1. Plasticizer function and mechanism
Plasticizer refers to a class of substances that can improve the rationality of polymers. Plasticizers are mainly used in PVC resins, and their usage can account for more than 98% of the total plasticizer usage. The addition of plasticizers can reduce the interaction between PVC molecular chains, reduce the glass transition temperature, flow temperature and melting point of the contained microcrystals of PVC plastics, and improve the plasticity of the resin, making the products soft and resistant to low temperatures.
When the plasticizer is less than 10 parts, the effect on mechanical strength is not obvious. When about 5 parts of plasticizer are added, the mechanical strength is the highest, which is the so-called anti-plasticization phenomenon. It is generally believed that the anti-plasticization phenomenon is that after adding a small amount of plasticizer, the activity of the macromolecular chain increases, so that the molecules are ordered and produce microcrystal effects. The impact strength of hard products with a small amount of plasticizer is smaller than that without plasticizer. However, after a certain dosage, the impact strength increases with the increase of dosage, which meets the universal law. In addition, with the addition of plasticizer, the heat resistance and corrosion resistance of the product decrease. For each additional portion of plasticizer, the Martin heat resistance decreases by 2~3. Therefore, generally, hard products do not add plasticizer or add less plasticizer. Sometimes, a few portions of plasticizer are added to improve processing fluidity. Soft products need to add a large amount of plasticizer. The larger the amount of plasticizer, the softer the product.
The total amount of plasticizer should be different according to the softness requirements of the product and the use, process and use environment. Generally, the total amount of plasticizer used in the production of PVC film by calendering process is about 50 parts. Blown film is slightly lower, generally 45~50 parts.
The main functions of plasticizer are as follows:
1) Reduce the melting temperature and melt viscosity of the polymer, thereby reducing its molding processing temperature;
2) Make the polymer product soft, elastic and resistant to low temperature.
Plasticizers and resins have different polarities and different mechanisms of action.
Introduction to PVC technical knowledge (39-44) Comprehensive performance of PVC plasticizers
1) Volume effect This effect is mainly for non-polar plasticizers. Its mode of action is to insert into the middle of the resin molecular chain, increase the distance between molecules, thereby weakening the interaction between molecules, reducing melt viscosity, and increasing the flexibility of the molecular chain. The more such plasticizers are added, the greater their volume effect, and the volume effect of long-chain plasticizers is greater than that of ring-shaped plasticizers.
2) Shielding effect This effect is mainly for polar plasticizers. Its mode of action is to replace the polar attraction between polymer molecules with the interaction between polar plasticizers and polar polymers, thereby weakening the interaction between molecules. This plasticizing effect is called shielding effect.
2. Performance of plasticizers
The performance of plasticizers mainly includes the following items: plasticizing efficiency, compatibility, cold resistance, durability, heat resistance, insulation, flame retardancy and hygiene, etc.
2.1. Plasticizing efficiency
Refers to the plasticizing ability or plasticizing effect of plasticizer on polymer. The plasticizing efficiency of different plasticizers is a relative concept. Plasticizing efficiency is generally evaluated by the amount of plasticizer added to change the quantitative flexibility index. The less plasticizer is added, the higher the plasticizing efficiency. Plasticizers with general molecular weight have higher plasticizing efficiency for PVC than plasticizers with high molecular weight. As the polarity of plasticizer molecules increases, the degree of alkyl branching and the increase of aromatic ring structure will reduce the plasticizing efficiency.
The order of plasticizing efficiency of commonly used plasticizers is: DBS>DBP>DOS>DOA>DOP>DIOP>M-50>TCP>PCl-50.
2.2. Compatibility
Refers to the compatibility of plasticizers with polymers. The same plasticizer has different compatibility with polymers of different polarity.
The order of compatibility between common plasticizers and PVC is: DBS>DBP>DOP>DIOP>DNP>ED3>DOA>DOS>chlorinated paraffin.
2.3. Cold resistance
Refers to the low temperature resistance of plasticizer plasticized products. The cold resistance of plasticizers is related to their structure. Aliphatic dibasic acid esters with methylene (—CH2—) as the main body have the best cold resistance and are the most commonly used cold-resistant plasticizers. Plasticizers with cyclic and branched structures have excellent cold resistance because they are difficult to move in polymers at low temperatures.
The order of cold resistance of common plasticizers is: DOS>DOZ>DOA>ED3>DBP>DOP>DIOP>DIDP>DNP>M-50>TCP.
2.4. Durability
Refers to the ability of plasticizers to persist in plasticized products. Mainly includes migration resistance, extraction resistance and volatility resistance.
2.5. Migration resistance
The migration of plasticizers includes two aspects: one is the plasticizer seeping from the inside of the product to the external surface; the other is the plasticizer transferring to the solid in contact with it. The migration of plasticizers is related to their compatibility. The worse the compatibility, the greater the migration. In addition, plasticizers with large molecular weight and branched or cyclic structures have good migration resistance.
2.6. Extraction resistance
Refers to the tendency of plasticizers to diffuse into the liquid medium in contact with it. The media mainly include water, solvents, detergents and lubricating oils. In terms of oil resistance and solvent resistance, plasticizers with a large proportion of non-polar alkyl groups have poor extraction resistance, while plasticizers with phenyl, ester groups and high branching degrees have good extraction resistance; while water resistance is just the opposite of the above. Polyesters are plasticizers with excellent extraction resistance.
2.7. Volatility resistance
Refers to the tendency of plasticizers to diffuse from the surface of the product into the air when heated. The volatility of plasticizers is related to their molecular weight. Generally, the smaller the molecular weight, the greater its volatility. From the perspective of molecular structure, plasticizers with straight-chain alkyl structures have better volatility resistance than those with branched alkyl structures; plasticizers containing large-volume groups such as rings in the molecule have better volatility resistance. Generally, plasticizers such as polyesters, epoxies, DIOP, TCP and pentaerythritol have good volatility resistance.
2.8. Insulation
The insulation of plasticizers is not as good as that of PVC resins. As the amount of plasticizer added increases, the electrical insulation of PVC decreases. When selecting plasticizers for PVC insulation products, pay attention to their insulation. The order of insulation of commonly used plasticizers is: TCP>DNP>DOP>M-50>ED3>DOS>DBP>DOA.
2.9. Hygiene
Refers to the toxicity of plasticizers. Most plasticizers are non-toxic or low-toxic. Epoxy and citrate esters are non-toxic plasticizers, phthalates and dicarboxylic acids are low-toxic plasticizers, but DOP and DOA are suspected of being carcinogenic; most phosphate plasticizers are toxic, with the exception of 2-ethylhexyl diphenyl phosphate; chlorinated paraffin is also a toxic plasticizer.
2.10. Flame retardancy
Phosphate esters and chlorinated paraffins are flame retardant plasticizers, and the order of flame retardancy is: TCP>TPP>TOP>DPOP.