ORIGINAL PAPER

Optimization of initiator and activator for reactivethermoplastic pultrusion

Ke Chen1& Mingyin Jia1 & Sun Hua1 & Ping Xue1

Received: 30 August 2018 /Accepted: 11 January 2019# The Polymer Society, Taipei 2019

AbstractIn order to manufacture continuous fiber reinforced thermoplastic composites with high fiber content, reactive thermoplasticpultrusion was developed by using the anionic polymerization of polyamide-6 in this study. Firstly, based on the polymerizationtime tests, combination of activator of difunctional hexamethylene-1,6-di carbamoylcaprolactam (C20) and initiator of sodiumcaprolactamate (C10) were chosen as the most suitable formula combination for pultrusion. Effects of concentrations of theactivator C20 and the initiator C10 on the polymerization time, molecular weight of polymer and degree of conversion were alsoinvestigated. Then, the range of initial polymerization temperature was determined, which had a great influence on the impreg-nation of dense fiber reinforcement. Comparing with commercial polyamide-6, the anionic polyamide-6 has a higher modulusand lower elongation. Finally, continuous glass fiber reinforced polyamide-6 composites with 50% volume fraction weresuccessfully pultruded. Great interfacial adhesion between fiber and polymer was observed by the scanning electron microscope.

Keywords Thermoplastic composites . Pultrusion . Polyamide-6 . Formula optimization

Introduction

Pultrusion has become one of the most cost-effective andenergy-efficient processes of manufacturing composite pro-files with constant cross-section over the last 70 years [1, 2].Pultruded composites exhibit not only light weight and highstrength, but also corrosion resistance and electrical insulation.Thermosetting polymer was the first to be used in pultrusionof composites because of its fast curing and low viscosity atroom temperature. In recent years, a great number of researchrelated to pultrusion of thermosetting polymers have beenconducted by researchers, such as vinyl ester, epoxy [3–5],

unsaturated polyester [6] and polyurethane [7]. However, ther-mosetting polymers have several disadvantages. They are brit-tle, sensitive to impact and can not be recycled. What’s more,volatile compounds would be released during the processingof many thermosetting polymers.

Thermoplastic polymer completely avoids the above short-comings and offers improved impact strength, damage toler-ance, toughness and reparability [8]. Nevertheless, the appli-cation of thermoplastic polymer in pultrusion was somehowlimited. It was difficult for thermoplastics to penetrate thedense fiber reinforcement with a relative high viscosity(100–10000 Pa·s) [9]. In the past decade, shortening the flowpath during the impregnation was considered as the effectivesolution. Pre-impregnated materials, such as prepreg tapes,commingled fibers, and powder coated towpregs, have beenwell developed, by bringing the polymer and fibers in moreintimate contact prior to the final molding step [10–13]. Theflow path between the fiber bundles was greatly shortened,but there was still a large gap in achieving good im-pregnation between thermoplastics with thermosets. Dueto the non-online impregnation, the production efficiencywas also greatly reduced.

Reactive processing of thermoplastic composites with an-ionic polymerization of polyamide-12 (PA-12) has recentlygained interest of researchers due to the work conducted by

* Ping Xuexueping@mail.buct.edu.cn

Ke Chenchenke0903@outlook.com

Mingyin Jiajiamy@mail.buct.edu.cn

Sun Huareignlan@163.com

1 Beijing University of Chemical Technology, Institute of PlasticMachinery and Engineering, Beijing 100029, China

Journal of Polymer Research (2019) 26:40 https://doi.org/10.1007/s10965-019-1708-6

Luisier et al [14]. The excellent impregnation of fiber rein-forcement was achieved by the monomer with extremelylow viscosity. Then the polymerization was carried outin a heated forming die, which was similar to the pro-cessing of thermosetting composites. Compared to PA-12, polyamide-6 (PA-6) was a better choice for reactiveprocessing for its higher performance/cost ratio and low-er processing temperature. Van Rijswijk et al. studiedthe vacuum infusion of thermoplastic composites withanionic PA-6. However, the formula used in vacuuminfusion was not suitable for pultrusion [9]. Barhoumi1et al. developed reactive rotational molding via anionicpolymerization of PA-6 [15]. To the best of our knowl-edge, there were few studies on the optimization ofinitiators and activators for the pultrusion with anionicPA-6.

In this study, the utilize of anionic PA-6 in the pultrusion ofthermoplastic composites was discussed. The effects of acti-vator concentrations and initiator concentrations on polymer-ization time, molecular weight of polymer and degree of con-version were investigated. The rheological behavior of thereactionmixture was discussed at different initial temperaturesof the reaction. The mechanical properties of the anionic PA-6were also assessed. Finally, the glass fiber reinforced polyam-ide 6 composites with 50% volume fraction were successfullypultruded, demonstrating the availability of optimized activa-tor and initiator.

Experimental

Materials

Commercial grade caprolactam (CPL, melting point was 69°C) was purchased from BASF Co. The sodium hydroxide(≥96% by mass, ‘NaOH’) and toluene 2,4-diisocyanate(TDI) were kindly provided by Sinopharm ChemicalReagent Co., Ltd, China. Difunctional hexamethylene-1,6-dicarbamoylcaprolactam (2 mol/kg concentration in caprolac-tam, ‘C20’) and caprolactam magnesium bromide (1.4 mol/kg concentration in caprolactam, ‘C1’) were provided byBrüggemann Chemical, Germany. Sodium caprolactamate(2 mol/kg concentration in caprolactam, ‘C10’) was self-prepared by the reaction of caprolactam and sodium hydrox-ide under vacuum environment. C10 and C1 were used asthe initiators while C20 and TDI as the activators. Thechemical structures of these materials are presented inFig. 1.

The reinforcements were the undirectional E-glass roving(ECT 4301R) with 2400 tex, which were provided byChongqing international composite materials Co., Ltd,China. These undirectional E-glass rovings were treated witha silane coupling agent suitable for PA-6.

Processing

Preparation of the initiator C10

The initiator was difficult to reserve for a long time, soit had to be prepared before use to ensure reactivity. Athree-neck flask containing caprolactam and a certainproportion of NaOH was placed in the oil bath, whichwas maintained at 135 °C. The mixture was heated toboiling in a vacuum environment for 10–20 minutes,until there was no particle existing, and then the initia-tor C10 was prepared, as shown in Fig. 2. The liquidinitiator was poured into a 500 ml beaker and cooled tosolidify for later use. Nitrogen gas was blown into thebeaker to isolate the air before being covered with aglass lid.

Mixing and injection of reaction mixtures

A lab-scale resin injection moulding (RIM) unit wasspecially designed for the preparation of two liquid re-action mixtures, which is illustrated in Fig. 3. The cap-rolactam and initiator were in tank A while the capro-lactam and activator were in tank B. The reaction mix-tures in the tanks were heated to 135 °C in a vacuumenvironment. After degassing for 15 minutes, the reac-tion mixtures were cooled to the appropriate injectiontemperature by stirring, and the nitrogen was introducedinto the tanks. Then the two kinds of reaction mixtureswere fed according to a certain ratio (1:1 in this study)and mixed by using a static mixer. All the pipes andtanks were set in the hot air oven to avoid solidificationof caprolactam in advance. The injection pressure ofRIM was set at 0.5 bar to ensure the impregnation,which was controlled by a pressure-control system.

Fig. 1 Structures of materials used in this study: a sodiumcaprolactamate; b hexamethylene-1, 6-dicarbamoylcaprolactam; c capro-lactam magnesium bromide; d toluene 2,4-diisocyanate

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Polymerization time tests

In order to reduce trial and error time for optimizing the acti-vator and initiator with pultrusion line, a polymerization timetest was conducted to obtain the polymerization time of acertain formulation. A test tube (15×100 mm) was placed inthe oil bath and heated at certain temperature. A K-type probethermocouple (1.5×150 mm) was inserted into the test tubeand the temperature and the time were recorded using a pa-perless recorder. The mixed reaction mixture was injected intothe test tube through RIM device, and then the recording timewas started immediately.

Reactive thermoplastic pultrusion line

The developed lab-scale thermoplastic pultrusion line is pre-sented in Fig. 4. In this study, samples with a rectangular crosssection of 20×4 mm2 were pultruded. The glass fibers got

sorted and dried by 160 °C hot air before entering the die toavoid inhibition of polymerization by moisture on the coldfibers. In the injection die, a tapered injection chamber withan inclination of 8° was designed to aid in impregnation andcontrol back flow. The total length of the heating die and thecooling die was determined to be 800 mm.

Characterizations

Degree of conversion

The degree of conversion (X0) of the samples polymerized bydifferent formulas was measured. The samples were cut intopieces (thickness less than 1mm), weighed (m1) and refluxed12h in demineralised water and then dried in a vacuum ovenand weighed again (m2). Since the caprolactam can be dis-solved in the water, the X0 was determined by Eq. (1).

X 0 ¼ m2

m1⋅100% ð1Þ

Fig. 4 The developed reactive thermoplastic pultrusion line

Fig. 2 Diagram of the preparation of the initiator C10

Fig. 3 Schematic diagram of RIM device

J Polym Res (2019) 26:40 Page 3 of 10 40

Viscosity average molar mass

The viscosity average molar mass (Mη) of samples were de-termined by using an Ubbelohde viscometer (diameter is 0.9-1mm) and a single-point measurement was adopted accordingto method in the literature [16]. The samples were dissolved inaqueous H2SO4 (40%) to obtain a solution with a concentra-tion of 0.5 g/dl. The inherent viscosity (ηinh) can be calculatedaccording to Eq. (2).

ηinh ¼ln t=t0ð Þ

cð2Þ

In which t is the flow time of the polymer solution, t0 is theflow time of the pure solvent and c is the concentration of thepolymer solution. Both t and t0 were averaged over three mea-surements.Mη can be be obtained byMark-Houwink equationfrom the following Eq. (3).

ηinh ¼ K ¶⋅Mαη ð3Þ

In which,K' and ɑ are theMark-Houwink conatants. In thisstudy, K'=5.92×10-4 dl/g and ɑ=0.69 [17].

Gel permeation chromatography

A Waters-1515 Gel Permeation Chromatography (GPC) in-strument was used to measure the weight-average molecularweight (Mw), the number-average molecular weight (Mn) andthe polydispersities (Mw/Mn) of the PA-6 samples. A differen-tial refractive index detector using HFIP as the eluent at 35 °Cwith a flow of 1 mL/min. The instrument was calibrated with10 PS standards, and chromatograms were processed withWaters Breeze software.

Rheology

The real-time viscosity of the reaction mixture was mea-sured by a rotary viscometer (NDJ-9S, Shanghai Lichen,speed was 6 r/min, rotor type 3). The reaction mixturewas injected into a 250 mL glass beaker in the oil bathat various temperatures. Then, the change in viscositywas recorded over time.

Mechanical properties

The mechanical properties of anionic PA-6 were tested byInstron 1121 universal testing machine. The anionic PA-6samples were polymerized in the heated metal moulds, whichhad the shapes of tensile and flexural specimens. Tensile prop-erties were carried out according to ASTM D-638. A threepoint bending test was performed in accordance with ASTMD-790 and the rectangular specimens dimension was

80mm×10mm×4mm. The span was 64 mm. At least fivespecimens per conditions were tested. The density of the sam-ple was measured according to ASTM D-792.

Scanning electron morphology

Scanning electron morphology (SEM) study was carried outby an electron microscope (S-4700, Hitachi, Japan). Thepultruded sample for SEM characterization was freezed inliquid nitrogen before fracturing.

Results and discussion

Choosing the suitable combination of activatorand initiator

The reactive thermoplastic pultrusion of PA-6 composites wasbased on the anionic ring-opening polymerization of caprolac-tam. The reaction would be completed from a few tens ofseconds to 60 min depending on different activator-initiatorcombinations [18]. Isocyanate or acyllactam and metal-basedcaprolactamate were mostly used as the activator and the ini-tiator, respectively. Since the polymerization was an exother-mic process, there would be a temperature rise of 20–40 °C[19]. Therefore, the degree of polymerization can beexpressed by the temperature rising according to the followingequation [20]:

X 0 ¼ T−T0

Tmax−T 0ð4Þ

Where T is the instantaneous temperature of the reactionmixture. T0 and Tmax are the initial andmaximum temperature.

Figure 5 shows the change in temperature with time duringthe polymerization with four activator-initiator combinationsat 150 °C, which all start at 100 °C. The discussion of thefollowing two sections had the same polymerization condi-tions. Since both activators were difunctional species, the con-centration of activator was the concentration of functionalgroups involved in the reaction. It could be seen that the po-lymerization was much faster with activator C20 and the tem-peratures reached the peak at around 6 minutes (C10 and C20)and 14 minutes (C1 and C20), respectively. The combinationswith activator TDI had not even reached the peak, whichmeant that the polymerization couldn’t be ended within 20minutes. It was because activator C20 contained N-acyllactam end group, which was the active point for chainfast growing, while activator TDI had to react with the mono-mer to generate N-acyllactam end group under certain temper-ature [21], as shown in Fig. 6. According to the polymeriza-tion mechanism, another factor for accelerating the reactionwas the caprolactam anion. The combination of C10 and C20

40 Page 4 of 10 J Polym Res (2019) 26:40

was able to lead to complex formation, which causedthe anions to initially be produced at a high rate [18].However, for combination of C1 and C20, the anionwas only provided in small amount due to the dissoci-ation of the initiator. This could explain why the com-bination of C10 and C20 had a faster reaction rate oncethe polymerization started.

As for pultrusion process, since the molding die was800 mm long, the polymerization need to be accomplishedwithin 1–5 minutes, at the pultrusion speed in the wide rangeof 16–80 cm/min. Although increasing the concentrations offormula of C1 and C20 could also make the reaction timewithin the range, the formula of C10 and C20 was more op-erational and offered more possibility of high speedpultrusion, which was the best choice.

Influence of the activator concentration

Concentrations of activator and initiator had great effects onthe polymerization time, molecular weight of polymer anddegree of conversion. Different ratios of C10 and C20 were

tested to find the suitable formula. Firstly, the influence of theactivator concentration on the polymerization time wasstudied.

The content of C20 increased from 1.0 to 3.0 mol% whileC10 content remained at 1.0%. The results are shown inFig. 7. Increasing the amount of activator, the polymer-ization time was shortened. It was because that moreactivators provided more active points of molecular chaingrowth at the very beginning of the polymerization and there-fore the reaction rate was risen. But the influence of the acti-vator on the reaction rate was not so significant. The activatorcontent increased to 3 times the original, the reaction time wasmerely shortened from 6 minutes to 4.5 minutes.

The effects of the activator concentration on the viscosityaverage molar mass and degree of conversion are presented inFig. 8. There was a sharp drop in viscosity average molar masswith the increase of activator concentration and the color ofthe synthesized sample changed from pale cyan to milky. Itwas due to the higher amount of activator, which caused moremolecular chains grew simultaneously. The number of avail-able monomers for each molecular chain was reduced and

Fig. 5 Conversion-time relationsfor four common activator-initiator combinations (1.0 mol%activator, 1.0 mol% initiator) at150 °C

Fig. 6 Reaction mechanism of TDI with caprolactam

J Polym Res (2019) 26:40 Page 5 of 10 40

then the viscosity average molar mass of polymer decreased.The final degree of conversion showed the same downwardtrend, but it was not so significant. There was still over 95% offinal degree of conversion, when activator concentration in-creased to 2.5%, which was able to guarantee the quality of theproducts. It was well known that the higher viscosity averagemolar mass and degree of conversion were, the better mechan-ical properties of the polymer had. In the case where there wasnot such a large gap between reaction times at different acti-vator concentrations, 1 mol% activator concentration wouldbe the best choice, with the sample having around 3.4×105 g/mol of viscosity average molar mass and 98% of degree ofconversion.

Influence of the initiator concentration

The initiator C10 acted as the catalyst during the reaction. Thereaction time was greatly shortened as the initiator con-centration was increased while the activator concentra-tion was set to 1 mol%, as shown in Fig. 9. Comparedwith activator C20, initiator C10 had a greater impacton the reaction rate. When the initiator concentrationincreased to 3.0 mol% and the polymerization time re-duced to about 2 minutes. The reason was that increas-ing the concentration of C10, more complexes could beformed and more anions were introduced at the beginning ofthe polymerization.

Fig. 7 Temperature-timerelations for different C20concentrations at 150 °C

Fig. 8 Effects of activatorconcentrations on the viscosityaverage molar mass and degree ofconversion (1.0 mol% initiatorC10)

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Figure 10 shows that adding more initiator also has effectson the final degree of conversion and viscosity average molarmass. Increasing the initiator concentration resulted in a de-crease in viscosity average molar mass and final degree ofconversion. When the initiator concentration increased above2.5%, the final degree of conversion dropped below 95%.There were white spots remaining on the inner wall of the testtube after demoulding. The surface of the samples were alsonot smooth and felt sticky. It was caused by the fact that higheramount of initiator led to faster reaction rate, the temperaturebuild-up increased and ring-chain equilibrium shifted to themonomer side [18]. Unreacted monomer diffused to the sur-face of product and stuck to the inner wall of the test tube. Inthe case of pultruded composites, it might diffuse to the sur-face of the glass fibers where it could significantly weaken thebonding of fibers and polymer. Ring-chain equilibrium shifted

to the monomer side and the final degree of conversion wasreduced, which meant that the amount of monomer participat-ing in the reaction was reduced. It could also explain the slightdrop in viscosity average molar mass. Therefore, the initiatorconcentration was optimized within the range of 1.0–2.0 mol% to ensure the quality of the product.

Influence of the temperature parameters

The polymerization temperature, which was also the temper-ature of the moulding die in pultrusion line, had a huge impacton the anionic polymerization of PA-6. The reaction can becarried out from 130 to 180 °C, according to the literatures[22–24]. Since PA-6 was a semi-crystalline polymer, its me-chanical properties were mainly determined by the crystallin-ity. It had been demonstrated that APA-6 would have thehighest crystallinity at approximately 145–150 °C [25].Below this temperature, the reaction rate was relative slow.Fast crystallization rate would cause the monomer to betrapped inside the crystal, resulting in a decrease in conversion[26]. Increasing the polymerization temperature, the reactionrate would be accelerated, but the high temperature wouldlower the tendency of crystallization, leading to a slight de-crease in mechanical properties. It was not acceptable for po-lymerization temperature above 180 °C because the tempera-ture of the synthetic product would exceed the melting pointof PA-6 (Tm=220 °C). The pultruded products cannot beshaped at this temperature. So, in this study, the polymeriza-tion temperature was set to 150 °C, in order to facilitate thestudy of the effects of activators and initiators on the polymer-ization. The polymerization temperature range of 150–180 °Cwas within the processing window so as to achieve high speedpultrusion.

Fig. 10 Effects of initiatorconcentrations on the viscosityaverage molar mass and degree ofconversion (1.0 mol% activatorC20)

Fig. 9 Temperature-time relations for different C10 concentrations at 150 °C

J Polym Res (2019) 26:40 Page 7 of 10 40

Another significant temperature parameter was the initialtemperature, which was also the temperature of reaction mix-ture in injection die during fibers impregnation. The initialtemperature during polymerization had a great influence onthe rheological behavior of the reaction mixture, which deter-mined the processing window of injection. A higher initialpolymerization temperature indicated a faster reaction rateand a faster viscosity growth. However, in the injection dieof the pultrusion line, a relatively low initial temperature wasneeded to keep the low viscosity of the reaction mixture torealize fully impregnation. In general, the viscosity should beless than 1 Pa·s to allow the reaction mixture to penetrate thefiber reinforcement thoroughly [18].

The real-time viscosity of the reactive mixture was inves-tigated with the initial temperature changing from 90 °C to120 °C with 1.0 mol% activator and 2.0 mol% initiator, asshown in Fig. 11.When the initial polymerization temperaturewas at 90 °C, the reaction hardly proceeded. It was not easy tokeep the viscosity under 1 Pa·s for long time at the temperatureabove 100 °C. However, 3–10 minutes would be needed forkeeping low viscosity from mixing to impregnation, accord-ing to the pultrusion speed and fiber content. Below about80 °C, the reactive mixture was in a solid state. So the initialtemperature was preferably in the range of 80–100 °C.

Comparison with a commercial PA-6

In order to check out whether anionic PA-6 synthesized by theoptimized activator and initiator had reasonable properties, acommercial PA-6 of extrusion grade (YH3400, SinopecGroup, China) was compared with anionic PA-6 and resultsare presented in Table 1. The anionic PA-6 was polymerized at150 °C with 1.5 mol% initiator C10 and 1.0 mol% activatorC20. It can be seen that the anionic PA-6 had a lower degree ofconversion due to the relative higher concentrations in

activators and initiators, causing not all caprolactams involvedin the reaction as the commercial PA-6. Through GPC analy-sis, compared with commercial PA-6, the anionic PA-6 had awider molecular weight distribution and there were more olig-omers in anionic PA-6. As for mechanical properties, the an-ionic PA-6 had better tensile strength, while the commercialPA-6 had better flexural strength. The higher modulus and thelower elongation indicated that anionic PA-6 had higher de-gree of crystallinity than the commercial PA-6. In short, themechanical properties of anionic PA-6 were acceptable tomake the anionic PA-6 as an alternative matrix for pultrudedthermoplastic composite.

Pultrusion with optimized activator and initiator

To verify the availability of the optimized activator and initi-ator in pultrusion process. Pultrusion experiment of compos-ites was carried out. Based on the above discussion, 1.0 mol%activator C20 and 1.5 mol% initiator C10 were chosen. Inorder to avoid premature inactivation of the initiator bycontacting with the moisture in the air, concentration of

Fig. 11 Viscosity-time relations at different initial polymerizationtemperature

Table 1 Properties of anionic PA-6 and commercial PA-6

Anionic PA-6 Commercial PA-6

Density (g/cm3) 1.14 1.15

Degree of conversion (%) 97.2 99.7

Mη (g/mol) 33398 35041

Mn (g/mol) 16843 19690

Mw (g/mol) 36910 38087

Polydispersity (Mw/Mn) 2.191415 1.934332

Oligomer (%) 8.95% 4.16%

Tensile strength (MPa) 75.4 60.7

Elongation at break (%) 24.3 48.1

Flexural strength (MPa) 69.2 86.6

Flexural modulus (MPa) 2944 2087

Fig. 12 Pultruded sample of glass fiber reinforced PA-6 composites

40 Page 8 of 10 J Polym Res (2019) 26:40

initiator was slightly higher than that of activator. The temper-ature in RIM device and the injection die was set to 100 °Cand the temperature in the heating die and cooling die wereadjusted to 150 °C and 130 °C, respectively. Glass fiber rein-forced PA-6 composites with 50% volume fraction wereachieved by using 43 glass fibers roving, as shown inFig. 12. Below this volume fraction, the matrix PA-6 in thecomposites was dominated, which would cause dimensionalshrinkage after cooling. Pultrusion speed of 30 cm/min couldbe achieved, which was faster than the polymerization of neatanionic PA-6. It was because the glass fibers had a higherthermal conductivity than the anionic PA-6 and the polymer-ization was accelerated, which would be discussed in detail infuture research. The cross-section of pultruded sample wasalso observed by SEM after fracturing in liquid nitrogen.Great interfacial adhesion between fiber and polymer is shownin Fig. 13. PA-6 matrix was tightly bonded to the fibers. Somevoid were also observed because that the fibers were pulledout during the fracture.

Conclusions

Optimization of initiator and activator for reactive thermoplas-tic pultrusion was investigated in this study. Combination ofactivators C20 and initiators C10 would be the most suitableformula. The effects of activator concentrations and initiatorconcentrations on the anionic polymerization have beenassessed. By taking the polymerization time, degree of con-version and viscosity average molar mass of the synthesizedPA-6 into consideration, the concentrations of activators andinitiators were optimized to have a fast process and desirablefinal properties. Initial polymerization temperature was also

optimized at the range of 80–100 °C, to ensure the resin hadthe enough low viscosity to impregnate the dense fiber rein-forcement. Glass fiber reinforced PA-6 composite sample wassuccessfully pultruded with a smooth surface with the opti-mized activator and initiator and has a great bonding betweenthe fibers and matrix. A relative high pultrusion speed wasobserved. The effects of process parameters of pultrusion onthe mechanical properties of the pultruded composites wouldbe assessed in future research.

Acknowledgements The authors would like to appreciate Mrs. JianingGeng (School of International Education, Beijing University of ChemicalTechnology) for her help in modifying the language of the manuscript.

Publisher’s Note Springer Nature remains neutral with regard to juris-dictional claims in published maps and institutional affiliations.

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