Modification of PLA and Its Foam

It has been reported [107] that 1,4-butanediol (BD) and 1,4-butane diisocyanate (BDI) can be used as chain extender to modify commercial PLA in two steps. In first step, the commercial PLA melt is allowed to first react with BD in the presence of trace amount of tin (11) 2-ethylhexanoate to obtain hydroxyl terminated PLA (OH—PLA) [Fig. 5.20(a)]. And then, in the second step, BDI is added to react with hydroxyl end groups of OH— PLA to achieve chain-extended PLA, leading to the formation of urethane [Fig. 5.20(b)]. 

The — NCO groups of isocyanate readily react with every active hydrogen in the reacting system. Consequently, besides engaging in urethane formation, BDI can also react with carboxyl group residuals (because of equilibrium reaction) leading to amide bond [Fig. 5.21(a)]. The urethane and amide groups can fiirther react with additional isocyanate. Such reactions can cause crosslinks in PLA [Fig. 5.21(b) and (c)]. 

In the lab, starch/PLA composite foams have also been prepared by extruding. First, the modified starch is prepared by mixing the starch with maleic anhydride and montmorillonite in a homogenizer and PLA with Si02, maleic anhydride (MAH), BPO and white titanium dioxide are extruded as the master batch by using 

As shown in Fig. 5.22, it is observed that melt strength (MS) reduces with an increase in content of the starch. Addition of maleic anhydride can remarkably make MS increase (Fig. 5.23). But when the content rises to above 2%, MS would decrease. This is because that when maleic anhydride is excessive, the graft reaction is completed. The remaining maleic anhydride functions as lubricant that causes MS to decrease. 

Cell density reflects the compact degree of the cells. High cell density means more excellent mechanical and thermal properties in the same apparent density. As shown in Fig. 5.24 and Fig. 5.25, cell density increases as the content of starch increases. Modified starch has no different influence on cell density compared to starch. As can be seen from Fig. 5.25, the bubbles become larger when the content of starch increases.. This is because bubbles can grow more easily due to low MS. 

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