Heat shrink film is also known as shrink film, as the name suggests it is a film that shrinks significantly in size when it is heated to a certain temperature. Shrinkable film is an important packaging film, the application of more functional films, mainly for shrink packaging, but also in some aspects of special applications (such as as bottle labelling), shrink film than other packaging materials also has incomparable advantages.
1. Shrinkage mechanism of heat shrinkable film
Shrink film by heat shrinkage function, from the film has a high elasticity forced deformation, that is, in the process of manufacturing film, so that the plastic macromolecules in the high elasticity state (i.e., the temperature interval between the glass transition temperature and the melting temperature) by the tensile directional action, resulting in high elasticity forced deformation and in the cooling of the high elasticity forced deformation will be frozen down; in the temperature rises, the shrink film of the macromolecule chain of the activity of the capacity to continue to enhance, when the temperature reaches a high enough temperature, the film will be frozen down. Enhanced, when the temperature reaches a high enough time, the intermolecular movement is enough to make the high elasticity forced deformation disappears and de-orientation, and then produce macro size contraction.
According to the mechanism mentioned above, it is easy to see that the necessary condition for obtaining the shrinkable film is that the material used in the production of seafoam has a certain high elastic deformation temperature range, in which the stretching orientation is carried out and the molecules orientated to be obtained can be well frozen down through a specific cooling process, so as to confer the function of heat shrinkage. In order to obtain the actual use of the value of the heat shrinkable film, to meet the needs of many practical applications, shrinkage film should have a large enough heat shrinkage in addition to shrinkage film should also have a wide enough temperature range of heat shrinkage, moderate heat shrinkage stress, as well as good physical and mechanical properties and other necessary performance indicators, in addition to the application of heat shrinkable film as a commodity packaging, the price of the right amount of money is also a very important factor.
In principle, all the thermoplastic film may be made of shrink film, but at present has been a large-scale industrial application of shrink film, mainly limited to polyethylene, PVC and other general-purpose plastics, in addition to polyvinylidene chloride, nylon, EVOH and other ‘barrier class shrink film’, in recent years, polyester, polystyrene and other heat shrink film In recent years, polyester, polystyrene and other heat shrinkable films have made great progress in research and development and application.
Shrinkable film ideal raw materials should have a wide range of high elastic deformation temperature range, which is not only conducive to the production of heat shrinkable film manufacturing - easier to get a high deformation of the film, and its shrink packaging performance is also better (to solve the forced deformation of the temperature range is broader, the temperature of the shrink wrapping machine to control the accuracy of the requirements of the lower packing is also more easily operation). This is the reason why polyethylene with lower density and lower melt flow rate is generally used in the production of polyethylene shrink films, and why co-polyester resins are usually used in the manufacture of polyester shrink films.
3. Manufacturing methods for heat-shrinkable films
The current production process of heat-shrinkable film for packaging: the first thicker blank film, and then heated to the appropriate temperature for stretching, that is, according to the different blank film, you can use the flat sheet stretching (flat sheet method), you can also use the adjusted second blowing account stretching (i.e., double-bubble stretching) method for production. Flat sheet stretching is currently BOPP, BOPET and other pull the common production method of the barrier with which to produce a variety of packaging heat-shrinkable film, from the control of the film's heat-shrinkage performance is ideal; the method of investment in equipment is larger, but the output is high, and thus the cost of the product but lower, while the thickness of the product uniformity is good, high mechanical strength, can be applied to the thickness of the thinner film, thus reducing the unit area of film The cost of the film per unit area, improve market competitiveness, so in recent years the development of faster. Double bubble stretching method compared with the flat sheet stretching method, energy consumption is larger, lower yield, and thus higher production costs, and thickness uniformity is not good, its main advantage is that the investment in equipment is less, is mainly used for polyvinyl chloride shrink film, co-extruded polyolefin heat shrinkable film production. For polyethylene heat shrinkable film can also be used for ordinary blown film equipment, equipment (mainly mouth mould) to make the necessary improvements, the selection of specific grades of resin and specific process parameters, the application of similar to ordinary blown film method of direct manufacturing heat shrinkable film.
4.Classification of heat shrinkable film
Heat shrinkable film has a variety of classification methods, now the more commonly used classification methods are introduced as follows.
(1)Classification according to the different types of plastics
Such as common polyethylene heat shrinkable film, PVC heat shrinkable film, polyolefin heat shrinkable film and polystyrene heat shrinkable film, polyester heat shrinkable film.
(2)Classification according to the different structure of the film
Such as single-layer heat-shrinkable film, multi-layer heat-shrinkable film (also often specify the layer structure, such as five-layer co-extruded heat-shrinkable film containing EVOH).
(3) Classification according to different shrinkage conditions
For example, bi-directional shrinkage film (shrinkage in the longitudinal direction when the film shrinks is comparable to that in the transverse direction) and uni-directional shrinkage film (shrinkage in the transverse direction when the film shrinks is significantly greater than that in the longitudinal direction, or the shrinkage in the longitudinal direction is significantly greater than that in the transverse direction); high shrinkage shrinkage film (shrinkage of 50% or more) and low shrinkage shrinkage film (shrinkage of 20% or so) and so on.
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