Plasticizers refer to a class of substances that can improve the rationality of polymers. Plasticizers are mainly used for PVC resins, and their amount can account for more than 98% of the total amount of plasticizers. The addition of plasticizers can reduce the force between PVC molecular chains and reduce the glass transition temperature, flow temperature and melting point of the contained microcrystals of PVC plastics. Plasticizers can increase the plasticity of the resin and make the product soft and resistant. Good low temperature performance.
When the plasticizer is less than 10 parts, the mechanical strength is not obvious. When about 5 parts of the plasticizer is 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 a small amount of plasticizer is added, the mobility of the macromolecular chain increases, so that the molecules are ordered to produce the effect of microcrystals. The impact strength of hard products with a small amount of plasticizer is lower than that without time, but after increasing to a certain dose, the impact strength will increase with the increase of the amount, which meets the universal law. In addition, the increase of plasticizer will reduce the heat resistance and corrosion resistance of the product. For each additional part of plasticizer, Martin's heat resistance will drop by 2 to 3. Therefore, general hard products do not add plasticizers or add less plasticizers. Sometimes several parts of plasticizer are added in order to improve processing fluidity. However, 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 requirements of the softness of the product and the use, process and use environment. The general calendering process produces PVC film, and the total amount of plasticizer is about 50 parts. The blown film is slightly lower, generally 45-50 parts.
The main functions of plasticizers are as follows:
1. Reduce the melting temperature and melt viscosity of the polymer, thereby reducing its molding processing temperature;
2. Make polymer products have softness, elasticity and low temperature resistance.
The mechanism of action of plasticizer:
Plasticizers and resins have different polarities, and their mechanism of action is also different.
1. Volume effect This effect is mainly for non-polar plasticizers. Its mode of action is to insert between the molecular chains of the resin to increase the distance between the molecules, thereby weakening the force between the molecules and reducing The melt viscosity increases the flexibility of the molecular chain. The more such plasticizers are added, the greater the volume effect, and the volume effect of the long-chain structure plasticizer is greater than that of the cyclic structure plasticizer.
2. Shielding effect This effect is mainly for polar plasticizers, and its mode of action is to replace the polar attraction between polymer molecules with the interaction between polar plasticizers and polar polymers. As a result, the forces between molecules are weakened, and this plasticizing effect is called the shielding effect.
Plasticizer performance:
The properties of plasticizers mainly include the following items: plasticizing efficiency, compatibility, cold resistance, durability, heat resistance, insulation, flame retardancy and hygiene.
Plasticizing efficiency: refers to the plasticizing ability of a plasticizer to the polymer or the plasticizing effect. The plasticizing efficiency of different plasticizers is a relative concept. The plasticizing efficiency is generally evaluated by the amount of plasticizer required to change the quantitative flexibility index. The less plasticizer is added, the higher the plasticizing efficiency. General molecular weight plasticizers. Higher molecular weight plasticizers have a high plasticizing efficiency for PVC. As the polarity of the plasticizer molecule increases, the degree of alkyl branching and the increase in the aromatic ring structure reduce the plasticization efficiency.
The order of plasticizing efficiency of commonly used plasticizers is:
DBS>DBP>DOS>DOA>DOP>DIOP>M-50>TCP>PCl-50.
Compatibility: refers to the compatibility of plasticizers with polymers. The same plasticizer has different compatibility with polymers of different polarities.
The compatibility sequence of commonly used plasticizers and PVC is:
DBS>DBP>DOP>DIOP>DNP>ED3>DOA>DOS>Chlorinated Paraffin.
Cold resistance: refers to the low temperature resistance of plasticizer-plasticized products. The cold resistance of plasticizers is related to its structure. Aliphatic dibasic acid esters with methylene (—CH2—) as the main body have the best cold resistance. They are one of the most commonly used cold-resistant plasticizers, and contain cyclic and The branched plasticizer has excellent cold resistance due to its difficulty in moving in the polymer at low temperatures.
The order of cold resistance of commonly used plasticizers is: DOS>DOZ>DOA>ED3>DBP>DOP>DIOP>DIDP>DNP>M-50>TCP.
Durability: refers to the ability of plasticizers to persist in plasticized products. It mainly includes migration resistance, extraction resistance and volatility resistance.
The migration of plasticizers with resistance to migration includes two aspects:
1. The plasticizer seeps out from the inside of the product to the outside surface;
2. The plasticizer transfers to the solid in contact with it.
The migration of the plasticizer is related to its compatibility, and the worse the compatibility, the greater the migration. In addition, plasticizers with large molecular weight, branched chains or cyclic structures have good migration resistance.
Extraction resistance: Refers to the tendency of the plasticizer to diffuse into the liquid medium in contact with it. The medium mainly includes water, solvent, detergent and lubricating oil. In terms of oil resistance and solvent resistance, plasticizers with a larger proportion of non-polar alkyl groups have poor extraction resistance, phenyl groups, ester groups, and plasticizers with a high degree of branching have good extraction resistance; while water resistance is Contrary to the above, polyesters are plasticizers with excellent extraction resistance.
Volatility resistance: refers to the tendency of the plasticizer to diffuse into the air from the surface of the product when heated. The volatility of a plasticizer is related to its molecular weight. Generally, the smaller the molecular weight, the greater its volatility. From the point of view of molecular structure, straight-chain alkyl structure has better volatility resistance than branched-chain alkyl structure plasticizers; plasticizers containing cyclic and other bulky groups in the molecule have better volatility resistance. Generally, plasticizers such as polyester, epoxy, DIOP, TCP and pentaerythritol have good volatility resistance.
Insulation: The insulation of plasticizer is not as good as that of PVC resin. As the amount of plasticizer increases, the electrical insulation of PVC decreases. When choosing plasticizers for PVC insulation products, pay attention to its insulation properties.
The insulating order of commonly used plasticizers is:
TCP>DNP>DOP>M-50>ED3>DOS>DBP>DOA.
Hygiene: refers to the toxicity of plasticizers. Most plasticizers are non-toxic or low-toxic. Epoxy and citric acid esters are non-toxic plasticizers, and phthalic acid esters and dicarboxylic acids are low. Poisonous plasticizers, but DOP and DOA are suspected of causing cancer; most phosphate plasticizers are toxic, with the exception of diphenyl 2-ethylhexyl phosphate; chlorinated paraffins are also toxic plasticizers.
Flame retardancy: Phosphate esters and chlorinated paraffins are flame retardant plasticizers, and the order of flame retardancy is: TCP>TPP>TOP>DPOP.
Varieties of commonly used plasticizers:
There are many types of plasticizers. According to their compatibility with polymers, they can be divided into main plasticizers and auxiliary plasticizers. The main plasticizers include phthalic acid esters and phosphate esters, and the auxiliary plasticizers include aliphatic dibasic acid esters and epoxy resins. According to the structural classification of plasticizers, it can be divided into: phthalic acid esters, aliphatic dibasic acid esters, epoxides, polyesters, etc.
Di(2-ethylhexyl) phthalate (DOP) has high plasticizing efficiency and good volatility resistance, migration resistance, water extraction resistance, weather resistance and other comprehensive properties. Production of soft PVC calendered agricultural film, with DOP as the main plasticizer, and di(2-ethylhexyl) adipate (DOA) as the cold-resistant plasticizer. Epoxy fatty acid ester plasticizers have good thermal stability and cold resistance. Commonly used varieties are epoxy stearic acid 2-ethylhexyl, epoxy soybean oleic acid 2-ethylhexyl, etc.
High barrier polyvinylidene chloride (PVDC) film for food packaging needs to add an appropriate amount of 8-10phr non-toxic plasticizer. The main varieties are acetyl tri-n-butyl citrate, acetyl tri-(2-ethylhexyl) citrate, di(2-ethylhexyl) sebacate (DOS), etc. These plasticizers are also non-toxic plasticizers commonly used in PVC.