1. The manufacture of Polylactic acid
The overall route of the manufacture of PLA [25] :
1. Slice drying: Like PET, PLA slices must be dried before melt spinning. PLA is a polyester product, because its polymer will be degraded by hydrolysis of ester bond breaking in active and humid environment, which will greatly reduce the molecular weight, thus seriously affecting the quality of finished fiber. Therefore, the water content of PLA polymer should be strictly controlled before spinning. It is particularly important to control the water content and intrinsic viscosity of PLA chips after drying, because the molecular weight loss caused by improper control of water content will bring difficulties to normal melt spinning.
2. Melt spinning：Because of its high crystallinity and orientation, PLA fiber has high heat resistance and high strength, and does not need special equipment and operation technology.
3. Cooling and oiling.
4. Winding overfeeding: in the production process, in order to ensure a certain degree of orientation of PLA fiber, and hope that tensile stress and winding stress can be eliminated in time and effectively in the spinning process, effective control of winding tension is the key.
5. Hot plate stretching：The temperature of hot plate is the drawing temperature. As one of the important conditions affecting fibers, choosing the appropriate drawing temperature is the key to improve the physical-mechanical properties of fibers.
Figure 1. The crude PLA crystal [25]
2. Industrial preparation of calcium carbonate
It is prepared by passing carbon dioxide gas through calcium hydroxidewhich is commonly known as slaked lime. When it reacts with a dilute acid, it gives carbon dioxide as a by-product. [26]
At 1200K, calcium carbonatedecomposes and liberates carbon dioxide along with calcium oxide. Calciumcarbonate is used extensively in many industrial processes.
Figure 2. The prepared calcium carbonate powder [26]
3.Mix and fuse the PLA and PCC
The mixing process of PLA (polylactic) and PCC( precipatated calcuim carbinate) is completed by the 'twin-screw extrusion'method, of which the main experimental apparatus is the twin-screw extruder. The fundamental operating principle of the apparatus is shown in figure 3. below.
Figure 3. The twin-screw extrusion apparatus[27]
Basically, the working process of the equipment can be divided into three stages.
(1) Raw materials (PLA and PCC) are injected into the machine barrel from the feeding hopper, along the direction of the screw groove forward, which is known as the ‘feeding conveyor’ process. As indicated in the figure, the clean material composed of PLA is made into flakes and injected into the barrel together with the reinforced agent PCC by the auger feeder. For this PLA+, the ratio of PLA and PCC is 3:1, in order to give a maximum shock strength of the mixture.[27] Meanwhile, the liquid combustibles used to bind two incompatible polymers together is added through a metering pump.
(2) Due to the intense mixing and shearing effect of the mixture between the screw and the barrel, and the external heating of the barrel, the mixture heats up and begins to melt until it melts completely. This process is called the 'compression and molten’ stage. In this stage, solid polymers are gradually compacted due to resistance from the head of the barrel, and then the mixture of PLA and PCC is melt blended and enters the ‘vacuum sizing tank’.
(3) This stage is referred to as' homogenization 'stage, in which the polymer mixture is further homogenized, and finally by increasing the temperature and pressure, the product is extruded uniformly from the head passage with a fixed amount and pressure.[28] The vacuum pumps in the equipment are used to balance and control the pressure of the mixing system.
To enhance flexibility, it has been inferred that some PLA Plus filaments contain a healthy dose of TPU. [29] In the case of colorFabb, they are quite forthcoming in describing their PLA/PHA filament, a combination of PLA and PHA (polyhydroxyalkanoate). With more toughness and flexibility, this has become the standard PLA filament of the color Fabb brand.
Figure 4. The final state of refined PLA+ product[29]
4. Shaping process of mixture
The mixture of PLA and PCC made by ‘twin-screw extrusion’ is shaped into a dump bell type by the hot pressing process.
The apparatus for hot process is shown in figure 5, which includes some core equipment, such as the upper and lower plate, base plate, heating cooling plate and the heat insulating plate[30].
Figure 4. The real figure of apparatus for hot pressing[31]
The solid polymer mixture of PLA and PCC are contained in the machining model, which is shown in figure 6. In addition, high accuracy and sensitivity are controlled by the temperature and pressure sensors.
Figure 5. The component schematic diagram of the apparatus[32]
In the hot pressing process, the machining model is firstly preheated for 20 minutes at about 190 degrees Celsius, and then the mixed specimen is injected into the model after which the model is fixed on the heating plate with the pressure of 15MPa for approximately 20 minutes. Control the melting temperature and time of the sample, so as to achieve hardening and cooling after melting, and take out the finished model with the sample.
5.The final properties
Firgure 6. The product of PLA+ fiber[33]
1. A low glass transition temperature is one of the weakest points of standard PLA. With PLA Plus, you can gain a boost in thermal stability by up to 15 °C. This puts PLA Plus at par with more heavy-duty filaments such as PETG.
2. A common goal of combining standard PLA with additives was to reduce its brittleness and make it more flexible. This makes PLA Plus a little more robust and versatile in terms of applications.
3. Printing functional parts with PLA is often not a good idea. Not only is PLA brittle, but it lacks the toughness to withstand sustained loads and repeated motion. Many PLA Plus filaments address this by improving metrics such as tensile strength and impact strength.
4. Some brands promise that their PLA Plus filaments naturally create a more radiant or glossy finish compared to standard PLA. This can surely be a selling point if you’re the type to finish your PLA prints until they look as good as possible.
5. With improved strength, you can also expect PLA Plus to perform better for overhanging features or bridges in your model. This is precisely because the improved material supports its own weight better and can develop more strength as it cools down. This is a property of PLA Plus you likely will have to experiment with to confirm.[33]
Reference
[25] Flynt and Flynt, 2021. What is PLA? Polylactic acid properties, uses, & melting.point-3DInsider.[online]3DInsider.Availableat:<https://3dinsider.com/what-is-pla/> [Accessed 17 May 2021].
[26] W3spoint. 2021. Industrial uses of Calcium Carbonate - W3spoint. [online] Available at: <https://www.w3spoint.com/industrial-uses-of-calcium-carbonate> [Accessed 17 May 2021].
[27] Xu W H, Yang Z T, Yin X C, Xu B P, et al. Study on the mechanical properties of toughened PLA/PBS blend under tensile deformation [J]. China plastics,2016,30(01):34-38.
[28] Deng J H, Tan X H, Li Y X. Performance analysis and application of twin screw extruder [J]. Food and machinery,2004(06):41-43.
[29] Flynt and Flynt, 2021. PLA vs PLA+ : What’s the Difference?. [online] 3D Insider.Available at: <https://3dinsider.com/pla-vs-pla-plus/> [Accessed 17 May 2021]
[30] Zhang Z Z, Qiu J Z. Development of hot-press molding machine based on polymer micro-nano manufacturing technology [J]. Manufacturing automation,2017,39(02):106-108.
[31] The hot pressing apparatus: Modification of PLA. Retrieved from xtm888.skxox.com/
[32]Budakci, Mehmet & Sonmez, Budakci, Mehmet & Sonmez, Abdullah & Pelit, Hüseyin. (2015). Effects of Thermomechanical Densification and Heat Treatment on Density and Brinell Hardness of Scots Pine (Pinus sylvestris L.) and Eastern Beech (Fagus orientalis L.). Bioresources. 10. 3097-3111.10.15376/biores. 10.2.3097-3111.
[33] Flynt and Flynt, 2021. PLA vs PLA+ : What’s the Difference?. [online] 3D Insider.Available at: <https://3dinsider.com/pla-vs-pla-plus/> [Accessed 17 May 2021].