The incorporation of carbon nanofibres to enhance the properties of self reinforced, single polymer composites

This paper describes the incorporation of carbon nanofibres (CNF) into polypropylene (PP) single polymer composites, materials where both the reinforcing phase and the matrix phase are PP. The CNF/PP composites were produced from an assembly of highly oriented tapes. The process of making the composites involves heating the tapes to a critical temperature such that a small fraction of the surface of each tape is melted; on cooling this recrystallises to form the matrix of the composite. The production of the composites required optimisation of three stages; incorporation of CNF into PP tapes, orientation of CNF/PP tapes by tensile drawing and hot compaction of the tapes. Results are presented to describe the research and findings in each of these key stages.Preliminary studies showed that the introduction of small amounts of carbon nanofibres (CNF) significantly improved the properties of isotropic PP. For example, 5% volume addition of CNF gave a 60% increase in the room temperature Young's modulus and a reduction of 35% in the thermal expansion coefficient. Moreover, the percentage enhancement of properties was greater at high temperatures where the stiffness of the PP is much reduced. These results can be very well understood in terms of conventional composite modelling.In unidirectional CNF/PP hot compacted composites the major improvements in mechanical behaviour are in the direction transverse to the orientation direction, where the CNF can make a proportionately greater contribution to the properties, and as shown by dynamic mechanical behaviour, this is most marked at high temperatures. Composite modelling based on uniform strain with appropriate allowance for the CNF aspect ratio predicts the behaviour extremely well. A very interesting result is that the peel strength of composites produced by hot compaction of woven CNF/PP shows a four-fold increase over woven PP composites and this is increased by another factor of two by the addition of a maleic anhydride compatibiliser. A further interesting result, of some practical significance, is that although the incorporation of CNF into PP causes voiding and some loss of molecular orientation during drawing, the hot compaction process closes and seals the voids, so that the original PP density is recovered.     

The use of an interleaved film for optimising the properties of hot compacted polyethylene single polymer composites

It is shown that the incorporation of interleaved films has major advantages for the production of polyethylene single polymer composites by the process of selective melting (termed hot compaction). The key issue is to choose a compaction temperature which melts the minimum amount of the original oriented elements whilst achieving acceptable bonding within the compacted structure. Utilising an interleaved film, excellent interlayer peel strengths can be achieved at lower compaction temperatures giving greater retention of the oriented fraction of the original fibres or tapes and a wider processing window. For example, using a very high modulus, ultra-high molecular weight, polyethylene tape, together with an interleaved film, resulted in an in-plane modulus of 25 GPa, an in-plane strength of 500 MPa, and an interlayer strength of >10 N/10 mm. These are amongst the highest values reported for a single polymer composite. Other important factors have been investigated including fabric weave style and whether it is better to use fibres or tapes as the oriented reinforcement.     

The hot compaction behaviour of woven oriented polypropylene fibres and tapes. II. Morphology of cloths before and after compaction

The morphology of woven oriented polypropylene tapes and fibres has been studied both before and after processing by hot compaction. In this technique bundles of oriented tapes or fibres are subjected to suitable conditions of temperature and pressure so that just sufficient of each tape or fibre is selectively melted; on cooling, this material recrystallizes to bind the whole structure together. Three different polymers were studied, woven into various individual weave styles, in relation to optimum processing conditions. The weave style, the adhesion between neighbouring interfaces after compaction and the direction of crack propagation along the neighbouring interfaces on peeling were examined leading to clear correlations between the observed morphology and mechanical peel strength data.     

Novel composites by hot compaction of fibers

The production of solid section products from highly oriented fibers by a novel compaction procedure is described for melt-spun and gel-spun polyethylene fibers, poly(ethylene terephthalate) and polypropylene fibers and Vectran liquid crystalline copolyester fibers. Differential scanning calorimetry and electron microscopy have been used to study the structure of the compacted polymers. For the most successful compaction, selective surface melting of a small fraction of each fiber enables the formation of a fiber composite of high integrity, where the matrix phase is formed by epitaxial crystallization of the melted fraction on the initial fibers, retaining a high proportion of their initial strength and stiffness. A wide range of potential applications is envisaged for the composites produced by hot compaction. In many cases these composites will be produced by thermoforming. In addition to the obvious advantages of high stiffness and strength, in several instances the unrestricted exploitation of unique properties of the fibers such as transparency to microwave radiation or low thermal expansion coefficients offer additional incentives for the use of these hot compacted materials rather than conventional fiber/resin composites.     

The hot compaction behaviour of woven oriented polypropylene fibres and tapes. I. Mechanical properties

The aim of this work was to establish the important parameters that control the hot compaction behaviour of woven oriented polypropylene. Five commercial woven cloths, based on four different polypropylene polymers, were selected so that the perceived important variables could be studied. These include the mechanical properties of the original oriented tapes or fibres, the geometry of the oriented reinforcement (fibres or tapes), the mechanical properties of the base polymer (which are crucially dependant on the molecular weight and morphology), and the weave style. The five cloths were chosen so as to explore the boundaries of these various parameters, i.e. low and high molecular weight: circular or rectangular reinforcement (fibres or tapes): low or high tape initial orientation: coarse or fine weave.A vital aspect of this study was the realisation that hot compacted polypropylene could be envisaged as a composite, comprising an oriented ‘reinforcement’ bound together by a matrix phase, formed by melting and recrystallisation of the original oriented material. We have established the crucial importance of the properties of the melted and recrystallised matrix phase, especially the level of ductility, in controlling the properties of the hot compacted composite.     

Rheological Analyses of Carbon Nanofiber/Polypropylene Composites

Rheological properties of carbon nanfiber/polypropylene composites were investigated. CNFs synthesized at 600°C were thermally modified at 2200°C. These CNFs with a curl/twisted morphology were incorporated into polypropylene (PP) by intensive mixing. The untreated CNF composites showed higher dynamic moduli, G′ and G″, than the heat-treated CNF composites. Also, stress relaxation results indicated that untreated CNF composites were relaxed in a longer time than heat-treated CNF composites. These results support that the untreated CNF composites needed less CNFs to perform CNF-CNF network than the heat-treated CNF composites did. It is suggested that structural changes can be verified by rheological analyses.     

Mechanical and thermal transmission properties of carbon nanofiber‐dispersed carbon/phenolic multiscale composites

The present article reports the development and characterization of carbon nanofiber (CNF)‐incorporated carbon/phenolic multiscale composites. Vapor‐grown CNFs were dispersed homogeneously in to phenolic resin using an effective dispersion route, and carbon fabrics were subsequently impregnated with the CNF‐dispersed resin to develop carbon fiber/CNF/phenolic resin multiscale composites. Mechanical and thermal transmission properties of multiscale composites were characterized. Elastic modulus and thermal conductivity of neat carbon/phenolic and multiscale composites were predicted and compared with the experimental results. It was observed that incorporation of only 1.5 wt % CNF resulted in 10% improvement in Young's modulus, 12% increase in tensile strength, and 36% increase in thermal conductivity of carbon/phenolic composites. Fracture surface of composite samples revealed the formation of stronger fiber/matrix interface in case of multiscale composites than neat carbon/phenolic composites. Enhancement of above properties through CNF addition has been explained, and the difference between the predicted values and experimental results has been discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Hot compaction of woven nylon 6,6 multifilaments

We describe a study of the hot compaction of woven nylon 6,6 multifilaments produced by a patented procedure, developed at the University of Leeds, for creating novel single‐polymer composites. In this process, an assembly of oriented elements, often in the form of a woven cloth, is held under pressure and taken to a critical temperature so that a small fraction of the surface of each oriented element is melted, which on cooling recrystallizes to form the matrix of the single‐polymer composite. This process is therefore a way of producing novel high‐volume‐fraction polymer/polymer composites in which the two phases are chemically the same material. Nylon is an obvious candidate material for this process because oriented nylon multifilaments are available on a commercial scale. The aim of this study was first to establish the conditions of temperature and pressure for the successful hot compaction of oriented nylon 6,6 fibers and second to assess the mechanical properties of the manufactured hot‐compacted nylon sheets. A crucial aspect of this work, not previously examined in hot‐compaction studies of other oriented polymers, was the sensitivity of the properties to absorbed water, with a significant change in the properties measured immediately after hot‐compaction processing and 2 weeks later when 2% water had been absorbed by the compacted nylon sheets. As expected, the water uptake had a greater effect on those properties that depended on local chain interactions (e.g., the modulus and yield strength) and less effect on those properties that depended on the large‐scale properties of the molecular network (e.g., strength). The only negative aspect of the properties of the hot‐compacted nylon sheets was the elevated‐temperature performance of the wet sample, with the modulus falling to a very low value at a temperature of 80°C. However, apart from the elevated‐temperature performance, the majority of the measured properties of the hot‐compacted nylon sheets were comparable to those of hot‐compacted polypropylene and poly(ethylene terephthalate). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 991–997, 2006     

Orientation development in retensilized,biaxially oriented polypropylene films

A basic study of orientation development in retensilized, biaxially oriented polypropylene films was performed. Wide-angle X-ray scattering (WAXS) and birefringence measurement techniques were used to examine films prepared under specific processing conditions. Biaxial orientation factors were determined for these films and were correlated with end-use properties of the materials. In particular, the dispensability of oriented polypropylene films was found to correlate with crystalline-phase and amorphous-phase orientation factors. Dispensability is a relatively important property of pressure-sensitive tapes; it is defined as the force necessary to cut a film backing over a serrated blade of the type used in commercially available adhesive-tape dispensers.     

Hot compaction of polyethylene naphthalate

In this article, we describe the production of single polymer composites from polyethylene naphthalate (PEN) multifilaments by using the hot compaction process. In this process, developed at Leeds University, highly oriented tapes or fibers are processed at a critical temperature such that a small fraction of the surface of each oriented element is melted, which on cooling recrystallizes to form the matrix of the composite. This process is, therefore, a way to produce novel high‐volume fraction polymer/polymer composites where the two phases are chemically the same material. A variety of experimental techniques, including mechanical tests and differential scanning calorimetry, were used to examine the mechanical properties and morphology of the compacted PEN sheets. Bidirectional (0/90) samples were made at a range of compaction temperatures chosen to span the melting range of the PEN multifilaments (268–276°C). Measurement of the mechanical properties of these samples, specifically the in‐plane modulus and strength, allowed the optimum compaction temperature to be ascertained (～ 271°C), and hence, the optimum mechanical properties. The optimum compacted PEN sheets were found to have an initial modulus close to 10 GPa and a strength of just over 200 MPa. The glass transition temperature of the optimum compacted sheets was measured to be 150°C, nearly 40°C higher than compacted poly(ethylene terephthalate) (PET) sheets. In previous work on polypropylene and PET hot compacted materials, it proved instructive to envisage these materials as a composite where the original oriented multifilaments are regarded as the reinforcing phase, and the melted and recrystallized material are regarded as the matrix phase. Dynamic mechanical bending tests (DMTA) were used here to confirm this for PEN. DMTA tests were carried out on the original fibers and on a sample of completely melted material to determine the fiber and matrix properties, respectively. The composite properties were then predicted by using a simple rule of mixtures and this was found to be in excellent agreement with the magnitude and measured temperature dependence of the hot compacted PEN material. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 796–802, 2004     

Plasma‐induced surface modification and adhesion enhancement of polypropylene surface

Unoriented (UPP) and biaxially oriented (BOPP) polypropylene films were treated under radio frequency plasma of air, nitrogen, oxygen, and ammonia. Surface modification of polypropylene films was investigated by using surface energy measurement and attenuated total reflection (ATR)‐FTIR spectroscopy. Surface energy of air and nitrogen plasma‐treated polypropylene film increased for shorter treatment time and then decreased and attained an equilibrium value. Such changes in surface energy were not observed for oxygen and ammonia plasma‐treated polypropylene film, which increased to an equilibrium value. ATR‐FTIR studies revealed characteristic differences in the absorption spectra for short‐duration and long‐duration treatments. From the relative intensity change in the C—H stretching vibration, the mechanism of surface chemical reaction could be inferred. Studies regarding the durability of surface modification due to plasma treatment were evaluated by investigating surface energy of samples aged for 2 months. Treated films subjected to peel strength measurement showed improvement in bondability for UPP and BOPP film by hydrophilic surface modification accompanied by surface crosslinking. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 925–936, 2002     

Interfacial properties of highly oriented coextruded polypropylene tapes for the creation of recyclable all‐polypropylene composites

The creation of highly oriented, coextruded polypropylene (PP) tapes allows the production of novel, wholly thermoplastic, recyclable “all‐polypropylene” (all‐PP) composites, which possess both a large temperature processing window (>30°C) and a high volume fraction of reinforcement phase (highly oriented PP tapes: >90%). This large processing window is achieved by using coextruded, highly drawn PP tapes. To achieve coherent all‐PP composites the interfacial characteristics following consolidation must be understood. This article investigates the interfacial characteristics of these coextruded tapes by using microcomposite models to create interfaces between tapes of varying draw ratios, drawing temperatures, skin/core ratios, and skin layer thicknesses. The tape drawing parameters are seen to control the interfacial properties in subsequent microcomposite models. The failure mode of these specimens, and hence bond strength, varies with consolidation temperature, and a model is proposed describing and explaining this behavior. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 118–129, 2007     

Structural description of self-reinforced polypropylene composites

Self-reinforced polypropylene composites (SR-PP) possess an exceptional property spectrum and are predestined for use in a multitude of structural or semi-structural applications. However, the underlying macromolecular orientation can only be transferred into the layered composites consolidated out of highly stretched fibres and tapes of semi-finished textile products. Specific preservation of the self-reinforcement throughout processing and beyond is necessary and creates special challenges for processing technology. Depending on the processing temperatures and pressures selected, the highly oriented fibres and tapes are influenced by their own degree of self-reinforcement, which, in turn, affects the microstructure and remaining composite properties. In this publication, three different semi-finished textile products are introduced as basic materials. Exemplary selected test samples, which display a low degree of compaction on the one hand and a high degree of compaction on the other, were subject to wet chemical etching to enable confocal laser scanning microscopic images to be created. These images were then used to compare the microstructures of the semi-finished textile products that were used.     

Synthesis of composite materials of carbon nanofibres and ceramic monoliths with uniform and tuneable nanofibre layer thickness

Composite materials consisting of ceramic monoliths and carbon nanofibres (CNFs) have been synthesized by catalytic growth of CNFs on the γ-alumina washcoating layer covering the walls of a ceramic monolith. The composites possess a relatively uniform mesoporous layer of CNFs of relatively small diameter. The thin alumina washcoating (ca. 0.1 μm) prevents the CNFs from being trapped inside the alumina pores and hence the CNFs grow freely throughout the washcoating layer to form a uniform layer of CNFs that completely covers the surface of the monolith walls. The growth temperature is found to control the thickness of the CNF layer (2-4 μm), the growth rate of the nanofibres, and the mechanical strength of the resulting CNF-monolith composite. At ideal conditions, a complete adhesion of the CNF layer and higher mechanical strength than the original cordierite monolith can be obtained. The CNF layer has an average pore size of 17 nm with absence of microporosity which renders these monoliths promising candidates for the use as catalyst supports, especially for liquid phase reactions. The CNFs have small diameters (5-30 nm) due to the high dispersion of Ni particles in the growth catalyst and the CNFs exhibit an unusual branched structure.     

Impact of processing method and surface functionality on carbon nanofiber dispersion in polyimide matrix and resulting mechanical properties

Composites were produced with functionalized carbon nanofibers (CNF) and polyimide (PI) matrix using either in situ polymerization or blending processes. The impact of the composite processing method, CNF surface chemistry, and fiber loadings on the dispersion of fibers and mechanical properties of composites were investigated. Specifically, functionalization of oxidized CNF with a diamine and polyimide oligomer that mimicked the structure of the base polyimide led to improved dispersion of CNF in the matrix polymer. Samples produced using precipitation blending from hot solvent and in situ polymerization exhibited improved dispersion and reduced agglomeration of CNF relative to samples made using direct blending. While SEM images showed poorly dispersed pristine CNF in PI in the form of agglomerations and thick deposition layer on the bottom of composite film, there was clearly better dispersion for functionalized CNFs. Composites produced with functionalized CNF exhibited improvements in modulus, glass transition temperature and tensile strength relative to the base polyimide. POLYM. COMPOS. 35:1473–1485, 2014. © 2013 Society of Plastics Engineers     

High performance PP/SEBS/CNF composites: Evaluation of mechanical,thermal degradation,and crystallization properties

Nanocomposites comprising carbon nanofibers (CNF) were prepared and evaluated in terms of morphology, mechanical performance, thermal stability and crystallization properties. It was found that addition of CNF reinforced polypropylene (PP) matrix by marginally increasing the strength and modulus, but at the expense of toughness and ductility. To improve the toughness of the composites, polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) was used. Presence of SEBS remarkably improved the toughness and ductility of the composites. The optimum level of reinforcement was observed at 0.1 wt% of CNF in the composites. Phase morphology studies revealed that at this concentration, CNF were well dispersed in polymer phases and beyond it, agglomeration occurred. PP/SEBS/CNF (0.1 wt%) nanocomposites exhibited good strength, excellent toughness and decent modulus, which make them suitable for cost effective, light‐weight, tough and stiff material for engineering applications. It was observed that thermal stability of composites is only marginally improved whereas crystallinity of PP drastically reduced by the addition of CNF. POLYM. COMPOS., 38:2440–2449, 2017. © 2015 Society of Plastics Engineers     

纤维增强热塑性复合材料新动向--从GMT-LFT到All-PP

随着人们环保意识的增强，对材料的开发研究提出了一些新的要求。All-PP复合材料可全部回收利用，低密、高强，低温下良好的耐冲击性，可替代GMT和LFT，有很大发展前景。本文将介绍热压法和共挤带法两种All-PP生产工艺，并对GMT、Twintex、All-PP的性能进行了比较。     

Effects of corona treatment on composite formation. Adhesion between incompatible polymers

Polypropylene–nylon 6 10 composites were prepared by the in situ polymerization of the nylon monomers on polypropylene films. The adhesion between the nylon and the polypropylene was markedly improved by a brief corona discharge treatment of the films in nitrogen prior to coating. This improvement was demonstrated by an increase in the peel strength of the nylon coating and a decrease in brittleness of photo-oxidized compesites when corona treatment was used. Adhesive bonding between the nylon and substrate was sufficiently strong to cause cohesive failure in the corona-treated polypropylene. Only interfacial failure was observed at untreated surfaces. These effects were demonstrated by electron microscopy of the surfaces produced in peel tests. The effects of corona treatment on adhesive bonding characteristics of surfaces are discussed in terms of the chemical and physical changes observed in treated surfaces.     

非结晶性水性聚氨酯油墨连接料的合成

以端羟基聚丁二烯(HTPB)、聚己二酸新戊二醇酯(PNA)、异佛尔酮二异氰酸酯(IPDI)为主要原料制备了一系列水性聚氨酯乳液.采用FTIR、XRD、SEM对胶膜的结构及形貌进行了表征,对乳液性能、胶膜的吸水率、力学性能及胶膜在双向拉伸聚丙烯(BOPP)薄膜上的剥离强度进行了测试,通过胶膜与水、乙二醇、二碘甲烷的接触角测试计算了胶膜的表面能.结果表明,随着HTPB含量的增加,乳液粒径逐渐增大、胶膜表面能大致呈逐渐减小趋势.当HTPB含量为PNA物质的量的30％时,水性聚氨酯乳液综合性能最好,平均粒径为287.3 nm,胶膜表面能为31.77 mJ/m2,拉伸强度为14.20 MPa,断裂伸长率为977.72％,在BOPP薄膜上的剥离强度为4.4 N/25 mm.以该乳液配制的油墨满足GB/T 26394—2011《水性薄膜凹印复合油墨》对油墨黏度13～50 s的要求,且具有良好的初干性和抗粘连性,可满足日常使用.