3.2 Handling application characteristics
3.2.1 Technological equipment and mold
A, PVC processing equipment
PVC recycling usually consists of two stages: first stage is mixing, and second is processing.
Commonly used mixing equipment includes an internal mixer, a high and low frequency mixing device, and in some cases an open mixer is used for post-mixing. It should be noted that use of high-low mixing units for mixing is unique to PVC processing, and other materials are usually mixed at high or low blend levels.
Product processing equipment uses flat vulcanizing machine, single screw extruder, twin screw extruder, double-stage extruder, calender, injection molding machine, etc. according to different products.
Despite fact that proportion of calendered products in PVC products is small, due to large investments required, this is not main direction of production. At present, production is still dominated by extruded pipes, blow molding, wire and cable, and injectables.
In general purpose equipment, single screw extruder used for PVC requires use of a mutation screw, purpose of which is to minimize PVC processing shear and reduce chance of PVC overheating and degradation.
When choosing a twin screw extruder for rigid PVC, a conical twin screw extruder should be used, due to high viscosity of rigid PVC, which requires extruder to have a certain back pressure in order to advance material has a higher torque that a parallel twin screw cannot provide. But it's not that parallel twin screws can't produce rigid PVC, it's that there is a risk of screw breakage.
There is another option for PVC production, namely use of a two-stage extruder. The two-stage extruder uses a combination of twin screw mixing and single screw extrusion, which makes it possible to combine mixing and pressure extrusion seamlessly. best choice for PVC production.
Especially in production of PVC cable materials, in order to obtain excellent materials, length of extruder screw should be as long as possible.
Regarding injection molding machines, due to high viscosity of PVC and large amount of filler commonly used in PVC, injection molding machines for production of PVC injection molding products must provide higher back pressure than conventional machines for injection molding. Therefore, most of injection molding machines that produce rigid PVC are specialized injection molding machines.
B. Featureand forms
When it comes to characteristics of PVC molds, let's talk about characteristics of extrusion molds first. Here we should talk about unique ability of PVC to be melt processed. Highly syndiotactic PVC, in which chlorine atoms are arranged alternately on either side of plane of carbon atoms, can form an almost infusible crystalline structure. Therefore, some small syndiotactic sequences and some fringed microvesicle crystals are found in industrial grade PVC (Fig. Crystalline region, intermicellar region is an amorphous region. Micelles are composed of many neatly arranged polymer chain segments, and their length is much less than total length of polymer chains , so polymer chain can pass through several micellar and intermicellar regions.This structure is very similar to situation when yarn is randomly tied into bundles (Fig. 2.15).
Figure 2.14. Microbeams with tassels
Figure 2.15. Schematic diagram of understanding microbes with brushes
This structure will not fully melt PVC during processing. Because PVC cannot be completely melted during processing, it flows like a bundle of tens of millions of molecules. The primary particle stream cell has a diameter of approximately 1 µm. This property is very rare in world of polymers, where most polymers flow as completely molten molecules. This feature has some unique advantages in PVC extrusion die design. Other polymers have greater elastic recovery and swell at lower melt temperatures, while PVC compounds have less swelling. In fact, swelling of PVC compounds is so low that cavity opening is sometimes cut to exact size of desired profile. That is, if melting temperature of PVC particles is not too high, they can only melt to size of primary particles or agglomerates, and PVC melt is composed of some particles. Thus, particles cannot be oriented during flow of melt in tube, and outlet does not expand. The general melt is oriented due to velocity gradient during flow in pipe, and due to entropy elasticity of polymer, oriented polymer deorients and expands upon exiting it.
See rice. 3.16 for a schematic representation of description.
Figure 3.16, Schematic diagram of PVC particle processing
That is, when designing a PVC extrusion die, it is not necessary to consider melt expansion, so a more accurate die design can be developed.
Injection mold. There is no difference between structure of mold and ordinary injection mold, difference is mainly design of gate and runner, but it should be noted that due to fluidity of PVC, it is difficult to form large-scale injection molding products, so when choosing materials, even if PVC fully meets needs of product, but it cannot be chosen as a material for larger injection molding products. All common gates in PVC molds can be used. Pin sprues or dip sprues are best for small parts, fan sprues are best for thicker parts. The minimum diameter of a point or immersion valve must be 1 mm, thickness of a fan valve must be at least 1 mm. The sprue should be as thick and short as possible.
Depending on amount of plasticizer used, PVC products are divided into three categories: rigid PVC, semi-rigid PVC and soft PVC. There are many types of PVC products with different hardness. We cannot cover everything here, but we can only present them according to types of products we manufacture and are familiar with. Only representative PVC cable materials, PVC plastic toys and plastic doors and windows are shown.
3.2.2 Characteristics and application of soft and hard PVC
A. Characteristics and application of soft PVC
Soft PVC is mainly made from high molecular weight PVC resin, that is, most of them are made from SG4 resin and above.
Soft PVC contains more plasticizers, it has increased softness, elongation at break, frost resistance, soft texture, good airtightness and water resistance. Although electrical insulation is reduced due to addition of plasticizers, it still retains very high electrical insulation. Reduced brittleness, hardness and tensile strength.
Soft PVC is mainly used in production of wire and cable, agricultural film, packaging film, industrial film, hose, artificial leather, plastic shoes.
Etc. Due to advantages of PE film over PVC film in processing and anti-aging, and development of resin technology and processing technology, agricultural PVC film is gradually leaving market.
For processing of soft PVC, extrusion, calendering and other processing methods are mainly used, and a small part is processed by injection molding.
B. Characteristics and application of rigid PVC
Most of production of rigid PVC isare made of low molecular weight PVC, and mainly use PVC resin grade SG5 and below.
Rigid PVC does not contain or contains a small amount of plasticizer, has good tensile, bending and compression properties, therefore it can be used as an independent structural material, for example, corrugated board for construction, doors and windows. , and wall decorations. please wait.
PVC has excellent chemical resistance and can be used as chemical anti-corrosion materials; It is mainly used for anti-corrosion pipes, pipe fittings, oil pipelines, centrifugal pumps, blowers, etc. PVC rigid boards are widely used in chemical industry for lining various tanks.
Excellent electrical insulating properties, suitable for insulating materials, therefore used in plugs, switches and sockets.
Liquid resistance is low. Experiments have shown that PVC-U pipe wall is very smooth and liquid resistance is very low. Its roughness coefficient is only 0.009. Concrete pipes increase by 40%. Therefore, PVC pipes and fittings were once in vogue, however, due to high and low temperature resistance of PVC pipes, with development and use of PPR pipes, PEX pipes, aluminum-plastic composite pipes and other pipes, PVC pipes have been removed from production. In most applications, but due to price factor, PVC pipes still have a significant market in some areas.
At present, amount of rigid PVC is about 70% of total PVC production, but most of them use extrusion technology to process products. Due to poor flowability of PVC, it cannot be introduced into large-sized products, which also limits some PVC applications.
Speaking of this, some people may wonder why soft PVC mainly uses SG4 and above resins, while hard PVC mainly uses SG5 and below resins. Why do you use PVC resins in this way?
This is because:
A. Generally speaking, PVC dehydrochlorination temperature has nothing to do with molecular weight. Because high molecular weight PVC has a higher melting point, more plasticizers must be added during processing to lower its melting point below its decomposition temperature. • Adding too much plasticizer will reduce hardness of product. On other hand, low molecular weight PVC can be easily processed with addition of less plasticizer or without it, and only with appropriate amount of stabilizer.
B. In terms of polymerization, when PVC polymerizes, probability of intermolecular chain collision increases with increasing temperature, and rate of transfer to monomer also increases, resulting in a decrease in molecular weight and a higher temperature.The temperature leads to formation of aggregates in combination of primary particles. During solidification, melting occurs, as a result of which number of voids between primary particles decreases. Absorbs less when absorbing plasticizers. Plasticizers that absorb less are more likely to lead to anti-plasticization, that is, with addition of a small amount of plasticizer, although mobility of macromolecular chain increases, crystallization of syndiotactic PVC that occurs during polymerization increases. at same time, more plasticizer is not enough to plasticize amorphous region, so that product becomes more solid than without plasticizer.
The interaction of these two effects determines that soft products are made mainly from high molecular weight PVC resins, and hard products from low molecular weight PVC resins. Of course, this use is not absolute, and it is sometimes possible to select a high or low molecular weight resin in terms of performance.
Source: "Practical Guide to Rubber and Plastic Technology"
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