What Is Paraldehyde?
Polyformaldehyde (English: polyformaldehyde) is also known as acetaln resins; polyoxymethylene (polyoxymethylenes). Full name polyformaldehyde resin, referred to as polyformaldehyde, thermoplastic crystalline polymer. Known as "super steel" or "saigang". The structural formula is as follows, the English abbreviation is POM. The polymer obtained through formaldehyde polymerization has a low degree of polymerization and is susceptible to thermal depolymerization.
- Chinese name
- Polyoxymethylene
- Foreign name
- polyformaldehyde
- Known as
- "Super Steel" or "Steel"
- Also known as
- Polyoxymethylene
- cas number
- 30525-89-4 [1]
- EINECS number
- 200-001-8 [1]
- Chinese alias
- Solid formaldehyde; polyformaldehyde; paraformaldehyde [1]
- Polyformaldehyde (English: polyformaldehyde) is also known as acetaln resins; polyoxymethylene (polyoxymethylenes). Full name polyformaldehyde resin, referred to as polyformaldehyde, thermoplastic crystalline polymer. Known as "super steel" or "saigang". The structural formula is as follows, the English abbreviation is POM. The polymer obtained through formaldehyde polymerization has a low degree of polymerization and is susceptible to thermal depolymerization.
Introduction to POM
- Around 1955, DuPont of the United States obtained formaldehyde homopolymer from formaldehyde polymerization. POM is easy to crystallize, and the crystallinity is above 70%. The melting temperature of homopolyformaldehyde is about 180 ° C.
- It is another engineering plastic with excellent comprehensive properties after polyamide. It has high mechanical properties, such as strength, modulus, wear resistance, toughness, fatigue resistance and creep resistance. It also has excellent electrical insulation. , Solvent resistance and processability, is one of the five general engineering plastics.
- Acetal polymer, or polyoxymethylene, is formed by the polymerization of formaldehyde, and it is often called polyoxymethylene (POM). The preparation of polymers from formaldehyde has been studied as far back as the 1920s, but it was not until 1950 when DuPont developed Delrin that thermally stable materials were made. Homopolymers are made by anion polymerization using very pure formaldehyde. The polymer formed is insoluble. Precipitation continues as the polymerization proceeds. As the acetal resin selected for formaldehyde is pulled apart, thermal degradation occurs. The thermal stability of polymers can be improved by esterification of hydroxy-terminated groups with acetic anhydride. Another method to improve thermal stability is copolymerization with a second monomer, such as ethylene oxide, and the polymer is prepared by a cationic polymerization method.
- There are four mechanisms for thermal degradation of acetal resins. The first is thermal or alkali-catalyzed chain depolymerization; the result is the release of formaldehyde, and the polymer's end group cleavage can reduce this tendency; the second is that the oxygen attacks the polymer's random position. The use of antioxidants can reduce the occurrence of this degradation mechanism, and copolymerization also helps reduce this tendency; the third mechanism is that the acetal resin chain is broken by acid. The fourth type of degradation is thermal depolymerization when the temperature exceeds 270 ° C. This is important. It warns operators to keep the processing temperature below 270 ° C to avoid polymer degradation.
- Acetal resin is highly crystalline, with a typical crystallinity of 75% and a melting point of 180 ° C. Compared with polyethylene (PE). Because the CO bonds are shorter, the molecular chains are more densely packed, with the result that the polymer has a higher melting point. The high crystallinity gives the acetal polymer good solvent resistance. If the polymer is linear. Its molecular weight (Mn) is between 20,000 and 110,000.
- Acetal resin is a strong (hard) thermoplastic with good fatigue and thermal stability. It has a low coefficient of friction and good heat resistance. It is considered that acetal resin is similar to nylon, but its resistance to Fatigue, creep resistance, hardness and water resistance are better than nylon. But acetal resins are not as resistant to creep as polycarbonate. As mentioned earlier, the acetal resin has excellent solvent resistance, and no organic solvent that can dissolve the acetal resin below 70 ° C has been found; however, it can swell in some solvents. Acetal resins are sensitive to acids, bases and oxidants. Although the CO bond is polar. But it has been balanced. And the polarity is much smaller than the carbonyl group in nylon, as a result, the acetal resin has relatively low hygroscopicity. The small amount of moisture absorbed may cause swelling and dimensional changes, but it will not cause the polymer to hydrolyze and degrade. The effect of moisture is much smaller than nylon polymers. Ultraviolet light can degrade the polymer from the polymer. To reduce this degradation, carbon black is added. Copolymers usually have similar properties to homopolymers, but the mechanical properties of homopolymers are slightly higher than copolymers. Its melting point is also higher, but its thermal stability and alkali resistance are worse than copolymers. Homopolymers and copolymers are filled with fillers (glass fibers, fluoropolymers, aramid fibers, and other fillers) made of toughened, ultraviolet (UV) stable materials. Acetal resins are blended with polyurethane elastomers to improve their toughness. These materials are all commercially available.
- Acetal resins are available for injection molding, injection molding, and extrusion molding. It is important not to overheat or severe overpressure caused by the production of formaldehyde during processing. The polymer should be cleaned before shutdown to avoid overheating during startup. The acetal resin should be stored in a dry place. The apparent viscosity of acetal resins is less dependent on shear stress and temperature than polyolefins, but its melt has low elasticity and low strength. Low melt strength is a problem when applying blow molding. For blow molding, copolymers with branched structures are more suitable. The crystallization speed is very fast, and the shrinkage after molding can be completed within 48 hours after molding. It is difficult to obtain transparent films due to rapid crystallization.
- The market demand for acetal resin in the United States and Canada in 997 was 368 million lbs. Applications of acetal resins include: gears, rollers, pipe parts, pump parts, fan blades, aerosol containers made of blown film, molded sprockets and chains, which are often used to directly replace metals. Acetal resin is mainly used for injection molding, and secondly used for extruding plates and rods. The low coefficient of friction of the acetal resin makes it useful for making good bearings.
Physicochemical properties of polyoxymethylene
- POM is a linear polymer with no side chains, high density and high crystallinity, and has excellent comprehensive properties.
- POM is a smooth, shiny, hard and dense material with a pale yellow or white color. It can be used for a long time in the temperature range of -40-100 ° C. Its abrasion resistance and self-lubricity are also superior to most engineering plastics, and it has good oil and peroxide resistance. Very resistant to acids, alkalis and sunlight.
- POM has a tensile strength of 70 MPa, small water absorption, stable size, and gloss. These properties are better than nylon. POM is a highly crystalline resin and the toughest among thermoplastic resins. With heat resistance, bending strength, high fatigue resistance, excellent wear resistance and electrical properties.
POM structure
- The molecule of polyoxymethylene is a high density, high crystallinity linear polymer without side chains. Because the bond length of the CO bond is shorter than the CC bond, the packing density in the direction of the axis of the polyacetal chain is large. Compared with polyethylene, polyoxymethylene has short carbon-oxygen bonds, high cohesive energy density, and high density.
- According to the different chemical structures in their molecular chains, they can be divided into two types: homo-formaldehyde and co-formaldehyde. The important differences between the two are: homopolyformaldehyde has high density, crystallinity, and melting point, but has poor thermal stability, a narrow processing temperature range (about 10 ° C), and slightly lower stability to acid and alkali; while copolyformaldehyde density, crystallinity, The melting point and strength are both low, but the thermal stability is good, it is not easy to decompose, the processing temperature range is wide (about 50 ), and the stability to acid and alkali is good. It is an engineering plastic with excellent comprehensive properties. Has good physical, mechanical and chemical properties, especially excellent friction resistance. Commonly known as Saigang or Gaogang, it is the third largest general-purpose plastic. Suitable for making wear-resistant and wear-resistant parts, transmission parts, and chemical, instrumentation and other parts.
- The flexibility of the polyacetal molecular chain is large, and the structure of the chain is high, so the crystallinity is high and the crystallizing ability is strong. The crystallinity of homopolyformaldehyde is 75% to 85%, and the copolyformaldehyde is 70% to 75%. Even if it is rapidly quenched, the crystallinity can reach more than 65%. Completely amorphous polyformaldehyde can only be obtained at -100 ° C.
- High density and high crystallinity are the main reasons for the excellent performance of polyoxymethylene, such as high hardness and high modulus, good dimensional stability, outstanding fatigue resistance, and resistance to corrosion by chemical media. Although the CO bond in the polyformaldehyde molecular chain has a certain polarity, the high density and high crystallinity restrict the movement of the dipole moment, so that it still has good electrical insulation and dielectric properties.
- The polyacetal end group contains a hemiacetal structure. When heated to about 100 ° C, it can gradually depolymerize from the hemiacetal of its end group, so its heat resistance is low. When heated to about 170 ° C, formaldehyde can be released from any place in the molecular chain by auto-oxidation reaction. Formaldehyde is oxidized to formic acid at high temperature and aerobic, and formic acid will automatically accelerate the degradation reaction of polyformaldehyde, so Thermal stabilizers, antioxidants, and formaldehyde absorbents are often added to homopolyformaldehyde resins to meet the needs of forming and processing. Since the copolyformaldehyde molecular chain contains a certain amount of CC bonds, it can prevent the oxidative degradation of the polyformaldehyde molecular chain, so the copolyformaldehyde has much better thermal stability than the homoformaldehyde. However, whether it is homo-formaldehyde or co-formaldehyde, the disadvantages of thermal stability and poor thermal oxygen stability should be fully taken into consideration during processing and application.
Performance value of POM
- Acetal products 1
- Melting point 175 ° C [1]
- Tensile strength (yield) 70MPa
- Elongation (Yield) 15%
- (Fracture) 15%
- Impact strength (no gap) 108KJ / m 2
- (With notch) 7.6 KJ / m 2
- Homopolyformaldehyde is generally synthesized by condensation polymerization of an aqueous solution of formaldehyde in the presence of an acid. A-polyformaldehyde with a polymerization degree of 100 or more is obtained, and then heated to decompose it into formaldehyde gas. After purification and dehydration, the monomer is usually purified by partial prepolymerization, and then passed into a dry solvent containing a small amount of initiator for polymerization. Due to the presence of water, the molecular weight is significantly reduced. As the initiator, a Lewis acid or a base can be used. However, most tertiary amines are used for negative ion addition polymerization. The reaction is as follows: The end group of polyoxymethylene is hemiacetal (-CH2OH). When the temperature is higher than 100 ° C, the end group is easily broken. Generally, it needs to be treated by end group to stabilize Into. Heat-resistant to 230 ° C after stabilization. Paraformaldehyde can be processed at temperatures ranging from 170 to 200 ° C, such as injection, extrusion, blow molding, etc. It is mainly used as engineering plastics for automobiles and mechanical parts.
POM properties
- POM is a tough and elastic material that has good creep resistance, geometric stability and impact resistance even at low temperatures. POM has both homopolymer and copolymer materials. Homopolymer materials have good ductility and fatigue resistance, but they are not easy to process. Copolymer materials have good thermal and chemical stability and are easy to process. Both homopolymer and copolymer materials are crystalline materials and do not easily absorb moisture. The high degree of crystallization of POM results in a relatively high shrinkage rate, which can be as high as 2% to 3.5%. There are different shrinkage rates for different reinforced materials.
Use of polyoxymethylene
- Polyoxymethylene (POM) is an engineering plastic with excellent properties.
- Acetal products 2
- Widely used in the manufacture of various sliding and rotating mechanical parts, making various gears, levers, pulleys, sprockets, especially suitable for bearings, hot water valves, precision metering valves, chain rings and rollers of conveyors, flow meters, automotive interiors External window turning machinery such as handles, cranks, oil pump bearing seats and impeller gas switching valves, electronic switch parts, fasteners, terminal mirror covers, electric fan parts, heating plates, instrument buttons; bearings for audio and video tapes; various pipes and Agricultural sprinkler irrigation systems and parts of valves, sprinklers, faucets, and bath tubs; switch keyboards, buttons, audio and video tape reels; temperature-controlled timers; power tools, garden finishing tool parts; can also be used as surfboards, sailboats and various sled parts Various kinds of buckles, fasteners, lighters, zippers, and buckles for watch micro gears, frame accessories for sports equipment, and backpacks; cardiac pacemakers in medical devices; artificial heart valves, apical vertebrae, prostheses, etc.
- It is used in chemical synthesis of chemical industry and pharmacy, as well as synthesis using anhydrous formaldehyde as raw material.
Production method of polyoxymethylene
- 37% of formaldehyde is evaporated under reduced pressure, and solid formaldehyde is obtained by catalytic condensation, filtered, washed with water, and dried under vacuum to obtain a finished product, the content is generally 93-95%. Each ton of paraformaldehyde consumes 3180kg of 37% formaldehyde. [1]
POM parameters
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- Its comprehensive performance is:
- High fatigue resistance.
- Good abrasion resistance and excellent friction performance.
- Low water absorption.
- Large surface hardness and good rigidity.
- Good dimensional stability and high dimensional accuracy.
- Good glide.
- POM environmental performance
- POM is not resistant to strong alkalis and oxidants, and has certain stability to enoic and weak acids. POM has good solvent resistance, can withstand hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oils and weak bases, and can maintain considerable chemical stability at high temperatures. Low water absorption and good dimensional stability.
Conductive modification of polyacetal
- Adding conductive carbon black is a common method for manufacturing conductive POM. The so-called conductive carbon black refers to a type of carbon black with a small particle size, a large surface area, and a lot of lock-like structures.
- Carbon black is generally made of various organic hydrocarbons by incomplete combustion method or thermal decomposition method. It is insoluble and insoluble microsphere particles. In addition to lone pair electrons and aromatic rings on the surface, there are quinoyl carbonyl groups. And polar functional groups such as phenolic hydroxyl. The addition amount of conductive carbon black is generally 0.5% -20%. If the conductivity of carbon black is good, the surface resistivity or volume resistivity of POM can be reduced to 1 * 10 & sup2; However, due to the effect of the higher functional groups on the surface of carbon black, the thermal stability of POM tends to decrease, resulting in a decrease in physical and mechanical properties. To overcome this shortcoming, a combination of conductive carbon black and a hydrophilic polymer compound (such as PEG) can be used to reduce the amount of carbon black used, or a method of adding a thermal stabilizer mainly based on formaldehyde trapping agents can be used Improve system thermal stability.
- In contrast, the use of carbon fibers can not only greatly improve the various properties of POM (including self-lubricating properties), but also achieve good antistatic properties. For example, when 20% carbon fiber with good conductivity is added, the surface resistivity and volume resistivity of POM can both reach the order of 1 * 10 & sup2;