Introduction

Several substrates are severely exposed to aggressive outdoor environments (abrasion, corrosion, resistance to chemical or solvents, degradation by moisture or water), and to prevent the reduction of their properties due to degradation several coatings of different nature are commonly used. In the particular case of piping for the oil and mining industry, degradation is a key concern as inside the pipeline corrosive fluids containing solid particles are circulating. For increasing the durability of the pipelines they are internally coated to increase their lifetime and increase the time for maintenance [1]. These coatings are commonly based on polymeric materials, more specifically semi-rigid polyurethanes [2]. Polyether diol-based polyurethane coatings are currently used for pipelines due to their relatively good water resistance but their wear resistance and stability against oils and solvents, as well as thermal stability are not fully satisfactory [3]. Typically, the drawbacks of these polyurethane coatings have been solved by incorporating additives particularly fillers of different nature and size (nano-silica, zinc oxide, alumina particles [4-6]) for increasing their abrasion resistance.

As compared to the polyurethanes obtained with polyether or polyester, due to the higher molar attraction constant of the carbonate groups, the polyurethanes prepared with polycarbonate diol can be a feasible alternative for improving the mechanical properties and increase the hydrolytic stability, imparting additionally good elastomeric properties and adequate behavior at low temperature [7,8] (Figure 1). Because these particular and unique features of the polycarbonate diol, in this study several polyurethane coatings were synthesized by using different polyether diol + polycarbonate diol mixtures as polyol for preparing polyurethane coatings with improved hardness and wear resistance in pipeline internal coatings, additional to improved mechanical properties and hydrolytic stability.


Experimental Materials
In this study two different polyols of different nature were used in the formulations of polyurethanes. One of the polyols was polytetramethylene glycol (PTMEG) with molecular weight 1000 Da and it was chosen to obtain polyether diol-based polyurethanes (the typical formulation in the current internal coating for pipeline), and the other polyol was a copolymer of polycarbonate of 1,6-hexanediol and 1,5-pentanediol of molecular weight 500 Da – PCD - (Eternacoll® PH50, UBE Chemical Europe S.A., Castellón, Spain). Different mixtures on PTMEG and PCD were prepared for obtaining synergistic properties, i.e. combining the advantages of the polyether polyurethanes (i.e. good flexibility) and of the polycarbonate diol polyurethanes (i.e. high abrasion and mechanical resistance, high hydrolytic resistante). Polymeric diphenylmethane diisocyanate (pMDI) with 24% free NCO content and 2.1 average functionality was used.

Polyurethanes were prepared by using the one shot method. Prepolymers were obtained by reacting pMDI with the polyols, and 1,4-butanediol was used as chain extender.

Experimental techniques
Thermal properties were measured using thermal gravimetric analysis (TGA) in TGA system by heating from room temperature to 800ºC at 10ºC/min under nitrogen atmosphere. Furthermore, the structure of the polyurethanes was analyzed by differential scanning calorimetry (DSC) using DSC system by heating from -70ºC to 100ºC at 10ºC/min under nitrogen atmosphere followed by cooling down to -70ºC and carrying out a second heating from -70ºC to 100ºC at 10ºC/min.

Abrasion resistance was evaluated using rotational abrameter using abrasion wheel according to ISO 54701 standard. Surface topography of the eroded polyurethane coatings after abrasion was qualitatively analyzed by optical microscopy.
Shore A hardness of the polyurethane films was measured with durometer, equipped with pin load according to standard ISO 868:2003.

Mechanical properties of the polyurethane films were obtained by stress-strain tests and resistance to tear. The stress-strain tests were carried out in dog bone test specimens of polyurethanes obtained according ISO 37 standard. The resistance to tear of the polyurethane films was obtained from tear strength tests according ISO 34-1 standard. In both cases, the experiments were carried out in universal testing machine using a pulling rate of 50 mm/min (stress-strain test) and 500 mm/min (tear test).

The hydrolytic resistance of the polyurethane films was estimated from stress-strain and tear strength tests of aged polyurethane films carried out by soaking in water at 70ºC for 15 days, according to ASTM D-471 standard. After degrading the polyurethane films, they were also characterized by using TGA and DSC.

Experimental methodology
The effect of the amount of polycarbonate diol in the polyols mixture (PCD + PTMEG) and the NCO/OH ratio on the properties of the polyurethane films were studied. In order to find the optimal formulation, a statistical experiments design methodology was applied for analyzing the combined effect of the two variables simultaneously; the abrasion resistance was chosen as response variable.

A Doehlert experimental plan was chosen for experiment design due to its spherical domain and the small number of experiments required to obtain a second degree response, as well as because of the high number of levels of study: 5 levels for PCD weight content in the polyols mixture (0, 25, 50, 75 and 100wt%) and 3 levels for NCO/OH ratio (1.05, 1.20 and 1.35). Figure 2 shows the distribution of experiments in the Doehlert experimental domain.

Results and Discussion
Thermal properties of the polyurethanes prepared with PTMEG, PCD or PTMEG+PCD mixtures
The values of the glass transition temperature of the polyurethanes obtained with NCO/OH ratio of 1.20 and varying the PCD content between 0 and 100%wt (Figure 3) are different because of the different degree of phase separation between the hard and soft segments. As the PCD (PH50) content increases the value of the glass transition temperature increases too.

Figure 4 shows the variation of the weight loss and the derivative of the weight loss as a function of the temperature of the polyurethane films. Several decomposition steps are found. For the polyurethane synthesized with PTMEG only, the decomposition of the soft segments is produced at higher temperature (410ºC) than that of the hard segments (364ºC).However, in the polyurethanes prepared with polycarbonate diol (PH50), the weight loss due to hard segments is produced at lower temperature (327-328 ºC) than the soft segments derived from polycarbonate diol (354-367 ºC). Interestingly, the polyurethane prepared with 50 wt% PTMEG + 50 wt% PH50 shows the differentiated thermal decompositions of both polyols.
Mechanical properties of the polyurethanes prepared with PTMEG, PCD or PTMEG+PCD mixtures

Polyurethanes obtained with polyether diol only show poor abrasion properties (Table 1), as notable weight loss is obtained in the coatings by friction. However, in the polyurethanes with polycarbonate diol in the formulation, the abrasion resistance was highly improved, even when small amount of polycarbonate diol is added. Furthermore, when the polycarbonate diol content in the polyurethane was higher than 50wt%, the abrasion properties remain almost constant. The increase in abrasion in the polyurethanes can be ascribed likely to the higher interactions between polycarbonate chains with respect to these in the polyether chains (Figure 1).

Using lower values of NCO/OH ratio, a slight improvement in the wear resistance of the polyurethane films is obtained. This can be ascribed to higher stiffness of the polyurethanes containing higher number of hard segments, and thus when soft segments are dominant (such as for low NCO/OH ratios) better resistance to abrasion with low weight loss is produced. Figure 5 shows the values of weight loss after abrasion for the polyurethanes prepared with different NCO/OH ratios and different contents of polycarbonate diol in the polyol mixtures. The better wear resistances are obtained in the polyurethanes prepared with higher content of polycarbonate diol (PCD) in the polyols mixture and lower NCO/OH ratio value.

The influence of the content of polycarbonate diol in the polyols mixture can be better observed in the response surface plots (Figure 6a) and its corresponding curve-level 2D plot (Figure 6b). Both plots show that the weight loss after abrasion is lower than 40 mg in the polyurethanes with PCD content higher than 20wt%, irrespective of the NCO/OH ratio, and thus the polycarbonate diol content has a dominant effect on the abrasion resistance of the polyurethanes. Furthermore, the response surface and curve-level 2D plots show that 60wt% PCD in the polyurethanes produces the highest abrasion resistance.

Figure 7 shows some optical microscope images of the surfaces of the polyurethane coatings before and after abrasion test was carried out. The images corresponding to the polyurethane prepared with polyether show marked changes in surface morphology caused by important abrasion leading to a highly roughened surface. On the contrary, the surfaces of the polyurethanes prepared with polycarbonate diol and 50wt% polycarbonate diol + 50wt% polyether obtained after abrasion show similar and lower roughness.
Shore A hardness values of the polyurethane films prepared with different content of polyether (PTMEG) and polycarbonate diol (PH50) increase by increasing the amount of PCD, i.e. the higher is the content of PCD, the higher is the Shore A hardness value of the polyurethane. Figure 8a shows the Shore A hardness values as a function of the PCD content in the polyol mixtures (for NCO/OH ratio of 1.20), and an almost linear increase of hardness in the polyurethane as a function of the PCD content in the polyol mixture is produced. On the other hand, by increasing the NCO/OH ratio in the polyurethane the Shore A hardness value also increases (Figure 8b). Both, the increase in the NCO/OH ratio and the polycarbonate diol content in the polyols mixture increases the Shore A hardness due likely to the increase in the hard segments in the polyurethane; however, for high content of PCD in the polyols mixture, the effect of the polycarbonate diol on Shore A hardness values of the polyurethane is more important than the incidence of the NCO/OH ratio.
The mechanical properties of the polyurethanes were also obtained from stress-strain tests of polyurethanes prepared with polyether only and with 40wt% polyether + 60wt% polycarbonate diol. Figure 9 shows that the polyurethane prepared with 40wt% polyether + 60wt% polycarbonate diol has higher Young’s modulus and tensile strength than the one prepared with polyether only, although the elongation-at-break is lower. The increase in tensile strength in the polyurethane prepared with 40wt% polyether + 60wt% polycarbonate diol can be ascribed to higher cohesive forces between carbonate groups in the polyol chains in the soft segments of the polyurethane. On the other hand, a similar trend was obtained in tear strength values of the polyurethanes (Figure 10).

Properties of the polyurethanes after hydrolytic degradation
The resistance to hydrolysis of the polyurethanes was determined by measuring their mechanical properties before and after immersion in water at 70ºC for 15 days. Noticeable changes in the polyurethane structure are found (Figure 11). After immersion in hot water the color of the polyurethane was clearer in the polyurethane prepared with polyether only although it becomes soft and has gum-like appearance.

Figure 12 (next page) shows the TGA thermograms of the polyurethanes prepared with PTMEG only and with a mixture of PTMEG and PH50 (PU- 40wt% PTMEG + 60wt% PH50). The TGA thermograms of the polyurethanes are quite similar before and after hydrolysis, although in the polyurethane prepared with PTMEG only no residue is obtained at the end of the experiment (2wt% remained in the polyurethane before hydrolysis). For the polyurethane prepared with 40wt% PTMEG + 60wt% polycarbonate diol mixture similar residue is obtained (4.7-5.1 wt%). There are fewer residues because the structure of the PCD is more compact than the polyether one; and the variation of the residue after degradation is more notable for the PTMEG PU (no residue left) due to the defragmentation of the backbone polymer chains produced by polyether hydrolysis.

Figure 13 shows the stress-strain curves of the polyurethanes prepared with PTMEG only and with 40wt% PTMEG and 60wt% polycarbonate diol mixture, before and after immersion in water at 70ºC for 15 days. After hydrolysis a decrease in stress and an increase in elongation-at-break are obtained in the polyurethanes, the changes in the mechanical properties are more pronounced in the polyurethane prepared with polyether only. On the other hand, the mechanical properties of the polyurethanes prepared with 40wt% PTMEG + 60wt% polycarbonate diol mixture after water immersion are higher than in the polyurethane prepared with polyether only before hydrolytic degradation.

Table 2 shows the values and the percentages of variation of different mechanical properties in the polyurethanes before and after water immersion at 70ºC during 15 days; for sake of clarity, the percentage of variation for each mechanical property is given in Figure 14 (red bars: polyurethane prepared with polyether only; blue bars: polyurethane prepared with 40wt% polyether + 60wt% polycarbonate diol). In general, the decrease in the Young’s modulus, elastic limit, toughness and resilience values is more important (up to 70% lower) in the polyurethane prepared with polyether only (except for the yield point); however, for the polyurethane prepared with 40wt% polyether + 60wt% polycarbonate diol the reduction in mechanical properties is lower than 40%. It is interesting that after water immersion an increase in elongation-at-break is produced in both polyurethanes.

Conclusions
Polyurethane coatings prepared with polycarbonate diol shows noticeable improvement in abrasion resistance, Shore A hardness, mechanical properties and hydrolytic resistance than in the polyurethane prepared with polyether only. Even with a minor content of polycarbonate diol in the polyols mixture, the improvement in the properties of the polyurethane is noticeable. The addition of polycarbonate diol affects more the mechanical properties and hydrolytic resistance of the polyurethane than the increase in the NCO/OH ratio does. The better mechanical properties and higher hydrolytic resistance of the polyurethanes prepared with polycarbonate diol can be ascribed to the particular properties of the carbonate group leading to stronger interactions between the soft segments in the polyurethanes and therefore favoring the miscibility between the hard and soft segments.

Acknowledgements
Financial support of UBE Chemical Europe S.A. (Contact person in UBE: Manuel Colera; e-mail: m.colera@ube.es) is acknowledged.

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