Analysis on the specific application of the packag

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Analysis on the specific application of dairy food packaging technology III

(3) polyvinyl chloride (PVC) accounts for the second place in the total output of plastics in the world (second only to polyethylene), and pure PVC is non-toxic. However, vinyl chloride, the monomer of synthetic PVC, has carcinogenic effect, so it has not been allowed to be used in food packaging for a long time. Now, the content of vinyl chloride monomer in PVC has fallen below the safety line. Developed countries such as the United States and Japan have already allowed it to be used in food packaging. The PVC non-toxic transparent sheet produced in China passed the identification in 1984. Most of the graft copolymers used on the packaging are PVC, and various processing aids (plasticizers, stabilizers, lubricants, colorants, etc.) must be added during molding. The characteristics are as follows:

① it has bacteria resistance and anti microbial decomposition or digestion. But processing aids will affect this characteristic

② good coloring. Products with various colors can be made after adding colorants

③ PVC films used for food packaging are mainly hard and semi-hard films with little plasticizing dose. They are produced by blow molding method, with small elasticity, high transparency and easy to break at low temperature

④ the processing aids during molding may penetrate into the content. It is possible to interpret the carbon fiber industry development policy thoroughly, so we should choose carefully

(4) polyester (PET), whose full name is polyethylene terephthalate, is formed by condensation of terephthalic acid and diols (such as ethylene glycol). Its characteristics are as follows:

① polyester film has good electrical insulation, tensile strength and bending fatigue properties. The tensile strength can reach the order of magnitude of metal, surpassing that of duralumin (150 MPa)

② it is sensitive to notch, has low tear strength, and is difficult to bond at the crack

③ polyester film is relatively stable to dilute acid, dilute alkali and ordinary organic solvents, but unstable to concentrated acid, concentrated alkali and some chlorinated hydrocarbons. It can be dissolved by heating in tetrachloroethane formic acid mixture and nitrobenzene

④ no shrinkage in a wide temperature range (-180 ~ 220 ℃)

⑤ pet is mainly used to manufacture PET bottles to replace glass bottles. This kind of bottle has high melting point, can withstand high-temperature sterilization, high impact toughness, good transparency, high mechanical strength, wear resistance, light weight, but high cost. Polyester bottle is used for carbonated drinks, and polyester film can be used for cheese packaging

(5) polyamides (PA), commonly known as nylon, is a typical crystalline polymer, which is mainly used as a layer of composite materials. Its characteristics are strength, toughness, heat resistance, oil resistance and oxygen resistance; However, it has high hygroscopicity, high water vapor transmission and high heat sealing temperature. The tear strength of biaxial tensile PA is increased by 3 times, with good toughness, high puncture strength, and particularly good optical properties. It can be printed without pretreatment. Moreover, the oxygen sealing performance is greatly improved. The combination of PA and LDPE can greatly improve the gas tightness, but the cost is high

(6) the manufacturing technology and overall strength of polyvinylidene chloride (PVDC) have been further improved. PVDC plastic for packaging, commonly known as yarn, is a copolymer of monomers such as vinylidene chloride, vinyl chloride, vinyl acetate and acrylic acid. Among all packaging plastics at present, its gas barrier performance is the best, close to metal; It has high heat sealing performance, impact resistance, high flexibility and toughness, wear resistance, hard and smooth surface, and cannot be penetrated by ultraviolet light. It can be used alone or in composite packaging. It is suitable for non corrosive (no harm to human body) products with high sealing requirements

(7) cellophane is made of natural cellulose. The advantages are transparent, airtight, non-toxic, cheap, good printability and good mechanical operability. The disadvantage is that water absorption is easy to expand, and the permeability increases after expansion. In order to overcome this shortcoming, various plastics are often coated. For example, coating with polyvinylidene chloride can be moisture-proof, airtight, non-volatile, heat sealing and smooth; Coated with saran resin, it has excellent oil resistance, can be heat sealed, and low gas permeability. It can be used in products with much grease; It is coated with nitrocellulose and cannot be wrapped with oily food. It is mainly used for fruits and vegetables

(8) both polystyrene and polyethylene can be used to make foamed plastics, but polystyrene has good performance, so it is widely used

foamed plastic is a semi-rigid foam material, with many closed foam granular structures on the surface. Its appearance is a white opaque packaging material, which can be dyed into various colors. With low cost and good shockproof performance, it is used in ice cream packaging in dairy industry

3. Glass bottle

in 1950, many people thought that as long as the metal was electroplated, it could achieve a uniform and dense surface effect. Years ago, almost all sterilized milk was packed in glass bottles, which were later partially replaced by plastic bottles. The shape of glass bottles is almost cylinder wide mouth bottles. Square bottles once appeared in the United States because the square body takes up less space. China has always used round colorless wide mouth bottles, and Finland uses brown bottles. The bottle cap is divided into two layers, the inner layer is made of waxed cardboard, and the outer layer is made of aluminum foil, LDPE, waterproof cellophane, etc. Characteristics of glass containers:

(1) stable chemical properties, no reaction with contents, no deformation

(2) transparent, consumers can clearly see the state of the contents from the outside, but it is unfavorable to photosensitive substances

(3) it can be reused, but at the same time, it increases the investment cost and brings secondary pollution opportunities

(4) heavy weight and high transportation cost

(5) it is easy to be damaged, and the damage rate of each cycle is 0.5% - 0.8%

(6) the price of bottles is low, but the investment cost of bottle washing room is high. The labor intensity of recycling and washing bottles is very high

(to be continued)

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