Properties and selection of stretched films

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stretch film has the following important performance indexes.

(1) The adhesion after the contact between the self-adhesive films can make the packaged goods tight and not loose during the stretching and winding process and after the levee bag. Self-adhesion is affected by various flashover cables in the external environment, such as humidity, dust and pollutants. There are two main methods to obtain self-adhesive films: one is to process films with smooth and glossy surfaces; The second is to use additives that increase adhesion to make the surface of the film moist, thereby improving adhesion.

(2) Toughness, toughness is a comprehensive property of film puncture resistance and tear resistance. Tear Resistance refers to the tear resistance of the film after being subjected to tension and punctured. It should be pointed out that the risk value of tear resistance must be horizontal (That is, perpendicular to the machine operation direction) Because tearing in this direction will make the package loose, but tearing in the longitudinal direction will still keep the package firm.

(3) Stretching is the ability of the film to produce elastic elongation after being pulled. The increase of longitudinal stretching will eventually make the film thinner and shorten its width. Although longitudinal stretching is beneficial, excessive stretching is often undesirable. Because the film becomes thinner, it is easy to tear, and the tension added to the package is increased.

(4) Stress retention refers to the degree to which the tension applied to the film can be maintained during the stretching and wrapping process. For ethylene- The common stress retention degree of vinyl acetate copolymer, low density polyethylene and linear low density polyethylene films is to stretch the original length of the films to 130% and relax to 60% in 16 h-65%; Relaxation of polyvinyl chloride film to 25%.

(5) Allowable stretching allowable stretching refers to the maximum stretching that can be applied to maintain various necessary characteristics under certain uses. Allowable stretching varies with different uses. Of course, the larger the allowable stretch, the less the film used, and the lower the packaging cost. In addition to the above performance indicators, other properties such as optical properties and heat sealing properties may be important for some special packages.

In addition, the following factors should be considered in the selection of stretch packaging film.

① the important properties of the stretch film are higher tensile rate, minimum width shrinkage, vertical ultimate tensile strength of the fan, high puncture strength, high shear resistance and low stress. relaxation and good viscosity, etc: these properties mainly depend on the combination of basic resin and additives.

② suitable ambient temperature. Higher ambient temperature will cause the stretch film to relax and lose the retention force under standard conditions. Under standard conditions, within 24 hours after wrapping, low-density polyethylene and linear low-density polyethylene films can maintain 65% of the original retention force, which is true, while polyethylene can only maintain about 25% at normal temperature. Low temperature can reduce viscosity, toughness and tensile properties. Generally, most films are suitable-29 ~ Use between 54 ° C.

③ humidity adaptability. In high humidity environment, the viscosity of the film can sometimes be improved, because some tackifiers function by absorbing moisture in the atmosphere. Therefore, if there is moisture between the two smooth surfaces, it is difficult to separate the two films. If in low humidity; It can form higher electrostatic viscosity. In the process of storage and transportation, this kind of electrostatic viscosity cannot be relied on. This is because static charges disappear with time. In order to improve humidity adaptability, some film manufacturers produce composite stretch films, and apply tackifying agents on one side to improve viscosity.

④ film manufacturer. Users must know that the performance of the film supplied in large quantities should be completely consistent with the pre-test and quality appraisal results of the film samples. Because on the actual tensile equipment, the stress and rate encountered by the film are not necessarily the same as those of the test equipment. The actual wrapping and transportation tests should be carried out with real equipment and real cargo loads, and different films should be compared to make the final decision on film selection.