Films made of polyvinyl chloride, polyethylene, polypropylene, polystyrene and other resins are used for packaging and as laminations. Plastic packaging and plastic packaging products occupy an increasing share of the market, especially composite plastic flexible packaging, which has been widely used in food, medicine, chemical industry and other fields. Among them, food packaging accounts for the largest proportion, such as beverage packaging., quick-frozen food packaging, retort food packaging, fast food packaging, etc. These products have brought great convenience to people’s lives.
History
Early exploration stage (late 19th century to early 20th century)
The origin of plastic film can be traced back to the late 19th century, when the development of the chemical industry laid the foundation for the birth of plastic materials. In 1862, British chemist Alexander Parkes demonstrated the first artificial plastic – “Parkesine”, which is a cellulose based material. Although it is not a true plastic film, it provides ideas for subsequent research.
At the beginning of the 20th century, with the advancement of chemical synthesis technology, scientists began to attempt to manufacture thinner plastic products. In 1908, Belgian chemist Leo Baekeland invented phenolic resin, which was the first fully synthetic plastic. Although phenolic resin is mainly used to manufacture hard objects such as electrical components and tableware, its emergence has driven the development of the plastic industry, and people have begun to think about how to make plastics into thinner forms for other purposes.
Initial application stage (1930s and 1940s)
In the 1930s, polyethylene (PE) was discovered and began industrial production. Scientists from Imperial Chemical Industries (ICI) in the UK accidentally synthesized low-density polyethylene (LDPE) while studying high-pressure reactions. This material has good flexibility and transparency, making it very suitable for making thin films. In 1933, ICI Company began producing LDPE film, initially mainly used for packaging and electrical insulation materials. This marks the beginning of plastic film entering the market as an independent product.
During World War II, the application of plastic film was further expanded. Due to the war requiring a large amount of material packaging and waterproof materials, the production of plastic film has rapidly increased. For example, polyethylene film is used to package military equipment, food, and medical supplies, and its lightweight and waterproof properties are fully utilized. Meanwhile, plastic film also plays an important role in the insulation of military communication cables.
Rapid development stage (1950s to 1970s)
In the 1950s, with the industrial production of plastics such as polyvinyl chloride (PVC) and polypropylene (PP), the types of plastic films became more diverse. PVC film has good flexibility and processability. By adding different additives, various performance films can be produced, such as soft packaging films and hard pipe films. PP film has a high melting point, good chemical resistance, and mechanical properties, and its application range is constantly expanding.
During this period, significant breakthroughs were made in the production technology of plastic films. Modern production processes such as extrusion blow molding and extrusion casting have gradually matured, resulting in significant improvements in both the yield and quality of plastic films. Plastic film has been widely used in the agricultural field, such as the large-scale use of greenhouse covering film and plastic film, which has greatly changed the agricultural production mode and improved the yield and quality of crops. Meanwhile, in the field of packaging, plastic film has gradually replaced traditional packaging materials such as paper and glass, becoming one of the mainstream packaging forms used for packaging various products such as food, daily necessities, and electronic products.
Diversification and High Performance Stage (1980s present)
Since the 1980s, with the continuous improvement of people’s requirements for the performance of plastic films, functional plastic films have emerged. For example, plastic films with high barrier properties have been developed to effectively prevent the permeation of gases such as oxygen, water vapor, and carbon dioxide. These films play an important role in packaging fields such as food and medicine, extending the shelf life of products. At the same time, anti-static plastic films and anti fog plastic films have also emerged, meeting the special needs of different industries.
The enhancement of environmental awareness has also had a profound impact on the plastic film industry. On the one hand, R&D personnel are committed to developing biodegradable plastic films, such as photo degradable plastic films, biodegradable plastic films, etc., to reduce the environmental pollution caused by plastic films. On the other hand, the recycling technology of plastic film has developed rapidly, and the recycling system has gradually improved, which has increased the resource utilization rate of plastic film and promoted the sustainable development of the plastic film industry. In addition, the application of plastic film in high-tech fields is becoming increasingly widespread, such as in the electronics, aerospace, medical and other industries, where plastic film plays a key role as a high-performance material.
Film types
PVA coated high barrier film
PVA coated high barrier film is made by coating PVA with nano- inorganic substances on polyethylene film and then printing and compounding it. Without significantly increasing the cost, its barrier performance is not only significantly better than that of EVOH five-layer squeeze film, and the packaging cost has also been greatly reduced. This not only ensures that the packaged objects meet all the quality requirements for aseptic packaging, but also greatly reduces the cost of aseptic packaging for food processing enterprises. It can be used to package beverages, juices, milk, Soy sauce, vinegar, etc.
Biaxially oriented polypropylene film (BOPP)
Biaxially oriented polypropylene film is made by co-extruding polypropylene particles into a sheet and then stretching it in both longitudinal and transverse directions. Due to the orientation of the stretched molecules, this film has good physical stability, mechanical strength, air tightness, high transparency and gloss, and is tough and wear-resistant. It is the most widely used printing film, and is generally used with a thickness of 20 to 40 μ m, the most widely used is 20 μ m. The main disadvantage of biaxially oriented polypropylene film is poor heat sealability, so it is generally used as the outer film of composite films. For example, after compounding with polyethylene film, its moisture resistance, transparency, strength, stiffness and printability are all better, and it is suitable for Contains dry food. Since the surface of the biaxially oriented polypropylene film is non-polar, has high crystallinity and low surface free energy, its printing performance is poor and its adhesion to inks and adhesives is poor. Surface treatment is required before printing and lamination. deal with.
Low density polyethylene film (LDPE)
Low-density polyethylene films are generally made by two processes: blow molding and casting. Cast polyethylene film has a uniform thickness, but is the most widely used due to its higher price and lower cost. Low-density polyethylene film is a translucent, glossy, soft film with excellent chemical stability, heat sealability, water resistance and moisture resistance. It is resistant to freezing and can be boiled. Its main disadvantage is that it has poor oxygen barrier properties. It is often used as the inner film of composite flexible packaging materials. It is also the most widely used and used plastic packaging film, accounting for more than 40% of plastic packaging film consumption.
Since the polyethylene molecule does not contain polar groups, has high crystallinity and low surface free energy, the film has poor printing performance and poor adhesion to inks and adhesives, so it needs to be printed before printing and lamination. Perform surface treatment.
Polyester film (PET)
Polyester film is a film material made of polyethylene terephthalate as raw material, extruded into thick sheets, and then biaxially stretched. It is a colorless, transparent and shiny film with excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, chemical resistance, oil resistance, air tightness and fragrance retention. It is one of the commonly used barrier composite film substrates. However, polyester film is more expensive and generally has a thickness of 12 μm. It is often used as the outer material for retort packaging and has good printability.
Nylon film (PA)
Nylon film is a very tough film with good transparency, good gloss, high tensile strength and tensile strength, and good heat resistance, cold resistance, oil resistance and organic solvent resistance. Excellent wear resistance and puncture resistance, It is relatively soft and has excellent oxygen barrier properties, but has poor water vapor barrier properties, high moisture absorption and moisture permeability, and poor heat sealability. It is suitable for packaging hard items, such as greasy foods, meat products, and fried foods., vacuum packaged food, steamed food, etc.
Cast Polypropylene Film (CPP)
Cast polypropylene film is a polypropylene film produced by the casting process. It can be divided into ordinary CPP and retort-grade CPP. It has excellent transparency, uniform thickness, and uniform longitudinal and transverse properties. It is generally used as the inner layer of composite films. layer material. The thickness of ordinary CPP film is generally between 25 and 50μm. After compounding with OPP, it has better transparency, bright surface and firm feel. This material is generally used in gift packaging bags. The film also has good heat sealability. The thickness of retort-grade CPP film is generally between 60 and 80 μm. It can withstand high-temperature cooking at 121°C and 30 minutes. It has good oil resistance and air tightness, and has high heat sealing strength. The inner layer of general meat packaging is Use cooking grade CPP film.
Aluminized film
The most widely used aluminum-plated films are polyester aluminum-plated film (VMPET) and CPP aluminum-plated film (VMCPP). Aluminized film has the characteristics of both plastic film and metal. The function of aluminum plating on the surface of the film is to block light and prevent ultraviolet radiation, which not only extends the shelf life of the contents, but also improves the brightness of the film. It replaces aluminum foil to a certain extent. It is also cheap, beautiful and has good barrier properties. Therefore, Aluminized film is widely used in composite packaging.
The current development status of China
With the development of the economy and the adjustment of the rural industrial structure, the market demand for plastic films in various industries in China continues to rise.
In 2005, the cumulative industrial output value of all plastic film production and supply enterprises nationwide reached 79,910,504 thousand yuan, an increase of 29% over the previous year. The cumulative number of enterprise units was 1,223, and the cumulative product sales revenue for the year was 77,417,653 yuan. thousand yuan, an increase of 25.89% over the same period last year, and the total cumulative profit reached 2,384,871 thousand yuan. The industry-wide per capita sales rate is 488,266.94 yuan.
In the first half of 2006, the cumulative industrial output value of all plastic film production and supply enterprises nationwide reached 43,959,811 thousand yuan, an increase of 17.31% over the same period of the previous year. The cumulative number of enterprise units was 1,299, and the cumulative product sales revenue was 44,040,148 thousand yuan. yuan, an increase of 22.11% over the same period last year, and the total cumulative profit reached 1,213,127 thousand yuan. The industry-wide per capita sales rate is 889,069.85 yuan. Overall, the operating conditions of plastic film production and supply companies across the country are good.
The output of plastic films in China accounts for about 20% of the total output of plastic products, and it is one of the categories with rapid growth in output of plastic products. Judging from the application fields of plastic films (thickness 0.06mm ~ 0.26mm) in China, the packaging industry uses the largest amount, has the most varieties, and is the most widely used. Its consumption accounts for about 2/3, followed by agriculture, which accounts for about 30%, and then They are functional membranes, such as microporous membranes, shielding membranes, geomembranes, etc. Theoretically, almost all synthetic resins can form films, but those that have economic significance, become commodities, and are used in the largest quantities are polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), and polyethylene terephthalate. Polyester (PET), polystyrene (PS), ethylene / vinyl acetate ( EVA ), polyamide (PA) and other resins. If appropriate plastic additives are added to the resin matrix, various functional films required can be prepared. The production methods in the plastic film industry include calendering and extrusion. The extrusion method is divided into extrusion blown film, extrusion casting, and extrusion stretching (also known as secondary molding).), etc., the extrusion method is the most widely used, especially for the processing of polyolefin films, while the calendering method is mainly used in the production of some polyvinyl chloride films.
China’s plastic film industry is in a stage of vigorous development. It is reported that the demand for China’s plastic film will grow at a rate of more than 9% every year. Moreover, with the continuous emergence of various new materials, new equipment and new processes, China’s plastic films will be driven towards the development of diversified, specialized and multi-functional composite films.
With the development of China’s economy and the adjustment of rural industrial structure, the market demand for plastic films in various industries in China continues to rise. It is predicted that the global plastic film and sheet market consumption is expected to reach 50.7 million tons in 2015, of which the Asia-Pacific region is expected to become the fastest growing regional market. China’s plastic film industry is currently in a stage of vigorous development. From 2001 to 2011, the average annual growth rate of the market size reached more than 18%, and it is expected to reach 350 billion yuan in 2012.
Surface properties
Plastic films are the most widely used in the packaging field. Plastic films can be used for food packaging, electrical product packaging, daily necessities packaging, clothing packaging, etc. They have one thing in common, that is, plastic films must be color printed, and as food packaging, multi-layer lamination or vacuum aluminum plating and other processes must be performed. Therefore, the surface free energy of the plastic film is required to be high and the wet tension is large to facilitate the firm adhesion of the printing ink, adhesive or aluminized layer to the plastic film; during the production, winding and high-speed packaging process of the plastic film, The film surface is required to have certain friction properties to prevent the film from adhering or slipping; when used for packaging electrical appliances, electronic products, etc., the film is required to have certain anti-static properties, etc.
Surface tension of plastic film
The surface tension of the plastic film depends on the surface free energy of the plastic film, and the surface energy of the film depends on the molecular structure of the film material itself. Most plastic films such as polyolefin films (LDPE, HDPE, LLDPE, PP) are non-polar polymers with small surface free energy and low surface wet tension, generally around 30 dynes/cm. Theoretically, if the surface tension of an object is lower than 33 dynes/cm, ordinary ink or adhesive cannot adhere firmly, so its surface must be treated. Polyesters (PET, PBT, PEN, PETG) are polar polymers with high surface free energy and surface wet tension above 40 dynes/cm. However, for high-speed color printing or to increase the bonding force between the vacuum aluminum plating layer and the BOPET film surface, the BOPET film also needs to be surface treated to further increase its surface wet tension.
Surface treatment methods for plastic films include: corona treatment, chemical treatment, mechanical roughening, coating, etc. Among them, corona treatment is the most commonly used.
The basic principle of the corona treatment method is to apply high-frequency and high-voltage power between the metal electrode and the corona treatment roller (usually a high-temperature-resistant, ozone-resistant, high-insulation silicone rubber roller ) to cause a discharge. The air ionizes and large amounts of ozone are formed. At the same time, high-energy electric sparks impact the film surface. Under their combined effect, the surface of the plastic film is activated and the surface energy is increased. Corona treatment can increase the wet tension of the polyolefin film to 38 dynes/cm; the surface wet tension of the polyester film can reach more than 52-56 dyne/cm. The surface wet tension of corona-treated plastic films is related to the voltage applied to the electrode, the distance between the electrode and the corona treatment roller and other factors. Of course, corona treatment should be moderate. It does not mean that the higher the intensity of corona treatment, the better. It is worth noting here that air should be avoided between the plastic film and the corona treatment roller, otherwise the reverse side of the film may also be corona treated. The consequences of reverse corona are: 1. The back-sticking phenomenon of ink printing may occur; 2. Aluminum plating layer transfer will occur during aluminum plating, and glue coating layer transfer will occur during gluing. The main measure to prevent corona on the reverse side of the film is to adjust the pressure of the rubber pressing roller in front of the corona treatment roller. The pressure at both ends of the pressing roller must be consistent and the pressure must be appropriate. In addition, the corona roller and the pressure roller must undergo strict dynamic and static balance tests, and the radial runout is required to be less than 0.05 mm. The purpose is to ensure that the plastic film enters the corona roller smoothly and prevents air from being pinched, thereby avoiding the occurrence of reverse corona.
Feature comparison
Commonly used plastic films include polyethylene (PE), polyvinyl chloride (PVC), polystyrene ( PS), polyester film (PET), polypropylene (PP), nylon, etc. Various plastic films have different properties, the ease of printing is also different, and their uses as packaging materials are also different.
Polyethylene film is a colorless, tasteless, odorless, translucent, non-toxic insulating material. It is widely used as packaging bags; food bags, and can also be used to make various containers. It is an inert material, so it is difficult to print. It must be processed before it can print with better results.
PVC film has good light resistance and aging resistance and has good tear resistance. It is breathable. It is a clean, colorless and transparent film. Plasticizer is usually added and it is soluble in acetone. Cyclohexanone and other solvents. Therefore, it is possible to print with ink made of polyvinyl chloride resin. Suitable for packaging bags, book covers, etc.
Polystyrene film is a soft and tough film, clean, colorless and transparent. When it does not contain plasticizers, the film layer is always soft, resistant to freezing, and does not age when stored. When printing, it uses oxidative polymerization synthetic binder ink, which can make the polystyrene film soft and tough. The imprint fastness is good.
Polyester film is a material that is colorless, transparent, moisture-resistant, airtight, soft, strong, resistant to acids, alkalis, oils, esters and solvents, and is not afraid of high and low temperatures. After EDM treatment, it has relatively good surface fastness to ink.. Used in packaging and composite materials.
Polypropylene film has good gloss and transparency, is heat-resistant, acid-base resistant, solvent-resistant, friction-resistant, tear-resistant, and breathable. It cannot be heat-sealed below 160°C.
The strength of nylon film is greater than that of polyethylene film. It is odorless, non-toxic, impermeable to bacteria, resistant to oil, ester, boiling water and most solvents. It is generally used for heavy-duty, wear-resistant packaging, and cooking packaging (reheating of food), it can be printed without surface treatment.
Polyethylene molecules basically have no polar groups and are a non-polar polymer. In polypropylene molecules, each structural unit contains a methyl group, which is a weakly polar group. Basically They are also non-polar polymers. Therefore, they have poor affinity for ink, so they must be processed before printing to obtain a satisfactory print.
Most treatment methods are through oxidation to increase polarity and change the surface structure. Specific treatment methods include discharge (commonly known as corona, spark) method, flame method, ultraviolet radiation method, acid ( sulfuric acid, chromic acid ) treatment method, etc. The discharge method is relatively simple and popular.
Production
The production of plastic film is a relatively complex process, mainly involving the following key links:
Raw material preparation
1. Polymer selection
The most commonly used raw materials for producing plastic film are various polymer resins. For example, polyethylene (PE) is a widely used raw material. It is further divided into low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). LDPE has good flexibility and transparency, and is commonly used in the production of food packaging films, agricultural films, etc; LLDPE has excellent tensile properties and toughness, and is widely used in the fields of packaging and geosynthetic films; HDPE has higher strength and is commonly used in the production of industrial packaging films and building waterproof films that require higher strength.
Polyvinyl chloride (PVC) is also a common raw material. It can change its performance by adding different additives, such as plasticizers that can increase the flexibility of PVC, making it suitable for producing soft films, such as PVC films used for packaging. However, due to the potential release of harmful hydrogen chloride gas during the production and use of PVC, it is limited in some application scenarios that require high environmental and safety requirements.
Polypropylene (PP) has a high melting point, good chemical resistance, and mechanical properties. It can produce biaxially oriented polypropylene (BOPP) film, which has high transparency, high glossiness, and good barrier properties, and is widely used in packaging fields such as food and cigarettes.
2. Additive addition
In order to improve the performance of plastic film, various additives need to be added to the raw materials. Plasticizers can lower the glass transition temperature of polymers and increase their flexibility. For example, in the production of PVC film, phthalate plasticizers were once widely used, but due to their potential health risks, some environmentally friendly plasticizers such as citrate esters are gradually replacing them.
Antioxidants can prevent plastic films from degrading due to oxidation during processing and use. For example, hindered phenolic antioxidants can capture free radicals and inhibit the progress of oxidation reactions. Light stabilizers can improve the light resistance of plastic films, which is very important for some outdoor films such as agricultural films and building films. UV absorbers can absorb ultraviolet radiation and convert it into heat energy, thereby avoiding damage to plastic films caused by ultraviolet radiation.
Lubricants can improve the flowability of plastic films during processing and reduce the friction between the melt and equipment. For example, calcium stearate is a commonly used lubricant that can make plastics easier to process in equipment such as extruders, improve production efficiency, and enhance the surface quality of films.
Processing technology
1. Extrusion blow molding method
This is one of the most commonly used methods for producing plastic film. Firstly, mix the polymer resin and additives and add them to the hopper of the extruder. In the extruder, the raw material moves forward under the push of the screw, and gradually melts under the action of high temperature and shear force, forming a uniform melt.
The melt is extruded through a die to form a tubular thin film blank. Then, compressed air is introduced from the center of the die to inflate the tubular billet into a bubble like thin film. During the inflation process, the film is stretched in both the circumferential and axial directions, resulting in a thinner thickness and improved performance.
The inflated film is cooled by a cooling device, usually air-cooled or water-cooled, to solidify and shape the film. Finally, the film is pulled out by a traction device and rolled up to obtain the finished plastic film. This method can produce films of various thicknesses and sizes, suitable for the production of various plastic films such as polyethylene and polyvinyl chloride.
2. Extrusion casting method
In extrusion casting, the raw materials are melted and mixed by an extruder, and then extruded through a narrow slit die to form a thin layer of molten film. The molten film is directly cast onto a cooling roller, and through contact with the cooling roller, the molten material rapidly cools and solidifies, forming a plastic film.
The film produced by this method has high transparency and flatness, as the melt is not inflated during the cooling process, resulting in better thickness uniformity of the film. Extrusion casting method is commonly used to produce films with high requirements for thickness accuracy and transparency, such as high transparency films used in food packaging and films used in the electronics industry.
3. Bi directional stretching method
Bidirectional stretching is a method that can significantly improve the performance of plastic films. Firstly, thick unstretched films, known as base films, are produced using extrusion blow molding or extrusion casting methods. Then, stretch the base film in both longitudinal and transverse directions.
During the longitudinal stretching process, the film passes through a series of stretching rollers, whose speed gradually increases, causing the film to be stretched in the length direction. Next, during the lateral stretching process, the film enters a stretching machine and is stretched in the width direction by a fixture. Bi directional stretching can make the molecular chain arrangement of the film more regular, thereby improving the strength, transparency, and barrier properties of the film. For example, biaxially oriented polypropylene (BOPP) film is produced through this method and is widely used in the high-end packaging industry.
Quality control
1. Thickness control
The thickness of plastic film is an important quality indicator. During the production process, the thickness of the film is monitored and controlled through an online thickness measurement system. These systems typically use non-contact measurement methods, such as infrared thickness gauges or capacitive thickness gauges.
According to the feedback from the thickness gauge, the thickness of the film can be ensured to meet the requirements by adjusting parameters such as the screw speed, traction speed, or inflation ratio of the extruder. For some high-precision films, such as those used in the electronics industry, the thickness tolerance requirements are very strict, with a deviation range of only a few micrometers.
2. Performance testing
Testing the properties of plastic film, including tensile strength, tear strength, barrier performance, transparency, etc. The tensile strength and tear strength can be tested by a tensile testing machine. The film is made into a standard sample and stretched on the testing machine until the film breaks. The force and elongation at break are recorded.
The testing of barrier performance mainly focuses on the barrier ability of the film to gases such as oxygen and water vapor. For example, oxygen permeability meter and moisture permeability meter are used to measure the oxygen and water vapor permeability of the film. Transparency can be measured by a haze meter, and the lower the haze, the higher the transparency of the film, which is very important for some films used in packaging and optical fields.
Production equipments
1. Extruder
Single screw extruder:
This is one of the most basic equipment in plastic film production. It mainly consists of a hopper, screw, barrel, heating and cooling system, and transmission system. The hopper is used to store and supply plastic raw materials, and the screw rotates inside the barrel to push the raw materials forward. During the pushing process, the heating system of the barrel heats the raw materials, gradually melting the plastic. The screw structure of a single screw extruder is simple, usually consisting of equidistant and equally deep threads. Its working principle is to melt and plasticize plastic through friction and shear. For example, in the production of some small plastic film products, such as thin plastic film used for packaging small accessories, a single screw extruder can meet the basic extrusion needs.
The length to diameter ratio (the ratio of screw length to diameter) of a single screw extruder is generally between 20-30. The larger the length to diameter ratio, the longer the plastic stays in the screw and the better the plasticizing effect. The rotational speed can be adjusted according to production requirements, generally around 10-100 revolutions per minute. The speed will affect the extrusion speed and quality of the plastic.
A twin-screw extruder has two screws that mesh with each other and rotate in the same or opposite direction. The advantage of twin-screw extruder is that the mixing effect of materials in the screw is better. Due to the synergistic effect of the two screws, plastic raw materials and various additives can be more thoroughly mixed and plasticized. It is suitable for processing plastic formulas containing multiple components, such as the production of plastic films that require the addition of various additives (such as plasticizers, antioxidants, etc.).
The screw of a twin-screw extruder can have a modular structure that can be combined and replaced according to different production requirements. Its aspect ratio can reach 30-50, and the speed is relatively high, reaching 100-300 revolutions per minute. This equipment plays an important role in producing high-performance and high-quality plastic films, such as functional plastic films used in aerospace, electronics, and other fields.
2. Mold head
Blow molding die head:
Used for producing plastic film by extrusion blow molding method. Its structure is usually circular with a vent hole in the middle. After the plastic melt is extruded from the extruder, it forms a tubular melt through the blow molding die. The design of the mold head is crucial for the quality of the film, as its inner and outer diameter dimensions, as well as the width of the annular gap, can affect the thickness and uniformity of the film. For example, in the production of agricultural plastic film, a well-designed blow molding die can ensure uniform thickness of the film in the circumferential direction, avoiding situations where it is locally too thin or too thick.
The material of blow molding dies is generally high-quality alloy steel, which has good high temperature resistance and wear resistance, and can withstand the high temperature and high-speed erosion of plastic melts. Some high-end blow molding dies are also equipped with automatic adjustment devices, which can automatically adjust the width of the die gap based on feedback information during the production process, such as changes in film thickness, to ensure stable film quality.
Casting die head:
Applied to the production of thin films using extrusion casting method. The casting die is generally narrow slit type, which can evenly extrude the plastic melt to form a thin layer of melt film. The key is to ensure uniform distribution of the melt throughout the width direction of the mold head. There is usually a special flow channel design inside the mold head, such as a hanger style flow channel, which is shaped like a hanger and can evenly distribute the melt to the narrow slit outlet of the mold head in the flow channel.
The precision requirements for the casting die are very high, with a slit width typically between 0.5-2 millimeters, and it needs to maintain extremely high flatness to ensure uniform thickness of the extruded melt film. For the production of high transparency and high-precision films, such as plastic films used in electronic touch screens, the quality and performance of the casting die directly determine the quality of the film.
3. Cooling device
Air cooling device:
In the production of plastic film, the air cooling device is mainly used to cool the blown film. It usually consists of a fan and a duct. The cold air generated by the fan is blown towards the film through the air duct, rapidly cooling and solidifying the film. The advantages of air cooling are that the equipment is simple, the cost is low, and it does not cause water stains on the surface of the film like water cooling.
The fan power and wind speed of the air cooling device can be adjusted according to the production speed and thickness of the film. For example, when producing thinner plastic films, lower wind speeds are required to prevent the films from being blown apart; When producing thicker films, the wind speed can be appropriately increased to accelerate the cooling rate. The number and location of fans also need to be arranged reasonably according to the actual production situation to ensure that all parts of the film can receive uniform cooling.
Water cooling device:
The water cooling device is mainly used for cooling thin films produced by extrusion casting method. It generally includes a cooling water tank and a circulating water system. After the film is extruded from the die, it directly enters the cooling water tank and achieves rapid cooling through contact with the cooling water. The cooling effect of water cooling is better than that of air cooling, which can cure the film faster and is suitable for producing films with high-speed and high-quality requirements.
The water temperature of the cooling water tank needs to be strictly controlled, usually between 10-30 ℃. Excessive water temperature can affect the cooling effect and lead to poor film formation; Low water temperature may cause condensation on the surface of the film, affecting the quality of the film. The circulating water system can ensure a continuous supply of cooling water and stable temperature, while also filtering and purifying the cooling water to prevent impurities from contaminating the film in the water.
4. Traction device
The main function of the traction device is to pull out the film extruded and cooled from the die at a certain speed and tension, so that the film can be continuously produced. It usually consists of one or more pairs of traction rollers. The surface of the traction roller generally has a certain roughness to increase the friction with the film and prevent the film from slipping during the traction process.
The speed of the traction device can be precisely adjusted according to production requirements, with an adjustment accuracy of up to 0.1 meters per minute. By controlling the traction speed, the thickness and stretching degree of the film can be adjusted. For example, in the production process of biaxially oriented films, the traction speed plays a key role in the longitudinal stretching stage, which, in conjunction with the speed of the stretching roller, determines the stretching ratio of the film in the longitudinal direction. At the same time, the traction device can also be linked with other equipment such as thickness measuring devices to achieve automated control of film production.
5. Coiling device
The winding device is used to neatly roll up the produced plastic film for convenient storage and transportation. It mainly includes a winding core, a drive motor, and a tension control system. The core is the foundation of film winding, and its material and size need to be selected according to the type and specifications of the film. The drive motor provides the power for winding, allowing the film to be smoothly wound onto the core.
The tension control system is a key component of the winding device, which ensures that the film maintains appropriate tension during the winding process. If the tension is too high, it may cause the film to stretch, deform or even break; If the tension is too low, the film roll will become loose, which is not conducive to subsequent use and transportation. The tension control system monitors the tension of the film through sensors and adjusts the speed of the driving motor based on feedback information, thereby achieving automatic tension control.
Application of plastic film
Plastic film is a thin and soft plastic product that has a wide range of applications in many fields:
In the field of agriculture
1. Greenhouse covering materials
Plastic film can be used as a greenhouse covering material for growing crops such as vegetables, flowers, and fruits. For example, polyethylene (PE) film is one of the most commonly used greenhouse covering materials. It has good transparency, allowing visible light from sunlight to pass through the film, ensuring the normal progress of plant photosynthesis. At the same time, it can also serve as insulation, reducing the loss of heat inside the greenhouse, making the indoor temperature higher than the external environment temperature, and creating suitable temperature conditions for plant growth.
Some functional plastic films, such as those with anti fog properties, are also important in greenhouse applications. This type of film can prevent water vapor in the greenhouse from condensing into water droplets on the inner surface of the film, as water droplets can affect the transmission and distribution of light, and may also cause diseases when dropped on plants. The anti fog droplet film can form a uniform water film on the surface of the film, ensuring good transparency.
2. Plastic film covering
Plastic film plays a crucial role in plastic mulching cultivation. It can cover the soil surface and increase soil temperature. For example, when planting crops in early spring, plastic film can raise soil temperature in advance, promote seed germination and seedling growth.
Plastic film can also play a role in preserving soil moisture and reducing the evaporation of soil moisture. It can effectively inhibit weed growth because after being covered with plastic film, weeds do not receive sufficient light and their growth is inhibited, thereby reducing the competition between weeds and crops for nutrients and water.
Packaging field
1. Food packaging
Plastic film is widely used in food packaging. For example, polyethylene terephthalate (PET) film has good barrier properties and has good barrier effects on oxygen, water vapor, etc. It can be used to package puffed foods such as potato chips, prevent food from getting damp, oxidizing and deteriorating, and extend the shelf life of food.
Plastic films used for packaging meat, such as polyvinyl chloride (PVC) films, have good flexibility and stretchability, can tightly wrap meat, maintain its shape, and to some extent prevent microbial contamination, ensuring the freshness of meat. At the same time, some plastic films can also be printed with information such as food brand, ingredients, shelf life, etc., making it convenient for consumers to purchase and use.
2. Packaging for daily necessities
In daily necessities packaging, plastic film can be used to package various products. The film used for packaging soap can prevent contamination and damage during storage and transportation. It can be transparent, allowing consumers to see the appearance of the product, or decorated with color printing based on the characteristics of the product to enhance its appeal.
For some small electronic products, such as headphones, memory cards, etc., plastic film packaging can play a role in dust and moisture prevention, and can be labeled with product specifications, usage instructions, and other labels on the packaging, making it convenient for sales and user use.
Industrial sector
1. Waterproof and moisture-proof packaging
In the packaging of industrial products, many precision instruments, electronic components, etc. require waterproof and moisture-proof protection. Plastic film, such as aluminum-plastic composite film, is made by compounding plastic film with aluminum foil and has excellent barrier properties. It can effectively block external moisture, oxygen, etc. from entering the interior of the packaging, preventing the product from getting damp, oxidizing, and rusting. For example, for some high-precision circuit boards, using this composite film packaging can ensure their safety during storage and transportation.
2. Protective film
Plastic film can be used as a protective film in industrial processes such as metal processing and plastic molding. For example, in the processing of stainless steel plates, a layer of plastic protective film is applied to the surface of the plate to prevent scratches and contamination during cutting, stamping, and other processing. This protective film usually has good adhesion and can be firmly attached to the surface of the protected material. After processing, it can be easily peeled off without leaving any residue on the material surface.
Medical field
1. Medical packaging
Medical plastic film is used for packaging medical devices and drugs. For example, some sterile medical devices, such as syringes, infusion sets, etc., are sealed and packaged with plastic film to ensure that they are in a sterile state before use. This packaging film needs to have good barrier properties to prevent microorganisms and external pollutants from entering the interior of the packaging.
For drug packaging, plastic film can be used to make blister packaging. Bubble wrap packaging is the process of sealing drugs within plastic film formed bubbles, each bubble can hold one or more individual drugs. This packaging method facilitates the counting and retrieval of drugs, while also protecting them from environmental factors such as light and humidity.
2. Medical protective equipment
Plastic film is also used in medical protective equipment. For example, the outer layer of disposable medical protective clothing is usually made of waterproof and breathable plastic film material. This material can prevent bodily fluids, blood and other pollutants from coming into contact with the bodies of medical staff, while also allowing sweat and other water vapor to dissipate, ensuring the comfort of medical staff wearing it. In addition, plastic film can also be used as isolation material in some simple isolation wards to provide isolation and protection.
Film recycling
The Importance of Recycling
Resource saving: Plastic film is mainly made of various polymers, such as polyethylene, polypropylene, etc. These polymers come from non renewable resources such as petroleum. Recycling plastic film can reduce the demand for new plastic raw materials, thereby saving resources. For example, recycling 1 ton of plastic film is equivalent to saving about 1-2 tons of oil resources.
Environmental Protection: If plastic film is discarded indiscriminately, it will cause serious pollution to the environment. They are difficult to degrade in the natural environment and may remain in the soil for decades or even centuries, affecting the soil’s permeability and permeability. Moreover, after plastic film enters the water, it will float on the surface, affecting the living environment of aquatic organisms, and may even be ingested by aquatic animals, leading to their death. Recycling plastic film can effectively reduce these environmental problems.
Energy saving: Compared to producing new plastic films, recycling plastic films consumes much less energy for reuse. Generally speaking, the energy required for recycling plastic film and reprocessing is only about 20% -60% of that for producing new plastic film, which helps to reduce energy consumption and greenhouse gas emissions.
Recycling method
Mechanical Recycling:
Collecting and categorizing: This is the first step in mechanical recycling. Plastic film needs to be collected from various types of waste, such as household waste, industrial waste, etc. After collection, the plastic film should be classified according to its type, color, and degree of pollution. Different types of plastic films (such as PE and PP films) have different physical and chemical properties, and classified recycling can improve the quality of recycling. For example, food packaging plastic film may be contaminated with food residues and needs to be separated from industrial packaging film to avoid contamination.
Cleaning and drying: The classified plastic film should be cleaned to remove surface dirt, labels, and other adhered substances. The cleaning process usually uses specialized detergents and cleaning equipment, such as drum cleaning machines. The cleaned plastic film needs to be dried to remove moisture and avoid quality issues during subsequent processing.
Crushing and granulation: The dried plastic film will be crushed into small pieces and then pelletized using equipment such as extruders. During the granulation process, plastic fragments are remelted and extruded into granules, which can be used as raw materials again to produce plastic films or other plastic products. For example, some recycled PE film particles can be used to produce garbage bags, plastic pipes, etc.
Chemical recycling:
Chemical recycling mainly targets plastic films that are difficult to process through mechanical recycling methods, or for obtaining higher quality recycled raw materials. The depolymerization reaction is an important way of chemical recycling, which can decompose the polymer molecules of plastic films into monomers or oligomers. For example, PET film can be chemically depolymerized into terephthalic acid and ethylene glycol, which can be re polymerized to produce new PET plastics.
Thermal cracking: Thermal cracking is the process of decomposing the polymer of plastic film into small molecule compounds under high temperature and oxygen free or oxygen deficient conditions. These small molecule compounds can be liquid oils, gases, or coke, etc. Liquid oil can be used as a chemical raw material, and gas can be used for energy production. For example, by thermally cracking waste PP film, liquid products containing olefins and other components can be obtained, which can be used to produce new plastics or as fuel.
Recycled applications
Production of recycled plastic film: Recycled plastic film particles can be directly used to produce recycled plastic film. However, due to the possibility of impurities or decreased performance in recycled plastic film, the quality of recycled plastic film may not be as good as brand new plastic film. In order to improve the quality of recycled plastic film, additives such as antioxidants and plasticizers are usually added during the processing. Recycled plastic film can be used in some fields where performance requirements are not particularly high, such as the secondary use of agricultural films (such as plastic films), ordinary packaging films, etc.
Manufacturing other plastic products: Recycled plastic film can also be used to manufacture other plastic products. For example, using recycled PE film to produce plastic trash cans, plastic tables and chairs, etc. These plastic products have relatively flexible requirements for appearance and performance, which can better utilize the characteristics of recycled plastics. In addition, recycled plastic film can also be used to produce building materials such as plastic insulation boards, providing environmentally friendly materials for the construction industry.
