How to improve the heat resistance of polylactic acid?

PLA has poor impact resistance and heat resistance, and is a brittle thermoplastic material at room temperature. The addition of crystallization nucleating agents, inorganic fillers, and other biodegradable polymer blends can improve the impact resistance and heat resistance of PLA. After finishing, changing the orientation and crystallinity through stretching may improve the impact resistance and heat resistance of the film or sheet, while maintaining its transparency.

Improving the Crystallization Performance of PLA

PLA is a crystalline polymer with a high melting point of 160 ℃~180 ℃, but the thermal deformation temperature of PLA products obtained by injection molding method is only about 58 ℃, far lower than that of general plastics PS and PP. The main reason is that although PLA is a crystalline polymer, it hardly crystallizes during the actual molding process. The poor heat resistance of PLA is due to its very slow crystallization rate. The crystallization rate of general polymers is related to the mobility of molecular segments and the probability of secondary nucleation.

There is only one methyl carbon atom between the ester groups of PLA, and the molecular chain forms a helical structure. Compared with PBT and PET, which are both polyester, the activity of the molecular chain is very low. Therefore, apart from increasing the probability of secondary nucleation through stretching orientation in film and fiber forming processing to promote PLA crystallization, PLA can hardly crystallize in simple extrusion molding, injection molding, or thermoforming.

1、 Add crystallization nucleating agent

Crystallization nucleating agents are commonly added in the industrial application of PLA, which can greatly increase the nucleation density, accelerate the crystallization rate, improve crystallinity, and also enhance the heat resistance, formability, and other properties of the product. The melting point of the nucleating agent should be higher than that of PLA and have some compatibility with PLA.

01. Talc powder, silica, and other inorganic compounds are commonly used nucleating agents. At 90 ℃, the crystallization rate of PLA increases with the increase of talc content from 2% to 21%, while at 105 ℃, the crystallization rate of PLA increases and then decreases with the increase of talc content. The nucleation effect reaches its maximum value at a talc content of 11%.

02. Among organic compounds, metal phosphates have the best nucleation effect, especially sodium 4,6-di-tert-butylphenyl phosphate or sodium 2,2-methylbis (4,6-di-tert-butylphenyl) phosphate hydroxide and aluminum salts.

The PLA crystallization promoters that synergistically interact with nucleating agents mainly include higher fatty acid esters and fatty amides. Some of these compounds also have toughening, plasticizing, and demolding effects, which can improve the surface glossiness of the product. When using, one or more combinations can be selected.

2、 Add filler

In the glass fiber reinforced PLA system, as the glass fiber content increases, the material modulus increases and the thermal deformation temperature of the reinforced PLA also increases. Research has found that if long glass fibers are used to reinforce PLA, and the average length of glass fibers in the molded product is above 2.0mm, high rigidity, high strength, high impact resistance, and high heat resistance PLA materials can be prepared.

The optimal amount of talc powder added is 5% -25% of the total mass. If the talc powder content is too low, the effect on improving heat resistance is not significant; If the content of talc powder is too high, the material becomes brittle and the impact resistance decreases. The talcum powder should be surface treated by epoxy resin lotion or polyurethane resin lotion, which can ensure strong bonding between talcum powder and the matrix PLA resin, improving both the thermal deformation temperature of the material and the impact strength.