3.1.3 Structural features
A, PVC structure
Figure 3.5 PVC Structural Diagram
Figure 3.5 (a) is molecular formula for polymerization of vinyl chloride monomer in PVC, and (b) is a model of three-dimensional structure of PVC. (c) The figure shows planar structural formula of PVC.
In structure of PVC, carbon atoms are arranged in a zigzag pattern, and all atoms are connected by σ-bonds. All carbon atoms are in state of sp3 hybridization.
We usually think that PVC has an amorphous structure, but studies have shown that PVC molecular chains contain short-range syndiotactic structures, and other parts can be considered as random structures. It is this short range syndiotactic structure that causes polymer to contain 5-10% fine crystallites. The existence of these 5-10% crystallites should not be underestimated. It is presence of this small amount of crystallites that makes characteristics of PVC different from other plastics. The reason why PVC has ability to absorb a large amount of plasticizers and can maintain a certain degree of serviceability after absorbing a large amount of plasticizers is due to existence of this small amount of microcrystals. These crystallites act as crosslinks holding polymer together, property that gives PVC its exceptional creep recovery and is responsible for "rubber-like" properties of plasticizers.
B, PVC classification
Since PVC produced by suspension method accounts for 80% of market, and with development of polymerization technology, degree of polymerization of PVC expands up and down, so national standard GB/T5761-2006 "General target polyvinyl chloride by suspension method" "Resin" expands original six categories classification methods up to 10 categories of classification methods. Since PVC resins used in market are mainly loose PVC resins, we will talk about loose PVC resins later.
Depending on viscosity number (or K value, average degree of polymerization) of different types of resins, use of different models is also different. additives. Uses or special cases, relevant information of SG9 has not been reported, so only main applications of SG1-8 are described, and main applications of the classification are shown in Table 3.2.
Table 3.2 Main objectives of SG1-8
The above uses are just a general classification based on intrinsic viscosity of PVC. In specific cases, users must select appropriate grade of resin to use according to specific requirements of their products.
C, PVC characteristics
PVC is one of most productive varieties among general-purpose plastics, has a wide range of applications and, of course, has irreplaceable characteristics.
（a）PVC chemical properties
PVC has an extremely high fire resistance (its oxygen index reaches 43.6) due to Cl content up to 58%, insolubility in water, alcohol, gasoline, gas and low water vapor leakage; any concentration of hydrochloric acid, sulfuric acid up to 90%, nitric acid 50-60% and sodium hydroxide solution up to 20%, have a certain chemical corrosion resistance; quite resistant to salts, but can be dissolved in ether, ketones, chlorinated fats. Organic solvents such as hydrocarbons and aromatic hydrocarbons. In addition, due to presence of side group Cl, light and thermal stability of PVC is poor. At temperatures above 100°C or prolonged exposure to sunlight, it will decompose to form hydrogen chloride and further autocatalyze decomposition and discoloration, resulting in a rapid decrease in physical and mechanical properties.
In industry, PVC resin mainly has an amorphous structure, that is, an amorphous structure, but it also contains some crystalline areas (about 5%-10%), so PVC does not have an obvious melting point.
（b）Physical properties of PVC
Density 1380 kg/m3
Young's modulus 2900-3400 MPa
Tensile strength 50–80 MPa
Elongation at break 20-40%
Notched impact strength 2–5 kJ/m2
Glass transition temperature 87℃
Vicat softening point 85℃
Thermal conductivity 0.16 W/mK
Coefficient of thermal expansion 10-5/k
Heat capacity 0.9 kJ/(kg.K)
Moisture absorption 0.04–0.4 (%)
It can be seen from chemical properties and physical properties of PVC that although PVC has advantages of high strength, its disadvantages are also obvious, especially thermal decomposition temperature of PVC is much lower than processing temperature, which hinders use of PVC, so it is said that PVC without modification has almost no use value.
(c) Mechanical properties of PVC
(1) Effect of different molecular weight on properties
PVC has good mechanical properties, chemical corrosion resistance and fire resistance, and is widely used in various fields of plastic products. There are various requirements for plastic products, but modification alone cannot meet requirements, which encourages PVC manufacturers to develop PVC resins with different molecular weights.(degree of polymerization) to meet market demand. Resins with different degrees of polymerization often have different properties, which are main factors influencing processing and use characteristics. Regarding effect of different molecular weight PVC on properties, study showed that as degree of polymerization of PVC resin increases, mechanical properties such as toughness, tensile strength and elongation at break increase and yield strength decreases. The specific impact is shown in Figure 3.6.
Figure 3.6. The relationship of characteristics of PVC with different molecular weights
(2) Mechanical properties of PVC filled with filler
Figure 3.7. Mechanical properties of PVC filled with calcium carbonate
Filler modification is most basic PVC modifier, such as talc, calcium carbonate, calcined kaolin, barium sulfate, white soot, coal fly ash, red mud, etc., which can be used as PVC filler modifiers. The author used most commonly used calcium carbonate for PVC (1750 mesh, treated with stearic acid) for testing (see Figure 3.7). Other fillers have a similar effect on PVC fill.
(3) Mechanical properties of PVC reinforced with hardener
Commonly used reinforcing additives for PVC include ABS, CPE, MBS, EVA, ACR, NBR, etc. Among these commonly used reinforcing additives, author conducts an applied study of most commonly used properties associated with reinforced CPE PVC (Figure 3.8 ). Other hardeners mixed with PVC have same effect and all increase toughness of PVC matrix causing a significant reduction in tensile strength.
Figure 3.8. Mechanical Properties of CPE Modified PVC
The rheology of a polymer material is most important guideline when choosing a processing technology. PVC is no exception as a polymeric material. So let's talk about PVC processing rheology.
A study of rheological properties of PVC with different degrees of polymerization showed that with an increase in degree of polymerization of PVC resin, mechanical properties such as impact strength, tensile strength and elongation at break increase, and yield strength decreases. ; with an increase in degree of polymerization of PVC resin, plasticization time of material gradually increases
The long balancing moment gradually increases. And apparent viscosity of material is characterized by a pseudoplastic fluid, which decreases with increasing shear rate, and shear stress increases with increasing shear rate; influence of shear viscosity and shear stress is greater; at high shear rates, effect of degree of polymerization of PVC resin on shear viscosity and shear stress becomes smaller, at same shear rates, shear viscosity and shear stress, shear stress decreases as degree of polymerization of PVC decreases. The correlation can be represented in Figure 3.9.
Figure 3.9 Rheological curves of a PVC capillary with different degrees of polymerization
The author conducted a study of rigid PVC, including PVC and various systems of its mixtures (see Fig. 3.10, 3.11). It can be seen from figure that with an increase in shear rate and temperature High, apparent viscosity of any PVC mixing system decreases to varying degrees. Among them, decrease in apparent viscosity with increasing shear rate is much higher than decrease in apparent viscosity with increasing temperature. That is, PVC blend system is a shear-sensitive material; Since decomposition temperature of PVC is close to processing temperature, this point is very important for PVC processing and is a very important factor in selection. equipment and technology for processing of PVC materials.
Figure 3.10, relationship between shear rate and apparent viscosity
Figure 3.11, relationship between temperature and apparent viscosity
Research on soft PVC rheology shows that: PVC plasticized resin melt is a non-Newtonian pseudoplastic fluid, looser structure of resin particles, higher shear sensitivity; as shear rate increases, activation energy gradually decreases. As amount of plasticizer increases, apparent viscosity of system decreases. As amount of filler increases, apparent viscosity of system increases. The influence becomes smaller and smaller. On fig. 3.12 shows relationship between apparent viscosity and shear rate after addition of calcium carbonate filler.
Figure 3.12. Relationship between apparent viscosity and shear rate of soft PVC system after addition of calcium carbonate filler
Studies on addition of various additives to PVC have shown that: processing aids and internal lubricants can promote plasticization of PVC, but external lubricants prolong plasticization time, and amount of metal soap stabilizers based on internal lubricant increases plasticization time. decreases with increasing, and plasticizing time increases with increasing amount of metal soap stabilizer mainly for external lubrication.
Because in processing of PVC, according to different requirements, tri-salt, salt-free or organotin systems are usually chosen as main stabilizer, so processing rheology of various stabilizing systems is also very important. The rheological properties of system, and it was found that viscosity of PVC mixture stabilized with organotin stabilizers is higher than that of stabilizers based on lead salts (see Fig. 3.13), that is, organotin stabilizers are processed under same conditions. PVC stabilized with tin stabilizers is more difficult to process than PVC stabilized with lead salt stabilizers, so try not to use organotin stabilizers in non-critical cases.
Figure 3.13, relationship between shear rate and apparent viscosity of various stable systems
The above analysis of PVC processing rheology is a very important guide for selecting PVC with different degrees of polymerization according to customer requirements in actual processing and production, applying best processing technology and formulating with most suitable requirements and lowest value of material costs.
Source: "Practical Guide to Rubber and Plastic Technology"
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