Introduction of EVOH High Barrier Rigid Packaging Composite Material Processing Technology and Equipment

Shantou Dacheng Environmental Protection Precision Machinery Technology Co., Ltd.

Wan Songzhou   Yang Weiguang

Abstract: This paper introduces the performance characteristics and application of EVOH resin for high-barrier rigid packaging. It focuses on the processing technology and processing equipment of EVOH composite materials, as well as the intelligent improvement content and future development direction of EVOH multi-layer co-extrusion sheet equipment.

Key words: EVOH; high barrier; composite material; processing equipment; intelligent manufacturing.

Introduction

Plastic packaging sheets are often used to produce disposable plastic cups, plates, bowls, dishes, boxes and other thermoforming products, which are widely used in the packaging of food, vegetables, fruits, beverages, dairy products, industrial parts and other fields. It has the advantages of softness, good transparency and easy to be made into popular styles of various shapes. Compared with glass, it is not easy to break, light in weight and convenient for transportation. However, the packaging industry usually requires plastic packaging sheets to have both mechanical strength and heat resistance. performance, cold resistance, gas and water vapor barrier, glossy transparency, heat sealability, printability and non-toxicity. Single-layer plastic sheets obviously cannot meet all these properties. For example, the barrier property of oxygen-sensitive commodity packaging is not as good as that of metal and glass containers. Therefore, multi-layer composite sheets need to be produced. The advantages of a variety of resins make the performance of the products further meet the requirements of various packaging materials.

1. EVOH packaging application market status

In the field of cold chain food packaging, people used metal or glass materials as food packaging to effectively isolate the penetration of various gas components inside and outside to ensure the quality of the contents and commodity value. Because there are three major factors that cause food spoilage: biological factors (biological enzyme reactions, etc.), chemical factors (mainly oxidation of food components) and physical factors (hygroscopic, drying, etc.). These factors play a role in environmental conditions such as oxygen, light, temperature, moisture, etc., which cause food spoilage. Preventing the deterioration of food is mainly to inhibit the proliferation of microorganisms in food, prevent the oxidation of food components by oxygen, and prevent moisture and maintain the original flavor of food. [1]

Ethylene-vinyl alcohol copolymer, referred to as EVOH, is known as the world's three largest barrier resins together with polyvinylidene chloride (PVDC) and polyamide (PA) [2]. EVOH can greatly inhibit the intrusion of oxygen in the air into food, thereby inhibiting the production of toxins and other harmful substances due to the proliferation of microorganisms, and can also prevent composition changes caused by oxidation, while maintaining fragrance and preventing external odor pollution. Moreover, the lack of moisture barrier properties can be compensated by other polyolefin layers. Therefore, EVOH multilayer packaging materials can effectively prevent food spoilage and extend shelf life. In addition, it is easy to process and form, and has good environmental protection performance. Due to the excellent gas barrier properties, transparency, processability and solvent resistance of EVOH resin, its application fields are getting wider and wider, and the demand is also growing rapidly.

2. High barrier EVOH resin

1) Material properties

The barrier properties of EVOH The barrier properties of polymer materials refer to the shielding ability of products to small molecular gases, liquids, water vapor, etc. Currently commonly used resin varieties with good barrier properties include: EVOH, PVDC, PAN, PEN, PA and PET. Table 1 shows the oxygen transmission rates of various materials [1]

As shown in Figure 1, the oxygen barrier properties of EVOH vary with relative humidity. When the relative humidity is below about 80%, it is significantly higher than other materials, and at high humidity such as above 85%, the oxygen barrier property of EVOH is significantly reduced. However, in practice, barrier packaging materials are almost always multi-layered, and polyolefin (PE or PP) with good moisture barrier properties is used on both sides. Even if the package is close to water-based food, when the surrounding environment is air, the relative humidity of EVOH in the middle layer is about 80% . The barrier properties of EVOH are still better than any other materials under this condition. Therefore, EVOH is widely used not only for dry food packaging, but also for water-based food such as milk, juice drinks, condiments, beer, wine, and mineral water. In addition to oxygen, EVOH also has excellent barrier properties to other gases such as carbon dioxide, nitrogen, helium, and ammonia. [1]

2) Application of hard packaging EVOH compound

When EVOH is used as a high barrier material, it usually adopts a multi-layer composite structure. Commonly used composite materials are: PP, HIPS, PE, EVOH, AD, and AD is the adhesive in the structure. The multi-layer composite structure can give full play to the properties of each material, improve the water resistance of EVOH, and obtain a high-barrier material with excellent comprehensive properties. In the past, most of them were used in flexible packaging, but composite resins such as PP, PE, PA, etc., due to their good toughness and poor rigidity, are not easy to punch, which limits their application in the field of rigid packaging. PP and impact-resistant polystyrene HIPS have good rigidity and excellent molding properties, and are suitable for punching and hard packaging materials. Therefore, it is particularly urgent to vigorously develop EVOH high-barrier composite materials suitable for hard packaging.

III. Rigid packaging EVOH processing equipment and processing technology

1. Processing equipment

The high-barrier multi-layer co-extrusion composite material has a multi-layer structure, so special multi-layer co-extrusion extrusion equipment is required to form, and the appropriate extrusion type is selected according to the material properties of each layer. For example, Shantou Dacheng Environmental Protection Precision Machinery Technology Co., Ltd. recently designed and manufactured EVOH multi-layer co-extrusion sheet extrusion equipment, using a combination of 5 single-screw extruders, the specific model is WJPG120/105/75/65/65-1200, Matching metering pump, the maximum extrusion output is 800-1000KG/H, and the width is 800MM, among which: 120 screw is suitable for the main material and secondary PP material, 105 screw is suitable for new PP material, 75 screw is used for edging material PP material, two Bar 65 screw is suitable for EVOH and AD material. After the die head comes out, the sheet is equipped with a tablet pressing machine, a roller constant temperature device, an online thickness gauge, a natural cooling device, a trimming device, a double-sided silicone oil coating machine, a pulling device, a waste edge winding device, and a double station. Coiler, PLC industrial control cabinet, etc.

The basic flow chart of the equipment is usually as follows:

Raw material system: raw material formula ratio and transportation, broken material recovery device;

Extrusion system: each extruder, filtration, metering, co-extrusion are distributed to the die for sheet output;

Three-roll calendering system: sheet calendering and trimming device;

Online thickness gauge, online display of sheet thickness

Traction system: power traction device;

Rewinding system: the sheet is wound into rolls;

Water circulation system: roller water temperature control;

Electrical control system: electrical control of the whole set of equipment.

WJPG120/105/75/65/65-1200 EVOH high barrier sheet machine

The EVOH high-barrier sheet produced by this equipment has been inspected by the National Packaging Product Quality Supervision and Inspection Center (Guangzhou), GB/T31354-2014 "Packages and Containers Oxygen Permeability Test Method Coulomb Counter Test Method", test conditions: T =(23±0.5)℃, RH=(50±2)%, air environment, oxygen concentration: 21%, oxygen permeability: 5.5×10-4 cubic centimeters/bag for 24 hours.

2. Processing technology

The main process route of the equipment: the material of the main material layer passes through the 120 and 105 screw PP main extruders, the 65 screw (AD raw material) is heated and melted and extruded, passes through the hydraulic quick screen changer, and reaches both sides of the distributor, and the 65 screw extrudes the EVOH raw material directly. Entering the middle of the distributor, the 75-screw PP material is heated, melted and extruded through the extruder, and directly enters the mold to meet through the screen changer to produce two edge-wrapped sheets. The five-roll tablet press is shaped and cooled, and after cooling, the thickness uniformity is measured by an automatic thickness gauge to adjust, trimmed, and the sheet is sent to the winding device through the traction roller, and is packaged as a finished product in rolls, and the trimmed PP material is discarded. After directly entering the pulverizer, it is returned to the extruder hopper for reproduction, and the edge material can also be wound into a bundle by the waste edge winding device.

3. Key technical points

Due to the poor compatibility between EVOH resin and PP resin, and the large difference in resin rheology rate, the bonding strength between the substrate and EVOH, the requirements for the tensile properties of EVOH during secondary molding, and the EVOH layer distribution during calendering to produce composite sheets The uniformity of composite materials are all key issues affecting the performance and use of composite materials, and are also difficult problems that need to be solved when producing this type of composite materials.

The key to the multi-layer co-extrusion technology is the adhesive (AD). The composite packaging materials of EVOH usually include PP\EVOH, but PP and EVOH cannot be directly thermally bonded, and an adhesive (AD) must be added between PP and EVOH. When selecting the adhesive, it is necessary to consider the adhesive of PP as the matrix material, the second is the matching of the melt viscosity of PP and EVOH, and the third is the requirement of tensile properties, so as to avoid delamination during secondary processing. Therefore, the co-extruded sheets are mostly five-layer co-extruded sheets (PP\AD\EVOH\AD\PP). /AD/EVOH/AD/R/PP, the outermost layer is PP new material, and the other two layers are PP crushed recycled material R(PP). Asymmetric structure can also be used, and other materials (PE/HIPS, etc.) extruders can be added for co-extrusion. The principle is the same, and the same multi-layer co-extrusion method can be achieved.

Schematic diagram of multilayer barrier packaging structure [4]

For rigid packaging, PP has also changed to impact-resistant polystyrene (HIPS) as a composite layer. The following figure structure [3]:

EVOH multi-layer co-extrusion composite material processing process Figure 2

Figure 2 is the process diagram of EVOH multi-layer co-extrusion composite material. As shown in the figure, the composite material is divided into 5 layers from the inside to the outside. The middle layer ③ is the EVOH layer, and a single extruder does not need to be layered directly.The middle layer is extruded into the stratifier; the two sides of the EVOH layer are the AD layers ② and ④, which are divided into two layers by a single extruder through the stratifier and compounded on both sides of the EVOH; the two outer surfaces of the composite material are PS Layers ① and ⑤ also use a separate extruder, which is divided into two layers by a layerer and compounded on the upper and lower surface layers of the material. The composite structure is used to improve the water-blocking and barrier properties of EVOH. The adhesive layer improves the adhesive strength between the PS layer and the EVOH layer.

The key component of the multi-layer co-extrusion technology is the distributor. The hot melts of PP, AD and EVOH extruded from the three extruders A\B\C pass through a distributor to form PP/AD/EVOH/AD/PP The key to the structure layer and the distributor is the design of the flow channel, so that the PP layer is evenly divided into one or five layers, the AD layer is divided into two or four layers, and the EVOH layer is the middle layer. The distributor flow channel is designed with adjusting blades to adjust the thickness uniformity between the two layers. The thickness ratio of each layer is controlled by the respective extrusion volume. After the machine, the melt gear pump is used for quantitative control. The melt viscosity of PP, AD and EVOH at the set temperature of the die is different, which will make the thickness distribution of each layer section of the sheet uneven. The material flow is corrected, and the thickness in the middle of the material layer is adjusted.

To obtain high-barrier multi-layer co-extrusion composites with excellent properties, the following key issues need to be paid attention to during processing:

(1) Adjustment of component structure: In order to strengthen the moisture resistance of the substrate and improve the oxygen resistance of EVOH, the composite material is designed with an asymmetric structure. The asymmetric structure can be realized by adjusting the middle layer or the outer layer of the layerer. If the middle layer of the layerer is adjusted asymmetrically, it is easy to make the adhesive layer resin form an asymmetric structure together with the surface layer resin, resulting in a thin side. The reduction in the amount of resin in the adhesive layer reduces the bond strength, resulting in delamination. Therefore, only the outer layer regulator can be adjusted, and the actual production can be selectively adjusted according to the thickness of the product, but it must be ensured that the thin layer does not break during the secondary molding.
(2) Adjustment of process temperature: Due to the difference in the rheological rate of EVOH resin, PP resin and bonding resin, and EVOH resin is in the middle layer, the fluidity of the resin will be affected to a certain extent by the pressure of multiple layers of resin. Therefore, the EVOH layer on both sides of the composite material will be reduced accordingly. The uneven distribution of resin will directly affect the barrier performance of the material. The heat will also be uneven during secondary molding, which will make the molding difficult. lost. Therefore, the temperature at both ends of the mold should be increased by about 5 °C during production to improve the fluidity of the EVOH layer on both sides of the material and ensure the uniformity of the EVOH layer distribution.
(3) Selection principle of extruder: EVOH resin is more sensitive to temperature and shear, so when selecting the model of extruder, the principle of small model and small aspect ratio is the principle. The residence time of the resin in the barrel should not be too long, the processing temperature should not exceed the upper limit temperature of the resin, the shearing of the screw should not be too large, and the rotation speed should not be too fast.
(4) Stop treatment method: Since EVOH resin is prone to gel phenomenon and has poor compatibility with other resins, the screw and barrel should be cleaned before and after production, and it is best to use the cleaning method. For polyolefin resins with high melt index, the cleaning time should not be less than 30 minutes. The cleaning process adopts the wave cleaning method. Only after observing that the extruded resin is free of impurities, it can be officially produced or stopped.

(5) The form of waste edge addition: the multi-layer structures of different materials generally need to be trimmed on both sides of the edges, and the trimmed residues include multiple layers of different raw materials, which makes it difficult to recycle the trimmed residues. . In order to achieve the recovery of the remaining material of the hemming, by adding a hemming extruder and adjusting the structure of the die, there are hemming fluid channels on both sides, and the fluid in the hemming fluid channel and the composite layer are combined at the outlet to form a finished product. The fluid in the side fluid channel is wrapped around the edges on both sides of the product, and when trimming the edges, the trimmed residual materials are all of the same fluid material, which is beneficial for recycling.

Fourth, the intelligent improvement of EVOH multi-layer co-extrusion sheet equipment

The industrial revolution characterized by the integrated development of the new generation of information and communication technology and the manufacturing industry is emerging globally, and intelligent manufacturing has become the main direction of the development of the manufacturing industry. The promotion and application of Internet + industrialization promotes the upgrading and upgrading of products, and the production of products from raw material modification, process optimization, equipment precision manufacturing, operation mode and other intelligent applications, breaking through the original mode, by adding intelligent units (including mechanical, sensor, electrical and intelligent software, with memory, perception, calculation and decision-making functions), making equipment operation more convenient and improving product quality and added value. It can realize automatic detection of equipment health status; can automatically manage various process parameters according to product and raw material requirements, realize dynamic setting and storage; can monitor production operation status and automatically adjust; can intelligently optimize and process various operating parameters. The intelligent unit has built-in artificial intelligence algorithms and has the ability to learn process parameters. It can find the optimal process parameters of equipment suitable for specific products through limited test sample training. The coordination of various functional devices ensures product quality, reduces energy consumption, and highlights green and intelligent features.

The intelligent improvement of EVOH rigid packaging multi-layer co-extrusion sheet equipment:

1.  The raw material supply system of each extruder can be equipped with an automatic batching system, which can be automatically measured according to the ratio requirements of each combination, which is suitable for the control of the proportions of new materials, returned materials, and color masterbatch. The accuracy of ingredients and the labor intensity are greatly reduced, which can give early warning and feedback compensation measures when the material level is reduced;

2.  Optimize the design of high-efficiency extrusion screw, combine computer simulation design with experimental data, and achieve high-efficiency screw with good plasticization, low energy consumption and high output through different raw material combinations and process parameters; 

3.  Optional high-performance gearbox to ensure high torque, high speed, low energy consumption and low noise;

4.  The extruder is equipped with a metering pump for quantitative and regulated transportation. Generally, a pressure sensor is equipped in front of the filter screen and before and after the metering pump, and the pressure and speed are closed-loop controlled. The first stage is the closed-loop control of the metering pump and the metering pump motor. After all the pressure signals enter the PLC, the operation output controls the frequency conversion, drives the motor, and is equipped with early warning, feedback control, and display functions;

5.  Precision multi-layer distributor, adopts double adjustment technology of flow ratio and blade clearance ratio, and the layering is more uniform;

6.  The high-efficiency and energy-saving barrel heating ring is selected to ensure the energy saving and stability of the heating system; and it is equipped with high and low temperature alarm and compensation devices.

7.   Equipped with static mixer, which is conducive to more uniform extrusion of materials, especially for the production of color material sheets;

8.  Optional automatic adjustment die head, supporting thickness gauge, realize automatic feedback control of thickness, realize automatic control of thickness of extruded sheet;

9.  Three-roller calendering roller press adopts servo motor control device, matched with roller gap grating measurement, closed-loop feedback, compact structure, automatic adjustment of the thickness of the production sheet online, which makes the operation simple;

10.  The water flow channel of the roller adopts the form of interlayer multi-channel, and it is necessary to pay attention to the water resistance and distribution uniformity of the flow channel to ensure that the temperature difference of the roller surface is consistent. Usually, the structure of the inlet and outlet channels with large flow scattered from the middle to both sides is adopted;

11.  The roller water temperature control system adopts servo proportional valve automatic water temperature adjustment, heating and cooling temperature double PID control technology, etc.;

12.  It is equipped with an online trimming, crushing, conveying and recycling system to realize automatic waste recycling, environmental protection and hygiene;

13.  Equipped with a storage rack before coiling, which can continuously store sheets to meet the requirement of changing the roll without slowing down when changing the roll;

14.   The winding structure design adopts the online non-stop roll changing and unloading device, which saves a lot of operation time and labor consumption. At the same time, it is necessary to pay attention to the pressing when changing the roll to make the sheet roll flat;

15.  In terms of function, an online coating compound system can be selected. It is necessary to ensure that the surface is uniform after coating. After passing through the drying device, the surface of the sheet is completely dried and no obvious oil spots appear. The double-sided film can be applied according to different functional requirements. Use hot lamination or cold lamination, pay attention to the temperature resistance of the film during hot lamination, and select an appropriate lamination temperature, such as directly entering the molten sheet for lamination or entering the lamination through a suitable cooling position of the fixed roller to ensure the lamination is smooth;

16.   Equipped with a double-sided surface defect detection system to detect the surface quality of products online (such as crystal spots, black spots or defects);

17.  The whole machine adopts PLC control and data-based information acquisition system to realize automatic control of parameter setting, data operation and feedback, alarm and other functions;

18.  Advanced dual PID control algorithm technology, real-time statistical analysis of technical data, real-time provision of improvement measures, to ensure that the production process is always in a state of statistical control to improve product quality, accuracy, energy consumption and other performance indicators.

19.  Remote control technology, through the server center industrial data center and industrial data acquisition and analysis software, remote monitoring network equipment, standard motor health detection and analysis system, equipment energy management optimization system, etc., to achieve supplier equipment in customer factories Remote monitoring and collection and storage of various process data, and analysis of process data to optimize production process formula.

V. Future development direction of intelligent sheet equipment market

With the increasing domestic demand, the amount of composite high-barrier fresh-keeping packaging sheet is also increasing year by year. It has better performance in food packaging such as high-end meat food and fast food rice, especially medicines and cosmetics, and has a promising future. In recent years, with the continuous increase of domestic labor costs, people have put forward higher requirements for equipment output, energy consumption, environmental protection and automation. There are few high-end sheet equipment manufacturers in China. Joint research and development with many advanced raw material suppliers, research institutes and product manufacturers at home and abroad, make full use of modern high-precision equipment, improve the machining accuracy of equipment parts, design and optimize the structure of various parts, optimize the processing technology, and improve the overall equipment performance.

At present, new-generation information technologies such as mobile Internet, big data, cloud computing, Internet of Things, fifth-generation mobile communications (5G), and blockchain are constantly making breakthroughs, especially the depth of new-generation artificial intelligence (AI) technology and advanced manufacturing technology. The new generation of intelligent manufacturing technology formed by integration has become the core driving force of the new round of industrial revolution [5]. Second, the demand for high-performance packaging in the food, beverage and new energy industries continues to rise, which promotes the continuous innovation of multi-layer co-extrusion technology; third, the development of various supporting parts processing fields of extrusion equipment, new technologies The pace of application and expansion is accelerating, promoting the rapid improvement of the overall equipment technology level, and the market demand for extrusion equipment is showing remarkable characteristics: energy saving, high efficiency, dataization, automation, and intelligent extrusion sheet machines are the direction of technological innovation, and intelligence leads the way. The future of product development.

Because of this, the active development of rigid packaging multi-layer co-extrusion intelligent sheet machines is an important measure for the industry to seize market opportunities. Create high-efficiency, energy-saving, green, environmentally friendly, intelligent and high-quality multi-layer co-extrusion sheet extrusion equipment, and advance towards digitalization, automation, and intelligence, while meeting the direction of green product packaging, technological innovation, and sustainable development, and constantly expanding the equipment application market. Increase the added value of products, with comprehensive cost-effective advantages and excellent quality assurance, help Shengwei for China's intelligent manufacturing.

references:

[1]   Zhou Bin, EVOH resin with high barrier properties and its application in food packaging, Plastic Packaging, 2012, Volume 22, Issue 3

[2] Song Yi, Li Jianhua, Han Xiaojie, Zhang Qiang. Development of EVOH five-layer co-extrusion composite film special unit, Plastic Packaging, 2005, Volume 15, Issue 2

[3] Luo Zhanping, Wei Longqing, Processing Technology of HIPS/EVOH High Barrier Composite Materials, Plastic Packaging, 2011, Issue 5

[4]  Song Yuan, Application of High Barrier Resin EVOH in Meat Packaging, Meat Industry, No. 12, 2008, No. 332

[5] Yuan Qingtang, Yin Ruiyu, Cao Xianghong, Liu Peicheng, Strategic Research on Intelligent Goals, Characteristics and Paths of Process Manufacturing Facing 2035 , China Engineering Science 2020 Vol.22 No.3