*3.3. Acid Value Determination*

The determination of the acid value is a method to verify if chemical reactions at the end groups of PLA occurred. Table 1 shows the results of the acid value of the different PLA compounds before and after the foaming.

the highest complex viscosity and is stable over the time of 1800 s.


**Table 1.** Determined acid values in mmol/kg.

The higher the acid value, the higher the amount of carboxyl end groups of PLA and therefore more possibilities for a hydrolytic reaction are given. PCDI can react with these end groups and lead to a small acid value. This means that PCDI is capturing the carboxyl end groups, which hinders the hydrolytic reaction. The results before indicated that a reaction between PLA and PCDI happened. The acid values before foaming were determined after the compounding step. The addition of 0.5 wt% PCDI leads to the smallest acid value. It confirms the other results and the expected reaction of PLA and PCDI. The combination of both modifiers has a higher acid value of 1.15 but it is still in a range where hardly any end groups exist in the whole polymer chains. As a comparison the neat PLA showed an acid value of about 18.8 mmol/Kg. The conditions during the foaming led for all materials to an increase of the acid value indicating a chain scission during the processing. This is not surprising, looking at the processing conditions (PLA pellets in water at 120 ◦C for 30 min). As it is known from literature, hydrolysis of PLA takes place around 60 ◦C and under humidity. Therefore, it can be concluded that the processing conditions for the foaming contain the ideal surroundings for hydrolysis.
