Polylactic acid (PLA) is a widely recognized biodegradable polymer material, which has excellent biodegradability and biocompatibility, high modulus and good processibility. However, compared with traditional polymer materials, PLA also has some obvious shortcomings, such as poor toughness, hard and brittle PLA resin, and generally the elongation at break is less than 10%. The crystallinity of PLA resin is generally low, and its glass transition temperature (Tg) is about 60℃, therefore the heat resistance of PLA is unsatisfied.
Biological additives refer to a kind of additives derived from biomass or bio-based materials.
As biological additives originated from organisms, the products prepared by modifying PLA resin with them are environmentally friendly and conform to the concept of green environmental protection.
Therefore, the use of biological additives to modify PLA has been widely concerned by researchers.
In order to improve the hard and brittle mechanical properties of PLA, it is often necessary to use additives to toughen it. At present, the bio-based additives used to toughen PLA include citrate, lignin, natural cellulose, glycerol, bio-based elastomer, starch, and oligomeric polylactic acid. For example, Chaos et al. plasticized PLA with tributyl citrate plasticizer, and found that the Tg of plasticized PLA decreased significantly, the ductility was improved and the excellent mechanical properties of PLA were retained. Jiang Aiju et al. prepared PLA composites filled with jute fiber by extrusion blending. The addition of alkali-treated jute effectively improved the tensile properties and impact toughness of PLA. Eucommia ulmoides Oliv. (EUG) is an isomer of natural rubber, a natural polymer extracted from Eucommia ulmoides Oliv., which has dual characteristics of rubber and plastic. Kang et al. prepared PLA/EUG blends with complete bio-base and high toughness by melt blending. The research showed that the elongation at break of PLA/EUG blends reached to 81% and the impact strength reached to 21.1kJ/m2.
The low crystallinity and Tg of PLA resin lead to the poor heat resistance of PLA.
When PLA is applied at higher temperature, it is necessary to modify it for heat resistance. Generally, there are two ways to improve the heat resistance of PLA. Firstly, to improve the heat resistance of PLA by increasing its crystallinity; Secondly, to add some specific substances, the chain movement of PLA is restricted when heated, which greatly improves the heat resistance of the material. Sorbitol is a bio-based nucleating agent. Zhang X et al. added sorbitol as a small molecular nucleating agent into PLLA, and obtained a nucleated PLLA sample with high crystallinity and high heat resistance by melt blending. The thermal deformation temperature (HDT) of PLLA can be up to 132℃. Natural fiber can also be used as an additive to improve the heat resistance of PLA. Wootthikanokkhan et al. blended PLA with kenaf fiber, and then annealed for 10 min to prepare the composite material. The results show that the HDT of the composite material can reach 120℃, as kenaf fiber plays a skeleton supporting role in the composite material, and kenaf fiber promotes the crystallization of PLA.
In fact, the biological additives used in the process of PLA modification will also affect the processing and molding process of PLA products. Some bio-based plasticizers will be used in the toughening modification of PLA. The existence of plasticizers can reduce the melt viscosity of PLA, and make the processing performance of PLA better. However, the crystallinity of heat-resistant modified PLA is improved, which will increase the melting point of PLA and the corresponding processing temperature. The addition of bio-based additives with rigid structure will hinder the movement of PLA chain segments, which will both increase the processing temperature and melt viscosity of PLA.