Polyvinylidene chloride is abbreviated as PVDC. Due to the symmetry and polar polarity of the two chlorine atoms in the monomer, the homopolymer has high crystallinity, so that the solubility of the gas is low and the diffusion is slow. Good barrier to oxygen, water and water vapor. In addition, due to its high chlorine content, it also has the advantages of flame retardancy, corrosion resistance and biodegradability. Therefore, PVDC films are now widely used as packaging materials and protective fabrics. However, two symmetrical chlorine atoms in the vinylidene chloride molecule are extremely unstable to heat and can only be used at 100 ° C. When heated to 125 ° C, HCl is released to form a conjugated double bond, which turns the polymer into yellow or brown. Turns black. In order to improve the thermal stability of PVDC, improve the processability and performance, a variety of additives are usually added directly to PVDC before processing, and plastic additives commonly used for film formation include plasticizers, stabilizers and lubricants.
Plasticizer
Plasticizers commonly used in PVDC film formation are dibutyl sebacate (DBS), dioctyl sebacate (DOS), dioctyl adipate (DOA), tributyl citrate (TBC), acetyl. Tributyl citrate (ATBC). Its main role is to reduce the intermolecular forces to facilitate plasticization. When selecting, it is mainly required to have good compatibility with the resin and meet the needs of food hygiene. Different plasticizers have different characteristics. Therefore, in practice, two different plasticizers are often used together to complement each other to meet the processing performance and product performance. The addition amount is generally 5-7 phr, and the plasticizer is added. If the amount is too low, it will produce an anti-plasticization effect, making it rigid; if the amount is too much, the barrier property and mechanical properties of the product are greatly reduced.
2. Stabilizer
PVDC is usually added with some heat-stable epoxy compounds such as epoxidized soybean oil (ESO), epoxy linseed oil (ESL), and the like. As a heat stabilizer, the epoxy structure in the molecules of these epoxy compounds acts with HCl to prevent or retard thermal decomposition and has a significant plasticizing effect.
3. lubricant
The role of the lubricant is to reduce the mutual friction between the PVDC resin and the extruder barrel and the screw due to mechanical shearing force during film formation, reduce the stagnant PVDC material in the screw and the die, and effectively prevent thermal decomposition, which is beneficial to the process. The film formation rate is improved while also giving the film surface a gloss. Commonly used lubricants are metal salts of stearic acid, stearic acid glycerides, stearic acid amides, etc., generally added in an amount of 011 to 1 phr, too much lubricant will affect the self-adhesiveness of the film.
4. Inorganic filler
When PVDC is formed into a film, an inorganic filler is usually added in a small amount. Commonly used are calcium carbonate, talc, magnesia, kaolin, calcium hydroxide, titanium dioxide and the like. In addition to adjusting the different barrier properties, the addition of fillers can also improve processability and mechanical properties of the film. For example, when the printability of the film and the smoothness of the surface are improved, the addition amount is generally from 0101 to 3 phr, which is based on the transparency of the film. In addition, the addition of titanium dioxide prevents the film from generating bubbles and white spots due to thermal decomposition of the resin, and does not affect the barrier properties of the film. The addition of magnesium oxide can also form a film at a relatively high temperature, so that a PVDC resin having a broad molecular weight distribution can have a large extrusion rate. In addition, the addition of talc powder can act as a nucleating agent to accelerate the crystallization rate.
5. High molecular polymer modifier
Although PVDC has excellent barrier properties, it still has defects in the processing properties, thermal stability and film strength of the resin. In order to better improve a certain performance or adapt to some special purposes, in addition to the commonly used, the molecular weight is small. In addition to the additives, macropolymers are often added as needed to improve their properties.
(1) Improve thermal stability and processability
PVDC is easily decomposed due to its poor thermal stability, which deteriorates the processing performance. In order to improve the processing performance, it is necessary to improve the thermal stability. The modifiers to be added for this purpose are: an elastomer or a rubber body of an ethylene alkane copolymer, which is added in an amount of 2% to 20%; a vinyl acetate content of 5% to 18%, and a melt flow rate of 011 to 110 dg/min. EVA copolymer, added in an amount of 5% to 40%. Modifiers for improving the processability of the resin include: a copolymer of styrene, a copolymer of vinylidene chloride and methyl acrylate; in addition, it is also possible to blend vinylidene chloride of different molecular weights with a vinyl chloride copolymer resin [16]. ], if 100 phr of a vinylidene chloride polymer, a styrene content of >20%, a softening temperature of 50 to 180 ° C, a polymer of 0101 to 5 phr, a plasticizer or a stabilizer of 0.1 to 3 phr, a high barrier property can be obtained. A composition with good processing properties. A PVDC resin having a molecular weight of 50,000 to 300,000 is blended with another PVDC resin having a molecular weight of ≥5000 to obtain a copolymer excellent in processability.
(2) Improve the strength of the film
The blending modifier used to increase the strength of the film is a copolymer of styrene, such as styrene-isobutylene, styrene-acrylonitrile, styrene-methyl acrylate, styrene-butadiene, etc.; butadiene-A A copolymer of methyl acrylate, ethylene-vinyl acetate, polyamide, grafted acrylate or the like, or an elastomer or the like. For example, the addition of polyamide can improve the impact strength at low temperatures and has a high barrier property.
(3) Improve the softness of the film
The PVDC film itself is poor in flexibility, and although the addition of an ordinary plasticizer can be improved, it is not suitable to use a special heat seal such as a high frequency heat seal due to the migration of the plasticizer during film formation. When 10% to 15% of the ethylene-alkyl acrylate copolymer is used, the film can be made soft, cold-resistant, heat-sealable, and barrier-stable, and the processability is improved.
(4) Improve the transparency of the film
Using ethylene chloride-vinyl acetate copolymer, the chlorine content is 1% to 50%, the amount is 1% to 40%; or 1% to 40% with ethylene chloride-methyl methacrylate copolymer and PVDC Mixing; or blending with two different molecular weights of PVDC can produce a film with high transparency.
(5) Improve the self-adhesiveness and cohesiveness of the film
Adding a liquid plasticizer (such as citrate) can improve self-adhesiveness and cohesiveness, and can also be used as a plasticizer. However, liquid plasticizers have limited solubility in PVDC and often migrate to the surface of the film and even to the vessel. Although this method sometimes increases the self-adhesiveness and cohesiveness of the film, there is also a problem of adhesion between the film and the film. For example, an ethylene-vinyl acetate copolymer having an ethylene content of 50% to 65% and a molecular weight of 1,000 to 4,000 is used, and the amount is 011% to 70%, and can be used together with a plasticizer.
Zhejiang minmetals huitong import and export Co., Ltd , https://www.zjminmetals.com