Chapter 24

Control of the Protective Properties of Polyethylene Coatings Using Molybdenum Disulphide Filling

V. Yu. Barinov, V. E. Panasyuk, and S. R. Prots

Department of Mechanics of Composite Materials, Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Lviv 290601, Ukraine

Control of the powder polyethylene coatings' properties using molybdenum disulphide filling has been studied. A powder blend of high-density polyethylene and filler was applied on a steel substrate by electrostatic spraying. The coatings as well as the isolated composite films 0.15 mm in thickness were formed. The influence of molybdenum disulphide on the stress-strain relationship of films as well as on adhesion, thermoplastic and electrical properties of coatings has been investigated. The majority of tested parameters achieve their maximum values in the narrow range of fillers concentrations. The highest resistance of coatings to an acid effect is achieved at the same content of additive.

Molybdenum disulphide filling is successfully used to increase the wear resistance of polymer coatings. The lubricating effect of such additive is determined by its lamellar structure (/). To select a filler concentration for practical use, it is necessary to take into consideration the diverse effect of molybdenum disulphide on the structure of polymer. Introduced additive must maximally optimise the definite composite properties. Other operational characteristics of coatings must be improved on or kept constant.

This article describes the possibilities of regulating the corrosion-protective and mechanical properties of powder polyethylene coatings using molybdenum disulphide filling.

Experimental Procedures

The Samples. High-density polyethylene (PE) and molybdenum disulphide were used as the subjects of research. The powder compositions of polymer and filler at

302 ©1998 American Chemical Society

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concentrations within the range up to 2.5 volume percent (%) were obtained by mixing the powders. The coatings 0.15 mm thick were formed from the mentioned composite powders. The powders were applied by electrostatic spraying over a steel substrate with the following fusion. The isolated films of filled PE were obtained from melt in hot moulding.

Electrical Measurements. The corrosion-protective resistance of coatings was studied by measuring the electrical characteristics. The 10% solution of hydrofluoric acid (HF) was served as a liquid electrode. The choice of such a medium was connected mainly with its high activity to PE. The changes in conductance (G) / resistance (R) and capacitance (Q of coatings under an acid effect were determined as a function of / d, where / is an exposing time; d is a coating thickness. The changes in the values of G and C characterize the existent specificity of liquid penetration into a polymer sample. Procedure based over (2, 3) describes such changes as follows

Οι Κ c i

where G 0 ,# 0 , and CQ are the values obtained by extrapolation, which correspond to the moment of contact beginning between electrolyte and coating; G,, Ri, and C, are the values measured in time / after the contact has begun. The measurement was carried out at the frequency of i kHz.

Tension of the Films. Yield point (ay ) and elongation at failure (Xf ) of filled PE isolated films were calculated from the data, which were obtained by registration of the stress-strain relationship (tension at the rate of 4 · 10"2 s - l ).

Adhesion Strength (A) was measured at peeling a steel foil strip (0.1 mm thick and 10 mm in width) from a coating.

A Flow Temperature ( TF ) was assessed from a flow curve as a function of temperature (weight pressure is 25 kPa; rate of temperature increase is 0.25 °C/s). It assumed to be equal to the temperature at the inflection point in this curve. Analysis of the way of changing in the value of TF makes possible to apprise the influence of filler concentration on the thermochemical destruction of macromolecules.

Results and Discussion

Protective Properties. The results of measurement of the electrical characteristics of coatings are shown in Figure 1. The values of G and C rise during the effect of 10% solution of HF. However, the rates of their changes differ from one another. The considerable increase in G is observed during the first one to two hours a sample is affected by the liquid medium. The value of G can be used as a porosity index of coatings. According to this inference, the conclusion may be drawn concerning presence of pores in PE. In this case the quantity of pores is sufficient for alteration in polymer's conductivity. Within the time range of 40-400 hours the influence of electrolyte becomes stabilized. Partial blocking of coating pores with some

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decomposition products (4) may be the cause of such stabilization. The second stage of corrosion activation, leading to the final loss of corrosion-protective properties of coatings, is observed after 400 hours of a sample staying under the acid effect. The variations in the value of C are comparatively less intensive. Nevertheless, the quick increase in C at the initial stages of medium effect is also observed as in the case with G. Since the value of C may be presented as an index of integral liquid penetration into the polymer volume. Its change reflects the swelling processes.

The filling influences on the protective properties of coatings. The liquid penetration into the composite reduces with the enhancement in additive content up to 0.8%. The reduction spreads over both the above-mentioned schemes. At a higher filler concentration the degradation of corrosion-protective properties is observed.

An introduction of molybdenum disulphide leads mainly to the physical modification in PE. From this point the corrosion-protective properties of such coatings should be examined just in the context of physical characteristics of composite itself and its specific interaction with the substrate.

Mechanical Properties. As shown in Figure 2, for the composite with the filler content more than 0.4-0.6% the value of Xf is far less than this value for the initial

polymer. Simultaneously the maximum value of ay is achieved in the area of the upper limit of this concentration range. To a slight degree these changes are connected with the influence of molybdenum disulphide on the crystallization in PE, as well as with the presence of additive in the deformed specimen. The presence of filler particles raises the amount of crystallization centers in the polymer. An increase in this amount is followed by a reduction in the average spherulite dimensions. Moreover, molybdenum disulphide acts as an internal lubricant in the volume of fused composite. This causes a plasticizing effect in the area of temperatures approximating to the crystallization temperature of PE. As a result of these factors more homogeneous spherulitic structure is formed in such composite, what is followed by the enhancement in σν. Filler particles are acting as defects in

the supermolecular structure of polymers. The dependence of deformation on the existence of these defects becomes essential for PE containing more than 0.6-0.8% molybdenum disulphide. Such dependence leads to lesser stresses during deformation of the composite. The result is the peak of σν versus concentration is existing on the curve. Besides that, introduction of fillers leads to an increase in polymeric chains' stiffness (5), which is reflected in the sharp decrease of À F .

Thus, the strength and straining properties of composite are basically defined by the diverse and opposite influence of additive on the structure and properties of PE. On the one hand, filling leads to more homogeneous spherulitic structure forming and to a plasticizating effect during crystallization, as well as to a rise in macromolecules' stiffness. On the other hand, the presence of molybdenum disulphide raises the amount of faults in the supermolecular structure. Specifically, the effect of increase in the polymeric chains' stiffiiess becomes dominant when the filler content in PE is in the range of 2.0-2.5% or more. The failure of such

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Ci ' Ct

0 100 200 iT/dC^-mm1)

G — G C — C Figure 1. ° and — versus 4t I d curves for a polyethylene

Gt Ct

coating affected by the 10% solution of hydrofluoric acid as a function of molybdenum disulphide concentration: 0% (1, 5); 0.2% (2, 6), 0.8% (3, 7); 1.4,o (4, 8).

6y (MPa) A f

Figure 2. The dependence of the values of yield point (a) and elongation at failure (b) of polyethylene film on the concentration of molybdenum disulphide. The point for the film with the tiller content of 2.5% is out of the graph a (see text).

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composites occurs at σ still before achieving the value of ay (see the point situated out of the graph a in Figure 2).

As shown in Figure 3 (curve a\ the value of A has a complicated dependence upon the additive content in polymer. The maximal value of A is achieved by filling within the concentration range of 0.6-0.8%. At the filler content near or more than 2.0% the non-continuous coatings are formed. The lower limit of this range can be established visually at the appearance of considerable amount of cavities still at the stage of the powder spraying. Heat treatment fixes these defects. Adhesion of such samples cannot be measured by the method used.

A peeling of polymer coatings depends on the ratio of cohesion forces to internal stresses (6\ The enhancement in cohesion forces leads to the ay rise and in such a

way becomes a reason for increase in the cohesive constituent part of adhesion. Besides, as shown in Figure 3 (curve b\ the value of TF reduces when the filler content goes up. The increase in molybdenum disulphide concentration heightens the fraction of heat energy that is absorbed by a powder coating under the definite conditions of heat treatment. The thermochemical destruction of macromolecules is intensified as a result of changes in the fusion conditions. This process also contributes to a rise in adhesion.

Conclusions

The existence of concentration range in which the corrosion-protective and mechanical properties of filled PE reach the optimal values is established. Considering the data of electrical and adhesive experiments, the conclusion is made about increased homogeneity and continuity in coatings with the additive content of 0.6-0.8%. The study of the mechanical properties of isolated films resulted in showing that the specificity of filler's influence is typical not only for coatings, but, probably, could be such a basic characteristic of the present material. An introduction of molybdenum disulphide in PE allows one to choose the production

Figure 3. Adhesion strength (a) and flow temperature (b) versus the concentration of molybdenum disulphide in a polyethylene coating.

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conditions of the powder composite coatings with heightened resistance to environment effect as well as with better adhesive and strength properties.

Literature Cited

1. Vadasz, E. The Plastic Coatings Used for Carrying Out and Restoring the Machine Details; Books: Budapest, 1978; chapter 3. 2. Emanuel, N. M.; Buchachenko, A. L. Chemical Physics of Ageing and Stabilization in Polymers; Science: Moscow, 1982; chapter 2. 3. Panasyuk, V. E.; Prots, S. R.; Barinov, V. Yu. Phys.-Chem. Mech. Mater. 1995, vol. 31, p. 118. 4. Frechette, E.; Compere, C.; Ghali, E. Corrosion Sci. 1992, vol. 33, p. 1067. 5. Vettegren', V. I.; Bashkarev, A. Ja.; Lebedev, A. A. Mech. Comp. Mater. 1990, vol. 26, p. 978. 6. Farris, R. J.; Goldfarb, J.; Maden, M. Makromol. Chem. Macromol. Symp. 1993, vol. 68, p. 57.

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In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.