(one) Extrusion and Particle Extraction Process of PLA with Chain extenders and Zinc stearate BASF Chain extenders in Biodegradable Materials-PLA PBS、PHA、PHB、PCL

Currently produced domestically and internationallyThe most common problem with PLA is its low molecular weight,The main reason is that the purity of lactide is too low to achieve the desired purity99.5% L-lactic acid, causing poor mechanical properties; Another situation is that the two ends of polylactic acid molecules contain-COOH and - OH water absorbing groups inevitably absorb water in open processing. According to investigations, it has been found thatPLA moisture content cannot reachAt two00ppm,The main reason for the performance degradation of PLA is water degradation, with thermal oxidative degradation being a secondary factor.

1、 PLA moisture degradation moisture content andPLAThe equilibrium relationship of degradation is as follows:chart1

2、 The effect of ADR chain extenders on the molecular weight of PLA According to the chain extension principle of ADR chain extenders, PLA increases its own molecular weight (molecular chain growth) after chain extension, which inevitably results in a decrease in - COOH and - OH water absorbing groups and a decrease in its water absorption rate.

Chain extenders help reduce performance degradation caused by a decrease in Mw. The molecular weight of PLA increases after ADR modification, as demonstrated by BASF's experimental results. The effect of ADR content on the molecular weight of PLA is shown in the following figure: Figure 2

ADR-4368C/CS and ADR-4370S maximize their molecular weight through branching reactions to adapt to high melt strength applications, suitable for blown films, sheets, and low-density foaming; ADR-4300 and ADR-4385 increase molecular weight through moderate branching reactions to adapt to high-speed processing applications, suitable for strip, low-density foaming, unidirectional and biaxial stretching films; ADR-4380 achieves an ideal balance of rheological and mechanical properties through the main chain and low branching chain extension, and is suitable for high-density foaming, sheet materials, long articles, fibers (short fibers, single fibers), etc.

3、 Performance of ADR chain extender modified PLA

In situations where high melt strength is required, ADR-4368C/CS and ADR-4370S utilize branching reactions to produce PLAThe melt strength reaches its maximum, solving problems such as homogenization of plastic embryos, sagging of profiles, closed cell structure/low-density foaming, etc; In situations where high elasticity is required, ADR-4300 and 4385 improve the strength of PLA melt through appropriate branching reactions, solving the problems of deep concave shaped products hot forming processing, balancing orientation/performance, etc; ADR-4380 relies on linear chain extension to increase melt strength, adapt to changes in shear rate, and solve problems such as highly oriented fibers, reduced crystallization by shear, and injection molding.

(1) The Effect of ADR on Melt Strength

(2) The Effect of Shear Rate on the Viscosity of PLA

The effect of shear rate on the viscosity of PLA is shown in Figure 3.

The effect of ADR on melt strength is shown in Figure 4 and Figure 5.

4、 The method of ADR chain extender for PLA chain extension

This method is simple to use and does not require complete drying, catalysts, or vacuum pumping. ADR chain extenders can significantly improve molecular weight and mechanical rheological properties; In the processing, post-treatment, and recycling of low-grade materials, solid-state polymerization reaction is not required to increase higher value. This method needs to pay attention to the following issues:

(1) PLA resin needs to be dried before processing, otherwise it will affect the chain extension effect;

(2) The focus of the screw combination before the vacuum exhaust port is to disperse and mix ADR chain extender and PLA, avoiding any reaction and preventing local overheating caused by shear heat; And between the natural exhaust port and the nose die, ADR chain extender and PLA need to undergo sufficient reaction to achieve the chain extension effect; There are two main methods for ADR chain extenders to extend the chain of PLA. The first method is post-processing reaction extrusion chain expansion, which is simple and easy to implement, and is the main method used; The second method is to expand the chain during polymerization reaction, which is difficult to control, has many variable factors, and has fewer users.

1. The most commonly used reaction basic equipment for reaction extrusion chain expansion is the twin-screw extruder, as shown in Figure 6.

(3) The melting point temperature of solid ADR is between 120-140 ℃. To complete 99% of the reaction, it takes about 30 seconds at 200 ℃. Therefore, when selecting a twin-screw extruder, the influence of the screw aspect ratio and screw speed should be considered. When necessary, a rough calculation should be performed first.

It is recommended that the aspect ratio L/D of the screw should be greater than 36, and it is best to use L/D=48:

1. This can ensure the best chain extension effect at lower temperatures and higher yields (higher screw speeds); If the aspect ratio of the screw is too small, it is necessary to increase the extrusion temperature to ensure the chain extension effect, which may result in the degradation of PLA;

(4) The temperature setting should be based on the premise that PLA can melt sufficiently and ADR can react quickly. Under this principle, the temperature setting should be set at the bottom limit;

(5) Do not add antioxidants such as 168 that contain phosphites;

(6) In order to achieve uniform chain extension effect of ADR chain extenders, it is recommended to add a filter screen to the mold head to increase reflux; (7) At lower temperatures, it is recommended to add 150PPM zinc stearate.

2. Chain extension through polymerization reaction

This chain extension method is rarely used and there is currently no successful experience or data to illustrate it, but the prospects are promising. A manufacturer once conducted a small pot test to increase the molecular weight of PLA from 14000 to over 60000, and the effect was very significant. Our company suggests that if it is a two-step process, namely propylene glycol ring opening polymerization to polymerize PLA, should a chain extender be added after the polymerization is completed to carry out the chain extension reaction? This is theoretically feasible, but there has been no successful experimental demonstration yet.

5、 Experimental Effect 1: Formula

Zine Stearate:Zinc stearate

2Processing technology:

(1) Extrusion and granulation process of PLA with chain extender and zinc stearate

1) PLA was baked at 70 degrees for 4 hours

2) High speed mixer for mixing

3) Twin screw extruder, with a diameter of 26mm and an L/D ratio of 40. The temperatures in each zone are: (from the feeding zone to the die head) 150 ℃, 180 ℃, 200 ℃, 200 ℃, 200 ℃, 200 ℃, 195 ℃, 195 ℃; Pressure 36, torque 54%, feeding rate 23s

(2) Pure PLA blown film process