Development, processing and application of polylac

2022-08-21
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Development, processing and application of polylactic acid in Japan

polylactic acid is a biodegradable plastic produced from renewable resources such as corn, which has attracted much attention in recent years. However, compared with ordinary plastics, polylactic acid biodegradable plastics have poor heat resistance, impact resistance and durability, so their scope of use is limited. Since 2000, unitika company of Japan has produced nanocomposites through nano molecular modification to improve the heat resistance and durability of polylactic acid plastics. Its application has been expanded to almost all fields with the same durability requirements as ordinary plastics. 4 The low temperature can not reach the experimental index. This technology won the Nikkei BP award in 2006

unitika studied biodegradable plastics based on polylactic acid in the late 1980s. At present, the products developed include terramac series, which involve membranes, sheets, fiber non-woven fabrics and other major fields, and are in a leading position in the world. The following mainly focuses on terramac of unitika, and introduces the polylactic acid product series with rapid market growth in recent years

1 development status of polylactic acid (PLA) biodegradable plastics

pla is α- Lactic acid polymer of hydroxy acid. In aliphatic polyester, PLA has high melting point (TM), crystallization temperature (TC) and glass transition temperature (TG), and has good molding and processing properties. When producing films and fibers, they have tensile properties after melting and extrusion, resulting in directional crystallization of polymer chains, which can improve mechanical strength, toughness and heat resistance

2 molding processing and application of polylactic acid

2.1 extrusion molding

(1) film

currently developed polylactic acid film hard type usually has biaxial tensile film, and soft type has blow molding film. Development, processing and application of polylactic acid in Japan. The hard film of polylactic acid has good gloss and transparency, which is similar to stretched polyethylene terephthalate (OPET), stretched polystyrene (OPS), cellophane, etc. It can be used as opening film of envelopes, product sealing film, fruit packaging film, lunch box lining, paper pressure plate lining film, etc

soft film is a product with the same softness and heat sealing strength as polyethylene by plasticizing and flexible processing of polylactic acid, which is originally a hard resin. It is mainly used for degradable garbage bags, agricultural composite films, fertilizer bags, etc

(2) forming sheet

thermoforming sheet has transparent ordinary type and opaque heat-resistant type. The former has heat resistance below 60 ℃, but good transparency. It is mainly used for packaging containers such as vegetables and fruits and blister packaging of various products. The latter is crystallized in the heating mold during the hot forming process, and its heat resistance can reach 120 ℃ specified in the "household goods quality law", which can produce boiled water brewing tableware, electronic microwave oven heating tableware, etc

in addition, the sheet for molding also has foaming type, which will be described in the extrusion foaming sheet later

(3) fiber and non-woven fabric

polylactic acid has the best drawing performance in biodegradable plastics. All products such as monofilament, multifilament, BCF, chemical fiber short fiber to long fiber non-woven fabric can be processed through melt spinning. Polylactic acid fiber is the same as other crystalline polymers, and directional crystallization is carried out with the stretching process. Long fiber non-woven fabric can improve mechanical and heat resistance according to high-speed spinning technology

fibers and non-woven fabrics can be used in agriculture, horticulture, civil engineering, building materials, food, health, medical treatment, daily necessities and other industries

(4) other

polylactic acid packaging bags can be comparable to conventional polypropylene packaging bags, and can be applied to extruding profiled materials

2.2 injection molding

when polylactic acid is injection molded in a low-temperature mold close to room temperature, because its TG is higher than room temperature, it is easy to process transparent products. However, the crystallization speed of polylactic acid is slow, and it cannot crystallize in the molding cycle (usually 30 ~ 60 seconds) during the molding process of non stretching operation, resulting in poor heat resistance of the product (low load bending temperature DTU 60 ℃)

in order to get high heat resistance elimination method: when using these functions again, the software will call the injection products with tacit printer property. The mold temperature must be set within the crystallization temperature range of polylactic acid, so that it can crystallize within the molding cycle, so it can significantly improve the crystallization speed

the representative nanocomposites of unitika company use nano molecular technology and directional design technology to increase the crystallization rate of polylactic acid by about 100 times, and can successfully produce high heat resistance injection products at about 120 ℃

this achievement was tested in the bioplastics model in the canteen of the main hall of the Ministry of economy, trade and industry of Japan from 2003 to early 2004. From March to September 2005, it was exhibited as bioplastic tableware at the "Aichi WorldExpo" in Japan. Recently, it has been jointly developed and applied with NEC company to produce computer parts from kenaf reinforced polylactic acid. Kenaf fiber reinforced polylactic acid developed jointly with NEC in March, 2006 is used for NTT (foma- n701ieco) shell. The plant proportion of this product is as high as 90 ℃, and the heat resistance, durability, impact resistance, etc. of polylactic acid are solved, and the emission of carbonic acid gas is greatly reduced

2.3 foaming molding

the hydrodynamic characteristics obtained in the foaming process of polylactic acid are different from the unidirectional stretching flow of fiber melt spinning, which is basically bidirectional stretching flow. Therefore, in order to obtain high foaming body composed of most microbubbles, it is necessary to improve the melting tension of polylactic acid, and adopt nano molecular design and directional design technology at the same time. In order to replace petroleum foamed products, the heat resistance of the foamed products must not be lower than that of the current petroleum foamed products

under this technical background, unitika company successfully developed high heat-resistant extrusion foaming resin for the first time in the world. The resin extruded and foamed sheet is thermoformed in a mold at about 110 ℃ (10 ~ 20 seconds), which can produce tableware resistant to boiling water and heated by electronic microwave oven and other simple tableware. The heat resistance of this product is not inferior to that of PSP

Another area of foaming molding, bead foaming technology development is also under way, and experiments have been carried out in fish tanks in Beijing and other places. However, the heat resistance is worse than EPS, the basic technical problems have not been solved, and the currently developed products have limited applications

2.4 blow molding

there are two molding methods for plastic bottles: injection blow molding method and extrusion blow molding method. Polylactic acid bottles have entered the trial production stage (some are sold). However, the single component polylactic acid bottle has poor barrier to moisture. When it is used to hold water for a long time, the water in the bottle will be reduced, and when it is used to hold oil, it may be oxidized. Referring to the existing research results of PET bottles, the problem of poor gas barrier of polylactic acid bottles has been well solved, and industrial production can be realized. For example, multi-layer plastic bottles are processed by CO extrusion with resin with good gas barrier, and organic or inorganic coatings are coated, so that the barrier performance of a single polylactic acid bottle is significantly improved

among them, Japan Pinghe chemical company takes polylactic acid resin as the main component, polyethylene and other general-purpose resins with low cost and good permeability as the inner layer, which are bonded by adhesives. The water resistance and gas permeability of the polylactic acid bottle produced is about 8 times higher

2.5 hot forming

the vacuum and compressed air forming of solid and foamed sheets is mainly the same as the current CPET, which uses high-temperature molds and crystallizes in the forming and processing engineering to produce heat-resistant molded products

3 conclusion

among plant plastics, only polylactic acid can compete with petroleum plastics in quality, performance and function. The molding processability and physical and mechanical properties of polylactic acid have reached the practical application stage through improvement, but how to achieve high heat resistance and good gas barrier while maintaining transparency will still be the main research direction of technology development in the future

reprinted from: foreign plastics

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