Chemorheology of Polymers: From Fundamental Principles to Reactive Processing

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Need help? For some specific tribological applications bearing, gear cast PAs are still unavoidable. Nowadays, however casting has received a powerful competitor, viz. Pioneering activities on the application of anionic polymerization of lactams for AM versions started in the s. Khodabakhshi et al. The solidification half-time, linked with the productivity of this AM method, could be reduced to below one minute. The authors compared the characteristics of their ink-jetted PA-6 with those of cast PA-6s. It can be predicted that the next step in AM via the anionic polymerization of lactams will be their modification, especially with nanofillers [ 43 ].

In these processes polymerization and shaping occur at the same time net-shape processing. It is noteworthy that the related process simulations are of practical interest [ 44 ]. In these operations the polymerization takes place below the T m of the final product thereby reducing the cycle time fast demolding owing to solidification. Rusu et al. Barhoumi et al. To support the reactive rotational molding of in situ polymerized PA-6 the authors summarized the chemorheological results isothermally heated in TTT diagrams. Table 1 clearly shows the benefits of reactive rotational molding over traditional ones [ 46 ].

Processing and material parameters of specimens produced by classical and reactive rotational molding [ 46 ]. Considering the fact that the anionic polymerization of lactams is much faster than the hydrolytic one, several attempts were made to synthesize PAs through reactive extrusion.

Supplementary files

Credit in this field should be given to the groups of Michaeli [ 47 ] and White [ 48 , 49 ]. The related works covered not only homopolymers but also other modifications. Wu et al. Interestingly, in situ foaming of anionically polymerized lactams was not yet a topic of academic research. The patent literature [ 51 ] suggests, however, that this is a feasible option.

When the polymerization is performed below T m then the decomposition temperature of chemical foaming agents should be carefully selected. A peculiar feature of the anionic polymerization of lactams is that it represents the only suitable method to produce PAs in powder form [ 1 ]. To get powder the polymerization proceeds in suitable organic liquids acting as precipitants. Oily polyisobutylenes proved to be very suitable dispersing fluids for the micronscale PA powders [ 52 ].

The major advantage of this process was that PA-6 particles in nanoscale 15—30 nm could be received. This was a big achievement as the traditional techniques resulted in particles ranging from a few to hundreds of microns [ 1 ]. Note that the applications of PA-6 powders, especially the nanoscaled ones, are manifold medical, sensoric, environmental [ 54 ].

Extensive research was dedicated to the modification of anionically polymerized PA-6 and PA through various copolymerization strategies.

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The property modifications mostly aimed at reducing the crystallinity and melting temperature, enhancing the ductility and toughness, as well as improving the thermal and hydrothermal resistances. Next, we give a brief survey on various copolymerization techniques classified according to the comonomer pairs or molecular build-up. Anionic copolymerization is focused on the combination of CL and LL as industrially available lactams. Based on the topic-related summary of Roda [ 1 ], the activated anionic polymerization with NaCL initiator yielded random copolymers. Kinetic investigations confirmed that in the initial stage of polymerization PA-6 homopolymer forms, while random chain, consisting of CL and LL, appears in a later stage [ 1 , 55 ].

Ricco et al. This work was focused on changes in the thermal and structural properties thereby considering the products of side reactions. De-oligomerization by extraction affected T g , T m and the crystallinity of the samples. Nauman et al. The authors reported on the formation of a gradient copolymer because CL was incorporated into the polymer backbone preferentially in the starting phase of polymerization. It was found that the conversion, M v , and degree of crystallinity were all reduced with increasing LL content.

Parallel to that, however, the flexural modulus and water uptake decreased while the Izod impact strength increased. Vygodskii et al. The above mentioned results underline that the copolymerization of lactams is a very complex process. The relative amount of the comonomer built-in and their position along the polymer chain depend on several factors reactivity of each activated monomer, acylation of the lactam anions, possible side reactions, crystallization reactions etc.

In this case the initiation step is the acylation of lactone by the lactam anion [ 1 , 59 ]. At higher concentrations, lactone may work as activator and comonomer at the same time. The acylated lactone growth center alkoxide anion prefers the polymerization of lactone resulting in a polyester chain segment because the incorporation of lactams is much slower. Therefore, one would expect the formation of a block copolymer. Nevertheless, lactam—lactone block copolymers can be synthesized when the copolymerization is kinetically controlled. The block length in this poly amid-block-ester could be controlled by the feeding ratio of each monomer during extrusion.

Fang et al. In this study only initiator was used, namely LiCL produced in situ from lithium-tri-tert-butoxyaluminohydride. The structure of these copolymers was random and their T g could be well predicted by the Fox equation. The preparation of copolymers di-, triblock with blocks composed of lactam or non-lactam chains requires the incorporation of suitable prepolymers in the molten, anionically polymerizable lactam.

Such prepolymers should be soluble in the lactam melt and, in addition, bear reactive end groups, such as hydroxyl —OH and amine —NH 2. The most important step of the block copolymerization is to convert the end groups of the polymer to N -acyllactam or N -carbamoyllactam moieties. Afterward, they function as non-ionic growth centers, i. The converting precursors are polyisocyanates N -carbamoyllactam or bis-acyllactams undergoing exchange reactions alcoholysis, aminolysis [ 1 ].

The related block copolymers are often referred to as nylon block copolymers NBC and another widely used name is nylon reaction injection molded RIM. This nylon reaction injection molded Nylon RIM, NBC block copolymer is obtained in the activated anionic polymerization of CL in the presence of polyols polyether- or polyester-types and bis-acyl derivatives of CL [ 65 ]. The initiators are usually NaCL or Grignard reagents e.

Although the water uptake of NBC is lower than that of PA-6, efforts were made to reduce it further and to increase its resistance to hydrothermal aging. Improvements in this relation were achieved by incorporating phenolic resin [ 66 , 67 ]. According to the related patent [ 68 ] the MW of the polyol is higher than 2 kDa. The reaction scheme of NBC preparation is given in Figure 5. In the NBC formulations mostly polyether and polyester appeared as block segments. However, their end groups were not always hydroxyls but amines. This change is well understandable considering the fact that such amines e.

The amine end groups of such polyethers were transformed to the necessary macroactivator via aminolysis with carbamoyl compounds before starting with the anionic polymerization of CL. The resulting block copolymers exhibited high MW and polydispersity which were traced to the onset of Claisen-type condensation reaction. Amines were the functional groups of the growth center generating polyether compounds of the group of Ye [ 69 , 70 ].

This group synthesized, however, the macroactivator with carbamoyl moieties by reacting the amine end groups with polyisocyanates. The block copolymers showed excellent toughness and even antistatic properties. Toughness improvement was the target of the work of Kim et al. It was found that differences in the block forming polymers and related macroctivators strongly affect the conversion, crystal structure and thermomechanical properties of the outcoming block copolymers [ 71 ].

The majority of the block forming prepolymers were polyether-types. Polyether-type diols, such as polycaprolactone PCL were also used to prepare poly amide-block-ester copolymers. Kim and White [ 72 ] end capped PCL with di-isocyanates to generate the macroactivator supporting the anionic polymerization of LL. As block segments other polymers than polyethers and polyesters may be inserted. Sobotik et al. The final product was poly amide-block-butadiene. The block soft segment may show rubbery characteristics. Note that liquid butadiene rubbers with suitable functionality, such as amine, were developed for the toughening of epoxies and they have been tried as prepolymers in the anionic polymerization of lactams, as well.

Rached et al. PDMS is water repellant and has a very low T g. Therefore, its incorporation seems to be the right selection for toughening and to reduce water sensitivity. In this work the silanol end groups of linear PDMS were reacted with diisocyanate that was converted into the macroactivator, viz. The latter activated the anionic polymerization of LL. There are some further possibilities to create block copolymers. Phenylester groups may also work as latent activators.

Phenylester groups were generated on a polyimide PI backbone by end-capping with phenylaminobenzoate in N -methylpyrrolidinone solvent [ 75 ]. The thermal stability, moisture resistance, and impact strength of the PAPI-PA-6 block and grafted—see later copolymers were markedly enhanced [ 75 ].

Copolymers are rarely formed by interfacial reactions. The anionic polymerization of lactams may open up a bright horizon, however, in this direction as outlined below. To graft PA onto a polymer, the latter has to be transformed into a macroactivator through suitable chemistry. The other aspect to be considered is whether this polymeric macroactivator is soluble only in a common solvent with lactams or in the melt of the latter which is strongly preferred. Hu et al. Recall that the target with copolymerization was to improve some disadvantageous properties of PA low T g , low toughness, high water uptake, moderate thermooxidative stability.

To reduce the moisture sensitivity of PA-6 it is often blended with polyolefins, such as PP. Zhang et al. The basic difference compared to that of Hu [ 76 , 77 ] was that PS itself was a copolymer containing styrene and an isocyanate group bearing styrene derivative. The strategy was similar to the above one in the work of Liu [ 79 ] who synthesized a copolymer composed of styrene and an allyl monomer having a carbamated caprolactam moiety.

The above examples underline that copolymerization is a versatile route to incorporate a monomer with suitable groups i. A further example in this direction can be taken from the work of Xu et al. These authors copolymerized styrene with N -phenyl- and N - 4-hydroxyphenyl - maleimides yielding poly styrene- co -maleimide copolymers. The other —NCO group was reacted with the lactam resulting in the macroactivator for the anionic polymerization of CL.

Macroactivation with pendant phenylester groups may work for CL polymerization as shown by Pay [ 75 ]. Latent CO 2 -protected N -heterocycle carbene based initiators were also applied for the anionic copolymerization of CL with LL [ 81 ]. Recall that this is a single initiator that is incorporated into the lactam or lactam blends directly.

The M v values of the copolymers were at about 30 kDa along with a polydispersity close to 2. Bouchekif et al. Tunc et al. This copolymer exhibited enhanced T g , higher resistance to thermo-oxidative degradation but reduced T m compared to PA The presence of the fluorinated groups rendered this copolymer hydrophobic. Volkova et al. It was quoted that through selection of PI chemical build-up, amounts the mechanical and tribological properties of the resulting copolymers can be controlled.

The work of Hou et al. The reason why this approach is reported here and not in the following blend section is, that the authors proposed the onset of a copolymerization reaction. According to their polymerization pathway TPU undergoes thermal dissociation in the alkaline CL melt. The authors found that with increasing content of TPU up to 10 phr the stiffness and strength dropped and the notched impact strength prominently increased. Bakkali-Hassani et al. The preparation method of the copolymers is identical—both in laboratory and industrial scales—with those repeated for PA homopolymers.

Copolyamides were produced by casting [ 87 , 88 ], melt spinning [ 87 ], reactive extrusion [ 89 ], centrifugal [ 45 ] and rotational molding [ 90 ]. Several reports [ 89 , 91 ] have dealt with the effects of processing parameters on the thermomechanical behavior of the copolyamides. Literature data support the fact that reactive extrusion may be the most efficient industrial technology when granulated, pelletized copolyamides for further processing, are the target materials.

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Blending of immiscible polymers envisages creation of blends with superior properties to those of the blend components. Preparation of the blends depends prominently on their morphology and adhesion between the phases of the blend. To improve the adhesion between the blend, constituent compatibilizers, compatible with both phases, are used.

Blending of polymers combined with the in situ AROP of lactams is a very promising route, because the morphology development may be kinetically controlled and copolymerization, supporting the formation of a strong interphase, may be triggered at the same time. With respect to morphology development attention should be paid to some similarities with the toughening of thermosets.

Tougheners, reactive or non-reactive with the actual thermosetting resin, are first dissolved in the resin or its hardener. The final morphology is given by reaction-induced phase separation RISP since the solubility of the toughening agent becomes less and less with the advanced curing of the resin [ 92 ]. When the modifier has co-reactive groups with those of the resin the separated phase may well be coupled to the matrix.

Blending of polymers by simultaneous anionic polymerization of lactams is similar. The polymer modifier, as blend component, is dissolved in the lactam melt prior to the start of the anionic activated ROP of the latter. Even the compatibilization can be achieved in one-shot making use of the various copolymerization options introduced above. As a consequence, blending may involve copolymerization and vice versa. Therefore, it is not always a simple task to differentiate between blending and copolymerization when AROP of lactams is adapted.

The distinction is, however, simple when the compatibilizer is introduced separately, being a previously synthesized polymer. The blend components in the in situ anionic polymerization of lactams are almost exclusively thermoplastics, though trials were made also with the combination of thermosets, such as epoxy resin [ 93 ]. A large body of work was done with the in situ blending of lactams with many different thermoplastics. Some of them were even produced in situ in the lactam melt before the AROP of the latter, others were premade and usually dissolved in the molten lactam.

The related achievements are briefly summarized below. Fang and Yang [ 94 ] investigated the structure-property relationship of low density polyethylene LDPE and in situ polymerized PA-6 in the presence and absence of a compatibilizer. The M v values of PA in the blends were between 11 and 22 kDa as a function of blend preparation batch mixing or reactive extrusion.

The crystallization of both the high MW polyethylene oxide and the PA-6 formed was influenced mutually. The particles of polyethylene oxide, well dispersed in the PA-6 matrix, supported the toughening via the enhanced ability of PA-6 for crazing and shear yielding. The work of Teng et al. As compatibilizer, PP- g -MA was selected. For its action, imide formation between the MA group and primary amine of the PA-6 was supposed which is likely incorrect. Pei et al. In the former case styrene was polymerized in the CL melt, whereas in the latter work a premade PS was dissolved in LL melt.

Above this threshold phase inversion PA-6 and PA became the disperse phase took place. Since the morphology is governed by the kinetics of phase separation, it is unstable and changes upon annealing, i. This is a common feature of blends prepared by AROP of lactams. Omonov et al. It was introduced into the CL melt with and without a previously prepared triblock copolymer PAblock-ethylene-butylene-block-PA This triblock copolymer was synthesized by the above introduced macroactivator route. Yan et al. The Izod impact strength was improved along with a slight loss in stiffness and strength.

This unusual behavior was traced to the peculiar formation of the dispersed domains in PA These core-shell droplets were dispersed in submicron range. PA-6 blends with styrene-acrylonitrile SAN and acrylonitrile-butadiene-styrene ABS copolymers have been developed and commercialized to reduce the moisture sensitivity, to improve the toughness and to reduce the shrinkage and warpage of PAs [ ].

Therefore, trials were made to incorporate these polymers into PA-6 via in situ anionic polymerization of the latter. In order to improve the phase coupling, additional styrene-maleic anhydride SMA was introduced which was converted into a macroactivator. For comparison, purpose blends were produced by classical melt blending.

The dispersed particles in the PA-6 matrix were smaller than those obtained by classical melt blending. The phase change formation of bicontinuous morphology and phase inversion was strongly affected by the relative amount of the micro- isocyanate compound and macro-activator SMA-based growth center. The dispersion of SAN was much finer than in classical melt blending. Wu and Yang [ ] incorporated a polymethacrylic ionomers in 0.

These ionomers, capable to compete with the H-bonding between the PA chains, strongly decelerated the crystallization of the resulting PA Parallel to that also the thermo-oxidative stability of the blends was reduced. Polyphenylene oxide polyphenylene ether, PPE is a traditional modifier of PS being miscible with it [ ]. PPE was transformed into a macroactivator first by solution grafting with 4-methoxyphenyl acrylate followed by the reaction with NaCL in CL melt.

Irrespective of the fact that CL was the major phase, the blends slowed an inverse morphology, i. Ahmadi et al. Since in the etched cryofractured surface of the samples no voids could be revealed, the authors concluded the formation of a single phase structure, i. This conclusion was supported by dynamic-mechanical analysis DMA. Interchange reactions, such as transamidation between PAs [ ], may be useful tools to improve their compatibility.

This strategy was adopted by Wang et al. The crystallized spherulites became finer by this modification. This resulted in improved toughness at the cost of stiffness and strength of these copolyamides. The processing techniques of the blends are similar to those listed to copolymers.

It is noteworthy that research works have started also with the extrusion of different blends [ ]. Different definitions exist for polymer nanocomposites which are also referred to as nanostructured and organic-inorganic hybrid composites. It is, however, generally accepted that the phase separated units, domains, and particles should be on nanoscale, at least in one space direction. Okada and Usuki [ ] quoted that the term nanocomposite appeared from the s. Interestingly, the first matrix of the related nanocomposite was hydrolytically polymerized also in situ PA The vast majority of polymer nanocomposites contain different fillers of inorganic origin.

Their common feature is the high specific surface area. The nanofillers are usually grouped into quasi spherical, acicular or needle-like, and flaky or platelet-like types. These groups are frequently termed as 0 direction 0D , 1D, and 2D nanofillers, respectively. For their incorporation into polymers different methods may be adopted. From the viewpoint of technological applications in situ polymerization, melt compounding, and suspension-assisted techniques seem to be most suited. Since many nanofillers are of polar character and bear polar functional groups, such as —OH, —COOH, their surface should be rendered organophilic.

Besides the traditional surface modification, AROP of lactams offers further possibilities. For example, the hydroxyl groups can be transformed into isocyanate which may form with the lactam an N- acetyl or N -carbamoyl group thereby generating an activator, growth center for the AROP. It is noteworthy that most of the related works synthesized the nanocomposites in casting and other melting techniques rotational and extrusion molding. An important exception is given by the microcapsule technique shown in Figure 7.

Morphology development of the nanocomposite during in situ solvent-assisted microcapsulation top and the related chemistry bottom. Notes: when different fillers were used all of them are mentioned at the related reference. Relevant works using micron-scale fillers are also included. Due to the preferred applications of cast polyamides tribological use some microcomposites are also covered. It should be born in mind that the targets with the incorporation of nanofillers reinforcements are besides the improvements in structural properties stiffness, strength, toughness also the creation of functional ones e.

Therefore, the present summary is likely the most exhaustive one in this field.

Extensional Rheology in Polymer Processing

Due to a very low viscosity of cyclic lactams and superior mechanical properties of polymers obtained from them, these materials have a great potential for application in different liquid composite molding LCM techniques. It is not surprising that within the last 20 years vast academic research has been conducted to investigate possible industrial applications of anionically polymerized thermoplastic composites TPC reinforced with glass, carbon, aramid, or natural fibers.

The first shots to produce such composites dated back to the s and are related to the easiest LCM technique—casting. Further development resulted in more advanced technologies and some of them can be treated as new ones. Subsequently, nowadays a number of techniques are available for the production of anionically polymerized TPC parts. It is important to mention that huge progress has also been achieved in the development of machineries and materials [ ].

Next we summarize the progress in different LCM techniques for the production of discontinuous fiber, continuous fiber Table 3 , and textile fabric Table 4 , reinforced anionically polymerized TPCs. The possibility of engineering the composite impact failure behaviors by using rubber-toughened matrices to achieve a higher toughness is illustrated. A processing window has been defined in terms of pultrusion line speed and mold temperature. Initial T mold has a great effect on the X C and crystal size. The IS of Nyrim parts decreased as filler was added while the flexural properties are improved.

Caprolactone was selected as activator best compromise between void content, reaction rate and polymer quality. Benzyl acetate and benzyl benzonate produced very slow reaction, though without voids. Tert-butyl acetate caused rapid reaction and a very tough polymer, but with many voids. Phenyl acetate worked for fast reaction yielding good polymer product, but it can terminate the reaction if used in excess.

Casting is probably the earliest manufacturing technique used for the in situ polymerized composites Figure 9. The main advantage of this manufacturing resides in simplicity and low capital costs. This process can be successfully used for the production of large parts. One of the very first attempts to produce PA-6 reinforced with carbon fibers CF by casting was implemented by Litt and Brinkmann [ ] in The more detailed description of the results is presented in Table 3. The study dealing with the incorporation of short glass fibers GF into NBC matrix produced by casting was introduced by the Monsanto company in [ ].

The purpose of the reinforcement application was improvement of mechanical and thermal properties of the resulting material. The authors used polyesteramide prepolymer with the acyllactam end groups acting as macroactivator for CL for the matrix and different types of reinforcement materials 1. Another positive effect of GF incorporation was the increased resistance to expansion from moisture absorption. Horsky et al. To polymerize CL, the authors utilized two initiators: sodium dihydrido-bis 2-methoxyethoxo aluminate and sodium tetra 6-caprolactamo aluminate , while the cyclic trimer of phenyl isocyanate PIC was used as an activator.

When the alternative system was used the polymerization rate was somewhat higher. The poor adhesion between fibers and matrix when using GF without sizing was also noted in the paper. Engelmann et al. The authors also noted that the processability of the reinforced PA-6 was limited by the viscosity of the fiber containing monomer melt. Rotational molding Figure 10 is a polymer processing technique that has been widely used for the production of different tubes, wheels, pulleys, etc.

The most commonly utilized materials for this process are dry powders. However, liquid resins have also been successfully used, thus enabling shorter cycle time and better properties of the resulting product. In that respect anionically polymerized lactams have high potential [ 45 ]. Incorporation of the fibers gives them the certain pros and cons of reinforced thermoplastics.

However, proper processing parameters of the rotary molded reinforced PA-6 are an issue of great importance. Thus, Harkin-Jones and Crawford [ 90 ] investigated the influence of the initial mold temperature on the crystallinity degree and crystal size of rotationally molded Nyrim parts reinforced with GF. The authors mentioned that the incorporation of 5 mm length GF enabled improvement of the flexural properties, but with a decrease in impact strength. Thermoplastic reaction injection pultrusion TRI-pultrusion Figure 11 has become an issue of particular interest only at the present time.

The interest is fueled by recent advances in material and equipment developments, as well as by the high commercial potential of this technique [ , ]. Their use allows us to almost double the pultrusion speed. Several successful development projects in this field prove the feasibility of its industrial application. Nonetheless, no commercial TRI-pultrusion is available at present. Note that 0. This front crash beam contains unidirectional GF in anionically polymerized PA-6 matrix [ ].

Apart from certain commercial benefits, the TRI-pultrusion allows unlimited length composite product with superior properties to be obtained, determined by the thermoplastic matrix and reinforcement. For example, transmission line conductor, traditionally consisting of steel, is subject to sag and failure because of the steel creep and hard operational conditions such as high temperature gradients, icing, storms, etc.

Composite conductor made of anionically polymerized lactams or their copolymers and high modulus and strength long fibers can solve the described problems [ ]. Nevertheless, the technological process development and process parameters optimization are essential issues and require comprehensive research activities. The development covered the fabrication of a mixing and injection unit, reinforcement pre-heating oven design, and evaluation of various die geometries, as well as determination of the processing window.

Optimization of the process was aimed at maximizing the line speed, while achieving well impregnated TPC profiles. For detailed information, see Table 3. In Rijswijk et al. The scheme of processing set-up is shown in Figure Rijswijk stated that it is possible to produce huge parts with anionically polymerized PA6 matrices if the concentration of the activating system [ ] and process parameters [ ] are properly defined and controlled to reach the highest DOC, crystallinity, and mechanical properties [ , ].

It was proved that siloxyl groups on GF have an inhibiting effect and affect the interlaminar shear strength ILSS negatively. Therefore, proper aminosilane based sizing must be applied. It was also found that melt degassing has a crucial influence on the final void content. This issue has been investigated in depth already in the in situ polymerization of LL by Zingraff et al. This is due to high risks imposed by the matrix system sensitivity to moisture and UV irradiation , sophisticated tooling achieving fast heating and cooling rates , and joining techniques for the blade halves as stated by Prabhakaran [ ].

The work of Yan et al. As long as the reduction of the production cycle time is the crucial parameter for the industry then two approaches are usually followed. Although the second approach shows a possibility for cycle time reduction, it has a much shorter processing window which can lead to a not fully impregnated preform. There are also some results with natural fiber reinforcements. Kan et al. The main challenge was related to the inhibition of the system due to the byproducts generated by the peeling reaction of cellulose in highly alkaline environment.

Therefore, the less alkaline CLMgBr initiator was used and proved to be most suitable. The investment into equipment is a key issue for any composite parts manufacturer. Clamping units and dosing systems for T-RTM technology are high-end machines and can be very expensive.

The more lines belonging to the dosing system the more expensive is the related unit more sophisticated mixing control system and mixing head design. Thus Barfknecht et al. However, an extremely low DOC in the middle of the TPC plate was found as the result of the wash out effect generated by the monomer flow. Successful industrial production of TPCs with textile-reinforced in situ polymerized CL was demonstrated by some companies, including KraussMaffei [ , ]. The roadster project involved not only an injection but also a compression step, when excess amount of polymerizable CL was injected into the mold after the fiber impregnation was finished.

It was foreseen to reduce shrinkage and void content, hence improving the surface quality. Such an approach was also followed in by Wakeman et al. Another equipment producer, Engel St. Valentin, Austria successfully demonstrated its own developed CL dosing system with a unique in-mold mixing unit [ ]. In their demonstration process also an overmolding step was implemented to enhance parts functionality [ ]. For impregnation only 0. The latter are more cost efficient because of cheaper pumps and smaller clamping units. The compression molding phase is relevant for surface quality, especially when automotive parts are targeted e.

Many research works have dealt with different testing methods to investigate mechanical properties, and especially the fracture and failure behaviors of TPC composites produced via AROP of lactams. In this respect non-destructive techniques such as acoustic emission and infrared thermography were used for detailed failure sequence analyses [ , , ].

In single-polymer or self-reinforced composite SPCs both the reinforcing and matrix phases are given by the same polymer one constituent , or by polymers belonging to the same family two constituents. SPCs are lightweight the density of polymers is usually below that of traditional reinforcement composites with ultimate recyclability via remelting.

In respect of LCM the AROP of lactams features two major benefits: i the melt viscosity of the polymerizable lactam is very low, and ii the polymerization can be performed below the T m of the resulting PA. Low viscosity is helpful to wet-out and impregnate the reinforcing structures. This large temperature difference is of great importance because the reinforcement otherwise loses its stiffness and strength with increasing temperature and dwelling time the closer the polymerization temperature is to the melting of the reinforcement.

Last but not least, the fact that polymerization below T m is accompanied by crystallization, allowing a faster demolding, is the guarantee for high productivity. Gong et al. This was attributed to the following effects: i impeded movement of CL toward the growth center within the reinforcing textile, and ii presence of H 2 O and —COOH groups on the PA-6 reinforcement surface which consumes —NCO groups of the activator used. Dencheva et al. To study the effect of sizing, they were removed by washing with acetone.

Based on detailed WAXS studies the authors confirmed the appearance of a transcrystalline layer interphase between the matrix and reinforcement. Transcrystallinity is caused by laterally impeded spherulitic growth on closely spaced nuclei on a heterogeneous substrate here the PA-6 or PA-6,6 reinforcements. Generally, the presence of a transcrystalline layer is considered to be a controlling factor of the stress transfer from the matrix to the reinforcement [ , ].

Interestingly, the thickness of this layer became smaller when the sizing was removed. The ultimate tensile strength and strain were enhanced with increasing reinforcement content, while only small changes were found for the stiffness. This was expected due to the low amount of the reinforcement and to the rather long dwelling time at the polymerization temperature.

The E -modulus stiffness and strength of the reinforcing PA-6 and PA-6,6 diminished with increasing temperature and holding time. This is often accompanied by considerable shrinkage that can be limited by processing under pressure when applicable see below. In the recent paper Dencheva et al. Note that the compression molding temperature was below the T m of the PA-6,6 reinforcement.

The beauty of this technique is that the MMT particles are well and uniformly dispersed. By contrast, direct impregnation of the reinforcement with a polymerizable CL melt containing MMT may result in a filtering-off of the particles owing to the dense mesh structure of the fabric layers. In this way an MMT-rich surface layer may appear which is undesirable. At higher PA-6,6 fabric content the tensile mechanical properties dropped owing to poor impregnation. Unexpectedly, this had only a marginal effect on the toughness.

Solvent-borne, liquid initiators and activators may likely be preferred. The copolymerization strategy will further focus on the toughness improvement of the related PAbased block copolymers. Besides the traditional block segments polyether- and polyester-based diols others, like polycaprolactone, polylactic acid etc.

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The in situ blending via AROP will hardly achieve industrial breakthrough. This development will target the production of new tribological compounds, containing novel carbonaceous nanofillers, which will be most likely produced still by casting. The toughness of such nanocomposites will be a key factor and thus the related works will be supported by extensive modeling [ ].

According to our view, novel and adapted manufacturing methods will be the real future drivers of the development of thermoplastic composites with AROP-produced matrix. Additive manufacturing via ink jetting should be mentioned among the emerging novel techniques. This claim is based not only on the straightforward recyclability of the related composite parts, but also on other beneficial design- and post-processing-related features, such as part integration, overmolding with and without additional reinforcements , surface coating and finishing, and welding.

The related research and developments works will run parallel with extensive modeling especially via finite element codes studies. The potential of PAbased single-polymer self-reinforced composites has been strongly underestimated, therefore in this field interesting developments may be expected. National Center for Biotechnology Information , U.

Journal List Polymers Basel v. Polymers Basel. Published online Mar Author information Article notes Copyright and License information Disclaimer. Received Feb 26; Accepted Mar Abstract This paper presents a comprehensive overview of polymers and related nano composites produced via anionic ring opening polymerization AROP of lactams. Introduction Polyamides PAs belong to the engineering thermoplastics with a broad range of applications.

Homopolymers 2. Chemistry The polymerization capability of cyclic monomers depends on both thermodynamic relative stability of the monomer and the resulting linear polymer and kinetic initiation, propagation, termination reactions factors. Open in a separate window. Figure 1. Figure 2. Properties The anionic polymerization of lactams can be performed in two temperature ranges: below or above the melting temperature T m of the resulting polylactams. Figure 3. Manufacturing PA-6 and PA homopolymer products in forms of plaques, pipes, rods, and various half-fabricates for further machining, are produced by casting see the related scheme in Section 6.

Figure 4. Table 1 Processing and material parameters of specimens produced by classical and reactive rotational molding [ 46 ].

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Copolymers Extensive research was dedicated to the modification of anionically polymerized PA-6 and PA through various copolymerization strategies. Chemical and Structural Aspects 3. Block Copolymers The preparation of copolymers di-, triblock with blocks composed of lactam or non-lactam chains requires the incorporation of suitable prepolymers in the molten, anionically polymerizable lactam. Figure 5. Graft Copolymers Copolymers are rarely formed by interfacial reactions.

Manufacturing The preparation method of the copolymers is identical—both in laboratory and industrial scales—with those repeated for PA homopolymers. Blends Blending of immiscible polymers envisages creation of blends with superior properties to those of the blend components. Nanocomposites Different definitions exist for polymer nanocomposites which are also referred to as nanostructured and organic-inorganic hybrid composites. Figure 6. Figure 7. PA-6 nanocomposites exhibited enhanced E -modulus and tensile strength.

No electric percolation observed. Potential for energy storage deduced. Nucleation and crystallization affected by the silica presence. The latter surface treatment improved the strength. Mechanical and dielectrical properties tailored upon amount, type and combination of the additives. At high macroactivator content polymerization rate and yield are influenced by the filler B 4 C and graphite.

AROP performed better than the hydrolytic route. Capsules filtered and dried prior to compression molding Optical microscopy, viscosimetry M v DSC, TGA, synchrotron WAXS electrical, dielectrical behavior All fillers enhanced the stiffness and reduced the deformation at break with increasing content. The volume resistivity above 0. Volume resistivity decreased with 6 order of magnitudes at a fullerene content of 0.

The coefficient of friction was halved in presence of fullerenes. Crystallinity slightly reduced. Effect of the dose of electron beam irradiation was moderate for nanoscaled CB. Graphite worked as heterogeneous nucleant during crystallization. Notched Charpy IS improved only at 0. Polyether-urethane as macroactivator yielded high MW with crosslinking. All nanocomposites showed increased tensile modulus and strength compared to neat PA Fibers produced at different stretching ratios. MWCNT worked as nucleating agent and also improved the thermal stability. Tensile modulus and strength were markedly improved at cost of the elongation at break.

DOC was simulated. M v values between 10 and 41 kDa along with polydispersity in the range of 1. MWCNT delayed the polymerization. M v data scattered between 54 and 59 kDa. Initiator added in N 2 atmosphere. The zero shear viscosity was prominently higher in CNC presence compared to the neat PA-6, suggesting the onset of a percolated structure that was prone for breaking upon shear. Samples produced by extrusion. For comparison purpose classical melt blending served.

Tensile stiffness and strength strongly improved at cost of elongation at break. Melt elasticity and strength enhanced by CNC reinforcement. OMMT introduced directly or in acetone—assisted dispersion. M n and M w values were at about 20 and 50 kDa respectively. Above this intercalation took place. The thermal stability was prominently improved by NaMMT.

Clay added differently. The intercalation was reduced with increasing LL content. In the block-type copolymer the intercalation of clay remained the same with increasing LL content. Crystallinity strongly reduced by LL content. Then initiator and activator added. Micronscale OMMT agglomerates also revealed. OMMT then decreased. OMMT intercalation was supported by the polymerization in solvent.

Nanocomponents displayed higher thermooxidative stability than PA The formed poly ester amid was random type. GO acted as nucleating and reinforcing additive. E -modulus increased while impact strength decreased with increasing GO content. Composites Due to a very low viscosity of cyclic lactams and superior mechanical properties of polymers obtained from them, these materials have a great potential for application in different liquid composite molding LCM techniques. In both cases a special aminosilane sizing was used. Melt degassing in a buffer vessel. An organosilane activator was deposited on the GF surface N -[5- trimethoxysilyl azaoxopentyl]caprolactam , and different isocyanate-based activators used.

Increasing of the crosshead speed resulted in increased Kc that is untypical. IS went through a maximum as a function of temperature. Failure sequence analysis was performed using AE and optical microscopy simultaneously. Tensile strength and stiffness of epoxy based and PA12 based laminates were compared; Testing in ILSS demonstrated plastic deformation instead of failure.

ILSS could not be assessed due to plastic deformation of specimens. At the equilibrium 1. To minimize the VC the N 2 content in the melt should be minimized and an optimal capillary number set for infusion. Bleeding was used to reduce the VC in parts with degassed matrix. Reconsolidation of the composites after preheating remained incomplete.

Recycling and overinjection molding strategies presented. Discontinuous Fiber Reinforced 6. Casting Casting is probably the earliest manufacturing technique used for the in situ polymerized composites Figure 9. Figure 9. Rotation Molding Rotational molding Figure 10 is a polymer processing technique that has been widely used for the production of different tubes, wheels, pulleys, etc. Figure Continuous Fiber Reinforced. Pultrusion Thermoplastic reaction injection pultrusion TRI-pultrusion Figure 11 has become an issue of particular interest only at the present time.

Scheme of pultrusion exploiting the in situ AROP of lactams. Single-Polyamide Comosites In single-polymer or self-reinforced composite SPCs both the reinforcing and matrix phases are given by the same polymer one constituent , or by polymers belonging to the same family two constituents. Outlook and Future Trends Based on this review the following conclusions can be deduced: -. Figure 8. Conflicts of Interest The authors declare no conflict of interest. References 1. Roda J. Handbook of Ring-Opening Polymerization. KGaA; Weinheim, Germany: Polyamides; pp.

Joyce R. Process for Making Polymeric Materials. Principles of Polymer Design and Synthesis. Springer; Berlin, Heidelberg: Ring-opening polymerization; pp. Lactam polymerization. Reimschuessel H. Nylon 6. Chemistry and mechanisms. Endo T.