VARIETIES OF STRETCH FILM DEPENDING ON THE INITIAL RAW MATERIALS
Stretch films are a group of polymer films with selective adhesive properties. That is, during packaging operations, the layers of the film adhere to each other, but do not adhere to the contents of the package. Stretch films have a pronounced ability to restore their original shape after stretching.
Stretch films can be divided into two groups:
Kling films. These are stretch films of small thickness. Their areas of application are mainly limited to individual food packaging. The thickness of the cling films is from 5 to 11 microns. They can be categorized as stretch wrap films.
Thicker polymer films. They are used in industrial enterprises and in agriculture for packaging non-food products, group packaging and for the transportation of consignments of goods. Their thickness ranges from 13 to 30 microns. Such stretch films are classified as packaging.
All stretch adhesive plastic films - PVC, polyethylene, polystyrene or ethylene vinyl acetate (EVA) copolymers - fall under the category of polymer stretch films.
Films made of polystyrene or EVA films are distinguished by high barrier properties, which opens up very broad prospects for their use as packaging materials. However, these films still make up a very insignificant share of the total production and consumption of stretch films.
Kling film made of PVC has some advantages over similar products made of polyethylene. Among these advantages, first of all, it is worth noting the increased positive gas permeability, which contributes to the long shelf life of food products, as well as the properties of smoothness, transparency and gloss, which increase the aesthetic advantages of the package. PVC film quickly regains its original shape, is more durable and resistant to fogging. The disadvantages of PVC cling film include its high price, therefore, preference is often given to plastic wrap. However, the choice is often made in favor of PVC film, for example, when choosing packaging for expensive food products that are displayed in the public domain.
CLASSIFICATION OF STRETCH FILMS DEPENDING ON THE PRODUCTION METHOD
1. Production of stretch films by blow (sleeve) extrusion.
PE stretch film is one of the modern packaging materials. It is based on linear low density polyethylene (LLDPE) and some of its copolymers. A distinctive feature of this packaging material is the ability to stretch up to 280-330% of the original length, as well as adhesion to its opposite surface. At the same time, the material does not stick to the packaged goods.
Currently, there are two methods of stretch film production. The first of them appeared along with linear low density polyethylene in the 70s of the last century. It requires less investment in production, but in terms of efficiency it is inferior to the second method. This method is called the method of blown (or tube) extrusion. Using the blown coextrusion method, a 2-3-layer film is obtained.
Blown extrusion polyethylene films are characterized by significant strength and elasticity under light loads. The ability of films made by this method to pre-stretch does not exceed 75-85%. At the same time, they are distinguished by a high elongation coefficient. They are mainly used in agriculture, especially when packing hay. Such films are even called haylage films. Their use is especially widespread in the Baltics and Finland, Poland, Denmark, Norway and a number of other countries.
2. Production of stretch films by means of flat-slit extrusion.
This method involves obtaining products in the form of a polymer web, which is then wound onto cylindrical bushings. In this way, for example, a polyethylene cleaning film is produced. Originally, flat-slit extrusion was used to produce a three-layer film that has long been considered standard. However, over time, in order to reduce production costs while maintaining film performance, manufacturers began to produce films of 5-7 layers.
The method of flat slit extrusion is also called the cast method. The film obtained by this method consists of layers A-B-C-D-E, which are arranged in order from the core to the outside surface.
These layers perform the following functions:
A - the layer containing adhesive additives is responsible for the adhesive properties of the film;
B and D - are made of elastic copolymers, are responsible for maintaining the original shape when stretched and the property of elongation;
C - consists of the most stable copolymers and acts as a supporting frame of the film;
E - with the addition of anti-adhesive and antistatic components, ensures that the material does not stick to the packed surface, and also prevents the formation of an electrostatic charge.
The main parameters of modern cast lines are the width of the produced web, the line productivity (passport and real), the thickness of the produced stretch film. All lines for the production of polyethylene stretch films provide a constant size of the film web, which can be in sizes from one and a half to 4 m. to the category of experimental.
For a polyethylene stretch film made in this way, the actual elongation capacity can be from 600 to 800%. This value should be distinguished from the guaranteed tensile capacity, which includes a certain margin of safety, as well as its ability to withstand a powerful initial force. This force (jerk) is present in all modern models of packaging mechanisms - pallet wrappers. The presence of a breakthrough makes increased demands on the stretch film, which must be taken into account by the developers and manufacturers of the material.
The main advantages of stretch films:
In addition to the ability of the film to restore its original shape (reversibility of stretching), the difference between the stretch film and the usual one lies in its increased puncture resistance, as well as the ability of the film layers to stick together, but not to stick to the load.
Another important parameter of stretch film is transparency, which makes it possible to visually access the packed product.
The barrier properties of the stretch film are also important, thanks to which it protects the cargo from pollution, damage and moisture, as well as preventing unauthorized access to the cargo.
The convenience of using stretch film as a packaging material is that it reliably fixes the contents of the cargo, which is especially important when it is packed together with pallets. This helps to ensure the safety of the cargo during transportation in any conditions, including during transshipment.
An important advantage of the stretch film is that it practically does not change its packing characteristics over a wide temperature range.
POLYETHYLENE STRETCH FILM SUBSTITUTES
In some cases, the polyethylene stretch film can be replaced with PVC film. In addition, EVA, PS films, weldable polypropylene films, composite packaging films, etc. can be used as substitutes.
When moving and storing certain types of raw materials in food production, so-called polyethylene stretch hoods can be used. These are bags that stretch and fit over the load, including the pallet, and then shrink to securely grip the pallet and its contents. This method is used when transporting goods without sharp hard corners.
Another way to replace plastic stretch films is with polyethylene shrink hoods. They are made of a sleeve or half sleeve made of heat-shrinkable polyethylene. They are also pulled over the load and pallet and then shrink by heating with gas burners or electric heaters.
TYPES OF STRETCH FILMS DEPENDING ON THE METHOD OF PACKAGING
Hand stretch. These are films intended for hand winding. They are used as individual and transport packaging in small businesses. Supplied in rolls weighing from 2 to 2.5 kg (with sleeve). The difference in weight should not be more than 50-70 g. Weight control carried out for randomly selected samples of the supplied goods helps to avoid dishonest suppliers and to be sure that the winding length is in accordance with the standard.
The thickness of the hand stretch is most often 17 microns, less often 15 or 20 microns. The guaranteed ultimate elongation is usually 75-100%, rarely 140%. The rollers are available in widths of 500 mm or 450 mm (for films with a thickness of 17 microns). The production possibilities of this film depend on the availability of butene polymers on the market, the share of which in the material is high.
Machine stretch. Its main difference is the value of the guaranteed ability to pre-stretch. In addition, films differ in thickness. The weight of the rolls is 17 kg with a sleeve, deviations in weight are determined according to the supply agreement. The width of the rolls is 500 mm.
The KristalPack machine stretch is available in standard 17 micron, 20 or 23 micron thicknesses, with guaranteed pre-stretch properties up to 280%. With an increase in the value of the guaranteed ability to pre-stretch, the strength of the film, the ability to restore shape, as well as the density of coverage of the load increase. That is, the higher the pre-stretching ability, the less film is spent on packing the cargo. However, as the guaranteed pre-stretch ability increases, so does the price of the film. However, when using it, there are real savings in packaging costs.
In order to make an economically feasible decision on the choice of the purchased type of stretch film, it is necessary to have competence in the field of production requirements for the used film, as well as the main parameters of various types of stretch film. There are special methods for calculating the effectiveness of using stretch films.