materials

Article Co-Bonded Hybrid Thermoplastic-Thermoset Composite Interphase: Process-Microstructure-Property Correlation

Jamal Seyyed Monfared Zanjani * and Ismet Baran *

Faculty of Engineering Technology, University of Twente, 7500AE Enschede, The Netherlands * Correspondence: [email protected] (J.S.M.Z.); [email protected] (I.B.)

Abstract: Co-bonding is an effective joining method for fiber-reinforced composites in which a prefabricated part bonds with a thermoset resin during the curing process. Manufacturing of co- bonded thermoset-thermoplastic hybrid composites is a challenging task due to the complexities of the interdiffusion of reactive thermoset resin and thermoplastic polymer at the interface between two plies. Herein, the interphase properties of co-bonded acrylonitrile butadiene styrene thermoplastic to unsaturated polyester thermoset are investigated for different processing conditions. The effect of processing temperature on the cure kinetics and interdiffusion kinetics are studied experimentally. The interphase thickness and microstructure are linked to the chemo-rheological properties of the materials. The interdiffusion mechanisms are explored and models are developed to predict the interphase thickness and microstructure for various process conditions. The temperature-dependent diffusivities were estimated by incorporating an inverse diffusion model. The mechanical response of interphases was analyzed by the Vickers microhardness test and was correlated to the processing condition and microstructure. It was observed that processing temperature has significant effect on the interdiffusion process and, consequently, on the interphase thickness, its microstructure and mechanical performance.



Keywords: polymer interdiffusion; co-bonding; interphase; thermoplastics; thermosets curing;
 microhardness; hybrid composites Citation: Zanjani, J.S.M.; Baran, I. Co-Bonded Hybrid Thermoplastic- Thermoset Composite Interphase: Process-Microstructure-Property 1. Introduction Correlation. Materials 2021, 14, 291. Advanced fiber-reinforced polymeric composites have emerged as structural materials https://doi.org/10.3390/ma14020291 with a very wide range of applications due to their significant weight reduction, extended durability, and higher design flexibility compared to other traditional material alternatives. Received: 3 December 2020 However, there are ever-pressing industrial needs for further improving the performance Accepted: 29 December 2020 of polymeric composites and to meet the requirements of demanding and cutting edge Published: 8 January 2021 applications. Combining various advanced materials and creating hybrid or multi-material

Publisher’s Note: MDPI stays neu- composites (MMCs) have the potential to push forward the mechanical performance of the tral with regard to jurisdictional clai- manufactured part. The laminated MMCs consist of two or more different combinations of ms in published maps and institutio- layers, which offer a design versatility to place each specific material, where they can exploit nal affiliations. their best characteristics and properties to the benefit of the overall integrity of the structure. Hence, it is considered that the laminated MMCs have the potential to develop future structural materials with high performance [1,2]. Thermoplastic-thermoset hybrid polymer- based composites are one of the possibilities of MMCs with significant potential to address Copyright: © 2021 by the authors. Li- challenges for many state of the art applications [3]. Thermoplastic-thermoset MMCs censee MDPI, Basel, Switzerland. combine the damage tolerance characteristics, and welding capability of thermoplastics This article is an open access article with the strength and stiffness of thermoset composites resulting in more reliable structures distributed under the terms and con- with an extended lifetime [4–6]. However, joining thermoplastics to thermosets remains a ditions of the Creative Commons At- challenging step despite the development of various joining technologies for composite tribution (CC BY) license (https:// structures including (i) mechanical fastening [7–9], (ii) welding [10–13], (iii) adhesive creativecommons.org/licenses/by/ bonding [14–16], (iv) co-curing (parts are cured at the same time) [17,18], and (v) hybrid 4.0/).

Materials 2021, 14, 291. https://doi.org/10.3390/ma14020291 https://www.mdpi.com/journal/materials Materials 2021, 14, 291 2 of 17

joints (combination of two or more of previous methods) [19–21]. Nevertheless, all these joining methods have disadvantages in terms of being unreliable, labour-intensive, time- consuming and expensive [22]. An alternative and effective joining method for composites is co-bonding (an uncured part is joined with one or more cured parts) [23,24]. The co-bonding method can be applied for jointing both similar or dissimilar materials including thermoplastics to thermosets. Co-bonding eliminates the needs for mechanical fasteners, and adhesives resulting in a more effective and applicable joining method for many applications. The interface between two parts in the co-bonding process is controlled by thermodynamic affinity and physical interactions in between and also the curing process of the reactive resin [10]. Several studies have been conducted on co-bonding process of thermoplastic-thermoset for epoxy ther- moset resins and various thermoplastics. In [25], the difference in diffusion of a bisphenol A type epoxy resin and the corresponding diamine curing agent into polysulfone (PSU) was studied. The diffusivity of amine was found to be an order of magnitude larger than that for epoxy at different temperatures. As an extension to the aforementioned study, Rajagopalan et al. [26] showed that the amine enhanced the epoxy diffusivity into PSU by a factor of three due to amine swelling effect. In addition, they identified three chrono- logical processes of diffusion, reaction, and phase separation which derive the interphase formation. In another study, Sonnenfeld et al. [27] improved the damage tolerance of epoxy matrix composites through manufacturing of thermoplastic/thermoset multilayer com- posites using semi-crystalline poly-ether-ether-ketone (PEEK) and polyphenylene sulfide (PPS). They demonstrated that the introduction of a layer which was compatible with both thermoplastic and thermoset sections such as poly-ether-imide (PEI) increased the adhesion energy by a factor of 15. Velthem et al. [28] studied the influence of the interdiffusion of two thermoplastics of poly(ether sulfone) (PES) and phenoxy in the epoxy system and they correlated the resulting interphase morphologies to the interlaminar fracture toughness. Nevertheless, all these studies were limited to epoxy as the thermoset resin and focused on the interdiffusion process and morphology of interphase. Processing condition plays an essential role in interphase formation between ther- mosets and thermoplastics during co-bonding by altering the cure reaction kinetics, and interdiffusion process. The curing reaction of a thermosetting resin is a multi-stage complex and highly temperature-dependent process. In the curing of a thermoset resin, many reactions take place simultaneously such as the decomposition of the initiator, the poly- merization of different monomers and oligomers, and termination reactions with changes in the physical behaviour of the resin. All these reactions influence the interdiffusion process and interphase formation since the degree of cure or degree of cross linking of the molecules increases as a function of process time and temperature as shown in [29,30]. Therefore, the interdiffusion kinetics and, consequently, the interdiffusion termination time as determining parameters on the interphase formation are closely linked to curing kinetics of the thermoset resin [31]. More specifically, as the cure reaction progresses the molecular weight and crosslinking density of the resin increase after which no polymer interdiffusion takes place. Regarding the cessation of the polymer interdiffusion at the thermoset-thermoplastic interface, the most accepted assumption is that diffusivity of the reacting resins terminates at the gel point defined as a time in which the resin switches its state from a viscous liquid to a semi-solid rubbery gel and loses its fluidity [12,32–34]. Accordingly, the kinetics of curing and interdiffusion determines the interphase thickness, its quality, and the microstructure. It is known that the mechanical performance and strength of interphase in hybrid composites are primarily determined by the interphase thickness [35,36]. Therefore, a physical and mechanical understanding of the interphase is critical to determine the load transfer efficiency across dissimilar materials and to over- see the overall integrity of the structure. To the best of the authors’ knowledge, there is no comprehensive study elaborating on the effect of processing conditions and cure kinetics on the micro-structure and mechanical response of the interphases in co-bonded thermoplastic-thermoset parts in the literature. Materials 2021, 14, 291 3 of 17

In this study, unsaturated polyester resin (UPR) for the first time is used to be co- bonded to a thermoplastic part. UPR is one of the widespread used thermoset resins in various industry due to its low cost and excellent processability in many applications such as wind energy and ship manufacturing industries [37,38]. On the other hand, a terpolymer of acrylonitrile, butadiene, and styrene with commercial name of acrylonitrile butadiene styrene (ABS) known to have excellent mechanical properties including high toughness, ductility, and very high impact strength is selected as the thermoplastic ma- terial [39]. Herein, the effect of processing temperature on the co-bonding of ABS and UPR and resultant interphase morphology and mechanical response are investigated. The interdiffusion kinetics and mechanisms during co-bonding are explained and linked to the chemo-rheological properties of UPR. Models are developed and utilized to predict the interphase thickness at various processing conditions and to provide information on interphase microstructure. The experimentally characterized gel time and interdiffusion thickness are used in a one-dimensional (1D) inverse diffusion model to estimate the diffu- sivity at different temperatures. Subsequently, the diffusivities are fit to an Arrhenius type of model. Vickers’ microhardness test was used to study the mechanical behaviour at the interphase of the specimens at various temperatures. Finally, a methodology is provided to assemble process-microstructure-property correlations at interphases which are essential to the design and manufacturing of reliable thermoplastic -thermoset MMCs.

2. Materials and Methods 2.1. Materials An industrial medium reactive orthophthalic unsaturated polyester resin (UPR) de- signed for resin transfer moulding and vacuum injection moulding processes as described in [40] was used as thermoset resin. This resin contained 45% styrene with an acid number of 25 mgKOH/g. A liquid peroxide system specialized for curing of the unsaturated polyester and vinyl ester resins at room temperature with low peak exotherm, and long working time (gel time) was used as an initiator. ABS in the sheet form with grade name of VIKUREEN ABS PLAAT GLANS WIT 0291 (Epsotech, Jülich Kirchberg, Germany), with strong resistance to corrosive chemicals and/or physical impacts, was used as thermoplastic material in this study.

2.2. Interphase between Thermoset and Thermoplastic To study the interdiffusion of thermoset resin into thermoplastic, ABS specimens were prepared by cutting them using a wet saw into parts with estimated dimensions of 15 mm × 15 mm × 3 mm followed by edge sanding with #500 SiC foil to remove any debris. Next, an isopropyl alcohol and a mixture of water were utilized to wash ABS specimens to remove any remaining particles and contamination. Specimens were dried in the oven at a temperature of 60 ◦C for 24 h and then left in a desiccator to reach room temperature. Afterward, ABS specimens were placed vertically in the middle of a cylindrical embedding mould with a diameter of 25 mm by using a couple of metallic clamping rings. The degassed UPR resin mixture was poured onto the ABS specimens until the resin completely covered them. Effect of temperature on the interphase was studied by preparing the samples and placing of the moulds in the preheated oven fixed at the set temperatures of 25 ◦C, 35 ◦C, 40 ◦C, 50 ◦C, and 60 ◦C and curing for 24 h followed by a post-curing process at 60 ◦C for 24 h. After the de-moulding of samples, Struers polishing system (Teramin-30, Struers, Cleveland, OH, USA) was employed to polish the cross-sections of specimens with SiC foil of #500, #1000, #2000 and #4000, in the given order. The final polishing of the embedded specimens were done by using OP-S NonDry. A schematic view of the cross-section for the co-bonded ABS-UPR sample and the resulting interphase due to polymer interdiffusion are shown in Figure1. Materials 2021, 14, 291 4 of 17 Materials 2021, 14, x FOR PEER REVIEW 4 of 17

UPR Interphase UPR 25 mm 25

thickness mm 3 ABS ABS Polymer interdiffusion . FigureFigure 1. SchematicSchematic view of the interphase region inin thethe acrylonitrileacrylonitrile butadienebutadiene styrene-unsaturatedstyrene-unsatu- ratedpolyester polyester resin (ABS-UPR)resin (ABS-UPR) co-bonded co-bonded specimen specimen [31]. [31].

2.3.2.3. Characterizations Characterizations 2.3.1. Microscopic Analysis of Unsaturated Polyester Resin-Acrylonitrile Butadiene 2.3.1. Microscopic Analysis of Unsaturated Polyester Resin-Acrylonitrile Butadiene Styrene (UPR-ABS) Interphase+ Styrene (UPR-ABS) Interphase+ The polished cross-section of interphases formed between the co-bonded ABS and UPRThe systems polished at various cross-section processing of conditionsinterphase weres formed analysed between by using the aco-bonded Keyence VHX-5000 ABS and UPRdigital systems microscope at various (Keyence, processing Osaka, conditions Japan) equipped were analysed with a VH-100UR by using lens.a Keyence 2D stitching VHX- 5000function digital of themicroscope microscope (Keyence, software Osaka, was used Japan) to jointequipped several with captured a VH-100UR images tolens. obtain 2D stitchinga sufficiently function large of cross-section the microscope of thesoftware interface. was Theused interphase to joint several thickness captured values images were toreported obtain fora sufficiently each temperature large cross-section as the average of the measurements interface. The at interphase mid-section thickness of the interface values werefar from reported the clamps for each and temperature obtained from as atthe least average three measurements specimens for eachat mid-section temperature. of the interface far from the clamps and obtained from at least three specimens for each temper- ature.2.3.2. Surface Swelling Measurements Furthermore, the surface swelling of ABS by UPR resin were examined at room 2.3.2.temperature. Surface Swel Forling this, Measurements a parallel plate apparatus of the rheometer (in stationary mode) (AntonFurthermore, Paar Germany the surface GmbH, swelling Ostfildern, of ABS Germany) by UPR wasresin employed were examined to confine at room the resintem- perature.between theFor headthis, a plate parallel and plate ABS apparatus sheet (as theof the bottom rheometer plate) (in as shownstationary in Figuremode)2 (Antona. The Paarnominal Germany normal GmbH, force was Ostfildern, set to 0.1 Germany) N in the rheometer. was employed The normal to confine force the was resin kept between constant theto ensure head plate the contact and ABS of resinsheet to(as the the rheometer bottom pl headate) as and shown surface in Figure of the specimen2a. The nominal and to normalprevent force resin starvationwas set to due0.1 N to diffusionin the rheomete at the studyr. The surface. normal The force capillary was kept forces constant between to ensureUPR and the thecontact plates of whichresin to might the rheometer cause a possible head and resin surface flow of out the was specimen prevented and to by pre- the ventplate-plate resin starvation configuration. due to Herein, diffusion 50 mm at th×e 50study mm surface. ABS plates The werecapillary cut, edgeforces sanded between as UPRmentioned and the in plates detail inwhich Section might 2.2, cause and then a poss wereible glued resin on flow the surfaceout was of prevented the bottom by plate the plate-plateof the rheometer. configuration. The plate-plate Herein, distance 50 mm × was 50 setmm to ABS 0.5 mmplates and were was cut, filled edge with sanded UPR, i.e., as mentionedwithout mixing in detail it with in Section the liquid 2.2, and peroxide then were system, glued and on kept the forsurface 60 min. of the Next, bottom Keyence plate ofVK-9700 the rheometer. confocal The microscope plate-plate was distance used to was capture set to the 0.5 changes mm and in was the filled surface with topology UPR, i.e., of withoutABS due mixing to swelling it with by UPR.the liquid The heightperoxide and system, spatial resolutionsand kept for were 60 min. 1 and Next, 120 nmKeyence in the VK-9700confocal confocal microscope microscope which had was a laserused withto ca 408pture nm the wavelength. changes in Asthe seensurface from topologyFigure2 ofb, Materials 2021, 14, x FOR PEER REVIEWABSthe confocal due to swelling microscopy by UPR. measurements The height and were spatial performed resolutions on the were path 1 which and 120 covered nm5 inof the17

confocalregion with microscope and without which UPR had toa laser define with the 40 surface8 nm wavelength. swelling of As APB. seen The from length Figure of the2b, thescanned confocal length microscopy by the confocal measurements microscopy were was performed approximately on the 8 path mm. which covered the region with and without UPR to define the surface swelling of APB. The length of the scanned length by25 mmthe confocal microscopy was approximatelyIn contact 8 mm.Measurement length to UPR

0.5 mm thick Rheometer head ABS UPR ABS

(a) (b)

FigureFigure 2. 2. (a()a )Schematic Schematic view view of of the the rheometer rheometer setup setup for for the the swelling swelling experiments experiments by placing UPR on onABS ABS surface, surface, and and (b ()b measurement) measurement direction direction in in confocal confocal microscopy. microscopy.

2.3.3. Chemo-Rheology of UPR The chemo-rheological properties of the UPR were characterized by using the Anton Paar-Physica MCR 501 rheometer (Anton Paar Germany GmbH, Ostfildern, Germany) in “plate–plate” mode. The effect of processing temperature on the viscosity, storage modu- lus (G′) and loss modulus (G″) were investigated. All the measurements were performed by using circular aluminium plates of 25 mm diameter in oscillatory mode at a 0.5% strain and a 1 Hz with a plate-plate spacing of 0.3 mm. The temperature-dependent gel time of the resin, as an indication of the cure kinetics of UPR resin, was estimated from the cross- ing point of storage modulus (G′) and loss modulus (G″) of UPR mixture including the liquid peroxide [41]. The rheometer measurements were performed at seven different iso- thermal temperatures of 25 °C, 35 °C, 40 °C, 50 °C, 60 °C, 80 °C and 100 °C and final gel time at each temperature was reported as the average of at least three repetitions. In the gel-time measurements, the rheometer was maintained at the set temperature for 30 min before placing the resin to achieve a uniform and stabilised temperature in the measure- ment chamber. The relation between temperature-dependent gel time values and temper- ature was describe by employing an Arrhenius relationship as [42]: 1 1 −∆ = (1)

where tG was gel time, t0 was the pre-exponential time constant, R was the universal gas constant, and ΔE is the activation energy of the gelation. Herein, gel time as an index of cure kinetics was employed to monitor the changes during the curing reaction and to de- termine the cure kinetics effect on interphase formation. The viscosity measurement was performed on the UPR (without initiator) with a ramp of 1.5 °C/min between room tem- perature to 85 °C where measurements were taken every 10 s and resulted in 240 meas- urement points.

2.3.4. Diffusivity Coefficient of UPR in ABS

1D Inverse Diffusion Model The 1D interdiffusion of the UPR into ABS as schematically shown in Figure 3 was modelled using the Fick’s second law [43] as: (, ) (, ) = (2) where (, ) was the UPR concentration in ABS, D was the diffusivity or coefficient of diffusion. By assuming D is constant at a certain temperature and the volume of ABS does not change with the diffusion of UPR into ABS, Equation (2) can be solved analytically as [44]: (, ) −(0,) =erf ( ) (3) (, 0) −(0,) 2√

Materials 2021, 14, 291 5 of 17

2.3.3. Chemo-Rheology of UPR The chemo-rheological properties of the UPR were characterized by using the Anton Paar-Physica MCR 501 rheometer (Anton Paar Germany GmbH, Ostfildern, Germany) in “plate–plate” mode. The effect of processing temperature on the viscosity, storage modulus (G0) and loss modulus (G”) were investigated. All the measurements were performed by using circular aluminium plates of 25 mm diameter in oscillatory mode at a 0.5% strain and a 1 Hz with a plate-plate spacing of 0.3 mm. The temperature-dependent gel time of the resin, as an indication of the cure kinetics of UPR resin, was estimated from the crossing point of storage modulus (G0) and loss modulus (G”) of UPR mixture including the liquid peroxide [41]. The rheometer measurements were performed at seven different isothermal temperatures of 25 ◦C, 35 ◦C, 40 ◦C, 50 ◦C, 60 ◦C, 80 ◦C and 100 ◦C and final gel time at each temperature was reported as the average of at least three repetitions. In the gel-time measurements, the rheometer was maintained at the set temperature for 30 min before placing the resin to achieve a uniform and stabilised temperature in the measurement chamber. The relation between temperature-dependent gel time values and temperature was describe by employing an Arrhenius relationship as [42]:

 1   1   −∆E  = exp (1) tG t0 RT

where tG was gel time, t0 was the pre-exponential time constant, R was the universal gas constant, and ∆E is the activation energy of the gelation. Herein, gel time as an index of cure kinetics was employed to monitor the changes during the curing reaction and to determine the cure kinetics effect on interphase formation. The viscosity measurement was performed on the UPR (without initiator) with a ramp of 1.5 ◦C/min between room temperature to 85 ◦C where measurements were taken every 10 s and resulted in 240 measurement points.

2.3.4. Diffusivity Coefficient of UPR in ABS 1D Inverse Diffusion Model The 1D interdiffusion of the UPR into ABS as schematically shown in Figure3 was modelled using the Fick’s second law [43] as:

∂c(x, t) ∂2c(x, t) = D (2) ∂t ∂x2 where c(x, t) was the UPR concentration in ABS, D was the diffusivity or coefficient of diffusion. By assuming D is constant at a certain temperature and the volume of ABS does not change with the diffusion of UPR into ABS, Equation (2) can be solved analytically as [44]: c(x, t) − c(0, t)  x  = erf √ (3) c(x, 0) − c(0, t) 2 Dt where erf was the error function, c(0, t) was the concentration at x = 0 and c(x, 0) was the initial concentration. Since the interdiffusion distance (xint) was experimentally determined, the corresponding D can be obtained from Equation (3) by considering the conditions in the following:

• c(0, t) = 1 for t ≤ tgel and c(0, t) = 0 for t > tgel where tgel was the gelation time of the polyester resin which was determined experimentally in Section 2.3.3. • The concentration was assumed to be relatively small by using a value of 10−5 at   −5 t = tgel and at x = xint, i.e., c xint, tgel = 10 (see Figure3). Materials 2021, 14, x FOR PEER REVIEW 6 of 17

where erf was the error function, (0, ) was the concentration at =0 and (, 0) was the initial concentration. Since the interdiffusion distance () was experimentally determined, the corresponding can be obtained from Equation (3) by considering the conditions in the following: • (0, ) = 1 for ≤ and (0,) = 0 for > where was the gelation time of the polyester resin which was determined experimentally in Section 2.3.3. • The concentration was assumed to be relatively small by using a value of 10−5 at = and at =, i.e., ,=10 (see Figure 3). Accordingly, was obtained using Equation (4). Note that was estimated for dif- ferent and which were obtained at different process temperatures. Materials 2021, 14, 291 1 6 of 17 = (4) 2erf (1 − ,)

FigureFigure 3.3. Schematic view of the the 1D 1D diffusion diffusion of of UPR UPR into into ABS ABS (left). (left ).The The resulting resulting concentration concentration distributiondistribution ((right).right).

TemperatureAccordingly, DependentD was Diffusivity obtained using Model Equation (4). Note that D was estimated for differentThe tobtainedgel and xint which values were from obtained the 1D inverse at different diffusion process model temperatures. for different tempera- tures were fit with an Arrhenius type of equation as follows [44]:  2 1 xint − D =  =exp ( )  (4)(5) t −1   gel 2erf 1 − c xint, tgel which was the diffusivity model also used in [25,26,44]. In Equation (4), D0 was the pre- Temperatureexponential constant, Dependent E1 was Diffusivity the activation Model energies, R was the universal gas constant and T wasThe the obtained temperatureD values in K. from A linear the 1D regressi inverseon diffusion was employed model for for different the fitting temperatures procedure wereby transforming fit with an Arrhenius the non-linear type ofrelation equation in Equation as follows (5) [44 into]: a linear one as:  −E  Dln=(D) =lnexp ()−1 (5)(6) 0 RT

which2.3.5. Resin was the Uptake diffusivity model also used in [25,26,44]. In Equation (4), D0 was the pre- exponential constant, E was the activation energies, R was the universal gas constant and The kinetics of the1 interdiffusion was investigated by resin uptake experiments T was the temperature in K. A linear regression was employed for the fitting procedure by where ABS specimens were immersed into a UPR bath (without initiator) and the weight transforming the non-linear relation in Equation (5) into a linear one as: of the absorbed resin by specimens were measured at different times of 60, 300, 600, 1200, 1800, 3600 and 7200 s after immersion and temperaturesE of 25 °C, 35 °C, 40 °C, 50 °C, and ln(D) = ln(D ) − 1 (6) 60 °C. The nominal dimensions of the ABS specimens0 RT were 30 mm × 30 mm which were

2.3.5. Resin Uptake The kinetics of the interdiffusion was investigated by resin uptake experiments where ABS specimens were immersed into a UPR bath (without initiator) and the weight of the absorbed resin by specimens were measured at different times of 60, 300, 600, 1200, 1800, 3600 and 7200 s after immersion and temperatures of 25 ◦C, 35 ◦C, 40 ◦C, 50 ◦C, and 60 ◦C. The nominal dimensions of the ABS specimens were 30 mm × 30 mm which were measured by using a micrometre with a precision of ±0.01 mm. An analytical balance was used to measure the dry weight of specimens. The immersion of the specimens in to the resin was carried out by using the metallic clamps. The excess resin on the specimen surface after the immersion step was washed by using an ethanol. In addition, non-sticking and absorbent fabrics were used to clean the specimen surfaces. After drying, resin uptake Materials 2021, 14, 291 7 of 17

weight were measured by the analytical balance and the uptake values per contact area (M*) were calculated using the below formula:

W − W M∗ = i d (7) A

where Wi and Wd were the resin uptake weight and dry ABS specimen weight, respectively, M* was the mass uptake per unit area and A was the surface area. As the length and width of specimens were considerably (over 10 times) larger than the thickness, the resin uptake from the edges of ABS samples was neglected. The measured data were fitted to Equation (8) in order to evaluate the transport type and associated diffusion kinetics (resin uptake) [45]. M∗ = ktn (8) where k was an empirical rate constant, t was the time and n was the transport exponent of diffusion. The type of diffusion is indicated by n in Equation (8), i.e., n = 0.5 for Fickian diffusion, 0.5 < n<1 for an anomalous diffusion, and n = 1 for Case II diffusion [45]. Nevertheless, to obtain a kinetics model for a continuous range of temperatures based on Equation (8), it was needed to have a model for n and k over experimental temperatures. Herein, n and k are approximated by Equations (9) and (10) as:

 N  n = A exp 0 (9) n T

k = k1 + k2T (10)

where An, N0, k1, k2 were defined as constants to be determined through fitting of experi- mental results.

2.3.6. Resin Volume Fraction at Interphase To correlate the diffusion kinetics to resin curing kinetics and consequently interphase formation, Equations (1) and (8)–(10) are combined into Equation (11). This eliminated the time term from resin uptake equation (Equation (8)) for the reactive resin and yield to a model for amount of diffused reactive resin per unit area (M*) into ABS at given temperature. N0 (An exp ( )) ∗ t0 T M = (k1 + k2T)( ) (11)  −∆E  exp RT

Moreover, the relation between M* and Vr (the volume of the diffused resin) was obtained by using the following relation:

M ρV M∗ = = r = ρV∗ (12) A A r

where ρ and A were the density of uncured resin (1.05 gr/cm3) and contact surface area, ∗ Vr respectively. Herein, Vr* was defined as Vr = A and so-called reduced resin volume (diffused volume per unit of contact area). Vr* values at any given temperature were obtained from combining Equations (11) and (12) as:

N0 (An exp ( )) ∗ 1 t0 T Vr = ( )(k1 + k2T)(   ) (13) ρ −∆E exp RT Materials 2021, 14, x FOR PEER REVIEW 8 of 17 Materials 2021, 14, 291 8 of 17

Accordingly, the ratio of Vr* to reduced volume of interface Vint* which corresponds to interphaseAccordingly, thickness the ratio provided of Vr* the to reduced volume volumefraction ofof interface resin at theVint interphase* which corresponds (φ) as an essentialto interphase microstructure thickness providedindicator in the binary volume interphases fraction of defined resin at as: the interphase (ϕ) as an essential microstructure indicator in binary interphases∗ defined as: = ∗∗ (14) Vr ϕ = ∗ (14) Vint 2.3.7. Hardness Measurement at Interphase 2.3.7. Hardness Measurement at Interphase The Vickers microhardness test was employed to analyse the changes in the mechan- The Vickers microhardness test was employed to analyse the changes in the mechanical ical response at the UPR-ABS interphase. The hardness obtained by this method was de- response at the UPR-ABS interphase. The hardness obtained by this method was described scribed as the resistance of the surface to indentation [46]. Vickers microhardness map- as the resistance of the surface to indentation [46]. Vickers microhardness mapping was ping was conducted using the LECO LM100AT micro-hardness tester by applying a load conducted using the LECO LM100AT micro-hardness tester by applying a load of 10 gr and of 10 gr and an indent spacing of 100 µm. The Vickers diamond pyramid hardness num- an indent spacing of 100 µm. The Vickers diamond pyramid hardness number, HV, was ber, HV, was defined as the ratio of the applied load, P, to the pyramidal contact area of defined as the ratio of the applied load, P, to the pyramidal contact area of the indentation: the indentation: P = Hv =α 2 (15)(15) d wherewhere dd waswas the the diagonal diagonal length length of of the the resultant resultant impression, impression, and and αα waswas defined defined as as 1.8544 1.8544 forfor the the Vickers Vickers indenter indenter [47]. [47]. Figure Figure 4 showsshows an an example example pattern pattern and sample sample configuration configuration usedused in in the the microhardness microhardness measurements inin thethe vicinityvicinity of of interphase interphase for for ABS ABS and and UPR. UPR. A Atotal total of of 25 25 indentations indentations for for each each line line werewere conductedconducted withwith 100 µµmm intervals in the the width width ofof interphase interphase as as shown shown in in Figure Figure 1 in whic whichh the initial 1000 µmµm was was for for the the UPR UPR and and the the restrest covered covered the the interphase interphase and and ABS ABS regions. regions. The The hardness hardness profiles profiles were were reported reported for for each each temperaturetemperature determined determined from from the the average average meas measurementsurements for for at at least least three three specimens. specimens.

FigureFigure 4. 4. VickersVickers microhardness microhardness measurement measurement pattern pattern and and sample sample configuration. configuration. 3. Results and Discussion 3. Results and Discussion 3.1. Interphase Formation and Processing Temperature Effect 3.1. Interphase Formation and Processing Temperature Effect Figure5 reveals the optical microscopic observations of ABS-UPR interphases indi- catingFigure the average5 reveals interphase the optical thickness microscopic formed observations at different of ABS-UPR temperatures interphases ranging indi- from cating25 ◦C tothe 60 average◦C. It was interphase observed thickness that an interphase formed at wasdifferent formed temperatures between ABS ranging and UPR from at 25 all °Ctemperature to 60 °C. It rangeswas observed with no that visible an gradientinterphase of was diffusion. formed Nonetheless, between ABS the and interdiffusion UPR at all temperaturefronts in both ranges sides ofwith formed no visible interphases gradient were of slightlydiffusion. different Nonetheless, from inner the sectionsinterdiffusion which frontsimplies in theboth contribution sides of formed of various interphases phenomena were inslightly interphase different formation. from inner Therefore, sections an whichoverview implies image the of contribution interphase formedof various around phen theomena ABS in specimen interphase at 25formation.◦C was constructed Therefore, anby overview stitching severalimage overlappingof interphase microscopic formed around images the as ABS shown specimen in Figure at6 25to illustrate°C was con- the structedeffect of by boundary stitching conditions several overlapping and constraints microscopic such as metallic images clamps as shown on thein Figure interphase 6 to for-il- lustratemation. the In Figureeffect 6of, aboundary dash-lined conditions frame is inserted and constraints in the image such to as estimate metallic the clamps boundaries on the interphaseof the original formation. ABS specimen In Figure before 6, a dash-lin interactionsed frame with is UPR.inserted The in frame the image size and to estimate location thewere boundaries approximated, of the based original on ABS the location specimen of metallicbefore interactions clamping rings with and UPR. nominal The frame thickness size andof the location ABS sample were approximated, before embedding. based Figure on th6 ereveals location that of the metallic interphase clamping formed rings around and nominalthe ABS thickness noticeably of exceeded the ABS the sample boundaries before of embedding. the original Figure ABS sample 6 reveals (dashed that frame).the inter- To phaseelaborate, formed it was around expected the toABS have noticeably the diffusion exceeded in both the directions boundaries of UPR of intothe original ABS and ABS ABS sampleinto UPR. (dashed Nevertheless, frame). To diffusion elaborate, of ABSit was into expected UPR was to have at a muchthe diffusion lower level in both compared direc- tionsto UPR of UPR into ABSinto ABS due toand the ABS higher into moleculeUPR. Nevertheless, size and lower diffusion molecular of ABS mobility into UPR of ABSwas at as compared with liquid UPR [48,49]. However, the difference in the interphase thicknesses

Materials 2021, 14, x FOR PEER REVIEW 9 of 17 Materials 2021, 14, x FOR PEER REVIEW 9 of 17

Materials 2021, 14, 291 9 of 17 a much lower level compared to UPR into ABS due to the higher molecule size and lower a much lower level compared to UPR into ABS due to the higher molecule size and lower molecular mobility of ABS as compared with liquid UPR [48,49]. However, the difference molecular mobility of ABS as compared with liquid UPR [48,49]. However, the difference in the interphase thicknesses inside and outside of the frame was not as significant as it in the interphase thicknesses inside and outside of the frame was not as significant as it wasinside anticipated. and outside Therefore, of the frame swelling was of not ABS as with significant UPR was as it a was contributing anticipated. parameter Therefore, in swellingwas anticipated. of ABS with Therefore, UPR was swelling a contributing of ABS with parameter UPR was in thea contributing interphase thickness.parameter In in the interphase thickness. In this regard, the swelling behaviour of ABS in contact with thisthe regard,interphase the swellingthickness. behaviour In this regard, of ABS the in contact swelling with behaviour UPR was of studied ABS in by contact placing with the UPR was studied by placing the UPR on the surface of the ABS sample in the rheometer UPRUPR on was the studied surface by of placing the ABS the sample UPR in on the the rheometer surface of as the mentioned. ABS sample Figure in the7 depicts rheometer the as mentioned. Figure 7 depicts the confocal microscopy results of the surface profile of confocalas mentioned. microscopy Figure results 7 depicts of the the surface confocal profile microscopy of ABS after results being of in the contact surface with profile UPR inof ABS after being in contact with UPR in the rheometer. It is clearly seen that the ABS sur- theABS rheometer. after being It in is clearlycontact seen with that UPR the in ABSthe rheo surfacemeter. was It swelled is clearly and seen its heightthat the increased ABS sur- face was swelled and its height increased approximately 70 µm as compared with the rest approximatelyface was swelled 70 andµm asits comparedheight increased with the approximately rest of the specimen. 70 µm as The compared increase with in the the ABS rest of the specimen. The increase in the ABS sheet thickness confirmed the swelling behaviour sheetof the thickness specimen. confirmed The increase the in swelling the ABS behaviour sheet thickness of ABS confirmed in contact the with swelling UPR. behaviour of ABS in contact with UPR. of ABS in contact with UPR. 25 °C 35 °C 25 °C 35 °C

40 °C 50 °C 60 °C 40 °C 50 °C 60 °C

FigureFigure 5. 5. MicrographsMicrographs showing showing the the interphase interphase formation formation be betweentween ABS ABS and and UPR UPR at at different different temperatures. temperatures. Figure 5. Micrographs showing the interphase formation between ABS and UPR at different temperatures.

Figure 6. Overall view of the interphase formed between ABS and UPR at 25 °C. FigureFigure 6. 6.Overall Overall viewview ofof thethe interphaseinterphase formed formed between between ABS ABS and and UPR UPR at at 25 25◦ °C.C. Moreover, it can be seen in Figure 5 that the process temperature influenced the in- Moreover,Moreover, itit cancan be seen in in Figure Figure 5 that the the process process temperature temperature influenced influenced the the in- terphase thickness considerably. Figure 8 displays the changes in interphase thickness interphaseterphase thickness thickness considerably. considerably. Figure Figure 88 displaysdisplays thethe changeschanges in in interphase interphase thickness thickness with respect to the process temperature. It is seen that the thickest interphase was withwith respect respect to to the the process process temperature. temperature. It is seenIt is that seen the that thickest the interphasethickest interphase was achieved was achieved for specimen prepared◦ at 25 °C with an interphase thickness of± 710 ±µ 20 µm while forachieved specimen for specimen prepared prepared at 25 C withat 25 °C an with interphase an interphase thickness thickness of 710 of 71020 ±m 20 while µm while the the thinnest interphase was observed at◦ 35 °C as± 635 µ± 10 µm. It can be seen in Figure 8 thinnestthe thinnest interphase interphase was observed was observed at 35 Cat as35 635 °C as10 635m. ± 10 It canµm. be It seencan inbe Figureseen in8 thatFigure the 8 thatinterphase the interphase thickness thickness sharply sharply decreased decrease by increasingd by increasing the temperature the temperature from 25 from◦C to 25 35 °C◦C that the interphase thickness sharply decreased by increasing the temperature from 25 °C tofollowed 35 °C followed by a gradual by a gradual increase increase after 35 after◦C up35 °C to 50up◦ toC and50 °C slightly and slightly drops drops from from 50 ◦C 50 to to 35 °C followed by a gradual increase after 35 °C up to 50 °C and slightly drops from 50 °C60 to◦C. 60 The °C. controversy The controversy effect of effect temperature of temperature on interphase on interphase thickness underthickness and aboveunder 35and◦C °C to 60 °C. The controversy effect of temperature on interphase thickness under and can be described by the effect of temperature on the cure kinetics and diffusion kinetics of the resin. An increase in temperature was expected to accelerate the curing kinetics leading to a decrease in the time available for the interdiffusion process (decreasing factor for interphase thickness). However, the rate of change in the cure kinetic is different at low

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aboveabove 35 35 °C °C can can be be described described by by the the effect effect of of temperature temperature on on the the cure cure kinetics kinetics and and diffu- diffu- Materials 2021, 14, 291 sionsion kinetics kinetics of of the the resin. resin. An An increase increase in in te temperaturemperature was was expected expected to to accelerate accelerate the the curing curing10 of 17 kineticskinetics leading leading to to a a decrease decrease in in the the time time available available for for the the interdiffusion interdiffusion process process (decreas- (decreas- inging factor factor for for interphase interphase thickness). thickness). However, However, the the rate rate of of change change in in the the cure cure kinetic kinetic is is dif- dif- ferentferent at at low low and and high high temperatures. temperatures. At At higher higher temperature, temperature, the the peroxide peroxide was was already already and high temperatures. At higher temperature, the peroxide was already over its thermal overover its its thermal thermal activation activation temperature temperature thre thresholdshold (the (the employed employed initiator initiator was was designed designed activation temperature threshold (the employed initiator was designed for application in forfor application application in in room room temperature temperature and and above above it). it). Therefore, Therefore, the the acceleration acceleration rate rate was was room temperature and above it). Therefore, the acceleration rate was lower while at the lowerlower while while at at the the lower lower temperature temperature an an increase increase in in temperature temperature can can provide provide the the neces- neces- lower temperature an increase in temperature can provide the necessary activation energy sarysary activation activation energy energy for for peroxides peroxides and and boost boost the the reaction reaction kinetics kinetics significantly significantly [50]. [50]. To To for peroxides and boost the reaction kinetics significantly [50]. To obtain a comprehensive obtainobtain a a comprehensive comprehensive understanding understanding of of the the effect effect of of temperature temperature on on the the interphase interphase for- for- understanding of the effect of temperature on the interphase formation, cure kinetics and its mation, cure kinetics and its mechanical properties, and interdiffusion process were in- mechanicalmation, cure properties, kinetics and and its interdiffusion mechanical processproperties, were and investigated interdiffusion and furtherprocess described were in- vestigated and further described in the next section. investigated the next and section. further described in the next section.

8080

6060

4040

2020

00 0246802468 LenghtLenght [mm] [mm]

FigureFigure 7. 7. Confocal Confocal microscopy microscopy measurement measurementmeasurement of ofof surface surfacesurface swelling swellingswelling of ofof ABS ABSABS in inin contact contactcontact with withwith UPR. UPR.UPR. m] m] Interphase thickness [ Interphase Interphase thickness [ Interphase

FigureFigure 8. 8. Interphase Interphase thickness thickness formation formation formation between between between ABS ABS ABS and and and UPR UPR UPR for for for different different different processing processing processing tempera- tempera- tempera- tures.tures.tures.

3.2.3.2. Resin Resin Curing Curing and and Diffusion Diffusion Kinetics Kinetics FigureFigure 9a9 9aa shows showsshows the thethe gel gelgel time time obtained obtained with with with respect respect respect to to toisothermal isothermal isothermal rheometer rheometer rheometer tests. tests. tests. It It isItis seen isseen seen that that that the the thegel gel time gel time time changed changed changed linearly linearly linearly in in the the in logarithmic logarithmic the logarithmic scale scale with scale with temperature withtemperature temperature indi- indi- catingindicatingcating that that the thatthe gel gel the time time gel was was time considerably considerably was considerably shortened shortened shortened by by an an increase increase by an increasein in temperature. temperature. in temperature. Herein, Herein, EquationHerein,Equation Equation (1) (1) is is reorganized reorganized (1) is reorganized as as Equation Equation as Equation (16) (16) to to determine determine (16) to determine the the constants constants the constants from from the the from fit fit line line the in fitin thelinethe graph graph in the ln( ln( graphtGt)G )vs. vs. ln( 1000/ 1000/tG) vs.TT graph 1000/graph asT as graphpresented presented as presented in in Figure Figure in 9b. 9b. Figure 9b. 111 11 lnln((ln( ))=ln =)=lnln (( ()−∆/))−∆/− ∆E/RT (16)(16)(16) tG t0 TheThe slope slope and and intercept intercept for for Equation Equation (16) (16) of of the the fit fit line line were were calculated calculated to to be be − −8.0058.005 KK and and 17.77, 17.77, respectively, respectively, which which correspond correspond to to the the pre-exponential pre-exponential time time constant constant of of 2 2 × × 1010−8− 8s sand and activation activation energy energy of of 66.5 66.5 kJ/mol. kJ/mol.

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Moreover, the changes in the temperature were associated with the alterations in in- terdiffusion kinetics by increasing the molecular mobilities indicated by a reduction in the viscosity of the resin as seen in Figure 9c [28,51]. In Figure 9c, it was found that the viscos- ity of UPR changed linearly with temperature in logarithmic scale and a small increase in temperature reduces the viscosity considerably which can promote the interdiffusion and result in thicker interphase. It is worth noting that to eliminate the effect of curing on the viscosity of the resin, initiator was not added to the resin in viscosity measurement tests [30]. These observations regarding the changes in cure kinetics via the gel time and resin viscosity by temperature clearly described the drop in the interphase thickness between 25 °C and 35 °C, i.e., the gel time decreased significantly in this region. On the other hand, after 35 °C the change in gel time took place at much lower levels. To elaborate, the gel time dropped approximately from 12500 s at 25 °C to 4000 s at 35 °C. This indicated that 8500 s less time was available for the polymer diffusion. The gel time difference between 40 °C and 50 °C was only about 900 s and it further declined from 50 °C to 60 °C as the gel time difference was obtained as approximately 360 s. On the other hand, at higher tem- peratures viscosity of the resin dropped to much lower values compared to the viscosity at lower temperatures. This low viscosity at high temperatures promoted the interdiffu- sion process with limited change in the gel time leading to a gradual increase in the inter- Materials 2021, 14, 291 11 of 17 phase thickness between 35 °C to 50 °C. Furthermore, the significantly shorter gel time at 60 °C compared to 50 °C is the reason to its lower interphase thickness. ) G [Sec] G ln (1/t ln Gel time t

(a) (b) 101

100 [Pa.s]

10-1 Viscosity Viscosity

10-2 20 40 60 80 100 Temperature [°C] (c)

FigureFigure 9. 9. (a()a )change change in in gel gel time time vs. vs. temperature, temperature, ( (bb)) ln(1/ ln(1/tGt)G vs.) vs. 1000/ 1000/T andT and fitted fitted line line based based on on Equation Equation (16), (16), and and (c ()c ) viscosityviscosity change change as as a afuncti functionon of of temperature temperature for for UPR. UPR.

3.3. DiffusivityThe slope Coefficient and intercept and Model for Equation (16) of the fit line were calculated to be −8.005 K −8 andThe 17.77, diffusivity respectively, of UPR which into correspond ABS was estima to the pre-exponentialted by using Equation time constant (4) and of the2 × results10 s obtainedand activation are shown energy in Figure of 66.5 10. kJ/mol. The inverse diffusion model resulted in an exact value for theMoreover, interdiffusion the thickness changes in as theseen temperature in Figure 10b were for the associated corresponding with the diffusivity alterations val- in ues.interdiffusion It can be seen kinetics in Figure by increasing 10a that thethe molecularfitted diffusivity mobilities was indicated found to by be a in reduction the range in of the viscosity of the resin as seen in Figure9c [ 28,51]. In Figure9c, it was found that the viscosity 10−12–10−10 (m2/s) for the temperature range of 25–60 °C. The estimated magnitudes of the of UPR changed linearly with temperature in logarithmic scale and a small increase in diffusivities matched with the reported diffusivities of other systems such as epoxy-PSU temperature reduces the viscosity considerably which can promote the interdiffusion and result in thicker interphase. It is worth noting that to eliminate the effect of curing on the viscosity of the resin, initiator was not added to the resin in viscosity measurement tests [30]. These observations regarding the changes in cure kinetics via the gel time and resin viscosity by temperature clearly described the drop in the interphase thickness between 25 ◦C and 35 ◦C, i.e., the gel time decreased significantly in this region. On the other hand, after 35 ◦C the change in gel time took place at much lower levels. To elaborate, the gel time dropped approximately from 12500 s at 25 ◦C to 4000 s at 35 ◦C. This indicated that 8500 s less time was available for the polymer diffusion. The gel time difference between 40 ◦C and 50 ◦C was only about 900 s and it further declined from 50 ◦C to 60 ◦C as the gel time difference was obtained as approximately 360 s. On the other hand, at higher temperatures viscosity of the resin dropped to much lower values compared to the viscosity at lower temperatures. This low viscosity at high temperatures promoted the interdiffusion process with limited change in the gel time leading to a gradual increase in the interphase thickness between 35 ◦C to 50 ◦C. Furthermore, the significantly shorter gel time at 60 ◦C compared to 50 ◦C is the reason to its lower interphase thickness.

3.3. Diffusivity Coefficient and Model The diffusivity of UPR into ABS was estimated by using Equation (4) and the results obtained are shown in Figure 10. The inverse diffusion model resulted in an exact value for the interdiffusion thickness as seen in Figure 10b for the corresponding diffusivity values. It can be seen in Figure 10a that the fitted diffusivity was found to be in the range Materials 2021, 14, 291 12 of 17

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of 10−12–10−10 (m2/s) for the temperature range of 25–60 ◦C. The estimated magnitudes of the diffusivities matched with the reported diffusivities of other systems such as epoxy- [26,52],PSU [26 amine-PSU,52], amine-PSU [26,52], [26 toluene-PEEK,52], toluene-PEEK (for 0% (for initial 0% initialcrystallinity crystallinity of PEEK) of PEEK) [53], CS2- [53], PEEKCS2-PEEK (for 0% (for initial 0% initial crystallinity crystallinity of PEEK) of PEEK) [53] [and53] andtoluene-PVAc toluene-PVAc (for (for mass mass fraction fraction of tolueneof toluene of 0.1) of 0.1)[54] [as54 ]seen as seen in Figure in Figure 10a. The10a. trend The trendof the oftemperature the temperature dependent dependent diffu- sivitydiffusivity was captured was captured well wellwith withthe temperature the temperature dependent dependent model model given given in Equation in Equation (5) 21 2 4 with(5) with =1.69×10D0 = 1.69 × 10m/sm and/s and =7.52×10E1 = 7.52 × cal/mol.10 cal/mol. The diffus Theivity diffusivity coefficient coefficient mag- nitudemagnitude obtained obtained and its and trend its trendwith temperature with temperature was in was conformity in conformity with available with available litera- ture.literature. The relation The relation between between the experimentally the experimentally obtained obtained viscosity viscosity and the and temperature the temperature de- pendentdependent diffusivity diffusivity obtained obtained by the by theinverse inverse diffusion diffusion model model is shown is shown in Figure in Figure 10c. 10It c.can It becan seen be seenthat thatthe diffusivity the diffusivity was wasfound found to be to inversely be inversely proportional proportional which which supported supported the aforementionedthe aforementioned observations observations regarding regarding the inte therdiffusion interdiffusion thickness thickness and andviscosity. viscosity. In ad- In ditionaddition the thetrend trend obtained obtained for forthe thediffusivity diffusivity vs viscosity vs viscosity also also matches matches with with the thefindings findings in [55]in [ 55for] forthe thediffusion diffusion of organic of organic molecules molecules in sucrose–water in sucrose–water solutions. solutions. m] /s] /s] 2 2 Diffusivity [m Diffusivity [m Diffusivity Interdiffusion thickness [ thickness Interdiffusion

(a) (b) (c)

FigureFigure 10. 10. ((aa)) Diffusivity Diffusivity vs vs temperature temperature for for UPR-ABS UPR-ABS in in comparison comparison to to material material pairs pairs from from literature, literature, ( (bb)) interphase interphase thicknessthickness change byby temperaturetemperature through through fitted fitted inverse inverse model, model, and and (c )( viscosityc) viscosity dependency dependency of theof the diffusivity diffusivity for UPR-ABS.for UPR- ABS. 3.4. Kinetics of Interdiffusion and Its Correlation to Interphase Microstructure 3.4. Kinetics of Interdiffusion and Its Correlation to Interphase Microstructure Figure 11a depicts the resin uptake per surface area (M*) vs. time at the different Figure 11a depicts the resin uptake per surface area (M*) vs. time at the different temperatures ranging from 25 ◦C to 60 ◦C. It can be seen that increasing the temperature temperatures ranging from 25 °C to 60 °C. It can be seen that increasing the temperature enhanced the resin uptake value for all the measurement times. A line fitting was employed enhanced the resin uptake value for all the measurement times. A line fitting was em- for the relation between Ln (M*) and Ln (t) seen in Figure 11b to determine k and n (see ployed for the relation between Ln (M*) and Ln (t) seen in Figure 11b to determine k and Equation (8)) and the results obtained are listed in Table1. It was observed that n value n (see Equation (8)) and the results obtained are listed in Table 1. It was observed that n changed with a change in temperature however it remained between 0.5 and 1 which value changed with a change in temperature however it remained between 0.5 and 1 indicated the anomalous nature of the diffusion process. More specifically, the value of n whichin Equation indicated (8) sharply the anomalous dropped nature from 0.7001 of the at diffusion 25 ◦C to 0.5717process. at 35More◦C andspecifically, thereafter the it valuefollowed of n a in gradually Equation decreasing (8) sharply trend dropped by an increasefrom 0.7001 in the at temperature25 °C to 0.5717 down at to35 0.5251 °C and at thereafter◦ it followed a gradually decreasing trend by an increase in the temperature 60 C. Herein, An and N0 as the constants in Equation (9) were calculated as 0.048 and 775 K, downrespectively to 0.5251 by at linear 60 °C. fitting Herein, on theAn and projection N0 as the of nconstantsvalues on in LnEquation (n) vs 1/ (9)T weregraph calculated as shown asin 0.048 Figure and 12 a.775 Similarly, K, respectivelyk values by at linear any given fitting temperature on the projection in our experimentalof n values on range Ln (n were) vs 1/T graph as shown in Figure 12a. Similarly, k values at any given temperature−5 in 2ourn evaluated by Equation (10) where k1 and k2 were calculated to be −4.13 × 10 gr/mm . s experimentaland 3.86 × 10 range−7 gr/mm were2 evaluated·sn. K by line by Equation fitting on (10) the kwherevs. T kgraph1 and k as2 were shown calculated in Figure to 12 beb. −4.13 × 10−5 gr/mm2. sn and 3.86 × 10−7 gr/mm2·sn. K by line fitting on the k vs. T graph as shown in Figure 12b.

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10-4 6 -7 10ABS-25°C-4 6 ABS-35°C -7 5 ABS-25°C ABS-40°C -8 ABS-35°C ] 5 ABS-50°C 2 4 ABS-40°C -8 ABS-60°C ) * -9 ] ABS-50°C 2 4

3 ABS-60°C ) * -9 [gr/mm Ln (M

* -10 3 ABS-25°C

M 2 [gr/mm Ln (M ABS-35°C

* -10 ABS-40°CABS-25°C

M 2 1 -11 ABS-50°CABS-35°C ABS-40°C 1 -11 ABS-60°C 0 -12 ABS-50°C 0 2000 4000 6000 8000 456789ABS-60°C 0 time [Sec] -12 0 2000 4000 6000 8000 456789Ln(t) (a) (b) time [Sec] Ln(t) FigureFigure 11. 11.( a()a) Resin Resin uptake( auptake) vs. vs. time time for for ABS, ABS, and and ( b(b)) Ln( Ln(MM*)*) vs. vs. Ln( Ln(t)t) based based on on Equations Equations(b) (7) (7) and and (8). (8). Figure 11. (a) Resin uptake vs. time for ABS, and (b) Ln(M*) vs. Ln(t) based on Equations (7) and (8). Table 1. Empirical rate constant (k), and transport exponent (n) for ABS-UPR at different tempera- Table 1. Empirical rate constant (k), and transport exponent (n) for ABS-UPR at different tempera- tures. tures.Table 1. Empirical rate constant (k), and transport exponent (n) for ABS-UPR at different tempera- Temperaturetures. (°C) 25 35 40 50 60 Temperature (◦C) 25 35 40 50 60 Temperaturen (°C) 0.700225 0.571735 0.553840 0.525450 0.5251 60 n 0.7002 0.5717 0.5538 0.5254 0.5251 k (gr/mmn 2·sn) 3.860.7002 × 10 −7 1.880.5717 × 10 −6 2.620.5538 × 10 −6 4.120.5254 × 10 −6 5.25 0.5251 × 10 −6 k (gr/mm2·sn) 3.86 × 10−7 1.88 × 10−6 2.62 × 10−6 4.12 × 10−6 5.25 × 10−6 k (gr/mm 2·sn) 3.86 × 10−7 1.88 × 10−6 2.62 × 10−6 4.12 × 10−6 5.25 × 10−6

] -n s ] -2 -n s -2 Ln(n) Ln(n) k [gr mmk k [gr mmk

(a) (b)

Figure 12. Line fitting on (a)( an) and (b) k changes by temperature to obtain predictive models(b) based on Equations (9) and (10).Figure 12. Line fitting on (a) n and (b) k changes by temperature to obtain predictive models based on Equations (9) and Figure 12. Line fitting on (a) n and (b) k changes by temperature to obtain predictive models based on Equations (9) and (10). (10). Table 2 summaries the φ vales calculated based on Equation (14) for ABS-UPR inter- Table2 summaries the ϕ vales calculated based on Equation (14) for ABS-UPR inter- phases obtained at different temperatures. It can be seen that the volume fraction of resin phasesTable obtained 2 summaries at different the temperatures.φ vales calculated It can based be seen on thatEquation the volume (14) for fraction ABS-UPR of resininter- at the interface decreases by increasing the process temperature. This can be explained by atphases the interface obtained decreases at different by increasing temperatures. the process It can be temperature. seen that the This volume can be fraction explained of resin by a difference in the progress of diffusion front line which was accelerated by an increase in aat difference the interface in the decreases progress by of increasing diffusion frontthe process line which temperature. was accelerated This can by be an explained increase inby the temperature while such an increase in temperature led to a faster curing and, there- thea difference temperature in the while progress such an of increasediffusion in front temperature line which led was to a accelerated faster curing by and, an increase therefore, in fore, lower diffusion time limiting the volume of diffused resin in the interphase area. lowerthe temperature diffusion time while limiting such thean increase volume ofin diffusedtemperature resin led in theto a interphase faster curing area. and, there- fore, lower diffusion time limiting the volume of diffused resin in the interphase area. Table 2. Volume fraction of UPR (φ) at the ABS/ UPR interphase obtained from Equation (14). Table 2. Volume fraction of UPR (ϕ) at the ABS/ UPR interphase obtained from Equation (14). TemperatureTable 2. Volume (°C) fraction 25 of UPR (φ) at 35the ABS/ UPR interphase 40 obtained 50 from Equation 60 (14). Temperature (◦C) 25 35 40 50 60 Temperature (°C) 0.40 25 0.34 35 0.26 40 0.21 50 0.20 60 ϕ 0.40 0.34 0.26 0.21 0.20 0.40 0.34 0.26 0.21 0.20

Materials 2021, 14, x FOR PEER REVIEW 14 of 17

3.5. Interphase’s Mechanical Response by Microhardness Figure 13a shows the variations of Vickers microhardness numbers in the vicinity of the interphase prepared at different temperatures. It was observed that the hardness num- bers at the interphase for all the samples were lower than for both pure UPR and ABS. It can also be seen that there was a transition region from the UPR to ABS (between 1000 µm and 1200 µm) as a sharp drop in the hardness value followed by a minimum value plateau indicating the interphase area and ended by an increase to ABS hardness value. The plat- Materials 2021, 14, 291 eau lengths were correlated with interphase thicknesses obtained by the microscopic14 ofob- 17 servations. The drop in the hardness at the interphase regions can be explained by the plasticizing effect of unreacted UPR monomers due to the limited access to reaction com- ponents3.5. Interphase’s at the interphase Mechanical area. Response It can by be Microhardness seen from Figure 13a that the interphase hardness is correlated with the processed temperature science a higher cure temperature results in Figure 13a shows the variations of Vickers microhardness numbers in the vicinity an increase in interphase hardness. of the interphase prepared at different temperatures. It was observed that the hardness To relate the microhardness at the interphase to the processing condition and micro- numbers at the interphase for all the samples were lower than for both pure UPR and structure of interphase, Figure 13b exhibits the averaged harness at the interphase (aver- ABS. It can also be seen that there was a transition region from the UPR to ABS (between age1000 ofµ m1200 and µm–1700 1200 µm) µm) as a and sharp volume drop infraction the hardness of resin value at the followed interphase by a(φ minimum) calculated value by Equationplateau indicating (14) on different the interphase processing area andtemperatures. ended by an It increasecan be seen to ABS that hardness samples value. prepared The atplateau higher lengths temperatures were correlated had a lower with φ interphaseand hence thicknessesa less resin presence obtained at by interphase the microscopic to act asobservations. a plasticizer Theand dropconsequently in the hardness this resulted at the in interphase a higher hardness regions canvalues be explainedcompared byto specimensthe plasticizing prepared effect at oflower unreacted temperatures. UPR monomers On the other due hand, to the samples limited prepared access to reactionat lower temperaturescomponents atshowed the interphase a lower hardness area. It candue be to seenhigher from volume Figure fraction 13a that of resin the interphase at the in- terphase.hardness Figure is correlated 13b provides with the a processedclear correlati temperatureon on the scienceprocess-microstructure a higher cure temperature and prop- ertiesresults relation in an increase at the interphase in interphase of ABS hardness. with UPR.

16 9 Average Interphase Hardness #, H v 0.4 Resin volume fraction v v 14 8.5 0.35 8 12 0.3 7.5 [] 10 0.25 25 °C 7 35 °C 8 40 °C 6.5 0.2 Vickers Hardness #, H Vickers 50 °C Hardness #, H Vickers 60 °C 6 6 0.15 0 500 1000 1500 2000 2500 20 30 40 50 60 L [ m] Temperature [°C] (a) (b)

Figure 13. ((aa)) Vickers Vickers microhardness microhardness values at the interphase interphase vicinity for UPR-ABS UPR-ABS interphase, interphase, and ( b) correlation correlation between interphase hardness andand resin volumevolume fraction.fraction.

4. ConclusionsTo relate the and microhardness Future Work at the interphase to the processing condition and mi- crostructureThe interphase of interphase, of a co-bonded Figure 13ABS-UPRb exhibits was the investigated averaged harness in this work at the by interphase critically correlating(average of the 1200 processingµm–1700 µconditions,m) and volume chemo-rheological fraction of resin behaviour, at the interphase resulting (ϕ microstruc-) calculated tureby Equation and mechanical (14) on different performance. processing The swelling temperatures. of ABS It with can be UPR seen was that identified samples preparedas a con- tributingat higher parameter temperatures to hadthe interphase a lower ϕ andsize. hence It was a lessobserved resin presencethat the atinterphase interphase size to changedact as a plasticizer with the processing and consequently temperature this resultedand it was in acorrelated higher hardness with thermal values changes compared in theto specimens viscosity and prepared cure kinetics at lower of temperatures. UPR through the On thegel time. other The hand, diffusivity samples of prepared UPR into at ABSlower was temperatures estimated by showed using the a lower 1D inverse hardness diffusion due to model higher in volume which the fraction experimentally of resin at characterizedthe interphase. temperature-dependent Figure 13b provides a cleargelation correlation time and on interdiffusion the process-microstructure thicknesses were and utilized.properties The relation diffusivities at the interphaseobtained were of ABS fit to with an Arrhenius UPR. relationship which can be used further for process simulations with various temperature ranges. More characterization 4. Conclusions and Future Work experiments must be carried out to validate the diffusivities obtained and the developed The interphase of a co-bonded ABS-UPR was investigated in this work by critically correlating the processing conditions, chemo-rheological behaviour, resulting microstruc- ture and mechanical performance. The swelling of ABS with UPR was identified as a contributing parameter to the interphase size. It was observed that the interphase size changed with the processing temperature and it was correlated with thermal changes in the viscosity and cure kinetics of UPR through the gel time. The diffusivity of UPR into ABS was estimated by using the 1D inverse diffusion model in which the experimentally characterized temperature-dependent gelation time and interdiffusion thicknesses were utilized. The diffusivities obtained were fit to an Arrhenius relationship which can be used further for process simulations with various temperature ranges. More characterization experiments must be carried out to validate the diffusivities obtained and the developed Materials 2021, 14, 291 15 of 17

diffusivity model. It was shown that the diffusivity was inversely proportional with the viscosity. The resin uptake experiments were employed to understand the kinetics of UPR diffusion into ABS at different temperatures. Accordingly, a resin uptake model was fitted to the experimentally obtained data by taking the processing temperature and cure kinetics of UR into account. The developed model was used to determine the volume fraction of the resin at the ABS-UPR interphase. The diffusivity coefficient obtained here is pro- vide substantial information for future studies on the process modelling of multi-material composites and complex shaped hybrid structures. Microhardness tests showed that the hardness of interphase was lower than both ABS and UPR regions due to the plasticization effect of UPR molecules at the interphase. A correlation was created between the processing condition, resin volume fraction and its microhardness at the interphase which can be used for designing such a hybrid MMCs in the future. In the current work, the effect of the normal force used in the swelling experiments with the rheometer on the polymer interdiffusion at the ABS-UPR interface was not taken into account which is considered as a future work. Similarly, the effect of processing temperature on the viscosity up to the gelation is also defined as future work because the change in viscosity might influence the interphase thickness which was assumed to be negligible in this study.

Author Contributions: Conceptualization, J.S.M.Z. and I.B.; methodology, J.S.M.Z. and I.B.; valida- tion, J.S.M.Z. and I.B.; investigation, J.S.M.Z. and I.B.; resources, I.B.; data curation, J.S.M.Z. and I.B.; writing—original draft preparation, J.S.M.Z. and I.B.; writing—review and editing, J.S.M.Z. and I.B; visualization, J.S.M.Z. and I.B.; funding acquisition, I.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the research program Talent Scheme (Veni) with project number 15897, which is (partly) financed by the Netherlands Organization for Scientific Research (NWO) and TKI-Wind op Zee Topsector Energy subsidy from the Ministry of Economic Affairs, the Netherlands with the reference number TEWZ118008. Data Availability Statement: The data presented in this study are available on request from the corresponding authors. The data are not publicly available due to legal and privacy issues. Acknowledgments: Authors gratefully acknowledge LM Wind Power for consultations during the project and supply of polyester resin. Conflicts of Interest: The authors declare no conflict of interest.

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