Introduction to PVC and Various Additives (III)

The PVC plastic formula is mainly composed of PVC resin and additives, which are divided into heat stabilizers, lubricants, processing modifiers, impact modifiers, fillers, aging resistant agents, colorants, etc. according to their functions. Before designing PVC formulas, one should first understand the properties of PVC resin and various additives.

Raw materials and additives

Impact modifier

An important aspect of polymer material modification is to improve its impact resistance. PVC resin is a polar amorphous polymer with strong intermolecular forces, making it a hard and brittle material; Low impact strength. After adding an impact modifier, the elastic particles of the impact modifier can reduce the total stress caused by crazing, and use the deformation and shear band of the particles themselves to prevent the expansion and growth of crazing, absorb the impact energy transmitted into the material body, and thus achieve the goal of impact resistance. The particles of the modifier are very small, which is beneficial for increasing the amount of modifier per unit weight or volume, thereby increasing its effective volume fraction and enhancing its ability to disperse stress. Currently, organic impact modifiers are widely used.

According to the molecular internal structure of organic impact modifiers, they can be divided into the following categories.

1. Intended Elastomer (PDE) type impact modifier is a polymer with a core-shell structure. Its core is a soft elastomer, giving the product high impact resistance. The shell is a polymer with a high glass transition temperature, and its main function is to isolate the modifier particles from each other, form component particles that can flow freely, promote their uniform dispersion in the polymer, and enhance the interaction and compatibility between the modifier and the polymer. The modifiers for this type of structure include MBS, ACR, MABS, and MACR, all of which are excellent impact modifiers.

2. Non predetermined elastomeric type (NPDE) impact modifier, which belongs to a network polymer, and its modification mechanism is to modify plastics through solvation (plasticization) mechanism. Therefore, NPDE needs to form a network structure coated with resin, which is not very compatible with the resin. The modifiers for this type of structure include CPE and EVA.

3. Excessive impact modifier, with a structure between two types, such as ABS. The specific varieties used for PVC resin include:

(1) Chlorinated polyethylene (CPE) is a powdery product that uses HDPE to suspend and chlorinate in the aqueous phase. As the degree of chlorination increases, the originally crystalline HDPE gradually becomes an amorphous elastomer. C used as a toughening agent? E. The C1 content is generally 25-45%. CPE has a wide range of sources and low prices. In addition to its toughening effect, it also has cold resistance, weather resistance, flame resistance, and chemical resistance. At present, CPE is the dominant impact modifier in China, especially in the production of PVC pipes and profiles, and most factories use CPE. The addition amount is generally 5-15 portions. CPE can be used in conjunction with other toughening agents, such as rubber and EVA, to achieve better results, but rubber additives are not resistant to aging.

(2) ACR is a copolymer of monomers such as methyl methacrylate and acrylate. ACR is a well developed impact modifier in recent years, which can increase the impact strength of materials by dozens of times. ACR belongs to the impact modifier of core-shell structure, consisting of a shell composed of methyl methacrylate ethyl acrylate polymer, and a rubber elastomer cross-linked with butyl acrylate as the core chain segment distributed in the inner layer of particles. Especially suitable for impact modification of PVC plastic products for outdoor use, using ACR as an impact modifier in PVC plastic door and window profiles has the characteristics of good processing performance, smooth surface, good aging resistance, and high welding corner strength compared to other modifiers, but the price is about 1/3 higher than CPE. Commonly used brands abroad, such as K-355, typically require 6-10 servings. At present, there are few manufacturers and users of ACR impact modifiers in China.

(3) MBS is a copolymer of three monomers: methyl methacrylate, butadiene, and styrene. The solubility parameter of MBS is between 94 and 9.5, which is close to that of PVC. Therefore, it has good compatibility with PVC. Its notable feature is that after adding PVC, it can be made into a transparent product. Generally, adding 4-6 parts to PVC can increase the impact strength of PVC by 6-15 times. However, when the amount of MBS added exceeds 30 parts, the impact strength of PVC decreases instead. MBS itself has good impact performance, good transparency, and a transmittance of over 90%. While improving impact resistance, it has little impact on other properties of the resin, such as tensile strength and elongation at break.

Qihong BS-156 is an MBS product carefully developed using advanced synthetic resin technology. It is a ternary copolymer of methyl methacrylate, butadiene, and styrene, and is a comprehensive resin that improves the impact strength and processing performance of PVC products. Adding BS-156 to the PVC formula can significantly improve the impact strength of the product and have no adverse effects on the inherent properties of PVC. It is widely used in PVC films, transparent sheets, pipes, pipe fittings, bottle pellets and other products.

(4) ABS is a ternary copolymer of styrene (40% -50%), butadiene (25% -30%), and acrylonitrile (25% -30%). It is mainly used for engineering plastics and also for PVC impact modification, and has good low-temperature impact modification effects. When the amount of ABS added reaches 50 parts, the impact strength of PVC can be equivalent to that of pure ABS. The addition amount of ABS is generally 5-20 parts, and its weather resistance is poor, making it unsuitable for long-term outdoor use in products. It is generally not used as an impact modifier in the production of plastic door and window profiles.

(5) EVA is a copolymer of ethylene and vinyl acetate. The introduction of vinyl acetate changes the crystallinity of polyethylene, resulting in a significant difference in vinyl acetate content. Additionally, EVA and PVC have different refractive indices, making it difficult to obtain transparent products. Therefore, EVA is often used in combination with other impact resistant resins. The amount of EVA added is less than 10 parts. The comparison of the performance of PVC modified by the above-mentioned impact modifiers is shown in Table 5.

4. Rubber impact modifiers

It is a toughening agent with excellent performance, mainly including ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), styrene butadiene rubber, natural rubber, cis-1,4-polybutadiene rubber, chloroprene rubber, polyisobutylene, butadiene rubber, etc. Among them, EPR, EPDM, and NBR are commonly used, which improve low-temperature impact resistance, but are not resistant to aging. Plastic door and window profiles generally do not use this type of impact modifier.

Other commonly used additives

1. Light stabilizer

PVC products are mostly exposed to sunlight and various other light sources. Adding a certain amount of light stabilizer according to the application environment of the product can prevent and delay its decomposition and aging, and extend the service life of the product.

Light stabilizers can be broadly divided into four categories:

(1) Light shielding agent. Titanium dioxide and carbon black can block ultraviolet rays from entering the interior of the profile to prevent the photodegradation of the polymer. If 2% carbon black is added to LDPE sheets, their aging resistance is 20 times higher than that of LDPE sheets without carbon black. Titanium dioxide has a significant improvement in the aging resistance of profiles. Titanium dioxide should be of the rutile type, with a usage rate of 3-6 parts in PVC plastic door and window profiles.

(2) UV absorber. It can strongly absorb 280-400nm ultraviolet light and convert it into visible light or heat. Commonly used products include UV-531, UV-327, UV-326, UV-p, etc., with a dosage of 0.1-0.5%. But the price is higher.

(3) Quenching agent. Mainly to eliminate the energy of excited polymer molecules and return them to the ground state. The specific varieties are nickel and cobalt complexes, including light stabilizer 2002 and light stabilizer 1084. Generally used in combination with other light stabilizers, with a dosage of 0.1-0.5%.

(4) Free radical scavenger. It is a light stabilizer that captures free radicals released by photodegradation and terminates the degradation reaction. Generally used in LDPE agricultural film. The main varieties include: light stabilizer GW-540, GW-544, CW-310, BW-10LD, light stabilizer 744, light stabilizer 622, light stabilizer 944, etc., with a dosage of 0.02-0.5%.

2. Filler

The main purpose of using fillers is to occupy space to reduce costs. Of course, some fillers also endow materials with special properties such as flame retardancy, conductivity, thermal conductivity, rigidity, etc.

The main indicators of fillers are: whiteness, particle size, particle shape, and particle surface activity.

Its main varieties include:

(1) Carbonates mainly include heavy calcium carbonate, light calcium carbonate, and active calcium carbonate. Generally, active lightweight calcium carbonate is used in PVC plastic profiles;

(2) Carbon black, such as natural gas tank black, mixed gas tank black, high wear furnace black, hot cracking method carbon black, acetylene carbon black, etc. Mainly used for reinforcing rubber, some varieties are also used as fillers, such as in conductive and anti-static polymer material products.

(3) Sulfates include barium sulfate, calcium sulfate, lithopone (lithopone), etc. They are mainly used as fillers and also have coloring effects. Barium sulfate can reduce excessive X-ray transmission.

(4) Metal oxides such as alumina, iron oxide, manganese oxide, zinc oxide, antimony oxide, magnesium oxide, iron oxide, magnetic powder, etc., are used as fillers and colorants.

(5) Metal powders such as aluminum, bronze, zinc, copper, lead, etc. are used for decoration and to improve thermal conductivity. In the production of plastic profiles, copper powder and aluminum powder are sometimes used to produce profiles resembling aluminum windows.

(6) Kaolin is commonly used as a filling agent in siliceous compound clay. Hard clay has a reinforcing effect. Talcum powder is used as a filler.

(7) Fibers such as glass fiber, boron fiber, carbon fiber, etc., are used as reinforcing agents.