Selection and treatment of plastic film for the ho

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Selection and treatment of plastic film for salt packaging bags

the State advocates residents to eat iodized salt, and then puts forward new requirements for the production and packaging of salt. Salt packaging bags belong to food packaging and have high requirements for hygiene. Therefore, we chose non-toxic or low toxic PE, pet, PP and PVC (only used as the outer composite film) films to study. Their characteristics are introduced below

performance comparison of plastic film

1 Polyethylene (PE) film

PE film has good air permeability, but with the increase of film density, its air permeability decreases. In addition, PE film has poor moisture permeability, so it has good moisture resistance

according to different manufacturing methods and control means, PE film can produce products with different properties such as low-density, medium density, high-density polyethylene and cross-linked polyethylene

(1) low density polyethylene (LDPE) film. Its density is about 0.92g/cm3, with good transparency and heat sealing, and can be waterproof and moisture-proof; However, the tensile strength is low, the tensile elongation is large, and it is easy to wrinkle. For films with a thickness of less than 0.03mm, the tension control should be small, and the tension everywhere should be constant; It is easy to deform when heated, resulting in difficult printing chromaticity. Therefore, when drying, the surface temperature of the film should not be too high (below 55 ℃). LDPE film can be divided into blow molding (IPE) film, casting (CPE) film, low foaming film, etc. according to the different film forming processes. IPE film has better tensile strength and opening than CPE film. It is printed on the front and can be used as food bags, clothing bags, etc. CPE film has uniform thickness, better surface gloss, transparency and heat sealing than IPE film, but the production cost is high. It can be printed on the front and back. It is mainly used as the inner layer of composite bags for the packaging of cosmetics, pickles and cakes. The low foaming film has good decorative property, thick texture and is not easy to stretch and deform. It is printed on the front and used as New Year pictures, trademarks and handbags

(2) high density polyethylene (HDPE) film. The density is 0.94 ~ 0.965g/cm3, and the thickness is generally more than 0.03mm. Its heat resistance and mechanical strength are better than LDPE film, with small tensile elongation and poor transparency. It is printed on the front, mainly used as vest bags, garbage bags and lined bags

(3) medium density polyethylene (MDPE) film. The density is about 0.93 ~ 0.4 Gb/t6147 ⑼ 2 "technical requirements for packaging, packaging marking, storage and transportation of experimental machines" 94g/cm3, its performance is between HDPE film and LDPE film

(4) cross linked polyethylene (CLPE) film. Because it has higher heat resistance, tensile strength, heat shrinkage and barrier than other polyethylene products, its use is expanding, and it is mostly used as heat shrinkable packaging film at present

generally speaking, with the increase of the density of the film, the mechanical properties and barrier properties will be improved accordingly, and the heat resistance will be better. Polyethylene with the same density has different properties due to different film-forming processes. Because the tape casting method can quickly cool the film, making it low crystallinity, high transparency and low turbidity, but the molecular arrangement tends to be more irregular, so the barrier property of this film is small (i.e. high transmittance), low elongation and poor tear resistance

2. Polyester (PET) film

biaxially stretched PET film has extremely high mechanical strength and rigidity, which is very straight; Good heat resistance, high transparency, good gloss, good drug resistance; The permeability of water vapor and oxygen is low, and it has good fragrance retention, which is unmatched by other plastic films; The surface energy is low, and surface treatment is required before printing; It is not easy to stretch and deform, has good printability, and has lower requirements for tension control than PE film, but has poor heat sealing performance. It can be used as cooking bags, cosmetic packaging bags, etc

static electricity is easily generated in the process of printing. Due to the excellent insulation of PET film, static electricity is not easy to disappear. In serious cases, the film will be adsorbed on the drum and cannot be printed normally. Therefore, static electricity elimination equipment must be set during high-speed printing. The drying temperature of PET film can be set higher, but its surface temperature should not exceed 85 ℃. When winding, the film should be fully cooled to room temperature to prevent ink adhesion

3. Polypropylene (PP) film

PP film has better transparency, heat resistance and printing operability than PE film. However, its ink adhesion is poor, and surface treatment is required before printing to improve the adhesion of the ink. The critical surface tension of surface treatment is controlled within (4.0 ~ 4.4) × N/m is appropriate

bopp film has small tensile deformation, not easy to wrinkle, good printability, high transparency and gloss, excellent moisture resistance, but poor heat sealing and tear resistance. CPP film has good heat sealing property, but it is easy to stretch and deform during printing, so the tension control is smaller than BOPP film, which can be used as the heat sealing layer of candy paper and composite materials. The tensile deformation and heat sealing property of iPP film are between BOPP film and CPP film, with good transparency and gloss, but the thickness uniformity is worse than BOPP film and CPP film. It is printed on the front and can be used as the packaging of textiles

4. Polyvinyl chloride (PVC) film

PVC film has good transparency, gloss and printability. Generally, surface treatment is not required before printing, and the adhesion of ink is very good. The disadvantage is that it has poor heat and cold resistance, and most of them are toxic. When making food packaging, we should pay attention to its safety and hygiene. According to the amount of plasticizer, PVC film can be divided into hard, semi-hard and soft, and its properties are slightly different

soft PVC film contains a lot of plasticizers, which will precipitate into the printing ink layer, causing discoloration, adhesion and ink migration. And it is very easy to stretch and deform, resulting in wrinkles, causing inaccurate overprint, so the tension should be small rather than large, and it is easier to stretch after heating, which brings difficulties to the printing operation. Soft PVC film has good self adhesion when it is thin enough, and is often used in fruit and vegetable packaging

from table 1, we can see that various films can be used respectively by comprehensively considering the requirements of various parts of salt packaging for tensile strength, moisture resistance, heat sealing and impact strength. It is better to use CPP and HDPE for the inner membrane of the package, which not only has good heat sealing property, but also can provide sufficient strength and moisture resistance; Pet and OPP are better used as outer film because of their high strength, moisture resistance and low elongation. Considering the price factor, PVC can also be used as the outer film of packaging

surface treatment and detection of plastic film

1 Surface treatment of films

most plastic films (such as polyolefin films) are non-polar polymers, and the surface tension is generally (2.9 ~ 3.0) at the selected load × N/m。 Theoretically, if the surface tension of an object is less than 3.3 × N/m, the current ink and adhesive cannot adhere firmly, so corona treatment should be carried out on its surface. The processing principle is: apply high-frequency and high-voltage electricity to the processing equipment to produce high-frequency and high-voltage discharge, resulting in small, dense purple blue sparks. Various ions produced after air ionization accelerate and impact the plastic film in the treatment device under the action of a strong electric field. The energy of these plasma particles is generally a few to dozens of electron volts, which is similar to the chemical bond energy of plastic molecules, so it can induce the chemical bond breaking and degradation of plastic surface molecules, increase the surface roughness and improve its specific surface area. When discharging, a large amount of ozone will be produced. Ozone is a strong oxidant, which can oxidize plastic molecules to produce carbonyl, peroxide and other polar groups, so as to improve its surface energy

the advantages of corona treatment are as follows:

(1) there is a wide range of processing materials, which can be used for polyethylene, polypropylene, polyvinyl chloride, polyamide, polyester, polycarbonate fluoroplastics, and various corresponding copolymers

(2) the processing time is short and the speed is fast. It can be processed on the production line

(3) simple operation and easy control

(4) 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

(5) no waste liquid discharge, basically no environmental pollution

corona treatment is widely used in the surface treatment of thin films before printing, coating and lamination, as well as the surface treatment of sheets with a thickness of less than 0.6mm

the degree of film treatment directly affects the quality of post-processing, which must be strictly controlled. If the treatment is insufficient, the surface wettability of the film is not significantly improved, resulting in poor adhesion of the ink and small peeling force of the composite film. On the contrary, if the treatment is excessive, the surface of the film will be aged and the gloss will become worse; The heat sealing property becomes poor due to excessive cross-linking of surface molecules; The film is easy to adhere (especially in summer, there is always one suitable for you in high temperature), and it is difficult to cut, and it is difficult to separate when using. In short, on the premise of meeting the requirements of post-processing, the degree of treatment should be reduced as much as possible to avoid excessive treatment. In actual production, the critical tension is often detected to control the degree of treatment. The critical tension required for different film printing and lamination is shown in Table 2

the effect of film treatment will fade with time, as shown in Figure 1. The rate of extinction 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 in an environment below 20 ℃, the critical surface tension of PE film has been maintained at (3.8 ~ 4.2) for 1 month or even half a year × N/m; If the ambient temperature rises above 36 ℃, no matter how deep the original film is treated, it will fall back to 3.8 after one month × Below n/m. Therefore, it is best to use up the film within 1 month after treatment, otherwise its printing or composite effect will be significantly reduced

2. Detection of treatment effect

(1) detection by critical surface tension method. Ethylene glycol ether (surface tension 3.0 × N/m) and formamide (surface tension 5.7 × N/m), the surface tension range is (3.0 ~ 5.7) × A series of test fluids between n/m. The surface tension value of the film is estimated in advance, and then the test solution with the same tension value is coated on its surface. Each coating area is about 40mm × 15mm, coating should be completed in 0.5 seconds. If the coated liquid film remains unbroken for more than 2 seconds (see Figure 2a), test with a test solution with a high surface tension value; If the liquid film breaks into many small droplets within 2 seconds (see Figure 2b), use the test solution with low surface tension value for testing; If the critical surface tension tested is less than 3.6 × N/m, indicating that the treatment quality is poor and needs to be reprocessed

(2) tape sticking method. Stick the adhesive tape on the printed film, press it flat with your fingers to make it closely fit with the film, and then lift the film slowly to observe the adhesion of the printing ink by the adhesive tape. If the stripping rate of printing ink is less than 1%, it can be considered that the treatment quality is good; The stripping rate of printing ink is between 1% and 10%, which can be considered as poor treatment quality; If the stripping rate of printing ink is greater than 10%, the treatment quality is poor and needs to be reprocessed

pe, PP, pet and even PVC film as printing film and composite film should be surface treated to make the surface tension of printing film reach (4.0 ~ 4.4) × N/m to meet the requirements of ink adhesion fastness; The surface tension of the composite film is (4.4 ~ 4.8) × N/m, so as to achieve sufficient composite fastness

Author: Zhao Dongbai

reprinted from: Printing Technology

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