What is the ratio of PBAT to cornstarch, the best preservation performance of the film?
The high cost and insufficient tensile strength of PBAT limit its large-scale application in food packaging and other fields. Some scholars have tried to strengthen PBAT with corn starch to reduce the cost of fully biodegradable films. Corn starch has high yield and wide application, and because of its special structural unit composition, it can play the effect of particle reinforcement. Therefore, PBAT/corn starch biodegradable film is a good choice to replace the traditional polyethylene fruit and vegetable packaging film, in which the ratio of PBAT and corn starch is very important to the film performance.
Preparation of PBAT/Corn Starch Films
PBAT and corn starch were vacuum-dried at 80 °C for 12 h before use. After stirring and mixing the dried PBAT and corn starch in a certain proportion, add the twin-screw extruder, circulate for 3 minutes at 130 °C, and extrude into a strip-shaped compound, and cut into pellets. The particles were added to a single-screw extrusion film blowing machine, and PBAT/starch films with different mass ratios were prepared at 120 °C with a film thickness of 15 μm.
Tensile properties of PBAT/cornstarch films The tensile properties of plastic packaging films are important parameters to evaluate their mechanical properties. The longitudinal and transverse tensile strengths and tensile rupture rates of films with different PBAT/corn starch ratios are shown in Figure 1.
With the decrease of PBAT content, the tensile strength first increased and then decreased in both longitudinal and transverse directions, and reached the maximum value of 20.6 MPa and 15.8 MPa when PBAT/corn starch was 70∶30.
Corn starch is crystalline spherical particles due to a large number of hydrogen bonds in the molecular chain, so adding it to the PBAT matrix can play a role in particle strengthening. With the increase of the addition ratio, the strength of the composite film increases.
When the corn starch content is too high (>30%), the dispersibility of corn starch granules is insufficient, resulting in stress concentration and fracture. This is similar to the research results of polylactic acid and polypropylene particle reinforcement. When the reinforcements such as bamboo particles and SiO2 exceed 20% to 30%, the strength will decrease significantly. When the cornstarch content is too high, it is easy to agglomerate, resulting in stress concentration and film rupture, which is also the reason why the tensile rupture rate decreases in both the longitudinal and transverse directions. Thermal
Properties of PBAT/Corn Starch Films
The thermal characteristic data of PBAT/corn starch film, including glass transition temperature Tg, melting temperature Tm, crystallization temperature Tc and cold crystallinity Xcc are shown in Table 2.
After the addition of cornstarch, the glass transition temperature and melting temperature do not change significantly, which ensures that the film can be used at room temperature and 100 °C without phase change affecting the mechanical properties.
After blending with corn starch, the crystallization temperature of PBAT increased from 69.6 °C to more than 80 °C, which may be caused by the heterogeneous nucleation of starch granules in the PBAT matrix, which promoted the crystallization of PBAT.
With the decrease of the overall content of PBAT, the cold crystallinity did not increase, but gradually decreased with the increase of corn starch content. This is because the bicontinuous structure will hinder the ordering of macromolecules, resulting in the higher starch content, which hinders the The more obvious the effect.
Barrier properties of PBAT/corn starch films
The barrier properties of the film include oxygen transmission rate and water vapor transmission rate. Barrier properties are extremely important for the preservation of fruits and vegetables, because appropriate barrier properties can inhibit cellular respiration, control water loss, and adjust the gas concentration of film packaging bags. The oxygen transmission index (OP) and water vapor transmission index (WVP) of PBAT/corn starch films are shown in Fig. 2.
It can be seen from the figure that the PBAT/corn starch film has a large oxygen permeability, and the OP is about 2.0 cm3 m/(m 2 ·d ·Pa), especially when the starch content is 30%, the OP value is the largest, which is 2.21 cm 3 ·m/(m 2 ·d·Pa). The larger oxygen permeability is helpful for the preservation of Agaricus bisporus. This is because the Agaricus bisporus respiration is vigorous, and the oxygen consumption in the packaging bag is fast. Alcoholism and other harm to their own tissues and cells.
At the same time, the WVP of PBAT/corn starch film can reach 210.0×10 -7 g·m/(m 2 ·d·Pa) , and the better moisture permeability can effectively dissipate unnecessary water, prevent fruits and vegetables from rotting and microbial growth.
Therefore, when the PBAT content is 70%, the fresh-keeping ability of the film is effectively improved.
The barrier ability of the film is closely related to the microscopic morphology of PBAT and starch. Due to the granular distribution of starch, oxygen and water vapor can pass through the interface between starch granules and PBAT, especially starch has good affinity for water vapor, so Improves the passage of oxygen and water vapor.
