POLYMERISATION OF ALIPHATIC POLYAMIDES
As already indicated, the fiber forming polyamides are produced commercially by reacting diamines with dibasic acids, by self condensation of an amino acid or by opening of a lactam ring. Whatever method is chosen it is important that there should be equivalence in the number of amines and acid groups for polymers of the highest molecular weight to be obtained.
NYLON 66,69,610,& 612
The Nylon 66 salt is prepared by reacting the hexamethylenediamine and adipic acid in boiling methanol, the comparatively in soluble salt (melting point 190-191*C) precipitating out.
A 60% aqueous solution of the salt is then run in to a stainless steel autoclave together with a trace of acetic acid to limit the molecular weight (9000-15000). The vessel is sealed and purged with oxygen free nitrogen and the temperature rose to about 220*C. A pressure of 1.7 Mpa is developed. After 1-2 hours temperature is raised to 279-280*C and steam
bled off to maintain the pressure at 1.7 Mpa. The pressure is then reduced to atmospheric for one hour, after which the polymer extruded by oxygen free nitrogen on to a water cool casting wheel, to form a ribbon, which is subsequently disintegrated. Nylon 610 is prepared from the appropriate salt (melting point 179*C) by a similar technique. Nylon 612 uses decane-1, 10-dicarboxilic acid. Azelaic acid is used for 69.
NYLON 6
Both batch and continuous processes have been used for the manufacture of nylon 6. In a typical bath process caprolactam, water (which acts as a catalyst) and a molecular weight regulator, e.g. acetic acid are charged in to the vessel and reacted under a nitrogen blanket at 250*C for about 12 hr. The product consists of about 90% high polymer and 10% low molecular weight material such as the monomer. In order to achieve the best physical properties the low molecular weight materials may be removed by leaching/ vacuum distillation. In the continues the reactants are maintained in reservoirs which continuously feed reaction columns kept at a temperature of about 250*C.
The polymerization casting of Nylon 6 in suit in the mold has been developed in the recent years. Anionic polymerization is normally employed; a typical system uses as a catalyst 0.1-1 mol.% of caprolactam and 0.15-0.50 mol.% of the sodium salt of caprolactam. The reaction temperature initially is normally between 140 and 180*C but during polymerization this rises by about 50*C. Moldings up to one ton in weight are claimed to have been produced by these casting techniques.
NYLON 11
This polymer may be prepared by stirring the molten w-aminoundecanoic acid at about 220*C. The reaction may be followed by measurements of electrical conductivity of the melt and the intrinsic viscosity of solutions in m-cresol 4. During condensation 0.4 - 0.6% of a 12-membered ring lactam may be formed by intramolecular condensation but this is not normally removed since its presence has little effect on the properties of the polymer.
NYLON 12
The opening of the caprolactam ring of Nylon 6 involves an equilibrium reaction that is easily catalyzed by water. In the case of Nylon 12 from dodecanelactam, higher temperature, i.e. above 260*C, are necessary for opening the ring structures but since in this case the condensation is not an equilibrium reaction the process will yield almost 100% of the polymer.
PROCESSING CONDITIONS
There is no foolproof way of predicting the best operating conditions for a particular Nylon composition in a specific injection molding process. The topic, processing conditions include following under mentioned aspects, which have to followed while processing Nylon.
MATERIAL HANDLING
Efficient material handling is the basic to operating the Nylon molding system because of the sensivity of Nylon to moisture and contamination. A well-engineered system will do the following:
- Maintain a uniform and low level of moisture.
- Avoid contamination.
- Maintain a uniform and adequate temperature.
- Minimize waste and spillage.
- Maintain a uniform speed to the molding system in terms of virgin
and regrind ratio, particle size, and material level in hopper.
Oil and/or grease contamination usually occurs after ejection of the molding and should be corrected immediately; it should be kept under control of preventive maintenance. A common cause of grease contamination is grease leaching from around the ejector pins, the mold should be striped and cleaned thoroughly and then, reassembled using a white PTFE based lubricant.
CONTROL OF MOISTURE
Control of moisture requires consideration of the plant envoirment, the polymer storage, and the effects of varying degrees of exposure in handling and processing. Any system that controls the total plant envoirment offers the best safeguard against moisture-related problems and hence increases downtime and improves quality. Plants can control their envoirment by installing air conditioning and dehumidifying facilities. The objective is to keep relative humidity below 35% and temperature below 75*F even in the worst of summer days.
Nylon should be stored in dry places at temperatures near to that of the operating area. Cold material can condense moisture and carry it into the molding machine. When storage in a cold location can’t be avoided, the container must be allowed to come to the temperature of the molding area before opening. It is good to practice to dry materials whenever either virgin or regrind contains over 0.3% of water and processes that involves high temperature or long cycle time may require moisture content below 0.2%. In general, exposure times of polymers at the loading station, through tubes, and in conveyers cannot be controlled unless there is a uniform demand and the accurate metering. These systems are open to the atmosphere and unless the envoirment is controlled, material is vulnerable to them. The optimum situation is naturally total and prompt feedback of the runner system and selection of the container size that will be consumed in an adequate time.
CONTROL OF FEED
Control of feed involves control of the ratio of virgin to regrind resin of particle size, and of the level of the material in the hopper. Change in processing characteristics (melt viscosity due to change in mol. weight, melting rate because of variation in particle size, and freezing point via nucleation by chance contamination) can be caused by random introduction of the virgin and regrind materials. These changes can result in quality fluctuations in the final product. In general, 50% regrind levels will not impair the performance of engineering parts made of Nylon. It is only in very specific situations, such as the molding of the glass-reinforced Nylons, critical appearance applications, or tight dimensional control molding.
Grinding equipment should be selected and maintained to produce regrind with an average particle size close to that of virgin Nylon. They absorb moisture and melt faster and can cause discoloration and loss of properties. A constant level of material in a hopper ensures uniformity of envoirmental exposure and of feed.
Hopper should be streamlined to allow free flow of the material into the machine barbell. Sometimes it is helpful to bias the hopper discharge in the direction of screw rotation. The hopper capacity should be such as to allow at least 1hr.of machine production.
DRYING
Most plastic materials are supplied dry and ready for use although some are wet and must be dried before being molded. It makes sense however, to prevent water contamination occurring, as, even if the material is going to be dried, this will reduce energy consumption. A measure of how much water a material will absorb is given how much it will absorb in 24hr at room temperature. If greater than approx. 0.2%, drying is usually necessary; if less, this means that drying is not normally necessary.
The time required to dry Nylon depends on the initial moisture contents, the goal level of moisture, the thickness of polymer particles, and the temperature and humidity in the drying device. Bed thickness in drying trays is normally not a factor if kept below 1inch(2.5cm). The type of Nylon affects the initial moisture level and drying rate and hence the drying time, but this problem has not been carefully quantified because the degree of variation has not been a significant factor with the currently available aliphatic.
PROCESS VARIABLES AND THEIR EFFECTS
In order to judge performance, there must be a reference to measure performance against. In the case of a plastic mold, the cavity pressure profile is a parameter that is easily influenced by variation in the process. It is selected as a reference for this discussion. This section points out how the variables affect this parameter and their effect on the part being molded.
There are four groups of variables that when lumped together have similar influences:
Group-1: Melt Viscosity & Fill Rate. Typical non-process-control machines apply a fixed injection hydraulic pressure to the ram piston. The resultant force in turn is contracted by the speed of the ram in the viscous plastic melt. The lower the viscosity or higher the hydraulic injection pressure, the faster the fill rate. If the fill rate is too fast, the cavity pressure increases long before boost time out. The result is over packing of the part. Some of the effect are flashed and or out of tolerance parts on the (+) side. If the fill rate is too slow, just opposite happens; cavity pressures indicate un packed parts, resulting in poor surface finish, voids, and dimensional problems.
Group-2: Boost Time. Typical non-process-control machines have a boost timer to terminate the fill and pack cycle. Even with the good fill rate repeatability; variation in peak cavity pressures can result from variations in the time the ram is in the boost mode. These variation typically result from
valve and solenoid response times from one cycle to the next, as well as long term drifts of these components. Cavity pressure variations that occur when coming out of boost have the same effect on the part as the Group-1 variables. The problem is addressed separately here because its solution is different from the solution for Group-1 variables.
Group-3: Pack & Hold Pressure. Typical non-process-control machines use the same ram pressure setting during the packing of the mold as was used during the filling of the mold. The level of the pressure setting is that which gives good mold fill-out without flashing the mold. Variations in this pack pressure result in cavity pressure profile variation. These cavity pressure variations indicate an inconsistency that can be causing dimension and surface finish problems. These pressure variations are a result of relief valve repeatability problems cause by the valve wear and temperature conditions as well as shot-to-shot variations.
After the part has been packed, the boost timer reduces the applied hydraulic pressure to a hold pressure while the part cools. At this point the cavity pressure sensor starts to lose accurate plastic pressure reading because the part surface is being to harden.
Group-4: Recovery or Plastication. The variables that are involved during recovery do not appear on the cavity pressure profile until the next fill cycle. Recovery has much to do with the viscosity of the melt. Recovery variable can be identified, however these variables have to do with how much energy is added to the plastic material: this energy and the resulting viscosity will vary.
The three main variables in descending order of importance are:
- Screw torque times speed product.
- Back pressure time rate of ram with drawl product.
- Barrel temperature.
Efforts to control these variables typically have to do with flow and or relief valves, which have their own short and long-term problems.
TIPS FOR MACHINING NYLON
Storage: Nylon has a high coefficient of thermal expansion (about three times that of aluminum) and low heat conductivity. Make sure that it has been exposed to normal room temperature for several hours before it is machined into finished parts.
Sawing: Nylon can be easily sawed on standard metal working equipment. Wood working equipment may be suitable but the high cutting speeds may cause excessive heat build-up. A blade that has been used for cutting metal is usually not sharp enough for Nylon. Use a new coarse tooth blade with good set. Coolant may be used to control heat buildup and to prevent melting the Nylon.
Holding: Keep in mind that Nylon is not as strong as metal and can be deformed by improper chucking methods. On small accurately sized rod, use standard spring collets. On larger parts, use a 6-jaw universal chuck instead of a conventional 3-jaw chuck to distribute the holding force more uniformly. For thin walled tubular shapes, machine soft jaws so that the part is almost entirely confined.
Turning: Satisfactory finishes can be easily obtained on Nylon over a wide range of surface speeds. Use tools that are honed sharp and have high rake and clearance angles, to minimize cutting force and reduce heat build-up. Chips will be continuous and tough. They should be directed away from the cut and prevented from winding around the work piece. Coolants are generally not necessary for lathe work unless there is excessive heat build-up.
Milling: Milling cutters should be honed sharp and should have high positive cutting angles. Care should be taken in clamping the part to prevent distortion. Double-faced pressure sensitive tape can be used to hold down flat parts. Cutting speeds and will be determined by the finish required and will be limited by heat build-up.
Drilling: Use conventional twist drill or flat type drills. Polished flutes will aid in the removal of chips. Do not use metal cutting reamers with Nylon. They do not cut freely enough. Drill small holes to size in one operation. Rough drill large holes and finish by single point boring.
Threading: Use only sharp taps and dies on Nylon parts. Don't use tools that have been used to cut metal. H5 or even larger oversized taps may be required because a threaded hole in Nylon closes in when the tap is removed. Threads to close tolerances can be easily single point chased.
Grinding: The large amounts of heat generated by grinding, together with the low heat conductance of Nylon, usually dictate that liberal amounts of coolant he used in most grinding operations. Thru-feed center less grinding of long, flexible parts of Nylon can be easily accomplished, and tolerances as close as 0.0005" are possible. Cylindrical grinding on Nylon is usually not required because it is easy to get good finishes and close tolerances on a lathe. Surface grinding of Nylon is usually not necessary. If a flat surface with close tolerances and good finish are required, the best approach is fly cutting in a milling machine. No, not cutting a fly on your milling machine, FLY cutting.
Stamping: Thin pieces may be stamped with standard equipment. Thick sections will require high shear angles if good edges are needed. Steel rule dies may be used for some parts.
Measuring: Use ordinary measuring equipment. However, use a light touch because the material is not as hard as metal. A micrometer anvil can deform a Nylon surface as much as several thousand. Homemade, soft plug and ring gauges are useful on thin walled parts. If extremely close tolerances are involved, make sure any temperature changes that the part will see are taken into account.
DRYING OF RESIN
Polyamides are characterized as hygroscopic plastics. Polymers of this type absorb moisture, which has to be removed before they can be converted into acceptable finished products. Very low moisture concentrations can be achieved through the utilization of an efficient drying system and proper handling of the dried material prior to and during the molding operations. Drying hygroscopic resin shouldn’t be taken casually. Simple tray dryers or mechanical convection hot air dryers, while adequate for some materials, simply are not capable of removing water to the degree necessary for proper
processing of hygroscopic polymers, particularly during periods of high ambient humidity.
The effect of excess moisture content in Nylon molding depends upon the process being employed. Splays, nozzle drool between shots, foamy melt, bubbles in the part, poor shot size control, or lower physical properties are the results of high water content during processing operations.
The most efficient and effective for hygroscopic polymers is one that in-corporate an air dehumidifying system in the material storage/handling network, which can consistently and adequately provide moisture free air in order to dry the wet polyamides. Although this kind of equipment is expensive initially, it results in improved production rate and lower rejects levels in the long run. There are variety of manufacturers and system from which to choose. While all systems are designed to accomplish the same end (i.e. dry polymer), the approaches to regeneration of the desiccant beds vary widely. Years of field experience with these systems have shown that break down in performance are not usually the fault of the equipment, but due to the user's lack of attention to preventive maintenance details as outlined by the manufacturers.
Manufacturers ref.
- CACTUS MACHINERY, INC. CANADA, suppliers of desiccant drying equipments.
- DRI-AIR INDUSTRIES, INC. Deals in solutions for drying plastic resins, mixing, blending and conveying virgin, regrind and plastic colorants.
DETERMINING MOISTURE CONTENT
In order to determine the effectiveness of the system, some methods of determing the moisture contents of the air in the drying system is recommended. The installation and monitoring of dew point meter in the drawing arrangement is a worthwhile investment. Both visual signals and recordings can easily monitor equipment performance. Dew point monitors can be purchased from most of the dryer manufacturers and be installed at the time of purchase or retrofitted at a later time. Also available portable types that can be used to spot-check various sections of the material-handling network. Although the investment is somewhat high ($500-$3000), the payback, when there are problems during productions is in incalculable
time and material savings. As the type of installation, the processor must decide what is best for his particular needs as well as his pocket book.
In addition to instruments designed for dew point determination, moisture analyzers are available that are capable of determining moisture content of either gases or solids to as little as 0.01% water. This type of equipment is relatively easy to use, and prices vary from around $2000-$8000.
Manufacturers ref.
- PANAMETRIC P.C.I, INC. Suppliers of moisture sensing applications.
- APPL, U.S.A. Suppliers of dew point monitors.
DRYING SYSTEMS FOR POLYAMIDES
Since drying and keeping the resin below the adequate moisture level (0.2%) is the basic necessity processing Nylon, especially Nylon 6 and 66, which have the moisture absorption level, range (1.3%-1.9% , 1.0%-2.8%) respectively.
More efficient dryers are now available that have been designed to meet the drying specifications of Nylon resins and keep energy consumption to a minimum. Since Nylon is hygroscopic and these plastics absorb moisture within the pellets or in the granules and form a molecular bond within the material. These highly efficient drying systems used is dehumidifying air to dry the material. More efficient drying results if, plastics material can be dried while a vacuum is applied during the drying process. However, because of the practical difficulties involved, vacuum drying is not often used in molding industry. A more popular method is desiccant drying .
According to experts at :
“Most manufacturers supply Nylon in moisture-resistant packaging, with a moisture content of 0.2 % or less. If the packaging is unopened and undamaged, it should be ready to process if transferred to the molding machine with minimal contact with the atmosphere. Most molders prefer to load the material into a hopper dryer for an added margin of safety. The hopper dryer must be a dehumidifying dryer. Air temperatures less than 180*F(80*C) are recommended to prevent yellowing of the pellets. Reground material and pellets from open containers need to be dried to below 0.2 % (or even less, depending upon the manufacturers' recommendation). Without knowing the initial moisture content, a drying time cannot be established that gives any certainty.”
DEHUMIDIFYING DRYERS
Using a dehumidifying dryer must dry hygroscopic plastics. Dehumidifying dryers absorb the moisture within the plastic material by using dry heated air brought down to a dew point of –40*F. This is done by use of desiccant beads. Desiccant beads are molecular sieves, which are synthetically produced crystalline metal aluminosilicates. All moisture is removed from the crystals during their manufacture.
There are two classifications for drying systems: single bed absorption system, which use one desiccant bed, and multibed absorption system, which use two or more desiccant beds. Dehumidifying dryers operate in a closed loop system. Air is brought in through a filter on the initial startup and sends to the desiccant bed to absorb the water out of the air when the desiccant beads absorb the water molecules. Approximately 1800 Btu per pound of moisture is released causing the air temperature to rise approximately 19*F. The air then travels to the heating unit where the air temperature is brought up to drying temperature specifications the dehydrated air is than circulated through the plastic in the drying hopper and recycled back through the unit, and the process is repeated.
Eventually the beads become saturated with moisture and have to be regenerated. This is done by blowing air heated to a temperature of 550*F through the desiccant beds. The elevated temperature drives the moisture out of the beads and into the ambient air. A multiple desiccant bed absorption system is the most efficient method for drying. A common absorption bed setup is double bed system. In double bed system, one bed is online drying material, while the other bed is in regeneration cycle. There are two types of air flow direction to regenerate desiccant bed: counter-current and co-current. When the desiccant bed is in working mode, the beads act like a sponge with water poured on one side of it. The water does not get dispersed evenly through the bed. Beads that make contact with the wet air will become moist first. Once these beads reach the saturation point, other beads in close proximity become saturated. This process continues until all the beads are saturated. In counter-current regeneration the air flows through the desiccant bed in the opposite direction of working airflow. This forces the moisture out of the desiccant bed opposite the direction in which it entered. The advantage of this is that the bed can be regenerated faster.
Dehumidifying dryers are sized similar to the hot air drying system. The hopper is sized by the production rate multiplied by the residence time. The dryer is then sized by the corresponding figures from the dryer sizing chart. The dryers are sized on a flow rate of 50ft/min. If the flow is more than 50ft/min the material will be blown around in the hopper. Any flow rates considerably less than 50ft/min may not have enough velocity to dry the plastic material.
Table: Drying times & air velocities for carrousel-type dryers at 160*F (71*C)
Table: Requirements for drying Nylon at 160*F ( 71*C) assuming 30% heat loss
PROPERTIES OF POLYAMIDES
MECHNICAL PROPERTY
Most mechanical properties data quoted by manufacturers and suppliers of polyamides refer to measurements taken over a comparatively short period using standard temperatures and strain rates. In this type of testing the load is normally applied at constant rate of strain, and the method is generally used for laboratory tensile, compressive, flexural and shears specification testing.
Tensile properties
In tensile testing strain rate in the range 1mm/min to 500mm/min are commonly used. Within this range appreciable differences in the stress strain relationships show up with increasing strain rates, the higher rates indicating greater moduli and yield stresses. Testing speeds should therefore always be quoted in specification and control testing, and account must be taken of this factor when comparing test results. Most quality control testing standards specify a number of testing speeds from which a selection can be made.
The increase of yield stress with strain rate is associated with an increase of the proportional (elastic) range where stress is proportional to strain, and usually with a decrease in the elongation at break and a decrease in the long linear elastic range before yield. Subjection of the polyamides to sufficiently high strain rates, such as encountered in impact loading, eliminate the yield zone and brittle fracture results. Decrease in temperature has a similar effect to increase in strain rate on modifying the shape stress strain curves of polyamides with increase in temperature the proportional elastic zone decreases and the yield stress is reduced. The effect of temperature on the tensile stress strain curve of dry Nylon 66 is shown in figure.
Compressive properties
In design polyamides components information on the behaviors of the material under compression may be as important as knowledge of the tensile properties.
At low strain the moduli of elasticity of Nylons in compression and tension are approximately equal. At high strains the compressive stress is larger than the corresponding tensile stress, indicating that the compressive yield stress is the greater. The difficulty of locating the yield point on the stress strain curve has let to the common practice of 0.1% or 1.0 % offset yield stress as indicative of the behavior of a particular Nylon in compression. Figure shows short-term stress strain curve for a typical Nylon 66 intension and compression.
Flexural properties
The determination of the short term flexural strength and modulus of polyamides in most conveniently and accurately carried out using one of variety of the standard method such as those describe in ASTM D790. Using the later method the flexural modulus and yield stress characteristics of the Nylon under standard conditions of the test are measured. The flexural mode of deformation has the advantage of allowing accurate measurement of modulus at low strains. As with tensile and compressive test flexural modulus and yield stress both decrease progressively as the test temperature is raised.
Hardness
Although the term hardness is sometime used to denote scratch resistance, the definition is restricted in the present context to describe resistance to indentation, i.e. the response of the material to a compressive deforming load applied in a particular way. In contrast, to the method used for short term tensile, compressive and flexural test, indentation hardness test are generally carried out under condition of constant load; also these tests major the properties of the material at or near the surface only, and not through out the bulk of the specimen. Usually the load is applied normal to the surface to the ball or needle indenter. Penetration or compression of the surface continuous until stress is raised beyond the yield point of the material. The tensile yield stress can infect be used to derive approximate values of hardness. Account must be taken of the temperature and moisture content of the material tested. The two most popular for polyamides being the Rockwell hardness tester (normally used on the R or M scale) and the shore durometer type D indentation tester.
These two instruments work on slightly different principals, but both are used in specifying material and also in quality control of fabricated parts. Since the shore durometer can be made portable it may be used locally on complete parts at the side of fabrication.
Table shows typical for the Rockwell and durometer hardness of some common polyamides.
Table: Typical Hardness Values For Commercial Nylons at 20*C; Dry As Molded
Since both the indentation test mentioned are virtually non destructive, the hardness method of test is favored in production inspection of injection molded parts.
Impact properties
The impact properties of the polyamides article depend on a number of factors for e.g. temperature of the material, moisture contents, speed of impact, stress concentration effects and anisotropy.
Standard impact test such as izod test of BS 2782 method 306A,D and E, the drop impact test of BS 2782 method 306B, and the tensile impact test of ASTM D1822 all, in general terms allow comparison of impact behavior of different Nylon types, and the same types subjected to different conditions. Generally the anticipated impact behavior of a material deduced from these tests is confirmed by its subsequent behavior INS service. The test is; however, used more frequently in specifying material quality. As might be expected the impact strength of a polyamide increases with temperature and moisture contents. In the absence of any second order transition, increasing stiffness and decreasing impact strength are exhibited with reduction in temperature below ambient. Moisture in plasticizers in the polyamides partially offset this effect, but there is no certain onset of brittleness.
Mechanical Damping
Polyamide can, without falling, be subjected to higher dynamic loads then the majority of the other engineering plastics. This is due to the high capacity of the material to absorb energy. In rapid cyclic stressing this result in superior vibration damping. When the frequency of application of stress on a component exceeds a critical value (which depend on the polyamide molecular weight and other factors) heat is generated in the bulk of the material, and ultimately failure occur due to excessive heat build up.
The damping capacity of a polyamide increases with a temperature and moisture content. The degree of damping deduced by observing the shear modulus, and conveniently measured by evaluating the mechanical loss factor.
MOISTURE ABSORPTION
Polyamides as a class are more hygroscopic then most thermoplastics. Liquid water or water vapor can be absorbed from the surroundings in proportions approaching 10% by weight of polymers, depending on the type. This property raises problems in processing and design of component made from polyamides, for not only are most important properties considerably are affected by the water absorption but the dimensional changes may occur that can affect the functioning of components.
For process such as injection molding, it is usually necessary to use the resins dried to low specified moisture content. The raw material must be supplied in sealed container, which shouldn’t be opened until just before processing operations. Absorption can be considered from a kinetic or thermodynamic point of view; i.e. by studding either the rate of process or the final equilibrium of the polyamide with the envoirment. Polyamides can absorb small and large amount of moisture depending upon CH2 /CONH ratio.
Effect Of Moisture Absorption
As we know that polyamide can absorb small or large amount of moisture according to type. Those types with a low CH2/CONH ratio (such as Nylon 66 or Nylon 6) can absorb over 9% of moisture, and the consequent effect on the mechanical properties can be profound. The moisture is not necessarily absorbed to saturation level and there often exist a moisture gradient across the section at right angles to the exposed surface, which result in a corresponding gradation of properties. The moisture in polyamide usually acts like a plasticizer and it facilitates molecular chain movement. This decreases stiffness and increases flexibility. Tensile and other moduli are reduced and elongation increased, therefore, with increasing moisture contents.
Some important properties of different polyamides are as follows:
INTRODUCTION TO NYLON 6
Nylons are the highly crystalline members in polymeric material. They are rigid and robust, but somewhat more expensive than the commodity plastics like, PE, PP, PS, PVC.
The family of Nylons consists of several different types. Nylon 6/6, Nylon 6, Nylon 6/10, Nylon 6/12, Nylon 11, Nylon 12, and Nylon 6-6/6 copolymer are the most common of these, Nylon 6 dominates the market. The numbers refer to how many methyl units (-CH2-) occur on each side of the nitrogen atoms (amide groups). The difference in number of methyl units influences the property profiles of the various Nylons.
Nylon-6 is manufactured by the ring opening polymerization of caprolactam; still contains 6% monomer at equilibrium, which is removed by extraction. Melting temperature of PA 6 is 225*C. PA 6 are frequently of low molecular weight; the resulting low viscosities requires special precautions during processing.
Properties: the properties of crystalline PA 6 are differing from those of polyolefin. PA 6 have high crystalline melting temperature as noted above, and are water attractive, the water absorption ( and the melting temperature) decreases as the methylene : amide ratio increases, ranging from 8-10%. The absorption of water effects dimensional stability, each 1% of water of water absorbed resulting in an increase in linear dimensions of 0.3% although this may be partially compensated by post molding shrinkage. The deformation behavior is considerably affected by moisture, the effective ‘modulus’ being reduced to 20-25% of its dry value for equilibrium ‘wet’ material.
PA 6 has good strength and toughness and excellent fatigue resistance, the toughness being increased markedly by absorbed water. Electrical properties are not outstanding even for dry polymer, and deteriorate further on absorbing moisture. In chemical properties PA 6 are attacked by acids, but are stable to alkalis; and are resistant to hydrocarbons, esters and glycols, but are dissolved by strongly hydrogen bonding solvents, e.g. phenol.
PA 6 are used mainly in textile but find many plastics uses, usually toughness is a prerequisite: some examples are oil filler caps for road vehicles; teeth in plastics zip fasteners; castors for light furniture; hopper barrels for good minces; radiators tanks for cars; hose connector for Electrolux vacuum cleaners, gears and especially in food processing equipment.
PRACTICAL WORK SEQUENTIAL
MATERIAL SELECTED
Nylon 6 was chosen for carrying out research and practical work due to, the market importance of the material. As Nylon 6 situate the most consumed polyamide around the world.
Now an important decision, to choose a specific supplier and its grade available was to be made. In that regard a specific grade of BASF PA 6 (Ultramid B35) was available at our own center, Plastics Technology Center. After a thought, that resin on hand was decided to be followed for further research work, because it satisfied and apt all our required necessities. The specifications provided by the supplier are mentioned below for a reference to generate.
Subcategory: Nylon; Nylon 6; Polymer; Thermoplastic
Key Words: Polyamide 6; PA6
Material Notes:
Low-to-medium viscosity injection molding grade for fast processing. Uses include rollers and thick-walled parts with very high impact strength. Product is externally lubricated.
Data was collected by ISO methods and provided by BASF
MOISTURE CONTENT IN RESIN
Definition: The amount of moisture in a material determined under prescribed conditions and expressed as a percentage of the weight of the moist specimen, that is, the original weight comprising the dry substance plus any moisture present.
How To Determine The Moisture Content: Weight about 1gm of resin in a small dish and note down the total weight by adding the weight of dish plus 1gm of resin. Place the dish in oven for an hour at 105*C. After hours confiscate the dish from oven and place it in silica gel jar (for the removal of moisture after heating) for almost 30 min and again weight the dish with resin. Calculate the difference by weight and multiply the difference by 100, which finally would give the percentage of moisture content in resin.
Moisture Calculated In Ultramid B35 (Undried):
Weight of dish 50.68gm
Weight of resin 1gm
Total weight of dish with resin 51.68gm
Weight of dish with resin after heating in oven for an hour 51.65gm
Difference in weight 0.03gm
Total moisture content 0.03 x 100 = 3.0%
DRYING OF ULTRAMID B35
Take the resin and spread it in the tray in the form of layer at maximum 1.5inch thick layer. After that put it in the dryer for at least 4 hours at 71*C.
Succeeding 4 hours take off the tray from the dryer and take at 1gm of resin for moisture content test in the fashion mentioned earlier.
Moisture Calculated In Ultramid B35 After Drying:
Weight of dish 44.405gm
Weight of resin 5.003gm
Total weight of dish with resin 49.411gm
Weight of dish with resin after heating in oven for an hour 49.364gm
Difference in weight 0.047gm
Difference in weight (5gm resin) is 0.047 therefore difference in weight (1gm resin) is 0.0094
Total moisture content 0.0094 x 100 = 0.094%
PROCESSING CONDITIONS FOR TEST DUMBLES
DRIED AND UNDRIED
Injection pressure 50 kg/mm2.
Nozzle temp. 95-100*C.
Feed Zone temp. 235*C.
Compression Zone temp. 255*C.
Metering Zone temp. 265*C.
MECHANICAL TEST METHODS &THEORIES
TENSILE TEST (ASTM D-638)
General Considerations: This test determines normal stress-strain relationships. Usually the sample is shaped so that the part will break out side the grip region. The most common shape for plastic sample is called a “dog bone” shape and is illustrated in figure 1-1. The increased width of the sample in the griping zones ensures that the sample will break with in the failure zone.
Griping Zone Griping Zone
Failure Zone
½ inch ¾ inch
8 ½ inches
Figure 1-1: Typical tensile specimen shape (dog bone) for plastics samples.
The major plastic tensile properties determine by this test are tensile strength, tensile yield, tensile modulus, and elongation. Because of visco- elastic nature of plastic materials, care should be taken to ensure that sample result that are to be compared are taken at the same rate of pull at the same temperature, as these parameters can strongly affect the tensile results. The results of the tensile test are usually expressed as tensile strength (in psi or Mpa), and elongation (in inch/inch, mm/mm, or %).
According to the Hook’s law, for an ideal elastic solid stress is proportional to strain (Figure: 1-2). Even when we ignore the effect of temperature and most of the effect of time, no plastics material comes very close to ideal. A plastic exhibits a whole spectrum behavior, which, in qualitative terms, may be expected from such dissimilar materials as soft PVC, PS, PA, and unplasticized PVC.
Figure 1-2: Hookean Behavior
Carswell and Nason have imagined five possibilities, which they express in the form of stress-strain diagrams (Figure 1-3). It is immediately obvious that such term as “ tensile strength “ and “ elongation at break” can be misleading or gross oversimplifications even when used with the knowledge that such properties themselves have strictly limited application. For instance, in Figure 1-3a, is the correct value for tensile strength that deriving from the stress at break or that at the peak of the graph? Again,
What is significant that elongation at break from a behavior such as that depicted in figure 1-3e, increase in extension with no increase in stress and finally a rapid extension with relatively little increase in stress?
To this already complex situation must be added the further complication of the viscoelastic nature of polymer, which causes the material’s response to an applied force to vary according to the time-scale and temperature of the experiment. Thus a tensile test carried out at various testing speed on a sample.
(b) Hard, Brittle
(a) Soft, Weak
(c) Hard, Strong (d) Soft, Tough
(e) Hard, Tough
Figure 1-3: Stress-Strain behavior of various types of plastics.
FLEXURAL TEST (ASTM D-790)
General Consideration: Flexural test is also called three point bending test. Three-point bending is the type of bending most commonly used in standard tests: for a rectangular beam supported at the mid point, the flexural stress is given by:
Flexural strength = 3FL
2bh2
Where,
F = Force applied on the mid point.
L = Span length of the sample.
b = Width of the sample.
& h = Height of the sample.
Many plastics parts are used in applications where flexural properties are important. For instance, plastic seat must have a minimum flexural strength and modulus or the seat will bend excessively or break. The flexural is a simple rectangular shaped beam that is placed over two rests or supports and then load in the middle of the beam between the supports (see Figure 2-1).
Load
Figure 2-1: Flexural tensile & modulus test
The flexural stress at the conventional deflection; which is defined in ISO 178 (1975) as the flexural stress at a deflection of 1.5 times the thickness of the test piece. This parameter is particularly useful for materials, which do not fracture under test.
Flexural strength and modulus also use the same symbols as those associated with tension and compression forces. In fact, the bending forces, which are used to determine the flexural strength and modulus, induce tension and compressive forces in the sample as illustrated in Figure 2-2. It is not surprising, therefore, that flexural strength and modulus are often thought of as combinations of tension and compression.
Bending (flexural) force
Compressive forces
present Flexural sample
Tension forces present
Figure 2-2: Forces present in flexural (bending) test
HARDNESS TEST (ASTM D2240)
General Consideration: Shore hardness is a measure of the resistance of material to indentation by 3 spring-loaded indenter. The higher the number, the greater the resistance.
The hardness testing of plastics is most commonly measured by the Shore (Durometer) test or Rockwell hardness test. Both methods measure the resistance of the plastic toward indentation. Both scales provide an empirical hardness value that doesn't correlate to other properties or fundamental characteristics. Shore Hardness, using either the Shore A or Shore D scale, is the preferred method for rubbers/elastomers and is also commonly used for 'softer' plastics such as polyolefin’s, fluoropolymers, and vinyl’s. The Shore A scale is used for 'softer' rubbers while the Shore D scale is used for 'harder' ones. The shore A Hardness is the relative hardness of elastic materials such as rubber or soft plastics can be determined with an instrument called a Shore A durometer. If the indenter completely penetrates the sample, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 results. The reading is dimensionless.
The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as 'Durometer hardness'. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the hardness reading my change over time - so the indentation time is sometimes reported along with the hardness number. The ASTM test number is ASTM D2240 while the analogous ISO test method is ISO 868.
The results obtained from this test are a useful measure of relative resistance to indentation of various grades of polymers. However, the Shore Durometer hardness test does not serve well as a predictor of other properties such as strength or resistance to scratches, abrasion, or wear, and should not be used alone for product design specifications.
Model No. 18811
Shore D Type
IZOD IMPACT TEST (ASTM D256-02)
General Consideration: Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics.
Scope: These test methods cover the determination of the resistance of plastics to "standardized" (see Note 1) pendulum-type hammers, mounted in "standardized" machines, in breaking standard specimens with one pendulum swing (see Note 2). The standard tests for these test methods require specimens made with a milled notch (see Note 3). In Test Methods A, C, and D, the notch produces a stress concentration that increases the probability of a brittle, rather than a ductile, fracture. In Test Method E, the impact resistance is obtained breakage by flexural shock as indicated by the energy extracted from by reversing the notched specimen 180° in the clamping vise. The results of all test methods are reported in terms of energy absorbed per unit of specimen width or per unit of cross-sectional area under the notch. (See Note 4.)
Note 1—The machines with their pendulum-type hammers have been "standardized" in that they must comply with certain requirements, including a fixed height of hammer fall that results in a substantially fixed velocity of the hammer at the moment of impact. However, hammers of different initial energies (produced by varying their effective weights) are recommended for use with specimens of different impact resistance. Moreover, manufacturers of the equipment are permitted to use different lengths and constructions of pendulums with possible differences in pendulum rigidities resulting. (See Section 5.) Be aware that other differences in machine design may exist. The specimens are "standardized" in that they are required to have one fixed length, one fixed depth, and one particular design of milled notch. The width of the specimens is permitted to vary between limits.
Note 2—Results generated using pendulums that utilize a load cell to record the impact force and thus impact energy, may not be equivalent to results that are generated using manually or digitally encoded testers that measure the energy remaining in the pendulum after impact.
Note 3—The notch in the Izod specimen serves to concentrate the stress, minimize plastic deformation, and direct the fracture to the part of the specimen behind the notch. Scatter in energy-to-break is thus reduced. However, because of differences in the elastic and viscoelastic properties of plastics, response to a given notch varies among materials. A measure of a plastic's "notch sensitivity" may be obtained with Test Method D by comparing the energies to break specimens having different radii at the base of the notch.
Note 4—Caution must be exercised in interpreting the results of these standard test methods. The following testing parameters may affect test results significantly:
EFFECTS OF MOISTURE ON PROPERTIES OF NYLON 6
Many polymers are moisture sensitive, they undergo certain changes in properties due to the influence of moisture. Because some of these polymers (particularly Nylon) are used in highly demanding applications, their physical and mechanical properties are of considerable importance.
Moisture Conditioning the Finished Product immediately after molding the part will then slowly pick up moisture depending upon the relative humidity of the surrounding atmosphere. For most applications the rate and amount of moisture absorption shown are permissible.
Thickness has considerable influence on the time required to reach a certain moisture content and upon dimensional changes caused by moisture absorption as product thickness increases, the time required to reach a certain moisture content increases and dimensional changes caused by moisture absorption becomes less significant.
Some Effects of Moisture on Product Properties
Properties of products made from Nylon6 vary with moisture content. The amount of moisture in Nylon6 as received is negligible (less than 0.1% moisture), and the amount of moisture absorbed during processing is also low. Moisture is absorbed by the end product in greater amounts but this is usually too low. After reaching a moisture content of 2% to 3% at 65% relative humidity (about 9% moisture at 100% relative humidity), there are no further significant changes in moisture content of the product.
The effect of moisture on some important properties of Nylon6 is shown in Figures , and . The shaded areas for each property represent the range within which Nylon6 falls. At higher moisture levels, the tensile strength, modulus of elasticity, flexural strength and Shore-D hardness of Nylon6 products all decrease (this is true of products made from all Nylons). Impact strength of Nylon 6 increases as moisture content increases. Thus, toughness is increased as moisture is picked up from the initial state.
Moisture also imparts a high abrasion resistance, a low coefficient of friction and a high degree of elasticity, which makes Nylon6 particularly flexible to such applications as gears, bearings and mechanical parts where shock loads are important factors.
TESTING FOR UNDRIED SAMPLES
Room Temp. = 31*C.
Humidity = 59%.
Flexural Strength
Testing Speed = 12.5 mm/min
1 N = 1 Mpa
Kgf /mm2 x 9.80665 = N/mm2
2.122 Kgf /mm2 x 9.80665 = 20.8114Mpa
Tensile Strength
Gauge Length = G.L = 25mm.
Testing Speed = 100 mm/min.
Tensile Strength = Force / Area.
1 N = 1 Mpa
Kgf /mm2 x 9.80665 = N/mm2
2.46 Kgf /mm2 x 9.80665 = 20.8114Mpa
3.59 Kgf /mm2 x 9.80665 = 20.8114Mpa
Izod Impact Strength
Hardness
TESTING FOR DRIED SAMPLES
Room Temp. = 31*C.
Humidity = 59%.
Flexural Strength
Testing Speed = 12.5 mm/min.
1 N = 1 Mpa.
Kgf /mm2 x 9.80665 = N/mm.
3.0124 Kgf /mm2 x 9.80665 = 29.54 Mpa.
Tensile Strength
Gauge Length = G.L = 25mm.
Testing Speed = 100 mm/min.
Tensile Strength = Force / Area.
1 N = 1 Mpa
Kgf /mm2 x 9.80665 = N/mm2
5.24 Kgf /mm2 x 9.80665 = 20.8114Mpa
5.23 Kgf /mm2 x 9.80665 = 20.8114Mpa
Izod Impact Strength
Hardness
DISCUSSIONS
- Decrease in mechanical properties.
A great extent of descend was observed in the mechanical properties of undried resin (moisture content of 3%) as compared to dried Nylon. The difference was prominent, especially considering results of tensile strength, but elongation was increased by an immense margin in high moistured content resin. The flexural strength also decreased not as much as tensile decreased, the possible reasons for such factors are stated below;
- The water content in the molecules of Nylon certainly reduces the polarity, in intermolecular segments, narrowing the crytallinity of Nylon and hence resulting in decrease of tensile properties. Selecting fiber filled resin grade can decrease this action, since the volume of Nylon molecules will be reduced.
- Since water settled in Nylon’s molecule acts like a plasticizer, increases chain mobility in the polymer and hence resulting in greater elongational phenomena.
The decrease in hardness of undried resin was simply due to moisture highly involved in it. The water molecules introduce softness to resin and bring about a change in a network of amide group.
The Izod Impact Test was a failure to be carried for both dried and undried resin test samples, the notch created wasn’t affected much by the pendolum to complete the specified requirements of test. Even the highest weight of pendolum was applied which didn’t made its impact as well. The possible reason are stated as below
- The resin a high co-efficient of toughness.
- The thickness of test sample is too much to carry out izod impact for this particular tough polymer.
- Observations made during processing of test samples.
- While processing specially dried material, it was observed that resin needed a high injection pressure to fill in the cavity, although melt temperature was checked time and again and a bit was increased when needed. This could have been due to over drying of material, which had moisture content of less than 0.094% (which automatically makes resin more viscous). The other possible reason could be the screw used in the injection-molding machine was a conventional type screw, which might regret such hard and crystalline material in flow case.
- Splash marks were observed on the molding of undried resin, which might have been because of the volatiles (water) trapment in the barrel. Since the water content level for undried resin was a higher one (3%). The other possible reason could be air trapment in barrel while the hopper feeding wasn’t proper or the hopper cover was putted off for a longer period.
- A slight color change (yellowish) was observed on the few of the moldings, although they were discarded due to slight over heating. The reason was the same less flow ability shown, especially by dried material.
- A local market survey was carried out which made an impression that Nylon wasn’t easily available in market and if they do, there isn’t enough
range of grades and diversity exists. Few of the grades available and their prices are quoted as under:
TRADE NAME RATES
- ZYTAL (DuPont plastics) 3500/-
- NOVAMID (Mitsubishi Plastics) 3750/-
- TORAY (1017) 3550/-
- U.B.E (1013) 3400/-
- Ultramid B35 (BASF Plastics) 3675/-
CONCLUSION
Polyamides are very much moisture attractive and this moisture content affects a great deal in reducing the mechanical properties of material. The moisture content in polyamides dealt appropriately before processing, in order to achieve the desired and naturally possessed properties of Nylon.
REFERENCES
Following references were made in order to compile this report and survey’s carried out.
- SPE handbook
- ‘Nylon Plastics’ (W.E Nelson)
- ‘Injection Molding Handbook’ (R. Donald)
- ‘Nylon plastics’ (M.I. Kohan)
- ‘Injection Molding Of Engineering Thermoplastics’ (Hanscher publc.)
-
‘S.D plastics’ ()
-
‘G.E plastics’ ()
-
‘American Standard of Testing Materials’ ()
A word of appreciation must be mentioned here for few personalities, which encouraged and helped us to come to an end of this project, ‘Mr. Shabir Ahmad’ and ‘Mr. Irfan’ (library asst.).
In the end but not the least, ‘Mr.Yasser Jaffer,’ our project advisor who made it looked so easier as it wasn’t ever. “He encouraged and guided us all the way in such a manner that we dedicate this project report to his name.”
TABLE OF CONTENTS
