Polymers (Polyvinylpyrrolidones) As Active Additives Modifying the

Oct 23, 2012 - ... as tests on antiseizure abilities under a constant velocity increment of load (409 N/s) were carried out using a four-ball tester (...
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Polymers (Polyvinylpyrrolidones) As Active Additives Modifying the Lubricating Properties of Water Marian W. Sulek,*,† Witold Sas,† Tomasz Wasilewski,† Anna Bak-Sowinska,† and Urszula Piotrowska† †

Department of Chemistry, Technical University of Radom, 26-600 Radom, Chrobrego 27, Poland ABSTRACT: Surface active compounds may act as effective additives modifying water lubricity. Their action mechanism consists in the formation of a lubricating film enriched with an active compound. The reasons for the choice of polyvinylpyrrolidones as additives were their physiological inertness and a lack of harmful environmental effects. The tests were carried out on the solutions of two polyvinylpyrrolidones with degrees of polymerization of 450 (PVP 450) and 14400 (PVP 14400). A significant effect of the polymer chain length on physicochemical (viscosity, surface tension, wettability of steel) and tribological properties (resistance to motion, wear, scuffing) has been found. PVP 14400 showed a considerably greater ability to thicken solutions. Over 10 times higher coefficients of viscosity relative to an analogous PVP 450 solution were observed already at a concentration of 5%. A slightly higher ability of PVP 14400 to reduce surface tension was observed. In the case of wettability of steel, a more effective decrease in the wetting angle was observed for PVP 450. However, the differences obtained were not large and usually did not exceed 20%. The measurements of resistance to motion and wear at three constant loads (2000, 3000, 4000 N) as well as tests on antiseizure abilities under a constant velocity increment of load (409 N/s) were carried out using a four-ball tester (T-02 tester). All friction pairs were made of bearing steel. Introducing additives to water significantly affects a decrease in resistance to motion and wear as well as an increase in the ability of a tribological system to prevent seizure. The system did not undergo seizure even at a load of 4 kN. A 2-fold decrease in the coefficient of friction (μ) and an over 30% decrease in the value of wear scar diameters (d) relative to water were observed for PVP 450 and PVP 14400 solutions at 2.0 kN. The use of water as a lubricant led to seizure of the tribological system already at above 2 kN. Addition of the polymers to water also results in a significant improvement in its antiseizure properties. The values of scuffing load (Pt) and limiting pressure of seizure (poz) for individual solutions were even 5 and 15 times higher, respectively, than the values for water. Seizure load (Poz) was also relatively high, and its level was almost two times higher than that for water. The results obtained indicate that polyvinylpyrrolidone used as an additive considerably improves the tribological characteristics of water as a lubricant base. generates a lubricating film with surfactant molecules which can separate mating parts of machines and devices. Polymers are an interesting group of additives modifying lubricating properties of water used as a base.8−10 Results of the studies of the following polymers in water-based lubricants have been presented so far: poly(vinyl alcohol) (PVA),12,18−23 poly(2-hydroxyethyl)methacrylate (polyHEMA),18,21 poly[2(methacryloyloxy)ethyl phosphorylcholine (pMCP),24 sodium carboxymethyl cellulose,30−32 polyethylene glycols,26 and their derivatives.25 In the search for a suitable kind of polymer, attention was focused on polyvinylpyrrolidone which is nontoxic, biocompatible, and water-soluble. The choice of PVP was preceded by a comprehensive analysis of the literature and by preliminary tests.12−14 The compound has been used as a serum albumin substitute.11,28 Currently it is being used, among others, as a component of artificial tears protecting the eyes against irritation and drying. Complexes of PVP with a strongly bactericidal iodine are commonly used wound disinfectants due to the fact that they are less toxic than iodine.11,29 There are also few literature reports describing attempts to apply PVP as a

1. INTRODUCTION Surface active compounds and polymers used as additives may significantly improve tribological properties of lubricant bases.1−10 Therefore, studies on solutions of those compounds may provide a significant input in the development of fundamental and applied research in the field of tribology. Aqueous solutions of surface active compounds are used in many applications of lubricants where ecological criteria are important.1−5 Water is one of the oldest lubricant bases. At present, compositions of mineral or synthetic oils with appropriate additives are used in friction nodes of machines and devices. These substances are often hazardous to human beings and the environment. The application of water as a lubricant base seems attractive for a number of reasons - it is inexpensive, easily available, nonflammable, and poorly compressible. However, water has a number of disadvantages, such as insufficient lubricating properties, high corrosivity, low boiling point, and high freezing point. These disadvantages can be minimized or simply eliminated by application of appropriate additives. It has been shown that a number of surfactants in aqueous solutions can be used as lubricity or anticorrosion additives.1−7 The compounds form micellar and liquid crystalline structures both in the bulk phase and in the surface phase. The formation of the surface phase at the solid - solution interface is of particular importance. Under friction conditions this phase © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14700

May 31, October October October

2012 13, 2012 23, 2012 23, 2012

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component of synovial fluids.12,18,22,27 Polyvinylpyrrolidone has also been widely used in the pharmaceutical, medical, food, and cosmetics industries.11,30,33,34 In living organisms PVP does not undergo chemical changes, is highly biologically resistant, and accumulates in the environment.35,36 High biodegradation resistance is beneficial from the viewpoint of its potential applications in tribology. The compounds will be stable as lubricant components. From the point of view of environmental protection, an increase in the degree of biodegradation of the vinylpyrrolidone polymer is possible, among others, with participation of the γlactamase enzyme produced by bacteria. The transformation product obtained is utilized by microorganisms as a carbon source. There are also literature reports on the studies on increasing the activity of the γ-lactamase enzyme in the presence of acrylamide which results in an increase in a PVP biodegradation rate.37,38 The start of systematic research on aqueous solutions of PVP from the viewpoint of its tribological applications is a novelty in the literature. PVP is water-soluble; therefore, we think that aqueous solutions of polyvinylpyrrolidones can be used as safe model lubricants in technological applications (lubricants in the pharmaceutical and cosmetics industries) and in medical applications (lubrication in the tissue - medical instrument system).

mixer (propeller agitator, rotational speed of 200 rpm) was then turned on. A preweighed amount of one of the two kinds of PVP was slowly added to mixing water. The whole was mixed for about an hour at 60 °C. The dissolution process was carried out until a completely clear solution was obtained (visual assessment). The mixing time was prolonged if a heterogeneous solution was obtained. After the dissolution process the mass of the solution was monitored, and, in case of water loss (evaporation), a suitable amount of water was added in the final stage of mixing. Foaming of the solutions obtained was not observed in the PVP dissolution stage. The other solutions were obtained by diluting the 5% solution. Physicochemical Tests. Kinematic viscosity (ν), surface tension (σ), and wetting angle of steel (θ) were determined for aqueous solutions of two polyvinylpyrrolidones with concentrations of 10−3, 10−2, 10−1, 0,5, 1, 2, 3, 4, and 5 wt %. The results given in Figures 2, 3, and 4 are arithmetic means from

2. MATERIALS AND METHODS Materials − Polyvinylpyrrolidones. Polyvinylpyrrolidone (PVP), also called Povidone, can be obtained in the reaction of vinylpyrrolidone as shown in the following scheme (Figure 1).

Figure 2. Friction pair in a four-ball tribotester: 1 - top ball, 2 - lower balls, 3 - ball chuck, 4 - ball pot.

Figure 1. Polyvinylpyrrolidone preparation reaction scheme.

A polyvinylpyrrolidone molecule contains polar amide groups and nonpolar ethylene groups. Such a structure determines its power of complex-formation with low-molecular mass compounds (e.g., dyes, vitamins) or with surfactants.11−14 Polyvinylpyrrolidones do not decompose into simpler compounds due to the action of microorganisms such as bacteria, protozoa, algae, or fungi. This is a beneficial characteristic because, as lubricant components, they exhibit stability.11−14 Additionally, they are nontoxic, and as such they can be treated as ecological additives. They exhibit good adhesion to various materials, and so they may be applied as additives modifying lubricity of water. The investigation was carried out using two commercial products manufactured by BASF which are characterized by relatively high purity (95−100%). The polymers used differ in their molecular mass. In the case of the polymer with a degree of polymerization of 450 the molecular mass is 50 000 g/mol, whereas the mass of the polymer with a degree of polymerization of 14400 (PVP 14400) is 1 600 000 g/mol. Both the degree of polymerization and molecular masses are averaged values, and polymer distribution can be expected. Aqueous 10−3, 10−2, 10−1, 0.5, 1, 2, 3, 4, and 5% solutions of PVP 450 and PVP 14400 were prepared for the investigation. A 5% solution was prepared in the first stage. Weighed distilled water was warmed in a water bath up to 60 °C. A mechanical

Figure 3. Schematic dependence of friction torque on load.

three independent measurement series. The measure of experimental error was the standard deviation of arithmetic mean based on Student’s t test (confidence level 0.95). Kinematic Viscosity. Kinematic viscosity was determined by means of an Ubbelohde capillary viscometer. The measurement consists in determining the time of passing of a given volume of the liquid tested through a viscometer capillary, due to gravitational forces, at 25 °C. The value of the coefficient of viscosity (ν) was determined using the following equation τ ν = ·ν0 τ0 (1) 14701

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where Poz is the seizure load, and d is the wear scar diameter measured after the test.

3. RESULTS 3.1. Viscosity. Viscosity is an important physicochemical quantity characterizing lubricants. While analyzing the effect of additives on lubricating properties of compositions, it is possible to compare only lubricants with comparable coefficients of viscosity, because the action of individual components in a friction zone can be ″masked″ by rheological properties of the solutions. The aim of this study was to determine the effect of the concentration of two polyvinylpyrrolidones on viscosity of their aqueous solutions. Preliminary studies have shown that the coefficient of viscosity values will not be really high. Therefore, a method for the determination of kinematic viscosity (ν) was selected. The results obtained are shown in Figure 4 in which the compounds are denoted as PVP 450 and PVP 14400. Up to the concentration of 10−1% the viscosities of the solutions of the two compounds were comparable and close to the values obtained for water (from 1.0 to 1.1 mm2/s). But at the concentration of 0.5% the viscosity value for the PVP 14400 solution was 1.5 times higher than the value for PVP 450. At 5% the value was about 11 times higher (Figure 4). Therefore, the action mechanism of additives and their effect on physicochemical and tribological properties can be comparable up to the concentrations of the order of a few tenths of one percent. Above this concentration the effect of viscosity should be taken into account when interpreting results. Based on the results shown and on literature data14 it can be expected that PVP 14400 can also be a viscosity modifier already at the concentrations of the order of a few percent. 3.2. Surface Activity. Surface tension at the solution - solid (platinum) interface and wettability of a steel surface with polyvinylpyrrolidone solutions were accepted as a measure of surface activity of polyvinylpyrrolidones from their aqueous solutions. It should be added that the same type of bearing steel was used both in wettability and tribological measurements. Surface Tension. Changes in surface tension (σ) as a function of load are shown in Figure 5. The points in this diagram are arithmetic means from three independent measurements. It has been found that the differences between the solutions of the compounds used are visible but do not exceed 20%. The

Figure 4. Dependence of kinematic viscosity on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Measurement temperature 20 °C. Kinematic viscosity of water ca. 1.0 mm2/s.

where ν is the kinematic viscosity of the liquid tested, ν0 is the kinematic viscosity of the reference liquid (water), τ is the flow time of the liquid tested, and τ0 is the time of passing of the reference liquid. Surface Tension. The measurement of surface tension (σ) is one of the basic criteria for the evaluation of surface activity of compounds from their solutions. The σ value was determined by means of the ″ring-tear-off″ method. The method consists in measuring the force needed to tear a ring made of a thin platinum wire off the surface of a liquid. The measurements were made using a TD 1 C LAUDA tensiometer. Wetting Angle. Wetting angle of a steel surface was measured using the sitting drop method. The measurement unit consisted of a microscope, a camera, and a computer with an installed image acquisition and digital image processing system MultiScanBase 808. The measurements were carried out at 20 °C. Tribological Studies. A four-ball tribotester (Figure 2, T02 tester) produced at the Institute for Sustainable Technologies in Radom (Poland) was used to evaluate resistance to motion and wear as well as antiseizure properties of aqueous solutions of polyvinylpyrrolidones. Friction pairs were made of bearing steel of surface roughness of Ra = 0.32 μm and hardness 60÷65 HRC. The description of the tester and testing methods can be found in the literature.15−17 The coefficient of friction (μ) was a measure of resistance to motion, whereas a mean value of wear scar diameters of the three lower balls (d) measured after the test was a measure of wear. The measurements were made at 3 constant loads: 2000, 3000, and 4000 N at the rotational speed of 200 rpm. The duration of the test was 900 s. The quantities characterizing seizure were determined by analyzing changes in friction torque (MT) as a function of load (P) and their schematic plot is shown in Figure 3. Scuffing load (Pt) and seizure load (Poz) were determined directly from the diagram (Figure 3). The Pt value corresponds to the load at which friction torque increases drastically. These changes can be equated with breakage of a lubricating film. The Poz value, however, is equal to the load at which MT ≥ 10 N·m. This value has been accepted conventionally as seizure load. The third quantity characterizing antiseizure properties is limiting pressure of seizure which was calculated using the following dependence P poz = 0.52 oz2 (2) d

Figure 5. Dependence of surface tension on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Measurement temperature 20 °C. Surface tension of water ca. 72 mN/m. 14702

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PVP 14400 solutions show higher surface activity in a range of lower concentrations (≤3%). As the polymers studied contain identical monomers, the results obtained can be interpreted in terms of an effect of the degree of polymerization. Changes in the gradation of surface tension at higher concentrations (4 and 5%) may result from higher viscosity of PVP 14400 solutions relative to PVP 450 solutions. Wettability of a Steel Surface. The affinity of additives for surfaces can also be determined by measuring the wetting angle (θ). A larger wetting angle corresponds to lower wettability, while higher wettability corresponds to a smaller angle. The dependence of wetting angles as a function of concentrations of individual polyvinylpyrrolidones is shown in Figure 6.

Figure 7. Dependence of friction coefficient (μ) on friction time (t). Measurement conditions: T-02 tribometer, stationary test, rotational speed of the spindle 200 rpm, test duration 900 s, load 2 kN.

solutions did not undergo seizure, and the coefficients of friction for the solutions of both polymers were over 2 times lower than those for water. The character of changes in the coefficient of friction as a function of concentration (c) and load (P) was analogous for solutions of the two compounds. Resistance to motion decreases with increasing concentration and load. The μ values for the two polymer solutions are practically comparable. However, they are slightly lower for PVP 14400 solutions for most of the concentrations and loads applied. The effect of concentration and load on wear scar diameter values (d) for polyvinylpyrrolidone solutions is shown in Figure 9. Wear decreases to a small degree with an increase in concentration of the additives but increases with a load increase. At P = 2 kN at which the system did not undergo seizure in the presence of water, the coefficients of friction decrease 2 times for PVP 450 (c = 5%) solutions and about 1.6 times for PVP 14400 (c = 5%) solutions relative to water. An analysis of the differences in wear scar diameters for longchain and short-chain polymers shows that wear is comparable for the two polymers. At concentrations of under 3% most long-chain polymers have lower values, while at c > 3% they have higher values compared with short-chain polymer solutions which can be explained by the observed changes in viscosity (Figure 4) and surface tension (Figure 5). The test results obtained by means of the T-02 tester indicate that polyvinylpyrrolidones can be treated as effective additives reducing resistance to motion and wear at high loads. Seizure does not occur even at a load of 4 kN, while, in the case of water, seizure takes place at 2 kN. No measurements were made at higher loads (P > 4 kN) to avoid damaging the tester. The polyvinylpyrrolidone solutions show relatively low resistance to motion and wear above 2 kN and do not undergo seizure even at 4 kN. The coefficient of friction for aqueous polyvinylpyrrolidone solutions at 4 kN is over 3.5 times lower, and wear scar diameter is about 1.5 times smaller than the same quantities for water at 2 kN. 3.4. Antiseizure Properties. Typical changes in friction torque (MT) as a function of linearly increasing load (P) are shown in Figure 3, whereas the MT(P) dependence for polyvinylpyrrolidones and water is given in Figure 10. The dependence of friction torque (MT) on load (P) for polyvinylpyrrolidone solutions is different from the one for water (Figure 10) and for model systems (Figure 3). The system does not undergo seizure in the presence of the polymer solutions within the whole range of concentrations

Figure 6. Dependence of wetting angle of steel on concentration of polyvinylpyrrolidonewith DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Measurement temperature 20 °C. Wetting angle for water ca. 89°.

As follows from Figures 4 and 5, the gradation of changes in surface tension and wettability is different for the solutions of both compounds. Long-chain PVP shows a higher ability to reduce surface tension but a lower wettability. The solutions of the polyvinylpyrrolidones tested show lower values of surface tension (σ) and wetting angle (θ) relative to water (Figures 5 and 6). The results point to their surface activity and an ability to form a film on the surface. However, a decrease in the two quantities relative to water is relatively small in comparison with other surfactants.1−5 Surface activity of these compounds cannot be the sole criterion determining tribological properties, as was the case with nonpolymeric surfactants.1−5 3.3. Resistance to Motion and Wear under Constant Load. As mentioned above, resistance to motion and wear were measured by means of a T-02 tribotester at various loads. 1, 2, 3, 4 and 5% aqueous solutions of polyvinylpyrrolidones were used as lubricating substances. The quantities measured were the coefficient of friction (μ) and wear scar diameter (d). The dependence of the coefficient of friction (μ) on time (t) is shown in Figure 7. The μ values given in the diagram are averaged values after 30 s time intervals. Changes in μ(t) for water and polyvinylpyrrolidones are different. Resistance to motion as a function of time increases in the presence of water but decreases, with a tendency to stabilize after ca. 100 s, in the presence of polyvinylpyrrolidone solutions. Averaged values of coefficients of friction (μ) from three independent measurements for three different concentrations and loads are given in Figure 8. In water medium it was possible to carry out measurements exclusively at the load of 2 kN at which the coefficient of friction reached a high value of 0.47. Above this load the system underwent seizure. Thus, water can serve as a reference system only up to the load of 2 kN. At this load the system with PVP 14703

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Figure 8. Dependence of coefficient of friction on concentration of polyvinylpyrrolidones with DPsof 450 (PVP 450) (part a) and 14400 (PVP 14400) (part b) in their aqueous solutions. Measurement conditions: T-02 tribometer, stationary test, rotational speed of the spindle 200 rpm, test duration 900 s, loads 2, 3, and 4 kN.

Figure 9. Dependence of wear scar diameter on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450)- part a and 14400 (PVP 14400) - part b in their aqueous solutions. Measurement conditions: T-02 tribometer, stationary test, rotational speed of the spindle 200 rpm, test duration 900 s, loads 2, 3, and 4 kN.

significant increase in the value of Pt of its aqueous solutions (Figure 11).

Figure 10. Changes in friction torque as a function of linearly increasing load for 1% polyvinylpyrrolidone solutions with DPs of 450 (PVP 450), 14400 (PVP 14400), and water. Figure 11. Dependence of scuffing load on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Limiting pressure of seizure for water is 200 N/mm2. Measurement conditions: T-02 tribometer, test duration 18 s, load increment 409 N/s.

used. The maximum MT value (Figure 10) obtained for the PVP solutions is about 2.5 times lower compared with the 10 N·m value which, if exceeded, signals seizure. This is the first sign indicating the formation of an effective lubricant film protecting against seizure. The quantities characterizing antiseizure properties of lubricants have been determined on the basis of an analysis of the plot shown in Figure 10. These are as follows: scuffing load (Pt), seizure load (Poz), and wear scar diameter (doz). Limiting pressure of seizure has been calculated from eq 2, on the basis of known Poz and doz values. Scuffing load (Pt) determines the value of maximum load at which a lubricating film is destroyed. Water does not form a lubricating film, and scuffing load for this base has low values (ca. 200 N). Introduction of polyvinylpyrrolidone results in a

Scuffing loads for PVP 450 solutions are by about 15% higher than those for PVP 14400. Compared to water, solutions of lower molecular mass polymers show a 5-fold increase in the Pt value, whereas this increase is 4-fold higher in the case of higher molecular mass polymers. Based on the results obtained, it can be postulated that the polymers tested form stable lubricating films at relatively high loads: ca. 1000 N (PVP 450 solution) and about 850 (PVP 14400 solution). Surprisingly high seizure load (Poz) values of 7.2 kN were obtained for all the solutions tested (Figure 11). This is the 14704

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boundary, maximum value for the T-02 tribotester, and it is approximately 2 times higher than the one observed for water. Limiting pressure of seizure (poz) characterizes a high loadcarrying capability of a tribological system, and it was determined on the basis of eq 2. An interpretation of this quantity will be preceded by an analysis of changes in wear scar diameter (doz) measured at a load of 7.2 kN. The dependence of doz on concentration is shown in Figure 13. An analysis of the dependence in Figure 13 shows that wear scar diameter values are even 3 times lower than those for water. In the case of the PVP 450 solutions the d values practically do not depend on concentration, whereas for the PVP 14400 solutions the values decrease slightly with an increase in concentration. It can thus be said that the d values for the solutions of individual polymers are comparable in a wide range of concentrations. As the Poz values were identical at all concentrations (Figure 12), changes in poz are determined by changes in doz (Figure

Figure 14. Dependence of limiting pressure of seizure on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Limiting pressure of seizure for water is 200 N/mm2. Measurement conditions: T-02 tribometer, test duration 18 s, load increment 409 N/s.

4. SUMMARY AND CONCLUSIONS The presented results are the first stage of the investigation of tribological properties of aqueous solutions of polyvinylpyrrolidones. The measurements of surface tension (σ) and wetting angle (Θ) show their specificity compared with the nonpolymer surfactants studied so far.1−7 The similar σ and Θ values for water and aqueous solutions of PVPs (Figures 5 and 6) indicate that surface activity of these compounds will not have a decisive influence on the tribological properties of their aqueous solutions. The aim of the two different kinds of tribological tests was to determine high load carrying capacity (P) of the order of several kN. At constant loads (2, 3, 4 kN) 1−5% PVP solutions show excellent tribological properties. For P = 2 kN the solutions have even 2.5 times lower friction coefficient values (μ) and 2 times smaller wear scar diameters (d) than the ones for water (Figures 8 and 9). At the loads of over 2 kN in the presence of water the system undergoes seizure, whereas in the presence of PVP solutions the μ values decrease with an increase in the P value (2, 3, 4 kN). At P = 4 kN the μ value of 0.14 is about 3.4 times lower than the one for water at 2 kN. The tests determining the resistance of a tribological system to seizure were carried out under extremely adverse conditions at high velocities of load increment (ca. 0.4 kN/s) and for a wide range of loads (0−7.2 kN). The seizure characteristics determined indicate that polyvinylpyrrolidone is an effective additive which considerably increases the ability of aqueous solutions to prevent seizure of the system. The scuffing load (Pt) of the solution is about 5 times higher than the one for water and practically does not depend on concentration (Figure 10). However, the Pt values for long-chain polymer solutions are about 15% lower than those for short-chain polymer solutions (Figure 11). These observations are in contradiction with the results obtained for nonpolymer surfactant solutions for which an improvement in tribological properties occurred with an increase in the alkyl chain length. Seizure load (Poz) does not depend on the kind and concentration of polymer (Figure 12) and has the maximum value of 7.2 kN. These results (Figure 12) do not fully reflect the abilities of polymers to prevent seizure. The analysis of the MT(P) dependences, including the examples presented in Figure 10, indicates that 1% PVP 450 solutions did not practically exceed the value of MT = 3.5 N·m, and PVP 14400 solutions did not exceed the value of MT = 5.0 N·m. Thus, in this case we cannot talk about seizure but only about the

Figure 12. Dependence of seizure load on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Seizure load for water is 3700 N/ mm2. Measurement conditions: T-02 tribometer, test duration 18 s, load increment 409 N/s.

Figure 13. Dependence of wear scar diameter on concentration of polyvinylpyrrolidones with DPs of 450 (PVP 450) and 14400 (PVP 14400) in their aqueous solutions. Limiting pressure of seizure for water is 200 N/mm2. Measurement conditions: T-02 tribometer, test duration 18 s, load increment 409 N/s.

13). The differences between the poz values for individual polymers are greater than in the case of doz (Figure 14). This results from the poz value calculation procedure based on the equation in which wear scar diameter (doz) is squared and considerably affects the measuring error. Limiting pressure of seizure for solutions of both polymers has very high values -almost 16 times higher than those for water. These results confirm a high ability of polyvinylpyrrolidones to prevent seizure. 14705

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solutions reaching maximum friction torque values (about 3.5 and 5 N·m), 3 and 2 times lower, respectively, than the value of 10 N·m above which we can talk about seizure. The results obtained for aqueous PVP solutions are comparable with or even more beneficial than those obtained for oil lubricants used in lubrication technology.15 Concluding, the tribological test results obtained show that polyvinylpyrrolidones are effective additives which modify the adverse lubricating properties of water used as a lubricant base. The fact that polyvinylpyrrolidones can be safely applied allows for application of their aqueous solutions as safe innovative lubricants not only in traditional tribological systems but also in frictional contacts of engineering and biological materials (medical applications).

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 (048) 3617538. Fax: +48 (048) 3617589. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financed by The National Centre for Research and Development in the years 2010−2013 − Project No. N R15 0103 10.

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REFERENCES

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