Polymers 2021 , 13 , 2485
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( a ) ( b ) Figure9. ( a ) Elastic modulus (MD) of hot-pressed sheets with and without nip pressure follows the density as expected. ( b ) Correlation between tensile strength index (MD) and density is still rather weak. The solids contents prior to pressing are indicated in the figures. Note the highest value, which appears like an outlier here, comes from the same measurement as the corresponding point in ( a ). The points represent an average of 10 data points and their 95% confidence intervals.
The deterioration temperatures of cellulose and hemicellulose differ slightly, and some differences for the high-temperature behaviour of elastic modulus and dry tensile strength could be expected. The hemicelluloses degrade at 230–315 ◦ C, whereas lignin decomposes over a broader temperature range of 200–500 ◦ C [30,31]. However, as seen in Figure 10 for varied pulp types, both elastic modulus and dry strength peak around 150–200 ◦ C, followed up by a decrease for most cases when further increasing the temperature. In other words, the above slight differences in polymer degradation do not seem to change the big picture concerning the mechanical behaviour of materials with different pulp types. The only exceptions are a few kraft pulp samples, which contain some lignin that might shield hemicelluloses, and for which a similar decrease of mechanical properties beyond 200 ◦ Cis not observed.
Figure 10. Elastic modulus and dry tensile strength index for a wide data set of sheets with uniform fibre orientation and different furnishes. One should notice that samples above 200 ◦ Chavebeen pressed using the steel-belt press, and those in the range of 20 − 200 ◦ C have been pressed with cylinder press. The dashed trend lines describe the average behaviour for varied pressing temperatures.
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