Mechanical properties of nano-MMT reinforced polymer composite and polymer concrete

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Construction and Building

MATERIALS

Construction and Building Materials 22 (2008) 14–20

www.elsevier.com/locate/conbuildmat

Mechanical properties of nano-MMT reinforced polymer composite and polymer concrete Byung-Wan Jo

a,1

, Seung-Kook Park

b,*

, Do-Keun Kim

a

a

b

Department of Civil Engineering, 2303 Structural Engineering Laboratory, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea Korea Research Institute for Construction Policy, Specialty Construction B/D, 14F, 395-70 Sindaebang-dong, Dongjak-gu, Seoul 156-714, Republic of Korea Received 15 December 2006; received in revised form 12 February 2007; accepted 23 February 2007 Available online 19 April 2007

Abstract Unsaturated polyester (UP) resin is widely used for the matrix of composites such as fiber reinforced plastic (FRP) and polymer concrete. Consequently, inexpensive and high performance resins are important for the future of polymer composites. One recent method for enhancing the performance of polymer composites is the manufacture of MMT (montmorillonite)-UP nanocomposite synthesized by intercalating the UP resin into the silicate layers of MMT. This study investigates the mechanical and thermal properties of MMTUP nanocomposites, and those of polymer concretes using these nanocomposites. Test results indicate that the mechanical properties and thermal stability of MMT-UP nanocomposites are better than those of pure UP. The glass transition and main chain decomposition temperatures of the MMT-UP nanocomposite exceed those of pure UP. The compressive strength, elastic modulus, and splitting tensile strength of the polymer concrete using MMT-UP nanocomposites exceeded those of polymer concrete using pure UP. Also, the polymer concrete made with MMT-UP nanocomposite has better thermal performance than that of pure UP. The improved performance of UP is very important for the future of polymer concrete.  2007 Elsevier Ltd. All rights reserved. Keywords: Polymer concrete; Unsaturated polyester resin; MMT-UP nanocomposites; Mechanical properties

1. Introduction Polymer composites are increasingly considered as structural components for use in civil engineering due to their excellent strength-to-weight ratios. Due to its excellent adhesion properties, unsaturated polyester (UP) resin is widely used for the matrix of composites such as FRP and polymer composites. However, compared to other resins, unsaturated polyester (UP) resin has relatively poor mechanical properties, thermal stability, and fire retardant properties, which limits its use in advanced composites. The modification of polymers is of considerable significance from a material science and engineering point of *

1

Corresponding author. Tel.: +82 2 2220 0327; fax: +82 2 2292 0321. E-mail address: [email protected] (S.-K. Park). Tel.: +82 2 2220 0327; fax: +82 2 2292 0321.

0950-0618/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.02.009

view. The performance of unsaturated polyester (UP) resin may be enhanced by the addition of inorganic fillers [5,7]. Conventional particulate polymer composites, often called filled polymers, are of significant commercial importance as materials in industrial applications. Polymer nanocomposites are a new class of composites derived from nano-scale inorganic particles. Their dimensions typically range from 1 to 1000 nm and they are homogeneously dispersed in the polymer matrix. Owing to the high aspect ratio of the fillers, the mechanical, thermal, flame retardant and barrier properties of polymers may be enhanced without a significant loss of clarity, toughness or impact strength. The layered silicate is generally made organophilic by exchanging the inorganic cation, which is located between the layers (d-spacing), with an organic ammonium cation. Clay–polymer composites can be classified into three types: conventional composite, intercalated nanocomposites and

B.-W. Jo et al. / Construction and Building Materials 22 (2008) 14–20

- 1nm

Sorbed cations, H2O

Fig. 1. The oxygen framework (solid circles) of smectite clay nanolayers.

exfoliated nanocomposites (Fig. 1). In a conventional composite the registry of the clay nanolayers is retained when mixed with the polymer, but there is no intercalation of the polymer into the clay structure (see Fig. 2a). Consequently, the clay fraction in conventional clay composites plays little or no functional role and acts mainly as a filling agent for economic considerations. An improvement in modulus is normally achieved in a conventional clay composite, but this reinforcement benefit is usually accompanied by a sacrifice in other properties, such as strength or elasticity. Two types of clay–polymer nanocomposites are possible [1,3,6]. Intercalated nanocomposites (Fig. 2b) are formed when one or a few molecular layers of polymer are inserted into the clay galleries with fixed interlayer spacings. Exfoliated nanocomposites (Fig. 2c) are formed when the silicate nanolayers are individually dispersed in the polymer matrix, where the average distance between

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segregated layers is dependent on the clay loading. The separation between the exfoliated nanolayers may be uniform (regular) or variable (disordered). Exfoliated nanocomposites show greater phase homogeneity than intercalated nanocomposites. More importantly, each nanolayer in an exfoliated nanocomposite contributes fully to interfacial interactions with the matrix. This structural distinction is the primary reason why the exfoliated clay state is especially effective in improving the reinforcement and other performance properties of clay composite materials. The key to the extraordinary performance of polymer– clay nanocomposites is dependent on the complete dispersal (exfoliation) of the clay nanolayers in the polymer matrix. The structure of the montmorillonite clay used as the filler comprises an octahedral alumina sheet sandwiched between two tetrahedral silica sheets. Alkylammonium ions lower the surface energy of the clay so that monomers and polymers with different polarities can enter the space between the layers and cause further separation of the silicate layers to form the nanocomposite [2,4]. The objective of this study is to enhance the performance of polymer composites using unsaturated polyester (UP) resin based recycled PET (poly ethylene terephthalate) [8]. Therefore, this work investigates the mechanical properties and thermal stability of MMT-UP nanocomposites and polymer concrete using the MMT-UP nanocomposite. The results are supported by mechanical testing, X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermo gravimetric analysis (TGA). 2. Research significance This study contributes to the understanding of the properties of MMT-UP nanocomposite and polymer concrete using MMT-UP nanocomposite as follows.

polymer

polymerization

a)

Conventional Composite

b)

Intercalated Nanocomposite

c)

Exfoliated Nanoomposite

Layered clay mineral monomer

Fig. 2. Schematic illustrations of the structures.

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B.-W. Jo et al. / Construction and Building Materials 22 (2008) 14–20

1. It proves possible the manufacture of a higher grade polymer concrete using nano-MMT. 2. It posits methods for enhancing the performance of polymer concrete with the addition of nano-MMT. 3. It suggests that polymer concrete made with recycled PET and nano-MMT may be useful materials for producing polymer concrete products.

plasticity and cohesiveness. The better gradation obtained with calcium carbonate also resulted in a hardened material with improved strength properties and surface appearance. The properties of the aggregate and resin are shown in Tables 2 and 3, respectively.

3. Experiments

There are two steps for manufacturing the UP-MMT nanocompostite. First, in the mixing process, the UP linear chains are mixed with styrene monomers and layered silicate. Second, in the curing process, the crosslinking reaction is started by decomposing the initiators. The unsaturated polyester chains, styrene monomers and nano-MMTs were mixed for 3 h at 60 C. The weight percentages of MMT in UP-MMT nanocomposite used were 2%, 5%, 8% and 10%, respectively. The mixture was then cooled to room temperature. 1% by weight of initiator (MKPO) was added and the mixture was stirred for 2 min. The mixture was poured into molds, cured at room temperature for 12 h and post-cured at 120 C for 4 h. X-ray diffraction (XRD) patterns were obtained using a Rigaku X-ray diffractometer equipped with CuKa radiation and a curved graphite crystal monochromator. Samples were prepared by applying the pre-intercalated mixture and nanocomposite of UP-MMT in sheet form on a slide. All XRD data were collected with an X-ray gen˚ ). Bragg’s law (k = 2d/sin h) was used erator (k = 1.5406A to compute the crystallographic d-spacing. In order to evaluate the change in the glass transition temperature, Tg, associated with increases in the MMT content, a differential scanning calorimeter (DSC) analysis was carried out using a General V4.1C DuPont 2000. The measurement was carried out from 30 C to 300 C using a heating rate of 10 C/min in a nitrogen atmosphere. The thermal behavior was determined with a thermogravimetric analyzer (TGA). Microscopic investigation was performed with a transmission electron microscope (TEM) with an acceleration voltage of 100 kV.

3.1. Materials Three different kinds of MMT were used. Southern Clay Products Inc., USA, supplied non-treated Na+-MMT and organophilic treated MMT under the trade names of Cloisite 30B and 25A. Cloisite 30B is a montmorillonite modified with methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium chloride; Cloisite 25A is a montmorillonite modified with dimethyl, dehydrogenated tallow, 2-ethylhexyl, quaternary ammonium chloride. Table 1 shows some of the manufacturer’s data on these MMTs. The unsaturated polyester resin based on recycled plastic (PET) was used as the matrix [8]. A styrene content of 40% in unsaturated polyester was chosen for its low viscosity (1300 mPa s at 25 C) and to achieve improved resin diffusion into the galleries of the MMT. To start the curing process, 1% (by weight of resin) of 10.7% active oxygen methylethy ketone peroxide initiator and 0.1% (by weight of resin) of 8% solution cobalt octoato promoter (used as an accelerator) were added to the resin. The following coarse and fine inorganic aggregates were used in the experimental study of polymer concrete: 8 mm pea gravel; siliceous river sand with a fineness modulus of 2.48, and CaCO3 (calcium carbonate). The aggregate was oven-dried for a minimum of 24 h at 200 C to reduce its moisture content to less than 0.3% by weight, thus ensuring a perfect bond between the polymer matrix and the inorganic aggregates. The use of calcium carbonate greatly improved the workability of the fresh mix. The fine and spherical calcium carbonate particle provided the fresh mix with better lubricating properties, thus improving its Table 1 Properties of modified montmorillonite Properties

Na+

Cloisite 30B

Cloisite 25A

Organic modifier Specific gravity (g/cc) % Weight loss on ignition (%) ˚) X-ray results (d001) (A

None 2.86 7 11.7

MT2EtOH 1.87 34 18.5

2MHTL8 1.98 30 18.6

MT2EtOH (methyl, tallow, bis-S-hydroxyethyl, quaternary ammonium), 2MHTL8 (dimethyl hydrogenated tallow 2-ethylhexyl ammonium).

3.2. UP-MMT Nanocomposites

3.3. Polymer concrete using UP-MMT Nanocomposites Tensile tests were performed according to ASTM D638M-91a at a crosshead speed of 5 mm/min. The polymer concrete cylinders used for compression and splitting tensile tests were 76 mm in diameter and 152 mm in length. Specimens were tested in a hydraulic load machine at a constant loading rate of 44,500 N/min. The mix design of polymer concrete, proportioned by weight, was as follows: 11% resin (MMT-UP), 45% oven-dried coarse aggregate, 33% oven-dried sand, and 11% CaCO3. The compressive

Table 2 Properties of the aggregates Type

Size

Specific gravity

Fineness modulus

Absorption (%)

Coarse aggregate Fine aggregate

Maximum size 8 mm 0.1–0.6 mm

2.63 2.60

6.42 2.48

0.08 0.05

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Table 3 Unsaturated polyester resin formulation Components

Recycled PET

Propylene glycol, diethylene glycol, dipropylene glycol

Terephthalic acid, maleic anhydride

Styrene monomer (SM)

Percentage by weight (%)

29.1

16.0

14.9

40.0

4. Results and discussion 4.1. UP-MMT Nanocomposite Silicate layer dispersion in the MMT-UP nanocomposite was analyzed by XRD. As shown in Fig. 3, the XRDs of MMTs and MMT-UP composites investigated different peak with the types of MMTs. The peaks for Na+, Cloisite 25A, Na+-UP and Cloisite 25A-UP nanocomposite are shown at 7.5, 3.5, 5.2 and 2.6, respectively. These 2h values correspond to interlayer spacings of 11.7, 18.6, 17.0 and ˚ , respectively. A new peak was observed in the Na+34.6 A UP composite. This indicates that UP by polymerization is intercalated between the MMT layers. However, for the

(f) 30B - UP 18.5

Relative Intensity

modulus of elasticity was first obtained using a compressometer with a 76-mm gauge length using two diametrically opposite sides. The compression elastic modulus was calculated where the stress was 40% of the maximum strain on the stress–strain (load–deflection) graph. Flexural specimens were mixed and compacted in a steel mold with dimensions of 50 · 50 · 305-mm. The beams were loaded in third-point loading at a uniform rate of 2225 N/min. The specimens were cast, cured, and tested at room temperature. Testing of the specimens was performed at 7 days. Tests were performed to determine the effect of temperature on the PC compressive strength, splitting tensile strength, modulus of elasticity, and flexural strength. After curing, specimens were put in an environmental chamber at the desired temperature for a period of 2 days prior to testing. Selected temperatures were 15 C, 25 C, and 65 C. Actual testing was performed at room temperature immediately after removing the specimens from the environmental chamber.

(e) 30B 34.6 (d) 25A - UP 18.6 (c) 25A 17.0

(b) Na + - UP 11.7

2

4

6

8

(a) Na +

10

12

Degrees (2Θ) Fig. 3. XRD data for MMT-UP nanocomposites with MMT contents of 5%: (a) Na+, (b) Na+-UP composite; (c) Cloisite 25A-UP composite, (d) Cloisite 25A-UP composite; (e) Closite 30B, (f) Cloisite 30B-UP composite.

Cloisite 30B-UP composite, the peak at the lower angle disappeared, suggesting that either the silicate layer platelets were exfoliated in the polymer matrix or they disappeared because the spacing between the layers was too large. It is important to note that polymerization of Na+-UP and Cloisite 25A-UP composite led only to an intercalated structure, while Cloisite 30B promoted the delamination process of layered silicates to achieve exfoliation. More direct evidence for the formation of a nanocomposite is provided by the TEM. The dark lines in the TEM image in Fig. 4b are individual silicate layers. In the case of the MMT (Cloisite 30B) layers, some irregular dispersions exist in the silicate layer. Also, the relatively exfoliated and well-dispersed portion of the nanocomposite was

Fig. 4. Transmission electron micrographs (TEM) of MMT-UP nanocomposite: (a) Na+-UP, (b) Cloisite 30B-UP.

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B.-W. Jo et al. / Construction and Building Materials 22 (2008) 14–20 88 UP Na+ -UP 25A-UP 30B-UP

4000

87 86 85

3500

84

Tg (ºC)

Tensile modulus (Mpa)

4500

3000 2500

83 82 81 80

2000 1500 0%

79 78

2%

4%

6%

8%

10%

12%

77

MMT contents (%)

-1

1

3

5

7

9

11

MMT contents (%)

Fig. 5. Tensile modulus of nanocomposites with MMT contents.

made with Closite 30B, as shown in Fig. 4b. However, the MMT (Na+) layers shown in Fig. 4a are more regular and some of the silicate layers maintained their original order. With the addition of the MMT, the tensile modulus of the composites shown in Fig. 5 increases up to 5% loading since MMT is more rigid than the matrix resin. The modulus of the Cloisite 30B and Cloisite 25A composites exceed that of the Cloisite Na+ composite owing to their higher degree of exfoliation and better adhesion at the MMT-UP interface. Above 5% MMT content, the tensile modulus starts to decrease with MMT content in both types of composites, due to a lower degree of exfoliation and a lower degree of polymer–MMT surface interactions at higher MMT content. For the Cloisite Na+ composite, the increase in the tensile modulus is not significant compared to the tensile modulus of pure unsaturated polyester. This is because there is little or no intercalation/exfoliation between the silicate layers of the Cloisite Na+ composite, so these materials act as conventional composites, especially at high MMT contents. Also, the cross-link density might be lower with a higher MMT content, leading to a lower modulus. The increase in tensile strength associated with increases in MMT content is demonstrated in Fig. 6. The variation in tensile strength of the composite with MMT contents is sim-

Fig. 7. Glass transition temperature of Cloisite 30B-UP nanocomposite.

ilar to that of the tensile modulus. The maximum tensile strength emerged at 5% MMT content. Work done on the thermal properties of polymers has shown that the glass transition of polymer–MMT nanocomposites increases with increasing MMT content. The effect of nano-MMT (Closite 30B) content on the Tg is shown in Fig. 7. The Tg increases with increasing of MMT (Closite 30B) content. This implies improved adhesion between the UP and MMT surfaces. Also, the nanoMMT prevents segmental motions of the polymer chains. It is known that the primary factor affecting the Tg of cured UP is the crosslinked density in the same UP resin. Therefore, it can be concluded that the UP-MMT nanocomposite has a high crosslinking density. However, beyond a certain MMT content (approximately in the range of 5–7%), Tg decreases with increasing MMT content. Thus, the crosslinking density might decrease at a high MMT content. TGA curves of the pure UP and the Closite 30B-UP nanocomposite are shown in Fig. 8. The onset of degradation is slightly but progressively hastened in the pure UP compared to the Closite 30B-UP nanocomposite. Thermal degradation of pure UP and MMT-UP has three distinct

120 100

100

Tensile strength (MPa)

90 80

residual weight (%)

UP Na+ -UP 25A-UP 30B-UP

70 60

pure UP 30B-UP

80 60 40 20

50

0

40

-20

30 20 0%

0

200

400

600

800

1000

Temperature (ºC) 2%

4%

6%

8%

10%

12%

MMT contents (%) Fig. 6. Tensile strength of nanocomposites with MMT contents.

Fig. 8. TGA thermal degradation profiles for Cloisite 30B-UP nanocomposites and pure UP Samples were heated from 23 C to 1000 C in nitrogen.

4.2.2. The effect of temperature on strength and modulus of elasticity The effects of temperature on the compressive strength, modulus of elasticity, and splitting tensile and flexural strength of the polymer concrete using UP-MMT nanocomposite are shown in Figs. 9 and 10. The modes of failure of polymer concrete differed depending on the temperature at which the materials were tested. Compression cylinders had a sudden, brittle failure when tested at 15 C and 25 C. Conversely, cylinders tested at 65 C had a slow, ductile failure resulting in an excessive bulging of the specimens. This behavior arises from decreases in the modulus of the resin binder in the polymer concrete specimens under increasing temperature. That is, the modulus of the polymer concrete specimen decreases with increases in temperature, as shown in Fig. 9. Increase in temperature effected a loss in strength and modulus of elasticity in the polymer concrete specimens because the resin binder decreased in strength with an increase in temperature. In the case of the polymer concrete

50

100

40

80

30

60

20

40

10

0

Modulus of elasticity of PC using pure UP Modulus of elasticity of PC using MMT-UP nanocomposite Compressive strength of PC using pure UP Compressive strength of PC using MMT-UP nanocomposite

-15 ºC

25 ºC

20

0

65 ºC

Temperature Fig. 9. Compressive strength and elastic modulus of polymer concrete using UP- MMT (Closite 30B) nanocomposite.

4.2. Effect of UP-MMT (Closite 30B) nanocomposite on polymer concrete

45

Strength (MPa)

4.2.1. Testing temperature 25 C The strength of polymer concrete specimens cast with Cloisite 30B-UP nanocomposite containing 5% MMT content was estimated. The compressive strength, elastic modulus, and splitting tensile strength of polymer concrete using Cloisite 30B-UP nanocomposite exceeded the corresponding properties of polymer concrete using pure UP, suggesting that the use of exfoliated MMT-UP nanocomposite enhances polymer concrete strength. The flexural strength of the polymer concrete does not significantly increase with the use of the Cloisite 30B-UP nanocomposite. The compressive strength, elastic modulus, and splitting tensile strength of polymer concrete were found to be correlated with the tensile strength and tensile modulus of the MMT-UP nanocomposite. However, the flexural strength of the polymer concrete was not significantly correlated with the tensile strength and tensile modulus of the MMT-UP nanocomposite.

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Compressive strength (MPa)

steps. The first is the decompostition of relatively weak head-to-head linkages, impurities, and styrene monomers in the UP. The second is the decomposition of the UP chain-end, and the third is the decomposition of the UP main chains. The three degradation steps occurred at 161 C, 272 C, and 321 C in the pure UP and at 224 C, 326 C, and 408 C in the MMT-UP nanocomposite. The temperature of the main chain decomposition of the Cloisite 30B-UP nanocomposite exceeds that of the pure UP by about 80 C. Pure UP is completely decomposed at 400 C. The nanocomposites show slower degradation above 400 C since only inorganic MMT is left in the system at that stage. This demonstrates that the MMT-UP nanocomposite has better thermal stability than pure UP.

Modulus of elasticity (GPa)

B.-W. Jo et al. / Construction and Building Materials 22 (2008) 14–20

Splitting tensile strength of PC using pure UP Splitting tensile strength of PC using MMT-UP nanocomposite Flexur al st re ngth of PC us ing pure UP Flexural strength of PC using MMT-UP nanocomposite

35

25

15

5 -15 ºC

25 ºC

65 ºC

Temperature Fig. 10. Splitting tensile and flexural strength of polymer concrete using UP-MMT (Closite 30B) nanocomposite.

specimens using pure UP, an increase in temperature from 25 C to 65 C decreased the compressive strength by about 33%, modulus of elasticity by about 36%, splitting tensile strength by about 31%, and flexural strength by about 38%. In the case of the polymer concrete specimens using UP-MMT nanocomposite, an increase in temperature from 25 C to 65 C decreased the compressive strength by about 18%, modulus of elasticity by about 22%, splitting tensile strength by about 18%, and flexural strength by about 22%. This result demonstrates that the polymer concrete made with MMT-UP nanocomposite has mechanical properties that are better than those of pure UP. The improved performance of UP is very important for the future of polymer concrete. Therefore, the enhancement of the mechanical and thermal performance of polymer concrete afforded by the use of nano-MMT is remarkable. 5. Conclusions The main objective of this study was to enhance the performance of polymer concrete using unsaturated polyester resin. This work investigated whether MMT-UP nanocomposite can be used to produce polymer concrete that exhibits

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B.-W. Jo et al. / Construction and Building Materials 22 (2008) 14–20

excellent mechanical and thermal performance. The following conclusions can be drawn from the results of this study: 1. The mechanical and thermal properties of the composites, which have their maximum tensile strength, tensile modulus, and Tg with 5% nano-MMT, were dramatically improved by the addition of nano-MMT dispersed in the polymer matrix. Also, the elastic modulus of MMT-UP nanocomposite was enhanced by the addition of nano-MMT. However, beyond a certain MMT content (approximately in the range of 5–7%), the mechanical and thermal performance of the composites decreased with increasing nano-MMT content. 2. In composites with Na+, the mechanical and thermal properties did not show a significant change because the degree of exfoliation is less than that of Cloisite 30B-UP nanocompostites. 3. The strength and elastic modulus of the polymer concrete was enhanced by the use of exfoliated MMT-UP nanocomposite. It is important to note that the exfoliated MMT-UP nanocomposite greatly affects the performance of the polymer concrete. Also, the strength and elastic modulus of polymer concrete was found to be positively correlated with the tensile strength and tensile modulus of the MMT-UP nanocomposite.

4. Unsaturated polyester resins made with recycled PET and nano-MMT may be used to greatly enhance the performance of polymer composites at a relatively low cost. References [1] Pinnavaia TJ, Beall GW. Polymer–Clay Nanocomposites. John Wiley & Sons Ltd; 2000. p. 127–49. [2] Hasegawa N, Kawasumi M, Kato M, et al. Preparation and mechanical properties of polypropylene-clay hybrids using a maleic anhydridemodified polypropylene oligomer. J Appl Polym Sci 1998;67(1):87–92. [3] Aranda P, Ruiz-Hitzky E. Ionic conductivity in layer silicates controlled by intercalation of macrocyclic and polymeric oxyethylene compounds. Electrochim Acta 1992;37(9):1573–7. [4] Giannelis EP. Nanoscale, two-dimensional organic–inorganic. Adv Chem Ser 1995;245:259. [5] Okada A, Usuki A, Kurauchi T. Polymer–clay hydrids. ASC Symposium 1995;585:55. [6] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites; preparation, properties and uses of a new class of materials. Mater Sci Eng R 2000;28(1/2):1–63. [7] Kornamann X, Berglund LA, Sterte J. Nanocomposites based on montmorillonite and unsaturated polyester. Polym Eng Sci 1998;38(8): 1351–8. [8] Farahat MS, Abdel-Azim A, Abdel-raowf M. Modified unsaturated polyester resins synthesized from poly (echylene terephthalate) waste 1 synthesis and curing characteristics. Macromol Mater Eng 2000;283: 1–6.