Polyurethane and unsaturated polyester hybrid networks - 2

Polyurethane and unsaturated polyester hybrid networks - 2

(Parte 1 de 2)

Polyurethane and unsaturated polyester hybrid networks: 2. Influence of hard domains on mechanical properties

Ludovic Valette, Chih-Pin Hsu*

Cook Composites and Polymers, Corporate Research and Development, 820 E. 14th Avenue, North Kansas City, MO 64116, USA Received 9 June 1997; revised 17 April 1998; accepted 28 May 1998

Abstract

The influence of hard domains on the mechanical and thermomechanical properties of polyurethane and unsaturated polyester hybrid networks has been investigated. The hybrid networks consist of a polyurethane linkage formed by reacting unsaturated polyester polyol with polymeric 4,49-diphenylmethane diisocyanate (MDI) and free-radical crosslinking through styrene monomer and vinylene groups in the unsaturated polyester. Hard segments were formed by condensing two different types of chain extender, ethylene glycol (EG) and 1,6- hexanediol (HD), with MDI. Incorporation of chain extenders in the hybrid networks varied from 0% to 12% by weight based on the weight of unsaturated polyester polyol. The thermomechanical properties of the polyurethane and unsaturated polyester hybrid networks were characterized by heat distortion analysis and by dynamic mechanical analysis. Flexural three-point bend tests and unnotched Izod impact analysis were used to investigate mechanical properties at ambient temperature. Hybrid networks with hard segments formed by MDI and EG showed an increase in the glass transition temperature. A second glass transition was found with incorporation of more than 6 wt% EG due to the formation of phase-separated hard domains. The rubber plateau of the hybrid networks decreased owing to the lower crosslinking density when chain extenders were incorporated. Phase-separated hard domains enhanced the rubber plateau by acting as physical crosslinks in the hybrid network until the glass transition was reached. The hybrid network had improved mechanical properties when more hard segments were added into it without creating the phase-separated hard domains. A dramatic drop in mechanical properties was observed for the sample with a two-phase structure. q1999 Elsevier Science Ltd. All rights reserved.

Keywords: Unsaturated polyester; Polyurethane; Hybrid networks

1. Introduction

Unsaturated polyester resins are one of the most widely used thermosetting materials in the composites industry. They offer reasonably good mechanical properties and are relatively inexpensive. The properties of the cured resins can be enhanced by adding various additives to the unsaturated polyester. The fracture properties of cured resins can be improved by blending these materials with reactive liquid rubbers [1,2]. The shrinkage that occurs during the crosslinking reaction of the unsaturated polyester resins with styrene can be eliminated by the incorporation of low-shrinkage or low-profile agents [3,4]. Improvements in the thermal properties and elastic modulus have been investigated recently by creating a polymer network of unsaturated polyester and bismaleimide resins [5]. The mechanical properties of the unsaturated polyester resin can be greatly improved by incorporating the polyurethane linkage into the polymer network. The mechanical properties of polyurethane and unsaturated polyester hybrid networks also can be altered with the techniques used in segmented polyurethanes. Segmented polyester polyurethanes are well-known materials that have been studied extensively by Cooper and co-workers [6–1] as well as other researchers [12–14]. The polyurethane and unsaturated polyester hybrid networks consist of soft segments from the crosslinked unsaturated polyester and hard segments from the extension of a diisocyanate with a low-molecular-weight diol. The structures of hybrid networks are different from those of segmented polyurethanes. The polyurethane and unsaturated polyester hybrid networks are thermosetting materials, while segmented polyester polyurethanes are usually thermoplastics.

The unique structure of the polyurethane and unsaturated polyester hybrid networks mentioned in this study was discussed in a previous paper [15]. The soft segment is composed of a styrene/unsaturated polyester crosslinking network and forms the continuous phase for most of the

samples. The hard segment comprises a polymeric 4,49- diphenylmethane diisocyanate (MDI) condensed either with ethylene glycol (EG) or 1,6-hexanediol (HD). The hard segment can be either the overall polyurethane segment or simply the chain-extended polyurethane [15]. If two or more hard segments stay next to each other through hydrogen bonding, they could form a region rich in hard segments called a hard domain. The hard domains may be dispersed in the soft domain, forming a phase-mixed structure, or they may coagulate and create phase-separated hard domains. The phase-separated hard domains may also contain soft segments because they are linked to the hard segments through urethane linkages in the hybrid network.

The previous study investigated the influence of hard domains on phase structure and its influence on thermal properties [15]. This paper aims to determine the influence of hard domains on the mechanical behaviour of polyurethane and unsaturated polyester hybrid networks. Flexural tests and impact analysis were run at ambient temperature. Thermomechanical properties were investigated by heat deflection analysis and dynamic mechanical analysis.

The elastic behaviour of polymer networks can be described by either the affine or the phantom network model. For the affine network model [16], the shear modulus is given by:

where n is the molar number of elastic chains per unit volume of network, R is the gas constant and T is absolute temperature. The phantom network [17] also considers the effect of elastically active junctions. The shear modulus of the phantom network is lower than that of an affine network and is given by:

where m is the molar number of elastically active junctions per unit volume of the network. The phantom network usually describes the elasticity of perfect networks.

However, most of the networks have less than perfect elasticity in real networks. Non-idealities such as pendant chains will decrease n and entrapped entanglements will increase n [18]. The molecular chains will also interact with each other and reduce the junction fluctuations. In the case of strong interactions, the junctions do not fluctuate at all and are displaced affinely with macroscopic strain

[19]. The shear modulus is equal to Gaff. Because strong interaction among molecular chains exists in the hybrid network, the shear modulus G of the hybrid network will be correlated by the affine network model as:

G¼ rRT where r is the network density at the given temperature T and hMci is the number-average molecular weight between crosslinks.

2. Experimental 2.1. Materials

Polyurethane and unsaturated polyester hybrid networks were made with the same materials as described in a previous paper [15]. The unsaturated polyester polyol resin was made by reacting an excess of diethylene glycol with 50 mol% of isophthalic acid and 50 mol% of maleic anhydride. It was dissolved in styrene monomer with a 76% solids content. The modified 4,49-diphenylmethane diisocyanate (MDI) was provided as a 90% solids content solution in styrene. The isocyanate content of the MDI solution was 20.5%. Chain extenders were two a,q-aliphatic diols: ethylene glycol (EG) and 1,6-hexanediol (HD). Various amounts of chain extender were blended into the unsaturated polyester polyol in styrene solution before preparing the hybrid network. All materials were used as-received without further purification.

The polyurethane and unsaturated polyester hybrid networks were prepared by reacting the unsaturated polyester polyol solution with MDI solution and about 1.5 wt% of benzoyl peroxide at ambient temperature. The reaction between MDI and hydroxyl groups took place first, forming the polyurethane linkage. The crosslinking reaction eventually occurred through the unsaturated polyester and styrene monomer. Two sets of samples were prepared with various EG or HD contents and their compositions are listed in Table 1. Clear castings about 3 m in thickness were prepared by moulding the samples between two glass plates. The clear castings were cured at ambient temperature for 24 h to convert the isocyanate groups fully and then postcured at 1208C for 1 day to complete the unsaturated polyester/styrene crosslinking reaction. The clear castings were cooled slowly for 2 h to ambient temperature after postcure. Quenched samples were prepared by heating postcured samples to 2308C and then quickly cooling them in cold water.

Table 1 Composition of the samples used in this study

Chain extender Wt%a Sample name Molar ratiob

aWt% calculated with respect to the unsaturated polyester polyol solution. bMolar ratio based on pure products (without styrene): (x:y:z) ¼ unsaturated polyester resin:modified MDI:chain extender.

2.2. Heat distortion analysis (h.d.a.)

A Tinius Olsen model HD 94 automatic deflection temperature tester was used to measure the heat deflection temperature. A beam, 127 m 3 12.7 m 3 3.2 m in size, was cut from the cast panel. The beam was placed in the test set-up, which consists of two supports with 102 m span and a loading nose midway between the supports. The setup was submerged in an oil bath, and the oil bath heated from 258C to 2008C with an initial soak time of 5 min at a heating rate of 28C min¹1. The heat distortion temperature (HDT) was detected by a linear variable displacement transducer as the temperature at which a 0.254 m deflection (2%) under a load of 1.8 MPa occurred. The reported HDT was an average of two measurements. Experimental variation was usually lower than 6 18C.

2.3. Dynamic mechanical analysis (d.m.a.)

A Rheometrics dynamic analyser RDA I in the oscillatory mode was used to measure the storage shear modulus (G9), the loss shear modulus (G0) and the phase angle as expressed by tan d. Torsion bars about 40 m 3 12 m 3 2 m in size were cut from the cast panel. D.m.a. tests were conducted at a constant frequency of 1.1 Hz and a strain of 0.05 or 0.2%. The first series of tests was run from ¹ 1408C to 2508Ci n 58C steps to get an overview of the thermomechanical behaviour of the polyurethane and unsaturated polyester hybrid networks. Each sample was soaked for 1 min at the measuring temperature before each measurement was taken, to allow the torsion bar to reach thermal equilibrium at the measuring temperature. A second measurement was conducted from ambient temperature to 2208Ci n4 8C steps to provide better resolution. The averages of results from both measurements are used in the discussion.

In order to apply the time–temperature superposition principle and to build master curves, d.m.a. tests were also conducted at the frequency range from 1 Hz to 100 Hz and the temperatures from 468C to 1468C. Ten data points were measured for each frequency decade, and the temperature step was 108C. Data were shifted according to the Williams–Landel–Ferry equation to form a master curve with a frequency range from 10¹2 Hz to 1012 Hz. Data shifts were based on a reference temperature of 1068C. The storage shear modulus was chosen as the shifting variable. Data shifts were completely automated by the computer software provided by the Rheometrics RDA I in the Guess shift mode.

2.4. Unnotched Izod impact test

The unnotched Izod impact strength of each sample was tested following the test procedures described in ASTM D 256-8 [20]. All samples were tested unnotched so they would be more sensitive to the transition between ductility and brittleness. As mentioned by Nielsen [21], a notch tends to decrease the apparent ductility of a material and often has a greater effect on ductile materials than on brittle ones. Specimens about 64 m 3 13 m 3 3 m in size were tested at ambient temperature (238C). The specimen was held as a cantilever beam and was broken by the single swing of a pendulum. A 2 lb pendulum was used for most of the samples, but samples with higher impact resistance were tested with a 10 lb pendulum. At least 10 specimens were tested for each of the samples to ensure the accuracy of the results. The fracture surface of the tested samples was examined under an optical microscope and micrographs were taken with a video imaging system.

2.5. Flexural three-point bend test

The standard flexural test method, ASTM D 790-86 [2], was used to determine the flexural properties of the hybrid networks on an Instron Universal Tester model 1125. Specimens were cut and tested at ambient temperature (238C). A constant crosshead speed of 1.3 m min¹1 was used with a sampling rate of 3 points s¹1. At least five specimens were tested for each sample. Stress at yield and strain at yield were defined at the maximum load point. Calculation of the flexural Young’s modulus was performed by the computer software provided by Instron from the initial linear region of the testing curve.

3. Results and discussion 3.1. Thermomechanical analysis

Plots of tan d and storage shear modulus G9 as a function of temperature for the polyurethane and unsaturated polyester hybrid networks with EG incorporation are shown in Fig. 1(a), Fig. 1(b). Four peaks were observed on the tan d curve: a, b, ga, and gb, listed in order of appearance with decreasing temperature, as described in Fig. 1(a). Two small relaxations, ga and gb, were present at about ¹ 808C and ¹ 1408C respectively for all samples. The lower-temperature relaxation, gb, was assigned to localized motion in the methylene sequences [9,23], whereas the higher-tempera- ture relaxation, ga, was related to some motion of the phenyl groups in the styrene sequences [24]. The b transition was due to the glass transition of the soft domain. The hightemperature a transition was from the phase-separated hard domains, which existed in hybrid networks with EG contents above 6 wt% only. The higher the amount of EG incorporated, the bigger the shoulder from the a transition.

The transition temperatures determined by h.d.a., d.m.a. and modulated differential scanning calorimetry (d.s.c.) [15] are listed in Table 2. The glass transition temperature determined by the b peak of tan d was about 208C higher than the Tg found by modulated d.s.c. for all samples. However, the temperature of the b peak of loss modulus G0 was very close to the Tg determined by modulated d.s.c. and by h.d.a. This temperature also represented the yield point of storage modulus G9. The a transition temperature of tan d measured by d.m.a. agreed with the second Tg from the phase-separated hard domain as measured by modulated d.s.c. [15]. EG increased the b transition temperature of the hybrid network by 16.88C between REF and EG12, whereas HD-extended segments showed an increase of less than 18C between REF and HD10. A greater increase in the b transition temperature was observed before the phase-separated hard domain was formed than after the phase-separated hard domain was formed. A 10.38C increase was observed between REF and EG6, while the increase between EG6 and EG12 was only 6.58C. The phase-separated hard domains act as crosslinking and/or filler for the soft domain until the temperature is higher than its glass transition [25]. This could explain the increase of the b transition temperature even when the weight fraction of hard segments dissolved in the soft domain remained constant [15].

Plots of the storage shear modulus G9 vs. temperature for various EG samples are given in Fig. 1(b). The glassy

Fig. 1. D.m.a. curves of REF, EG3, EG6, EG8 and EG12: (a) tan d; (b) storage shear modulus, G9.

plateau was a constant for all HD and EG samples at 1.5 GPa. This value agrees with the storage shear modulus of segmented polyurethanes found in the literature [26,27]. The G9 rubber plateau varied with the content of hard segments. The more chain extender incorporated, the lower was the ultimate rubber plateau due to the change in crosslinking density. The weight fraction of unsaturated polyester polyol in the resin mixture decreased when more chain extender was added to the system. Since crosslinks come from the reaction between unsaturated polyester and styrene, a lower crosslinking density is expected with higher incorporation of chain extender. To correlate the rubber plateau modulus with the hybrid resin composition, the number-average molecular weight between crosslinks, hMci, was chosen for the calculation. hMci was calculated from:

where wUPR is the weight fraction of pure unsaturated polyester polyol resin. The value of 422.4 is the number-average molecular weight between unsaturations as calculated from the polymer composition. Eq. (4) does not consider the effect of pendant chains which exist in the hybrid network.

Table 3 lists the hMci values of all samples calculated from Eq. (4).

The relationship between G at the rubber plateau and hMci as given by Eq. (3) was examined at 120 and 1808C. The entire hybrid network was in the rubbery state at 1808C, while only the soft domain of the phase-separated hybrid network reached the rubbery state at 1208C. The hybrid network density was calculated from the specific thermal expansion of 5 3 10¹4 cm3 g¹1 K¹1 [28] and a network density of 1.20 g cm¹3 at 258C. The calculated hybrid network density was equal to 1.14 g cm¹3 at 1208C and 1.10 g cm¹3 at 1808C. The rubber storage shear modulus, G0, was used in the comparison since the phase angle was small at both temperatures. The comparisons of measured and calculated rubber storage shear modulus, G9, as a func- tion of 1/hMci are shown in Fig. 2. A very good correlation between the measured and calculated results was observed for the phase-mixed system. However, the predicted results were slightly higher than the experimental results. The difference is due to pendant chains which were not included in the hMci calculation. The pendant chains should decrease n and result in a lower G0 as described by Patel et al. [18]. The intermediate storage shear modulus measured at 1208C for phase-separated samples was much higher than the value predicted by Eq. (3). This is due to the existence of phaseseparated hard domains. The phase-separated hard domains act like physical crosslinks within the rubber soft domain

when the temperature is lower than their Tg. Fedors described the phenomenon as ‘virtual crosslinking’, although no chemical crosslinks are present [29]. Therefore, the number-average molecular weight between crosslinks was smaller than the value calculated by Eq. (4).

The phase structure of the hybrid network can be changed by quenching the sample from the rubbery state to the glassy

Table 2 Transition temperatures determined by h.d.a., d.m.a. and modulated d.s.c. (in 8C)

HDT D.m.a. D.s.c.

Peak b of tan d Peak a of tan d Peak b of G0 Peak a of G0 Peak height of b trans. in tan d Tg aSecond Tg.

Table 3 Average-number molecular weight between crosslinks, hMci (in g)

Incorporation of chain extender (wt%) EG HD

Number-average of molecular weight between unsaturations for pure unsaturated polyester polyol ¼ 422 g/CyC. aREF.

state as described in the previous paper [15]. Fig. 3 shows G9 and tan d vs. temperature for EG12 before and after quenching. The hybrid network changed from a phaseseparated system into a phase-mixed system after quenching. The high-temperature a transition was not present on G9 nor tan d curves after quenching. G9 yield as well as tan d peak were much broader for the quenched samples. The half-height width of the tan d peak changed from 408Ct o6 08C for EG12 after quenching. The height of the tan d peak also increased after quenching. These results indicate that the volume fraction of the soft phase was higher since the hard domains were dissolved in the soft domain instead of forming phase-separated hard domains.

(Parte 1 de 2)

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