The aim of this literature review is to give a general introduction to this project and review published work that related to this project. In this literature review, the basic properties of nylons, especially nylon 6, are given first. The synthesis and processing of nylon 6 are then reviewed. After that, lubricant filled nylons, particularly, oil filled nylons are introduced. Finally, the influences of oil on the properties of oil filled nylons, together with characterisation techniques, are reviewed. These reviewed published works may help to understand this project and to plan practical work.
CONTENT
Introduction
Cast nylon 6 has a wide range of applications due to its significant properties including: high modulus, good frictional properties, good chemical resistance (especially to oil, solvents), good weather resistance and high melting point. Oil filled cast nylon 6 was developed to replace classic cast nylon 6 in some applications, such as gears and bearings, for its superior frictional properties [2]. This project is supported by Nylacast. This company would like to use some novel oils as additives to modify cast nylon 6 for better self-lubricating properties compared to their currently existing oil filled nylons. In this project, both laboratory and production scale oil filled cast nylon 6 samples will be produced. Physical properties of prepared samples will then be tested and compared to pure cast nylon 6 and Nylacast currently existing oil filled nylon 6.
Nylon
Polyamides, though were first synthesised at the end of the 19th century, were originally used for commercial applications and given the name nylons by DuPont company in 1935 [2, 3]. Nylons are recognised by the amide group
existing in the polymer chain. The properties of different nylons are determined by the nature of the repeating units between these amide groups. Generally, nylons can be divided into two groups according to the reactants: (a) copolymers of a diamine and a diacid and (b) homopolymers of an amino acid or lactam. Nylon 66, prepared using hexamethylene diamine and adipic acid and nylon 6, prepared from -caprolactam, are the two representative commercial products for the two nylon groups, respectively [4, 5].
Basic properties of Nylon
As the first engineering thermoplastics, nylons were well known due to their outstanding mechanical properties and excellent resistance to chemicals [3, 4].
Mechanical properties
Crystallinity, molecular weight and moisture content are the main factors that control the mechanical properties of nylons [6]. High crystallinity (40-60%) of nylons contributes to their high modulus, high creep resistance and good abrasion resistance. With higher molecular weight, nylon can have better mechanical properties [2]. Nylons are sensitive to water and absorb water. The water absorbed by nylons may play a function similar to a plasticiser and, hence, give an effect of modulus reduction and toughness improvement [5]. The water absorption of a nylon material is determined by its amide group content, since the amide groups can form amide-water hydrogen bondings with water. The higher the amide ratio is, the higher the water absorption [4, 5]. This is displayed in Figure 1 [6].
Figure 1 Water absorption of aliphatic polyamides [6].
Chemical properties
Because of the polarity of the amide group, nylons are resistant to chemicals including: oils, some organic liquids, benzene, grease, esters and so on. But, nylons can be attacked by acids, bases, phenols, oxidising agents [5, 6]. The chemical resistance of nylons decreases with the increasing temperature. When the crystallinity of a nylon increases, its impermeability increases leading to a higher chemical resistance [5].
Nylon 6
Nylon 6, which, as mentioned before, is obtained by the polymerisation of -caprolactam, is one of the dominating commercial nylons. The superior mechanical properties and low cost allow nylon 6 to be a successful engineering polymer [5].
Properties and applications of nylon 6
Nylon 6 has quite similar properties to nylon 66. The excellent mechanical properties of nylon 6 include: fatigue resistance, wear resistance, creep resistance, good toughness (especially at low temperature) and low frictional coefficient. Nylon 6 shows good frictional properties with the coefficient of friction ranging from 0.1-0.3, which may be affected by reinforcement or tribological additives [5]. From Figure 2 [5], the yield stress and modulus of nylon 6 can be clearly observed. The yield stress of nylon 6 and nylon 66 range from 50MPa to 80 MPa, while the modulus of the two nylons fall in the range 1.0-1.5 GPa, approximately.
PA66 or PA11 or PA610 PA46 PA transparent
PA 6 PA12 (a)
PA66 or PA11 or PA610 PA46 PA transparent
PA 6 PA12 (b)
Figure 2 (a) Pure PA: stress at yield examples (MPa); (b) Pure PA: modulus examples (GPa) [5].
Though, nylon 6 maybe the cheapest nylon, it is more expensive than commodity polymers including polyethylene (PE), poly(vinyl chloride) (PVC), polystyrene (PS) and polypropylene (PP). As a nylon, nylon 6 has the common disadvantage of absorbing water, which will decreases the dimensional stability and electrical properties [5].
Nylon 6 has a wide range of applications including [5]:
Self-lubricating parts: gears, bearings and so on;
Automobile: door handles, grilles, headlight parabolas and so on;
Electrical parts: junction boxes, fuse holders, castings of projectors and so on;
Sports applications: crank gear for bicycles, wheels, ski boots and so on;
Films and fibres.
Among all these applications, selfâ€"lubricating products, perhaps, are the most commonly involved applications [5]. Nylon 6 has been used for bearings, gears and rollers for decades due to its good frictional properties [7].
Synthesis of nylon 6
To prepare nylon 6, two main routes may be involved: hydrolytic polymerisation of -caprolactam and anionic polymerisation of -caprolactam [2, 8].
Hydrolytic polymerisation of -caprolactam
As a classical manufacture route, hydrolytic polymerisation of -caprolactam still takes the dominating place and is widely applied for industrial production of nylon 6 [8, 9]. The stages and mechanisms of the hydrolytic polymerisation of -caprolactam are displayed in Figure 3 [9]. The first step of this polymerisation is the ring-opening reaction of -caprolactam (). In the presence of water, -caprolactam hydrolyses to -aminocaproic acid () (reaction 1). After that, the condensation of the -aminocaproic acids produced before (reaction 2) takes place. At the same time, ring-opening polymerisation of -caprolactam (reaction 3) happens as well [2, 9, 10]. The reaction speed of the first stage is very slow, hence, controls the whole polymerisation speed. To overcome this problem, -aminocaproic acid is usually added before the polymerisation starts. Byproducts such as cyclic dimer () can be found in the polymerisation. Reaction 4 and 5 show the ring-opening reaction of the cyclic dimer and the ring-opening polymerisation of the cyclic dimer, respectively [9, 11, 12]. Oligomers and monomers may also be found after the polymerisation since all the reactions in the process are equilibrium reactions. The content of these oligomers and monomers may be controlled by choosing a proper polymerisation temperature [2, 9].
Figure 3 Stages of the hydrolytic polymerisation of -caprolactam [9].
Anionic polymerisation of -caprolactam
Nylon 6 can also be obtained by anionic polymerisation of -caprolactam. Compared to the hydrolytic polymerisation method, the reaction rate of the anionic polymerisation of -caprolactam is much higher [2, 13]. This polymerisation is normally chosen for some special products manufacturing such as bulky parts [2, 8]. Besides, with anionic polymerisation, the average molecular weight of nylon 6 can be improved [14].
Anionic polymerisation of -caprolactam is an anionic ring-opening polymerisation. Generally, ring-opening polymerisation, shown in Figure 4, has the advantage of over normal condensation polymerisation of avoiding byproducts. Catalysis is usually used to initiate the polymerisation. For anionic ring-opening polymerisation, two end activations may be found, exhibited in Figure 5 [15].
Figure 4 Ring-opening polymerisation [15].
Figure 5 Anionic ring-opening polymerisation involving different end activations [15].
The anionic polymerisation of -caprolactam is normally initiated by a catalyst system containing: caprolactam anions (or precursors) and N-acyllactam (or similar activators), shown in Figure 6 [16-18].
(b)
Figure 6 (a) caprolactam anion; (b) N-acyllactam [16].
As is displayed in Figure 7 [16], the first stage of the polymerisation is the caprolactam anion forming process, during which the catalyst pulls a proton from the caprolactam monomer. In the polymerisation, the caprolactam anion, firstly, attacks the endocyclic (part of a ring system) carbonyl group in the N-acyllactam (or activators), forming an amidate anion. This amidate anion, then, draws a proton from another caprolactam monomer and creates a new caprolactam anion. This process repeats and leading to the chain growth [16, 19].
Figure 7 Anionic polymerisation of caprolactam [16].
The reason of adding activator is that the transfer speed of a proton from the endocyclic carbonyl group in the activator to the caprolactam anion is much faster than that from carbonyl group in the monomer, displayed in Figure 8 [16]. Hence, in the presence of activator, the initiation speed can be greatly improved [16, 20].
Figure 8 The nucleophilic attack of the caprolactam anion on the carbonyl group in the monomer [16].
Processing of nylon 6
Commonly used methods for nylon 6 processing include: injection moulding, extrusion and casting [7]. Monomer casting processing is now chosen by more and more manufacturers for nylon 6 processing due to its simple procedure and superior final properties such as dimensional stability, high molecular weight and high crystallinity [21, 22]. High crystallinity leads to the excellent mechanical, thermal and chemical resistance properties of nylon 6 [21, 23]. During the casting process, molten -caprolactam will be mixed with catalyst and poured into a heated mould. Anionic ring-opening polymerisation of -caprolactam then happens producing nylon 6. Dry nitrogen gas environment is normally required to prevent the living reaction being stopped by oxygen [24, 25]. Manufacturers usually obtain blocks of nylon 6 through this casting process and machine the nylon blocks into required products [26].
Additives Modified Nylon
Being an important group of engineering materials, nylons are always modified by various additives to for properties improvement or to meet some special properties. The most widely used additives for nylon modification can be classified into five groups: functional additives, reinforcements, fillers, impact modifiers and plasticisers [4, 6].
Functional additives
Heat stabilisers protect nylons from oxidation at an increasing temperature, hence, prevent the mechanical properties decreasing. Lubricants allow the frictional properties improvement of nylons. Flame retardants are added for fire resistance. Nucleants are used to control the crystallinity of nylons, consequently, the product properties [4, 6].
Reinforcements
With reinforcements, the performance of nylons can be greatly enhanced including: improved tensile strength, hardness, dimensional stability, chemical and hydrolysis resistance; decreased water absorption and heat capacity. Commonly used reinforcements include glass fibres, carbon fibres and clay [6, 27].
Fillers
Fillers play an important role of improving hardness, abrasion resistance, scratch resistance, tear strength and reducing cost. Typical fillers are silicon dioxide, calcium carbonate and mineral fillers [6].
Impact modifiers
Elastomers, such as ethylene propylene diene monomer (EPDM) rubber, butadiene rubber (BR) and styrene butadiene rubber (SBR), are often used as impact modifiers to improve the impact properties of nylons [4, 6, 28].
Plasticisers
Plasticisers are applied to nylons to meet the needs of improving flexibility and reduce stiffness for some special applications [1, 4].
Lubricant Filled Nylon
As self-lubricating materials, nylons are among the most popular engineering materials for gear and bearing applications [7, 25, 29]. Compared to metals, nylons show advantages of light weight, corrosion resistance, low frictional coefficient, flexible formulation and processing, mass production, low cost and the ability to work in dry friction conditions [29, 30]. However, low heat conductivity may limit the performance of nylons. This problem becomes obvious since frictional heat can be produced during the working of the bearing product. With frictional heat, nylon products can be easily worn and exhibits much shorter bearing life [7, 24, 29]. Lubricant filled nylons were developed to deal with this problem. With lubricant added, the frictional coefficient can be reduced, hence the frictional heat that may be induced [24].
Classification of Lubricant Filled Nylon
According to the lubricants used for nylon modification, lubricant filled nylons may be divided into two types: solid lubricant filled nylons and oil filled nylons [24, 25]. Cast nylons dominate the nylons selected to be lubricant-modified due to the easy manufacturability and their wide applications in gear and bearing products [25]. Internally lubricated cast nylons can be simply prepared by anionic ring-opening polymerisation of -caprolactam with lubricants dispersed in the monomers [24].
Solid lubricant filled nylon
There are varieties of solid materials that are suitable to be processed into nylons and play the function of internal lubricants including: self-lubricating polymers, reinforcements and fillers.
Self-lubricating polymers as internal lubricants
One commonly used polymer lubricant is PTFE (polytetrafluoroethylene), also known as Teflon. PTFE has a relatively low frictional coefficient from 0.03 to 0.1 due to the fact that the interaction forces between molecule chains are very small which allows the chains slide easily over each other. The chemical resistance of PTFE is very good and its working temperature of PTFE may go up to 300 [31]. Literatures [31, 32] show that, filled with PTFE, the frictional coefficient of nylon is reduced resulting in improved bearing performance.
Another well known polymer lubricant that may be used to improve the frictional properties of nylon is ultra-high molecular weight polyethylene (UHMWPE). Both nylon and UHMWPE are self-lubricating materials. However, UHMWPE has lower friction, better impact resistance and higher dimensional stability. With UHMWPE added, the wear life of nylon products become longer [33, 34]. Figure 9 [34] displays the lubricant effect of UHMWPE on nylon.
Figure 9 Dynamic frictional coefficient of PA, UHMWPE and UHMWPE filled PA specimens with the sliding distance at contact pressure 1 and 2.5 MPa. The sliding speed is 0.5 m/s. Frictional coefficient at zero sliding distance is the static frictional coefficient [34].
Reinforcement as internal lubricants
Reinforcements such as carbon nanotube (CNT) and carbon fibre may also perform as internal lubricants [35, 36]. Meng et al. [35] found that, as reinforcement, carbon nanotubes (CNTs) can increase the tensile strength, Young’s modulus, thermal conductivity and crystallinity of PA 6. Besides, PA 6 reinforced by CNTs absorbs less moisture than pure PA 6. It was also found that CNTs are able to reduce the frictional coefficient of PA 6 and works as lubricant. In addition, for the CNT reinforced PA 6, the frictional heat accumulated during the sliding can dissipate faster because of the higher thermal conductivity. In the presence of CNTs, the friction and wear properties of nylon 6 can be markedly enhanced [35]. The lubricating function of carbon fibre was confirmed by Zheng et al. [36] already. Research showed that the frictional coefficient of the carbon fibre reinforced PA 6 met a minimum value when the carbon fibre content (volume fraction) went to 35% [36].
Fillers as internal lubricants
Adding lubricating fillers is another way of improving the frictional properties of nylons. Molybdenum disulphide () is such a filler that is commonly applied for frictional coefficient reduction. But, an obvious frictional coefficient reduction need a high content (e.g. 50 wt%) [37]. Nowadays, is usually mixed with PTFE to form a wax to act as internal lubricant for nylons [25, 38, 39].
Oil filled nylon
Oil filled nylons were first developed for commercial applications in 1970s [26]. As liquid lubricant, oils may provide more efficient lubricating effect than solid lubricants and this can be affected by the contact pressure and testing scales (contact area sizes) [25, 38, 39]. Data (Table 1) given by Samyn and Tuzolana [25] is a good example which compares the frictional properties of oil internal-lubricated nylon and solid lubricant filled nylon, confirming the superior lubricating ability of oil. Drawback is that nylons filled by oil have less stable sliding than solid lubricated nylons, which means a larger difference in static friction and dynamic friction [25, 39]. The most used oils, currently existing, for lubricant filled nylon manufacturing are mineral oils, for example: white oil and synthetic oils such as silicone oil [24, 25, 38-40].
Table 1 Static and dynamic frictional coefficients (,) for internal-lubricated PA [25].
Normal load
1.15 N
3.15 N
5.15 N
Sliding velocity (mm/s)
PAo1
0.12
0.30
0.25
0.25
0.22
0.23
0.20
0.5
0.32
0.24
0.27
0.22
0.24
0.18
2.0
0.35
0.22
0.30
0.20
0.28
0.19
8.0
0.37
0.22
0.32
0.21
0.30
0.19
PAs1
0.12
0.27
0.26
0.30
0.25
0.29
0.25
0.5
0.35
0.26
0.32
0.25
0.30
0.25
2.0
0.37
0.26
0.37
0.24
0.36
0.24
8.0
0.47
0.25
0.42
0.23
0.40
0.23
*PAo1 presents oil (silicone oil) lubricated PA, PAs1 presents solid lubricant (wax which is consists of PTFE and ) filled PA.
Adding oil will affect the polymerisation of nylon. Influence of lubricant on the anionic polymerisation of -caprolactam has been investigated by Kang and Chung [24] before. Experiments showed that the presence of oil may increase the solidification time when casting nylon, which means the polymerisation rate is reduced. A saturation point for lubricants in nylon, 8 wt%, was discovered, above which cast nylon will finish with an inhomogeneous lubricant dispersion [24]. This is illustrated in Table 2 [24].
Table 2 Solidification time for nylons with different lubricant contents [24].
Lubricant content (wt%)
Solidification time (min)
Oil
Wax
0
6
2
7
7
4
7
9
6
9
10
8
15
18
10
N.A.
N.A.
*N.A.: not available (polymerisation does not occur in this situation).
Properties of Oil-filled Nylon
Compared to pure nylon, oil filled nylon has better frictional and impact properties, but lower tensile properties. Thermal properties before and after lubricant adding have no significant changes [24, 41]
Mechanical Properties
Coefficient of friction
The frictional coefficient of nylons can be obtained using tribotesters. Researchers showed that different testing scales (contact area scales) may give different friction results [38, 39]. Figure 10 [39] presents three commonly used tribotesters for frictional properties characterisation.
Figure 10 Tribotesters for different test scales: (a) flat-on-flat meso-scale, (b) cylinder-on-plate small-scale, (c) flat-on-flat large-scale [39].
Figure 11 [39] displays the results from flat-on-flat meso-scale (with contact area around 600 ) tribotesting. It can be easily observed that there is an obvious coefficient of friction decrease, from 0.42 to 0.25, when nylon is internally lubricated by oil [39]. Kang and chung [24] pointed out that the releasing of the contained oil may reduce the shear strength on the contact surface, hence reduces the coefficient of friction.
(a)
(b)
Figure 11Meso-scale friction (1.15N, o.125 mm/s) for (a) PA 4, (b) PA 6 with 4-6 wt% silicone oil [39].
Tensile properties
The tensile properties of oil internal-lubricated nylon can be simple obtained via tensile testing. Oil droplets in nylon will cause the decreasing in tensile strength and modulus. The decreasing is greater as the oil content increases [24, 25, 38-40]. Figure 12 [24] is an example showing the tensile strength decreasing when the lubricant content increases.
Figure 12 Influence of lubricants on tensile strength of nylon 6 [24].
Impact properties
Different from tensile properties, within certain content, the impact properties of nylons are improved when oil is impregnated [24, 25, 39], displayed in Figure 13 [24]. One possible explanation for the toughening effect of oil is that oil plays a function similar to plasticisers. With low molecular weight, oil dispersed in the amorphous region may decrease the Tg of the amorphous phase of nylons, hence increased the toughness of the nylons [1].
Figure 13 Influence of lubricants on impact strength of nylon 6 [24].
Thermal Properties
Available techniques for thermal properties characterisation of oil filled nylon are thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). TGA is mainly for thermal stability characterisation. DTA is able to give the melting point of oil filled nylon. DSC plays an important role of finding out the effect of oil on the crystallinity of cast nylon [38]. An example of thermal characterisation is exhibited in Figure 14 [38]. Results show that PA-Na and oil-filled PA-Na have the same softening point, 95 and all exhibit a melting peak around 220 , which means the thermal properties of this nylon will not be affected by adding oil [38].
Figure 14 Thermal analysis (DTA-TGA) of different PA: (i) PA 6 with sodium catalyst (PA-Na), (ii) PA 6 with magnesium catalyst (PA-Mg), (iii) PA-Na containing 4-6 wt% synthetic oil (PAo1), (iv) PA-Na containing 8 wt% wax (+PTFE) [38].
Morphology
Since the oil droplet size and the dispersion state of the oil droplets have notable effect on the mechanical properties, especially frictional properties, of oil filled nylon, it is essential to investigate the morphology. Besides, morphology of the oil added nylon is helpful to understand the influence of oil on the polymerisation of nylon. Powerful methods to obtain the morphology of oil lubricated nylons are optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [24, 41].
Project Outline
In this project, nylon 6 will be prepared via casting process involving anionic polymerisation of -caprolactam. Some novel oils are going to be applied as internal lubricant. The properties of the new oil filled nylons will be investigated and compared with pure nylon and currently existing nylon.
In practical work, the catalyst and activator for the anionic polymerisation of caprolactam are DILACTAMATE and INCOREZ 501, respectively. Referring to literatures, the oil content of the lubricated nylon should be controlled within 8 wt% for a homogeneous oil dispersion and successful polymerisation.
Once the oil filled nylon samples are prepared, physical properties of these samples are going to be tested. The frictional properties characterisation of prepared samples will be done at Nylacast. The tensile and impact properties of the oil filled samples are going to be obtained via tensile testing and impact testing, respectively. DSC will be involved in this project to investigate thermal properties, such as Tg, and crystallinity. Optical microscopy, SEM and TEM may be applied to study the morphology of oil filled nylon samples, like oil droplet size and oil dispersion.
Conclusion
As one of the earliest engineering materials, nylons have a wide range of applications. Nylon 6 can be prepared via either hydrolytic polymerisation of -caprolactam or anionic polymerisation of -caprolactam. Cast nylon 6, involving anionic polymerisation of -caprolactam, is chosen for gear and bearing applications for its excellent mechanical properties, especially self-lubricating property. Lubricants, as additives, can be used to improve the frictional properties of cast nylon. Studies on oil filled nylon have been done by other researchers before. Filled by oil, cast nylons have improved frictional properties, but reduced mechanical properties such as tensile strength. Synthesis and physical properties characterisation of some novel oil filled cast nylon 6 are going to be done in this project.
