I. Questions raised
Figure 1 Antistatic polyethylene film bag
At the end of the 1990s, China's packaging industry developed a single substance antistatic polyethylene film bag (Figure 1). Because it is a polar material and has low double surface resistance (ρs is in the range of 105Ω ~ 109Ω), it is difficult to corona. It has low surface energy and cannot be coated with adhesive, so it cannot be combined with other plastic substrates to form a layered packaging material. The mechanical and physical properties of the elementary film, such as water vapor permeability, tensile strength, breaking force, and puncture resistance, cannot meet the requirements of packaging materials for precision mechanical and electronic products. It can only be used for the inner lining of powder products such as static-sensitive chemical products. package. Some use extrusion compounding or cast compounding process to make layered packaging materials, but the disadvantages are poor cast film formation, low peel strength after compounding with other substrates, and sealing strength after bag making cannot meet the requirements for use.
The packaging industry urgently needs to develop an antistatic polyethylene film that has a high surface energy and can be reliably compounded with other substrates.
Second, process change
We know that in order to obtain a certain conductivity of plastic materials, it is necessary to mix conductive materials such as conductive carbon black with thermoplastic resin in a certain proportion, and add coupling agents, plasticizers, slippers, stabilizers, antioxidants, etc. A variety of additives are mixed, plasticized, and pelletized to obtain semi-conductive particles with high MFR (melt flow rate) and good plasticization. After full demonstration and experimental verification, the latest 3-layer (ABC) co-extrusion blow molding process is used to add semi-conductive particles and low-density polyethylene resin to layer A (heat-seal layer, directly contact the inner packaging product) in a certain proportion, and High voltage corona treatment was performed on the C surface to obtain a high surface energy antistatic polyethylene film (the wetting tension of the C surface surface was ≥38dyn / cm, and the surface resistivity of the A surface was 105Ω ~ 109Ω).
This process change focuses on the following three points:
First, the semi-conductive particles and polyethylene (LDPE, LLDPE) resin are mixed uniformly and added to the A-layer hopper of a three-layer (ABC) co-extrusion film blowing machine. To explore the critical amount of semi-conductive particles added, if the added amount is higher than the critical amount, when the corona treatment is performed, the thin film is easily broken by electric sparks, and the product cost is increased; if the added amount is far below the critical amount, The resistivity of the film is higher than 1012Ω, which is approximately insulated and does not have an antistatic function. The critical amount is determined according to the amount of conductive carbon black added during the mixing and the extrusion speed of the layer A extruder. Polyethylene (LDPE, LLDPE) mixed resin is added to the B and C layers of the three-layer (ABC) co-extrusion film blowing machine.
The second is to appropriately control the extrusion amount of layer A (heat-sealing layer) according to the actual conditions of the equipment. If the extrusion amount of layer A is too large, the thickness of the inner layer film increases, and the overall film's electrical conductivity is enhanced. When corona treatment is performed, the film is likely to be punctured by electric spark, which increases the cost of the product, and the sealing strength becomes poor; If the extrusion amount of the layer A is too small, the thickness of the inner layer film decreases. According to the conductive mechanism of conductive carbon black, the formation of the conductive network is three-dimensional, and the conductivity is related to the thickness of the film. The thickness of the inner film is too low to form a conductive network and has no antistatic effect.
The third is to corona the C layer of the film. Corona treatment effect is related to input high voltage, film thickness, and distance between electrodes. Because the surface resistivity of layer A is low, the volume resistivity of the film is low, the dielectric constant is low, and the withstand voltage is low, the spark breakdown phenomenon is prone to occur during corona treatment. Therefore, the distance between the electrodes and the electricity should be appropriately reduced Halo processing voltage. However, if the corona treatment voltage is too low, the surface wetting tension of the film is low, and it cannot be reliably combined with other substrates, or the peel strength and heat seal strength of the composite material cannot meet the requirements, thereby reducing the mechanical and physical properties of the material.
The main performance test is performed on the produced high surface energy antistatic polyethylene film (see Figure 2). The test results are shown in Table 1:
Figure 2 High surface energy antistatic polyethylene film coil
Table 1 Main properties of high surface energy antistatic polyethylene film
It is concluded from the above table that this product has antistatic performance, and its technical indicators meet the requirements of GB / T4456-2008 "Polyethylene Blown Film for Packaging". Due to its high surface energy of C layer, it can be coated with adhesive. It can be combined with other plastic substrates into a layered packaging material (see Figure 3). The material has excellent mechanical and physical properties such as water vapor permeability, tensile strength, breaking force, and puncture resistance, and can meet the special requirements of packaging materials such as precision mechanical electronics and military products.
Figure 3 Dry-laminated laminated rolls
High-surface-energy antistatic polyethylene film manufactured by three-layer co-extrusion (ABC) blown film technology. Due to its low surface resistivity on the A side (ρs is 105Ω ~ 109Ω), it has the function of antistatic accumulation, and the surface energy on the C side High (surface wetting tension ≥38dyn / cm), can be coated with adhesive, dry-laminated with several materials in polyester (PET), nylon (PA), metal foil, high-strength fiber cloth, non-woven cloth After that, anti-static soft plastic layered packaging materials can be prepared and formed into bags to protect the inner packaging from moisture, mildew, salt spray, corrosion, rust, antistatic accumulation, and electromagnetic radiation. effect.
This process solves the technical problems of low surface resistivity and high surface energy of plastic film, which are difficult to achieve at the same time, and the antistatic film cannot be bonded to other substrates. It also solves the difficulty of film formation, low bonding strength, and high temperature in the original casting process. The quality problem of easy carbonization and low sealing strength after bag making can be called a "small change" in the process to solve the "big problem" of quality.