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-pen-
tanediol 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 abram-eter 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
Figure 2. Doehlert experimental design for polycarbonate diol content and
NCO/OH ratio of the polyurethanes at 5 and 3 levels of study respectively.
