Polyamide, commonly known as Nylon or PA in English, has a density of 1.14g/cm3. It is a general term for thermoplastic resins containing repeated amide groups - [NHCO] - on the molecular main chain, including aliphatic nylon and aromatic nylon (aromatic nylon also includes semi aromatic nylon and fully aromatic nylon).
Among them, there are many varieties of aliphatic nylon, with large production and wide applications. Their names are determined by the specific number of carbon atoms in the synthesized monomers, such as PA66, PA12, etc; Most aromatic nylon is high-temperature nylon, and its usage is not as large as aliphatic nylon. However, due to its excellent properties, there are increasingly more valuable applications for development, such as PA9T, PA6T, etc. Today, Xiaowei will discuss the nylon family in engineering plastics based on two parts: aliphatic nylon and aromatic nylon.
**Part: Aliphatic Nylon
Classification of Aliphatic Nylon
1. According to the number of carbon atoms in diamines and diacids, nylon synthesized from two monomers includes PA46, PA66, PA610, PA612, PA1010, etc
2. According to the number of carbon atoms contained in the monomer, they are named PA6, PA11, PA12, etc
Characteristics of Aliphatic Nylon
1. Nylon has excellent toughness, self-lubricating properties, wear resistance, chemical resistance, gas permeability, oil resistance, non toxicity, and easy coloring, so it is widely used in industry. Nylon has high impact strength (higher than ABS, POM but lower than PC). Low thermal deformation temperature, high moisture absorption, and poor dimensional stability.
2. The commonly used PA66 has a strong structure in nylon materials and has excellent processing performance.
Process characteristics of aliphatic nylon
1. Rheological properties of nylon
Nylon is mostly a crystalline resin, and when the temperature exceeds its melting point, its melt viscosity is low and the melt flowability is good, so the occurrence of edge overflow should be prevented. At the same time, due to the fast condensation speed of the melt, it is necessary to prevent material blockage of nozzles, channels, gates, etc., which may cause insufficient products. The overflow value of the mold is 0.03, and the viscosity of the melt is sensitive to temperature and shear force changes, but more sensitive to temperature. To reduce the viscosity of the melt, we should start with the temperature of the barrel.
2. Water absorption and drying of nylon
Nylon has a high water absorption capacity. During the molding process, wet nylon shows a sharp decrease in viscosity and the presence of bubbles, resulting in silver threads on the surface of the product and a decrease in mechanical strength. Therefore, the material must be dried before processing.
3. Crystallinity
Except for transparent nylon, nylon is mostly a crystalline polymer with high crystallinity. Its properties such as tensile strength, wear resistance, hardness, and lubricity have been improved, while its thermal expansion coefficient and water absorption tend to decrease. However, it is not conducive to transparency and impact resistance.
The mold temperature has a significant impact on crystallization, with high mold temperature resulting in high crystallinity and low mold temperature resulting in low crystallinity.
4. Shrinkage rate
Similar to other crystalline plastics, nylon resin has a problem of high shrinkage rate. Generally, the shrinkage of nylon is closely related to crystallization. When the crystallinity of the product is high, the shrinkage of the product will also increase. During the molding process, reducing the mold temperature, increasing the injection pressure, and reducing the material temperature will all reduce the shrinkage rate. However, the internal stress of the product will increase and it is easy to deform. For example, the shrinkage rate of non glass fiber reinforced PA6 and PA66 is 1.5-2%, and adding glass fiber can reduce the shrinkage rate to 0.3% -0.8%.
5. Forming equipment
When molding nylon, the main attention should be paid to preventing the phenomenon of "nozzle drooling", so self-locking nozzles are generally used for processing nylon materials. Additionally, it is advisable to choose an injection molding machine with a slightly larger plasticizing capacity.
6. Products and molds
(1) Wall thickness of the product
The flow length ratio of nylon is between 150-200, and the wall thickness of nylon products should not be less than 0.8mm, generally ranging from 1-3.2mm. Moreover, the shrinkage of the product is related to its wall thickness, and the thicker the wall thickness, the greater the shrinkage.
(2) Exhaust gas
The overflow value of nylon resin is about 0.03mm, so the exhaust hole groove should be controlled below 0.025.
(3) Mold temperature
For molds with thin walls that are difficult to form or require high crystallinity, cold water temperature control is generally used for molds that require a certain degree of flexibility. The diameter of the runner and sprue should not be less than 0.5 * t (where t is the thickness of the plastic part). Using a submerged gate, the small diameter of the gate should be 0.75mm.
Injection molding process of aliphatic nylon
1. Barrel temperature
As nylon is a crystalline polymer, its melting point is obvious. The temperature of the barrel selected for injection molding of nylon resins is related to the properties of the resin itself, equipment, and the shape of the product.
Excessive material temperature can cause color change, brittleness, and silver threads in the rubber parts, while excessively low material temperature can make the material hard and potentially damage the mold and screws. The melt temperature of nylon 6 is generally as low as 220 ℃, while that of nylon 66 is 260 ℃. Due to the poor thermal stability of nylon, it is not suitable to stay in the barrel at high temperatures for a long time to avoid discoloration and yellowing of the material. At the same time, due to the good fluidity of nylon, it flows rapidly when the temperature exceeds its melting point.
2. Injection pressure
Nylon solution has low viscosity and good fluidity, but its condensation speed is fast. It is prone to deficiency problems in products with complex shapes and thin wall thicknesses, so higher injection pressure is still required, especially when glass fiber is added.
Usually, if the pressure is too high, the product may experience edge overflow issues; If the pressure is too low, the product may produce defects such as ripples, bubbles, obvious fusion marks, or insufficient products. The injection pressure of most nylon varieties does not exceed 120MPA, and is generally selected within the range of 60-100MPA to meet the requirements of most products. As long as the product does not have defects such as bubbles and dents, it is generally not desirable to use a higher holding pressure to avoid increasing internal stress in the product.
3. Injection speed
For nylon, it is beneficial to have a fast injection speed to prevent ripples and insufficient filling caused by slow cooling. The fast injection speed does not have a significant impact on the performance of the product.
4. Mold temperature
Mold temperature has a certain impact on crystallinity and molding shrinkage. High mold temperature results in higher crystallinity, wear resistance, hardness, increased elastic modulus, decreased water absorption, and increased molding shrinkage of the product, making it suitable for thick products; Low mold temperature has low crystallinity, good toughness, high elongation, and decreased shrinkage, making it suitable for thin products with good transparency. If the wall thickness is greater than 3mm, it is recommended to use a low-temperature mold of 20-40C. For glass reinforced material molds, the temperature should be greater than 80C.
Precautions for Processing Aliphatic Nylon
1. Use of recycled materials
*No more than three times should be used to avoid discoloration of the product or a sharp decrease in mechanical and physical properties. The application amount should be controlled below 25%, as excessive use can cause fluctuations in process conditions. The mixture of recycled and new materials must be dried.
2. Safety Notice
When starting up nylon resin, the nozzle temperature should be turned on first, and then the feeding cylinder should be heated. When the nozzle is blocked, avoid facing the spray hole to prevent the molten material in the cylinder from suddenly releasing due to pressure accumulation, which may cause danger.
3. Use of release agent
The use of a small amount of release agent sometimes has the effect of improving and eliminating defects such as bubbles. The release agent for nylon products can be selected from zinc stearate and white oil, or mixed into a paste for use. When using, the amount must be small and uniform to avoid surface defects on the product.
4. During shutdown
Clear the screw to prevent it from twisting and breaking during the next production.
Post treatment of aliphatic nylon products
The post-treatment of nylon products is to prevent and eliminate residual stress or dimensional changes caused by moisture absorption in the products. There are two post-processing methods: heat treatment and wet conditioning.
1. Heat treatment
The commonly used method is to heat treat high boiling point liquids such as mineral oil, glycerin, and liquid paraffin at a temperature 10-20 ℃ higher than the operating temperature. The treatment time varies depending on the thickness of the product, with 10-15 minutes for thicknesses below 3mm and 15-30 minutes for thicknesses between 3-6mm. Products that have undergone heat treatment should be slowly cooled to room temperature to prevent stress regeneration caused by rapid cooling.
2. Humidity control treatment
Humidification treatment is mainly carried out for products with high humidity in the usage environment, and there are three methods: one is boiling water humidification; 2、 Room temperature water immersion method III. Steam method.
**The first method has a faster processing speed, but is prone to oversaturation. The third method has better efficiency and uniformity, but requires the addition of steam equipment.
Part 2: Aromatic Nylon
As mentioned earlier, aromatic nylon includes semi aromatic nylon and fully aromatic nylon. Fully aromatic nylon, such as poly (p-phenylene terephthalamide) (PPTA) and poly (m-phenylene terephthalamide) (PMIA), has excellent mechanical properties, insulation, chemical stability, and ultra-high thermal performance due to the aromatic groups in the molecular chain. At present, fully aromatic polyamide is mainly used in industries such as aerospace, atomic energy, and electronics and electrical. However, the ultra-high melting point of fully aromatic polyamide makes it impossible to melt extrude and inject, and can only be processed using special methods, which limits its application in daily engineering plastics and makes it difficult to recycle and reuse.
Semi aromatic polyamide contains both aromatic and methylene groups in its molecular chain, giving it the characteristics of both aliphatic polyamide and fully aromatic polyamide. It has excellent mechanical and thermal properties and can be used for conventional melt extrusion and injection molding, making it suitable for widespread use in the field of high temperature resistance. With the rapid development of the automotive industry and electronic and electrical industry in recent years, as well as the demand for environmental protection, the market demand for semi aromatic nylon has continued to increase, and there have been many advances in application development. Semi aromatic nylon mainly includes PA9T, PA6T, PA10T, etc. Let's take a look at them separately below.
High temperature resistant nylon PA6T
PA6T is a typical representative of semi aromatic nylon, which is formed by the condensation of hexamethylenediamine and terephthalic acid. Pure PA6T has a melting point as high as 370 ℃. At this temperature, nylon has already degraded and cannot be thermoplastic formed. Therefore, the PA6T circulating on the market is a copolymer or composite that has been copolymerized with other monomers to lower its melting point.
PA6T introduces a large number of benzene rings on the basis of the fatty chain. Compared with traditional PA6 and PA66, PA6T has higher Tg, lower water absorption, better dimensional stability, and heat resistance.
Due to the need to introduce other monomers for copolymerization of PA6T to reduce the melting processing temperature, different monomer ratios have become the key to PA6T modification. Therefore, it can be said that the high-temperature resistance modification of PA6T has great development space.
Application scope:
Due to its excellent welding resistance, low water absorption, and excellent flowability, it is widely used in automotive parts, mechanical parts, and electrical/electronic parts such as mobile phones and drones.
PA9T high-temperature resistant nylon
PA9T is independently developed by KURARAY, a Japanese company, and is formed by the condensation of nonanediamine and terephthalic acid. Its trade name is Genestar.
Although both are semi aromatic nylon, PA9T does not require copolymerization modification to lower its melting point before processing, as pure PA9T has a melting point of 306 ℃. The high glass transition temperature (125 ℃) and high crystallinity of PA9T endow it with good toughness in high-temperature environments. At the same time, it also has unparalleled chemical resistance compared to other PA materials, second only to PPS, and its water absorption rate is only 0.17%, which is the lowest among all PAs. The comprehensive performance of PA9T is undoubtedly one of the better traditional heat-resistant nylon, and with the continuous expansion of production scale, its cost will be close to that of ordinary PA. Therefore, PA9T is a variety with great development potential.
Application areas:
Mainly applied in three fields: electrical and electronic industry, automotive industry, and fiber industry.
High temperature resistant nylon PA10T
PA10T is made by condensation polymerization of terephthalic acid and decanediamine as monomers, and has excellent heat resistance. Its melting point is 316 ℃, chemical corrosion resistance, low water absorption, good dimensional stability, and glass fiber reinforcement modification can withstand lead-free soldering temperatures exceeding 280 ℃, with excellent comprehensive performance.
Compared to other short chain high-temperature nylon such as PA46 PA4T、PA6T、PA6I、 Compared to other materials, PA10T has a longer diamine flexible long chain, which gives the polymer a certain degree of flexibility, resulting in a higher crystallization rate and crystallinity. It is suitable for rapid prototyping and the production of small electronic components such as LED reflector brackets and connectors.
Due to the excellent properties of rigidity and corrosion resistance brought by the benzene ring structure in its molecular main chain, the modified products of PA10T can also be applied to some chemical reagents and/or heat-resistant environments, such as water treatment, nanoinjection molding NMT, engine peripherals, etc.
Application scope:
LED reflector bracket, connector, water treatment, nano injection molding NMT, engine peripherals, etc.
PA4T High Temperature Nylon
PA4T is the first synthesized high-temperature nylon since the 21st century, launched by DSM, which holds the global industrialization plan for butanediamine. Similar to PA6T, its melting point is also very high (430 ℃), and it is generally copolymerized with other nylon, such as nylon 66 or nylon 6.
This is the first new polymer of the 21st century, with excellent spatial stability, lead-free soldering compatibility, high melting point, high hardness and mechanical strength under temperature rise, and compared to DSM's original PA46 products, even PA9T, it exhibits ultra-low water absorption.
Application scope:
PA4T can be applied in the electronics, electrical, and automotive industries. In the field of electronics and electrical engineering, PA4T can be used to produce electronic components with small size, light weight, and long service life, such as circuit boards, memory card connectors, memory module connectors, CPU sockets, current transformers, high-temperature resistant wire spools, and other products. It is applied in fields such as smartphones, tablets, PCs, and wearable devices. In the automotive industry, PA4T can be used to produce high-temperature resistant automotive components such as electrical systems, fuel systems, cooling systems, engine covers, etc. In addition, there is also a demand for PA4T in the aerospace industry.
High temperature resistant nylon PA5T
The melting point of PA5T will be lower than that of PA6T. The main reason that previously limited its development was that the industrialization of pentanediamine was still in the research stage. However, in the second half of 2018, a domestic company successfully put 50000 tons of bio based pentanediamine into production, indicating that it is one step closer to the industrialization of PA5T.
High temperature resistant nylon PA12T
PA12T is a product of homopolymerization of dodecanediamine and terephthalic acid. Henan Junheng in China has built a 1000 ton/year PA12T production demonstration plant and is currently constructing a 10000 ton/year long-chain dicarboxylic acid production line.
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