Volume shrinkages and mechanical properties of various fiber-reinforced hydroxyethyl methacrylate-polyurethane/unsaturated polyester composites

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Volume shrinkages and mechanical properties

of various ﬁber-reinforced hydroxyethyl

methacrylate–polyurethane/unsaturated polyester composites

S.P. Lin

a

, J.H. Shen

a

, J.L. Han

d

, Y.J. Lee

c

, K.H. Liao

a

, J.T. Yeh

e

,

F.C. Chang

f

, K.H. Hsieh

a,b,*

a_{Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan} b_{Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan}

c_{Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei 106, Taiwan} d_{Department of Chemical and Materials Engineering, National I-Lan University, I-Lan 260, Taiwan}

e_{Department of Textile Science, Nanya Institute of Technology, Taoyuan 414, Taiwan} f_{Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu 300, Taiwan} Received 19 March 2007; received in revised form 28 August 2007; accepted 13 September 2007

Available online 1 October 2007

Abstract

In this study, two kinds of hydroxyethyl methacrylate–polyurethane (HEMA–PU) were used as volume shrinkage modifiers, and sev-eral hydroxyethyl methacrylate–polyurethane modified unsaturated polyester (HEMA–PU/St./UP) resins crosslinked with styrene were synthesized and characterized using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and in terms of their mechanical properties, including their tensile strengths and Izod impact energies. The properties of the HEMA–PU–modified St./ UP and St./UP were compared for potential applications as matrices for glass fiber-, aramid fiber-, and UHMWPE fiber-reinforced com-posites. The effects of the HEMA–PU content and the type of polyol in the HEMA–PU resin matrices of the composites were investi-gated through testing of their mechanical properties, bulletproof testing, and observations of their morphologies.

Keywords: A. Fibres; A. Polymer-matrix composites (PMCs); B. Mechanical properties; B. Infrared (IR) spectroscopy

1. Introduction

Although unsaturated polyester resins are used in many industrial applications because of their good mechanical properties, high processability, and high chemical resis-tance, they often undergo undesirably high volume shrink-age. To reduce the volume shrinkage of the matrix system, many researchers[1–3]have attempted to prepare modiﬁed polymer reins, for use as matrices in composites, from unsaturated polyester resins. Cao et al. [4,5] found that the addition of low-proﬁle additives (LPA) reduced the

UP resin’s shrinkage, but increased the residual styrene (St.) content; they also found that the use of an unsaturated polyester at a low MMA/styrene ratio led to a lower amount of styrene residue with almost unchanged shrink-age, whereas a high MMA/styrene ratio reduced the resid-ual styrene adequately but with poor volume shrinkage control. Li et al. [6–9] studied the low-temperature curing shrinkage control mechanism of LPA; they demonstrated that the shrinkage behavior of the resin mixture is depen-dent upon the competition between the shrinkage arising from polymerization and the expansion arising from microvoid formation. Huang et al. [10–12]found that the addition of LPA enhanced the degree of phase separation between LPA and a styrene-crosslinked polyester resin, such that microgel particles could be identiﬁed throughout

*

Corresponding author. Address: Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan. Tel.: +886 2 3366 5314; fax: +886 2 3366 5237.

E-mail address:[email protected](K.H. Hsieh).

www.elsevier.com/locate/compscitech Composites Science and Technology 68 (2008) 709–717

COMPOSITES

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the reaction; they also observed the morphologies of sty-rene-crosslinked polyester containing various LPA content. Zhang et al. [13]described how compatible LPAs yielded more-homogeneous matrices than did immiscible ones.

Fiber-reinforced polymer composites are used widely in material applications. Several kinds of ﬁbers have been applied as reinforcing materials, including carbon ﬁbers

[14,15], glass fibers [16–19], aramid fibers, and ultrahigh-molecular-weight polyethylene (UHMWPE) fibers[20,21]. In this study, we synthesized eight sets of HEMA–PU/ St./UP resin systems possessing various HEMA–PU con-tents and polyol types to investigate their volume shrink-ages and mechanical properties, including their tensile strengths and Izod impact energies, and compared them with those of traditional UP resins. For the manufacture of the fiber-reinforced composites, we incorporated four types of fiber reinforcement material (i.e., E- and P-type glass fibers, aramid fiber, and UHMWPE fiber) into two different HEMA–PU/St./UP matrices and then determined the bulletproof nature and mechanical properties of the systems, including their tensile strengths and elongations.

We obtained SEM images to observe the fracture behavior and wet-out properties of these composites. The bullet-proof properties of the UHMWPE fiber reinforced HEMA–PU/St./UP and aramid fiber reinforced HEMA– PU/St./UP composites of different thickness were also investigated in this study[22,23].

2. Experimental

The materials used are listed in Table 1. Two polyols, PBA 700 and PPG 400, were heated and degassed under vacuum overnight prior to use.

2.1. Preparation of Polyurethane prepolymer and HEMA–PU The preparation of PU prepolymer was mentioned else-where before [23,24]. The HEMA–PU polymer was pre-pared by reacting HEMA (2 equiv.) with PU prepolymer (1 equiv. based on PPG 400 or PBA 700). The molecular structures of the HEMA–PU polymers (based on PPG 400 or PBA 700) are presented below.

O C N NCO MDI + Polyester-type polyol HO H2 C O C O 4 H2 C 4C O O H2 C ₄ OH n

PU prepolymer (Polyester-type polyol)

2 H2C 1 Polyether-type polyol HO C 1 CH3 H H2 C O C CH3 H H2 C OH n or PBA type PPG type O C N H2 C N C O O H2 C O C O 4 H2 C 4C O O H2 C ₄ O n C O N NCO H2 C H H O C N H2 C N C O O OC O N NCO H2 C H H C CH3 H H2 C O C CH3 H H2 C n

Hard Segment Soft Segment Hard Segment

PU prepolymer (Polyether-type polyol)

PU prepolymer (Polyester-type polyol) O C N H2 C N C O O H₂ C O C O 4 H₂ C 4C O O H2 C 4 O n C O N NCO H2 C H H O C N H2 C N C O O O C O N NCO H2 C H H C CH₃ H H2 C O C CH₃ H H2 C n

Hard Segment Soft Segment Hard Segment

PU prepolymer (Polyether-type polyol)

N R1 N C O O R2 O C O N R1 N C O H H C O PU prepolymer

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2.2. Curing the HEMA–PU/St./UP (or St./UP reins) Methyl ethyl ketone peroxide (MEKPO, 5 phr, initiator) and the HEMA–PU/St./UP system which containing a particular HEMA–PU content (or the UP resin with vari-ous styrene contents) were mixed individually, and then poured into an aluminum mold coated with polytetraﬂuo-roethylene (PTFE) and pressed at 100 kg/cm2 and 60C for 1.5 h before post-cured at 150 kg/cm2 and 80C for 1.5 h.

2.3. Preparation of glass ﬁber-reinforced HEMA–PU/St./ UP and St./UP composites

Each 0.125-mm-thick glass ﬁber mat was impregnated with HEMA–PU/St./UP and St./UP resins containing var-ious St. contents. The impregnated mats were fabricated using eight plies; the dimensions of the compression mold were 200· 200 · 1 mm (length · width · thickness). The pre-pregs were stacked and compression-molded at 100 kg/cm2 and 60C for 1.5 h prior to post-curing at 150 kg/cm2and 80C for 1.5 h.

2.4. Preparation of UHMWPE and aramid ﬁber-reinforced HEMA–PU/St./UP composites

The preparation of UHMWPE and aramid ﬁber-rein-forced HEMA–PU/St./UP composites was described else-where[22,23]. The MEKPO initiator (5 phr) and HEMA– PU/St./UP systems with various HEMA–PU contents were chosen as the matrices in this study. Unidirectional ﬁber reinforced composites ([0]8) and cross-ply ones ([0, 90]2S)

were fabricated using eight plies of manufactured pre-pregs. For mechanical testing, the various composite specimens were cut into smaller sizes using a hydraulic power cutting machine (UHO Enterprise Corporation, Taiwan).

2.5. Testing methods

Infrared spectra were recorded using a BIO-RAD FTS-40 FT-IR spectrophotometer operated at 4 cm1

resolu-tion. Morphological studies were performed using a Tescan 5136 MM scanning electron microscope (SEM). The tensile strengths and Izod impact energies of the various HEMA– PU/St./UP and St./UP samples were measured according to the ASTM-638 and ASTM-256 protocols. The stress– strain properties of the various UHMWPE fiber-reinforced composites were measured according to the ASTM-D3039 protocol. The method of bulletproof testing was performed under National Institute of Justice ballistic standards (NIJ 0108.01 IIA and NIJ 0108.01 IIIA). The UHMWPE and aramid fibers were knitted using the Plain Weaves method of the Super Textile Corporation, Taiwan. Every 0.5-mm-thick UHMWPE and aramid fiber knit was impregnated with HEMA–PU/St./UP (20/20/60, weight ratio) resin. The impregnated knits were fabricated using 6, 10, and 20 plies; and the compression method was the same as men-tioned before.

3. Results and discussion 3.1. FTIR spectroscopic analysis

Fig. 1 displays the FTIR spectra of HEMA–PU(PBA 700) resin. A broad peak for the –OH groups

curing the HEMA-PU/St./UP (or St./UP Reins) 2 + H2C C H CH CH3 C O O H₂ C H₂ C OH R2 O O C O N H R₁ C O N H R1 N N H C O O H₂ C H C O O H₂ C O C O H C CH₃ C H CH2 H₂ C H₂ C O C O H C CH3 C H CH2 HEMA-PU Table 1 Experimental materials Designation Description UHMWPE ﬁber Spectra@_{900 Denier 1200}

Aramid ﬁber Technora@ T-200

Glass ﬁber 450 g/m2(E-type), 300 g/m2(P-type) (Taiwan Glass Industry Co., Taiwan)

PBA 700 (polyol)

Poly(tetramethylene adipate)glycol; MW = 700 (Tai Gin Co., Taiwan)

PPG 400 (polyol)

Poly(oxypropylene)glycol; MW = 400 (Tai Gin Co., Taiwan)

MDI 4,40_{-Diphenyl methane diisocyanate (TCI Chem.)} St. Styrene (Acros Organics)

HEMA Hydroxyethyl methacrylate (Acros Organics) UP Unsaturated polyester (Eternal Co., Taiwan) MEKPO Methyl ethyl ketone peroxide (Eternal Co., Taiwan) Acetone 2-Propanone (Acros Organics)

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(3500 cm1) of the polyol is evident inFig. 1a. The spec-trum of the sample obtained after mixing MDI (2 equiv.) and PBA 700 (1 equiv.) in a purged reaction kettle is pre-sented in Fig. 1b. During the middle stages of the reac-tion, the intensity of the signal of the –OH groups (3500 cm1) gradually disappeared, while that of the sig-nal for the –NCO groups (2270 cm1) reduced to half of its original value (based on the signal for the p-pheny-lene moieties at 840 cm1), as presented in Fig. 1c. Finally, HEMA (2 equiv.) was added to the reaction sys-tems, as indicated in Fig. 1d. During the ﬁnal stages, the peak intensity of the residual –NCO groups (2270 cm1) continued to reduce until ﬁnally it remained constant, as indicated in Fig. 1e.

3.2. Volume shrinkages and mechanical properties of various St./UP systems

The compatibility between the styrene and the unsatu-rated polyester was high because they both contain benzene rings. Thus, the addition of styrene acted as a diluting modiﬁer in the St./UP system; it diluted the viscosity of the neat unsaturated polyester resin, and enhances the mechanical properties by increasing the cross-linking den-sity in the ﬁnal St./UP networks.Table 2lists the volume shrinkages and tensile properties of St./UP systems con-taining various styrene contents.

As presented in Fig. 2, the values of volume shrinkage increased linearly upon increasing the styrene contents in

Fig. 1. FTIR spectra recorded at ﬁve diﬀerent reaction stages during the synthesis of PU prepolymer based on PBA 700: (a) Initially: PBA 700 only; (b) MDI added in PBA 700; (c) PU prepolymer based on PBA 700; (d) HEMA added in PU prepolymer; (e) Finally: HEMA–PU(PBA 700).

Table 2

Physical properties of UP resins with diﬀerent St. contents and HEMA–PU/St./UP resin systems with various HEMA–PU contents Modiﬁed UP resin systems PU content

(wt%)

Styrene content (%)

Tensile strength (MPa)

Izod impact energy (J/m) Volume shrinkage (%) Styrene/unsaturated polyester 0 12.6 13.7 52.0 4.1 0 19.6 15.1 53.5 5.9 0 26.4 17.3 54.5 6.7 0 30.5 23.3 57.7 8.2 0 40.0 25.3 58.2 11.3 0 44.8 25.2 58.3 12.1 0 54.9 21.3 56.6 13.6 HEMA–PU(PPG)/Styrene/unsaturated polyester 8.0 32.0 18.6 57.6 7.3 10.0 30.0 17.0 57.9 7.0 13.3 26.6 15.1 60.1 5.5 20.0 20.0 12.9 60.8 3.5 HEMA–PU(PBA)/Styrene/unsaturated polyester 8.0 32.0 24.0 58.2 7.1 10.0 30.0 14.1 58.8 6.9 13.3 26.6 12.6 63.0 5.6 20.0 20.0 5.0 82.3 2.9

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the St./UP system, and the regression equation, y = ax + b, had values of a and b of 0.2375 and 1.0802, respectively (R2=0.9815). Extending the regression equation to the Y axis (the styrene content = 0%), the value of b (1.0802%) represents the volume shrinkage of the neat unsaturated polyester. The positive slope value of a, increased upon increasing the styrene content. The tensile strength increased upon increasing the styrene content in the St./ UP system up to 40 wt%, but then it decreased to 21.3 MPa at a styrene content of 54.9 wt%. Thus, the opti-mal styrene addition for the St./UP system to provide the strongest material was 40 wt% (25.3 MPa). Thus, for subse-quent experiments, the HEMA–PU/St./UP resin materials were prepared with the modiﬁer (HEMA–PU/St.) content of 40 wt% in the unsaturated polyester resin.

3.3. Volume shrinkages and mechanical properties of various HEMA–PU/St./UP systems

Table 2 lists the volume shrinkages and mechanical properties, including tensile strengths and Izod impact energies, of HEMA–PU/St./UP systems containing vari-ous HEMA–PU contents and two types of polyol (PPG 400 and PBA 700). The values of tensile strength decreased upon increasing the HEMA–PU content in the HEMA–PU(PPG 400)/St./UP and the HEMA– PU(PBA 700)/St./UP systems. The HEMA–PU(PPG 400)/St./UP system exhibited higher values of tensile strength than did the HEMA–PU(PBA 700)/St./UP sys-tem when the HEMA–PU content was greater than 8 wt%. This situation arose presumably because the HEMA–PU(PBA 700)/St./UP system contained a greater number of soft segmental structures, causing the

cross-linking density in the HEMA–PU, styrene, and the unsaturated polyester network to be lower than that in the HEMA–PU(PPG 400)/St./UP system at the same HEMA–PU content.

The values of the volume shrinkage decreased upon increasing the HEMA–PU content in the HEMA–PU(PBA 700)/St./UP and the HEMA–PU(PPG 400)/St./UP sys-tems; these two types of HEMA–PU-modified St./UP resin systems exhibited seemingly identical volume shrinkages. The lowest volume shrinkages were 3.5% for the 20 wt% HEMA–PU(PPG 400)- and 2.9% for the 20 wt% HEMA–PU(PBA 700)-modified St./UP resin system. As mentioned above in the discussion of the St./UP system, the volume shrinkage of the neat unsaturated polyester resin was 1.0802%; therefore, these two polyol-containing 20 wt% HEMA–PU-modified St./UP resin systems pos-sessed highly improved volume shrinkage properties. It seems that the soft segmental structures of the two pol-yol-modified HEMA–PU systems had been dispersed uni-formly in the microvoids during the course of UP cross-linking, such that the HEMA–PU units reacted with the styrene and the unsaturated polyester resins to result in the formation of network structures during the cross-link-ing reaction.

In the HEMA–PU(PPG 400)/St./UP system, the Izod impact energy decreased to 57.6 J/m at an HEMA–PU content of 8 wt%, but increased to 60.8 J/m at an HEMA–PU content of 20 wt%, relative to that of the St./ UP (40/60 in weight ratio) (seeTable 2). This result is con-sistent with the SEM images of the fracture surfaces of the HEMA–PU(PPG 400)/St./UP systems. For the 10 wt% HEMA–PU-containing HEMA–PU/St./UP system as shown inFig. 3a, the fracture surface revealed many

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tinuous narrow shear band structures, indicating rapid crack propagation[22]. In the HEMA–PU/St./UP system containing 20 wt% HEMA–PU (Fig. 3b), the crack propa-gated from the surface across the cross-sectional area, and fracture surfaces revealed non-continuous, broader cracks. This ﬁnding suggests that the HEMA–PU(PPG 400)-modiﬁed St./UP systems exhibit better load bearing

charac-teristics. For the HEMA–PU(PBA 700)/St./UP system, the value of Izod impact energy increased from 58.2 J/m at an HEMA–PU content of 8 wt% to 82.3 J/m at an HEMA– PU content of 20 wt%, relative to that of the St./UP (40/60 in weight ratio) resin. This result is consistent with the SEM images inFig. 3c and d. At higher HEMA–PU content in the HEMA–PU(PBA 700)/St./UP systems, the

Table 3

Mechanical properties of various UHMWPE fiber-, Aramid fiber-, and glass fiber-reinforced HEMA–PU(PBA 700)/St./UP composites

Matrix Reinforcement Packing way Tensile strength (MPa) Elongation (%)

HEMA–PU(PBA 700)/St./UP = 20:20:60 UHMWPE ﬁber [0, 90]2S 330.8 31.9

[0]8 586.8 39.7

Aramid ﬁber [0, 90]2S 270.8 26.2

[0]8 526.5 28.5

Glass ﬁber A Mat type 102.4 21.3

Glass ﬁber B Mat type 74.8 20.6

HEMA–PU(PBA 700)/St./UP = 8:32:60 UHMWPE ﬁber [0, 90]2S 399.9 30.3

[0]8 739.6 33.1

Aramid ﬁber [0, 90]2S 314.8 15.3

[0]8 647.0 23.8

Glass ﬁber A Mat type 114.9 18.6

Glass ﬁber B Mat type 79.7 14.7

Fig. 3. SEM images of side-view fracture surfaces with various PU content in HEMA–PU/St./UP reins: (a) HEMA–PU(PPG 400) content = 10 wt%; (b) HEMA–PU(PPG 400) content = 20 wt%; (c) HEMA–PU(PBA 700) content = 10 wt%; (d) HEMA–PU(PBA 700) content = 20 wt%.

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fracture paths exhibited discontinuous, convoluted, and large feather-like features; the fracture surfaces that spread from the bottom upward are presumably responsible for the higher toughness and load bearing characteristics.

Thus, a lower HEMA–PU content (8 wt%) in the HEMA–PU/St./UP resin system led to a higher tensile strength, whereas a high HEMA–PU content (20 wt%) improved the Izod impact energy and led to a smaller vol-ume shrinkage for both types of polyol-containing HEMA–PU/St./UP systems. The HEMA–PU(PBA 700)/ St./UP systems exhibited superior values for their tensile and Izod impact energies relative to those of the HEMA– PU(PPG 400)/St./UP systems at both 8 and 20 wt%

HEMA–PU content. Thus, for manufacturing subsequent ﬁber-reinforced composite materials, we incorporated 8 and 20 wt% of HEMA–PU into the HEMA–PU(PBA 700)/St./UP systems.

3.4. Mechanical properties of various ﬁber-reinforced composites

Table 3lists the mechanical properties, including tensile strength, elongation, and Izod impact energy, of various glass fiber-, aramid fiber-, and UHMWPE fiber-reinforced HEMA–PU(PBA 700)/St./UP composites containing two different contents of HEMA–PU(PBA 700). The values

Table 4

Bulletproof tests of UHMWPE fiber/HEMA–PU/St./UP composites and Aramid fiber/HEMA–PU/St./UP composites System (fiber content; vol.%) Standard testing

method

Results Fiber weight/unit composite area (g/cm2)

Result photographs

Front face Rear face 3 mm UHMWPE ﬁber composite

(30.93 vol.%)

NIJ 0108.01 II A (380 m/s)

Penetrated 0.09

3 mm Aramid ﬁber composite (31.18 vol.%)

NIJ 0108.01 II A (380 m/s)

Penetrated 0.13

5 mm UHMWPE ﬁber composite (26.80 vol.%)

NIJ 0108.01 II A (380 m/s)

Penetrated 0.13

5 mm Aramid ﬁber composite (27.34 vol.%)

NIJ 0108.01 II A (380 m/s)

Penetrated 0.19

10 mm UHMWPE ﬁber composite (28.87 vol.%) NIJ 0108.01 III A (433 m/s) Not penetrated 0.28

10 mm Aramid ﬁber composite (28.78 vol.%) NIJ 0108.01 III A (433 m/s) Not penetrated 0.40

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of tensile strength and elongation of the UHMWPE fiber-and aramid fiber-reinforced HEMA–PU(PBA 700)/St./UP composites incorporating HEMA–PU content of 8 and 20 wt% were clearly higher than those of the two corre-sponding glass fiber-reinforced composites. These findings suggest that the UHMWPE and aramid fibers have poten-tial for application as next-generation fiber-reinforcement materials.

3.5. Bulletproof properties

Because the HEMA–PU(PBA 700)/St./UP (20/20/60 in weight ratio) system exhibited the lowest volume shrink-age and highest Izod impact energy, we manufactured UHMWPE fiber knit- and aramid fiber knit-reinforced HEMA–PU(PBA 700)/St./UP (20/20/60, weight ratio) composites and subjected them to tests of their bullet-proof nature. Table 4 lists the results of bulletproof test-ing of these two kinds of fiber knit-reinforced HEMA– PU(PBA 700)/St./UP (20/20/60, weight ratio) composites. From standard testing using the NIJ 0108.01 IIA method at thickness of 3 and 5 mm, neither the UHMWPE fiber knit/HEMA–PU/St./UP composite nor the aramid fiber knit/HEMA–PU/St./UP composite was bulletproof (i.e., they were penetrated). In contrast, from standard tests using the NIJ 0108.01 IIIA method at a thickness of 10 mm, both the UHMWPE fiber knit/HEMA–PU/St./ UP and aramid fiber knit/HEMA–PU/St./UP composites were bulletproof (i.e., they were not penetrated). The fiber weight per unit composite area played an important role in affecting the bulletproof nature. For example; using the NIJ 0108.01 IIIA method at a thickness of 10 mm, the fiber weight per unit composite area of 10 mm thick-ness composites were greater than other two smaller thickness (3 and 5 mm), and they could passed the bul-let-proof testing via produced the deformation and de-limination of composites to adsorb the high-speed impact energy.

4. Conclusion

The optimal addition of styrene to the St./UP system to provide the strongest material was 40 wt%, albeit with a high volume shrinkage. After the addition of two kinds of HEMA–PU as volume shrinkage modiﬁers for the St./ UP system, we found that a lower HEMA–PU content (8 wt%) led to a higher tensile strength, whereas a higher HEMA–PU content (20 wt%) increased the Izod impact energy and decreased the degree of volume shrinkage in both types of polyol-modiﬁed HEMA–PU/St./UP systems. The HEMA–PU(PBA 700)/St./UP system exhibited higher values of tensile and Izod impact energies relative to those of the HEMA–PU(PPG 400)/St./UP system at both 8 and 20 wt% HEMA–PU addition.

The UHMWPE ﬁber- and aramid ﬁber-reinforced HEMA–PU(PBA 700)/St./UP composites displayed

clearly higher values of tensile strength and elongation than did the two corresponding types of glass fiber-rein-forced composites. Thus, it seems that the UHMWPE and aramid fibers have the potential for application as next-generation fibers reinforcement materials. The mechanical properties of the UHMWPE fiber- and ara-mid fiber-reinforced composites were different, suggesting that these two kinds of fibers were suited to applications in different fields.

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