Processing °C 1 : 2.0 -
Processing °C 1 : 2.0 - 3.5 1 : 3.0 - 5.0 Sequential BO stretching line - TDO inlet Typical temperatures during BOPLA process technology, which uses linear motors driving clips without chain connections. This drive principle, also used on the “Transrapid” (Germany’s high-speed monorail train using magnetic levitation), allows a new level of freedom to be obtained when manufacturing high quality film in fast and extremely flexible production. Biodegradable oriented film is gaining great interest Excellent properties in stiffness, transparency, gloss, and dead-fold retention, combined with the environmental benefits clearly emphasize the breadth of its appeal for use in consumer packaging applications. • As an alternative to cellophane in: confectionery twist wrap, premium wrapping for flowers, toiletries and prestige gifts • Bags for compost and garden refuse, as well as agricultural mulch films to replace paper (when wet strength is required) • Multi-layer films for packaging uses, especially food • Lamination films where cellulose acetate can be replaced • Co-extruded structures with low temperature heat seal layers and/or flavor and aroma barriers where properties allow layer simplification or replacement of nylons • Shrink sleeve films and high modulus label films • Non-fogging films for fresh produce packaging Key performance indicators of Brückner’s stretching lines for biodegradable film Due to the fact that PLA resin is sensitive to humidity, special raw material handling and extrusion technology is needed. Particularly in the storage system, Brückner’s line layouts include silos with humidity protection. Resin dryers guarantee the resin’s low moisture content prior to extrusion. For the extrusion, Brückner utilizes their over ten year’s worth of experience in twin screw technology. Benefit: no additional material drying is needed. The special screw design creates uniform melt properties. Stainless steel melt pipes, polymer filters with short dwell times, static mixers and a three-layer adapter block designed for PLA are essential to the entire process. The pinning and stretching properties of PLA are very similar to the behavior of PET. Therefore, an electrostatic pinning device is needed to fix the extruded sheet to the chill roll. A shockless speed variation is realized with a drive motor concept, which assures a constant take off speed. A special roll design guarantees a temperature accuracy of ± 1 K (Kelvin) across the roll surface. The MDO (machine direction orienter) is equipped with the same drive technology. Special drive functions called MSD (MD soft drive system) assure a “scratch-free” surface of the MD stretched film. The elongation ratios in machine direction (MD) are in a range of 2 - 3.5. The stretching ratios within the transversal direction (TD) are around 3 to 5. The TDO (transversal direction orienter) is characterized by accurate and adjustable air distribution and a reliable chain track system. Stretching profile 1.767 mm Die Neck-in 80 mm Neck-in 120 mm Clip Range 30 mm 7. 435 mm 1.687 mm Castfilm 1.567 mm MD Film 6.600 mm Edge trim 150 mm 26 bioplastics MAGAZINE [07/04] Vol. 2
CF-PLA BOPLA (2,2 x 6,0) s-BOPLA (4,5 x 5,2) 220 180 3510 4110 5155 4616 Processing 180 180 120 3000 3050 90 100 70 53 52 17 6 MD TD MD TD MD TD Tensile Strength [N/mm²] Elongation at Break [%] E-Modulus [N/mm²] Mechanical Properties Improvement of mechanical properties by biaxial stretching Sequential BO streching line - Winder The stretched film needs to be surface treated and the thickness gauge must be measured. Brückner’s stateof-the-art treatment systems and scanning devices combined with a ultra-fast die-bolt system guarantee highest surface properties and a constant gauge throughout the complete production. The winder winds up the final treated film on steel reels. Criteria like winding tension, roll profile, roll density and perfect roll build-up are essential for the further process steps. Technologies like LIWIND ® (winding technology with linear motors), tension control systems and winder oscillation systems are essential for perfect further processing. The output of such a state-of-the-art stretching line is 1,300 kg/h, with the end film having a thickness range between 15 and 50 µm. Unique technology center Brückner‘s experience in processing PLA using an orientation process is based on their worldwide unique technology center. The variety of stretching methods, ranging from monoaxial to sequential and simultaneous stretching, gave the possibility to run PLA in each production mode. Produced PLA film: • MOPLA (mono-axially oriented PLA) shrink film for sleeves applications • BOPLA (bi-axially oriented PLA) thin film • BOPLA thick film for thermoformapplications (190 – 350 µm) All produced film types displayed improved film properties after the stretching process. Considering that cast – PLA is brittle and inflexible the stretching process made the final film flexible, while at the same time improving its tensile strength and E-modulus. The final film exhibited excellent dead fold and twist wrap properties. The simultaneous stretching mode also presented amazing results. Sequentially, PLA can be processed with a MD ration of 2.2 and a TD ration of 6.0. The main challenge in this line configuration is the maximum pinning speed. The use of simultaneous technology is completely changing the situation. Stretching ratios of 4.5 times 5.2 are no longer a problem. Therefore, line output can be dramatically increased. In addition, film properties were better when compared to the sequential stretching process. • Mechanical properties • Sealing properties (by using a low sealing skin layer) • Adjustable shrink properties due to the special process • Better optical quality (no scratches, higher gloss) Overall, the PLA tests at Brückner’s technology center were surprisingly successful and they gained a lot of new experience and insight from these trials. The successful thermoform application tests for the thick film were carried out at Brückner’s new Group Company Kiefel GmbH in Freilassing, Germany. More PLA technology from Brückner Group Brückner Formtec, supplier of cast film and sheet extrusion lines, developed a process to produce PLA film and sheet for rigid packaging in a very cost effective way by increasing productivity through high outputs and high speeds. Highlights of the line concepts are: Twin screw extrusion for highest efficiency, outputs of up to 2,000 kg/ h, speeds of up to 75 m/min, thickness range from 250 µm up to 1,200 µm, proven pinning technology, especially suitable for inline processed high volume applications. www.brueckner.com bioplastics MAGAZINE [07/04] Vol. 2 27
