POLYAMIDE (PA)

HISTORY

1.
1889 Gariel and Maass synthesized polyamide in the laboratory first.
2.
1939 DuPont commercialized PA66 with the trade name of Zytel (first is Nylon).
3.
1941 DuPont invented PA610 and commercialized it.
4.
1937 P.Schlack of IG Farben (now joint-venture of BASF) invented PA6 and commercialized it in 1942.
5.
1958 Sailient Chemical of China developed PA1010 based on castor oil and commercialized it in 1961.
6.
1963 Huls of Germany started to manufacture PA12 and commercialized it in 1966.
7.
1984 DSM of Holland succeeded in development PA46 and commercialized it in 1990.




PROPERTIES
The chemical structure of PA6 and PA66 as below:

PA is a semi-crystalline thermoplastic engineering polymer which is the resin with amino group - [NHCO] - in the repeating units of macromolecules main chain. PA is a tough angular translucent or milky white translucent crystalline resin with the molecular weight from 15,000 to 30,000. The main kinds of Nylon are PA6 and PA66, which are in the absolute dominant position. The next are PA11, PA12, PA610 and PA612.Besides, there are PA1010, PA46, PA7, PA9 and PA13. New kinds are PA6T, PA9T and special nylon MXD6 (barrier resins), etc. Modified kinds include: Enhanced nylon, nylon casting monomer (MC nylon), reaction injection molding (RIM) nylon, aromatic nylon, transparent nylon, high impact (Ultra tough) nylon, electroplating nylon, conductive nylon, flame-retardant nylon, blends of nylon and other polymers and alloys, etc.

1.
General properties: Nylon has good overall performance: Wear-resistant, ductile, lightweight, resistance to chemicals, heat and cold, easy processing, self-lubricating, non-toxic and easy coloring. Thermal expansion and inaccuracy caused by water absorption should be paid attention to use nylon products; Poor resistance to acid and light; Stiffness, wear resistance and strength can be improved after reinforced by glass fiber. The biggest feature of Nylon is good toughness which increases along with the molecular weight increases. Among nylon, PA66 is the hardest and stiffest, but of worst toughness. The order of toughness is: PA66 <PA66 / 6 <PA6 <PA610 <PA11 <PA12. Good fatigue resistance, low friction coefficient and good wear resistance. Resistance to chemicals, such as hydrocarbons, ketones and esters, etc. Nylon is a yellow translucent solid with the relative density of about 1.13 to 1.7.
2.
Mechanical properties: Nylon has the properties of high mechanical strength, high softening point, high heat resistance, low friction coefficient, wear resistance, self-lubricating, shock absorbing and silencing and conspicuous fatigue resistance. The affinity of nylon and glass fiber is very good, thus easily enhanced. But the tensile strength, stiffness, hardness and accuracy of Nylon will be decreased after the absorption of water. Will have thermal oxidation with oxygen in the long-term high-temperature environment and at the beginning, become brown and then crack which is not suitable for outdoor use. Outdoor use can only be achieved by adding additives. Poor creep resistance which can be improved by glass fiber reinforced.
3.
Thermal performance: Excellent heat resistance with high HDT. HDT of GF reinforced PA6 & PA66 are 220°C & 250°C while unfilled are 63°C & 70°C, RTI of unfilled PA6 & PA66 are 105°C while GF reinforced are 220°C & 250°C, RTI of high crystalline nylon, such as PA46, is 150°C, Heat resistance of aromatic nylon can be up to 260°C.
4.
Electrical properties: Can be used for electrical insulations owing to its better electrical properties in the low temperature and low humidity environment. However, it can’t be used as insulating material after water absorption at high frequency and humidity environment owing to its high water absorption.
5.
Flame retardance: The flammability of Nylon is UL94V-2 with oxygen index of 24~28. The decomposition temperature of Nylon is more than 299°C and it will ignite itself at 449°C~499°C. Flame retardance can be achieved by adding flame retardants.
6.
Weather resistance: Excellent weather resistance.
7.
Resistant chemical substances: Resistant to most organic solvent, especially oil. Resistant to organic solvents, such as alcohol, aromatic, esters, ketones, the majority of organic acids, aromatic hydrocarbon and alcohol (methanol), etc. It is a material of first choice for automotive owing to its outstanding oil resistance. Resistant to alkali and weak acid. Inert to aromatic compounds. Can be used as packaging materials of lubricants and fuels, etc.
8.
Non-resistant chemical substances: Strong acid solvents.
9.
Hydrolysis resistance: Excellent hydrolysis resistance.
10.
Radiation resistance: Poor radiation resistance.


MOLDING CONDITIONS

PA can be molded in the following ways:
1. Injection 2. Extrusion 3. Blow molding 4. Spray forming 5. Casting 

Injection molding conditions:

1.
Predrying: Nylon must be predried before processing since it is very easy to absorb water and the absorption is as high as 3-5%. Vacuum drying is the best (especially for light-colored and flame-retardant grades) with the temperature of 90°C for 4-6 hrs and 110°C for 6-10 hrs by hopper dryer.
2.
Use of regrind: Less than 10% of regrind can be added so long as it is well predried and not degraded for flame retardant grades. 20% of regrind can be added for non-flame retardant grades.
3.
Thermal stability: PA can’t stay in the cylinder for more than 20mins, otherwise it is easily degraded.
4.
Injection machine: Standard screw, special geometry with strong plasticating capability; Non-return ring and through nozzles; High wear-resistant bimetallic barrel for GF reinforced PA.
5.
Cylinder capacity: Weight of one shot must be 35%~65% of the cylinder capacity.
6.
Nozzle and locking system: Shut-off nozzle is desired since Nylon has very good flowability and is very easy to drool, thus the material is wasted and processing is influenced.
7.
Screw: Single-ring, equidistance and all thread types. L/D ratio is 12 ~ 20 and the geometric compression ratio is 3 to 4 with chrome nitrided steel better.
8.
Cylinder temperature: Different temperatures for different nylons. Usually 240°C~290°C.
9.
Mold temperature: Different temperatures for different nylons. Usually 60~100°C.
10.
Injection pressure: 100~160MPa (1000~1600bar). It must be 180MPa (1800bar) for thin-walled and long distance products, such as cable ties, etc.
11.
Injection speed: High.
12.
Back pressure: 2~8MPa (20~80bar). Accurate adjustment will be needed since too high a back pressure can cause uneven plasticating.
13.
Screw speed: High screw speed at 60~100rpm.But it is better to set up a little lower as long as the plasticating can be completed before the end of the cooling time with low screw torque.
14.
Molding shrinkage: Unfilled: 0.7~2.0%; 30% GF reinforced: 0.3~0.8%.
15.
Secondary processing: PA can be machining, welding and bonding. If heat treatment is needed, it should be 10-20°C higher than its use temperature and the time is decided by the wall thickness, usually 10-30mins. Moisture treatment is needed to realize the toughness of PA and the procedure is: Parts will be immersed in boiling water or potassium acetate solution (potassium acetate and water ratio of 1.25:100, the boiling point of 121°C), the immersion time decided by the wall thickness of the biggest parts with 1.5 mm for 2 hours, 3 mm 8 hours and 6mm 16 hours. Moisture treatment can improve the plastics crystal structure, parts toughness and distribution of internal stress.



MOLD DESIGN

1.
Mold material: SKD — 11, NAK — 55 and S50C are recommended.
2.
Runners: The diameter of the runners should be around 30mm for unfilled PA since it has low MFR.5~8mm diameters for GF reinforced PA and all the runners should be round.
3.
Gate: Sprue must be as short as possible. Various gates can be used. Small gates can’t be used for large parts in order to avoid excessive pressure or too much shrinkage. It is the best for the gate thickness to be equal to the thickness of plastic parts. The minimum diameter of pinpoint gate is 0.75-0.8 mm. Hot runner mould is very effective but it must be accurately adjusted to prevent leakage or solidification of materials in the nozzle. In hot runner, gate size should be smaller than the cold runner. Cold slugs must be set up at the end of the sprue.
4.
Release taper: At least 1/2~1°.
5.
Air vent: Air vent must be reasonable with the depth of 0.01 — 0.03mm.


APPLICATIONS


1.
Automotive industry: The intake system, cooling system, fuel system and transmission system, etc.
2.
Electrical and Electronics: Contactors, small circuit breakers, fuse switch boxes, leakage protections and relay boxes, etc.
3.
Home-use Electric Appliances: Rice cookers, electric vacuum cleaners and high-frequency electronic food heaters, etc.
4.
Mechanical industry: Machine tools, reducers, printing machinery, manufacturing textiles, building materials of construction machinery, cutting machines, cement machinery, pipe joints, bearings cages, various valves, shells, gaskets and power tool enclosures, etc.
5.

Medical industry: Medical blood transfusion tubes, blood collectors and transfusions, etc.



AUTOMOTIVE PART
ELECTRIC PART




MANUFACTURERS AND BRANDS


MANUFACTURER
BRAND
Lanxess Durethan
AsahiKASEI Leona
BASF Ultramid
DuPont Zytel
UBE UBE Nylon
Toray Amilan
Solutia Vydyne
DSM Akulon
LG Chem Lumid
Rhodia Technyl
MEP Novamid
KEP Kepamid
EMS Grilon/Grilamid