LT6761B - A method for manufacturing of ldpe film from post consumer recycled ldpe waste material - Google Patents

Abstract

The invention relates to a process for regenerating used articles of LDPE film, such as plastic waste bags, to reusable raw material, in particular for a method for manufacturing the recycled bags from 100% post consumer recycled LDPE waste. The process comprising the steps of sorting the post consumer LDPE waste material to exclude heavily printed film of dark colours, paper labels, and metal foil labels; preparing the blend mixture comprising 70% of transparent film no more than 35 µm thick and 30% of transparent stretch film; washing the sorted LDPE waste material using cold washing methods combined with such technologies as grinders, float-sink separators and friction washers; shredding the washed material into small pieces and melting the shreds to get a homogeneous polymer melt; filtering the melt using a metal plate with cone-shaped holes of 90-110 µm or smaller size; mixing the granules thus obtained with supplements and re-granulating again to reach the final homogenization and filtration; conveying the granules to the extrusion line to produce a final recycled LDPE film in form of a layflat tube having a wall thickness of 5 to 7 µm.

Description

FIELD OF THE INVENTION

The invention relates to the recovery of used LDPE film products, such as plastic waste bags, into reusable raw materials, in particular to a process for the production of bags from 100% recycled LDPE waste.

Level of the invention

The market for plastic waste bags can be divided into two main product categories:

- garbage bag. They are often made from 100% polyethylene waste films. The wall thickness of these bags typically ranges from 23 to 35 pm, although some bags are made up to 15 pm thick, usually using special blends of polyethylene waste;

- garbage bags. These bags are most commonly used as liners for various types of trash cans. Mostly used in domestic kitchens and bathrooms, as well as in offices, hotels and so on. Trash bags are usually made of high density (HDPE) polyethylene film with a thickness of 5.0 to 9.0 pm.

Thus, although thicker bags were produced from plastic waste for a second life, the production of thinner bin liners was continued from primary plastic products without the use of waste resources.

It is desirable that thinner bin liners be made from 100% recycled materials, as are thicker garbage bags. LDPE, as a thermoplastic material, can be remelted and deformed due to its molecular structure, making it suitable for reuse. However, any processing to obtain high-quality LDPE films with defined properties is hampered by the addition of various additives to used products, in particular LDPE films, such as antistatic agents, lubricants, labels, metal foils, which remain from in their processed products. For technical and economic reasons, it is not possible to sort used products by ingredient.

Attempts have already been made to solve this problem. For example, US5368796 describes a process and apparatus for recovering used LDPE products into a high quality raw material for reuse. According to the invention, this object is achieved by extracting the shredded and intensely moving spent products in a solvent bath containing an organic solvent. LOPE waste can be shredded in commonly used shredders. The components are extracted from the pieces by washing them in an organic solvent, where the ingredients and decomposition materials are separated and concentrated in the solvent without dissolving the plastic material. The rags move intensely in the solvent bath. In this way, the rags rub against each other and the printing ink adhering to them is removed due to high friction. The printing ink washed in this way enters the solvent bath. The combination of simultaneous extraction in a solvent bath and intense motion, such as mechanical agitation, results in the removal of printing inks and the separation or extraction of components and decomposition materials from LDPE waste. Once in a continuously stirred solvent bath for a predetermined time, the pieces thus treated in one cycle can be melted to form granules or agglomerates and reused to make high quality polyethylene products.

The essence of the invention

The present invention offers an alternative to the regeneration of used LDPE film products for the production of garbage bags up to 7.0 μm thick to replace the current imported primary products. The invention relates to the entire production technology of such recycled bags made from 100% recycled LDPE waste: starting with the choice of material, the recycling technology and the film blowing technology, and the operating parameters. The process of making a film with a thickness of 5.0-7.0 pm is complicated because it is made of primary LDPE, which is monolithic in terms of molecular branches, bulk density, melt flow index (MFI), and so on. It is therefore necessary to maintain stable production and technological parameters.

Brief description of the drawings Fig. illustrates the process of extruding a blown film;

fig. the parameters of the blown film process are shown;

fig. the design of the extruder propeller is shown.

Detailed description of the invention

Material

The selection of the material is very important for the whole process in order to obtain the perfect mixture for the further production of LDPE bags with a thickness not exceeding 7.0 pm. Any sorting process - manual, optical or by weight - can be selected to obtain a pure 99% LDPE mixture without any additional impurities. The mixture must meet high requirements, such as:

- mechanical properties: tensile strength, stiffness, abrasion resistance, temperature stability, formability, elongation;

- ability to seal, such as package seals, security seals and closure / resealing devices;

- optical properties such as transparency, surface gloss.

The properties of the film will depend on the MFI and the density of the polyethylene used. To ensure good impact resistance, small MFIs are desirable, but in this case, extrusion becomes more complicated. Increasing the density increases the stiffness and tensile strength, but decreases the resistance to directional tearing (multiple cracks in the film occur). Our goal is to obtain a film with a density of -0.925 g / cm and an MFI of -1.

There are two ways to obtain a good raw material for an LDPE film of different molecular weights: either by layering the system, or by optionally dissolving-precipitating the different components. Although layering methods may be based on the chemical decomposition of the interlayer or adhesive layer, methods based on selective dissolution and methods based on the combined treatment of different components may be based on the thermodynamics of polymer solutions.

To allow the spontaneous solubility of polymers in solvents or other polymers, the Gibbs free mixing energy - AG _m j _x - must be zero or negative. AG _m j _{x for} solution preparation is calculated according to the Gibbs-Helmholtz equation (Equation 1):

AG mix ⁼ AH mix T'AS _m ix (1)

Since the entropy gain AS _m ix is insignificant due to the high molecular weight of the polymer chains, the enthalpy difference ΔΗ _{mix of the} mixing process must be negative, which means that the process must be exothermic.

Flory and Hugins described AG _m ix by determining the entropy of mixing polymer chains and solvent. This is done by calculating the number of solvent molecules and polymer segments, or two different polymer segments can be placed on the grid. According to Flory-Hugins theory, AG _m ix is generally described as:

AG = RT [^ LNY + ^Φ2 / ηφ + φφχ] (2) Mix _Ν Ί 1Ν ₂ 21,212 where Nj represents a polymerization degree (i represents the number of monomer substances line), φ ₁₂ is the molecular volume of × ₁₂ is the Flory-Huginso interaction parameter, R is the gas constant and T is the absolute temperature. The first two logarithmic terms of the FloryHugins equation give a combinatorial mixing entropy that, depending on φ, is negative and always promotes mixing, and the third term is mixing enthalpy.

In polymer blends, Nj is high in both cases, thus, the combinatorial entropy becomes very low. Therefore, the mixing capacity of the system is determined by the value of the last term, i.e. the mixing enthalpy. Exothermic heat of mixing can be caused by specific interactions between the components of the mixture, such as covalent and ionic bonds. Weak, non-binding interactions such as hydrogen bonding, ion-dipole, dipole-dipole, donor-acceptor, or Van der Walsh interactions often do not result in polymer mixing. This is why polymer blending is the exception rather than the rule.

When Nj = 1, the general equation becomes the equation for polymer solutions:

AG = RT [ylny + ^Φ2 / ηφ + φφχ] (3) m / x 11N ₂ 21212

In addition, for polymer solutions, combinatorial entropy promotes the solubility of the system. In the case of a solvent-solvent mixture, all three parameters of the Flory-Hugins equation contribute to solubility. This explains why there are many systems of miscible solvents and why the solubility of polymers is generally lower than that of low molecular weight molecules.

As described in the following sections, the formation of polymer solutions in both solvents and other polymers plays an important role in the recycling of multilayer polymeric packaging. Approximate percentage of the material mixture: 70% transparent film up to 35 μm thick and 30% transparent elastic film.

table. Types of films problematic in the further processing process

Type of film Problem Meaning of circumstance / Problem solving Multilayer constructions Non-polyethylene (Pe) resins have different melting points than LDPE Resins other than LDPE can contaminate all resins during the granulation process Film with disintegrants Depending on the application, additives can damage the recycled product Film with durability additives usually cannot be identified and separated from the film without additives Metallized film Aluminum film cannot be separated from LDPE film Screen lock an option that can affect filtering quality Color film Color film may have another additive that degrades the quality of the film A transparent film should be used in the recycling process Printed film Printed films can cause excess gas by recycling and using colored recycled resins Avoid severe printed films Film with label Labels are difficult to separate from the film during washing. As a result, the label may burn out later in the granulation process and give the film a yellow tint. This can contaminate the film resin Avoid films with labels

Direct printing is preferred. Of all the possible labeling methods, direct printing is the least polluting. A small amount of suitable dyes is distributed in the final polymer without significantly affecting its quality. High-embossed dark-colored films can cause problems because dark colors affect a large amount of polymer, limiting its reusability. The amount of imprints should be limited because large amounts of ink are released in the extruder and this can cause gels in the final product, even if most processors use ventilated extruders. Large amounts of ink exceed the ability of extruders to remove volatile components.

Polyethylene labels are preferred. Labels made of the same or a compatible polymer as the film are not contaminated and recycled together with the film. Paper labels are detrimental to recycling. Paper labels soak and become a problem for water filtration and contamination if they are recycled by wet recycling.

Individual paper fibers are very difficult to remove from the film, resulting in stains and irregularities in products processed from such film. In addition, by treating them both wet and dry, they break down in the extruder and create an unwanted burning odor that cannot be removed from recycled plastic. This significantly limits its reuse. Packages with metal foil labels and their layers are not suitable for recycling according to the APR definition. These labels should not be confused with metallized film.

Labels with metal foil are particularly problematic for two reasons. First, they trigger metal detectors that are used early in the recycling process to protect the machines. In this case, all packaging containing metal foil is disposed of in a landfill. Second, if such packages pass through the process to the extruder, they can quickly block the melting filter, creating a pressure that automatically stops the process for safety reasons.

Washing, extrusion, mixing

The washing quality is also very important to obtain a 7.0 pm thick film. LDPE is usually a soft or film-type plastic to which various plasticizers are added to make it elastic and flexible, and is commonly used in packaging. Although LDPE is abundant in household waste, it is a type of plastic waste that is difficult to recycle. The surface of LDPE plastic film is usually large and its thickness is less than 0.2 mm, making it very difficult to laminate and laminate. As a result, LDPE plastic waste is usually heavily contaminated with dirt, organic matter, glue, etc., so it must be intensively treated during the recycling process, grinding, washing, drying. In addition, high contamination causes significant processing problems and impairs the properties of the final product, such as appearance and color.

Current state-of-the-art LDPE plastic waste recycling methods are based on cold washing methods combined with proven technologies such as milling, floating-sinking particles separation and friction washing. LDPE plastic waste is shredded into smaller pieces, cleaned, heated and extruded for reuse. Due to the poorer appearance and quality of the finished product, recycled LDPE plastics are often blended with higher grade plastics to achieve higher quality standards, or used in low quality products such as irrigation pipes, garbage bags, black films and films for the agricultural and construction industries. With current methods, recycling facilities can produce regranulate with a quality of> 99% primary plastic.

One of the key factors in obtaining a good quality resin from recyclable materials is homogenization in the extrusion process. For this reason, it is preferable to use Erema TVEplus® technology with a 90 μm laser filter filtration system and a ring head granulation system.

The extrusion system with TVEplus® technology is specially optimized for difficult-to-process materials, such as washed, mixed, especially contaminated waste film waste with a high residual moisture content of 5 to 10%. As written in the previous paragraph, a 7.0 pm thick film technology requires the use of a transparent material. In this case, homogenization with this technology will be maximal.

The factory multifunctional cutter compactor with air purge module optimally recycles materials with relatively low residual moisture and contamination. Cutting, homogenizing, heating, degassing, compacting, buffering and dosing are all done in one step.

The advantage of the TVEplus® system is that the melt is filtered before the extruder is degassed, which has two important advantages: one is that the minimum shear effect during the melting process does not further reduce the particle size before filtration and thus increases the filtration process efficiency. A metal plate of special construction is used for filtration, which is treated with laser beams to form cone-shaped openings. A filter with 90-110 μm orifices should be used for filtration. Another advantage is that contaminants cannot form gases prematurely due to their early removal from the recycling system, thus minimizing the accumulation of odors in the regranulation process.

After the addition of the additives, the granules are poured into a mixer and mixed evenly. The mixing time depends on the type of mixer. The mixing process ends when the supplements are evenly distributed in the selected volume. If mixed unevenly, the process is repeated. The mixing process usually takes 1 hour.

The mixture is then re-granulated to finally homogenize and filter. An extruder with a screw (at least 120 mm) is used for homogenisation. A backwash system with a complex of five wire meshes is used for filtration: two external (weave spacing is 150 pm), which are supportive, two internal (100 pm), and one middle final filtration network (50 pm).

The MFI of the reconstituted beads is tested in a laboratory. Also, by blowing a 7.0-thick film, the cleanliness and the possibility of opening the opening are checked to allow stable production. After blowing the film, it is checked whether the film can stick together and whether anti-blocks have been added.

Description of the film extrusion technology process

The extrusion process of the blown film is shown in Figure 1. The polymers are usually fed to the extrusion line by a vacuum conveying system, but in our case this is not necessary because all the resins are first mixed and regranulated. Gravimetric systems are preferred for controlling the mixing process and dosing the resins into the extruder. In the extrusion process, a homogeneous polymer melt fed at a constant rate from one or more extruders 1 is extruded through an annular extruder head 2 to obtain a sleeve 3 of controlled diameter and wall thickness. it is blown into a bubble 5 of the required diameter with air introduced through the middle of the extruder head. The film is compressed between two rolls 6 so that the bubble 5 formed between the rolls 6 and the extruder head 2 has a constant volume of inflated air.

At a certain diameter of the extruder head 2, the diameter of the bubble 5 and together the films 7 (width of the flat sleeve) are determined by the amount of inflated air. The film thickness depends on the output of the extruder 1, the bubble blow ratio and the film tension rate. Thus, by changing these parameters, a film having different widths and thicknesses can come out of one extruder head 2.

After the bubble 5 has been compressed in the frame 17 and compressed in rolls 6, the film 7 is wound under constant tension, either as a sleeve or as a sheet film. If the film is to be printed, the surface of the film must be corona-treated before winding to obtain a good adhesion surface for the printing ink.

The process parameters of the blown film are shown in Figure 2. The final characteristics of the film are its thickness in microns (t) and its width in millimeters (LFW). The parameters of the extrusion process are the mass flow rate from the extruder, expressed in kilograms per hour (G), the diameter of the bubble 5 in millimeters (D), and the line speed or production rate in meters per minute (V). Attached to the extruder is an annular head 2 with fixed diameters in millimeters (d) and a head gap in millimeters (h).

Some useful calculations for this process are expressed in the equations below. The meaning of these terms will be explained in later sections.

Blow ratio (BUR) = bubble diameter (D) = 2LFW (4)

LFW should be measured before any cutting or cutting. The blow ratio should not be confused with the less commonly used term "blow ratio":

Blow ratio (BR) = LFW (5)

Traction ratio (DDR) is a measure of the degree of direction of the machine:

Traction ratio (DDR) = 1000h (6)

The approximate yield can be calculated from the film dimensions and extrusion parameters:

Yield = 2tLFW 60 V p (7), where p is the average density of the resin in grams per cubic centimeter. It must be borne in mind that stretching or loosening of the film may result in slight yield discrepancies between the line speed and the film dimension measurements. In co-extruded films, the yield formula can also be applied to individual layers.

Tensile flow

During the bubble blowing process, the polymer melt exiting the head is drawn or pulled by subjecting the melt to a tensile flow. Typically, the drawing time during extrusion of the blown film is 0.01 to 3 seconds. This tensile flow also contributes to the molecular orientation of the melt, which is “frozen” at the freezing line, and affects the final properties of the film, such as tear resistance. The viscosity of a polymer subjected to tensile flow is also an important extrusion factor because it is related to the strength of the melt.

The dependences between the tensile viscosity and the elongation curve of the flow were calculated. Lower MFIs tend to have higher tensile viscosities at the same tensile strength. However, the type of polymer is also important. Of particular importance is that the LDPE resin, due to its wide molecular weight distribution and long chain branches, hardens upon stretching, i. the tensile viscosity increases with higher tensile force. At high tensile forces, the tensile viscosity of LDPE increases significantly. This is described as higher melt strength.

Melt surface fraction

The difficulty in processing narrow molecular weight distribution polymers, such as LDPE, is that at a critical flow rate, they tend to form a surface roughness defect called shark skin or melt surface fraction. This roughness is primarily manifested by a loss of gloss. As the flow rate increases, the shark skin defect becomes even more pronounced. It is characterized by parallel waves on the surface of the extruded product and spiral elongated edges in the film. The critical shear rate at which this problem is first observed decreases (i.e., the problem only increases) as the polymer MFI and temperature decrease.

Shark skin is a phenomenon that results from a change in the flow of polymer through the extruder head and the nature of the outlet. There are two possible theories that explain its cause. The first states that the flow of melt at the head wall occurs at a critical shear stress due to loss of direct contact or when the melt loses adhesion to the surface of the head. This creates a slip-adhesion effect on the contact surface and this results in said melt defect. In fact, measurements of slip rate at the surface of the extruder head show a sudden increase in flow rate or shear stress, and this slip is always related to the surface roughness of the LDPE. The adhesion of the melt to the surface of the extruder head depends on the nature of the surface (roughness), the type of metal of the extruder head and the polymer itself.

The second theory attributes the formation of shark skin to the sudden acceleration of the melt surface layer due to the change in velocity distribution that occurs when the melt leaves the extruder head and its exit velocity equals the tensile velocity. If the surface stresses exceed the tensile strength of the melt, the surface ruptures and this forms the surface roughness of the shark skin. Extrusion can be performed below a critical shear rate using wide extruder head gap technology. Alternatively, the melt temperature may be increased. The metal used to make the extruder head can be selected to reduce the chance of slipping. Another way is to insert a suitable adhesion promoter or excipient that changes the flow pattern on the surface of the extruder head and delays the onset of melt rupture. Adding LDPE or increasing the amount of LDPE also delays the onset of melt fracture.

Extrusion process

Extrusion is the main process in which the granular raw material is converted into a homogeneous melt for supply to the extruder head and to form the final product, in this case a blown thin film. The extruder 1 essentially consists of a heated cylinder 8, inside which a Archimedean screw 9 is precisely fitted. strip heaters 11, and the heat of friction caused by the cutting action of the propeller by melting part of them. The molten and compacted polymer is then extruded through the annular extruder head 2 to form a thin tubular film 3, cooled and extracted by compression rolls 6.

Extruder drive

The power of the motor driving the extruder screw 9 must be sufficient to generate the heat and torque required to extrude the most viscous types of polymer. This is especially true for LLDPE (linear low density polyethylene) extrusion, which, being much more viscous than LDPE polymers, will require more power, create higher pressures, and produce higher melting temperatures. Thus, extruders should be designed for LLDPE. With extruders of the right capacity, almost adiabatic operation is possible, i. the heat energy produced by the propeller is almost sufficient to generate and maintain the melting temperature. The engine power should be at least 20 kW.

Cylinder heating and cooling

The extruder cylinder 8 shown in Figure 1 has a continuous, wear-resistant hardened or alloy steel liner. The cylinder is heated in several zones along its housing, usually by means of electric heater strips 11, controlled by automatic temperature controllers. The cooling of the entire cylinder housing is designed to remove excess heat, and the cooling medium is water or air, in the latter case fans 12 are used. The front of the cylinder is also cooled with water to prevent premature partial melting of the pellets in the hopper.

To ensure that the temperature conditions are met, it is necessary to measure the melting temperature with a thermocouple inserted in the melt in the extruder head adapter compartment. It is also recommended to install the melting pressure transducer 13 in front of the extruder breaker plate 14 in order to create a back pressure and to evaluate the homogenization capacity of the screw. When using LDPE polymers, extrusion is usually performed by gradually increasing the temperature over the entire length of the cylinder 8. Reverse or uniform temperature profiles are often recommended to maintain a higher temperature in the feed zone and a lower temperature in the dosing zone. Product specifications often indicate the optimal melting point and the temperature profile at which the polymer is to be processed. However, the temperature profile needs to be set specifically for each specific application, in which case it is recommended to have at least seven heating zones with a stable temperature.

table. The extrusion temperature of the material is recorded

1 zone Zone 2 Zone 3 Zone 4 Adapter Head no. 1 Head no. 2 Degrees ° C 190 195 195 200 210 215 220

Propeller design

The most important part of the extruder 1 is the screw 9, and its construction must be such that a large part of the homogeneous melt can be fed to the extruder head 2 at a constant speed and pressure at the same temperature and viscosity. The function of the auger is to collect the polymer granules from the feed hopper 10, to move them forward, to melt them, to simultaneously compress and homogenize the melt, and then to feed it to the extruder head 2.

A conventional propeller design, as shown in Figure 3, includes three different zones or compartments along its length: a feed zone (zone 1), a compression zone (zone 2), and a dosing zone (zone 3).

Most extruder screws have a length to diameter (L / D) ratio of at least 20: 1 to achieve maximum mixing, although some older equipment with a lower L / D ratio is still in use. The L / D ratio of most modern devices ranges from 24: 1 to 30: 1. When producing 5.0-7.0 pm film from recyclable material, the (L / D) ratio should not be less than 27: 1.

The polymer is compressed in a screw, gradually reducing the volume of the screw channel between the supply area and the dosing area. The degree of compression is the ratio between the volume of the first channel in the supply zone and the volume of the last channel in the measuring zone. The compression ratios of standard screws range from 2.5: 1 to 4.5: 1, and the compression ratio of grooved screws is lower, e.g. 1.5: 1. Appropriate screws must be selected to ensure a good degree of compression to obtain a homogeneous melt. The compression ratio for obtaining the film of the present invention should be at least 3.5: 1.

Various manufacturers have made improvements to the standard propeller, which include double groove steps 15, variable step 16, mixing pins, decompression zones between the two compression zones, intermittent step structures, and other improvements to improve mixing efficiency. Such improvements reduce shear stresses and required power, as well as reduce overheating problems.

Barrier screws are recommended for best material homogenization and product yield. The main purpose of the barrier step is to separate the already molten resin from the unmelted solid polymer. The layer of molten resin on the wall is kept thin, thus ensuring high shear rate and high temperature formation in the melt. Separation of the melt into a deeper channel for this purpose also prevents over-treatment of the melt by a screw and unnecessary shear temperatures. The screws may also form a high shear mixing portion behind the screw end to improve homogenization, increase performance, and ensure a more uniform melting temperature. With this type of screws with an additional mixing zone, a film thickness of 5.0-7.0 pm should be produced.

Extruder head

The extruder head at the end of the cylinder has a screen consisting of one or more wire meshes. It is recommended to use a 4050 pm wire screen for the production of the film of the present invention. The screen is used to increase the back pressure in the measuring zone and thus increase the melt homogenization, as well as to prevent contaminants from entering the extruder head and the final product. Typically, a screen changer is used to melt new screens and prevent the build-up of excess pressure created by contaminants. These screen converters can be interchangeable with flat panel or permanent screens, depending on the system used. The adapter attached to the head acts as an attachment point for the extruder head. The head and adapter are heated to maintain a constant melting temperature and are designed to trap any unwanted material that could enter the melt stream.

The small grooves in the cylinder feed compartment, which run substantially longitudinally but sometimes spirally, are designed to improve the power of the propeller feed compartment. The grooves extend from the feed zone below the hopper 10 to a point along the extruder approximately 4 lengths of the extruder diameter. The resulting high pressure at the end of the groove part also greatly facilitates the melting of the polymer. For optimal feeding, this type of feeding area should not be too hot, as this can cause the pellets to soften prematurely and clog the grooves and outlet. The groove areas have water cooling to prevent overheating.

Film extrusion heads

The function of the blown film extruder head is to supply the polymer melt at a constant pressure and constant speed and turn it into a thin-walled sleeve while maintaining a uniform temperature. To blow the film, the head 2 has an outer metal body and an inner core to form an annular opening through which the molten polymer exits to form a tubular extrudate. The lips of the extruder head are in the form of hardened steel rings, which can be made together with the core and the body, but are usually fastened with screws so that they can be replaced. The extruder head is heated to maintain a uniform melting temperature. Air to inflate the film bubble is supplied through a duct in the core.

The film heads of the extruder are fed with plastic either from the side or from below. In both cases, the lip positions of the extruder head can be adjusted relative to the fixed core to ensure an even feed of molten plastic from the extruder head. Adjustment of the extruder head orifice is traditionally performed manually before the extruder is turned on, using centering screws arranged around the perimeter of the head, and measuring the thickness of the resulting film with a micrometer. Computer feedback on the thickness of the film to be measured is used in more complex lines to automatically adjust the gap of the extruder head, for example by using electrically heated thermal expansion elements arranged around the perimeter of the extruder head.

The thickness of the film or the length of the bubble circumference may vary if the extruder head is not centered, if it is worn, or if the melt flows from the extruder head unevenly. If not corrected, these dimensional variations or irregularities will affect the quality and appearance of the film, and the roll of film on the roll will become tapered or widened at the top or have protrusions or edges. These edges can later cause problems with operations such as gluing, printing, and sealing. The thicker film will have no edges and the appearance of the roll will improve. This problem can be solved by using a slowly rotating extruder head, which will cause the belt or bubble to spiral.

To produce a very thin film, an extruder head with a very narrow gap should be used. It is recommended to use a head with a gap of 0.65-0.85 mm. Obviously, when using an extruder head with a wide gap, the extrusion process will take place at a significantly lower shear rate, which is lower than the critical melt fracture onset level. Higher melting points are also beneficial. The causes of melt surface fracture have been discussed previously. The melt surface fraction is not only affected by the shear rate or melt flow rate through the extruder head or extrusion temperature, it is also affected by the nature (roughness) of the extruder head surface, the type of metal used for the head and the polymer itself. The construction material of the extruder head has been found to have a significant effect on the flow profiles of molten plastic at the polymer-metal junction and on the subsequent melt fraction and film appearance. An extruder head made of an alpha brass alloy allows extrusion to be carried out at a much higher speed than using heads made of steel or chrome-plated before melt fracture begins. Thus, by selecting the right metal for the production of the head, a much higher rate of yield through the head gap can be achieved without melt fracture.

Film bubble

The extruded polyethylene sleeve exits the extruder head with air supplied at a pressure of 15-35 kPa through the core of the head, which is blown into a bubble 5 of the desired diameter and film thickness. The bubble diameter to head diameter is known as the blow ratio. In the present invention, it is recommended to use an exhaust ratio of 2: 1 to 3: 1. Higher blow-out ratios may result in bubble instability and film wrinkling.

The film cooling process is very important because cooling can affect the yield rate, film thickness uniformity, film density, and many other film properties. The cooling system has four main functions:

- removes heat from the melt exiting the extruder head and cools the film bubble to a solid state so that it can be later leveled and wrapped;

- stabilizes and maintains the bubble as it exits the extruder head and minimizes film dimensional changes;

- controls the film density and many other properties of the film, including impact resistance, abrasion resistance and visual properties;

- determines the maximum output speed at which the film can be produced without blocking the head. To avoid blockage of the head, the temperature of the film should be less than 40 ° C before reaching the compression rolls.

When the film exits the extruder head, it is melted and must be cooled as soon as possible to stabilize the bubble and achieve a low degree of crystallinity. The freeze line or cold line is an annular zone where the molten polymer hardens and the bubble reaches its final diameter. Slow cooling, i. high freezing line, causes high crystallinity and high film density, and this can adversely affect both the optical properties of the film and its impact resistance.

The main element of the cooling unit is an air ring mounted above the extruder head. The air ring is designed to direct the flow of air evenly into the film bubble around its perimeter as the bubble exits the head. For this purpose, a combined cooling system is used, supplying a large amount of low-pressure air at the highest possible speed. It should not be a gust of air that would damage the bubble or cause it to vibrate. Any changes in air temperature or flow rate around the perimeter of the air ring will cause variations in cooling rate, which will affect the quality of the bubble film.

Bubble formation

Before entering the compression rolls 6, the film bubble 5 is compressed into a flat sleeve by means of compression plates 17. They can come in a variety of shapes and can be either solid, hollow, or roll. The friction between the film and the surfaces of the compression plates 17 should not be too great, as then the plates 17 could "grab" the film, thus forming wrinkles in it. This effect becomes more noticeable if the film is too hot or the contact area between the plates and the film is too large. This can be largely overcome by roughening the surfaces or by using coatings that do not accumulate excess heat and do not cause the film to stretch.

Each compression plate 17 faces at an angle to the compression rollers 6 and is arranged symmetrically with respect to the vertical. The angle between the plates 17 does not exceed 45 degrees. This angle is suitable for the production of a small-thickness film, because the short contact between the film and the plates does not create wrinkles in it. For thicker films with a relatively low degree of tension, the angle between the plates 17 can be significantly smaller. Air compression systems can be used to create almost friction-free surfaces and to minimize wrinkling of film products.

Laminated film is often used to make polyethylene bags. Layering is obtained inside narrow wedges or plates placed at an angle formed by compression plates; these plates are directed to opposite sides of the bubble before the film enters the compression rolls 6 so that it folds itself. Layering elements that can be made of wood or metal should have a smooth surface and rounded edges to avoid scratching the film. All this is very important in the production of 5.0-7.0 pm thick recycled film.

The compressed bubble passes through compression rollers 6 which compress the tubular film before its winding step, trap the inflation air in the film bubble and regulate the film feed rate. The compression roller assembly 6 consists of two rolls, usually one made of steel and the other covered with an elastic material such as rubber to remove film defects. The compression rollers 6 are mounted vertically above the extrusion nozzle 2, at a height of at least two meters and directly above the compression plates 17. When only a conventional air cooling ring 3 is used to cool the film, the compression roller height above the extruder head affects the film production speed. using the compression rollers installed above. However, when the compression rollers are very high, bubble instability can occur, especially when extruding LDPE polymers.

The pressure between the two rolls 6 should be uniform along their length and simply sufficient to maintain a constant linear velocity and prevent air from escaping from the film bubble 5. If the compression pressure of the rolls is too high, the film may stick together. The film can also stick together if the film entering the rolls 6 is too hot. The rollers must be in good condition and their position must be properly adjusted. Worn or improperly adjusted rolls can cause a variety of problems, including:

- external scratching of the film;

- air outflow from the bubble caused by worn and misaligned rollers.

As a result, the bubble will lose its diameter. Any air remaining between the two layers of the film sleeve will damage the outside of the film, including:

- appearance of wrinkles;

- wrinkling and creasing of the film due to poor alignment of the rolls with the extrusion head and compression plates.

Film wrapping

The last operation of the blown film process is the winding of the film. Since the wrapped film always shrinks slightly, the tension of the film wrap must be controlled so that the film is not wrapped too tightly on the roll. If the tension is too high, the layers of the wrapped film may stick together, the film may be crumpled or even torn. A film that is too slightly wrapped can slip telescopically from the roll, especially if a slip material is used.

As the LDPE film is inherently elongated, its overstretch should be avoided. Film wrapping systems are designed to ensure uniform film tension when wrapping film on a roll. Both surface (contact) and central wrapping methods can be used, but contact wrapping is standard.

Claims (15)

A method of producing LDPE film from used and recycled LDPE material, comprising sorting used products from LDPE material; shredding the sorted LDPE products into pieces; placing the fragments in a solvent to remove printing ink adhering to the fragments; washing and heating the pieces to obtain a molten plastic mixture; regranulation of the mixture; and feeding the reclaimed material into an extrusion line to produce a sleeve-shaped LDPE film, characterized in that: (a) used LDPE waste is sorted by removing the film with dark ink, paper and metal foil labels; (b) a mixture is prepared containing 70% transparent and not more than 35 pm film and 30% transparent elastic film; (c) sorted LDPE waste is washed out by cold washing using techniques such as grinding, floating-separating particles and friction washing; (d) the washed materials are crushed into pieces and melted to form a homogeneous polymer melt; (e) filtering the melt using a metal plate with conical orifices between 90 and 110 μm; f) the granules thus obtained are mixed with the additives and regranulated to obtain the finally homogenized and filtered granules; g) The granules are fed to an extrusion line to obtain a final recycled sleeve-shaped LDPE film with a wall thickness of 5.0 to 7.0 μm. 2. The process according to claim 1, characterized in that the mixing of the granules in step f) lasts for 1 hour. 3. The method according to claim 1, characterized in that an extruder with a screw of at least 120 mm is used for homogenization in step f). 4. A method according to claim 3, characterized in that barrier type screws are used. 5. The method of claim 1, wherein the filtration in step f) comprises a backwash system with five wire mesh screens comprising two external screens with 150 μm weave gaps, two internal screens with 100 μm weave gaps and one central final filtration screen. with 4050 pm weave gaps. 6. The method of claim 1, wherein the power of the extrusion engine is at least 20 kW. 7. The method of claim 1, wherein the extruder in step g) uses a screw having a length to diameter (L / D) ratio of at least 27: 1. 8. The method of claim 7, wherein the compression ratio between the volume of the first channel in the auger feed zone (zone 1) and the volume of the last channel in the auger dosing zone (zone 3) is at least 3.5: 1. 9. The method of claim 1, wherein the extrusion line uses a narrow gap extruder head to produce a 5.0 to 7.0 μm thick LDPE film. 10. The method according to claim 9, characterized in that a head with a gap of 0.65-0.85 mm is used. 11. The method of claim 1, wherein the pressure of the air supplied in the extrusion line through the head core ranges from 15 to 35 kPa. 12. The method of claim 1, wherein the blowout ratio in the extrusion line, i. y. the ratio between the diameter of the bubble and the diameter of the head is between 2: 1 and 3: 1. 13. The method according to claim 1, characterized in that compression plates (17) are used in the extrusion line, wherein the angle between the plates (17) does not exceed 45 degrees. 14. The method according to claim 1, characterized in that compression rollers (6) are used in the extrusion line, mounted vertically above the extrusion head (2) at a height of at least two meters directly above the compression plates (17). An LDPE film having a density of 0.925 g / cm 3, a melt flow index (MFI) of 1.0 and a wall thickness of from 5.0 to 7.0 μm, produced by the process according to claims 1-14 from used and recycled LDPE substances.