Crystallinity of nylon

Ergungor describes the application of on-line Raman spectroscopy and neural networks to the simultaneous prediction of temperature and crystallinity of nylon-6 nanocomposites as a function of cooling rate. The authors prefer their neural network approach because they make use of information in the entire spectrum rather than from a few bands as most studies have done.84 Van Wijk etal. of Akzo Nobel obtained a patent on the use of a Raman spectrum of a polymeric fiber to determine dye uptake and other structural or mechanical properties based on previously developed models.85... 

Ergungor, Z. Batur, C. 8c Cakmak, M. On Line Measurement of Crystallinity of Nylon 6 Nanocomposites by Laser Raman Spectroscopy and Neural Networks /. Appl. Polym. Sci. 2004, 92, 474-483. 

The increase in both yield strength and modulus, and the decrease of ultimate elongation suggest that the degree of crystallinity of Nylon 66 has been increased during the treatments at both conditions. No significant difference has been found between two treatment conditions in terms of these three mechanical properties. 

Due to the presence of the polar groups (CO-NH), nylon attracts water. Hence, water absorption by nylon is relatively high, and higher than by any other thermoplastics, considered in this book. Compared to water absorption (24 h) of HDPE and PP (less than 0.01%), PVC (0.1%) and ABS (0.3%), Nylon 6 absorbs 1.2% water in 24 h (underwater), and 9% water when reached saturation. The lower the degree of crystallinity of nylon, the higher the water absorption. Due to this hygroscopicity, nylons must be dried before melt processing. 

The crystallites are destroyed upon melting and reform upon cooling. Their type and quantity depends on the sample s thermal history. Crystallinity values have been determined [26] for poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate) from both heat of fusion and heat capacity measurements by DSC. DSC has also been used to study the degree of crystallinity of Nylon 6 [27, 28] and crosslinked PVOH hydrogels submitted to a dehydration and annealing process [29]. 

It has been noted [11] that when unique vibrational bands can be associated with each phase, it is not necessary in principle to calibrate against other standards, although it is often desirable to do this. All that is required is a set of samples of widely varying composition for which the relative band intensities can be determined and extrapolated to the 100% and 0% crystallinity levels. Raman measurement of crystallinity in polyethylene illustrates this approach [229], as does the IR measurement of crystallinity of nylon [224]. Extrapolation of the spectroscopic calibration has sometimes been used to determine the amorphous and crystalline densities of a polymer [224, 225], thereby adding to the information available from the primary calibration standard. However, one must note that vibrational spectroscopy, DSC, X-ray and density actually measure different physical parameters and so a crystallinity determined solely by vibrational spectroscopy may well differ from that obtained by other techniques [230]. Also, with anisotropic materials, molecular orientation can alter band intensities and invalidate calibrations developed using isotropic standards [48, 227, 231]. 

El-Zaher, N.A., Kishk,S.S. Study of the effect of ultraviolet radiation on the chemical structure, mechanical properties and crystallinity of nylon-6 films, Colourage, 43 (1996) 11, p. 25-30... 

Nylon. The high degree of crystallinity in nylon means that plasticization can occur only at very low levels. Plasticizers are used in nylon but are usually sulfonamide based since these are generally more compatible than phthalates. DEHP is 25 phr compatible other phthalates less so. Sulfonamides are compatible up to 50 phr. 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

Commercial engineering thermoplastic nylons are mainly crystalline resins. Nylon-6,6 [32131 -17-2] is the largest volume resin, followed by nylon-6 (48). Other commercially available but much lower volume crystalline nylons are -6,9, -6,10, -6,12, -11, and -12. The crystallinity of the molded part decreases with chain size (49). A few truly amorphous commercial nylon resins contain both aromatic and ahphatic monomer constituents (50). For example, Trogamid T resin is made from a mixture of 2,2,4- and 2,4,4-trimethylhexamethylenediamines and terephthahc acid (51). 

The properties of the nylons are considerably affected by the amount of crystallisation. Whereas in some polymers, e.g. the polyacetals and PCTFE, processing conditions have only a minor influence on crystallinity, in the case of the nylons the crystallinity of a given polymer may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled and subsequently annealed, may be 50-60% crystalline, while rapidly cooled thin-wall mouldings may be only 10% crystalline. 

The suppliers of nylon 46 have laid particular emphasis on the fact that this polymer, with its highly symmetrical chain structure, leads to both a high level of crystallinity and a high rate of nucleation. In turn the high nucleation rate leads to a fine crystalline structure which in this case is claimed to lead to a higher impact strength (dry as moulded) than with nylons 6 and 66. 

In a partially crystalline homopolymer, nylon 6, property enhancement has been achieved by blending with a poly(ethylene-co-acrylic acid) or its salt form ionomer [24]. Both additives proved to be effective impact modifiers for nylon 6. For the blends of the acid copolymer with nylon 6, maximum impact performance was obtained by addition of about 10 wt% of the modifier and the impact strength was further enhanced by increasing the acrylic acid content from 3.5 to 6%. However, blends prepared using the salt form ionomer (Sur-lyn 9950-Zn salt) instead of the acid, led to the highest impact strength, with the least reduction in tensile... 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

In the PP-PA system, the DSC thermograms showed two peaks corresponding to nylon and PP. For the compatibilized system the crystallization peak of nylon remains unaltered, while that of PP shifted toward a higher temperature in the case of PPacr, and for PPmal, the shift was to lower temperatures (Fig. 1). This may be due to the fact that PPacr was acting as the nucleating agent. The average crystallinity of the blend was also decreased by the incorporation of compatibilizer. The mechanical properties of these blends was improved by the addition of PPmal and PPacr as compatibilizers (Table 1). 

Also common is the occurrence of transitions between different crystalline forms under tensile stresses. We recall here, for instance, the solid-to-solid transitions under stress of nylon 6 [76], PVDF [77, 78], and polybutylene terephtalate (PBT) [79-83]. 

The diamine and diacid monomers used to make type AABB nylons are typically rather difficult to handle in their pure form. Diamines are liquids or semisolids at room temperature, while the diacids are crystalline solids. These monomers become much more manageable when they are combined to form nylon salts, as shown in Fig. 23.7 a). Nylon salts are solids that can be easily handled and ensure a stoichiometric balance between the diacid and diamine, which is necessary to produce high molecular weight polymers. In the case of nylon 66, the precursor salt is made by boiling adipic acid and hexamethylene diamine in methanol, from which the nylon salt precipitates. 

We can manipulate the properties of nylon products by changing the conditions under which we crystallize them. The degree of crystallinity is increased by slow cooling, annealing, and by crystallization from highly oriented melts. As we increase the crystallinity level, stiffness and yield strength increase at the expense of impact strength. 

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