Improving impact, tensile and thermal properties of thermoset unsaturated polyester via mixing with thermoset vinyl ester and methyl methacrylate

Polymer Testing 81 (2020) 106193

Available online 24 October 2019

0142-9418/© 2019 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Material Properties

Improving impact, tensile and thermal properties of thermoset unsaturated polyester via mixing with thermoset vinyl ester and methyl methacrylate

Hairul Abral

^a,*

, Rahmat Fajrul

^a

, Melbi Mahardika

^a

, Dian Handayani

^b

, Eni Sugiarti

^c

, Ahmad Novi Muslimin

^c

, Santi Dewi Rosanti

^c

aLaboratory of Biocomposites, Department of Mechanical Engineering, Andalas University, 25163, Padang, Indonesia

bLaboratory of Sumatran Biota/Faculty of Pharmacy, Andalas University, 25163, Padang, Sumatera Barat, Indonesia

cLaboratory of High-Temperature Coating, Research Center for Physics Indonesian Institute of Sciences (LIPI) Serpong, Indonesia

A R T I C L E I N F O Keywords:

Toughness

Polyester/vinyl-ester blends Impact strength

A B S T R A C T

The brittleness of thermoset unsaturated polyester (UP) limits its usefulness in many applications. Use of appropriate additives may improve the toughness of this material. The aim of this present study is to characterize the mechanical and thermal properties of the toughened UP and to relate these properties to the fracture surface morphology of the UP before and after adding various loadings of thermoset vinyl ester (VE) with 10% methyl methacrylate (MMA). VE loadings chosen were 10, 20, 30, and 40 wt%. UP mixed with 30%VE and 10%MMA displayed the best performance with a maximum impact strength of 314 kJ/m², a 17.6% increase compared to that of neat UP. This sample also had the highest tensile strength of 64 MPa (an increase of 45.5%), higher elongation at the break of 13% (an increase of 27%), and higher thermal resistance. Addition of 30% VE and 10%

MMA significantly improves a range of important properties of UP. This blend has a high potential to be used in UP resin applications where toughness is required.

1. Introduction

UP resin is thermosetting and widely used as coatings and in an automotive, aeroplane, and ship construction [1]. This resin has rela- tively high tensile strength, is economical and has high chemical and water resistance [2]. However, UP has a low toughness, it is relatively brittle due to its rigid cross-linked polymer chain structure [3]. This weakness limits the commercial applications of UP resin. Many attempts have been made to improve the toughness of this material with fillers including water hyacinth fiber [4,5], pristine halloysite nanotubes [6], screw pine fiber [7], and rubbers [8]. Another approach to toughening UP is the reduction of rigidity by blending it with chemicals that disrupt the cross-linked UP chain structure [9]. Previous studies have explored this possibility using VE [9–13]. Blend with VE is a reasonable approach for disruption of the cross-linking of the UP chain network due to a similar chain structure between these thermosets [10]. As reported previously that addition of 40% VE resin in UP resin has been found to result in a 63.6% increase in tensile strength and 85.7% elongation at break of the blend [11]. Although there are many works on the effects of VE on toughening PU, there is hardly any information on how VE

dispersions modify the cross-linked UP chain structure which can be observed from the fracture surface morphology of the UP/VE blends.

Therefore the aim of this study is to characterize properties the VE toughened polyester resin and to relate the measured differences in the properties to the changes in polymer structures. In this present work, MMA was also used to disperse VE in UP matrix. Properties measured were the viscosity, tensile and impact properties, transparency and thermal resistance. Further characterizations conducted were Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM) observation of morphological fracture surfaces of tensile samples.

2. Materials and method 2.1. Materials

All materials for preparing matrix were purchased from Justus Sakti Raya Inc, Indonesia. They consisted of UP resin (YUKALAC BQTN 157- EX type, specific gravity 1.1 g/cm³ at 25 ^�C and viscosity 4.5–5.0 P at 25 ^�C), VE resin (RIPOXY R-802 EX-1, specific gravity 1.05 g/cm³ at

* Corresponding author.

E-mail address: abral@ft.unand.ac.id (H. Abral).

Contents lists available at ScienceDirect

Polymer Testing

journal homepage: http://www.elsevier.com/locate/polytest

https://doi.org/10.1016/j.polymertesting.2019.106193

Received 22 August 2019; Received in revised form 4 October 2019; Accepted 23 October 2019

25 ^�C), MMA, and methyl ethyl ketone peroxide (MEKP) as a catalyst.

2.2. Preparation of UP/VE blends and viscosity measurement

The UP, VE resin and MMA were mixed in the ratios shown in Table 1. This composition ratio was modified from previous works [9, 11,13]. Each mixture was homogenized using a magnetic stirrer (Daihan MSH-20D) for 10 min at 400 rpm and about 25 ^�C. The viscosity of each mixture was measured five times using a viscometer (NDJ-8S Digital Rotary Viscometer) at 60 rpm at room temperature for 12 s. 4% MEKP was added to the homogenized solution as a catalyst and the mixture stirred for a further 5 min. This liquid was then cast onto a silicon-based die for curing at about 25 ^�C for 72 h. The dimension of the die was made in accordance with ASTM D256-04 standard (64 mm (length) x 13 mm (thickness) x 13 mm (width), notched) for the impact test and ASTM D638-14 standard (Type I) for the tensile test [14,15]. Five test samples for each ratio of additives were prepared for characterization.

2.3. Characterizations 2.3.1. Transparency of sample

A spectrophotometer (Shimadzu UV 1800, Japan) was used for determining the transparency of the pure resin and UP/VE blends. All samples were cut into 10 mm �35 mm rectangles and placed in a spectrophotometer. ASTM D1003-00 was used as a testing standard [16]. Transmittances were measured at wavelengths from 400 to 800 nm. The process was repeated three times for each film.

2.3.2. FTIR analysis

An FTIR spectrometer (Frontier, PerkinElmer, USA) was used to characterize FTIR spectra of pure resin and UP/VE blends. The scanning was performed within the wavenumber range of 4000–600 cm ¹ with 4 cm ¹ resolution.

2.3.3. X-ray diffraction

The degree of crystallinity of a material can be determined with X- ray crystallography [17]. X-ray diffraction pattern of all samples was recorded using a PANalytical Xpert Pro diffractometer with Cu Kα ra- diation at 40 kV and 30 mA. All film samples were scanned between 2θ ¼10^�–60^�. The crystallinity index (CI) percentage was measured using Eq. (1) [18]:

CIð%Þ ¼ðIT IAÞ IT

x100 (1)

where IT is the intensity of the main peak at 2θ ¼19.5^�in Fig. 4, and IA is the lowest intensity before growing up the main peak.

2.3.4. Thermogravimetry analysis (TGA) and derivative (DTG)

The thermal properties of the samples; TGA and DTG were measured using a DTG-60 instrument from Shimadzu (Japan). The heating rate

was 10 ^�C/min with 50 mL/min nitrogen flow from 30 ^�C up to 550 ^�C.

2.3.5. FESEM observation

The fracture surfaces of the tensile samples were observed with JIB 4610 FE-SEM model from JEOL with an accelerating current of 5 kV and probe current of 8 mA to optimize observation of the surface morphology of the sample. The sample surface was coated with carbon followed by gold for 30 s.

2.3.6. Impact test

The impact strength of the sample was measured using equipment electronic impact testing machine (Chengde Precision Testing Machine CD, LTD. XC-5.5D, China) according to ASTM D 256–04 [15]. The impact test was repeated five times for each sample.

2.3.7. Tensile test

The procedure for tensile testing was similar to a previous study [19]. Tensile testing was performed using a Universal Testing Machine Gotech (GT-7001-LC-30) using a tensile test speed 4 mm/min to mea- sure tensile strength (TS), tensile modulus (TM) and elongation at break (EB). The samples were prepared according to the ASTM D638-14 standard, Type I [14]. Before the test, samples were conditioned for 48 h under 50 �5% relative humidity at 25 ^�C. Tensile tests were repeated five times for each sample.

2.3.8. Statistical analysis

The experiment data were analyzed using IBM SPSS Statistics 25.0 (IBM Corporation, Chicago, USA). Analysis of variance (ANOVA) and P- test were applied to identify the effect of various treatments on the tensile test, and impact strength of the pure resin, and UP/VE blends.

Detection of differences amongst the mean values of the film properties was based on Tukey’s multiple range test using a 95% confidence level (p �0.05). The measurements were replicated at least three times for each sample tested.

3. Results and discussion 3.1. Fracture surface

Fig. 1 displays the fracture surfaces of tensile samples. Neat UP (Fig. 1a) and VE resin (Fig. 1b) have smooth surfaces corresponding to their low toughness. This smooth appearance results from the rupture of rigid cross-linked chain network ahead of the crack leading to a fracture surface perpendicular to the tensile stress direction. A growing crack cleaves the polymer structure along the weakest atomic bonds. Adding MMA or/and VE to the UP results in a rougher fracture surface (Fig. 1c and d). This is attributed to nonhomogenous UP polymer chain network (see the yellow circle in Fig. 1e in region 1) resulting from the different chemical structure of the blended materials [10]. The blending resulted in a disruption of the UP’s chain network links leading to a decrease in the structural rigidity, and an increase in the number of the nano-sized voids (see yellow arrow in Fig. 1d). These voids result in a dimpled appearance of the fracture surface and become points of high-stress concentration from which cracks can initiate (see inset in Fig. 1f).

When subjected to a sudden load, many cracks can grow simultaneously forming beach marks where the cleavage from several cracks meets (see red arrow in Fig. 1d and g). The higher the number of these defects the rougher the surface. Fig. 1e shows the characterization of the fracture surface of UP/VE30%/MMA with three different regions; a smooth zone corresponding to slow crack growth rate (1); a transition zone, where the surface roughness steadily increases (2); a final failure zone with much rougher marks (3). Region 1 has the smoothest surface while Region 2 and 3 are much rougher corresponding to the crack propaga- tion mechanism in each region. The rougher appearance in Region 2 and 3 is caused by rapid radial crack growth radiating from nano-sized voids (yellow arrow) and resulting in beach marks. In contrast, the smooth Table 1

Composition of samples with abbreviations used.

Samples UP (mL) VE (mL) MMA (mL) MEKP (mL)

UP100% 100 – – 4

VE100% – 100 – 4

UP/VE10% 90 10 – 4

UP/VE20% 80 20 – 4

UP/VE30% 70 30 – 4

UP/VE40% 60 40 – 4

UP100%/MMA 100 – 10 4

VE100%/MMA – 100 10 4

UP/VE10%/MMA 90 10 10 4

UP/VE20%/MMA 80 20 10 4

UP/VE30%/MMA 70 30 10 4

UP/VE40%/MMA 60 40 10 4

surface in Region 1 is a result of slow tortuous crack growth along the zones of lowest rigidity in the disrupted cross-linked polymer chain structure. The size of Region 1 reflects the amount of plastic deforma- tion. The higher this zone the higher amount of plastic deformation with typical beach marks as seen in Fig. 1e, Region 1. The boundary between Region 1 and 2 (black arrows in Fig. 1e) was a result of the termination of slow speed crack growth. This boundary formed when the driving energy for growing crack from nano-sized voids had been exceeded.

With further loading, the crack growth rate increased until the final rupture occurred.

3.2. Viscosity

Average viscosity value for each studied sample is shown in Table 2.

Addition of MMA reduced the viscosity. Sample without MMA was more viscous than the sample with MMA. The decreased viscosity results in a more homogeneous mixture, and fewer number of micro- and nano- sized voids. The viscosity for the neat UP resin and UP/VE30% blends was 462.2 mPa s, and 464.8 mPa s decreased by 68.6, and 69.3%, respectively after addition of MMA of 10%. This is because the polymer chain density of sample without MMA is higher than that with MMA causing the chains slower to move. When the MMA was added into neat UP resin or UP/VE blends, this additive can be polymerized readily and incorporated into the thermoset chains leading to an increase in distance between themselves. Consequently, the thermoset polymer chains were more mobile due to their lower resistance to gradual deformation by shear stress or tensile stress [8].

3.3. UV–vis spectroscopy

Fig. 2 displays transparency of the pure resin, and the blends. UP transmits more light (61%) than VE (43.8%) at 800 nm. With the addition of MMA transparency increased. For instance, transparency of UP100%/MMA at 800 nm (74.4%) was 22% higher than UP100%. This is because of the homogenous structure of the resin after adding MMA.

The presence of this the reduction of viscosity due to MMA (Table 2) facilitated the migration of micro and nanovoids out of the liquid resin surface. With a decreased number of air voids to reflect and deflect light, the resin transmittance increased [20]. Homogeneous biopolymer structure in starch film has also been reported to lead to increased op- tical transparency [21,22].

3.4. FTIR spectroscopy

FTIR functional groups of studied samples without and with MMA are shown in Fig. 3a and b respectively. Peaks at 3440–3475, 2940–2946, and 1712 cm ¹ correspond to OH groups, CH aliphatic group, and C––O carbonyl bond respectively [23,24]. FTIR pattern can be used to monitor mixture homogeneity of VE in UP using shifts in the

Fig. 1. FESEM of the fractured surface of the tensile sample.

Table 2

The crystallinity index (CI from Fig. 4), the temperature at maximum decom- position (Tmax from Fig. 5) of samples without and with MMA.

Samples CI (%) Tmax (^oC) Viscosity (mPa.s)

UP100% 33 381.4 462.2 �2.7

VE100% 62 433.4 484.1 �4.6

UP/VE10% 28 403.6 484.1 �8.8

UP/VE30% 50 412.4 464.8 �5.1

UP/VE40% 36 418.0 486.9 �2.2

UP100%/MMA 31 385.4 145.1 �3.7

VE100%/MMA 55 432.4 176.8 �2.5

UP/VE10%/MMA 28 386.3 138.5 �0.3

UP/VE30%/MMA 38 407.3 142.6 �0.7

UP/VE40%/MMA 39 413.3 158.5 �0.9

transmittance (T) of the peaks [12]. As shown in Fig. 3a, pure VE dis- plays the peak of OH group at 3440-3475 cm ¹, and this peak is not present for pure UP. But, after adding the VE, the corresponding peak did not appear in the UP/VE30% sample probably indicating inhomo- geneous dispersion of VE in UP during sample preparation [25]. A similar result was also displayed by CH stretching at 2940 cm ¹ which did not change consistently with increased VE loading. In contrast, the samples with MMA showed a consistent decrease in the T value for both OH and CH functional group peaks as VE loading was increased (see Fig. 3b). This result suggests that MMA enabled the VE to be dispersed throughout the UP homogeneously.

3.5. XRD pattern

Fig. 4 shows XRD curve of samples without (a) and with (b) MMA. All samples show a similar pattern with mean intensity peak at 2θ ¼19^� corresponding to similar thermoset structures [10]. Height of these peaks is a measure of the crystallinity index (CI) of polymer [26]. Table 2 displays CI value for each sample. Pure VE has higher CI (62%) than pure UP (33%). After addition of MMA, CI for both VE and UP decreased, to 55% and 31% respectively. The CI of UP also decreased after adding 10%VE. These reductions in CI are evidence of MMA and VE disrupting of the three-dimensional network of polymer bonds (crosslinking) in UP and consequent reduction in the order and density of the cross-linked chain structure. Disruption probably occurs as MMA incorporates into thermoset polymer chain network leads to an increase in the distance among chains [22]. The structural disruption of the crosslinking in UP can also be the reason for changes molecular structure of UP due to the presence of VE. Interestingly, with each increased addition of VE i.e., 10, 30, and 40% into UP with MMA the crystallinity index increased (28, 38, and 39% respectively in Table 2). Again, this result suggests that the VE was dispersed homogeneously. However, this phenomenon did not happen for samples without MMA where homogeneous dispersion was unable to occur. For example, CI for UP/VE40% was 36% lower than UP/VE30% (50%). This result is consistent with the FTIR pattern (Fig. 3b).

3.6. Thermal analysis

Fig. 5 shows TG and DTG curves of samples without and with MMA.

Pure VE has the highest thermal stability probably attributed to the highest CI value. The temperature of maximum decomposition (Tmax) for each sample is shown in Fig. 5 (c, d) and Table 2. T_max for VE100%

was 433.4 ^�C higher than that of UP100% (381.4 ^�C) corresponding to the more abundant presence of the thermally stable aromatic benzene ring structure in VE than in UP. Hence, adding VE to UP increased the thermal properties of the blend. In this case, Tmax for UP100%MMA sample was only 385.4 ^�C, which increased to 413.3 ^�C (increased by 7%) after adding 40%VE. This result is consistent with the XRD curve (Fig. 4b) showing the increasing CI for higher VE loadings in UP mixed with MMA. A similar phenomenon has also been reported for improvement in thermal resistance of the unsaturated polyester after mixing with vinyl ester oligomer [9,27]. Tm for all UP/VE blends without MMA has higher values than those with MMA (see Table 2). This is probably because the presence of MMA disrupts the crosslinking network structure in UP, and reduces the polymer structural rigidity and, thus, decreases thermal stability.

3.7. Tensile strength

Fig. 6a and b shows the stress-strain curve of each sample without and with MMA and VE. Pure UP and VE stress-strain curves which are linear correspond to their brittle nature. This is consistent with a smooth fracture surface (Fig. 1a and b). In contrast, the UP/VE30%/MMA stress- strain curve start linearly but then curves downward close to maximum stress implying that plastic deformation has occurred (red arrow in Fig. 2. The transmittance of each sample without and with MMA.

Fig. 3. FTIR curves for samples mixed without (a) and with MMA (b).

Fig. 4. XRD patterns of the samples without (a) and with (b) MMA.

Fig. 5. TG of the sample without (a) and with MMA (b); DTG curve for the sample without MMA (c) and with MMA (d).

Fig. 6b). This sample had a fracture surface as shown in Fig. 1e (Region 1). With further tensile loading, the tensile strength of the sample decreased as crack growth rate increased. The appearance of the fracture surface for this sample is shown in Fig. 1e (Region 2). Finally, the crack propagates at a rapid rate to cleavage as shown in the fracture surface in Fig. 1e (Region 3). TS (64 MPa) and EB (13.3%) for UP/VE30%/MMA are higher than those for UP/VE30% (39.3 MPa and 10.5%). The pres- ence of MMA led to increases in the polymer chain flexibility resulting from the reduced polymer chain cross-linking in the UP structure. A similar result was also reported for increasing the TS of the UP resin after mixing with VE [11]. However, TS and EB values for 40% VE without or with MMA were lower. This is probably due to a disrupted structure as VE loading exceeds that which could be incorporated into the UP cross-linked network.

3.8. Izod impact strength

Fig. 7 displays the impact strength of each UP based sample as a function of MMA and VE. The addition of various VE loadings to UP did not result in a significant increase (p �0.05) on the impact strength of the UP blend as shown in Fig. 7a. Impact strength for the pure thermoset

and blends without MMA are relatively similar showing the low impact resistance that would be expected of a highly rigid cross-linked polymer structure. When subjected to a sudden load, the samples experience brittle fractures due to rapid crack growth through structural defects in UP. After adding 30%VE and 10%MMA, impact strength increased significantly (p �0.05) in comparison to pure UP. The highest impact strength was measured on UP/VE30%/MMA (314 kJ/m²), a 17.6% in- crease over UP resin. This result is in agreement with the tensile strength of this corresponding sample (Fig. 6b) which shows the most significant plastic deformation. A similar phenomenon has been reported for improvement of the toughness of the UP resin after adding fillers [28, 29]. Addition of 40% VE however reduced impact strength probably due to an increase in the disruption of UP polymer structure and the fraction of nano-sized porosity in UP as shown in Fig. 1d (Region 2).

4. Conclusion

This work reported successful fabrication of a tough UP blend and related its tensile properties to fracture surface morphology. The UP mixed with 30%VE and 10%MMA had the highest impact strength (an increase of 17.6%), the higher tensile strength (increased by 45.5%) and Fig. 6. Stress vs strain curves of samples (a, b) and the average value of TS (c, d), TM (e, f) and EB (g, h) without and with MMA. Different letters a, b, c, d, e in the vertical bar chart indicate significant differences at p �0.05.

Fig. 6. (continued).

Fig. 7. Impact strength of pure resin and UP/VE resin blends without (a) and with (b) MMA. Different letters a, b, c, in the vertical bar chart indicate significant differences at p �0.05.

the higher elongation at the break (an increase of 26.9%) compared to pure UP. Adding VE and MMA to the UP resulted in an increase in UP toughness as a result of disruption of the UP’s chain network links leading to a decrease in the structural rigidity, an increase in the fraction of plastic deformation zone and the number of the nano-sized voids. A plastic deformation zone displayed the smooth surface resulting from slow tortuous crack growth along the zones of lowest rigidity in the disrupted cross-linked polymer chain structure. After termination of the slow speed crack growth, the final rupture occurred due to rapid radial crack growth radiating from nano-sized voids.

Declaration of competing interest

The author declares that there is no conflict of interest.

Acknowledgement

Acknowledgement is addressed to Andalas University for supporting research funding with project name “Skim Klaster Hibah Riset Guru Besar”, grant number T/21/UN.16.17/PP.IS-KRP1GB/LPPM/2019.

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