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Effect of a Bio-Based Dispersing Aid (Einar^{®} 101) on PLA-Arbocel^{®} Biocomposites: Evaluation of the Interfacial Shear Stress on the Final Mechanical Properties

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## Abstract

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## 1. Introduction

^{2}[13,14]) and low production cost if compared to other biodegradable materials [15,16,17]. These features make PLA adaptable in many areas of Engineering and Technology [18,19,20,21]. However, there are some PLA drawbacks (such as low flexibility, excessive brittleness, low thermal stability, low crystallization rates) that highly restrict its possible applications [22,23].

^{®}600 BE/PU) was further investigated. In particular, due to the agglomeration tendency of these fibers inside the polymeric matrix, the effect of the addition of a bio-based dispersing aid (Einar

^{®}101) was evaluated. This liquid dispersing aid should coat the fibers, guaranteeing a homogeneous dispersion, and at the same time, should decrease the interaction between polymer matrix and the fibers. In this way, it should be possible to obtain a SFRP composite containing a high quantity of fibers and at the same time, improve the final ductility and impact resistance of the material. The effect of Einar

^{®}101 on this composite system was also evaluated using different existing analytical models. In particular, the Pukánszky [56] model and Sato and Furukawa [57] models were applied. Furthermore, to estimate the IFSS value, the B-B models (the classical version and the modified ones) were adopted, and it has been shown how for the case of ultra-short fiber composites, it is convenient to use the modified approach of the B-B model.

## 2. Materials and Methods

#### 2.1. Materials

- Poly (lactic) acid (PLA) 2003D, purchased from NatureWorks (Minnesota, Minneapolis, MN, USA), was used. It derives completely from renewable resources and this grade contains about 4% od D-lactic acid units in order to lower the melting point and the crystallization tendency. PLA2003D is a transparent general-purpose extrusion grade biopolymer that can be used naturally or as part of formulated blends or composites. Thanks to its high molecular weight, this PLA grade can be easily processed on conventional extrusion equipment. According to the producer’s data sheet, PLA 2003D has a density of 1.24 g/cm
^{3}, a melt flow index (MFI) of 6 g/10 min (210 °C, 2.16 kg) and a nominal average molar mass of 200,000 g/mol. - Arbocel
^{®}600 BE/PU, provided by J Rettenmaier and Söhne (Rosenberg, Germany), are 100% natural ultra-short, highly pure white micro-cellulose fibers (mean diameter 20 µm, mean fiber length 60 µm, and consequently, mean aspect ratio of 3, bulk density: 200–260 g/L, fiber density 1.44 g/cm^{3}). In the following paper, these fibers will be named Arbocel. - Einar
^{®}101 provided by Palsgaard (Juelsminde, Denmark), is yellowish viscous and food-grade, entirely based on vegetable oils dispersing aid (density: 1.62g/cm^{3}, viscosity: 7 Pa s). It is a Polyglycerol ricinoleate oil with a maximum acid value of 3 mg KOH/g and a saponification value of 170 mg KOH/g. The addition of Einar^{®}101 should enable a better fiber dispersion and at the same time, should enable to reach higher fiber loads, decreasing the melt viscosity during the processing. In the following paper, this dispersing aid will be named Einar.

#### 2.2. Composites’ Preparation

#### 2.3. Melt Flow Rate

#### 2.4. Specimens Preparation

#### 2.5. Mechanical Tests

#### 2.6. Scanning Electron Microscopy Analysis (SEM)

#### 2.7. Thermo-Mechanical Analysis

^{®}DMTA (Gabo Qualimeter, Ahiden, Germany) equipped with a 100 N load cell. At least three specimens were tested for each formulation. Test bars (width: 10 mm, thickness: 4 mm, length: 50 mm) were obtained by cutting the dog-bone tensile specimens. The samples were mounted on the machine in tensile configuration. The used temperature range varied from 20 to 120 °C with heating rate of 1.5 °C/min and at a constant frequency of 1 Hz.

## 3. Theoretical Analysis

#### 3.1. Indirect Approach

_{c}and σ

_{m}are the strength at break of the composite and matrix respectively, and V

_{f}is the volume fiber fraction. The term (1 – V

_{f})/ (1 + 2.5V

_{f}) is correlated to a decrement of the effective load-bearing cross-section caused by the fibers’ addition, while the parameter B is an interaction parameter that considers the capacity of stress transmission between the matrix and the filler [61]. By writing Equation (1) in a linear form (Equation (2)), a linear correlation can be obtained in which the interaction parameter, B, is found as the slope of the Pukánszky’s plot (obtained plotting the natural logarithm of Pukánszky’s reduced strength, σ

_{red}, against the volume filler fraction):

_{m}are the Young’s modulus of the composite and the matrix, respectively.

#### 3.2. Direct Approach

_{0}, and, in first approximation, it is retained constant and equal to the maximum stress value reached by the matrix alone [24].

## 4. Results and Discussion

^{2}with Einar addition). The action of the Einar is similar to what happens when a plasticizer is added to PLA, where not only the processability but also the toughness is enhanced [23,72].

_{g}deviation (tan δ peak) can be encountered when Einar is added.

_{m}is the stress of the matrix at the composite elongation at break for the composite without Einar, in which the final elongation at break of the composite is inferior to the matrix elongation at break. While, for the system with Einar, in which the elongation at break of the matrix is inferior to that of composites, the final stress at break of the matrix was considered. An orientation factor equal to 3/8 valid for randomly oriented fiber composites was considered for both the systems analyzed. Because IFSS is an interface property that does not depend on the volumetric filler fraction [24,62,64,80], a mean value among the values obtained for the two composites systems at different volumetric percentages was considered and reported in Table 5.

_{m}/√3) [81].

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Stress-strain representative curves for pure PLA and for PLA/Arbocel fiber composites containing fibers content from 10 to 25 wt.%: (

**a**) Composite system without additive, (

**b**) composite system with the Einar addition.

**Figure 2.**Mechanical properties for the two PLA/Arbocel composite systems: (

**a**) Young modulus, (

**b**) stress at break, (

**c**) elongation at break, (

**d**) Charpy impact resistance.

**Figure 3.**Storage modulus of PLA-Arbocel biocomposites as a function of temperature: (

**a**) composite system without Einar, (

**b**) composite system with Einar.

**Figure 4.**Tan δ of PLA-Arbocel biocomposites as a function of temperature: (

**a**) composite system without Einar, (

**b**) composite system with Einar.

**Figure 5.**Comparison between the predicted elastic modulus obtained with the Sato and Furukawa model and the experimental data.

**Figure 7.**Scanning electron microscopy (SEM) images, at different magnification (300x and 1600x) of some of the composites investigated: (

**a,b**) PLA_10, (

**c**,

**d**) PLA_10_E, (

**e**,

**f**) PLA_25, (

**g**,

**h**) PLA_25_E.

Name | PLA-Arbocel wt.% | Einar 101 wt.% |
---|---|---|

PLA | 100–0 | - |

PLA_10 | 90–10 | - |

PLA_15 | 85–15 | - |

PLA_20 | 80–20 | - |

PLA_25 | 75–25 | - |

PLA_E | 100–0 | 5 |

PLA_10_E | 90–10 | 5 |

PLA_15_E | 85–15 | 5 |

PLA_20_E | 80–20 | 5 |

PLA_25_E | 75–25 | 5 |

Main Injection Molding Parameters | Composites without Einar | Composites with Einar |
---|---|---|

Temperature profile from feeder to injection zone (°C) | 180–185–180 | 170–175–170 |

Mold temperature (°C) | 60 | 60 |

Injection holding time (s) | 5 | 5 |

Cooling time (s) | 25 | 25 |

Injection pressure (bar) ^{1} | 90–110 | 90–110 |

^{1}Increasing the fiber content, the injection pressure was incremented.

Symbol | Meaning |
---|---|

ε_{c} | Composite strain |

η_{0} | Fiber orientation factor |

σ_{c} | Composite stress at break |

σ*_{f} | Fiber stress at break |

σ’_{m} | Stress of the matrix at ε_{c} |

E_{f,m} | Elastic modulus of fiber (f) or matrix (m) |

a_{r} | Fiber aspect ratio |

V_{f} | Volume fraction of the fibers in the composite |

τ | Interfacial Shear Strength (IFSS) |

D | Average fiber diameter |

L_{c} | Critical fiber length |

L_{i,j} | Fiber length below (i) and above (j) the critical fiber length |

**Table 4.**Values of the calculated parameter correlated to the fiber/matrix adhesion according to the Sato and Furukawa and Pukánszky models for the PLA/Arbocel composite system with and without Einar.

Composite System | Sato and Furukawa ξ Parameter | Pukánszky B Parameter |
---|---|---|

PLA/Arbocel | 0.13 | 2.58 |

PLA/Arbocel/Einar | 0.37 | 1.98 |

Composite System | IFSS (MPa) According to Classic B-B Model (Equation (7)) | IFSS (MPa) According to Modified B-B Model (Equation (8)) | IFSS (MPa) Von Mises Threshold |
---|---|---|---|

PLA/Arbocel | 55.92 | 34.44 | 37.20 |

PLA/Arbocel/Einar | 23.20 | 8.20 | 25.98 |

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**MDPI and ACS Style**

Aliotta, L.; Gigante, V.; Cinelli, P.; Coltelli, M.-B.; Lazzeri, A.
Effect of a Bio-Based Dispersing Aid (Einar^{®} 101) on PLA-Arbocel^{®} Biocomposites: Evaluation of the Interfacial Shear Stress on the Final Mechanical Properties. *Biomolecules* **2020**, *10*, 1549.
https://doi.org/10.3390/biom10111549

**AMA Style**

Aliotta L, Gigante V, Cinelli P, Coltelli M-B, Lazzeri A.
Effect of a Bio-Based Dispersing Aid (Einar^{®} 101) on PLA-Arbocel^{®} Biocomposites: Evaluation of the Interfacial Shear Stress on the Final Mechanical Properties. *Biomolecules*. 2020; 10(11):1549.
https://doi.org/10.3390/biom10111549

**Chicago/Turabian Style**

Aliotta, Laura, Vito Gigante, Patrizia Cinelli, Maria-Beatrice Coltelli, and Andrea Lazzeri.
2020. "Effect of a Bio-Based Dispersing Aid (Einar^{®} 101) on PLA-Arbocel^{®} Biocomposites: Evaluation of the Interfacial Shear Stress on the Final Mechanical Properties" *Biomolecules* 10, no. 11: 1549.
https://doi.org/10.3390/biom10111549