaldehyde is characterized by a low friction coefficient and good wear characteristic. The coefficient of friction of polyacetal varies between 0.2 and 0.5 [8,20]
The tribological tests were carried out for all produced composites under different sliding speeds. The obtained results are shown in Fig. 4 and 5.
The lowest coefficient of friction and average specific wear rate, in comparison with other composites, was observed for polyacetal containing silicone addition in a volume of 5 wt% (POM5M), while the highest coefficient of friction and the high wear rate was exhibited by a composition with thermoplastic polyurethane. The remaining compositions had similar friction coefficient values. Noteworthy is the composition of polyacetal with aramid fiber (POMAR), which showed the lowest specific wear rate and low friction coefficient compared with other compositions. This is interesting because the introduction of fillers in the form of fibers or particles into polymeric materials may in some cases improve wear resistance and dimensional stability of the sliding element, but it may also increase the coefficient of friction. The literature describes the four wear processes of individual components of the composites during friction: wear of the polymer matrix, abrasive wear of the fiber filler, cracking and chipping of fibers and separation of fibers from the polymer matrix, the last two processes occur periodically . The fibers removed from the composite matrix and remaining in the frictional area act as a loose abrasive on both cooperating surfaces, and the weakening of the matrix material at the fiber removal sites contributes to its increased wear. The low wear of the composite on the polyformaldehyde matrix with aramid fiber indicates good adhesion between the fiber and the matrix.
Determination of the influence of sliding speed is quite difficult due to the attendant increase in temperature, which in polymers is of great importance because of the limited thermal conductivity. From the literature data, no unequivocal influence can be stated, which is also related to different measurement methods and various test's parameters. The change of the coefficient of friction in polymers at variable sliding speed is determined by rheological properties, and thus the temperature and the impact of fillers addition. Two opposing dependencies are met: it is time-temperature equivalence and the dependence of mechanical properties on temperature. The growth in the sliding speed considered as the load causes an increase in the elastic modulus. The raise in temperature caused by growth in the sliding speed and low-thermal conductivity of polymers lead to a decline in the elastic modulus. As the Young's modulus decreases the wear resistance also decreases. Additionally, during friction process occurring in the POM SL, the spherolites increase in size as the temperature rises, and the value of crystallinity falls off which may provide the higher the coefficient of friction and wear rate. The influence of the sliding velocity is therefore the sum of the opposing effects that can be seen in Figs. 4 and 5.Analyzing the value of the coefficient of friction in relation to the sliding speed, the composites with the addition of aramid fibers and polyurethane showed a downward tendency with the increase of the sliding speed. POM with the addition of silicone (POMSO) presented an upward trend (0.08-0.117). The addition of ultra-high molecular weight silicone provided an initial growth in the coefficient of friction with an increase of the speed to 0.3 m/s, at 0.4 m/s the coefficient remained almost the same level. The wear test showed that the increase in the sliding speed decreased the specific wear rate of each material, increasing the sliding speed to 0.3 m/s resulted in reduction of wear rate, further increasing the sliding speed raised the rate of wear; however, these values were still lower than those obtained at a speed of 0.2 m/s.
All compositions except POM with aramid fiber reduced tensile strength. The addition of silicones has effect on the strength properties of POM-based composites. The flexural properties and tensile modulus increased for each of the tested compositions in comparison with pure POM; however, additives caused a decrease in tensile strength. Hysteresis loops showed that the addition of polyurethane causes a significant increase in the energy dissipation coefficient. SEM images showed the nature of the structure of the studied materials. The silicone additives made an irregular form with the diameter approximately 1-6 [micro]m. The composition with polyurethane showed a two-phase nature. The friction coefficient test showed that each selected additive reduces the coefficient of friction and wear rate in relation to pure POM. The lowest coefficient of friction is characterized by the composition with the addition of 5% silicone (POM5M), the composite with polyurethane has the highest coefficient of friction. The sliding speed has a significant effect on the coefficient of friction and wear. Composites with aramid fiber and thermoplastic polyurethane are characterized by a decrease in the coefficient of friction as the sliding speed increases. The other compositions show an upward trend.
Patrycja Bazan (iD), Stanistaw Kuciel (iD), Marek Nykiel (iD)
Faculty of Mechanical Engineering, Cracow University of Technology, al. Jana Pawfa II37, 31-864, Cracow, Poland
Correspondence to: S. Kuciel; e-mail: stask @mech.pk.edu.pl
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Caption: FIG. 1. The principle of friction coefficient measurement by means of the "ball on disk" method.
Caption: FIG. 2. The comparison of the first mechanical hysteresis loops of the examined composites.
Caption: FIG. 4. The influence of changing sliding speed on the friction coefficient (lines serve as a guide for an eye only).
Caption: FIG. 5. The influence of changing sliding speed on the specific wear rate (lines serve as a guide for an eye only).
TABLE 1. Description of tested materials. Index Additives POM5M 95 wt% Tarnoform 300 (POM) + 5 wt% Dow corning MB40-006 Masterbatch- POMSO 100 wt% Tarnoform 300 SO NAT POMAR 80 wt% Tarnoform 300 + 10 wt% aramid fiber AR POM2U 80 wt% Tarnoform 300 (POM) + 20 wt% TPU POMBK5M 95 wt% Tarnoform 300 BK + 5 wt% Dow corning MB40-006 Masterbatch Index Description SEM images POM5M POM with an ultra-high molecular weight silicone additive POMSO Ready mix in the form of POM granules with silicone for injection to reduce friction coefficient and abrasion in plastic-plastic systems POMAR Ready mix POM with aramid fiber to improve abrasion resistance and reduce coefficient of friction POM2U POM with the addition of thermoplastic polyurethane to improve the damping of mechanical vibrations and noise reduction POMBK5M POM with addition of special chalk and silicone lubricant to reduce abrasion TABLE 2. Parameters marked in a static tensile test and static bending for the tested materials. Properties Tarnoform POM5M T300 (1) (POM) Density, g/[cm.sup.3] 1.140 1.401 Tensile strength, MPa 62 54.0 (0.5) Tensile modulus, MPa 2.800 2.966 (102) Strain/elongation at break, % 50 37.0 (11.5) Flexural strength 61 80.9 (1.8) at 3.5% of stain, Mpa Flexural modulus, MPa 2,500 2.719 (37) Impact strength, kJ/[m.sup.2] 200 96.5 (14.7) Properties POMSO POMAR Density, g/[cm.sup.3] 1.400 1.413 Tensile strength, MPa 51.3 (0.7) 70.4 (0.2) Tensile modulus, MPa 2.894 (82) 3.821 (458) Strain/elongation at break, % 37.8 (3.8) 5.5 (0.2) Flexural strength 77.9 (0.4) 92.1 (3.4) at 3.5% of stain, Mpa Flexural modulus, MPa 2.645 (5) 3.286 (264) Impact strength, kJ/[m.sup.2] 176.1 (11.4) 51.5 (5.4) Properties POM2U POMBK5M Density, g/[cm.sup.3] 1.360 1.426 Tensile strength, MPa 41.3 (0.2) 51.3 (0.8) Tensile modulus, MPa 1841 (306) 3.105 (118) Strain/elongation at break, % 63.0 (-) 41.5 (8.5) Flexural strength 50.4 (0.8) 82.8 (1.5) at 3.5% of stain, Mpa Flexural modulus, MPa 1.713 (52) 2.947 (47) Impact strength, kJ/[m.sup.2] Not break 105.9 (13.9) (1) Date of manufacturer of Tarnoform T300. FIG. 3. The comparison of dispersed energy of the examined composites. POM5M 1th hysteresis loop 14.5 20th hysteresis loop 4.9 POMSO 1th hysteresis loop 16.1 20th hysteresis loop 4.7 POMAR 1th hysteresis loop 14.2 20th hysteresis loop 4.8 POM2U 1th hysteresis loop 49.0 20th hysteresis loop 27.3 POMBK5M 1th hysteresis loop 17.0 20th hysteresis loop 4.5 Note: Table made from bar graph.
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|Author:||Bazan, Patrycja; Kuciel, Stanistaw; Nykiel, Marek|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2019|
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