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Common problems and solutions in the production of PVC transparent sheet

Common problems and solutions in the production of PVC transparent sheet

2021-09-13

There are two production processes of PVC transparent sheet, one is conventional calendering method, the other is extrusion calendering method, that is, it is produced by twin-screw extruder. It is the main hard PVC with the advantages of high light transmittance, smooth and bright surface, high physical and mechanical strength, corrosion resistance and high fineness. It is widely used in the fields of food, medicine, daily necessities and so on. There will be many problems in the production process of PVC, and the quality problems and solutions of calendering method and extrusion calendering method are also different due to different processes and equipment. This article pe wax manufacturer will discuss the common problems in the production process of PVC and put forward the corresponding solutions.

Main problems and analysis of PVC transparent sheet production

1. The PCV transparent sheet has crystal point impurities, and the black and yellow points exceed the standard. Due to different production processes and equipment, the crystal impurity points in PVC sheet products produced by extrusion method are much less than that of transparent sheet produced by calendering method. Because the extrusion method has a filter screen in the melting section before the extruder material enters the die, large unplasticized particles and impurities are filtered out. However, if the resin particles in the production process are not plasticized, the plasticization is not standard or difficult to plasticize, there are still small non plasticized particles passing through the filter screen.

The main causes are:

1.1 There are problems in the polymerization formula of PVC resin suspension production, wrong process conditions and wide molecular weight distribution, resulting in the failure of plasticization of large molecular weight particles, aging and yellowing of some small molecular weight particles, forming yellow spots and black spots;

(2) The variety, quality and dosage of processing aids;

(3) The operation level of processing technology includes feeding sequence, heating rate and mixing type, drying equipment and process, which will affect the number of crystal impurity points. The processing formula of PVC resin is wrong or the processing conditions are improper. In the process of processing, the temperature, time, impurities, the amount and properties of additives used in processing and the problems of processing equipment are related to the generation of crystal impurity points. During the plasticization process, it is difficult to plasticize due to no absorption of plasticizer, so as to form crystal points. Some particles with large relative particle size and high degree of polymerization can form crystal points.

1.2 bubbles in PVC transparent sheet

The main reasons for bubbles are that there are many volatile components with low molecules in the material, the moisture content of resin and additives may increase due to moisture, the gas in the melt cannot be well excluded during processing, the residual material between rollers is not well adjusted, the temperature is too low, and the external lubrication amount is too high, resulting in high viscosity of molten material, and the volatile components cannot be completely discharged in the melt.

1.3 the color of PVC transparent sheet is yellow

The molding temperature range of PVC is very narrow. The melting temperature is 136 ℃, and it begins to decompose at 140 ℃. Therefore, it is necessary to add stabilizer to improve its thermal stability and plasticizer to reduce its forming temperature. There is a problem of adding process control in both sheet forming methods.

The main reasons for the yellowing of sheet color are:

(1) The processing temperature of PVC transparent sheet is high, the resin has no good stability, the whiteness and aging whiteness are low in performance indexes, and the double bond content of PVC resin is high. The amount of stabilizer in the formula is insufficient. The main reason for this phenomenon is that the manufacturer controls the product cost and reduces the amount of additives, resulting in quality problems in the processing process.

(2) The processing equipment has the problem of temperature control. When the displayed temperature is lower than the actual temperature, the PVC material will decompose, which will also make the PVC sheet yellow.

(3) In order to reduce the cost, some manufacturers mix type 5 powder with type 6 powder or even type 7 powder, resulting in too wide molecular weight distribution. In order to achieve high molecular weight plasticization in the processing process, under the condition of high operating temperature, the low molecular weight part has been decomposed, resulting in yellowing of the sheet.

(4) In the process of equipment processing, there are dead corners in the molten logistics channel, resulting in the decomposition and yellowing of molten plastics due to too long residence time, and occasional outflow in the production process, resulting in yellowing. For example, in the calendering method, the extruder head, the edges and corners on both sides of the calendering roll that are not cleaned in time, the dead corners and turnover heads that are not cleaned by the plasticizer, the dead corners on both sides of the die head of the extrusion method, the screw grooves and flow channels that are not reasonably designed, etc.

1.4 flow pattern (water ripple)

The flow pattern of transparent sheet, also known as flow mark or water ripple, is a process defect in calendering production. Due to the rotation of residual materials between rolls and the anisotropy of plastic viscous flow, the flow and flow velocity in the roll width direction in the roll gap are inconsistent, resulting in the formation of flow pattern on the sheet surface after passing through the sheet exit roll due to the memory effect of high scores. The extrusion method does not have this phenomenon, because the extrusion method uses the sheet quenching process, so that the polymer of the molten material has been frozen without orientation, so as to show a surface consistent with the roller surface finish. The product surface has no flow lines or very slight, and the cleanliness is very high.

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2. Solution to production problems of PVC transparent sheet

2.1 for the problems of crystal impurity points and black and yellow points

(1) Mixing formula and process: fine particles of PVC resin are evenly mixed with additives in the high-speed mixer, and at the same time, water and volatile matter are volatilized. The surface of PVC resin is covered by the melt of low melting point additives, and even part of the melt is absorbed by PVC resin. Plasticizer penetrates into the interior of PVC resin particles, Properly extending the mixing time and increasing the mixing temperature can effectively improve the crystal point problem. Due to the extension and increase of the mixing time and mixing temperature, the additives will be more uniform. The mixing speed should be switched between high speed and low speed to prolong the curing time of the mixture;

(2) In addition, adding a small amount of ACR to the PVC transparent sheet material can effectively accelerate the melting of the material, improve the melt strength, smooth the melt, strengthen the fluidity, reduce the material adhesion, decompose the sintering, significantly reduce the crystal point, and improve the physical properties of the sheet and the extrusion production efficiency;

(3) If the problem is the equipment and extrusion process, the screw shear force and head pressure can be increased, and the set temperature of the barrel in the melting zone can be increased. There are various extruders for PVC resin processing, such as single screw extruder, twin-screw extruder, planetary extruder, etc. the extrusion process includes head pressure, traction speed and extrusion temperature. The processing formula and auxiliary varieties of products are generally specific and general. Because the product type, technical index and proportion in the formula of auxiliary need to be set for specific processing, Therefore, the determination of processing equipment, additives and raw materials should have a specific formula. In the calendering method, because there are multiple roller gaps, appropriately increase the surplus material, and effectively improve the plasticizing effect due to the shear effect between the two rollers.

2.2 solution to bubble problem

In case of bubbles in the sheet, the mixing process adjustment test shall be carried out. The mixing mode shall be high and low speed mixing. The mixing time shall be appropriately extended and the discharge temperature shall be increased to remove moisture from the material. After adjusting the mixing process, the volatiles in the materials can be fully discharged by prolonging the mixing time of the materials. If the water content of raw materials is high, the main measures are to prolong the mixing time of materials or replace materials. Some additives will have incomplete reaction and high volatile content in production. Such raw materials should be avoided in production. In the calendering method, the amount of plasticizer or lubricant should be increased or reduced to reduce the melt strength, so that the low molecular volatile can be discharged smoothly when the molten material is sheared between rollers.

2.3 sheet yellowing strain

By comparing the PVC resin with the normal resin, the whiteness of the resin and the aging whiteness of the resin can be observed, and the problems can be analyzed. The cause of yellowing can be judged by detecting the iron ion content of the PVC resin, because the high iron ion content of the resin will affect the color of the product, resulting in adverse phenomena such as yellowing or darkening of the sheet. Therefore, the lack of stabilizer is the main reason for the yellowing of sheet color, and the main solution is to replace the resin. Increasing the amount of stabilizer in PVC formula can effectively reduce the impact of processing process fluctuation on product quality, regularly overhaul extruder and other equipment, and calibrate the temperature control of each area.

In order to reduce the generation of hydrogen chloride, prevent the formation of conjugated multi dilute structure, increase the smooth progress of extrusion processing and prolong the service life of products, a heating stabilizer can be added. The heat stabilizer can stabilize the chlorine atoms on the carbon chlorine chain in PVC. The heat stabilizer should not only have excellent thermal stability and good compatibility, but also have no effect on the physical properties of the products. Granular materials with good compatibility with PVC shall be selected to prevent agglutination. In order to promote the decomposition of materials, the mixing process can be checked. When selecting granular materials, it shall be dominated by strong coloring force, good dispersion and good cushion resistance. Adding lubricant to PVC plays a significant role in improving the surface properties of products. It is mainly divided into two kinds: internal lubrication and external lubrication. Internal lubrication can obviously help reduce friction between materials, increase the gelatinization of powdery materials in the cylinder, increase melt fluidity, reduce equipment load, and enhance extrusion processing ability. The function of external lubricant is to reduce the adhesion of materials, reduce the adhesion of materials to the surface of screw, barrel and die lip, decompose and sinter the materials in the extruder, resulting in the improvement of smoothness and transparency of transparent sheets.

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Summary

PVC transparent sheet calendering method and extrusion calendering method will encounter various problems in the production process, but the two methods have the same treatment and formula of raw materials. Due to the differences in subsequent process equipment, the treatment methods are different. All kinds of problems should be carefully studied to find out the root causes of the problems and deal with them in time, In addition, in the production process, the raw materials, equipment and environment should be in strict accordance with the requirements of operating procedures and operating specifications, so as to effectively avoid the quality problems of PVC transparent sheets. At the same time, preventive measures should be taken according to theoretical knowledge to reduce the possibility of problems.

Qingdao Sainuo Chemical Co., Ltd. We are manufacturer of  PE wax, PP wax, OPE wax, EVA wax, PEMA, EBS, Zinc/Calcium Stearate…. Our products have passed the REACH, ROHS, PAHS, FDA testing. 

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