The Effect of Adding ETBP on the Thermal, Mechanical, and Rheological Properties of PLA/PBAT Blends

Tech 2023-07-11 11:31:21 Source: Network

prefaceBiobased polymer polylactic acid (PLA) is a widely used material in the fields of packaging and medical materials, which is favored for its good biocompatibility, biodegradability, and processability. However, PLA has some drawbacks, such as high brittleness, high hardness, and low impact strength, which limit its application in other potential fields

preface

Biobased polymer polylactic acid (PLA) is a widely used material in the fields of packaging and medical materials, which is favored for its good biocompatibility, biodegradability, and processability. However, PLA has some drawbacks, such as high brittleness, high hardness, and low impact strength, which limit its application in other potential fields.

In order to overcome these limitations, PLA is often mixed with other polymers with good toughness and biodegradability, such as Polycaprolactone (PCL), Polyhydroxybutyrate (PHB), polypropylene carbonate (PPC), Polybutylene succinate (PBS) and poly (butylene Adipic acid terephthalate) (PBAT).

Previous studies have shown that the interfacial compatibility between PLA and PBAT is poor, so the introduction of compatibilizers is necessary. One method is to use Glycidyl methacrylate (GMA) to improve the interfacial compatibility between them, thus improving the impact strength of the blend. Some researchers have introduced three block copolymers containing PLA PBAT PLA and PLA PEG PLA into PLA/PBAT blends to improve the compatibility between PLA and PBAT.

Other researchers used Cottonseed oil based derivatives to improve the compatibility of PLA/PBAT blends, and found that it can effectively enhance mutual fusion. In order to improve the thermal performance of PLA/PBAT mixtures, some research teams added volcanic rock particles (VPS) to the mixture, and the results showed that the thermal performance of the mixture was optimal when the amount of VPS was 5wt%. The introduction of nano clay also has a significant impact on the crystallization of PLA/PBAT blends, increasing the tensile strength and modulus of solid and microporous components.

Among various compatibilizers, end epoxy group compatibilizers are widely used due to their high reactivity. These substances act as chain extenders through ring opening reactions with the peripheral epoxy groups and terminal hydroxyl groups within the PLA/PBAT molecular chain. For example, researchers used styrene acrylic acid copolymer (ESA) with epoxy functionality to modify PLA/PBAT blends and found that ESA only reacted with the PLA matrix, thereby improving the toughness and melt elasticity of the blends.

Similarly, the use of multifunctional epoxides and the addition of organomontmorillonite/epoxy functionalized graphene blends also help to improve the rheological and mechanical properties of PLA/PBAT blends. Researchers have also improved the mechanical properties of blends by using different types of epoxy POSS.

Compared to linear polymers, branched polymers have many active end functional groups that can form strong hydrogen bonds and specific types of chemical reactions with other polymers. Due to its highly branched structure, branched polymers can effectively prevent entanglement between molecular chains, thereby reducing viscosity. Therefore, it is a feasible strategy to use branched polymers to enhance the compatibility between polylactic acid (PLA) and polybutylene adipate (PBAT).

The following three figures show the mechanical properties of PLA/PBAT mixtures with different ETBP contents in terms of tensile strength, elongation at break, and impact strength. Observing the curve, it was found that the tensile strength of the mixture fluctuated within a certain range, but remained relatively stable overall, even though the ETBP content changed.

With the increase of ETBP content, the elongation at break of the mixture shows a trend of first increasing and then decreasing. The elongation at break of pure PLA/PBAT mixture is 45.8%. When adding 3phr of ETBP, the elongation at break reached its peak at 272%, which is 5.9 times that of the pure mixture. However, when excessive ETBP (5phr) was added, the elongation at break decreased to 167%.

The improved mechanical properties can be attributed to the strong physical hydrogen bonding and chemical micro crosslinking reactions between the epoxy functional groups at the end of ETBP and the end groups (- OH and/or - COOH) inside PLA and/or PBAT. These reactions promote the compatibility of the two phases at the molecular level, making the PLA/PBAT mixture more similar to elastomers. In this system, ETBP can serve as a rigid part, while PBAT can serve as a flexible segment, thereby improving the fracture elongation and impact strength of the mixture.

The physical hydrogen bond and Chemical bond between ETBP and PLA/PBAT strengthen the adhesion of the interface between PLA and PBAT, and increase the thickness of the PLA/PBAT interaction layer. This further enhances the stress absorption ability between the continuous phase of PLA and the dispersed phase of PBAT, thereby enhancing the toughness of the PLA/PBAT mixture.

The combined effect of strong physical and chemical micro crosslinking transforms the structure of PLA/PBAT mixture from a linear structure to a three-dimensional micro crosslinking network structure, which helps to improve the elasticity and impact strength of the mixing system.

When the ETBP content exceeds 3.3 phr, the elongation at break and impact strength of the PLA/PBAT mixture begin to decrease. This is because when the threshold value is exceeded, the excessive physical hydrogen bond and Chemical bond reaction between ETBP and PLA/PBAT mixture limit the movement and rotation of macromolecular chains.

In this case, only the Chemical bond between PLA and PBAT breaks, rather than the molecular chain movement of both. This resulted in a decrease in the elongation at break and impact strength of the PLA/PBAT/ETBP mixture. From the above indications, it can be seen that when the ETBP content is 0.3 phr, the mechanical properties (elongation at break and impact strength) of the mixture can reach the highest value.

In order to verify the formation of chemical micro crosslink network, the gel content of PLA/PBAT blends with different ETBP content was measured. During the dissolution process, only macromolecular chains with physical cross-linking structures can be completely dissolved in the solvent, while macromolecular chains with chemical cross-linking structures are insoluble and only undergo swelling.

According to the data in the table, after adding a certain additive, the pure PLA/PBAT blend cannot be completely dissolved in the solvent. The gel content of the pure mixture was 1.8%. This may be due to the presence of a small amount of insoluble inorganic nucleating agents in PLA. With the increase of additive content, the content of gel in the blend gradually increased. When the additive content reached 5.0 phr, the gel content of the blend was about 17.8%. Through the analysis of gel content, it can be determined that the introduction of additives led to the formation of chemical micro crosslinking structure.

According to the TGA and DTG curves shown in the figure, the Thermogravimetric analysis analysis results of PLA/PBAT mixtures with different ETBP contents are presented.

From the observation graph, it can be observed that the initial weight loss temperature (T5%) and 50% weight loss temperature (T50%) are basically consistent between PLA/PBAT mixtures with different ETBP contents and pure PLA/PBAT mixtures. More detailed results can be found in Table 3. These results indicate that the addition of ETBP did not significantly reduce the thermal stability of PLA/PBAT blends. The thermal stability of the blends remained relatively stable, which meant that the addition of ETBP had no obvious effect on the Hot working properties of PLA/PBAT blends.

In the following two figures, it can be seen that the DSC curves and parameters of PLA/PBAT mixtures with different ETBP contents in the first heating cycle. With the gradual increase of ETBP content, the cold crystallization temperature of PLA/PBAT blend shows a trend of first decreasing and then increasing. The decrease in cold crystallization temperature of PLA/PBAT/ETBP (0.5phr) mixture is caused by the lubrication effect of ETBP.

As a branched polymer, ETBP has no intermolecular entanglement and low viscosity. It plays a lubricating role in PLA/PBAT blends, increasing the movement and orientation arrangement of macromolecular chain segments. This makes the mixture easier to crystallize, thereby reducing the cold crystallization temperature. In addition, after adding ETBP, there was no significant change in the melting temperature of the PLA/PBAT mixture.

According to the results of the above table, as the ETBP content increases (from 0 to 3.0 phr), the crystallinity percentage (Xc) of PLA gradually increases from 3.0% to 7.3%. This indicates that some amorphous regions of PLA transform into crystalline regions. However, when the ETBP content exceeds 3.0phr, Xc begins to decrease, indicating that some crystalline regions have re transformed into amorphous regions.

In the figure, it can be observed that PLA exists as a continuous phase in the sample, while PBAT is dispersed in the continuous phase as droplets. The image clearly shows the phase separation structure between PLA and PBAT, and the interface between the two phases presents an island like structure.

In Figures b to e, it can be observed that the interface between PLA and PBAT becomes blurred, and more PLA and PBAT are combined with each other.

Figures a to c show two incompatible or partially compatible mixtures, exhibiting a distinct island like structure; In Figures d and e, only one phase can be observed, indicating that after sufficient addition of ETBP, the compatibility of PLA and PBAT is improved. These results are consistent with previous research findings, indicating that the introduction of ETBP improves the interface compatibility between PLA and PBAT.

The following figure shows the SEM image of the tensile fracture surface morphology of PLA/PBAT/ETBP blend. From Figures a and b, brittle fracture characteristics and obvious island structures can be observed.

From Figures c to e, it can be seen that the morphology of the tensile fracture surface exhibits ductile fracture characteristics and obvious wire drawing state, with the island structure almost disappearing. This indicates that adding ETBP exceeding 1.0 phr to PLA/PBAT blends can significantly improve the compatibility between PLA and PBAT, as well as the toughness of the blends.

Observing the two figures, it can be concluded that as the ETBP content increases, the G 'and G' 'values of the PLA/PBAT/ETBP mixture also increase. This can be explained by the addition of ETBP, which forms a small cross-linked network structure, strengthening the entanglement and interweaving between PLA and PBAT molecular chains. This structural change and increased intermolecular friction result in an increase in the elasticity and melt viscosity of the material, resulting in an increase in the storage modulus (G ') and loss modulus (G' ').

The addition of ETBP can enhance the interfacial compatibility between PLA and PBAT, and improve the performance of PLA/PBAT blends by generating chemical micro crosslinking structures. The reaction between the epoxy group in ETBP and the hydroxyl group in PLA/PBAT helps to form a chemical micro crosslinking structure, thereby improving the performance of the blend.

The analysis of gel content further confirmed the formation of chemical micro crosslinking structure. The study found that when the ETBP content was 5.0 phr, the gel content of the blend was about 17.8%. The SEM observation results showed that ETBP improved the dispersion of PBAT particles in the PLA phase and blurred the interface morphology between the two phases.

Rheology analysis shows that after the addition of ETBP, the G 'value at low shear frequency increases with the increase of ETBP content, while at high shear frequency, the G' value almost overlaps. The G '' value of the blend is influenced by the micro crosslinking effect and lubrication effect.

Increasing the crystallinity of PLA and forming a chemical micro crosslinking structure can effectively slow down the movement of PLA and PBAT molecular chains, thereby reducing the free volume of PLA and PBAT macromolecules. Therefore, in PLA/PBAT mixtures with ETBP content not exceeding 3.0phr, these changes reduce the transport of gas molecules such as O2 and CO2. This is very important for the gas barrier performance in food packaging.

References

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3. Ding Yong, Feng Wei, Lu Bo, Wang Ping, Wang Gang, Ji Jun, PLA-PEG-PLA triblock copolymer: an effective compatibilizer for promoting the interface structure and mechanical properties of PLA/PBAT blends. Polymer, 2018146179-187.
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