Postcure processes in polyurethane elastomers

The evolution of PUs postcure processes was compared by us by introducing and following a reduced 300% tensile stress function (a) which we calculated according to eq. (4.21) as follows  

The function reproduces the percent level of the 300% tensile stress (erf ), achieved at a time (t) by comparing it to the final 300% tensile stress achieved 

30 days after synthesis. The a—time dependence is a function of the chemical structure of the PUs hard constituent polymer blocks. The relationship between the function a and the isocyanate (NCO) excess was followed (Fig. 4.64). Decreasing the NCO groups excess (I) from the value of I = 110 to I = 105 and consequently to 1 = 100 resulted in a decrease of a from 33% to 13% and finally to 1.5%. Shown in Fig. 4.65 and 4.66 is the evolution in time of the tensile strength and residual elongation for the series of PUs hsted in Table 4.28. 

As seen in Fig. 4.65 and 4.66, regardless the type of diisocyanate (DBDI or MDl), the tensile strength and residual elongation developed towards stabilized values within specific periods of time, ranging from about 7 to 30 days. The higher the index values, the longer the postcure process[331]. 

The PUs groups of samples were kept under different conditions (a) - ageing at 25°C and 50% RH (b) - under vacuum of 0.1 mm Hg in sealed ampoules (c) -maintained in closed P2O5 anhydrous atmosphere. PU postcuring reactions in vacuum and especially in PjOj anhydrous atmosphere were found to be very poor as compared to the case when after synthesis, the materials were kept under atmospheric humidity at room temperature. This is shown in Fig. 4.67 for the polymer PUl derived from DBDI and with the index 1 = 110. 

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