Stretch film, also known as the stretch film, the earliest in China to PVC as a substrate, DOA for plasticizers and self-adhesive production of PVC film. Due to environmental issues, high cost (relative to a large PE ratio, small unit packaging area), poor stretchability, and other reasons, when China began to produce PE stretch film in 94 to 95 years, it was gradually eliminated. The PE stretch film is first made of EVA as a self-adhesive material, but its cost is high and it has a taste. After the development, PIB and VLDPE are self-adhesive materials. The substrate is now LLDPE square, including C4, C6, C8 and metallocene PE. (MPE).
In the early LLDPE stretch film, there were many blown films, from single layer to two-layer and three-layer. Now LLDPE stretched film is mainly produced by casting method. The flow is shown in the figure below, because the production of flow line has a uniform thickness. The advantages of high transparency, can be applied to high magnification pre-stretching requirements. Since single-layer casting does not achieve one-sided adhesion, the application field is limited. The single and double casters do not have a three-layer cast width in material selection, and the formulation cost is also high, so it is still better to use a three-layer co-extruded structure. High-quality stretch film should have high transparency, high longitudinal elongation, high yield point, high transverse tear strength, and good puncture performance.
2Production process
22 Process Conditions Casting Process Because the flow path is long and narrow, the flow rate is fast, the melt temperature range is generally controlled at 250 °C ~ 280 °C, the temperature of the cast cooling roll is controlled at 20 °C ~ 30 °C, winding tension To be low, generally less than 10kg, in order to facilitate the viscous agent to move out, while reducing the finished film stress.
23 Abnormalities and countermeasures that often occur in production
3 Sticky Control
Good stickiness makes the packaging film layer outside the goods stick together with the layer to make the goods firm. There are two main methods for obtaining the stickiness: one is to add PIB or its masterbatch to the polymer; the other is to blend VLDPE. . PIB is a translucent viscous liquid, and the direct addition requires special equipment or modification of the equipment. PIB masterbatch is generally used. There is a process for the transfer of PIB. It usually takes three days. It is also affected by temperature. When the temperature is high, the viscosity is strong; when the temperature is low, it is not sticky and the viscosity is greatly reduced after stretching. Therefore, the finished film is preferably stored in a certain temperature range (recommended storage temperature is 15°C to 25°C). Blended with VLDPE, the viscosity is slightly worse, but there are no special requirements for the equipment, the viscosity is relatively stable, not controlled by time, but also affected by the temperature, the temperature is higher than 30 °C is relatively viscous, less than 15 °C, the viscosity is slightly poor, can be By adjusting the amount of adhesive layer LLDPE to achieve the desired viscosity. Three-layer co-extrusion uses this method.
4 Control of physical and mechanical properties
High transparency facilitates the identification of the goods; high longitudinal elongation facilitates pre-stretching and saves on material consumption; good puncture performance and transverse tear strength allow the film to encounter sharp corners of the goods at high draw ratios or The edges do not break; the high yield point tightens the packaged goods.
Films produced by the cast method are all highly transparent and are not discussed here. As the number of C atoms in the comonomer of the material increases, the length of the branches increases, and the degree of crystallinity decreases, and the resultant "coiling or kinking" effect of the copolymer increases, so the elongation increases, and the puncture strength and the tear strength also increase. The MPE is a highly stereoregular polymer with a narrow molecular weight distribution and can accurately control the physical properties of the polymer. Therefore, the performance is further improved. Because the MPE has a narrow molecular weight distribution, the processing range is also narrow, and the processing conditions are difficult to control. Add 5% LDPE to reduce the melt viscosity and increase the flatness of the film. The price of MPE is also high. In order to reduce costs, MPE is usually used together with C4-LLDPE. However, not all C4-LLDPEs can be used together, and they should be selected. Machine stretch film uses C6, C8 materials, easy to process, to meet a variety of packaging requirements. For manual packaging, C4 material is often used due to its low stretching ratio.
Material density also affects the performance of the film. As the density increases, the degree of orientation increases, the flatness improves, the longitudinal elongation increases, and the yield strength increases. However, the transverse tear strength, puncture strength, and light transmittance decrease, so the performance in various aspects is often non-sticky. Add appropriate amount of medium density linear polyethylene (LMDPE) to the layer. The addition of LMDPE also reduces the coefficient of friction of the non-stick layer and avoids sticking of the packaged tray to the tray.
The effect of cooling roll temperature. As the temperature of the cooling roll increases, the yield strength increases, but the remaining properties decrease. Therefore, it is advisable that the temperature of the cooling roll is generally controlled at 20°C to 30°C. The tension of the cast line affects the flatness and the winding tension of the film. If PIB or its masterbatch is used as the adhesive layer, it also affects the migration of the PIB and reduces the final viscosity of the film. Tension is generally not more than 10kg, too much stress remains in the film roll, so that the elongation and other properties decreased, easily lead to the phenomenon of broken film.
5 Application form of stretch film
Stretch film has a wide range of applications. It is mainly used in conjunction with trays to package bulk goods and replace small containers. Because it can reduce the cost of bulk cargo transportation and packaging by more than 30%, it is widely used in hardware, minerals, chemicals, pharmaceuticals, food, machinery and other products of the whole set of packaging; in the storage area of the warehouse, more foreign use Stretch wrap film trays for dimensional storage to save space and space.
The 5使用1 main use form is as follows:
• Sealed Packaging This type of packaging is similar to a shrink wrap. The film wraps the tray around the tray, and the two hot grippers heat the ends of the film together. This is the earliest form of wrap film and has resulted in the development of more packaging formats:
Full-width packaging This kind of packaging requires that the film width is enough to cover the tray, the shape of the tray is regular, so its use has its limitations, suitable for film thickness of 17 ~ 35μm;
· Manual packing This kind of packaging is the simplest type of wrapping film packaging. The film is installed on a shelf or hand-held, and the tray rotates or the film rotates around the tray. Mainly used in repackaging damaged pallets, and ordinary pallet packaging. This kind of packing speed is slow, suitable film thickness is 15~20μm;
·Pallet mechanical packaging This is the most common and most extensive form of mechanical packaging. It rotates around the tray or the film rotates around the tray. The film is fixed on the support and can be moved up and down. This packaging capacity is very large, about 15 to 18 discs per hour. Suitable film thickness of about 15 ~ 25μm;
· Horizontal mechanical packaging is different from other packaging in that the film rotates around the goods and is suitable for long goods packaging, such as carpets, plates, fiberboards, special-shaped materials, etc.
· Packaging of paper tubes This is one of the newest uses of stretch film, and it is better than the old-fashioned wrapping packaging for paper tube packaging. As shown in the right figure, suitable film thickness is 30~120μm;
· Packaging of small items This is the latest form of wrapping film, which can not only reduce material consumption, but also reduce the storage space of the tray. In foreign countries, this kind of packaging was first introduced in 1984, and after only one year, it appeared on the market. With many such packages, this form of packaging has great potential. Suitable for film thickness 15 ~ 30μm;
· Packaging of tubes and cables This is an example of the application of a wrap film in a special field. The packaging equipment is installed at the end of the production line. The fully automatic stretch film can replace the tape and protect the material. The suitable thickness is 15 to 30 μm.
52 Stretching of Pallet Mechanism Packaging
Stretch film packaging must be stretched, and the tray mechanical packaging stretches directly stretched and pre-stretched. Pre-stretching is divided into two types, one is roller pre-stretching, and the other is electric stretching.
Direct stretching completes the stretching between the tray and the film. This method has a low draw ratio (about 15% to 20%). If the draw ratio exceeds 55% to 60%, the film's original yield point is exceeded, the film width is reduced, and the puncture performance is also lost. Easy to break. And at 60% elongation, the pulling force is still large, and for light cargoes, it is likely to deform the cargo.
Pre-stretching is accomplished by two rollers that are next to the film roll. Roller pre-stretched two rollers are connected by a gear unit. The stretching ratio can be different according to the gear ratio. The tension is generated by the turntable. Since the stretching is generated within a short distance, the friction between the roller and the film It is so large that the film does not shrink and the original puncture performance of the film is maintained. No stretching occurs during actual winding, reducing breakage due to sharp edges or corners. This pretensioning can increase the draw ratio to 110%.
The pretensioning mechanism of electric pretensioning is the same as that of pretensioning, except that the two rollers are electrically driven, and the stretching is completely independent of the rotation of the tray. Therefore, the adaptability is stronger, light, heavy and irregular goods are all applicable. Due to the low tension during packaging, the pre-stretching magnification of this method is up to 300%, which greatly saves materials and reduces costs. Suitable for film thickness 15 ~ 24μm.
