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Description of Polyvinyl chloride(PVC)

By Thomas Huang December 8th, 2023 96 views
Photo of PVC products

I, What is PVC? 

Polyvinyl chloride shorten as PVC. It is a polymer of vinyl chloride monomer (VCM) polymerized by initiators such as peroxides and azo compounds or by free radical polymerization mechanism under the action of light and heat. 
Vinyl chloride homopolymer and vinyl chloride copolymer are called vinyl chloride resin.

1.1 General description

 Color  Yellowish, translucent, and shiny    CAS login number   9002-86-2
 Chemical formula  (C2H3Cl) n
picture of Chemical formula
 Structure   -(CH2-CHCl) n-
picture of STRUCTURE
 Density  1.38 g/cm ³  Melting point  212 ℃
 Softening temperature  85 ℃  Glass transition temperature  87 ℃
 Young's modulus  2900-3400 MPa  Molecular weight  50000 ~ 110000

Industrial PVC has high polydispersity and the molecular weight increases with the decrease of polymerization temperature. 
Without a fixed melting point, it began to soften at 80 ~ 85 ℃, changed into a viscoelastic state at 130 ℃, and changed into a viscous flow state at 160 ~ 180 ℃. 
It has good mechanical properties, tensile strength of about 60 MPa, impact strength of 5~10kJ/m2. 
It has excellent dielectric properties. 
PVC is insoluble in common solvents but expands in monomers and some chlorinated hydrocarbon solvents.

1.2 PVC history and yield

PVC is the third largest synthetic polymer plastic in the world (after polyethylene and polypropylene), with production of about 40 million tons annually.
PVC was once the largest general-purpose plastic in the world with lots of applications. 
PVC was industrialized in the early 1930s. 
The output of PVC has been ranked first in the consumption of plastics for a long time.
In the late 1960s, polyethylene replaced polyvinyl chloride. 
Although PVC plastics retreated to second place, the output still accounted for more than 1/4 of the total plastic output. 
Before the 1960s, the production of monomer vinyl chloride was mainly calcium carbide acetylene, because the production of calcium carbide needed a lot of electric energy and coke, and the cost was high. 
phto of calcium carbide   Calcium carbide is an inorganic compound, chemical formula is CaC2. It is a white crystalline powder. Its industrial products are grayish-black lumps, the cross-section is purple or gray. 
It immediately reacts violently with water, producing acetylene and releasing heat. 
Calcium carbide is an important basic chemical raw material, which is mainly used to produce acetylene. 
It is also used for organic synthesis, oxyacetylene welding, and so on. 
After the industrialization of vinyl chloride production by ethylene oxychlorination in the early 1960s, countries turned to cheaper oil as raw materials. 
In addition, a large part of the raw materials of polyvinyl chloride (about 57% by weight) is the inevitable by-product of chlorine in the alkali industry. It is not only rich in raw materials, but also one of the very important products for the development of chlor-alkali industry and balance of chlorine. 
Therefore, although the proportion of PVC in plastics decreased, it still maintained a high growth rate.

II, PVC structure. 

Polyvinyl chloride (PVC) is a kind of polymer material that gets a hydrogen atom replaced by a chlorine atom in polyethylene. It is an amorphous polymer with a small amount of crystal structure. 
The structure of this material is as follows: (CH2-CHCl) n -. 
PVC is a linear polymer in which most VCM monomers are connected by a head-tail structure. 
The carbon atoms are arranged in a zigzag shape, and all atoms are connected by a σ bond. 
All carbon atoms are sp3 hybridized.
There is a short syndiotactic structure on the PVC molecular chain. 
The tacticity increases along with the decrease in polymerization temperature. 
There are some shortcomings in the macromolecular structure of polyvinyl chlorides, such as head structure, branched chain, double bond, allyl chloride, tertiary chlorine, and so on. 
The shortages can be eliminated after cross-linking.
picture of PVC structure types

III, Types of crosslinking

3.1  Radiation crosslinking:

This type of crosslinking is generated by using high-energy rays and crosslinking auxiliaries.
The high-energy rays are produced by cobalt 60 radiation sources or electron rays accelerated by electrons. It mainly uses electron rays. 
Cross-linking auxiliaries are monomers with two or more carbon-carbon double-bond structures. 
This method requires skillful operation on high-end equipment. 

3.2 Chemical crosslinking:

This type of crosslinking uses triazole di mercapto amine salt (FSH). Its mechanism is the amine and sulfhydryl groups combine to attack the carbon chloride polar bond to carry out a substitution reaction. 
This method could improve product properties, including UV resistance, solvent resistance, temperature resistance, impact toughening, and so on.

IV, PVC main classification.

4.1 According to applications:

4.1.1 General-purpose PVC resin

General-purpose PVC resin is formed by the polymerization of vinyl chloride monomer under the action of initiator. 

4.1.2 High degree polymerization(DP) PVC resin

PVC resin with a high degree of polymerization is a resin polymerized by adding a chain growth agent to the polymerization system of vinyl chloride monomer.

4.1.3 Crosslinked PVC resin

Crosslinked PVC resin is the resin polymerized by adding a cross-linking agent containing diene and Polyene to the polymerization system of vinyl chloride monomer.

4.2 As per the method of obtaining vinyl chloride monomer

4.2.1 Calcium carbide method.

4.2.2 Ethylene method.

4.3 As per the polymerization methods

4.3.1 Suspension method

4.3.2 Emulsion method

4.3.3 Bulk method

4.3.4 Micro-suspension method.

Suspension polyvinyl chloride is the most productive variety, accounting for about 80% of the total PVC output. 
Suspension polyvinyl chloride is divided into six types according to absolute viscosity: XS-1, XS-2. 
to XS-6;XJ-1, XJ-2 to XJ-6 . 
The meaning of the letters in the model: X-suspension; S-loose; J-tight.

4.4 As per the content of the plasticizer

Non-plasticized PVC.  The plasticizer content is 0. 
Rigid PVC with plasticizer content less than 10%; 
Semi-rigid PVC with plasticizer content of 10-30%. 
Soft PVC with plasticizer content of 30-70%. 
PVC paste plastic, plasticizer content is more than 80%. 
The difference between hard PVC and soft PVC is shown in the table.
 Properties   Unit  Rigid PVC  Soft PVC
 Density   g/cm3  1.3–1.45  1.1–1.35
 Thermal conductivity   W/(m·K)  0.14–0.28  0.14–0.17
 Yield strength   psi  4500–8700  1450–3600
  MPa  31–60  10.0–24.8
 Young's modulus    psi  490000  —
  GPa  3.4  —
 Bending strength   psi  10,500  —
  MPa   72  —
 Compressive strength   psi   9,500  —
  MPa   66  —
 Coefficient of thermal expansion (linear)  mm/(mm·°C)  5×10−5  —
 Vicat softening temperature  °C  65–100  —
 Resistivity  Ω·m  10^16  10^12–10^15
 Surface resistivity  Ω  10^13–10^14  10^11–10^12

V, Preparation of PVC.

5.1 Routes for the Synthesis

5.1.1 Crude oil route:

Crude oil (ethylene) produces dichloroethane (EDC)→ vinyl chloride (vcm) and polyvinyl chloride (PVC) from ethylene. 

5.1.2 Coal routes:

Coal →calcium carbide→acetylene →vcm→polyvinyl chloride (PVC). 

5.1.3 Natural gas acetylene route:

Natural gas→acetylene→vinyl chloride (vcm) →polyvinyl chloride (PVC)

5.2 preparation method.

Polyvinyl chloride can be made from ethylene, chlorine, and catalyst by substitution reaction. 

5.2.1 Suspension polymerization: introduction
Suspension polymerization is a mature process. The typical process of suspension polymerization is to add non-ionic water and suspending agent to the polymerization kettle, seal the polymerization kettle after adding the initiator, remove oxygen, and add monomer vinyl chloride (VCM) for polymerization. Overview of the polymerization process.
At room temperature, vinyl chloride (cm) is a gas. Gas-liquid coexists under sealing and self-pressure. 
Suspension polymerization is when the liquid VCM is dispersed into droplets and suspended in the aqueous medium dissolved with dispersant under the action of stirring. Each droplet is equivalent to a small bulk polymerization system. The initiator dissolved in vinyl chloride monomer decomposes into free radicals at the polymerization temperature (45-65 ℃), initiating VCM polymerization. 
The suspension polymerization of industrial PVC is carried out according to the mechanism of free radical polymerization, including elementary reactions such as chain initiation, growth, transfer termination, etc. 
During the polymerization of PVC, the macromolecular radical was significantly transferred to the monomer chain, which became the main elementary reaction to determine the molecular weight of PVC resin. Basic formula.
It is composed of vinyl chloride monomer (VCM), water, oil-soluble initiator, dispersant, pH regulator, molecular weight regulator (mainly used for a low degree of polymerization varieties), anti-sticking agent, defoamer, and so on. 
According to the different requirements of loose and compact polychlorenes, the ratio of water to monomer in the formula varies from 1.2:1 to 2: 1. 
Most of the initiators are organic peroxides and azo compounds, such as diisopropyl bicarbonate peroxide, dicyclohexyl bicarbonate peroxide, diethylhexyl peroxide dicarbonate, azo diisoheptyl nitrile, azo diisobutyronitrile, and so on. 
Dispersants include gelatin, polyvinyl alcohol, methylcellulose, hydroxyethyl cellulose, and so on. Factors affecting the quality of PVC.
The quality of PVC resin is characterized by particle size, particle size distribution, molecular weight, molecular weight distribution, apparent density, porosity, fisheye, thermal stability, color, impurity content, powder-free fluidity, and so on. 
photo of check PVC Degree of polymerization (DP).
In the polymerization of vinyl chloride, transfer to monomer is the main chain termination mode, so the DP of PVC (600 ~ 1600) has nothing to do with the concentration of the initiator, but is only controlled by temperature (45 ~ 65 °C). The temperature fluctuation should be controlled within 0.2 ~ 0.5 °C. The amount of initiator mainly regulates the polymerization rate.
As the heat transfer performance of the polymerizer is good, people mainly choose highly active initiators such as peroxide carbonate with a dosage of 0.02% to 0.05%. 
If the combination of high activity and low activity initiator is used properly, for example, the half-life is 2 h, it is expected to be close to a uniform reaction. 
The uniform reaction is beneficial to the control of heat transfer and temperature. Phase state.
Polyvinyl chloride and vinyl chloride is a partially miscible system, forming two phases: 
One is the rich phase of polyvinyl chloride swollen with vinyl chloride (about 30%), which becomes the main place of polymerization. 
The other is a monomer phase dissolved with a small amount of polyvinyl chloride (< 0.1%), which is close to the pure monomer. 
When the conversion is more than 70%, the monomer phase disappears. The system pressure begins to be lower than the saturated swallow pressure of pure vinyl chloride, and vinyl chloride continues to be polymerized in the rich phase of polychloroprene. 
When polymerization reaches 85% conversion, the reaction needs to be ended, so as not to affect the loose structure of the resin particles. Particle morphology.
The properties of dispersants play an important role in the morphology of PVC particles. 
When gelatin is selected, the surface tension of its aqueous solution is larger (68 mN ·m-1 at 25 °C), and a compact resin will be formed. 
When preparing loose PVC, requiring the surface tension to be less than 50 mN ·m-1, we could combine part of the hydrolyzed polyvinyl alcohol (50-55 mN ·m-1) and light propyl methylcellulose (45-50 mN ·m-1), sometimes can add a third component into it. 
Although surface tension can be used as a partial reference for the coordination of composite dispersants, it is necessary to decide as on practical experience. Molecular weight.
Vinyl chloride monomer should be removed from the resin as much as possible. 
The content of free monomer in PVC used for food packaging should be controlled below 1 ppm. 
To obtain the resin with specified molecular weight and molecular weight distribution and to prevent explosive polymerization, the temperature and pressure of the polymerization process must be well controlled. Particle size and Particle size Distribution.
The particle size and particle size distribution of the resin are controlled by the stirring speed and the suspension stabilizer's selection and dosage. Brief description of the production process of PVC
(1)Add the soft water and suspending agent to the polymerization kettle. 
(2)Close the polymerization kettle after the initiator is added. 
(3)Vaccum the air from the polymerization kettle and the oxygen dissolved in the material.
(4)Heat, stir, maintain the temperature at about 50 °C, and keep the pressure at 0.88-1.2MPa after the start of the reaction.  
(5)Reduce the pressure when the conversion rate reaches about 70%. 
(6)Stop the reaction when the pressure drops to 0.13-0.48MPa. 
(7)Extract the unreacted monomer after polymerization.
(8)Strip the slurry and recover the the vinyl chloride monomer. 
(9)Centrifuge the slurry has been extracted from the gas, making the water content of vinyl chloride to be 10% to 15%.
(10)Dry the water content to 0.3% and 0.4% via the dryer.  
(11)Sieve and get the product.
In the polymerizer, the use of a dispersant with small surface tension and dosage, and high-speed stirring in the later stage of polymerization is beneficial to the formation of loose resin particles with large volume, surface expansion, porous surface, and loose interior. 
On the contrary, a compact resin is formed. Operation steps of PVC production process
PVC plant photo
(1) Preparatory stage
 in the cleaned stainless steel polymerizer, add soft water, polyvinyl alcohol, vinyl chloride monomer, and initiator diisopropyl peroxide. 
For the anti-fisheye agent and anti-sticking agent, the jacket is filled with steam to raise the temperature in the polymerizer to 50-60 to carry out the polymerization reaction. 
Put the industrial water into the jacket to adjust the polymerization temperature after the exothermic phenomenon starts the reaction. 
(2) Reaction and Separation
85% to 90% of the monomer vinyl chloride is polymerized and gets into a settling tank by the effect of pressure in the kettle after  6-9 hours of the reaction. 
The residual vinyl chloride in the kettle is recovered to the gas tank by irrigation and exhaust methods. The monomer vinyl chloride is recovered after the resin-settling liquid enters the sedimentation tank. 
The suspension is dehydrated by a centrifuge to obtain a wet resin containing 10% of 15% water, which is fed into an air flow dryer through a spiral feeder and sent to a cyclone separator with hot air of 150-160°C for separation. 
After separation, the water content of the resin is reduced to about 1%. 
(3) Dry and packing
The material column valve is added to the boiling dryer to dry at 100-120°C and the water content is about 0.3%. The overflow material is cooled to 40-50 °C through the cold air tube inhaled by the feeder, and the coarse material is removed by the cyclone separator and packed by a drum sieve. Suspension polymerization process of vinyl chloride.
Water, dispersant, other auxiliaries, and initiators were successively added to the polymerization kettle, emptied, and filled with nitrogen and oxygen, and then monomers were added to polymerize at a predetermined temperature. 
The monomers are suspended and dispersed in the aqueous phase in droplets, and the selected oil-soluble initiators are dissolved in the monomers, and the polymerization is carried out in these droplets. 
The heat of polymerization is absorbed by water in time. 
To ensure that these droplets are beaded in water, suspension stabilizers need to be added. 
The polymerization is carried out in a polymerizer with an agitator. 
The temperature and pressure remain constant during the polymerization process. 
In the later stage, the pressure decreases by 0.1~0.2MPa, which is equivalent to an 80%-85% conversion rate, ending the polymerization. 
If the blood pressure is reduced too much, it will make the resin dense. 
After polymerization, the material flows into the monomer recovery tank or stripper to recover the monomer. 
Then flow into the mixing kettle, wash, and then centrifugal dehydration, drying to get the finished resin. Major equipment.
The polymerization kettle is the main equipment, which is made of a steel kettle body lined with stainless steel or enamel, equipped with an agitator and heat transfer jacket for temperature control, or internal cooling tube, reflux condenser, and so on. 
The volume of the reactor is several cubic meters, more than ten cubic meters, and the maximum has reached 200 cubic meters (kettle reactor). 
The high heat transfer capacity of the polymerizer ensures the constant polymerization temperature, and stirring not only helps to mix materials and heat transfer but also has a significant effect on liquid-liquid dispersion and resin particle properties. 
Heat transfer and stirring are two major engineering problems in vinyl chloride polymerization. 
The polymerizer should be descaled after being used many times.

5.2.2 Emulsion polymerization:

One of the earliest methods of industrial production of PVC. 
photo of PVC paste resin
Emulsion polymerization is a method to produce paste resin, which usually uses a water-soluble initiator, vinyl chloride monomer, water, emulsifier, and nonionic surfactant.
The emulsion is made of water, vinyl chloride monomers, the emulsifiers with surfactants such as sodium alkyl sulfonate.
The initiator can use water-soluble potassium persulfate or ammonium persulfate. The "redox" initiation system can also be used. 
Polyvinyl alcohol is also added as an emulsion stabilizer, dodecyl mercaptan as a regulator, and sodium bicarbonate as a buffer. 
There are three polymerization methods: batch method, semi-continuous method, and continuous method. 
The polymerization product is latex, and the particle size of the emulsion is 0.05 ~ 2 μ m, which can be directly applied or spray dried into powdered resin. 
The polymerization period of emulsion polymerization is short and easy to control. The obtained resin has a high molecular weight and a uniform degree of polymerization, which is suitable for polyvinyl chloride paste, artificial leather, and impregnated products. 
The formula of emulsion polymerization is complex and the impurity content of the product is high.
The relative molecular weight of PVC resin prepared by emulsion polymerization is 12500 to 125000, which is a white powder with fine particles (generally 0.1 ~ 1 μ m). It is loose, odorless, non-toxic, stable to acids, bases, and salts at room temperature, and has good plasticizing properties. It can be mixed with plasticizers and other auxiliaries to form a paste, and the thickening viscosity does not exceed 20% for 24 hours at room temperature. 
The disadvantages are poor electrical insulation, poor transparency, and high product cost. 

5.2.3 Bulk  polymerization: Advantage
This method has the advantages of a simple production process, good product quality, and low production cost.
(1)This method doesn't use water as the medium, nor need any auxiliary such as dispersants. The only materials needed are vinyl chloride and initiators, so the production process is very simple. 
(2)The performance of bulk PVC resin is better than that of suspension PVC resin. 
The resin contains few impurities, the monomer content is less than 1mg/kg, and the particle size distribution is concentrated. 
(3)The structure is regular, the porosity is high and uniform, the amount of adsorbed plasticizer is large and fast, and the plasticized resin mixture is the same as the dry powder, which is easy to store and transport. 
(4)It is also easy to process without a plasticizer. 
(5)The products have good transparency, thermal stability, and electrical insulation. Produce equipment
photo of polymerizer reactor
The polymerization device is special and is mainly composed of a vertical prepolymerizer reactor and a horizontal polymerizer reactor with a frame agitator. 
Because the particle characteristics of bulk PVC are similar to those of suspension resin, which is loose, but without skin film and more crystalline. In addition to heat dissipation and anti-sticking, bulk polymerization also needs to solve the problem of maintaining the loose structure of particles, which is guaranteed by two-stage polymerization. 
(1) The first stage is pre-polymerization, which is carried out in a vertical kettle. 
A small part of vinyl chloride and a limited amount of highly active initiators (such as ethyl peroxide sulfonate) are added to the kettle and prepolymerized to 7%-11% conversion at 50-70 °C to become dead-end polymerization, to prevent excessive conversion. 
Stir quickly to form a loose particle skeleton. 
The conversion rate is estimated by the heat taken away by jackets and condensers. 
(2)The prepolymer, more monomers, and another part of the initiator are added to another low-speed stirring (30r/min) kettle. The monomers continue to polymerize on the pre-formed particle skeleton to make the particles grow and keep the morphology unchanged. 
The polymerization is ended when reaches 70% to 90% conversion,  
The residual monomer is discharged, and the finished product is obtained by crushing and sieving. 
The prepolymerization only takes 1 ~ 2 h, but the polymerization takes 5 ~ 9 h. 
One prepolymerizer can be equipped with several polymerizers. 
The particle size and shape of the resin are controlled by stirring speed, and the reaction heat is taken out by monomer reflux condensation. 

5.2.4 Micro-suspension method.

The micro-suspension method has high production efficiency and stable product quality. 
The particle size of the obtained resin is between suspension and emulsion polymerization resin, which can replace paste resin. 
The dosage of plasticizer absorbed is big, and the product transparency is good. 

5.2.5 Comparison of three polymerization processes:

 Items  Suspension polymerization  Bulk polymerization  Emulsion polymerization  Micro-suspension method
 Formula composition  Monomer, initiator, dispersant, water  Monomer, initiator  Monomer, initiator, emulsifier, water  Monomers, initiators
 Polymerization location  Monomer droplet  Inside the body  Micelle and latex particles  In solution
 Temperature control  Easy  Hard  Easy  Easy
 Polymerization speed  Larger  Medium  High  Low
 Molecular weight control  Medium difficult  Difficult  Easy  Easy
 Production characteristics  Batch  Batch  Continuous  Continuous
 Main characteristics and uses  Suitable for injection molding or extrusion resin  Pure and rigid injection molded products of polymer  Paints, adhesives  Paints, adhesives

5.3 Basis for Process Plan Selection.

photo of engineer designing

5.3.1 Technological process.

(1)The bulk polymerization process is simple, the occupation area is small, and there is no waste discharge, the exhaust can reach the lowest degree, the environmental pollution is less, the product quality is good and the purity is high, especially suitable for transparent packaging materials and cable lines. 
(2) The suspension process is a relatively mature process, which occupies an absolute share of PVC production in the world at present, with a high product conversion rate, the largest variety of products, and ease of adapting to the market. 
(3) The emulsion method is a method to produce paste resin with a complex process, high cost, and poor resin quality. 
(4)Micro-suspension method has the problem of monomer pollution in solvent recovery and residual use, and the high cost is only used for special purposes.

5.3.2 Economic aspects.

Bulk polymerization is the most economical scheme, but the viscosity of the system in the later stage of polymerization is very high, which can only be effectively implemented when there are engineering solutions for stirring and heat transfer in the polymerization reactor. 
If the later processes such as refining, recovery, and drying are included, the economy of each polymerization process decreases in the following order: 
suspension polymerization > bulk polymerization > Micro-suspension polymerization > emulsion polymerization.

VI, New material - modified Plastics.

At present, the total annual demand for modified plastics is about 50 million tons. 
Modified plastics are an important part of the field of new chemical materials. 
Because 1 kg plastic can replace heavier materials such as 2-3 kg steel, fuel consumption can be reduced by 6% -8% for every 10% weight reduction in the car. 
Therefore, the more modified plastics are applied in the automobile, the more energy-saving can be achieved. 
photo of application of modified Plastics.

6.1 Replacing Steel with plastic material of PVC

The application of using internal plasticizing process and additive formula can ensure the mechanical and electrical properties, and improve the flame retardancy of PVC plastic steel. The product can have the advantages of high strength, corrosion resistance, non-flammability, good insulation, lightweight, convenient construction, and so on. This material can replace steel pipe in the electrical wiring system.

6.2 Replacing Wood with plastic material PVC

PVC wood-plastic composite is a new type of composite prepared by a hot pressing process using waste wood fiber and plastics as the main raw materials and appropriate processing aids. 
This product fully meets the concept of recycling renewable resources and petroleum products and is of great significance in alleviating the current shortage of wood and oil resources, serious environmental pollution, and other problems. 

VII, Recycling.

Polyvinyl chloride (PVC) is recyclable and its resin identification code is "3".
picture of resin identification code is

VIII, Main products and uses of PVC.

 Model  Average polymerization temperature / ℃  Viscosity number/(mL/g)  K value  Degree of polymerization P  Reference use
 SG-1  482  154-144  77-75  1800-1650  Advanced insulating material
 SG-2  505  143-136  74-73  1650-1500  Insulating materials, general soft products
 SG-3  530  135-127  72-71  1500-1350   Insulating materials, clothes, plastic shoes
 SG-4  565  126-119  70-69  1250-1150  General film, artificial leather, hose 
 SG-5  580  18-107  68-66  1100-1000  High hardness rigid pipe
 SG-6  618  106-96  64-63  950-850  Record, transparent film, hard glue, fiber, welding rod
 SG-7  655  95-87  62-60  850-750  Blow-molded bottles, transparent sheets
 SG-8  685  86-73  59-55  750-650  Peroxyethylene resin

Name Explanation:
Soft water (soft water) refers to water that contains no or less soluble calcium and magnesium compounds.
Hydrogen chloride and hydrochloric acid
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