Good adhesion makes the packaging film layer on the outside of the goods stick together with the layer to make the goods firm. There are two main methods for obtaining the viscosity: one is to add PIB or its masterbatch in the polymer; the other is to blend VLDPE. . PIB is a translucent viscous liquid. Direct addition requires special equipment or equipment modification. PIB masterbatch is generally used. There is a process for the removal of PIB, which 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. After stretching, the viscosity is greatly reduced. Therefore, the finished film is preferably stored in a certain temperature range (recommended storage temperature is 15 ° C ~ 25 ° C). Blending VLDPE, the viscosity is slightly worse, but there is no special requirement for the equipment, the viscosity is relatively stable, and it is not controlled by time, but it is also affected by temperature. When the temperature is higher than 30 °C, the viscosity is relatively sticky. When the temperature is lower than 15 °C, the viscosity is slightly worse. The desired viscosity is achieved by adjusting the amount of the adhesive layer LLDPE. This method is used in the three-layer coextrusion.
2. Control of physical and mechanical properties
High transparency facilitates the identification of the goods; high longitudinal elongation facilitates pre-stretching and saves material consumption; good puncture performance and transverse tear strength allow the film to encounter sharp angles of the cargo at high draw ratios or The edges do not break; the high yield point makes the packaged goods more tight.
The film produced by the casting method has high transparency and is not discussed here. As the number of C cations of the material comonomer increases, the length of the branch increases, the crystallinity decreases, and the effect of the resulting "coiled or kinked" copolymer increases, so the elongation increases, and the puncture strength and tear strength also increase. MPE is a high stereoregular polymer with a narrow molecular weight distribution, which can accurately control the physical properties of the polymer, so it has further improved performance. Because MPE has a narrow molecular weight distribution, the processing range is narrow, and the processing conditions are difficult to control. 5% LDPE was added to lower the melt viscosity and increase the flatness of the film.
MPE is also expensive. In order to reduce costs, MPE is usually used in combination with C4-LLDPE, but not all C4-LLDPE can be matched with it. The machine stretch film is made of C6 and C8 materials, which is easy to process and can meet various packaging requirements. Handmade packaging uses C4 material due to its low draw ratio.
Material density also affects the properties of the film. As the density increases, the degree of orientation increases, the flatness is good, the longitudinal elongation increases, and the yield strength increases, but the transverse tear strength, puncture strength and light transmittance decrease, so the performance of all aspects is often non-sticky. The layer was added with an appropriate amount of medium density linear polyethylene (LMDPE). Adding LMDPE also reduces the coefficient of friction of the non-stick layer and prevents the packaged tray from sticking to the tray.
The effect of the chiller temperature. When the temperature of the chill roll increases, the yield strength increases, but the rest of the performance decreases. Therefore, it is preferable to control the temperature of the I roll to be 20 ° C to 30 ° C. The tension of the casting line affects the flatness of the film and the tightness of the winding. If PIB or its masterbatch is used as the adhesive layer, it also affects the migration of PIB and reduces the final viscosity of the film. The tension is generally not more than 10 kg, and the stress remains too much in the film roll, so that the elongation and the like are degraded, and the film breakage phenomenon is likely to occur.
Application form of stretch film:
The application of stretch film is very wide, mainly used in conjunction with pallets, to complete the packaging of scattered goods, instead of small containers. Because it can reduce the cost of bulk cargo transportation and packaging by more than 30%, it is widely used in the whole package of hardware, minerals, chemicals, medicine, food, machinery and other products; in the field of warehouse storage, foreign countries also use more Stretch-wrapped film tray packaging for three-dimensional storage to save space and space.