Processing and performance testing of plastic film

2022-08-08
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Treatment and performance testing of plastic films for salt packaging bags (Part 2)

3 surface treatment and testing methods of films

3.1 surface treatment methods of films

most plastic films (such as polyolefin films) are non-polar polymers with low surface tension, generally 29 ~ 30 mN/m. theoretically, if the surface tension of an object with insufficient production capacity in recent years is lower than 33 Mn/m, At present, the known inks and adhesives cannot adhere firmly on it, so the surface should be treated by corona method. The processing principle is to apply high-frequency and high-voltage electricity to the processing equipment to produce high-frequency and high-voltage discharge and produce small and dense purple blue sparks. After air ionization, the scientific and technological content is improved, and various ions produced accelerate and impact the plastic film in the treatment device under the action of a strong electric field. The energy of these plasma particles generally ranges from a few electron volts to dozens of electron volts, which is similar to the chemical bond energy of plastic molecules, so it can induce the chemical bond fracture and degradation of plastic surface molecules, and increase the surface roughness and surface area. When discharging, a large amount of ozone will be produced. Ozone is a strong oxidant, which can oxidize plastic molecules and produce carbonyl, peroxide and other polar groups, thus improving its surface energy

The advantages of corona treatment are as follows:

the range of treatment materials will realize the survival of the fittest in the market, and can be used for polyethylene, polypropylene, polyvinyl chloride, polyamide, polyester, polycarbonate fluoroplastics, and various corresponding copolymers; The processing time is short and the speed is fast, which can be processed on the production line; Simple operation and easy control; Corona treatment only involves the range of extremely shallow plastic surface, which is generally only in the order of nanometer, and basically does not affect the mechanical properties of plastic; No waste liquid is discharged, and the environment is basically not polluted

corona treatment is widely used for surface treatment before film printing, coating and lamination, as well as surface treatment of sheets with thickness less than 0.6mm

the degree of film treatment directly affects the quality of post-processing, which must be strictly controlled. If the degree of treatment is not enough, the surface wettability of the film is not significantly improved, and there will be problems such as poor adhesion of the ink. On the contrary, if the treatment degree is too high, the surface layer of the film will be aged, the gloss will be poor, the surface molecules will be cross-linked too much, the heat sealing will be poor, the film will be easy to adhere (especially in high-temperature days in summer), it will be difficult to cut, it will be difficult to separate when using, etc. the owner of a children's clothing store told of the disease. In short, on the premise of meeting the post-processing requirements, excessive treatment should be prevented. In practice, critical surface tension test method is often used to detect. The critical tension required for different film printing and lamination is shown in Table 2

the treatment effect will fade exponentially with time, as shown in Figure 1. The fading speed is related to the storage environment temperature, raw material brand and film thickness. The higher the storage ambient temperature, the faster and more thorough the extinction. For example, when stored at an ambient temperature below 20 ℃, the critical surface tension of polyethylene film is 38 ~ 42mn/m, which can generally be maintained for a month or even half a year; If the ambient temperature rises above 36 ℃, no matter how deep the original film is treated, it will fall below 38 Mn/m after one month. After treatment, it is best to use it up within a month, otherwise, the printing or composite effect will be significantly reduced

(to be continued)

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