Polycaprolactam

Student Name: Syed Ali Shahid, ID Number # 20595181; email: sashahid@uwaterloo.ca

1. Introduction and Applications
Polycaprolactam or PA 6 is a synthetic polyamide commonly recognized by its trade name Nylon 6. Polyamides or nylons are polymers characterized by the amide group –CONH-as recurring part of their polymer chain. The original nylon was first made in a laboratory in DuPont, USA in 1935 by Wallace Carothers. It was recognized as the first semicrsytalline polymer, the first engineering thermoplastic, and the first synthetic fiber .Amongst various variants of polyamide, polycaprolactam is particularly different in that it is made from only 1 kind of monomer- caprolactam which will be discussed in this paper in further detail from various aspects. Schalak while working on polyamides in IG Fabraen laboratory synthesized polycaprolactam through ring opening polymerization of caprolactom. He heated caprrolactam in a flask to about 250oC with about 5-10% water which hydrolyzed a few percent of the caprolactam to aminocaproic acid. The NH2 group of the aminocaproic acid acted as ring opening initiator for polycaprolactam followed by a polycondensation reaction of the NH2 and COOH group of the low molecular weight product of the ring opening polymerization reaction resulting in a high molecular weight product-Polycaprolactam.

The key properties of polycaprolactam that have made them suitable for a variety of industrial applications are toughness, fatigue resistance, oil and abrasion resistance, high melting temperature, excellent fire resistance etc. The distinguishing feature that gives polycaprolactam these combinations of properties are the amide groups which are responsible for the strong hydrogen bonding between the adjacent polymer chains.These H- bonding brings the chains closer together increasing the crystallinity which accounts for the properties like high tensile and yield tress, high density, high stiffness.
These properties have proved advantageous to various industrial applications. The morpholgy of polycaprolactam allows for strain induced orientation and enhanced crystallinity. As such excellent drawability enable them to be drawn into fibers of high modulus hence finding use in industrial yarn, tire cord fabric, nylon strings for musical instrument, nylon fishing lines and many useful household items. It’s been seen that fiber industry constitutes about 60% of the polycaprolactam used in industry.
The resin form of polycaprolactam has been of much interest to the injection molding sector and transportation industry in particular has exploited the mechanical properties of polycaprolactam. For e.g. Fatigue resistance of Polycaprolactam has found use in gears, bearings, antifriction parts, valves etc. Toughness of Polycaprolactam is utilized in ski boots, racquet equipment, kitchen utensils etc. The resin form accounts about 30% of the Polycaprolactam used. The remaining 10% is deployed in making films that have been used in food packaging.

Despite the excellent set of properties of Polycaprolactam, one limitation that encounters any polyamide is its susceptibility to water and Polycaprolactam is no exception. The water which is present as moisture tends to absorb on to the polyamide groups and act as a plasticizer increasing the mobility of the chains which consequently deteriorates the physical properties of the nylon. Therefore this inevitable weakening is foreseen and is compensated by increasing the crystallinity at the time of its processing. Additives are used as stabilizer against such deterioration. Alternative techniques like copolymerization, adjusting moleular weight, conditioning to some moisture level, post treatment on surface have recently gained attention of polymer chemists and engineer.

2. Molecular Weight Measurement
Molecular weight measurement is one of the most important characteristics in discussion of properties of synthetic polymers, especially for homopolymers such as commercial polycaprolactam. Many different methods for determining Molecular weight of polymers have been used so far. Much has been established and published on methods to determine the molecular weight of polyamides. Most of these techniques rely on colligative properties in solution; and it has been found that the characterization of polyamide in solution is complicated because of their limited solubility. Polycaprolactam dissolves in concentrated acids, phenols fluorinated alcohols and concentrated solutions of salt but due to the possibility of hydrolytic degradation, polyelectrolytic effects and apparatus damage, molecular weight measurement becomes difficult. More over from the fact that most polycaprolactam molecular weight lies in the range for 10-15 K, therefore methods like light scattering and dilute solution viscometry become less favorable. Therefore end group analysis was generally used in the past for Molecular weight measurement of PA6, but it too was limited to PA6 with known end groups. Vapor osmometric had been therefore a most reliable technique for PA6.
But with the advent of GPC with advance detectors, molecular weight of nylons have become less tedious and results are more reproducible and capture the true molecular weight distributions predicted by theoretical models. However the problem of finding a suitable solvent that does not degrade the polyamide still remains a concern. Therefore to avoid these problems chemists Schuttenburg and Schulz1 have found a way of chemically modifying the polyamide with N-trifluroacetylation reaction. This modifies the amide group and makes the polyamide soluble in ordinary solvents like acetone, chloroform, tetrahydrofuran.
Therefore this modification allows chemist to conveniently use GPC and the calibration curves to determine molecular weight and distribution of the polycaprolactam. Since for calibration curve relationship between Log[n] and log M is needed, it has been established that if the samples are monodisperse we prepare let’s say 5 solutions of varying concentrations (0.2-1% w/v) and from the Mark Houwink equation find the corresponding Molecular weight from which we draw the calibration curve. However if the constants are not known than other methods like calibration with polydisperse standards2 are usually used.

3. Chemical Composition Measurement
A very wide range of analytical techniques are used to determine the microstructure of polymeric materials to decipher the chemical composition of a polymer chain. Elemental analysis by spectroscopy can distinguish specific chemical groups and help identify the chain structure.
Since the electromagnetic spectrum is made up of different wavelengths, the responses of the molecules with which they interact also differ. As a result spectroscopy measures these interactions to form a correlation between the interactions and the molecular structure. I will discuss two most common spectroscopies used for polymers; NMR and FTIR
In principle NMR detects the atomic nuclei and detects what sort of environment it is in within the molecule. A polymer sample is dissolved in a suitable solvent and placed in a strong magnetic field. It is then irradiated with a short pulse of radio frequency energy. Some nuclei absorb the energy and come in the excitation state. When they fall back they emit energy which is picked by a radio receiver. A complex computer algorithm coverts this information into a display of intensity against frequency. Correlation and empirical data helps analytical scientist to define the spectrum and it helps them identify information like sequence in copolymer, tacticity, structural and stereo isomerism. A typical NMR spectrum of polycaprolactam 6 is shown in figure 3
IR spectroscopy differs to NMR in that it provides a way of finding the functional groups that cannot be seen in an NMR spectrum. For e.g. C=O or C=C can be seen in the NMR because they contain carbon atoms while other groups like OH can be detected by the chemical shifts of the Carbon atom they are joined to. But functional groups like CONH2, NO2, COCl, CO2H, and NH2 cannot be seen on NMR at all. Infrared spectroscopy detects these because it detects the stretching and bonding of bonds rather than any property of the atoms themselves.
The sample is exposed to IR radiation and the wavelength scanned across the spectrum. When some specific wavelength is absorbed the intensity of radiation reaching the detector decrease momentarily and gets identified in the spectrum. This signal is a time domain signal and is converted to frequency domain signal through Fourier transformation. FTIR helps identify chain conformation, crystallinity, intermolecular interactions and structural isomerism.

4. Synthesis, Mechanism and Production
In today’s world synthetic fiber industry and engineering thermoplastic Polycaprolactam is a key polymer. It is mass produced through various routes for the production of technical yarn, textile filaments, film and extrusion application as well as resin for engineering thermoplastic. Production of Polycaprolactam begins with benzene which is used to produce cyclohexane which through different reactions is converted to caprolactam; the immediate precursor of polycaprolactam. The manufacturing of caprolactam from benzene has been shown in figure 1. After forming caprolactam it is polymerized by ring opening polymerization at 240-270 oC in the presence of water3. The ring opening and the reaction mechanism has been shown in the figure 2.
For the industrial production of polycaprolactam hydrolytic polymerization is performed in a batch or continuous tubular rector. Some of the characteristics of the either type are described below.
I) Semi batch reactor: From our understanding of the reaction kinetics and the mechanism of ring opening polymerization it is desirable to play around with the conditions of the polymerization during the initial and final phases of the reaction. Initially it is required to keep the water content high by applying high pressure in the reactor and facilitate ring opening reactions. Also the temperature is kept high to allow rapid conversion of the monomer. At the later stage since the reaction is exothermic the reactor is slowly brought to a lower temperature and low pressure to favor high conversion and a high molecular weight final product.
Alternative scheme used in industry is to carry out the reaction in 2 semi batch reactors. The first reactor is kept at high temperature and water content to achieve fast rate of initiation an propagation reaction whereas the second reactor in series have a lower temperature and lower water content to drive the polycondensation and the polyaddition reaction in the forward direction to yield high molecular weight product. ii) Continuous Tubular reactor:
Another common but sophisticated reactor used in industry for hydrolytic polymerization is the VK tubular reactor. It is a specially designed reactor with different zones with interbed heat exchangers4. Zones are separated by gratings which facilitates uniform heating and near flat velocity profile for the reaction stream. The feed which includes molten caprolactam, water, stabilizer and inert nitrogen enters from the top. The feed picks up heat passing through the heat exchanger in the tubular cavity of the reactor. Figure 4 shows the schematic of the reactor. As the temperature reaches the desired temperature of 240 oC the reaction takes place and the molten mass gets heated further by the exothermicity of the reaction and reaches a maximum of 270 oC when the mass reaches the bottom portion of the reactor where the second heat exchanger recovers the heat and the product polymer exits from the bottom of the reactor

5. Properties
The performance properties of any polymer are determined by the microstructure of its polymer chains which in turn depends on properties of polymers like molecular weight, its distribution, chemical composition, thermal stability, defects in morphology etc. Polycaprolactam as shown previously consists of strong hydrogen bonding between adjacent chains which give the polymer its useful combination of properties like high moduli, impact strength, chemical resistance, and melting temperature. Some of the properties are discussed below
Mechanical: Generally stress strain curves of a polymer give a good measure of the strength and toughness of a polymer like polycaprolactam. Mechanical properties are summarized for the different variants of the polycaprolactam in table 1.This strength gets weakened overtime by moisture absorption which disrupts the H-bonding
Electrical: Polycaprolactam owing its good surface and volume resistivity makes them excellent dielectric materials hence finds wide use in electrical components
Environmental: Polycaprolactam is attacked by substances such as acids, phenols, and gets plasticized with water, alcohol, chlorinated solvents. Conversely due to its highly polar nature it shows excellent resistance to common hydrocarbon and automobile fluids at temperature up to 140 deg C.
Thermal: It has low heat resistance and poor heat conductivity. Heat causes the breakdown of inter polymer bonding. It has glass transition temperature of 47 oC and a meting temperature of 220 oC

6. Research Advances
Nanocomposites of polymeric material are a fast growing area of industrial as well as scientific research. In fact they are called as the material of the 21st century. Work on nanocomposites of polycaprolactam has been going on since 1985 by Toyota and it is no surprise that in a short span Polycaprolactam nanocomposites have moved from lab products to industrial products. Examples of such are Nanalok, Nanosolve, and Ecobesta.
It is well known that polycaprolactam due to its polar nature have been recognized as a biologically significant polymer5; as such have found interesting use in biomedical applications recently. I will quote one such example where polycaprolactam has been successfully used for human osteoblastic cell culture applications6,7,8. Summarizing the research work of R. Niemal et al they prepared chitosan blend with polycaprolactam nanofibers via electrospinning process. Although similar work has been done in previous studies7 all recent but this research differs in that it produced very high aspect ratio nanofibers that can be seen as a breakthrough and found use in drug delivery, wound dressing and the most interesting application that of scaffold for tissue engineering. There high aspect ratio nanofiber was characterized by SEM images which showed that osteoblast cells were incorporated in to composite nanofibers which proved the success of this research work.

References
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2. Mahabadi, H. and O’Riscoll, KF, J. Appl. Polym. Sci. , Vol 21,1283-1287, 1977
3. Chavel, A., Lefebvre, G., “Monomers for polyamide synthesis in Petrochemical Processes2”, Editions Technip Paris, 233 & 28,1989.
4. K,Agrawal and K,Devika, Ind.Eng.Chem.Re, 2001, 40(12). 2563-2572
5. K. Fujihara, M. Kotaki, S. Ramakrishan, Biomaterials 26 (2005) 4139–4147.
6. R. Nirmala, Colloids and Surfaces B: Biointerfaces 83 (2011) 173–178
7. F. Yang, S.K. Both, X. Yang, X.F. Walboomers, J.A. Jansen, Acta Biomater. 5 (2009)
3295–3304.
8. H.W. Kim, J.H. Song, H.E. Kim, Adv. Funct. Mater. 15 (2005) 1988–1994.
9. H. Zhang, S. Li, C.J.B. White, X. Ning, H. Nie, L. Zhu, Electrochim. Acta 54 (2009)
5739–5745

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Figure 3
NMR spectrum of polycaprolactam

FTIR of polycaprolactam

Figure 1

Figure 2

Figure 4
Schemicatic diagram of VK tubular reactor

Table 1