Polyamide (PA, commonly known as nylon) is a resin first developed by DuPont in the United States for fiber, and industrialized in 1939. In the 1950s, it began to develop and produce injection molded products to replace metals to meet the requirements of lightweight and cost-reducing downstream industrial products. PA has good comprehensive properties, including mechanical properties, heat resistance, wear resistance, chemical resistance and self-lubrication, and has a low friction coefficient, a certain degree of flame retardancy, and is easy to process. It is suitable for filling and reinforcing modification with glass fiber and other fillers to improve performance and expand the scope of application. There are many varieties of PA, including PA6, PA66, PAll, PAl2, PA46, PA610, PA612, PAl010, etc., as well as many new varieties such as semi-aromatic nylon PA6T and special nylon developed in recent years.

Development

Nylon engineering plastics are widely used in electronics, automobiles, construction, office equipment, machinery, aerospace and other industries due to their high performance advantages in mechanical properties, durability, corrosion resistance, and heat resistance. Replacing steel and wood with plastic has become an international trend. As the fastest growing field in the world's plastics industry, the development of nylon engineering plastics not only supports the country's pillar industries and modern high-tech industries, but also promotes the transformation of traditional industries and the adjustment of product structure.

Looking to the future, the development prospects of China's nylon engineering plastics industry are still very broad. From the perspective of the home appliance industry alone, the annual demand for engineering plastics for refrigerators, freezers, washing machines, air conditioners and various small household appliances will reach about 600,000 tons in the future. The amount of engineering plastics used in communication infrastructure construction, railway and highway construction is even more amazing. It is expected that the total demand will reach more than 4.5 million tons in the next few years.

According to the data from Qianzhan.com, in 2010, China's consumption of engineering plastics reached 2.443 million tons, an increase of 11% year-on-year, making it the country with the fastest demand growth in the world; in 2011, it further rose to 2.72 million tons. It is expected that by 2013, China's consumption of nylon engineering plastics will reach 3.37 million tons, and in 2015, it will exceed 4 million tons.

Market Forecast

In order to enhance the competitiveness of PA with other materials (including engineering plastics), expand the scope of application, and increase market share, the performance of the product crystal level has been improved from three aspects: improving product performance, reducing costs, and being beneficial to the environment. Resin manufacturers such as Du Pont, DSM, Rhodia, and Toray have successively launched rapid prototyping grades, shortening the molding cycle and reducing production costs. It is more convenient to use metallocene polyolefins such as polyolefin elastomers (POE) for toughening and modification than using elastomers, and the adjustable range is larger. At the same time, the number of manufacturers that can produce flame-retardant (especially halogen-free flame-retardant) PA grades has increased, and the number of products available for users to choose has also increased. In addition, it is worth mentioning that PA nanocomposites are the largest industrial polymer nanocomposites in terms of production volume. The amount of nanoparticles filled is small, and the density of the modified product is almost the same as that of the basic crystal level. Its superiority is obvious.

Industrialists and consultants are generally optimistic about the future market of PA engineering plastics. Some believe that its use will increase at an average annual rate of 7% from 2000 to 2005. There are also reports that the average annual growth rate of the world PA engineering plastics market is expected to be 5%-6% from 2001 to 2006. The share of production capacity in Asia will increase. The hot spots and future potential markets of PA engineering plastics market are:

(1) Automobile engine intake manifolds. In order to reduce production costs, automobile manufacturers require the use of integrated components, the selection of high-performance materials and simplified designs. The production of PA intake manifolds can make the products lighter, reduce costs by 40%-50%, and have a good vibration reduction effect. European automobile manufacturers are at the forefront of the application of PA intake manifolds, and it is expected that the United States and other regions will soon follow.

(2) Heat resistance (especially welding heat resistance) Crystal grade will be very active in the electrical industry, and the development and application of halogen-free flame retardant PA will receive greater attention.

(3) Food packaging film centered on PA6 This product has a promising application prospect. Biaxially oriented (BO) PA film has good puncture resistance, barrier properties to oxygen and carbon dioxide, and resistance to boiling. It can be used as the core film of co-extruded multilayer film to extend the shelf life of food. The demand will grow steadily and expand from Japan, where it was initially developed and applied, to other countries and regions.

Production and Demand

Overview of PA Engineering Plastics

PA is a general-purpose engineering plastic with a long history and a wide range of uses. In 2000, the world engineering plastics market was divided into PA 35%, PC 32%, POM 11%, PBT 10%, PPO 3%, PET 2%, UHMWPE 2%, and high-performance engineering plastics (PPS, LCP, PEEK, PEI, PESU, PVDF, other fluorine-containing plastics, etc.) 2%. Due to the rapid growth of PC market demand, its market share has exceeded that of PA.

Taking into account the comprehensive performance and price, the market consumption of PA6 and PA66 still accounts for about 90% of the total PA, and they are in a dominant position. In 2001, the world's consumption of PA66 was 740,000 tons, slightly higher than the 680,000 tons of PA6. The consumption structure in Europe is 50% for PA6, 40% for PA66, and 10% for PAll, PAl2 and other homopolymer and copolymer PA. The consumption of PA66 in the United States exceeds that of other varieties. In Japan, PA6 consumption ranks first, at 52%, PA66 accounts for 38%, PAll and PAl2 account for 5%, and PA46 and semi-aromatic PA account for 5%. PA engineering plastics are mainly injection molded, and injection molded products account for about 90% of PA products. The molding processes of PA6 and PA66 are different. PA66 is basically processed by injection molding, accounting for 95%, and extrusion molding accounts for only 5%; injection molding products of PA6 account for 70%, and extrusion molding accounts for 30%.

In the past 10 years, the world's PA consumption has increased at an average annual rate of about 7.5%, while the average annual growth rate of PA resin for engineering plastics is about 8.5%. The use of fillers, reinforcing agents, elastomers, other resins or additives to modify it has made the PA engineering plastics industry full of vitality. Since the Asian financial crisis of 1997-1998 and the recovery of the European economy, the demand for PA engineering plastics has resumed growth. In 2000, it increased by 7% over 1999. In 2001, due to the recession in the world IT industry and the impact of the "9.11" incident in the United States, the world's PA6 and PA66 demand in 2001 showed negative growth, a decrease of 3% over 2000, from 1.46 million tons to 1.42 million tons. Among them, PA6 remained the same as in 2000, and PA66 was greatly affected by downstream industrial markets such as automobiles, electronics and electrical, and decreased by 5% year-on-year. The changes in the world demand for PA6 and PA66 are shown in the following table:

Table 1 Changes in the world demand for PA6 and PA66 (10,000 tons)

Variety 1999 2000 2001

PA6 63.5 68 68

PA66 72.5 78 74

Total 136 146 142

Supply and consumption of PA engineering plastics

Compared with general plastics, the production and consumption of engineering plastics, including PA, are more concentrated in developed countries. The PA production capacity of the three major countries and regions, including the United States, Europe and Japan, accounts for 90% of the world's total production capacity, and consumption accounts for 80%. The data provided by the German journal "Plastics" is: the PA production capacity of the United States accounts for 31% of the world, Europe accounts for 45%, and Asia accounts for 40%. Ube Industries of Japan reported that the production capacity of PA engineering plastics in the United States, Europe and Asia is 500kt/a, 760kt/a and 442kt/a respectively (of which Japan is 282kt/a). Although the data from different sources are different, the general view is consistent. The production and consumption of PA resin for engineering plastics are faster than that of PA resin for fiber, so its share in PA resin is also increasing year by year.

Production of PA6 and PA66 in 2001

The world production of PA6 and PA66 in 2001 is shown in Table 2.

Table 2 Production of PA6 and PA66 in various regions of the world (2002 data unit: 10,000 tons)

Country PA6 PA66 Total

USA 23 33 56

Europe 25 26 51

Japan 12 7 19

Others 8 8 16

Total 68 74 142

As can be seen from Table 2, the output of PA6 in Japan is higher than that of PA66, which is different from that in Europe and the United States. According to data provided by the American Chemical Systems Company, the total production capacity of PA engineering plastics in the world is 1,600 kt/a. The top four companies and their shares are: DuPont is 390 kt/a, accounting for 24.4%; BASF is 220 kt/a, accounting for 13.5%; GE-Honeywell is 200kt/a, accounting for 12.5%; Rhodia is 145kt/a, accounting for 9.1%; other companies are 645 kt/a, accounting for 40.2%. The leading manufacturers are all large-scale comprehensive petrochemical or chemical companies with strong strength. In addition to large-scale advanced polymerization equipment, they all have attached compounding plants to increase the number of grades available and provide special products that meet the market and user requirements. Other manufacturers include Allied Signal, Solulia-Dow, UBE, Bayer, DSM, RodiciPlastics, EMS, Toray, Asahi Kasei, Mitsubishi Engineering Plastics, etc. There are also some independent plastic compounding plants such as the famous American RTP and LNP companies, which provide brands with characteristics and user-required performance. A wide variety of products, flexible supply and short cycle are the characteristics of these suppliers.

The demand for PA in different regions of the world between 1996 and 2000

is shown in Table 3. The consumption in Europe and America accounts for 41.7% and 30.2% of the world total respectively, which is absolutely dominant.

Table 3 Demand for PA around the world (unit: 10,000 tons)

Region 1997 1998 1999 2000

America 37.3 38.8 41.5 45

Europe 52.2 53.6 58 62

Asia 28.6 28.2 30.5 32

Others 7.9 7.5 8 8

Total 126 128.1 138 147

Consumption structure of PA

PA has excellent properties. Its properties can be further improved after being modified by filling, strengthening, toughening, flame retardant, etc. It is widely used in automobile, electrical and electronic, packaging, machinery, furniture, building materials, sports and leisure products, daily necessities, toys and other industries. The automobile industry is the largest user of PA, followed by the electrical and electronic industry. Of course, the consumption structure of each country is different. The PA consumption structure of the United States, Europe and Japan in 1999 is shown in Table 4. As can be seen from Table 4, Japan's film share (mainly PA6) is larger than that of other countries and regions, while the proportion of PA used in the European electronic and electrical industry is relatively high.

Table 4 Consumption structure of PA resin applications in 1999 (unit: 10,000 tons)

Item United States Europe Japan

Automobile 18.3 21.9 8.2

Electronic and electrical appliances 4.1 11.5 2.5

Industry 3.7 3.9 1.9

Film 6.1 6 5.5

Others 9.3 14.7 3.5

Total 41.5 58 21.6

The PA engineering plastics market in Western Europe in 2000 was distributed as follows:

Automobile 31%, electronic and electrical appliances 21%, packaging 11%, daily consumer goods 11%, machinery 7%, construction 6%, sports and leisure 5%, and others 8%.

In 2001, the consumption structure of PA resin in Japan was: 37% for automobile and other transportation industries, 23% for electronics and electrical appliances, 10% for industrial crystals, and 30% for others (monofilaments, films, wires and cables, and pipes).

Although data from different sources vary, it is undoubted that automobiles are the largest consumer market for PA, and the industry with the most applications of PA alloys and blends is also the automotive industry. For decades, PA has successfully replaced metals for automotive interior and exterior parts, body and under-hood parts. In order to save energy and reduce consumption, developed countries have long begun to accelerate the pace of lightweighting and plasticization of automobiles, and regard the amount of plastic used in each car as a symbol of automobile modernization and technological progress. Some models use more than 20 ke per car. For example, BMW's BMW 328i uses 162 ke of engineering plastics per car, accounting for 11.6% of the total weight of the car, of which PA uses 21.8 kg; ", so it can be said that the automobile industry is one of the main driving forces for the development of the engineering plastics industry. PA has balanced mechanical properties, good thermal properties and flame retardancy, can withstand the long-term working requirements of electronic and electrical devices, and is suitable for making various switches, gears, household appliance components, electronic facilities, large automotive electronic connectors, wiring heads and hand-controlled power tool components. PA single-layer film and its multi-layer with other plastics Layer film and container can be used to package meat, sausage, cheese, peanuts, fish, liquid food, etc., which can extend the shelf life of goods.

The global PA6 consumption structure in 2001 was: 34% for automobiles, 8% for electronics and electrical appliances, 18% for machinery, 30% for monofilaments and films, and 10% for others; while the PA66 market was: 50% for automobiles, 30% for electronics and electrical appliances, and 20% for machinery'q. It can be seen that the two main PAs have different consumer markets.

Production method

Add a certain amount of dimethylacetamide to the reactor, and then add 4,4¡ä-diaminobiphenyl ether. After it is basically dissolved, add pyromellitic anhydride. The reaction temperature is controlled at about 50¡ãC to obtain a transparent polyamic acid top polymer solution. Prepolymer After removing the solvent, the polyimide is separated by high temperature dehydration cyclization at 300¡æ or salt precipitation by adding acetic anhydride (dehydrating agent) and triethylamine (neutralizing agent).

3. Physical and chemical properties

The properties of molding powder and molded plastic are as follows:

3.1 Molding powder

Appearance: light yellow powder

Fineness: ¡Ü250¦Ìm

Apparent density: ¡Ý0.35 (g/cm3)

(0.5% o-cresol solution, measured at 35¡æ)

3.2 Molded plastic

Appearance: amber translucent

Surface resistivity: ¡Ý1015¦¸

Volume resistivity: ¡Ý1016¦¸¡¤cm

Compression strength: ¡Ý160MPa

Flexural strength: ¡Ý180MPa

Impact strength: ¡Ý100kJ/m2

Dielectric loss tangent (106 Hz) 1¡Á10-3¡«5¡Á10-3

Dielectric constant (106 Hz) 3.0¡«3.5

Processing method

People are not unfamiliar with nylon. Nylon products are everywhere in daily life. It was developed by Carothers, an outstanding American scientist, and a research team under his leadership. It is the first synthetic fiber in the world. The emergence of nylon has given textiles a new look. Its synthesis is a major breakthrough in the synthetic fiber industry and an important milestone in polymer chemistry.

In 1928, DuPont, the largest chemical industry in the United States, established the Institute of Basic Chemistry. Dr. Carothers, who was only 32 years old, was hired as the head of the institute. He mainly engaged in research on polymerization reactions. He first studied the polycondensation reaction of bifunctional molecules, and synthesized long-chain polyesters with high molecular weight through the esterification condensation of diols and dicarboxyls. In less than two years, Carothers made important progress in the preparation of linear polymers, especially polyesters, and raised the relative molecular weight of polymers to 10,000~25,000. He called polymers with a relative molecular weight higher than 10,000 high polymers. In 1930, Carothers' assistants found that the molten high polyester prepared by the polycondensation reaction of diols and dicarboxylic acids could be drawn out like cotton candy, and the fibrous filaments could continue to stretch even after cooling, and the stretching length could reach several times the original. After cooling and stretching, the strength, elasticity, transparency and gloss of the fibers were greatly increased. The peculiar properties of this polyester made them feel that it might have great commercial value and it was possible to spin fibers from molten polymers. However, further research showed that obtaining fibers from polyester was only of theoretical significance. Because high polyester melts below 100oC, it is particularly soluble in various organic solvents, but it is slightly more stable in water, so it is not suitable for textile use.

Carothers then conducted in-depth research on a series of polyester and polyamide compounds. After many comparisons, he selected polyamide 66 (the first 6 represents the number of carbon atoms in the diamine and the second 6 represents the number of carbon atoms in the diacid), which he first synthesized from hexamethylenediamine and adipic acid on February 28, 1935. This polyamide is insoluble in ordinary solvents and has a melting point of 263oC, which is higher than the commonly used ironing temperature. The drawn fiber has the appearance and luster of silk, and is also close to natural silk in structure and properties. Its wear resistance and strength exceed any fiber at the time. Considering its properties and manufacturing costs, it is the best choice among known polyamides. Then, DuPont solved the problem of industrial sources of raw materials for the production of polyamide 66. On October 27, 1938, it officially announced the birth of the world's first synthetic fiber, and named polyamide 66 (PA66) nylon. Nylon later became the general term for all polyamides synthesized from coal, air, water or other substances, with wear resistance and similar protein chemical structure in English. The process of producing nylon is as follows:

1. Drying temperature and method

Including the drying method, temperature and time of the raw materials. The drying of raw materials mainly adopts ovens or hopper dryers. Studies have found that these two drying methods can only remove the surface moisture of the raw materials, but cannot remove the inherent moisture of the raw materials. Therefore, vacuum drum dryers should be used for drying. The advantage of vacuum drying is that it can not only remove the deep moisture of the raw materials, but also use vacuum extraction to improve the drying efficiency and prevent the raw materials from oxidizing and yellowing. The drying process should use steam with a pressure of 0.25MPa. The drying time can be reduced from 10 hours to 3 hours, and the drying temperature can be reduced from 105¡æ to 100¡æ. This drying process can control the moisture content of the raw materials below 0.2%, and also solves the degradation and yellowing of the raw materials in the drying process, so that the materials maintain their original flexibility.

The drying equipment and the injection molding machine barrel are generally not directly connected, and the dried raw materials need to be stored in the intermediate silo. Special attention should be paid to the sealing problem of the dried raw materials during storage. If secondary water absorption occurs during storage, bubbles or hollowing will still occur during production.

2. Injection method

A relatively ideal injection molding process was obtained through a large number of experiments. First, the injection temperature was changed from the previous 3 stages to 4 stages. The temperatures of stages 1 to 4 are: 270~275¡æ, 280~285¡æ, 285~290¡æ and 280~285¡æ. Then the injection rate was also changed from the previous 2 levels to 3 levels. The injection rates of levels 1 to 3 are 35, 60, and 50 (relative values), that is, the injection method was changed from fast to slow in the early stage to slow-fast-slow.

3. Nozzle temperature

The nozzle temperature should be lower than the barrel temperature, generally controlled at around 280¡æ. If the nozzle temperature is too high, the product will have pores or yellowing, and in severe cases, the product will be brittle; if the nozzle temperature is too low, the melt will solidify and block the nozzle or the condensate will be injected into the nozzle, and the product will be brittle or the weld mark will be obvious.

4. Mold temperature

The mold temperature should be controlled at 70~90¡æ. If the mold temperature is too high, the cooling rate will decrease, thereby extending the molding cycle, and the product will have dents or bubbles. On the contrary, if the cooling rate increases, the melt flow crystallization phenomenon is likely to occur, the product will be brittle, and the weld mark will be obvious.

5. Injection pressure

Injection pressure is a key factor in injection molding production. Injection pressure includes plasticizing pressure and injection pressure. The plasticizing pressure should make the screw plasticizing and feeding smooth, and it should generally be maintained at 60%~80% of the maximum plasticizing pressure of the injection molding machine; the injection pressure should be to ensure that the product completely fills the cavity without producing batch peaks. When producing nylon shuttle blanks, it is generally maintained at 40~60MPa. When the injection pressure is too high, the melt fills the mold too quickly, enters in the form of turbulence near the gate and causes "free injection", entraining air into the part, thus producing cloud spots or golden flash defects on the surface of the part. When the injection pressure is too low, the raw material enters the cavity slowly, and the raw material close to the cavity wall will increase in viscosity due to the rapid drop in temperature, and quickly spread to the flow axis, making the plastic flow channel very narrow in a very short time, greatly reducing the cavity pressure, resulting in defects such as ripples, lack of material, bubbles, etc. on the surface of the part, and sometimes brittle fracture.

6. Cooling, holding pressure and injection time

The length of time directly affects the quality and production efficiency of the part. The time for producing nylon shuttle blanks mainly includes cooling time, holding pressure time and injection time. If the cooling time is too short, it is easy to cause problems such as sticking to the mold in the main channel and rupture of the water nozzle. Generally, it is appropriate to set the cooling time at 8~10 s. The length of the holding time is directly related to the material temperature. When the melt temperature is high, the gate closure time is long and the holding time is also long; conversely, the holding time is short. When the holding time is too short, the product is easy to be brittle, the size is unstable, and it is easy to have dents and bubbles; when the holding time is too long, the product is easy to stick to the mold. Generally, it is appropriate to set the holding time at 10~15 seconds. The injection time is determined by the level of graded injection. In production, the previous 2-level injection process is changed to 3-level injection, and each level can be controlled at different times. When the mold filling time is appropriate, the internal and external stresses of the product are consistent, the shrinkage depression is small, the color is more uniform, and the fullness of the product can be better guaranteed. If the injection time is too short and the filling is insufficient, the product is prone to dents, delamination, poor bonding, brittleness and other symptoms. If the injection time is too long, the product is prone to slope peaks, yellowing, warping, and even burns and scorching. Generally, the total injection time is set at 20 seconds, which is allocated according to 3 levels. The levels can be adjusted according to actual conditions.

7. Shaping and post-processing of nylon shuttle blanks

The shaping and post-processing of nylon shuttle blanks are mainly to stabilize the size of the parts and eliminate the internal stress of the parts, so as to improve the dimensional stability of the parts. First, place the pre-prepared shaping block in the cavity of the shuttle blank, and then place it in water or potassium acetate solution (so that it is completely immersed), control the solution temperature to 100¡ãC, and reach moisture absorption equilibrium after 24 hours of treatment. Then take out the module, wipe it clean and dry it, and after passing the inspection, it is the finished shuttle blank and can enter the next process.

The annual output of nylon fiber in the world has reached millions of tons. Nylon has been widely used in civil and industrial fields due to its unique and superior properties such as high strength and wear resistance.

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