PVC foamed profiles - unbalanced characterization between various additives

October 11, 2019

PVC foamed profiles - unbalanced characterization between various additives
 

1, stabilizer
      Polyvinyl chloride (PVC) has good acid and alkali resistance, wear resistance, electrical insulation, flame retardant self-extinguishing and low price. It is a widely used thermoplastic. Although PVC has many of the above advantages, it has a fatal disadvantage, that is, thermal stability is poor. PVC usually begins to degrade when heated to 140 ° C, and its processing temperature is about 160-200 ° C. Therefore, heat stabilizer must be added during processing to inhibit the degradation of PVC. At present, the traditional heat stabilizers mainly mainly contain metal ions and heat stabilizers such as lead salts, organotins and metal soaps.
   

 The role of heat stabilizers is:
   1) Metal soaps neutralizing HCl organic acids, basic salts of certain inorganic acids, epoxy compounds, amines, metal alkoxides and phenols, and metal thiolates can be quickly liberated from PVC. The HCl reacts to form a harmless compound that acts as a stabilizing agent. Obviously, such stabilizers only inhibit by accepting and neutralizing HCl and do not prevent the decomposition of PVC.
   2) Replacing unstable chlorine ship atoms Unstable chlorine atoms play an important role in the degradation process of PVC. Therefore, the most effective stabilizer should be substituted with a weak link in the PVC molecule, the unstable chlorine atom, before the decomposition of the PVC, to replace the unstable chlorine atom which is likely to cause degradation to the more stable heat group. Heavy metal hydroxylates and thiolates can play this role. However, further studies have shown that these salts also react with HCl, thereby reducing the effective amount of the substitution reaction; moreover, the formation of heavy metal chlorides will promote the degradation of PVC. Therefore, the synergistic effect of heavy metal hydroxy acid salt and alkali metal or alkaline earth metal hydroxy acid salt and epoxy compound is utilized to improve the stability performance.
   3) reacting with an unsaturated site to form an addition reaction with an unsaturated double bond on the PVC chain by a certain stabilizing group, thereby suppressing the de-HC1 and achromatizing action by opening the double bond of the conjugated polyene. It is another effective stabilization effect. For example, a metal thiolate reacts with HCf to form a thiol, and a thiol can undergo an addition reaction with a double bond to form a stable thioether bond on the PVC chain.
   4) Deuterated impurities A class of stabilizers such as phosphite ternary ester neutralizes or deactivates certain resin impurities, trace metal contaminants, and initiator residues to improve the thermal stability of PVC resin.
A single stabilizer is often not as good as a variety of requirements, and multiple stabilizer combinations are often used. The most desirable composite effect is a synergistic effect.
     

The most widely used stabilizers on PVC foam products are lead salt stabilizers and calcium-zinc stabilizers. With the increasingly strict environmental protection requirements and the higher cost performance of calcium-zinc stabilizers, the application of calcium-zinc stabilizers has begun to change. It is getting more and more extensive. In the extrusion process, the molten material flows far in the mold and has a long residence time. This has high requirements on the thermal stability and fluidity of the formulation. As an outdoor user, it must also have a high degree. Weather resistance, therefore, in addition to excellent long-term heat stabilizers, calcium-zinc stabilizers for sheet materials need to contain high-efficiency auxiliary heat stabilizers for inhibiting initial coloration and light stabilizers for providing weather resistance. 

Insufficient stabilizer, yellow surface, paste, product brittleness, strength reduction, low foaming rate;
If there are too many stabilizers, the foaming agent will decompose in advance, and the gas will overflow from the feeding hole and the vacuum hole, and there will be cracks or shrinkage marks of the cavity structure;

 

2, the function of the lubricant
 

Increase the melt index
   

The melt index of a polymer melt is an important parameter to characterize its processing fluidity. Generally, as the temperature increases, the melt index increases and the flow rate of the melt increases. Due to factors such as the thermal stability of the material, the processing temperature cannot be increased indefinitely. Moreover, lowering the melt viscosity by increasing the processing temperature also increases energy consumption. At this time, adding an appropriate amount of internal lubricant can change the rheological behavior of the resin melt, effectively increase the melt index, achieve the purpose of reducing the melt viscosity and improving the melt fluidity.
 

2. Reduce endogenous heat
  

Endogenous heat is the heat generated by the internal friction of the polymer flow process. In the high-speed processing process, high shear converts a large amount of mechanical energy into heat energy, and the greater the melt viscosity, the stronger the shear force, and thus the more endogenous heat is generated.
   

The undesirable consequences of endogenous heat can cause local overheating of the polymer melt and a decrease in its thermal stability. The addition of an internal lubricant can effectively reduce the melt viscosity and effectively reduce endogenous heat. The possible disadvantage is that low melt viscosity may make processing such as injection molding difficult, and in this case, we should choose a lubricant that is sufficiently high to impart internal structural units to the resin. At the same time, it has a very small lubricant for the solvation of the polymer. Thus, it is possible to improve the processing stability by reducing the frictional heat generation at the internal interface of the resin or dissipating the generated endogenous heat as quickly as possible, and also avoiding problems such as a decrease in the melt viscosity and a decrease in the heat distortion temperature of the product.
 

3. Demoulding action
 

In the later stage of polymer processing, the product is molded. At this time, it is relatively easy to fall off from the abrasive tool to improve the processing efficiency and the quality of the product. At this time, a lubricant having a release action can be added. The lubricant for demolding is generally an external lubricant, mostly a polar compound, which has limited compatibility with the resin and can migrate from the melt to the surface, and the polar group has a certain affinity with the metal surface. This creates a relatively stable, mutually isolated molecular layer between the melt and the metal surface, thereby inhibiting adhesion between the polymer melt and the processing mechanical surface.
 

4. Delay plasticization
 

Delayed plasticization is another manifestation of external lubricity in plastics processing. In PVC processing, PVC resin is melted under shear deformation and uniformly mixed with various additives. In specific cases, the requirements for resin melting are not uniform. For example, in the primary stage of the molding process, it is often undesirable to prematurely melt the resin particles, and sometimes the resin is not required to be completely melted in order to obtain the optimum mechanical properties of the final article, which is particularly noticeable in high impact polyvinyl chloride processing.

 

By blending a less compatible lubricant in the resin, it is possible to migrate to the resin particles or the melt surface at the processing temperature, thereby forming a lubricating layer on the surface of the resin particles or the melt and the surface of the processing machine to produce a good slip. It weakens the shear deformation effect and achieves the purpose of delaying the plasticization of the resin.
 

5. Prevent melt fracture
High viscosity at high shear rates can lead to surface phenomena known as melt fractures because the precipitated material undergoes regular adhesion/sliding transitions within the flow channel.

 

6. Improve the self-lubricity of the product
Many plastic products require good surface lubrication, such as gears. Lubricants with external lubrication can often form a self-lubricating layer by migration or exudation to the surface of the product, resulting in permanent lubrication.

 

7. Slip and anti-blocking effect
For plastic products with large specific surface area, surface adhesion often brings many difficulties to its production or application. Many lubricants with external lubrication migrate to the surface of the product, and can form a uniform distribution of thin coating after cooling. Since the two interfaces are inserted into the separation layer, the surface slidability of the product is imparted without causing surface roughness. This functional additive is called a slip agent.

 

8, external lubricant
     

The most widely used external lubrication on foamed products is paraffin wax and PE wax. Paraffin wax is easy to precipitate, so PE wax is generally used.
     

Insufficient external slip, temperature in the 4 and 5 zones of the extruder is not easy to control, easy to overheat, large bubbles, string bubbles, yellowing, rough surface of the board surface; frictional shear heat increases, causing material decomposition, board surface Yellow, paste;
     

Excessive slippage, poor plasticization, resulting in scaling of the mold cavity and precipitation of the surface of the product, leucorrhea, uneven wall thickness and some symptoms of irregular movement on the surface;
 

9, internal lubricant
     

Commonly used internal lubricants are stearic acid, 60, monoglyceride, 316, and the like.
     

Insufficient internal lubrication, poor material dispersion, uneven plasticization, difficult to control the thickness of the product, the thickness of the foamed sheet is thin on both sides, and there may be leucorrhea, adhesion to the cavity and local overheating;
      

Excessive internal lubrication, brittle foaming products, reduced heat resistance, and converted to external lubrication under certain temperature and melt pressure, resulting in unbalanced lubrication;
     

The internal and external lubrication are insufficient, the melt viscosity is large, the plasticizing torque is large, the melt sticking phenomenon is serious, the material surface has a yellow decomposition line, the surface smoothness is poor, and the mechanical properties of the product are lowered;
     

The inner and outer lubrication are excessive, the plasticizing torque is small, and the melt plasticization is obviously insufficient. Although the product has good smoothness but poor pressure point bonding, it seriously affects the mechanical properties of the product;
     

Less internal lubrication, more external lubrication, significantly longer plasticizing time, reduced plasticizing torque, difficult molding and brittleness;
    

More internal lubrication, less external lubrication, significantly shorter plasticizing time, more serious sticking wall phenomenon, shorter heat stabilization time, and yellow line on the surface of the product;

 

4, calcium carbonate
     

Calcium carbonate has a molecular formula of CaCO3, a molecular weight of 100, is a white powder, colorless, odorless, non-toxic, and has a relative density of 2.71. The melting point is 1339 ° C. Slightly soluble in water, 10% suspension pH 8-10. In case of dilute acid or heating to 825 ° C decomposition, calcium oxide is formed to release carbon dioxide.
 

Calcium carbonate is the most commonly used filler, rich in resources and inexpensive. Calcium carbonate fillers are classified into light calcium carbonate and heavy calcium carbonate. The most commonly used fillers are light calcium carbonate, industrial precipitated calcium carbonate and ultrafine activated calcium carbonate series.
 

Light calcium carbonate is precipitated calcium carbonate, which is calcined into lime by natural limestone, and then digested into water. Carbon dioxide is introduced into the obtained lime milk (calcium hydroxide aqueous emulsion) to precipitate calcium carbonate, and the precipitate is collected. Dried out. The calcium carbonate of the precipitation method has uniform particle size, large specific surface area, and good dispersibility in the polymer base material.
       
Ultrafine calcium carbonate is a special crystalline type of calcium carbonate fine powder with an average particle size of less than 0.4um, all passed through a 325 mesh sieve, the oil absorption is mL40mL/100g, and the moisture is 0.4%.

 

Active calcium carbonate (colloidal calcium carbonate) refers to a calcium carbonate filler obtained by treating with a chemical substance such as a higher fatty acid. The surface treatment of the calcium carbonate with a fatty acid (such as stearic acid, lauric acid, citric acid, etc.) or a solution of a fatty acid salt, an organosilane coupling agent or the like can coat the surface of the filler fine particles with a layer of hydrophobic organic substance. Through modification, the compatibility and bonding force of the filler with the polymer can be improved, the water absorption of the filler itself can be reduced, and the system can be used for the polyurethane sealant, etc., the storage stability can be improved, and the amount of the filler can be increased without lowering the performance of the product.
 

The fine active calcium carbonate has an average particle diameter of um1.0 um, an oil absorption value of 85 to 110 mL/100 g, calcium carbonate (on a dry basis) of 6.096.0%, an activation rate of ≧96%, a moisture content of 0.50%, and a whiteness of ≧90. The fatty acid content was (1.0 ± 0.5)%. The bulk density is 0.40 to 0.60 g/mL, and the specific surface area (BET) is generally 25 to 50 m 2 /g.
 

Heavy calcium carbonate, also known as stone powder, is obtained by subjecting natural limestone to beneficiation, dry pulverization and classification. There are heavy active calcium carbonate, heavy ultra-fine calcium carbonate, heavy fine calcium carbonate and various kinds of fineness (120-600 mesh) of heavy calcium carbonate with whiteness (85-95). The heavy calcium carbonate is roughly divided into single fly powder, double fly powder, three fly powder and four fly powder according to the fineness. Three fly powder is commonly used, its calcium carbonate content is ≧95%, moisture ≦0.5%, and 45um sieve residue ≦0.5%.
   

Foamed products generally use light calcium with a particle size of about 1200 mesh. The calcium powder is easy to absorb moisture and form bubbles, silver streaks, etc. on the surface of the product, affecting the appearance and physical properties, so pay attention to the rainy season storage.
    

When the particle size of calcium carbonate is too large, the mixing is uneven, the plasticizing time of the mixture is delayed, and the screw torque is low;
When the particle size of calcium carbonate is too small, agglomeration easily occurs, and the small particles become large particles, which is similar to the excessive effect of the particles;
    

When the dose of calcium carbonate is too small, the cell lacks a core, the number of cells decreases, and the foaming rate decreases;
    

When the dosage of calcium carbonate is too much, the relative content of the resin in the component is decreased, the melt strength is lowered, and the cross section of the plate is prone to foaming;

 

5, foaming regulator
      

PVC foaming regulator is an acrylate processing aid. It has all the properties of ordinary processing aids. It has unmatched compatibility with other materials of PVC, and can realize the melt plasticization of PVC at a lower temperature. It can accelerate plasticization and form a gel network to improve melt flow and surface gloss. We know that PVC is an amorphous material, and the melt strength decreases as the melt temperature increases, whereas the melt strength decreases as the melt temperature decreases.
      

Can greatly improve the melt strength of PVC materials. The molecular weight of industrially produced PVC is generally between 50,000 and 110,000. The molecular weight of the foaming regulator greatly exceeds the molecular weight of PVC. The PVC molecules can be entangled very well. The dosage of the foaming core can be controlled within the controllable range. The expansion prevents the occurrence of bubbles and gas overflow, and can solidify the bubble body, preventing the bubble body from collapsing and forming a large number of uniform small bubbles. We know that when a large bubble is broken down into innumerable small bubbles, the comparative area of ​​the bubble will increase greatly, and the volume of the product will not change, so the density will be greatly reduced.
     

Of course, foaming regulators are divided into many varieties and are designed for more products. For example, foamed sheets, free-foamed sheets, which are relatively thin, require better fluidity and lower melt strength. In the selection, it is necessary to select a foaming regulator with faster plasticization and a slightly lower melt strength. The crust foaming cabinet board has a relatively high melt strength requirement, and a foaming regulator having a large melt strength should be selected. For the foaming regulator, the plasticizing is fast and the melt strength is a contradiction. The plasticizing is fast and the melt strength is large, which is bound to have a better effect on the foamed product.
     

The foaming regulator generally refers to an acrylate processing aid having a viscosity of 10 or more. Among them, the wood-plastic foaming products such as ecological wood and siding generally use a type of plasticizing, and the foaming board, especially the cabinet board, is generally selected. Plasticizing a foaming regulator with a high slow melt strength.
    

Insufficient foaming regulator, poor melt strength, uneven cell pores, cell openings in the cross section, and increased density;
     

Excessive foaming regulator, too much melt strength, the bubble in the melt can not expand, the product density is high, the surface of the board is prone to wave pattern, distortion and so on;

 

6, foaming agent
    

The most used foaming agent is AC yellow foaming agent and NC white foaming agent. AC will release a lot of heat during the decomposition process, resulting in yellowing of the product, and a small amount of ammonia smell in the decomposition; NC will be in the process of decomposition. It absorbs heat and is decomposed and tasteless. Therefore, two kinds of foamed products are generally used in coordination.
     

The amount of foaming agent is insufficient, the foaming ratio is low, the product has less cells, and the density is too large;
     

If the amount of the foaming agent is too much, the melt strength will become low, the product is difficult to form, and the cross section of the foamed sheet is likely to cause cells; when the range exceeds a certain range, the density of the product becomes larger;
     

The yellow foaming agent is more, the white foaming agent is less, and the cross section is prone to large and round cells.
     

Therefore, the amount of various additives has a degree of relationship and mutual constraints, the amount is not good, not the amount is not good, in the production practice must be repeatedly tested, give full play to the coordination effect of various raw materials, find the most Good balance point, fully improve melt strength and foaming efficiency, and extend boot time.

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