Interface Morphology and Mechanical Properties of Uni- directional Fibre Reinforced Nylon 6 BY T. BESSELL, D. HULL AND J. B. SHORTALL Department of Metallurgy and Materials Science, University of Liverpool, Liverpool L69 3BX Received 23rd June 1972 Unidirectional glass and carbon fibre reinforced Nylon 6 composites have been prepared by the in situ anionic polymerization of caprolactam on the fibres. In this way, it is possible to control the solidification and crystallization characteristics of the nylon. The morphology of the nylon around the fibres has been determined using sections of 0.15 Vf (volume fraction) composites and also thin films containing single fibres. The fibres nucleate directional crystallization (columnar growth) of the nylon around the fibres. The amount and form of the columnar growth depends on the type of fibre and the polymerization conditions.Preliminary studies on the mechanical properties and fracture characteristics of 0.15 Vf composites have been made and the results are discussed in terms of the matrix and interface morphology. One of the most efficient ways of making use of the properties of thermoplastics as engineering materials is by reinforcing them with fibres. With nylon 6 this has resulted in the production of a useful and well established engineering material. Reinforcement by short random fibres imparts an increase in rigidity and tensile strength with increased dimensional stability under load. These properties, together with the toughness, abrasion resistance and ductility of the nylon itself has meant that fibre reinforced nylon composites have found many applications particularly where components are subjected to high stresses.In the conventional approach, these composites are prepared by incorporating the fibres into the nylon by dry mixing and then either injection or compression moulding the mixture into the desired shape. The mechanical properties of a composite are a function of the aspect ratio of the fibre. Damage to the fibres during processing greatly reduces the aspect ratio so that the optimum improvement in mechanical properties which should be achieved with random fibre reinforcement is seldom reached. The composites then show a less than two fold increase in strength over that of the nylon alone. However, by using uniaxially aligned continuous fibres (in which the fibre loadings may be increased to over 70 % by volume), maximum benefit can be obtained from the high modulus fibres when the load is applied in the fibre direction.In a recent paper we have outlined a method of preparing uniaxial aligned fibrous composites involving the in situ anionic polymerization of caprolactam directly on the fibres. This method overcomes the difficult problem of incorporating uniaxially aligned fibres in a thermoplastic matrix, and preliminary work has shown that the composite has interesting fracture characteristics associated with the interfacial properties. EXPERIMENTAL The anionic polymerization of nylon 6 was carried out using dry, high purity 8-caprolactam with sodium hydride as the catalyst and acetyl caprolactam as the initiator.For polymer- ization, 0.01 molar proportions of both catalyst and initiator were used. To prepare samples 137138 FIBRE REINFORCED NYLON 6 of nylon, the polymerizing solution was cast into a steel mould having a rectangular cross section 4 x 8 mm. The nylon was polymerized at the desired temperature, usually between 410 and 480 K, for one hour in an atmosphere of nitrogen. The mould was then allowed to cool slowly in air. Full details of the methods used are given elsewhere.2 Nylon reinforced composites were prepared using either glass or carbon fibre. The glass fibre was supplied by Pilkington Brothers in the form of roving, in which the individual fibres had a mean diameter of 12 pm and a fibre tensile strength of between 0.75 and 3.4 GN m-2. The carbon fibres, used to make tensile specimens, were supplied by Le Carbone Ltd., and Courtaulds Ltd., and had diameters of 11 and 8.2 ,urn and tensile strengths of 1.5-2.2 and 1.72-2.41 GN m-2 respectively.The Courtaulds fibre had a surface treatment to give an increased interlaminar shear strength. Unidirectional continuous fibre composites with a nylon 6 matrix were fabricated using a modified leaky mould technique. The mould, which measured 140 mm long, consisted of a lower portion containing two 8 mm wide channels. Into this fitted a top platen which rested on stops leaving rectangular channels 4mm deep. The lower portion of the mould was supported by a base plate which held fibre tensioning attachments. The mould was sprayed with a P.T.F.E.mould release agent prior to assembly. Reweighed tows, of either glass or carbon fibres, giving the required fibre volume fraction were placed through the mould, tensioned and held in position. The mould and support plate with the fibres in position was placed in a vacuum oven maintained at the nylon polymerization temperature required. 8-caprolactam and sodium hydride (catalyst) were reacted under nitrogen at 393 K to produce sodium caprolactam. This was then heated to the desired polymerization temp- erature with the requisite amount of initiator. The solution was then poured onto the fibres in the mould, and the top platen placed in position. The in situ polymerization was allowed to continue under nitrogen for one hour. After this period the mould was cooled in air before disassembly. The resultant composite samples, measuring 140 x 8 x 4 mm, were stored under vacuum until tensile specimens were prepared.Rectangular specimens, 140 mm long with an 8 x 4 mm cross section, were prepared for modulus determinations according to Courtaulds’ specification^.^ The specimen ends were tabbed with aluminium tabs measuring 4Ox 8 x 4 mm, again prepared according to Court- aulds’ specification. The tabs were bonded to the composites using an Araldite adhesive which was allowed to cure at room temperature for seven days. Curved neck specimens with a radius of about 55 mm and having a minimum width of 4 mm were used for the fracture studies. These specimens were also tabbed. The surfaces of these specimens were polished on emery paper commencing with 600 grade and finishing with grade 410.The final polish was achieved with alumina (0.3 pm) powder. RESULTS MORPHOLOGY To establish the effect of the reinforcing fibres on the morphology of Nylon 6, it was first necessary to study the relationship between the spherulite structure of anionically polymerized nylon and variables such as catalyst and initiator concentra- tions and the temperature of polymerization. Morphological examinations were performed on microtomed sections, 5 ,urn thick, mounted in MS 550 silicone fluid. When viewed between crossed polars these sections showed the characteristic birefringence pattern associated with a spherulitic structure. The morphology was also studied by examining the surfaces of bulk specimens after polishing and etching in a 5 % m-cresol/methanol solution.The spherulite diameter was found to vary with polymerization temperature, as illustrated in fig. 1. The material polymerized at 473 K had a mean spherulite diameter of 60 pm and the spherulite boundaries were clearly defined. The spherulite size decreased with decreasing polymerization temperature, having a mean value of 50 pm at 453 K and 30 pm at 433 K. The spherulitic structure and spheruliteFIG. l ( a and b).-Spherulitic morphology of Nylon 6. (a) polymerized at 473 K, (b) polymerized at 413 K. Specimens etched in 2 % m-cresol in methanol. [To face page 138(4 FIG. 2(a-e).-Optical transmission micrographs using crossed polars of thin films of Nylon 6 showing columnar growth around carbon fibres.a, Grafil HM-S (type 1) ; b, Grafil A (type 11) ; c, Hitron HMG-50s surface treated carbon fibre ; d, Hitron HMG-50 untreated fibre ; e, Hitron HMG-50s fibre in matrix of small spherulite size.FIG. 3.-Section of a 0.15 Vf carbon fibre composite microtomed perpendicular to fibre axis showing the nucleatirig effect of the fibres. Optical transmission micrograph between fibre. crossed polars. FIG. 4.-Optical transmission micrograph using crossed polars of a thin film of Nylon 6 showing the very limited columnar growth around a glass FIG. 5(u and b).-Fracture specimens showing extent of pull out in (a) 0.15 Vf glass fibre composite, (6) 0.15 Vf carbon fibre composite.FIG. 6.-Transverse matrix cracks leaving the fibres intact FIG. 7.-Redirected crack in a fractured 0.15 P'f glass fibre/nylon composite. The tensile axis is parallel to the fibre axis.T .BESSELL, D. HULL AND J . B . SHORTALL 139 boundaries of nylon polymerized at 413 K were ill-defined, although this material showed birefringence effects when viewed through crossed polars. of the effect of catalyst and initiator concentrations on induction time and morphology of nylon 6, we have shown that the spherulite dia- meter is dependent predominantly on the polymerization temperature and moulding and cooling conditions and not on catalyst or initiator concentrations. These concentrations are known to have an effect on molecular eight,^ relative viscosity and polymerization conversion. 7-9 The moulding conditions will also have an effect on the resultant spherulitic structure, since it is known that the crystallization process of this polymer is lo on the morphology of nylon, which was polymerized in the presence of glass and carbon fibres, were made by preparing thin films of nylon containing single fibres.A small drop of liquid monomer, catalyst and initiator was placed onto a glass slide and the fibre laid across it. A second glass slide was then positioned on top to disperse the solution into a film. Polymerization was allowed to take place under nitrogen in an oven held at the required temperature. In the preliminary work,l we established that a columnar crystallization structure can be produced at the fibre interface. This work has been extended to determine the effect of different types of fibre on the columnar structure.Carbon fibres from different manufacturers were used and a range of columnar structures obtained are illustrated in fig. 2. Fig. 2a and 2b show the difference in structure arising from the type of carbon fibre. The crystalline structure around Grafil HM-S, Type 1 (Court- aulds Ltd.) fibre is much finer than the corresponding structure around Grafil A, Type 11. Two zones can be identified in the columnar region; an inner zone with a fine random structure and an outer zone with a fibrillar structure extending to the spherulitic region. The inner structure is attributed to rapid nucleation of the nylon on the carbon fibre surface and is more pronounced on Type I fibres. A similar effect has been observed on Modmor I and Modmor I1 (Morganite Ltd.) and Rigilor AG (Type I) and Rigilor AC (Type 11) (Le Carbone Ltd.) carbon fibres.No marked differences in the columnar growth were observed with surface treatment. Fig. 2c and 2d show a surface treated and an untreated Hitron HMG-50 (Hitco) carbon fibre. The inner zone is well defined around these fibres and is wider than the fibre diameter. The width of the columnar region is related to the spherulite size as illustrated in fig. 2e, which shows a surface treated Hitron HMG-50s carbon fibre in a matrix of small spherulites. The morphology of nylon in the composites was examined in thin films prepared by microtomy and in sections of bulk specimens prepared by mechanical polishing followed by etching. Unfortunately, it was only possible to produce thin sections of carbon fibre composites, microtomy of glass fibre composites being impossible. The columnar growth around carbon fibres in aO.15 V, uniaxially aligned composite was similar to that found in the thin films described above (fig. 2), although the width of the columnar region tended to be more uniform and to extend to about the fibre diameter into the matrix. In these aligned composites, the columnar crystalline region around adjacent fibres tend to impinge on each other resulting in a modification of the spherulitic structure. Microtomed sections, cut perpendicular to the fibre axis (fig. 3) show the nucleating effect of the carbon fibres. When observed from this direction the columnar growth has the characteristic spherulitic appearance with the carbon fibres acting as nucleating sites.As in the single fibre observations (fig. 2c and 24 it was found that the columnar growth was unaffected by surface treatment of the fibres, since both treated and untreated fibres gave rise to the same morphology. However, as indicated later, the In the investigation Preliminary experiments140 FIBRE REINFORCED NYLON 6 strength of the bond between the fibre and the columnar region is increased by surface treatment. The nucleating effect of the surface coated glass fibres was found to be considerably less than that of the carbon fibres. Fig. 4 shows a thin film specimen containing a glass fibre. The extent of directional crystallization normal to the fibre is very limited and at some points along the fibre length it is non-existent.Sections of specimens polished perpendicular to the fibre axis did not show a correlation between the posi- tions of the glass fibres and the spherulite nuclei, again indicating that the glass fibres do not exhibit the same nucleating effect as carbon fibres. MECHANICAL PROPERTIES The mechanical properties reported here form part of an initial evaluation of these composites. Only a limited number of specimens were tested and thus no confidence limits are presented for the values given. The tensile modulus (E) and the tensile strength determinations of both 0.15 Vf carbon and glass fibre composites were carried out on an Instron testing machine using tabbed specimens. The rectangular cross section specimens used for the modulus determinations failed prematurely at about half the fracture stress of the necked tensile specimens.This premature failure occurred at the aluminium tabbed area due to stress concentrations associated with gripping. In the case of glass fibre/nylon composites, the modulus agrees fairly well with that predicted from the simple law of mixtures, viz. The moduli and fracture stresses of the composites are given in table 1. where c, f and m refer to the composite, fibre and matrix respectively. This calcula- tion is based on the reported modulus of undamaged glass fibres. The values of modulus and tensile strength of the nylon 6 have been taken as 1 GN m-2 and 75 MN m-2 respectively. TABLE ELASTIC MODULUS AND TENSILE STRENGTH OF GLASS AND CARBON FIBRE NYLON COMPOSITES composite system elastic modulus/GN m-2 tensile strength/MN m-2 observed calculated observed calculated 0.15 Vf unidirectional glass fibre/ 8.2 12.1 450 320-575 nylon 0.15 Vf unidirectional carbon fibre 0.15 Vf unidirectional carbon fibre (Le Carbone) Nylon 6 30.7 56.3-62.3 3 80 290-397 (Courtaulds) Nylon 6 - - 300 3 2 6-429 The difference between the theoretical value of 12.1 GN m-2 and the observed value of 8.2 GN m-2 may be due to slight fibre misalignment which is known to cause an apparent reduction in modulus and to the additional deformation of the tab joints.The fracture strengths of the necked specimens were found to agree very well with the values calculated from the law of mixtures. The composites reinforced with surface treated Courtaulds fibres having a slightly lower fracture stress than that predicted.T . BESSELL, D.HULL AND J . B . SHORTALL 141 FRACTOGRAPHY Examination of the glass fibre composites after fracture showed that a large amount of pullout had occurred (fig. 5). It was also noted that in these specimens many cracks propagated transverse to the fibres, leaving the fibres bridging the gap between them as illustrated in the polished section in fig. 6. In some cases, the trans- verse cracks had been redirected parallel to the fibres at distance of about 2pm from the fibre (fig. 7). This indicates that there is a surface of weakness some distance from the fibre surface which may be related to the columnar region around the fibre (fig. 4). This suggestion is supported by the fact that, on examination of the fracture surfaces in the scanning electron microscope, some fibres were surrounded by sheaths of nylon (fig, 8).X-ray studies confirmed the presence of nylon on these fibres. In contrast, the fractured carbon fibre composites did not show the extensive pull out (fig. 56) that had been observed with the glass fibre composites. Some evidence for matrix cracking perpendicular to the fibres was apparent but on a very limited scale and restricted to an area very close to the actual fracture surface. Optical microscopy revealed that these matrix cracks are not redirected by the fibres to the same extent as in the case of glass fibre composites. Stereoscan microscopy of fractured 0.15 Vf surface treated carbon fibre specimens, fig. 9 and 10, showed that a limited amount of pull out of fibres had occurred, although there were many areas on the fracture surface in which the fibre fracture had occurred in the failure plane of the matrix (fig.9). In contrast, carbon fibres, which had not been surface treated, exhibit a much greater degree of fibre pull out on the fracture surface (fig. 11). This suggests that the bond strength between the surface treated fibre and the nylon matrix is improved by the surface treatment. No evidence for the sheathed type pull out has been observed with carbon fibre composites. DISCUSSION In composites prepared by anionic in situ polymerization of caprolactam on uniaxially aligned graphite and glass fibres, a system has been developed in which there is a layer between fibre and the bulk nylon. In thermosetting systems it has been shown that layers with an intermediate modulus can markedly affect the mechanical properties due to improvements in the stress transfer between the fibre and the resin.Thus, the elongation l2 and toughness l3 of composites have been increased. The changes in properties are sensitive to the thickness of the layer. Although such layers can be introduced, with some difficulty, into thermosetting systems this has not yet been demonstrated in nylon filled systems and the in situ polymerization route described here offers one possibility. The most significant difference in the fracture modes of the glass and carbon fibre composites is the propagation of longitudinal modes along surfaces some distance from the fibre surface in the glass fibre reinforced material.The limited evidence available in fig. 4,7 and 8 suggests that this is related to the columnar growth structure around the glass which results in a weak interface between the columnar structure and the main spherulitic structure. No corresponding effects were observed in carbon fibre composites, and it will be noted that the columnar region has a width in excess of one fibre diameter. Failure of these composites is brittle in nature and usually occurs by the catastrophic propagation of a single crack. The reasons for these fracture modes in nylon/glass and nylon/carbon systems can be understood by consideration of the energies associated with crack propagation. For the glass/nylon system, the fractographic studies revealed that some of the pulled142 FIBRE REINFORCED NYLON 6 out glass fibres were coated with a sheath of nylon.Energetically the removal of a fibre plus a sheath would be possible, provided the bond strength between the fibre and the sheath is much higher than between the sheath and bulk matrix. It is considered that the bond strength in a glass fibre nylon system is increased by the chemical bonding between physisorbed water (which invariably exists on the surface of glass fibres 14) and the fibre. This allows for the formation of a strong hydrogen bond between the fibre and the reactive -C=O and -N-H groups of the polyamide. Thus it is possible that less energy is required to pull the fibre with the columnar sheath from the matrix than to sever the strong chemical bond between fibre and polymer, even allowing for the increased frictional forces which have to be overcome in pulling the irregular surfaced sheath from the bulk matrix.The columnar growth in the carbon fibre system has been shown to be cylindrical in nature, the fibres acting as nucleating sites for row spherulites in the bulk material (fig. 3). With nylon/glass specimens, the fibres do not act as nucleants to the same extent and matrix cracking on a large scale, crack redirection and pull out is observed, The reason for the difference in nucleating activity between the carbon and glass fibres cannot yet be explained. Nucleation of polymers against surfaces is still not clearly understood and such consideration as wetting of the fibres, surface energies and super- cooling may be involved to a greater or lesser extent.It has been suggested l 5 that columnar growth against fibres from a thermoplastic crystalline polymer matrix may be determined to some extent by low energy sites on the surface of the carbon fibre and by coherency between the two crystal structures. In the glass fibre composites, the columnar growth is limited and localized, and in some areas is not apparent at all. The evidence in fig. 7 suggests that the glass fibre polymer interface is relatively weak in shear, so that transverse cracks can circumvent the reinforcement leaving the fibre bridging the gap. Fibre pull out and gross matrix and interface cracking will then occur. In the carbon fibre composites, the columnar crystalline region is large, and pull out of a sheath is not possible. The extent of pull out will be determined by the strength of the bond between the fibre and columnar region and the distribution of weak points along the fibres.16 By increasing the shear strength of the fibre/matrix bond by surface treatments fracture surface pull out is reduced.This is illustrated by comparison of fig. 9 and 11. In both these cases the extent of the columnar region appeared to be unaffected by the surface treatment. As a result of this preliminary work, several modifications to the nylonlfibre composites are being studied with the aim of optimizing the fracture mode. Control of the extent of the columnar growth on both graphite and glass surfaces is being investigated. Also by variation of the volume fraction of the fibres the columnar region will be restricted in growth by impingement on adjacent columnar regions and a system consisting of cylindrical rows of spherulites can be developed, nucleated around fibre cores. 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