*Communication* **Investigation of the Thermal and Hydrolytic Degradation of Polylactide during Autoclave Foaming Investigation of the Thermal and Hydrolytic Degradation of Polylactide during Autoclave Foaming**

**Julia Dreier <sup>1</sup> , Christian Brütting <sup>2</sup> , Holger Ruckdäschel <sup>2</sup> , Volker Altstädt <sup>2</sup> and Christian Bonten 1,\* Julia Dreier 1, Christian Brütting 2, Holger Ruckdäschel 2, Volker Altstädt 2 and Christian Bonten 1,\*** 


**Abstract:** Polylactide (PLA) is one of the most important bioplastics worldwide and thus represents a good potential substitute for bead foams made of the fossil-based Polystyrene (PS). However, foaming of PLA comes with a few challenges. One disadvantage of commercially available PLA is its low melt strength and elongation properties, which play an important role in foaming. As a polyester, PLA is also very sensitive to thermal and hydrolytic degradation. Possibilities to overcome these disadvantages can be found in literature, but improving the properties for foaming of PLA as well as the degradation behavior during foaming have not been investigated yet. In this study, reactive extrusion on a twin-screw extruder is used to modify PLA in order to increase the melt strength and to protect it against thermal degradation and hydrolysis. PLA foams are produced in an already known process from the literature and the influence of the modifiers on the properties is estimated. The results show that it is possible to enhance the foaming properties of PLA and to protect it against hydrolysis at the same time. **Abstract:** Polylactide (PLA) is one of the most important bioplastics worldwide and thus represents a good potential substitute for bead foams made of the fossil-based Polystyrene (PS). However, foaming of PLA comes with a few challenges. One disadvantage of commercially available PLA is its low melt strength and elongation properties, which play an important role in foaming. As a polyester, PLA is also very sensitive to thermal and hydrolytic degradation. Possibilities to overcome these disadvantages can be found in literature, but improving the properties for foaming of PLA as well as the degradation behavior during foaming have not been investigated yet. In this study, reactive extrusion on a twin-screw extruder is used to modify PLA in order to increase the melt strength and to protect it against thermal degradation and hydrolysis. PLA foams are produced in an already known process from the literature and the influence of the modifiers on the properties is estimated. The results show that it is possible to enhance the foaming properties of PLA and to protect it against hydrolysis at the same time.

**Keywords:** polylactide; biofoam; hydrolysis; degradation **Keywords:** polylactide, biofoam, hydrolysis, degradation

#### **1. Introduction 1. Introduction**

Degradation describes any type of mechanism in which there is a reduction in molecular weight and consequently a shortening of the polymer chains. These include hydrolysis, enzymatic oxidation, photooxidation and auto-oxidation. Mechanical and thermal as well as UV radiation-induced degradation can also occur. All these processes take place without the presence of microorganisms, which is why they are called abiotic degradation processes. This is their difference from biodegradation, which involves microorganisms. The abiotic degradation processes can lead to fragmentation and the formation of small microparticles, which in turn can be metabolized by microorganisms [1,2]. Degradation describes any type of mechanism in which there is a reduction in molecular weight and consequently a shortening of the polymer chains. These include hydrolysis, enzymatic oxidation, photooxidation and auto-oxidation. Mechanical and thermal as well as UV radiation-induced degradation can also occur. All these processes take place without the presence of microorganisms, which is why they are called abiotic degradation processes. This is their difference from biodegradation, which involves microorganisms. The abiotic degradation processes can lead to fragmentation and the formation of small microparticles, which in turn can be metabolized by microorganisms. [1,2]

PLA, as with many other bioplastics, is very sensitive to thermal and hydrolytic degradation, which is characteristic of all polyesters. In this process, cleavage takes place at the hydrolyzable groups, such as esters, by water molecules. The structural formula of PLA is shown below in Figure 1, with the functional groups color-coded. PLA, as with many other bioplastics, is very sensitive to thermal and hydrolytic degradation, which is characteristic of all polyesters. In this process, cleavage takes place at the hydrolyzable groups, such as esters, by water molecules. The structural formula of PLA is shown below in Figure 1, with the functional groups color-coded.

**Figure 1.** Chemical structure of PLA with its functional groups. **Figure 1.** Chemical structure of PLA with its functional groups.

In hydrolysis, a distinction must be made between two different types, acidic and basic ester hydrolysis. Both reactions can occur in PLA and it must be noted whether the reaction

**Citation:** Dreier, J.; Brütting, C.; Ruckdäschel, H.; Altstädt, V.; Bonten, C. Investigation of the Thermal and Hydrolytic Degradation of Polylactide during Autoclave Foaming. *Polymers* **2021**, *13*, 2624. https://doi.org/10.3390/ polym13162624 **Citation:** Lastname, F.; Lastname, F.; Lastname, F. Investigation of the Thermal and Hydrolytic Degradation of Polylactide during Autoclave Foaming. *Polymers* **2021**, *13*, x. https://doi.org/10.3390/xxxxx

Academic Editor: José Miguel Ferri Academic Editor: José Miguel Ferri

Received: 29 June 2021 Accepted: 30 July 2021 Published: 6 August 2021 Accepted: 30 July 2021 Published: 6 August 2021

Received: 29 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Polymers* **2021**, *13*, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/polymers

takes place within the chain or at the end groups and under which conditions. Hydrolysis is influenced by several parameters, including on the one hand the prevailing environmental conditions such as water activity, temperature, pH and time [3,4]. On the other hand, the degree of crystallization, molar mass, size and geometry of the samples, stereo complex formation, number of acid end groups and hydrophobicity also play a crucial role [5–8]. A special feature in PLA is also the occurrence of the so-called autocatalyzed hydrolysis. The mechanism is similar to that of acid ester hydrolysis. The proton of the carboxyl group catalyzes the hydrolysis reaction. The proton activates the carbonyl group and makes it more susceptible to attack by water molecules. Under neutral conditions, a slower degradation takes place and in alkaline media, a faster degradation takes place than in acidic environments [1,4,9]. Hydrolysis often occurs during processing (for example extrusion or injection molding) under the influence of high temperatures. One way to counteract this is to pre-dry the PLA pellets very well before processing or to add stabilizers during processing [6,7,10]. However, hydrolysis can occur not only at high temperatures, but also at relatively moderate temperatures around 60 ◦C and under the influence of increased humidity, such as under industrial compost conditions [11].

Hydrolytic degradation does not necessarily end in complete decomposition of the material. However, it must be considered in order to identify and minimize downgrading of the polymer properties. The literature contains numerous papers and patents that counteract undesirable degradation with chemical modifiers [3,6–8,10,12–17]. The substance classes epoxides, carbodiimides and phosphorous acid esters have shown the most promise to date [10,13]. Most of these modifiers react with the end groups of PLA to inhibit the hydrolysis. The determination of the acid value is a method to verify possible reactions between modifiers and PLA and degradation through processing. Although there are some ways to prevent the hydrolysis of PLA, to the best of our knowledge, this was not investigated for foaming. Especially in bead foaming, PLA has to undergo different processing steps, such as compounding, foaming and welding.

Often, biopolymers such as PLA are said to be potential alternatives in packaging applications. Here, expanded PS (EPS) and expanded Polypropylene (EPP) are the market leaders due to their possible complex geometries compared with their low densities. In order to compete with these materials, PLA bead foams have to be made and fused together. Standau et al. [18] showed different ways of producing bead foams. As an example, expanded PLA (EPLA) can be made by a stirring autoclave process described by Nofar [19]. This process is characterized by a water-polymer mixture which is processed at temperatures far above 100◦C, which leads to a tremendous degradation and therefore a loss in mechanical stability. To be competitive with polyolefine bead foams, the used polylactides need to be modified in order to prevent degradation during processing.

As a conclusion, PLA suffers from hydrolytic degradation during processing. Therefore, several modifiers have been used to increase the molecular structure and prevent the PLA from degradation during processing. Modified samples have been processed in a stirring autoclave process according to the literature to evaluate the stabilizing effect of the modifiers.
