PVC Heat Stabilizers: Enhancing the Thermal Stability and Performance of Polyvinyl Chloride
Polyvinyl chloride (PVC) is a widely used thermoplastic polymer, valued for its versatility, durability, and cost-effectiveness. However, PVC is susceptible to degradation when exposed to heat during processing or in high-temperature applications. This degradation can lead to discoloration, loss of mechanical properties, and the release of harmful hydrogen chloride gas. To overcome these challenges, PVC heat stabilizers are employed to enhance the thermal stability and performance of the polymer. This essay will discuss the role of PVC heat stabilizers, their types, and their impact on the properties of PVC products.
PVC heat stabilizers are additives that prevent or minimize the degradation of PVC during processing or in high-temperature applications. They work by either scavenging the hydrogen chloride (HCl) released during degradation or by promoting the formation of cross-links between PVC chains, which improves the polymer’s thermal stability. The choice of heat stabilizer depends on the specific PVC application, processing conditions, and desired product properties.
There are several types of PVC heat stabilizers, including metal soaps, organotin compounds, epoxy-based stabilizers, and mixed metal stabilizers.
Metal soaps: Metal soaps, such as calcium and zinc stearates, are the most commonly used PVC heat stabilizers. They act as HCl scavengers, reacting with the released HCl to form stable metal chlorides. Metal soaps are cost-effective and provide good thermal stability, but they may have limited performance in high-temperature applications or when exposed to moisture.
Organotin compounds: Organotin stabilizers, such as dibutyltin maleate and dibutyltin laurate, are highly effective in promoting the thermal stability of PVC. They work by both scavenging HCl and promoting cross-linking between PVC chains. Organotin stabilizers are particularly suitable for applications that require excellent transparency, electrical properties, and long-term heat resistance. However, their use is being phased out due to environmental and health concerns.
Epoxy-based stabilizers: Epoxy-based stabilizers, such as epoxy resins and epoxidized vegetable oils, are used in combination with metal soaps or other stabilizers to enhance the thermal stability of PVC. They work by reacting with the HCl released during degradation and forming cross-links between PVC chains. Epoxy-based stabilizers are particularly effective in improving the heat stability of PVC in high-temperature applications and in the presence of moisture.
Mixed metal stabilizers: Mixed metal stabilizers, such as calcium-zinc and barium-zinc systems, are a newer generation of PVC heat stabilizers. They offer several advantages over traditional stabilizers, including improved thermal stability, reduced environmental impact, and better performance in specific applications. For example, calcium-zinc stabilizers are widely used in PVC pipes, window profiles, and cable insulation due to their excellent long-term heat resistance and low extractability.
The choice of PVC heat stabilizer has a significant impact on the properties and performance of the final product. For instance, the type and concentration of stabilizer used can influence the polymer’s thermal stability, color, mechanical properties, and resistance to weathering. Therefore, it is crucial to carefully select and optimize the stabilizer system based on the specific requirements of the PVC application.
In addition to traditional heat stabilizers, there is ongoing research into developing new and advanced stabilizer systems for PVC. These efforts aim to improve the thermal stability and performance of PVC while addressing environmental and health concerns associated with conventional stabilizers. For example, researchers are exploring the use of bio-based stabilizers, such as vegetable oil-derived epoxies, and nanoparticle-based stabilizers, such as layered double hydroxides, to enhance the sustainability and performance of PVC products.
In conclusion, PVC heat stabilizers play a crucial role in enhancing the thermal stability and performance of polyvinyl chloride. By preventing or minimizing the degradation of PVC during processing or in high-temperature applications, these additives enable the production of durable and versatile PVC products. The choice of heat stabilizer depends on the specific application requirements, and ongoing research in the field holds promise for improving the performance and sustainability of PVC products.
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The effect of catalysts on foaming
Polyether, as the main raw material, reacts with isocyanate to form carbamate, which is the skeleton reaction of foam products. With the same functionality, the tensile strength, elongation and resilience of foam increase with the increase of molecular weight, while the reactivity of similar polyethers decreases; In the case of the same equivalent value (molecular weight/functionality), with the increase of functionality, the reaction will be relatively accelerated, and the crosslinking degree of the generated polyurethane will be increased, the hardness of foam will be increased, and the elongation will be decreased. The average energy switching off degree of polyols should be more than 2.5. If the average energy switching off degree is too low, the recovery of foam body after pressure is poor.
If the amount of polyether is large, it is equivalent to the reduction of other raw materials (TDI, water, catalyst, etc.), which is easy to cause cracking or collapse of foam products.
If the amount of polyether is small, the foam products are hard, the elasticity is reduced, and the hand feel is bad.
2. Foaming agent
Generally, when manufacturing polyurethane block foams with a density greater than 21, only water (chemical foaming agent) is used as the foaming agent, and low boiling compounds such as dichloromethane (MC) (physical foaming agent) are used as auxiliary foaming agents in low-density or ultra soft formulations.
The auxiliary foaming agent will reduce the density and hardness of foam. Because its gasification absorbs part of the reaction heat, it will slow down the curing, so the amount of catalyst needs to be increased. Due to the absorption of heat, the danger of burning the core is avoided.
The foaming ability can be reflected by the foaming index (the equivalent of water or water used in 100 parts of polyether): m – the amount of foaming agent used
Foam index IF=m (water)+m (F-11)/10+m (MC.)/9 (100% polyether)
Water, as a foaming agent, reacts with isocyanates to form urea bonds and releases a large amount of CO2 and heat, which is a chain growth reaction.
With more water, the density of foam decreases and the hardness increases. At the same time, the foam pillar becomes smaller and weaker, which reduces the bearing capacity and is easy to collapse and crack. The consumption of TDI increases, releases more heat, and is prone to heartburn. If the water content exceeds 5.0 parts, a physical foaming agent must be added to absorb some of the heat and avoid the occurrence of core burning.
With less water, the amount of catalyst used decreases correspondingly, but the density increases
3. Toluene diisocyanate
Generally, a mixture of TDI80/20, 2,4, and 2,6 isomers is used for soft foam. T100 can be prepared by cooling method, which is pure 2,4TDI.
TDI dosage=(8.68+m water x 9.67) x TDI index. The TDI index is generally 110-120.
When the isocyanate index increases within a certain range, the hardness of foam increases, but after reaching a certain point, the hardness no longer increases significantly, but the tear strength, tensile strength and elongation decrease. The foam forms large pores, the closed pores rise, the resilience decreases, the surface becomes sticky for a long time, and the curing time is long, leading to core burning.
Low isocyanate index will cause foam cracks, poor resilience, poor strength, large compression permanent deformation, and a sense of moisture on the surface.
4. Catalyst
Amine: Generally, A33 is used to promote the reaction between isocyanate and water, adjust foam density, bubble opening ratio, etc., mainly to promote foaming reaction.
Amine: foam products split, and foam has holes or blisters
Less amine: foam shrinks and closes pores, and the bottom thickness of the foam product is produced.
Tin: Generally used is stannous octanoate T-9; T-19 is a gel reaction catalyst with high catalytic activity, mainly promoting gel reaction, that is, late reaction.
Tin excess: fast gelation, increased viscosity, poor rebound, poor breathability, resulting in closed pores. If the dosage is properly increased, a good open cell foamed plastic with looseness can be obtained. Further increasing the dosage makes the foam gradually become compact, so that it shrinks and closes the pores.
Less tin: insufficient gel, resulting in splitting during the foaming process. There are cracks at the edges or top, and there are phenomena of detachment and burrs.
Reducing amine or increasing tin can increase the strength of the polymer bubble membrane wall when a large amount of gas occurs, thereby reducing hollow or cracking phenomena.
Whether polyurethane foam has an ideal open or closed cell structure mainly depends on whether the gel reaction rate and gas expansion rate are balanced during the formation of foam. This balance can be achieved by adjusting the type and amount of tertiary amine catalyst, foam stabilizer and other additives in the formula.
5. Foam stabilizer (silicone oil)
Foam stabilizer is a kind of surfactant, which can make polyurea disperse well in the foaming system, play the role of “physical cross-linking point”, and significantly improve the early viscosity of foam mixture to avoid foam cracking. On the one hand, it has the emulsification effect, which enhances the mutual solubility between the components of foam materials; on the other hand, the addition of organosilicon surfactant can reduce the surface tension r of the liquid, reduce the free energy required for gas dispersion, make the air dispersed in the raw materials easier to nucleate during the mixing process, help to produce small bubbles, adjust the size of foam pores, control the foam pore structure, and improve the foaming stability; Prevent the cell from collapsing and cracking, make the foam wall elastic, and control the foam pore size and uniformity. It stabilizes foam at the initial stage of foaming, prevents foam from coalescing at the middle stage of blasting, and connects the foam pores at the later stage of foaming. The more foaming agent and POP used, the greater the amount of silicone oil used.
More dosage: make the elasticity of foam wall increase in the later period, not crack, and the foam hole is thin. Causing closed pores.
Low dosage: the foam bursts, collapses after starting, and the pore size is large, which is easy to mix.
6. The influence of temperature
The foaming reaction of polyurethane accelerates with the increase of material temperature, which can cause the danger of core burning and fire in sensitive formulas. The temperature of polyol and isocyanate components is generally controlled to remain constant. When foaming, the foam density decreases and the material temperature increases accordingly. With the same formula, the same material temperature, the high temperature in summer and the accelerated reaction speed lead to the decrease of foam density and hardness, the increase of elongation and the increase of mechanical strength. In summer, the TDI index can be appropriately increased to correct the decrease in hardness.
7. The impact of air humidity
With the increase of humidity, the isocyanate based part in the foam reacts with the moisture in the air, resulting in a decrease in hardness. Therefore, the amount of TDI can be properly increased during foaming. When it is too large, it can cause the ripening temperature to be too high and cause heartburn.
8. The influence of atmospheric pressure
With the same formula, the density of foam products is small when foaming at high altitude. Note:
1. In the forming process of foam plastics, gel reaction and foaming reaction occur together, but there is competition among the reactions. Generally, the speed of foaming reaction is greater than that of gel reaction.
Gel reaction – carbamate formation reaction (reaction with – OH)
Foaming reaction – a reaction involving water to generate urea and produce bubbles
2. Nucleating agent – a substance that causes the formation of bubbles, such as fine solid particles or liquids in a system
Foam stabilizer or tiny bubbles dissolved in materials; Including air or nitrogen, carbon dioxide, foam stabilizer, carbon black and other fillers dissolved in polyols and isocyanates. But gas generates more bubbles in the material; The more stable and generated bubbles, the finer the pores.
The number of bubbles formed in the foaming system and the size of the bubbles in the foam plastic depend on the role of the external nucleating agent; There are many nucleating agents, many bubbles, and small pores.
When the temperature rises, the solubility of gas in the liquid decreases, resulting in more bubbles forming or causing the previous start to grow. Long milky white time is beneficial for the growth of large bubbles.
The opalescence time can be shortened by increasing the amount of catalyst. Because of the competitive reaction between gel reaction and bubble formation, microporous foam can be obtained.
3. Whether foam has an ideal open or closed cell structure mainly depends on whether the gel speed and gas expansion speed are balanced during the formation of foam. This balance can be achieved by adjusting the type and amount of tertiary amine catalyst, foam stabilizer and other additives in the formula.
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