Performance characteristics and common modification methods of POM
TIME：2021-10-20
Polyoxymethylene (POM) is a kind of engineering plastics with excellent properties. It is known as "seizing steel" and "super steel" abroad. POM has the hardness, strength and rigidity similar to metals. It has good self lubrication, good fatigue resistance and elasticity in a wide range of temperature and humidity. In addition, it has good chemical resistance.
POM, at a lower cost than many other engineering plastics, is replacing some markets traditionally occupied by metals, such as making many parts instead of zinc, brass, aluminum and steel. Since its inception, POM has been widely used in electronic and electrical, machinery, instruments, daily light industry, automobile, building materials, agriculture and other fields. POM also shows a good growth trend in the application of many new fields, such as medical technology and sports equipment.
Performance characteristics of POM
POM is a linear polymer with high density and high crystallinity without side chain. Because the bond length of C-O bond is less than that of C-C bond, the filling density in the direction of POM chain axis is large. Compared with polyethylene, POM has shorter C-O bond, higher cohesive energy density and higher density.
According to the different chemical structures in the molecular chain, it can be divided into homopolymer POM and copolymerization POM. The important differences between the two are as follows:
Homopolymerized POM has high density, crystallinity (75% ~ 85%) and melting point, but poor thermal stability, narrow processing temperature range (about 10 ℃) and slightly low acid-base stability;
The copolymerized POM has low density, crystallinity (70% ~ 75%), melting point and strength, but it has good thermal stability, is not easy to decompose, has a wide processing temperature range (about 50 ℃), and has good acid-base stability.
Both homopolymer materials and copolymer materials are crystalline materials and are not easy to absorb water. High density and high crystallinity are the main reasons for the excellent properties of POM.
POM has high thermal deformation temperature, and the continuous service temperature is about 100 ℃.
It has good low temperature resistance, and the glass transition temperature is - 40 ~ 60 ℃.
It has excellent creep resistance. After 3000 hours at room temperature and 21MPa load, the creep value is only 2.3%.
The fatigue resistance is good. Under the condition of 107 alternating times, the fatigue strength is still maintained at 35 MPa.
The wear resistance is good, the friction coefficient is only 0.21, which is lower than that of nylon, has self-lubricating effect and low working noise.
Excellent insulation performance, good arc resistance even in high temperature and high humidity environment.
Good oil resistance, high resistance to grease and organic solvents.
The water absorption is low, no more than 0.3%. The moisture has little effect on the molding process, and there is no need for pre drying.
Common modification methods of POM
POM also has some performance defects, such as large molding shrinkage, which makes the products easy to deform; Not resistant to ultraviolet light; The melting point is not very high and is easy to be decomposed by heat during processing; Low toughness and high notch sensitivity; Relatively high density, opaque, acid resistant, etc.
Therefore, under many harsh conditions, it still can not meet the use requirements, which is the reason for its continuous modification.
POM modification is mainly through physical and mechanical methods, that is, adding inorganic or organic substances into POM, blending different types of high molecular polymers, or realizing POM copolymerization, grafting and crosslinking by chemical methods, or combining the above methods to improve its fluidity, friction and wear properties, rigidity, strength, electroplating, dielectric, notch sensitivity Impact resistance, so as to reduce material cost, improve molding processing performance or final use performance, or be endowed with unique functions in electricity, magnetism, light, heat, sound, combustion and so on.
#01 chemical modification
The method of introducing the required segments into the POM molecular chain by means of grafting and block. While maintaining the balance of other properties of POM, strengthen or give some specific service properties to the material. For example, POM used in automotive industry parts often has high weather resistance and UV resistance stability. The color change of the modified material under the irradiation of 600kj / m2 xenon lamp is no more than 2 Δ E。
#02 chemical modification
Toughening modification
At present, the main toughening modification methods of POM at home and abroad are blending modification. There are generally two methods. One is to add elastomer to improve the toughness and impact strength of POM. Common elastomers include EPDM, Br, SBR, TPUR (thermoplastic polyurethane), acrylate elastomers, etc. TPUR is now recognized as the most effective elastomer impact modifier of POM.
Another toughening method is non elastomer toughening, that is, the toughness or rigid materials with excellent comprehensive properties are mixed with POM, such as nylon or copolymerized nylon. Elastomer is not the most ideal impact modifier for POM, because the improvement of impact toughness is at the cost of the decline of other mechanical properties. Therefore, in order not to damage other mechanical properties and improve toughness at the same time, the material of blending non elastomer has a good effect.
Reinforced filling modification
Filling modification is to add inorganic filler or organic filler in the plastic molding process to reduce the raw material cost of plastic products, achieve the purpose of increment, or significantly change the performance of plastic products, that is, while sacrificing some aspects of performance, people hope to significantly improve other aspects of performance.
POM is often reinforced with glass fiber, which can improve its tensile strength and dimensional stability, but the wear resistance will be reduced.
Other modification
It is mainly aimed at improving the wear resistance, flame retardancy and weather resistance of POM.
POM has a large friction coefficient and its application in many conductive parts is limited. POM self-lubricating composites have been developed by various modification methods at home and abroad. At present, the development of POM self-lubricating materials has become one of the research directions of its modification. Solid graphite powder, PTFE and aviation lubricating oil can be added to make oil-bearing and wear-resistant POM.
POM is relatively easy to burn. Many brands of POM flame retardants and flame retardant POM products have been developed abroad. At present, China has started late in this regard. Domestic research shows that the flame retardants suitable for preparing flame retardant POM are melamine, ammonium polyphosphate, pentaerythritol diphosphate and melamine ammonium salt. Flame retardant POM can be produced by extrusion granulation process, and its process conditions are the same as ordinary POM. It can also be formed by injection, blow molding, molding and other molding processes.
The poor photostability and thermal stability of POM lead to the rapid decline of mechanical properties and loss of use value, which limits its application in many aspects. It is of great significance to add antioxidants, heat stabilizers and light stabilizers to improve the weather resistance of POM.
