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POM

POM

POM (English: polyformaldehyde), also known as acetal resin (acetaln resins); polyoxymethylene (polyoxymethylenes). Full name Delrin, POM referred to, thermoplastic crystalline polymer. Known as the “super steel” or “match steel”. Structural formula, the English abbreviation for POM. Obtained by polymerization of formaldehyde polymer, the degree of polymerization is not high, and vulnerable to thermal depolymerization.


polyformaldehyde known as the “super steel” or “match steel”

Also known as polyoxymethylene
cas number
30525-89-4
EINECS No.
200-001-8
Alias ​​solid formaldehyde; poly ant aldehydes; paraformaldehyde


Brief introduction

After 1955 years ago, DuPont formaldehyde obtained from formaldehyde polymerization homopolymer. POM is very easy to crystallize, the degree of crystallinity of more than 70%. POM melting temperature of about 180 ℃.
It is the second polyamide has a comprehensive performance excellent engineering plastic with high mechanical properties, such as strength, modulus, wear resistance, toughness, fatigue resistance and creep resistance, also has excellent electrical insulating properties , solvent resistance and workability, is one of five general-purpose engineering plastics.
I.e. polyoxymethylene acetal polymer is formed by the polymerization of formaldehyde, it is also often referred to as polyoxymethylene (POM). Formaldehyde polymer prepared as early as the 1920s, it was studied, but until l950年杜邦开发出 Delrin (Dai Lin) material yet be prepared before thermally stable. Homopolymers is very pure formaldehyde by anionic polymerization. Polymer units formed is insoluble. As the polymerization continues precipitated. As elected formaldehyde acetal resin is opened, then a thermal degradation. Thermal stability of the polymer can be increased by esterification of terminal hydroxyl groups with acetic anhydride. Another method of improving the thermal stability with a second monomer: comonomer such as ethylene oxide, etc., which copolymer is prepared by cationic polymerization.
Hot acetal resin degradation mechanism has four. The first is solution heat chain or base-catalyzed polyethylene; the result is the release of formaldehyde, cutting off the end groups of the polymer can reduce this tendency; second random bit Xuan is oxygen attack of the polymer is also turned convergence depolymerization using antioxidants can reduce the incidence of this degradation mechanism, copolymer also help reduce this tendency; third mechanism is acetal resin acid chains are broken. The fourth degradation occurs when the thermal depolymerization temperature Cliff  270 ℃, this is very important, it warns the operator to maintain the following processing temperature 270 ℃, in order to avoid degradation of the polymer.
Is highly crystalline acetal resin, the degree of crystallinity is typically 75%, m.p. stars 180 ℃. Compared with the polyethylene (PE). Since the C-O bond is shorter molecular chains so stacked more closely, as a result of higher melting point polymer units. High crystallinity polymer acetal gives a good solvent resistance. If the linear polymer oct. Molecular weight (Mn) between 20,000 to 110,000.
Acetal resin is strong and hard and has good fatigue resistance and thermal stability of thermoplastic (sexual) plastic, which has a low coefficient of friction electricity and good heat resistance to think acetal resin similar to nylon, but its resistance fatigue resistance, creep resistance, hardness and water resistance better than nylon. But acetal resin creep resistance better than polycarbonate. As mentioned earlier, acetal resin having excellent solvent resistance, has not been found at the following 70 ℃ acetal resin soluble organic solvent; however, it can swell in certain solvents. Acetal resin acids, bases and oxidants sensitive. Although the C-O bond is polar. But it has been balanced. And polar than nylon carbonyl much smaller, which results in a polyacetal resin having a relatively low hygroscopicity. A small amount of moisture absorbed may cause swelling and dimensional change, but will not cause degradation of the polymer hydrolysis. The impact is much less moisture than nylon polymer. UV-cho might {f from biodegradable polymer units, town by adding carbon black to reduce this degradation. Copolymers and homopolymers typically have similar properties, but the mechanical properties of the homopolymer is slightly higher than some of the copolymer. Its melting point is higher, but its poor thermal stability and alkali resistance than copolymers. Homopolymers and copolymers are filled with fillers (glass fiber, a fluorine-containing polymer, aramid fibers and other filler) is made of toughened grade, ultraviolet light (UV) stabilized grade material. Acetal resins and polyurethane elastomer blends to improve the toughness of these materials can be purchased in the market.
For injection molding, injection molding and extrusion molding of acetal resin can buy. Important in the process is not over-temperature or serious overpressure caused due to the generation of formaldehyde. Polymer should be cleaned before shutting down to avoid overheating during startup. Acetal resin should be stored in a dry place. Apparent viscosity acetal resin for shear stress and temperature dependence than the polyolefin small, but it does have low melt elasticity and low intensity. Low melt strength is a problem when you blow molding applications. For blow molding, the copolymer having a branched structure more appropriate. Fast crystallization, shrinkage after molding can be completed within 48h after molding. Since it is difficult to obtain rapid crystallization transparent film.
US and Canada need one year 368 million lb. Applications acetal resins include: gears, roller, pipe parts, pump parts, fan blades, empty aerosol containers made of blown films, molded sprockets and chains, it is often used to directly replace the metal. Acetal resin is mainly used for injection molding, extrusion sheet and secondly bar. Low coefficient of friction acetal resin to make it available to make a good bearing.


Physical and chemical properties


POM is a side-chain, high-density, high-crystalline linear polymer with excellent overall performance.
POM is a surface smooth, shiny, hard and dense material, yellow or white, long-term use in the -40-100 ° C temperature range. Its wear resistance and self-lubricating plastic is superior than most, and good oil, peroxide-resistant performance. Very acid, alkali and impatience impatience solar ultraviolet radiation.
POM tensile strength up to 70MPa, water absorption, dimensional stability, glossy, these performance are better than nylon, POM is a highly crystalline resin, the thermoplastic resin is the toughest. With thermal strength, flexural strength, fatigue resistance are high strength, wear resistance and excellent electrical properties.


structure


Molecular POM is a side-chain high-density, high-crystalline linear polymer. Since the CO bond length is less than CC bond, therefore POM chain packing density axis direction. Compared with polyethylene, POM carbon-oxygen bonds short, high cohesive energy density, density.
Different according to their molecular chain chemical structure, can be divided into POM and acetal copolymer two. The difference is important: POM density, crystallinity, melting point are high, but poor thermal stability, narrow processing temperature range (about 10 ℃), slightly lower on the pH stability; acetal copolymer density, crystallinity, melting point, strength are low, but the good thermal stability, not easily broken, the processing temperature range (about 50 ℃), good stability on pH. Engineering plastics with excellent overall performance. Good physical, mechanical and chemical properties, especially those with excellent abrasion performance. Commonly known as the Race of steel or seize steel, is the third largest generic plastic. Suitable for the production to reduce mill wear parts, transmission parts, and chemical, instrumentation, and other parts.
Polyoxymethylene molecular chain flexibility large chain of high structural regularity, therefore high crystallinity, crystallization ability. POM crystallinity of 75% to 85%, acetal copolymer is 70% to 75%, even if rapid quenching, crystallinity can reach more than 65 percent. Completely amorphous POM only at -100 ℃ when to get.
High density and high crystallinity is mainly POM has excellent wins energy, such as high hardness and modulus, good dimensional stability, outstanding fatigue resistance, not easy to be chemical media corrosion. Although the molecular chain POM CO keys have a certain polarity, but due to the high density and high crystallinity tied the dipole moment of the campaign, so that it still has good electrical insulation properties and dielectric properties.
POM end group containing hemiacetal structure. When heated to about 100 ℃, hemiacetal group at the end from which gradually depolymerization, thus its low heat resistance. When heated to about 170 ℃, and emit formaldehyde can occur from any one of the molecular chain of auto-oxidation reaction of formaldehyde at high temperatures oxygen can be oxidized to formic acid, formic acid degradation of POM automatic accelerate catalysis, thus often in the POM resin added heat stabilizers, antioxidants, formaldehyde absorbers, etc., to meet the needs of forming. Because acetal copolymer molecular chain containing a certain amount of CC bonds, it can prevent polyoxymethylene molecular chain oxidative degradation than the acetal copolymer thus homo- thermal stability of formaldehyde is much better. But whether it is or POM acetal copolymer, processing and application should be in full attention to its thermal stability and thermal oxidation stability shortcomings of poor.

Performance Value

POM products 1

Specific gravity 1.43 [1]
A melting point of 175 ° C [1]
Tensile strength (yield) 70MPa
Elongation (yield) 15%
(Break) 15%
Impact strength (unnotched) 108KJ / m2
(Band gap) 7.6 KJ / m2
POM synthesis generally aqueous solution of formaldehyde in the presence of acid condensation polymerization. To obtain a degree of polymerization of 100 or more a- polyoxymethylene, and then thermally decomposed into formaldehyde gas, after refining and dehydration, generally using part of the pre-polymerization of purified monomer, and containing a small amount of initiator introduced into the drying solvent polymerization. Because the presence of water, so that the molecular weight is significantly reduced. Available initiator Lewis acid or alkali. But mostly with a tertiary amine anion addition polymerization reaction is as follows: POM hemiacetal end groups (-CH2OH), when the temperature is higher than 100 ℃, the end groups easy to break, generally subject to treatment so that stable end groups of. After the stabilization treatment can be resistant to 230 ℃. Paraformaldehyde may be at a temperature of 170 ~ 200 ℃ processing, such as injection, extrusion, blow molding and the like. Mainly used for engineering plastics for the automotive, mechanical components.

characteristic

POM is a tough elastic material, even at low temperatures are still very good creep properties, geometric stability and impact resistance. POM homopolymer materials have both copolymer material. Homopolymer material has a very good extension strength, fatigue strength, but not easy to process. Copolymers have good thermal stability, chemical stability and ease of processing. Both homopolymer material or copolymer materials are crystalline materials and are not easily absorb moisture. The high crystallinity of POM cause it has a fairly high shrinkage, can be high up to 2% to 3.5%. For a variety of enhanced materials have different shrinkage.

use

Polyacetal (POM) is an excellent engineering plastics, in foreign countries have “wins Steel”, “

POM products

Super Steel, “said .POM having similar metal hardness, strength and rigidity, in a wide range of temperature and humidity have a good self-lubricating properties, good fatigue resistance, and resilient, in addition it It has good chemical resistance .POM below the cost of many other engineering plastics are replacing traditionally been occupied by some of the metals markets, such as alternative zinc, brass, aluminum and steel production in many parts, since its inception, POM has been widely used in electronic, electrical, mechanical, instrumentation, daily light industry, automobile, building materials, agriculture, etc. used in many new areas, such as medical technology, sports equipment, etc., POM also showed good growth .
Widely used in the manufacture of various sliding, rotating machine parts, do all kinds of gears, levers, pulleys, sprockets, especially suitable as bearings, water valve, precision metering valve, ring and chain conveyor roller, flow meters, car external handles, window cranks and other rotating machinery, pumps gas bearing and impeller off valves, electronic switch components, tight solid, terminal mirror mask, fan parts, heating panels, instrument button; tapes bearing; a variety of channels and Agricultural Irrigation systems and valves, sprinklers, faucets, bath basin parts; switch keyboards, buttons, audio and video reels; thermostat timer; power tools, garden tools parts finishing; in addition can be used as a surfboard, sailing and various sled parts, Watch miniature gears, sporting accessories and backpacks framework for equipment used in a variety of buckles, fasteners, lighters, zippers, buckles; medical equipment Cardiac pacemakers; artificial heart valves, the top vertebra, prostheses, etc.
For the chemical synthesis of chemical, pharmaceutical and other and synthesis using anhydrous formaldehyde as a raw material.

Production method

37% formaldehyde evaporated under reduced pressure, the catalytic solid formaldehyde condensation, filtration, washed with water, dried under vacuum to obtain the finished product, the content is usually 93-95%. Paraformaldehyde consumption per ton of 37% formaldehyde 3180kg.

Its overall performance is:
High fatigue strength.
Good abrasion resistance, friction performance excellent.
Low water absorption.
Surface hardness, good rigidity.
Good dimensional stability, high dimensional accuracy.
Good sliding.
POM environmental performance
POM intolerance alkali and oxidant, acid and weak acid has a certain stability. POM good solvent resistance, can be resistant to hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricants and weak base, and maintain a considerable chemical stability at high temperatures. Water absorption, good dimensional stability.

Conductive modified

Add conductive carbon black is a common method of manufacturing a conductive POM, the so-called conductive carbon black refers to the smaller particle size, surface area is larger and more lock-like structure for a class of carbon black.
Carbon black is generally a variety of organic hydrocarbons to incomplete combustion method or thermal decomposition method made, as insoluble and infusible spherical micro-particles, in addition to the surface electron lone pair and the aromatic ring, there quinone carbonyl and a phenolic hydroxyl group and other polar functional groups. Amount of conductive carbon black is generally 0.5% to 20%, if the carbon black is preferably conductive, the POM surface resistivity or volume resistivity may be reduced to 1 * 10 & sup2; number of levels. However, due to the higher carbon surface functional groups, often resulting in decreased thermal stability of POM, which causes lower physical and mechanical properties. To overcome this drawback can be taken conductive carbon black and hydrophilic polymer compound (such as PEG) and the methods used to reduce the amount of carbon black can also be used to add heat stabilizers to formaldehyde scavenger based method, Improved thermal stability of the system.
In contrast, both the use of carbon fiber to make a variety of performance POM (including self-lubricating) has greatly improved, but also to achieve good antistatic properties. Such as adding 20% ​​better conductive carbon fiber, POM surface resistivity and volume can reach 1 * 10 & sup2; magnitude.